- Your most recent book titled Einstein's Unfinished Revolution. So I have to ask, what is Einstein's Unfinished Revolution and also how do we finish it? - Well that's something I've been trying to do my whole life. But Einstein's Unfinished Revolution is the twin revolutions which invented relativity theory, special and especially general relativity.

And quantum theory, which he was the first person to realize in 1905, there would have to be a radically different theory which somehow realized or resolved the paradox of the duality of particle and wave for photons. - And he was, I mean, people I think don't always associate Einstein with quantum mechanics 'cause I think his connection with it, founding as one of the founders, I would say, of quantum mechanics, he kind of put it in the closet.

Is it-- - Well he didn't believe that the quantum mechanics as it was developed in the late 19, middle late 1920s was completely correct. At first he didn't believe it at all. Then he was convinced that it's consistent but incomplete and that also is my view. It needs, for various reasons I can elucidate, to have additional degrees of freedom, particles, forces, something, to reach the stage where it gives a complete description of each phenomenon as I was saying, realism demands.

- So what aspect of quantum mechanics bothers you and Einstein the most? Is it some aspect of the wave function collapse discussions, the measurement problem? Is it the-- - The measurement problem. I'm not gonna speak for Einstein. (Lex laughing) The measurement problem basically and the fact that-- - What is the measurement problem, sorry?

- The basic formulation of quantum mechanics gives you two ways to evolve situations in time. One of them is explicitly when no observer is observing or no measurement is taking place and the other is when a measurement or an observation is taking place and they basically contradict each other.

But there's another reason why the revolution was incomplete which is we don't understand the relationship between these two parts. General relativity which became our best theory of space and time and gravitation and cosmology and quantum theory. - So for the most part, general relativity describes big things, quantum theory describes little things and that's the revolution that we found really powerful tools to describe big things and little things and it's unfinished because we have two totally separate things and we need to figure out how to connect them so it can describe everything.

- Right and we either do that if we believe quantum mechanics as understood now is correct by bringing general relativity or some extension of general relativity that describes gravity and so forth into the quantum domain that's called quantized theory of gravity or if you believe with Einstein that quantum mechanics needs to be completed and this is my view, then part of the job of finding the right completion or extension of quantum mechanics would be one that incorporated space time and gravity.

- So where do we begin? So first let me ask, perhaps you can give me a chance if I could ask you some just really basic questions. Well, they're not at all. The basic questions are the hardest but you mentioned space time. What is space time? - Space time, you talked about a construction.

So I believe that space time is an intellectual construction that we make of the events in the universe. I believe the events are real and the relationships between the events which cause which are real. But the idea that there's a four dimensional smooth geometry which has a metric and a connection and satisfies the equations that Einstein wrote, it's a good description to some scale.

It's a good approximation. It captures some of what's really going on in nature. But I don't believe it for a minute is fundamental. - So okay, we're gonna, allow me to linger on that. So the universe has events. Events cause other events. This is the idea of causality. Okay, so that's real.

In your view, it's real. - Or hypothesis or the theories that I have been working to develop make that assumption. - So space time, you said four dimensional space is kind of the location of things and time is whatever the heck time is. And you're saying that space time is, both space and time are emergent and not fundamental?

- No. - Sorry, before you correct me, what does it mean to be fundamental or emergent? - Fundamental means it's part of the description as far down as you go. We have this notion. - As real. - Yes. - As real as real it could be. - Yeah, so I think that time is fundamental and quote goes all the way down.

And space does not. And the combination of them we use in general relativity that we call space time also does not. - But what is time then? - I think that time, the activity of time is the continual creation of events from existing events. - So if there's no events, there's no time.

- Then there's not only no time, there's no nothing. - So-- - So I believe the universe has a history which goes to the past. I believe the future does not exist. There's a notion of a present and a notion of the past. And the past consists of, is a story about events that took place to our past.

- So you said the future doesn't exist. - Yes. - Could you say that again? Can you try to give me a chance to understand that one more time? So the events cause other events. What is this universe? 'Cause we'll talk about locality and non-locality. - Good. - 'Cause it's a crazy, I mean it's not crazy, it's a beautiful set of ideas that you propose.

But if causality is fundamental, I'd just like to understand it better. What is the past, what is the future, what is the flow of time, even the error of time in our universe, in your view? And maybe what's an event? Right? - Oh, an event is where something changes.

Or where to, it's hard to say because it's a primitive concept. An event is a moment of time within space. This is the view in general relativity, where two particles intersect in their paths or something changes in the path of a particle. Now we are postulating that there is, at the fundamental level, a notion, which is an elementary notion, so it doesn't have a definition in terms of other things, but it is something elementary happening.

- And it doesn't have a connection to energy or matter or exchange of any-- - It does have a connection to energy and matter. - So it's at that level. - Yes, it involves, and that's why the version of a theory of events that I've developed with Marina Cortes, and by the way, I wanna mention my collaborators because they've been at least as important in this work as I have.

It's Marina Cortes in all the works since about 2013, 2012, 2013 about causality, causal sets, and in the period before that, Roberto Manghibera-Anger, who is a philosopher and a professor of law. - And that's in your efforts together with your collaborators to finish the unfinished revolution. - Yes. - And focus on causality as a fundamental-- - Yes.

- As fundamental to physics. So-- - And there's certainly other people we've worked with, but those two people's thinking had a huge influence on my own thinking. - So in the way you describe causality, that's what you mean of time being fundamental, that causality is fundamental. - Yes. - And what does it mean for space to not be fundamental, to be emergent?

- That's very good. There's a level of description in which there are events, there are events create other events, but there's no space, they don't live in space. They have an order in which they caused each other, and that is part of the nature of time for us. But there is an emergent approximate description, and you asked me to define emergent, I didn't.

An emergent property is a property that arises at some level of complexity, larger than and more complex than the fundamental level, which requires some property to describe it, which is not directly explicable or derivable is the word I want, from the properties of the fundamental things. - And space is one of those things in a sufficiently complex universe, space, three-dimensional position of things emerged.

- Yes, and we have this, we saw how this happens in detail in some models, both computationally and analytically. - Okay, so connected to space is the idea of locality. - Yes. - So we've talked about realism. So I live in this world, I like sports, locality is a thing that you can affect things close to you and don't have an effect on things that are far away.

It's the thing that bothers me about gravity in general, or action at a distance, same thing that probably bothered Newton, or at least he said a little bit about it. Okay, so what do you think about locality? Is it just a construct? Is it us humans just like this idea and are connected to it because we exist and we need it for our survival, but it's not fundamental?

I mean, it seems crazy for it not to be a fundamental aspect of our reality. - It does. - Can you comfort me, sort of as a therapist? - I'm not a good therapist, but I'll do my best. There are several different definitions of locality when you come to talk about locality in physics.

In quantum field theory, which is a mixture of special relativity and quantum mechanics, there is a precise definition of locality. Field operators corresponding to events in space-time, which are space-like, separated, commute with each other as operators. - So in quantum mechanics, you think about the nature of reality as fields, and things that are close in a field have an impact on each other more than farther away.

- That's, yes. - That's very comforting. That makes sense. - So that's a property of quantum field theory, and it's well-tested. Unfortunately, there's another definition of local, which was expressed by Einstein and expressed more precisely by John Bell, which has been tested experimentally and found to fail. And this setup is you take two particles.

So one thing that's really weird about quantum mechanics is a property called entanglement. You can have two particles interact and then share a property without it being a property of either one of the two particles. And if you take such a system, and then you make a measurement on particle A, which is over here on my right side, and particle B, which is over here, somebody else makes a measurement on particle B, you can ask that whatever is the real reality of particle B, it not be affected by the choice the observer at particle A makes about what to measure.

Not the outcome, just the choice of the different things they might measure. And that's a notion of locality, because it assumes that these things are very far spaced, like separated, and it's gonna take a while for any information about the choice made by the people here at A to affect the reality at B.

But you make that assumption, that's called Bell locality. And you derive a certain inequality that some correlations, functions of correlations have to satisfy. And then you can test that pretty directly in experiments which create pairs of photons or other particles. And it's wrong by many sigma. - In experiment, it doesn't match.

So what does that mean? - That means that that definition of locality I stated is false. - The one that Einstein was playing with? - Yeah, and the one that I stated, that is, it's not true that whatever is real about particle B is unaffected by the choice that the observer makes as to what to measure in particle A, no matter how long they've been propagating at almost the speed of light, or the speed of light away from each other.

- It don't matter, so like the distance between them? - Well, it's been tested, of course. If you want to have hope for quantum mechanics being incomplete or wrong and corrected by something that changes this, it's been tested over a number of kilometers. I don't remember whether it's 25 kilometers or 100 and something kilometers.

- So in trying to solve the unsolved revolution, in trying to come up with a theory for everything, is causality fundamental and breaking away from locality? - Absolutely. - A crucial step. So in your book, essentially, those are the two things we really need to think about as a community, especially the physics community has to think about this.

I guess my question is, how do we solve, how do we finish the unfinished revolution? - Well, that's, I can only tell you what I'm trying to do, and what I have abandoned. - Yes, exactly. - As it's not working. - As one ant, smart ant in an ant colony.

Or maybe dumb, that's why, who knows? But anyway, my view of the, we've had some nice theories invented. There's a bunch of different ones, both relate to quantum mechanics, relate to quantum gravity. There's a lot to admire in many of these different approaches but to my understanding, none of them completely solve the problems that I care about.

And so we're in a situation which is either terrifying for a student or full of opportunity for the right student, in which we've got more than a dozen attempts, and I never thought, I don't think anybody anticipated it would work out this way, which worked partly, and then at some point, they have an issue that nobody can figure out how to go around, or how to solve.

And that's the situation we're in. My reaction to that is twofold. One of them is to try to bring people, we evolved into this unfortunate sociological situation in which there are communities around some of these approaches. And to borrow again a metaphor from Eric, they sit on top of hills in the landscape of theories and throw rocks at each other.

And as Eric says, we need two things. We need people to get off their hills and come down into the valleys and party and talk and become friendly and learn to say not no but, but yes and. Yes, your idea goes this far, but maybe if we put it together with my idea, we can go further.

- Yes. So in that spirit, I've talked several times with Sean Carroll, who's also written an excellent book recently. And he kind of, he plays around, is a big fan of the many worlds interpretation of quantum mechanics. So I'm a troublemaker, so let me ask, what's your sense of Sean and the idea of many worlds interpretation?

I've read many, the commentary back and forth. You guys are friendly, respect each other, but have a lot of fun debating. - I love Sean and he, no, I really, he's articulate and he's a great representative or ambassador of science to the public for different fields of science to each other.

He also, like I do, takes philosophy seriously. And unlike what I do in all cases, he's really done the homework. He's read a lot, he knows the people, he talks to them, he exposes his arguments to them. And I, there's this mysterious thing that we so often end up on the opposite sides of one of these issues.

- It's fun, though. - It's fun and I'd love to have a conversation about that, but I would want to include him. - I see, about many worlds. Well-- - No, I can tell you what I think about many worlds. - I'd love to, but actually on that, let me pause.

Sean has a podcast, you should definitely figure out how to talk to Sean. I actually told Sean I would love to hear you guys just going back and forth. So I hope you can make that happen eventually, you and Sean. - I won't tell you what it is, but there's something that Sean said to me in June of 2016 that changed my whole approach to a problem.

But I have to tell him first. - Yes, and that'll be great to tell him on his podcast. So. (laughs) - I can't invite myself to his podcast. - I told him, yeah, okay, we'll make it happen. So many worlds. - So anyway. - What's your view? Many worlds, we talked about non-locality.

Many worlds is also a very uncomfortable idea or beautiful, depending on your perspective. It's very nice in terms of, I mean, there's a realist aspect to it, I think you called it magical realism. - Yes. (laughs) - Just a beautiful line. But at the same time, it's very difficult to for our limited human minds to comprehend.

So what are your thoughts about it? - Let me start with the easy and obvious and then go to the scientific. - Okay. - It doesn't appeal to me. It doesn't answer the questions that I want answered. And it does so to such a strong case that when Roberto Manguibar-Angur and I began looking for principles, and I wanna come back and talk about the use of principles in science, 'cause that's the other thing I was gonna say, and I don't wanna lose that.

When we started looking for principles, we made our first principle, there is just one world, and it happens once. But so it's not helpful to my personal approach, to my personal agenda. But of course I'm part of a community. And my sense of the many worlds interpretation, I have thought a lot about it and struggled a lot with it, is the following.

First of all, there's Everett himself, there's what's in Everett. And there are several issues there connected with the derivation of the Born Rule, which is the rule that gives probabilities to events. And the reasons why there is a problem with probability is that I mentioned the two ways that physical systems can evolve.

The many worlds interpretation cuts off, one, the one having to do with measurement, and just has the other one, the Schrodinger evolution, which is this smooth evolution of the quantum state. But the notion of probability is only in the second rule, which we've thrown away. So where does probability come from?

You have to answer the question, because experimentalists use probabilities to check the theory. Now, at first sight you get very confused, 'cause there seems to be a real problem. Because in the many worlds interpretation, this talk about branches is not quite precise, but I'll use it. There's a branch in which everything that might happen does happen, with probability one in that branch.

You might think you could count the number of branches in which things do and don't happen, and get numbers that you can define as something like frequentist probabilities. And Everett did have an argument in that direction. But the argument gets very subtle when there are an infinite number of possibilities, as is the case in most quantum systems.

And my understanding, although I'm not as much of an expert as some other people, is that Everett's own proposal failed, did not work. There are then, but it doesn't stop there. There is an important idea that Everett didn't know about, which is decoherence, and it is a phenomenon that might be very much relevant.

And so a number of people post-Everett have tried to make versions of what you might call many worlds quantum mechanics. And this is a big area, and it's subtle, and it's not the kind of thing that I do well. So I consulted, that's why there's two chapters on this in the book I wrote, chapter 10, which is about Everett's version, and chapter 11.

There's a very good group of philosophers of physics in Oxford. Simon Saunders, David Wallace, Harvey Brown, and a number of others. And of course, there's David Deutsch, who is there. And those people have developed and put a lot of work into a very sophisticated set of ideas designed to come back and answer that question.

They have the flavor of, there are really no probabilities, we admit that, but imagine if the Everett story was true and you were living in that multiverse, how would you make bets? And so they use decision theory from the theory of probability and gambling and so forth to shape a story of how you would bet if you were inside an Everettian universe and you knew that.

And there is a debate among those experts as to whether they or somebody else has really succeeded. And when I checked in as I was finishing the book with some of those people, like Simon, who's a good friend of mine, and David Wallace, they told me that they weren't sure that any of them was yet correct.

So that's what I put in my book. Now, to add to that, Sean has his own approach to that problem in what's called self-referencing or self-locating observers. And it doesn't, I tried to read it and it didn't make sense to me, but I didn't study it hard, I didn't communicate with Sean, I didn't do the things that I would do, so I had nothing to say about it in the book.

And I don't know whether it's right or not. (mouse clicking) (mouse clicking) (mouse clicking) (mouse clicking) (mouse clicking) (mouse clicking)