Does the Many Worlds Interpretation make sense?

physics is fascinating because well because of many reasons of course not least that it works but one reason is certainly that it gives a touch of plausibility to ideas that otherwise seem like pure fiction strange objects that you think shouldn't exist wormholes invisibility cloaks or Birds but the physics idea that has captured imagination the most are probably parallel universes the many words interpretation of quantum mechanics hasz it that anything which can happen does happen in Another Universe really in this much asked for video I want to explain how the many words interpretation works is it science does it solve any problems and are the parallel universe is real that's what we'll talk about today when you look around yourself you see a lot of stuff stuff that's somewhere in some particular place or if you you're watching this in a pop maybe stuff that's walking around on two legs but legs are not everything around you has very definite properties it's not in two places at the same time or smeared out across the room or if it is maybe you've had enough beer for tonight if you look at very small particles however things are very different take for example our good old friend the electron an electron is described by a wave function usually denoted with the Greek letter a s but the wave function doesn't tell you where the electron is or where it's going from the wave function you just calculate the probability of what you'll find when you measure where the electron is or where it's going for example the wave function tells you the probability of finding the electron in a particular place but before you measure it it isn't in any particular place it could be in several places or indeed be smeared out across the room according to standard quantum mechanics often called The Copenhagen interpretation nothing has definite properties until you measure it not just small particles but also large objects it's just that large objects like that pint in front of you are constantly being measured in some sense because light and air molecules ping off them the cophen interpretation does not explain just why some interactions result in measurements and others not this is the measurement problem of quantum mechanics what makes quantum mechanics so weird is this switch from a particle that's described by a wave function without definite properties to a particle that has definite properties which you know because you just measured it say you take a Quantum of light a photon and you send that through a beam splitter no we don't take our friend the electron friends don't let friends go through beam splitters because if you send a photon through a beam splitter then its wave function says that the photon goes both ways it's what's called a superposition of both possibilities let's say that each has a 50% probability but once you measure the particle on the right side you know it's not on the left side this means you have to update the wave function since the wave function only tells you probability the outcome of a measurement can't be predicted with certainty this is why quantum mechanics is not deterministic this update of the wave function is sometimes also called the collapse or reduction of the wave function and it's a key element of quantum mechanics if you don't update the wave function you'll get wrong probabilities if you want to know for example what's the probability of measuring the particle on the left given that it was measured on the right the answer should should be zero but this only comes out correctly if you update the wave function the update of the wave function is instantaneous it happens at the same time everywhere and is the reason why quantum mechanics is non-local this wave function update is what Einstein called a spooky action at distance I did an entire video previously about what this means but here's the brief summary quantum mechanics in its standard version is non-local as a matter of fact that's just a property of the theory the key question is whether this non-locality is also a property of reality whether that's the case or not depends on whether you think quantum mechanics is fundamentally correct or just a description of an underlying reality we still don't have an answer to that if quantum mechanics is fundamentally correct then the word is nonlocal period but if there's an underlying reality in which the outcome of a measurement was determined we just didn't know of it then this reality could well be local this is called a hidden variables model think back to the example with the beam splitter if there was an underlying reality the photon went either left or right you just didn't know what it did until you measured it in this case the wave function would just describe you in complete knowledge and while in this case the update of the knowledge is still non-local that's not a problem because the photon itself traveled entirely locally to the place where it's being measured for this to work however you need something else more variables that tell you where the photon really went since it's a rather persistent myth let me add that hidden variables models have not been excluded by experiment but it's somewhat off topic and I explained all this already in in a previous video for today the important point is just to understand the reason why quantum mechanics is nonlocal the reason that quantum mechanics is nonlocal is a combination of a the observational fact that a measurement outcome in one place tells you something about another measurement outcome in another place if you measure the particle here you now know you won't measure it there fact not interpretation and B the absence of other variables in the theory that could have carried the information locally this is why quantum mechanics is nonlocal and this is also why hidden variables can restore locality but this is not a video about hidden variables the many wordss interpretation now is based on the idea that you can throw out the update of the wave function by reinterpreting what happens in the measurement according to this interpretation all all outcomes of a measurement happen each in its own universe but we can only over seee the result in one universe so for us it looks like the wave function collapses instead of the measurement update in many worlds we have what's called a branching or splitting of Worlds this branching makes it impossible for one Observer to see more than one outcome of a measurement the major challenge for many world is to explain why the thing we call an observer does not itself Branch with those words and therefore sees all the outcomes but somehow randomly only experiences one of those words I've never found a good explanation for that but in all fairness from a purely instrumental point of view if you only ask for the outcome it doesn't really matter why observers see only one outcome you just assume that this is somehow the case which is as unsatisfactory as the measurement in standard quantum mechanics but then many words makes the same predictions as standard quantum mechanics it also leaves you with the mindblowing idea that each time a Quantum particle bounces off another one which happens gazillions of times a second our entire universe splits and anything that can happen does happen had salad for lunch today well in some of universe you had pizza with Elon Musk on masss whatever you can think of so long as it respects the laws of nature it's real in some parallel universe since I get this question frequently energies conserve just fine in the many worlds Universe it's not like you duplicate all energies each time the worlds Branch if you calculate the entire energy of all those worlds then you must give weight to each World by the probability with which it came into existence and that works out the same way as it does in normal quantum mechanics so there's no problem with energy conservation no the biggest problem with many worlds is that its supporters believe their interpretation is somehow better than the standard interpretation with the collapse when it's really just as mediocre many world supporters often claim that their interpretation is simpler because it just does away with the collapse postulate but as we saw earlier you need the collapse postulate to calculate probabilities you can't just throw it out that doesn't work and indeed this is not how the many words interpretation works it's how many world supporters say that it works but it's not true at this point things get a bit murky because there isn't just one many words interpretation there are two original ones going back to you ett and Bryce dot but meanwhile there are dozens of slightly different versions they differ in how they deal with the branching of the worlds but they all have to make new assumptions about how a measurement works and under which circumstances it happens I guess it it better be called The Many Many Words interpretation hey there's untapped potential here how about the many many many words interpretation it should not be surprising that many words interpretations need new assumptions let's just leave aside all the talk about interpretations and me we'd have to write an algorithm for a computer if we take standard quantum mechanics and remove the collapse postulate the resulting algorithm simply does not give predictions that agree with observations the collaps postulate is there for a reason I don't mean to say that the collaps postulate is the only way you can do it in cubism for example which is another interpretation of quantum mechanics the update of the wave function is interpreted as a basion update of knowledge and this is just as well in terms of predictions as the collapse postulate and likewise to make the many worlds interpretation work one needs to add other axioms you can interpret this as something to do with observers in branching words but if you look at it algorithmically in the end they just do exactly the same thing as the collapse postulate so it's not correct that the many words interpretation is simpler they take out one axom but have to replace it with others by the way this video comes with a quiz on quiz withth it.com even better if you subscribe to quiz with it you can collect points from all our videos and you get free access to the transcripts with links to all references so go and check it out those who believe in many words excuse me many worlds also often claim that their theory is local I I'm guessing they believe this because they've thrown out the collapse postulate which is the non-local element of the standard interpretation but as we saw earlier you can't just throw out the collapse postulate it needs to be replaced with something else that's of course also the case in the many world interpretation and as a result it's exactly as non-local as the standard interpretation to see why remember that the reason quantum mechanics is non-local is a that a measurement in one place does as a matter of fact tell us something about what happens in another place and B the theory has no variables that could transport this information locally these two reasons are still fulfilled in many words consequently it's exactly as non-local as quantum mechanics with the collapse postulate I find it surprising how many physicists are confused by this lots of papers have been written about how many worlds can be made local but of course the only way to make it local would be to introduce some kind of hidden variable that transports information locally this was exactly the point of the famous paper by Albert Einstein Boris Podolski and Nathan Rosen now just known as the epr paper they said if you want reality to be local you need an element of reality that underlies quantum mechanics that for quantum mechanics is incomplete the papers now almost 90 years old but physicists still don't get it do they more worrying still I've recently noticed a curious development which is what actually triggered this video it's that many words adherence have managed to convince themselves that they avoid Bell's theorem they are claiming that Bell's theorem has a hidden assumption which is that a measurement has only one outcome they call it the One World assumption B's theorem as a reminder says that all theories which are local and fulfill measurement Independence predict correlations that are in conflict with observations these observations are what the 2022 Nobel Prize and physics was awarded for and since the many world's interpretation fulfills measurement Independence it must be non-local but many world supporters don't want to admit that their theory is exactly as mediocre the standard interpretation they want to believe it's local so they need an explanation for how they avoid conflict with B theorem and to get this done they have invented this supposed extra assumption that a measurement has only one outcome well first of all this assumption isn't exactly hidden but more importantly B's theorem is about correlations that we observe and it's an observation of fact that we only ever observe one outcome of a measurement if you throw out this assumption from B's theorem you have a theorem that isn't about what we observe for theory that doesn't explain what we observe as Einstein Podolski and Rosen said if you want to have a local Theory you need something to transport the information locally the wave function doesn't do it so you need something else the many words interpretation doesn't introduce anything new to get the job done so of course it's still nonlocal that said I don't mean to say that the many words interpretation is wrong if you replace the collaps postulate with suitable other assumptions about branching wordss it gives the same results as the standard interpretation so you can as well believe in it since we cannot observe the other words not even in principle we cannot ever prove that they exist but for the same reason we also can't prove that they don't exist one final thing because I've encountered this misunderstanding before the many wordss interpretation has nothing to do with the path integral yes you could think of the paths in the path integral as being in different worlds but the point of the many words interpretation is that the different branches exist after the measurement in the path integral approach they only exist in the period leading up to the measurement so the path integral approach works is not a justification for many words they're two different things in summary the many words interpretation is neither wrong nor unscientific but it's exactly as problematic as standard quantum mechanics whether you believe that all those parallel universes exist is up to you we can neither confirm them nor rule them out if you want to know more about superpositions and wave functions and all that a free and easy way to do it is brilliant.org who have been sponsoring this video brilliant is an amazing tool for learning they have interactive courses on a large variety of topics in Science and Mathematics and they are adding new ones each month all their courses come with visualizations and will challenge you with questions so you can check your understanding right away for this video for example I suggest you check out my course on quantum mechanics it's an introduction to the topics we talked about today it explains Concepts such as interference superpositions and entanglement the uncertainty principle and B theorem and after this you can continue maybe with their course on Quantum objects or Quantum Computing or maybe you're more interested in special relativity or astrophysics brilliant has you covered on this too to support this Channel and and learn more about brilliant go to brilliant.org Saina and sign up you'll get to try out all courses for free for 30 days and the first 200 subscribers using this link will get 20% off the annual premium subscription thanks for watching see you next week

Channel: Sabine Hossenfelder

Views: 297,733

Keywords: science without the gobbledygoook, hossenfelder, physics, quantum physics, many worlds theory, many worlds interpretation, many worlds, many worlds interpretation of quantum mechanics, copenhagen interpretation, philosophy, science, parallel universe, parallel world, are parallel universes possible, is parallel world real, do parallel universes exist, multiverse, science humor, science comedy, one world assumption, one-world assumption

Id: 433tAfO4dbA

Length: 18min 25sec (1105 seconds)

Published: Sat Oct 21 2023