Quantum Mechanics and General Relativity (Lee Smolin)

00:00:00.000 | - Your most recent book titled

00:00:04.280 | Einstein's Unfinished Revolution.

00:00:06.780 | So I have to ask, what is Einstein's Unfinished Revolution

00:00:11.240 | and also how do we finish it?

00:00:14.480 | - Well that's something I've been trying to do

00:00:16.600 | my whole life.

00:00:17.440 | But Einstein's Unfinished Revolution

00:00:20.240 | is the twin revolutions which invented relativity theory,

00:00:23.900 | special and especially general relativity.

00:00:27.200 | And quantum theory, which he was the first person

00:00:30.320 | to realize in 1905,

00:00:32.720 | there would have to be a radically different theory

00:00:36.960 | which somehow realized or resolved the paradox

00:00:40.360 | of the duality of particle and wave for photons.

00:00:43.360 | - And he was, I mean, people I think don't always

00:00:47.760 | associate Einstein with quantum mechanics

00:00:50.240 | 'cause I think his connection with it,

00:00:52.440 | founding as one of the founders, I would say,

00:00:56.480 | of quantum mechanics, he kind of put it in the closet.

00:00:59.520 | Is it--

00:01:00.360 | - Well he didn't believe that the quantum mechanics

00:01:03.040 | as it was developed in the late 19, middle late 1920s

00:01:07.200 | was completely correct.

00:01:08.600 | At first he didn't believe it at all.

00:01:11.200 | Then he was convinced that it's consistent but incomplete

00:01:14.200 | and that also is my view.

00:01:16.300 | It needs, for various reasons I can elucidate,

00:01:21.080 | to have additional degrees of freedom,

00:01:25.060 | particles, forces, something, to reach the stage

00:01:29.720 | where it gives a complete description of each phenomenon

00:01:33.000 | as I was saying, realism demands.

00:01:36.640 | - So what aspect of quantum mechanics

00:01:39.360 | bothers you and Einstein the most?

00:01:41.960 | Is it some aspect of the wave function collapse discussions,

00:01:46.960 | the measurement problem?

00:01:48.900 | Is it the--

00:01:50.700 | - The measurement problem.

00:01:53.160 | I'm not gonna speak for Einstein.

00:01:55.120 | (Lex laughing)

00:01:56.740 | The measurement problem basically and the fact that--

00:02:00.900 | - What is the measurement problem, sorry?

00:02:03.140 | - The basic formulation of quantum mechanics

00:02:05.900 | gives you two ways to evolve situations in time.

00:02:10.140 | One of them is explicitly when no observer is observing

00:02:13.940 | or no measurement is taking place

00:02:16.220 | and the other is when a measurement

00:02:17.740 | or an observation is taking place

00:02:19.500 | and they basically contradict each other.

00:02:22.980 | But there's another reason why the revolution

00:02:25.760 | was incomplete which is we don't understand

00:02:27.720 | the relationship between these two parts.

00:02:30.240 | General relativity which became our best theory

00:02:33.720 | of space and time and gravitation and cosmology

00:02:37.720 | and quantum theory.

00:02:40.600 | - So for the most part, general relativity

00:02:43.120 | describes big things, quantum theory describes

00:02:46.000 | little things and that's the revolution

00:02:48.720 | that we found really powerful tools

00:02:50.400 | to describe big things and little things

00:02:53.080 | and it's unfinished because we have two totally

00:02:57.000 | separate things and we need to figure out

00:02:59.120 | how to connect them so it can describe everything.

00:03:01.400 | - Right and we either do that if we believe

00:03:04.880 | quantum mechanics as understood now is correct

00:03:08.960 | by bringing general relativity or some extension

00:03:12.120 | of general relativity that describes gravity

00:03:14.520 | and so forth into the quantum domain

00:03:16.880 | that's called quantized theory of gravity

00:03:21.880 | or if you believe with Einstein that quantum mechanics

00:03:25.920 | needs to be completed and this is my view,

00:03:30.440 | then part of the job of finding the right completion

00:03:33.920 | or extension of quantum mechanics would be one

00:03:36.800 | that incorporated space time and gravity.

00:03:39.380 | - So where do we begin?

00:03:43.960 | So first let me ask, perhaps you can give me a chance

00:03:48.640 | if I could ask you some just really basic questions.

00:03:51.120 | Well, they're not at all.

00:03:52.560 | The basic questions are the hardest

00:03:53.900 | but you mentioned space time.

00:03:55.760 | What is space time?

00:03:57.920 | - Space time, you talked about a construction.

00:04:01.280 | So I believe that space time is an intellectual construction

00:04:05.440 | that we make of the events in the universe.

00:04:08.240 | I believe the events are real and the relationships

00:04:11.720 | between the events which cause which are real.

00:04:14.760 | But the idea that there's a four dimensional

00:04:19.520 | smooth geometry which has a metric and a connection

00:04:23.040 | and satisfies the equations that Einstein wrote,

00:04:26.360 | it's a good description to some scale.

00:04:29.420 | It's a good approximation.

00:04:30.840 | It captures some of what's really going on in nature.

00:04:34.120 | But I don't believe it for a minute is fundamental.

00:04:37.640 | - So okay, we're gonna, allow me to linger on that.

00:04:41.460 | So the universe has events.

00:04:43.800 | Events cause other events.

00:04:45.760 | This is the idea of causality.

00:04:48.080 | Okay, so that's real.

00:04:50.880 | In your view, it's real.

00:04:54.560 | - Or hypothesis or the theories that I have been

00:04:58.120 | working to develop make that assumption.

00:05:01.240 | - So space time, you said four dimensional space

00:05:04.360 | is kind of the location of things

00:05:06.240 | and time is whatever the heck time is.

00:05:11.280 | And you're saying that space time is,

00:05:16.280 | both space and time are emergent and not fundamental?

00:05:20.480 | - No.

00:05:21.320 | - Sorry, before you correct me,

00:05:23.500 | what does it mean to be fundamental or emergent?

00:05:27.820 | - Fundamental means it's part of the description

00:05:30.540 | as far down as you go.

00:05:32.220 | We have this notion. - As real.

00:05:33.460 | - Yes.

00:05:34.300 | - As real as real it could be.

00:05:36.300 | - Yeah, so I think that time is fundamental

00:05:39.220 | and quote goes all the way down.

00:05:41.500 | And space does not.

00:05:43.520 | And the combination of them we use in general relativity

00:05:46.720 | that we call space time also does not.

00:05:49.880 | - But what is time then?

00:05:53.160 | - I think that time, the activity of time

00:05:58.160 | is the continual creation of events from existing events.

00:06:03.440 | - So if there's no events, there's no time.

00:06:06.600 | - Then there's not only no time, there's no nothing.

00:06:10.120 | - So--

00:06:10.960 | - So I believe the universe has a history

00:06:14.840 | which goes to the past.

00:06:17.820 | I believe the future does not exist.

00:06:20.760 | There's a notion of a present and a notion of the past.

00:06:24.540 | And the past consists of, is a story about events

00:06:29.400 | that took place to our past.

00:06:32.680 | - So you said the future doesn't exist.

00:06:34.560 | - Yes.

00:06:36.400 | - Could you say that again?

00:06:39.240 | Can you try to give me a chance to understand that

00:06:43.160 | one more time?

00:06:44.320 | So the events cause other events.

00:06:47.520 | What is this universe?

00:06:48.440 | 'Cause we'll talk about locality and non-locality.

00:06:52.600 | - Good.

00:06:54.140 | - 'Cause it's a crazy, I mean it's not crazy,

00:06:56.160 | it's a beautiful set of ideas that you propose.

00:07:01.160 | But if causality is fundamental,

00:07:03.680 | I'd just like to understand it better.

00:07:06.140 | What is the past, what is the future,

00:07:09.020 | what is the flow of time, even the error of time

00:07:13.600 | in our universe, in your view?

00:07:15.700 | And maybe what's an event?

00:07:18.340 | Right?

00:07:20.080 | - Oh, an event is where something changes.

00:07:23.400 | Or where to, it's hard to say

00:07:28.400 | because it's a primitive concept.

00:07:31.240 | An event is a moment of time within space.

00:07:36.240 | This is the view in general relativity,

00:07:40.500 | where two particles intersect in their paths

00:07:44.480 | or something changes in the path of a particle.

00:07:48.760 | Now we are postulating that there is,

00:07:52.620 | at the fundamental level, a notion,

00:07:54.640 | which is an elementary notion,

00:07:56.260 | so it doesn't have a definition in terms of other things,

00:08:00.500 | but it is something elementary happening.

00:08:03.800 | - And it doesn't have a connection to energy or matter

00:08:06.400 | or exchange of any--

00:08:07.240 | - It does have a connection to energy and matter.

00:08:09.160 | - So it's at that level.

00:08:10.720 | - Yes, it involves, and that's why the version

00:08:14.040 | of a theory of events that I've developed

00:08:18.740 | with Marina Cortes, and by the way,

00:08:20.600 | I wanna mention my collaborators

00:08:23.080 | because they've been at least as important

00:08:24.960 | in this work as I have.

00:08:26.840 | It's Marina Cortes in all the works since about 2013,

00:08:31.840 | 2012, 2013 about causality, causal sets,

00:08:36.540 | and in the period before that, Roberto Manghibera-Anger,

00:08:40.260 | who is a philosopher and a professor of law.

00:08:43.820 | - And that's in your efforts together

00:08:45.940 | with your collaborators to finish the unfinished revolution.

00:08:49.140 | - Yes.

00:08:49.980 | - And focus on causality as a fundamental--

00:08:52.620 | - Yes.

00:08:53.620 | - As fundamental to physics.

00:08:55.580 | So--

00:08:57.100 | - And there's certainly other people we've worked with,

00:08:59.420 | but those two people's thinking

00:09:01.740 | had a huge influence on my own thinking.

00:09:03.960 | - So in the way you describe causality,

00:09:05.820 | that's what you mean of time being fundamental,

00:09:08.500 | that causality is fundamental.

00:09:10.580 | - Yes.

00:09:11.420 | - And what does it mean for space to not be fundamental,

00:09:16.380 | to be emergent?

00:09:17.220 | - That's very good.

00:09:18.040 | There's a level of description in which there are events,

00:09:21.660 | there are events create other events,

00:09:26.140 | but there's no space, they don't live in space.

00:09:29.780 | They have an order in which they caused each other,

00:09:33.060 | and that is part of the nature of time for us.

00:09:36.620 | But there is an emergent approximate description,

00:09:42.620 | and you asked me to define emergent, I didn't.

00:09:45.140 | An emergent property is a property

00:09:51.420 | that arises at some level of complexity,

00:09:55.500 | larger than and more complex than the fundamental level,

00:10:00.260 | which requires some property to describe it,

00:10:05.140 | which is not directly

00:10:06.980 | explicable or derivable is the word I want,

00:10:13.620 | from the properties of the fundamental things.

00:10:17.660 | - And space is one of those things

00:10:19.860 | in a sufficiently complex universe,

00:10:22.180 | space, three-dimensional position of things emerged.

00:10:27.180 | - Yes, and we have this, we saw how this happens in detail

00:10:31.980 | in some models, both computationally and analytically.

00:10:36.660 | - Okay, so connected to space is the idea of locality.

00:10:40.260 | - Yes.

00:10:41.100 | - So we've talked about realism.

00:10:44.260 | So I live in this world, I like sports,

00:10:49.860 | locality is a thing that you can affect things close to you

00:10:54.860 | and don't have an effect on things that are far away.

00:10:58.860 | It's the thing that bothers me about gravity in general,

00:11:01.900 | or action at a distance,

00:11:04.100 | same thing that probably bothered Newton,

00:11:06.420 | or at least he said a little bit about it.

00:11:09.120 | Okay, so what do you think about locality?

00:11:14.620 | Is it just a construct?

00:11:17.740 | Is it us humans just like this idea

00:11:20.700 | and are connected to it because we exist

00:11:23.100 | and we need it for our survival, but it's not fundamental?

00:11:26.140 | I mean, it seems crazy for it not to be

00:11:27.940 | a fundamental aspect of our reality.

00:11:30.940 | - It does.

00:11:31.780 | - Can you comfort me, sort of as a therapist?

00:11:34.880 | - I'm not a good therapist, but I'll do my best.

00:11:39.500 | There are several different definitions of locality

00:11:45.860 | when you come to talk about locality in physics.

00:11:48.660 | In quantum field theory, which is a mixture

00:11:54.540 | of special relativity and quantum mechanics,

00:11:58.660 | there is a precise definition of locality.

00:12:01.680 | Field operators corresponding to events in space-time,

00:12:06.600 | which are space-like, separated,

00:12:07.980 | commute with each other as operators.

00:12:10.500 | - So in quantum mechanics, you think about

00:12:13.260 | the nature of reality as fields,

00:12:15.220 | and things that are close in a field

00:12:17.820 | have an impact on each other more than farther away.

00:12:22.060 | - That's, yes.

00:12:23.260 | - That's very comforting.

00:12:24.620 | That makes sense.

00:12:25.460 | - So that's a property of quantum field theory,

00:12:27.420 | and it's well-tested.

00:12:29.380 | Unfortunately, there's another definition of local,

00:12:33.960 | which was expressed by Einstein

00:12:36.700 | and expressed more precisely by John Bell,

00:12:40.140 | which has been tested experimentally and found to fail.

00:12:44.780 | And this setup is you take two particles.

00:12:48.660 | So one thing that's really weird about quantum mechanics

00:12:53.040 | is a property called entanglement.

00:12:55.560 | You can have two particles interact

00:12:57.980 | and then share a property without it being a property

00:13:01.740 | of either one of the two particles.

00:13:04.500 | And if you take such a system,

00:13:07.540 | and then you make a measurement on particle A,

00:13:12.500 | which is over here on my right side,

00:13:15.140 | and particle B, which is over here,

00:13:17.300 | somebody else makes a measurement on particle B,

00:13:19.800 | you can ask that whatever is the real reality of particle B,

00:13:26.180 | it not be affected by the choice

00:13:31.020 | the observer at particle A makes about what to measure.

00:13:33.700 | Not the outcome, just the choice

00:13:36.140 | of the different things they might measure.

00:13:38.720 | And that's a notion of locality,

00:13:40.460 | because it assumes that these things are very far spaced,

00:13:43.540 | like separated, and it's gonna take a while

00:13:46.460 | for any information about the choice made

00:13:49.280 | by the people here at A to affect the reality at B.

00:13:53.140 | But you make that assumption, that's called Bell locality.

00:13:56.660 | And you derive a certain inequality

00:13:59.180 | that some correlations, functions of correlations

00:14:03.020 | have to satisfy.

00:14:05.080 | And then you can test that pretty directly

00:14:08.420 | in experiments which create pairs of photons

00:14:11.340 | or other particles.

00:14:13.060 | And it's wrong by many sigma.

00:14:15.860 | - In experiment, it doesn't match.

00:14:18.940 | So what does that mean?

00:14:20.900 | - That means that that definition

00:14:22.900 | of locality I stated is false.

00:14:25.500 | - The one that Einstein was playing with?

00:14:27.900 | - Yeah, and the one that I stated,

00:14:29.780 | that is, it's not true that whatever is real

00:14:33.820 | about particle B is unaffected by the choice

00:14:37.740 | that the observer makes as to what to measure

00:14:40.020 | in particle A, no matter how long they've been propagating

00:14:43.620 | at almost the speed of light,

00:14:45.020 | or the speed of light away from each other.

00:14:47.500 | - It don't matter, so like the distance between them?

00:14:51.020 | - Well, it's been tested, of course.

00:14:52.680 | If you want to have hope for quantum mechanics

00:14:56.580 | being incomplete or wrong and corrected

00:14:59.340 | by something that changes this,

00:15:01.500 | it's been tested over a number of kilometers.

00:15:04.920 | I don't remember whether it's 25 kilometers

00:15:08.460 | or 100 and something kilometers.

00:15:11.180 | - So in trying to solve the unsolved revolution,

00:15:16.180 | in trying to come up with a theory for everything,

00:15:19.220 | is causality fundamental

00:15:22.580 | and breaking away from locality?

00:15:26.580 | - Absolutely.

00:15:27.420 | - A crucial step.

00:15:29.460 | So in your book, essentially, those are the two things

00:15:33.060 | we really need to think about as a community,

00:15:36.860 | especially the physics community has to think about this.

00:15:41.140 | I guess my question is, how do we solve,

00:15:44.720 | how do we finish the unfinished revolution?

00:15:48.180 | - Well, that's, I can only tell you what I'm trying to do,

00:15:51.780 | and what I have abandoned.

00:15:53.400 | - Yes, exactly.

00:15:55.100 | - As it's not working.

00:15:56.900 | - As one ant, smart ant in an ant colony.

00:16:01.780 | Or maybe dumb, that's why, who knows?

00:16:04.980 | But anyway, my view of the,

00:16:09.300 | we've had some nice theories invented.

00:16:12.140 | There's a bunch of different ones,

00:16:16.260 | both relate to quantum mechanics,

00:16:18.660 | relate to quantum gravity.

00:16:20.940 | There's a lot to admire in many of these different approaches

00:16:25.820 | but to my understanding,

00:16:29.220 | none of them completely solve the problems

00:16:31.860 | that I care about.

00:16:33.120 | And so we're in a situation

00:16:37.060 | which is either terrifying for a student

00:16:40.860 | or full of opportunity for the right student,

00:16:44.000 | in which we've got more than a dozen attempts,

00:16:48.620 | and I never thought, I don't think anybody anticipated

00:16:51.180 | it would work out this way, which worked partly,

00:16:53.980 | and then at some point, they have an issue

00:16:56.660 | that nobody can figure out how to go around,

00:16:58.940 | or how to solve.

00:17:00.000 | And that's the situation we're in.

00:17:05.140 | My reaction to that is twofold.

00:17:08.580 | One of them is to try to bring people,

00:17:11.700 | we evolved into this unfortunate sociological situation

00:17:15.980 | in which there are communities

00:17:17.860 | around some of these approaches.

00:17:19.740 | And to borrow again a metaphor from Eric,

00:17:22.820 | they sit on top of hills in the landscape of theories

00:17:27.060 | and throw rocks at each other.

00:17:29.460 | And as Eric says, we need two things.

00:17:31.940 | We need people to get off their hills

00:17:34.500 | and come down into the valleys and party and talk

00:17:37.740 | and become friendly and learn to say

00:17:41.980 | not no but, but yes and.

00:17:46.900 | Yes, your idea goes this far,

00:17:48.740 | but maybe if we put it together with my idea,

00:17:50.860 | we can go further.

00:17:52.020 | - Yes.

00:17:54.260 | So in that spirit, I've talked several times

00:17:58.380 | with Sean Carroll, who's also written

00:18:01.780 | an excellent book recently.

00:18:03.300 | And he kind of, he plays around,

00:18:05.940 | is a big fan of the many worlds interpretation

00:18:08.060 | of quantum mechanics.

00:18:09.500 | So I'm a troublemaker, so let me ask,

00:18:13.500 | what's your sense of Sean

00:18:16.340 | and the idea of many worlds interpretation?

00:18:19.060 | I've read many, the commentary back and forth.

00:18:21.820 | You guys are friendly, respect each other,

00:18:24.860 | but have a lot of fun debating.

00:18:26.460 | - I love Sean and he, no, I really,

00:18:31.340 | he's articulate and he's a great representative

00:18:36.340 | or ambassador of science to the public

00:18:40.180 | for different fields of science to each other.

00:18:43.340 | He also, like I do, takes philosophy seriously.

00:18:48.500 | And unlike what I do in all cases,

00:18:53.420 | he's really done the homework.

00:18:55.500 | He's read a lot, he knows the people,

00:18:58.180 | he talks to them, he exposes his arguments to them.

00:19:03.180 | And I, there's this mysterious thing

00:19:06.580 | that we so often end up on the opposite sides

00:19:09.740 | of one of these issues.

00:19:10.860 | - It's fun, though.

00:19:12.100 | - It's fun and I'd love to have a conversation about that,

00:19:16.780 | but I would want to include him.

00:19:19.180 | - I see, about many worlds.

00:19:20.700 | Well-- - No, I can tell you

00:19:22.140 | what I think about many worlds.

00:19:23.100 | - I'd love to, but actually on that, let me pause.

00:19:25.260 | Sean has a podcast, you should definitely figure out

00:19:28.020 | how to talk to Sean.

00:19:29.780 | I actually told Sean I would love to hear you guys

00:19:32.100 | just going back and forth.

00:19:34.100 | So I hope you can make that happen eventually,

00:19:36.580 | you and Sean.

00:19:37.420 | - I won't tell you what it is,

00:19:38.640 | but there's something that Sean said to me

00:19:41.180 | in June of 2016 that changed my whole approach to a problem.

00:19:46.300 | But I have to tell him first.

00:19:48.300 | - Yes, and that'll be great to tell him on his podcast.

00:19:52.340 | So. (laughs)

00:19:53.180 | - I can't invite myself to his podcast.

00:19:55.340 | - I told him, yeah, okay, we'll make it happen.

00:19:57.600 | So many worlds. - So anyway.

00:19:59.780 | - What's your view?

00:20:01.580 | Many worlds, we talked about non-locality.

00:20:03.860 | Many worlds is also a very uncomfortable idea

00:20:08.860 | or beautiful, depending on your perspective.

00:20:12.460 | It's very nice in terms of,

00:20:17.460 | I mean, there's a realist aspect to it,

00:20:19.620 | I think you called it magical realism.

00:20:21.580 | - Yes. (laughs)

00:20:23.300 | - Just a beautiful line.

00:20:24.740 | But at the same time, it's very difficult

00:20:27.860 | to for our limited human minds to comprehend.

00:20:30.380 | So what are your thoughts about it?

00:20:32.240 | - Let me start with the easy and obvious

00:20:37.660 | and then go to the scientific.

00:20:39.740 | - Okay.

00:20:41.260 | - It doesn't appeal to me.

00:20:42.540 | It doesn't answer the questions that I want answered.

00:20:46.740 | And it does so to such a strong case

00:20:49.500 | that when Roberto Manguibar-Angur and I

00:20:52.300 | began looking for principles,

00:20:53.980 | and I wanna come back and talk about

00:20:55.420 | the use of principles in science,

00:20:57.620 | 'cause that's the other thing I was gonna say,

00:20:59.220 | and I don't wanna lose that.

00:21:00.620 | When we started looking for principles,

00:21:03.660 | we made our first principle, there is just one world,

00:21:06.700 | and it happens once.

00:21:08.900 | But so it's not helpful to my personal approach,

00:21:13.900 | to my personal agenda.

00:21:18.060 | But of course I'm part of a community.

00:21:20.340 | And my sense of the many worlds interpretation,

00:21:26.060 | I have thought a lot about it and struggled a lot with it,

00:21:29.840 | is the following.

00:21:31.900 | First of all, there's Everett himself,

00:21:36.860 | there's what's in Everett.

00:21:39.340 | And there are several issues there

00:21:42.620 | connected with the derivation of the Born Rule,

00:21:45.940 | which is the rule that gives probabilities to events.

00:21:49.980 | And the reasons why there is a problem with probability

00:21:54.460 | is that I mentioned the two ways

00:21:57.500 | that physical systems can evolve.

00:22:00.300 | The many worlds interpretation cuts off,

00:22:03.380 | one, the one having to do with measurement,

00:22:06.140 | and just has the other one, the Schrodinger evolution,

00:22:08.780 | which is this smooth evolution of the quantum state.

00:22:12.060 | But the notion of probability is only in the second rule,

00:22:17.740 | which we've thrown away.

00:22:19.860 | So where does probability come from?

00:22:21.620 | You have to answer the question,

00:22:23.980 | because experimentalists use probabilities

00:22:26.700 | to check the theory.

00:22:27.940 | Now, at first sight you get very confused,

00:22:34.060 | 'cause there seems to be a real problem.

00:22:36.540 | Because in the many worlds interpretation,

00:22:39.940 | this talk about branches is not quite precise,

00:22:42.500 | but I'll use it.

00:22:43.540 | There's a branch in which everything that might happen

00:22:48.260 | does happen, with probability one in that branch.

00:22:52.860 | You might think you could count the number of branches

00:22:56.460 | in which things do and don't happen,

00:22:59.260 | and get numbers that you can define

00:23:01.380 | as something like frequentist probabilities.

00:23:03.900 | And Everett did have an argument in that direction.

00:23:09.240 | But the argument gets very subtle

00:23:12.300 | when there are an infinite number of possibilities,

00:23:14.820 | as is the case in most quantum systems.

00:23:17.980 | And my understanding, although I'm not as much of an expert

00:23:22.260 | as some other people, is that Everett's own proposal

00:23:26.460 | failed, did not work.

00:23:29.740 | There are then, but it doesn't stop there.

00:23:34.460 | There is an important idea that Everett didn't know about,

00:23:37.580 | which is decoherence, and it is a phenomenon

00:23:40.300 | that might be very much relevant.

00:23:42.660 | And so a number of people post-Everett

00:23:48.180 | have tried to make versions of what you might call

00:23:51.340 | many worlds quantum mechanics.

00:23:53.300 | And this is a big area, and it's subtle,

00:23:58.740 | and it's not the kind of thing that I do well.

00:24:02.140 | So I consulted, that's why there's two chapters on this

00:24:05.260 | in the book I wrote, chapter 10,

00:24:07.300 | which is about Everett's version, and chapter 11.

00:24:09.800 | There's a very good group of philosophers

00:24:13.640 | of physics in Oxford.

00:24:15.160 | Simon Saunders, David Wallace, Harvey Brown,

00:24:20.140 | and a number of others.

00:24:21.820 | And of course, there's David Deutsch, who is there.

00:24:26.160 | And those people have developed and put a lot of work

00:24:30.560 | into a very sophisticated set of ideas

00:24:33.360 | designed to come back and answer that question.

00:24:36.640 | They have the flavor of, there are really no probabilities,

00:24:40.480 | we admit that, but imagine if the Everett story was true

00:24:44.800 | and you were living in that multiverse,

00:24:47.840 | how would you make bets?

00:24:50.040 | And so they use decision theory

00:24:53.720 | from the theory of probability and gambling and so forth

00:24:57.720 | to shape a story of how you would bet

00:25:02.120 | if you were inside an Everettian universe and you knew that.

00:25:05.600 | And there is a debate among those experts

00:25:11.000 | as to whether they or somebody else has really succeeded.

00:25:15.660 | And when I checked in as I was finishing the book

00:25:19.840 | with some of those people, like Simon,

00:25:21.840 | who's a good friend of mine, and David Wallace,

00:25:25.640 | they told me that they weren't sure

00:25:28.320 | that any of them was yet correct.

00:25:31.200 | So that's what I put in my book.

00:25:32.880 | Now, to add to that, Sean has his own approach

00:25:37.160 | to that problem in what's called self-referencing

00:25:39.760 | or self-locating observers.

00:25:42.480 | And it doesn't, I tried to read it

00:25:48.680 | and it didn't make sense to me,

00:25:51.540 | but I didn't study it hard,

00:25:53.160 | I didn't communicate with Sean,

00:25:54.520 | I didn't do the things that I would do,

00:25:56.020 | so I had nothing to say about it in the book.

00:25:58.360 | And I don't know whether it's right or not.

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