The Delayed Choice Quantum Eraser, Debunked

[@pbsspacetime]

Sabine, this is amazing. You are, as usual, 100% right. The delayed choice quantum eraser is a prime example of over-mystification of quantum mechanics, even WITHIN the field of quantum mechanics! I (Matt) was guilty of embracing the quantum woo in that episode 5 years ago. Since then I've obsessed over this family of experiments and my thinking shifted quite a bit. And then I procrastinated on filming the retraction! Thanks for laying it out more clearly than I could have. I have some thoughts to share that might add one more nail to the coffin ... coming to a video real soon!

[@rayfangrui]

This is the kind of mutual respect that real scientists have for each other!

[@AndronikosKoutroumpelis]

Congrats for your response Matt, others would be "insulted"

[@eewls]

I learned to trust my intuition from QE. If the particulars of the experiment design confuse me too much, there must be something off.

[@frede1905]

Huge respect for your honesty. I am looking forward to your extra info on the topic!

[@IncompleteTheory]

In the spirit of Space Time videos without the comment checks: "The 2nd part of Matt's response was quantum erased and will have been restored in some past comment."

[@mureebe1]

"Ackchyually", upon returning to point A, the captain can be any age 46 or above, as we do not know the length of the voyage.

[@nictamer]

The voyage is 100 km, you didn't pay attention! What we really need to know is how close is his birthday.

[@K9Megahertz]

Also need to know his speed, that wasn't specified. Could have taken him 2 years to go 100km. I'm sure that's not the case, but it is a possibility.

[@nictamer]

@@K9Megahertz a container ship will typically go at about 10 knots at their minimal cruising speed, so it will do 100 km in just a few hours. Unloading can take a few days depending on circumstances, so the speed is not significant.

[@jamiegagnon6390]

@@K9Megahertz You are still trying to parse all of the irrelevant information. The only thing of relevance is the stated age of the captain. It says the captain is 46 years old, not was.

[@jonathansturm4163]

@@jamiegagnon6390 Thanks. I used to teach my clients how to think. I’d give everyone in the group a double-sided page of questions. At the top of the first page was the instruction: read through all of the questions before proceeding to write answers. The last line of the the list stated: “do not answer any of the questions. Just write your name at the top of the page and sit quietly for a few minutes.”

[@woogemooge]

I’ve watched a dozen hours of your content. SO amazing. I got curious about double slit expt after coming across a Short from another channel. And then searched for double slit content and found your channel. You had the most accurate and precise explanation of things - the other videos were approximating some of the details and were therefore confusing me. Awesome channel, love it.

[@UnderTheGoldenGate]

The problem with many of these 2-slit explanations is that nobody I have ever encountered when talking about this has described what they mean by "measured which slit the particle went through". How do you do that? In order to "see" or "measure" a particle you need to have it interact with another particle. This in turn will change the particle, e.g. the wave function collapses. (e.g. when you see something with your eye, light is bouncing off the object). The mystery here is why nobody (nobody I've ever met) has yet answered the question how you measure the particle, not so much the mystery of quantum mechanics. We need to start there before proceeding to the quantum eraser issue. And then we can ask after the particle comes out of the slit and hits that material, it's similarly interacting with the material?

[@UnderTheGoldenGate]

@@schmetterling4477 Thanks, but I'm not following and I don't see how this clears this up. I still maintain that we need to very clearly answer this in order to understand the impact of "measuring".

[@bongoh2607]

Really late reply but yeah I also agree with you, have been thinking this all the time too without seeing anyone raise it. I'd also go further and add that I find statements like 'if you fire a single photon somewhere' very hard to believe as... Can we really control that?

[@manlyadvice1789]

Most professionals don't talk about it publicly because they don't want to ruin the sales pitch and lose their funding. Have you ever seen an interference pattern for yourself? I have. It's pretty easy to demonstrate. According to QM, that would be impossible if you count as a quantum observer. That's a really important point. The field around a "non-interfering" photo-sensor (placed next to the beam of light) counts as a quantum observer, but YOU looking through your eyes does not. Only hacks and salesmen talk about the "observer effect." Professionals talk about the "measurement problem." They know that, in the context of an experiment, the "observer" is operationally defined as a particular piece of equipment. It's an equipment effect. That's what ruins the sales pitch. Does QM have anything to do with consciousness? Nope. Stare at the slits all you want and it won't make any difference. If we think about the many worlds interpretation, the thing that would create new universes is turning the sensor on, not you observing something. If everyone knew that, would they get as much public support? I would guess probably not. It's an open secret.

[@victorsilvapacheco9630]

ChatGPT says you throw electrons and you measure them by using photons, those photons have to be very "weak" so that they don't have an effect on the electrons and can't "push" them

[@NivuagYdna]

How the measurement is made matters the most because that's where things go sideways. It's easy enough to figure out, just move the sensor around to see if the results change. If they do, then you know the sensor is interfering. Or, you can tweak the sensor in other ways. My suspicion is the measurement is done by blocking one slit randomly for each shot. In which case, the results make sense, even for a wave.

[@liesdamnlies3372]

I’m glad you recognized the most important thing here: You can never have enough videos of kittens in boxes.

[@Dziaji]

The prime rule of the internet.

[@spiralsun1]

✊🏻❤️‍🔥 Love wins! Unleash your kitties!

[@Lurieh]

As long as they're not Schrödinger's experiments.

[@KawazoeMasahiro]

She should make one!

[@NotAvailable_na]

😮 🤔 Lol 😆😉

[@sgalas1]

So the combined photons detected by D3 + D4 produce a non-interference pattern. But why, when viewed separately, do they each produce an interference pattern at all? Why not just a random subset of photons from the combined pattern (since the detections at D3 and D4 are randomly determined by a beam-splitter)?

[@The_Trojan]

Yes, I would love to have Sabine explain that. As far as I can tell it is a big hole in the explanation given in this video. Surely if Sabines logic is correct both measurements should result in similar blobs (that just take twice as many photons to produce as 50% are excluded).

[@kanein3d]

Exactly what I was thinking, D3s pattern would just be the same pattern as D4 except with half the photons removed. If it’s an interference pattern, combining both D3 and D4 would be the same pattern just twice as strong.

[@dunbabin]

@sgalas1 - I had the same question!! But I think I have it now... I don't know much about beam splitters, but most things can not make "random" choices. She says "the interference pattern really comes from selectively disregarding some of the particles". I think she is referring to the beam splitter causing the pattern...

[@argfasdfgadfgasdfgsdfgsdfg6351]

I thought the same thing. Needs clarification!

[@yourfutureself4327]

"The non-quantum delayed choice eraser" by craig gidney might help

[@hoagie911]

In case anyone's something, as of February 2024 PBS Spacetime still hasn't taken their video down. They haven't even edited the description or written a pinned comment to indicate their explanation was misleading. And they certainly haven't released a new video retracting what they said. Extremely disappointing behaviour.

[@johneagle4384]

"I think they are all wrong.." You've got to love Sabine. I believe the number of friends she has decays as fast as Fr-87.

[@Webfra14]

Maybe, but the ones that are still there, will last forever.

[@sjzara]

The thing about science is that disagreements are commonly friendly. What looks like an attack on someone is nothing more than a challenge to an idea.

[@rreiter]

@@sjzara Imagine if Nature allowed video submissions for the correspondence in "Matters Arising"...

[@johneagle4384]

@@sjzara I am only joking....But, some people are weird, and some do not like to hear things like "you are wrong". Some take offense, specially Physics PhDs who have been working on a particular field for many decades. By the way, I've seem mature scientists at each other's throats because of scientific disagreements. This behavior is becoming more common.

[@pedrolmlkzk]

friends are passing, truth and integrity is forever

[@DeSinc]

I've looked on several occasions for a video like this to describe what is actually going on but all I could find was videos of people saying the particles act differently based on if you observed them or not which just always frustrated me to hear. finally now I have a video that I don't completely understand but at least can point to when people bring this topic up.

[@Lucky10279]

If you're still struggling to understand, this video is also really good: kzbin.info/www/bejne/n6qxY2qIbLp3pKs

[@_whatthefacts_]

I always point my mates to "Glitches and tricks in Half Life 2" when I need to explain quantum weirdness.

[@sashas3362]

@@_whatthefacts_ The unexplainable disappearance of the wavelike interference pattern of the single particle double slit experiment when the which-way info is detected has been cited as proof of "simulation theory" (the theory that reality is a computer simulation). It's claimed this otherwise unexplainable phenomena can easily be explained/understood as a more detailed rendering. The wavelike interference pattern is explained as a less detailed rendering which appears when you aren't looking at which slit the particle went through. It's like a crappy rendering engine with noticeable "glitches".

[@pflaffik]

DeSinc, Sabine doesnt question that the particles act differently based on if you observed them or not, thats a fact and she confirms it in this video.

[@quattrocity9620]

@@pflaffik That's not what I got at all here... The particles didn't behave differently , we just observed them differently. I could be wrong but it seems she was saying that we observed the particles without the which way information. Don't murder me on here, I'm obviously not a quantum physicist and it's possible (if not likely) I misunderstood this video.

[@PSG_Mobile]

I still dont understand why D3 and D4 have complementary patterns that look like interference instead of complementary patterns that look like normal distribbution.

[@AdamJMilward]

Me too. Since the half mirror randomises the photons that go to D3 and D4 why do those randomly selected groups of photons each produce an interference pattern.

[@ThePinkus]

Below is the very same post as I used in the reply to a very similar question elsewhere. It is not a satisfactory answer. The reference is to equation 10 in arXiv:quant-ph/9903047v1 (figures 3 and 4 in the same). Note: in the article the eraser path has D1 and D2, which is opposite to the notation in the video. The setup is in figure 2. I would lie if I told You that I can really follow through the derivation. What I can sum up: the conclusion is that there is a phase shift between the two patterns in (10), one with cos^2( x pi d / lambda f) the other with sin^2([same]), so that when we sum the two patterns these two terms sum to one and only sinc^2(x pi a / lambda f) remains (which looks as a blob with no interference-like bands); in turn, this comes from eq. 4, where "the different sign between the two amplitudes [...] is caused by the transmission-reflection unitary transformation of the beamsplitter BS [ref. to rig. 1 and 2]". I think this is also to be considered in combination with the computations after eq. 4, which include the paths' lengths. If I understand it correctly, it is a matter of different path lengths combined with the different effects on the phase of transmission-reflection at the beam splitter that causes the shift between the patters, but I might be wrong.

[@PSG_Mobile]

@@ThePinkus thanks for answer! So, if you are right, this interference pattern has nothing to do with wave functions, it is just Optics.

[@ThePinkus]

@@PSG_Mobile Current optics is done by wavefunctions, in fact the computation we find in the article derives the results as a "standard quantum mechanical calculation" (pg.2 just before eq. 1), and the equations are not classical wave optics, but quantum field theory. So, correct, it is optics, hence, of course, it has everything to do with wavefunctions. Best regards!

[@PSG_Mobile]

@@ThePinkus thanks!

[@ebehdzikraa3855]

I don't know whether: 1. she fully understand about DCQE thus can explain it simply or 2. she fully didn't understand about DQCE down to conceptual level thus overly underestimate the complexity of it or 3. she is in the superposition of both

[@paulyberk]

"You can find any pattern you want if you're willing to ignore enough data." -Matt Parker Wonderful explanation of this experiment and eye opening to me.

[@theoldleafybeard]

And so do many keep on ignoring God within us for as long as they want. 😐

[@FrankJosephKodra]

3:24 you lost me, the difference between getting a wave function from the double slit expirment and getting direct hits is when you measure it going through the split and if you unplug the sensor than you will get a huge wave function, you still get a slight wave function with the sensor turned on because there's space between the split and the photoelectric sheet where you observe the proton which means that it's gets turned back into a wave once it passes the sensor but because there's no two slits after the sensor so it's less random and won't be as spread apart but every once and a while an atom or particle will shoot off to one of the sides more than most particles ,but it's just not as probable, you try to rationalize it by combining the two blobs using math but that just illogical because ofcourse most of the locations are gonna average in the middle because that's just the more probable outcome, and based on our current equations to model the collapse of the wave function this is true, so explain to me how you can rationalize that, the whole premise of your argument is wrong lmfao. Literally if you send particles through one at a time they will end up in different location each time it's just more probable for the to be in the middle but when there's two slits the probabilities collide with each to create a more random outcome lmfao so when you collapse the wave function before the split it's less random because there's less probable outcomes lmfao. Like I seriously don't know what this bitch is on. She still doesn't confront the fact that our observation cause the wave function to collapse which is pretty much what the double split and quantum eraser prove.

[@FrankJosephKodra]

@@theoldleafybeard ik right thank God another based person in this comment section

[@hook-x6f]

*****HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!******* The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. The The three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@dragoscoco2173]

@@FrankJosephKodra This is doable by a 1$ red laser and 2 human hairs on an scotch tape. 2 hairs makes a nice spaced interference pattern, 1 hair makes just a diffraction pattern, which is exactly like a double hair interference pattern just much much wider, as if the two slits were closer (because they are).

[@Disappearation]

Finally someone actually explains it instead of pretending to explain it while mystifying it further.

[@adrianflo6481]

These people rely on the youtube algorithm for money, not being truthful to science. Shrödingers cat has done absolutely nothing for the betterment of understanding in popular knowledge either. Some things should not be simplified down. If someone doesnt contain the requisit capacity to undersand you shouldn't try to teach them. You just end up with confused people who spread false information.

[@trucid2]

Goes to show that even most physicists don't understand the concepts they talk about, they merely parrot what they've heard others say.

[@angrymeowngi]

@@adrianflo6481 I really hate that especially those who present their interpretations and metaphors and then say "the math is correct" so what they are saying should also be true. What a B.S! The math maybe correct but your interpretation is wrong. There is only one type of interpretation which is useful and that science should trust: a geometric interpretation. If you have a mathematical formula, interpret it using geometry to communicate it to the public. And then, make those who are interested to learn more and know its application by teaching them how to do your math. Other than that is how we get multiverses and holograms that don't even make scientific sense. Where is Popper's science now?

[@circumsizedmind]

@@adrianflo6481 wow, now you are bringing the irreducible complexity. Btw, there are a lot of PhDs in photos that didn't grok the quantum eraser experiment. Or at least not many demonstrated a good comprehension

[@circumsizedmind]

@@trucid2 there was a joke about maths or physics professors explaining a topic and asking the students is they did understand everything. After they said yes, he looks at the board and says :and I after 20 years still don't fully. understand it....

[@feynstein1004]

Everyone: The delayed choice quantum eraser is impossible to understand. Sabine: Hold my Bier

[@jirijelinek2038]

More like "Hold my beer, I'll present Sean Carroll's explanation."

[@feynstein1004]

@@jirijelinek2038 Fair enough 😂

[@p-j-y-d]

Not fair. The didactic value here is higher.

[@Sharperthanu1]

@@jirijelinek2038 Richard Feynman says that Sean Carroll's explanation is crap so forget it.

[@brian.josephson]

Listen at 0.75 speed and it'll be clearer.

[@lorrygoth]

If particals move through dimensions in a wave then when they are measured we only see the point in the wave where we are measuring them from? The rest of the wave is obscured by time? So by making a choice of where and when to measure you aren't changing the outcome you are just limiting the possible outcomes to a single outcome based on time.

[@patrickegan8866]

Would love to see someone film the actual experiment rather than just seeing it animated. Thank you for the clear explanation

[@AashishVishwakarma]

I've seen the exact experiment, and I personally think the animation does more justice to explanation than the actual video. Personally I think animation works very well for College or even early PhD level students to get a better sense of what's going on. Of course, people working on this domain may want to know more about the set-up.

[@adventures_w_otto1370]

SHHHH. If you talk about that out loud ''THEY'' might hear you. ᶜᴾᵁ > ᴳᴾᵁ > ᵀᴴᴱ ᴰᴿᴱᴬᴰᴱᴰ ˢˢᴰ ᴬᴿᴱ ᴬᴸᴸ ᴬᴺᴵᴹᴬᵀᴱᴰ. [ᴮᴱᵂᴬᴿᴱ ᵂᴬᵀᶜᴴᴵᴺᴳ], ᴴᴼᵂ ᴬ ˢˢᴰ ᵂᴼᴿᴷˢˀ ʸᴼᵁ ᶜᴬᴺ'ᵀ ᶠᴵᴺᴰ ᴬᴸᴵᴱᴺˢ ᴮᴱᶜᴬᵁˢᴱ ᴬᴸᴵᴱᴺˢ ᴬᴿᴱ ᴺᴬᴺᴼᵎ ''ᵀᴴᴱ ᶠᴵᴸᴱˢ!'' kzbin.info/www/bejne/gpDSkIJtq5aYrdU

[@howdareu7529]

i would like to see "globe from space,not cgi ed

[@btd6vids]

I know this is super late but it’s actually possible to do at home, you just need a laser pointer as the only thing you might not already have, and you can get those for like 5 bucks. “I did this by taking a piece of glass and holding it over a candle to get a nice thick layer of black soot on it, then holding two x-acto knife blades parallel to each other and scraping two nice thin lines right next to each other in the soot.” You can probably do this with razor blades or anything similar to it, and you can even use aluminum foil instead of the glass (but be aware that the foil will tear super easily)

[@btd6vids]

As for the “detector” you just need to shine it at a wall or a piece of paper or something. Now for the “measuring individual photon paths” one, that’s a lot harder to do at home

[@blawrence42]

Sabine, love your channel, but I think some clarification is needed here. Yes, I think I understand the explanation being offered here that retro-causality need not be invoked to explain the outcome of the delayed choice quantum eraser experiment. And yes, I read Sean Carroll's article that offers essentially the same explanation that you do here. After all, "we can trust Sean with the quantum stuff" as you so aptly put it. But, Sean Carroll's explanation (and by extension yours) invokes the Everettian "Many Worlds" interpretation to explain away the need for retro-causality. The wave function doesn't "collapse" as it would in the Copenhagen interpretation, but rather there was a decoherence that branched the multiverse along all possible paths. But, the Many Worlds interpretation of quantum mechanics has been something that you've been very highly and very publicly critical of here in the past. If we can really trust Sean Carroll with the quantum stuff, and his explanation invokes Many Worlds to explain the delayed choice quantum eraser, then does it not follow that we should trust Sean Carroll's opinion of the Everettian "Many Worlds" multiverse upon which it depends? "There’s no need to invoke retro-causality to explain the delayed-choice experiment. To an Everettian, the result makes perfect sense without anything traveling backwards in time." - Sean Caroll, "The Notorious Delayed-Choice Quantum Eraser", Sep 21, 2019

[@martir.7653]

My understanding is that one can subscribe to the "many-worlds interpretation", while considering the other universes as just a tool for the explanation, not implying they are "real" in any sense. Wikipedia has this quote from Stephen Hawking: "But, look: All that one does, really, is to calculate conditional probabilities-in other words, the probability of A happening, given B. I think that that's all the many-worlds interpretation is. Some people overlay it with a lot of mysticism about the wave function splitting into different parts. But all that you're calculating is conditional probabilities."

[@xen2]

@@martir.7653 Here is a video about multiple time dimensions: kzbin.info/www/bejne/n5inn6SQfJVgrNU This improves on the "many-worlds interpretation" in some ways

[@markfernee3842]

There's no need to invoke the many worlds interpretation to explain the result. This delayed choice quantum eraser can be shown to be the same as Wheeler's delayed choice experiment. In that case, the detections at D0 just provide an alternative visual output for the experiment, which is nice, but nothing different.

[@blawrence42]

@@markfernee3842 One can always "shut up and calculate" and perhaps this is even one of those times. But, the whole point of having an interpretation is to try to understand what the math is telling us. Yes, different "visual outputs" at D0 is empirically observed, but that alone is just ignoring the most profound aspect of the results. That is that the different outputs seem dependent on the presence, or absence, or arrangement of detectors that are further downstream in the experiment (and therefore reached at a later point in time). Sabine here is accepting Sean Carroll's explanation for the most part and then stopping short at the point where Sean actually brings the argument home that under Many Worlds this doesn't necessitate retro-causality. Sabine is trying to have her cake and not eat it too (to belabor an analogy). Even Sean Carroll in his article on the same topic acknowledges that if you subscribe to other interpretations of quantum mechanics where wave functions are considered to actually collapse then it would be understandable to invoke retro-causality to explain the outcome (though he ultimately disagrees on this point). But Sabine here seems to want to invoke all the math that Sean does, but not the interpretation that Sean does to bring it all home. In essence, Sabine just talks around the point without actually addressing it. Again, from Sean's article cited in Sabine's video: "But alas, not everyone is an Everettian. In some other versions of quantum mechanics, wave functions really do collapse, not just the apparent collapse that decoherence provides us with in Many-Worlds. In a true collapse theory like GRW [...] you can convince yourself that retrocausality needs to be part of the story. Or you can accept the smooth evolution of the wave function, with branching rather than collapses, and maintain time-symmetry of the underlying equations without requiring backwards-propagating signals or electrons that can’t make up their mind." Sean Carroll, "The Notorious Delayed-Choice Quantum Eraser"

[@markfernee3842]

@@blawrence42 the problem is that this experiment purports to be a proof of retro-causality. Retro-causality in quantum theory is an interpretational aspect that can be applied to a range of experiments. It's not just shut-up and calculate that shows no retro-causality in this experiment. One can just run through the detection chain logic without to see this. The photon pair generated in the crystal are momentum-entangled. When D0 is detected, that detection is compatible with the photon having a certain momentum. However, there is more momentum information available from the entangled partner. We just have to wait for it to be detected. Once it is detected, the particular detector, D1-D4, will determine the additional momentum information that is provided. So after the second detection, we can say more about the momentum of the first detection at D0 because we have updated out information. Where is that retro-causal? One of the main problems with how people interpret this experiment is the complete disregard for the SPDC crystal and the effect it has on the interference pattern. It completely washes out the possibility of observing an interference pattern. That's due to the finite thickness (0.3 mm) of the crystal, the wavelength dispersion in the crystal, and the random point of down conversion within the crystal. These effects were included in the Scully's calculations in the paper, but otherwise not mentioned. However, these effects are well known to anyone who works with SPDC crystals in optical circuits. From that aspect, I find this experiment somewhat disingenuous. Furthermore, the paper didn't explain the origin of the two complementary interference patterns. There should only be one Young's double slit interference pattern, not two. This is should be red flag when interpreting this experiment. It is telling use that it is not working they way we expect it does. Finally, one can actually see that this experiment is actually a rather unique version of Wheeler's delayed choice experiment by identifying the interferometer circuit. There is no "eraser", just the delayed choice outcomes as already described by Wheeler. This is rather profound, but already known. I could easily rewrite this experiment titled as, "Wheeler's delayed choice experiment with entangled photons".

[@einfacherkerl3279]

This shows, information from unknown KZbin bloggers is not reliable. Thanks Sabine!

[@augustadawber4378]

In order for the Nature of the sub-atomic Particle to be known, all the results have to be obtained. In order for all the results to be obtained, both the sub-atomic particle and the observer has to 'know' what those results are. If the observer is not aware of all the results, how do we know if the nature of that sub-atomic was changed in the past or not ? If the observer is aware of all the results, how do we know the results are or are not from a particle being influenced in the Past ? As John Wheeler has postulated, wether a photon leaving a Star is a particle or a wave, depends on wether an observer from billions of years in the future observes it as a particle or a wave. Thats even though that particular Star still even exists when it is observed in the far far future.

[@kingo_friver]

I've already given up to understand this topic after watching all those videos several times. Finally, today I got satisfaction, even though I still need to rewatch this several more times. Thank you so much!

[@lubricustheslippery5028]

I also have to rewatch the video. Still don't understand why the detectors can change the interference pattern.

[@needmorespaceformyna]

@@lubricustheslippery5028 In the regular double slit you mean?

[@m.c.4674]

@@lubricustheslippery5028 It doesn't change the Interference pattern . It will help if I explain how the sector works . The detector , when on blocks the light , but when it is off allows the light to pass through , it is literally that simple. Next , inorder to create a interference pattern two beam of light need to over lap with each other and undergoes , destructive and constructive interference . Now I want you take another look at the experiment . You will see that the detector was on ,which prevents the second beam of light to interfere with the first beam of light , thus there is no interference pattern . Now move the detector further back than the screen . The dector being on doesn't affect the light pattern in the future . The light beam simply does what It would normally do when there is no other light to interfere with . At this point you should realize the word trick that was being played on you. I don't only disagree with the idea that causality is broken , I also disagree that the direction of the beams of light are erased , because as you can see the light is slit into two when the detector is off , so if I put a screen their I see light . It is literally that simple.

[@kingo_friver]

@@lubricustheslippery5028 We are enjoying to understand what we don't yet know😊 It's ok that I don't understand all the details, as these science experiment and math are not hands-on for me anyway. But I've just wanted a rational explanation for this fascinating topic.

[@louferrigno4712]

​@@lubricustheslippery5028 Don't over complicate it. On detectors D3 and D4 each detector receives two beams of photons. One of the beams goes through a beam splitter, the other beam does not. The splitter reflects 50% of the wave basically cutting a sine wave in half. The patterns that show up are the gaps in the data from the reflected portion of the wave. It seems to be a test set up to see if you can predict one photon by measuring it's entangled counter part that got misconstrued.

[@markpaterson2053]

Where have you been all these years, Sabine? You are truly the only scientist capable of translating/adapting/simplifying/whatever the problem of grasping things that others deliberately romanticise.

[@stefaniasmanio5857]

She has been working and studying quite hard …

[@massimiliano-oronzo]

S. Hossenfelder is not a superhero. In science some perform experiments and try to give an interpretation, others (rightly) raise criticisms. This is how it works. At the end of the 19th century, almost all physicists thought that light must necessarily propagate through an ether.

[@jeffriesmovies]

There are so many great science communicators beyond your youtube algorithm

[@cristianproust]

"deliberately romanticize" how can that be romanization if it is perfectly consistent with time not being experienced by a photon?. Wouldn't that be the expected result?. What it is a romanization it is to think that this experiment not appearing conclusive to reiterate the result that time is not real as we perceive it, and that it would mean things still have a chance to evolve unexpectedly. People who romanticize having free will have more interest in this experiment being not being conclusive than the other branch that see it as a result perfectly consistent with Relativity

[@justdave9610]

She's one of my favorites because of how she tends towards skepticism and isn't afraid to call bs when she sees it even despite how doing so risks creating animosity with others in science and physics. Also she's really good at explaining physics in a practical, straightforward, and understandable way without dumbing it down too much

[@CalamitousProphet]

I would very much appreciate clarification on the nature of a particle detector in reference to the double slit experiment. It is frequently implied that observation itself is the cause of wave function collapse, rather than any force imparted by the measurement device. This can be pretty frustrating when trying to understand the why behind these things.

[@DavidByrden1]

Relax. No need to be frustrated. Because "wave function collapse" does not exist. You don't need to worry about what causes it. When the particle hits the detector, its wave function becomes entangled with the detector, resulting in a whole continuum of detectors in parallel universes, each registering a "hit" in a different spot. And the effects propagate until there is a continuum of multiple copies of YOU, each seeing a "hit" in a different spot. Nothing collapses. Yes, I know that I sound insane here, but I'm simply telling you exactly what the Wave Function says. This was clarified by Everett many years ago. And once you get used to the "many worlds" interpretation, you become the thing that Feynman didn't believe in : a person who understands quantum mechanics.

[@maxbro16]

@@DavidByrden1 lol wut?

[@kdub9812]

collapse is an illusion, it is an artifact of perception. it only appears to happen given one’s limited appearance of reality

[@karlschmied6218]

You can actually see an interference pattern of a candlelight (which is romantic). Look at a burning candle in a dark room about 4 m from your face through the one gap between two fingers held directly in front of one of your eyes. With a little playing around you will see the pattern. But you'd better not do this on a first date.

[@atlanticx100]

You are the first one on youtube that I have seen to take the magic out with a fuller explanation. Thank you.

[@preppen78]

I'm just happy the kitten was alive

[@vladimirseven777]

The one that turned fuller explanation into working clickbait (I definitely clicked). Others are fine by using magic in their videos to get views and sponsors.

[@DrSharoyko]

not true: kzbin.info/www/bejne/fXfVhGOIq5uCiqc

[@atlanticx100]

@@DrSharoyko Never saw that one thanks.

[@fatfrumos1163]

Check Science Asylum video on this. I think he was the first to demystify the situation

[@NotRodShop]

Most of the experiments that I have seen show 2 distinct vertical lines on the film plate when the "which way" information is measured. The way you are showing this is that there is one big horizontal blur when the which way information is measured. This seems to be the big disconnect in information that needs to be established more than anything else.

[@ThePurza]

The thing I don't understand about Sabine's explanation, is that supposedly the fact that you can't see an interference pattern until you separate them by the path of their untangled pair, supposedly demystifies anything. They do result in an interference pattern when correlated with either of the paths that send the beams down a path that makes it impossible to determine which slit the photons took. it deserves more explanation as to how the merged 'interference' detections perfectly merge to a indiscernible pattern. Sabine has stated that she thinks determinism solves these things in a more recent video. That explanation makes sense to me, and would resolve this also, essentially arguing that 'Determinism' is the 'hidden variable' all along.. that wave functions take future measurements into account.

[@hook-x6f]

***HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!****** The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. The The three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.” What now?

[@samsam2235]

Sabine calling out the YT physics boys. You gotta love it.

[@andrearaimondi882]

Sabine adding Joe among the KZbin physics guys. That's an acknowledgement!

[@isonlynameleft]

@@andrearaimondi882 Joe is not a phisicist so I don't really expect him to have that level of knowledge but if you're a phD physicist you really should know better!

[@alphagt62]

@@isonlynameleft and that’s the thing. So many physicists don’t really understand what they are teaching. They’ve gone through the school, and passed the classes, but they fail to achieve Sabine’s level of understanding.

[@d.t.4523]

If you're any good at physics, you probably have better things to do than sensationalize video content on the web. 👍

[@jonathansturm4163]

@@alphagt62 This!

[@KarmaPeny]

Finally, a Quantum Physicist who appreciates that the obvious explanation to the D.C.Q. Eraser is probably the correct one, rather than adopting the absurd 'go-to' approach that most others use, which is "there must be a choice happening somewhere between acting as a wave and acting as a particle".

[@schmetterling4477]

Most people simply don't know quantum mechanics and they are projecting the state of their misinformation on it. That's no different from any other area of life.

[@Bodyknock]

Just FYI in the two PBS Spacetime videos after the one mentioned here they do talk more about how the two interference patterns combine to form the single blobby image you get from the which-way detectors. In particular he talks about how the interference patterns are actually partitions of the original set of photons based on the characteristics being measured by the combined detectors C or D at the end. So there's no way to know in advance which photon is going to be in which interference pattern until you actually get the measurements from those two detectors. He uses this as an example of why you can't build a "time machine" that sends morse code signals by placing or removing the mirrors at the which-way detectors to send a message back in time based on seeing or not seeing an interference pattern. The image you see is always the same, it's just whether you're categorizing the photons as two combined interference patterns or one blob and you can't know details about the interference patterns embedded in the image until you get the measurements.

[@AashishVishwakarma]

Indeed! Matt even made a competition our of it if I remember correctly!

[@somebodysomewhere5571]

People may get too caught up in the time aspect and fail to realise how mind blowing it is that just by measuring the data is collapsed which is the best part

[@Littleprinceleon]

But why does the separation of photons into those two groups even create a pattern which looks like as if there was interference? Why is it so that those photon-pairs that end up in one of the "after erasure" detectors can not hit certain locations on the screen? And the other half - in opposite - hits these bands on the screen, so the second pattern is shifted exactly to match the "negative" of the first: except - if I understood correctly from a real published study and not some interpretations - the two patterns are almost the same just shifted...

[@janPeja]

@@Littleprinceleon As far as I understand it, it's like releasing rabbits towards a carrot and a lettuce pile. Later you measure the color of their poop and realize all green poop rabbits interfered with lettuce pile and orange poop rabbits with carrot pile. It's not like measuring their poop caused them to retroactively choose a certain pile. :D

[@Littleprinceleon]

@@janPeja but why did half of them choose one pile at all? The paths to the "erasure" detectors are symmetric: only difference is that the photons leave the BBQ crystal at two different sites depending on the slit they "went through". That's what Sabine emphasizes: the which way information is "detected" and transferred by the crystal (absorption, reemission). That in the end we combine the information doesn't seem to "erase" anything. We only obscure the direct info on the path for ourselves. So another relevant Q: are the particles distributed in the same two patterns on the "screen" if we detect their entangled pairs at the earlier "which way" detectors? And we just cannot obtain the right information with this setup of the detectors? Is the result random and the pattern emerges because of some property (phase? polarization?) of the photons? Just like with coins: two possible outcomes inherent to the object, evoked by random influences... Is this property dependent on their path ? Even if, why are there discrete bands? The "bomb experiment" implies that whenever the photon has a choice, eg. at the semitransparent mirror (beam splitter) in reality it goes through both ways: even interferes with itself... But in the case of the DCQE this happens to that part of the entangled pair detected by the "erasure" sensors. Does the entanglement influence all the information contained in the wavefunction? How big/long can be one quantum of electromagnetic wave? It seems that amongst chlorophyll molecules a photon takes many, many different paths until it "finds" the energy center.

[@PADARM]

Finally! The fact that you are not afraid to debunk the videos of your colleagues speaks volumes of how much integrity you have as a Scientist. That's what science is about! Science is not about friendship, It's about the truth. Congrats Sabine!

[@donniseltzer7718]

IKR. Sabine Rocks!

[@TheBrightmanFan]

That's what I love about her

[@hook-x6f]

*****HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!******* The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. All three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@hook-x6f]

***HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!****** The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. The The three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@theshermantanker7043]

She's all fine and good until it's her theories that are on the line. You'll see a different side of her when that happens

[@X_Baron]

After reading Carroll's explanation, my understanding is that the error in the other videos is this: they show a stripe pattern appearing on the detector screen, but in actuality it doesn't. You can create a visualization after-the-fact, showing which parts of the blob pattern correspond to which measurements, and this way create an interference pattern.

[@ggg148g]

Would you please give the reference to the book or article by Carroll you're quoting? Thanks

[@X_Baron]

@@ggg148g The link to his blog post is in the video's description. It's apparently a section of his book that he had to edit out.

[@finelinerin]

Danke, vielen Dank! Ich kehre immer wieder hierher zurück, wenn ich verwirrt bin. Super Präsentation. Klar und einleuchtend. Und die Didaktik im Video - einfach genial 🙏🏻

[@BluesDoctor]

As usual, Sabine presents her deep insight in a conceptual context that is easier to grasp, just pay attention. A gifted communicator the likes of which are quite rare. These concepts make the math and statistical probabilities easier to grasp and interpret from the bigger picture. A delight for the young pup and old dog alike.

[@hook-x6f]

SABINE NONSENSE IS DEBUNKED BY NOBEL PRIZE 2022 WHICH PROVES ENTANGLEMENT IS REAL! The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. All three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@ericknight5014]

Thank You Sabine. I love you. I have been puzzling about this for years and can give my brain a rest and stop worrying about a patent application for long distance instant communication.

[@bloodswacky94]

Here is how I understand the weirdness of this experiment, which is in my view a little bit understated in this video: Assuming a single photon is emitted from the source travelling through the double slit. Then, following Sabine, either the blue path or the yellow path is taken, whereby these paths are decoherent. Now the wave function of the upper photon will look like a one slit diffraction pattern and hits the screen with the according probability at which point a second decoherence is taking place: Either the photon hits a crest of the "interference" pattern of D3 or of D4. This information is via "Einsteins spooky action at a distance" transferred to the second photon still travelling. Now the weird part: This means the second photon does not hit D3 and D4 with the same probability as the setup implies, instead it hits D3 with a greater probability if the photon was detected at a D3 crest and vice versa. If this is how the experiment actually works (I can't explain it otherwise), I would be greatly interested which state the second photon takes, such that the probability hitting the detector D3/D4 is no longer 50/50.

[@willhastings731]

Way late to this comment, but this is the same question I walked away with. Sabine simply stating that you can select a sample, just like picking pennies from a pile, doesn't explain how/why detectors 3 and 4 show this bias. This point can't be trivialized if the argument is meant to trivialize the "sending information back in time" perspective.

[@DarthHydrae]

And I'm there later still, and find myself with the same question. Does someone has found some ressource that could clarify that, or is it beyond layman explanation?

[@DavidByrden1]

I explained this in a separate comment. But, to summarise it here: the mirror (that selects between D3 and D4) causes a 180 phase shift in the blue beam but not the yellow beam. That's just how mirrors work. So, you get 2 groups of photons with a 180 difference in how Blue relates to Yellow. Naturally they produce interference patterns that are 180 different.

[@pedroproano4598]

​@DavidByrden1 thank you for your answer, I am having the same question... Could you please refrence a more detail explanation on this mechanism?

[@erinm9445]

Late to the party! But this drove me so nuts myself, that I thought you might want an answer! The answer is that both entangled photons are in a superposition of two beams (one from left slit, and one from right slit), but as soon as the first photon hits the D0 screen the relative offset of the phases of those two photons becomes set. The relative offset correlates to locations on the D0 screen in a way that cycles from 0 to 180 degrees and back to 0, repeatedly as you move across the screen horizontally. Hence anything that is correlated with values for the relative phase differential of the superposition will take on that stripey pattern. The beam splitter does exactly that. It sorts photons based on wave amplitude, which is random for an ordinary photon, but which is nonrandom for a beam with a superposition of two phases with a *set* relative phase offset. Waves with certain relative offsets will have higher probabilities of going to D3 (and correspond with entangled photon twins that landed on certain stripes on the D0 screen), and waves with the other offsets will have higher probabilities of going to D4 (and correspond with the inverse stripes on the D0 screen, created by entangled twins). It's all explained beautifully in a new video on the delayed choice quantum eraser by a youtuber called Diego Emilio.

[@danreach]

But you could do this with only D0 and D3. Why are you obligated to combine the results from D3 and D4? The whole point of the incidence counter is to disambiguate the results from each detector. The fact that the two interference patterns aren't perfectly aligned is the only reason they'd look like a blob on D0. It isn't two blobs, it is two misaligned interference patterns.

[@Lin_The_Cat_]

3:07 Joe’s face in that screenshot of his delayed choice quantum eraser video. 😂 Gotta love him. Love this video, too! I’ve heard about how the quantum eraser experiment doesn’t actually rewrite the past; however, I love the way you explained it in this video in that it was much easier to follow.

[@EM-bs4kt]

That’s because her experiment is nothing like the one done by skulls Kim et al

[@hook-x6f]

*****HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!******* The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. All three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@hook-x6f]

***HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!****** The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. The The three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@hckytwn3192]

10:17 this is a very poor analogy. You don't randomly/arbitrarily remove data in the Delayed Choice experiment. Instead, you're looking at two distinct data sets based on if their "which way" was measured or not. As a result, I don't think she explains much of anything here. Also, Carroll's paper neither supports what Sabine said here, nor disproves retro-causality-he just chooses to explain it via the Many Worlds theory. He actually says if you don't agree with Many Worlds, then Delayed Choice *can* show retro-causality: *"But alas, not everyone is an Everettian. In some other versions of quantum mechanics, wave functions really do collapse, not just the apparent collapse that decoherence provides us with in Many-Worlds. In a true collapse theory like GRW, the process of wave- function collapse is asymmetric in time; wave functions collapse, but they don’t un- collapse. If you have collapsing wave functions, but for some reason also want to maintain an overall time-symmetry to the fundamental laws of physics, you can convince yourself that retrocausality needs to be part of the story. Or you can accept the smooth evolution of the wave function, with branching rather than collapses, and maintain time-symmetry of the underlying equations without requiring backwards-propagating signals or electrons that can’t make up their mind."*

[@fkeyvan]

You are correct. I read Sean Carroll and his explanation is tied to the MWI which is not at all mentioned by her. Calling Carroll who believes in MWI a person who knows his QM is strange,

[@ThePinkus]

Important: it is not an analogy, it is how the data is actually partitioned. And it is correct: we start with a set of data points, then we partition this already given and fixed set of data points by considering how this set was generated in coincidence with other results, exactly as Sabine is showing. We get something interesting by this partition if the results are correlated, i.e., when the partition is not random respect to the data set, and this is exactly how the coincidences with D3 and D4 produce interference-like patterns as a partition of the overall pattern, again, exactly as Sabine is describing. Note, though, that she's considering only the "erasure" setup there. We can say exactly the same with the "non-erasure" setup using just D1 and D2 (though in this partition the two blobs might be quite less discriminating respect to D1 and D2, the process is the same). And we can also say the exact same thing for the combined setup (arXiv:quant-ph/9903047v1), which is probably the one You are referring to. The important point to make clear is that in this setup the discrimination of the "erasure" "non-erasure" path IS random, i.e., not correlated to the results on the screen. That is, the partition of the results on the screen between the (the coincidences with D1 + the coincidences with D2) and (the coincidences with D3 + the coincidences with D4) is just the original pattern with halved intensity (assuming equal probability between the paths). Then it is the further partition of this first partition that shows the patterns as we had them in the two non-combined setups (with halved overall intensities again), i.e. "wave-like" (from D3 and D4) and "particle-like" (from D1 and D2) as the wave-particle complementarity approach would like to call them. All these partitions are obtained exactly as she is describing it at that point in the video. The point she is making is that the position on the screen tells absolutely nothing about a wave-like, i.e. D3 or D4, or a particle-like, i.e. D1 or D2, result, and conversely that a wave-like or particle-like result causes absolutely nothing to the results on the screen. The core of the matter is that we are exploiting correlations, not causation, between the results. Note that these experiments are used in the context of the discussion on the wave-particle complementarity approach to the understanding of QM, and the "delayed choice" notion itself was born as a critic to this approach. "Delayed choice", similarly to "spooky action at distance", it is not intended to describe what is going on according to the theory (QM), but rather to point out some nonsensicality resulting from some narration/interpretation/approach tentatively used to understand the theory.

[@ScottLahteine]

Super-determinism has also been proposed. As I understand it, the whole experiment is one single complex distributed wave function, distributed in spacetime, which will only reflect possible outcomes. The way the whole experiment is designed will produce a characteristic result that says more about the nature of the experiment than it does about causality. Even though "what is possible" is temporally distributed, the fact is that we haven't looked at the data until the whole event has completed. Perhaps we can think of probability waves like a flowing river, and the physical experiment like a system of gates. A photon's wave function will not "flow" where it cannot, and (as with the phenomenon of refraction) a photon will take the "shortest" path to its destination simply as a consequence of its wavy self-interfering nature. Taking all this into account, the experimental result is not so counterintuitive.

[@ThePinkus]

@@ScottLahteine I do not know super-determinism in detail, but I agree with Your observation already in respect to correlations as encoded in entanglement in QM. Indeed correlations can only be assessed ("observed") considering the combination of results, which implies that the assessment itself can only occur in the intersection of the future cones of the events giving the results themselves (i.e. of the measurements). This explains how correlations can be a property of the combinations of systems (actually of potential results of measurements on those systems) defined irrespective of space-time separation and still be consistent with the local causality of relativity -- we can only check them in the intersection of the future cones from the measurements. It is rather surprising when/if this is not reliant on determinism, which is the point raised by EPR. I tend to consider (possibly even prefer) that it is NOT reliant on determinism, but this is VERY MUCH debatable.

[@randystegemann9990]

Yes, Peter is 46 years old, but that is at the start of the puzzle. How long did his voyages take and the loading and unloading of the ship? Especially now, with container ships waiting offshore for their turn and canals sometimes getting blocked. At the end of the puzzle we are asked how old is the captain, not how old he was.

[@rog2224]

Where did she say 'At the start of the voyage Peter is 46 years old' There's no implication of an actual timeline in the two bits of information - Peter's age with his job, and what happened on some unspecified voyage at some unspecified time. Peter *IS* 46 years old is the only concrete information in terms of a when, which is 'currently' the rest is just noise (unless you're in logistics, then it might be of interest)

[@johnnyzee383]

@@rog2224 Of course there is an implication of time between the 2 points unless of course you think the ship arrived instantaneously, therefore, Peter's age IS different from the start of the voyage then to its end, even if extremely slightly. "Noise" is simply someone's avoidance of measuring or determining a very very small subset of data that would be crucial to determine an outcome, otherwise the outcome would be flawed.

[@WhosBean]

Correct me if I'm wrong but isn't this just re-wording the same phenomena? Ok, so the interference not being able to form was established beforehand, agreed. But the prerequisite for that inability to hold is being fulfilled after the pattern already should have formed (in delayed choice). So haven't you just moved the goalposts back one step? Ok, so we're not changing the past in the sense that the pattern which was made is being retroactively altered, but we are changing the past in the sense that the fact that the quantum data is being erased is being "communicated" back to the past so that the pattern would have never formed. Saying that the interference never actually forms in any case doesn't work either, because unlike Sabine's coin-tossing example, the pattern being formed isn't due to bias from the observer but is an objective pattern based on which detectors one is considering.

[@PublicVoidFoo]

I kinda feel deceived now, a critical distinction was missing from popular explanations! I'm curious, if the experiment is not showing anything new really, then what' is its significance in the scientific community?

[@SabineHossenfelder]

That's an excellent question. Partly I think the reason is historical, the roots go back to an old idea by Wheeler. But I think it's also because it's more visual, as opposed to (say) Bell-type tests in which you calculate correlations. Here you have several paths on which particles travel resulting in a pattern on a screen -- lot of spatial information in this which is much more intuitive than other experiments. This is my guess in any case. I mean, the reason you see this so much on KZbin is exactly because it can be visualized with dots moving on paths etc.

[@_John_P]

@@SabineHossenfelder Would you be so kind to tap on the controversies of Bell's experiment?

[@DrDeuteron]

My brilliant and world famous advisor would say "nothing". In fact, what he did say was, "Hey look: quantum mechanics works, we've known that for 100 years". Literally does not care. Nevertheless, we now have experimental techniques that allow us to probe psi in new ways, and some ppl like that. The weird stuff starts with multiple particle states (e.g., the bomb), but it's still linear mathematics and theory alway predicts experiment.

[@ListenToMcMuck]

@@SabineHossenfelder Are you sure that nobody whispered the words: "Let's make it seem like it's pure magic...", while writing the paper. ^.^ ,

[@kenlogsdon7095]

@@SabineHossenfelder Historically, Wheeler and Feynman were heavily into modeling QM interactions involving temporally "advanced" and "retarded" fields. The idea you allude to by Wheeler is, of course, his self-excited universe. I attended one of his "Distinguished Lecture Series" lectures about it at the University of Louisville on 10 March 1980. He very kindly autographed my copy of his little book, "Gravitation".

[@terribleterrier1685]

Thank you Sabine. This experiment always confused me and I finally understand. You make it seem so effortless! Science educators on KZbin more and more chase the clickbait as opposed to actually trying to bring understanding. I never miss one of your videos and I always come away more enriched.

[@william_benckert]

How does this disprove the mysterious aspect? The first particles are still landing on the screen seemingly dependent on the future randomly chosen destination of their pair particles..?

[@Lestertails2]

it breaks the mysterious aspect because you have causality backwards. In reality, The particles are landing on the screen, and latter they are at either detector d3 or d4. But whether they land at d3 or d4 is already determined by where they land on the screen. This effect, while weird, is just a normal quantum entanglement effect. So, your choice of using detector d3 and d4 vs d1 and d2 is not retroactively changing where the particle lands on the screen, it is only changing whether or not you apparently see an interference pattern for that grouping of particles. Does that make sense?

[@william_benckert]

@@Lestertails2 Yes, the fact that the second particles "decide" whether to land on a detector with or without which-way-informatoon depending on where the first particle landed is the mystery, right? So I am not sure I get the point of the video.

[@Lestertails2]

@@william_benckert No not exactly. The apparent mystery is that you could change whether an interference pattern is observed after the first particle lands at the screen by changing between measuring with detector d1/d2 vs d3/d4. Because it is possible to make the choice of d1/d2 vs d3/d4 (hence the "delayed choice experiment") some people wrongly interpret this to mean that quantum information is sent backwards in time (spooky). Sabine points out that this is not the case. At the screen, there is no interference pattern, regardless of d1/d2 vs d3/d4. The interference pattern only apparently appears when using detector d3/d4 to match where a particle in the past landed on the detector screen. Thus where particle 1 landed on the screen will determine which detector (D3/D4) its entangled partner lands at, to make the apparent interference. This is an interesting property of quantum entanglement (and yes, quite mysterious), but it isn't "sending information back in time" mysterious.

[@william_benckert]

@@Lestertails2 Uhm, either the first particles are looking into the future or the second particles are looking into the past (apparently). This video doesn't debunk anything.

[@EM-bs4kt]

@@Lestertails2 D3 or D4 both correspond to an erasure state. Summing the total of both is nonsense. Would you attribute a coin that landed heads today to a coin that landed tails yesterday and say that it was completely determined and dependent ?

[@vijayshankar102]

I think the conclusion in this video is omitting a few points, if you read the link of Sean Carrol given here, it is clearly mentioned that retro-causality is false only if the Many Worlds interpretation is true and that retro-causality may be true if you follow collapse theories like GRW. In addition to what Sean Carrol has mentioned there is the transactional interpretation.

[@narfwhals7843]

If I remember correctly, Matt from PBS actually comes to the same conclusion, and explains the particular detector correlation in the followup video about the "winning the lottery with the quantum eraser" question. kzbin.info/www/bejne/aIbdqqeopNStapY

[@meleardil]

It sounds like a debunk, but it is NOT that simple. Because you do NOT change how you measure the "interfering - non-interfering" photons as it is done in the classical double slit... Nope! You change how you measure their entangled TWINS. If that measurement happens MUCH later, then the "interfering - non-interfering" measurement, than the conclusion is true that the effect of a decision seemingly propagates back in time. The fact that the 2 patterns can be combined into a non-interfering blob does not nullify the experience that the result of one measurement DO CHANGE based on the method of another measurement done LATER in time. If both detectors are working, the non-interfering blob can be seen, but the separate interference patterns can be "reconstructed" after post-correlating the measurement with the delayed measurement. This implies 2 things: 1. The entangled particles DO act in unison, which is very WEIRD 2. Einstein can rest in peace: information do not travel back in time, because the pattern can be restored by correlating the results of the 2 measurements. (this would mean that the entanglement is FTL, but the actual quantum information still obeys causality)

[@cristianproust]

@@meleardil I love finding a real physicist in the comments. Good job 👍

[@woowooNeedsFaith]

I guess he did not reach quite same conclusion because of the nature of his comment on this same video: kzbin.info/www/bejne/iILZZnaMerF6abc&lc=UgzX3BUTGug1AR1pabp4AaABAg

[@jamiegagnon6390]

See above where Matt actually admits he was wrong and says he is working on a video to fix it.

[@rtyzxc]

@@meleardil I'm very confused because these explanations are all so different from the traditional explanation, but if measurements do change based on another measurement later, it creates a paradox. You could create an ultra-delayed quantum eraser where you see the screen AND control whether to erase the which-way information later. Now, the system would either magically force your choice, or you could choose to get an impossible result (get interference pattern while also measuring which-way information). Another possibility would be that the mere possibility of this happening would create a clump pattern, which would all kinds of weird implications, it would become a reality predictor of "is it possible to press this button fast enough to enable collection of which-way information".

[@JonnesTT]

If there is one thing I love about your channel it's the statement "It's not as weird as you think". Because it always turns out not to be as weird as I've been told before.

[@MrCmon113]

@@dextermorgan4490 Because that is nonsense. It's not "observed" and it doesn't change. Instead the particle, the measurement device and you all synchronize, such that all possible you's see corresponding particles.

[@saulberardo5826]

What part of the experiment setup determines which of the two non-overlapping interference patterns related to D3 and D4 end up shifted more to the left or to the right? Can we change the experiment in order to change how the patterns overlap? These might be minor points, but I'll register here my doubts anyway so I can think about this later

[@schmetterling4477]

You can change the experiment any which way you like, but that won't change that you are really just chasing your own tail. The problems are not with the experiments, it's with how you are trying to describe the world. The world simply doesn't work that way.

[@wiedreist6405]

why can't I see the answer to this question?

[@celivalg]

I did find a debunking of this problem on youtube a few years ago, but I couldn't find it again when I wanted a refresher and found your video instead! thanks a lot!

[@FuriousGuineaPig]

OK, I have a question - what are the separate patterns from photons hitting D1 and D2 - before they are combined? Another question - beam separator at the end that lets 50% of photos pass and reflects another 50% is it like round robin thing 1 pass 1 reflect or is it random thing that over time makes up to 50% pass 50% reflect? I think you've also omitted some of the details.

[@The_Trojan]

Correct, there is no explanation as to why any pattern appears at all. Sure, half the photons are excluded, but why are the not excluded totally randomly? why are they excluded such that an interference pattern appears?

[@francescocannistra7915]

@Sabine Hossenfelder, I am afraid you didn’t get it right this time. What you say is partially correct: indeed you can select subsets of dots on the screen arbitrarily in order to create whatever pattern you want. However, in this case, the subsets of dots that give rise to the interference pattern (as opposed to those that create a smoothed-out distribution) are not selected with the purpose of creating that pattern. Rather it’s exactly the way around. The subsets of dots are not selected arbitrarily to show target patterns: on the contrary, dots are grouped based on physical properties (photons with and without the which-path info), and only then it’s shown that from certain properties (photons with which-path info resolved) some patterns do emerge and from other properties (photons with which-path still unresolved) other patterns emerge. It’s the coincidence counter in the experimental apparatus that makes the selection of the dots, grouping them based on what detector was hit by the entangled photons and thus distinguishing between dots originated by photons that have the which-path info and photons that don’t. Then it's just shown that an interference pattern emerges from the dots originated by photons without the which-path info. Therefore, I believe you are wrong in minimizing the meaning of the experiment. On the contrary, I believe that the experiment really shows what is claimed in the main descriptions that can be found in the literature (and on KZbin) that, instead, you contest: • The dots originated by photons that were subjected to a which-path measurement formed a smoothed-out distribution on the screen regardless of the fact that at the time they hit the screen they could still skip the which-path measurement. • The dots originated by photons that were not subjected to a which-path measurement formed an interference pattern on the screen regardless of the fact that at the time they hit the screen they could still undergo a which-path measurement. In other words, it's like if at the time they hit the screen they had already incorporated in the dynamic properties the info on an event that indeed would have happened later (i.e. undergoing or not a measurement of the which-path info), What this means then it’s another story. On the other hand, since we know that space and time are so tightly bound and that non-locality is a core feature of QM, it seems to me that this just a manifestation of non-locality in spacetime rather than just in space. Then you can even call it retro-causality to let it fit better into the categories of our perception (and our classical mindsets).

[@Azrael1486]

It's amazing that so many creators are willing to chat in the comments... Except for skeptic scientists. They never address good rebuttals like this... Because you're right.

[@francescocannistra7915]

@@Azrael1486 thanks. Not sure I am right but I think at least that my argument is well grounded upon logics.

[@Bombbashable]

@@francescocannistra7915 I think you're right! But then again I'm not a physicist. My father, who is a Nuclear Physicist refuses to discuss this with me because he doesn't believe it :)

[@AlexTorres-qv3hv]

There are 2 major flaws in this video, first, representing the process of pairing the hits at d0 with those at d1d2d3d4 as a random or purely selective process ... neglects the purpose of the coincidence counter that tells exactly within space and time when a hit at d0 coincides with any single hit at d1d2d3 or d4.....so this first mistake lead to another mistake, that is, to assume that graphs at d3 and d4 can be overlapped as if their plotted points came from the same source detector...again totally neglecting the mechanical process of obtaining the data. There's a step motor that moves the d0 screen in only one direction across the x axis so every single data point is unique and traceable to the hits by the signal photons twin at d1d2d3d4...since every single hit by the signal photon at d0 can be paired to its idlers twin at an specific detector d1d2d3d4 the graphs at d3/d4 are totally independent from each other and cannot be overlapped without impairment of the experiment results.....indeed this video is a total waste of time and debunks nothing...

[@jonatanblais957]

You're right. I have no idea where she got the strange notion that hits at D0 were paired with detectors D1, D2, D3 and D4 in an either random or deliberate way to produce a desired interference pattern. As you point out, a coincidence counter is used to match each photon hit at D0 to the actual detector of its idler twin in a completely objective and deterministic way. Therefore, the different patterns observed after this process between hits that were paired to D1/D2 vs those paired with D3/D4 must be explained. And what is really perplexing is that the only difference between this two kinds of detectors (D1/D2 vs D3/D4) is that in one case the which path information is available in principle, while in the other it is not. That's in fact the purpose of this experiment (see my earlier comment). So suggesting otherwise is not only false but completely ridiculous. No reviewer would have let someone publish (in the journal Nature...) such a ridiculous experiment where the experimenter would have discarded data has he pleases to obtain a given pattern !...

[@Merilix2]

"neglects the purpose of the coincidence counter" No. You probably missed Sabines point here. Emphasizing the purpose of the coincidence counters is just not very important for what Sabine was showing. I would say this video assumes everyone already knows what the purpose of those counters are. Its like teaching kids about multiplication assuming they already know addition^^ Same to d3/d4. Of course you should overlap those graphs in order to understand why there is indeed no interference pattern at d0. If you overlap the pattern d3/d4 AND if you overlap the patterns d1/d2, you cannot tell which one is which. They are indistinguishable and that's the important point here.

[@AlexTorres-qv3hv]

@@Merilix2 if you go to the 10:09 mark, she states that the fringes patterns produced at the eraser section results from selectively chosing some data points while disregarding others. There's plenty of peer reviewed papers in google, just link one here citing as correct Sabina's assertion, then we can have a constructive discussion.

[@Merilix2]

@@AlexTorres-qv3hv That's correct. She says that and what she says is very true. That's exactly what coincidence counters are for: selecting signals you are interested in from a bunch of signals you are not. Its a filter mechanism. Btw: google doesn't have peer reviewed papers at all. google only provides links to sites to find them. I already read some of them in order to understand this experiment better but none of them in disregard of what Sabine says. Sabine just highlight aspects some other publicists don't or completely miss. (e.g. the explanation I recently saw from fermilab :/ )

[@AlexTorres-qv3hv]

@@Merilix2 read the other answer to this thread...he gives an objective description of what the source papers of this experiment tells about the purpose of having a coincidence counter, or you can checkout this video which is very straightforward kzbin.info/www/bejne/q2rFiaKihKt0eJo

[@nitink9879]

Summary: If the quantum eraser actually erased which way the photons went through, then D3 + D4 combined must be an interference pattern. But actually this isn't the case which means the quantum eraser didn't erase the information which way the photon went through. i.e., The quantum eraser didn't make the collapsed wave function return back to the superposition state.

[@HSMAdvisor]

"Entangled particles are like a pair of brand-new socks. When you put a sock on your left foot, the other sock, no matter how far away will become the RIGHT sock!" - The analogies physicists use to explain entanglement without hidden variables :)

[@ThePinkus]

As long as You don't use incompatible observables You won't see a difference between entanglement and classical correlations (in measurements on separated systems). In fact, fixing the observable is equivalent to fixing a "slice" of classical logic within quantum logic. It is important to understand what is the same in quantum logic and classical logic, as much as it is important to understand what is different. That is why EPR and Bell use incompatible observables.

[@matthewcahill4475]

Sort of but not necessarily, let’s say that on the inside of the sock is an L or an R to denote which foot it goes on, you don’t know which is which until you check and when you see it is marked L, you know the other has to be marked right

[@HSMAdvisor]

@@matthewcahill4475 Your example, however literally has hidden variables.

[@matthewcahill4475]

@@HSMAdvisor well a simplified analogy will always lose detail it’s just a stepping stone to understanding, I just had to add the detail that you can’t choose the quantum state upon measurement you just know the other one when you do

[@stevekiley6121]

What are the hidden variables? Where are the hidden variables? It is amazing how far mainstream scientisits will go to try and explain something which doesn't fit in with the mainstream (funny about Bohm though, he went well out of the mainstream, and believed in hidden variables).

[@carlosmartinezbadia2532]

I don't see any debunking here. The interference patterns from detectors 3 and 4 are real, not created by the experimenters arbitrarily picking detections. And so are the non-interference patterns from D1 and D2. The fact that the screen is closer to the slits than to the detectors means that the photons that hit them reveal or hide the which-way information of their twins at the slits AFTER the fact, and at the same time determine exactly how this fact happenED! Of course you need to discriminate between the detections corresponding to each trajectory of the entangled photons, because they correspond to different events, some revealing interference (and so a simultaneous passing through the slits as a wave) and others revealing a particle-like behaviour. Of course again, if you don't discriminate between these different events you get a blob on the screen: all cats are grey in the dark! Would this prove retro-causality? Not necessarily, I'd say, but any other interpretation I can think of is just as weird. What I don't see is how this argumentation can dismiss the experiment as banal, the mere product of a silly confussion of its creators -- and their peer-reviewers

[@darkace37]

I am shocked. I feel like I could as likely won the lottery today without playing. As a quantum casual I feel somewhat betrayed by the simple explanation being disregarded. I today find myself more cynical than ever and appreciate people like you. I have neglected your channel over others and realized after the past few episodes that I have been wrong to do so.

[@argfasdfgadfgasdfgsdfgsdfg6351]

Sabine's explanation is incomplete, though. She does not explain why D3 and D4 individually would see an interference pattern at all - she says that for those detectors, scientists were "simply ignoring" 50% of the data, but that would just lead to D3 seeing a blob with half the count and D4 seeing a blob with half the count, but not in a certain pattern. Big flaw in the argument.

[@psiphisapiens]

@@argfasdfgadfgasdfgsdfgsdfg6351 you have misunderstood the video

[@argfasdfgadfgasdfgsdfgsdfg6351]

@@psiphisapiens It's strikingly clear that instead of giving us insight on how exactly we are wrong, you chose to criticize us without any factual substance, which is a strong sign of a lack of understanding from your side.

[@argfasdfgadfgasdfgsdfgsdfg6351]

@@psiphisapiens That's high school physics and it doesn't even explain what I mentioned.

[@psiphisapiens]

@@argfasdfgadfgasdfgsdfgsdfg6351 oh You’re asking something else to the first comment. I apologise. Yes, it does seem strange that a diffraction pattern results, but note that the diffraction and interference seen by monitoring the which way information of the double slit results in a DIFFERENT diffraction pattern than the unobserved double slit. Even without understanding the sampling bias, it’s clear that the fact these 2 patterns add up to give the exact same pattern as D₀, and that you ONLY get these patterns when special sampling, indicates that the sampling method and not the quantum effect is responsible. Since I have tried to understand the sampling bias (in a different case of electron spin delayed choice variants, and classical metaphors), I am fairly confident you’ve not come across the same material. But yes, I do apologise, I didn’t read your comment properly and just scanned and thought you were agreeing with the first comment. I thought the first comment was saying that Sabine hasn’t covered the quantum weirdness of the experient, but she had. She acknowledges the “weird” part of the experiment, the seemingly spontaneous collapse of the wave function, but this wasn’t about entanglement or superposition, this was specifically about retrocausality. So I was explaining the rudimentary understanding of these principles, and then I was going to explain how they can be understood with the double slit alone, and that the delayed choice adds very little bar talking about the limits of “decoherence” and sampling.

[@hakikataynas9769]

The screen has already received one-way information patterns before D3 or D4. The screen is placed before D3 and D4, right? So the pattern on the screen will always be the same, whether D3 or D4 is included or not. If not, this would still mean sending a message to the past!

[@Gerard1971]

Your representation of the experiment is a bit incomplete, in the original experiment there are beam splitters for D1 and D2, and there is a detector D0 which scans for the interference pattern, and all 5 detectors are connected to a coincidence counter. This is used to record correlations, but only when joint detection of two entangled photons occurs at both D0 and D1/D2/D3/D4, to create after-the-fact visualization showing which parts of the blob pattern correspond to which measurement. The amount of time between the measurements plays no role. As you mentioned in the beginning, an interference pattern is never seen at the screen, because the crystal destroys that. A detection on D1/D2 must correlate with a detection at D0 which has which-path information and therefore will not cause an interference pattern. A detection on D3/D4 must correlate with a detection at D0 without which-path information and therefore will cause an interference pattern. It is not clear to me why this even occurs in this experiment, and you also don't explain this. You mentioned that you could not find other explanations. I found pretty good explanation of what you described by Mark John Fernee, PhD in physics from the University of Queensland, but there are also a couple of YT-videos that explain it. Even the Wikipedia article mentions that consensus is that there is no retro-causality and the interference pattern can be seen when the experimenter looks at particular subsets of signal photons that were matched with idlers that went to particular detectors. 10:12 That this has nothing to do with quantum mechanics I don't understand, this experiment wouldn't work without entanglement?

[@jonatanblais957]

"A detection on D1/D2 must correlate with a detection at D0 which has which-path information and therefore will not cause an interference pattern. A detection on D3/D4 must correlate with a detection at D0 without which-path information and therefore will cause an interference pattern. It is not clear to me why this even occurs in this experiment, and you also don't explain this." You're right. This is the key point of the whole experiment and she doesn't even address that at all in the video ! See my earlier comment for why this is important. And this correlation is indeed observed in the experiment ! This is the reason why this experiment was done (to test whether the observed patterns at D0 would be different for photons detected at D1/D2 vs those detected at D3/D4, which would indicate that it is the in principle availability of the which path information that collapses the wave function), not to test for retrocausality ! And indeed, the patterns are different which does indicate that the in principle availability of the which path information changes the observed physical outcome ! That is, what an observer can or cannot in principle know about the system modifies the behavior of fundamental particles ! Very strange on her part to completely ignore this ground breaking finding, which is what the experiment was meant to test, and focus on a wrong tangent about retrocausality...

[@gelmir7322]

This is the reason why im baffled by this whole "debunking" attempt. There are many variations and improvement of this experiment because it's a very peculiar phenomen. How did the experiment reached to such level of interest and development when Sabine can easily explained it like exposing a cheap magic trick?

[@jonatanblais957]

@@gelmir7322 You're entirely justified in your skepticism. Because indeed, she didn't debunk anything relevant at all. The results of delayed choice quantum eraser experiments are truly mysterious and baffling for anyone clinging to a materalist/naturalist/physicalist ontology. And the most baffling nature of such experiments doesn't lie with its implication of retrocausality but in its implication that the availability (or not) of information about the system determines observed physical outcomes. She either doesn't understand what the experiment is actually testing or she does but doesn't want to talk about it (it seems to be taboo among many contemporary physicists...). Either way, it is sad.

[@Gerard1971]

@@jonatanblais957 Because Sabine says at 8:48 that everything after what she told us at 5:46 is completely irrelevant, I originally had the remark "It seems that you don't fully understand this experiment" in my comment, but thought it was a bit too bold for me as a lay person to say to someone like Sabine, but I'm glad to see I'm not the only one that is skeptical about this. Like many people in the comments, even Matt O'Dowd now seems convinced that Sabine is right. Can't wait to see his video on this. :-)

[@Littleprinceleon]

Do beam splitters differentiate between differently polarised EM wave packets? Or they "split" randomly regardless this property? ... the interference-like pattern: I denote it "like", because evidently the intensities of the bands are only roughly symmetric according to the central, most intense band - just look at the original experiment. More importantly: the classical single photon interference after the "wave" is "going" through the two SLITS has its physical explanation, but here after leaving the CRYSTAL how/why would the entangled photon - which encounters no physical obstacle on it's way to D0 - interfere with itself?! Maybe these two SHIFTED patterns are in reality the "which way" information manifesting in the form of the shifted bands thus proving that we get info on the path because that was "detected" (via absorption and reemission) by the crystal! The varying intensity of the bands also can be simply resulted by the crystal: if in certain direction much more photons leave it then to the sideways... This D0 detector bothers me more: they simulate a rigid detector screen with an apparatus which is able to receive photons only at ONE PARTICULAR LOCATION and then by moving the detector to a number of positions they get a "continuous" line of detection. But a huge bunch of photons is NOT detected all the time (only those going in the actual direction of the detector)! Perhaps this doesn't matter: but if the setup of experiment - as Sabine implies in some of her videos - DETERMINES the outcome (eg. a field ensuring nonlocality is at play), then maybe the position of the detector is relevant not only regards the phase, but also some other properties of the photons ... I don't know. Is this kind of moving detector also used to examine the classical (no entanglement) interference with individual photons, too?

[@scptime1188]

Craig Gidney's video on the DCQE experiment actually does clear up the misconceptions you covered here too!

[@francescoromano2857]

Wow, he really did!

[@hook-x6f]

****HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!****** The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. The The three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@paulkocyla1343]

I´m a bit confused. You say that the pattern on the screen will always be the blob and never an interference pattern. But at 7:30 you show that it changes, depending on what you do on the far side. And this WOULD imply a retro-temporal action. Or did I misunderstand something? Is D0 always showing the same pattern, no matter what happens on the far side?

[@alexartiushkin2865]

So do I. Why are they all wrong? I see no contradiction.

[@pet2226]

woww....so many great videos and explanations! I've watched PBS + Fermilab video of the quantum eraser + studied quantum mechanics at the university as an engineer but I've never got the essence of the quantum eraser in PBS + Fermilab....it led always to confusions. Now I know why....This explanation was really helpful! Danke schön for you and the team for this additional video..

[@schmetterling4477]

The essence of all of these videos is bullshit. ;-)

[@pet2226]

@@schmetterling4477 hm. It is not clear what YT channels you are referring to. Do you mean PBS or Fermilab or Sabine's channel? I think however "bullshit" is too strong ...All of these channels are put a lot of effort in quality content creations and how to try to explain physics to a broad audience who have no background of high level math (eg high school students) .....

[@TheBrightmanFan]

This is all I wanted to hear 9:03 .Thank you Sabine!

[@sofiarupil7746]

Exactly, that was what made the difference

[@badbgp]

Amazing vid and explanation! But why is the interference pattern at D3 & D4 not the same? Since they're both made from a mix of both slits would we not expect the patterns to be identical in position?

[@lphillss5914]

Same question here

[@gehtdichnixan4704]

Why do we see interference with D3 and D4 at all? The photons at the screen cannot interfere! Is what she said. --- Edit: I found the answer in the article linked in the description of the video. It's all about quantum weirdness, but no magic is involved.

[@yyyy-uv3po]

Isn't it because for each detector D3 and D4, one path gets 1 more reflection than the other, knowing that each reflection changes the phase by 180 degree ? I have some reminiscence of that, but would gladly accept a (dis)confirmation.

[@afrosymphony8207]

@@gehtdichnixan4704 quantum weirdness IS magic!

[@opstube]

isn't it intuitive to think that "sticking objects right in the path of something" (mirrors) does something? also when you have a look at how those "detection devices" actually work, it's not far off to imagine that stuff is filtered out in those measurements (for each device). the usual videos always show eyes or a camera being "outside" and just looking at everything, but they always omit those small details that Sabine showed here

[@petertrahan9785]

Imagine a "car detector." We want to know which gate the car goes through, so we build a "detector." The detector works like this: it contains a wrecking ball that smashes anything that passes through the gate. We can tell if the car went through the gate because any car that we "measure" with our "detector" no longer runs. So weird. The act of measuring which gate the car went through, changes the car's behavior. Incredible!

[@Google_Censored_Commenter]

it is weird, since no one has and probably never will explain what actually happens at the point of measurement. For your analogy to be complete, the chance that the car is hit when you launch the wrecking ball is only 50%, meaning when you inspect the wrecking ball, 50% of the time there's damage on it and by extension the car. But if you don't measure the damage on the ball post launch, somehow 100% of the time the car is undamaged.

[@petertrahan9785]

@@Google_Censored_Commenter That's a bit of an exaggeration but I do wish folks would spend more time explaining what, exactly, is involved when we "measure" and "detect" and "look at" a photon or electron, etc. And I think this should be stated up front: "we cannot look at, measure, or detect a subatomic particle without physically interacting with it and these interactions fundamentally change some of the properties and behaviors of the object." That's just a fact, and seems to be the sum of what the two slit experiment shows. But when stated plainly, it seems a lot less weird. A photon behaves differently after we physically interact with it. If you replace the word "measure" with "interact" it just seems to make more sense. The willful choice to keep using the word, "measure" in this context seems to be an intentional attempt to make the whole thing seem stranger than it needs to be. Which is the opposite of what an explanation is supposed to do. Where the audience seeks clarity, the presenter offers obfuscation. Bad form, folks. Bad form.

[@petertrahan9785]

@ช่องน้องจิมมี่ Not quite. Let's make sure we're talking about the same experiments because different issues can come up that we can discuss. In the traditional two slit experiment a single particle goes through the slit. Then another. Then another. Until a pattern emerges. Now we redo this experiment by adding a detector. We send one particle. Then another. Until a pattern emerges. Interestingly, the pattern is different. The detector changed the behavior of the particles because there is no such thing as "detecting" the particle without interacting with it.

[@petertrahan9785]

@ช่องน้องจิมมี่ my original response was highlighting the fact that if the author of the video really wants to get rid of the gobbledygook, she should stop using terms like "detect" without explaining that to "detect" a particle, we have to "interact" with it. When I first learned that "looking at" a particle changed it's behavior, I started asking questions like whether it mattered if I had my glasses on or not? What if I couldn't "see" it when I "looked" at it? Turns out, what was meant by this phrase, "looking at" was something more like "bombarding with a laser" or crashing the particle through a polarizing filter. But it took me decades of chasing my tail watching videos like this and reading books about this "mysterious" thing that was happening. It was very hard for me to find an explanation of what it meant "to look at" or to "detect" the particle. Once I did learn what physicists meant by these terms, I realized they meant something completely different than what a normal person would understand these terms to mean. Once I realized this, things became much, much easier to understand.

[@petertrahan9785]

@ช่องน้องจิมมี่ you're correct that I am making a general point about communication. Such critiques do not require a degree in physics. Secondly, arguments should live or die on their merits, not on the credentials of the person making the argument. Otherwise we fall victim to the argument from authority fallacy. Yes, my critique here is a pet peeves and marginal to the content, but the channel brands itself on getting rid of gobbledygook and I am all for that. A good place to start with regard to that mission statement is with the heart of the confusion about interference patterns and the the things we do to make them come and go in these experiments.

[@aaronmicalowe]

I still haven't heard any explanation of the double slit experiment that involes measuring the particle without interacting with it. If you interact with it, you're not measuring the particle but the relationship of the particle with the measuring device. There's no such thing as a totally passive measuring device.

[@eraldosantana5944]

Great video, but I'm still confused about the delayed nature of the selection of which subset of particle one can measure with D3 and D4. If the pattern shown on the screen still depends on a fact that happens latter than the photons hitting the screen, why is it relevant that it's not a true interference pattern? I just hope I can understand this one day.

[@hyperstone9]

The pattern on the screen is always a single blob no matter what you do afterwards. The measurements from D3 and D4 do nothing to change this. What does happen is that you can read your measurement from D3 or D4, find the corresponding photons on the screen and ignore the rest, revealing an interference pattern. So nothing on the screen actually changes, it's just that if you selectively ignore half of the photons on the screen, you can make it look like an interference pattern, and D3 and D4 tell you which photons to ignore.

[@eraldosantana5944]

@@hyperstone9 thank you for the answer. I thought it was impossible to differentiate between the two photons beams once you passed them through the semi transparent mirror.

[@jjhhandk3974]

@@eraldosantana5944 it is. You’re right.which is why he never responded.

[@mademedothis424]

I distinctly remember the Space-Time video on this clarifying that the interference pattern only showed up after correlating each subset of particles. It may or may not have been in the Q&A follow-ups they do to their videos, though, but he definitely acknowledged it. I'm not sure I understand why this is supposed to be a crucial flaw or not add anything to the double slit experiment. So is the argument here that all those photons were simultaneously showing a double slit interference pattern with 50% of each other AND two overlapping single slit interference patterns among all of them? Is the idea that any 50% of any photons coming out of a post-measurement double slit experiment will always show an interference pattern with each other if separated consistently in some way? I have way more questions about what this result implies now than before, frankly.

[@silentobserver3433]

I'd summarize it as this: if you use detectors D1 and D2, then partners of D1 particles form a "left blob" on the screen and partners of D2 particles form a "right blob". When you combine those two blobs you see the big blob that's actually on the screen. However, you can remove your detectors D1 and D2, and pass your entangled photons through that complicated mirror device. The complicated mirror device (specifically the semi-transparent mirror in the center) aligns the photon wavefunctions from both slits so that they can interfere normally (as they would without the initial splitting), so that mirror device "erases" the initial information of which slit the particles went, but only for those particles who actually pass through the mirror device (so not the screen photons). Therefore, by adding a mirror device all you do is allow D3 and D4 particles to interfere, so if the D3 and D4 detectors were actually a screen, you'd just get a single regular interference pattern. However, when you separate *the partners* of D3 and D4 particles, you get 2 interference patterns on the screen. And that shows that entangled particles carry enough information about the original ones so that by interfering D3 and D4 particles you can see the parts of the original wavefunction that *would have* formed the interference pattern, if not for the splitting crystal. So what the quantum eraser experiment actually shows is that if you combine two wavefunctions (for example in an interfering kind of way), then the wavefunctions of their entangled partners, if combined in the same way, would form the same pattern. Which is also a nontrivial result, but it can be shown much easier and without any "erasure" nonsense

[@admiralduy]

As I understand, this is just the double splits experiment but organized in a way to confuse people. As she said, the main idea is you get the result of what you measure. The whole ABCD detectors part is just for the selection of what you measure.

[@mademedothis424]

@@silentobserver3433 So the particles' wave function appears decohered if you measure them one way, but if you blend them back together and split them again randomly they "un-decohere", or at least interfere again and you can see that same interference having been present in the particles that hit the screen earlier? Is the interference pattern (*every* interference pattern) always there at all points in time and you're just retrieving the information about it by creating one later with the entangled particles to separate which particles belong to which pattern? That's... still super weird. Reading the additional documentation provided and your explanation makes it sound as if every particle encodes the state of every other particle at all points in time as decoherence happens and we just can't roll that process back after it happens for a bit, which may actually be *weirder* than the other explanation.

[@silentobserver3433]

@@mademedothis424 I'd say that's actually pretty close, yes. Like, any wavefunction is represented as a superposition of some basis wavefunctions. In the double slit experiment the most sensible choice of basis is "left slit photon" and "right slit photon", and a photon that passed through the slits is in a 50/50 superposition of those two basis wavefunctions (which then collapses into a single point or something, depending on your interpretation). In the eraser experiment you just add entanglement to that, so the states become "left slit photon and it's left partner" and "right slit photon and it's right partner". But a choice of basis is not unique, you can choose almost any wavefunctions as a basis and that'd work. For example, you can choose the D3 and D4 interference wavefunctions (if I'm correct, then one of them is "left+right" and the other is "left-right", up to a constant factor, probably 1/√2) as a basis, and then those two original wavefunctions can be represented as a superpositions of new basis ones: "left" = "D3"+"D4", "right" = "D3"-"D4" (again, up to a constant). That mirror device actually mixes pure left and pure right wavefunctions in such a way to get a pure D3 and pure D4 wavefunctions. But the same operation can be done mathematically with the actual screen photons. So yes, any wavefunction can be represented as a sum of some other wavefuctnions, and the choice of those wavefunctions is almost arbitrary. So any wavefunction "contains" all possible interference patterns, if you wish, you just have to extract them somehow. And the mirror device mixes "left partner" and "right partner" particles into "D3" and "D4" states, so by measuring the partners in the D3-D4 basis you can get information to split the original screen particles into a similar "left+right" and "left-right" interference patterns. So the quantum weirdness in this experiment can be separated into these parts: 1) Existence of superpositions (regular Schrodinger's cat things) 2) Ability to choose an arbitrary basis (regular linear algebra, also used in the Bell's experiment with spins) 3) Existence of entangled particles 4) The fact that mixing of states on one side of the entanglement somehow corresponds to the similar mixing of states on the other side (which is also pure mathematical thing with how entanglement represented as a product of states) Only the last of those is unique to the eraser experiment, and it doesn't seem particularly deep compared to the other ones.

[@Bad666Moon]

PBS just commented on this video about it

[@jehl1963]

This is why I watch Sabine's channel.

[@ThePinkus]

I just read Your article, with Bracken and Hance, "The Quantum Eraser paradox", and I want to congratulate You for the publication. It is very clear in the presentation of the experiments. It also makes it very clear that the "paradoxes" are not about what QM says, but about its interpretations: when we approach QM trying to interpret it in a specific way, we might have certain problems, paradoxes and even inconsistencies that the adherent narration of the formalism just doesn't have. It is in fact interesting to review the different classes of problems that the various approaches present. This is important in both the evaluation of different interpretations but also in relation to divulgation, that all too often ascribes the issues to QM itself forgetting to mention that they are instead dependent on the specific interpretation. This is of course the case for "delayed choice" (sic) experiments. It is very intriguing that You, Bracken and Hance have devised a set up that could experimentally discriminate between QM itself and a retro-causation approach to it, I hope someone will do the experiment.

[@david203]

Good comment. See the paper "Taming the Delayed Choice Quantum Eraser" by Johannes Fankhauser , University of Oxford, July 25, 2017 for a simple explanation based on the deterministic Bohm theory of quantum mechanics. Dr. Hossenfelder is mostly correct, she just doesn't as yet have a firm understanding of how simple explanations based on the Bohm theory really are.

[@ThePinkus]

@@david203 Thank You! I'll look into the article for sure! Best regards.

[@ThePinkus]

Stimulated by the article, I made some consideration on the topic. The theoretical context is that of establishing the possibility of various kind of HVTs to match QM statistics (or, otherwise, determine how they would experimentally differ), given results such as Bell's inequalities. In particular, this topic is addressed respect to the classification given by the three assumptions that lead to Bell's inequalities themselves -- locality, no-retrocausation, statistical independency. Note: Bell's inequalities were developed to address the possibility to match QM statistics of those HVTs that satisfy EPR's requirement, with a negative conclusion. They do not imply that other type of HVT couldn't match QM. The three hypothesis might not be a good classification of HVT that could match QM, as this was not their aim by design. Understanding what "matching QM statistics" means requires an understanding of how those statistics are produced by QM. The probabilities of QM are used as expectations about observable physical quantities, and they are produced (by Born's rule) with a very specific probability algebra, that is what constitutes quantum logic. The distinction between quantum and classical logic is effective and physical: if the observable physical quantities used in QM are confronted with classical counterparts, they would give different results, and the latter don't match experimental results. That is why classical physical quantities must be HV respect to the physical quantities of QM (conceding that this "hidden" adjective does not imply in principle that they cannot, eventually, be observed, it does mean that the HVs cannot be the physical quantities of QM). On the other hand, it is not always explicit for HVTs what logic-mechanics they should follow, namely, if they are classical, quantum, or some other type. But this is an essential point in determining the statistical matching between QM and HVTs, and should be most definitely addressed. There is, though, another way of stating that a physical theory (a mechanics) is classical, and that is by saying that is deterministic (I will not address this here, as this is already too long). As a consequence of how we use QM and its logic, states of QM encodes more than just "what is (determined)", as they also encodes for counterfactual alternatives and holistic correlations (holistic refers to the fact that the information on the combination of systems is encoded in a way that is not separable between the systems, which is the EPR basic observation). This yields as an input for the computation executed by the mechanics information on alternatives, as a direct effect of the superposition property which is a core characterization of the logic and the most basic distinction between quantum logic and classical logic. This is not an abstract property, as it is physically effective, as shown in the EV bomb experiment -- since the QM evolves computing over alternatives, in the specific sense that it does so as an expression of the requirement to maintain overall consistency of the correlations over all alternatives, when we test correlated results we obtain statistics that are consistent beyond the mere determination of facts. This is an expression of wave-mechanics (superposition property) applied to probabilities, rather than factual waves as those of classical mechanics. I mention this to emphasize how difficult it is to match quantum statistics for classical, or otherwise limited HVTs -- QM "works", "computes" considering information that is accessed, brought into the computation, by alternatives (why quantum computing is a powerful thing? this is why). A classical theory cannot have that information available -- determinism does not support modal logic, it trivializes to actuality, where modal logic opens up potency, namely, consideration of alternatives (this is a reason why objective and subjective probabilities are distinct by their logic, which is often unrecognized in the debate on that topic). These are the kind of limitations that needs to be addressed by the discussion on the possibility of HVTs to match QM, and they are essentially about which information is available to the mechanics (of the HVs) to determine (discriminate between) the results. That said, onto the article. My main doubt is if it employs a fair representation of retro-causality, where the latter is applied as a feed-back loop, the inconsistency of which is then used to argue for the exclusion of a set of results which are expected according to QM. If the argument is correct, this would yield a physical (experimental) discrimination between this RC-HVs model and QM. I think though, that this is not entirely fair to RC, and thus not sufficient to make the conclusion for RC in general. Given my bloated premise above, to consider Local+RC HVTs in general we need to deduce the limitations that the assumptions imply for the class of theories. HVs work as an ancillary notebook adding information to the physical quantities, so that results on the physical quantities of QM can be discriminated according to the complete set of information. The limitations of the different kind of HV are on how we are allowed to feed information into the notebook, and how we can organize and elaborate it. Locality (of the mechanics) means that we only add to the system the information that the environment (= the rest of the whole) offers locally (there is also a locality which is a consequence of trivial correlations, i.e., non-holistic correlations, which equates to the separability of the encoded information among the systems, note: QL is holistic, CL is trivial). Forward-only causation implies that the information is accessible only after it is acquired. This is an effective limitation in the sense that when systems interact they can exchange information, and then other results can be discriminated accounting for this shared information. If causation is forward-only, the discrimination of results can only be affected on successive events. But if instead we concede retro-causation, we concede that the information of an interaction can be exploited also in previous events, which in turn can be interaction events, occurred in the past of the one we started from, with other systems. At this point we can follow the other system forward in time, and see that retro-causation thus renders available into the hidden notebook information on spatially-separated results. This illustrates, I think, that retro-causation shows itself not as the logical deduction of the exclusion of inconsistent time-loops, but rather as a specified form of super-determinism, one that can exploit the histories of systems to share and synchronize the hidden information. This is why I think the argument in the article is not entirely fair to RC. But, the very same article shows that this form of local-RC-HVTs might not suffice to match QM. This is deduced by the consideration of the EV bomb setup, where the presence of D1 along one of the paths to D3 and D4 can be detected in the coincidence statistics of QM for the correlations of U3, U4 with D3, D4. IF we assume that the information in the hidden notebooks of the couple of photons was not already seeded with the information on the setup of the apparatus, then while retro-causation can synchronize the correlations of actual results across the whole apparatus, it still cannot be fed with information from parts of the apparatus that are not reached by the photons, and this is limited by the assumption that the HVs determines the paths of the photons. This is where QM can exploit the fact that it elaborates over alternatives -- it "explores" all the possible paths, and only later actual results, consistent over all alternatives, are produced. Our class of local-RC-HVTs cannot do that, and thus cannot distinguish the presence of D1 by the correlation in the statistics of only U3, U4 and D3, D4. We see, and this is typical, that the issue is not to discriminate the results that QM does not determine, but to distinguish those that QM actually discriminates! We already know that QM does this kind of distinction, and that this is experimentally tested. Therefore, we have already excluded this class of local-RC-HVTs, IF the information on the apparatus was NOT already fed into the HVs of the photons (e.g. if retro-causation is applied back to the big-bang, and assuming this means no separation between all existent systems, RC can account for the different setups of the apparatuses). Thus, other forms of super-determinism / retro-causation are not excluded.

[@david203]

Stefano, I tried to read your reply but kept getting lost. It is likely that your knowledge of QM greatly exceeds mine, such that I cannot follow your discussion. One problem for me is "determinism does not support modal logic, it trivializes to actuality", which makes no sense to me since "actuality" is very different in the very tiny domain as compared to our macroscopic statistical domain, and since you did not define "modal logic". I did not understand the non-standard terms "holistic correlations" and "counterfactual alternatives", which you did not define. As to your in-depth discussion of HVTs, I understand them to relate to experiments with simple radial symmetry (such as in atomic orbitals), which the double-slit experiment does not have. In short, you appeared to have either ignored or misunderstood the simple basic premise of the Bohm theory, which in short is that (for the double-slit) individual particle trajectories are determined by their initial position relative to the width of the slit and the Schrödinger equation (which describes the complete nonlocal experimental geometry), which determines the trajectory shapes.

[@ThePinkus]

​@@david203 It is more likely the combination of my writing being intrinsically confusing and the irreducible conflict between what I'd like to write about and YT limiting posts to a certain number of characters (something around 10000)! ;) Bohmian mechanics was not addressed at all in my post since I was trying to comment on the article by Sabine (I have not yet managed to read the one You referred to!!! My opening was quite misleading on this point!!!) in respect to its aims, and it considers retrocausation together with locality in a sense that does not apply to Bohmian mechanics (e.g. in the sense of the non locality You referred to). A foreword before I try to address those definitions, the issue of how we should intend quantum logic is valid irrespective of hidden variable theories because it is the logic of the physical quantities that we deal with. The issue of HVTs is how to deduce QM from them, but it is still the QM of physical quantities that we are dealing with (at least, today and as long as it is not falsified or integrated by other physical quantities). I state this because the notions I am using are meant to address the understanding of QM in itself, before any reference to possible HVTs. Indeed, I do not think that HVTs shifts the issue of understanding QM in itself. In a broad sense, modal logic accounts for the qualification of "is", e.g. "is actual", "is necessary", "is possible", etc. In its ontologically-styled narration, modal logic deals with different qualities of the modus essendi (the modality of existence), which can be exploited for a suggestive distinction of subjective and objective probabilities once we recall the old discussion of actuality and potency. Suggestions might be misleading, but there is a merit in these distinctions when establishing the significance of the various part of the theory as it is established by their different relation with physis (what is given to experience and comprehended by mean of the theory). For an example of a modal approach to QM see Bas van Fraassen "QM - An empiricist view". Now, I use "trivial" not in a negative sense, but just in the sense that it simplifies to the extent that it makes certain distinctions irrelevant. E.g. if our theory is entirely, or at least in its fundamental formulation, about actuality, then I would comment that modal logic is trivialized in the sense that we can discuss the theory without its terminology. This is the case for determinism already in the sense that it equates to getting rid of the probabilistic formulation of physics. Note that determinism is always about the fundamental (and ideal/metaphysical) level, whereas determination (with reference to limitations such as resolution of the determination) can be addressed at different scales. At perhaps a more significant level, the statement "determinism does not support modal logic, it trivializes to actuality" needs to go through the links between determinism, classical logic, and subjective-epistemic probabilities. Determination and subjective probabilities are dual: it is a subjective probability if and only if it is subjective uncertainty about an objective (possibly relational) determination. Uncertainty indicates determination. The identification of determinism and (fundamental) classical logic is due to the atomization property of the logic -- by having a unique atomization with always compatible observables, CL legitimizes the idealization of the indefinite removal of uncertainty which is what yields the deterministic formulation of classical physics. QM disallows such a formulation for its physical quantities because their QL does not have a unique atomization limit, the limits of atomization corresponds to incompatible observable and thus to their indeterminacy relations. Which is why they are called "indeterminacy", rather than "uncertainty", relations. Within fundamental CL, i.e., once we have warranted determinism, the talk about possibilities is "merely" epistemic, subjective, trivial (in the sense of the previous terms being entirely valid), and thus modal logic is trivialized in the sense that the fundamental physics is warranted to be deterministic and thus entirely actual. The sense in which QL, instead, "opens up" to potency is the extent in which it prescribes indetermination in the effective function of objective probabilities for the physical quantities (irrespective of HVTs, as long as they remain H, and then it's another physics). For the previous post, the relevance of "opening up" to potency, is the possibility for QM of having information about alternatives available within the state and thus to the mechanics yielding the results. These alternatives are not available to a fundamentally deterministic theory, in fact for the very reason that being fundamentally entirely actual it cannot consider alternatives to the one realization that it is following, so that such a theory has to have other means to yield the results that QM prescribes as experimentally discriminable as a result of its consideration of alternatives. Then, the general analysis goes, these other means might have to be constrained by the other assumptions that we might like to add, until the capacity of such class of HVTs is no longer capable to match QM, indicating that either we need to change our assumptions, or we set out to experiment which one is the right theory, if we don't know the results already. Note that, in the modal logic parlance, QM processes alternatives (of course in the sense of the superposition property) precisely in the sense that these are NOT actual -- there is one unique state and it can be represented as sums over alternatives, depending on the arbitrary choice of basis-alternatives. PS: as soon as we consider objective probabilities, we have to address their relation to subjective probabilities, because the latter establish the physical semantic of the theory. This is another topic, but within this perspective it is entirely the emergence of classical logic within quantum logic, which also happens to be the measurement problem. I use the term "holistic correlations" to state the that a logic, as an "algebraic" structure of probabilities, encodes the information about the combination of systems in a way that is not separable between the systems, thus it is (a name for) an analytical property of the logic. "Trivial correlations" are, in contrast, those of a logic that allows their separation. Entanglement is not separable (which is why it got its name, referring to the fact that entangled systems are not representable as separate pure states), and it expresses this not only in correlations but also as joint observables. Arguably, this might put statistical physics on a better footing than if it had a classical foundation. Classical correlations are instead trivial, the deterministic limit of classical logic implies that the correlations are always warranted by the deterministic separate state of the system, which is always given in the theory. "Counterfactual alternatives" is a bit emphatic, since the mere consideration of alternatives requires at least some form of counter-factuality. There is also the consideration that these "alternatives" are not actual/factual in QM, and this equates to the sense in which QM considers them, as mentioned above.

[@somethingsinlife5600]

My thoughts for long time on the Delayed Choice Quantum Eraser experiment have been like this "Sounds too much like magic, can't be true or we are doing something wrong". And I hate that everyone jumps on the opportunity to make it sound like magic, everyone makes the "Look" vs "No look" sound like magic as well.

[@ThingsYoudontwanttohear]

I had the same thoughts. Another thing that made no sense to me is this: Extrapolating from the double slit experiment that our consciousness somehow determines reality, because when you "look" at a foton the interacting wave pattern disappears.....but you cannot just 'look' at a foton. It literally has to hit you it the eye for you to register it. At quantum sizes everything that is measured is interacted with. (Influenced or even stopped dead in its tracks) As far as I understand this rule also applies to the censors used in these experiments. So the foton censors interact with the fotons that they measure. This does not sound like the magic of consciousness to me, but more like physically bumping into waves/particles.

[@freshbakedclips4659]

@@ThingsYoudontwanttohear waves become particle or particles become waves, doesn't it sounds more magic to you?

[@ThingsYoudontwanttohear]

@@freshbakedclips4659 If you put like that, then yes indeed, that sounds like magic too.

[@frozen_eclipse]

The result of the experiment is still influenced by the future. It is still magical.

[@angrymokyuu1951]

​@@frozen_eclipse The screen is merely a fifth detector. It detecting(interacting) with photons before the labelled detectors do the entangled ones just means it's the bit collapsing the wave function/entanglement.

[@andrewwotton3014]

Viewing D3 and D4 as a subset of particles hitting the screen seems to resolve causality discrepancies but what causes D3 and D4 to locally exhibit bias?

[@DavidByrden1]

I've explained this in a comment of my own; in short, the semi transparent mirror induced a 180 phase shift for D3 but not D4.

[@HakWilliams]

You neglected to mention a reason for the apparent "pattern". I heard it is from phase shifts introduced photon entangler. (Or electron entangler). You end up with 180 degrees out of phase photons or spin-up and spin-down electrons. Is that right? So when you select just one going through a single slit you get a diffraction pattern that looks sort of like it an interference pattern but it's not. Is that right?

[@FalcoGer]

Peter might have had a birthday during his trip, or multiple for that matter. The answer to "how old is peter?" is >= 46yr, because we lack information about the speed of the ship, his actual date of birth and any date on his travel.

[@josemarin359]

I still have a question. How the beam splitter is able to send always photons coming from one specific slide into D3 and the rest into D4?

[@dragoscoco2173]

It doesn't. It is hard to visualize. But reaching D3 or D4 supposes a specific quantum state, which is present in the partner. For photons and beam splitters it is a certain polarization. If you select for D3 or D4 you get 2 separate Interference patterns (as in the video). If you use D1 or D2 to select your photons you get half of each of those 2 interference patterns from D3 and D4 and select half of the non interference blob.

[@semh213]

@@dragoscoco2173 ı dont still understand. How did Sabine debunk this experiment?

[@dragoscoco2173]

@@semh213 She did not debunk it, she merely explained that it is not that cracked up as it is purported to be. Basically a more complex version of the double slit and nothing more.

[@semh213]

@@dragoscoco2173 thanks, Well, I want to ask something about the double slit experiment. As far as I know, if the detector behind the slit works after the photon passes through the slit, the interference pattern disappears, if it does not work, it becomes an interference pattern. Didn't the wave function depend on which slit it passed through? How is this possible?

[@dragoscoco2173]

@@semh213 Well, it is not really like that. The interference pattern is not affected by whatever happens after the double slit. And turning the detector on or off does nothing really. In order to affect the interference pattern one needs to affect one of the two pathways the photon could have taken at the double slit, via filters, polarizers or some device whatever complex to determine which path it took. As that device would require affecting the path with around as much energy as it would be needed to obtain the interference, it jumbles it or blurs it completely (instead of nice fringes you observe a smudged solid line or alike). You can easily see interference patterns with a scope (binoculars) through a mosquito net while looking at a very shiny far off source of light. It will appear as a rainbow dotted cross, as the net is like a double slit but in 2 directions. If you then focus your scope to look at the net instead, meaning you can in principle observe where the photon passed through or nor, then the cross becomes completely blurred. This might not seem like a quantum effect and truly very optical, but it is the very same thing. What QT keeps telling us is that a full prediction of where a single photon will pass through and eventually land is not possible and trying to determine it via observing the setup better affects the outcome.

[@juzoli]

“How many videos do you need of…” A lot more! This is a complicated subject, and it helps a lot by hearing multiple explanations, because all of these have a different angle. They complement each other, and I often understand it only after 3 or 4 such videos.

[@boo6237]

Agreed. even many available, admittedly this is one of the first to demystify it. Actually i have seen one "not so prominent" physicist explain this story before, but being ignored and overlooked by many as nonsense. A "Quantum Eraser" erases nothing! by Jeffrey H. Boyd MD Well, he is a MD afterall.

[@juzoli]

@@boo6237 To be fair, many other videos get to the same conclusion, that it doesn’t actually touch the past or anything like that. But they do it in such a complicated way that it is harder to understand. She has really cut the unnecessary information and made it simple.

[@hook-x6f]

*****HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!******* The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. All three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.”

[@kenczepelka6314]

Once again Sabine demonstrates her almost super natural ability to clearly think past what other scientists have accepted as reality, and not be afraid of what anyone thinks. She has easily become my favorite scientist.

[@VaShthestampede2]

It still creates an interference pattern though at d3/d4 - the which way info is still lost and that is reflected by the fact that there are interference patterns vs just two blobs on d3 and d4, that add up to a bigger blob.. what am i missing? This still seems remarkable to me.. someone please explain why this is not remarkable, that when you add a beam splitter, the which way info is lost and thus instead of blobs there are two interference patterns that show up?

[@hook-x6f]

***HOSSENFELDER HAS BEEN DEBUNKED****NOBEL PRIZE 2022 PROVES NON LOCALITY IS REAL!**** The trio’s experiments proved that connections between quantum particles were not down to local ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Instead, the phenomenon comes from a genuine association in which manipulating one quantum object affects another far away. German physicist Albert Einstein famously called the phenomenon ‘spooky action at a distance’ - it is now known as quantum entanglement. The The three winners are pioneers of the fields of quantum information and quantum communications, says Pan Jianwei, a physicist at the University of Science and Technology of China in Hefei who participated in some of Zeilinger’s landmark experiments as a graduate student in the 1990s. The recognition was long overdue, Pan says. “We have been waiting for this for a very, very long time.” What now Sabine?

[@AveiMil]

This video erased my previous misconceptions.

[@franks.6547]

So how do the photons get reshuffeled into D3 and D4 in just the right way to make their counterparts reveil an interference pattern that was neither in the blue or yellow track? How does the half-mirror preserve the correlation with the partner-photons in the blurry pattern when it is randomly chosing D3 and D4?

[@jjhhandk3974]

She honestly just doesn’t understand the experiment at all. I’ve never seen anyone get this more wrong

[@folepi7995]

unfortunately Sabine left out this part... my guess: The photons at D3 and D4 interferre as well. And when you look at interferrence in a more traditional way with lines (Huygens style), you see that the position and angle of the photons emmited by the BBO crystal is decisive attribute to determine constructive or destructive intefference. The positions are the same and the angles are probably linked (e.g. always 90degrees) for the photon pairs emmitted by the crystal. So when the photon pair shows constructive interferrence on the screen, they show constructive or destructive interferrence and D3 or D4 and vise versa. Sorry for my english. Just take it as a guess. But this is what came to my mind.

[@franks.6547]

@@folepi7995 Thank you, that makes sense to me! That way, it is a little bit less mysterious: one only delays the choice of either looking at which-way or interference - simultaneously in the near and in the far branch, which both somehow share the same "quantum information". And it probably doesn't matter if the geometry in the far branch is comparable. It just matters, that some blue and yellow track photons land on the same spot (detector, e.g. D3) in the far branch. All events formed on that far spot D3 have partner events in the near branch where yellow and blue track combine on the screen. These near events form a coherent pattern according to the particular near geometry. But they take part in such a coherent pattern if and only their correlated far branch events stem from a blue/yellow superposition in whatever pattern that far branch geometry allows for. So, the actual geometric pattern is less important than which photons to include: Blue/yellow superposition photons form interference patterns in the near and in the far branch. But photons that are "forced to appear/measured" in an insolated blue or yellow track (like D1, D2) in the far branch have only pure blue or yellow partners in the near branch. That means actually, whether you regard a near branch screen photon as mixed yellow/blue or as pure blue or yellow track photon is not objective. You reassign them according to whatever type of photon events you force to appear as their partners in the far branch (mixed or pure). The correlation between the partners is spread out space-like anyway, so no-one should expect a fixed time order anyway.. just rambling here... thanks again!

[@roflistganztoll]

@@jjhhandk3974 can you elaborate what she got wrong?

[@kdowey13]

​@@roflistganztoll She did not characterize the experiment properly-- it was a thoughtfully crafted experiment that did indeed erase which-path information. See this short vid, without narration it explains the part Sabine is mischaracterizing: kzbin.info/www/bejne/hmPGfnmojdaVj68 D1,2,3+4 are in operation simultaneously. D0 detector (supposed to be the same as the reference "screen" in the upper left in Sabine's video) correlates each entangled photon pair *in time* (time is tracked for each detector). By tracking each photon's arrival at the detector in time, we see that on D0 (the reference "screen") the photons that arrive either demonstrate single-slit (known path) or interference (unknown path) patterns, based on the *random* path that photon's entangled partner traveled through. This is the strange part, because the only opportunity for the original photon to self-interfere is at the double-slit, which is at the very beginning of the process. Her debunking boils down to the notion that there is "selective disregarding" between D3 and D4, but this doesn't make sense as @Frank S pointed out with the question: why are there distinct peaks and troughs (interference pattern) at D3 and D4 when the half-mirror is supposed to randomize the photon paths? Simply adding together D3 and D4 does not explain why an interference pattern was preserved on both of them. If anything, the fact that D3 and D4 patterns align begs an even stranger question about the trajectories of an unconnected string of individuals photon in relation to a half silvered mirror. @Fol Epi makes a fine attempt at explaining, but it breaks down and cannot answer Frank's question when you think of single photons passing through the system. The time-defying weirdness of this experiment that "everyone" has been talking about remains intact, and this is shown if you understand the video linked above. Cheers

[@davidnassau23]

This time, your explanation leaves me unsatisfied. Surely there is still some kind of retrocausality going on. The photons “knew” I was going to use D3 and D4 at the time they hit the screen; thus making it possible for me to find the interference pattern after the fact. Imagine I put D3/D4 in orbit around Neptune. That’s a substantial time lag. It seems one is left with only two possibilities: either retrocausation or superdeterminism. Both “ugly” concepts from our perspective because they destroy free will. The only way out of this (that I have thought of) is to think of time as an emergent property at a higher level that doesn’t actually exist at the quantum level. So at the quantum level, the photons didn’t hit the screen “before” their partners hit the detectors - that concept doesn’t apply. It only seems like that to us.

[@NerdENerd]

The random distribution pattern was laid down as the photons hit the screen, this is set in stone and nothing after the point can change the initial distribution pattern. But in that distribution pattern are an infinite amount of interference pairs we can can sperate out by correlating to which entangled partner went through a half mirror and which one reflected off a half mirror depending on the placement of that half mirror or the particular half mirror used.

[@davidnassau23]

@@NerdENerd It must be a finite number of pairs since it’s a finite number of points. But I see your point. The past didn’t get changed, only the interpretation of that past. Still feels like some kind of retrocausation to me though. I don’t think you can say it’s not very weird.

[@Ghostshadows306]

@@davidnassau23 I think it’s safe to say that anyone who thinks quantum mechanics isn’t that weird doesn’t understand that no one understands quantum mechanics. Ok, pretty poor attempt of being clever off Feynman. I’ll leave it though and just say that when anyone even slightly refers.to Quantum Mechanics as not being as weird as people have made it out to be, it raises a red flag regardless of who they are or what they are saying. Another flag is so many comments in here that basically say “ Oh, now I get it, no duh”. It’s an awfully complicated experiment and explanation to have such a “ah ha” revelation in my opinion. To be honest it makes think they don’t know what they’re talking about. When it comes to this experiment, I sure as hell don’t know what I’m talking about either. That’s what you people are for and I normally choose to listen to those who are most humble, most realistic and make the most sense.

[@NerdENerd]

@@davidnassau23 By infinite pairs I mean the continuous wave functions that represent the wave function of the initial random distribution and the infinite amount of pairs of wave functions that can be split out representing the interference patterns of each detector.

[@petersmythe6462]

Destroying free will is not an issue though.

[@Swerve-Online]

Didn't D4 or D5 revert to the interference pattern? Why overlap when you are trying to find out what is happening at D4 or 5?

[@JustXAshton]

Yeah, this lady doesn't understand the experiment, or science apparently. Pretty disappointing.

[@mygoogle424]

Interesting explanation, but i still dont get why D3 and D4 show a interference pattern. According to your explanation there should be a "blob" pattern. Am I missing something here?

[@pascalmestdagh5074]

I see no explanation about why the interference patterns for D3 and D4 are not on the same locations on the screen, but exactly seem to complement each other, could you elaborate on this please? Thanks.

[@opstube]

there is no screen for d3 and d4, there is a "coincidence circuit" that just tries to measure what slit was taken, and correlates with the screen detector. and this detector thingy splits up its "left/right detection" into a wave pattern

[@AlleyKatt]

Sabine always finds a way to debunk the mystical and add wonder. Science without the bull. Love it.

[@maecentric]

Mysticism is positing theres a physical reality outside of your conscious experiences - a reality you never have direct access to and one which breaks down in both quantum mechanics and general relativity - and of course the physicalist abstraction never accounts for the mind experienceing it

[@gwiz6278]

​​@@maecentric Through the use of ritual one can access this realm. The opposition of mysticism and science is a false dichotomy that is perpetuated by both in the current age.

[@MrCmon113]

Quantum mechanics is way worse than the mystical. No matter which way you put it, something people considered obviously true has to give.

[@ohedd]

What I don't get is why D3 and D4 form an "anti-pattern" with respect to each other. If I threw a bunch of coins on the ground, and then I separated them somehow, so half of them fell right through the carpet, then I wouldn't get an interference pattern. I would just get the same distribution of coins as I had before, only the pile would have half as many coins on it. They wouldn't randomly arrange themselves in a certain pattern. So what causes D3 and D4 to produce equal, but opposite, interference patterns?

[@schmetterling4477]

You don't get a pattern anywhere here. The pattern only "develops" in your data analysis.

[@yuryshulaev]

​@@schmetterling4477 The fact that the pattern is not apparent until the data is analyzed does not mean that it wasn’t there, just that it was hidden. Unless the analysis is shown to be flawed, this is irrelevant, and showing that the pattern is lost when the separation is undone doesn't count, because that’s to be expected.

[@schmetterling4477]

@@yuryshulaev That's the difference between correlation and causation. In causation A causes B. In correlation you can't tell if A causes B or if B causes A or if neither is the case.

[@yuryshulaev]

@@schmetterling4477 I didn’t make any claims about what causes the patterns - that’s a separate issue. I just don’t like the approach of making them disappear with "one simple trick" of jumbling stuff back together and suggesting they were made up or unrelated. And the coin "analogy" strikes me as hand-waving.

[@schmetterling4477]

​@@yuryshulaev What keeps happening here is that people are mistaking correlation for causation. For one thing, even if you analyze this classically in a Galilean setting the claim that the later event of a correlated pair caused the earlier one is complete nonsense. Why would it not have been the earlier event that caused the later one? What's wrong with that assumption and how would you differentiate between the two scenarios? You can't. It's a completely unsupported claim that this experiment violates causality even by that simple argument. That we are mixing some measurement magic in there by modifying the momenta distributions of the two quanta is just some additional level of obfuscation. Take the following example in a classical scenario: I separate a pair of cans with paint. One is always green, the other always red. I send one to Australia. In Australia somebody takes the unknown paint and mixes it with blue and I mix my remaining can also with blue. Now one of us has purple paint (red + blue) and the other one has a shade of cyan (green + blue). Did the mixing in Australia change the color here at home instantaneously? In this case the "color mixing" is quantum mechanical momentum mixing. The momenta of the entangled quanta are correlated and the double slit obfuscates the momentum distribution just like the color mixing was obfuscating the original color in the classical experiment. None of this gets you "back in time" in any way, shape or form.

[@michaelfox9675]

Sabine: At 4:40 you speak of the spins of the individual particles. The essence of the 2 spin states being entangled is that the combined spin state of the 2-particle system cannot be written as a simple combination (a tensor product) of individual spin states (wavefunctions) of the 2 particles - the individual spin states do not exist until the entanglement is broken by measuring the spin of one of the particles - until that time, the combined spin wavefunction isn't a tensor product of two individual spin wavefunctions.

[@Porquetegusta]

I love this video. I couldn't find another one that explained it so well. concrete and simple. Excellent, it shows that you know what you are talking about. please do not stop making these jewels!

[@TheDudeWithSome409]

The old 'Obfuscated Science' A good way to sound brilliant while republishing someone else's work.

[@abrisvegas]

I still don’t understand how the beam splitter just prior to detectors D3 and D4 can select which photons get reflected/sent through in order to correspond to the two different interference patterns. When picking up scattered coins, we can select which coins to pick up in order to get a pattern we want. But how does the beam splitter know a particular photon’s entangled pair corresponds to the D3 interference pattern and send it to D3 (and the same for D4)?

[@piercingspear2922]

Physics need more people like you, Dr. Hossenfelder! :D

[@nziom]

Agreed

[@stephenrichards5386]

Global warming needs more people like Sabine

[@aniksamiurrahman6365]

Don't put her in a pedestal. Science communicators have to over-simplify and mystify anyway. Being a researcher herself, she's just able to find out other's gap and make a nice niche for her KZbin presence.

[@mckanebullerlee3020]

Understatement! She is gold. Academic community needs help!

[@bonerici]

There's only one Sabine

[@gauharsingh7015]

Isn't it weird that if you only consider d3 and d4 and forget about the rest, you have two observers that see interference patterns individually but together they don't ?

[@doggo6517]

The observers don't see interference patterns [landing on the screen] because they aren't looking at the screen. What they're doing is recording yes/no information ("did I get a particle which was fired at time T on detector D4?") and then combining it with data on the screen retrieved later ("show me the position on the screen which the particle fired at time T landed. But I'll exclude the data point if I got a 'no' in my first question about D4")

[@FreestyleTraceur]

@@doggo6517 Oooo okay this makes sense. I was still a bit confused from Sabine's explanation. But I understand what she meant by the coins example now. Basically, whether you use D1,D2 or D3,D4 you won't be looking at patterns created by the same individual particles that would've/should've been in the same place.

[@artyomfomenko2232]

Haha, I actually remember how long I read the Wikipedia to understand these details and came to the same conclusion. Such a good feeling to be on the same page with Sabine :)

[@odomobo]

Same here!

[@Cjeska]

Sure you did.

[@CeezGeez]

🧢

[@AlexTorres-qv3hv]

Then you read it wrong...there's no reason to deduct that graphs at d3/d4 actually overlaps....if you go over the mechanical description, the screen at D0 has a step motor that moves the screen only in one direction across the x axis....also there's no such thing as "being selective" the coincidence counter tells exactly how to pair the photons absorbed at d0 with those absorbed at d1d2d3d4

[@localverse]

@@AlexTorres-qv3hv To confirm, are you saying Sabine's video is correct or incorrect?

[@user-zp4jt6vx6l]

I was deciding on which video should pop my q-eraser Cherry and selected yours first. I LAUGHED when you referred to all of the others. I chose well!

[@jonatanblais957]

I'm afraid this video completely misses the point of the experiment. Retrocausality is a red herring and has never been the real issue. The real issue to consider and explain is: why do we observe different results at D1/D2 vs D3/D4 ? Because this experiment in fact addresses the issue of the measurement problem. It was designed to test the hypothesis that, e.g. in the double slit experiment, it is the physical interraction between the wave/particle and the detector that causes wave function collapse rather than the observer obtaining knowledge of the particle's position. The design is very clever as it isolates the variable "observer obtaining knowledge" by setting up things such that, this is the case for detector D1 and D2 (the observer could in principle know which path the particle went by), but this is not the case for dectector D3 and D4 (the observer could never, even in principle, gain knowledge about which path was taken since this information has been "erased" by the use of a combination of mirrors and half-silvered mirrors). However, the nature of the physical interractions between wave/particle and detectors are the same (identical detectors and wave/particle in both D1/D2 and D3/D4) in all cases. 1) If wave function collapse is due to physical interractions between wave/particle and detector, one should observe the same patterns (a "collapsed" pattern) from the D1, D2, D3 and D4 detectors. 2) If wave function collapse is instead due to the observer getting knowledge about the system, the patterns should be different depending whether photons were detected at D1 and D2 ("collapsed pattern") or D3 and D4 ("interference" pattern). When one does the experiment and recover from the data which detector detected each particle hitting the screen D0 (with the use of a coincidence counter), the results agree with 2) above, ruling out the hypothesis that wave function collapse is the result of a physical interraction between wave/particle and detector. This thus implies, because it is the only difference between D1/D2 and D3/D4, that it is indeed the observer having the possibility in principle of getting knowledge of the trajectory of the particle that makes the difference between an interference pattern (wave behavior) at D3/D4 and a collapsed pattern (particle behavior) at D1/D2. What's really weird about this is that under a naturalist/physicalist/materialist metaphysics (ontology), there's no reason to think that whether or not an observer could in principle know something about the physical world should have any impact whatsoever about the behavior of the physical world, and yet it does... Meaning that these results are incompatible with such metaphysics... In the early days of quantum mechanics, physicists were willing to face this issue and talk about this problem openly. For some obscure reasons, it seems to have become taboo among many contemporary physicists...

[@neftu3588]

Isn't it extremely ironic that the "interaction" people have always dismissed other views as quantum woo, adopting a high horse attitude, yet now there is even experimental proof that the interaction explanation has also always been quantum woo. If anything, QM has shown us that the universe doesn't even have separable parts that could interact. "Interaction" is a tool from Newtonian physics, not a real process of nature.

[@jonatanblais957]

@@neftu3588 "Isn't it extremely ironic that the "interaction" people have always dismissed other views as quantum woo, adopting a high horse attitude, yet now there is even experimental proof that the interaction explanation has also always been quantum woo." Yes. From a psychological and sociological perspective, it is fascinating that such clear experimental refutation of the idea that some sort of physical interaction could possibility explain wave function collapse and the deep philosophical implications of this refutation, is not more widely acknowledged and discussed in the scientific community. In the philosophy of science community, this is probably more widely recognized but I would guess only among those specializing in the philosophy of quantum mechanics like Tim Maudlin and David Alberts...

[@elgatoconbolas]

Thanks Jonatan. I am no physicist but if some experiment (proposed by John Wheeler himself) has been repeated and verified through the years , it would be difficult to debunk it in a 20 minutes vídeo unless it is debunking something else.

[@jonatanblais957]

@@elgatoconbolas What I find very concerning about this kind of "move along folks, nothing to see here" video such as this one, is that it gives the layperson the completely false impression that quantum mechanics doesn't really have profund philosophical implications that are very difficult, if not impossible, to fit within a materialistic/physicalistic/naturalistic worldview. Delayed choice quantum eraser experiments are just impossible to make sense of without acknowledging that the way the physical world behaves depend on what an observer can or cannot know (in this case the which path information) about the physical world... This fact deserves to be known by the general public. Not hidden behind pseudo "debunking" videos...

[@PeaceTrainUSA-1000]

Thank you for pointing out the measurement problem aspect of this experiment, as it is critical. To do a video on the topic and not address it is misleading and suspect.

[@tomamberg5361]

'Nick Lucid' at The Science Asylum did an episode clarifying this as well. His take-home point is that you have to remember that the screen is a detector as well, influencing what happens at the 'formal' detectors of D1, D2, D3, and D4. kzbin.info/www/bejne/n6qxY2qIbLp3pKs

[@rodrigoappendino]

There is a guy in the comments in this Sabine's video that comment that Sabine didn't mention de D0 detector, while she should, since it's part of the experiment.

[@truthisthenewhatespeech9572]

@@rodrigoappendino what?

[@classicalphysic]

The screen or D-0 appears to be slightly ambiguous. It’s observed data is presented as a screen. But in the experiment it is a moving photodetector supposedly. Scanning the interference pattern over time to build up the pattern. Kind of like a reverse printer printing the image one line at a time. However it does pose questions. If the D-0 detector only measures the overall interference pattern in small increments. How does the fact that Detectors 4 measures the whole beam all at once? If that makes sense. Anyways what actually happens is D-0 observes 2 rapidly overlapping interference patterns. These are because the two coherent interfering beams going toD-0 are circularly polarised. ( as we all know this will produce 2 overlapping interference patterns) D1&2 only recieve 1/2 the cycle. And thus only each see one interference pattern. As observed, and confirmed by Sabine. D 3&4 on the other hand recieve both haves of the cycle and are matched to both interference patterns ( ie a diffraction pattern) . As observed and confirmed by Sabine. Not only is there no QT magic, It is also explained better as a classical experiment using waves and polarisation.

[@hesseldekraai]

@@classicalphysic >It is also explained better as a classical experiment using waves and polarisation. To be honest that is true for basically all derivations of the double slit experiment. They all make a lot more sence if you look at it through the lense of classical waves. The only consistent weird part about it is that in the end they turn back into particles at the detector. (if you are using electrons for example)

[@kdowey13]

@@classicalphysic She did not characterize the experiment properly-- it was a thoughtfully crafted experiment that did indeed erase which-path information. See this short vid, narration free, which you may have not seen before: kzbin.info/www/bejne/hmPGfnmojdaVj68 D1,2,3+4 are in operation simultaneously. D0 detector (supposed to be the same as the reference "screen" in the upper left in Sabine's video) correlates each entangled photon pair *in time* (time is tracked for each detector). By tracking each photon's arrival at the detector in time, we see that on D0 (the reference "screen") the photons that arrive either demonstrate single-slit (known path) or interference (unknown path) patterns, based on the *random* path that photon's entangled partner traveled through. This is the strange part, because the only opportunity for the original photon to self-interfere is at the double-slit, which is at the very beginning of the process. Sabine's debunking boils down to the notion that there is "selective disregarding" between D3 and D4, but this doesn't make sense due to the somewhat obvious question: why are there distinct peaks and troughs (interference pattern) at D3 and D4 when the half-mirror is supposed to randomize the photon paths? Simply adding together D3 and D4 does not explain why an interference pattern was preserved on both of them. If anything, the fact that D3 and D4 patterns align begs an even stranger question about the trajectories of an unconnected string of individuals photon in relation to a half silvered mirror. The time-defying weirdness of this experiment that "everyone" has been talking about remains intact, and this is shown if you understand the video linked above. Cheers

[@leogama3422]

Thank you, Sabina! These experiments and explanations were driving me crazy. Quantum physics has enough weirdness even without past-changing phenomena

[@mikekidd5975]

Sabine says that even if you detect which slit the wave function goes through then it’s still a wave function, only a single slit wave function. But surely the wave function is smeared across both slits. And the act of detecting which slit the photon goes through forces the wave function to collapse to a particle therefore it cannot continue as a wave. I’m confused?

[@schmetterling4477]

The wave function isn't going anywhere. A wave function is like a probability distribution: it doesn't even exist until we are performing an infinite number of experiments.

[@BeetMachine]

Thank you for this explanation, Sabine! I’ve struggled to make sense of this experiment for years and it’s honestly been driving me nuts. I feel like my sanity has just been restored!

[@Gunni1972]

Pretty much: "The sun is not bright enough to show me the world....unless i take off my 5 polarized glasses"

[@WylliamJudd]

I actually thought some of these videos that you mention already made this clear. Maybe I'm not understanding what it is they got wrong, but I remember them saying that you need the information in the present to know which particles to disregard and create the interference pattern in the past.

[@WylliamJudd]

@@dextermorgan4490 the issue is that in order to observe something you actually have to interact with it, so when you add an instrument to detect which slit a photon goes through, you interact with the photon at the slit and cause the wavefunction to collapse. Exactly why and how wavefunctions collapse is still not well understood (as far as I know).

[@somebodysomewhere5571]

The version of the experiment you present wasn’t the original one done in 1999 it’s a almost completely different experiment

[@jasonrosenblum6846]

Any chance of doing a vid that explains what’s actually weird -and not- about quantum computing? Thanks!

[@johngough2958]

Or, as Feynman put it, there is only one mystery - remember what goes on in the two slit experiment, that's it!