Understanding Quantum Mechanics #5: Decoherence
ฝัง
- เผยแพร่เมื่อ 27 พ.ค. 2024
- To check out the physics courses that I mentioned (many of which are free!) and to support this channel, go to brilliant.org/Sabine/ and create your Brilliant account. The first 200 will get 20% off the annual premium subscription.
The physics survey that I mention is here:
arxiv.org/abs/1612.00676
If you want to know more technical details, this is a really excellent review article that will tell you all you ever wanted to know about decoherence:
arxiv.org/abs/1911.06282 - วิทยาศาสตร์และเทคโนโลยี
![[TH] 2024 PMSL SEA W3D5 | Summer | ชี้ชะตา 16 ทีมสุดท้าย](http://i.ytimg.com/vi/PabKlXzcK8c/mqdefault.jpg)
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!
@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?
@@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.
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.
@Ron Maimon You know what exactly happens during the measurement? A Nobel Prize is awaiting you, IF you can prove it...😉
@@ThatCrazyKid0007 I have my own interpretation of the measurement and nobody can disprove it... Am I incompetent? If yes, who decides?
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!
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.
Happy to hear!
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.
@@skebess Yeah, Sabine's videos are not as educational as yours, but she's trying!
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?
That was a very coherent explanation of decoherence. Thanks! 😎
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?
@@moses777exodus In collapse models decoherence isn't necessary.You can just postulate for big systems superpositions do not apply since the wavefunction collapses.
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.
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.
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.
This Channel has been a great find! So much knowledge and with so much clarity!
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.
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.
Thanks. This is a very good point actually, I will keep this in mind.
@@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.
Yakir Aharonov's work might have something to say about this.
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?
@@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.
You are truly a gifted teacher - clear, vivid and direct. Thankyou! 🙏
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.
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.
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 TH-cam, 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.
Thanks for making these videos. I hope you enjoy making them as much as I enjoy watching them!
Hi, Sabine - I just love your videos. Nice to see that you're also into music. I recently completed a double degree in Physics and Music (at age 76). I find both subject very satisfying and I expect you do too.
With 76? Wow. I am also thinking to study music with 70. Just the probability of the musical wave functions to reach a degree may be beyond being measurable. ))) My congratulations 😀 .
@@RolandHuettmann Go for it!! - I found the Music somewhat easier (but still quite a challenge - I'm not very good at performing (piano) in front of people) than the Physics/Maths. The quantum section was quite a challenge; with the wave function, Bra / Ket notation and the 'strangeness' of the whole quantum world - but all of it was amazingly satisfying. Are you going for music performance or maybe orchestration? I'm sure you'll have a great time. I was graced with a wonderful bunch of talented young people (Late teens early twenties) who were very kind and embracing with their time and friendship.
@@Syzygy-21cm Thank you. Well, I am a medium advanced level studying piano with Elina Akselrud and Denis Zhdanov, both Lucerne, Switzerland. She is also performing as "Intertwining Arts" -- with a nice video playing the contemporary composer Carl Vine and sonatas from Skriabin including live performance by an artist. To me it is amazing how serious music practice enhances the functioning of the brain in all areas over time. I come from psychology. In math/science I am intrigued by the works of cognitive psychologist professor Donald Hoffmann, going beyond space-time and quantum field theory -- I am trying to understand the basic math. // Audiance -- I hope it will find you! Age is no excuse... I pretend to think to myself that the whole cosmos is listening anyway...)))
@@RolandHuettmann Hi, Amazing - I Studied Orchestration under Carl Vine at Sydney Conservatorium. I really like Carl Vine's music - he has written some great pieces of orchestral music and many piano pieces - all of which I have enjoyed. Neither Carl V's nor Scriabin's piano pieces are "easy pieces" aarrrggghhh! I have found that music in general and actual playing/practicing, certainly help with keeping the mind alert and attentive. The extra boost from really challenging yourself, mixing with younger folks, discussion with Uni Professors and other learned folks, I think, really opens one's horizons in all sorts of ways and provides a stimulus for further learning. Further more - it gives you that sense of child-like awe - again. Good luck with your studies.
Brilliant video, as always from Sabine. The best physics explanation on the web, in my opinion.
She goes to the heart of the issues without beating around the bush ...
When you showed me a Danish, I instantly thought: "It's been a long time since I had a nice cup of coffee and a pastry in a nice coffee shop." But then I remembered why...
scamdemic
You cleared a long standing doubt of mine. Thanks for the wonderful explanation Dr. Sabine!
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!
Please just let this series continue
You are doing a great job
Thanks a lot really
Thank you for making this digestible while still giving a thorough explanation. Amazing video🙌
Your videos are amazing, they are helping me with my studies
Happy to hear!
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 :)
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.
It's octopus that is intelligent. Squid are actually not bright at all.
I take it you have no objections to certain politicians, appropriately cooked! Not even amoeba-level problem solving abilities.
@@mdshett2 Indeed, I referred yo cephalopods in general.
@@davidwright8432 I would have no reservations, indeed I would consider many of them suitable for vegetarians.
@@hesitantjaguar7897 cuttlefish are very smart too.
Absolutely wonderful, I was wondering about this issue for some time.
Fantastic episode! This taught me more than any video I have seen about the subject so far.
Thanks for bringing up the density matrix in explaining decoherence. It gives the meat to the abstract concept of decoherence. Great work.
This is an absolutely EXCELLENT VIDEO! Thank you!!!
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
Soooo clear. Best on the net. Thx Sabina. That was very helpful.
Very clearly explained. Thank you, Sabine.
Awesome explanation! Thank you so much Sabine!
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.
Thank you Sabine! It is the clearest and most coherent explanation I have found of quantum decoherence. You gained a subscriber :)
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!!
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.
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.
Another great one helping improve my understanding. Mind you with physics I take a step forward and there is still so much more to understand.
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.
Thanks a lot ...keep making videos i have been anxious as no video explain any mathematics ...thank you so much
The transition between explaining science to the act of advertising at 11:50 is hilarious. A phase transition indeed. As always, a wonderful exposition.
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!
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.
"A problem must be fully understood if there is any hope of solution." - I think this applies in all areas of life.
Again, the best explanation on youtube, I will venture to say that is the best explanation I have seen in general.
Amazing! A notch up in my understanding of QM.
Finally an explanation that helps me!
Thank you!
Wonderful video!!!! I've never understood this before! If you can, please explain more about the phase of the wave function. Thanks again!
You are an excellent teacher, thank you for sharing this insight.
Very nicely explained Ma'am .....keep it up.
Great matching background there! I mostly appreciate the visual quality of Your videos, as a visual artist.
Wow, finally a good explanation of decoherence, I never got how it would affect the measurement itself....because it doesn't in the end.
A very lucid description of a very profound phenomenon. 👍
Truely one of your best videos, keep it up ❤
Sehr geehrte Frau Doktorin Hossenfelder,
vielen Dank für die hilfreiche Erklärung.
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.
Great. Commendable clarity.
Excellent series of talks.
Tks for your work!
It's surprisingly difficult to come across such a clear explanation of decoherence. Thanks Sabine!
Great video Sabine.
Perfect explanation........thank you very much for these videos.
Wonderfully described
I'm a simple man, I see Sabine I press like.
Beautiful explenation, briliant physicists!
That explanation is simply brilliant! :)
Thank you for this wonderful explanation! 😁
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.
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.
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
Thank you so much! really. With this video I remebered why I love the pyshics
Execellent and nicely described. Thank you.
Excellent video! Thanks!
Really nice, thank you sabine!
you explanation give a real sence about how wave function collapse.thank you so much
Thank you, Sabine, the videos on QM have been very helpful.
Could you do one on the Penrose interpretation of QM?
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).
Your videos are just awesome, thank you!
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!
Thank you madam. I think this is your best video, and the best youtube video I've seen on the topic of decoherence.
Thank you for doing the math to explain this. It made it a lot more clear for me
I've been waiting for this.
Thank you. I'd love to see a video on the Heisenberg cut.
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.
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.
Matt O'Dowd over at Spacetime once quipped phase as 'the angle of a wave's centre of mass off the horizontal axis as corresponding to the wave of a given frequency'. But this lesson today brings alive that previously gleefully dry proposition.
Great series
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.
Thank you Sabine !!
Brillant explanation of a complex issue.
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
My compliments for the videos, which are very interesting and well made. I would suggest adding a video about quantum teleportation and other quantum phenomena. Sincerely
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 . . .
Brilliant, Curiosity Stream and Square Space have taken over all my favorite channels now.
Absolutely brilliant like ever.
Awesome. I was looking for exactly the same content .
Excellent explanation, thanks a lot 🙂
Wow, that was a hell of a great explanation
Revisited after one year, again a brilliant explanation (better than my university QM). Thanks
The best explanation of decoherence I saw.
Thanks Sabine for another great video! I love your hair in this one, and your dress! I'm finally learning something!
Perfect explanation 😊