"Many Worlds" is a simplification of quantum mechanics
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- čas přidán 21. 04. 2022
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The many worlds interpretation of quantum mechanics is often criticised for being excessive. Isn't it crazy that there are an infinite number of worlds splitting all the time? In this video I give an introduction to what many worlds actually is, and why I think it's actually a simplification of standard quantum mechanics.
Book recommendations:
The best book on MW in general and the one that inspired this video is Emergent Multiverse by Wallace.
Another fantastic book is Decoherence: and the quantum-to-classical transition by Schlosshauer. It isn’t specifically about MW, and is useful to understand decoherence in general. - Věda a technologie
You finally made the video! If only people knew how long you've been thinking about this one.
Ahaha, at least you know and I’ve finally done it!
You should do a collab. 🙂
@The Science Asylum Oh you're here? Your channel is so good.
Your videos are better than Veritasium
There are 31 planes of existence from Hell to Brahma worlds according to Buddhism. The states of mind give rebith in those worlds.
Always happy to see a new video from you!!
Thanks Dianna!! I should try to do it more often
Always cool when other content creators share the same interest
@@LookingGlassUniverse I anthropomorphised quantum particle as students and the experiment as the curriculum of a particular subject. I the. Took the measurement as testing and exams that brought with it a intuition I never had before that. The way you lay out the problems seems to be a fit with my conceptualisation of it.
@@LookingGlassUniverse this is where I wrote it down in the feverdream that was lockdown as covid refugee in Europe. Rumour has it die article drove crazy according to some locals, I had fun, perhaps this will bring some ⛅️and luck
😘
You two and Jade from Up and Atom should do a collaboration.
But now I have other questions....
What do you two think about Dr. O'Dowd on Spacetime? (Truth be told, he is one of my heroes, but his content is a bit beyond me)
Sabine Hossenfelder is another hero too as she strikes me as a very grounded scientist.
Anyways, thank you both for great videos
As someone who knows almost nothing about quantum mechanics, this is the most sane explanation of "a measurement" I have ever heard. The fact that you took the time to mention the name of the instrument "Stern Gerlach" is brilliant, it makes the entire discussion seem a bit more real to me. I can actually search a picture of what an experimental setup might look like. It is so rare that actual instrumentation is discussed and I find that super frustrating. Often the mathematical models are presented as if they "are quantum mechanics" and the results from the math alone are supposed to be interesting somehow.
So glad you’re uploading again. You’re one of my favorite science CZcamsrs. I was inspired by your older stop motion videos to make one of a similar style on my own channel.
I checked out your channel! Amazing work! I’m excited to see more from you :)
@Unedited Life Of Daniel You have quite an imagination. You should work for Marvel Comics, seriously.
Oh I love this explanation! You've made this concept very approachable and yet I don't feel like I got a watered-down media version of it. Thank you so much! Subscribed.
Ok, you've convinced me. I'm sure you're right that the "many worlds" idea would have gone down better if it hadn't been given that name. The way you explain it suggests that it's really just a generalization of superposition to include the observer, and that really makes a lot of sense to me now. Thank you!
David Wallace calls it “the emergent multiverse” which I think is a much better name. I wish it had been called that instead!
Entanglement with the measurement device/observer seems fine and nice. But that is just replacing one problem with another with no predictive improvement. There is still the problem of explaining why we find the measurement device or the observer to be probabilistically in one state and not the other one. So, no. The many worlds interpretation doesn't solve the measurement problem. It just shifts it to a new one and ultimately had no new predictive value.
@@LookingGlassUniverse .
@@somasundaramsankaranarayan4592 umm, no. It's an interpretation, it's not supposed to increase the predictive ability of QM. it's just a more elegant explanation for what happens because it requires less axioms
@@LookingGlassUniverse It's Expanding Bubbles of Quantum Superposition...
I took calc 2 over summer school in university. And I had the greatest teacher ever. They loved the subject, they loved thinking about it, talking about it. Their passion was infectious. He made me want to learn. Made me believe I could learn. Your passion for knowledge and talent to explain, and you do it with charisma. It’s infectious. You’re a treasure and makes me want to go crack open any book and learn. I’m so glad I found your videos.
Thank you so much for the incredible work you're doing. As a layman with an interest in youtube physics that goes back a decade, this is probably the single best video I've seen yet for a couple of reasons-the hugeness of the subject matter plus the clarity of your explanation is absolutely mind-blowing. Thank you so much, and keep going!
Thank you for this. It may have felt haphazard in the making but I have to say, this filled in some really important gaps in my understanding of Many Worlds and answered some questions I've had about it. So I'm really grateful for this video. I've never heard it explained this way before and it makes so much sense. What looks like irreversibility is actually irretrievability, which is functionally equivalent. And we can see it happen on the scale of single particles. It's really quite brilliant.
One of the best science CZcamsrs around!
I hope to start my PhD next year and your approach of coming up with simple yet rigorous answers to very difficult questions in your field is incredibly instructive!
I stumbled across your channel very recently and it's pure gold! You are a very good teacher.
Please don't worry about the space between videos. I have been trying to jumpstart myself to do this youtube-physics-education thing for some time but it has taken a lot of effort to try to make the videos that I envision (not plugging myself, I literally don't have anything to plug, just testifying that it's harder than it looks, and it looks hard). Take your time to express the unique voice you have in this field.❤
Your probably the most PRECISE science educator on youtube. You don't just parrot what everybody else says you deep dive and find all the things that don't make sense. Your attention to detail is legendary but your also thinking outside the box. It's a crazy valuable mix! You and Science Asylum are the BEST!!🔬🧪🥼
What do you suppose is the significance of the word "world" in the so-called "many *worlds* interpretation"?
What you suppose that the word "world" signifies? Milieu, or just state of affairs?
@@vhawk1951kl A "universe".
@@ThePowerLover What is an universe?
You have absolutely no idea? - My left pocket is now betting my right pocket that you have no idea what you mean by "universe" and are about to demonstrate that by signally failing to set out what you mean by or defining an universe - whatever that might be, and plainly you have not the faintest idea as you are about to demonstrate - do you have any idea what the word "universe means?
Apparently not
@@vhawk1951kl You didn't read the quotation marks, didn't you?
@@ThePowerLover What do you suppose the quotation marks add to what you cannot even begin to define, as you are about to demonstrate by signally failing to do so - what does "many worlds" actually mean?
You have absolutely no idea whatsoever? - This you are about to demonstrate by signally failing to set out what you mean by "many worlds", and my left pocket bets my right pocket that you have absolutely no idea whatsoever what you mean by "world", singular, any more than you have any idea whatsoever what you mean by "worlds, pleural, as you are about to demonstrate by signally failing to set out what you mean by or define "worlds", pleural, it being an athlete incoherent and meaningless word as far as you are concerned, as you are about to demonstrate by signally failing to define either world or worlds, without reference to cognates and synonyms and substituting one undefined for another undefined
This is easily the most amazing explanation in layman’s term of MW I’ve ever seen! Thank you so very much!
She has some of the best explanations I've seen in years.
It's an excellent explanation of the measurement problem and its correct understanding as entanglement/interaction, all of which, as she explained (extremely well) is empirically demonstrated.
But her jump to MWI seems a magician's (mathematician's) trick to me, nothing else.
@@LuisAldamiz Out of curiosity how does this seem to be just a mathematical trick to you, i.e. what would it take to make you change your opinion on this?
As far as I can tell the logic holds for her jump its just you have to account for entanglement's role in information theory i.e. the change in entropy to define the measurement and reversibility i.e. decoherence. Aside from decoherence what is missing for you?
Now to be frank I also think it is likely to be a bit more complicated for various reasons beyond this topic (namely relating the establishment of formalism equivalence between the Schrodinger equation within the Feynman path integral with general relativity and the Einstein field equations and explaining the observed phenomenon known as the quantum Zeno effect) but I'm curious about what particularly bothered you in this description she gave.
@@Dragrath1 - Maths are a logical tool not reality. Often maths produce "solutions", "results" that, when faced with reality are proven false (or most unlikely), you could probably "demonstrate" gods and what-not with maths, it's full of logical traps because it's nothing but logic. I know that there's a lot of Pithagorics/Platonists who think otherwise but Kepler himself had to break with that kind of "philosophy" in order to open the gates to modern science. Science is mostly not based on ancient Greeks but on modern Europeans like Gallileo, Kepler and Descartes.
"Shut up and calculate" is ultimately not a good enough approach... unless those calculations are confirmed experimentally, empirically, by observational facts.
That's what it takes to prove something in terms scientific: evidence. Something that is not observable in principle, be it String Theory or Many Worlds, is not science, it's speculative philosophy.
I'm amiss on how you relate Schrödinger and Feynman to General Relativity, as so far, in spite of the best efforts by Dirac and QFT, these two branches of modern physics remain almost totally separated.
@@Dragrath1 PS - The "quantum Zeno effect" is interesting, because it implies that measurement is also "caging" of the "wavicle", what may help explaining the decay of quantumness with number of particles, as they "measure" (interact with, get entangled with) each other again and again and again. Thanks for mentioning that even if in passing because it was a bit I was unaware of.
It seems to imply that "measurement" (i.e. interaction/entanglement) reduces or totally quells the uncertainty or "superpositionness" of the wavefunction or wavicle, forging macroscopic, relatively stable, Reality as we usually perceive it. I still don't see how MW may emerge from that.
This is literally the best explanation of quantum mechanics, entanglement, and many worlds that I have ever seen. Subscribed!
Wow, this is incredibly clear and well-paced. It’s the first time I feel like I’ve grasped the many worlds concept.
You made this so easy to follow and understand: it's the first time that has ever happened to me on the subject of quantum mechanics. Now I'm actually interested in this subject that had previously seemed out of reach and elitist. Good job!
Please correct me if I am wrong or mistaken, but is it right to say that what is called "quantum mechanics" is in reality a series of guesses upon the basis of which various experiments are made the outcomes of which tend to suggest that the guesses are rather better or more than guesses? Would that sound better if we substituted for guess inference or deduction or supposition - or perhaps even just imagination?
Good to see you back again. I'm really impressed by your ability to put complex things so clean and simple. Just want to say thank you for all of your videos. Keep on your amazing work (but don't forget to enjoy your holiday either :)
Insanely good video, makes perfect sense - you really have a talent for explaining things like this!
This is wonderful as is! I look forward to your follow up videos!
I really enjoyed this explanation. So glad you're talking about the Everett interpretation as I've always found it to be the most fascinating one. You did an excellent job explaining it to someone without a degree in science (me!)
Could you add the books you mentioned to the description when you get a moment? And please make more videos about this! And also just more videos in general. Love your channel
I love how you explain in such a lucid manner Mithuna ❤️ I really look forward to your videos. Wish you could upload more often 😇✨
Thank you so much :) I will be trying to upload a lot more
Wow, absolutely incredible!! I swear to god, Nobody has explained quantum mechanics in such detail ever before and this made me understand soooo much more, such as the true nature of quantum entanglement (and the existence of uncertainty linked to missing entanglements.(I'd like to think.) ) But honestly, feeling lucky to have come across your channel and would be glad to support you if you have a patreon.!! Keep moving forward!
Hurray! I have been waiting for your new video!
Thank you for being patient with me!
Loved this video! I'm glad we got right to the math of it all instead of trying to fly off into space. I'd love to hear more about decoherence, especially/maybe in the context of macroscopic quantum states.
Great video!
I like that it explains MW without relying on branching (because that's basically the same as WF collapse)
In my view it makes more sense to assume an infinite but constant number of timelines, and measurements just select those timelines that are consistent with the outcome. Quantum probability then is just a measure on these timelines.
I don't think MW was ever about branching as you mean it. It was never about branches that don't yet exist until a measurement is made. It was always about existing branches which are just all the probabilities allowed by the wave function. You can think of it as each possibility is aware of itself but is unaware of all the others.
You look better, simply happy to see you are doing well. Great video as always, thanks for inspiring so many of us in these trying times.
Very nice video! I had always been sceptical of the Everettian’s appeal to Occam’s razor because it felt like they were substituting the measurement axiom for the axiom “I don’t experience superpositions” (which seems true, but brings in lots of philosophical complications of “I” and “experience”). Your argument for why the branches can no longer interfere is much more appealing, so thank you.
I also love how the whole thing was done with qubits. This is why I call quantum computing “experimental metaphysics”
“If you can't explain it simply, you don't understand it well enough.”
I just discovered your channel and it's absolutely amazing. You seem to have a genuine and even masterful understanding of your subject matter, and yet you're incredibly effective at making it accessible to amateur physics folk like myself. Time allowing...keep it up!!!
I can now understand MW but I cannot explain how she made it so that a six year old could understand.
If you please, and if that be right, explain "understand" simply.
What in practice you are suggesting is that if someone has a blurred or out of focus photograph of something the details and precise outline of which, cannot be identified, that they identify that which cannot be identified because the photograph is out of focus and blurred- In short inviting them to, or suggesting that they should (or you would like it if) focus what cannot be focused or clarified what cannot be clarified or define what cannot be defined or bring an unfocused photograph into focus, which is impossible.
In the all of mirrors or miasma or fog of the human mind, there are an untold number of vague generalisations, or you could perhaps call them unfocused photographs, which are words or ideas, and what in practice happens is that men (human beings come across these vague generalisations or unfocused photographs, and offer them as proxies or substitutes for yet another vague generalisation or unfocused photograph, or in the simplest possible language they produce unfocused photographs of something, or some vague generalised idea, and when the interlocutor looks at it and tries to discover from the one producing the unfocussed photograph and asks him of what it is a photograph, or to clarify or define or focus it or tell him of what it is a photograph, the one showing him the photograph produces yet another or other unfocused photographs, to which is all there interlocutor says exactly the same thing, and the one producing the unfocused photograph cannot tell of what it is a photograph either, because it is impossible to tell of what it is a photograph - and that is why men rarely understand one another, thus if I were to ask you to focus or clarify or define "understand" you simply cannot tell me of what it is a photograph or what you mean or seek to convey, because you don't have any idea yourself - you simply cannot, for you is impossible as you are about to discover, and when you discover that you yourself cannot focus it unfocused photograph "understand", you will almost certainly proffer me or anyone interlocutor another unfocused photograph, or a series of unfocused photographs, or in plain language you don't have the faintest idea what you mean by "understand".
Moreover you are about to demonstrate that by signally failing to define or set out what you mean by, "understand" - in your own terminology"Simply".
Explain Understand *Simply*.
Of course you cannot, because the word/photograph "understand" - for you (and for all I know all men) is an unfocused photograph, and neither you nor any of them can focus the unfocused, it is as if you came across something and took a photograph of it, and when you came to develop that photograph, you discover that you cannot discover of what it is a photograph because the photograph you have is out of focus and no more than ablur, or just a vague generalisation, simply cannot be focused, or you simply cannot determine of what it is a photograph, because that is what words/ideas are, are they are not?
When I ask you to set out clearly or focus or define "understand", and you go to look at that photograph, you discover that you cannot, and this you are about to demonstrate for precisely the reasons I set out.
It is not your fault that you cannot do the impossible, any more than it is your fault that you do not understand why it is possible, and you only discover that something is impossible when you come to try to do the impossible, and you discover that you *cannot* explain focus define clarify the word/idea/photograph "understand", it is only then that you discover that is completely impossible to do so, because no-one given an unlimited amount of time can focus an unfocused photograph or discover of what it is a photograph - you follow?
No matter how hard or for how long you look at the word/idea/photograph "understand", you still won't be able to bring it into focus or define "understand".
You have a vague unclear or unfocused idea that it is a photograph of something, but for the life of you you cannot tell me of what it is a photograph.
It is not your fault or a criticism of you, that you simply cannot do the impossible. Moreover producing to me a number of similarly unfocused photographs, or vague generalisations of whatever it is that you are trying to explain define or clarify or producing more word/idea/word/photographs (that themselves are unfocused) - or simply resorting to synonyms and cognates or psychological algebra, will not assist you in any way either.
Psychological algebra or X = Y = X where both X and Y are undefined, assigned no value, or simply substituting for one undefined term another or several other undefined terms, or cognates or synonyms, or simply substituting for one unfocused photograph another or several unfocused photographs.
It will be interesting for you and I to discover if in practice it is to precisely that that you may be forced to resort, but we shall see what is the outcome of that experiment
Thought it was really good on the whole. As someone who's studied QM, i think a brief explanation of the wavefunction coefficients and normalisation would make a lot of your hand-wavy bits more easy to accept.
Glad to see you back on CZcams! Loved every minute of this video and will def be watching again to make sure I understood
Thank you so much, that’s too kind!
That is the best explanation of Many Worlds I have heard. I have been trying to understand it for a while now. Thank you for this video!
An outstanding explanation, thanks for this. Very satisfying "A-ha!" moments from the single particle measurement and many-worlds interpretation explanations.
Thank you for making this video, it was difficult to get the concepts across, and sadly I as much as tried to understand it, I couldn't get all of the details... but I understand what you were saying about having rushed through things and that it might be a jumble, but not to worry, it is a complex subject and it has strange notation and concepts... maybe you'll have clarity to explain what it was you wanted to say, and I really think you're close to having the video that you wanted to achieve... so I hope that you do make another video and I'll happily watch it.
Excellent explaanation. I'm glad I found you. Keep up the great work!
Simply - AMAZING !
Love this channel !!
This was the first time someone explained the many worlds interpretation in a way that made sense… I’ve seen too many CZcams videos that talked about it, but only talk about the woo woo part. You actually explained it. Thanks!
If you were forced at gunpoint to set out exactly how you understand what is called the "many worlds interpretation", how would you go about doing that?
For the purposes of what is called the "many worlds interpretation", what exactly is the significance of the word "world" will it make any difference to you was called the many things interpretation?
Why not just call it the many things interpretation? - Or even perhaps just the lots of stuff interpretation. Does it matter what you call it
@@vhawk1951kl That's an interestingly aggressive way of phrasing such a simple question... I actually started working on an answer, but then I stopped. I'm honestly under the impression that you are just trolling me, and there are better ways to spend my time.
Really nice, never realized that the many worlds theory is an actual simplification. That immediately makes clear why it exists and many like it.
I so glad to hear it made some sense!
This is insane... with every particle interaction a new world would be copy pasted. Where shall all the Energy come from to copy paste a whole world all the time endlessly? Only explenation might be, that the worlds are run in a super computer where copy and paste in the RAM doesnt take much energy, so we are a simulation
@@WackyJackyTracky
From superposition.
1. Multiple states of reality are real.
2. A particle interaction happens
3. Multiple states of reality continue to be real.
The universe is one big single thing which contains these multitudes.
@@WackyJackyTracky you could argue that all states are already there in the beginning, so the full vector with an element for all the combination of all the possible existing particles. Just that in the beginning only a few elements have non zero weight. And from there, it is is only a rotation (means the length of the vector stays the same) in the high dimensional vector space.
And since the base of this vectors space is arbitrary (in the video, the up and down could be written as a combination of left and right), you could always find a new base, where the current state is use one base vector. In that base, only that element would have a weight of 1. So where is the branching/splitting of the world?
@@WackyJackyTracky
Quantum computers, for sure.
This is the best explanation I ever saw to this topic. Great! Thanks a lot.
Well, I won't claim that I understand all of this, and yet... you've given a more accessible explanation for some of these things than I can recall having yet experienced, so... thank you for that, and I hope you continue! Subscribed based on just this, with several others now queued up to watch next!
This was actually the best explanation of "many worlds" I've seen. All of the others sort of forced me to try to imagine a multiverse that continuously splits uncountably many times. Your explanation shows this through the concept of superposition. Maybe I'm strange, but for me that requires less of a leap of faith than the other explanations I've seen. Thank you! I look forward to seeing more videos from you.
This is a very clean, clear explanation of the many-worlds interpretation. I think leaving off the coefficients (but mentioning that they are there) helped keep everything more on topic. Excellent work for an off-the-cuff video 😃
Thank you so much!
A very good explanation of both the concepts and (simplified) math behind the many worlds interpretation. You even included the fact that it's possible to reverse the process, which is very cool. I've long agreed that the many worlds interpretation is the most sensible Occam's Razor view of quantum mechanics, so it's nice to see a well-argued and experimentally verified explanation.
One of the most comprehensive QM videos. Congrats times a million!!
This is really convincing! I've never seen a computational argument for MW so this is definitely something I'm gonna link to maths-type friends who are curious. I don't see the books you mentioned in the description btw, is there one you like that goes more indepth on decoherence? That's definitely the thing that interests me the most here and the thing I understand the least.
I’m sorry, I forgot to put the books in the description. They’re there now though! If you read either of them, let me know :)
I think the word “worlds” and the media portrayal of it as separate dimensions and universes is a huge reason for making MWI sound much more exotic than it really is. MWI is just the math, without the special affordance for measurement collapse
Listen to Deutsch, Carroll or almost any MWer. They say there exists countless, maybe infinite versions of all of us that share parts of the same past, but are now almost surely forever separate. This isn’t just math, these are other people that are no longer us, but are going on existing and living their separate lives. No other popular interpretation of QM has this feature. To me, MWI is clearly exotic, fantastical and any other awe-striking word you can think of and it has nothing to do with the media. Your take that MWI is “just math” makes me feel like I am living in a different many world than the one you are living in.
I agree. The MWI just moves the measurement problem to the wave function's branching, which suffers almost exactly the same issues. So I see the MWI as equivalent to the Copenaghen Interpretation (well, the version of the CI explaining measurement with wave function collapse) with just a change of terminology (branching instead of collapse) and a bunch of additional (and unnecessary) metaphysics.
Furthermore, it's not clear at all how the MWI would comply with special relativity.
@@francescocannistra7915
I don't think "branching" is just another word for "collapse". The latter throws away information about a particle, when it is measured, while the former keeps all of it. (The Schroedinger Equation doesn't loose information.)
Also, interactions with particles are still local, therefore MWI should not have a problem with special relativity. It is not that the "universe itself" gets branched instantly over billions of lightyears. It's just that the entanglement "spreads" through the universe, whenever the entangled particles interact with other particles, getting them entangled as well... and so on.
@@timjohnson3913 A theory isn't judged by what it entails but through its content.
The mathematics is not the physics. To think it _is_ is metaphysical overload. Besides which, the orthodox postulates of QM do not mention anything "collapsing". When you write the measurement postulate in linear algebra there is nothing collapsing. Ergo no need to invoke any metaphysical assumptions like MWI. There are more parsimonious ontologies besides, see my comments above.
I'm not an expert by any means, but I've been trying to understand as many qm concepts as possible, without having a super tight grasp of the underlying mat, etc.
I have sooo many questions, but instead, I'll just say, that the notion of macroscopic systems being entangled and part of a superposition, is something that I always thought could solve so many issues. The first I encountered this, was with the delayed choice quantum eraser. All this explains perfectly what you see in that experiment. (I think:))
It's great to see you've been uploading lately. We had a short email exchange back in the day, around 5 or so years ago. Congrats on all your achievements and keep it up! :)
Pretty much the best video I have seen about the topic, don't be too harsh with yourself. After all it's not that easy to explain it without using too much maths and in only 30 mins! Nonetheless I would still absolutely love to see a "deeper" version of it. Thank you for all your effort, it is really appreciated :)
Btw would really love to see which books you meant at the end...? Unfortunately there are none linked
I promise there's a world where I am watching the video before commenting! I will, but just excited to see you back!
Haha! Thank you for always watching!
Interesting. If I'm understanding this right, "wave function collapse" is nothing more than the measured particle getting entangled with something we don't have access to. It's not saying that the universe has split into multiple universes but that when we measure entangled particles in a state of superposition, we're going to get results that look like decoherence due to a loss of access to some of the state of the complete system. Did I get that right?
Yeah, that’s a great way to put it!
I don't think it is completely right. Let me try to explain why.
In brief: decoherence is not the same as wave function collapse and it's not enough to explain it; and the "splitting" actually corresponds to a dynamical fact (albeit as conventional as the choice of a basis), the fact that the state vector of the Universe becomes close to a superposition of terms which have certain "macroscopic" properties that are relevant to us. Anyway we'll never have empiric access to the whole state vector of the Universe, just to the component pertaining to us.
So, about decoherence. When we trace out the (unknown) environment state, the state of the system is no longer described by a state vector but by a density matrix, which incorporates the (epistemic) lack of knowledge about the environment. What decoherence does is transforming the density matrix of the system into a diagonal form (w.r.t. the eigenbasis of the observable being measured), which means the state _after decoherence/measurement_ is a classical probabilistic mixture of pure states (with probabilities given by the Born rule). This means that, if we have an already-measured system in our hands but we don't know the result of the measurement, when we do stuff with it, it will behave exactly (in the sense of the info we can extract from it) as if it were randomly picked from an ensemble of eigenstates prepared with ratios proportional to the Born amplitudes squared.
Now, how does decoherence cause the density matrix to become diagonal? I don't know the details, which depend on how you model the environment and the whole decoherence process. I only know that, the more decoherence occurs, the more the off-diagonal terms tend to look like the scalar products of the pointer/environment eigenstates, and the latter are orthogonal. I think the reason they're orthogonal is merely that they're position eigenstates of the set of all "particles" of the pointer of the detector: if you change the position of the pointer a little bit, it's already a different position eigenstate.
Summing up: the off-diagonal terms are responsible for interference; and decoherence, by killing off-diagonal terms, explains why different final states don't interfere with each other; and this in turn explains why they behave "classically", i.e. according to "classical probability" and not "quantum probability".
*But,* and this is important, the above does _not_ explain how the state seemingly passes from a diagonal density matrix to a pure state (rank 1 density matrix) during a "measurement" (i.e. why the wave function, as seen by an agent, seems to collapse). To explain (or, rather, explain away) measurement you need the Everett interpretation. According to this, it's not that the state vector falls to an eigenstate as per Copenhagen, instead it's that:
-The Schroedinger equation holds at all times.
-The state vector of the Universe evolves from a separable initial state like, say, (A+B)*(Ready) to an entangled final state A*(E1)+B*(E2).
-Each component, A*(E1) and B*(E2), evolves independently under unitary evolution because the Schroedinger equation is linear.
-From the point of view of the system (i.e. by tracing out the environment) the final state is a classical probabilistic mixture of A and B (because of decoherence and because E1 and E2 are orthogonal). So you recover the perception of the experiments' results being random classical states (no Schroedinger's cats and no weird statistical facts due to interference).
-Each component A*(E1), respectively B*(E2), of the Universe's wave function represents a classical world in which the experiment has resulted in A, respectively B.
-There's effectively no way to know anything about other worlds by computing expected values and other measurable stuff using only the component pertaining to "our world", say A*(E1). There's no empirical way to know whether the state vector of the Universe now, after measurement, is really A*(E1) as it would seem to an agent like us, or it is A*(E1)+B*(E2).
One thing that blows some minds about Everett is that you have to change your conception of "agent/observer" and the meaning of probability. Now the history of one "observer" is not linearly ordered anymore, but it's structured like a branching tree (The past history of each agent is still linearly ordered though). And probability is not the probability of some physical event happening: because they _all_ happen with probability one, each in their own "world". Probability is more like what philosophers would call indexical uncertainty: you don't know _which one you_ you are. So to speak...
I've probably been a bit too verbose. Also, warning: I've background in math, I'm not a physicist; so take what I write with a grain of salt.
No, wave function collapse is simply a physical interaction that effectively reduces a superposition to an eigenstate. MWI on the other hand argues that there is no such collapse.
@@rv706 thank you
@@rv706 I agree, "Many Worlds" interpretation of Quantum Mechanics is pure nonsense and there is no measurement problem
Even this quick and dirty video has clarified so much for me. Great work!
This is a wonderful video. It feels so good listening to a bright beautiful mind like yours.
Gives me something to ponder.
Many worlds interpretation already felt the most natural to me, but this story makes it so much clearer. Many thanks for this beautiful explanation!
This is a reasonably good short explanation of Many Worlds. I studied a bit under Dr. Wheeler in the late 70s early 80s and tried to grasp It from Bit and his other takes on QM when I was a teen/early 20s as an undergraduate physics student and it was fascinating but made good sense to me (A in his course and on my paper) and that made it easier. He was the best professor I had had at the university, a concerned and interested teacher that interacted with his students and made the complex as simple as possible. His recommendation helped me obtain my first teaching position.
Interestingly, as a member of the UT ballroom dance team at the time, I helped another student set up for his dance routine tryout for the team, an astrophysics student, said his name was Neil. ;) I've watched your videos over the years as you went through your schooling and appreciate your explanations and willingness to help present this emerging understanding of the universe. :)
Wow, you had Wheeler as a professor? That must have been such an honour!
I really appreciate your encouragement, thank you :)
@@LookingGlassUniverse Yes, he was very encouraging of those students that showed promise and worked hard, and an excellent instructor, which can't be said of many professors. Having Neil Degrasse Tyson become a member of the Ballroom Dance team I was a part of as just a fellow uni student was an interesting happenstance, as well. I look forward to more videos like this, your presentation and understanding approaches the directness and simplicity of Wheeler's instructional style!
I will tell you, he had an interesting way of handling students that were always looking for him to make a mistake, for instance, on the chalkboard. If someone pointed out an obvious scripting or math error, he would stop, look at them, then slowly pull out of his pocket a small change purse,, pull out a penny, then walk through the aisles directly to the student and hand it to them with much ceremony, without a word, then walk back down the aisle and continue his teaching. I used it with my students, I thought it was so great! ;)
One of the biggest reasons i subscribed you because your life, story is related to mine. I am not really good at maths and i love physics.
Keep at it :)!
People who aren't good at high school mathematics tend to just intuitively understand that the way they teach it is just brainwashing. Once they get exposed to the actual structures like Lie groups they start to actually like it.
this is really a fantastic point of view about measurement, collapse,interference, entanglement etc.
beautiful accompanying mathematical expositions. thanks.
Hello,
Yesterday evening I was rewatching your old videos after a long time, and I was wondering if "real world" has taken over.
And BAM!
Today i woke up and saw notification that new vid is up!
Quantum entaglement at work? 🤔
Seriously, I LOVE how you took the jist of the many worlds interpretation here and explained it super simply and without hype and BS unlike I'm sure 98 % of other YT would.
Keep up the good work.
Thank you so much!! Sorry for disappearing
Literally yesterday I was talking with my friend about how quantum state collapse is unnecessary because entanglement can explain the same observations, and today you upload a video explaining it to me!
This was pretty good. The best explanation of Many Worlds still comes from Sean Caroll but this actually filled out some of the gaps that Sean left me with.
Nonetheless many worlds is still the ONLY interpretation that ever made sense to me and it made enough sense that I could intuitively and correctly predict the results of common "baffling" quantum experiments.
The saying that "if you can't explain it simply, you don't understand it" goes both ways and I think the fact that many worlds simply explains (even to me) what happens means that it's the correct understanding of QM or at least on the right track.
First video of yours I have watched. Watched because I was possibly going to subscribe. 1:09 in and im subscribing. Your first words on many worlds sold me
I really liked this explanation! Very impressive you built up from nothing to decoherence and emergence of MW in such a short time! I’d love to see the born rule derived as well, because I feel like that’s the actual difficult part in MW. I think you maybe forgot to put the books you recommended in the description. (£5 says one of them is Sean Carroll’s one as you both seem to like the same emphasis that it’s about removing rather than adding postulates)
Nice explanation of the MW interpretation. Personally, I am kind of a true believer in the Copenhagen interpretation. By this I mean that I think there must be some kind of physical process that approximates to wave function collapse.
I think that the argument of this video can be boiled down to the following points.
a) Wave function collapse is so terrible that it must be wrong.
b) Environmental decoherence is sufficient to explain quantum mechanical measurements.
c) People don't perceive superpositions because our brains get entangled with the objects being observed.
There are two ways that people normally try to explain away wave function collapse. The first is to push determinism down to the microscopic level. The result is some kind of Bohmian Mechanics / hidden variable theory. These kinds of theories have many problems, but that is beyond the scope.
The second way of avoiding collapse is to allow superpositions to swell up to the macroscopic level. The result is the Everett / Many Worlds interpretation. In my opinion, the the MW interpretation is more viable than Bohmian Mechanics, for the reason explained in this video, namely, that it doesn't modify the basic formalism of quantum mechanics. The price paid for this way of getting rid of wave function collapse is an ever increasing infinity of unobservable branches of the wave function. If you want to get rid of these unobservable branches, you simply gotta have some kind of spontaneous decoherence (i.e. wave function collapse). This is the basic argument why Copenhagen is the best interpretation. In the end, you have to pick your poison.
The debate between Copenhagen and MW is not likely to be resolved experimentally. Actually, there is a way the question of collapse could be resolved experimentally. If spontaneous decoherence were ever observed in the laboratory, MW would be dead. Obviously, that hasn't happened so far, and may never happen.
Im only a layman with an deep interest in science, and physics, and I think your right here for what it's worth. From everything I have been able to gather on the subject, things like MWI only exist because many physicists hold to materialists ideals. And QM wavefunction collapse spits right in the face of that philosophical presupposition.
@@anthonypolonkay2681 Yes, materialism is a religion and the "Many Worlds" interpretation of Quantum Mechanics is their god and temple rolled into one and they worship unconsciousness and none free will and pure nonsense
@@anthonypolonkay2681 Mathematics, by is very nature, is materialistic. It's not about materialism, but rather, it's simply about having a mathematical mind, which is the definition of a good physicist.
@@JohnSmith-ut5th idk where your getting that from. Mathematics is conceptual. You can't destroy, or alter mathematics by destroying, or altering anything in the physical world. I'd like to know on what grounds you consider mathematics materialistic by its nature. Because that's going to be a hard case to make.
@@anthonypolonkay2681 Mathematics is our conceptual understanding of physical space and time. Look at group theory. It is a study of symmetries. Symmetries are physical things. All mathematics boils down to logic, which is fundamentally based on AND, OR, and NOT operations. What does AND mean? It's really a conceptual statement about a physical state of the universe. It means A AND B are both true. Same for OR. This is why we use the words "and" and "or" so frequently (look, I did it in this sentence).
This is why Muslims rejected mathematics as "evil" around the time of the Enlightenment. They thought a study of physical reality was placing the universe before God. Now, I'm a Christian, and I don't believe it has to be either/or. However, we do have to recognize facts for facts. All languages, including mathematics, are ways of communicating facts about the state of the world (or, in reality, our concept of it).
Great video! There certainly is one sense in which MWI is a simplification of QM, and you describe it well here: the mathematical structure of MWI is much simpler than the standard quantum formalism.
However (and this is something that is often glossed over) there is more to a physical theory than its mathematical structure. There is also the conceptual prescription of how that mathematical structure is supposed to correspond to physical reality. Otherwise you just have a bunch of equations and you're doing math, not physics. One of the central problems with standard QM is that is isn't really clear what the wavefunction is supposed to physically represent, and MWI does very little to resolve this problem. I think this is an apt comparison:
Standard quantum formalism: vague/unclear division between quantum and classical domains (but at least fairly clear how the classical part is supposed to correspond to observable reality)
Many worlds interpretation: much simpler mathematical structure, but now there is no clear prescription for what the equations physically represent
Pilot-wave theory: simpler mathematical structure than standard QM although not quite as simple as MWI (guidance equation is not as ad-hoc as the measurement postulates and doesn't conflict with the Schrodinger equation) and very clear prescription for what the equations physically represent
Those who take MWI to be the best formulation of QM probably want to say something like "fundamentally, the quantum state is all that exists" and that our observable reality is emergent from the behavior of the quantum state. But without a detailed specification of how that actually works - of what physical reality actually emerges from any given quantum state - the simplicity of MWI is bought at the price of essentially just handwaving away one of the most important parts of the theory, namely, how the equations relate to the real physical world. And if one does add a detailed specification of the emergence, you end up with something comparable to pilot-wave theory in terms of mathematical complexity, but with much greater complexity in terms of its physical ontology (since then it really would contain a multitude of physical universes instead of just one).
In fact, what a lot of the most notable MWI proponents say is that the quantum state is all that fundamentally exists, and that its really only the appearance of the physical reality which emerges - not only are there no multitude of physical universes, but there isn't even one, at least not anything like the (3+1)-dimensional space-time universe we perceive. Not only does this rely on some major philosophical assumptions (e.g., functionalism in the philosophy of mind), but it puts the MWI in some ways on the level of a "brain-in-a-vat" scenario - even though it looks like we live and move in a world made of all sorts of matter and energy, our perceptions really just emerge from various features of the quantum state and are not in any sense caused by the material things we think we perceive (just the like the brain-in-a-vat's experiences are caused by electrical impulses fed to it by mad scientists and not by anything in the simulated world it perceives, since that world isn't real).
Anyways, there's my rant about the Many Worlds Interpretation. :)
This is a great take on it! I agree that MW isn’t a complete description because it doesn’t make us any wiser about what the wavefunction actually is
Super interesting topic! I think this is the interpretation people should learn. I also think we should just call it Quantum mechanics. MW is simply the best set of notions to use the famous name for.
Your explanation highlights a few of the most important details and does so very well. There is definitely a lot more to say about it. But great video, great explanation!
Absolutely beautiful explanation. Loved it.
It's my belief that the many world's interpretation, is wild arsed speculation like many other philosophies in QM. As Feynman said "Just do the math if it's useful"
Adam Becker's "What is real?" is a great read about the history of QM including MWI.
I almost do not understand how could anyone not take MWI as the default interpretation of QM at the level of our current understanding.
That’s been on my reading list for a while- thanks for the suggestion!
I've also been leaning more on Many Worlds and getting away from Copenhagen the last few years, but I know nothing about the math so this was wonderful. Nicely done.
I'm very glad you're back. (:
I think this is a good try but honestly I think quantum mechanics is so reliant on the maths "under the hood" to make sense of things, that videos like this only ever give the semblance that the topic has been understood rather than dispensing actual understanding.
The math helps with inconsistencies in our speculations and finding proper footholds for further exploration. If it has enabled the popping of the hood, we all are still incapable of making sense of what we're truly looking at. But we've only been staring at it for 100 years. Demands for answers this early in the game is unreasonable.
I watch a lot of physics videos. Most of them are very similar. Your explanations usually teach me something new. Always a pleasure.
Hi, I just watched your video and it made my many hours of watching Quantum Physics videos all that worth while. It was actually beautiful to have you explain "many worlds" in such an understandable way, it made sense and took away that "billions of other universes" stuff. I will watch it again just for the snazzy artwork. I then started to watch your next one and it was about you and endometriosis, and I nearly fell out of my chair. My eldest daughter has had it all her life and it's now being acknowledge as a major illness.... a bit late for my daughter! Please keep up your excellent videos, I for one will follow them. Many thanks. Rick Carroll (Australia)
Many thanks Rick! I really appreciate it. I hope your daughter is doing ok now?
Great video, had just a little quantum physics at University (computer science) and I always like the idea that observer is not collapsing the wave function but rather being entangled with the measured system, always observing only one branch. It actually removes "non-determinism" in some sense, as evolution of the system is fully described by Schrödinger equation, but this evolution happens in a way we can't observe, the same way a particle cannot observe other versions of itself.
Best explanation of many worlds that I have seen! Thanks.
I'm so happy you're back to making these lovely educational vids! You have a unique style and grace in getting things across. I'll be thinking about this for awhile and just may get that decoherence book. If you return to this topic, a bit more clarification on the part at 14' would good; how up = R + L (in superposition?) and D = their difference (and thus interference) is non-obvious and hiccups the flow a bit. If one of your older vids hits that point, or close, then perhaps add a link? If not, then maybe link to another CZcamsr's treatment? Please stay at this, you have star power!
Thank you so much for this kind comment, I really appreciate it! I also really appreciate the question and I’ll try and fix it up in future videos. It definitely wasn’t clear enough! I kind of talk about this in my “experimental facts about spin” video I believe (not sure now) but I think I can do a better job in a new video
glad to have you back!
here are my several cents on this topic:
1. what if the violation of schrodinger eq ("unitarity") is gradual? we slowly lose the ability to predict earlier state as our measurement apparatus gets bigger
2. I can see another case why many worlds taken literally can sound absurd: imagine in our lab 2 stern-gerlach experiments being run at the same time. now we have *four* universes split. considering just how many quantum processes happening in the lab, in the device, and in our bodies, it seems like infinite number of universes are being created every second
3. Is decoherence the same as saying quantum mechanics is in a bigger theory which *includes* measurement apparatus?
4. Looking at the thought experiment with the extra particle, is it right to say that every interaction between microscopic particles result in an entanglement? can we "hit" two particles with each other *without* having them entangle?
5. If measurement is equivalent to information being carried away by particles we ignore, does that mean that schrodinger's equation only *fully* work for the *entire universe* ?
6. What do you think is the difference between measurement and preparation (how to obtain |up>+|down> state in the first place?) preparation might give us a clue to what measurement really is
fascinating subject!
Hey, you're back with a new QM video! Yay!
Astonishingly detailed yet concise deep dive and very accessible. What an incredible presentation! One thing I think might not be beyond the scope of what a surprising number of prospective viewers would like to see would be if you could explain the missing factors and terms in your mathematical explanation of how the directions combine in a nominal way with a minimal expression of their relations to just the extent required to get the desired algebraic properties. I know that a very good presentation was done by a gentleman from Microsoft who illustrates basic quantum computing using changes in state in the complex plane, which is easier to grasp but still exhibits enough of the same properties to make many of the key intuitions demonstrable. Don't be afraid to bring rotations about the origin as part of multiplication into the mix. I've had good success explaining that to college and highschool students by starting with the intuition of rotations to understand what i is and then from there how the rotations actually work.
Thank you for this clear explanation!
Thank you so much for this!! Amazing video
Hello! Thanks for the video. You're one of the best science communicators out there. I always look forward to your uploads. Could you please say what are your favourite books on quantum decoherence?
..You video was so understandable ..that i didn't knew when i felt for so calm-full sleep and both understood what you were saying at the same time.
Wow! ...I like this; makes the subject much clearer to me. I also like your drawings, which are just as good to describe quantum physics as anything else.
Very intriguing perspective! Thank you! Closely considering... :)
Very happy to hear your voice again! Came due to 'looking-glass' and kept on for your presentation style. -- especially as, if I recall, you were advancing as a student whilst making these videos(?) But still nifty!
I'm currently reading Sean Carroll's "Something Deeply Hidden" and it's a great read but I got totally lost in the part where he talks about adding up the probabilities but it clicked for my watching your video, thank you! Also gained more insight on how it relates to interference, decoherence, and entanglement here too.
I was captivated throughout this beautiful video explaining a beautiful topic!
My main confusion is with the particle that measures and then goes missing: when you replace the particle with yourself a ‘collapse’ is apparent however we don’t go missing, in fact it feels impossible to lose track of oneself! Also, I find it strange that losing the measuring particle (essentially losing information) corresponds to a measurement (something that feels like gaining information).
The red particle itself may gone missing, but the blue particle also carries the information, that it got entangled.
Therefore, the information makes it into the detector and from there to the observer's brain.
Like a footprint in the sand... 🙂
I could well be wrong (this is not my field), but I feel that under the Copenhagen interpretation, a measurement would not necessarily appear to be gaining information. It gains you classical information, but at the cost of quantum information. And maybe the amount it loses is more than it gains.
What a great explanation of the many worlds interpretation!
wow that was extraordinary well explained. I'm really happy when I find a Video explaining fascinating stuff like that and I'm able to follow to the end. (coming from someone who is unfortunately not well educated in physics...)
Brilliant explanation, thank you.
This video is so amazingly substantial.
I was happy to see this video pop up on my subs. more of whatever you wish to do. thx :)
Awesome explanation. I have no education in physics whatsoever and am just interested in these subjects, and still I was able to understand this, which shows how well the video explains these subjects.
brought my mind back ... wordless grateful
Great video! No bad explanations! Interesting to think about the consequences of "Wigner's friend" type experiments where entire human experimenters undergo coherent quantum operations (such as unitary reversal of a "measurement" like you describe at the end of your video) for Many Worlds and Copenhagen interpretations.
Thank you so much for your videos. As a youth, and throughout my life, people have been telling me that I am smart. And that's how it looked to me. Test taking, problem solving and so on... it al came so easy to me.... then I tried to make sense of quantum mechanics...frustration,,, anger even,. that there was a realm of knowledge that I could not understand.... I took some comfort in the Feynman quote: "If you think you understand quantum mechanics, you don't understand quantum mechanics." ... but it was cold comfort. Finally with your videos I have made a breakthrough...do I understand? Maybe. But at least I have a handle on the subject that I have never had before.
Well done, I really enjoyed your way to explain it 👍
Well done. This approaches the Feymann "QED: The Strange Theory of Light and Matter" level of understandability. I appreciated the use of kets along with the Stern-Gerlach diagrams. I hope you will do a slightly more in-depth video on that notation. Was the |u> + |d> and |u> - |d> formulae derived from first principles and then compared to experiment? Or was the math invented to describe the experimental results?
Simple, deep, informal and unassuming explanation of an existentially scary consequence of quantum rules. Given what you say in the last couple of minutes I’d campaign to rename Many Worlds ‘The Humpty Dumpty Interpretation’ - less scary. I’m now subscribed in this universe and possibly in infinitely many others!
You're back!
I like what you've done here. subscribed