The Trouble with Many Worlds

[@Jehannum2000]

A brilliant video which takes several difficult concepts and makes them perfectly understandable without losing any of the essential meaning.

[@bat2293]

@paul w Ok, fair enough. However, she does express them in a perfectly understandable way and the essential meaning is clear (to me at least). Sabine - Thank you for sharing your "interpretation and feelings" with us.

[@EffySalcedo]

Doc Sabine also has the term a 'place-holder' which I wanted to hear when there are currently no official nomenclature at the moment.

[@docholiday8029]

@paul w Good distinction Precise language is important in any dialogue. And, I do love these videos

[@docholiday8029]

@@bat2293 I agree with you

[@mintakan003]

She's one of the clearest and most concise "explainers" around. This video is 7 minutes 42 seconds long. (LOL. And I mean it as a complement.)

[@aleksandarignjatovic3130]

Thank you for your work. As a 55 year old man who is crazy about science, and having read many books or listemed to many lectures know that it takes a special talent for explaining stuff. You have it. We (serious laymen public) need you and especially critical thinker like you. Keep it up. And your music also.

[@SabineHossenfelder]

Thanks for the feedback, much appreciated!

[@pokeman123451]

Bertrand de Born You should read this book... it’s called Lost in Math: How Beauty Lead Physics Astray. 🧐

[@TheCabIe]

@Bertrand de Born "People should pause to think. Modern Physics is explaining less and less and creating more and more fancy words and more and more rabbit holes each contradicting itself more than the last theory. Worse they have lost the sense of all the singularities and absurdities that they have created." You make it sound like that's being done on purpose and there's some conspiracy going on. Yeah, we have seemingly figured out quite a lot of shit about the Universe so it makes sense that the last few bits may be REALLY hard to grasp and we need to think in radically different ways than before. It's possible we have evolved so differently that being able to understand the very foundations is next to impossible. It doesn't mean we should stop trying.

[@GonzoTehGreat]

@Bertrand de Born Modern Physics has just moved beyond the comprehension of the layperson, assuming it ever was understandable by the average person, which is questionable. Just because you don't understand it doesn't mean it's nonsense. That's an arrogant and ignorant position to take. Also, you're actually only talking about Theoretical Physics, which is so named because it focuses on Mathematics to improve our understanding of reality at the fundamental level by unify our existing knowledge into an underlying common theory (aka a TOE). However, modern physics also has wide applications in engineering, technology and medicine and most physicists work alongside engineers and technologists in these areas, designing lasers, optical fibers, semiconductors, thermal engines, power plants (nuclear and renewables), machine learning algorithms, medical scanners, weather and climate prediction models and much more...

[@ssm445]

@Bertrand de Born I don't think he assumed your a lay person to anything science. He could tell. As a demonstration, kinetic energy does only apply to objects of mass - hence the m - while your photon is mass-less. A non-lay-person would not even think in the direction your "argument" headed.

[@zicada7661]

Would love to hear you debate this with Sean Carrol over on his Mindscape podcast. Hope he invites you!

[@kjustkses]

zicada Carroll has missed the pee pot some time ago and is still pissing into the wind. It started with the duel arrows of time and it’s just getting worse.

[@dimitri1462]

Da Koos - I also agree, he stopped making sense some time ago

[@JuBerryLive]

@@kjustkses You don't believe in emergence? I think it's a good way to talk about the physical world.

[@sergeynovikov9424]

​@@dimitri1462 have you read his new book "Something Deeply Hidden" -- what is your impression? Sean doesn't insist on giving the final answers, but tells us (so as Sabine) that our present understanding of the world is incomplete and illusive, while trying to show some ways for resoving the fundamental problems of physics. surely he is brilliant in his ability of clear speaking about complex things from different areas and the idea of his talk with Sabine sounds promising and interesting to me.

[@kjustkses]

Ju Berry I prefer Freeman Dysons approach. If you can’t prove something in science then you don’t know anything about it scientifically. Carroll says that the many worlds interpretation should be taken seriously since it is the simplest explanation. I don’t think it simple at all. What mechanism causes the worlds to split?

[@scotthammond3230]

I dont like the measurement thing. We are not outside the universe. Everything is just a particle interacting with a particle. How is a measurement different from any other interaction?

[@SabineHossenfelder]

Exactly!

[@RolandOrre]

There is one case though, and that is the von Neumann-Wigner interpretation, which is my favourite, as it's almost the only physical hypothesis which deals with how physics and consciousness interact.

[@TheSimonScowl]

How can you prove the universe is inside you (and I'm totally serious)? It's kinda like a dog telling another dog that the tail he keeps chasing is inextricably attached to him. And that's not to say there's nothing to be seen here. I love science as much as anyone. Chasing our tails is fun, after all. But we keep learning everything from the inside out. What if reality is inherently outside-in... like dreams and hallucinations?

[@paulwaddy6061]

@@SabineHossenfelder I feel like the only sensible way to define a 'measurement' is as an instantaneous exogenous change to the Hamiltonian of the quantum field?

[@aneikei]

@@SabineHossenfelder Perhaps particles do not have have a wave-function to collapse. Perhaps we "conscious beings" are the wave-function and every time we take a measurement we collapse our individual wave-function and coalesce into a reality where those outcomes have become certain. Perhaps conscious beings are "quantum mechanics". Instead of the universe being in a constant state of superposition and splitting. It's we conscious beings who are always in a superposition state and thus are always surfing the multiverse. Hence every time we "collapse" we make a brief pit stop in that universe until our wave-function is reestablished and off we go until our next collapse in another universe.

[@nevis4567]

You're my new favorite physicist. The sheer amount of rationality you have while building an argument is more of what we need to see from the world.

[@q09876543]

I remember taking a course on the theory of learning. The biggest understanding I took away from it was, to find out if you have understanding of a subject, teach the subject. Believe me, when you have a debate with another individual over the same subject, you will truly learn if you know that subject or not. This woman truly knows her subject to the point of getting her point across to even people who have only a partial understanding.

[@rabeeet]

This lady is so underrated. Legit all the videos are clear, convincing and quiet practical.

[@quasarsupernova9643]

She is appropriately rated as brilliant and articulate by most who have seen her videos..

[@rabeeet]

Ron Maimon why so negative bro

[@rabeeet]

Ron Maimon just because she disagrees with them doesn’t mean she disrespects them comeon

[@rabeeet]

Ron Maimon you do have LQG and I’m not against string theory at all, I’m a fan but after all those years there still is no proof for it. Anyway I don’t even debate online so let it go

[@quasarsupernova9643]

@Ron Maimon I dont have to memorize the scriptures to criticize religion. String theory is religion with Popes and high priests. It describes a fantasy world we don't inhabit...

[@dillonkeller4477]

I like how quantum physics is so foreign to our way of thinking that the best technical explanation is "it's just not a thing"

[@seper159]

Her explanation is incorrect, many worlds is much more elegant than se understands

[@rjbeery3]

Dr. Hossenfelder's logic is razor-sharp and she explains the contentious issues in the intersection of physics and philosophy brilliantly. I look forward to each video she has made.

[@pmcgktr]

Your channel has grown so much since just a couple years ago. professional & informative

[@dariopalomba8420]

Gentle prof. Hossenfelder, I just began to follow your videos and I would like to congratulate you for the quality of exposition. As I'm not a native English speaker I have to watch twice your videos, but it's not a problem. Thank you again for your job and greetings from Athens.

[@bazoo513]

Very clear, dr. Hossenfelder, much clearer than most of the other presentations on the topic I have seen.

[@manfredadams3252]

Dr. Hossenfeffer has the best Physics videos on youtube. Very subdued gesticulation too. Some other physics chicks are out of control with that.

[@bazoo513]

@@manfredadams3252 I find PBS Space Time series presented (and mostly written) by Matt O'Dowd, himself a practicing physicist, also very interesting and well presented. But Sabine's series has some, umm, iconoclastic flair to it :o)

[@sumdumbmick]

are you deaf? it's nonsense.

[@bazoo513]

@@sumdumbmick What is nonsense?

[@sumdumbmick]

@@bazoo513 if you listen to what she's actually saying she contradicts herself quite a few times in quite grand ways. like when she asserts that the Copenhagen Interpretation is just an approach that accepts the functionality of the maths and doesn't require making up stories about how it might actually be working (or in other words, interpreting what the maths means), but then yaks on about the consequences of accepting that interpretation (which by her own definition that she just provided is not an interpretation, and thus not something that you even can accept). don't confuse your inability to follow her reasoning for her being smart.

[@EffySalcedo]

This presentation is world class. (If not many worlds ✌)

[@josephjohnson3738]

Ass kisser.

[@platinumedge7886]

@ Effy Salcedo -- A very intelligent and thoughtful response Yes it is a world class. Very good presentation By the way I notice you got a whopping about 70 thumbs up and only 2 ignorant idiots down with the use of unacceptable language -- Pay no attention to them for they are losers. Nobodies and will never achieve anything Continue your excelllent posts. Cheers

[@josephjohnson3738]

Almost all sciences are now very, very, very, far beyond what we actually know about things. Thanks to you science types lost in the world of magical bullshit that you WANT to believe you've discovered. Science today only exists in the realm of inferrences and speculation. Or you can call it mathematical suggestion science. But call it what you will, because of all the pretend discoveries, it is in the same quality and caliber of flat earther science. Equally valid in terms of evidence. For instance, what evidence is there for black holes or neutron stars? And I don't mean what speculations you have, but what real evidence do you have for your modern science crappola proclamations? None, just admit it!

[@platinumedge7886]

@ joseph shalosky -- there are literally thousands of examples but a recent famous one is the magical “ God” particle aka Higgs-Boson which was mathematically derived and speculated existed in 1960s and only proven to exist 1/2 century thats 50 years later in the LHC for which the Noble Prize There is an entire branch of Theoretical Physics indeed all Sciences at all Universities have Research Depts dedicated to pure research for future advances

[@quantumwater6582]

@ joseph shalosky -- All modern inventions are because someone researched then conducted an experiment

[@ggg148g]

Excellent video of an excellent series. On this particular topic Sean Carroll convinced me that the many world interpretation doesn't have the problem Prof Hossenfelder ascribes to it. There it goes a quick resume of Sean Carroll's explanation: we, conscious observer, are also quantum systems, we also live in a superposition. When we become aware of a measurement, a particular many-world branch of the quantum system that describes our body, i.e. the particular branch corresponding to the stream of consciousness that we call "self", becomes entangled with the system we are measuring, and that moment is the many-world counterpart of the wave function collapse. Is it sound? I would love to have prof Hossenfelder opinion on this, but I don't dare to hope for such a privilege.

[@vegn_brit5176]

I've just landed here. This has already become my favourite channel on KZbin.

[@peterhall6656]

I would have to say this is one of the most succinct and incisive explanations ever given on this subject in such a short video.

[@imjustpassinthru]

If that's your impression, you should read an article titled "Quantum mechanics and reality," authored by physicist Bryce DeWitt and published in the journal Physics Today in September of 1970. It was DeWitt who coined the term "other worlds" as a description of Everett's theory. The paper I'm referring to can be found on the internet.

[@tripleplonk]

Watching your videos is an exercise in humility. Keep them coming!

[@steveHoweisno1]

I just don’t get this. How is the detector definition equivalent to the wave function collapse hypothesis? I feel like she skipped something. In many worlds the wave function just keeps on evolving according to shrodinger but what we observe depends on which branch we are in and there is self localizing uncertainty which I think makes sense at least to me.

[@PavlosPapageorgiou]

The wave function doesn't branch. It's a continuous multi-dimensional space. When you "measure" you entangle the particle with a few particles in the detector, and those with more particles of the detector, and those with everything else (decoherence). The wave function continuously describes all the outcomes that are consistent with a finite amplitude, and the many more possibilities that are inconsistent like "half left" have near zero amplitude. It's a continuous space, not logical branches. And your amplitude at any point is minuscule given all the evolution since the start of the universe but you don't care, you experience relative amplitudes.

[@sergeynovikov9424]

@Pavlos Papageorgiou : When you "measure".. when speaking so, you're making somehow a transfer from the finite/infinite Hilbert space with the WF to our 3d+t world with gravity -- how do you do that? taking into account that there is no a complete quantum theory which includes gravity yet..

[@ollywright]

I am of similar mind, but then I've recently read work by Sean Caroll who is describing something similar to what you are saying. A way of thinking about it is the 'wave function of the entire universe' contains all possible (ie in any sense probable) descriptions of the 'many' worlds. No branching is occurring. Having said that, this is all up for debate, and we should remain open minded. I'm glad to see the subject is finally being addressed more widely, rather than it being relegated to the 'don't talk about it' vault of the (entirely insufficient) Copenhagen Interpretation.

[@faisaldebouni]

I'm not sure I understand the "it's not branching argument" correctly. But if each branch already exists, doesn't that mean that each branch by itself has a local hidden variable? how doesn't that violates bell inequality?

[@innocentsmith6091]

@@ollywright It seems like a relatively reasonable version of MWI, and, I believe, the actual original formulation of it. While I've heard his book is pretty good, the problem is Carroll has promoted in interviews and pop science articles the literal branching interpretation.

[@olegkorobkin6235]

If you don't mind... you seem to be arguing for the possibility of reducing the measurement postulate to peculiarities of the Schroedinger equation for many-particle systems. Why are you sure that such a reduction is always possible? For once, Schroedinger wavefunction evolution is a manifestly reversible process which cannot be said about the measurement.

[@mathewomolo]

I like your no-nonsense approach to facts. New sub

[@joachimelenador6259]

Critical thinking just flowing out of the video, like a watermark of Sabine, really extraordinary channel along other following this kind of topics. No sensation no drama, absolutely perfect.

[@MountThor]

I find your method of explanation of these difficult subjects to be quite excellent and helpful, please keep it up Sabine. Thanks!

[@seper159]

I find them to be incorrect xD

[@andyiswonderful]

Just finished your book, "Lost in Math". Greatly enjoyed it.

[@Thomas-gk42]

Just finished 'Existential Physics', just the same

[@aosteklov]

I like the phrase in your book Lost In Math: "In some of them (splitting world branches) you will understand this." I was laughing loudly when reading it. I think the many world interpretation is an "easy" solution to a really hard question. It doesn't teach us something new about our universe, but rather give an excuse why we can't understand the measurements. This is not science, it's more like a mythological/mathematical story that is used to reconcile frustration. Your explanations are top notch. Thank you for everything you do.

[@ThePinkus]

I disagree, in the sense that the multiverse notion is intended to provide a perspective that makes sense of the "measurement problem" as conditionalization, not an excuse why we can't understand it, quite the contrary. We can argue if it succeeds, but that is what it intends to do.

[@aosteklov]

It may be an elegant "solution" but it will be hard to prove this is what actually happens and it will not lead science to a fruitfull theory. It will not reduce complexity but rather make things even more unobtainable. I am afraid it will be dead end like string theory... But it's good to have a discussion about it because this is one of the biggest open question in science and it's good there are different approaches now to try and solve it

[@WALLACE9009]

YET IS THE TRUTH. Everything that can possibly be exists.

[@epajarjestys9981]

@@WALLACE9009 how do u kno? pls explain. thx.

[@denysvlasenko1865]

@@aosteklov > it will be hard to prove this is what actually happens It should not be possible to "prove" one interpretation over another, since they all predict the same outcome for any experiment. That's why they are "interpretations" of quantum mechanics, not separate theories.

[@edreusser4741]

This is, just like every other video by this presenter, absolutely brilliant. I have been sharing your videos with every technical-minded friend I have Sabine. Please keep it up. You do us all a tremendous service with your work. I can hardly wait until you get all the bugs worked out of your superfluid solution to mond. My guess is that there is a very well-earned noble prize in your future.

[@SnaFubar_24]

You are such an amazingly talented teacher of physics! Thank you for all these videos, the topics are explained in a way that I grasp much better than with other science educators.

[@urfinjuice1437]

I am not sure that Many-Worlds is represented accurately here. E.g., as I understand Sean Carroll, in his interpretation the wave function does NOT really collapse. In the experiment, e.g., the electron becomes entangled with the measurement apparatus.

[@davidwuhrer6704]

Of course the particle becomes entangled with the observer. That is true in any interpretation. What she is saying here is that after the observation, you are in a random world with one possible outcome, which is equivalent to wave function collapse: You have to re-normalise your probabilties for further predictions. The difference between CI and MWI is that with the latter, all other possibilities are also realised, but not measured. And while Everett has argued that his interpretation is testable in principle, nobody has managed to so far.

[@clemonsx90]

I agree, I was just about to type something to this effect. Wavefunction collapse is a deviation from the Schroedinger equation we use to talk about what we see as conscious observers locked in a particular basis. As a part of the system, we experience our entanglement with the system as probability, but the actual underlying reality is just smooth Schroedinger evolution. Usually people invoke DECOHERENCE as the way in which entanglement can be irreversible and thus measurement can result in permanent collapse. It's sort-of like a statistical mechanics argument where strangely quantum deviations from experience are just not going to be experienced. So far as I know, in experiments where measurement data is physically destroyed, such as by measuring the measuring device in an orthogonal basis, wavefunction collapse is reversed. Which is just to say that measurement as entanglement seems completely inline with experiment. A recent 2018 thought experiment by Frauchiger and Renner claimed to throw doubt on ALL interpretations, but to do this for the many-worlds they once again had to invoke this idea of observers OUTSIDE the system and insert the non-Schroedinger dynamic into the problem. The thought experiment just extends the Wigner's friend experiment slightly and asks for consistency between multiple observers. Quite frankly, MW is the only interpretation in which the universe is actually governed by the Schroedinger equation, rather than Schroedinger Equation + SOMETHING, so it will likely persevere over the centuries, while the other interpretations will not.

[@davidwuhrer6704]

@@clemonsx90 Decoherence is a phenomenon that you also see with MWI.

[@davidwuhrer6704]

@roki feler If there is no time, then what separates the past from the future? Quantum immortality is a thought experiment designed to point out the absurdity of how laymen get MWI wrong. It has nothing to do with reincarnation either.

[@davidwuhrer6704]

@roki feler The Big Bang is an ongoing proces. Feynman's interpretation that anti-matter moves backwards through time (and thus there is a mirror universe in the past) is untestable and more of a joke. The MWI is one interpretation of Schrödinger. Changing the equations would require different interpretations. One thing about the Schrödinger equations is that they treat time as symmetrical. That is not unual, most physics equations do. In statistical mechanics, time has a clear direction, because of entropy. In statistical mechanics, fhe change in entropy is the passing of time. Memory is irrelevant. Essential is the undeniability of not only the directionality of time, but also an absolute separation between past and future, which we experience. You are confusing a lot of things, and accumulate all kinds of misconceptions, to arrive at a conclusion that is blatantly wrong. And yet you think you can tell which thoroughly tested equations need to be changed to support your conclusion which blatantly contradicts the interpretation that you think you support. The MWI is meaningless without time.

[@jean-pierredevent970]

There is a variant on many worlds called "many interacting worlds" where there is no branching but all those slightly different parallel interfering worlds like slices are already there. It looks a lot like "modal realism" and it can even be combined with the block universe view.

[@aaroncurtis8545]

Hey, that's interesting. I was contemplating the fairly recent Russian nesting dolls paradox; and applying it to the Mandela effect phenomenon all the kids are talking about. I was saying to my daughter that the nesting dolls problem could be an explanation for the Mandela effect without the need for many worlds. Seems like that jives with what you're saying; in that sitting around smoking having fun and talking about it way; not in the provable way.

[@jean-pierredevent970]

@@aaroncurtis8545 Russian nesting dolls?? Mandela effect ?? Awtsj, I must look that up, I am left behind :- ). That many interacting worlds interpretation is described here. But I don't know how far they got with it meanwhile. It's hard to find much update. www.newscientist.com/article/mg22429944-000-ghost-universes-kill-schrodingers-quantum-cat/

[@RogerLuedecke]

I love this sooooo much. I've thought similarly for a long time about the Copenhagen interpretation, but you not only articulate it better but as a proper physicist you still manage to school me despite there being total agreement.

[@vincewatkins8439]

Stumbled onto your videos Friday and have been watching them for two days straight. Thank you!

[@garethbarry3825]

Can i just say Dr Hossenfelder that in addition to being brilliant, you a brilliant teacher? Thank you for taking the time to make these videos, bringing enlightenment to the rest of us

[@SabineHossenfelder]

Thanks for the kind words.

[@tim57243]

The version of MWI that makes sense to me claims that detectors are macroscopic objects that decohere when detection happens, and that is why you don't see a detector in a superposed state. Wikipedia says this too: en.m.wikipedia.org/wiki/Many-worlds_interpretation in the second paragraph. Basically the idea is that an object with many degrees of freedom will tend not to interfere with itself because when you multiply together a large number of numbers with norm less than one, you effectively get zero. The large number of numbers is one per degree of freedom for the macroscopic system. Sabine doesn't mention decoherence. I don't know if the intent was to mention it in some of the follow-up posts. Edit: She now has a video about it here: kzbin.info/www/bejne/n5jWpnyrfsR7d7M I watched this, and it is relevant, but she says it does not solve the whole problem. She seems to be planning to make at least one more video to finish the topic. Edit: the claim that decoherence doesn't solve the measurement problem is at 11:30 of the latter video "Understanding Quantum Mechanics #5". She isn't making use of the idea that if you multiply many numbers less than one you get approximately zero, and she is limiting to two basis vectors, so there is an opportunity to get it to come out right here. She is using the idea that if you add up a bunch of random complex numbers with norm 1, you get approximately zero. But measurement can happen when you have only one photon, so that can't be relevant.

[@Rykvp]

Do you know if she did finish the topic with another video ? I can’t figure why she thinks MWI + decoherence doesn’t “solve” the measurement problem.

[@tim57243]

She now has "Understanding Quantum Mechanics" numbered 5 through 8. I haven't watched 6 through 8.

[@Endymion4242]

Just happened across Sabine's presentations a few days ago and have watched quite a few. What precise thinking she uses; this is extremely impressive and critical to doing good science. I look forward to watching the rest of her presentations and learning to think more clearly about physics.

[@omp199]

"What precise thinking she uses" You spelt "muddled" wrong. Anybody who takes any interest at all in physics can spot very easily where her thinking goes wrong. She is assuming that an act of measurement is somehow outside the the realm of quantum physics: that the measuring apparatus is somehow exempt from obeying the Schrödinger equation itself. This is such an obvious mistake that I find it impossible to imagine how she misses it. When you make a measurement, the system that is doing the measuring interacts with the system that is being measured. The systems become entangled. The fact that you can observe only one part of the wavefunction in no way implies that anything has "collapsed". You can only see part of the system because you are now inside the system.

[@DiegoItzep2012]

I enjoy your insights and deep dives into the problems with theoretical physics. Easy to follow arguments. Great clarity. Thanks. As an aficionado photographer I also enjoy the upgrade of your videos. Backgrounds and outfits are tasteful. One suggestion. Photographers either show mid body shots or full body shots. We avoid the 3/4 shots by convention. Not sure why. Cutting feet is unorthodox. But hey. No fast rules.

[@daybertimagni4841]

Great video. Couldn’t agree more, I’ve always thought the Many Worlds interpretation is a ‘lazy’ way of explaining the gaps in the current model.

[@Tehom1]

I don't find MWI problematic as argued in the video. I think I understand what you're arguing: Why should a detector branch such that each branch of it agrees 100% with some respective branch of the universe, ie why should it do the branching right? In answer, first I'd point to would-be detectors that don't branch. One example is the part of the lab setup before the measurement; the part where we carefully maintain the particle in superposition. This part of the setup can be viewed as a would-be detector that doesn't branch and therefore doesn't detect. My second observation is that it's difficult to make it work that way. Absent a great deal of care, anything and everything behave likes the first type of detector, the type that does branch, albeit without convenient readouts. And thirdly, there is some leakiness in even the best lab setups. You're lucky if you can get 90% of your photons or w/e thru your setup without losing superposition. So you can picture the joint wavefunction of any detector and detectee as characterized by a degree between 0 and 1 of what we might call branchingness, regarding some measurement of interest. To make that more exact, we'll view both detector and detectee as superpositions, and a very branched wavefunction is one where there is some normalized component of the detectee that, projected onto some component of the detector, has amplitude of nearly zero. The rest of the definition follows along the obvious lines. So we can at least mathematically entertain the concept of a partially branched wavefunction regarding some measurement. Then we say, albeit in a less precise way, that almost every system amplifies branchingness for almost every measurement, excluding only systems like the careful laboratory setup above. Thus branchingness almost always converges to the 100% agreement that we asked for. So I don't find that problematic.

[@ivanfromunion3513]

"I don't find MWI problematic as argued in the video": What you said was very good, actually. But it doesn't convince me anymore than Carroll does: what if what is being interpreted as "many worlds" is just a fractal copy at a different resolution? (see my previous comments with suggested definitive tests somewhere on this page)

[@Google_Censored_Commenter]

If the detector doesn't branch, then you would find yourself in a universe without a detector 50% of the time. Unless of course you assume the detector interacts with the air molecules, the air with you, and the rest of the universe, and thus branching doesn't occur at all. And then we're right back to the measurement postulate, with no branching occuring. There is no such thing as "gradual branching", because once again, it's not what we observe. There is no "almost every measurement". We've done TRILLIONS of measurements, not once have we ever observed a deviation, 100% of the time we get what we expect.

[@quantumdave1592]

Are you high?

[@LucaAmadeus]

Thank you very much for this video on this interesting topic. I would like to discuss some points. "you already know that you can not derive this detector definition from the schrödinger equation, it's an additional assumtion, a derivation is not possible." (6:50) I'm sure you can derive the behavior of the detector aswell as the observation from the schrödinger equation without any new assumptions. The many worlds interpretation just assumes that everything in the universe obays the Schrödinger equation. Nothing more, nothing less. The MWI specificly does not say "the detector is by definition only the thing in 1 branch" (6:38). It says that the detector is described by the schrödinger equation which means its in a superposition (so it's not a single entity at all). The fact that we (as humans) only see one detector clicking is simply explained by the evolution of the schrödinger equation. We are also part of the superposition (decoherence). The MWI is so tempting because it does not need anything other than the schrödinger equation which makes it the simplest and most elegant interpretation that we have. Is it perfect? Not at all. But it gives rise to a number of fascinating conclusions about how probability works and what it actually is (no other interpretation does this so far). Why should a reality in which detector "A" clicks be "more real" than the reality in which detector "B" clicks if the porbabilites are the same? Where does this "breaking of symmetry" come from? In the MWI this symmetry stays unbroken (I recommand this lecture of David Wallace regarding this topic: kzbin.info/www/bejne/b3LTm3SPibecotE&ab_channel=PhilosophyOverdose) Thats why I think the MWI is the best we can do right now. Knowing it won't be the final answer to all of our questions.

[@tac6044]

I will simplify in my own worlds. For anything to be possible everything must be possible.

[@timoluetk]

Still seems artificial to me. All the branches upon subbranches just pile up into infinitys exponentially. And nothing does explain how to make the shift into one specific reality. I'll believe it if it's testable, like if somehow parallel dimensions are detectable. Otherwise it seems more philosophical in nature not scientific. If it can't be tested I don't trust it.

[@LucaAmadeus]

@@timoluetk Just to clear up some misunderstandings that often regard the MWI: There is not an infinite amount of worlds. Only states that can be derived by applying the schrödinger equation to the wavefunction of the universe when it came into existence are "realistic" worlds. The MWI does not "create" those countless worlds artificially. It is a straight forward conclusion without adding any extra assumptions into the Schrödinger equation. One that does not want to conclude these countless worlds exists needs to find a different theory. The birden of proof lies on those who don't trust the MWI and not the other way around. Having said that I can empathize with your standpoint only trusting theories that are falsifiable. I just think that shouldn't stop us from deriving possible answers to those questions based on a theory that is as good as QM.

[@timoluetk]

@@LucaAmadeus thanks for that expansion on your thoughts. I'm not a physicist so my assumptions are kinda superficial. Just from the outside looking in it seems like a big conclusion to make. And an unfalsifiable one too.

[@LucaAmadeus]

@@timoluetk I can totally see why it is difficult to jump on board with this theory.

[@MarkArandjus]

I'm five minutes in and have come to the conclusion that this is not the best video to watch at 4AM just before bed.

[@TheHsubh]

Thank you for vocalizing this. I struggled to put in words how I felt the multiverse theory was not actualized. I felt that probability doesn't equate to actually what happens.

[@Thomas-gk42]

exactly, how I felt too, Sabine makes it acurate🙂

[@dhoffheimerj]

I love Sabine. She is so articulate and to the point.

[@terrykosowick594]

This is the clearest explanation I have heard of the problems with these interpretations of quantum mechanics. Thanks you.

[@Alekosssvr]

Sabine this is one of your best videos. Excellent!

[@Thomas-gk42]

My thought with every lesson I see made by Sabine

[@discogodfather22]

Would be great if you could go into Bohmian mechanics and pilot wave De Broglie-Bohm theory in a video Sabine!

[@huntingape]

Chris Morlock Agreed! Would love to hear Sabine’s take on Bohmian mechanics!

[@simonjgriffiths]

Me too !

[@aleksandarignjatovic3130]

@@simonjgriffiths Perhaps you can give this clip a look. kzbin.info/www/bejne/jXrchY16fdqUqLM

[@devilisahomo]

I love these fake accounts promoting this propaganda. Hilarious

[@g.v.3493]

Aleksandar Ignjatovic That video is awesome! Fifty years ago I read an (even then) old book about “wave mechanics” and I knew that had to contain the answer. I never bought into the Copenhagen interpretation. Thank you for posting the video!

[@hervebozec7756]

In my opinion you should be granted physics Nobel Price for making things CLEAR

[@Thomas-gk42]

You're so right!

[@NextLevel-kv5kn]

My problem with MWI is that universe only splits in two when the probabilities are 50-50. In general, probabilities are irational numbers which means universe has to split in infinitely many branches all the time. That's a lot of branches.

[@chriscontact5857]

Oh god, three of you. I think I am going to die and explode from my happiness. Keep up the good work. Thank you. That will be all.

[@OtakuByNight]

Looking forward to a future pilot wave video. ;)

[@kjustkses]

Please share this link with Sean Carroll. His latest book had me at a loss for words... not in a good way.

[@planetnicky11]

Alois Raich Hi! Thanks you for shedding some light on this. I haven’t read Carrols book so I can’t contribute. But I did like how Sean presented many worlds. I would love to hear Sabine and Sean discuss! Please share your best material from this topic with me!

[@Luthiart]

I can't even get through one of his LECTURES without annoyance... I have no interest at all in reading any of his books.

[@rockets4kids]

Better still, get Sabine Hossenfelder on Sean Carroll's podcast!

[@myothersoul1953]

​@@aloisraich9326 What you said is false, Sean Carroll is fine with saying "we don't know". He recognizes all knowledge is tentative. "The truth is, scientific knowledge is inevitably tentative, not metaphysically certain. " See: www.preposterousuniverse.com/blog/2006/12/19/what-we-know-and-dont-and-why-2/

[@ollywright]

@@aloisraich9326 I have to you are attacking a straw man version of Sean Carroll there. The simple fact that he invites many physicists on his podcast that hold different opinions on the interpretation of quantum mechanics, suggests he likes them. He certainly is a big fan of his own theory, but that's hardly rare for a scientist.

[@zoperxplex]

The problem with Many Worlds is that in order for it to be consistent, i.e. avoiding the contradiction where in one world the cat lives and changes the entire atomic structure of that universe and in the other the cat dies and it's atoms and all that it represents in terms of its interaction with that universe, there would have to be an entire universe created for each quantum event.

[@ramonatila6277]

I'm really happy I discovered this channel

[@KeboEdan1]

Thank you. I am very pleased I subscribed to your channel, looking forward to many more fine presentations. Greetings from Bali.

[@atimholt]

I always understood “measurement” and collapse to be equivalent to a sort of integral of “knowledge” about the microstate of a system-the collapse is reflective of every microstate that can allow the particle to be detected there-a kind of hyperfocused constructive interference pattern. It can only happen because of the wide inharmonic spread of whole-system wave functions (i.e. entropy).

[@darektidwell1158]

Shots fired. Update: 100% probability it took the path to something deeply hidden within the Department of Physics at the California Institute of Technology.

[@JoachimVHW]

As I understand it, many worlds proponents consider their interpretation to rely on less postulates than for example the Copenhagen interpretation. They just say: "there is a wave function, which evolves according to the Schrödinger equation". This would make it, to them, not equivalent to interpretations that make an additional proposition of a collapse of the wave function. If we have an observation with two possible outcomes, the branching that happens I have heard formulated in terms other than "branching detectors" - the branching happens as "result A and observation of result A" and "result B and the observation of result B". The way they think of the probability of finding themselves in one branch versus another, after the system under observation has become entangled with the universe, is exactly given by the Born rule: they very much insist on calculating the probability of every branch. In fact, if I have that correctly, they insist on doing that and nothing else.

[@jehadrizk1934]

U make science complexity easy to understand. Thankyou

[@clarencegood7490]

Creating imaginary dragons to fill the dark void of our ignorance .

[@zenitovec2]

This is sooo amazing, oh my gosh! Thank you Sabine, you’re the best!

[@claironaut]

I absolutely love the interpretations of the double slit experiment. Quantum mechanics is so mysterious and exciting to think about. I'm not set on any answers, but making sense of these interpretations is great food for thought.

[@schmetterling4477]

That's cool, kid, except that the double slit is not even quantum mechanics. Feynman simply pulled a physics joke on you with that one.

[@schmetterling4477]

@@thesystem6246 There are no particles. There are only quanta. Quanta exist only when an irreversible emission or absorption process is involved.

[@schmetterling4477]

@@thesystem6246 No, that does not work, either, but I can't take your religious feelings about the universe away. Only you can do that by reading a textbook about quantum field theory.

[@psychohist]

This video misrepresents the Everett/MWI interpretation. MWI just says the Schrodinger equation continues to be true and that there is no quantum collapse. It does fix the problem that other interpretations have of making the predictions of macroscopic systems theoretically impossible; you can use the Schrodinger equation to make predictions for macroscopic systems just as for microscopic systems. What Hossenfelder describes as "branching" is simply different superposed Schrodinger solutions becoming macroscopically different. In the Schrodinger's cat thought experiment, it's that a triggering event of the timing of a nuclear decay gets magnified into a macroscopic cat. Two microscopic states which are superposed - nucleus decays now versus nucleus decays later - are set up by the experiment to evolve into two macroscopic states - cat dead versus cat alive, which then evolves into scientist sees and publishes finding that cat is dead versus scientist sees and publishes finding that cat is alive. These can be discussed as "branched" uniiverses, but in fact, they are simply superpositions of two Schodinger solutions. Both solutions - Hossenfelder's "branches" - still continue to be part of full, superposed Schrodinger solution that defines the universe; it's just that, like all superposed solutions, they can't generally "see" each other. They can interact with each other only through interference, and that's true for macroscopic solutions - Hossenfelder's "branches" - just as it is for microscopic solutions, such as in the two slit experiment on individual photons. Research into quantum entanglement and decoherence over the past decade or two continues to support Everett/MWI while falsifying Copenhagen style "collapse" for larger and larger systems.

[@cartossin]

Thank you so much for explaining this. No matter how much I read about many worlds, I can't find anything that convinces me that I should believe in it. Why must we say what happens to the unobserved? I always hear that waveform collapse requires more assumptions, so we must believe many worlds. But why? Can't we just say that we have an observed part and the other part either does not exist or is in another universe? My biggest problem is that we cannot differentiate "in another universe" from "does not exist". Both concepts are experimentally identical.

[@AlexanderFarley]

It would be interesting to have a video going into detail on how the MW interpretation is logically equivalent to wavefunction collapse. I didn't quite follow the probabilistic arguments regarding detectors on different branches.

[@DarwinsStepChildren]

You're not alone. How certain things are defined needs to be explained before any discussion involving the wave function collapse and the many worlds interpretation even begins. Is there only one wave function, is there one wave function that describes each universe, or is there more than one wave function in each universe? I can say that Hossenfelder is describing the collapse of THE wave function. Meaning that she is taking the view that there is only 1 wave function that describes all universes. Let's take a water molecule that exists in universe A. THE wave function had to collapse to form this water molecule in universe A. Even though the individual hydrogen and oxygen atoms could form bonds with other elements in universe A, meaning this particular water molecule is no longer classified as a water molecule anymore, and even though the atomic particles - neutron - in this water molecule could decay, the matter/energy of this water molecule in universe A will forever and always be conserved in universe A. Saying that the mass/energy will be conserved, is the same as saying that the wave function collapse that formed the water molecule will always and forever be conserved. Stating that the mass/energy of this water molecule cannot spontaneously disappear from universe A, is the same as saying that the subatomic particles that make up the water molecule cannot spontaneously go backwards and become a part of THE (non collapsed) wave function again. Because there is only 1 wave function, and because subatomic particles cannot become a wave function again, this is the reason why Hossenfelder states that the probability of a detector in a universe like this should always be 1. Hossenfelder is taking the position that it is possible for different detectors in the same universe to detect different directions, i.e. left or right, for a photon, and she is stating that it is possible for the "same" detector to detect different directions in different universes, however, it is not possible for the same detector in the same universe to detect different directions. Example - It is possible for detector z in universe A to detect a photon moving right in universe A, and it is possible for detector w in universe A, manufactured exactly the same as detector z, to measure a photon moving left in universe A, however, if detector z in universe A measures a photon moving right in universe A, then it should always detect a photon moving right in universe A, no matter how many measurements are made, and detector w in universe A should always detect the photon moving left in universe A, no matter how many measurements are made. Now, it is also possible for detector z to detect a photon moving left in universe B, however, If detector z in universe B measures a photon moving left in universe B, then it should always detect a photon moving left in universe B, no matter how many measurements are made, if only 1 wave function exists. Because, in our universe, the same detector has been observed to detect some photons going left and some photons going right, this is Hossenfelder's argument against the many worlds interpretation. Again, based on the view that only 1 wave function exists. However, many physicists take the view of many wave functions existing. Even though they call it the collapse of THE wave function, suggesting there is only 1 wave function, they actually mean the collapse of many wave functions. Some physicists take the view that each "object" has its own wave function. The observer, the detector and the particle being detected each has its own wave function describing it. Some physicists take the view that the wave function itself splits. If there is 1 wave function describing universe A, and a measurement is made to collapse this wave function, the wave function splits into 2, and there is now 1 wave function describing universe A, and a second wave function describing universe B. This splitting obviously continues exponentially. Essentially arguing that because there is more than 1 wave function, there should be many, many, many, many etc. worlds. I agree, each physicist's personal view, 1 wave function vs. many, 1 per universe vs. many per universe, needs to be defined before any discussion about the wave function(s) or many worlds even begins. Without initially stating one's personal view makes the explanation difficult, if not impossible, to completely understand.

[@AlexanderFarley]

@@DarwinsStepChildren Interesting - I think I am on the side of multiple wavefunctions. Which ensembles we are averaging over or integrating is a crucial detail.

[@angeldude101]

I personally take the stance that there is 1 wave function for the entire universe that includes the quantum state of every object in that universe. "Many worlds" then aren't separate universes, but rather just large scale superpositions shared by collections of particles. Since I don't view the whole universe as splitting all at once, I only see quantum events as making "bubbles" that expand at (a maximum of) the speed of light that grows by interacting with particles outside the bubble, which happens no faster than the speed of light. Multiple bubbles then can overlap just like ripples in a pond, with the intersection containing the product of the original two superpositions.

[@alvinlindsay2381]

This video clearly explains the Many Worlds Interpretation, while not supporting it. It finds problems with other interpretations as well. Overall, it is a brilliant presentation. Cheers, Professor Hossenfelder!

[@psychohist]

Sorry, but it doesn't explain MWI correctly at all.

[@AbeDillon]

My understanding is that an interpretation is different from a theory, it's a way of conceptualizing the math of a theory. Supposedly MWI is what you get when you "take the math at face value" as Sean Carroll puts it. From what I understand, this means: the math of probability theory involves calculating weighted sums of different possible outcomes. QM uses probability theory not because there is some underlying mechanism for which we have incomplete information, but because the evolution of the world is literally determined by sums of different possible outcomes. Supposedly, Copenhagen treats one particle's (the "observer") frame of reference as special and its interactions ("observations") as special (they can collapse waveforms), but if you consider the frame of reference of all particles to be equally valid, then observation is just entanglement and MWI is the result. Some people claim that makes MWI strictly better than Copenhagen by Occam's razor. I think I read (maybe skimmed) a paper a while back about self-localization uncertainty in Hilbert-space being the source of "uncertainty/hidden variables" in MWI. I think the gist was you can localize yourself in 3D Euclidean space by measuring your distance from 4 non-coplanar points, if the points are coplanar, they provide redundant information. Analogously, you can specify what world you reside in by the outcome of every un-entangled measurement you can possibly make (or something?) and entangled measurements provide redundant information. Again, as Sean Carrol says, the Hilbert space for a universal wave function is HUGE.

[@dmshunt]

great presentation -why have you and sean carroll not done a debate on this? - it would be fascinating

[@Kyrowebb]

But at 1:10 that is assuming that there is only one version of you observing the particle. What if there are multiple versions of you already existing observing their particular particle in their particular universe?

[@schmetterling4477]

There are not even particles, kid. ;-)

[@frankb2659]

@@schmetterling4477 I have a few questions I was wondering if you could answer. How does QFT explain/relate to violations of bell inequality/quantum entanglement and and how does it explain "spooky action" observed in such bell tests? (I'm a novice, if I'm not making sense or misunderstanding something horribly please forgive me). Also, does quantum mechanics mean that the universe is not deterministic/random and what do you think of Nigel Cook? Thanks for answering if and have a nice day.

[@schmetterling4477]

@@frankb2659 There is no spooky action, you simply don't understand either quantum mechanics or relativity.

[@frankb2659]

@@schmetterling4477 can you help yo understand the things I asked about? is entanglement something that actually happens? I know that I do not understand quantum physics which Is why I am asking you

[@schmetterling4477]

@@frankb2659 I told you the solution: there are no particles, there are only quantum fields. Entanglement is simply quantum mechanics plus conservation laws. All of your "problems" with this stuff are in your head.

[@rv706]

2:06 "The wavefunction collapse is not linear" - How's projection onto an eigenspace not linear? Maybe she meant "is not unitary".

[@Teleleco_do_ifood]

I think what she meant is that the wave function is linear and the projection is linear but the transition between the two linear functions, before (the wave function) and after (the dirac delta like function) the measurement is not linear.

[@amapolo7211]

Agreed, it has no sense

[@mikhailryzhov9419]

If you know what eigenvector to project onto that’s a linear function, but you don’t as a random one is chosen through an unknown, assumed purely random, mechanism, so it’s not even a function, let alone a linear one.

[@rv706]

​​​​​@@mikhailryzhov9419: Just saw the comment. Well, it is definitely a function: the fact that we don't _know_ which one doesn't matter. Look. Evolution under the Copenhagen interpretation works as follows (no matter what physicists who forgot their linear algebra tell you). There's a (nonlinear) function L from the positive real numbers to End(H), linear endomorphisms of your Hilbert space H. So in particular, for every t, L(t) is a linear function from H to H. Given an initial state ψ(0), the evolved state ψ(t) is given by ψ(t):=L(t)ψ(0). At all times t, L(t) is a composition (depending on t) of unitary operators (coming from usual Schroedinger evolution) and projection operators onto eigenspaces (coming from "measurements"). Despite the fact that L(t) is linear for every t, the weirdness comes from the fact that the map L is _not continuous_ from the positive real numbers to End(H) - it has jumps precisely at those t in which a measurement occurs. Another strange fact is that the theory doesn't allow us to know at a time T what L(t) will be for t>T (the theory is not deterministic). There are interpretations, such as the GRW interpretation, in which evolution is given by an actual stochastic differential equation. Nonetheless, at each time t you can write ψ(t) in the form L(t)ψ(0) where L(t) is linear (but in general non unitary) and we don't know L(t) deterministically for future t.

[@fallenlegacyz]

The other Me in another universe understands this... while I write this comment.

[@locutusdborg126]

My head is spinning, at least in this world. In another world I comprehend this video. Unfortunately, I'm stuck in the first world.

[@daves2520]

And in another world you are just finishing a bowl of ice cream before going to bed. Oh, the possibilities!

[@imankhandaker6103]

Hi Sabine - I'm confused! I thought that the 'many-worlds' interpretation; was a 'single wave-function' interpretation? No wave-functions ever collapse, but detector & detected (both quantum systems) simply become more deeply entangled? At least from DeWitt, quantum decoherence seems to (partially!) play the part of Copenhagen collapse - no physical mechanism described, but at least the quantum probabilities (with oscillatory form) fade into more smoothly varying classical probabilities. I once studied the minimum size of a copper 'block', for the specific heat capacities to behave as 'bulk'. It turned out that a 3x3x3 cube (27 atoms) was comfortably big enough - I still find it hard to believe that something that is all surface, behaves as bulk! My question is - should it not be possible to devise an experiment where two such 'nano-bulks' are entangled & then alternately used as detector & detected? 27 atoms is still small enough that the system is thermodynamically reversible - but wave-function collapse is irreversible. Is that why Copenhagen insists that detectors are a mysteriously 'super-classical'? How many atoms does the block need, to function as a detector, does its sampling rate improve as reversibility becomes less & less likely. Or do the blocks need to be distinct (say one atom of a different isotope?), for the experiment to work? As a physicist, I am ashamed to say that I have no intuition about these things at all. I thought that the essence of quantum mechanics, was that you could not observe a system without changing it. So in contrast to the beautiful symmetry of observers in relativity; the sheer asymmetry of a detector that is itself undetectable, leaves me perplexed!

[@sumsar01]

I think that in modern experiments you can measure currents from a single atom. So, in theory, you could probably use that as a detector. But tbh I think I prefer Feynman's interpretation: "Shut up and calculate". All this is unknowable with current theory and is thus just fantasy. Interpretations of quantum mechanics are unmeasurable and thus just religion. An interpretation is only worth talking about if it makes unique predictions otherwise it just speculations.

[@imankhandaker6103]

@@sumsar01 I COMPLETELY disagree. I think that was David Mermin BTW. 'Shut up and calculate' is not an interpretation at all - just an attempt to forcibly (& dishonestly) silence questions that cannot be answered. The shut up is a giveaway - that is low religious explanations start, not scientific ones. Why did we search for the Higgs boson - when can calculate anything with m in it ... no explanation needed. Engineering might be all calculation, but physics is understanding what is being calculated. "Look mum, I can ride with no hands!" "Son, where are you going?" "Shut up & watch me ride" What is the point of being able to ride around a campfire, when you are in complete darkness.

[@gabedude68]

@Iman Khandaker - Great posts! Shame this discussion has to be buried so deeply in the comments. (BTW anyone: is there a quick way to do @name in my reply so it shows up in blue?) Here's MY interpretation of the theory (I don't see how Sabine's argument presents a problem with this one either): There is no "decoherence".. its just whether you checked which exact "world" you're in, or not. If you have 10 Cat Boxes, then after an hour, each has a probability of each cat being dead.. theres also probabilities of ALL the cats being dead, or ALL alive, and every combination. Thats all. Suppose you wait another hour. The "Many Worlds" continue to "branch", you just don't know which you're in, so you only have Probabilities.. and none of the cats are ever "20% dead and 80% alive" so it doesn't even matter what they "add up to" at any point - the reality is always 100% dead or 100% alive... so NOW you check the boxes... nothing mystical happens, you simply find out (in my example) that all your cats are, in fact, dead (and you are a bad cat owner).. After this "measurement", nothing magical about Physics has happened, its just that if you seal the boxes and do your Quantum Math and think that, NOW, there is any chance of finding a Live cat from here on, then you are simply being stupid, or too blinded by the Math to apply common sense. I believe the "Shut up and Calculate" is a bit tongue in cheek, but really means: "this is a way to work out the Probability, not an complete final explanation of what is happening, exactly, and why" - or.. the "10 Cat Box" Teacher has her students work out the probability of, say, 10 cats being alive after 1 hour, and 5 cats, after 2 hours.. and a student keeps asking: "does that mean that some of the cats are slowly dying as the second hour passes?".. and the Teacher, frustrated, is saying: "you're not getting it. Do the darn Math and you will know the Probability, it won't tell you anything about the cats, just the % that this one Outcome is the one you find at 1, and 2, hours".. Anyone who can clearly explain why my reasoning is wrong, please, I would love to read it... Personally, I dislike "Many Worlds" because it sometimes seems to make everything we do, ultimately pointless..

[@imankhandaker6103]

@@gabedude68 Thanks.

[@gabedude68]

@@imankhandaker6103 It's an honour to be talking to an actual physicist, who is clearly passionate about the field, on a subject I am similarly passionate about. Sadly my brain was not built to navigate the actual mathematics at that level, but having spent a lifetime trying to gain a deep understanding of everything despite that, it would be nice to know if some of my thoughts are at least on the right track, before my time is up, or help someone with greater potential, fill in a missing puzzle piece that might help them. I love Sabine's videos, btw, but sometimes she doesn't take enough time to explain/explore a topic, and sometimes doesn't dig deep enough. You mentioned being ashamed that your intuition fails you - don't be! - human intuition doesn't work at these scales.. though I draw on something similar to search for answers, I try to expect logic and self-consistency rather than "common sense" or assumptions about -anything- even if space and time even exist, etc. You maybe should make your own videos, or some longer discussion format on Reddit etc, if you have time. I would also like to try to discuss this further, if possible. My email is adrian underscore speight at hotmail dot com, though I don't check it often.

[@timhourigan6257]

Danke sehr, Professor Hossenfelder! Ich genieße wirklich alle Ihre Videos!

[@jarahfluxman20]

*Professorin😊

[@Thomas-gk42]

​@@jarahfluxman20 Einverstanden, leider hat sie bis heute keine Professur, die sie längst verdient. Der Lohn einer Rebellin. I love her new book

[@JuBerryLive]

Can we replace the expression " every time a mesurement is made" with " every time a quantum interaction happens"? "Mesurement" sounds like a human needs to be involved. Quantum interactions happens everywhere, all the time, anywhere that isnt pure vacuum. The moon is still there when a human do not mesure it. Every single subatomic particule in the universe is being "mesured" by it's surrounding at every moment in time. The number of "worlds" it produces, if many world is correct, is ridicoulusly high.

[@SabineHossenfelder]

No, not every interaction constitutes a measurement. A measurement, roughly speaking, amounts to an amplification of the signal.

[@ThePinkus]

The motivation is not human needs, as shown by the fact that such motivation has no bearing on classical physics. Rather, that we are not confident in a clear cut distinction of what is objective and what is epistemic in the theory of QM. Yet, we still have to place ourselves on firm ground when using QM, the epistemic narration of our observations and the corresponding projection rule does that, and we cannot achieve the same result by only considering interactions, at least in a way that we agree upon (it's about the interpretation of QM). The problem with this practical solution is that, of course, it's awkward to have something that should be objective reliant on something epistemic. But as Sabine points out, to use QM we need the projection rule, because e.g. there is an observable difference between a photon going either left or right and a photon going through a superposition of the two paths, and we are confident to use the projection rule only when we made epistemic observations. Now, of course we have decoherence, but one thing is decoherence and another is deriving an interpretational solution to the measurement problem that clarifies the above issues. I like this, I deem it is exactly what we need as objective before/independently of our epistemic observations, but I have to point out there's disagreement on in at the interpretation level. PS: for a distinction of QM with or without the projection rule, it is interesting, though a bit technical, to follow Frauchiger & Renner's argument considering the pure interaction representation of their narration (as in their work) or the observation/projection representation (not in their text, it has to be worked out independently) (e.g. arxiv.org/abs/1604.07422v2 ). The conclusion is quite different. The decoherence treatment of the argument is more subtle, esp. in the v2 version of the article, as both conclusions have merits, each in its own sense/scope.

[@JuBerryLive]

@@SabineHossenfelder it's a bit clearer now. So, a mesurement is done only when something interact strongly with a particule?

[@JuBerryLive]

@@ThePinkus thank you

[@massecl]

An amplification of the signal doesn't make a measurement, there can even be a measurement without an amplification. I think that the difference between interaction and measurement is when there is a feedback on the system being measured. The equations then become non linear. That would point to a dynamical collapse, but it is excluded by Bell type experiments. Anyway the purported "success" of quantum mechanics has blinded most of the physicists, who believe this means there is some truth in it, however unsound it is. Non sequitur, mathematical coincidences are common in physics.

[@ScottJPowers]

another problem with the many worlds interpretation is, why would the universe split because something happened to allow for different outcomes? Do you have any idea the amount of energy and how that energy would have to be applied to split the whole universe? the reality is, everything that has happened was alway going to happen that way. you just didn't have enough information to determine that. Likewise, going forward, everything that happens was always gonna happen. Things may seem to be random or have certain probability to us because we don't have all the information needed to make such a determination of what's going to happen, but reality is made up of particles with particular mass, energy, and velocity and will interact in particular ways, so there is no room different outcomes or randomness. some of you just can't believe that you're not actually in control of anything. In your egocentric mind, you want to believe that, somehow, you are making decisions out of thin air that are uniquely yours.

[@pitdog75]

A discussion with prof Sean Carroll would be interesting.

[@Bgrosz1]

For me it wouldn't really be a discussion. I would listen and finish with "I agree".

[@lenn939]

@Bryon Grosz With whom? They obviously disagree about Many Worlds since Carroll is a big proponent of it.

[@yvesnyfelerph.d.8297]

Carroll is a many worlds moron. Can't stand him and his whiny voice. Never understood why he is always on top of everyone's list when it comes to popular physics scientists. I take Alan Guth, Brian Greene even Neil Tyson or Michio Kaku over him any day thank you. Carroll has zero to offer that the others don't but they are much more pleasant to listen to. I find him repulsive

[@thetruthchannel349]

I see Sean Carroll as more of pop philosopher than anything else. We could imagine the Universe to be anything just about and find some way to constitute it with something that SOMEONE has proposed or written about in the past 400 years.

[@thetruthchannel349]

@@yvesnyfelerph.d.8297 *Sean is a likable guy but I dont take him seriously at all.

[@redriver6541]

Thank you Dr. You have helped me understand these things better than anyone else.

[@attica7980]

Good, because nobody (other than you) understands quantum mechanics. The mathematics is the simple part; what it all means is the difficult part.

[@SquirrelASMR]

0:57 the wave function collups?

[@schmetterling4477]

There is no such thing. There are only people who don't understand physics. :-)

[@lsb2623]

The only mathematical confusion with Many Worlds is that Sabine is an absolute goddess in EVERY world.

[@discogodfather22]

Funny true story: my computer mouse is broken. When I clicked on this video, it spawned 4 windows and they all played at once. Many Dr. Hossenfelders......

[@yosefgomgom225]

Imagine how small the probability it happened

[@DukePaprikar]

Please let us know what she said in the other 3 worlds.

[@thetruthchannel349]

I think there are other realities relative to this one. But I dont view them as being multiverses or even 'copies' with different outcomes. Thats sci-fi nothing more. Ive seen too much to entertain Naturalism. I know theres more. What it is and how it is? Ive no clue and its most likely going to remain that way. Thats just the way it is. You wont get published writing that but its true anyway.

[@howaboutataste]

The wave function collops? Sounds tasty.

[@jjay6764]

Great video! I think the recent Wigner's Friend experiments helps gets us closer to an explanation. If you look at the recent Wigner's Friend experiment, it seems to support Carlo Rovelli's Relational Interpretation which says there's no collapse. Wigner's Friend carries out a polarization measurement. Before he does, the quantum system is in a superposition of horizontal/vertical polarization. He carries out a measurement and gets horizontal. He records that outcome. The record and the quantum system are sent to Wigner outside of the lab. Wigner does an interference measurement and sees interference between a record of his friends outcome and the photon. He concludes that his friend in the lab hasn't carried out a measurement. So at 1 P.M. Wigner's Friend can say,"I measured Horizontal polarization but Wigner outside of the lab can say,"At 1:10 P.M. I measured interference." So Wigner can say his Friend hasn't carried out a measurement yet but we know he did. This says the knowledge of the Observer is important. Once Wigner's Friend calls him up and says,"Hey Wigner, I measured Horizontal polarization." Wigner can no longer measure interference. Say Wigner's Friend(O) is going into the lab to measure the spin of an electron(S). Before the measurement, the spin is in a superposition of up/down. Wigner's Friend(O) becomes part of the S+O system when he observes the state of the electron. The S+O system now contains spin up/spin down, an observer observing both spin up and spin down, measuring apparatus that measures both spin up and spin down and a lab where spin up is measured and a lab where spin down is measured. O can only interact with one part of the system because he can't see interference, therefore he thinks he observes collapse. He can't measure interference because he's part of the S+O system and doesn't know it's own Hamiltonian or interaction Hamiltonian for the measurement. So a collapse never occurs. It's just O has a lack of information about the S+O system and sees what he thinks is a classical outcome but O' outside of the S+O system can see that there wasn't any collapse and measure interference. Now when Wigner(O') gains knowledge of the measurement outcome of his friend, he now becomes apart of the S+O system and can no longer measure interference.

[@elmundodeFreeman]

I love your explanations, you are everything good with the world.

[@greghansen38]

At university we used the "Shut up and calculate!" interpretation.

[@lepidoptera9337]

So you never went to the library to read up on the things that your teacher didn't tell you? I did.

[@greghansen38]

@@lepidoptera9337 The teachers talked a little about interpretations, and I had read about them on my own. But here's the thing--they're just not very useful. They don't add anything to the design and interpretation of an experiment, they don't help the student solve problems, they simply don't come up. Interpretations are ABOUT quantum mechanics, but they are not quantum mechanics.

[@lepidoptera9337]

@@greghansen38 I didn't say that you needed to read up on interpretations. As you said, they are not useful. They are also not necessary. Nature gives you a natural physical interpretation in quantum field theory. The problem with "quantum mechanics for beginners" is that they are not actually teaching you real physics. The world does _not_ behave anywhere close to what non-relativistic QM suggests. If you were to try to probe the location of an electron inside an atom with ever higher precision, for instance, then all you would achieve is to ionize the atom. If you then ramp up the precision of your measurement by another three to four or orders of magnitude, what you get is not one electron but pair production of electron-positron pairs. At a few hundred MeV of probe energy muons will pop out, then taus and eventually quarks and gluons and W and Z. We have no idea what cute stuff happens at 10TeV. Non-relativistic QM is not even a theory. It does not describe the world correctly except in a very narrow range of energies. It does have the advantage of being simple, but that comes at the disadvantage of being non-physical. Reality is much more complicated mathematically, but ontologically it's much, much less convoluted than non-relativistic QM suggests. That is what your teachers didn't tell you... they simply didn't know.

[@greghansen38]

@@lepidoptera9337 I don't know, everything is still there in QFT but it's hidden in scattering matrices. When you shoot an electron at an ion, you still get "Which path did the electron take? All of them!" and "How many new worlds were created in that interaction? As many worlds as there were distinct possible measurements." It's just easy not to think about that because you don't deal with QFT Stern-Gerlach experiments which are carefully designed to highlight that stuff. It also doesn't help that in QFT the student solves problems that he had never seen before in another context. How much time did you spend on CLASSICAL scattering theory? We had a section in an E&M class, but I didn't understand it, and I didn't see a connection at the time. But the methods in the two regimes are similar until you add up your states in the classical problem and choose just one of them in the quantum problem. I think the path integral approach is no resolution, either; it just restates the "which path?" problem.

[@lepidoptera9337]

@@greghansen38 Scattering matrices don't have ontological problems. They are describing the scattering of waves from infinity with the outgoing solutions also being probed at infinity. This eliminates questions like "what is a measurement" completely (not that that's hard to answer in non-relativistic QM but one has to drag thermodynamics into the mix, which is not necessary in relativistic theory). It eliminates the classical-quantum system boundary. Infinity always provides more than enough degrees of freedom for decoherence without having to construct "measurement objects". Phase doesn't matter at infinity, it all automatically boils down to amplitudes, just to name a few advantages. I do agree with you all the way that we are not teaching classical scattering theory nearly enough. It is extremely important in practice (optics, electromagnetic systems engineering, radar, medical imaging etc.) but the average physics student gets to see almost nothing about it. I certainly didn't, except for a trivial 2-d Coulomb toy problem. We also didn't learn relativistic dynamics enough to have a solid handle on collisions and high-boost systems. That leaves accelerator physics mostly in the dark, even from a classical perspective. I was taught some nuclear physics, but basically all in the non-relativistic approximation. High energy physics? Forget about it. That, however, is the real "footprint" of the universe. OK, it would have been different at a university that had an active high energy physics program, but even those lectures (I took them later as part of my PhD) were kind of very basic and insufficient to understand QFT on more than a surface level. One can, of course, learn important lessons about QM from atomic, molecular and solid state physics, but the fact that we are teaching the formalism independently of its applications makes connecting the dots harder, IMHO. I hope that younger professors are slowly getting a better handle on how to give students a _working_ knowledge of QM, because for sure mine didn't.

[@daltanionwaves]

It's unfortunate when someone invests years of their life, grant money, and then gets tenured while doing work on a hypothesis that doesn't pan out, and then continues to pursue it anyway in order to make a living and maintain relationships and prestige... Many Worlds is a hypothesis, not a theory.

[@daltanionwaves]

This happens a lot more than many people realize

[@JurekOK]

. . . so what? `Many worlds` is not a hypothesis, nor a theory. It is an interpretation. And the drive behind creating interpretations is legit. If by "theory" you mean something that has been tested, then the Copenhagen interpretation has the same amount of tests going for it - zero.

[@Levon9404]

Sabine I hope more scientists will join you to become more realistic like you

[@Angarsk100]

I never suscribed to the "multiverse" theory, didn't know why, It just didn't make sense to me. Now, you made it all clear. Thank you.

[@dsnow6578]

And I've just subscribed based off one video because of your non-biased approach to explaining the ins and outs of MWI and CI

[@ekklesiast]

"Once you have measured the particle..." How exactly do you measure it? What is "measurement"?

[@proteus5]

Measurement = any interaction with the environment. So something as simple as bouncing a photon off the particle constitutes a measurement.

[@bobfish7699]

Finally - a physicist who lives in the 'real' world. Great explanation

[@MrRolnicek]

How was it possible to make this video without saying the word "decoherence" even a single time?

[@milesscheetz9841]

I was waiting for “fungible” and “uncountably infinite.” Unfortunately it seems Sabine doesn’t understand the Many Worlds interpretation.

[@Alexander_Sannikov]

i agree that act of the observer ending up in one specific branch is exactly equivalent to the measurment postulate. however, i don't see a contradiction here, because this means that manyworlds explains measurment without using that additional postulate directly, which is exactly its goal. what is the problem here?

[@shelley-anneharrisberg7409]

Thanks so much - really interesting video, and I don't think anyone has been able to explain the wave function collapse as clearly as you have!

[@seper159]

She hasn't explained it clearly, she doesn't understand the point of many worlds.

[@stevegrant7762]

I like the many costumes interpretation of Sabine's wardrobe. Seriously, how does she have a different outfit every week?

[@zoltankurti]

In my experience physicists realise the inconsistency of the coppenhegan interpretation, and they don't treat it as an exact theory but a useful calculational trick, a placeholder.

[@bogdanovist]

To an extent, but then too many physicists treat asking how we could do better than a place holder as taboo. It's getting better but there's been many decades of neglect in this area within the physics community.

[@massecl]

The Copenhagen interpretation is not a calculation, it is, well, an interpretation. There is the Born rule for the probability, but that can be experimentally tested. In the Copenhagen interpretation, the wave function has no reality (although we calculate it, but it would be the same if it were real), it only represent the knowledge we have on a system. When a measurement is performed, our knowledge changes, so do the wave function which collapse, but nothing changes in the system.

[@massecl]

@@bogdanovist The rationale of the physicists has been: since there is no solution, there is no problem. Indeed in the first half of the previous century, they hadn't the tools to find the solution. Now we perhaps have, but the scientific community has changed, and have accepted as a fact that there is no solution, and that the problem is us.

[@zoltankurti]

@@massecl what you described in your first comment is definitely false. If the measurement doesn't change anything about the system, a hidden variable should exist. And really, hidden variable theories seem to be more and more unpheasable. I'm thinking about the experiments testing bells and leggetts inequality for example. Ps: do you now understand that field operators for different spacetime points operate on the same hilbert space in qft?

[@massecl]

@@zoltankurti The system change because there is an interaction, but there can be a measurement without interaction, check out counterfactual.

[@wilhelmsarasalo3546]

If we can't interact with the other worlds, they don't exist to us.

[@carlhopkinson]

Wha? Did u ever hear of a quantum computer??? A quantum computer enlists the resources of multiple universes to perform prodigious calculations in parallel.

[@joshyman221]

Carl Hopkinson no it doesn’t. I’m sorry if you read that on a pop science article or something, but the fundamental part of wavefunction collapse is that you cannot go back to the original wavefunction

[@theoldone51]

That's like saying there's no more space or matter beyond the observable (observable from any arbitrary point in spacetime) universe. You're conflating _what is_ with _"what a particular species of apes can interact with"._

[@joshyman221]

TheOldOne well it gets into philosophy. Does it exist if we can’t interact with it? I don’t think it’s as trivial as it may seem, but then again I’m no philosopher :/

[@michaeljones7465]

Wunderbares menschiches denken/Wonderful human thought.

[@davidfoss4365]

Hi Sabine, I was never comfortable with the Copenhagen probaility concept. I think it's an artifact of the Schroedinger equation which is a flawed model of the real world in that it predicts very well but doesn't really explain. And I think quantum experiments give counter intuitive results because the experiments force particles to behave as they do rather than causing a wave funtion collapse, new univers or something else. At least that's a perspective I'm comfortable with though I can't provide robust reasoning. In other words, I think physics is caught up in the math and would benefit from taking a fresh look at reality.

[@schmetterling4477]

The probability concept is not something we can get around easily. That's simply what the experiments show: there is no correlation between any two independent repetitions of the same quantum experiment. We can speculate that this isn't true any longer near the Planck scale (and people have), but we are far, far, far away from the Planck scale and ordinary measurements simply don't violate the statistical independence principle. This is not to say that there aren't experiments that can possibly uncover these hypothetical Planck scale correlations - quantum computers might actually be able to probe that regime and so might gravitational wave experiments. What we do know is that, so far, no such experiment has been carried out. That quantum mechanics gives counter-intuitive results is a consequence of the fact that we are not dealing with particles. Quantum mechanics it not a more precise version of atomism. Quanta are not material constituents of quantum systems. They are energy, momentum, angular momentum and charge exchanges between quantum systems. And since we are not dealing with "stuff" at all, it's not surprising that it doesn't behave like stuff (i.e. according to the "rules of logic", which mathematically speaking is a commutative algebra). Instead energy quanta are behaving according to the mathematical properties of non-commutative algebras, which are somewhat non-intuitive until one gets used to them.

[@johnp1]

Sabine did a great job providing "peer review" to other scientific arguments.

[@krm398]

I dont know anything about this myself, but after listening to you I feel like the process you use and the measurement you get would be 100% correct on ANY world, so if there are multiple worlds as some seem to think then...you are both right. because on this world the measurement is absolutely true, but on another world just like this one, perhaps some tiny difference is made, so you get a different answer that too is 100% true...for that world.

[@LucAnderssen]

Sean Carroll triggered

[@judgeomega]

i wanna see prof carroll review this video with one of his own and dissect it piece by piece.

[@ollywright]

@@judgeomega I'd rather see Sabine on his podcast. The two discussing quantum mechanics would i'm sure be fascinating.

[@manfredadams3252]

Every measurement creates an entire universe worth of energy out of thin air? Something doesn't quite seem right there...

[@LuisAldamiz]

My thoughts almost exactly. Also where's the evidence?

[@quasimobius]

Agreed. It sounds like "somebody" took too many mushrooms.

[@LuisAldamiz]

@@quasimobius - Mushrooms are fine. To me it looks more like someone was brainwashed at Sunday School and could never get over it.

[@quasimobius]

@@LuisAldamiz She doesn't come across as religiously motivated to me, just hasn't got a clue.

[@LuisAldamiz]

@@quasimobius - I never meant Sabine but Sean Carroll and people like him. She's obviously not a proponent of the Manyworlds hypothesis but quite obviously a strong detractor.

[@RaphiTheOne]

After studying for 2-3 months and doing my phd exam, the display of the schrodinger equation gave me a spasm...

[@MichaelOZimmermannJCDECS]

You are the example for everything being not just black and white... Thank you!