Mysteries of Modern Physics by Sean Carroll

I'm Sean Carroll, I'm a theoretical physicist at the California Institute of Technology, and I'll be talking about some of the

mysteries

of

modern

physics

. I will say a little about how much

physics

knows, which is a lot, but the

mysteries

. What we have left are extremely deep, so I'm going to talk about the nature of quantum mechanics, the nature of space and the nature of time, so welcome, welcome everyone to the second of the Darwin College lectures of 2020 and, as you know , our theme this year. They are enigmas, riddles or riddles tonight we took our instructions from Alexander Pope the poet who called humans glory joke and enigma of the world long after that he left calling us a riddle ordered us to go measure the air of the earth and declare the tides instruct to the planets in which orbs to function correctly in the old time and regulate the Sun, so following that instruction tonight we turn to physics, once so logical and orderly that the Pope declared nature and the laws of nature lay hidden in the night God said Newton be and all was light, but not now as the writer JC Squire once wrote, the cry of the devil did not last ho, may n Stein be restored to the status quo well, why where are we now?

I think

modern

physics can often leave us more bewildered than enlightened, it's not just that you already know us all. Non-experts, I remember a story circulating here in my college days, when a student left her class notes in the basket of her bicycle and returned to find an old, rather poorly dressed man rummaging through them for his quantum physics notes. . I wanted to know what they taught people these days, since older hobos don't usually read advanced math. She told him what for and he apologized and headed to the free school. Now that old man was Paul Dirac, who wrote about a physicist's painting. of nature and the different implications of quantum physics and relativity suggested at the time, perhaps a little earlier, that the situation could be described as if God were a very high-level mathematician and used very advanced mathematics to construct the universe.

Advances in mathematics would allow for a better understanding of the universe, so tonight we will discover what baffled Dirac, what led Stein to the paradox of eerie action at a distance, these deeper enigmas of the quantum world and the multiple universities. University, there are multiple universes that you can distinguish. but normally I think we have Professor Sean Carroll, a theoretical physicist versus a physicist at Caltech who, as you can see, is going to talk about the mysteries of modern physics. Thank you so much. Of course, it is a huge honor to be here as part of the Darwin conference. series, I like the bottle of vodka that they left me here, which is also something that is not available in the United States, but I don't know how familiar you are with the system, but when you are invited to give one of In the conferences, there is a topic for the year and they assign you a title.

You know they're suggesting what you should talk about in general terms, but then it's understood that you're going to change the title to be what you want it to be, so this was the title. was assigned and I changed it, but then I changed it again because I realized that instead of just giving a list of my favorite mysteries or just talking about my one favorite mystery, I'd actually like to make an interesting comment about the types of mysteries. facing physics today because we're in an interesting part of the history of physics that it's always hard to know when you're in it because you don't know what's going to happen next, but I think we can see some clues, so I'll start with this is a modern image, of course, but I want to make an old point: what is physics and how it talks about the world, you know, interpreted in general terms, that's physics, that's the world, that's what the world is made of and how . the world behaves and the point of this image I mean one of this image for the person who took it, no doubt it's the cute little dog driving the car right there, but for me I want to emphasize the difference between old physics and modern. physics, if you are Aristotle and you were thinking about physics, you had a story to tell and that story was one of causes and purposes, the right of the nature of things to be a certain way and, from the point of view of physics, one of the important points was that for something to move you had to push it there had to be a reason why it moved there was a natural state of being that was for everything more or less at rest until you pushed it well so here you see that It's a reminder that Aristotle was no fool in our everyday lives, it's generally true that if you want something to move you have to push it, and so it was really a huge step forward when we started to realize that wasn't the case.

How physics worked. Fundamentally there began to be a difference between the type of underlying laws and the phenomenological experience that we have in our daily lives. This turns out to be a very long history of people realizing this, even sometimes latinized as others. Sena was a scholar during the Islamic Golden Age, he was Persian around the year 1000, and physicists consider him extremely annoying because his work daily he was a doctor, he was a doctor and he wrote many volumes on health and medicine and then, in his free time on weekends, he invented. new fundamental laws of physics, right, this doesn't endear you to the physics community, as far as I can tell, even watching it, he was the first person to say out loud that if it weren't for these confusing things like friction and air resistance, what would really happen to a moving object is that it would just stay in motion that it is not necessary to keep something in motion things tend to move on their own is it just common or things would tend to move on their own alone but it's just that they were surrounded by dissipation and what we now call friction that prevents them now it was a long time ago even seen his point was not that this is what the world really was like he thought of this as a reduction against the idea that there could be empty space because surely Don't think that things just move forever, of course, now we know that there are things very, very close to empty space.

We fly spaceships towards them and they seem to move forever. There are no rockets propelling the Voyager probe into interstellar space. It's just moving. and that creates a huge and different vision of how the world works. On the one hand, this would be another talk, but it helps to naturalize our view of the world. You know, Aristotle wrote physics, but then he wrote more than that and used this. idea that you needed something to make the movement to prove the existence of God, it is correct to say that there must at some point be an unmoved mover if the universe can run on its own, that again raises a different conceptual framework, that's another talk entirely different from what I want What I'm saying is that this new paradigm not of causes, goals and purposes, but rather patterns, the way of thinking about physics and this codified in Galileo, Newton and Laplace is that if you give me the state of the universe at any time, then there will be equations that represent natural patterns that will tell you what happens next and they are not pushing, they are not causing the universe to happen, they are just telling you concisely what is happening, let me show you what those are. laws first in cartoon form.

This image basically sums up the laws of physics that underlie what I call everyday life and I'll explain what I mean by that in a minute, but you get the cartoon image of an atom, you've seen things like that, there is a nucleus in the middle. With protons and neutrons, we now know that protons and neutrons are made up of part quarks and down quarks that spin an electron and these different particles interact through different forces. There is the strong nuclear force that holds them together within the proton and the neutron holds the quarks. Together, there is the electromagnetic force that keeps the electron in the atom.

Sometimes, of course, almost everything interesting you have ever encountered in your life is due to electrons moving between and between atoms, and then, of course, there is the weak nuclear force that brings a fourth elementary particle to life. called neutrino because the up and down quarks can convert to each other by spitting out neutrinos and finally there is gravity that attracts everything towards the earth, everything is attracted towards everything else under the force of gravity at the bottom of all this while we Verified just a few ago years in 2012 at the Large Hadron Collider in Geneva, there is a Higgs field spread all over the place.

As I wave my hand, it moves through the Higgs field and that affects the properties, especially the masses of these particles, so it's a very simple picture of particles for forces, a background, Higgs field, it's possible that, If you've been around the wrong corners and learned a little physics, think well, aren't there other particles? Yes, there are, you know the strange quark and the top quark and the muon and things like that, but all these particles decay pretty quickly, you're not made of any of those particles, these are the particles that make you, this table, me, this laptop, and really, everything you've ever owned. seen with your eyes touched with your fingers smelled with your nose in your life and we also know how they interact with each other and even better than that the most impressive fact is that there will not be a discovery tomorrow or the next century or a million years from now, which says you know what there was another particle or another force that we didn't know, but now we realize that it plays a crucial role in our everyday life in regards to our everyday life, by which I really mean what what can you see. with your eyes, touch with your hands, etc., we're done, find me the underlying ingredients, it's a huge achievement in human history, one that doesn't get enough credit because of course, as soon as we do it, we move on to the Next is physics. not done I'm not saying that physics is done, but physics has understood certain things and those things include everything you encounter in your daily life unless you are a professional experimental physicist or unless you are looking, of course, outside our everyday life in the universe and other places where we don't know what's going on, so I know that simply showing you a cartoon may not be as persuasive as it could be, especially when the point is so great since the laws of physics underlying everyday life are completely known at some point.

Many of you are sitting there thinking I'm not going to believe that until I see an equation. Well, here they are. This is the equation. This is what Nobel Prize winner Frank will prove. He has called it the central theory of modern physics. It is expressed in a certain mathematical language. I just want to write everything down explicitly so you're prepared for the quiz at the end of the lecture, but the point is, remember I told you that the paradigm of physics is if I tell you everything about the world in a moment. moment there is a law that tells you what the world will be like at the next moment, both forward and backward in time, so this is the law of modern central theory for quantum field theory of all those particles that I just told you, it is a Path Integral that was solved by Richard Fineman and others and everything that is there, there is quantum mechanics, there is space-time, gravity, the other forces, so they are the weak and strong nuclear forces and electromagnetism, there is matter, by which I mean electrons and neutrinos. and the quarks and there is the Higgs boson in the background the only purpose of showing you this is to point out that it is a pattern, it is simply something relentless that the universe does over and over again, it is not moving towards any goal nor being pushed from the outside Anyway now there is another subset of you who say I sure like equations.

I'm glad you showed me that, but I really only like equations that fit on a t-shirt. Here you have proof that this fits. a t-shirt, okay, so the reason I'm going through this, although it's not the mystery yet, right, this is something we understand, is to point this out about the mysteries that I want to get to. We have many mysteries left. in physics, but among those mysteries is not how the things that make up this table behave or the earth beneath us or you and I or the planets and the Moon and the stars; the mysteries are a different type, so there is an obvious type of mystery that, in fact, in retrospect I really should have made a slide and I feel bad now what the collective behavior of all these things is.

I understand how electrons behave. I understand how protons behave. I don't understand how 10 to 25 electrons behave, especially when there are protons surrounding them that can have all kinds of emergent properties. I'll nod towards them later in the talk, but clearly that is, in fact, what most working physicists think about and it's incredibly important that there is another set of myths and mysteries that we have yet tounderstand. of the universe is made up of law and this is something that astronomers have taught us, bless them, when you look at the sky in various ways you realize that the types of particles that make up the central theory, the types of particles that we What we have detected in our laboratories and others are simply not enough to explain what we see in the upper left, there is the famous bullet cluster where two galaxy clusters intersect, the matter was heated because it somehow collided with the matter of a cumulus.

We run into matter from the other group which is red, but then you can use gravity and the lensing effect on light caused by gravity to get to where the mass is and it's not where ordinary visible matter is. Blue is where we rebuild the dough. So what you interpret this image is to say that these two galaxy clusters simply passed through each other and all the mass passed through almost all the mass, because almost all the mass is something called dark matter, something that does not interact with the model with we, the particles. the core theory we know and love and then use evidence from the Cosmic Microwave Background and high redshift supernovae to say that most of the energy in the universe isn't even made of matter, not just ordinary matter , it's not even dark.

It matters, the pie chart we built in the 1990s has 70% of the universe, something called dark energy that is distributed smoothly through space and, most importantly, does not dilute as the universe expands; there is a certain amount of energy in each cubic centimeter of space 10 to the power of minus 8 herbs if you want to know and it remains the same even when the total number of cubic centimeters increases, that phenomenon separates the universe and that causes an acceleration that we can detect using our telescopes, so 25% is dark matter leaves only 5% of the universe for the particles that were listed on the t-shirt right there, okay, clearly there are mysteries in physics that no one can claim otherwise, but None of this is dark matter and dark energy.

It affects our everyday lives, in fact, there is a sense in which it is kind of like every day, it is down to earth. I mean, after all, dark matter is matter and we know what matter is and what it does, there is probably some particle, in fact, it is full employment for graduate students in theoretical physics to invent their own Dark Matter particle, they haven't worked hard enough if they haven't invented their own dark matter particle in modern physics, likewise, dark energy, we have a perfectly good explanation of what Einstein's cosmology might be. Constantly filling all the space, we don't know the details, but it is easy to explain these phenomena in broad terms, so it is a kind of normal science.

Thomas Kuhns sends to find out what these things are, in addition to these kinds of mysteries, we have deeper mysteries. and that's why I want for the rest of the talk to bring you up to speed on some of the even deeper mysteries, some of the fundamental mysteries that call into question the whole paradigm that we tend to use, there's a secret. way of doing physics implicit in this equation, quantum field theory, which has gaps, has things that might not be up to the task in some ways and that's what I want to mention to you, so I'm going to have three mysteries.

The number one mystery is quantum mechanics. I already mentioned quantum mechanics, you may have heard of it, how could it be mysterious? Paul Dirac, whose name was already mentioned, traced here, one of the founders of quantum mechanics did many things, a Nobel Prize winner, we can use quantum mechanics with extraordinary precision to make predictions to build new technologies to understand why the table is solid, why the sun's rays, etc., however, my esteemed predecessor at Caltech, Richard Fineman, very famously said. I think I can safely say that no one understands quantum mechanics. This statement is still true if you interpret what he meant.

Like I think I can safely say that no one but me understands quantum mechanics, not because Fineman understood it or not because I understand it, but there are people who think they understand quantum mechanics but they don't think other physicists understand quantum mechanics. . There is no consensus on what is really happening and this is a long story that deserves many lectures on its own. Quantum mechanics was put into its final form around the 1920s and late 1920s and there's a bit of a fight between you and people like Einstein and Schrodinger, who said, "You know, so far, so good.

I'm impressed. but you can't say we're done, we have to move on, but a deeper understanding is required here and, on the other side, people like Niels Bohr." and Berner Heisenberg and Wolfgang Pauli who said no, no, we're fine, there are some mysteries there but we'll think we don't need to solve them, it's good enough for government work, the motto here is shut up and calculate. No one said that sincerely, but we accused each other of thinking that way. So what does Fineman mean? It's not that we can't use quantum mechanics, but when you ask what's really going on, I'll explain what I mean in a second, we can't give you a meaningfully agreed upon answer and that's okay, not understanding things that have mysteries is cool, so It's how you progress trying to solve mysteries.

What's not right is that we're not trying to because the field of physics over the last 90 years has largely abandoned the project of trying to understand what's going on in quantum mechanics. I compare it to this famous Aesop fable, the fox and the grapes, do you know that this one was walking by the fox and he sees this bunch of grapes on the vine and the fox says: I want these grapes, he jumps to get them, they are just outside out of his reach, he can't get the grape, so after a few tries, the fox says, you know, I never wanted those grapes, anyway, they were probably sour.

We're not supposed to explain your parables, but just to be clear the fox represents physicists the grapes represent understanding quantum mechanics we used to admit we wanted that and now we say no, no, we don't even want to understand quantum mechanics, what's happening? With that, let me give you a brief reminder of why quantum mechanics came about and what it says. Here's another Cambridge icon, Rutherford's atom. This is something I already showed you, but we'll boil it down to its essential core in the central electrons orbiting around you like they were planets in a solar system, I'm so sorry Cambridge, but I have to say this is nonsense, this image we know that this picture is not correct, they knew it very, very quickly after they invented it, why because another famous Cantabrian Isaac Newton came up with something called classical mechanics which was the predecessor of quantum mechanics and classical mechanics is quite simple about what happens when particles and forces move in the 19th century.

We put together this theory of electromagnetism Faraday and Maxwell and his friends. and it said the following if you have a charged particle like an electron there is an electric field pointing towards it so if I move the electron the electric field adjusts to point to the new position so if I move the electron up and down Down in the electric field it responds by waving and those waves move at the speed of light because they are light, that is what light is. All the light in this room is made up of electrons that move up and down and emit electromagnetic waves.

This is how light works. These electrons should emit light. They are spinning fine, this counts, orbiting like this counts as being shaken up and down, so classical mechanics along with the specific equations of classical electromagnetism predict that the electron should be losing energy and therefore should be spinning spiraling towards the center of the nucleus. not stay in constant orbits, you can even ask how fast it should happen and the answer is about 10 to the minus 11 seconds, about 1/100th of a billionth of a second, so I fear the Attica physicists, but it's always good to do experiments to see. what's up so let's do this didn't it happen this table you me the earth should collapse to a point in a small fraction of a second if classical mechanics were true now normally, when they come up with an anomaly like this, physicists The first step is to be conservative.

Can we change the rules a little to save the phenomenon? But eventually, if you try really hard, you will come up with something completely dramatic in the paradigm shift. That's what happened in the case of the atom. The idea that it occurred to people that electrons are not particles despite the image that they look a lot like particles, in reality the electron is spread out in a kind of cloud that we cleverly call a wave function, is not really very smart, so it is a boring name in the world for the The most interesting and important concept in the world is that the electron is stable with a certain size around the nucleus because it is like taking a rope that is tied at both ends and pulling it , there will be a lower frequency at which it can vibrate and there will be harmonics that will come in a discrete set, in the same way, the various forms of the wave function of electrons found near an atomic nucleus come in a set discrete, there is a lower energy place that the electron can be and it is not a point like something in the The center of the nucleus is a small cloud that extends over a certain size and then there is a rule that says that two electrons They can't do exactly the same thing, so if you put more and more electrons around the nucleus they become more and more baroque.

Ways any of you who have taken chemistry classes have been tortured by images like this orbitals right, you've heard of these words, okay this is a wonderful idea, it explains why atoms don't disintegrate, it even explains why certain types of radiation Atoms come and have discrete, predictable frequencies because you're jumping from one of these types of energy states to another. Best of all, there is an equation. I love the equations here. This is the Schrodinger equation. Okay, I'm not going to go into details. I can't explain this any more than the previous one, but the point is that this wave function for the electron, the Greek letter psy obeys an equation, which means we are back in the old physics paradigm where I tell you what is doing.

Right now, this equation is what tells you what will happen in the future. Again, it is one of these patterns that nature obeys. For a physicist, having an equation like this is the happiest place they can be because they can assign problems to their students to solve. the Schrodinger equation in all kinds of different situations this is a huge triumph at a huge cost because you dramatically changed what you meant you thought the electron was a particle now you say it's a wave there's a problem when you look at the electron which is that it doesn't It looks like a wave, so you can use Schrodinger's equation to make predictions, like when a radioactive substance emits a particle, how does that particle move away from the substance that emitted it like a nucleus decaying and spitting out an electron?

The answer is that if it looks a lot like one of these shapes, it will spread out in a roughly spherical configuration, a big puffy cloud that will move in all directions and essentially fade over time, so when you look, this is what What you see is a sample of real uranium in a cloud chamber, which means that when a particle moves, a charged particle will remove some electrons from nearby atoms and you will see it as a small cloud and what you see very clearly is not a big, swollen thing, but a set of individual trajectories, as if a particle had been emitted from the substance and moved away from it when radioactivity occurred.

This is really difficult to understand if we already said that electrons are not particles they are waves it is almost as if electrons are waves when we are not looking at them in particles when we look at them so the greatest minds in the history of physics got together and They thought about what was happening and came up with the following paradigm: Electrons are waves when you don't look at them and they are particles when you look at them. What can I say? This is still what we teach our students. This is called the Copenhagen interpretation or the textbook interpretation.

Quantum mechanics says that on the left you could have some extended wave function for an electron sitting there by itself, but when you measure it, when you take a picture of it, when you say where it is, it suddenly collapses at one point and it looks like a particle and also You can't even tell me where it's going to collapse, the best you can do is calculate the probability of it collapsing in different places and the probability is governed by the wave function when the wave function is larger there is more probability when the wave function is very small thereless probability, so I'm NOT making this up when we teach Kwon mechanics to our students, whether at Cambridge or Caltech or anywhere else, there are two different sets of rules that come along with quantum mechanics.

There is a set of rules for how quantum systems act when no one is looking, and those rules are very practical and sensible. There are quantum states that take the form of wave functions and have an equation, the Schrodinger equation, in which it is very present. the classical paradigm and then when you look at them, when you measure them, when you observe them, all hell breaks loose, you can't predict exactly what is going to happen, it's just a probability and the wave function changes suddenly and discontinuously, this is what that we teach our students. The problem with this is that it is clearly nonsense, this is clearly unacceptable as a fundamental theory of nature, so there are two problems here, one is the problem of measurement, which is if you look back at these rules, what do you mean? with when someone looks?

What counts as a measurement? Does it have to be a human being? I mean, could a cat count by looking at something? How about a video camera? Would that count? What if he just looked at her from the corner of his eye? How fast does it do it? it happens because it is probabilistic, etc. so none of these questions are answered by the conventional Copenhagen interpretation. This is why people like Einstein and furniture didn't say quantum mechanics is wrong, they said it's a good halfway idea that we need to do better. We need a more mechanistic underlying model and the other one I want to emphasize is the problem of reality.

I mean, I said it quite explicitly because I think it's true that the electron is a wave function. That's what a particle is not, but is it exactly like that? There are other people who will say, well, reality is partly a wave function, but there are other things happening too, maybe the reason electrons act like waves sometimes on particles, other times is because they are waves and particles. or maybe the wave function. It is the tool we use to calculate probabilities and there is no reflection of reality at all. The fact that we don't know ninety years after we came up with this theory is a huge shame for physics and if you tried to find out if you were a student in the back of the room who raised your hand and said, "I don't like what you're telling me "I'd like to try to do better.

He gently pushed you off the field physically or you know you're going to go voluntarily or not." It didn't stop people from trying, so here's my favorite version of how to solve these problems. Hugh Everett was a graduate student in the 1950s and worked with John Wheeler, who in turn worked with Niels Bohr, so he was in the tradition of the greats of quantum mechanics. and if anything, Everett's approach was both therapeutic and scientific, basically what he's saying is that guys, you're working too hard, you don't need to go all out with all these crazy rules just to understand what we see in the experiments that they want. to know what the reality is the wave function is precisely represented by the wave functions nothing extra there you will not know how the wave function behaves obey by abandoning your equation you know that it obeys the Schrodinger equation sometimes I say that it obeys the Schrodinger equation all the time that's all that happens so it says relax of course we know there's a problem with this but let me emphasize how simple the idea would be if it worked.

Here is Everett's version of quantum mechanics. There are not two separate types of rules. just a set of rules and it's just that there are quantum systems described by wave functions and they obey the Schrodinger equation and that's it, how much simpler would students' lives be when we taught them quantum mechanics if that were all. The problem, of course, is this question I mentioned: when we look, we don't seem to see wave functions, we seem to see particles, so how can we reconcile this very beautiful, pure, austere, rigorous framework with the evidence of our eyeballs when shall we look?

Actually, look, that's a little subtle, but it's worth explaining, so I'll evoke the thought experiment stylings of urban Schrodinger, who famously put a halfway-to-death cat in a box, so the Schrodinger's cat is inside a big box and there is a quantum event going on. like a radioactive source that is detected by a detector and if the Phi detector opened a small container and released gas throughout the box in Schrödinger's way of doing things, the gas was cyanide and if the gas was released, the cat died killing the cat it doesn't increase understanding of physics in any way so in my version it's just sleeping gas and when it's released the cat falls asleep but there's still a small smile on the cat's face, it's okay no harm done cat in these thought experiments. of Schrodinger's thought experiment that was developed in correspondence with Einstein because they were on the same side in this debate is not to say Wow look how cool quantum mechanics is to say surely you don't believe that because if you believe in the rules of quantum mechanics mechanics doesn't is that the radioactive source decays or not, it has a wave function that is a superposition of I have decayed and I have not decayed just as the electron does not have a defined position, it is a wave function extended over all the different possibilities , the quantum state of the radioactive source is not one or the other, it is a superposition and when it comes to tiny microscopic things like a radioactive nucleus, we think that's fine.

Schrodinger's thought experiment aims to amplify that quantum weirdness to the macroscopic. level, then if you believe the Copenhagen interpretation that wave functions obey the Schrodinger equation until you look at them and don't count the cat as an observer, then what happens is that the detector enters a superposition of I have detected a decay and the container doesn't go into a superposition of I've opened and I haven't and the cat goes into a superposition of I'm awake and I'm asleep so that's the real Schrodinger's cat situation, it's not that we don't know It's not Whether there is a cat in the box that is awake or asleep and we simply ignore the truth, is that there is a prediction from quantum mechanics that there is a cat inside the box that is in a superposition of both awake and asleep. and the story that the Copenhagen interpretation would tell about this is to say here is the superposition of the cat from awake to asleep but the observer in brackets is classical so I am putting quantum things in parentheses classical things in brackets here the role of the observer is played by Niels Bohr and the story they tell you is that when you open the box there is a collapse of the wave function, you never see the cat in a superposition of awake and asleep, with some probability you see the cat awake and the cat is awake. or you saw the sleeping cat and you see the sleeping cat and that's what the cat was doing here is Everett's version of exactly the same story remember that there is no collapse of the wave function but there is also no classical world that you don't deal with Everett's human formulation Humans treat electrons differently, they also have a state of quantum mechanics and Everett makes use of a crucial feature of quantum mechanics that was highlighted by Einstein and his collaborators in the 1930s called entanglement .

This is a longer story, but in quantum mechanics there are different subsystems of the The universe does not have its own wave functions, there is only one wave function for the entire universe. Everitt cleverly called it the universal wave function. Stephen Hawking called it the wave function of the universe. It's the same idea. Everything belongs to the same wave function and therefore you can't isolate the quantum state of one thing, the quantum state of another thing, the quantum state of a third thing, you have to talk about the whole universe at the same time, which allows for this thing called entanglement, where the observable features of one subsystem of the universe could be related to the observable features of another in a way that would be impossible in classical mechanics, so here's Everett's version of Schrodinger's cat: you start with a cat and the observer listens to the cat in a superposition.

The observer now played by Hugh Everett is treated quantum mechanically, so parentheses and measurement are no longer at all creepy, remember that Everett is simply saying that he just evolved everything according to the Schrodinger equation, there is no separate type of evolution and this is the interesting point, everyone agrees, no matter how they feel about quantum mechanics, everyone agrees that if you just take the observer and the cat and treat them using the rules of quantum mechanics. We know what they are going to evolve into. What they will evolve into is a superposition of an entangled state where part has the cat awake and the observers saw it. awake and in part he has the cat asleep and they saw him asleep everyone agrees on that part the question is what are you going to do about it and the question arises because you would think that if that happened to you you would feel like me I have seen the cat awake and I I've seen the sleeping cat, but I've never felt that way, no one has felt that way, therefore this cannot be correct, whatever its genius, which is to say that both parts of the wave functions exist and are real, but They go their separate ways.

This part of the wave function will never interfere with or otherwise affect that part of the wave function and there is math involved in this and you can argue for it etc. but the point is these two parts. From the wave function, once entanglement occurs, they behave as if they are separate worlds, so you can simply point and say that the self that is having the experience of looking at the cat is not the combination of both things, it is that there was one. Me and now there are two Me's, there is a me that saw the cat awake and for me that's all the cat asleep, so as time goes by the universe has a wave function that branches into more and more separated worlds .

That's why he's a good public relations man. In this case, Bryce DeWitt relabeled the Eveready in the theory of quantum mechanics as the many-worlds interpretation of quantum mechanics, but what I want to emphasize is that at no time does Evert, nor does he, nor anyone else put a Lots of extra worlds than the worlds always were. There, all Evert did was remove things, namely some clunky and unjustified extra rules about what happens to quantum systems when you observe them. If you believe that an electron can really be in a superposition here and there, then you should believe that a cat really can be. in a superposition of awake and asleep and therefore you should really believe that you can be in a superposition of having observed the awake cat and having deeply observed your sleeping cat and the universe can be in a superposition of those two things and then It's just math to show that those two parts of the superposition have a life of their own and no longer affect each other, our separate worlds, that doesn't mean this is true.

If I had a series of 10 lectures instead of just this one, we could mention all the alternatives to There are some very, very bright people in this room who don't quite believe this picture, no matter how convincing and obviously correct it is, but the lesson What I want to make clear is not that many worlds are correct, nor even that they ever are. true, that's true, but that's not the lesson I'm trying to get at. The least I'm trying to understand is that it matters. I mean, I'm sure what dark matter and dark energy are are perfectly good mysteries, but this is a previous mystery.

This is a mystery that has been around longer and is a deeper mystery. It is not about identifying any substance, it is the basic rules by which nature operates. We must not abandon the project of trying to solve this. Finally, perhaps it is right that we do so. We don't understand quantum mechanics, but we shouldn't be proud of it, we should be ashamed and move on, which brings us to mystery number two, which will be related. Mystery number two is how gravity and spacetime come to exist, so again, this is a situation. where there are no mysterious things, this young Albert Einstein everyone shows you a photo of when he was not a young man, true, but when he was young someone combed his hair, he was an elegantly dressed guy, he didn't wear sweaters in a bad mood and he was inventing theories that changed the way we thought about the universe, special relativity appeared in 1905, that is the theory according to which space and time are part of four-dimensional space-time, the speed of light is an absolute limit, etc., general relativity appears ten times.

Years later, that is the theory that says that space-time has a life of its own, it is curved, it is deformed, it is dynamic and you and I feel that deformation like the force of gravity. Einstein says that the reason why the pointerlaser falls is because the earth has energy, that energy bends the space-time around it and all the laser pointers try to do is describe as straight a line as possible in this curved background of space-time. This theory, as I'm sure you know, has been extraordinarily successful. Arthur Eddington, another Cambridge boy. demonstrated that space-time Einstein's prediction that space-time can bend light can be tested experimentally recently we tested it with much greater precision by observing gravitational waves emitted by spiraling black holes that's the good news the good news It's just that this is wonderfully amazing theory and you know, I don't feel bad approaching it so quickly.

Einstein wrote some equations in 1915 and implicit in those equations, although I didn't know it at the time, were the ideas of black holes and spiral holes and gravitational waves and gravitational wave detectors and in 2016, a hundred years later, we detected it like that. The power of the combination of human imagination and nature's stubborn insistence on obeying the laws of physics allows something scribbled in a notebook in 1915 to appear in a billion-dollar experiment a hundred years later, which I have never forgotten. I'll get tired of being surprised, the bad news is that I just told you about quantum mechanics.

Einstein's theory is decidedly classical. He says there is something called space-time. how it evolves can be measured, etc. There are no wave functions, no entanglements, nothing like that, but in some sense, in a very clear sense, quantum mechanics is more fundamental than general relativity. General relativity is in the classical paradigm. Quantum mechanics discards the classical paradigm. How are we going to unite them? People have tried to do this without much success so far, so what I want to suggest is that there is a way forward, but first remember this motto: general relativity reduced to two words: geometry, energy, the geometry of space. time is influenced and in turn influences the energy in it so here is my suggestion, even though physicists were trained in quantum mechanics or very smart cookies and know a lot about what they are doing, they still grow up thinking in ways classic that they are not babies, right?

Sorry, there's a baby quantum physics book, but I don't think it really works. I don't think babies really think in terms of wave functions in the Schrodinger equation, most of us have classical intuition and so when we try to invent quantum theories for new phenomena we traditionally start with classical theories and quantize them, this I think it has the potential to hold us back because nature doesn't do that. Nature doesn't start with a classical theory in quantified nature is just quantum from the beginning so maybe it works sometimes and in fact that image I showed you with The Grand Equation of Core Theory is a series of success stories where you start. with classical versions of electromagnetism and quarks and so on and then you quantize it, but when it comes to gravity that seems to have failed us if you quantize general relativity in the simplest way, what I want to suggest and other approaches like strings don't work.

The theory essentially proposes different classical theories that you then quantify. What I want to suggest is that maybe we should take nature seriously, take quantum mechanics seriously, and instead of trying to quantify gravity, try to find gravity within quantum mechanics, not start with curved space. -time, but starting with wave functions and entanglement and asking if we can point out the curved spacetime that emerges from those quantum notions so that we can cheat when we play this game, okay, we'll cheat using things that the world knows from where physics works, forget about gravity, think about quantum field theory as the central theory that I showed you.

This is another wonderful feature of modern physics: the world is actually not made of particles, but of fields, so you know. The magnetic field here is the magnetic field in action around a magnet, but in fact the electrons and protons that make up matter are also vibrations in quantum fields, the real matter of the world according to quantum field theory are these fields that They permeate everything. of space and where you think there is nothing where you say oh, there are no particles or forces or anything here, it's just that the quantum fields are there, but they are what we call in their vacuum state, they had the least amount of energy.

It is possible that this view changes what we mean and how we think about empty space, so if you were in a particle view of nature you would think that there are particles and that there is some distance between them, but in the middle only There is space. that's all there was and space was the same everywhere, but if you think about the quantum field view, a particle is just a field that vibrates a little more than it would like and all the other regions of space as well They have quantum fields. they are both fields, so they are there and they are quantum, so they can become entangled, so different parts of empty space have a complicated and very interesting quantum structure.

There are people there, particle physicists whose life is dedicated to understanding the vacuum. Understanding empty space, most physicists want to have at least a particle or two in space before analyzing it, but there is a lot of richness there in empty space. and there is a characteristic of this entanglement, so if you look at the nearby regions of empty space, here I removed all the particles, the space is empty, but I say hello to one region, another nearby region and there are quantum fields vibrating in those regions and guess what? So I can calculate the entanglement between them and the answer is that if the regions are close, they are very intertwined. if the regions are very far away, they are not very entangled at all, so when I am in this context where I have space and geometry etc, there is a relationship between the geometry of the space and the entanglement, what I want to suggest is that we can reverse that. relationship when I mention that relationship I was talking in the language of space already existing, but if we want to see if space can arise from quantum mechanics, all our initial vocabulary should be words like wave functions, entanglement, etc., instead to say when two regions. they are close, they are highly entangled, let's say that when two quantum degrees of freedom are highly entangled, they are close, that's what we mean by close, close is just a colorful way of talking about the amount of entanglement between abstract quantum degrees of freedom , so if you give me a complete collection of abstract quantum degrees of freedom and the amount of entanglement between them there can be an emergent geometry here is a three-dimensional geometry here is Euclidean geometry here is a sphere depending on the quantum state of the system you are looking at next That in mind and I will also mention something a little more subtle: this relationship between entanglement and energy.

Remember when we had an empty space. I could tell you how intertwined different things were. Now let me start putting particles in there. Let me add energy to a region of space by putting particles in it, but putting particles in it in this language actually just makes the field vibrate more there and what that does is break the entanglement between that region and the regions around it. , so when you put particles and therefore energy into a region of space, the entanglement between that region and the regions surrounding it decreases, so I want to play exactly the same game.

Let me go the other way when I have a collection of quantum degrees of freedom. If I reduce the entanglement, I interpret it as putting energy into that region, therefore giving me a relationship between entanglement and energy, so what I have, I know this is very fast, but you can trust me, what I have then is that by using entanglement I can relate it to geometry. I can relate too. to energy and therefore by the rules of the arrows I can relate geometry and energy to each other, but of course that is exactly what time did and in fact when you attach the equations to this story What I'm telling you, you can somewhat very, very limited contexts derive Einstein's equation for general relativity from purely quantum mechanical starting points;

In other words, we don't know because we have a very long and complicated program ahead of us, but we have just started this very ambitious program of starting with pure quantum mechanics and seeing whether or not, we can achieve curved spacetime and relativity. They generally emerge from it rather than quantifying a classical theory, so this remains a mystery at the moment, but it might be a mystery we can really make progress on if we take quantum mechanics seriously. The final mystery we just discovered. This is more or less understanding space, why don't we try to understand time too?

My favorite part of time is the arrow of time. The arrow of time is simply the fact that the past and the future are different in important ways, so yes I showed them these photos of these young people and told them which was the youngest photo and which was the oldest photo. Everyone would know that certain things happen to us as we get older, certain things happen to society as it goes on when I give this talk in the United States, that's Elvis, I show up there and this is the signing of the Declaration of Independence, but I thought it would be rude to show it here, so the Magna Carta is what represents the past.

Well, this, of course, everyone knows what it is, I guess. this is from Charles Darwin's notebook this is the first appearance of the Tree of Life going up so he didn't put an arrow saying that time goes this way but this is it this is him saying that from one species you can evolve to many different species, that usually happens how the arrow of time works at work the best the reason everyone loves this image, of course, is that it says "I think at the top it's a warning to the rest of us, you know , don't be too confident, why." Is the past different from the future?

That's not exactly the mystery I want to highlight. I think we mainly know why the past is different from the future and the answer is something called entropy. Entropy is a way of characterizing how messy, random, disorganized, messy, a physicist is. system is and there is a law of nature the second law of thermodynamics that if you leave a system to its fate if you don't clean it you fix it you organize it the entropy will increase over time it is very natural it is a terrible terrible illustration of what I chose because clearly putting a frying pan is not a closed system, but the point is that it is easy to turn an unbroken egg into scrambled eggs, it is very difficult to go back, the same goes for your rooms here at university, you may tend to leave Messier to his own devices. luck they don't spontaneously clear themselves law of nature behind that and this is a mystery this law of nature was a mystery 150 years ago because the underlying laws Newton's laws or that equation that I showed you for the first time The central theory has no no difference between the past and the future built in and yet the evolution of entropy has a difference, it increases as we go from the past to the future, which is happening in a nutshell, the arrow of time has the same thing. type of origin like the space arrow what do I mean by that if you are up there with a new astronaut suit in orbit there is no space arrow all the different directions if you are floating with your astronaut suit they are the same from top to bottom ? left right forward back but here on earth there is clearly an arrow of space when I drop the laser pointer everyone knew it was going to go down right there is a difference between down and up here on earth that is an arrow but you don't have the impression that either The reason the laser pointer falls is that, since the aerosol might have thought that there is something fundamentally different between up and down, we know that it is a local contingent fact because when we live in the vicinity of a very influential object, the Earth, Yes, we are.

In the astronaut suit that arrow would not exist in the same way, what I am suggesting here is that the arrow of time has the same characteristic, it is not embedded in the laws of physics, the reflection of the fact that we live after a very influential time. event something you may have heard of called the Big Bang fourteen billion years ago the universe was very, very ordered, okay, it had very, very low entropy, no one knows why it's a real mystery, it's a mystery about it's worth writing books and things like that, again, people in this.

Having done this, we thought that if you start with a low entropy, Boltzmann, Maxwell, Gibbs and others explained why the entropy will tend to increase. We don't know why it was so low in the past, so the general fact that entropy increases I think is the correct explanation for the arrow, but there are still two very deep mysteries associated with it. One is forwhat started so small. This is clearly a job for cosmologists, physicists, maybe theologians, if you're oriented that way. The other mystery. How exactly does entropy explain all these other time symmetries? Why do the beetles look different in 1963 and 1969?

Are you really going to tell me it's due to entropy? Yes, I'm going to tell you, so there is a lot of work. to do here this is the reason why it is one of the mysteries, let me give you enough argument so that you can think that it is reasonable why you remember the past but not the future, why do you have pictures of the past but not the future? it's a joke Mitch Hedberg joke, he says why my friends always show me pictures of themselves saying this is what I looked like when I was younger, not all pictures show us what we looked like when we were younger right, we have no records or photographs of the future why not like this here is a record here is an egg this is what we use egg and cream and coffee these are the only options allowed to illustrate the increase in entropy you come across an egg lying on the sidewalk broken you say which one It's the egg's future Maybe someone will clean it Maybe a dog will come Maybe a storm will blow it away There are many possible futures for the egg What was probably in the egg's past Most people would say I bet there used to be an intact egg egg and someone dropped it, so somehow just with that little bit of information we have a lot more influence in saying what the past of the egg was than we do about the future of the egg why isn't it because of the fundamental laws of physics? the fundamental laws of physics, given what you know, there is a broken egg, predict a number of possible futures and an exactly equal number of possible pasts, the difference is that we also have this belief called the past hypothesis that the universe It started near the Big Bang at low entropy. state, so adding that gives you an asymmetry between the past and the future, there is no future hypothesis, there is no low entropy future state, at least not in the next ones, so knowing both the current state and the fact The fact that it comes from low entropy allows us to reconstruct that the most likely way to get a broken egg is to get an intact egg and break it, so this is a paradigm, this is a way to get at the many different ways that the past and future are different from each other.

Add in the fact that entropy is increasing from a low entropy past and then they argue about the visible macroscopic phenomenological everyday world, that's where the arrow of time is so vividly observable but there's a lot of work left to do there, you know there's no equations in this image, okay, there are many. ways in which the past is different from the future not only memory not only aging but also biological evolution what about cause and effect? If I move my arm and knock the glass off the table, you will naturally say something like the glass.

I fell because I moved my arm, no one has ever been tempted to say that I moved my arm because the glass was going to fall. There is a temporal asymmetry here. I think it's due to entropy, but I haven't proven it mathematically yet. I'm literally working. In a paper about that, I think it's demonstrable, but this is an incredibly rich area of ​​thought for all young physicists and protophysicists, the fact that increasing entropy is enormously important, literally, to our existence. Here is an image by a famous artist named Roger Penrose, who also dabbled in mathematics and physics.

The point of this image is that when you think about the Sun, the Sun is this bright orange thing that we see. in California that you may have never seen here, okay, but what do we get from the Sun? Well, you could say that we get energy from the Sun. That's true, but that's not exactly the point, global warming of the Earth module, the Earth radiates back to the universe to the same extent. energy it receives from the Sun, the difference is that for every photon of light we receive from the Sun we radiate twenty photons back to the universe, we get visible light from the Sun and we radiate infrared light back to the universe with an average of one twentieth of N each again let's go to the math what that means is that we have increased the entropy of that energy that we get from the Sun by a factor of 20 the reason why life is possible is because the Sun is a hot spot in a cold sky that reflects the fact that we live very, very far from the balance of existence if the Sun were not there and the whole sky was the temperature of the night sky, we would all freeze and life would cease, but if the Sun, if the whole sky were the temperature of the Sun we would get much more energy and we would all burn out and all life would cease.

What keeps us going, what keeps the fish rolling in the tank is the fact that there is all this low entropy energy that we can then use to live. and return to the universe in a highly degraded form more things that we would like to understand better we have a sketch in our brain of what is going on but nowhere near a complete quantitative understanding finally sometimes people say when they see things like cows and people and fish , universities and countries, they say, you know, all this seems very organized to me, some countries are more organized than others, we can unite over the lack of sensible organization in our countries lately, but where did that come from if the natural law of nature is entropy? and disordered only to increase, how in the world could increasing disorder lead to things as exquisitely ordered as people and societies?

So the answer again, we don't know the full answer, but we have the clue, the clue is that entropy can increase, here's the cream. and the coffee goes from low entropy to high entropy all separated low entropy all mixed high entropy but you notice that this is very simple all the creamers on the top all the coffee on the bottom this is also very simple they are all mixed it is in between the moment when the cream and the coffee are just starting to mix, where there is a state of medium entropy, is when things seem complicated, is when there are swirls and tendrils and fractal patterns within what literally happens if you take a photograph of this and you put it on your computer, this image and this image would take up fewer bytes on your computer than this one because there is more information to store in the image, which seems to be not an absolute rule, but a very common feature in the universe.

Entropy simply increases, but complexity does. it first goes up and then disappears as you get closer to equilibrium, so the point is that it is not despite the fact that entropy is increasing that we can get ordered things like you and me, but it is due to the fact that entropy is increasing. entropy is increasing, if entropy was not increasing, we would. not being able to use this resource that we have to live and think and do interesting things and this idea that complexity first comes and then goes away is not just cups of coffee, the universe shows exactly the same behavior in very early times, it was very, very simple and low entropy in recent times will be very, very simple again and high entropy is in the middle is right now cosmically speaking the universe is at its most complex and most interesting we live on the fun Friday night let's go crazy part of The history of the universe is something to be grateful for so to conclude my last slide is that there are many ways in which these mysteries could still be solved or remain to be solved.

I'm trying to highlight that there are some kind of famous mysteries that everyone talks about. What I have tried to do in the talk is to highlight some of the lesser known but in some sense deeper mysteries that we have and I firmly believe that focusing on these deep mysteries opens up the possibility of understanding a lot of things there is no quantum gravity of course here There is an artist's conception of a black hole, this much less interesting image is an attempt to apply quantum mechanics to the universe as a whole to think of the entire universe as a quantum computer in some sense in a very different sense, this is a possible starting point for life itself.

Life is not something anti-entropic. Life is something that uses increasing entropy to come into existence. Filling in the details will take time and then once life begins, how life works is also based on increasing entropy. We have some crazy ideas, myself and others, about why the early universe had low entropy, which might take us back to square one, trying to understand quantum mechanics and the emergence of spacetime, so I think you know the lesson. These mysteries are the good part of science. Mysteries exist at all levels of complexity and solubility. Physics is nice enough to be in a situation where some questions have been answered and yet the ones we still face are very, very deep.

Thank you so much. thank you, thank you very much, in fact, John, thank you for taking us on this guided journey from everyday core physics that seemed quite simple the way you laid it out to the mysteries of quantum entanglement, many worlds, space, time and gravity, time , entropy, etc. I mean, in some ways, you know, for all of us who are not physicists, it's really encouraging that even physicists don't have a common understanding of these difficult issues and I just note that Max Planck once commented that science can't solve the problem. ultimate mystery of nature. I wonder and I'm sure it will be a question for hours and hours of discussion tonight.

You know, could there ever be a common understanding of how far we can go in these mysteries? So next week we'll go back in time to the technology and astronomy of ancient Greece, when dr. Joe Marchant, journalist and also an intellectual on decoding the heavens, the Antikythera mechanism, and we hope to see you there, then you'll hear it, so finally, Sean, thank you very much for coming to Cambridge to talk to us this afternoon, a fascinating lecture . Thank you.