Could we reveal quantum gravity in a tabletop experiment?

Daniel and Jorge talk about how clever experiments might provide a breakthrough in the effort to understand the quantum nature of space-time.

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, who do you think would win in a
fight theoretical or an experimental physicist?

Speaker 2 (00:13):
That depends are we're talking arm wrestling or like integration
competitions into what mathematical race?

Speaker 1 (00:22):
Then I think I would put my money into theoretical physicist.
I mean, no offense.

Speaker 2 (00:27):
Maybe we have to do the experiment, or maybe you.

Speaker 1 (00:29):
Should keep this theoretical. I don't know if you want
to pick a fight.

Speaker 2 (00:33):
Well, maybe the two sides of the field just compliment
each other beautifully.

Speaker 1 (00:36):
Is that all it takes us? Some compliments and you
guys are back as friends.

Speaker 2 (00:40):
Theories are cheap, right, They don't need money for experiments,
They just need compliments in theory.

Speaker 1 (00:44):
In my experience, hi am Warham, a cartoonist and the
creator of PhD comics.

Speaker 2 (01:04):
Hi, I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, And back in the day, I did
want to be a theorist.

Speaker 1 (01:11):
Back in the day, How old were you.

Speaker 2 (01:13):
When I started grad school? I wasn't sure if I
wanted to do experimental or theoretical physics, So I guess
I was in my early twenties, which by now is
pretty far back in the day.

Speaker 1 (01:22):
Did you actually get a choice, like they offer you
an option of which way to go, or do you
have to, like, I don't know, test into it.

Speaker 2 (01:28):
You definitely have to kind of try out and work
with the theorists if you want to be a theorist.
But you have all the options when you start grad school.
You could end up being an experimental particle physicist or
a theoretical cosmologist or whatever. All those paths are available.
You just gotta like it enough and be good at it.

Speaker 1 (01:46):
So what happened? Why didn't you pick the theory?

Speaker 2 (01:48):
I discovered I just didn't like writing down equations as
much as a theorist. They would sit there and like
develop several different mathematical fonts to write their equations in,
and I was like, wow, I'm just not loving this
as much as they're loving this.

Speaker 1 (02:00):
It sounds like you were against the idea of it
in theory.

Speaker 2 (02:04):
My experience was the experiments were more fun.

Speaker 1 (02:07):
But anyways, Welcome to our podcast Daniel and Jorge Explain
the Universe, a production of iHeartRadio.

Speaker 2 (02:11):
In which we try to blur the line between theory
and experiment. We want to talk about all the concepts
in theoretical physics that try to explain what's going on
in our world, but we also try to touch back
on the ground and understand what experiments are telling us
about the nature of reality, what is Nature actually saying
to us as she spins the story of the universe,

(02:34):
And then we try to explain all of it to you.

Speaker 1 (02:36):
That's right, because it is a pretty storied universe, full
of amazing little details and facts and things to discover
out there that we are still puzzling over and which
require all kinds of scientists to figure out, theorists and experimentalists.

Speaker 2 (02:49):
And in the history of physics we have made progress
in lots of different ways. Sometimes the theorists have come
up with a clever idea, a suspicion about how the
universe might work, with lots of cool directions for experimentalists.
Go out and check this thing, Measure how light bends
around the sun, see if you can find the Higgs boson.
Those can be wonderful directions to help unravel the mysteries

(03:11):
of the universe. But sometimes the experimentalists lead the way,
turning on particle smashers and discovering gobs and gobs of
new particles that nobody expected.

Speaker 1 (03:20):
I guess my question, Daniel is why can't you be both?
Why can't you be a theoretical and an experimental physicist.

Speaker 2 (03:26):
I'm doing my best. Actually i'm doing my best. But
the reality of academia these days is to get one
of these jobs, you have to be the world's expert
in some subfield. And that makes it really hard to
sort of live between two fields because you have to
be like the top person in that field that year.
And so if the theorists aren't sure, if you're a

(03:46):
theorist and the experimentalist aren't sure if you're an experimentalist,
nobody's going to give you that job. So you've got
to sort of get the job in one category and
then inch your way over to the other one if
you're interested. That's kind of cliquish, it's definitely very cliquiche. Absolutely,
these fields form and then they protect themselves and it
can be hard for new kinds of subfields to emerge,

(04:07):
Like right now we have the emergence of physicists who
are experts in machine learning, and people aren't sure is
that theoretical is it experimental? Because you're running a bunch
of calculations nobody's really sure. Everybody knows that it's valuable,
but we aren't quite sure where to put them.

Speaker 1 (04:21):
That's because they're robots? Are they in disguise?

Speaker 2 (04:27):
We're all just biological robots?

Speaker 3 (04:29):
Man?

Speaker 1 (04:30):
Oh, there you go.

Speaker 2 (04:31):
Aren't you the expert in squishy robots?

Speaker 1 (04:33):
I am, Yeah, Well I used to be, at least
a lifetime ago. We're a couple of lifetimes ago.

Speaker 2 (04:38):
Now back in the day. Is there such a thing
as a theoretical roboticist?

Speaker 1 (04:42):
Uh? Yeah, there's a lot of theory in robotics as well.
But no, as we I guess we're not as clique
as you're just a roboticist. If you're into robots, you're
just a roboticist.

Speaker 2 (04:51):
New York because you just build your own friends. You're like, hey, look,
I don't need people's friends. I can build my own.

Speaker 1 (04:56):
Yeah. But as you said, I guess you need both
kinds of endeavors or search. You need experimental research and
you need theoretical research in order to figure out how
things work in the universe. Because I guess you need
to come up with a theory so that you can
prove it with an experiment, and you need an experiment
to prove the theories. Otherwise there's no science.

Speaker 2 (05:13):
That's sort of a theoretical way of thinking about it,
that we come up with the theories and improve them
with experiment. Remember that sometimes experiments don't just prove theories.
They blow up theories and tell us that the universe
is different from the way we understand it and operates
in some other way we don't yet understand. Like the
photoelectric effect was a demonstration that boy, we really don't

(05:33):
understand at light and how it works, and it took
a few years before the theorist came up with any sort
of explanation for it.

Speaker 1 (05:39):
Yeah, but I guess experimenter's lunches kind of experiment blindly, right.
You usually have some sort of theory at hand when
you design your experiments, when you go out there and
turn stuff on.

Speaker 2 (05:50):
It's a bit of a raging debate right now in
experimental physics whether we should be focused on searching for
the ideas that theoretical physicists are suggesting, or whether we
should be developing strategies that are more just exploratory that
leave us open to surprises. Like when you turn on
the Hubble Space telescope and look out into space. Sure,
you want to see the things that you had in

(06:11):
mind to look at, but you're also open to like
seeing aliens waving at you, or seeing new kinds of
stuff you didn't even expect to see.

Speaker 1 (06:19):
But I guess also at the same time, we're getting
to a spot where you know, things are so complex
and so subtle and so hidden that you kind of
need to know what you're looking for in a way, right,
it's kind of hard to just like look for everything.

Speaker 2 (06:31):
It is really hard to look for everything you really
put your finger on it, especially when your data is
very statistical. If you do like a single experiment and
you get some weird result, you might be able to say, hey, look,
there's definitely something new here. But if the data are subtle,
if the new things appear as like trends in your data,
then you're right, it can be hard to know how
to find them. So then you have to play some

(06:52):
clever statistical arguments and say, well, you're the kinds of
things that we could see, and here are the ways
that we could search for them. So you have to
do a little bit more work to define the kinds
of things you might be able to see. Even if
you aren't sure which specifically might pop up in your data.

Speaker 1 (07:07):
Well, sometimes there are cases where both the theories and
the experimental lists are stumped. And that is the case
where non physics. There's kind of a big hole in
physics in terms of our knowledge of how things work
in the universe.

Speaker 2 (07:19):
That's right, at the most fundamental level, we still don't
really understand the basic rules of physics. We have two
pillars of modern physics relativity that tells us about space,
time and gravity, and quantum mechanics that tells us about
particles and forces, and we just don't know how to
bring them together. And it's important because it has to
do with one of the most basic questions in physics,

(07:41):
which is what is the universe made out of? What
is the fundamental fabric of reality? After all?

Speaker 1 (07:47):
Yeah? And is it soft and comfortable? Is what I
want to know.

Speaker 2 (07:51):
It seems to have a little bit of spandex in it.

Speaker 1 (07:53):
Here, you guys, long is a stretchy that can accommodate
all sizes.

Speaker 2 (07:56):
Because my waste is not the size it was back
in the day.

Speaker 1 (08:00):
You want the universe to kind of expand with you,
your mind and your waste. But yeah, there's kind of
a big hole in our understanding of the universe, and
it has to do with gravity. We're not quite sure
where gravity falls, whether it falls or it fits with
quantum mechanic skill theory, or whether it works the way
that Einstein envisioned in special relativity.

Speaker 2 (08:20):
Right, that's right. Einstein's special relativity tells us about light
and how it propagates. His theory of general relativity tells
us about space time and how it bends. And these
two theories are in conflict and tell us very different
stories about the nature of the universe. But so far
we haven't been able to figure out a way to
test them without building a solar system sized particle collider

(08:42):
or peering inside a black hole. So experimental physicists have
not really been able to contribute to this conversation until now.

Speaker 1 (08:50):
So the deal in the podcast, we'll be asking the question,
can we test quantum gravity in a tabletop experiment? And
right here the word tabletop I think of board games.
Is this what we're talking about? Like a little cardboard
unfolding thing with pieces, And then you test quantum gravity exactly.

Speaker 2 (09:14):
You can download the schematics from the internet and print
out your own Nobel Prize winning experiment. Do you go?

Speaker 1 (09:19):
Is it called settlers of quarks or quantum ton.

Speaker 2 (09:24):
I'll leave you to do the branding of it. But
when we say tabletop experiment in physics, we basically mean
something not like the large Hadron collider or something that
doesn't require a ten billion dollar facility staffed by thousands
of people. We mean the kind of thing a single
physicist could do in their laboratory, in the basement of
your nearby university.

Speaker 1 (09:44):
I see you're talking about a million dollar tabletop, not
a billion dollar tabletop.

Speaker 2 (09:48):
Exactly, just like everybody has a million dollar table in
their kitchen. No, it's really like a single physicist experiment,
something you could do in a reasonable physics lab, not
something people are going to be doing on their kits table.

Speaker 1 (10:00):
Well, as usually, we were wondering how many people out
there had thought about this question or perhaps have any
ideas about how to do it.

Speaker 2 (10:07):
So thanks very much to everybody who participates in this
segment of the podcast. If you've been listening for years
and would like to hear your voice speculating about the
topic of the day, please write to us two questions
at Danielandjorge dot com everybody's welcome.

Speaker 1 (10:21):
So think about it for a second. Do you think
we can test quantum gravity on somebody's table? Here's what
people had to say.

Speaker 4 (10:28):
Well, since quantum gravity is, you know, with the gravity
of the really small, I don't see why the experiments
with it couldn't be done on a tabletop. I just
have no idea what those experiments would even begin to
look like.

Speaker 3 (10:40):
Though.

Speaker 2 (10:40):
If yes, then it will come to our table soon.

Speaker 1 (10:43):
But till then, I don't think it is possible at all.

Speaker 2 (10:46):
Uh, yeah, you probably could, but probably not today.

Speaker 3 (10:49):
I do not feel like we could test quantum gravity
in a tabletop experiment because you.

Speaker 1 (10:55):
Need a lot of gravity for it to work, and
I don't think the Earth has that kind of gravity.
All right, not a lot of optimism, I like the
person who said the tabletop, I don't think so. But
maybe a desktop or on the floor, or on a shelf,
maybe mountain top maybe, yeah, tabletop on a mountain. There

(11:16):
you go, lower gravity.

Speaker 2 (11:18):
Well, we talked recently about how to measure big g
and that experiment was definitely done on a mountain side
swinging pendulums next to a big mountain in Scotland. So yeah,
you can do funny gravity experiments on tops of mountains.

Speaker 1 (11:30):
And I like the person who said probably, but not today,
Like is today a bad day for that? Were they
busy that day? How about next week? Next week work?

Speaker 2 (11:40):
Please fill out this doodle pole for when we will
win a Nobel prize.

Speaker 1 (11:44):
There you go. Yeah, I guess people didn't feel like
it could work. But let's find out, Daniel step us
through this. What is quantum gravity?

Speaker 2 (11:53):
So when we say quantum gravity, what we mean is
a theory that explains both the quantum mechanical behavior super
tiny particles, the way like electrons and photons do things
that baseballs and basketballs and mountaintops don't do. You know,
they don't move in smooth paths. They have weird quantized
energy levels. They can be in a superposition of different states,

(12:15):
like maybe they're here, maybe they're there. They can interact
with each other and interfere in all sorts of complicated
ways described by their wave function. And we want a
theory that explains gravity as we know it. That things
seem to move in these inertial pass through curved space time,
and that mass in space tends to bend the path
which affects the motion of other mass. So we have

(12:37):
these two very different theories of the universe, and so
far we can't bring them together. So quantum gravity would
be a theory that explains both these things somehow harmoniously.
But it's not a theory that we have today.

Speaker 1 (12:50):
Well, I guess maybe step us through a little bit
of what we haven't been able to bring these two
things together as far as I understand it. It's kind
of due to two things, right, Like, one is that
we haven't measure the gravitational force at the level of
quantum particles, right, that's one thing. And also we don't
know what happens to general relativity when you get down

(13:11):
to that small level.

Speaker 2 (13:12):
Too exactly, I think you put your finger on it. Really,
we don't know what the gravity is for little particles.
The gravity for a baseball or for a moon. We
think we understand and we've been able to test that.

Speaker 3 (13:22):
Right.

Speaker 2 (13:23):
We see moon's orbiting planets, we see planets orbiting suns.
We see how gravity works. But that's all really really
big stuff. What we don't know is what happens when
you have gravity for particles, because particles are super duper tiny,
which makes it really complicated. For two reasons. One is
that they have almost no gravity. Remember that gravity is
like the weakest force in the universe, and so the

(13:46):
other forces overwhelm it. You try to do experiments with electrons,
then their charge is much more powerful than their mass. Right,
the electromagnetic force is much more powerful than the gravitational
force on an electron. So it's basically possible to measure
the gravitational force on an electron.

Speaker 1 (14:03):
Can I ask why that is?

Speaker 3 (14:04):
Though?

Speaker 1 (14:04):
Like, couldn't I shoot an electron from here to London
and see if it curves with the curvature of the Earth.

Speaker 2 (14:11):
You could try that, absolutely, I think you probably shouldn't
shoot beams across the surface of the Earth without getting
signatures from everybody who might live in between. But say
you did that, the electron would be effected by all
sorts of charged particles between here and London, right, There'd
be lots of other effects on the electron which would
swamp out any gravitational effects.

Speaker 1 (14:31):
But I guess maybe, like from a satellite, I'm thinking,
you know, I just shoot a whole bunch of them,
and wouldn't the effects from other things kind of even
out If you shoot a bunch of them out, Like,
don't we have like quantum drives or like electron cannons.
What happens if I just shoot them out there in space?
Do they keep going straight or do they bend?

Speaker 2 (14:50):
Yeah, you could build an electron gun and put it
in space and shoot them out, but still it would
be dominated by the effects of other particles. Remember space
is not totally empty. There's cosmic microwave background photons there,
there's other charge particles from the Sun, and all of
these would dominate the fate of that electron. Really, the
problem is that the charge is more powerful than the mass.

(15:11):
We talked about this once, and this is either because
gravity itself is just weaker than the other forces for
reasons we don't understand, or because electrons are just packed
with a lot of charge compared to how much mass
they have. You can think about it sort of either way.
But that just means that the effect of gravity is
tiny compared to the effect of electromagnetism. So to do
that experiment, you'd need to isolate those particles from any

(15:33):
sort of effect. And today we'll talk about an experiment
that's going to try to do that, all.

Speaker 1 (15:37):
Right, So then that's where quantum gravity comes in. It's
kind of a is it a theory or an idea
that tries to bring these two big ideas together.

Speaker 2 (15:44):
It's not a theory. It's like a category of theories.
It's like a dreamt of theory. What we want is
a theory that bring these two things together. We don't
have one. We don't know what the theory of quantum
gravity is. You know, sometimes you have like ten different
theories that describe the universe. The experiment has to go
off and tell you which one is correct. Right now,
we have zero. We have zero theories that explain quantum

(16:06):
mechanics and gravity at the same time. So we sort
of need an experimental result to be like, hey, this
is the right direction though where hey, here's something to
grab on to, here's a clue. But there's the second
reason why these experiments are difficult that we didn't get
to yet. One is just that gravity is so weak,
and the other is that these particles do things that
we don't know how to explain with gravity, Like particles

(16:28):
don't have smooth paths. It's not like the electron is
always somewhere, has some velocity. You know, you want to
calculate the gravity of an electron, you have to know
where it is, so you know how far away it is,
you can calculate it's gravity. But electrons don't have specific
locations that have probabilities, so we don't know. For example,
if an electron, when it has probabilities to be in

(16:50):
multiple places, does it mean it has like multiple different
possible gravities. We just don't know how to do gravity
for things that have uncertainties in their locations.

Speaker 1 (17:00):
You mean, we don't know how to do that if
gravity was not a quantum force, right, Like you're sort
of assuming that. I guess you want gravity to be
like a quantum force like the other forces that we
know about, right.

Speaker 2 (17:13):
Yeah, that's sort of one of the basic questions when
you want to build the theory of quantum gravity, like
is it a quantum force? If so, then two electrons
interacting gravitationally wouldn't like collapse each other's wave functions. Some
bits of one wave function would interact with some bits
of another wave function, and they could do all sorts
of weird quantum interactions. But if gravity is actually a
classical force and not a quantum force, then it would

(17:35):
collapse the wave function sort of like when you use
a detector in a double slit experiment, it forces the
particle to pick one of the options instead of the
other one. So we just don't know, like is gravity classical,
is it quantum mechanical. We just don't even know where
to begin.

Speaker 1 (17:50):
And when you say classical, you mean like basically not
quantum mechanical, like not fuzzy, not uncertain.

Speaker 2 (17:57):
Yeah, exactly, we mean not quantum is sort of an
overused word. Some people say classical to me, not relativistic,
like Newtonian, but today we mean not quantum mechanical. So
we don't know if gravity, like really is just classical
the way Einstein described it, thinking about space as smooth
and continuous and everything having passed, or if it is

(18:17):
a quantum effect, in which case it could either be
a force like you suggested, mediated by weird gravitons, or
maybe like space itself is quantum mechanical and uncertain. If
gravity is the curvature of space, maybe space itself can
be like maybe bent here and maybe bent there in
some weird quantum mechanical way. There's so many possible directions

(18:38):
for quantum gravity, nobody really knows which one is going
to build a viable theory that even can do calculations.

Speaker 1 (18:45):
All right, well, let's get a little bit deeper into
quantum gravity and whether or not we can test it
and test it for under a billion dollars, because I
guess the cheaper the better. We'll dig into that, but first,
let's take a quick break. All right, we're talking about

(19:11):
quantum gravity and whether or not that is a thing
at all, whether it will bring together quantum mechanics in
general relativity to give us one theory of the universe,
and whether or not we can even design experiments to
test such a theory.

Speaker 2 (19:25):
I like your threshold of a billion dollars.

Speaker 1 (19:27):
Yeah, well though these days with inflation, maybe that's more
like ten billion.

Speaker 3 (19:31):
Though.

Speaker 2 (19:33):
You know, if we could spend a billion dollars and
get the answer to quantum gravity, I'm pretty sure we
would do it. The truth is, the experiments might cost
a lot more than one billion dollars.

Speaker 1 (19:43):
All right, well, let's dig into the cost of these experiments.
How can we test quantum gravity and figure out whether
or not it's a real thing or not.

Speaker 2 (19:50):
Well, you had sort of the right idea, which is like,
let's just zoom in on a quantum particle and look
at its gravity somehow. But remember the scale of things
we're talking about here, Like these particles are super duper tiny,
and the effects we're talking about what happened on really
really short distance scales. Like gravity gets more powerful when
things get closer together. In order for gravity to be

(20:10):
powerful enough for us to really test it, you need
to get things together to like the Plank scale distances
we're talking about, like ten to the minus thirty five meters.
So until recently it seemed like, well, the only way
to test quantum gravity is to have like a microscope
that can see effects at the scale of ten to
the minus thirty five meters, which felt almost impossible.

Speaker 1 (20:31):
Now, I guess, pain me a picture here of what
it is that you would be trying to do. Like,
for example, what if I just take a bunch of
hydrogen atoms. Like a hydrogen atom is just an electron
and a proton, so it's perfectly balanced in terms of charge.
And I know that if I stick up bundle I'm
in a container, they'll sort of tend to fall down
because of gravity, right, They'll sort of accumulate the pressure

(20:52):
of the hydrogen tank will be higher at the bottom
than at the top. That means gravity is working on
them and it is pulling them down. Can I build
some sort of model or theory that kind of models
or tells me how it's working at the quantum level.

Speaker 2 (21:05):
Well, there's the theoretical difficulty, and then there's the experimental difficulty.
On the theoretical side. Like we've tried to build those theories,
they just don't work. Gravity is complicated because everything is
affected by it. It's not like electromagnetism where you can
like shoot out photons and those photons themselves don't feel electromagnetism, right,
Photons don't interact with other photons. Gravity interacts with everything

(21:28):
with energy. So when you try to build a quantum
theory of gravity, like including the exchange of gravitons, those
gravitons amid other gravitons which feel those gravitons, and it
gets very hairy, very quickly. We talked once about the
strong nuclear force, which has a similar property that it's
gluons amid other gluons which affect other gluons, and it's

(21:49):
a nightmare to do any calculations. Gravity is even more
complex than that, and that's sort of one of the
reasons why it's been so difficult to build a theory.
So anytime people build a theory of quantum gravity, it
just sort of predicts nonsense. We just can't mathematically make
it work. And then there's the experimental challenge. And what
you're talking about is like trying to build a setup
where you can see the gravitational effects on particles. But

(22:12):
the experiment that you describe like a bunch of hydrogen,
you know, those are classical effects. The fact that those
hydrogen atoms are quantum particles is irrelevant to the fact
that they have more pressure on the bottom of the
tank than the top of the tank. Oh, I see.

Speaker 1 (22:24):
You're trying to kind of like see what happens to
gravity at the quantum distance level. Right, that's kind of
the problem, right, Like you might be able to design
a hydrogen gun something that shoots hydrogen atoms and you
can track how the gravity affects its path, maybe, but
that doesn't necessarily tell you whether or not there's like
uncertainty or whether the there's fuzziness at the you know,

(22:48):
really small distance.

Speaker 2 (22:50):
Exactly in the same way that like every time you
toss a baseball, and principle you're tossing quantum objects right
at a baseball. That just a bunch of quantum objects, and
definitely they're feeling gravity. We're not asking like, do electrons
and protons feel gravity? We're pretty sure they do. We're
asking is how does their quantum mechanicalness interact with their
gravitational attraction, you know, when they're doing their weird quantum stuff.

(23:11):
How does gravity play a role with that? You know,
if you have a particle that like has a possibility
to be here and they're simultaneously, what is its gravity?
So you've got to get something to be showing as
quantum effects, which means really small distances and revealing its
gravitational interactions, which requires really really large masses, which is
why some people are excited to see inside black holes,

(23:33):
because that's where you have like, really really really big
masses squeeze down to quantum distances, and so what's going
on inside a black hole would really tell us about
the nature of quantum gravity and therefore the deepest nature
of space time itself. Of course, we can't see inside
black holes, so those secrets are hidden from us.

Speaker 1 (23:53):
Yeah, you might want to let that one go. It
seems like we're never going to find out what's inside
of a black hole.

Speaker 2 (24:00):
There's even a theory called cosmic censorship that suggests will
never be able to answer this because the answers are
always going to be hidden behind some weird horizon. It's
sort of a pessimistic approach.

Speaker 1 (24:10):
WHOA, I didn't know there was a ratings board for
the universe.

Speaker 2 (24:15):
And there are even some theorists that suggests this whole
enterprise is a waste of time. Like Freeman Dyson, the
guy who thought of Dyson's fears. He likes to think
that we live in a dualistic universe, that quantum mechanics
and gravity just sort of like rule in different regimes
and they never actually overlap at any place where they
come into contact is hidden from us by these event horizons.

Speaker 1 (24:35):
Like maybe gravity is classical and it's not quantum. But
you're saying, or he's saying that at the quantum level,
there's things that are happening that you will never find out.

Speaker 2 (24:46):
Yeah, exactly that maybe you don't have a single theory
at the universe. You like two theories and each have
their own regime and they never overlap anywhere we could
test them. But a lot of people don't really like
that theory. I really don't like that theory because I
want there to be one theory of the universe, one
thing that explains everything, and I'd love to see these
two different concepts battle it out. I want to force

(25:08):
the universe to show us what the answer is.

Speaker 1 (25:10):
But I wonder couldn't they Couldn't he be right though, Like,
couldn't it just be the gravity, you know, bend space
and quantum fields and quantum particles exist in that bent space.

Speaker 2 (25:21):
Yeah, he could be right. But if we can come
up with some experiments that force quantum mechanics and gravity
to speak at the same moment, to say, like, all right,
here's what happens when you have a particle that has
two possibilities and it has some gravity, then we'll know.
And maybe he's right. Maybe gravity really is classical. And
what happens when particles interact gravitationally is that their wave

(25:44):
functions collapse, because that's what happens when classical objects interact
with quantum objects. But it sure would be nice to know.

Speaker 1 (25:51):
Yeah, gravity is pretty classic. So talk to us a
little bit about how we've been trying to study this
or get answers to this question.

Speaker 2 (25:59):
So, other than wishing we could see inside a black hole,
the other typical tool in our toolkit is a particle collider.
You build a big particle smasher, you pour a lot
of energy into one tiny little spot. Then you can
like break open bonds. You can see how the pieces interact.
But in order to see gravitational effects, you would need
so much energy. You'd basically need like the Plank energy.

(26:23):
It would require building a collider that's like the size
of the galaxy in order to get enough energy into it.
Or some calculations suggest if you build a particle collider
that big, it would collapse into a black hole.

Speaker 1 (26:33):
Wait, why do you need so much energy?

Speaker 2 (26:35):
Because gravity is really really weak, which means it operates
on really small distance scales. In order to get to
small distance scales, you need a lot of energy. It's
sort of like the de Burgly wavelength, right, Like the
wavelength of a particle is inversely proportional to its momentum,
and so the more momentum an object has, the smaller
its wavelength. And you want to see like really really

(26:56):
short distance effects, you need really really high energy probes.
So you need like super duper high energy particle collisions
in order to see things happening at really short distance scales.

Speaker 1 (27:07):
Why because I guess the more energy two particles have
when they smash into each other somehow, that gives you
more resolution in space.

Speaker 2 (27:15):
Yeah, exactly, The more momentum the particle has, the smaller
the wavelength of their wave function. You can think of
the motion of every particle is described by a little
wave function that determines what happens to it, the same
way you can think of like light as a wave, right,
it's wiggling around, And if you're using light to see things,
you can only really see things that are the wavelength
of that light or larger. Anything smaller than that wavelength

(27:38):
the photon sort of can't interact with it. And so
you want to see really really small effects, you need
really really high energy photons or in our case, we
need really really high energy particle beams to see very
very short distance interactions.

Speaker 1 (27:51):
Right, Because I guess you need things with mass, right
to test the quantum gravity or gravity at the quantum level.
And so that's also true for things with mass, Like
the faster they're going, the smaller they are, is.

Speaker 2 (28:03):
That what you're saying effectively, the smaller their wavelength. Is.
Another way to think about it is that you need
enough energy in those collisions to make gravity stronger, Like
you want to overcome the electromagnetic force and the strong
force and make gravity as powerful as those other forces
so that you can see its effects. You need to
pour a lot of energy into those collisions, because the
power of gravity is linked to the mass and to

(28:25):
the energy of these things. So you pour enough energy
into one little location, you'll get a very strong gravitational interaction.
So if we want to see the gravitational effects on
quantum particles, you need to pour a lot of energy
into one little spot.

Speaker 1 (28:39):
And is that the only way to do it through
particle colliders? Isn't there some like I don't know, like
aim your beams better approach or make smaller wavelength particles.
I don't know, Like can we do this without making
a black hole in our solar system?

Speaker 2 (28:52):
The short answer is no. I mean, we do our
best with particle beam aiming already. But really the limitation
is the energy of the particles, and we have them
going as fast as we can, and the wavelength of
the particle is determined by its energy. So really sort
of at the limit there. We talked recently about other
strategies for accelerating particles that might make it smaller, faster, cheaper.

(29:12):
So there might be a breakthrough in accelerator technology which
could leap us up like a factor of ten or
one hundred. But we are like a factor of a
trillion away from being able to test quantum gravity in
particle colliders. So really, nowhere in the near future will
particle colliders be able to answer this question?

Speaker 1 (29:29):
All right, well, I think part of what we're going
to be talking about here today are experiments that have
kind of ideas about how to test this without destroying
the Solar system. And they involve diamonds and lasers and
space lasers. No tabletop lasers in space. No, no tabletop
lasers in Pasadena. Oh that take up space in Pasadena.

Speaker 2 (29:52):
A table near you?

Speaker 1 (29:54):
All right, Well, to dig into it, Daniel, What is
the first of these experiments?

Speaker 2 (29:57):
So the first of the experiments involves diamonds, and the
goal here essentially is to create a situation where a
particle has the probability of being in two places at once,
and then you test its gravity. You see, if it's
gravity really is sort of like split between its two
possible locations, or if when you probe it with gravity,

(30:18):
it somehow collapses into just having one possible location. Because
remember this, quantum particles can do this weird thing. They
can be in a superposition like if there's two possibilities
for an electron, it doesn't have to choose A or B.
It can maintain both possibilities until something interacts with it classically,
which forces it to choose. That's the weird thing about
quantum mechanics and something we don't understand. So this is

(30:41):
very hard, of course, because particles are very small and
they're very delicate. But they've come up with a clever
way that they think might be possible, and it involves
electrons embedded in falling diamonds.

Speaker 1 (30:52):
Sounds like a rap video where there's like money and
diamonds falling from the sky. Break it down for us.
How does his experiment work?

Speaker 2 (31:00):
Is you take a very tiny diamond and has a
nitrogen added inside of it, like embedded inside the diamond.
And this has a cool property, which is that if
you zap it with a laser, the electrons have a
probability to absorb that photon, which case they flip their
spin to be up, or to ignore that photon and
flip their spin to be down. So you shoot a
laser at this diamond, and now it's in a quantum

(31:22):
superposition of two possibilities. Maybe the electron inside there on
the nitrogen is spin up, and maybe it's spin down.
So you have your particle now in a quantum superposition.
But what you need is for it to be in
a quantum superposition of two locations rather than two spins.
So then you pass it to a little magnetic field.
The magnetic field will push it left or right based

(31:43):
on the spin. So you have this falling diamond which
passes through a magnetic field and it either moves left
or it moves right. Now if it's in a quantum superposition,
and then it has both possibilities to move left and
to move right. So now it's location depends on this quantumness.
It's the same thing for another diamond nearby. Now you
have this pair of falling diamonds, each of which has

(32:06):
the possibility to be in two slightly different locations, and
you see how they interact. Do the possibilities for one
diamond interact with the possibilities for the other diamond, or
do the two diamonds like collapse each other's wave functions.

Speaker 1 (32:18):
I see, So you embed a little nitrogen atom into
the diamond Ezaly with a laser, and now the nirogen
atom has quantumn certainty, which kind of extends to the
whole diamond. Is basically what you're saying, right like if
I don't know, there's quantum certainty about the electron into nitrogen,
and that means there's quantum certainty about the whole diamond
because it could be swinging right or left. And now

(32:40):
if you put two of them together really close, they
should interact with gravity. And so now you have the
system where you have two quantum objects interacting with gravity exactly.

Speaker 2 (32:50):
And they have some really clever mathematical way to tell
if the two diamonds interacted in a quantum way or
if the two diamonds interacted in a classical way, Like
if they interacted in a quanti when you measure the
spins of those electrons after they fall far enough in
your experiment, they'll have some cool correlation to them, and
if they interact it in a classical way, then they'll

(33:10):
be uncorrelated, like whether they're spin up or down will
just be random, and so because of the weird rules
of quantum mechanics, you can tell whether quantum mechanics has
been at play in the gravitational interaction, Like did gravity
cancel out the quantum mechanic effects because it's really just
a classical force, or did it allow the quantum uncertainty
to be maintained, meaning that gravity would be a quantum

(33:32):
mechanical effect not a classical effect.

Speaker 1 (33:34):
Well, I guess quantum mechanics aside. Can you measure gravity
the force of gravity by just dropping two diamonds together
and seeing if they attract each other? Is that like
a real thing you can do.

Speaker 2 (33:44):
It's a real thing you can try to do. That's very,
very difficult because little diamonds have very very gentle gravity,
and so this is not something we think we can
do today. There's a group in the UK that thinks
that they can figure out how to do this, and
there's lots of complicated steps involved and they're hoping to
maybe pull this off sometime in the next ten years.

(34:05):
There's a lot of really complicated moving parts involvement getting
the nitrogen inside the diamond, flipping its spin with a laser,
beam getting two pairs of diamonds to fall simultaneously close
enough each other that maybe they have a gravitational interaction.
Now you don't actually have to see any sort of
like gravitational pull. You're not measuring like how far did
the diamond move because of gravity. You're just bringing them

(34:27):
close enough together that you think gravity is at play.
The gravity like wakes up and says, ooh, there's something
going on here. You don't have to measure the gravity.
You just have to see if gravity messes up the
quantum state. I see.

Speaker 1 (34:39):
But to measure the quantum state at the end, wouldn't
you be doing something like measuring that whether or not
the two diamonds were attracted to each other gravitationally or not.

Speaker 2 (34:47):
No, all you need to do is measure the spins
of those electrons embedded inside the nitrogen atoms in the diamonds.
You don't have to see the gravitational effects directly. It's
sort of like in the double slit experiment when you
add a detector and that ruins the interface spearance. We're
adding gravity to a quantum interaction and seeing if it
ruins the interference or not.

Speaker 1 (35:05):
M But would that necessarily tell you anything about quantum
gravity or gravitons.

Speaker 2 (35:12):
It wouldn't tell you that much, but it would be
a powerful clue. It would tell you if gravity is
classical or quantum mechanical. Like, if gravity is classical, it'll
act like a detector and it'll collapse those wave functions
and destroy this interference. If gravity is quantum mechanical, it won't,
and everything quantum mechanical will stay quantum mechanical, and you
get all sorts of weird interference. So that just tells

(35:32):
you if gravity is classical or quantum mechanical. It doesn't
tell you like, oh, space is quantized, or oh there
are gravitons. It doesn't tell you which theory of quantum gravity,
but it is a powerful clue. It would mean, for example,
if we know gravity is quantum mechanical, the Freeman Dyson
is wrong about classical gravity and quantum mechanics being able
to play together.

Speaker 1 (35:52):
Yeah, he could be wrong, in which case he might
need to stick to them making vacuum tees. All right, Well,
that's one experiment, and I guess it's in progress. I
guess they're designing it or making it. Where are they
with that?

Speaker 2 (36:05):
This physicist at University College London who's leading a team
of researchers who are trying to make this work. And
there's folks in Santa Barbara as well, and they're trying
to work on this. But you know, there's a lot
of complicated steps and making this thing do its stance
and being sure you know, what they're doing is a
lot of pieces involved, lots of complicated experimental cleverness really
required just to be able to do this test. So

(36:26):
they're hoping sometime in the next ten years to be
able to pull this off.

Speaker 1 (36:30):
All right, Well, let's get to the second of these
potential experiments to measure quantum gravity. We'll dig into that,
but first let's take another quick break. All right, we're

(36:50):
talking about quantum gravity and whether or not it's a thing,
whether gravity is quantum mechanical or is it pretty and
classical and doesn't care about quantum mechanics and this weirdness
of things being uncertain, And so we talked about one
possible experiment that it might look at that using falling diamonds.

(37:10):
And there's another interesting potential experiments happening also, right.

Speaker 2 (37:15):
That's right, And this one is being developed and built
in your.

Speaker 1 (37:18):
Backyard, like literally my backyard.

Speaker 2 (37:21):
Look at your window.

Speaker 3 (37:22):
Man.

Speaker 2 (37:22):
You ever wonder what those people are told you?

Speaker 1 (37:23):
What?

Speaker 2 (37:25):
No, it's at cal Tech. Both the theorists and the
experimental list are at Caltech, and it's a really cool idea.
And what they're trying to do in this experiment's completely
different from the other one is try to see if
space itself is quantum mechanical. Like, if gravity is quant
mechanical and there are gravitons, then that would mean that
graviton should be like popping out of the vacuum all

(37:48):
the time, the same way that other quantum particles are.
Like if you go out in the middle of empty space,
there's nothing there, there's still always a little bit of
energy in the quantum fields, which means that like those
fields can turn into particles briefly and then back into
potential energy. So if space itself is quantum mechanical, if
gravity is quantum mechanical, then gravitons should also be popping

(38:10):
out of the vacuum. There should be like effectively tiny
little ripples in space making quantum size gravitational waves.

Speaker 1 (38:19):
WHOA wait, I think you just confused me a little bit.
So I thought there were two possibilities. Either gravity is
quantum mechanical or space is quantized. Which one are you
talking about here?

Speaker 2 (38:28):
Here, we're talking about gravity being quantum mechanical, that there
exist gravitons which mediate the force of gravity, which in
this theory would be a quantum force like the other
forces in the universe.

Speaker 1 (38:39):
Okay, so we're not talking about quantizing space itself.

Speaker 2 (38:42):
That's right. We're not talking about quantizing space and like
a space foam. But we're talking about space being filled
with a quantum force of gravity, which would have fluctuations
in it, right, And those fluctuations would be like quantum
gravitons popping in and out of the quantum gravitational field.

Speaker 1 (39:00):
I see. So anything that is quantum or has a
quantum field, by its nature, by its kind of statistical
random nature, has these particles popping out of nothingness. But
doesn't it need some sort of like energy to it.

Speaker 2 (39:11):
It does, But quantum fields always have energy to them.
They can never relax down to zero because the uncertainty principle,
the minimum energy level of a quantum field is always
above zero energy, which is why there's always energy in
quantum fields, which is why there's energy in all of
space because of its quantum nature.

Speaker 1 (39:28):
Well, that's kind of an odd idea, Like what happens
if a graviton appears out of nothingness, well.

Speaker 2 (39:33):
Mostly almost nothing, because gravitons would be super duper tiny,
gravity is super duper weak, and so it would be
basically impossible to see these things what have effects on
super tiny distance scales we typically can't probe. But a
theorist that Caltech, Catherine Zurich, came up with this idea
that maybe gravitons can all work together. Instead of just
looking for one graviton, maybe you can look for like

(39:55):
larger effects of graviton sort of working together to make
something else emerge from this quantum craziness. And she designed
an experiment to maybe see that.

Speaker 1 (40:05):
Hmmm, interesting, Well, we actually have an interview of Daniel
talking with professor Catherine Zurich from cal Tech about her
idea for this experiment.

Speaker 2 (40:14):
That's right. Kathy and I have known each other since
we were POSTOCX, and so I called her up and
asked her about her crazy idea to not build a
black hole in Pasadena.

Speaker 1 (40:22):
I feel like it's a little says, you have to
throw that disclaimer in there, it's like, what are you
guys doing? I am not destroying your town if that's
what you're asking. First of all, let's get that clear.

Speaker 2 (40:35):
That didn't make you feel any better.

Speaker 1 (40:37):
Nobody asked I wasn't something I was concerned about before.

Speaker 3 (40:41):
All right.

Speaker 2 (40:41):
In that case, I'm also not testing any nuclear weapons
in Pasadena.

Speaker 1 (40:44):
Oh good, thank you. What else are you not doing
in Pasadena? Let's go down the list. All right. Well,
here is Daniel's interview with Professor Catherine Zurich from cal Tech.

Speaker 2 (40:55):
All right, so it's my pleasure to welcome Professor Katherin Zurchod.
Thank you very much for chatting with us.

Speaker 3 (41:02):
It's my pleasure to join you.

Speaker 2 (41:04):
So help me understand, first of all, how it's possible
at all to see effects of quantum gravity. We understood
for a long time that these things were just on
the plank scale. How do they sort of work together
to emerge to some signal we can see experimentally.

Speaker 3 (41:19):
So it's just like smoke. So if you ask yourself
the question how to smoke spread? So there are interactions
of molecules on very short distance scales, much shorter than
what we can observe and yet you can see the
effects of those short distance interactions simply by waiting a
while for the effects of those short range interactions to

(41:44):
accumulate over time. So that's a physical analogy for what
we're interested in doing. So we have these quantum fluctuations
on very short distance scales. So in this case it's
the plank length, which is about to into the minus
thirty five meters. And the idea is that if those

(42:06):
quantum fluctuations accumulate over long times, then we can observe them.
They're still very small, but we can observe them then
with sufficiently precise instruments.

Speaker 2 (42:21):
So what makes quantum fluctuations add up to make a
microscopic effect and what makes them not because sometimes they don't. Right,
you have like a bunch of electrons in a baseball,
they have all such fluctuations those average out to nothing.
You can see what makes these guys add up over
longer distance scales.

Speaker 3 (42:39):
So it's really the fact that you're losing information. Any
measurement that you make is over a finite time. So
you know, I turn on my instrument, let's say it's
in an intraferometer, and the light goes out. It comes
back and I make a measurement of it. And so
what it does, what an instrument does, is it defines

(43:00):
what we call a horizon. So there's a region of
the space time that I measure and there's a region
of the space time that I don't, and so that
leads to a quantum mismeasurement effect that accumulates over time.
So you're absolutely right that, you know, normal systems, where
we can confine all of our information to a particular region,

(43:22):
there's no information that's going to accumulate over time. But
in this case, we can't actually confine quantum fluctuations. There's
just part of the space time that we can't measure,
and so what we're doing now is quantifying how much
of that information is lost over the period of time
that I make that measurement.

Speaker 2 (43:42):
Very cool, and so what kind of models of quantum
space time is as sensitive to generally any kind of
model where space time has quantum fluctuations or only specific
sort of kinds of ideas.

Speaker 3 (43:55):
So what we're trying to show is that this effect
occurs very generally across the space of theoretical ideas that
people explore, you know, commonly, So what do I mean
by that exactly, So we're still trying to understand a

(44:18):
precisely what are the minimal sets of requirements that you need.
At minimum, we need quantum fluctuations at the plank scale,
so that has to be there, and those quantum fluctuations
have to accumulate into the infrared. And there are various
ways that we can see that. We can see it

(44:39):
actually coming out in a quite broad range of theories
where we can just write down some general properties of
the theory and then crank through it and we see
this effect come out. So we think it's pretty generic.

Speaker 2 (44:53):
Wonderful, And so why can't existing interchometers like LEGO, which
is already very very precise, why can't it's signatures of
this quantum fluctuation.

Speaker 3 (45:02):
So we actually think that LEGO is not very far
from being able to see it. But one of the
reasons why LEGO is not optimally sensitive to this signal
is because they recycle their light by which I mean
the light beam goes out and it comes back, and
they don't make a measurement of it. After one round trick,

(45:24):
it actually goes out and comes back many times before
they make a measurement of it. And so for the
signals that they're interested in, which come from you know,
let's say black holes merging, that's fine because there's a
classical source that generates a wave at some frequency. In
this case, we're also interested in gravitational waves, but they're

(45:45):
gravitational waves that come from the vacuum fluctuating, and they're uncorrelated.
If I measured the system over time scales that are
long in comparison to the light crossing time, So the
fact that ligo weights and beam goes out and comes
back many times before they measure it means that they're
actually averaging down their signal, and so they have a

(46:07):
reduced sensitivity to it in comparison to if they had
this same that they could measure the same space time fluctuation.
But they did it after the light just went out
and came back. Then we claim that you can actually
see this signal.

Speaker 2 (46:20):
Do these space time fluctuations look different from a gravitational
way you could get from black hole collisions, for example.

Speaker 3 (46:26):
Yeah, they do. So one thing that's different about this
signal in comparison to what you would get from let's
say black hole mergers is in that case, the signal
is the signal. It doesn't depend on my measuring apparatus.
If I have a gravitational way of coming in at
some frequency. It's like your radio station is broadcasting something
and it has a frequency, and that's just you know,

(46:46):
you tune it to some station, and that's what it is.
In this case, what you measure actually depends on your apparatus,
like your interferometer. So if I have a smaller apparatus,
my signal is going to be coming in at a
higher number radio station. Then if I have a bigger apparatus,

(47:07):
then it comes in at a lower frequency station. The
reason for that is because it's the quantum mismeasurement. And
of course how much you're mismeasuring the space time depends
on how big you know, the volume of space time.

Speaker 2 (47:18):
You're measuring affects your horizon.

Speaker 3 (47:21):
Yeah, it depends on the size of your horizon. That's
another way of saying it. It depends on the size
of your horizon, depends on how many quantum degrees of
freedom are fluctuating inside your volume, which depends on how
big your horizon is. And so as a result, you know,
you would really know about this signal. First of all,
it would have a very particular shape, but it would
depend on your measuring apparatus, So you could compare between

(47:44):
different instruments and then start to tell what the source
of it would be.

Speaker 2 (47:47):
And can you also see things unexpected, like if there's
a general enough detector that you might see things that
aren't these quantum fluctuations, then aren't gravitational away some black
holes but something else, you know, surprise?

Speaker 3 (48:00):
Yeah. Sure, So these instruments that were interested in building,
they can be sensitive to anything that's generating gravitational waves
in that same frequency range. So the signal definitely has
to be predictive enough to be able to tell apart
different sources. And our claim is that the signal has
very particular you know, frequencies that it's peaked at. It

(48:22):
has angular correlations, like it depends on the angle between
the arms and your interferometer. So therefore you'll be able
to tell what the source of these gravitational waves.

Speaker 2 (48:33):
Are and what's the sort of timeline like best case
scenario when you guys can build this thing and discover
quantum gravity.

Speaker 3 (48:39):
Yeah yeah, So we've got the first bit of funding
to come in and my colleague Lemacullor who's spearheading this
effort here at Caltech. You know, his lab is ramping
up on this. There are some technological objectives that they

(49:00):
have to demonstrate, and they have to do R and
D because they're proposing a novel readout scheme for these interferometer.
What we have proposed is to have a demonstrator apparatus
that would kind of scrape the signal. Okay, we're talking
about two sigma kind of sensitivity in five years, so

(49:24):
I think to really start to see this, you know,
like five sigma, you're just really confident you can start
to test various aspects of it. I think we're probably
talking the ten year timescale.

Speaker 2 (49:37):
So I've read your paper. There's a lot of nice
theoretical maneuvers in there. My question to you is, do
you believe this is going to be real? Like you
turn this thing on in ten years? Nature tales you
an answer. What's your confidence that this is out there
that you're going to see it?

Speaker 3 (49:52):
Yeah, so it doesn't seem to be going away. Let's
put it that way. When you see something in a calculation,
you know, you try to test it by doing a
different calculation that behaves differently. You know, it has different
theoretical systematics, and the kinds of things that you could
mess up in the calculation are different, so on and

(50:14):
so forth. And then you also check for whether it's
in conflict with anything that you know. And through the
process of doing this, you know, based on my experience,
when you try to build a theory, oftentimes it'll fail
and then you try to fix it up by adding
other things to it. This has not been like that.

(50:35):
If it seems like it's going to fail for some reason,
it means that you should just stop and wait and
try to understand what's there better, because it fixes itself.
So to me, that's an indication that there's something there.
It hangs together in a very self consistent way, and
so from that point of view, I find it theoretically

(50:57):
very attractive, very interesting. It's right now, I don't want
to tell nature what to do. Right. Nature gets to decide.
You know, there are some things that go in right,
there's this fundamental fluctuations and then space time you know,
needs to remember right, so there needs to be the
sense in which you're losing information. And if those two
things are there in nature, and we certainly know lots

(51:19):
of analogous physical systems where that happens, then we'll see it.
But at the end of the day, nature decides. And
that's one of the things I really like about this
problem is I can write these things down on paper
and they're beautiful, and I'm understanding more things about it
from a mathematical perspective. But at the end of the day,
nature gets to decide.

Speaker 2 (51:40):
All right, Well, we look forward to hearing nature's side
of the story. Thanks very much for joining us today.

Speaker 1 (51:45):
All right, pretty interesting. I'm super impressed you can talk
to a theorist. I thought you guys spoke different languages
and didn't like each other.

Speaker 2 (51:52):
They mostly speak in Greek symbols exactly, but sometimes I
can translate. These days, I'm trying to move a little
bit in the direction of theoretical physics, so it's really
fun for me to talk to these folks. But yeah,
they think on a whole different plane of existence. But
what's really cool are theorists who propose experiments, who develop
new techniques and new ideas that allow experimentalists to maybe
force the universe to reveal something about its nature. And

(52:16):
the story of this one is similar to the story
of a very similar experiment, which is LIGO, the innerferometer
that looked for classical gravitational waves. That was originally just
a theoretical idea, and experimentalists were like, all right, let's
try to build it, see if we can find it,
and they did. This is like the quantum version of it,
which would look for little quantum ripples in space time,

(52:37):
basically little quantum gravitational waves. And the experiment itself is similar.
It's a little innerferometer. Like shoot laser beams back and forth,
see how they overlap, and see if you can catch
a graviton interfering with those laser beams.

Speaker 1 (52:51):
Hmmm, because the gravit times would be sort of like
bedding space, is that the idea? Because gravity can't interact
with footon or candy.

Speaker 2 (53:01):
Gravity doesn't interact with photons in a sort of Newtonian
way because photons have no mass, but gravity does bend space,
and photons move through that bend space, and so yeah,
you're exactly right, Like a little gravitational quantum fluctuation the
kind she's looking for, would affect the shape of space
for one of these beams and would sort of knock
a photon out of the path. And that's what they're

(53:23):
looking for.

Speaker 1 (53:24):
The idea is that like a graviton would pop out
of nowhere, it pops out, it bends space around it,
and maybe it will deflect the photon. Is that the idea.

Speaker 2 (53:32):
That's the idea. But it's not one single graviton that
would be totally invisible. It's this effect where a lot
of gravitons are working together. And the super duper weird
thing is that this effect only happens when you're making
a measurement. It's a quantum effect. It comes from not
being able to see the whole universe. So she's imagining
space filled with all these gravitons, and when you make

(53:53):
this measurement, it can only be affected by like a
certain bubble of the universe, a bubble of the universe
that's like close enough to you that light can travel
to you. Because you create this information horizon, you limit
like the wavelengths of these gravitons, and so only some
of them can talk to your experiment, and that's what
creates this weird effect. And I'll be totally honest, there's
a lot of math there that I just don't even understand.

(54:15):
But she's been trying to prove to herself that this
works or that this doesn't work, and the math just
keeps holding together no matter how she probes it. So,
as you heard maybe in the interview, she really believes
this is real.

Speaker 1 (54:26):
And so the idea is that you could maybe build
this experiment on a tabletop like it could be, you know,
a small experiment to prove a huge thing like quantum
gravity exactly.

Speaker 2 (54:35):
Lego classical gravitational wave experiment is like kilometers long and
cost billions of dollars. This would be like meters long.
You literally could build it in a lab in the
basement at Caltech, and if it works, they could see
quantum gravitational effects on these beams of light and they
could prove that gravitons are out there and that they're

(54:55):
dancing together to make these little tiny ripples in space time.

Speaker 1 (54:59):
Cool. Well, she's welcome to hang out in my backyard
and do the experiment here. That could be exciting.

Speaker 2 (55:07):
I don't think she wants her experiment it sprayed by
the hose or like doused with water.

Speaker 1 (55:11):
Balloons, yeah, or have screaming kids running all around it
that you usually it tends to make gravitons shy.

Speaker 2 (55:19):
I tend to dampen the effects of your experiment.

Speaker 1 (55:21):
All right, well, pretty exciting. Thank you to doctor Catherine
Zurich for talking about her research. What does this all mean, Daniel,
Are we far or near proving the idea of quantum gravity?

Speaker 2 (55:33):
I think we're still pretty far from figuring anything out.
The theorists are working hard and making progress all the
time about building their theories. But now it's exciting that
we have experimental efforts which maybe in the next five, ten,
fifteen years could provide us with really valuable clues to
tell us, Oh, gravity is classical or nope, gravity is
quantum mechanical. You better figure it out. That would be

(55:53):
really powerful indication for sort of which direction to go theoretically.
And I love this dance between experimental and the radical physics.
You know, the ideas flourish and then experiments kill them,
or sometimes experiments discover something weird which inspires lots of
new theoretical ideas. It's really beautiful to see the interplay
of these two different avenues of exploration. It's like a

(56:14):
theoretical tango exactly. Even the physicists don't really know how
to flirt. And I think the tango is pretty flirtatious.

Speaker 1 (56:20):
All right, Well, it sounds like the answer is stay tuned.
In theory, it might be ten to fifteen years, but
in reality, who knows. It could be that we may
never answer this question, or it could be that we'll
answer it within our lifetimes.

Speaker 2 (56:33):
That's right, we could be flirting with understanding or confusion.

Speaker 1 (56:36):
We hope you enjoyed that. Thanks for joining us, See
you next time.

Speaker 2 (56:48):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeart Radio. For more
podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or
wherever you listen into your favorite shows.

Speaker 3 (57:06):
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