Sean Carroll: Quantum Mechanics and the Many-Worlds Interpretation | Lex Fridman Podcast #47
the following is a conversation with Sean Carroll part 2 the second time we've spoken on the podcast you can get the link to the first time in the description this time we focus on quantum mechanics and the many-worlds interpretation that he details elegantly in his new book titled something deeply hidden I own and enjoy both the ebook and audiobook versions of it listening to Sean read about entanglement complementarity and the emergence of space-time reminds me of bob ross teaching the world how to paint and his own television show if you don't know who Bob Ross is you're truly missing out look him up he'll make you fall in love with painting Sean Carroll is the Bob Ross of theoretical physics he's the author of several popular books a host of a great podcast called mindscape and is a theoretical physicist at Caltech and the Santa Fe Institute specializing in quantum mechanics arrow of time cosmology and gravitation this is the artificial intelligence podcast if you enjoy it subscribe on YouTube give it five stars of iTunes supported on patreon or simply connect with me on Twitter Alex Friedman spelled Fri D ma N and now here's my conversation with Sean Carroll Isaac Newton developed what we now call classical mechanics that you describe very nicely in your new book because you do with a lot of basic concepts in physics so was classical mechanics I can throw a rock and can predict the trajectory of that rocks flight but if we could put ourselves back into Newton's time his theories work to predict things but as I understand he himself thought that they were their interpretations of those predictions were absurd perhaps he just said it for religious reasons and so on but in particular sort of a world of interaction without contact so action at a distance it didn't make sense to them in a sort of a human interpretation level does it make sense to you that things can affect other things at a distance it does but you know that so that was one of Newton's worries you're actually right in a slightly different way about the religious worries he he was smart enough this is off the topic with still fascinating Newton almost invented chaos theory as soon as he invented classical mechanics he realized that in the solar system so he was able to explain how planets move around the Sun but typically you would describe the orbit of the earth ignoring the effects of Jupiter and Saturn and so forth just doing the earth and the Sun he he kind of knew even though he couldn't do the math that if you included the effects of Jupiter and Saturn the other planets the solar system would be unstable like the orbits of the planets would get out of whack so he thought that God would intervene occasionally to sort of move the planets back into orbit which is how you could only way you could explain how they were there presumably forever but the worries about classical mechanics were a little bit different they worried about gravity in particularly wasn't it worried about classical mechanics worried about gravity how in the world does the earth know that there's something called the Sun 93 million miles away that is exerting gravitational force on it and he said he literally said you know I leave that for future generations to think about because I don't know what the answer is and in fact the people under emphasize this but future generations figured it out Pierre Simone Laplace in circa 1800 showed that you could rewrite Newtonian gravity as a field theory so instead of just talking about the force due to gravity you can talk about the gravitational field or the gravitational potential field and then there's no action at a distance it's exactly the same theory empirically it makes exactly the same predictions but what's happening is instead of the Sun just reaching out across the void there is a gravitational field in between the Sun and the earth that obeys an equation Laplace's equation cleverly enough and that tells us exactly what the field does so even in Newtonian gravity you don't need action at a distance now what many people say is that Einstein solved this problem because he invented general relativity and general relativity there's certainly a field in between the Earth and the Sun but also there's the speed of light as a limit in Laplace's theory which was exactly Newton's theory just in a different mathematical language there could still be instantaneous action across the universe whereas in general relativity if you shake something here its gravitational impulse radiates out at the speed of light and we call that a gravitational wave and we can detect those so but I I really it rubs me the wrong way to think that we should presume the answer should look one way or the other like if it turned out that there was action at a distance in physics and that was the best way to describe things that I would do it that way it's actually a very deep question because when we don't know what the right laws of physics are when we're guessing at them when we're hypothesizing at what they might be we are often guided by our intuitions about what they should be I mean Einstein famously was very guided by his intuitions and he did not like the idea of action at a distance we don't know whether he was right or not it depends on your interpretation of quantum mechanics and it depends on even how you talk about quantum mechanics within any one interpretation if you see every forces of field or any other interpretation of action at a distance he's just stepping back to sort of caveman thinking like do you really can you really sort of understand what it means for a force to be a field as everywhere so if you look at gravity like what do you think about I think so it's just something that you've been can and by society to think that to map the fact that science is extremely well predictive of something to believing that you actually understand it like you can intuitively under the how as the degree that human beings can understand anything that you actually understand it are you just trusting the beauty and the power of the predictive power science that depends on what you mean by this idea of truly understandings right right you know I mean can I Lily understand four months Last Theorem you know it's easy to state it but do I really appreciate what it means for incredibly large numbers right yeah I think yes I think I do understand it but like if you want to just push people on well you put your intuition doesn't go to the places where Andrew Wiles needed to go to prove Fermat's Last Theorem and I can say fine by something I understand the theorem and likewise I think that I do have a pretty good intuitive understanding of fields pervading space time whether it's the gravitational field or the electromagnetic field or whatever the Higgs field of course one's intuition gets worse and worse as you get trickier in the quantum field theory and all sorts of new phenomena that come up in quantum field theory so our intuitions aren't perfect but I think it's also okay to say that our intuitions get trained right like you know I have different intuitions now that I had when I was a baby that's okay that's not an intuition is not necessarily intrinsic to who we are we can we can train it a little bit so that's where I'm gonna bring in norm Chomsky for a second who thinks that our cognitive abilities are sort of evolved through time and so they're they're biologically constrained and so there's a clear limit as he puts it to our cognitive abilities and it's a very harsh limit but you actually kind of said something interesting and nature versus nurture thing here is we can train our intuitions to sort of build up the cognitive muscles to be able to understand some of these tricky casas so do you think there's limits to our understanding that's deeply rooted hard-coded into our biology that we can't overcome there could be limits to things like our ability to visualize okay but when someone like Ed Witten proves a theorem about you know hundred dimensional mathematical spaces he's not visualizing it he's doing the math that doesn't stop him from understanding the result I think and I would love to understand this better but my rough feeling which is not very educated is that you know there's some threshold that one crosses in abstraction when one becomes kind of like a Turing machine right one has the ability to contain in one's brain logical formal symbolic structures and manipulate them and that's a leap that we can make as human beings that that dogs and cats haven't made and once you get there I'm not sure there are any limits to our ability to understand the scientific world at all maybe there are there's certainly a ability limits on our ability to calculate things right you know people are not very good at taking cube roots of million digit numbers in their head but that's not an element of understanding it's certainly not a little bit in principle so of course there's a human you would say that doesn't feel to be limits to our understanding but sort of hey have you thought that the universe is actually a lot simpler than it appears to us and we just will never be able to like it's outside of our okay so us our cognitive abilities combined with our mathematical prowess and whatever kind of experimental simulation devices we can put together is there limits to that is is it possible there's limits to that well of course it's possible there is or is there any good reason to think that we're anywhere close to the limits is a harder question look imagine asking this question 500 years ago to the world's greatest thinkers right like are we approaching the limits of our ability to understand the natural world and by definition there are questions about the natural world that are most interesting to us that are the ones we but yet understand right so there's always we're always faced with these puzzles we don't yet know and I don't know what they would have said five hundred years ago but they didn't even know about classical mechanics much less quantum mechanics so we know that they were nowhere close to how well they could do right they could do normally better than they were doing at the time I see no reason why the same thing isn't true for us today so of all the worries that keep me awake at night the human minds inability to rationally comprehend the world is low on the list well put so one interesting philosophical point and quantum mechanics bring up is the that you talk about the distinction between the world as it is and the world as we observe it so staying at the human level for a second how big is the gap between what our perception system allows us to see and the world as it is outside our minds I sort of so if not at the quantum mechanical level yeah as just are these particular tools we have which is a few senses and cognitive abilities to process those senses well that last phrase having the cognitive abilities to process them carries a lot right I mean there is our sort of intuitive understanding of the world you don't need to teach people about gravity for them to know that apples fall from trees right that's something that we figure out pretty quickly object permanence things like that the three dimensionality of space even if we don't have the mathematical language to say that we kind of know that it's true on the other hand no one opens their eyes and sees atoms all right or molecules for ourselves for that matter forget about quantum mechanics so but we got there we got to understanding that there are atoms and cells using the combination of our senses and our cognitive capacities so adding the ability of our cognitive capacities to our senses is adding an enormous amount and I don't think there's a hard and fast boundary you know if you believe in cells if you believe that we understand those then there's no reason you believe we can't believe in quantum mechanics just as well what to you is the most beautiful idea in physics conservation of momentum can you elaborate yeah if you were Aristotle when Aristotle wrote his book on physics he made the following very obvious point we're on video here right so people can see this so if I push the bottle let me cover this bottle so we do not have a mess but okay so I push the bottle it moves and if I stop pushing itself moving yes and this is this kind of thing is repeated a large number of times all over the place if you don't keep pushing things they stop moving this is a indisputably true fact about our everyday environment okay and for Aristotle this blew up into a whole picture of the world in which things had natures and teleology x' and they had places they wanted to be and when you were pushing them you were moving them away from where they wanted to be and they would return and stuff like that and it took a thousand years or fifteen hundred years for people to say actually if it weren't for things like dissipation and air resistance and friction and so forth the natural thing is for things to move forever in a straight line there's a constant velocity right conservation of momentum and that is the reason why I think that's the most beautiful idea in physics is because it shifts us from a view of nature's and teleology to a view of patterns in the world so when you were Aristotle you needed to talk a vocabulary of why is this happening what's the purpose of it what's the cause etc because you know it's nature does or does not want to do that whereas once you believe in conservation of momentum things just happen they they just follow the pattern you give me you have Laplace's deamon ultimately right you give me the state of the world today I can predict what's gonna do in the future I can predict where it was in the past it's impersonal and it's also instantaneous it's not directed toward any future goals it's just doing what it does given the current state of the universe that I think even more than either classical mechanics or quantum mechanics and that is the profound deep insight that gets modern science off the ground you don't need nature's and purposes and goals you just need some patterns so it's the first moment in our understanding of the way the universe works where you branch from the intuitive physical space to kind of the space of ideas and also the other point you said which is conveniently most of the interesting ideas are acting in the moment you don't need to know the history of time or the future then of course this took a long time to get there right I mean the conservation momentum itself took hundreds of years it's weird just like someone would say something interesting and then the next interesting thing would be said like 150 or 200 years later right they weren't even talking to each other there was reading each other's books and probably the first person to directly say that in outer space in the vacuum projectile would move at a constant velocity was Avicenna even Sina and the Persian Golden Age circa 1000 and he didn't like the idea he used that just like furniture used Schrodinger's cat to studies freely you don't believe that right even Sina was saying surely you don't believe there really is a vacuum because if there was a really vacuum things could keep moving forever right but still he got right the idea that there was this conservation of something impetus or mile he would call it and that's 500 years 600 600 years before classical mechanics and Isaac Newton so you know Galileo played a big role in this but he didn't exactly get it right and so it just takes a long time for this to sink in because it is so against our everyday experience do you think it was a big leap a brave or a difficult leap of sort of math and science to be able to say that momentum was conserved I do you know I think it's a example of human reason in action you know even Aristotle knew that his theory had issues because you could fire an arrow and it would go a long way before it stopped so if his theory was things just automatically stopped what's going on and he had this elaborate story I don't know if you've heard the story but the arrow would push the air in front of it away and the molecules of air would run around to the back of the arrow and push it again and anyone reading this is going like really that's that's what you thought but it was that kind of thought experiment that we got people to say like actually know if it weren't for the air molecules at all there I would just go on by itself and it's always this give-and-take between thought and experience back and forth right theory and experiment we would say today another big question that I think comes up certainly with quantum mechanics is what's the difference between math and physics to you to me you know very very roughly math is about the logical structure of all possible worlds and physics is about our actual world and it just feels like our actual world is a gray area when you start talking about interpretations of quantum mechanics or no I'm certainly using the word world in the broadest sense all of reality so I think the reality is specific I don't think that there's every possible thing going on in reality I think there are rules whether it's the Schrodinger equation or whatever so i think i think that there's a sensible notion of the set of all possible worlds and we live in one of them the world that we're talking about might be a multiverse might be many worlds of quantum mechanics might be much bigger than the world of our everyday experience but it's still one physically contiguous world in some sense but so if you look at the overlap of math and physics it feels like when physics tries to reach for understanding of our world it uses the tools of math to sort of reach beyond the limit of our current understanding what do you make of that process of sort of using math to so you start maybe with intuition or you might start with the math and then build up an intuition or but this kind of reaching into the darkness into the mystery of the world would math well I think I would put it a little bit differently I think we have theories theories of the physical world which we then extrapolate and ask you know what do we conclude if we take these seriously well beyond where we've actually tested them it is separately true that math is really really useful when we construct physical theories and you know famously Eugene Wigner asked about the unreasonable success of method Mattox and physics I think that's a little bit wrong because anything that could happen any other theory of physics that wasn't the real world with some other world you could always describe it mathematically it's just it might be a mess the surprising thing is not that math works but that the math is so simple and easy that you can write it down on a t-shirt right I mean that's what is amazing that's an enormous compression of information that seems to be valid in the real world so that's an interesting fact about our world which maybe we could hope to explain or just take as a brute fact I don't know but once you have that you know it there's this the indelible relationship between math and physics but but philosophically I do want to separate them well we what we extrapolate we don't extrapolate math because there's a whole bunch of wrong math you know that doesn't apply to our world right we extrapolate the physical theory that we best think explains our world again an unanswerable question why do you think our world is so easily compressible into beautiful equations yeah I mean like I just hinted at I don't know if there's an answer to that question there could be what would an answer look like well an answer could look like if you showed that there was something about our world that maximized something you know the the mean of the simplicity and the powerfulness of the laws of physics or you know with maybe we're just generic maybe in the set of all possible worlds this is what the world would look like right like I were I don't really know I tend to think not I tend to think that there is something specific and rock-bottom about the facts of our world that don't have further explanation like the fact of the world exists at all and furthermore the specific laws of physics that we have I think in some sense we're just gonna at some level we're gonna say and that's how it is and you know we can't explain anything more we I don't know how if we're anywhere close to that right now but that seems plausible to me and speaking of rock bottom one of the things so your book kind of reminded me a reveal to me is that what's fundamental and what's emergent it just feels like I don't even know anymore what's fundamental in in physics if there's anything it feels like everything especially with quantum mechanics is revealing to us is that most interesting things that I would as a he as a limited human would think are fundamental or it can actually be explained as emergent from the the more deeper laws I mean we don't know of course is you had to get that on the table like we don't know what is fundamental we do know we do have reason to say that certain things are more fundamental than others right atoms and molecules are more fundamental than cells and organs quantum fields are more fundamental than atoms and molecules we don't know if that ever bottoms out I I do think that there's sensible ways to think about this the if you if you describe something like this table as a table it has a height and a width and it's made of a certain material and has a certain solidity and weight and so forth that's the very useful description as far as it goes there's a whole nother description at this table in terms of a whole collection of atoms strung together in certain ways the language of the atoms is more comprehensive than the language of the table you could break apart to the table smash it to pieces still talk about it as atoms but you could no longer talk about it as a table right so I think of this comprehensiveness the domain of validity of a theory gets broader and broader as the theory gets more and more fundamental so what do you think Newton would say maybe write in a book review if you read your latest book on quantum mechanics something deeply hidden would take a long time for him to think that any of this was making any sense you catch him up pretty quick in the beginning yeah give him a shout out that's right I mean he used the man I'm happy to say that Newton was the greatest scientists who ever lived I mean he met in calculus in his spare time which would have made it the greatest mathematician just all by himself right I'll buy that one thing but of course you know it's funny because Newton was in some sense still a pre-modern thinker rocky Kolb who is a cosmologists at at the University of Chicago said that you know Galileo even though he can be for Newton was a more modern thinker than than Newton was like if he got Galileo and brought him to the present day you take him six months to catch up and then he be in your office telling you while your most recent paper was wrong whereas Newton just thought in this kind of more mystical way you know he wrote a lot more about the Bible and alchemy didn't he ever did about physics and but he was also more brilliant than anybody else and and way more mathematically astute than Galileo so I really don't know you know he might have he might just yeah say like give me the textbooks leave me alone for a few months and then be caught up but he but he or he might have had mental blocks against against seeing the world in this way I really don't know or perhaps find an interesting mystical interpretation of quantum mechanics very possible yeah is there any other scientists or philosophers through history that you would like to know their opinion of your book that's against a good question um I mean Einstein is the obvious one right y'all and he was not that long ago but speculate at the end of my book about what his opinion would be I am curious as to you know what about older philosophers like Hume or Conte right like what would they have thought or Aristotle you know what would they thought about modern physics because they do in philosophy your predilections end up paint playing a much bigger role in your ultimate conclusions cuz you're not as tied down by what the data is in physics you know physics is lucky because we can't stray too far off the reservation as long as we're trying to explain the world that we actually see in our telescopes and microscopes but it's it's just not fair to play that game because the people were thinking about didn't know a whole bunch of things that we know right like we lived through a lot that they didn't live through so by the time we got them caught up they'd be different people so let me ask a bunch of basic questions I think it would be interesting useful for people are not familiar but even for people who are extremely well familiar let's start with what is quantum mechanics quantum mechanics is the paradigm of physics that came into being in the early part of the 20th century that replaced classical mechanics and it replaced classical mechanics in a weird way that we're still coming to terms with so in classical mechanics you have an object it as a location has a velocity and if you know the location and velocity of everything in the world you can say what everything's gonna do quantum mechanics has an aspect of it that is kind of on the same lines there's something called a quantum state or the wave function and there's an equation governing what the quantum state does so it's very much like classical mechanics the wave function is different it's sort of a wave it's a vector in a huge dimensional vector space rather than a position in a velocity but okay that's a detail and the equation is the Schrodinger equation not Newton's laws but okay again a detail where quantum mechanics really becomes weird and different is that there's a whole nother set of rules in our textbook formulation of quantum mechanics in addition to saying that there's a quantum state and it evolves in time and all these new rules have to do with what happens when you look at the system when you observe it when you measure it in classical mechanics there were no rules about observing you just look at it and you see what's going on that that was that right in quantum mechanics the way we teach it there's something profoundly fundamental about the act of measurement or observation and the system dramatically changes its state even though it has a wave function like the electron in an atom is not orbiting in a circle as sort of spread out in a cloud when you look at it you don't see that cloud when you look at it it looks like a particle with a location so it dramatically changes its state right away and the effects of that change can be instantly seen and what the electron does next so that's the again we need to be careful because we don't agree on what quantum mechanics says that's what I need to say like in the textbook view etc right but in the textbook view quantum mechanics unlike any other theory of physics places uh gives a fundamental role to the act of measurement so maybe even more basic what is an atom and what is an electron sure this all came together you know in a few years around the turn of the last century right around the year 1900 Adams predated then of course the word Adam goes back to the ancient Greeks but it was the chemists in the 1800's that really first got experimental evidence for atoms they realized you know that there were two different types of tin oxide and in these two different types of tin oxide there was exactly twice as much oxygen in one type as the other and like why is that why is it all why is it never 1.5 times as much right and so Dalton said well it's because there are 10 atoms and oxygen atoms and one form of tin oxide is one atom of tin and one atom of oxygen and the other is one atom obtained and two atoms of oxygen and on the basis of this is you know speculation a theory right a hypothesis but then on the basis of that you make other predictions and the chemists became quickly convinced that atoms were real the physicists took a lot longer to catch on but eventually they did and I mean Boltzmann who believed in atoms was God he had a really tough time his whole life because he worked in Germany where atoms were not popular they were popular in England but not in Germany and there in general the idea of atoms is it's the most the smallest building block of the universe for for them that's the kind of how the Greek idea but the chemists in the 1800's jumped the gun a little bit so these days in atom is the smallest building block of a chemical element right hydrogen tin oxygen carbon whatever but we know that atoms can be broken up further than that and that's what physicists discovered in the early 1900's Rutherford especially and and his colleagues so the atom that we think about now the cartoon is that picture you you always seen of a little nucleus and then electrons orbiting it like a little solar system and we now know the nucleus is made of protons and neutrons so the weight of the atom the mass is almost all in its nucleus protons and neutrons or something like 1,800 times as heavy as electrons are electrons are much lighter but they're because they're lighter they give all the life to the atoms so when atoms get together combine chemically when electricity flows through a system it's all the electrons that are doing all the work and we're quantum mechanic steps in as you mentioned with position or velocity with classical mechanics and quantum mechanics is modeling the behavior of the electron I mean you can model the behavior of anything but the electron because that's where the fun is the electron was it was the biggest challenge right from the start yeah so what's a wavefunction you said it's an interesting detail yeah but in any interpretation what is the wave function in quantum mechanics well you know we had this idea from Rutherford that atoms look like little solar systems but people very quickly realize that can't possibly be right because if an electron is orbiting in a circle it will give off light all the light that we have in this room comes from electrons zooming up and down and wiggling and that's what electromagnetic waves are and you can calculate how long would it take for the electron just to spiral into the nucleus and the answer is 10 to the minus 11 seconds okay a hundred billions of a second so that's not right meanwhile people had realized that light which we understood from the 1800s was a wave had properties that were similar to that of particles right this is Einstein and plunk and stuff like that so if something that we agree was a wave had particle-like properties then maybe something we think is a particle the electron has wave-like properties right and so a bunch of people eventually came to the conclusion don't think about the electron as a little point particle orbiting in like a solar system think of it as a wave that is spread out they cleverly gave this the name the wave function which is the dopiest name in the world for one of the most profound things in the universe the there's literally you know a number at every point in space which is the value of the electrons wave function at that point and there's only there's only one wave function that yeah they eventually figured that out that took longer but when you have two electrons you do not have a wave function for electron one in a wave function for electron two you have one combined wave function for both of them and indeed as you say there's only one wave function for the entire universe at once and that's where this beautiful dance can you say what is entanglement it seems one of the most fundamental ideas of quantum again well let's temporarily buy into the textbook interpretation of quantum mechanics and what that says is that this wave function so it's very small outside the atom very big in the atom basically the wave function you take it and you square it you squared the number that gives you the probability of observing the system at that location so if you say that for two electrons there's only one wave function and that wave function gives you the probability of observing both electrons at once doing something okay so maybe the electron can be here or here here here and the other electron can also be there but we have a wave function setup where we don't know where either electron is going to be seen but we know they'll both be seen in the same place okay so we don't know exactly what we're gonna see for either electron but there's entanglement between the two of them there's a sort of conditional statement if we see one in one location then we know the other one's going to be doing a certain thing so that's a feature of quantum mechanics that is nowhere to be found in classical mechanics in classical mechanics there's no way I can say well I don't know where either one of these particles is but if I know if I find out where this one is then I know where the other one is that just never happens they're truly separate and in general it feels like if you think of a wave function like as a dance floor it seems like entanglement is strongest between things that are dancing together closest so there's a there's a closeness that's important well that's not that that's another step we have to be careful here should cause in principle if you if you're talking my the entanglement of two electrons for example they can be totally entangled or totally unentangled no matter where they are in the universe there's no relationship between the amount of entanglement and the distance between two electrons but we now know that you know the reality of our best way of understanding the world is through quantum fields not through particles so even the electron not just gravity and electromagnetism but even the electron and the quarks and so forth are really vibrations in quantum fields so even empty space is full of vibrating quantum fields and those quantum fields in empty space are entangled with each other in exactly the way you just said if they're nearby if you have like two vibrating quantum fields that are nearby them it'll be highly entangled if they're far away they will not be entangled so what do quantum fields in a vacuum look like empty space just so like empty space it's as empty as it can be but there's still a field it's just yeah it uh what is nothing just like right here or this location in space there's a gravitational field which I can detach by dropping something yeah I don't see it but there it is so we got a little bit of idea of entanglement now what is Hilbert space and Euclidean space yeah you know I think that people are very welcoming over their lives not knowing what Hilbert space is but if you if you what I dig in a little bit more into quantum mechanics it becomes necessary you know the English language was invented long before quantum mechanics or various forms of higher mathematics were invented so we use the word space to mean different things of course most of us think of space as this three dimensional world in which we live right I mean some of us just think of it as outer space okay but space around us it gives us the three-dimensional location of things and objects but mathematicians use any generic abstract collection of elements as a space okay a space of possibilities you know momentum space etc so Hilbert space is the space of all possible quantum wave functions either for the universe or for some specific system and it could be an infinite dimensional space or it could be just really really large dimensional but finite we don't know because we don't know the final theory of everything but this abstract hilbert space is really really really big and has no immediate connection to the three-dimensional space in which we live what what do dimensions in hilbert space mean you know it's just a way of mathematically representing how much information is contained in the state of the system how many numbers do you have to give me to specify what the thing is doing so in classical mechanics I give you the location of something by giving you three numbers right up down left likes XYZ coordinates but then I might want to give you its entire state physical state which means both its position and also its velocity the velocity also has three components so it's state lives in something called phase space which is six dimensional three dimensions of position three dimensions of velocity and then if it also has an orientation in space that's another three dimensions and so forth so as you describe more and more information about the system you have an abstract mathematical space that has more and more numbers that you need to give and each one of those numbers corresponds to a dimension in that space so in terms of that amount of information what is entropy this mystical word that's overused in math and physics but has a very specific meaning in this context sadly it has more than one very specific meeting this is this is reason why it is hard and roomy means different things even to different physicists but one way of thinking about it is a measure of how much we don't know about the state of a system right so if I have a bottle of water molecules and I know that okay there's a certain number of water molecules I could weigh it right and figure out I know the volume of it and I know the temperature and pressure and things like that I certainly don't know the exact position and velocity of every water molecule right so there's a certain amount of information I know certain amount that I don't know that is that is part of the complete state of the system and that's what the entropy characterizes how much unknown information there is the difference between what I do know about the system and its full exact microscopic state so when we try to describe a quantum mechanical system is infinite or finite but very large yeah we don't know that depends on the system you know it's easy to mathematically write down a system that would have a potentially infinite entropy an infinite dimensional hilbert space so let's let's go back a little bit we said that the hilbert space was the space in which quantum wave functions lived for different systems that will be different sizes they could be infinite or finite so that's the number of numbers the number of pieces information you could potentially give me about the system so the bigger hilbert spaces the bigger the entropy of that system could be depending on what I know about it if I don't know anything about it then you know as a huge entropy right but only up to the size of its hilbert space so we don't know in in the real physical world whether or not you know this region of space that contains that water bottle has potentially an infinite entropy or just a finite entropy we have we have different arguments on different sides so if it's infinite how do you think about infinity is this something you can your cognitive abilities are able to process or is it just a mathematical tool it's somewhere in between right I mean we can say things about it we can use mathematical tools to manipulate infinity very very accurately we can define what we mean you know for any number n there's a number bigger than it so there's no biggest number right so there's something called the total number of all numbers that's infinite but it is hard to wrap your brain around that and I think that gives people pause because we talk about infinity as if it's a number but it has plenty of properties that real numbers don't have you know if you multiply infinity by 2 you get infinity again right that's a little bit different than what we're used to okay but are you comfortable with the idea that in thinking of what the real world actually is that infinity could be part of that world are you comfortable that a world in some dimension and somehow comfortable with lots of things I mean you know I don't want my level of comfort to affect what I think about the world you know I'm pretty open-minded about what the world could be at the fundamental level yeah but infinity is a is a tricky one it's not almost a question of comfort it's a question of is it an overreach of our intuition sort of it could be a convenient almost like when you add a constant to an equation just because it'll help it just feels like it's useful to at least be able to imagine a concept not directly but in some kind of way that this feels like it's a description of the real world think of it this way there's only three numbers that are simple there's zero there's one and there's infinity a number like 318 it's just bizarre like that that you need a lot of bits to give me what that number is yeah but zero and one infinity like once you have 300 things you might as well have infinity things right otherwise yet to say how when to stop making the thing that's right so there's a sense in which infinity is a very natural number of things to exist that I was never comfortable with it because it's just such a kick but it was a too good to be true mmm because in math it just helps make things work out when things get very it's when things get very large close to infinity things seem to work out nicely it's kind of like because of my deepest passion it's probably psychology and I'm uncomfortable how in the average the the beauty of the very very the how much we vary is lost in that same kind of sense infinity seems like convenient way to erase the details but the thing about infinity is you it seems to pop up whether we like it or not right right like you're trying to be a computer scientist you ask yourself well how long will it take this program to run and you realize well for some of them the answer is infinitely long it's not because you tried to get there you wrote a five line computer program it doesn't halt 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 the 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 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 wave function that 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 wave function 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 out 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 worlds 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 some subsystem of it so how many worlds are there you want to 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 a very very big number one of you talked 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 the you know overlap a duplicate world that you return 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 over time 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 a 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 faster than 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 10 to 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 thing 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 once you read branched 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 between 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 1 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 ok 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 but the length is the same I just I'm 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 of energy yes it does not great and I say this definitively because there are other questions I think there's answers too 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 think 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 as 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 on to 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 on to 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 that 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 and 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 I don't think we touched on it so the the Copenhagen interpretation and the many-worlds maybe there's a difference between the effort I've already in many worlds and many worlds 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 family of 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 would 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 wave function the wave function 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 invariable 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 around 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 wave function and always a basis reading your equation hidden variables there's a wave function that always the basis 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 in 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 way of making predictions for experimental outcomes it's not mapping on to an element of reality in any real sense 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 are gonna be but the other the other interpretation is kind of think that the wavefunction is real yes that's right so that's an antic interpretation of the wavefunction ontology being the study of what is real what exists as opposed to an epistemic interpretation of the wavefunction 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 that's all you do okay awesome I'll link it and 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 understanding 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 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 should start from the most quantum theory that you have which is really many worlds so start with the quantum theory and try to build up a model of space-time the emergence of space-time that's okay so I thought space-time was fundamental yeah I know so this sort of dream that Einstein had that everybody had and everybody has of you know the theory of everything so how do we build up from many worlds from quantum mechanics a model of space-time model of gravity well yeah I mean let me first mention very quickly why we think it's necessary you know we've had gravity in the form that Einstein bequeathed it to us for over a hundred years now like 1915 or 1916 he put general relativity in the final form so gravity is the curvature of space-time and there's a field that pervades all the universe that tells us how curved space-time is and that's a fundamentally classical that's totally classical right exactly but we also have a formalism an algorithm for taking a classical theory and quantizing it this is how we get quantum electrodynamics for example and it could be tricky I mean you think your quantizing something so that means taking a classical theory and promoting it to a quantum mechanical theory but you can run into problems so they ran into problems and you did that with electromagnetism namely that certain quantities were infinity and you don't like infinity right so Fineman and tominaga and Schwinger won the nobel prize for teaching us how to deal with the infinities and then Ken Wilson won another Nobel Prize for saying you shouldn't have been worried about those infinities after all but still that was the it's always the thought that that's how you will make a good quantum theory you'll start with the classical theory and quantize it so if we have a classical theory general relativity we can quantize it or we can try to but we run into even bigger problems with gravity than we ran into with electromagnetism and so far those problems are we've not been able to get a successful theory of gravity quantum gravity by starting with classical general relativity and quantum it and there's evidence that there's a good reason why this is true that the whatever the quantum theory of gravity is it's not a field theory it's something that has weird non-local features built into it somehow that we don't understand and we get this idea from black holes and Hawking radiation and information conservation and a whole bunch of other ideas I talked about in the book so if that's true if the fundamental theory isn't even local in the sense that an ordinary quantum field theory would be then we just don't know where to start in terms of getting a classical precursor and quantizing so the only sensible thing is at least the next obvious sensible thing to me would be to say okay let's just start intrinsically quantum and work backwards see if we can find a classical limit so the idea of locality the the fact that locality is not fundamental to the nature of our existence sort of you know I guess in that sense modeling everything's the field makes sense to me stuff that's close by interact stuff that's far away doesn't so what's what's locality and why is it not fundamental and how is that even possible yeah I mean locality is the answer to the question that Isaac Newton was worried about back in the beginning of our conversation right I mean how can the earth know what the gravitational field of the Sun is when the answer has spelled out by Laplace and Einstein and others is that there's a field in between and the way a field works is that what's happening to the field at this point in space only depends directly on what's happening at points right next to it but what's happening at those points depends on what's happening right next to those right is you can build up an influence across space through only local interactions that's what locality means what happens here is only affected by what's happening right next to it that's locality the idea of locality is built into every field theory including general relativity as a classical theory it seems to break down when we talk about black holes and you know Hawking taught us in the 1970s the black holes radiate they give off they'll eventually evaporate away they're not completely black once we take quantum mechanics into account and we think we don't know for sure but most of us think that if you make a black hole out of certain stuff then like Laplace's Demon taught us you should be able to predict what that black hole will turn into if it's just obeying the Schrodinger equation and if that's true there are good arguments that can't happen while preserving locality at the same time it's just that their information seems to be spread out non locally in interesting ways and people should you talk about holography with the Leonard Susskind on your - a podcast yes I have a podcast I didn't even mention that I'm terrible no I'm gonna I'm gonna ask you questions about that too and I've been not shutting up about it's my favorite science podcast so or not it's a famous science podcast it's like it's a scientist doing absolutely yes yeah anyway yeah so holography is this idea when you have a black hole and black hole is a region of space inside of which gravity is so strong that you can't escape and there's this weird feature of black holes that again is a totally a thought experiment feature cuz we haven't gone and probed any yet but there seems to be one way of thinking about what happens inside a black hole as seen by an observer who's falling in which is actually pretty normal like everything looks pretty normal until you get the singularity and you die but from the point of the view of the outside observer it seems like all the information that that fell in is actually smeared over the horizon in a non-local way and that's puzzling and that's a lot so holography because that's a two-dimensional surface that is encapsulating the whole three-dimensional thing inside right still trying to deal with that still trying to figure out how to get there but it's an indication that we need to think a little more subtly when we quantize gravity so because you can describe everything that's going on in the in three-dimensional space by looking at the two-dimensional projection of it that means that locality doesn't it's not necessary well it means it's somehow it's only a good approximation it's not really what's going on how we're supposed to feel about that supposed to be liberated you know space is just a good approximation and this was always gonna be true once you started quantizing gravity so we're just beginning now to face up to the dramatic implications of quantizing gravity is there other weird stuff that happens to quantum mechanics in black hole I don't think that anything weird happen with quantum mechanics thinking weird things happen with space-time I mean that's what it is like quantum mechanics is still just quantum mechanics but our ordinary notions of space-time don't really quite work and there's a principle that goes hand in hand with holography called complementarity which says that there's no one unique way to describe what's going on inside a black hole different observers will have different descriptions both of which are accurate but sound completely incompatible with each other so depends on how you look at it you know the word complementarity in this context is borrowed from Niels Bohr who points out you can measure the position or you can measure the momentum you can't measure both at the same time in quantum mechanics so a couple questions on many worlds how does many worlds help us understand our particular branch of reality so ok that's fine and good that is everything is splitting we're just traveling down a single branch of it so how does it help us understand our little unique branch yeah I mean that's a great question but that's the point is that we didn't invent many worlds cuz you thought it was cool to have a whole bunch of worlds right we invented it because we were trying to account for what we observe here in our world and what we observe here in our world are wave functions collapsing ok we do have a position as a situation where the electron seems to be spread out but then when we look at it we don't see it spread out we see it located somewhere so what's going on that's the measurement problem of quantum mechanics that's what we have to face up to so many worlds is just a proposed solution to that problem and the answer is nothing special is happening it's still just the Schrodinger equation but you have a wave function - and that's a different answer than would be given in hidden variables or dynamical collapse theories or whatever so the entire point of many world's is to explain what we observe but it tries to explain what we already have observed right it's not trying to be different from what we've observed because that would be something other than quantum mechanics but you know the idea that there's worlds that we didn't observe they keep branching off it's kind of it is uh you know it's stimulating to the imagination so is it possible to hop from you mentioned the branches are independent yes is it possible to hop from one to the other no it's so physical limit then the theory says it's impossible there's already a copy of you in the other world don't worry yes leave them alone no but there's a there's a fear of missing out form oh yes that I feel like immediately start to wander if that other copy is having more or less fun well the downside in many worlds is that you're missing out on an enormous and that's always what it's gonna be like and I mean there's a certain stage of acceptance and that yes in terms of rewinding you think we can rewind the system back sort of the the nice thing about many worlds I guess is it really emphasizes the maybe you can correct me but to determine it the deterministic nature of a branch and it feels like it could be a while back is it is do you see is this something that could be perfectly won back rewinded back yeah you know if you're at a fancy French restaurant yeah there's a nice linen white tablecloth and you have your glass of Bordeaux and you knock it over and the wine spills across the tablecloth if the world were classical okay it would be possible that if you just lifted the wine glass up you'd be lucky enough that every molecule of wine would hop back into the glass right but guess what it's not going to happen in the real world and the quantum wave function is exactly the same way it is possible in principle to rewind everything if you start from perfect knowledge of the entire wave function of the universe in practice that's never gonna happen so time travel not possible nope at least one mechanic says no help what about memory does the universe have a memory of itself where we could in in it's a not time travel but peek back in time and do a little like replay well it's exactly the same in quantum mechanics as classical mechanics so whatever you want to say about that you know the fundamental laws of physics in either many worlds quantum mechanics or Newtonian physics conserve information so if you have all the information about the quantum state of the world right now your Laplace's demon-like in your knowledge and calculational capacity you can wind the clock backward but none of us is right and you know so in practice you could never do that you can do experiments over and over again starting from the same initial conditions for small systems but once things get to be large Avogadro's number of particles write bigger than a cell no chance we we've talked a little bit about arrow of time last time but in many worlds that there is a kind of implied arrow of time right so you've talked about the arrow of time that has to do with the second law of thermodynamics that's the arrow of time that's emergent or fundamental we don't know I guess no it's emergent it's a is that there's everyone agree on that well nobody's everything so that area of time is that different in the arrow of time that's implied by many worlds it's not different actually no in both cases you have fundamental laws of physics that are completely reversible if you give me the state of the universe at one moment in time I can run the clock forward or backward equally well there's no arrow of time built into the laws of physics at the most fundamental level but what we do have are special initial conditions 14 billion years ago near the Big Bang in thermodynamics those special initial conditions take the form of things were low entropy and entropy has been increasing ever since making the universe more disorganized and chaotic and that's the arrow of time in quantum mechanics these special initial conditions take the form of there was only one branch of the wavefunction and the universe has been branching more and more ever since ok so if time is emergent so it seems like our human cognitive capacity likes to take things that are emergent and assume in feel like they're fundamental so what it sequence of times emergent and locality like space emergent yes ok but I didn't say time was merge and I said the arrow of time was emergent those are different what's the difference between the arrow of time and time are using arrow of time to simply mean this the synonymous with the second law thermodynamics no but the arrow of time is the difference between the past and future so all right there's space but there's no arrow of space you don't feel that space has to have an arrow right you could live in thermodynamic equilibrium there be no arrow of time but there'd still be time it'd still be a difference between now and the future also ok so if nothing changes there's still time well things could even change like if the whole universe consisted of the earth going around the Sun yeah ok it would just go in circles or ellipses right that's not every things would change but it's not increasing entropy there's no arrow if you took a movie of that and I played you the movie backward you would never know so the arrow of time can theoretically point in the other direction for brief briefly so intent that it points in different directions it's not a very good arrow I mean the arrow of time in the macroscopic world is so powerful that there's just no chance of going back when you get down to tiny systems with only three or four moving parts then entropy can fluctuate up and down what does it mean for space to be an emergent phenomena it means that the fundamental description of the world does not include the word space it'll be something like a vector in Hilbert space right and you have to say well why is there a good approximation scription which involves three dementia space and stuff inside it okay so time and space or immersion we kind of mentioned it in the beginning but can you elaborate what do you feel hope is fundamental in our universe a wavefunction living in hilbert space wave function and hilbert space that we can't intellectualize or visualize really we can't visualize it we can intellectualize it very easily like well how do you think about it's a vector in a 10 to the 10 to the 120 2-dimensional vector space it's a complex vector unit norm it evolves according the Schrodinger equation got it when you put it that way what's so hard really yep quantum computers there's some excitement actually a lot of excitement with people that it will allow us to simulate quantum mechanical systems what kind of questions do about quantum mechanics about the things we've been talking about do you think do you hope we can answer through quantum simulation well I think that there are there's a whole fascinating frontier of things you can do with quantum computers both sort of practical things with cryptography or money privacy eavesdropping sorting things simulating quantum systems right so it's a it's a broader question maybe even outside of quantum computers some of the theories that we've been talking about what's your hope what's most promising to test these theories what are what are kind of experiments we can conduct whether in simulation or in the physical world that would validate or disprove or expand these theories well I think for the there's two parts of that question one is many worlds and the other one is sort of emergent space-time for many worlds you know there are experiments on going to test whether or not wavefunction spontaneously collapse and if they do then that rules out many worlds and that will be falsified what if there are hidden variables there's a theorem that seems to indicate that the predictions will always be the same as many worlds I'm a little skeptical this theorem I'm not completely I haven't internalized it haven't made it in part of my intuitive view of the world yet so there might be loopholes to that theorem I'm not sure about that I part of me thinks that there should be different experimental predictions if there are hidden variables but I'm not sure but otherwise it's just quantum mechanics all the way down and so there's there's this cottage industry in science journalism of writing breathless articles that say you know quantum mechanics shown to be more astonishing than ever before thought and really it's the same quantum mechanics we've been doing since 1926 whereas with the emergent space-time stuff we know a lot less about what the theory is it's in a very primitive state we don't even really have a safely written down respectable honest theory yet so there could very well be experimental predictions we just don't know about yet that is one of the things that we're trying to figure out me before emergence space-time you need a really big stuff all right well or really fast stuff or really energetic stuff we don't know that that's the thing you know so there could be violations of the speed of light if you have a version space-time not going faster than the speed of light but the speed of light could be different for light of different wavelengths right that would be a dramatic violation of physics as we know it but it could be possible or not I mean it's not an absolutely prediction as that's that's the problem the theories are just not well developed enough yet to say is there anything that quantum mechanics can teach us about human nature or the human mind do you think about sort of consciousness and these kinds of topics is there it's certainly excessively used as you point out the word quantum is used for everything besides quantum mechanics but in more seriousness is there something that goes to the human level and can help us understand our mind not really is the short answer you know minds are pretty classical I don't think we don't know this for sure but I don't think that phenomena like entanglement are crucial to how the human mind works or about consciousness so you mentioned I think early on in the Commerce you said it would be I'm a it would be unlikely but incredible if sort of the observer somehow a fundamental part yeah so observer not to romanticize the notion but seems interlink to the idea of consciousness so if consciousness as the pant site because believe is fundamental to the universe is that possible is that wait I mean every possible just say Joe Rogan likes to say it's entirely possible but ok but is it on a spectrum of crazy out there how how the statistic is speaking how often do you ponder the possibility that consciousness is fundamental or the observer is fundamental to personally don't at all there are people who do I'm a thorough physical estate company side I do not think that there are any separate mental states or mental properties I think they're all emergent just like space-time is and you know space-time is hard enough to understand so the fact that we don't yet understand consciousness is not at all surprising to you as we mentioned have an amazing podcast called mindscape it's as I said on my favorite podcast sort of both for your explanation of physics which a lot of people of and when you venture out into things that are beyond your expertise but it's just a really smart person exploring even questions like you know morality for example it's very interesting I think you did a solo episode and so on I mean there's a lot of really interesting conversations that you have what are what are some from memory amazing conversations that pop to mind you've had what did you learn from them something that maybe changed your mind and just inspired you or just what do this whole experience of having conversations what stands out to you it's an unfair question totally unfair that's okay that's alright you know it's often the ones I feel like the the ones I do on physics and closely related science or even philosophy ones are like I know this stuff and I'm helping people learn about it but I learned more from the ones that nothing to do with physics or philosophy right so talking to Wynton Marsalis about jazz or talking to a Master Sommelier about wine talking to will Wilkinson about partisan polarization and the urban-rural divide talking to psychologists like Carol Tavarez about cognitive dissonance and and how those things work scott derrickson who is the director of the movie dr. strange I had a wonderful conversation with him where we went through the mechanics of making a blockbuster superhero movie right and he's also not a naturalist he's a he's a evangelical Christian so we talked about the nature of reality there I want to have a couple more you know discussions with highly educated theists who know the theology really well but I haven't quite arranged those yet I would love to hear that I mean that's how comfortable are you venturing into questions of religion well I'm totally comfortable doing it you know I did talk with Alan Lightman who is also an atheist but he he is trying to rescue the sort of spiritual side of things for atheism and I did talk to very vocal atheists like Alex Rosenberg so I need to talk to some I talked to some religious believers and he talked more how have you changed through having all these conversations you know part of the motivation was I had a long stack of books that I hadn't read and I couldn't find time to read them and I figured if I interviewed their authors forcing me to read them right and that's that is totally worked by the way now I'm annoyed that people write such long books I think I'm still very much learning how to be a good interviewer I think that's a skill that uh you know I I think I have good questions but you know there's the give-and-take that is still I think I could be better at like I want to offer something to the conversation but not too much right I've had conversations where I barely talked at all and I have conversations where I talked half the time I think there's a happy medium in between there so I think I remember listening to without mentioning names some of your conversations where I wish you would have disagreed more yeah as a listener it's more fun sometimes it well this is that's a very good question because you know my everyone has an attitude toward that like some people are really there to basically give their point of view and the they're their guest is supposed to you know respond accordingly I I want to sort of get my view on the record but I don't want to dwell on it when I'm talking to someone like David Chalmers who I disagree with a lot you know I want to say like here's why I disagree with you but you know I want we're here to listen to you like I have a Pepa sowed every week and you only own once a week right and so I have enough coming podcast episode with Philip golf who is much more dedicated pants I kissed and so there we really get into it I think that I probably have disagreed with him more on that episode than I ever have with another podcast guest but that's what he wanted so it worked very well yes yeah that kind of debate structure is beautiful when it's done right like when you're when you can detect that the intent is that you have fundamental respect for the person yeah that and that's for some reason it's super fun to listen to when two really smart people are just arguing and sometimes lose their a little bit if I may say well there's a fine line because I have zero interest in bringing I mean like I mean it may be maybe you implied this I have zero interest in bringing on people for whom I don't have any intellectual respect like I constantly get requests like you know bring on a flat earth or whatever and really slap them down or a creationist like I'm at zero interest I'm happy to bring on you know a religious person a believer but I want someone who's smart and can act in good faith and can talk not a charlatan or a lunatic right so I will only I will happily bring on people with whom I disagree but only people from who I think the audience can learn something interesting so let me ask the idea of Charlton is an interesting idea you might be more educated on this topic than me but there's um there's folks for example who argue of various aspects of evolution sort of try to approach and say that evolution sort of our current theory of evolution has many holes in it as many flaws and they argue that I think like Cambridge Cambrian explosion that which is like a huge added variability of species doesn't make sense under our current description of evolution to a theory of evolution sort of if you had were to have the conversation with people like that how do you know that they're the difference being outside the box thinkers and people who are fundamentally unscientific and even bordering on charlatans it's a great question and you know the further you get away from my expertise the harder it is for me to really judge exactly those things and you know yeah I don't have a satisfying answer for that one because I think the example you use of someone who you know believes in the basic structure of natural selection but thinks that you know this particular thing cannot be understood in the terms of our current understanding of Darwinism that's a perfect edge case where it's hard to tell right and I would have to I would try to talk to people who I do respect and who do know things and I would have to you know given that I'm a physicist I know that physicists will sometimes be too dismissive of alternative points of view I have to take into account that biologists can also be too dismissive of alternative points of view so yeah that's a tricky one have you gotten heat yet yeah all the time like there's always something I mean it's it's hilarious cuz I do have I try very hard not to like have the same topic several times in a row I did have like to climate change episodes but they were from very different perspectives but I like to mix it up that's the whole puzzle I'm having fun and every time I do an episode someone says oh the person you should really get on to talk about exactly that is this other person like well I don't but I did that now I want to do that well I hope you keep doing it you're inspiring millions of people your books your podcasts Sean it's an honor to talk to you thank you so much thanks very much like you