Understanding Quantum Mechanics #5: Decoherence

[@wholenutsanddonuts5741]

This is brilliantly described. So many physics teachers I had did such a poor job and in 12 minutes you finally got me to have a good intuition of decoherence. Thank you!

[@ThatCrazyKid0007]

@Ron Maimon She is literally a theoretical physicist that works in the field of quantum gravity research, which means she has a strong formal education in quantum mechanics and you call her incompetent in talking about an aspect of her own field?

[@francescomunizmiranda4425]

@@things_leftunsaid Yes, I read what Ron Maimon says about himself: "I have no PhD, I am almost entirely self taught. I like physics, but I think the professionals are, for the most part, completely incompetent. I have a lot of my own personal theories about physics which I like to spread online. I am unemployed and not by choice. Despite this, I consider myself to be the next Isaac Newton." kew1beans.wordpress.com/2014/10/20/167/ I think I don't need to comment this further. I understand that this is the era of so-called "disintermediation", but luckily this approach doesn't work with physics.

[@benheideveld4617]

Ron Maimon Dear Dr Maimon. I am seriously considering that you are right on this point, because your reputation is frightfully great. But please do go easy on Sabine, even while she does not go easy on the physics community. Please make your point in a constructive way, no need to be overly polite, just friendly would do. I personally am very happy with female physicists, which probably paints me as a sexist, however it was terrible to see that in Utrecht in the late seventies there were like 3 women and 80 men in the freshmen physics course. I do get that being wrong and aggressive at the same time is an unfortunate combination, but to quote a famous saying attributed to a then obscure Jewish rabbi, let a person clean of sin throw the first stone. Please tell us what Sabine should have said, Sir.

[@tomashull9805]

@Ron Maimon You know what exactly happens during the measurement? A Nobel Prize is awaiting you, IF you can prove it...😉

[@tomashull9805]

@@ThatCrazyKid0007 I have my own interpretation of the measurement and nobody can disprove it... Am I incompetent? If yes, who decides?

[@rc5989]

Wow! If Sabine had dropped this a year ago, I would be lost. However, she has provided so many great videos building up to this that I can receive a valid familiarity with this important open question in physics. Thank you very much Sabine!

[@Melomathics]

Not to burst the bubble but the familiarity is just an illusion - this topic is much more complicated than this, and unfortunately this video gives a totally false sense of understanding.

[@SabineHossenfelder]

Happy to hear!

[@rc5989]

Stephen No bubble to burst. I have obviously chosen my words carefully. If you posses a working mastery of this topic, I would be happy to watch a video of yours.

[@stevenkelby2169]

@@Melomathics Yeah, Sabine's videos are not as educational as yours, but she's trying!

[@moses777exodus]

Does this mean that a wave function is decoherence and is described by a probability; furthermore, that coherence occurs after wave function collapse and with value 1?

[@ernestuz]

Finally somebody explained this. A friend of mine had already told me decoherence didn't solve the problem of measurement, but I got lost in his explanation... now I can go to him and tell him that decoherence half solves the problem :). Thanks Sabine.

[@mrgadget1485]

I've never seen decoherence explained as simple and beautifully as you did. This concept was introduced to me in Quantum Statistics course in the context of ensembles - very different kind of approach.

[@progra_kun4331]

you are the best physics teacher in You Tube that I have seen, from Latin America a scientific greeting for your videos so precise and concise

[@itsawonderfullife4802]

Great video. You should write a book on quantum mechanics. From elementary to the advanced level and covering decoherence and interpretation matters in detail. You did a great job of explaining an advanced topic in such easy terms and using simple math, whereas many established texts with their full math machinery fail to do so and only confuse the student along the way without giving the essence of the matter and the big picture.

[@EugeneKhutoryansky]

The survey mentioned at the beginning of the video was biased based on the list of options given to the respondents. If it had instead given a reasonably comprehensive list of philosophical interpretations of Quantum Mechanics, and asked them to pick their favorite, or to pick "other", then the percentage who would have picked "decoherence" would be much smaller.

[@l0_0l45]

Hello! You make awesome videos Eugene! Glad to see you here

[@SabineHossenfelder]

How do you know that? Serious question. If there are any better polls, I would be curious to see them!

[@mwk22bath]

"It is/will be solved some other way" = Other

[@Jehannum2000]

@@SabineHossenfelder Why not make one? It could be good research material for a new book - outstanding problems in quantum physics. The poll could simply include the question: "Which interpretation of QM most successfully solves the measurement problem?" with a possible answer as "None of above".

[@Bambabah]

Well hello there Eugene. Need more physics videos :)

[@musicalfringe]

Nitpicks/clarifications: 7:31 - The animation actually illustrates e^(i*theta) with theta decreasing from pi/2 to -3*pi/2 . Ranging from theta to 2*pi would have the vector starting/ending parallel to the R axis and rotating anticlockwise. 9:12 - I believe she's saying that for one of the coefficients to be zero means that one or more PRIME-diagonal elements must also be zero, which it plainly is not in the wave function.

[@special-delivery]

I have a request. In your upcoming videos about the measurement problem, please shed some light on how such measurements are irreversible and might have something to do with the direction of time. I don’t know much about this but would love to know what our current stage of knowledge is about this issue. Thank you for your amazing videos.

[@SabineHossenfelder]

Thanks. This is a very good point actually, I will keep this in mind.

[@nerdomania24]

@@SabineHossenfelder What if I have my own interpretation of QM, Which laws and experiments I have to follow to verify it mathematically? If you are intrested in details. My interpretation says that time flows in all directions at once and only when particle interacts with smth it gets vector of time directed in relation to its partner, and all such vectors get biased towards each other which makes gravity to appear. Also, measurement problem is solved in my interpretation, it is just generic act of a system getting determined state from hidden parameters and mechanics which we cannot observe but are able to guess. All in my interpretation relies on smart guesses, like causal dynamics but much more deterministic with no statistical data, only rigid and solid structures with zero randomness, only chaos is allowed. Also in a system time can flow in two opposite directions at once and then suddenly turn in only one direction.

[@musicalfringe]

Yakir Aharonov's work might have something to say about this.

[@timbeaton5045]

Yes. good question. I have always wondered why both classical and quantum mechanics, seem to insist upon time reversibility. It seems that only the 2nd law of thermodynamics actually implies a "direction" to time. You would have thought that somewhere in QM or whatever might "replace" it (in the sense of GR "replacing" Newtonian mechanics) there would be something that is asymmetric with respect to time. Is it actually "merely" statistics that state that entropy increases with time? at least without a situation like here on Earth, where the sun's output supplies the energy to temporarily drive entropy seemingly backward, locally?

[@musicalfringe]

@@timbeaton5045 As I understand it, it's simply that no experiment has ever found evidence of time asymmetry so it's not in the theory and the latter hangs together nicely without it. However: the weak force, at least, seems to suffer from broken symmetry. The conserved symmetry there is the compound CPT IIRC, so even there there's no actual time asymmetry.

[@yagvtt]

Such clarity, thank you so much for demystifying not only this word, but also why the use of density matrix is mandatory here, and in such a short time. Many thanks for your work on this channel.

[@ThatCrazyKid0007]

Finally some simplified maths to explain how physicists explain the phenomena they observe in experiments. I know it's not very layman friendly, but as someone with an electrical engineering education, it makes it much more approachable and understandable for me. Now I understand why decoherence does not fully resolve the measurement problem. Thank you for another quality video Sabine, you have an excellent channel on your hands and I really enjoy the way you present these topics. Been binge watching your videos since yesterday and I have to say your videos are some of the best scientific ones on KZbin, and I've seen a lot of them. Very clear, concise, diction and pacing easy to keep up with and a non-distracting style of editing, straight to the point, as well as keeping the tone serious, but not too serious. Keep it up, really looking forward to future content.

[@jarnoldp]

I’m a physicist MS. You do a very good job breaking down the topics and illustrating the key points. Please keep up the good work. Also, not to be too forward, but that dress works very well for you. Goes well with your eyes. I hope you have nice day.

[@quasarsupernova9643]

She goes to the heart of the issues without beating around the bush ...

[@Ralph85Williams85]

This Channel has been a great find! So much knowledge and with so much clarity!

[@harshitabhuyan8892]

My master's thesis is about environmental decoherence and it took me one full year of reading up vague literature available and still understand only 80% of the concepts in this video. This video in just 12 minutes cleared all my conceptual doubts.

[@cmilkau]

There is one question remaining about decoherence that I would like to understand. While it explains the absence of interference, and entanglement explains the single-outcome, there is a third phenomenon you could call "probableness" or "typicality". Everettians are often criticised that it doesn't make sense that everything possible does happen, while they shrug it off as "why not". Now let's look at decoherence more closely. If you take very small contributions to the wave function and very improbable outcomes into account, you can't gloss over some things anymore. The nature of decoherence is statistical (rare interference effects are still allowed) The averaging is representing a physical process (entanglement). What I wonder is: could this explain "typicality" of observations even in many-worlds? Might "improbable worlds" actually be very short-lived, constantly created and eradicated by sensitivity to interference? Particularly if the total number of basis states is actually finite (which I believe is an open question), fast scrambling over a long time could cause all states to be populated (more precisely, start with a basis state, *fix the basis* and evolve the system over a long time, and the resulting state will have mostly nonzero coordinates), so there is always a small amount of interference, like a background noise. That noise would only allow reasonably probable/typical worlds to look classical and roast the others to a quantum soup.

[@algonte]

Great explanation, thank you. Please let me add: A quantum state can be pure (isolated system) or mixed, pure states are described by a single wave function but mixed state are not. Density matrices allow to describe them both. A density matrix is the result of the addition of ket-bra products of one wave function (pure state) or several wave functions (mixed state) multiplied by probability factors as a result of entanglement between your quantum system and the outside world: Decoherence.

[@stefanvennberg9539]

Great as usual! Some of your videos bring back the feeling I had in the early 80:s when I read "The Dancing Wu-Li Masters". Unfortunately this is one of the videos when I'm at a loss because of my lacking in math. But that doesn't matter. The important thing is that you are more successful than most scientists in explaining quantum mechanics at a popular level that common people can actually understand. Keep doing it!!

[@b43xoit]

Leonard Susskind will give you the math you need. Check his series on entanglement, not the whole thing to the end, but the beginning few lectures where he explains the math.

[@guribuza2007]

The transition between explaining science to the act of advertising at 11:50 is hilarious. A phase transition indeed. As always, a wonderful exposition.

[@mobilephil244]

What a beautiful explanation. KZbin has done some bad harm to jobs and careers (including my own) but one really good thing it has done is to bring the world's great explainers out of their closets.

[@michaelblacktree]

That was a very coherent explanation of decoherence. Thanks! 😎

[@moses777exodus]

Does this mean that a wave function is decoherence and is described by a probability; furthermore, that coherence occurs after wave function collapse and with value 1?

[@Vld45]

@@moses777exodus In collapse models decoherence isn't necessary.You can just postulate for big systems superpositions do not apply since the wavefunction collapses.

[@sjzara]

This is a fantastic video. It’s the first time I have seen decoherence clearly and accurately explained for an amateur like me. I had a vague understanding of the measurement problem’s relationship with decoherence, but now I really understand. Thank you!

[@ideliversoftontario4976]

Brilliant video, as always from Sabine. The best physics explanation on the web, in my opinion.

[@hesitantjaguar7897]

I stopped eating squid when I realized they had simian level problem solving abilities. It seems now that my love of pastries has come to an end, Danish are obviously intelligent enough to participate in surveys.

[@mdshett2]

It's octopus that is intelligent. Squid are actually not bright at all.

[@davidwright8432]

I take it you have no objections to certain politicians, appropriately cooked! Not even amoeba-level problem solving abilities.

[@hesitantjaguar7897]

@@mdshett2 Indeed, I referred yo cephalopods in general.

[@hesitantjaguar7897]

@@davidwright8432 I would have no reservations, indeed I would consider many of them suitable for vegetarians.

[@Krmpfpks]

@@hesitantjaguar7897 cuttlefish are very smart too.

[@2tehnik]

There are two crucial premises I didn't understand: 1. If the phase factor is just a formality which doesn't effect the probability distribution, on what ground do we reason that the object of interest is perturbed by its environment? Why should this be taken as true? 2. Why would the true measurement be an average of all the different phased variants?

[@schmetterling4477]

It is not "just a formality". While absolute phases don't matter, relative phases do. Measurements are not averages. Measurements are irreversible energy transfers. She is simply trolling you with physics bullshit.

[@2tehnik]

@@schmetterling4477 the idea that measurements are averages of phase variants is bs or sth else as well?

[@schmetterling4477]

@@2tehnik We can't measure a phase on its own. We can only measure a quantum, which is a finite bit of energy. We don't have to measure all of the energy of the system at once, which is called a weak measurement. Weak measurements make quantum systems look more classical. This has been known since at least 1927. One doesn't need the density matrix for that.

[@Littleprinceleon]

I think when SH mentions measurement she means a concrete setup of the apparatus and not one data point gained by a measurement. Interference pattern is a statistical phenomenon. But if each datapoint making up that pattern is randomly shifted (even if a little bit) then the experimenters do get a smeared blob instead of the pattern. What results counts still as interference I don't know...

[@uvofsam]

Please just let this series continue You are doing a great job Thanks a lot really

[@wenzhengli6716]

Hi Sabine. I'm not a physicist, but I thought about this back in university, and came up with an intuition that I was satisfied with at the time. Would like to briefly run by you if you don't mind: Given that 1) Measuring a system changes it's state. 2) Measuring it multiple times yields the same results. 3) Possible outcomes consist of only eigenstates of the measurement operator. The most simple mathematical process that could exhibit this behaviour is fixed point iteration. Ie take any state, and repeatedly apply the measurement operator on it until it converges (to an eigenstate by definition), with probability proportional to how similar the initial state is compared to the outcome. Since act of measuring something "once" isn't well defined or distinguishable from the myriad of continuous processes it consists of. It would make sense that what we finally observe is the converged state after repeated measurements.

[@philippecassagne3192]

Thank you for that very interesting video. Il have a question, maybe naive, but I never found a satisfactory answer to it. I understand that quantum systems must be carefully preserved from interactions with their environment in order not to lose their quantum properties through decoherence (see quantum computers for example). But how do you explain that some quantum systems and phenomena can be easily observed at our scale (radioactivity, laser etc...) ? Is it because they are protected from interactions with their environment ? Many thanks for your answer !

[@philippecassagne3192]

@@schmetterling4477 Thanks for your answer. That's clearer for me now (and surely for others too, because it is a difficult point !).

[@Littleprinceleon]

@@philippecassagne3192 what I have learned from an Arvin Ash video on entanglement is: decoherence is definitely not necessarily an all or nothing process, there are physical experiments even with types of REcoherence. However, the states of the Q-bits for an effective PC have to be precise enough, and as far as I understood, even lower degrees of decoherence lead to more frequent errors...

[@thelocalsage]

Wow! What a succinct way of conveying this, I’d never even heard of a real mechanism for decoherence, and being able to peak at a bit of the math was very helpful. Fascinating!

[@jamesblank2024]

An excellent video that frames the decoherence problem well. Thank you. That so many physicists don't understand there is a problem exemplifies apparently simple effects require detailed study. A problem must be fully understood if there is any hope of solution. Perhaps this is one reason fundamental physics discoveries have stalled. Note: the ket-bra notation are touching so they look like a big X, which is visually confusing at first.

[@musicalfringe]

"A problem must be fully understood if there is any hope of solution." - I think this applies in all areas of life.

[@Villaboy78]

Really clever video showing the effect of considering the complex conjugate and phase , whilst leaving the open question about WHY a wavefunction collapses into a single observed result (with classical probability) Also that Euler dude was a genius

[@gabrielvian3210]

Your videos are amazing, they are helping me with my studies

[@SabineHossenfelder]

Happy to hear!

[@ChadEnglishPhD]

Thanks for this great explanation, but it brings up more questions than answers for me, as does any great learning experience. 1. Why does interactions with other particles cause "averaging" rather than each interaction as a stand alone event. 2. Why does averaging of phase occur at the level of the density matrix (rho) rather than at the level of the wave function, Psi. That is, why don't we get the e^i(thetha) going to zero in the |Psi> wave function @7:00? I suspect the reason has something to do with the coefficients of the bases functions having to add up to 1, but that is more a mathematical dictum rather than a physics description of why wave functions "behave" one way and density functions "behave" in a different way. Why? 3. Why did you only add the e^i(theta) to the second coefficient and not the first? Does that change anything? (I will carry the math through myself to check and understand, but it might be helpful to explain that from the start.) 4. Fundamentally it appears the physics of wave functions and density matrices is different. The density matrix here is written as a Ket-Bra derivative of the wave function, but if the density matrix can have off-diagonals go to zero and not have a corresponding wave function, then the density matrix has to represent something physically independent from wave functions, with the Ket-Bra relationship only being a special case where they are related. Or have I missed something? 5. This implies it is possible to partially decohere by bumping once or twice, but not enough for the statistics to average the off diagonals to zero yet. What physically would partial decoherence look like? 6. If I understand the thesis if this video, it is that decoherence and measurement are two different phenomena, meaning "bumping into particles along the way to the detector" (decoherence) is fundamentally different from "bumping into particles in the detector causing the detector measurement to occur" (measurement). I mostly get the math of this difference, that "bumping" causes the phase to "collapse" to zero (off diagonals) without affecting the probabilities, and "measurement" causes the probabilities to "collapse" to all zeros and one 1, corresponding to the event that is measured. What I don't understand is what the different physical processes are for "bumping along the way" with other particles vs "bumping with detector particles" causing measurement. What is the difference in how the particles interact in those two different scenarios? To me this is highly reminiscent of regular probabilities, such as the "boy-girl" problem. Suppose I have a dime and a nickel and I flip both in a closed box, then peak in. I tell you that at least one of them is heads and ask you what are the odds that the other is also heads. You ask me how many coins I saw when I looked: (A) If I saw both coins then the options of (nickel, dime) are HH, HT, and TH since at least one is heads, so the chance of the second being heads is 1/3. (B) If I only saw one coin, the options are (coin I saw, unseen coin) = HH or HT, and therefore 1/2. The "bumping" here is the reduction of possible states from 4 (HH, HT, TH, TT) to 3 with the information "at least one is heads", and another bump from 3 to 2 possible states with "I only saw one coin". Then "measurement" occurs from reducing probability to 1 state by determining the state of the second coin. I know it is different, but seems similar in the reduction of possibilities. Thanks again for this video. Despite my questions, it did make some things click a lot more clearly.

[@Rykvp]

+1 to the question 5 : What physically would partial decoherence look like? As to the difference between "bumping along the way" and "bumping into particles in the detector" : my guess is the only difference is wether you, as the observer, are being affected by the bumping. If you were a particle along the way, each side of your wave function would be measuring 100% each state of the particle that's bumping into you, with each side of your wave function representing 50% of your wave function.

[@hossainpezeshki6964]

Sehr geehrte Frau Doktorin Hossenfelder, vielen Dank für die hilfreiche Erklärung.

[@anonymouswombat2354]

I'm so happy to find videos that delve into the mathematics behind quantum mechanics, so I'm not limited to a simplified explanation in layman's terms. As someone close to getting their bachelor's degree in mechanical engineering, I am able to understand the mathematics she's talking about and it's a much more "coherent" explanation than some weird metaphor. I think this video gave me some insight into *how* and *why* quantum tunneling is a problem for the tiniest transistors now. I would like it if she could talk about transistors and the R&D being done to counteract that problem. I read a short article about it and couldn't really understand what they're trying to do.

[@DouglasVieira91]

8:59 An alternative explanation, for those who are familiar with linear algebra, is that the ket-bra product must be a rank 1 matrix and the diagonal matrix with no zero entries in the diagonal has full rank. Therefore, the density matrix that suffered decoherence must be a sum of at least n ket-bra products, where n is the number of states. I was a bit confused by the fact that averaging -- which is sort of a superposition -- could destroy the ket-bra structure of the density matrix. The fact is that the averaging is done to the density matrices directly, and not to the coefficients of the states -- which would then correspond to a true superposition.

[@joshuascholar3220]

Maybe you can explain this to me. The sum of two wave functions is a wave function, but decoherence is because the sum of a bunch of wave functions is a density matrix that isn't a wave function. I don't understand the difference.

[@Raphael4722]

Thank you madam. I think this is your best video, and the best youtube video I've seen on the topic of decoherence.

[@jakeadams2562]

Thank you for making this digestible while still giving a thorough explanation. Amazing video🙌

[@fluffy_tail4365]

Wow, finally a good explanation of decoherence, I never got how it would affect the measurement itself....because it doesn't in the end.

[@isonlynameleft]

I guess it depends on how you are defining the measurement problem. If you're speaking about it in terms of wave function collapse then decoherence does "solve it" because wave function collapse is not required to tell you the particle will end up in a definite state. Although it does not tell you why you got one state instead of another so I guess you could say it just kicks the can down the road.

[@Rykvp]

Or you got both, but your 2 states that each got one of the 2 different states are decoherent, they cannot interfere with each other, so you are not aware that your wave function taken as a whole did get the 2 states. In the case of Sabine she is also incapable of admitting that she could be only a small part of the wave function of all the Sabines that are experiencing the whole density matrix, the one making the video is only experiencing one result, so she says there is still a measurement problem ... maybe it's an ego problem :)

[@DanielL143]

Best physics educator ! Thanks very much. The scope and level of your videos are perfect. You hit the key ideas and treat them in a serious and appropriate fashion while also making quantum mechanics comprehensible.

[@l0_0l45]

You cleared a long standing doubt of mine. Thanks for the wonderful explanation Dr. Sabine!

[@peterwan9076]

Thanks for bringing up the density matrix in explaining decoherence. It gives the meat to the abstract concept of decoherence. Great work.

[@DumblyDorr]

I do enjoy that you are one of the few science communicators who i) presents this extremely important idea (and presents it well!) and even more so that ii) you are one of the not-too-many physicists who take conceptual & methodological problems - and in general philosophy of science seriously (I was very glad about the references wrt Popper and Feyerabend during the recent two discussions - would have loved to throw Lakatos, Duhem-Quine, Sneed and Suppes in there). Thank you for both of those things - they are very much appreciated :)

[@blinkingmanchannel]

Oh one question I keep forgetting to ask: I thought wave energy was stated in the "amplitude" of the wave. The frequency, I had as simply the oscillations per unit of time. Thus, radio is "tuned in" as amplitude modulation (AM) or frequency modulation (FM)... Or in the old days "UHF" TV stations. Indeed, there was a whole transmission interference discussion based on higher frequencies hitting more raindrops, so though they had more bandwidth, higher frequency signals had to be boosted to send them very far. AM stations from Chicago can get picked up as far as St Louis sometimes. Did I get that mixed up?!

[@lepidoptera9337]

That is true for classical waves. The energy density is proportional to the square of the classical amplitude. In quantum mechanics these classical waves don't exist, so one can't take the squares of their amplitudes. We can, however, recover them for the case of electromagnetism by using a theory of positive definite energy transfers alone. These small amounts of energy are called "photons". This is a special case, though. Other quanta (like electrons) do NOT lead to a relatively simple classical mean field theory like electromagnetism.

[@profkingthing]

Thanks for making these videos. I hope you enjoy making them as much as I enjoy watching them!

[@slash196]

I seem to recall a quote being attributed to Freeman Dyson that the measurement problem can be boiled down to whether you put you place yourself in time before a quantum observation or after it. If you look at a quantum observation that's happened, you see one result. If you look at a quantum observation that you haven't done yet, you see only probabilities. In other words, the measurement problem reduces to the problem of time's arrow. In your mind, does this line of thinking go in the right direction?

[@LuisAldamiz]

Sounds like a spinning roulette and like a stopped roulette to me, nothing else. You can't know the state of the previous system from just the observation of the ball being in the box labelled 13: it could have spinned right or left, at this or that speed, it could have last bounced on the box to the right or to the left, it could have been mischievously placed there by a croupier, you only know that final situation and you cannot reconstruct from that the previous history or state of the system.

[@slash196]

@@LuisAldamiz Precisely my point. The quantum measurement problem is not any different in principle from the measurement problem we're familiar with in normal life. Will the Yankees or the Red Sox win the World Series? Before you play the games, you can only ever give probabilities; after you play, there is only one answer. I'm not saying I have an answer for the problem, but it's a much more familiar problem.

[@bh5606]

Got my weekly Sabine. Have no clue what she is talking about but like listening to her earnestness.

[@jonyeawright]

Yea, me too

[@happygimp0]

7:33 Should that arrow not start at 1 and go the other way around?

[@mobilephil244]

Yes, I spotted that too :)

[@DrKnowitallKnows]

This is amazing. Thank you! A question: if you put an imaginary term in the main diagonal (ie e^itheta for term 1) what happens then? What does this actually mean for the probability density of your system (eg, spin up/down)?

[@SabineHossenfelder]

It doesn't make a difference because the random kicks really only change the relative phase. You can therefore just chose to put the phase entirely into the second factor. You may have read this somewhere as the statement that the global phase of the wave-function is unobservable: it's the same statement.

[@DrKnowitallKnows]

@@SabineHossenfelder Ahhh. That makes sense that's it's all relative rather than absolute. Thanks so much for explaining that point. I wish your videos had existed when I was but a wee physics student. I'd probably still be doing physics rather than AI for my job now :)

[@paulmichaelfreedman8334]

@@DrKnowitallKnows I dunno but AI sounds like an exciting thing too, with the massive breakthroughs lately, and soon to come.

[@j.lo.5784]

There could be a time reverse option in addition to all superpositions. The direction of time could change at random. On the "re"-reverse of time another messurement outcome could be taken. Since all messurements on a time reverse are lost, you couldn't prove or falsify it. This would lead to time loops wich are cut out of history. Would a time reverse superposition fit into QM?

[@ChuckCreagerJr]

Decoherence can't really solve the measurement problem. This is because there are many Which Way path experiments usually versions of the double-slit experiment that do not disturb the particles. In such cases, they usually use entanglement as a way of detecting Which Way path information and whether or not you get an interference pattern depends upon the preservation of that information. Dose decoherence occur in these cases, yes, can decoherence itself explain it. The answer is no because these experiments are specifically designed to prevent the problem of disturbing the particles.

[@pushkarchoudhary8006]

Thanks a lot ...keep making videos i have been anxious as no video explain any mathematics ...thank you so much

[@lunafoxfire]

Wow what a good explanation! I'm not a physicist so this is something I actually wondered about -- whether the measurement problem was a real thing or just scientists still stuck using bad metaphors to communicate quantum mechanics, and it was actually just solved by decoherence. Now I understand clearly that I don't understand just as much as everyone else doesn't understand :D

[@tommylee2894]

Wonderful that some of us...Sabine Hossenfelder within the Scientific Community are pointing out and deconstructing the flawed foundational paradigms. Dispassionate examination of the passionate flawed scientific paradigm!

[@stephenpuryear]

These videos continue to fascinate me and obviously, many others. At 8:43 you say "but the terminology is not the interesting bit". To me the terminology is almost always central, because it links current knowledge to a previous historic era, even if it's dumb or opaque. For example the use of the term "random" has not only seen many ups and downs. There have been periods in which no one believed in in it at all. Laplace thought that we had only our own ignorance to blame when we did not know something. Thank you once again for these short videos. They are quite wonderful.

[@jaimeduncan6167]

Again, the best explanation on youtube, I will venture to say that is the best explanation I have seen in general.

[@david_porthouse]

After watching this video it occurred to me that the simplest model of decoherence is to identify the Uncertainty Principle with classical Brownian motion for all objects heavier than the Planck mass. Reinhold Fuerth wrote a paper comparing the two UPs. Roger Penrose advocates looking at the Planck mass as a boundary between the micro- and macroscopic worlds, but I feel we might have guessed it anyway. I've just put two and two together. This model can inspire a computer simulation, though it does not explain everything. It's one to explore and we have something to get going. My thanks to Dr Hossenfelder.

[@markuspfeifer8473]

Measurements seem to me like the operation of conditioning your probability distribution. HMM theory may shed some light on that, even though quantum processes are not themselves Markovian.

[@schmetterling4477]

Measurement is simply irreversible energy transfer. If you are confused about that, then you need to go back to studying classical physics for a couple more years. :-)

[@Dr_LK]

7:31 , I thought the e^itheta phasor starts from the horizontal (+ve x-axis) and then rotates anticlockwise?

[@rv706]

@Sabine Hossenfelder: wonderful video! I have a couple of questions. *Why* does interacting with the surrounding environment give each time a little shift in the phase? Also, I gather, the phase that is changed in these little interactions is not the overall phase of the state vector (otherwise it will be the same ray in Hilbert space, i.e. the same state). So how does Nature know that she has to administer a phase shift to an eigenvector *of that specific observable that you're about to measure* and not another one? Thank you!

[@rv706]

@Dr Deuteron: I am not sure your comment answers one of my question(s). My second question was about this: looking at the video, it seems that the component that gets the phase shift is always one (or several) eigenvector *of the same observable* (in this case, the vectors named |1> and |2>). Why is that so? Remember that one can expand a given state vector |Ψ> in any (Hilbert) basis, not necessarily a basis of eigenvectors of the observable you're about to measure in your experiment.

[@Jim-uq1mc]

What a great and concise explanation of decoherence issues. The ‘measurement problem’ may require decades, if not centuries of further research. I am somewhat under the impression that events at the scale of the Planck length may play a decisive role, requiring some substantial extension of current quantum mechanics. The intricacies of the measurement problem may be in a similar ballpark like issues of particle creations and annihilations. These events are currently handled via creation and annihilation operators, yet what ‘really happens’ when particles are created or annihilated is currently way beyond the range of current physics . . .

[@5ty717]

Soooo clear. Best on the net. Thx Sabina. That was very helpful.

[@souvikdas]

Thank you for such a lucid explanation, especially on why we need density matrices instead of wavefunctions to understand this. But, isn't the set of basis vectors arbitrary? That is to say, if |0> and |1> are the basis vectors for a physical system, then |+> = |0> + |1> and |-> = |0> - |1> are also a legitimate set of basis vectors for it. What decides which set of basis vectors Nature decides to collapse along? I suspect it may have something to do with the interaction Hamiltonian with the environment or the observer directly, i.e. it must be eigenstates of that. But please let me know what you think.

[@manuelperez4004]

Thank you Sabine! It is the clearest and most coherent explanation I have found of quantum decoherence. You gained a subscriber :)

[@project.eutopia]

Has there been any research that considers looking at whether the measurement problem is an emergent behavior from the other axioms of quantum mechanics? Perhaps wavefunction collapse could be due to the special property of a measuring device that it tends to amplify small scale information to large scale macroscopic states?

[@atandritabhattacharyya3882]

Very nicely explained Ma'am .....keep it up.

[@HidekazuOki]

This is an absolutely EXCELLENT VIDEO! Thank you!!!

[@Mikostan19]

6:11 Why do the real coefficients (in this case 1/2) remain untouched in the density matrix, while the phase (theta) changes with each "little bump" (eventually averaging to zero)?

[@schmetterling4477]

Because that is the assumption similar to the assumptions of statistical mechanics about micro and macro states. It's just intellectual nonsense without which one can't calculate a thing and which can be shown not to be true for e.g. systems with mass gap. You can observe the superconductivity on a superconductor all day long and it will stay a superconductor. Unless you go beyond the critical field or the critical current density the macroscopic quantum effect is perfectly stable. All of this nonsense comes from theorists who can't even be bothered to re-read their undergrad textbooks on solid state physics.

[@special-delivery]

Absolutely wonderful, I was wondering about this issue for some time.

[@nias2631]

Sabine, in the example you gave, we had a matrix in R^2. For a particle in our reality with states 1 and 2 would it be wrong to consider the particle as |psi> = a|1> + b|2> +...+ n|N> so that the density matrix in R^N has Diag(a^2, b^2 , ..., n^2) where all coefficients beyond letter b have values almost surely zero? This would be an ill-conditioned matrix that leaves open the possiblity for other states beyond 1 and 2 that could be transitioned to given interaction with the world over time? Thanks, phenomenal decoherence explanation by-the-way.

[@MrHotlipsholohan]

Hi, does the position of a particle in an atom behave like a distribution similar to the normal distribution curve, ?

[@arctic_haze]

I hope you mean the electrons (the atom nucleus is a different and much more complicated story). The answer is no. The electrons avoid the nucleus so their phase function have a minimum (zero) in the very middle, unlike the normal distribution. Also they avoid each other (Pauli exclusion) so every orbital has a different pattern of where the electron can be (the phase function squared is probability). See the beautiful figure in the "Atomic orbital" article on Wikipedia.

[@MrHotlipsholohan]

@@arctic_haze Tks,

[@lepidoptera9337]

@@arctic_haze The ground state solution of the hydrogen problem does not have a zero in the center. The overlap of the electronic and nuclear wave functions can be easily measured in spin-spin coupling experiments and it leads to effects like electron capture in heavier atoms.

[@FractalTeapot]

You are truly a gifted teacher - clear, vivid and direct. Thankyou! 🙏

[@alexaserna8330]

Thank you so much! really. With this video I remebered why I love the pyshics

[@jakevikoren]

It's surprisingly difficult to come across such a clear explanation of decoherence. Thanks Sabine!

[@jjeherrera]

For a better insight into this, a good reference for the general public, with references to the technical literature, is Lee Smolin's "Einstein's Unfinished Revolution: The Search for What Lies Beyond the Quantum." While no previous knowledge of quantum mechanics is necessary to read the book, the technical literature is for specialists.

[@georgerevell5643]

I must give credit where it is due, this is a brilliant explanation I feel I understand decoherence so much more clearly now.

[@lepidoptera9337]

That's cool, but it doesn't solve the problem. Decoherence makes for longer recurrence timescales, but it can't produce irreversibility, which is the fundamental requirement for a measurement.

[@luisramrod9121]

I'm a simple man, I see Sabine I press like.

[@sanskarjain9455]

I have a rather silly question to be honest: Suppose we have a double slit experiment setup where we shoot one high frequency photon through at a time. To my understanding (which could be completely wrong) the 'measurement problem' in this setup is that though the photon is a wave and interferes with itself, it still appears as only a single point on the detector screen. Now I don't know exactly how the screen works, but I assume the photon would excite one of the electrons of one of the atoms and the photon would be registered as being detected at the location of that atom. But if I understand correctly, then it should perfect sense that the photon is only detected at one place, because a single photon can only excite one electron at a time. We wouldn't be able to see it spread out over the screen because a photon cannot partially excite many electrons, it is the lowest denomination of energy transfer and thus is either entirely absorbed by one electron or it passes through. Also I assume that it is more likely for an electron to be excited wherever the electromagnetic field is stronger i.e the crests of the interfered photon wave, producing the overall interference pattern. Sorry for rambling, but if someone could tell me what's wrong with what I said (because it has to be wrong), it would be great.

[@Jehannum2000]

Nothing wrong in your explanation. Measurement is only a 'problem' because some physicists don't like the fact that from the many choices available to the photon (the crests of the wave), the universe chooses only one. But this is what happens, so this should be the end of it: the photon was emitted and then it is absorbed. When there are equiprobable absorbers, the universe randomly chooses one of them. The deterministic evolution of the wave function according to the rules of the Schrodinger equation comes to an end and we get an indeterminate 'collapse' to a single outcome. The wave function and any superpositions disappear because they are waves of possibility and the possibility of the photon being elsewhere after it's absorbed is zero.

[@michaelkaliski7651]

The double slit experiment is a classic gotcha. When a particle hits the detection screen the uncertainty is resolved. Any attempt to measure or determine the path of the particle before it hits the detection screen counts as a detection and any uncertainty is resolved at that instant. Hence you either get the classic interference bar pattern, or a circle of decreasing brightness towards the edges if the photons are disturbed by any attempt at measurement during flight from the source to the detector screen. The classic interference pattern arises even if single photons are fired through a diffraction slit because each photon has a 50:50 chance of passing through either slit. Diffraction gratings need to be of the order of the wavelength of light in order to work. The photons of light are influenced by the diffraction grating as they pass through and it is this influence that creates the interference pattern that builds up over time. If any attempt is made to identify which slit photons are passing through, this also influences the photons and you just end up with a roughly circular pattern of intensity, bright in the middle and darker towards the edges, so roughly speaking a classic beam of light. This is (very approximately) a way in which we can determine whether a given quantum signal has been intercepted prior to being received, i.e. is someone spying on our quantum communications?

[@NightWanderer31415]

Gotta love how Sabine roasts her colleagues without mercy when they deserve it, lol. Not even 30 seconds in and she's already at it. Savage.

[@degautaborg]

Great matching background there! I mostly appreciate the visual quality of Your videos, as a visual artist.

[@inanconur9220]

Sometimes it occurs to me that if I watch enough Sabine videos, I'd understand Quantum Mechanics without a major in Physics.

[@garnier6098]

Suggest we have a pure case. There are many Hilbert basises we can use fore represent the density matrix. It does not depend on the choice of the basic. In the beginning we have a pure case, decoherence causes a mixed case, which depends on the base. I can not catch that in case of decoherence the result dependens on the base. But that should not be, should it?

[@bhangrafan4480]

I'm a non-specialist so unsurprisingly there are things about this I don't understand. The one thing I really do not understand is why the random shifts of phase have to add up to zero. I would have thought that they would tend to zero if a very large number of such interactions happen, but that if only a few interactions happen the sum could be non-zero. Does this just mean that the time scale for measurements is enormously great compared to the time interval between interactions of the particle with its environment? In theory if you had a very fast method for measuring could you get a particle which has not decohered, or is there something about the process of measurement which enforces decoherence in some way? For example in order to amplify the information of the measurement from a quantum scale to a macroscopic scale of the observer the particle must undergo vast numbers of interactions.

[@SamB-gn7fw]

Thank you for doing the math to explain this. It made it a lot more clear for me

[@saeiddavatolhagh9627]

A very lucid description of a very profound phenomenon. 👍

[@jamesbra4410]

Can you really differentiate a slight phase offset from a slight frequency delay? When you measure a photon you use some of its energy and decrease the frequency. In the double slit experiment case, there is superposition resembling amplitude modulation such that there are two near impulse Gaussian distributions suggesting filtering through the slits.

[@lepidoptera9337]

The photon IS the energy that we are absorbing in the measurement system. One can not absorb half a photon. This is forbidden by angular momentum quantization. The explanation with Gaussians for the double slit is incorrect, even in the classical case. A single slit has sinc (sin(x)/x) functions in the Frauenhofer approximation as scattering amplitudes. That function still has minima, they are just not as easily visible as those of the double slit. The cause for the sharp minima has nothing to do with physics. It's a feature of the Fourier transformation. As a rule of thumb, if the original function has "sharp edges", then the Fourier transform can contain these first order zeros. The details when these zeros are occurring are complicated, though, if I am not mistaken. Anyway, if you want a nice Gaussian diffraction pattern, then you have to have a smooth, Gaussian slit response.

[@jamesbra4410]

@@lepidoptera9337 right I mean for the wave function collapse the probable path is a Gaussian directly from the point where the slits are in front of

[@lepidoptera9337]

@@jamesbra4410 There is no such thing as a wave function collapse. That's a word that you won't even find in well written quantum mechanics textbooks like Sakurai. It's online. Go look it up and do a search in the pdf. Energy also doesn't have a path. It never had a path. It never even had a location. We never taught you that nonsense in high school. Energy is the ability of a system to perform work on another system. It's a SYSTEM property. A system is an arbitrary partition of nature by the physicist. Depending on how YOU chose your system partitions you will find the energy in there... with a probability that depends on YOUR choices. Dude, all of this is easily understandable by students who are actually listening in high school... which none of you did. ;-)

[@jamesbra4410]

@@lepidoptera9337 high school? I’m a college graduate dude. Charges accelerate they define an EM field which propagates to determine the probability amplitude of where the particle will be when many are fired. You detect something and perturb this field and the wave interference merely goes out of phase and boom no sinc function and you’re left with a Gaussian.

[@MrCmon113]

Do I understand the end of this video correctly in that decoherence + the many world interpretation are a possible solution to the measurement problem?

[@meahoola]

This is exactly what I thought, I scanned the comments hoping I wasn't alone with that. Decoherence explains how in the measurement process a randomized sequence of unitary interactions turns a quantum system into a statistical one. The multiple worlds interpretation then reflects on the random nature of the result.

[@Rykvp]

@@meahoola You need to add that the observer is part of the wave function which is why each version of the observer sees each version of the result as if it was the only result.

[@meahoola]

@@Rykvp Thank you, that's right !

[@sandropollastrini2707]

Thank you Sabina for your videos, I always enjoy to see you clear up some physical ideas. In this video, however, there is something I don't quite understand of your argument. You asserted: "Decoherence only partially solves the measurement problem. It tells you why normally do not observe quantum effects for large objects. It does not tell you how a particle ends up in one and only one possible measurement outcome." Is this really a problem? For what I understand the framework of decoherence is actually avoiding the collapse of the wave function all together: there is no collapse. That should mean that an isolated system never shows collapses; but a subsystem of it can experience a classical reality by just the decoherence mechanism just explained. Isn't decoherence just an approximation to the fact that we can't follow too many interactions between the observed system and the environment? The framework of the density matrix then is just handy, because it can handle both pure and mixed states, and therefore lends itself to represents this loose of information induced by the interaction with the environment.

[@sandropollastrini2707]

Or using other words, decoherence solves the measurement problem by showing a mechanism by which we can explain the raising of one random state out of the possible ones during measurement with a "classical apparatus" (I.e., a measurement system so big compared to the measures system, that is acts as an environment) without ever speaking about collapses.

[@Achrononmaster]

@7:05 "...what you actually measure is the average over all those random changes... " you could _say_ that, but I won't believe you. It's consistent with QM to say _what you actually measure is likely to be very _*_close_*_ to the average._

[@Achrononmaster]

The difference in emphasis is important, since the whole grift of the decoherence people is to ignore the fact that strictly the superposition is never destroyed. The whole measurement apparatus is in superposition, according to strict QM the whole universe is in superposition. I do not buy any of that (I'm more of the mind of Penrose in thinking somehow gravity can "destroy" superpositions, I just do not think Penrose's collapse business is right --- I do not think there is "wavefunction collapse" as mythically believed, but that's another story). Zeilinger, Gell-Mann and that crowd want to think we just cannot detect the tiny departures from exact decoherences, but I find that highly metaphysical. I mean, how likely is it that you get enough perturbations to reduce the off-diagonal element _precisely_ to zero, but then not further kicks to make them a bit off-zero? The perfection of the zeroing makes no physical sense to me, it's fantasy. Hence there is still probably always a superposition of all states. Arkani-Hamed seems to think this is no problem, and that our brain just cannot _perceive_ those tiny amplitude states, hence the world always looks classical. But I find that incredibly naïve. It seems more realistic to me to suppose those states just cannot be realized any more. The maths of QM tells us how likely, but not why there is superposition. If we can ever find out why there are superposition, and not just claim "that's how the world is," then I think all of this will be better understood. I have some ideas on that, but the comments section is too small to contian them.

[@schmetterling4477]

@@Achrononmaster Dude. This has a trivial (as in 19th century) solution, but what you are doing here is a theological he-said-she-said routine. What anybody says is of absolutely no consequence in science. Either you know how to be smarter than these people (which doesn't take much in this case), or you don't. Arkani-Hamed is correct. None of this is a problem, but not really for the reason you cite. It is not a problem because relativity resolves it for you for open systems. The math tells you that even for completely isolated systems with finite volume this turns into a straight forward time scale separation problem. The state spends most of its time near one of the solutions and almost no time in the other. If your physics intuition doesn't get you there within a couple of hours, then you are no smarter than Schroedinger was in 1936 or something. That is a very long time to be confused about something this trivial.

[@jamesruscheinski8602]

Classical particle observed or measured separate from wave function (decoherence); conscious awareness of observed or measured particle not the wave function. Conscious awareness may require particle measured by observer instead of quantum wave function.

[@compellingpoint7802]

Amazing video! As I see it, the problem of decoherence is a problem for all fundamentally classical systems. In nature there are no "fundamentally" quantum systems. All objects that have ever existed or will exist in the future are fundamentally classical and obey the laws of physics. Thus if we say that something is happening because it is subject to decoherence then something else must be happening because its not subject to decoherence; namely our universe itself. The question is whether any process that we might think of as decoherence can happen to the universe itself. I give you two examples with opposite answers. One answer is that the universe can not experience decoherence. It must be fundamentally classical, and thus it will never subject to any laws other than those of physics. If this is the case, then there are only two ways for something to happen in our universe: either because some fundamental process results in it happening or because a sentient being causes it. If the first case is true, then things must happen because they are part of a fundamental process. If there were no sentient beings or any other classical objects to decohere something (which is impossible), then nothing would ever happen in our universe. If the second case is true, then there is a third way for things to happen. Sentient beings must not be fundamentally classical systems. I don't mean this in the sense that they are made of different stuff than atoms, but rather that their internal workings obey a different set of laws from those which describe fundamental physics. If things happen because sentient beings cause them to happen, then the universe is not a fundamentally classical system. Thus any process that we might imagine happens subjectively must also objectively be true. I am curious your take. Thanks!

[@waitinginberniesbreadline922]

Fantastic and excellent point!!

[@compellingpoint7802]

@@waitinginberniesbreadline922 Thanks for your kind words. Have a nice day!

[@goddardwb]

Can you lecture regarding why Quantum Mechanics is a spooky subject excluding the wave mechanic math. One Prof used a parking garage example of watching vehicle parking places moving from one place to another in his discussion of solid state hole conduction. My impression of QM is that some mechanism is happening that acts like a wave and a particle perhaps at the same time but in reality it is something that seems to defy description.

[@johnnynuuma7094]

Immensely appreciate your videos. I have a suggestion. I find reading captions along with listening to your narration makes my learning experience more efficient & satisfying ... stickier, I suppose. However when equations are presented on bottom of screen the equations/captions mask each other and can't make sense of either ( type of superposition? ). Thinking if you stood off center ( say to the left when viewing video ) and edit in equations in the open space from top to about your waist or elbow when arms by your side this collision could be avoided in most instances. Kind of picky, embarrassing request but perhaos others feel similarly -- john

[@hleiqawi]

You can avoid that by putting the caption option off at the moment you like to view the equation.

[@johnnynuuma7094]

@@hleiqawi-- Suppose that is the Copenhagen interpretation response to my request. :)

[@jamesruscheinski8602]

After updating probability to 100% for measurement in classic space, could the other quantum probability of 50% form a new superposition in quantum wave function, such that there would be quantum branching for the wave function only and not in classic space? This could allow for more measurements in classic space over time, but not create alternate realities in classic space at the same time. There might be many worlds over time in the same classic space, rather than many worlds over classic spaces at the same time.

[@malongfan1948]

Could you do a video on how quantum mechanics describe a macroscopic system? For example, if an apple is moving back and forth on a spring, does it have discrete energy levels? When i look at the apple is that a measurement causing its wavefunction to collapse? If its wavefunction collapses, then why does the apple still move?

[@schmetterling4477]

You can find an explanation for that in a paper of Heisenberg from around 1927, I believe. He didn't discuss the linear oscillator but Rydberg atoms, but it's pretty much the same once you understand it intuitively. ;-)

[@Nathillien]

The best explanation of decoherence I saw.

[@alphalunamare]

All up and until 11:25 isn't this just saying that the miraculous disappearance of the complex diagonal terms is entirely equivalent to the collapse of the wave function, and hence is no solution to the measurement problem? just a different view.

[@MrDecato]

I thought it meant that the 'collapse' is actually the sampling of the distribution of the density matrix, i.e. choosing one out of the many possible outcomes. Disappearance of the diagonal terms is the loss of quantum phase due to interaction with the other quantum things, but no measurement/sampling (i.e. selection of one outcome) has occurred yet.

[@alphalunamare]

@@MrDecato How does the sampling process manage to select only those without complex diagonal terms? I suppose one could imagine the system 'popping' in and out of the Real (classical) World through its evolution and 'somehow' the very next 'popping in' after a measurement is deemed selected? As a continuous process I could live with that.

[@MrDecato]

@@alphalunamare I don't know if I really understand your question, nor the topic fully :) But as far as I can tell this video basically says the complex terms have already averaged out to zero before the sampling/collapse happens. The former (decoherence) happens due to interactions and the latter (selecting the one outcome out of many) is the observation/'measurement.The two are independent/unrelated.

[@alphalunamare]

@@MrDecato I am not sure I understand it either , hence my questions. If the video says the complex terms average out to zero before sampling then the video is wrong. How do they know they are going to be measured? Wouldn't that imply that the system is in a single yet unknown state until measured? - that seems fanciful to me. In my mind I see a phase 'arrow' rotting as the state evolves and on occasion it results in a Zero phase and hence a potential candidate for measurement. These candidates being thrown up continuously until one immediately follows a measurement attempt. I am not sure, but I think I might be arguing against decoherence?

[@MrDecato]

This is awesome. Thanks for these videos. Just a small nitpick: I think the complex phase angle is measured from the +ve real axis and in counterclockwise direction (there is an animation that shows it from the complex axis and clockwise direction).

[@Bruno_Haible]

What is the experiment that best proves that the wave function lives in a Hilbert space over the complex numbers, nor in a Hilbert space over the real numbers?