Sean Carroll: Many-Worlds Interpretation of Quantum Mechanics

so coming back to the textbook definition of quantum mechanics this idea that we I don't think we talked about can you this one of the most interesting philosophical points we talked at the human level but at the at the physics level that it that at least a textbook definition of quantum mechanics separates what is observed and what is real one how does that make you feel and and two what does it then mean to observe something and why is it different that what is real yeah you know I my personal feelings such as it is is that things like measurement and observers and stuff like that are not going to play a fundamental role in the ultimate laws of physics but my feeling that way is because so far that's where all the evidence has been pointing I could be wrong and there's certainly a sense in which it would be infinitely cool if somehow observation or mental cogitation did play a fundamental role in the in the nature of reality but I don't think so I can I don't see any evidence for it so I'm not spending a lot of time worrying about that possibility so what do you do about the fact that in the textbook interpretation of quantum mechanics this idea of measurement or looking at things seems to play an important role well you you come up with better interpretations of quantum mechanics and there are several alternatives my favorite is the many-worlds interpretation which says two things number one you the observer are just a quantum system like anything else there's nothing special about you don't get so proud of yourself you know you're just a bunch of atoms you have a wavefunction you obey the Schrodinger equation like everything else and number two when you think you're measuring something or observing something what's really happening is you're becoming entangled with that thing so when you think of there's a wavefunction for the electron it's all spread out but you look at it and you only see it in one location what's really happening is that there's still the wave functions the electron in all those locations but now it's entangled with the wave function of you in the following way there's part of the wavefunction says the electron was here and you think you saw it there the electron was there and you think you saw it there the electron was over there and you think you saw it there etc so and all of those different parts of the wavefunction once they come into being no longer talk to each other they no longer interact or influence each other it says if they are separate worlds so this was the invention of Hugh Everett the third who was a graduate student at Princeton in the 1950s and he said basically look you don't need all these extra rules about looking at things just listen to what the Schrodinger equation is telling you it's telling you that you have a wavefunction that you become entangled and that the different versions of you no longer talk to each other so just accept it it's just he did therapy more than anything else you know he said like it's okay you know you don't need all these extra rules all you need to do is believe the Schrodinger equation the cost is there's a whole bunch of extra worlds out there so how the worlds being created whether there's an observer or not the worlds are created anytime a quantum system that's in a superposition becomes entangled with the outside world what's the outside world it depends let's back up yeah whatever it really says what his theory is is there's a wave function of the universe and a base the Schrodinger equation all the time that's it that's the full theory right there okay the question all of the work is how in the world do you map that theory on to reality on to what we observe right so part of it is carving up the wavefunction into these separate world saying look look it describes a whole bunch of things that don't interact with each other let's call them separate worlds another part is distinguishing between systems and their environments and the environment is basically all the degrees of freedom all the things going on in the world that you don't keep track of so again in the bottle of water I might keep track of the total amount of water and the volume I don't keep track of the individual positions and velocities I don't keep track of all the photons or the air molecules in this room so that's the outside world the outside world is all the parts of the universe that you're not keeping track of when you're asking about the behavior of sub subsystem of it so how many worlds are there you don't know that one either there could be an infinite number there could be only a finite number but it's a big number one way or the other it's just a very very big number in one of their talk somebody asked well if it's a if it's finite so actually I'm not sure exactly the logic you used to derive this but is there you know going to be a you know overlap a duplicate world that you returned to so you've mentioned and I'd love if you can elaborate on sort of idea that it's possible that there's some kind of equilibrium that these splitting worlds arrive at and then maybe overtime maybe somehow connected to entropy you get a large number of worlds they're very similar to each other yeah so this question of whether or not Hilbert space is finite or infinite dimensional is actually secretly connected to gravity and cosmology this is at the part that we're still struggling to understand right now but we discovered back in 1998 that our universe is accelerating and what that means if it continues which we think it probably will but we're not sure but if it does that means there's a horizon around us there there's because the universe not only expanding but expanding faster and faster things can get so far away from us that from our perspective it looks like they're moving away fast from the speed of light you'll never see them again so there's literally a horizon around us and that horizon approaches some fixed distance away from us and you can then argue that within that horizon there's only a finite number of things that can possibly happen the finite dimensional hilbert space in fact we even have a guess for what the dimensionality is it's 10 to the power of 10 to the power of 122 that's a very large number yes just to compare the age of the universe is something like 10 to the 14 seconds time of the 17 or 18 seconds maybe the number of particles in the universe is 10 to the 88th but the number of dimensions of Hilbert space is 10 to the 10 to the 120 - so that's just crazy if that story is right that in our observable horizon there's only a finite dimensional hilbert space then this idea of branching of the wavefunction the universe into multiple distinct separate branches has to reach a limit at some time once you branch that many times you've run out of room in Hilbert space and roughly speaking that corresponds to the universe just expanding and emptying out and cooling off and and entering a phase where it's just empty space literally forever what's the difference which means splitting and copying do you think like in terms of a lot of this is an interpretation that's that helps us sort of model the world so perhaps shouldn't be thought of as like you know philosophically or metaphysically but in even at the physics level do you see a difference between two generating new copies of the world or splitting I think it's better to think of in quantum mechanics in many worlds the universe splits rather than new copies because people otherwise worry about things like energy conservation and no one who understands quantum mechanics worries about energy conservation because the equation is perfectly clear but if all you know is that someone told you the universe duplicates then you have a reasonable worry about where all the energy for that came from so a pre-existing universe splitting into two skinnier universes is a better way of thinking about it and mathematically it's just like you know if you draw an x and y axis and you draw a vector of length one at 45 degree angle you you know that you can write that vector of length 1 as the sum of two vectors pointing along x and y of length 1 over the square root of 2 okay so I write one arrow as the sum of two arrows but there's a conservation of air Onis right like if now two arrows the length is the same I just been describing it in a different way and that's exactly what happens when the universe branches the the wave function of the universe is a big old vector so to somebody who brings up a question of saying doesn't this violate the conservation of energy can you give further elaboration right so let's just be SuperDuper perfectly clear yeah there's zero question about whether or not many-worlds violates conservation energy yes it does not great and I say this definitively because there are other questions that I think there's answers to but they're legitimate questions right about you know where does probability come from and things like that this conservation of energy question we know the answer to it and the answer to it is that energy is conserved all of the effort goes into how best to translate what the equation unambiguously says into thing plain English right so this idea that there's a universe that has that that the universe comes equipped with a thickness and it sort of divides up into thinner pieces but the total amount of universe is is conserved over time is a reasonably good way of putting English words to the underlying mathematics so one of my favorite things about many worlds is I mean I love that there's something controversial in science and for some reason it makes people actually not like upset but just get excited why do you think it is a controversial idea so there's there's a lot of it's actually one of the cleanest ways to think about quantum mechanics yeah so why do you think there's a discomfort a little bit among certain people well I draw the distinction of my book between two different kinds of simplicity in a physical theory there's simplicity in the theory itself right how we describe what's going on according to the theory by its own rights but then you know a theory is just some sort of abstract mathematical formalism you have to map it onto the world somehow right and sometimes like for Newtonian physics it's pretty obvious like okay here is a bottle and it has a center of mass and things like that sometimes it's a little bit harder with general relativity curvature of space-time is a little bit harder to grasp quantum mechanics is very hard to map what you're the language you're talking into wave functions and things like that onto reality and many worlds is the version of quantum mechanics where it is hardest to map on the underlying formalism to reality so that's where the lack of simplicity comes in not in the theory but in how we use the theory to map onto reality and in fact all of the work in sort of elaborating many-worlds quantum mechanics is in the this effort to map it on to the world that we see so it's perfectly legitimate to be bugged by that right to say like well no that's just too far away from my experience I I am therefore intrinsically skeptical of it of course you should give up on a skepticism if there are no alternatives and this theory always keeps working then eventually you should overcome your skepticism but right now there are alternatives that are that you know people work to make alternatives that are by their nature closer to what we observe directly can you describe the alternatives identically touched on it so that the Copenhagen interpretation and the many-worlds maybe there's a difference between the effort of already in many worlds and many worlds as it is now like has the idea sort of developed and so on and just in general what is the space of promising contenders we have democratic debates now there's a bunch of candidate well candidates on stage what are the quantum-mechanical candidates on stage for the debate so if you had a debate between quantum-mechanical contenders there'd be no problem getting 12 people up there on stage but there would still be only three frontrunners and right now the frontrunners would be Evert hidden variable theories are another one so the hidden variable theories say that the wavefunction is real but there's something in addition to the wavefunction the wavefunction is not everything is part of reality but it's not everything what else is there we're not for but in the simplest version of the theory there are literally particles so many world says that quantum systems are sometimes are wave-like in some ways and particle-like in another because they really really are waves but under certain observational circumstances they look like particles whereas hidden variable says there they look like waves and particles because there are both waves and particles involved in the dynamics and that's easy to do if your particles are just nonrelativistic Newtonian particles moving they get pushed around by the wave function roughly it becomes much harder when you take quantum field theory or quantum gravity into account the other big contender are spontaneous collapse theories so in the conventional textbook interpretation we say when you look at a quantum system its wavefunction collapses and you see it in one location a spontaneous collapse theory says that every particle has a chance per second of having its wavefunction spontaneously collapse the chance is very small for a typical particle it will take hundreds of millions of years before it happens even once but in a table or some macroscopic object there are way more than a hundred million particles and they're all entangled with each other so when one of them clacks it collapses it brings everything else along with it there's a slight variation of this that's a spontaneous collapse theory there are also induced collapse theories like Roger Penrose thinks that when the gravitational difference between two parts of the wave function becomes too large the wavefunction collapses automatically so those are basically in my mind the three big alternatives many worlds which is just there's a wavefunction and always a basis reading your equation hidden variables there's a wave function that always the base of Schrodinger equation but there are also new variables or collapse theories which the wave function sometimes obeys the Schrodinger equation and sometimes it collapses so you can see that the alternatives are more complicated in their formalism than many worlds is but they are closer to our experience so just this moment of collapse do you think of it as so is a wave function fundamentally sort of a probabilistic description of the world and its collapse sort of reducing that part of the world into something deterministic or again you can now describe the position and the velocity in this simple classical model well there is a hard thing about collapse there is a fourth category is a 4th contender there's a mayor Pete of quantum mechanical interpretations which are called epistemic interpretations and what they say is all the wavefunction is is a wave making predictions for experimental outcomes it's not mapping onto an element of reality in any realsense and in fact two different people might have two different wave functions for the same physical system because they know different things about it right the wave function is really just a prediction mechanism and then the problem with those epistemic interpretations is if you say okay but it's predicting about what like what is the thing that is being predicted and I say no no that's not what we're here for we're just here to tell you what the observational outcomes you're gonna be but the other the other interpretation is kind of think that the wave function is real yes that's right so that's an antic interpretation of the wave function ontology being the study of what is real what exists as opposed to an epistemic interpretation of the wave function epistemology being the study what we know now actually just love to see that debate on stage there was a version of it on stage at the world science festival a few years ago that you can look up online I need you yep it's on you do okay awesome I'll link it watch it anyone i won there was no vote those there's Brian Greene was the moderator and David Albert stood up for spontaneous collapse and Shelley Goldstein was there for hidden variables and Ruettiger shock was there for epistemic approaches why do you I think you mentioned it but just why do you find many worlds so compelling well there's two reasons actually one is like I said it is the simplest right it's like the most bare-bones austere pure version of quantum mechanics and I am someone who is very willing to put a lot of work into mapping the formalism onto reality I'm less willing to complicate the formalism itself but the other big reason is that there's something called modern physics with quantum fields and quantum gravity and holography and space-time doing things like that and when you take any of the other versions of quantum theory they bring along classical baggage all of the other versions of quantum mechanics prejudice or privilege some version of classical reality like locations in space okay and I think that that's a barrier to doing better at understanding the theory of everything and understand quantum gravity and the emergence of space-time whenever if you change your theory from you know here's a harmonic oscillator oh there's a spin here's an electromagnetic field in hidden variable theories or dynamical collapse theories you have to start from scratch you have to say like well what are the hidden variables for this theory or how does he collapse or whatever whereas many worlds is plug-and-play you tell me the theory and I can give you as many worlds version so when we have a situation like we have with gravity and space-time where the classical description seems to break down in a dramatic way then I think you can start from the most quantum theory that you have which is really many worlds you

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Keywords: physics, mindscape, something deeply hidden, quantum mechanics, caltech, theoretical physics, jre, many-worlds, black holes, arrow of time, sean carroll, artificial intelligence, ai, ai podcast, artificial intelligence podcast, lex clips, lex fridman, lex podcast, lex mit, lex ai, mit ai, ai podcast clips, ai clips

Id: kxvQ3Wyw2M4

Length: 18min 45sec (1125 seconds)

Published: Tue Nov 05 2019