To get there we are going to bring up the filings of irwins. Youve heard of this one right. He says imagine i have a box that is sealed in some radioactive source like the ukrainian we stopped before. They can detect when the source permits up particle. When it does, it will open a box. Gas will come out of the box. It is cyanide in the cat dies. As his daughter once said i think i. Dot my dad just didnt like cats. I see no reason to kill a cat. I think im gonna undo the decades of cat killing the has been a favorite pastime. When you havent yet opened the box. He invented his equation. At the time he didnt have this interpretation of the equation that you got the proper ability of something. I wish i have nothing to do with this. He did not return the nobel prize the wave function of the detectives is in the superposition. And having open and have a knot. His results in conclusion from this surely you dont believe that thats what really happened that thats what his own equation says. Lets use us to illustrate the difference in reasoning. I will introduce notation. I know this is everyones favorite part of the science talk. And i have things in square brackets that means they are classical somehow. They have definite value and if there they are quantum. Classically the cat is alive or dead. I might not know what it is. But there is a fact of the matter that the cat the cat is awake or asleep. It says no no no theres a whole another set of possibilities. If you have taken a little bit of quantic mechanics. What youre taught is that there are quantum mechanical subsumes but then there are classical observers the observer lives somehow in the classical world they do have a wave function. They just observe things and get answers. They have definite values. Have you ever been in the superposition. There is something definite about you this is a picture of the godfather of Quantum Mechanics. You open the box the way function would collapse and either you would have the cat awake and you have the observer see the cat awake i tell you this which is all obvious because i want to contrast it with the ever ready world of this. There is no collapse there is only one wave function. That includes you. Ever ready Quantum Mechanics. You are part of the wave function of the universe. You can be in superposition. The experiment starts with the cat being a superposition. And an observer in this case. Wasnt anything mystical. It doesnt have anything to do with consciousness. I have a physical interaction with what was outside the box and what was inside the box. What does the equation predict when i open the box. Here is what we know if we open the box in the way function was entirely awake you would see in awake cat. If it was entirely asleep. He would see a sleep cat. Of the cat was in the superposition. Namely it evolves into a superposition of the cat was awake in the cat saw them awake. Were literally nobody in the world disagrees with the fact that this is the prediction of the equation. They might disagree that you should be using the equation but that is the prediction. This is why nobody believes if you seem to follow that it sounds like i myself evolved into superposition. And i have observed quantum things before. I felt like i hopped left. I think everett got it right. But he didnt have any right to get it right. This was in figured out until the 70s and 80s. Namely, there is only one wave function for the entire universe and you notice we didnt include the entire universe here. What about the rest of the universe. Technically we should put the rest of the universe in. We call the rest of the university environment. Is just a technical term we mean. We are not keeping track of everything that happened were in a room with lights coming. I dont know the position and velocity. I would not even know if they changed a little bit. The environment is both inside and outside the box. If the cat starts out in the superposition of awake and asleep it interacts with the environment right away. Before you open the box there is a process called decoherence in which the state in and the quantum system become entangled with the environment. It evolves into a superposition of. Then you open the box and that is the measurement process. This is the true wave function of the universe. Likewise there there. Why would it matter. In the slightest. That i keep they keep the environment in there. Its not anything to do with the human being. Measurement occurs at the very scientific mechanism thing. When a system that is in a superposition becomes entangled with the outside world. He becomes entangled with the environment. And when that happens the following wonderful thing happens. The person in the part of the wave function who saw the cat awake says, is there a whole another universe or part of the wave function. Is there an experiment i can do to detect that. The answer is no. We have an environment that is so dramatically different that this part of the wave function cannot effect affect and is not affected by the second part of the wave function. Just for 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 to get more publicity we should call this the many world interpretation. They originally called it a theory of a universal wave function. Now the problem is that it makes you think if you havent gone to nice talks like this. It makes it think that im adding extra worlds to Quantum Mechanics to help make sense of it. At no point did i add worlds. All i did was force the wave function to evolve. Every other formulation is a disappearing world. All he wants to do is say that the world is there. That is a therapeutic move. There are many questions and i refer you to this excellent book that just came out. The very well could be an infinite number of worlds. We dont know. We have not been for the last several decades of doing the work that we should be doing trying to understand Quantum Mechanics. Go by the wayside. One way or another there are many of them. Time is going from the bottom to the top. Its much more like there is an existing amount of universe and it gets subdivided. That stays the same. According to the equation. It remains the same amount. As branching happens. The individual branch becomes thinner and thinner. I want the greatest good for the greatest number. You shouldnt do that. Because that should be balanced by the fact that you are making the universe universes thinner and thinner. Its happening so often and so frequently every time and adam inside you decays roughly 5,000 times per second. You split the universe into. You make to it to the power 5,000 universes just in your body and that is a lower limit because every adam that doesnt decay also contributes a. Rather than individual to street you should think of this as an ongoing constant process. Dont worry. Thats not really what i want to talk about here. Once we start thinking about it more carefully maybe it helps us answer more questions that we went puzzling with for a long time. Let me get some objections out of the way. Quickly. That thats too many universes. Where can we fit them all. There is the same amount of quantum mechanical stuff and and every other interpretation. Dont worry if you believe the superpositions are real there are plenty of room for the universe to be in a superposition. The only question is whether you let those super positions describe reality. You might also worry that it cant be tested. If you read even a smidgen of philosophy of science you probably have seen. If they have only read once. They havent actually read karl popper but that he once said to make a scientific theory needs to be falsifiable. I could in principle imagine an experiment that if i got a certain outcome it would show me that the theory is wrong. In principle its falsifiable. We know better now that was a good attempt that there are details that popper didnt exactly get right. That is not an objection to the Quantum Mechanics. I cant falsify the existence of all these other universes but the point is not the other universes. They are a projection of 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 even though its not been entangled with anything else. There are other experiences in these predict that there are different physical consequences. He thought the copenhagen interpretation was a mistrust see. He did not love the many worlds interpretation because it is own interpretation. As well as scientific testability. Here, i want to emphasize as much as i like the many worlds interpretation we are not done. We have not done the work that we need to do. There are good worries to have that many worlds might not be right. They have given me this six hour slot so i can tell you about all the other interpretations. I will tell you about what the worries are. Why do we get probability in this theory at all. In the copenhagen interpretation there is a postulates when you observe the thing it collapses with a certain probability. Everett just says there is an equation. It is deterministic. If you give me the quantum state at one time. I know the quantum state of the universe at all other times. The other one is how does the classical world emerge. I think its much more interesting and important problem very few people have paid attention to because we cheat our way around. We live in a classical world and we put it in by hand. I think we need to do better. And by doing better we can actually learn something. For probability part of the answer is because theyre perfectly they are perfectly deterministic. Whatever probability is its not a truly its what they have to be. It has to be our ignorance. You might say if i knew the exact wave function they know everything. That is the wondrous nest. Even if you know the exact wave function. Therefore you have to assign a probability too. Where you are in the wave function. The first step was decoherence. The important thing about the steps is. They have not become entangled with the cat. The observer has not yet become. I can still treat it as two different branches of the wave function. There was one observer. Even know they know the perfect way function of the universe. The best they can do is to say i dont know. There is a probability. And then you write papers and philosophy journals in philosophy journals and say what is the probability. Its exactly the rule. Again not everyone agrees those people are wrong but thats okay they can fight my papers when they are disagreeing with them. Thats called the scientific method. The problem that i think is richer in some sense. What do you mean the classical version. The classical world is all around us. 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 theres part of our brain that says the electrons are making a little track. If you were out there hunting your alliance and you said i thought i see the line there. Youre to be selected away from the gene pool. As a result we tend to privilege what we see. We start with cats and leaves of grass and people. We do what is called quantitative in it unit we have some classical system on there is a set of rules that physicists could imply. The way function of the cats or the grass. Mathematically that wave function as an is an element of a vector space. Thats what it is. This is what we teach our students this is what we do every day. We start with the classical system we quantize it. Presumably nature doesnt do this reality doesnt start with the classical system and then quantize it. Thats just quantum. From the start. Start with the quantum system and say in certain circumstances under the right condition there is a good story we can tell which we call classical mechanics. What we should be able to do is just a start with the mathematical object think about what its doing and how its evolving and what the dynamics are and then say yes thats a cat. Finding that classical world. This is super hard and no one thinks about it. Were just really beginning on this path but 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 so special and all the laws of physics. Things bump into each other whether they are at the same location in space. There is no spooky action at a distance as einstein says. You cannot actually teleport things faster than the speed of light. Think about why when you open the box you see the cat is awake or asleep. After all these are just quantum states. These are perfectly good quantum states. Awake plus asleep. Yet when we open the box we only ever see awake or asleep. These are the preferred states of the cat. Why is that. The wave function branches when it bumps into the cat. For the asleep cat on the ground. Here is a picture of the photon that would hit the cat were it awake. The way the wave function branches is into systems of a 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 dash rec wave function you would ask if you could divide it up into little pieces some of which interact with each other and some of which dont. And i would call the pieces nearby in space. Just from the way that the wave function evolves over time. This is not what we do because we live in space and we think its already there given to us. We are can have to find space. Rather than saying interactions are local in space space is the set of variables in which the reactions are local. The other thing we know about space is that it is curved. Everyone shows you pictures of einstein when he was older. When he was a young man. He was very sharply a draft addressed. He didnt look frumpy like your uncle. Einstein had this idea that you and i think of as gravity the curvature of space time. It is on a static stage. The stage as a player in the game. By getting curved. The problem is in every problem of course is an opportunity in the gravity that gravity is the one part of our current understanding that we have not made fully quantum mechanical. Everything else we have a good a theory that theory that seems to work to arbitrary accuracy. Not in important cases. My strong feelings that the reason why we have it been able to find the quantum theory of gravity. As because we keep starting with the classical theory and try to quantize it. I think rather than trying to quantize it. We should just define the quantity within mechanics. Why in the world should we be surprised that we dont understand quantum gravity. Maybe we are just lucky and me are misled. How in the world are we gonna do that its not that were moving completely blindly we are gonna need to get some hints from what we know work. The quantum field theory is our best current understanding of non physics. Its not even particles particles that you are quantizing. In a particle point of view. Its nothing that is happening. The quantum field theory point of view. Something is happening everywhere. We can precisely characterize this. And if you if you take one of them. It looks like a particle there. The same story the particle is moving. And guess what the little regions of empty space they are entangled with each other. There is literally nothing there. There is still a little vibrating function field there. They are entangled with each other. In distant regions they are only a little entangled with each other. That is a lesson of quantum field theory as we know and love it and use it. We dont know what space is. We dont know what difference distance is. Maybe we can define it. Distance is it preexisting conception. When regions are untangled with each other they are far away they are very close by. There entangled. They are close by. And if you can imagine doing that for all of the little systems that make up the wave function. An entire geometry if i know the distance if i know of the the size of that triangle. I know that geometry of space. I claim this is a way we are exploring the consequences. There is something that we do know and that it that it is related to empathy. The quantum version has certain special properties. When two systems are entangled with each other either one of them separately. How much you dont know about the state of the system. And we said with two systems a and b are entangled. Neither one of them are in one state by itself. Thats kind of like we dont know. What state a is in. There is a formula when two systems are entangled. You can quantify the amount of entanglement. There is a relationship between empathy and energy. When you see particles you picked out a little vibrating quantitive field. Youve also brokered the amount of entanglement between that little field in the rest of the field around it. You decrease the entanglement to increase the empathy. There is another equation i wont show you. A mathematical relationship between the amount of entanglement in a region in the amount of energy in that region. I know this is all going fast. But here is what we learned. There is a relationship between geometry. If they emerged from the quantitive wave function. There is a relationship between energy and empathy. And something that is close to a quantum field theory. There is a relationship between geometry of space and the energy in it. That should sound familiar from your general course. When he invented the theory of general relativity. It is governed by the amount of energy in that region broadly construed. What i am saying now is that rather than being einstein we can posit that the idea of geometry emerges from the wave function. And it is driven and sourced by the energy in the universe. We can hope to derive it from fundamental features of Quantum Mechanics. We can imagine alternative history. And we developed all of the quantitive field theory. And we discovered general relativity. By thinking really hard about Quantum Mechanics. There is a lot that happens there a lot of stuff went by very quickly. My advice is. It is explained in greater detail here. If my goal were to make you perfectly understand i would be crazy because no one perfectly understands it rather i want to get you thinking in a new way i started by saying they had been making mistakes for the last century. And not trying as hard as they could to understand Quantum Mechanics. My particular strategy for reconciling these two things may or may not work about what i want you to do is whatever thing you do for a living. Stop doing that. You have a leg up on the professional physicist. And theyre doing the wrong thing. 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. Understanding how nature works and theres only one way to make progress on that. And thats what i think we are doing. Thank you very much. Thank you to sean carroll. We are in a going to come around and get your questions dont forget the book is for sale and sean will shauna will be signing them after. Hi, the most agreedupon theory of cosmology i can fill that in. My question was a does the many worlds interpretation make this problem worse because now there is many superposed quantum states. For those of you that dont hang out on the wrong street corners the idea is that if we lived in the hightech equivalent of a box of gas that lasted forever the gas that is the atmosphere in this room basically stays smooth and spread out. The individual molecules are bumping around. If we waited long enough all of the molecules are moved to one side of the room or they would make a little catch. If the universe was like that and it was number one made out of little tiny pieces and lasted literally forever there are mineral things. It would be enough where adams were created that made a brain that was conscious enough to go and look around and say all empty space and then it would die. The question is there is so many more of those than there are two of us. How can this possibly be right. Most people say therefore one of the hypotheses is wrong even either the universe does not last forever or its not made of randomly fluctuating things. What i would say about the challenge is the guidelines to reflect on those theories. It doesnt make the problem any worse. Is not the idea that everything happens. The wave function always obeys the equation. Theres always things that dont happen. And even more things that happened but only very rarely. So all of the predictions for typical occurrence are exactly the same as they wouldve been in a single universe theory. It seems to me that you have discovered a new conservation level in the sense that the previous explanation seems hard to believe. And so does this one in a way. When it be easier to not let go of the equation and just think of a classical mechanics. Be my guest is the short answer. Everyone in the world would be thrilled to find a physical theory that reproduced without having extra worlds. Im not into the existence of the world. Eggs accepted that there there. But it turns out that constructing such another theory is hard. Its okay. Dont worry about it. Everything will be fine. He if the two electrons. They go far apart. That is like the spooky action. If the geometry is emerging because things are entangled with each other. Is that how you are solving that problem. Its a very good question. If i have more time would be more careful about this. The whole story of when things are more entangled in closer together as a story of the vibrational modes of space. Its not a story of particles. They can be equally entangled. No matter how close or far away they are. But the number of things in the universe it was overwhelmingly dominated the empty space. Even as the center of the sun. Its empty space. The number of particles there are an enormously smaller you first had this whole conversation that we been having about the entanglement. You can just tack particles on top of that. That is the good news. It doesnt help you with any puzzles you may have. See mac you had mentioned in order for the branching universe some property of it has to become dinner. I was just wondering what property you were referring to. Remember back in the days when we believed the copenhagen interpretation. The probability of getting an answer is the wave function squared that quantity over every possible outcome is concerning. When you branch into two different possibilities it is a half you are splitting one thickness of universe into to half thicknesses. It is can served. In Quantum Mechanics there is the death of the universe. Are you waiting for that yes. Its can happen. Most of the traditional projections are good to be just as true in Quantum Mechanics as anything else. We live in a universe thats not only expanding but accelerating. Its emptying out and at a rate which we think is never gonna go away the best current theory we have says weve had 14 billion years in the past and we have infinity years in the future all of which will be a story of getting emptier and emptier. The universe will go to a heat death. The quantum universe is that it will settle down to the vacuum quantum state. Live your life now. Live your life now. You only had ten or 15 years so the last star burns out. I was wondering what you thought of the Norman Wagner interpretation and what the role of consciousness and experiences were. When i went back here and i mentioned one of them was his consciousness really part of it. We are told there is Something Special about when you observe and measure and look at a system. The Nobel Prize Winning novelist. There is a relation nest between the Quantum Mechanics. Nobody understands consciousness but maybe theyre the same thing somehow. They tried to make it more respectable than that. Many of them looked at that. There are still people who tried to draw the connection its not the craziest thing in the world because they have a lack of understanding and both domains. I think there is zero evidence that we should be connecting quantum nests. There is no need to do it by anything we know about any consciousness. My question is do you give any simulation theory possibility and do you think the nature of that is something that could increase the conversation of that. Large amount of information without having to what render everything. Do you ever entertain that possibility. We are Building Computers they are Getting Better and better you will get Artificial Intelligence at some point. You can imagine that we will build them so good that they are conscious of creatures creatures in the computers not knowing that we exist. Theyre just talking to each other and living their lives. We would count them as conscious. And then you save me thats me. Maybe i am a little creature in someone elses computer. You might imagine that most living creatures in the universe are in a simulation and probably we are. I dont buy it. I think its to the extent that you would have predictions for what they would look like. Those predictions do not come true. But its possible my personal feelings as it has zero to do with Quantum Mechanics. If i were simulatinin