For everything airing over the next 48 hours. Check the program guide. We kick off the weekend with a look at Quantum Mechanics. Good evening everyone. Welcome. Thank you for coming out to the secret Science Source north. We are a series that brings science test of all the disciplines out to public sages where they can be part of the cultural life of the city where people like you can come in and interact directly with them. And be informed at the same time. Our regular hangout is at the bell house in brooklyn. The secret science club north we are were back here tonight it is our kick off of our many season here are fit many season of secret science club north. Were also happy to be helping to kick off the new season i think we are just right behind the new republic as always. We want to thank all of the staff of the symphony space. They are the best. A very special thanks they have created our fabulous libation of the night. It has in it bourbon, maple syrup in a smoky ginger ale topped with lemon. Very delicious. You can get at the bar which is out to the left there. If youre bored by me nows a good time to go do it. We want to give a heads up we will be back on december 3 she is a former nasas or not. You may have read the thoughts like this in the paper. If you would like to know more about the secret science club and the Upcoming Events you can visit our website at secret science club. Dot blogspot. Net. Onto tonights event. We want to say a wonderful night. Sean carroll has a new book out. And in fact it came out today. And that may be why you need to have a cocktail. Its a book party. Its a lovely folks at books on call nyc. They are at book sellers tonight. They will be signing copies after the talk. We are pretty excited to have sean carroll with us tonight. Were also pretty excited to have a very special guest here to introduce sean carroll. Brain gearing is a physicist. A professor at Columbia University in director of the center for theoretical physics. Hes also a playwright sometimes an actor. And the bestselling author of several science books. He has hosted two Emmy Award Winning miniseries. Please welcome dr. Brian greene. Thank you. The quantum cocktail. I was shocked that dorian did not follow it up by saying you can get it at the h bar. Youve been even a great audience. Tonights focus with sean carroll is on Quantum Mechanics. And i said think many of you know we have a wonderful quite straightforward understanding of the universe. You tell me how things are today and i use the equations of newtons or the equation of einstein. That all changed. In the early decades of the 20th century beginning with the work of einstein itself the paradigm change and now the new way of thinking about the world is the best we can do is predict that probability of what they will be like tomorrow. That was not an easy transition for physicists in that era to take on board. It was an emotional upheaval for many of them. But here is the thing. As sean no doubt will describe for you in more details the laws of Quantum Mechanics even though they are probabilistic. Can be used to make decimal point onward to nine or ten decimal places in the agreement between theory and experiment is so potent and powerful that people began to stop thinking about what the theory actually means. Whats actually telling us about the nature of reality and begins to just use it as a wonderful structure to make predictions about the world to begin to control the micro world. To yield all of the wonderful technological gadgets that we walk around with all the time. I got to a point where anybody who really is a physicist wanted to think about the meaning of Quantum Mechanics what but it was really telling us about how the world works they were looked on as someone who had lost their way. It wasnt something that we were supposed to do. They are a few people that say that how we transition from the description of the world to the deaf not reality that we all experience. And many ideas were put forward. The most wild of all of those ideas. The possibility that we are part of a multi verse. Many worlds. Parallel universes. A very strange idea. I will say this. Ever since the events at least i had found in talking about these people have a much easier time understanding the concept of parallel universes. And with that it gives me great pleasure to introduce tonight speaker you know him the bestselling books on the nature of time. We physicists know and love sean as one of the great leaders and pioneers in understanding quantum physics. Tonight you will have a great treat here from one of the profound thinkers who are willing to keep pushing through. Give a warm welcome to my friend and colleague sean carroll. [applause]. Thank you brian. Who do i need to think. Symphony space. All of you folks for figuring out the secrets. Im happy that it does. When you become as best to do demonstrations to bring it down to earth. Im in a start by doing a demonstration. I will do push a button on my iphone. It will send a photon down the tube and then it will hit a beam splitter and go left or right with 5050 probability. And then depending on if its observed to go left or right the signal will come back here and it will tell me other i should happen to the left or to the right. Quantum event. I should happen left. Ready is the demonstration. Why is it worth anything at all. It predicts actually believe its true. There is an entire other reality where i hopped over there and am currently standing right there gesturing to my left. There is a different version of me that did something different. I actually believe this is true. What you should be same as what kind of a crazy person would believe that this is true. The reason why we are talking about these things is because Quantum Mechanics is enormously successful we need it to understand lasers in transistors. Even white chairs and tables dont collapse. As my predecessor says i think i can safely say that nobody understands Quantum Mechanics i usually dont like to appeal to authority by bringing up other physics. When the claim is that nobody understands something the person that understood understood the thing the best is a Good Authority how can it be possible. We can make predictions but if you ask what is going on what is the be at reality beneath that prediction i will say i dont know. Theyll say i dont want to know. I just want to make predictions. This is a terrible attitude. And my fellow physics should be embarrassed of themselves. Its only since Quantum Mechanics came on the scene. The people who specialize in this would be the superstars of fedex dash mike physics. These people should be millionaires living and pent House Apartments and theyre not. Its not that we dont understand that we havent even really tried. That is a whole different talk. Right now i just want to highlight the degree of embarrassment. The fable of the fox they look juicy and delicious. They are too high. They cant get them. I didnt want those grapes anyway they are probably a sour. The fox represents physicist. We used to try we couldnt do it. We never really wanted to understand Quantum Mechanics anyway. Everyone is going to understand Quantum Mechanics. And why it led us to these crazy propositions. Historically accurate to her. I will tell you a story that gets there in a relatively brief time. I hundred years ago or a little bit more. They were putting together of the adam everyone has seen this picture. There is a nuclears at new keyless at the center. Today we know they knew there was a new keyless. And orbiting around the new keyless like planets orbit the sun are the little particles called electrons. It also cannot possibly be right the reason my before Quantum Mechanics came along. We have a successful theory called classical mechanics. The classical mechanics makes up prediction. When you send it as aiming around is aiming around in circles. It admits late. And infrared electric waves. Here you have them zooming around being accelerated. What they should do they should spiral into the center and collapsed into collapse into the new keyless. The timescale for this to happen maybe it takes a long time. Less than a picosecond. Trust me, im a scientist. They should not be stable. They should just collapse. It apparently doesnt. Did a good theory. It was conceptually inadequate. It was hard to do better. There is a long set of steps that i am skipping. Maybe the electron is not a particle. May the way to think about electrons is as waves not as particles. Someone eventually said the electron which is being a particle. We call that the wave function. One of the dumbest and least significant names. There is a wave function that spreads out across the new keyless. It then fades away. You can do that math problem seen what are the different ways that the waves could settle down into an atom. And even though the weight itself was going back and forth. There was certain discrete positions that they could have. It could just sit there and not change. The electrons settle into this. This is data. This is a picture of an electron around the helium atom. It is spread out and what we in what we now call the wave function. Thats why they dont decay because theyre not actually moving its not just a little point going in a circle. It really is this function, this cloud and the waves could be absolutely static and unchanging. Thats progress. More progress came a couple years later. Who gave us an equation. I need to show you the details of the equation because of the quiz that we are having at the end. Conceptually its not that hard. The greek letter represents the wave function. You have a question that is asked of the wave function. On the right you have an answer which says how fast are you moving. A typical wave function well had various different parts to it. In the equation simply said that high energy parts change. Scientists love equations. An equation is full. Both for the scientist and student. The equation says now that i know not just what its doing in this particular circumstance i can figure out what they will do in any circumstance. So the equation gave so does the ability to say how it evolves with time. There is one obvious huge dramatic flaw. When you look at it electron. Usually when you look at electrons they dont look like waves. They look like particles. This is actually a little chunk of uranium. This is in a Cloud Chamber when the particle gets through it leaves a string of bubbles behind it. I should you mostly because of school. What i want you to learn from this picture is the prediction of the equation. Its not what you see when you look at it. What you see is a particle like thing. Thats why would come particles in the first place. The wave sort of bunches up and looks like a particle. They found thats not what happened. It spreads out all over the place. There is a dramatic mismatch between the formalism. In the observation which says it localizes and looks particle like. So people invented the awful no good idea which is that ways dash mike wave functions can be there. You think im kidding you this is what we teach our students. This is what i got taught. There is theres the wave function on by itself. And on the right it says when you measure when you say that. The word went out. It is the collapse of the wave function. Theres something that happens. The copenhagen and interpretation. And this is what we teach our students today. If you open up a textbook in Quantum Mechanics you will find what do they do their two sets of rules. Wave functions do one kind of thing when youre not looking at them they do another kind of thing when someone looks at them. When you have something spread out like that on the left that means the probability of measuring the electron is on. This is what we teach our students. See mike this is Utter Nonsense but let me be more kind. This is clearly unacceptable. It clearly cant be the right answer. Nobody in their right mind thinks that this is the ultimate way that reality works. I will highlight two of the problems with the cookbook that we are given. What is the world not what do you observe even when youre not observing it. We kind of slid around on the question. There is a wave function is the wave function. Is it a representation of a complete story of reality or its only part of reality. Maybe there is particles as well. Maybe it has nothing to do with reality. He was just a Little Black Box that we can use to make predictions. There are people who believe all of these things. We do not agree about what reality is according to Quantum Mechanics. What do you mean look at, looking at things changes them. Could a cat look at it. What about something non living. What if i just glance at it. Does it imply that somehow my agency and consciousness. Has something to do with the fundamental recipe of reality there are people that believe all of these things. At least we dont agree on the answer so there is a problem. See mac mike i have ideas thats why were here. Peruse perusal will still position of highlighting how weird and spooky everything is. I want to tell you my favorite version of how to get it right. They basically acted as a quantum therapist what he said was look, even working too hard. You have some good looking goodlooking rules for whats going on. Their functions clearly. The wave function is everything. Theres nothing else. That was his prescription. Naturally he left right away. He tried to convince other people that what he said was interesting. And here was the interpretation. There is one set of rules. It is parallel to every other theory of physics. There are states, they evolve according to an equation. And all that about measurements and other things. You dont need it. It sounds like it would be good if it worked. But the stuff was invented for a good reason. How in the world could this simple version of Quantum Mechanics possibly fit the data. The seeker is a little feature of Quantum Mechanics. Entanglement. Let me get it in modern terms. I also mentioned that i wrote a book about it. I would be hurt if you took the book out. Has zero spence. Its very much similar in spirit to just the ordinary the spin is zero. It can decay in various ways. Two electrons. We will just call those two electrons. They do have a spin. They had spin one half for historical reasons. But it is a spineless particle. Angular momentum. It is concerned. It must be the case that the spin of the two electrons canceled out and had to add to zero to give you the same spin they have before decayed. You can spin up or spin down. It could be spin up or spin down. But they must be opposite. Thats what we know. The way that that happens in Quantum Mechanics is that they decay into super position. Either electron one as is spin up or spin down. It is everything now. What this means is that it decays into a quantum wave function. The two electrons were entangled with each other. We dont know whether it will be spin up or spin down. We know of the spin of electron to will be opposite. They will have to cancel each other out. Theyre related in they are related in some way. In the lesson here is in Quantum Mechanics. There is not separate states. There are separate wave functions. There is only one wave function. These days we call it the wave function of the universe. There is a state for particle one which is a plus down. But thats not right. He put the cat up in the box. The uranium that you found before. That probability that it clicks. And when it does. It will open the box. And in the original telling. It is poison gas. I think my father just didnt like that. Thats really true. I didnt make that one up either. The point of it though. Apparently. Didnt like any of this. That they squared and got the probability of something. Thats when the on equation was for. According to my equation. If the wave function of the radioactive source was a super position of having decayed or not. The wave function of the box is in a super the super position and having not. Lets use this to realize the difference in reasoning. I will introduce notation. There is a fact of the matter. At the cat is awake or asleep. It says no know theres a whole other set of possibilities. Whether you are taught. This case. But there is classical observers. They just observe things and gets answers. They have definite value. Have you ever been in the superposition. You think that you can do that. The godfather of Quantum Mechanics. The text version is you would open the and either you would have it awake. I tell you this because i want to contrast it with the ever ready many world version of this. There is no collapse. Only one wave function. There is no dividing line. You are part of the wave function of the universe. The experiment starts with the cast being the superposition. In an observer is played by hugh everett. And then they opened the box. It is not anything mystical. Opening the box just means i solved the equation. And have a physical interaction with what is outside the box obeying the equation at all time. What does it predict. When i open the box. Heres what we know. If we opened the box. And it was entirely awake he would see in a week text. If it was entirely asleep you would see it and that has all the information you need to know exactly what you would observe. Namely the world and involves a superposition of the cat was awake. Literally nobody in the world disagrees with the fact that this is the prediction of the equation. They might disagree it you should be using that. But thats the prediction. This is why no one believed. If you seem to follow that. It sounds like and i observed quantum things before. I think like everything. He didnt really had any right to get it right. It stems from the same philosophy. There is only one wave function. We did not include the entire universe here. We have a cat and we have an observer. What about the rest of the universe. Technically we should put it in. You might think that doesnt matter. But you would be mistaken. We call the rest of the universe. The technical term we use. We are not keeping track of everything that happens. Were any room here with light coming. I dont know the position i would not even know if they changed a little bit. The environment is both inside and outside. If it starts out. It interacts with the environment right away. Before you open the box. It becomes entangled with the environment. The environment is what ever it would be. And its whatever it would be then. And then you open the box that is the measurement process. And then you see an answer this is a true wave function of the universe. And like ways. Why would it matter. In the slightest. That i keep the environment in their its because thats what i really mean by measurement. Its not anything to do with the human being. Measurement occurs at the very he becomes entangled with the environment. And when that happens the following wonderful thing happens. The person in the part of the wave function. Is there a whole another universe. Is there another part of a wave function. Is there an experiment i can do to detect that. The environment is so dramatically different. Just to the inevitable movement of the wave function. According to the equation. These two possibilities have become not interacting with each other as if they have become separate worlds. He said you know what we should do. We should call this the many worlds interpretation of Quantum Mechanics. He got to admit many worlds is a much sexier name. They make you think to help make sense of this. All i did was force the wave function to evolve. The worlds were always there. As they once put it. Every other formulation is a disappearing role. That is a therapeutic move. There are many questions. About the details of many worlds Quantum Mechanics. There is a lot. Thats all i can say. The truth is we dont know. This is part of the embarrassment that we have not been for the last several decades doing the work that we should be doing trying to understand Quantum Mechanics. Go by the wayside. One way or the other there are many of them in the way i want you to think about the world in the branching as time is going from the bottom to the top when this happens its not to double the universe they get worried about questions like energy conservation. It is really much more like an existing amount of universe and it gets subdivided. The wave function squared in total. That stays the same. According to this. It remains the same amount. As it happens in the universe splits. Its becoming thinner and thinner. If i am both an ever ready and in the utilitarian i want the greatest good for the greatest number should i spend all of my waking days with the universe. You shouldnt do that. Think of the universe splitting and its happening so often and frequently. It is an atom that decays inside your body roughly 5,000 times a second. 5,000 times a second you split the universe into. You make to to the power 5,000 universe. Just in your body. Every atom in your body that doesnt decay also contributes. Rather than individual discrete events. We think of this as an ongoing constant process that the universe is splitting and differentiating. But there doesnt have to be plenty of room. Its not really what i want to talk about here. Some of the good news about those worlds. Once we start thinking about it more carefully maybe it helps us answer other questions that we been puzzling with for a long time. Lets get some objections out of the way it really quickly. Where can we put them all. I already answered that. There is the same amount of quantum mechanical staff and the interpretation and in every other interpretation. There is room for a very large number of universes. The only question is whether you let those super positions describe reality. You might also worry that it cant be tested. I cant see any of these other universes. If you read even sp vision of science. Scientists, bless their hearts had only ever read a smidgen. They havent actually read karl popper but they read that they once said to make a scientific theory you need to be falsifiable. An experiment if i got a certain outcome. That would show me that the theory is wrong. Not that its false. But in principle its falsifiable. We know better now that was a good attempt at there are details that they did not exactly get right but even at that naive level that is not an objection to Quantum Mechanics. You might say, i cant falsify the existence of all these other universes. They are prediction of the theory. They are not input to the theory. They evolve according to the equation. In order to falsify the many worlds interpretation. I have to do is show a wave function involving in quantum addiction. There are other alternatives to the many worlds that predict that there are different physical consequences in fact one of the fans of the many worlds interpretation was karl popper. He thought it was a monstrosity. He didnt love the many worlds interpretation. As far as scientific testability. It was testable as you would want the theory to be. Here, i want to emphasize as much as i like the interpretation. We are not done. We havent done the work that we need to do. There are purple dash mike perfectly good worries that we have to have. Happily, the secret science club has given me the six hour slot so i can take all of the things about Quantum Mechanics. One of the worries is why do we get probability in this theory at all. In the copenhagen interpretation when you observe it. It is put in. There is an equation. And as deterministic. There is nothing probabilistic about it. I know the universe at all other times. Why is there any talk of the probability of getting a certain outcome for different measurements. And the other one is how does the classical world emerge. I think its an important problem. Its one very few people have paid attention to because we cheat our way around. We live in a classical world and we put in by hand. By doing better we can actually learn something. Let me rush through question one. And then i will chat a little bit about question two. For probability, because many worlds perfectly deterministic whatever probability is is not a truly stochastic element. They has to be what they call epistemic. Has to be our ignorance is somehow causing us to use probability talk. And you might say will, if i knew the exact wave function i know everything and there you are wrong. Even if you know the exact wave function. There might be something you dont know. And therefore you have to define a probability. Where you are in the wave function. Remember the story we told where you have the cat, the first step was eco. They became entangled with the environment. The observer observer has not yet become entangled with the cat. The cat became entangled with the environment. I can still treat it as two different branches of the wave function. They are exactly identical. They dont know which branch they are on. Now there too. Neither one of them know which branch they are on. Even though they know the perfect way function of the universe the best they can do if they ask them. You say i dont know. Theres a probability. And you say what is that probability it is is the wave function squared. Not everyone agrees. This people are wrong but thats okay. They can cite my papers when they are disagreeing with them. The problem i think that is richer in some sense and less salt than a probability issue is finding the classical world. This is one of the things i need to convince you there is a problem and then convince you that we are making progress towards it. The chairs, tables. When youve defined it. And thats because its very natural as human beings to think about the world classically. We tend to privilege what we see when we see the electronic making a little track. Its not the dumbest thing in the world. Maybe its spread out in the cloud of probability. You can be as a result we tend to privilege what we see. And in the physical literature we quantize it. We have some classical system. Whether its cats or magnetic waves. And there is a set of rules they can apply and say im going to invent a wave function. Mathematically that function as an element of a vector space. Really its a billion ways. We start with the classical system. Presumably nature doesnt do that. It doesnt start with the classical system. If our for emulating how nature works. In certain circumstances under the right conditions there is a good approximate story you can tell. Which we call classical mechanics. We should be able to start with the mathematical object think about what its doing. And how its evolving. And then say that as a cat. Finding the classical world and expressing it from the wave function and nobody thinks about it. Were just really beginning on this path. I think we can see a way forward most of the work is being done by the idea of locality the idea that space is threedimensional is so special and all the laws of physics. And the ways it special is things bump into people. There is no real spooky action at a distance if you want to actually communicate you send a signal place by place. Why is this important. Think about the cat why when you open the box. After all, these are just quantum states. There is another perfectly good quantum state. And all of the different combinations. And we open the box we only ever see awake or asleep. These are the preferred states of the cat. The wave function branches when the photon bumps into the cat. What do they bump into. They bump in to the cats differently. In the awake cats crawling around trying to get out. Here is a picture of the photon that would hit the cat. The way they branch out. Is into systems of definite physical location. That is how interactions work in physics. They pick out locations in the world. Even if i handed you a very abstract wave function and said what is it. The first thing you would do is can i divide up this function to little pieces some of which interact with each other and some dont. And i would call the pieces nearby in space. And i would call the ones that did interact far away in space. You can imagine building up the concept of space just because had how it evolves over time. This is not what we do because we live in space. And all the rest of physics. Space is the arena. That the world plays out on. And the advanced informal listen. Well had to find space. Rather than seen space interactions are local in space space is a set of variables in which they are vocal. Let me see if we can make progress with that. It is curved. Everybody shows you pictures of einstein. The hair is gone. He was very sharply dressed. He looked smarter than you. He did not look frumpy like your uncle. And he have this idea that you and i would think of as gravity its actually a manifestation of the curvature of space. Even if it is a stage. Its not a static stage. The stage is a player in the game. They respond to matter and energy by getting curbed and then they feel that curvature and do orbits around the sun or whatever other things people do in the presence of gravity. The problem is, and every problem as an opportunity gravity is the one part of our current understanding that we have not made fully quantum mechanical. Everything else the nuclear forces. We have a good quantum mechanical theory that seems to work. For gravity we have another theory that works with the moon moving around the earth. My strong feeling is that the reason why we have been able to find the quantum theory of gravity is because we keep starting with the classical theory and trying to quantitate it. I think its a mistake. I think rather than quantitative easing that gravity. Why in the world should we be surprised that we dont understand quantum gravity and we dont even understand Quantum Mechanics. Maybe we just got lucky and we were misled by the fact that we could find quantum theories. How in the world are we get to do that. Its not that we are moving completely blindly. It is the best current understanding. Of a non gravitational business. You are quantitate is in these fields and stretching to space. We think of as a particle is just a place where the field is more intergenic than you thought it was. In a particle point of view you might think that there is empty space and nothing is happening in empty space. But where there are particles something is happening in the quantum field theory point of view. Something is happening everywhere even in empty space. There are fields that are vibrating. We can characterize those vibrations. If you take one of them and vibrate up more it looks like a particle there. Its a different ontology different way of a different way of looking at the world. These little regions of empty space they are entangled with each other. Of course they are. Even in empty space. What we call the vacuum there is literally nothing there. Its empty. Quantum field theory as we know, love it and use it. What i want to do is turn it around, because im saying we dont know what space is, we dont know what distance is. Maybe we can define distance using entanglement. Entanglement is a feature of the function in its own right. Theres no reliance on conception. Distance is a preexisting classical conception, so maybe we can say when things, when regions of the quantum wave function are unentangled with each other, theyre far away. When theyre close by sorry, when theyre very entangled, theyre very close by. And if you can imagine doing that for all the little substances that make up the wave function, you woulding define an swire geometry on this emergent space. If i know the distance between any two points in space, if i know the size of a triangle and the angles inside, etc. , i know the geometry of space. And thats what einstein wants us to talk about. So i claim, and this is just chutzpah. This is not something we know is true, this is a guess, this is a hypothesis that were exploring the consequences of. But to imagine that we can extract geometry e from entanglement. Theres something that we do know for sure, that entanglement is related to entropy. I love entropy, but the quantum version has special certain qualities. A wonderful physicist shows that when two systems are entangled with each other, either one of them separately has entropy. You can roughly think of entropy as how much you dont know about the space of a system. And we said those two systems, a and b, are entangled, either one of them is in one definite space all by itself. Thats kind of like we dont know what state a is in. Its not true that we dont know. So its kind of like its an analogy, but theres a formula when two systems are entangled, you can quantify the amount of entanglement by the end to my of entropy of the different substances. Finally, theres a relationship between entropy and energy. Remember i said that when the particles in quantum field theory, you pick out a little vibrating quantum field and youve made it vibrate more, youve also broken the amount of entanglement between that little field and the rest of the field around it. So you decrease the entanglement, youve increased the entropy, and youve increased the energy in that region. Theres another equation, i wont show you, but theres a mathematical relationship between the amount of entanglement in a region in a quantum field theory and the amount of energy in that region. So i know this is all going fast, this is all going by, but here is what we learned. A relationship between geometry and entanglement that were positing because we think it would be cool if somehow space emergessed from features emerged from features of the quantum wave function. Theres a relationship between energy and entropy that we can derive under the right circumstances if something is close to a quantum field. Therefore, theres a relationship between geometry of space and the energy in it. That should sound familiar with your general relativity course. That is simply what einstein said when he invented the theory of general relativity, that the geometry of spacetime is governed by the amount of energy in that region, broadly construed. Theres more math than that. There are greek letters and so forth. This is einsteins equation of general relativity, but the moral of this equation, the more you have a region, the more it curves the spacetime around it. What im saying now is rather than being einstein and positing this relationship directly, we can posit that the idea of geometry emerges from the quantum wave function and derives the fact that geometry is governed, is driven, is forced by the energy in the universe. So we could hope to derive einsteins equation for general relativity from fundamental features of Quantum Mechanics. We can imagine a history where einstein never lived and we can discovered general relativity just by thinking really, really hard about Quantum Mechanics. Now, i know were done, this is the end of it. Theres a lot that happens there, a lot of stuff went by very quickly. My advice is [laughter] its explained this in greater detail here. But thats okay, you know . If my goal was to really make you perfectly understand this, then id be crazy, because nobody perfectly understands this. Rather, i want to get you thinking in a new way, or okay . I started by saying that physicists have been making mistakes for the last sently and not trying century and not trying as hard as they could to understand Quantum Mechanics. Also without a lot of progress there. My particular strategy for reconciling these two things may or may not work. What i want you to do is whatever thing you do for a living, stop doing that and start trying to figure out where spacetime comes from in Quantum Mechanics. [laughter] will youre a professional physicist or not, they know too much and are doing the wrong thing, okay . So we are trying to think in new ways. It may or may not work, but it is our responsibility to figure these things out, because what matters is not actually making predictions for experiments. What matters is understanding how nature works, and theres only one way to make progress on that, and thats what i think were doing. Thank you very much. [applause] thank you to sean carroll. Something deeply hidden. Were going to come around and get your questions, if sean will indulge. So were going to come one on each side. And dont forget, the book is for sale back there where youre heading, and sean may be signing them after. You can even get them inscribed, i believe. All right. So were going to head down and take your questions. Should i pick people or do you want to pick people . You can pick them, if you want. [inaudible] all right. Hi. So the most agreed upon theory of cosmology predicts that were [inaudible] right . Well, no. But go ahead. Go on. [laughter] i can fill that in. Keep going. So my question was does the of Quantum Mechanics make this problem worse because now theres many who propose quantum states that have many [inaudible] yeah. So for those of you who dont hang out on the wrong street corners, the idea of a boltsman brain is if we lived in the hightech equivalent of a box of gas that lasted forever, you know, the gas that is the atmosphere in this room basically stays smooth and spread out in something called equilibrium, but the individual molecules are bumping around in random patterns, and if we waited long enough, all the molecules would move to one side of the room. Or all the molecules would make a little path or a little brain. So if the universe were like that, if the universe were, number one, made out of little, tiny pieces that fluctuated around randomly and, number two, lasted literally forever, then over the course of eternity there would be random fluctuations into all sorts of things, planets and people, and this minimal thing to the conscious observer would be enough fluctuations where atoms were created that made a brain, and then it would die. These are called boltsman brains. And the question is theres so many more of those boltsman brains than there are of us, how can this possibly be right. And people argue over whats right and whats wrong. But the short answer is most people say, therefore the, one of the hypotheses is wrong. Either the universe does not last forever, or its not made of randomly fluctuating things, it settles down to a true, static vacuum state. So what i would say about the challenge of boltsman brains is its a guideline to constructing cosmo logical theory because that predicts most living beings are random fluctuations. Throw that one out. It doesnt make the problem any worse. Many worlds is not the idea that everything happens, its the idea that the wave function always [inaudible] theres plenty of things that dont happen in in many a world, and things that happen only very, very, very rarely. So all the predictions for a typical occurrence in many worlds are exactly the things they would have been, you know, in a ingle universe theory single universe theory. We have one over here. It seems to me that you have maybe discovered a new conservation level of embarrassment. In the sense that, you know, the previous explanation seems hard to believe and so does this one, in a way. So wouldnt it be easier to not just let go of maybe the equation and try to think of just classical mechanics get superseded by relativity . Can we think of theres Something Else maybe makes less embarrassing explanations . Yeah. Look, be my guest, is the short answer. [laughter] the little, glib answer to that. Everyone in the world would be thrilled to find a version of, a physical theory of fundamental nature of reality that reproduced all the predictions of Quantum Mechanics without having extra worlds. Like im not into the existence of the world. I accept that theyre there because my theory predicts that. But it turns out that constructing such another theory is hard. So its okay. [laughter] dont worry about it. [laughter] everything will be fine. [laughter] yeah. Okay. Youve got the two electrons, and theyre entangled. Yeah. And they go far aa part. Right. You measure one, and the other one has yes. So they say, yes. If the geometry is emergent because things are entangled with each other so things that are entangled a lot are closer together . Is that how youre solving the problem . No. Therefore, theyre geometrically closer together . Its a very good question. If i had had more tile, i would have been more careful about this. The whole story of when things are more entangled theyre closer together, is a story of the vibrational mode of quantum fields in empty space. Its not a story of particles. Once you have two particles, they can be equally entangled no matter how close or far away they are. But the number of things in the universe, the number of quantum mechanical degrees of freedom is overwhelmingly dominated by the degrees of freedom of empty space. Like, even at the center of the sun, quantum field theory standards the center of the sun is empty space. The number of particles there is enormously smaller than the number of quantum field modes. So you first have this whole conversation that weve been having about the entanglement between molds and empty space, and you can just tack particles on top of that. Thats the good news. The bad news is it doesnt help you with any puzzles or feelings you might have about [inaudible] hi. You had mentioned that in order for a universe to be consistent with conservation of energy, some property has to become the center and the branches, and im just wondering what property yeah, i can tell you that. Remember back in the days when we believed the copenhagen interpretation. You would say the probability of getting an answer is the wave function squared. Thats the thickness of a branch. So when you branch into two different possibilities, both of which have a wave function one over square root two, which is a half probability each, youre splitting one thickness of universe into two half thicknesses and so forth. This is the quantity which is conserved over the evolution of the wave function. In Quantum Mechanics is there such a thing as the thermodynamic [inaudible] sure. Are you waiting for the thermodynamic death of the universe . Yes, its going to happen. Most of the traditional predictions are going to be just as true in Quantum Mechanics as in anything else. It is relevant to the previous question about boltsman brains. We live in a universe that is not only expanding, but accelerating. Its emptying out at a rate which we think is never going to go away. So the best current theory we have of cosmology says we have 14 billion years in the past between the big bang and today, we have infinity years in the future, all of which will be a story of getting emptier and emptier. So the universe will go to a heat death x. The quantum version of that is the universe will settle down to a vacuum quantum state and just sit there quietly together, and there will be no living beings around to think about it, so live your life now [laughter] because you only have about [inaudible] until the last star burps out. [laughter] burns out. Sorry. We have one over here. Hi. I was wondering what you thought of the von neumann interpretation vigner, yeah. And what the role of consciousness is and experiences in quantum physics. Right. So when i went back here, you know, i mentioned the puzzles here, one of them was is consciousness really part of it. Because in the textbook interpretation of Quantum Mechanics, we are told that theres Something Special about when you observe, when you measure, when you look at a system that sounds like consciousness might be involved. And vigner took that to its logical conclusion and said, well, maybe consciousness is really crucial to understanding Quantum Mechanics. Participant of this is an old joke that this relationship between consciousness and Quantum Mechanics comes from the fact that nobody understands Quantum Mechanics, and nobody understands consciousness, so maybe theyre the same thing somehow. Vigner tried to make it more respectable than that, but honestly, he later said he was wrong. He said, no, that was a badded idea, stop thinking that bad idea. Many more people listened to him when he suggested it rather than when he said he was wrong, stop thinking that. Its not the craziest thing in the world because we have lack of understanding in both domains, but i think that theres zero evidence that we should be connecting consciousness to Quantum Mechanics. Theres room to do it because consciousness is complicated, but theres no need to do it by anything we know about either consciousness or quantum physics. Yes. My question is do did you give any, i guess, do you entertain any simulation theory possibilities . And do you think that the problem listic probablistic [inaudible] that this is all a simulation because you could, i guess, consume large amounts of information without actually having to render everything and do things in much larger scales. Did you at all entertain that possibility . Look, were building computers, theyre Getting Better and better, well get Artificial Intelligence at some point. You could imagine that we will build computers that are so good that there are little conscious beings in the computers not knowing we exist, right . Theyre talking to each other, living their lives, and for all possible criteria we would count them as copps, and then you go, oh, my goodness, maybe im a creature in someone elses computer. And, in fact, you might imagine that most living creatures in the universe probably are. I dont buy it. I think to the extent that you would have predictions for what a universe would look like were we to live in a simulation, those predictions do not come true. But its possible. You know, i certainly am open to the possibility of it. My personal feeling is that it has zero to do with Quantum Mechanics. I think that if i were simulating things, i would just make them class call. Thats really classical. Thats really the best way to stay computational about it. The information about a quantum state with all these entanglements is enormously bigger than the information in a classical state. Thats why Quantum Computers might be much more powerful than classical computers. So if your only criterion was saving cpu cycles and doing your simulation, i would not use the simulation of Quantum Mechanics at all. Okay. We have one in the center here. Thank you very much for the talk. The two puzzles that you mentioned were, one was the measurement problem and the other was where do we find the classical world in Quantum Mechanics. So i do a lot of statistics, and i thought of an analogy which is probably wrong, im just trying to understand why its wrong. But with respect to the measurement problem, when we do something simple like a survey, you know, we have a point estimate and theres a confidence around it, and we dont consider that controversying bejust controversial, we dont think theres thousands of oh worlds in which it could be different. So why is that an incorrect analogy . Why cant we just say the same thing about wave function . And to your second point about where do we find the classical world, again in Probability Theory we have the concentration of measured phenomenon, that the sample size goes infinity, variance goes to zero, so maybe the classical world is just an infinite example of the world and so thats why [inaudible] are so stable. Yeah. No, thats a very natural way to go, and thats along the lines that einstein would have been thinking back in the 1920s. It doesnt work. It doesnt work in the most straightforward way, and the easiest way to see that is that the probability of getting an outcome is not the wave function, its the wave function squared. The wave function can be negative. It can even be a complex number, and you might is ask yourself why not just work with the probability . And the answer is the wave function can interfere with itself. Different parts of the wave function can add up either constructively or destructively. A true, simple probability distribution is made of a set of numbers that are nonnegative. Probability distributions dont interfere with each other. But in quantum experiments, its very easy to see the wave function interfering with itself. So that is the first clue that its really real in some sense. There are plenty of people that till try to avoid this conclusion, but that would be my answer. Im a past of least resistance kind of guy. We have one here in the back. Hello. Hi. So i had a question, you were saying that you really think that things are splitting off based on different actions. So youve im wondering if you visualize with your own life that there are other versions of your life taking place and what the impact is. I mean, you seem very at peace with that [laughter] yeah. You know, contributing to your ideas and stuff. But is it something that could freak people out, or could it make their lives better or, like, if this gains cultural if your ideas gain more cultural traction, whats the impact of that . Yeah. My own view is that freaking people out is not a bug, its a feature. [laughter] in the sense that what were trying to do is understand reality at its most fundamental, foundational, central piece. And what we have been trained on is our everyday experience which is a tiny, unsenate sliver of reality insignificant sliver of reality. And the whole history of science has been expanding our view of reality, and the total story we tell about reality becomes weirder and weirder because it becomes further and further away from our everyday experiences. The fact that the world of Quantum Mechanics predicts something that is so weird and freak out, i think thats good. Nevertheless, despite the weirdness, this is the best fit, this is the simplest theory that fits the data, thats a reason to be pro that theory, not givens it. Not against it. Of course it does freak you out. Of course it does. What are you talking about . When i was a kid, the thing that freaked me out was thinking about what if the universe never existed in that was what was keeping me up at night. There are other versions and is so forth, its hard to get it right, its hard to come up with the exact right way to think about these different things, but i dont think theres really exist,ly worried. Existent,ially worried. How you should live your life in this view of reality is exactly the same as how you should live your life if the copenhagen interpretation had been right. We have a centrist back here. Centrist. I didnt know that there were any. [laughter] you used in this concept to construct the concept of space. How would you use this concept to construct the concept of time . Oh, man, thats a whole other lecture, but you did catch me. Very good. Einstein says that gravity is the curvature of spacetime and, in fact, space and time are two different aspects of one underlying, fourdimensional thing. But the thing that i talked about emerging out of the wave function was really space. Now, there are ways that you can also get time to emerge, and the short version is, again, you use end tanglement. You say, well, what if time werent fundamental . What if it werent real . What could that possibly mean . What it possibly means is theres a subsystem of the universe which is entangled with the entire rest of the universe and has the property that the subsystem acts like a clock. That is to say depending on what the subsystem is doing, depending on what its measured to be, its entangled with something the whole rest of the universe is doing in such a way that its as if rest of the universe is evolving with time according to what the clock actually reads. So it can be done, but its not at all necessary. Its entirely possible that deep down space and time are different from each other, and the fact that they appear to be part of one underlying fourdimensional manifold to us is kind of an e emergent phenomenon in the macroscopic world. Its the kind of thing we should have figured out 50 years ago, but were just tackling it right now. Were going to give sean carroll a break from the public questions now, but he will be signing copies of something deeply hidden in the back, and you can talk to him there. How about a hand for sean carroll . Hopefully, hell be back talking about time next time. Hell be signing the books, and you can get it inscribed. [applause] the city of toledo has been called the glass capital of the world. Its referred to as glass city. And thats because of the influence of four Major International corporations all producing some aspect of glass. And that has been the case since 1888 when Edward Drummond libby brought the first Glass Factory to toledo. This weekend the cspan cities tour takes you to toledo, ohio. I think places like toledo are often taken for granted, and thats a shame because theres such wonderful history here. We serve the Steel Industry here. Without this region of america, we could not go to war, nor serve the vast automotive and Railroad Industry which is also a part of our heritage here. Located in the northwest corner of the state, it has a population of about 300,000, and its waterways connect to lake are eerie. Theres lake erie. Theres a lot that happens on this river in toledo, ohio, as a shipping port that many people probably arent even aware of, even people who live here. Join us today at 9 30 a. M. On booktv and sunday at 2 p. M. On americanly tv as the cspan cities tour take you to toledo, ohio, as we explore the american story. The new cspan online