The Trouble with Many Worlds

In today's video I want to tell you why I am not a fan of the many worlds interpretation of quantum mechanics. It's not the many worlds. I don't a priori have a problem with those. It's that I think you do not gain anything from this reinterpretation.
First, I briefly explain the basics of quantum mechanics: everything (yes, really everything) is described by a wave-function, and this wave-function changes according to the Schrödinger equation. Then, we have in the Copenhagen interpretation (and Copenhagen-like interpretations) an additional assumption: the measurement postulate.
In my video, I explain what the problem is with this measurement postulate. I then tell you why the many worlds interpretation - contrary to what its supporters claim - does not solve the measurement problem, but just sweeps it under the rug.
Support me on Patreon: / sabine

[@Jehannum2000]

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

[@bat2293]

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

[@EffySalcedo]

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

[@docholiday8029]

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

[@docholiday8029]

@@bat2293 I agree with you

[@mintakan003]

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

[@zicada7661]

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

[@kjustkses]

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

[@dimitri1462]

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

[@JuBerryLive]

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

[@sergeynovikov9424]

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

[@kjustkses]

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

[@aleksandarignjatovic3130]

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

[@SabineHossenfelder]

Thanks for the feedback, much appreciated!

[@pokeman123451]

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

[@TheCabIe]

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

[@GonzoTehGreat]

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

[@ssm445]

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

[@nevis4567]

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

[@pmcgktr]

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

[@scotthammond3230]

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

[@SabineHossenfelder]

Exactly!

[@RolandOrre]

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

[@TheSimonScowl]

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

[@paulwaddy6061]

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

[@aneikei]

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

[@rabeeet]

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

[@quasarsupernova9643]

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

[@rabeeet]

Ron Maimon why so negative bro

[@rabeeet]

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

[@rabeeet]

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

[@quasarsupernova9643]

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

[@RogerLuedecke]

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

[@q09876543]

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

[@MountThor]

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

[@seper159]

I find them to be incorrect xD

[@dillonkeller4477]

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

[@seper159]

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

[@peterhall6656]

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

[@imjustpassinthru]

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

[@tripleplonk]

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

[@ggg148g]

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

[@andyiswonderful]

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

[@Thomas-gk42]

Just finished 'Existential Physics', just the same

[@vincewatkins8439]

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

[@edreusser4741]

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

[@SnaFubar_24]

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

[@dariopalomba8420]

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

[@rjbeery3]

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

[@zenitovec2]

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

[@EffySalcedo]

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

[@josephjohnson3738]

Ass kisser.

[@platinumedge7886]

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

[@josephjohnson3738]

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

[@platinumedge7886]

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

[@quantumwater6582]

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

[@vegn_brit5176]

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

[@teacherslatinamerica8672]

Thank you!! This explanation has been sorely lacking for a long time now

[@KeboEdan1]

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

[@bazoo513]

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

[@manfredadams3252]

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

[@bazoo513]

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

[@sumdumbmick]

are you deaf? it's nonsense.

[@bazoo513]

@@sumdumbmick What is nonsense?

[@sumdumbmick]

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

[@Endymion4242]

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

[@omp199]

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

[@josiah42]

I knew Many World was messed up, but I didn't realize it was quite as unproductive as you described. Great job!

[@jean-pierredevent970]

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

[@aaroncurtis8545]

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

[@jean-pierredevent970]

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

[@aosteklov]

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

[@ThePinkus]

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

[@aosteklov]

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

[@WALLACE9009]

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

[@epajarjestys9981]

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

[@denysvlasenko1865]

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

[@shelley-anneharrisberg7409]

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

[@seper159]

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

[@jacobbailey2331]

Thank you for explaining this so clearly and concisely.

[@Alekosssvr]

Sabine this is one of your best videos. Excellent!

[@Thomas-gk42]

My thought with every lesson I see made by Sabine

[@Tehom1]

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

[@ivanfromunion3513]

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

[@Google_Censored_Commenter]

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

[@quantumdave1592]

Are you high?

[@berniv7375]

Thanks for the video. Difficult keeping track of your explanation but mesmerising.

[@redriver6541]

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

[@attica7980]

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

[@terrykosowick594]

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

[@philg8556]

Looking forward to a future pilot wave video. ;)

[@ericfleet9602]

I just discovered this channel and am very impressed. I have to admit that some of this goes over my head and I am not qualified to determine who is right and who is wrong, but I am getting my eyes opened to the fact that many theories that are considered the accepted model of how things works are not totally settled science. I appreciate that very much. Just subscribed....

[@dopaminecloud]

I believe right now it is too early for anyone to claim who is wrong, we can only seem to use logic to guess which interpretation may actually help to arrive at some clarity.

[@cmck1777]

Your videos are the best. Thank you for making them!

[@garethbarry3825]

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

[@SabineHossenfelder]

Thanks for the kind words.

[@atimholt]

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

[@mathewomolo]

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

[@joachimelenador6259]

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

[@LucaAmadeus]

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

[@tac6044]

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

[@timoluetk]

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

[@LucaAmadeus]

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

[@timoluetk]

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

[@LucaAmadeus]

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

[@MarkArandjus]

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

[@DiegoItzep2012]

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

[@abdoolmoron6430]

Thanks for the explanation. Sharp and clear.

[@tim57243]

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

[@Rykvp]

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

[@tim57243]

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

[@alvinlindsay2381]

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

[@psychohist]

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

[@dsnow6578]

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

[@timhourigan6257]

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

[@jarahfluxman20]

*Professorin😊

[@Thomas-gk42]

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

[@chriscontact5857]

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

[@AlexanderFarley]

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

[@DarwinsStepChildren]

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

[@AlexanderFarley]

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

[@angeldude101]

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

[@elmundodeFreeman]

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

[@TheHsubh]

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

[@Thomas-gk42]

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

[@discogodfather22]

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

[@huntingape]

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

[@simonjgriffiths]

Me too !

[@aleksandarignjatovic3130]

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

[@devilisahomo]

I love these fake accounts promoting this propaganda. Hilarious

[@g.v.3493]

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

[@claironaut]

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

[@schmetterling4477]

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

[@schmetterling4477]

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

[@schmetterling4477]

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

[@lespukh]

this one is so cool, thanks for the explanation!

[@daybertimagni4841]

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

[@lsb2623]

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

[@ScottJPowers]

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

[@jermsbestfriend9296]

So, I thought you never described the measurement problem very well, but I found your debunking of quantum regression and it's one of the best descriptions I've ever seen. Thank you so much.

[@schmetterling4477]

There is no measurement problem. There are only people who don't understand physics. ;-)

[@jermsbestfriend9296]

@@schmetterling4477 whatever, dude.

[@schmetterling4477]

@@jermsbestfriend9296 See, a curious person would now ask "Why is that?". It doesn't seem like you are a curious person, after all. ;-)

[@johnmanderson2060]

Great video ! Very good points and very good arguments, thanks for that ✅👍🏻👍🏻👍🏻

[@krm398]

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

[@PavlosPapageorgiou]

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

[@sergeynovikov9424]

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

[@ollywright]

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

[@faisaldebouni]

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

[@innocentsmith6091]

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

[@olegkorobkin6235]

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

[@matrixate]

Brilliantly presented. Clear and concise.

[@rv706]

I agree with Sabine that phrasing the probabilities appearing in the MWI in terms of mathematical Decision Theory is equivalent to reintroducing the mysterious anthropocentrism of observers/agents/etc that plagued the Copenhagen interpretation. *BUT* there are two things: 1) there's probably a way of expressing the same probabilistic content via symmetry considerations, that get rid of "agents" etc. 2) The need of justifying the use of a given probability distribution rests upon a more fundamental problem: what even _is_ (physical) probability? This is a deep problem in the foundations/philosophy of physics, and it's a problem that has nothing to do specifically with the MWI but is shared by classical physics and by other interpretations of QM. So, "how do you justify the Born rule?" is not really a well-defined criticism of the MWI per se.

[@schmetterling4477]

There are no observers in Copenhagen. There are only people who didn't pay attention in high school when quanta were explained to them. ;-)

[@darektidwell1158]

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

[@danielforrest3871]

I would like to hear your thoughts on the many pizzas interpretation. From the Chicago, New York and Napolitan perspective.

[@jrbleau]

@Zeke Bean I concur. Been there many years ago and I still remember the pizza!

[@LuisAldamiz]

You can't have pizza if you adhere to the paleodiet interpretation.

[@Bgrosz1]

Are you sure? Get ready to accept that there is only one pizza.

[@Antares070]

Excellent explanation of that quantum mechanics interpretation. I like Everett's many worlds interpretation but you point out pretty well its holes..

[@ramonatila6277]

I'm really happy I discovered this channel

[@kjustkses]

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

[@planetnicky11]

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

[@Luthiart]

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

[@rockets4kids]

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

[@myothersoul1953]

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

[@ollywright]

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

[@locutusdborg126]

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

[@daves2520]

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

[@jjay6764]

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

[@hdufort]

I really like these video series. Will you do a video on Pilot Waves? Thanks in advance.

[@urfinjuice1437]

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

[@davidwuhrer6704]

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

[@clemonsx90]

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

[@davidwuhrer6704]

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

[@davidwuhrer6704]

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

[@davidwuhrer6704]

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

[@howaboutataste]

The wave function collops? Sounds tasty.

[@tonylikesphysics2534]

Please do a video on the quantum eraser experiment, Dr. Hossenfelder. Your explanations are second to none.

[@jehadrizk1934]

U make science complexity easy to understand. Thankyou

[@clarencegood7490]

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

[@AbeDillon]

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

[@billreeves5949]

Love your videos Sabine thank you.

[@gmotionedc5412]

Wow I think I just about understood that. Great job!!

[@rickeisenberg4091]

Sabine, how do the Many Worlds believers explain how these other parallel universes are created? Wouldn't such a process require the creation, from nothing, of an enormous amount of matter and energy with each branching?

[@ian_b]

A wizard does it :)

[@SabineHossenfelder]

No, because the probability for each branch is small, so if you add all energies up, weighted by the appropriate probabilities for each branch, energy is conserved as usual.

[@epajarjestys9981]

@@SabineHossenfelder But the branches all supposedly _exist_ (with probability one from the omniscient point of view of Quantum God), so I don't see how this would counter the above argument. Is it not rather the case that energy conservation is simply supposed to hold in each individual branch, separately, according to many worlds the0ry?

[@dsa3df3]

@@epajarjestys9981 If you cut a carrot in half, where did the two halves come from? You only had one carrot. Obviously not a perfect analogy because you can "cut" the universe infinitely many times (I guess) unlike a carrot.

[@greghansen38]

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

[@lepidoptera9337]

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

[@greghansen38]

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

[@lepidoptera9337]

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

[@greghansen38]

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

[@lepidoptera9337]

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

[@SmithnWesson]

Beautiful Sabine. I love your videos.

[@alexander_nunezf]

great vid, it would be awesome to have one regarding bohmian quantum interpretation.

[@discogodfather22]

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

[@yosefgomgom225]

Imagine how small the probability it happened

[@DukePaprikar]

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

[@thetruthchannel349]

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

[@tobiashallingstad1107]

Unless you mean "observe" by "calculate the probability for", I don't understand what is meant by this: "The many worlds people explain this as follows: Of course you are not suppose to calculate the probability for each branch of the detector." Because according to many-worlds the question of probability is a question of which branch. The Schrödinger equation calculates a probability distribution, which according to many-worlds, is objective reality, not just probabilities. This view conserves the information of the wave function. It does not claim that the worlds suddenly pop into existence as seems implied in this video. I do agree that many-worlds is not a "complete theory", but I don't believe that is a fair standard for an interpretation of quantum mechanics.

[@j-trainchannel4626]

I agree. This video completely misses the point of MWI.

[@thetruthchannel349]

I agree its not a complete theory. Its not even half a theory. Its just pure stupidity and the problem with academia. I get a PHD and then I start imagining things nobody ever heard of and get my thing published first and then other people take it seriously and start writing papers and getting published because its like a new hit on the Billboard top 40 & so everyone trying to emulate it and before you know it you have a WHOLE NEW GENRE of JUNK SCIENCE that ONLY exists because some guy got some grant money and some publisher wanted to make money & it was good for the University so they allowed it and round and round and round and round we go. This is NOT what science is supposed to be.

[@Jupiter__001_]

@@thetruthchannel349 Yup. Academia is toxic.

[@valmarsiglia]

@@thetruthchannel349 Lol. "The Truth Channel" hath spoken (run by a guy who'd probably have trouble passing a 7th-grade science test).

[@valmarsiglia]

@@Jupiter__001_ "Yup. Academia is toxic." ...he bitches to the world using technology only made possible by Academia.

[@hervebozec7756]

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

[@Thomas-gk42]

You're so right!

[@dhoffheimerj]

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

[@pitdog75]

A discussion with prof Sean Carroll would be interesting.

[@Bgrosz1]

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

[@lenn939]

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

[@yvesnyfelerph.d.8297]

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

[@thetruthchannel349]

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

[@thetruthchannel349]

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

[@cmilkau]

Started out thorough, then comes up with the same shortcut as everyone else, which prevents understanding. You don't split the whole universe, you entangle the detector with the experiment. Thing is, you cannot derive any predictions without reintroducing the "measurement", at least on universe scale. That's the "universe split". And that's already beyond the model parameters, (just like a black hole singularity is for GR): because you can't really entangle the entire universe, causal horizons are everywhere. Maybe if we understand what happens at those horizons (and there's quite a lot of research activity), we also find a way to resolve the "measurement-vs-wave" mystery.

[@BANKO007]

This is possibly the most brilliant and somewhat thrilling thought experiment applied to the interpretation of the wave equation I have heard since I learned about Einstein's musing on light in university several decades ago. This reasoning is a measurement of the Many Worlds wave function, which appears to have collapsed.

[@josephdestaubin7426]

Just wanted to say thank you for your work.

[@Levon9404]

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

[@fallenlegacyz]

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

[@dougyates7218]

I enjoy and try to understand everything that you cover in your videos. I hope that you will someday cover the topics of Free Energy and anti-gravity, especially, electrogravatics(sic).

[@danielbuchta1063]

This is very clear and nice explanation:) I have a question: "What is, in fact, the measurement?" What can act as an observer, under what ciscumstances?" Thank you for your videos:)

[@seper159]

your question is the exact reason this explanation isn't very clear and nice XD the interpretations of measurement are the FUNDEMENTAL difference between all the different quantum mechanics interpratations (many wolrds being the only one without any forced postulates)