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Gravity Might Not Work The Way We Think — Transcript

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0:00I think we're going wrong at trying to
0:02quantize space time. On one hand, we
0:04have quantum theory [music] and on the
0:06other hand, we have general relativity
0:08and we know that both theories are
0:11[music] correct, but we also know of
0:13scales where we should expect them to
0:15interplay and we don't know how to
0:18describe [music]
0:18the physics at at those scales. The way
0:21to resolve the problems of quantum
0:23mechanics is by introducing the
0:25gravitational effects. How could we use
0:28quantum systems like Bose-Einstein
0:30[music]
0:30condensates to measure general
0:33relativistic effects?
0:40>> Thanks so much for, yeah, meeting me
0:42today, Eva. The first question I had was
0:44around well, quantum theory and general
0:47relativity. And you know, these are two
0:49big pillars of modern physics, yet
0:53for over a century, we've struggled to
0:55reconcile them. So,
0:57where you know, where do you think we're
0:58going wrong?
0:58>> I'm going to say things that are quite
1:00bold, I think. I [snorts] think we're
1:02going wrong at trying to quantize space
1:05time. Well, in quantum theory, we have
1:07that time is absolute, so all the clocks
1:10tick at the same rate. Doesn't matter
1:12the state of the observer and in general
1:14relativity, we have that time is
1:16relative, no? So, clocks at different
1:19observers tick at different rates.
1:21So, let let's think about a simple
1:23situation where you when
1:26the way you describe time becomes
1:28relevant. So, if I have a mass and I
1:30believe general relativity, the mass
1:32curves space time. And what happens is
1:34that at every point, a clock will tick
1:37at a different rate. But now I have a
1:40mass in a superposition of two different
1:42locations. So, choose any point in time
1:45in the space time, you will have a clock
1:48that ticks at two different rates at
1:50each point.
1:51So, this is incompatible with quantum
1:54mechanics because in quantum mechanics,
1:56the equation that we have, the
1:58Schrödinger equation, has one time.
2:00So, which one would you use? I like how
2:02Roger Penrose sometimes poses the the
2:05question, which is well, on one hand, we
2:07have quantum theory and its principles,
2:11and on the other hand, he had uh we have
2:13general relativity and for example, the
2:14equivalence principle. And then he sees
2:17that well, there's two routes.
2:19And one route would be to consider the
2:23principles of quantum mechanics as
2:25fundamental.
2:26And then we modify general relativity in
2:29order to unify them.
2:31And that would be like what modification
2:33of general relativity are we thinking
2:34about? Well, it basically would be
2:37quantizing space-time. But that has not
2:40really worked out as we hoped it would.
2:42Then the other approach would be to keep
2:45the foundations of general relativity as
2:49fundamental and then change quantum
2:52mechanics in order to unify them. And I
2:55agree with Roger that that's the right
2:57way to go. Uh
2:59because well, quantum mechanics already
3:01has uh big problems. We don't know what
3:04the quantum state is and we don't
3:07understand where probabilities come from
3:08and why the wave [clears throat]
3:10function collapses.
3:11And so, why would you take space-time
3:13and now try to quantize it where you
3:16already have a theory that's
3:17problematic? Whereas I think that the
3:19way to solve the problems of quantum
3:21mechanics and explain where
3:23probabilities come from is by
3:25incorporating uh gravity into the
3:28picture.
3:29And explaining how gravity produces the
3:32collapse of the wave function. But that
3:34really, it's very interesting because
3:35that depends a lot on what is your
3:38philosophical standing on, you know,
3:41kind of nature. If you think that there
3:44are many worlds, many universes, then
3:48that incompatibility is actually not
3:50there.
3:52But if you want to have like a single
3:54universe, then that's when you're forced
3:57to see that there are inconsistencies
3:59between the theories.
4:01>> Is Is it fair to say that most
4:03physicists are trying to tackle this big
4:06problem through
4:08quantizing gravity as as you say uh
4:11>> If you look at things like we have uh
4:13the electromagnetic uh force and and
4:16then the weak and the strong and if you
4:19see um gravity as you know the piece of
4:23the puzzle that should fit in, I guess
4:26perhaps that would be um
4:29a way that makes sense to get started
4:31with.
4:32But
4:33>> sort of seen as the anomaly perhaps and
4:35so it makes sense to try and squeeze
4:36that into the box of the other forces
4:39that that we know about.
4:40>> Well, I mean when you have two theories
4:42and um and they're both right in their
4:44own scale. So, if you go to large
4:46scales, general relativity um has been
4:49tested many times. Now, we have the
4:52amazing success of LIGO that discovered
4:54gravitational waves that were predicted
4:56by the theory.
4:58And we've done so many experiments also
5:02testing the theory and and it's uh the
5:05predictions uh and the comparison with
5:07experiments is super accurate. So, it
5:10works really well. So, we know that both
5:12theories are correct. But, we also know
5:15of scales where we should expect them to
5:17interplay and that's the problem where
5:21we don't know how to describe uh physics
5:23at at those scales.
5:25>> And that's where perhaps the opportunity
5:27is to really figure out what what is
5:29going on by exploring this this boundary
5:31between between the two.
5:32>> Yes.
5:33>> Can you just Yeah, pull back and tell me
5:35the different approach you're taking in
5:37answering this question?
5:39>> When I met Roger and we started to talk
5:41about um different ideas, he told me
5:45about um
5:47his work on testing
5:50um if gravity collapses the wave
5:52function.
5:53>> But what would it mean for gravity to
5:54collapse the the wave function? And what
5:56I maybe this pulls us back to this idea
5:58of the the measurement problem perhaps
6:00as well.
6:00>> Yes, it touches on on almost all the
6:03fundamental questions in the interface
6:05of quantum theory and and and and
6:07general relativity. Um and we I was
6:11telling you before that sometimes like
6:13your philosophical standing on what the
6:16universe is or um the fabric of reality
6:19is um makes you take either one route or
6:22or another.
6:23>> Mhm.
6:23>> Right? So um Roger was interested in
6:26gravity collapsing the wave function um
6:30because he's not very uh supportive of
6:33many worlds for example. So
6:35>> And many worlds is is the idea that
6:37yeah, there are many almost almost
6:38parallel universes and and that's a way
6:40to make sense of the measurement
6:42problem.
6:42>> Yes.
6:43>> Right?
6:43>> So let's go to the simple situation
6:45where you have one atom that can be
6:48either in the left or or the right. So
6:52um in quantum mechanics we describe that
6:54with um the wave function. But the
6:56problem that we have in quantum
6:58mechanics is that we don't know what the
7:00state of the quantum system is in terms
7:02of physics. We can write it down
7:05mathematically and it has a a very
7:07precise mathematical meaning. But how do
7:10we connect that to physics? So you would
7:13say to object uh uh
7:16um elements of reality
7:17>> Mhm.
7:18>> we haven't been able to do that uh for
7:20100 years. So then there are different
7:23um
7:24ways of uh trying to make sense of what
7:27the wave function is.
7:28>> Yeah, because it has all of these
7:30possibilities within it and we seem to
7:33exist in one reality.
7:35>> Well, because like so you you have the
7:37the wave function and then you need to
7:40add another ingredient out of your
7:42pocket, which is the measurement.
7:44>> Mhm.
7:44>> So, this is the Born rule. So, then you
7:47say, "Okay, I have this uh state,
7:49but when I measure, I collapse the wave
7:52function.
7:53>> Mhm.
7:53>> So, suddenly probabilities come out of
7:56the blue.
7:57>> Mhm.
7:57>> Nothing explains where these
7:58probabilities come from. They're just
8:01postulated by the Born rule. So, these
8:04are two different things that we don't
8:06understand. We don't understand what is
8:09the quantum state exactly, and we don't
8:11understand why we need to put in the
8:13probabilities in this way.
8:15>> Mhm. There seems to be some inherent
8:18element of chance in nature, and that's
8:21something which you don't know what you
8:22know, it's hard to explain what where
8:24where it where it comes from, and some
8:26people really dislike that.
8:28>> Yes, and I I spend a lot of time talking
8:31to my students about what is uh we know
8:34so far about quantum mechanics. I love
8:36to discuss these issues with them, and
8:39they get very confused about uh
8:41probabilities because we're very used to
8:44talking about probabilities in in life,
8:48right? I mean, even if you're talking
8:50anything, we talk about uh uh
8:52probabilities, but they're not really
8:54fundamental. So, let me explain what I
8:57mean by that. Uh let's say you have a
8:59coin, and you toss the coin, and then
9:01you have 50/50 probability that it's
9:03heads or tails. This is not like a real
9:05probability. Why? Because if I hit the
9:08coin, imagine I have a supercomputer.
9:10Now, we have like, you know,
9:12>> [laughter]
9:13>> and and I could tell you exactly what
9:16happened with the way I hit it, um the
9:19state of the molecules in the air. I had
9:22control of everything. This is not a
9:24quantum system. This is dictated by
9:27classical physics.
9:28So, um it looks probabilistic because
9:31I'm ignoring all the details.
9:33>> Mhm.
9:33>> So, if I ignore all the details,
9:36it's 50/50. But, if we took all the
9:38details into account,
9:41then there would be no mystery. It would
9:43I could tell you exactly it's going to
9:45be heads because of
9:47all the equations and and so on.
9:51What happens with quantum mechanics is
9:52that we have probabilities that are not
9:56obtained by a deeper understanding of
10:00the system, the details.
10:01>> Yeah, no matter how well you know it,
10:02you're not going to be able to explain
10:04Yeah, the origin of the system. So it
10:06seems to be some intrinsic part of
10:09reality, but that again is part
10:11>> It looks like there is a part of reality
10:12that is missing.
10:14Or at least in our account of it.
10:16>> So So then how did Penrose, for example,
10:19try and Yeah, what you know, what was
10:21his take on that and the best way to try
10:23and make sense of that?
10:24>> He wants also to explain why in the
10:26classical world we don't see these
10:29superpositions. We do see them in the in
10:32microscopic scales because you can take
10:35an atom and hit it with a laser,
10:39put the state of the atom to go left and
10:41right in a quantum superposition, then
10:44reflect it back with the some fields,
10:46and this is called an interferometer,
10:48and you can see measurements that show a
10:51wave type behavior. So we call this an
10:54interference pattern. So we can see an
10:56interference pattern if we make an atom
11:00go through a superposition of two
11:02different, say, trajectories.
11:05We are sure about this because we've
11:07made many experiments that confirm this.
11:10But in the classical world we just see
11:12this glass either here or there.
11:14So it
11:15So the idea is that and it comes also
11:18from Lajos Diósi, which two years before
11:23also independently proposed this this
11:26idea and that mass is is relevant here.
11:29In the microscopic world we have very
11:31little mass.
11:32And in the classical world, we have a
11:35lot more mass. So, I think it makes a
11:37lot of sense to
11:39uh propose that it's actually you know,
11:41mass is playing a role. But, how is mass
11:44playing a role? Well, through the
11:46gravit- the the gravitational effects
11:48that it produces.
11:49>> Mhm. So, so we sort of know that, you
11:51know, gravity works very well in this
11:54more macroscopic realm realm rather, and
11:57quantum theory is working very well in
11:59yeah, with very small things. But, then
12:01if you're trying to make sense of what
12:03happens when in the in this middle
12:04ground here, then mass would seem to be
12:06the
12:07>> Yes.
12:07>> the way to enable that.
12:09>> No, we know the classical world works
12:11cuz we can make many experiments here
12:13and Newton's uh laws is going are going
12:16to explain very well. And Maxwell's
12:19equations are going to explain very well
12:20what what happens. And if we go to the
12:23microscopic world, we can do all these
12:25atom interferometers, and and that's
12:27going to the predictions that the theory
12:30makes are going to be tested, and
12:31they're going to be correct.
12:33The interface is a problem, and how do
12:36you go from a world where you have
12:38superpositions to another world where
12:40things only are in one place at the
12:42time. So, I think Roger found that we
12:44need to learn to understand how gravity
12:47affects quantum mechanics,
12:49>> Mhm.
12:49>> not how like let's say we change general
12:53relativity
12:55to unify it with gravity. So, he calls
12:57it more like gravitize
12:59um a quantum theory, which is
13:03uh learning how gravity affects quantum
13:06mechanics.
13:07>> So, it's not sort of saying that one is
13:09necessarily more right than the other.
13:10You're just saying that they have they
13:12have to find a way for them to to yeah,
13:14align it at at some point.
13:17>> I think in this middle place, you have
13:19to change them somehow both.
13:21>> Mhm. Okay.
13:22>> But, but here you have GR.
13:23>> Yeah.
13:24>> And then here you have quantum
13:25mechanics, but there is a place where
13:28scales at which both
13:31interplay.
13:32And
13:33you you you have to change both somehow
13:37at the scales, but you need to have a
13:39theory, the new theory has to
13:42reduce to these two theories at the
13:44right scales because we know those are
13:46right.
13:46>> Um um what do you think is is
13:48essentially happening there at that
13:50scale?
13:50>> I think that the definitely gravity is
13:53the culprit.
13:55I think gravity
13:57>> this collapse of of the superposition?
13:59>> Yes. Yes. And also what I like about it
14:03is that when you get, let's say again,
14:05the atom in a superposition of left and
14:08right.
14:08>> Mhm.
14:09>> People that work in many worlds, how
14:11they would understand this is that they
14:13would say, "Okay, there is a universe
14:16where the particle is on the right, and
14:19you and I see the particle on the right.
14:22So we're entangled in that
14:24state." And there's another branch of
14:26the universe where the particle is on
14:29the left, and you and I agree, and if we
14:32measure that it's on the left. So that's
14:34how you get these two universes.
14:36>> Mhm.
14:37>> But what collapse would
14:40would say is that, "Well, but if the
14:42particles are big enough, then this will
14:45collapse the wave function into one
14:48possibility, either left or right. Then
14:51we have one single universe."
14:53>> Mhm.
14:53>> And I like the idea of one single
14:56universe. And this is why I was saying
14:58like your philosophical standing has
15:01a big effect in the physics that you do
15:04because if you wake up and you say, "I
15:06want a single universe." You know,
15:09>> [laughter]
15:09>> you will try to build theories and that
15:13that explain. You might be wrong, you
15:15know, and and that's what that's why
15:17it's good to have other people that say,
15:19"No, I I'm super happy with the idea of
15:22having
15:23many worlds.
15:24>> So, you're sort of accepting that
15:26you have this singular quantum world,
15:28but within that world, you can have
15:30these strange, seemingly strange to us,
15:32things happening where you have,
15:34um, you know, particles existing in many
15:37states, but you would then also have
15:40your single general relativity universe,
15:43and you find a way to link them
15:44together.
15:45>> Yes.
15:46>> Rather than saying, "Oh, because of this
15:48weirdness in the quantum world, that
15:50means we have to invoke many, many
15:52realities to try and make sense of
15:54this."
15:55>> When you have, um,
15:57things are not very massive,
15:58>> Mhm.
15:59>> you can have them in these quantum
16:00superpositions, where we know you you
16:02have them. But, when they become more
16:04massive, then these superpositions are
16:07unstable, and they collapse into one
16:10classical outcome, and that explains why
16:13we don't see superpositions in the
16:15classical world. For me, this idea makes
16:17a lot of sense.
16:18>> How do you propose that we test who is
16:21right, the the men the many worlds camp,
16:24or the gravitizing quantum theory camp?
16:27>> I I don't know how you test if, um, many
16:31many worlds, um,
16:34but I do know that if you test that
16:36gravity collapses a wave function, this
16:39would, um, imply that there are not many
16:42worlds. So, you would falsify it in that
16:44sense. Now, but let's say that nature is
16:46not like that.
16:47>> Yeah.
16:48>> And that, uh, actually there are many
16:50worlds. For every single possibility,
16:54you have all these different, uh,
16:55branches.
16:56>> Mhm.
16:57>> So, how would you test that you have,
17:00indeed, like an infinite number of yous
17:04>> Mhm.
17:05>> living different experiences at the
17:07time, because what many worlds also says
17:09is that quantum mechanics doesn't only
17:11apply to the world where you have little
17:13mass and microscopic scales and so on,
17:16but applies universally.
17:18>> Mhm.
17:18>> So, it applies also to scales where you
17:20and I live, and the consequences of that
17:23it would be that there are an infinite
17:25number of us living different
17:27experiences.
17:28>> Mhm.
17:28>> And although it sounds super cool and a
17:30lot like science fiction, and now being
17:33serious, I don't like it. I don't like
17:35the idea of a a universe that's like
17:38that.
17:39>> a lot of baggage there. Yeah,
17:40potentially.
17:41>> So, I I mean, I wanted to just go back
17:43to what ultimately led
17:45>> you to study or physics, yeah, become a
17:47a physicist.
17:48>> I started to think that I would like to
17:50understand how from the interactions of
17:53atoms
17:54uh we got thought.
17:57And I started to ask people around me,
18:00what do I have to study to understand
18:02how thought emerges from the
18:04interactions of atoms? And everybody was
18:07like, what?
18:08>> Yeah.
18:08>> And nobody could tell me what to do, and
18:11eventually I gave up, and I thought,
18:12well, physics is also interesting.
18:14[laughter]
18:15>> So, then you studied physics, and then
18:18you when you started to to specialize,
18:21how did that then lead you towards these
18:23uh questions around how to connect
18:26quantum theory to gravity?
18:28>> I studied in uh UNAM in Mexico.
18:30>> Mhm.
18:31>> And I
18:32uh was very inspired by a teacher of
18:34mine. She was an astronomer,
18:36>> Mhm.
18:36>> uh Deborah Dultzin. And she was working
18:38with Seyfert galaxies, astrophysics, and
18:40I ended up working with her, and my
18:42first paper is on Seyfert galaxies and
18:45black holes in the middle of galaxies
18:47and and so on. But then I I felt that um
18:51I was not really going into the
18:54fundamental questions. So, I then turned
18:57into quantum optics. I was um uh in the
19:01cafeteria and a friend of mine told me
19:03about the experiments that Anton
19:05Zeilinger had just done or Haroche and
19:09Wineland trapping single atoms, uh and
19:12and I thought, this is going to get very
19:14interesting because these experiments
19:17are going to give us answers to some of
19:20these questions.
19:21>> You've also spoken about
19:23you know, feeling almost lost in maths
19:26at some point in your
19:27career. Was that you know, what were
19:29what was happening there and then how
19:31did you find your way back to trying to
19:34you know, make sense of all of this?
19:36>> So, at the moment you you really
19:38sometimes some things you're not really
19:40aware of, but when you look back, that's
19:42when sometimes everything makes sense.
19:45And what I can see that the approach
19:47that is important for me has always been
19:51that physics has
19:53the possibility of being tested in the
19:55experiment. This is what really like I
19:58find
19:59very exciting. And if you can't propose
20:02an experiment or
20:04you maybe you can, but you know, it's
20:07really
20:0925 years plus
20:12for one to maybe be able to test things
20:16and so on, then I lose interest because
20:19what really motivates me is that we can
20:21propose an experiment and test if these
20:23ideas are right or not, hopefully in my
20:25lifetime.
20:26>> [laughter]
20:27>> I started to do things that I wasn't
20:29aware they were quite new of applying
20:32the techniques that I had learned from
20:33quantum information, measures of
20:35entanglement, classical correlations
20:38versus quantum correlations
20:40to situations in black holes or the
20:43expanding universe. This is what now
20:46people call relativistic quantum
20:48information. But now these beautiful
20:50questions on
20:52the interaction between space-time and
20:54quantum fields, I can then combine them.
20:57I wasn't aware that was quite like a new
21:00thing to do.
21:01>> Yeah, you just saw this possibility for
21:03this connection.
21:04>> What happens with quantum optics is that
21:06it's a test bed to
21:09learn about the basic interactions
21:11between
21:12matter and fields
21:14treating them both quantum mechanically,
21:17but it's also back in the day it was
21:19like year 2000. People were very
21:22interested in can we compute with
21:24quantum systems so that are the
21:25beginnings of quantum computing of
21:27quantum cryptography of teleportation.
21:31So how can we learn to manipulate the
21:35systems using quantum optics that was
21:38quite interesting.
21:39>> But then you saw that actually some of
21:41these similar ideas might have
21:44application in this other area.
21:47>> In in gravity, yes.
21:48>> So yeah, tell me about what questions do
21:50you think we might be able to to ask
21:52through combining these two?
21:53>> Yes, I was working on this sort of
21:56problems and I learned how to calculate
21:58entanglement and other other aspects of
22:01these interactions.
22:03And then I was at the Perimeter
22:05Institute I went into a talk of a
22:07quantum field theory in curved space
22:09time and people were looking at things
22:11like the expansion if you consider a
22:14field living on space time and the space
22:17time expands then that creates particles
22:22for example or what happens with a black
22:24hole if you describe a field in the
22:28space time of a black hole from
22:29different observers and so on so then I
22:33noticed that there was an analog between
22:37the space time and in quantum optics a
22:40crystal.
22:40>> What do you mean in an analog they they
22:43just had similar characteristics?
22:45>> Mathematically the mathematics
22:46underpinning the two type of problems
22:48were exactly the same. So let's say in
22:50quantum optics you have a crystal could
22:53be like a crystal like a beam splitter
22:56and then you have a quantum field that
22:58arrives to it. So let's say when the
23:01field is arriving it's in the vacuum
23:03state or in we have one photon or some
23:06quantum state. And then inside the
23:08crystal is very difficult to describe
23:10what's happening to the field because
23:12it's very complicated, but then when the
23:15light comes out, you can all describe it
23:18mathematically well. So then there are
23:20all these techniques called Bogoliubov
23:22transformations that allow you to
23:23describe what's coming in from what's
23:25going out.
23:27And that depends on the type of crystal
23:29you have. You can also have uh something
23:31called a non-linear crystal that
23:33produces pairs like in a black hole.
23:36So I started to see that, you know, the
23:38way that you could treat in quantum
23:39optics the interaction of light with a
23:42crystal was actually mathematically the
23:44same as
23:46um having
23:47um some uh quantum fields in in
23:51space-time. So you also have uh
23:53Bogoliubov transformations. So you think
23:55about the past infinity having a flat
23:58space-time, let's say. And some the
24:01state could be in the vacuum, then the
24:03universe expands and then you can
24:05describe what happens in the future with
24:08a Bogoliubov transformation.
24:09>> And that's that transformation it is an
24:11attempt to try and apply quantum theory
24:13to general relativity, but not in a in a
24:16precise way. Is that is that fair to
24:18say?
24:19>> Well, we call it like you could say like
24:21it's a semi-classical approach.
24:23It allows you to answer a lot of
24:25questions. That's why I got very
24:26interested in it.
24:27>> You're sort of seeing how these two
24:28things maybe rub up against each other,
24:30but it's not it's not like a
24:32perfect theory. And then you saw there
24:35was this mathematics that was used in
24:37this very testable experimental field,
24:40and that might be able to help to find a
24:42way to like yeah.
24:43>> Yeah, so I I noticed that I had learned
24:45things that I could apply now to these
24:47questions about black holes.
24:49I got very excited about that. I made
24:51some progress with that. Actually, my
24:53most cited work is the work that I did
24:56with this sort of ideas. But then I
24:58decided to leave the field.
25:00>> You changed your mind.
25:02>> So I opened it and then I thought I'm
25:04going to leave the field because I I
25:06noticed that you could not test things.
25:09So when I first it was like a
25:12learning curve. And I tried to like
25:17learn things about what happens with
25:20entanglement from the perspective of
25:22different observers.
25:23For example.
25:25Um but then when I wanted to
25:28propose an experiment, I started to
25:30notice, oh my god, none of these That's
25:33when I was lost in maths.
25:34>> Hm, okay.
25:35>> None of these
25:37effects that I am finding, that I'm
25:40predicting, are there, are really like
25:43anywhere close to being tested in the
25:45experiment.
25:46>> Hm.
25:46>> And that's when I decided to
25:48to leave, let's say, the field that I
25:51helped
25:52develop. I I thought this I I don't I I
25:55don't want to move in that direction.
25:58>> Hm. So then what has been your main then
26:00con- concern since then?
26:02>> So evolving in parallel to quantum
26:04information, there was another field
26:06called quantum metrology.
26:08>> Okay.
26:08>> So quantum metrology is again about
26:10using quantum systems, but now to
26:13measure parameters that come into your
26:17Hamiltonian, let's say, in your
26:18interactions, like frequencies,
26:21time, gravitational field strength,
26:24magnetic field strength. So they're not
26:26observables, they're not like quantized
26:30quantities, but they're parameters that
26:33come up in in your system.
26:34>> So the this is a a new way of sort of
26:37Yeah, testing quantum systems very
26:39precisely in ways that go go beyond
26:42Yeah, measuring that go beyond, say,
26:44like quantum optics, if that makes
26:45sense. So this new field of quantum
26:48metrology allowed you to start uh
26:51questions around
26:53where quantum theory and general
26:55relativity might overlap. How were you
26:57able to to use these different
27:00techniques to really start asking this
27:02core question that you were interested
27:04in?
27:04>> Yeah, so I thought there was a lot to
27:06learn in the interplay of
27:09uh general relativity and quantum
27:11mechanics
27:12through quantum field theory in curved
27:13spacetime, no? Which basically assumes
27:16like the spacetime is classical, it's
27:19not a quantum thing. And then you study
27:22how fields behave when you know they're
27:26like lying on that as a spacetime.
27:29>> So you're able to kind of see how that
27:30that sort of work start start to like
27:32prod a bit how how they might work
27:34together.
27:35>> So it allows you like that say it would
27:37be a theory that allows you to take the
27:38first steps in order to understand how
27:41they interact. But the sort of things I
27:44was doing could not be tested in the
27:46experiment. And then I thought
27:49well, maybe we can apply now all the new
27:52techniques of quantum metrology and try
27:55to think about how could we use quantum
27:57systems
27:59like Bose-Einstein condensates, that we
28:01can talk about that what they are later
28:04to measure general relativistic effects.
28:07So for example, gravitational waves
28:10or dark energy, dark matter. Um so and
28:15and I've noticed that that was also not
28:17something that was there and that we had
28:21the chance in my group to
28:23make some first steps in that direction.
28:25And then I I I I moved in in that
28:28direction.
28:28>> And you mentioned there gravitational
28:30waves, dark matter, dark energy. That
28:32these are phenomena that we observe in
28:34some sense out in the universe, but we
28:36don't maybe have a good understanding
28:38necessarily of what what all of them
28:39are. So this allows you to maybe think
28:41about whether they might have a quantum
28:44origin and to test that using these
28:46quantum experiments.
28:48>> Yes.
28:49>> And and you mentioned Bose-Einstein
28:51condensates and that it's been something
28:52which comes up quite a lot in the
28:54research that you do. So, what you know,
28:57what are they and why does that allow
28:59you to ask these questions?
29:01>> Yes.
29:02Yes, I I think it's a
29:03beautiful system. So, when we learn
29:06quantum mechanics in the second year,
29:08one of the first sort of um
29:11uh problems that we solve with quantum
29:13physics is thinking about having a a
29:16potential. So, you can imagine like an
29:18electromagnetic field forming some sort
29:21of um cup or something like this that
29:23could [snorts] hold atoms.
29:25And and so, you study like how an atom
29:29behaves inside this uh potential. So,
29:31it's trapped in there.
29:33And if you cool down the atom, so you
29:35take all its energy as much as you can,
29:38then it goes into what we call the
29:41ground state. So, the state with lowest
29:43energy. But the beautiful thing about
29:46this system is that the atom becomes it
29:49has a almost no energy. You can never
29:51have zero energy, but it has very little
29:54energy, but the atom becomes delocalized
29:58in the whole potential.
29:59>> What what does that mean?
30:00>> Yeah. So,
30:01>> [laughter]
30:03>> So, in quantum mechanics, you Well, in
30:05classical physics, you can have the
30:06glass being either here or there.
30:10But we can have an atom being in a
30:12superposition of being here and there
30:15in in the quantum states. Well, now we
30:17go into foundations of quantum mechanics
30:19because for 100 years, we haven't been
30:21able to understand what that is. But I'm
30:23not going to go in that direction now.
30:25Just let's assume that you can have an
30:28atom in two different places at the same
30:30time. But now in this potential, when
30:33you cool down the atoms to the ground
30:35state, it can be everywhere
30:38>> Okay.
30:39>> Okay, so it's not just this state and
30:41that state, but it's
30:43>> And that what I I I mean like it's
30:45delocalized within the potential.
30:48>> Do these
30:49Bose-Einstein condensates behave in a
30:51very strange way because of that?
30:54>> Well, that's where just talking about
30:55one atom that is delocalized. Now think
30:58about putting I don't know 10 to the
30:59five, 10 to the six atoms
31:02in in the ground state. So atoms behave
31:06under some circumstances like bosons.
31:09That just means let's say to put it easy
31:12that they can occupy the same
31:14uh quantum state.
31:16So you can cool them all down to the
31:19ground state of that uh potential.
31:22And that is what you would call sort of
31:23the the bulk of the BEC. So imagine it a
31:27little bit like the ocean maybe.
31:29But then um the atoms because they're in
31:32a in a potential, they always interact.
31:35You cannot get rid of the interaction.
31:37So you that's one of the reasons why you
31:38can't get absolute zero.
31:41So when they interact, they create like
31:42little waves. So you can imagine the
31:45system as this ocean of everything in
31:48the ground state, but then little
31:50quantum excitations that are waves
31:53living on top of it.
31:55So what I found with my team is that the
31:57waves are relativistic quantum field.
32:01They're super sensitive to anything
32:03gravitational. So if you have a
32:05gravitational wave,
32:07those excitations, if you prepare them
32:09in squeezed states or some special uh
32:12they will be they will change due to
32:14what the space-time is doing.
32:16>> So you you have these systems that are
32:20you know hundreds of thousands if not
32:21millions of of the of these particles
32:24because of this special quantum state
32:26they're in, and because you have so many
32:29particles, then they're also going to be
32:32affected by gravity. Is that the kind of
32:34idea?
32:35>> Yes.
32:35>> So you then have this quantum field as
32:37you say, but it also is susceptible to
32:41gravity and and so you're able to start
32:43seeing how they yeah how how one affects
32:47the other. Is that is that broadly kind
32:48of right or wrong?
32:49>> we saw that it was very sensitive.
32:51Exactly. Yeah, no it's completely right.
32:54So then we applied the techniques of
32:55quantum metrology to try to find what
32:58states are the best states that you can
33:00prepare so that you can detect
33:03gravitational waves. And at the time
33:05this was a very crazy idea because um um
33:09people were
33:10actually we wrote this paper and I think
33:14a year later uh LIGO detected
33:16gravitational waves for the first time.
33:18But the proposal that we have is for
33:20high-frequency gravitational waves where
33:22the LIGO loses uh sensitivity. So it's
33:24not like a competing thing is there like
33:26different scales.
33:28But at the beginning it was very
33:29difficult to convince the community that
33:32this made sense because LIGO is so big.
33:34It's Each arm is like 3 km. So then I
33:38was saying in 100 micrometer
33:41where the BEC lives you can detect
33:43gravitational waves
33:45people were finding this uh
33:47hard to to believe at the time.
33:50>> could be that sensitive to it?
33:52>> be that yes. So um
33:54it took me a lot of effort to explain
33:57that I'm not using a normal
33:59interferometer which is an
34:00interferometer in space. So you have to
34:03be make it very big to make it
34:05sensitive, but I'm building a new type
34:08of interferometer which is an
34:10interferometer in frequency or if you
34:13want to see it as in time. So instead of
34:16having to making it very big to have
34:18sensitivity, I need the states to live
34:20long.
34:21And this is like now at the heart of a
34:24application to minimi- miniaturize um
34:28gravimeters and and detectors for
34:31um
34:32anything that's is of gravitational
34:34origin.
34:35>> How much progress has been made in in
34:39using them to try and answer this this
34:41key issue of how you make these two
34:42theories
34:44>> So many of the ingredients are out
34:46there. So in order to use a
34:49Bose-Einstein condensate to test some of
34:51the things I proposed like high
34:54frequency gravitational waves or
34:55searches for modifications of gravity,
34:58we actually done a lot of work. I have a
35:00patent also on how to use it to measure
35:03the local gravitational field and its
35:04gradient. It's been like 10 years of
35:06many different applications. But the
35:08ingredients that all of these
35:10applications need is a very big BEC. So
35:13how big? I don't know. Like things like
35:1510 to the 8, 10 to the 9
35:18rubidium atoms in
35:21in the condensate.
35:22>> So it's like a practical challenge of
35:23how you build a big enough Bose-Einstein
35:25condensate to really start to notice the
35:29effects of
35:29>> To see the effects, to get the the
35:31sensitivities that we predict, you need
35:33a lot of atoms.
35:34But
35:36people have already done for example in
35:38hydrogen, they reached 10 to the 10.
35:41Or in sodium, 10 to the 8. The thing is
35:44that making very big BECs didn't really
35:47have an application
35:49when people were using Bose-Einstein
35:52condensates to the detect, let's say,
35:54gravitational, well, more like atom
35:57interferometry where the atoms are
35:59falling
36:00to
36:02to detect the local gravitational field.
36:05The experiments that people were
36:06proposing didn't require many atoms. So
36:10people could do them, but there was no
36:12interest in them. But, you know, that's
36:15why I'm saying the ingredients were
36:16there.
36:16>> you you sort of saw the potential.
36:18>> Yes, and because there were like one
36:20paper or a few papers on very big BECs,
36:23then you also needed another type of
36:26characteristics that other experiments
36:28had. So, all the ingredients were there,
36:31but no no experiment that put everything
36:33together.
36:34>> Mhm.
36:35>> So, the potential was there, but still
36:36very challenging.
36:37>> But then how do you
36:39are you proposing to use Bose-Einstein
36:41condensates to
36:44test the idea of Yeah, that you know,
36:47gravity that mass is what causes the
36:50>> Yes. So, let let me let me go to that
36:52because there's been many experiments
36:54for many years now trying to test that
36:57gravity collapses the wave function. And
37:00they use many different systems. For
37:02example, nanobeats, diamonds, membranes.
37:06Actually, one of the experiments is in
37:08this very building. My colleague Henry
37:11Goldrich has an experiment with a
37:13nanobeat. So, these are silicia like
37:16little silicia marbles, let's say, and
37:18they try to put them in a quantum
37:19superposition. The record is by Markus
37:22Arndt who he's using molecules. And he
37:26very recently has an amazing paper where
37:28he reports being able to put a molecule
37:31that has around maybe
37:3310,000 atoms or 7,000 atoms or long
37:36around that in a in a in a
37:38superposition.
37:39>> But is that still not big enough to
37:41create the effect that you're proposing?
37:43>> Rogers' formula predicts that you would
37:47see the collapse of the wave function
37:48due to gravity when you have around 10
37:50to the nine atoms. So, look at how many
37:53orders of magnitude it's like a very
37:55very far away from
37:58>> Yeah, we're nowhere near that.
37:59>> from nowhere near Yeah. And Markus has
38:02Arndt from the University of Vienna has
38:04always had the record.
38:06>> Mhm.
38:06>> And I think he goes like one order of
38:08magnitude, you know, an increase on mass
38:12like every five years or the order of
38:14that. So, we're really really far away
38:16from it. And I wanted to think if we
38:19could find an alternative way to test
38:21it. So, all of these experiments so far
38:24use solids.
38:25>> Mhm.
38:26>> So, let's say the bead. So, you want to
38:27put the bead in the superposition. All
38:30the atoms are also in the molecule,
38:32they're bounded to each other. So, the
38:34only possible state that you have is
38:36like left plus right.
38:39>> Mhm.
38:39>> But, a Bose-Einstein
38:41condensate has some advantages and
38:43disadvantages. It's not a solid, it's
38:46more of a gas or a fluid. All the atoms
38:49are not bounded between them. So, you
38:52could create with electromagnetic fields
38:54what we call a double well potential.
38:56So, we talked before about having a
38:58potential like a little pot made of
39:01electromagnetic fields. Now, think about
39:03having like a double one. And you would
39:06want to prepare a state where all the
39:08atoms are on the left plus all the atoms
39:11on the right.
39:13And well, what are the advantages and
39:15disadvantages of doing it with a BC?
39:18First, the problem why people have not
39:20been able to reach those big masses is
39:24because solids are very difficult to
39:26cool down.
39:27>> Mhm.
39:27>> So, the big problem is temperature.
39:29Solids are always vibrating and they
39:32always have large temperatures and it's
39:34very very difficult to
39:36to cool them down. Where a Bose-Einstein
39:39condensate is the coldest thing we can
39:41produce in the lab. It reaches
39:43temperatures of half a nano Kelvin.
39:46>> And why is the cold temperature useful
39:48in this case?
39:49>> When it's very cold, you can ensure that
39:52gravity's producing the collapse. When
39:54it's very hot, it could be like
39:56>> some other yeah, effects.
39:58>> So, you want to have it very cold so you
39:59know it's gravity and not something
40:01else.
40:02So, well, so the Bose-Einstein
40:04condensates have that advantage that
40:06there's nothing in any experiment that
40:09we can cool down to those temperatures.
40:12But, now it has a problem because
40:13imagine that I can do the superposition.
40:16But, the moment that I lose one atom,
40:19it collapses because you lost one atom.
40:21So, they're incredibly fragile.
40:23>> So, you need to Yeah, you have to build
40:25this thing in a very controlled way and
40:28under vacuum.
40:28>> it is impossible to make a superposition
40:31in a Bose-Einstein Well, impossible for
40:33like time scales of my life.
40:35>> [laughter]
40:36>> To have a and I want to see, you know,
40:38the results of these experiments one
40:39day. So, of having like left plus right.
40:42But, the BEC has another very beautiful
40:45feature that also the Nobel Prize has to
40:48do something with the recent Nobel
40:49Prize. So, you have this double well,
40:52and you can have atoms here and there.
40:55But, the atoms can tunnel. So, that is
40:57something that we get in quantum
40:59mechanics as well that does not occur in
41:02classical physics that if you have a
41:04potential um and you throw a ball, let's
41:07say, classically it will always bounce
41:09back. But, if you have a quantum
41:11potential, there's a probability that
41:13the energy is enough to let the particle
41:17tunnel through the potential barrier.
41:20>> Penrose has also proposed that this uh
41:23the way in which gravity enters into
41:26quantum theory might have something to
41:28do with our conscious experience as
41:31well. And that seems to connect quite
41:33nicely to your original question that
41:35you asked. You know, I just wondered
41:37whether you could briefly explain that
41:39idea and and also what your kind of view
41:41on it is.
41:42>> Yes, I think that Roger's um idea about
41:47uh the connection between sort of mind
41:49and
41:50um matter is very important. When when
41:53he came up with this idea, I think uh
41:56people
41:57thought it was a taboo to talk about
41:59these things. So, Roger was breaking a
42:02taboo, and I really admire him for doing
42:04that. It must have been very, very
42:06difficult. However, now with artificial
42:09intelligence and with um computers um
42:13reaching these incredible intelligence,
42:16the question about
42:18can a computer become conscious, can it
42:21understand, can it be creative,
42:24became very very relevant.
42:26>> Mhm.
42:26>> But I think that Roger's ideas are very
42:29misunderstood.
42:31And I
42:33I can understand why because they're
42:35actually not so easy to understand. So
42:39what he does is that he
42:42uses the incompleteness theorem of
42:46Gödel. I pronounce it very badly.
42:49Um so I'm not going to go into the
42:51details of that cuz it's very intricate,
42:53but I think very roughly it would be
42:55that if you have some
42:58some that you want to compute a
43:00mathematical axioms, some rules, the
43:03theorem tells you that understanding the
43:06rules is not within the rules.
43:09So the rules are an algorithm. The rules
43:12are a computation.
43:14And they're all classical deterministic.
43:17But Roger shows that like understanding
43:21is not a deterministic thing.
43:23>> Okay. How does that explain this?
43:25>> So what's very interesting, and I think
43:27this is where people really don't
43:28understand what he says, is that he's
43:31then saying, "Okay, where in nature
43:33things are not deterministic?"
43:35>> Mhm. Okay.
43:36>> Okay. Everything in physics is
43:38deterministic. All of classical physics,
43:40uh
43:41Maxwell's equations, even the
43:43Schrödinger equation is deterministic.
43:45>> Apart from the measurement, okay. So he
43:48recognizes this as an aspect of uh you
43:51know, our experience perhaps, at least
43:54according to the way he's done the study
43:55it, that there's a kind of a logical
43:58reason why this gap within our kind of
44:01theories of of nature could be where how
44:04experience enters into the
44:06>> So so then Roger thinks, "Okay, when
44:09when you have the collapse of the wave
44:10function, you have this kind of element
44:13of understanding." And I I think that
44:17all of what he says, I think is um very
44:20relevant, and I agree with his like
44:23logic. But, I uh disagree a little bit
44:27towards the end
44:28because um I think I have been with time
44:32developing a different philosophical
44:34standing when it comes to mind and
44:37matter. Um right now we live in the
44:40materialistic paradigm, where we agree
44:43that everything is made out of atoms and
44:45fields. But, recently thinking about
44:48these things um
44:50uh
44:51more and and deeper, I think I'm slowly
44:54coming into maybe like a dual
44:56um point of view in which I think
44:59>> dualism, the mind and matter, there's a
45:01separation, perhaps.
45:02>> Well, not not separated, but that that
45:04they're it's like they're both
45:06fundamental. So, I
45:08Look, I I change, so uh because I'm
45:11starting to think about these things.
45:12So, this is where I'm standing right
45:14now.
45:15Uh but, that might change later. But, I
45:18think that matter
45:19uh is fundamental, but that also um
45:23consciousness
45:25uh and thoughts are fundamental, and
45:27they interact. I mean, clearly they
45:29interact because our thoughts can
45:30produce a stomach ache.
45:33So, I uh but, we haven't found the way
45:35to make a model that lets you know what
45:37the interaction is. And maybe Roger's um
45:39ideas are a first step to making a mind
45:43mind-matter model in the future.
45:46But, in one way that I differ is that I
45:48somehow think that consciousness because
45:51consciousness is not a thought. Uh
45:54I mean, if you think, "Oh, I'm conscious
45:56like now." That's a thought, but 3
45:58minutes later we're still conscious,
46:00although we're having different
46:01thoughts. I think uh consciousness is
46:04a background very similar to space-time.
46:07So, you I think you also asked me about
46:09like what I I thought about time being
46:12fundamental or not. I think time is
46:14fundamental and actually I think that
46:16space-time
46:17is the arena where atoms and fields
46:21interact.
46:22And then consciousness is the arena
46:25where sort of thoughts and perceptions
46:28and all of these things take place.
46:30And now, you know, the thing I don't
46:32even think we have a good understanding
46:35of is how would these interact. Although
46:38lately there's been become a popular uh
46:41question and more and more people start
46:44working on trying to make uh
46:47sense of of of both. Something is
46:49happening in our brains, right? Because
46:51well, our brains are made of matter,
46:53they're made of atoms and fields and we
46:56know that. But we also know that we're
46:59aware and that we have experience. We're
47:01actually interesting because in order to
47:03measure an atom,
47:05you need an apparatus and actually
47:07I've never measured an atom. I trust my
47:09colleagues who claim who measured atoms
47:12in the lab, right?
47:13>> But we've all had experiences.
47:16>> And we think other people do.
47:18>> And we think other people do, which I
47:19think it's a fair assumption to make.
47:22So,
47:23somehow in our brains, I think there's a
47:26interface between
47:29um matter and whatever we call
47:31experience.
47:33But we might be very far away or I don't
47:35know, maybe not from understanding how
47:38that interaction takes place. But that's
47:40why I was telling you that I um slowly
47:43moving at the moment towards like a
47:45dualist um
47:47uh point of view.
47:48>> Yeah, that's a a really fascinating way
47:51to to think about how, you know, you're
47:53making sense of this question which has
47:55has driven you um yeah, since you were a
47:57teenager. So yeah, thank you so much for
48:00taking the the time to yeah to speak
48:02with me.
48:03>> No, thank you. On the contrary, I'm very
48:06grateful for
48:07coming here to Southampton to speak to
48:09me. Thanks a lot.

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This page contains the full transcript of Gravity Might Not Work The Way We Think by New Scientist, generated from the public captions YouTube serves with the video. The transcript has 7,993 words across 1,296 segments, with the original timestamps preserved so you can click any line to jump to that moment in the embedded player.

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