Quantum Mechanics (an embarrassment) - Sixty Symbols

there was this gathering I think in 2011 fairly recently of experts in quantum foundations what quantum mechanics really means and they took a poll they took a poll there like 16 different questions to ask the experts and one of them was simply what is your favorite interpretation of quantum mechanics or approach to thinking about what quantum mechanics really means and the answers was that there wasn't even any one approach that got 50 percent of the vote the results were all scattered across different possibilities the traditional Copenhagen interpretation where the quantum state collapses when you look at it a more modern many-worlds interpretation where the universe splits into different possibilities interpretations that add something new to quantum mechanics like random stochastic changes of the wavefunction or hidden variables that we can't see and I said it was extremely embarrassing to physics that here it is 80 years after we sort of established how quantum mechanics works we still don't know what it really means well it's absolutely great that we have unknown questions that's not the embarrassing part the embarrassing part is this particular question has been unanswered for 80 years with very little by the way of immediately demonstrable progress even though it's such an important question it seems to me that we have not been trying to answer this question with as much vigor as we should what is quantum mechanics really I mean that's what's saying what is the universe really I mean what more important question is there than that my favorite interpretation is the many-worlds interpretation of quantum mechanics this goes back to a guy named Hugh Everett who was a graduate student in the 1950s at Princeton and he said you know he had this brilliant insight that all the complicated ill-defined parts of quantum mechanics they're all having to do with when you measure something when you observe something and he said you know what what if we just erase those parts from quantum mechanics so nothing different happens there's no special role for observation or anything like that the trick is that you the observer or a quantum mechanical system just like anything else so what you need to do everett said was just a trace carefully what the world would look like if there were quantum systems interacting with other quantum systems and what he claimed happened is that every time these systems interact with each other the world splits you get one world that looks like one answer was achieved in another world in which another answer was achieved and these rules will never talk to each other ever again it bugs people they have all those universes going around but that's what the math seems to imply so is this like a car getting to a t-junction and instead of turning left or right it splits into and two cars go both ways it's basically like that there are just as many parts of the universe before and after it's not like you're creating more stuff in the universe it's kind of like and this is just an analogy it's not exactly like this but it's like there were exactly two copies of the car all along but they were precisely the same and then they diverged when this quantum event occurred so there's sort of all sorts of universes sitting on top of each other and they're splitting apart in differentiating as time goes on is there a finite number of these or this is a very simple kind of basic question to which we don't know the answer it could be a finite number or an infinite number it's probably an infinite number we I say that because if there were only a finite number of possibilities quantum mechanically then in an infinitely old universe all of those possibilities would happen over and over again an infinite number of times and this relates back to another important question in cosmology you know the arrow of time why is the past different from the future so I think we need an infinite number of possibilities in quantum mechanics to explain a universe that keeps evolving and that's what we seem to observe does it make sense to you you're telling me and you know it's baffling me but is it baffling you I think it does make make sense actually um it is an amazing consequence of some simple equations right but that's exactly what we should expect when we do physics that we should follow what the equations tell us we should take them seriously up until they are not compatible with the data right until they violate experiments so quantum mechanics in order to fit the data back in the 1920s people had to come for these dramatically different ideas you know you can't make predictions with 100% certainty and so forth and they boiled it down to a very simple set of equations and whatever it said in the 1950s is just take those equations seriously just believe them they already fit the data you just have to become comfortable with the fact of the world you see is a tiny tiny slice of the whole world that exists to me equations can have a number at the end of them or a letter at the end of them or a combination of letters and numbers what can be on the other side of the equal sign that says to you multiple universes what's the result that says that what is that equation look like well there there's the the equation is the Schrodinger equation this is the equation that tells you how the quantum state of the universe changes with time and the point is that ever it's suggested that we don't need two different equations to talk about the change of the evolution of the universe we only need one we only need the trigger equation it in the Copenhagen interpretation the the thing that we teach our students the thing that's in the textbooks there's two different ways that the quantum state evolves it obeys Schrodinger's equation when you're not looking at it and then when you look at it it suddenly discontinuously changes and that should really bug you that bugs me much more than a whole lot of universes these splitting events these moments when the universe splits what causes that what's the tipping point what counts as an observation that causes this split there's a very good question it is the major I would say progress we have made over the past 50 years in quantum mechanics since Everett what we understand now is that in order to so Everett said it's not enough to think about the quantum system you also have to think about the observer well we've said since then is you also have to think about the entire rest of the universe okay and we call this the environment and secretly you are interacting with your environment all the time light is reflecting off of you you're making noise and so forth and in quantum mechanics that means you are becoming entangled with the environment your quantum state is becoming correlated with the quantum state of everything else and that's a process that takes time and you can tell exactly how long it takes and it's that interacting with the environment that leads to this splitting of universes if there were no environment then universes would not split it's that if I have an electron here an electron there a quantum state where it could be there or there even before I look at it you could say there's a universe in which it's here and there's a universe in which it's there but those universes can interact with each other right I mean they can interfere with each other in a quantum experiment but once I look at it and I see the electron there or there and then all of my degrees of freedom all my information get entangled with the environment around me then these two possibilities can't interfere with each other anymore they've gone their separate ways decoherence has occurred in the technical jargon and that's something we think we understand on the basis of equations when you and I talked we talked about this in analogies like turning the steering wheel of a car and going left right but these aren't the events we're talking about are there these events are something much smaller and much more fundamental yeah the kinds of events that lead the universe to split or simply observations of quantum systems and that's a little bad way to put it because observation makes it sound like the existence of a conscious observer is somehow important to quantum mechanics and it's completely not that that's an utterly bogus road to go down what an observation simply is is bringing one system into contact with another so that what this system is doing becomes entangled becomes related to what this system is doing so if this system is spin up or spin down and this system is a little arrow then you bring them in contact and the arrow becomes correlated with what the spin is doing that's when the universe splits you've told me where you stand on this you favor this you know the multiple worlds thing that didn't win the survey did it no it did not I'm in the plucky minority on this one the winner although again nothing got a majority but the plurality 42% of the vote was the Copenhagen interpretation which is sort of the traditional textbook interpretation and it's good enough for government work the Copenhagen interpretation that's the one that says that when the cat is in the box and I open it up then suddenly the state of the cat changes to either being alive or dead depending on which one I observe and if I observe it alive then the part of the cat that was dead ceases to exist it is erased from the books of reality and in that approach all of these questions we've just been talking about become very very difficult to answer if I open the box but I didn't look in does the that branch of the wavefunction go away what if a virus observes the cat you know is the cat observing it's all these questions that sort of make our minds confused they're really difficult questions in the Copenhagen interpretation in the Everett interpretation every quantum system is on equal footing there's nothing special that causes wave functions to collapse forty-two percent forty-two percent believe it and I suspect you know now I'm going beyond saying true things and saying my personal opinion which I happen to is true I think it's just because people haven't thought about it very hard and that's not to say if you think about it very hard you're led to the many-worlds interpretation but you are letting the Copenhagen interpretation when you think about it very hard you know the more people worry about how quantum mechanics really works the less likely they are to accept the Copenhagen interpretation these are physicists being so many these are people who do this for their job how can they not think about it well it's the difference between being a racecar driver and a mechanic right in order to drive the car really well and really fast you don't necessarily need to know all of its inner workings it might be helpful you might make an argument that knowing the best knowing the car the best you can makes you a better driver but not necessarily right so working physicists don't always need to understand the interpretation of quantum mechanics they just need to know how to work with it right how to solve the equations that it gives you no one disagrees about what the predictions of quantum mechanics really are unless you have an interpretation that changes those predictions in some way so I think it's embarrassment personally because why are you doing physics if you're not interested in how the world really works also a lot of bars on that graph they're certainly yeah there's a handful let me just very quickly mention some of the possibilities one is something of an ingenious but sort almost certainly wrong idea called the gr W theory after the three people who invented it whose names I can't pronounce so gr W says that wave functions do collapse but not when you look at it there's just a certain chance per unit time that the wave function of any one particle will spontaneously collapse randomly and unpredictably and then if that particle is part of a bigger system it brings the whole system along with it and so basically it's a way that you can enforce big systems to always look classical even when small systems obey the rules of quantum mechanics it's an ingenious idea another famous one is the Bohm interpretation of quantum mechanics which actually goes back all the way to Louis de burrell who was one of the founders of quantum mechanics and that's a hidden variables theory in other words they say that the wavefunction is not all the information there is you can only make statements and predictions about probabilities because you don't have all the information there is to have there are sneaky variables that you don't know about people recoil at the idea of that but it is a theory that that makes sense it makes unambiguous predictions and finally I mean not maybe not finally but one other approach is a more epistemic approach which means an approach that treats the wavefunction not as something real but just as a bookkeeping device for talking about your information your knowledge about the system so it is a purely informational approach to quantum mechanics I'm not an expert in those areas I don't see why that would help me understand when wave functions collapse even if you say that it's not real but it's certainly one of the things that got votes in that surveying you think this is embarrassing what's it going to take to fix it are we looking for evidence or proof or a great communicator or evangelist who's going to make people sing a lot yeah I need to be clear the thing that is embarrassing is not that people disagree with my favorite interpretation but that we as a community don't agree what it will take well that's a very good question it's going to I think it depends on what the right answer is if the right answer is that we really need a better theory that some new dynamics like in the grw theory is actually involved then maybe what we need is an experiment you know these are theories that are truly different from conventional quantum mechanics and therefore make experimentally testable predictions and we're testing them very few people expect to see the quantum mechanics is wrong but if it is that would be a huge step forward obviously if it's not that then a lot of it is just you know convincing a bunch of skeptical hard-nosed physicists that the unpalatable consequences of your favorite interpretation of quantum mechanics need to be taken seriously whether it's hidden variables or many many universes out there that you will never see everyone agrees on which direction of time is yesterday in which direction is tomorrow for example we can remember things that happened yesterday none of us can remember things that happen tomorrow even though things will happen

Channel: Sixty Symbols

Views: 883,017

Rating: 4.8405056 out of 5

Keywords: sixtysymbols, sean carroll, quantum mechanics, many worlds, mwi, copenhagen

Id: ZacggH9wB7Y

Length: 14min 7sec (847 seconds)

Published: Tue Jan 22 2013

Reddit Comments

[u/Platypuskeeper]

I don't see it how the failure of physics to adjudicate metaphysical interpretations is an 'embarassment'. I think it's more embarrassing when physicists fail to realize they're actually advocating a philosophical position, rather than talking about physics.

Maybe Carroll hasn't 'thought about it hard enough' to realize it, but he's starting from a philosophical premise: That the goal of physics is to 'find out what's really going on'. In other words, he's clearly taking a realist stance, that physical theories work when/because they reflect an objective underlying reality, and the distinction between observable and unobservable quantities is not so strict.

As opposed to the anti-realist or instrumentalist stance - that what matters most is that your theory correctly describes what's actually observed. Unobservable quantities might represent underlying things, but this isn't really important as long as they're unobservable. There may be any number of additional 'layers to the onion' than you'll never know about. It's a model of reality, not reality.

Then he goes on to state his problems with the Copenhagen Interpretation - which (in its original form) is non-realist. The wave function itself is not a physical thing; it merely represents what you know, what you can know, about the system. Its 'collapse' is not a physical process, but merely a metaphysical representation of the eigenselection that's actually observed. The details of how this happens physically were omitted, intentionally.

So the problem of unitary vs non-unitary evolution of it doesn't really matter. Not unless/until someone has figured out the Measurement Problem in terms of observable things. But Copenhagen also isn't the only non-realist interpretation; Consistent-Histories is another, which does deal with decoherence more explicitly.

MWI and Bohm (among others) are realist interpretations. The wave function is a real thing, and it does not 'collapse' but only evolves unitarily, which is sensible - if you accept the premise that it's real. While in Bohm the wave function is ultimately a statistical ensemble of the 'real' underlying states of classical particles acting through a mysterious non-local potential.

So the statement: "The more you think about how quantum mechanics really works, the more you're lead away from the Copenhagen interpretation" is disingenuous and circular. He's posing a realist question and finding a non-realist answer unsatisfying. Copenhagen never purported to describe how QM 'really works'!

The reason Copenhagen got popular and remains popular isn't merely because Bohr was a famous guy, or because people haven't thought about it. It's that physicists tend towards Logical Positivism. That debating the experimentally-unobservable is metaphysics, not science. It doesn't matter how simple or elegant your interpretation is; if you can't falsify it experimentally, it's a metaphysical interpretation, not physics. Which is also why many if not most physicists simply don't care that deeply about interpretational questions either.

Personally I'm interpretation-agnostic, but Carroll's argument is circular and he doesn't even seem to realize it.

[u/eternauta3k]

Oh man oh man, when he asks

What could be on the other side of the equal sign that says to you, multiple universes?

This is great! How would you explain this to a layman without basically teaching him QM?

[u/xxx_yyy]

Sean Carroll says he likes the many worlds interpretation. I wish he had said something about the probability problem that this interpretation (like all other interpretations) faces. See this discussion.

[u/ididnoteatyourcat]

I'm very glad to see this sort of thing coming from Sean. We need big names like him to be more public about the sadness that is the Copenhagen Interpretation.

[u/aroberge]

Listening to Carroll brought back to me the very first problem I saw with the MWI. According to what Carroll said (at one point in the video), some people believe that all multiverses (and infinite number...) always exist and that they branch out as they evolve. I gather that this is to get around the objection that if you had new universes created at each branching point, at a fundamental level, you'd have a problem with conservation of energy....

However, the problem is there in a different way. In QM, we have interference (which, I gather, is handled in MWI as interference between these different universes). This means that they are somehow coupled. And, if they are coupled, how can they not be gravitationnally coupled? And, if so, you suddenly have magnified infinitely the problem of the non-vanishing cosmological constant...

I believe that what is needed is a more fundamental description of the world, perhaps something like noncommutative algebras as fundamental elements from which spacetime is derived will lead into something that will bypass the need for all those different interpretation of QM.

[u/JCrossno]

I cant say I quite buy into the many-worlds interpretation, but I completely agree that something smells fishy with the standard Copenhagen.