What is post-quantum gravity?

Daniel and Kelly talk to Jonathan Oppenheim about his unusual theory of unpredictable gravity

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Episode Transcript

Speaker 1 (00:05):
Humans have been working on the mysteries of the universe
for quite a long time now, and you know, we've
made some pretty good progress. But about one hundred years
ago we kind of got stuck. We have two big
theories of physics, general relativity that describes space and time
and how stuff moves, and quantum mechanics that describes probabilities

(00:27):
and how tiny particles behave when you're not looking. Bringing
these two ideas together into one harmonious explanation of everything
has been a real puzzle that it's stumped some of
the greatest thinkers in history. We have some famous ideas
that try to tackle it, string theory, loop quantum gravity.
Then there's some fringe ideas that very few people take

(00:47):
very seriously from Wolfram or Weinstein. But what if everybody's
taking the wrong approach. What if instead of attacking the problem,
the right strategy is to actually sidestep it and reframe
the question. That's what we'll do today when we talk
about post quantum gravity. Welcome to Daniel and Kelly's Extraordinary Universe.

Speaker 2 (01:22):
Hello, I'm Kelly Wiener Smith. I'm a parasitologist, and today
we are going way out of my comfort zone. To
discuss post quantum gravity.

Speaker 1 (01:31):
Hi, I'm Daniel. I'm a particle physicist, and I don't
even understand pre quantum gravity, not to mention a post
quantum gravity.

Speaker 2 (01:38):
You know, Daniel. One thing I appreciate about my field
of study that I don't think I really appreciated enough
before this conversation that we had is how great it
is that I know where fish are and I can
count them.

Speaker 1 (01:51):
That's because you've never met a post quantum fish yet, Kelly.
They're very unpredictable.

Speaker 2 (01:55):
Uh, you know, No, I think that the fish are
going to be way more predictable. And Zachowy likes to
Joe that Kelly's job is to figure out what the
fish are up to, and I think that's way easier
than figuring out what the electrodes are up to. So
what kind of personality type do you think ends up
in both of these fields?

Speaker 1 (02:09):
Hmmm, Yeah, that's a great question. You know, I think
people who like to deal with the tiniest bits of
the universe or people have to give up the concreteness
of being able to see what you're working on, you know,
being able to like look at your experiment and say, oh,
I see what it's doing, or I can take pictures
of it at least. So this is like leap into
the abstract realm where you just have this like mathematical scaffolding.

(02:31):
You have to just sort of trust and you've got
to be sort of into that, you know, mental puzzles
and mathematical mazes. But it's often unsatisfactory, right, Like we
build these huge machines, we collide these particles. We can't
even really look at what's happening at their core.

Speaker 2 (02:45):
So I agree with you and I disagree. So like
I study fish, but what I'm really interested are the
parasites that are inside of them that I can't see
and that I have to like try to study in
indirect ways. Maybe I split the difference. Still, I guess
at the end of the day, I humanely euthanize those
fish and open them up to see the parasites, which
is much easier than seeing the particles.

Speaker 1 (03:05):
I have this fantasy sometimes about solving most of science
just by having like perfect visualization. Like if you could
watch anything anywhere in the universe at any time, you
could like zoom in arbitrarily and just look at stuff,
you would understand what happens, like how do viruses kill
a bacteria? We'll just watch it, you know. But most
of our science is so limited by not having the instrumentation,

(03:27):
but not being able to see what we want to see,
and having to have all these indirect probes growing bacteria,
seeing if they die, speculating about the mechanisms. It'd be
amazing to be able to just like X ray the
whole universe and know what's happening, because then the explanations
I feel like they would just fall out.

Speaker 2 (03:43):
Well, so to be the wet blanket we've all come
to know and love, hopefully know and love. When we
were doing the Human Genome project, we felt like as
soon as we could like get down to that level,
suddenly we'd be able to like solve all cancer. And
now we're working on the Human Connectome project, where we're
going to like know about all the connections in our brains,
like all of our neurons connect. But I don't feel
like immediately being able to see things at a higher

(04:06):
level gives us the answer. And I guess you're saying,
once we can see everything, I still feel like there
would be decades of us trying to be like, well,
what the heck does it all mean?

Speaker 1 (04:15):
I just need one more level of zoom, bro, just
one more level of zoom, and I'll figure it all out,
all right.

Speaker 2 (04:20):
Well, the biologists have been saying that for a long time,
and so far we're discovering it just makes things more complicated.
But I hope you're right.

Speaker 3 (04:26):
All right.

Speaker 1 (04:27):
Well, a big group of physicists have been zooming in
on the fundamental nature of the universe, trying to understand
how it all works, what is the universe really made
out of it as smallest scale, what is the bedrock
foundation of the universe itself? And they've all run into
a problem trying to understand how gravity and quantum mechanics
come together. So we've had a few episodes about various

(04:48):
solutions to one of the biggest puzzles in modern physics,
quantum gravity. And today we have another episode talking about
the theory of post quantum gravity, which is an intriguingly
compact name for.

Speaker 2 (04:59):
A Yeah, and you know, I've really been enjoying looking
at this problem of quantum mechanics in general relativity and
how we make them work together from a couple different angles,
And you know, it's exciting to get to hear yet
another angle today.

Speaker 1 (05:13):
Soon we'll be interviewing fish about it, right, to see
what is the fish perspective on quantum gravity.

Speaker 2 (05:17):
I mean, I hope, but you know, as an experimentalist,
I love that at the end there are actually experiments
that can be tested to figure out if this one
is correct or not. So everyone's just going to have
to hold their breath, that's right.

Speaker 1 (05:29):
And so at the beginning, as usual, we ask people
what they think about post quantum gravity or if they
even know what it is. Thank you very much to
everybody who sends in their thoughts. If you'd like to
be part of this crew, just email us to questions
at danielon Kelly dot org. You can play along at home.
It's very easy and fun. So think about it for
a minute. What do you think the theory of post

(05:50):
quantum gravity is. Here's what our listeners had to say.

Speaker 4 (05:55):
Post quantum gravity, I would guess, is the products of
finding out theory that explains quantum gravity. So what the
benefits of such a theory would be.

Speaker 5 (06:08):
The name suggests that there is two types of gravity,
one that does before quantum effects and one that does
after quantum effects, and post quantum gravity would be that
it only appears if quantum effects are done with their work.

Speaker 2 (06:29):
I would guess that the gravity that happened after the
Big Bang turned everything from actual energy into matter.

Speaker 6 (06:37):
When you spill your cereal in the morning for your coffee.

Speaker 4 (06:41):
The reality inside a black hole where the quantum realm
is affected by gravity at its most extreme.

Speaker 7 (06:50):
Gravity that happens when particles drink their milk and grow
up from being quantum particles to regular sized particles, and
then they have regular size gravity. Pop quiz Tricia, what
is post quantum gravity gravity?

Speaker 2 (07:06):
After children?

Speaker 1 (07:09):
I think that's postpartum.

Speaker 8 (07:11):
Post quantum gravity is like post punk rock, the notion
of that the idea is not working.

Speaker 3 (07:16):
We should just move on.

Speaker 2 (07:18):
I thought that gravity was basically irrelevant at the quantum level,
so I can't say I have any idea.

Speaker 1 (07:27):
I would say the new physics that we find after
we find quantum gravity.

Speaker 6 (07:32):
Post quantum gravity is the conceptualization of what we think
of is gravity, taking into account that neither general relativity
nor quantum mechanics completely explains the phenomenon that we experience.

Speaker 3 (07:48):
Maybe post quantum gravity is the state of society after
we figure out what quantum gravity is, and we're bored
because we don't have any more problems to solve.

Speaker 1 (07:57):
David Letterman's theory of gravity. What you start looking for
when you give up on the idea of quantum gravity.

Speaker 9 (08:06):
I believe it is a term that refers to the
facts of gravity beyond this scale of particles, maybe a
theory that tries to explain gravity in an ever smaller scale.

Speaker 10 (08:18):
Post quantum gravity is a theory of gravity that finally
reveals that gravity actually does not obey quantum rules.

Speaker 8 (08:29):
I think post quantum gravity might be the effects of
gravity at quantum scales, down at the particle level, where
it's got to fight it out with all the other
forces down there.

Speaker 1 (08:43):
All right, Kelly, did these responses align with your thoughts
of post quantum gravity.

Speaker 2 (08:47):
That we were all apparently comparably confused? Yes, I feel
like we have a particularly intelligent audience. It made me
feel better that I did not know what post quantum
gravity was when I heard these answers.

Speaker 1 (08:59):
So well. It's sort of a niche idea in a
niche field of quantum gravity, but it's one that a
listener wrote into me and said, Hey, can you explain
what this is? I was trying to read an article
about it and I couldn't understand it. So as an
invitation to everybody out there, if you're reading about some
niche theory in physics and it doesn't make sense to you,
please write to us. We will break it down for you.

(09:20):
And we're lucky enough on this episode to have as
a guest the guy who then did post quantum gravity
and does a pretty solid job of explaining it.

Speaker 2 (09:27):
Totally solid. Yes, it's so nice when you can have
the experts come in to tell you what's up.

Speaker 1 (09:31):
And he's got a beautiful voice as well. So here's
our interview with Jonathan Oppenheim. So it's my pleasure to
welcome to the podcast professor Jonathan Oppenheim, a physics professor
at University College London and proponent of the theory of
post quantum gravity. Jonathan, thank you very much for talking

(09:52):
to us, Thanks for having me. So first I'd like
to set the stage and understand what is it we
are trying to solve. Why is everybody after quantum gravity.
Why can't we just have general relativity to describe big
stuff and quantum mechanics to describe small stuff and be
happy with that. Why do we need one unified theory
of the universe.

Speaker 3 (10:13):
Well, the two theories are frameworks general relativity and quantum mechanics.
They're inconsistent, so they can't really live together in a
mathematically consistent way, and so we know that either one
of them is wrong or both of them are wrong,
and so we know that they cannot be a correct
description of nature.

Speaker 1 (10:30):
What if they always describe different domains, Do they need
to give us a single, unified sort of conceptual understanding
of the universe or do they actually disagree about what
happens in the universe. Do they conflict in terms of
their predictions.

Speaker 3 (10:44):
Yeah, they conflict. I mean the regimes that we're used to,
it doesn't matter so much. But now that we're exploring
the quantum realm, and we're exploring the quantum realm, when
it comes to larger and larger particles than it does
start to matter. If you think of like a very
small gold atom, which is put in a superposition of
two places at once, which is something we can do

(11:06):
with gold atoms, then gold atoms gravitate in a very
small way. But we don't have a theory which would
describe that, and maybe that's not a realm that we
care that much about. But we know from the history
of physics that when we have these inconsistencies and contradictions
in our theory, that once we start to try and
fix them, everything just starts to unravel. I mean, if

(11:27):
you look at the history of quantum mechanics, there was
only one or two little places where things were seemed strange.
And once we started really looking at how we could
reconcile and explain certain phenomena back in the beginning of
the nineteen hundreds, we realized that all of our laws
of physics were wrong and there was the whole quantum revolution.
So we know from history that when there are these contradictions,

(11:48):
they usually spell the beginning of a new era of physics.

Speaker 2 (11:51):
Could you for the biologists dig in a little bit
more to the gold superposition atoms.

Speaker 3 (11:58):
So quantum mechanics is strange. And in the world that
we inhibit, we think of things as being in definite places.
So a coffee mug is in one place or it's
in another place. But as you go to smaller and
smaller particles, in smaller and smaller systems, then things behave
very differently. So gold atom, which is not like a
coffee cup. We'd say that it's in a superposition of

(12:21):
being in many places at the same time. So in
some sense, it doesn't have a definite position, it doesn't
have a definite velocity. As we get smaller and smaller,
things just behave very weirdly and that's what quantum mechanics is.

Speaker 2 (12:33):
And so quantum mechanics can explain the weirdness of what
the gold atom is doing, but general relativity cannot, is
that right, That's right, that's right.

Speaker 3 (12:42):
I mean general relativity is our theory of gravity, and
it describes planets and the Solar system and the way
the universe evolves and the Big Bang, and so we
used to it explained those very big things, but well
contradicts what's happening on the small scale.

Speaker 1 (12:58):
So we have this theory of the very large gener relativity,
which requires us to know where things are and when
they're there in order to describe how space is bending
and how things gravitate. But then we have quantum mechanics,
which says things can have an uncertainty in their location,
and these two things are fundamentally in contradiction.

Speaker 3 (13:16):
That's a good way of putting in it, and maybe
just to say that you mentioned space time, and the
thing to say maybe, is that our theory of gravity.
What Einstein one of the massive contributions to science is
he taught us that gravity is really space time bending.
So we know that large objects bend space time, that's
what gravity is. And very small objects, because they don't

(13:38):
have a definite position, as you said, we don't know
how space time should bend because space time doesn't even
know where they are. So that's why you get these contradictions.

Speaker 1 (13:49):
And you have to cook up this example of a
gold atom because typically general relativity applies to the very
large things planets and galaxies, and quant mechanics are the
very small. So you need something sort of on the
edge there where it's large enough where we can measure
its gravity, but it's small enough the quantum mechanical effects
are still relevant. Is that why you come up with
this idea of a gold at them? And why gold

(14:09):
in particular would it work also for a lead atom.

Speaker 3 (14:13):
I mean, I love gold because it's it's actually because
gold is very dense, and so it turns out that
you run into trouble actually with very dense objects rather
than very heavy objects. That's one of the things we've
found out. But you know, it's true that we generally
think of the gravitational field as being caused by the
Moon and the Earth and things like that. But one

(14:33):
kilogram mass will bend. We can feel that gravitation pool
of a one kilogram mass. And now we're doing experiments
where we can feel the pull of gold, which is
a millimeter sphere. We can feel the gravitational pull of
that kind of an object. But when they get much
smaller than that, then we don't know what happens. And
we haven't even been able to really perform the experiments

(14:54):
that will tell us what happens at that scale.

Speaker 1 (14:56):
Right, So we need some sort of unified picture of
gravivity and quantum mechanics. Why don't we just do that?
I mean, we did it for electromagnetism. We had classical
theory of electromagnetism, and then folks quantized it and gave
us a theory of quantum electrodynamics. Why can't we also
just do that for gravity? Take space time, consider that
a field, quantize it. Bob's your uncle.

Speaker 3 (15:19):
Yeah, I mean that's what everyone has thought, and that's
what we've been thinking for the last one hundred years,
which is probably the amount of time that we've been
failing to quantize gravity. So yeah, you have this contradiction.
You have gravity which is not quantum. You want to
make it quantum to fit in with everything else. That
would be I think the thing that everyone thought that's
what we should be doing. But we're finding that really troublesome,

(15:41):
and there's reasons for that. And one of the I
think big reasons is that gravity is really different from
the other forces. So we're used to electromagnetism, which is
what keeps my two fingers from being able to push
through each other. They're repulsed by the electromagnetic force. Gravity
just behaves very differently. And what Einstein taught us about
gravity is that, unlike the other forces, gravity is spacetime bending.

(16:04):
So the reason that the Earth goes around the Sun
is that the Sun causes space time to bend in
just such a way that the Earth instead of rolling
past it orbits around it. So it's slightly weirder than
that actually. I mean, you often in science centers and stuff,
the vehicle demonstration where you see a big planet like

(16:25):
the Sun sitting in the middle of a cone and
then the Earth goes around the Sun as if it's
caught in this vortex or funnel. It's the kind of
demonstration we often see to kind of give us a
sense of why gravity as space time bending is causing
the Earth to orbit the Sun.

Speaker 1 (16:43):
Since you brought that up, do you find that to
be an accurate description, like a useful mental model of
how gravity works? Because I find it to be very confusing.
You have like a two D situation, and then you're
adding curvature in a third dimension, where general relativity has
the curvature to be intrinsic in the three D space.
Do you find those demonstrations to be misleading or do

(17:03):
you find them to be a useful description of what's
happening in general relativity?

Speaker 3 (17:07):
All our descriptions are misleading. So what we do is
we tell ourselves lies and each other lies, and they're
useful for a bit, and then we replace it with
a better lie. And I feel like the funnel is
a reasonable lie because it gives you a partial sense
of what's happening. And like when I say a gold
at them can be in a superposition of two places

(17:29):
at once, that's also a bit of a lie. Because
you could just as well say it's in a superposition
of being in neither place at once. This is a
mental picture that we're using and it has some truth
to it. Also, the are parts where it fails and
the funnel that you see at many sigence centers, which
is meant to explain why space time bending causes the

(17:51):
Earth to go around the Sun. Well, a better lie
is that what's really happening is that time is slowing
down you get closer to the Sun, and the reason
that the Earth is going around the Sun is because
time slows down closer to the Sun, and the Earth
wants to travel in a path which causes clocks to

(18:13):
tick the least number of times. And so it's actually
time that's bending and traveling at a different rate at
different places in space, and that's why the Earth goes
around the Sun.

Speaker 2 (18:23):
Why does Earth want that?

Speaker 1 (18:27):
Earth is laid on its deadlines, Kelly, that's why I
hear you Earth, I hear you.

Speaker 3 (18:34):
Yeah, Earth is traveling the shortest distance between two points.
And why it wants to do that, I don't know.
And there's lots of principles in physics where things want
to do. The principle least action, which is how we
derive most of our physical laws and was famous The
movie Arrival is about that principle, and we don't really
know why that is, but that just seems to be

(18:56):
the way it is.

Speaker 1 (18:56):
I think it's at the level philosophically of if you
may this assumption, you get a model which works and
describes the universe very well, and then you can come
back to, well, what does that mean about the universe. Well,
we're still shrugging our shoulders over that one.

Speaker 3 (19:09):
I think the thing you learn in physics when you're
in grade school, which is that you know when you
are in outer space and there's no force acting on you,
you move in a straight line. That's an example of that.
Why do we move in a straight line when there's
no force applied to it? Well, we're just taking the shortest.

Speaker 1 (19:27):
Distance in some set.

Speaker 3 (19:28):
We're taking the shortest path, the easiest path in some sense,
And that's an particular example of that. But things could
be different, but they're not.

Speaker 1 (19:37):
It's also interesting to me how some things, when you
say then, people will just accept them because they sound intuitive,
and other things demand an explanation. Why do clocks tick
slower when you see them moving at high speed. That
needs an explanation. Why would clocks always tick at the
same speed. Why doesn't that need an explanation. So there's
an imbalance there because some things confront or intuition and

(19:57):
some things support it.

Speaker 3 (19:59):
I mean, maybe this is not for this podcast, but
I could give you a good reason why we think
that the Earth is traveling the shortest distance in curved space.
Please do if you're willing to accept Newton's law, which
is that in empty space, my velocity should stay the
same if no force is acting upon me, and if
I should just move in a straight line in empty space.

(20:20):
If you accept that, then you can now ask the question, well,
if there's no push or pull on me, and I'm
in a curved space, how should I move? I think
it's free one answer, which is you should take the
shortest path, because that's what the satellite is doing an
empty space, when it's going around the Earth or when
it's going in a straight line, it's just taking the

(20:40):
shortest path.

Speaker 1 (20:41):
I see. So you're making the argument that in flat space,
a straight line is the shortest path, and so if
you generalize to any space, including curved space, you should
always still take the shortest path, which in this case
is not a straight line. That's right, fascinating. So I
think we interrupted you as you were explaining why gravity
is weird and different and why it's harder to quantize

(21:01):
than electromagnetism. And you were explaining how gravity is actually
the curvature of space. Why does that make it harder
to quantize it. Why can't we take the same tricks
we applied to the electromagnetic field and apply them to
the metric of space time or the curvature of space time.

Speaker 11 (21:17):
I mean, there's a bunch of technical reasons why we
run into trouble, but I think great conceptually is this
idea that gravity is about time flowing at a.

Speaker 3 (21:29):
Different rate in different points in space, and it's about
the flow of time how fast it flows. And if
you think about it, it's almost like what are we
doing as physicists.

Speaker 11 (21:40):
What we're doing, is physicists, is we're describing how things.

Speaker 3 (21:43):
Change relative to our clock. So we have a clock
and it's telling the time, and we say, okay, at
nine am, the particles were in this configuration, and then
I predicted some later time they're going to be in
some other configuration. And so we're predicting the future based
on the past, and in order to do that, we
need to know how our clocks and our rods are

(22:06):
all behaving. And if we're going to quantize that, if
we're going to make our clocks and our rods and
the speeds at which the clock ticks and the length
of our rods, if that is going to be quantum,
then personally, I don't know how to describe physics anymore,
because you cut my legs out from underneath me, and

(22:27):
I no longer am able to really, I think, talk
about how things evolved relative to some time. If the
rate at which time is flowing, if that itself becomes
the thing I have to quantize and doesn't have a
definite value, because I guess remember that the thing that
distinguished classical things and quantum things was that classical things

(22:50):
have definite values. The coffee mug is at a definite position,
and quantum things don't. The gold atom that its quantum
can be as if it's in any places at once.
And so if your time can be running at many
different speeds, how do I tell you what is happening
relative to the time If I can't even really tell

(23:10):
you what time my clock is running.

Speaker 1 (23:12):
Now, I see. So for an electron, if we just
ignore gravity and we think about it quantum mechanically, we
can handle its uncertainty and we can propagate that forward
in time. We have the Shorteninger equation, and we can
even allow for that uncertainty to create new uncertainties and
more uncertainties, because we always agree on a clock and
we can say what time is, and we can propagate
things forward and calculate how this uncertainty envelope is going

(23:34):
to change. But if I'm now talking about something that
has gravity, the additional complexity that time itself is changing
in a way that depends on what's happening. And so
time is bent by the object, and its motion then
depends on time. And so this this back and forth
interaction between the motion and time itself. Is that the complexity.

Speaker 3 (23:55):
That's definitely part of it. Yeah, that's a big part
of it. It's not even clear what I mean by
the past and the future. It's not even definite that
something is to the past or to the future of
some other event. If I have two events that are happening,
you know, the mere act of setting initial conditions, which
is what we do in physics, we say, here are
the initial conditions, this is what's going to happen in
the future. The mere act of doing that, I think

(24:17):
becomes problematic if you're trying to quantize the space time itself,
or at least the flow of time and the distances.
If those are being quantized, then I think it's very
difficult to even ask the questions that we're used to
asking as physicists.

Speaker 1 (24:51):
We often hear the quantum gravity is hard because there
are infinities, and like, maybe the universe is infinite, maybe
there's an infinite number of locations between me in this microphone.
Can you help us understand where the infinities come from
and also why they're a problem. Why can't we have
infinities in our theories.

Speaker 3 (25:07):
Well, so this is something which is called renormalization, and
what that really just means is that most of our
physical theories they break down at very short distances. With
most of the theories that we have at the moment,
they are valid at very short distances, but gravity is not.
And actually, one of the things which gives me some

(25:29):
faith in this post quantum theory is that it turns
out we've just recently shown that it's formally renormalizable. So
even though quantum gravity has the property that it's not
valid to very short distances, this post quantum theory does
seem to be valid at very short distances. And that's
important because this description of gravity in terms of geometry,

(25:50):
if you think that that really is what gravity is,
that gravity really is geometry, then you believe that this
picture that it's geometry should hold up to the very
smallest scale. And so that's one of the reasons why
we want to have our theories be predictable at short distances.

Speaker 2 (26:07):
So renormalizable means it doesn't matter what scale you're looking at,
the theory still works. Would that be a fair okay?

Speaker 6 (26:16):
Thanks?

Speaker 2 (26:16):
The biologist needs simplification every once in a while.

Speaker 1 (26:20):
And what do you mean when you say it doesn't work?
I mean, we have a theory of particles that allows
us to smash electrons together tiny distances to describe the
creation of very heavy particles. Why doesn't that work with gravity?
Why does it break down? What happens?

Speaker 3 (26:36):
Well, there are two breakdowns that happen with gravity. One
is something called the black hole singularity, which is that
black holes, which are the strongest gravitational fields we know
and cause light to bends to the point where they
can't get out those You have something called the singularity,
which means we just don't know what happens, and the
center of a black hole that is maybe related but

(26:58):
maybe not related to another kinds of infinity that we get.
You know, the reason that gravity has this problem and
our other forces don't have this problem, it's for a
reason that I don't know that I have a good
podcasting way of saying other than that's saying that the
Newton's contents has, you know, the wrong dimension. But there

(27:20):
is a technical reason why the usual thing you would
do for other forces just doesn't work with gravity. The gravity,
we say, has these infinities which you cannot get rid of.
And therefore the quantum theory of gravity, at least in
the three spatial dimensions that we live in or appear

(27:40):
to live in in three special dimensions, it has these infinities,
and we don't know what to do. And that's actually
the reason why we have approaches like string theory and
loop one and gravity theories in which you imagine that
space time lives on a lattice or you know, instead
of having point particles, we have these extended strings. Those

(28:02):
are solutions to the problem that gravity has these infinities,
and that's why these other approaches have been born.

Speaker 1 (28:10):
So your background is in string theory, right, You were
once a string theorist. Is that a fair description?

Speaker 3 (28:16):
Well, no, I hang out with a lot of string theorists.
I have string theorist friends, and I've written some papers
that are string theory adjacent.

Speaker 1 (28:24):
So you don't want to be described as a string theorist.
But you know something about string theory and you decided
to take a different path than the string theory crew.
Why did you not follow the crowd? What is it
about string theory that doesn't satisfy your desire to unify
quantum mechanics and gravity.

Speaker 3 (28:40):
Well, I think one of the things is that I
think we should take this geometric picture of gravity very seriously.
I mean, maybe that's just a description which breaks down
at some scale, but let's just try and assume that
that is actually what's happening, that gravity is actually spacetime bending.
And I think if you have that as your picture
as to what is happening, then quantizing it becomes a

(29:01):
lot more problematic. And string theorist for example, or you know,
I would say almost everybody else the various approaches to
quantizing gravity, they somehow need to have it at some
small scale. That picture is not true and breaks down
the geometrical picture, you mean, the geometrical picture of gravity,
And so if you want to hold true to this

(29:24):
geometrical picture, then I don't think you can quantize it. Now,
maybe that's the wrong approach. It could be that the
geometric picture does break down and we get to the
smallest scale and space time is emergent in some way,
or you know, we're all just fuzzy dots in a
lattice of space and time. All those things are possible,
and I'm attracted to them. They sound sci fi and great,

(29:47):
but I don't know how to make them work. I
don't know how to think about them. And I think
it's worth trying the kind of conservative imagines geometry all
the way down.

Speaker 2 (29:55):
Approaching and so of all of the things that you
could have decided, let's assume that this this is true
and then work around that. Why did you pick the
geometry thing?

Speaker 3 (30:05):
Yeah, I mean, I guess there's an aesthetic element to it,
But I think also I have a feeling that I
don't think we should all be doing the same thing.
I think that's really important for science. I have a
plot and cheer my string theory friends when they make
breakthroughs and my new port of gravity friends, et cetera,
et cetera. I think we should be supporting each other.
But I also think we should be diversifying and trying

(30:27):
as many approaches as possible. And so I'm going to
pick this kind of from the slightly lonely route, but
I think it's important that we try these different things.
And I think it would be dangerous to put all
our eggs in one basket. We're looking for a needle
in a haystack, and we shouldn't all be looking in the.

Speaker 1 (30:42):
Same place, thank you. And I think that's very valuable
contribution to science more broadly, So let's talk about more
deeply your idea post quantum gravity. How is it that
we can avoid quantizing gravity, to take the geometry seriously
all the way down and still somehow handle the uncertainty
of quantum mechanics, I mean classical gravity. Geometric gravity says

(31:03):
we need to know where a particle is to know
how it bends space time. But quantum mechanics says, sometimes
that knowledge doesn't exist, not just that it's not known,
but the gold atom is partially here and partially there,
or has a probability to be here or there or neither.
So in your picture where you take geometry very seriously,
what happens to space in that situation? How do you

(31:24):
avoid quantizing it?

Speaker 3 (31:25):
Yeah, you have to give up something, and the thing
you give up is predictability.

Speaker 1 (31:30):
Didn't you say earlier that you needed predictability, that was
the problem you're doing to solve.

Speaker 3 (31:35):
Yes, I did. Yeah, I mean it's very strange that
people are willing to accept that you can't predict exactly
where the particle is and has a certain probability of
being found in a certain place. So people accept that,
or maybe they don't accept that. You could do a
whole show on the different interpretations of quantum theory that
you haven't. Maybe you've done the show on this. But

(31:57):
quantum mechanics does have this lack of predictability, whereas classical
physics doesn't. Right like in classical general relativity. In Einstein's
theory of classical general relativity, there is only one space time, right,
and it is definite and space time has a definite configuration.
So if you're going to redd these two theories together

(32:19):
and you want to keep space time as classical in
the sense that it has definite features, the only way
to do it is and this is a difficult concept
to kind of get your head around. It has to
be both definite and unpredictable.

Speaker 1 (32:38):
That was definitely unpredictable. Yeah, what do you mean by that?
What happens when you have a particle and it's potentially
in two different places? Does it bend space time in
both places? Is space time probably bent in both places?
Or is it random where it gets bent?

Speaker 3 (32:56):
Well, space time has to be undergoing these random f suctuations.
Some people have said it's like it's wobbly. It's undergoing
these random fluctuations. The particle then will kind of bend
it in all the places that it's in, but you
won't be able to really tell where it's bending it.
So if I were to look at space time, I

(33:17):
wouldn't be able to tell where the particle is because
space time will be undergoing all these random fluctuations. It's
kind of wobbly and jumping around all over the place.
And so if I looked at the space time, even
though it was being bent by the particle, I wouldn't
be able to tell where the particle was. And that's
what you need in order to reconcile quantum theory with

(33:40):
general relativity, if you're going to keep general relativity as
a really theory of definite geometry.

Speaker 1 (33:47):
I see, so space time is not emerging from some
deeper string theory. It fundamentally is the geometry of the universe.
But it's also fundamentally random. And when you say that,
you mean that it's and unknowable, the way it is
in quantum mechanics, where the information just does not exist
it's not determined until you measure it, or that it's

(34:09):
random but it's unknown. Like in classical physics. You know,
if I flip a coin and I don't look at
the answer, in principle, I could have calculated the outcome
of that coin. The information exists, and it is either
heads or tails under my hand, even if I haven't
looked at it. Is your space time random and unknowable
or random and unknown.

Speaker 3 (34:27):
It's knowable in principle, I could know exactly what configuration
is at the present time, but it's unpredictable in that second.
Later I will not be able to predict oh, it
will evolve too, So it's more like a breakdown in
predictability rather than a breakdown in unknowing.

Speaker 4 (34:44):
Nice.

Speaker 3 (34:44):
And I think you're really right to make that distinction,
because you know, when people first learn quantum mechanics, they
might learn the Heisenberg and certainty principle where they say,
you know, you don't know the position of the particle,
you don't know where the gold ATM is. But I think,
as you're hinting, it's weirder than that the gold particle
doesn't have a position that does not exist at all.

(35:04):
And so that's maybe the difference between a kind of
a classical breakdown and predictability and a quantum one. A
classical object can be definite but unpredictable, whereas a quantum
system it's not knowable, but it can be predictable. Because
I know that's the strange thing about quantum mechanics. I

(35:26):
can actually predict with certainty how something called the wave
function will evolve, But the position of the particle doesn't
have definite value. So this notion of predictability, definiteness and
noable that they're all kind of tied in a very
strange not They're quite complicated anyway, in both classical mechanics
and quantum mechanics, and they really get jumbled around here.

Speaker 1 (35:48):
So let me see if I understand the distinctions. So
in a classical theory, we have something which is definite,
spacetime has values and locations and bending, and it's also
predictable in that the past controls the future. The future
is determined by the past, whereas some kind of mechanics
we can predict the possible outcomes very precisely. The Shortener
equation tells us how to describe the possible outcomes, but

(36:09):
we don't know which one will actually be selected when
we interact with it. But in your theory you have
something which is definite but unpredictable. Does that mean that
the past doesn't completely control the future, that space time
in one moment is not determined by space time in
the past.

Speaker 3 (36:26):
That's right.

Speaker 2 (36:27):
Does that allow time travel? Let's get to the important part.

Speaker 1 (36:31):
Here, Kelly working on her deadlines again, that's the question.

Speaker 3 (36:36):
On everyone's mind. Can get somewhere faster? There's a lot
of time travel.

Speaker 1 (36:41):
So if I imagine like an empty universe, right, no mass,
no radiation, nothing, Space is completely flat in your conception.
Is that fluctuating even though there's nothing happening to it,
Nothing is being inserted, nothing is moving. Is space time
still fluctuating just like randomly changing?

Speaker 3 (37:00):
Yeah, but it's not that there's nothing there. There's always
something there. There's the vacuum. I mean, in quantum mechanics,
nothing's complicated. It might look like there's nothing there, but
at the small scale there are actually in a quantum
realm also these fluctuations, but in some weird way in
the quantum realm they look random, but they're not. They're
only random if you make a measurement. You know, if

(37:23):
you don't make a measurement, then they're not that random.
You can kind of predict how things evolve precisely. It's
a bit why this whole theory is very tied up
with this whole measurement problem in quantum mechanics, because in
this theory you don't need somebody called the measurement postulate.
In quantum mechanics, you know, you don't know where the
atom is. It could be anywhere, and then you make

(37:44):
a measurement and you find that it's here or there
with certain probabilities. That's called the measurement postulive. It says
that the particle when you measure it, will appear to
be somewhere with some probability. And in this theory you
don't need the measurement posture. That's where unpredictability comes in
in theory. That's the boning place that comes in, and
it's a very artificial way that it comes in, and

(38:05):
we don't really understand it. We kind of put that
on top of the rest of quantum theory. We put
in this measurement posture that which tells us that, okay,
you might not know where the particle is, but then
you measure it and you see it there with probability.

Speaker 1 (38:18):
You have because when we measure things, we don't get
answers that are like it's half here and it's half there.
That's we get it's here or it's there. So we
have to somehow reconcile the spread and predictions of quantum
mechanics with the specific observations.

Speaker 3 (38:31):
That's right, and that's where the unpredictability comes in in
quantum theory. But if you remove the measurement postulate and
in many interpretations of quantum theory. They don't like the
measurement postulate and they remove it. If you remove the
measurement postulate, then there is no unpredictability in quantum theory.
And that's what I've done here. So in this post
oneum gravity, there is no measurement postulate. So quantum theory

(38:53):
by itself would be predictable but not definite. So you know,
the particle doesn't have a definite position, but everything's predictable.
The particle is predictably in a superposition of being in
many places at the same time. Right right.

Speaker 2 (39:08):
I feel like I might understand it better if there
were like an experiment that we could describe for how
we would test this or is this one of those
things that can't be I think string theory we can't
do an experiment to test either. Yeah. Where are we
here with experiments?

Speaker 3 (39:21):
Yeah, So at the moment, there's a few experiments that
people are doing to test theory. So it turns out
that you can test both the theory specifically and then
just the proposition more generally about whether space time is quantum.
So maybe we must quantize gravity, and we now have
actually experiments which can test whether space time should be quantum,
and then those experiments will also test this particular theory,

(39:44):
because this is a particular theory in which space time
is not quantum. So there are experiments to do that.

Speaker 1 (39:50):
And so these gold atoms you were talking about earlier,
do we have enough control of gold atoms so that
we could potentially measure their gravity and understand how there's
super position affects their bending of space time? Are we
still years away from being able to do that actual experiment.

Speaker 3 (40:05):
We can measure the gravity produced by at least millimeter
sized gold spheres. But strange enough, you can test these
theories in different ways and which don't involve having to
measure gravity. They do require us to measure gravity very precisely,
but we do have that already. So we have these

(40:25):
satellites in empty space which have done very precise measurements
of gravity, and so we can use that, and then
we can also use you know, people are taking gold
atoms and they're putting them in this superposition of being
in many places at once, and we can use those experiments,
and then the combination of those two experiments can be

(40:46):
used to rule out a theory in which gravity is
not quanta.

Speaker 2 (40:52):
So could you have an answer about whether or not
you're right, like in the next five years or something
that would be exciting.

Speaker 3 (40:58):
Yeah, so I think it's possible in the next five years.
So we're still doing calculations to figure out how close
we are. I think five years is probably reasonable. And
then there's another set of experiments which my colleagues to
Goatto Bosa has proposed as well as a number of
other people, which is about producing something called entanglement using gravity,

(41:19):
and those are going to require probably you know, a decade,
maybe two decades, who knows. I mean, those are very
difficult experiments. They are as difficult as building a quantum computer.
And that's another test that will require a huge effort,
but which excitingly, we can use to determine the quantum
nature of space time. So there are experiments, and I

(41:39):
think that's what's exciting about this field.

Speaker 1 (41:41):
Well, are you excited or terrified to see the outcome
of those experiments?

Speaker 3 (41:46):
I mean, I think there's a sense that you're going
to spend a lot of time doing something you want
to find out as quickly as possible if it's quite wrong.
It's not really personal in the sense that okay, you're
going to spend some time working out of theory, and
so because you've been invested yourself in it, you kind
of wanted to be true. And I think that's true
of everyone, that the strength theory is one string theory
to be true. But at the end of the day,

(42:08):
we're making an assumption, we're seeing with the theory critics,
and then we're testing it. And I think that's a
good thing, regardless of the outcome.

Speaker 5 (42:14):
Yeah.

Speaker 2 (42:14):
Absolutely, even if the answer is no, that's a good answer.
Then people can stop looking down that particular path, Like
I think it's still worthwhile no matter what the answer is.

Speaker 3 (42:23):
Yeah.

Speaker 1 (42:23):
Yeah, But before we can do those experiments, we can
try to tackle some like stubborn conceptual problems that arise

(42:47):
from interactions of quant mechanics and gravity. I was reading
your comments on solutions to the black hole information paradox
and how your approach might untangle that problem. Can you
tell us briefly how post quantum gravity solves the black
hole information paradox? What happens to information as it falls
into a black hole.

Speaker 3 (43:06):
I wouldn't just as far as to say that, What
I would say is that the paradox it kind of
loses its bite. And it's this weird thing that probabilities
seem to emerge in physics in a few different places.
As physicists, we don't like it because we believe everything
should be predictable. So the measurement problem that we've discussed
as an example of that. You know, physics only lets

(43:28):
us predict probabilities of a particle being in certain places,
and that is kind of information destruction, right, because we
start off with something which is in a definite state
and we end up with some indeterminism like where the
particle is, and that seems that's a kind of information loss.
And so probabilities come up in quantum theory in terms

(43:49):
of this measurement problem that we talked about. And then
the other place where these probabilities come up and information
loss comes up is in black hole. So in a
black hole, you throw something into the black hole which
is in a definite state. The black hole sacks that
information in and then it slowly evaporates away, and when
it finishes evaporating, it appears that we're left with just

(44:11):
a bunch of noise, a bunch of thermal radiation, and
so predictability seems to be lost. And physicists don't like that.
I'm okay with that. I think we've just lost predictability
in measurements, So why are we so unhappy about losing
predictability in black holes? And in fact, I think they're
probably related, and in this theory they are related. So

(44:32):
this theory allows for information loss because it has this
probabilistic nature. And so I spent a lot of my
time previously trying to construct a theory which allowed for
information loss. I wasn't able to do it within quantum theory,
and I don't think it's possible to do it within
quantum theory. But within this theory it's possible. And in fact,
it's a feature of the theory that you have this

(44:53):
information loss. So if you have information loss, then there
is no black hole paradox. The paradox arises in black holes.
If you insist that information is preserved, then you get
the paradoxes. So if you insist that information is not lost,
then you run into a whole bunch of paradoxes. And
so in this theory, information is lost, no paradox.

Speaker 1 (45:16):
It sort of sounds like you solve the problem by
saying it isn't actually a problem like information is lost
by a black hole, but that's okay.

Speaker 3 (45:23):
Well that was always the case for the information lost paradox,
So there was always this debate in the community where
usually general relative business to people who think space time
geometry is really what's going on, who study space time geometry,
they were always or generally tended to be okay with
information loss. Information is lost, get over it, not a problem.

(45:44):
It was the string theorist usually or the high endy
physicists who insisted that information should be preserved, and that's
when you get a paradox. So it's only in that
case that you run into various paradoxes. If you're willing
to accept information loss, then you probably don't call it
the black hole information paradox. You call it something else,
like the black hole information and annoyance or something.

Speaker 2 (46:08):
That sounds like a better title anyway.

Speaker 3 (46:10):
Yeah, not as catchy.

Speaker 1 (46:13):
So to accept that, you have to accept something kind
of weird about the universe, that there is a randomness,
that it's not predictable, that one moment is not determined
by the previous moment, or even the probabilities aren't determined,
that there's something fundamentally random about gravity itself. You must
hear a lot of sort of philosophical objections. Even if
the mathematics of your concept work and you can make predictions,

(46:35):
you can describe experiments, you must hear philosophical objections to
having a universe that works this way. Do you hear
those objections? And what are your answers to them?

Speaker 3 (46:44):
So I think as business we believe we can predict everything,
and so how can we not predict something? It must
be So it's interesting. People are okay with a breakdown predictability,
as we discussed when it comes to the measurement problem
in quantum accounts. But for some reason, I think it's
part sociological, they're not willing to accept it in black holes.
I think there's a good reason that you might not

(47:05):
accept it in black holes. And the good reason is
that you say, well, okay, but I can just imagine
that there is this randomness somewhere else which determines you know,
I can imagine that there's this hidden environment we call
it a hidden variable, you know, a hidden system, and
if I knew what that system was, then everything would
be predictable. So everything is predictable. It's just something that

(47:28):
I'm not looking at and that's I think the philosophical objection.
But you know, we know that doesn't work for the
measurement problem in quantum theory. So we know that there
could be no hidden variables, no hidden environment which could
explain the randomness in quantum theory, at least not a
local hidden variable theory. There's some very famous theorem called
Bell's theorem which tells us that, and in this theory,

(47:51):
although we haven't proven it, I think there is no
way to construct a hidden environment or a hidden system
which would everything and make everything predictable. You know. I
think the philosophical objection is you could always imagine some
other system which, if you knew about it, you would
be able to predict things. And I think that we

(48:12):
know from quantum theory, and I think this is also
true in this theory that that may not be true.
But yeah, I would say that philosophically, that's probably the
biggest stumbling block if you're going to have a problem
with this theory. I think that's the reason you're going
to be skeptical.

Speaker 1 (48:28):
So then my last question is a philosophical one. Imagine
that aliens have arrived on Earth and they're scientific and
we get to talk to them, and you get to
meet with their physicists. What do you think the chances
are that they have a community that does string theory
and a community that does loop quantum gravity, and a
community that does post quantum gravity, or that they even
think quantum gravity is a hard problem or an interesting problem.

(48:51):
Do you think that we're probing something about the universe
here or exploring questions that have to do more with
the way humans organize our thoughts.

Speaker 3 (49:00):
I feel like we understand so little about the universe.
I guess I like to think of ourselves as like these,
you know, these little single cell organisms that are kind
of slowly moving towards something they think is light. If
my dog met my cat, they would have some similar
theories about the world, but they would be pretty different,
I guess. I mean, maybe my dog and my cat

(49:22):
would have similar theories, but maybe my dog and my
amoeba would have really different conceptions of the universe. And
I imagine that's how it would be. I mean, you know,
maybe they're more advanced civilizations were just we would look
quite foolish to them.

Speaker 1 (49:38):
Let's hope when they arrived that they don't treat us
like Amba and just.

Speaker 3 (49:43):
Well they would say, okay, right, because you ignorro meba.

Speaker 2 (49:47):
That might be the best outcome.

Speaker 1 (49:49):
Yeah, all right, Well, thank you very much for your clear,
encouraging explanations of your your post quondum gravity. My actual
last question is why did you call it post to
quantum gravity.

Speaker 3 (50:01):
Oh, because one of them theories is modified in this theory,
so you have to modify it in order to make
it consistent with geometry. I mean, there's a whole kind
of literature of post quantum theories modifications to quantum mechanics,
and this fits in there, but probably in the most
gentle way you can imagine. I mean people myself, you

(50:23):
would have spend a lot of time trying to imagine
theories that were really different, you know, that would go
well beyond quantum theory, and they're actually almost impossible to construct,
and they may not exist. So this may be the
only kind of modification to quantum theory that we can make.
I don't know.

Speaker 1 (50:40):
Wonderful. Well, thanks again very much for your time and
your explanations. Really appreciate it.

Speaker 3 (50:44):
Thank you, Thanks, thanks very much.

Speaker 2 (50:54):
Daniel and Kelly's Extraordinary Universe is produced by iHeart Reading
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Really would.

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