Why is quantum gravity so hard?

Daniel and Jorge explain why the two theories of physics are so at odds with each other.

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, how smart do you have to be to
be a physicist?

Speaker 2 (00:12):
You know, it's not actually about being smart. It's more
about thinking that these kind of particular challenges are really fun.

Speaker 1 (00:19):
So if you like having fun, you shouldn't be a physicist.
What do you mean?

Speaker 2 (00:24):
I mean, you know, science is a very personal thing,
so some people might think doing integrals is really boring,
and somebody else might do them to relax.

Speaker 1 (00:31):
Well, are you saying math can be relaxing.

Speaker 2 (00:35):
It can be relaxing, and it can also be exciting.

Speaker 1 (00:38):
You know.

Speaker 2 (00:38):
Sometimes you're like bush whacking through the math and you
make amazing discoveries and you don't even have to risk
your life to jaguars.

Speaker 1 (00:47):
Well you do have to worry about paper cuts, right.

Speaker 2 (00:52):
Yeah, you know. I think there's a reason they didn't
make Indiana Jones a physicist.

Speaker 1 (00:55):
Yeah. I don't think physicists could pull off the Indiana
Jones hat.

Speaker 2 (01:00):
And we all have daddy issues.

Speaker 1 (01:16):
Hi. I'm Jorge make, cartoonist and the author of Oliver's
Great Big Universe.

Speaker 2 (01:20):
Hi, I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I always wanted to have physics
adventures ooh.

Speaker 1 (01:29):
Like real adventures, like in your couch, like, oh no,
I spilled my coffee.

Speaker 2 (01:35):
I don't think you have to risk your life and
like get into a spaceship or even become an astronaut
to have physics adventures. You know, you can explore the
universe in your mind, make amazing discoveries, and feel like
you are connecting yourself to the universe.

Speaker 1 (01:49):
I guess technically aren't like real adventures physical adventures. But
I guess maybe you don't want physical adventures, want physics adventures.

Speaker 2 (01:57):
Yeah, exactly, physics versus.

Speaker 1 (01:59):
Physical phone You don't want to make any physical exertions
or efforts, just like the mental kind.

Speaker 2 (02:07):
Yeah, but you know, sometimes thinking really hard can make sweat.
I've definitely perspired while doing intergrals before.

Speaker 1 (02:15):
M I see, have you ever wiped out doing integrals?

Speaker 2 (02:19):
I've never injured myself doing math, that's for sure.

Speaker 1 (02:24):
Well, I guess some people headaches. I guess that's sort
of cow injury.

Speaker 2 (02:28):
Exactly. Migraines are a hazard of doing physics.

Speaker 1 (02:31):
Yeah, no, I find math very relaxing, actually puts me
right to sleep. In fact, you should make an a
f for like a sleep relaxation app. And now we're
going to do integrals people do like the all time.
There you go, that's very ASMR and you will be

(02:55):
subliminally learning math.

Speaker 2 (02:57):
The billion dollar idea right here.

Speaker 1 (03:00):
Yeah, I can finally quit this podcast job. But anyways,
welcome to our podcast Daniel and Jorge Explain the Universe,
a production of iHeartRadio.

Speaker 2 (03:11):
In which we do our best to make the complexities
and the challenges of physics accessible to you and to everybody,
because we think that the whole point of physics is
to understand the universe, and not just by a select
few group of people who can understand nineteen dimensional string
theory integrals, but by everybody. Because the end science is

(03:32):
a bunch of stories, mathematical or intuitive or in English,
and we want to tell those stories to you.

Speaker 1 (03:38):
That's right. We take you through the adventures of science,
the wipeouts, the close calls, and the headaches of trying
to find out how the universe works and what are
place in it is.

Speaker 2 (03:49):
Sometimes these integrals are more annoying than mosquito bites, though
never as dangerous as jaguars.

Speaker 1 (03:54):
I don't think anything is this annoying is mosquitobiites. Jaguars
are pretty pesky too. Have you ever seen a jaguar
in the wild in the wild? Uh no, no, thankfully not.
Or do you mean like a car the car? I
think plenty of jaguars around here in South Pasady.

Speaker 2 (04:14):
That's true, and those drivers can be pretty dangerous.

Speaker 1 (04:16):
Yeah, yeah, and annoying like mosquitos.

Speaker 2 (04:19):
You just want to swat all the jaguar drivers.

Speaker 1 (04:22):
No, no, that's a you can get arrested for that
kind of thing. Yeah, exactly, Well, let me insult them
on a podcast.

Speaker 2 (04:30):
When you make a billion dollars on your sleep math
app that you can buy yourself a jaguar and you
can be the same one.

Speaker 1 (04:36):
I can buy several jaguars and a mosquito repellent.

Speaker 2 (04:43):
Well, I was just wondering, because I know you grew
up in Panama, if you'd ever seen a jaguar or
maybe just jaguar sized mosquitoes.

Speaker 1 (04:50):
Oh, I see, I see. That's your your perception of
how I grew up. I grew up in the hut,
in the jungle, barefoot, with my little pet jaguar next
to me. Exactly. I think people will live.

Speaker 2 (05:01):
No, I'm asking paint us the picture.

Speaker 1 (05:05):
I've seen a lot of coyotes around here in the wild. Yes, yeah,
those are pretty dangerous too. I guess if you're the
if you're small, you're a small baby or something.

Speaker 2 (05:14):
In our neighborhood, everybody with a small dog puts these
spiky vests on them so the coyotes can't just snatch
them up and have a snack.

Speaker 1 (05:22):
Wait, what you turn your dog into a weapon? What
do you mean is like spy? How dangerous are these spikes?

Speaker 2 (05:30):
They're pretty big because there's a whole epidemic of coyotes
jumping into people's backyards and like grabbing these little dogs.

Speaker 1 (05:36):
Oh wow, maybe just not keep your dog out at night.
Don't turn it into dangerous weapon. Like what if the
dog runs at a little kid or something wearing this
killer jacket.

Speaker 2 (05:47):
Yeah, these coyotes are pretty brazen. It's not just that night.
During the full daylight, they will hop in people's backyards
and make off with their little shit.

Speaker 1 (05:54):
Sues M. Sounds like me. You just just make the
coyotes your pets, and then that's also the problem, doesn't it. Well, last,
then you get a problem of jaguars coming in here
and eating your coyotes.

Speaker 2 (06:06):
And somehow we have to connect all of this back
to physics.

Speaker 1 (06:10):
Yeah, no, there's a physics zero of sizes right right, yes, sizes. Yes.

Speaker 2 (06:16):
Some problems are hard, like how to protect your little
pet from neighborhood coyotes, and other problems are hard, like
how do you figure out the mathematics of the universe?

Speaker 1 (06:26):
It's all connected, yeah, because I guess. Figuring out the
math of the universe has been one of the goals
of physics, to understand what's underlying everything that we see
around us, and all of the mechanics and the motion
and the energy that is swirling around us, what is
at the core of the universe.

Speaker 2 (06:42):
Physics has two great theories, two incredible ideas that have
been very very successful, quantum mechanics and general relativity. But
bringing them together into an idea of quantum gravity has
been a challenge that has stood for over a century.
The greatest minds in physics have tried to take a
bite out of it, but it seems to be protected

(07:03):
by a spiky vest that's right.

Speaker 1 (07:05):
It's been one of the hardest problems to solve in
physics for the last one hundred years, which makes you
wonder why is it so hard? What's taking so long
to solve this fundamental problem in physics.

Speaker 2 (07:18):
It's because us physicists haven't just gotten off our couch
and bush whacked our way into the mathematical jungle.

Speaker 1 (07:23):
Yeah. Yeah, I think that's the problem, Daniel. You need
to get off your couch an experiment with real gravity,
not just like imaginary gravity.

Speaker 2 (07:32):
I'm waiting for my Indiana Jones hat to come. You
can't do it without the right cand of hat. You
have to be prepared.

Speaker 1 (07:37):
Oh that's right, Yeah, that's right. You don't want to
get sunburned. It's not like there are other ways to
protect yourselves from the UV rays.

Speaker 2 (07:46):
I mean, I want to understand the universe, but I'm
not willing to make physical sacrifices.

Speaker 1 (07:50):
Do you also need a whip? Also?

Speaker 2 (07:53):
I want to whip those integrals into shape for sure.

Speaker 1 (07:56):
Yeah, there you go, crack them into shape. But anyway,
to be on the podcast, we'll be tackling the question
why is quantum gravity so hard?

Speaker 3 (08:09):
Now?

Speaker 1 (08:09):
Is this like hard like difficult or hard like tough?

Speaker 2 (08:13):
If your floors are made of quantum gravity and you
drop your glass, man, is it gonna shatter? Yeah?

Speaker 1 (08:18):
But how is it going to fall without gravity? Quantum mechanics,
I answered everything, it's gonna fall and not fall. No.

Speaker 2 (08:28):
I love that physics has a name for this theory
quantum gravity, but it's just kind of like a placeholder.
We don't know what it is, or how it works,
or what the mathematics of it are. People argue about
different approaches. We already have a name for it, but
we haven't even figured it out yet.

Speaker 1 (08:42):
Sounds unbrand for how physicists name thinks. Well, anyways, we're wondering,
as usual, how many people out there had thought about
this question of why quantum gravity is so hard? As usual,
Daniel went out there and God answers from real people.

Speaker 2 (08:56):
Thank you to all the real people and definitely not
made up had GPT inspired bots who answered this question.
If you are a real person and you'd like to
answer future questions, please write to me two questions at
Danielandjorge dot com.

Speaker 1 (09:10):
So think about it for a second. Why do you
think quantum gravity is so hard to solve? Here's what
people had to say.

Speaker 3 (09:18):
I know that.

Speaker 4 (09:20):
Quantic mechanic is very good explaining things that happen in
very smallst scale, and gravity is not very strong and
small as scale.

Speaker 1 (09:34):
I don't know.

Speaker 4 (09:35):
I think that's the reason, but I don't really I
don't really know.

Speaker 1 (09:40):
I honestly don't have an intelligent answer for that. But
I will say this, if Albert Einstein was afraid of it,
that I am too.

Speaker 4 (09:49):
It's hard because we are trying to apply our comparatively
super massive vantage point to these tiny particles in the
quantum realm that probably don't even know the difference.

Speaker 2 (10:05):
They probably don't even know that gravity is a force.
Did you mean real people as opposed to chat GPT
or real people as opposed to physicists. I'm just now
realizing that was a jab.

Speaker 1 (10:17):
I mean, like not paid actors. Okay, although maybe you
should include a chatgypt answer every every time we do this. Interesting. Yeah,
that's right now. What does chat GPT say about quantum
gravity being so hard?

Speaker 2 (10:34):
Chatchipt says quantum gravity is consider challenging because it involves
the attempt to reconcile two fundamental theories of physics, quantum
mechanics and general relativity. Each of these has been incredibly
successful in its own but it becomes problematic when combined.

Speaker 1 (10:48):
Whoa it just did the podcast for us? Done? AI
did replace? In our job? Does a chat GPT have
like a like a voice out. Can you have it
read the answer?

Speaker 2 (11:01):
I think the free version that I have access to
you can't do images or voices. So no, you cannot
be replaced by chat jipt just yet.

Speaker 1 (11:08):
Oh right, you still need us to read the answer
chat GPT gives out. I see.

Speaker 2 (11:15):
Also, I would never rely on chat CHEPT. I asked
some hard physics questions sometimes and it just makes up balogney. Oh.

Speaker 1 (11:21):
I think the news here is that you're asking chat
GPT for answers.

Speaker 2 (11:25):
Yeah, I like everybody else, I was curious when it
came out, what you like to talk to the average
of the internet wisdom and follies, And the answer is
it's not very reliable.

Speaker 1 (11:35):
Well, technically, neither are we, Daniel. I don't think we're
gonna have a hard answer here today.

Speaker 2 (11:42):
No, but we're not going to make stuff up.

Speaker 1 (11:43):
Well, So let's dig into this question. Why is quantum
gravity so hard? Why is it so difficult? Why has
it puzzled physicists for over one hundred years? So let's
start with the basics, Daniel, what is quantum gravity?

Speaker 2 (11:56):
So chatchapt got this bit right. Quant gravity is an
attempt to bring together are two great theories of physics.
Quantum mechanics that describes things like electromagnetism and how particles
work and gives us a probabilistic picture of the universe,
and general relativity that described space and time and gravity

(12:18):
and explains things like the expansion of the universe and
how things move through space and time. And both of
these work really really well in their own regime. Quantum
mechanics for the small stuff, general relativity for the big stuff.
Quantum gravity is an attempt to have a single consistent
theory that works for all the stuff.

Speaker 1 (12:37):
And these were developed independently, sort of right like, while
Einstein was coming up with general relativity, other people were
thinking about things at the smallest level and why they
were it quantized right exactly.

Speaker 2 (12:51):
They were developed independently, though, around the same time, and
both were actually sparked by Einstein. Quantum mechanics really got
its kickoff from Einstein's realism that the photoelectric effect, what
happens when you shine a bright light at a piece
of metal, can only be explained by the fact that
photons were little packets. They were quantized. They weren't just
continuous beams of energy because what you saw was as

(13:14):
you turned up the energy of that beam of light,
you didn't get electrons with more energy boiling off. You
got more electrons because each one just gets one servant,
one photon. That was explained by saying the beam of
light had more photons in it, not just a brighter beam.
So Einstein kicked off quantum mechanics around the turn of
the century, and at the same time he developed his

(13:36):
theory of special relativity and general relativity that explained the
apparent force of gravity. And these two have been in
parallel development over the last hundred years, but nobody's been
able to bring them together into one picture of how
the universe works.

Speaker 1 (13:51):
I guess maybe the question is why do you want
to bring them together? Like, if you have one that
works really well for some things and the other one
works well for other things, what's the need to unify them?

Speaker 2 (14:01):
Yeah, it's a fair question.

Speaker 1 (14:02):
You know.

Speaker 2 (14:03):
Sometimes in life we have things that are separate, Like
you got one group of friends and another group of friends,
you bring them together, it's awkward. You don't do it again, right, Yeah,
they're usually a bad idea, And I guess in this
case there are two answers. One is philosophical and the
other really is experimental. Philosophically, we just think that the
universe probably does have a single set of laws. You know,

(14:24):
there should be one explanation for why something happens. You know,
the same way like when a computer program runs, it's
running with one source code. It's not like there's two
codes there battling it out. There should be one explanation.
And this is just sort of like a philosophical preference.
It would be nice if the universe had a single,
unified theory. It would sort of make sense to our brains.

(14:46):
That doesn't mean it has to happen. It's just sort
of like a philosophical preference.

Speaker 1 (14:51):
Well, maybe explain to folks how they're separate. So, for example,
quantum mechanics works to describe what exactly, like the motion
or of little tiny particles or their interactions, or what
exactly does quantum mechanics do.

Speaker 2 (15:07):
Quantum mechanics describes everything about tiny little particles, their motion,
their interactions, what's going to happen, what's not going to happen.
If you have, for example, two electrons and they're interacting
with each other, quantum mechanics tells you what's going to happen.
You make two electron beams, you shoot them at each other.
Quantum mechanics tells you the probabilities of what will come
out of those collisions, or if you replace an electron

(15:30):
with a muon, or you put in a quark or
a proton or whatever. Quantum mechanics is the rules of
all of those interactions, and the standard model of particle
physics what we talk about on this podcast all the
time that has been super successful in explaining the structure
of matter deep down inside the atom and why everything's
bound together and how that all works. That's all quantum mechanics.

(15:51):
It's all fundamentally quantum mechanics. Every little bit of it
is quantum mechanical, and it's quantum mechanical because it paints
this picture of how the universe works that's very different
from the way that our universe seems to work, the
one on our level you know about baseballs and planets
and basketballs, where things have like smooth paths. It tells
us that fundamentally the universe follows very different rules. That

(16:13):
quantum objects, tiny little bits, only have probabilities to go places.
They don't have smooth.

Speaker 1 (16:19):
Paths, right, thinks are kind of fuzzy down at the
microscopic level. What does general relativity do exactly?

Speaker 2 (16:25):
So general relativity explains space and time and gravity, So
it says that what Newton described as a force of
gravity is actually just objects moving through curved space. Newton
imagined space was absolute as this backdrop of the universe,
and then things with mass had a force between them.

Speaker 1 (16:44):
You know.

Speaker 2 (16:44):
He famously explained that apple dropping and also the Moon
orbiting the Earth unified in his law of gravity as
an attraction between mass. But Einstein tells us that that's
not the case. General relativity tells us that actually things
are just moving through the curvature of space. Space itself
is curved when mass is nearby, and that changes the

(17:04):
natural inertial path of objects. Objects will move in what
looks like curves even without accelerating.

Speaker 1 (17:11):
And space gets spent by gravity, right or gravity is
the bending of space.

Speaker 2 (17:15):
Right, base gets spent by energy in a very complex way. Essentially,
mass is a kind of energy, so it helps bend space,
but it's not the only way you can bend space.
And gravity is sort of a fuzzy term. Now, it's like,
are you referring to the Newtonian force, which isn't really
part of our picture anymore, or you're talking about the
whole theory of general relativity as an explanation for it.
But you know, what we describe as gravity things seeming

(17:38):
to fall down, is explained by Einstein, is things just
following the curvature of space.

Speaker 1 (17:44):
Which which gets a curved because of the presence of energy. Right,
that's the basic and that effect. Basically you can sort
of lump it into the idea of gravity.

Speaker 2 (17:54):
Yeah, exactly. And we had a whole podcast digging deep
into like why gravity isn't the force and how if
you're moving along with the curture of space time you
don't feel any acceleration even if other people see you,
like moving in circles or moving towards the center of
the Earth, all that kind of stuff. It's a really
fascinating different way to think about how the universe works.
It tells a very different story from Newton's, but it

(18:17):
mostly describes really really big stuff because you need a
lot of mass to curve space, and that curvature is
kind of gentle, so the effect of that curvature is
hard to measure, especially compared to these quantum forces, which
are extraordinarily powerful in comparison.

Speaker 1 (18:32):
All right, so now maybe paint us a picture of
how they are not unified, Like can I just have
some quantum particles interacting in a gravitational field? Or can
I just have the path of a quantum particle bent
by the bending of space and time. What doesn't these
two things do together? Like, what are scenarios in which

(18:52):
they exclude each other?

Speaker 2 (18:53):
Yeah? Great, And so this is sort of like number
two reason why we want to unify them, because in
some situations they disc Like we talked earlier about having
separate theories of the universe, Maybe that's cool, but it's
not cool if they're talking about the same phenomenon. If
you're asking them the question what happens here? Most of
the time you can keep them separate because for tiny,
little particles you can ignore gravity. Gravity is very very

(19:15):
weak for little particles, and for really big stuff, quantum
effects mostly average out. You don't need to know quantum
mechanics to predict the path of a baseball. But in
some scenarios they do disagree, things like what happens inside
a black hole. That's a scenario where you have really
powerful gravity so gravity can no longer be ignored. And
things are very very small because we think that things

(19:37):
are super compressed inside a black hole, so quantum effects
are important. So super duper massive, very very tiny objects,
quantum mechanics and general relativity disagree about what happens there,
and so the universe can't have a contradiction. Two theories
tell different stories. They can't both be right.

Speaker 1 (19:55):
But I guess I mean, in like an everyday scenario,
do they work together? Sort of like a man imagining
say a microscopic particle, like an electron out there in
the near Earth orbit and it's floating out there in
space close to the Earth. Does it get pulled by gravity?
Is it going to fall down to Earth? Is there
a problem with me trying to use quantum mechanics to

(20:16):
model how it falls to Earth?

Speaker 2 (20:17):
Yeah, so you might think, can't we just test quantum
gravity and figure out what the answer is, which one's
right by looking at the gravity of a tiny quantum particle, Right,
So that's what you're asking. What happens for the Earth's
gravity on an electron?

Speaker 1 (20:30):
Yeah? Like, do the two theories break down or do
they agree on their normal conditions that are not inside
of a black hole.

Speaker 2 (20:36):
So the two do not agree about what happens to
an electron in the Earth's gravitational field.

Speaker 1 (20:41):
They don't.

Speaker 2 (20:42):
They don't, but they're not both relevant at the same time.
It's a little tricky, and the issue is how do
you calculate the gravitational field of the electron, or even
the gravitational force on the electron, or the effective curve
space however you want to say it, because that depends
on where the electron is. Electron is a little quantum
particle with quantum effects. Then maybe it has like a

(21:04):
fifty percent chance to be at this altitude and fifty
percent chance to be at that altitude, in which case
it would feel different amounts of gravity. And so how
do you calculate the gravity on a quantum particle, We
don't know. The theory of quantum gravity would tell us
how to do that, but general relativity doesn't tell us
how to do that. General relativity requires that you know
where the electron is, and so it ignores its quantum nature.

(21:29):
So if the quantum nature of the electron is important,
it's doing quantumy stuff, then we don't know how to
calculate the force of gravity.

Speaker 3 (21:35):
On it.

Speaker 2 (21:36):
But also we can't measure the force of gravity on
a tiny little object because the force is so small
because its mass is so small.

Speaker 1 (21:44):
Sounds like a great situation and maybe one that we
need a little bit more time on. So let's dig
into that scenario and dig into well, how exactly these
two theories don't match up, and then we'll get into
a little bit of the math that makes it so
hard to integrate the tube. So let's do that. But
first let's take a quick break. All right, we're talking

(22:16):
about why quantum gravity theory that unites quantum mechanics and
general relativity it's so hard to come up with and
to make these two theories play well together. And so Daniel,
we're talking about a scenario in which I have an
electron in near Earth orbit. It's out there in space
above the atmosphere, and I'm trying to figure out what's

(22:37):
going to happen to this electron. Is it going to
fall to Earth? What path is it going to take
as it falls to Earth? And you're saying that it's
hard to theoretically predict what's going to happen, right, because
it's definitely going to fall if I put an electron
on your Earth, right, like it's going to do something,
but we don't really have a good theory to predict
what it's going to do.

Speaker 2 (22:57):
Is that what you're saying, we can't be very very
precise about its predictions. We can be approximate. Like, there's
two approaches you can take. You can say, I'm going
to ignore the quantum mechanical part of it. I'm just
going to treat the electron like it's a tiny little
rock or a tiny little ball. I'm going to calculate
its gravity and I think about how it's basically in
orbit around the Earth. And you can do that and

(23:17):
you get very good predictions, and you can calculate how
things boil off the top of the atmosphere or they
fall to Earth, or whether they're in stable orbits or not.
So basically, ignore the quantum nature of the electron, treat
it like a tiny classical object and do gravity on it.
That's one approach.

Speaker 1 (23:33):
But then you're saying it's hard to know how much
gravity is applied to the electron because of quantum mechanics,
or is it hard? Can you just say, like the
electron has this much mass and it's a little tiny rock,
and so that's how much gravity's going to feel or
is that at some level wrong?

Speaker 2 (23:49):
Well, that's at some level wrong because you're ignoring the
quantum nature of the electron. You're treating it like a
tiny rock, and it's not a tiny rock.

Speaker 1 (23:56):
Yeah, but I guess. I mean, like, if you do
treat it like a rock, do you get something wildly
off or do you get something that seems to be
pretty exact?

Speaker 2 (24:04):
You get something that works pretty well. As long as
it doesn't have any interactions. As long as that electron
is not interacting with any other particles, it's mostly just
ignoring them, then yeah, you get something that's correct.

Speaker 1 (24:15):
Like it's going to follow the same path as a
little rock.

Speaker 2 (24:18):
Yes, as long as it's not interacting. But if it's
in a soup of other electrons and charged particles and
it's interacting with those, then boom, it's quantum nature becomes
important and those quantum effects dwarf gravity. They completely take over.
So you can either ignore the quantum effects and just
do the gravity, or you can ignore the gravity and
just do the quantum effects. For an electron, only one

(24:41):
of those is relevant at a time, and that's why
you don't need quantum gravity to think about electrons. You
can do either quantum mechanics or gravity. They're never both
relevant at the same time.

Speaker 1 (24:52):
But I guess to an approximation. So then when do
you get into trouble? Like when is it a problem
that these two are not unified? But what's the scenario?
Like it's in gravitaational orbit around the Earth, the electron
is and it's sort of a little bit interacting with
another electron. Then it's like we don't know what to do.

Speaker 2 (25:07):
As long as it has a quantum interaction, that's just
going to dominate because the quantum forces are so much
more powerful than gravity. You know, they're like ten to
the thirty times as powerful as gravity.

Speaker 1 (25:18):
But like what if it's like ten to thirty one
times further away, wouldn't it be at the same level
of gravity.

Speaker 2 (25:25):
I mean gravity falls with distance just like quantum forces do. Right,
So it's not a matter of distance, it's a matter
of the mass to charge ratio. Like if you have
two electrons, they feel a very strong electromagnetic repulsion because
of their charge. They don't feel a very strong gravitational
attraction because of their mass. The electromagnetic force there is
always more powerful at any distance.

Speaker 1 (25:47):
I thought maybe the scenario you're trying to paint was like,
I have an electron out there in space, and it's
been attracted by gravity to the giant Earth, but maybe
it's also sort of being repelled by another electron that's nearby,
and so then we don't know what's going to happen.

Speaker 2 (26:01):
You have a scenario we have a single electron orbiting
the Earth, and then some very distant electron is very
gently pushing on it with the same power as the
gravity of the entire Earth.

Speaker 1 (26:11):
Yes, Is that like the scenario that you run into
problems or can you still handle it?

Speaker 2 (26:16):
No, that's a cool idea. That's a scenario where gravity
and quantum forces are at the same level, and so
you can't ignore one of them. You have to take
both into account, and we don't know how to do
that prediction. That kind of experiment is also pretty hard
to realize because you need an isolated electron affected by
only one other electron that's super far away, So it's

(26:37):
not like practically something we could set up. Otherwise that
would be really awesome. It would tell us something about
quantum gravity.

Speaker 1 (26:43):
Yeah, yeah, But that's a physical problem about a physics problem,
so you know, are couch surfing and Dana Jones doesn't care.

Speaker 2 (26:49):
The other way to tackle this is to say, well,
what if you have a really really massive quantum particle
particle that is feeling quantum forces but actually has enough
mass that it's gravity can't be ignored, and that's when
you end up at.

Speaker 1 (27:00):
A black hole? All right? So can you be more
specific about what the problem is, like we don't know
how to tell what the particle is going to do next,
or we can't predict how it's going to interact. Can
you describing words what the problem is at the indecentnarios?

Speaker 2 (27:14):
The problem is that our two theories, general relativity and
quantum mechanics make different predictions about what's going to happen.

Speaker 1 (27:21):
What do you mean, like one theory says that the
electron is going to turn right and the other theory
says the electric is going to turn left.

Speaker 2 (27:26):
Yeah. For example, general relativity is a classical theory, and
so it assumes electrons have a very definitive location at
every point in time, whereas quantum mechanics says no, this
is probabilistic, and you can get things like interference. General
relativity says, I'm ignoring all that interference stuff, and it's
going to make a prediction based on treating the electron
like it's a little rock flying through space. So they're

(27:46):
going to come up with very different predictions for what's
going to happen. General relativity doesn't allowed for like entanglement
or any of the other important quantum effects that totally
control what happens to an electron.

Speaker 1 (27:57):
I see, because quantum mechanics also might change which direction
the electron goes.

Speaker 2 (28:01):
Absolutely, yeah, Electromagnetism is a quantum effect, right, Those forces,
all the forces in the universe that cause acceleration, the
weak force, electromagnetism, the strong force, These are all quantum effects.
They all operate on the probabilistic wave functions that control
these particles. General relativity ignores those. So you ask quantum
mechanics and gr what's going to happen to this electron.

(28:23):
They disagree about what's going to happen.

Speaker 1 (28:25):
Well, maybe this is the point where we have to
get more into the math, because you know, as a
lad person, I might just say, like, can you just
add these two things, like why can't just have a
particle that gravity pulls on its average position, and so
the average position curves according to gravity, but then where
it is exactly might be fuzzy due to quantum mechanics.

Speaker 2 (28:46):
Yes, so what you're trying to do right now is
come up with the theory of quantum gravity. You're trying
to say, can I get all the good bits of
general relativity and all the good bits of quantum mechanics
and smooth them together to make a theory of quantum
gravity that makes one prediction? Right? And that's the topic
of the episode. Why is that so hard? And people
have been working on it for one hundred years. It
sounds straightforward, but there's a bunch of reasons why it's

(29:08):
actually quite tricky. One of them is the problem you
just mentioned, which is this question of like space and
time and probability. You know, quantum particles don't have definitive locations,
and general relativity doesn't allow for probabilities in space time.
What you just described is like, well, what if the
electron is allowed to have a probability being here or there?

(29:28):
And so we just say, like space has a probability
of being curved here and a probability of being curved there. Right,
that's like a pretty deep change to how general relativity works,
and the mathematics of it breaks down, Like general relativity
doesn't allow for those probabilities?

Speaker 1 (29:45):
What do you mean, like doesn't allow it? Like you
just don't know how to write it down, or like
you get nonsensical answers, or it's like trying to fit
a square peg in a round hole. You know, am
I trying to use fractions to you know, compute things
to certain decimal point or something.

Speaker 2 (30:00):
Yeah, the mathematics of general relativity is hard.

Speaker 1 (30:02):
You know.

Speaker 2 (30:02):
It took Einstein like ten years to figure out how
to wrangle these equations to make any sense of them.
He had this idea that maybe space was curved and
that was explaining what gravity really was. But to make
the math work to even Einstein in a decade, and
it takes a lot of people a lot of time
to understand and to wrestle with the equations, which turn
out to be really really complicated. It's not like one

(30:24):
equation for a general relativity that says more mass means
more curvature. It's a matrix of equations. There's like sixteen
coupled equations which are really hairy, and if you add
to those the probability that space is may be curved
here and maybe curved there, it increases the complexity exponentially,
and it makes those equations impossible to even write down.

(30:44):
We don't know how to write down equations that both
describe space as a curvature of this differential geometric manifold
and allow for probabilities, like we just don't have the
tools for it. It might be that somebody out there
is developing some cool theory probabilistic manifolds that later will
be able to slip in to build the theory of
quantum gravity. But it's like we need a power tool

(31:07):
and all we have is a handsaw.

Speaker 1 (31:08):
Yeah, Like you say, like if we just don't have
the right tools that will both fit a square peg
and a round hole.

Speaker 2 (31:14):
And we don't know if we're missing the right tool,
Maybe that's just the wrong direction, right. It could be
that that's not the right way to try to build
the theory of quantum gravity. But a lot of times
it is the case that progress in mathematics is limiting
progress in physics. Einstein was only able to build general
relativity because the theories of differential geometry had been developed
like ten fifteen years earlier by mathematicians who didn't care

(31:37):
at all about gravity or physics. They just thought it
was cool to think about like wiggly shapes in their mind,
and so this kind of stuff happens all the time.

Speaker 1 (31:45):
I see you're saying it's all the mathematicians fault, like
they're holding you back. Man.

Speaker 2 (31:49):
Math is the language of physics, and in the end,
it's mathematical problems that are preventing us from building theories
of quantum gravity. And what you described is basically trying
to make space quantum mechanical. You can also go the
other direction, and you can say, well, what if we
try to make gravity itself quantum mechanical. What if we
try to describe the theory of gravity as a quantum

(32:10):
force instead of this whole crazy curvature of space and time,
And then you run into a completely different kind of
mathematical mm I see.

Speaker 1 (32:18):
I think what you're saying is like each of these
two theories work, but only if they ignore each other.
Like quantum mechanics assumes that space doesn't bend and there
is no gravity. Basically gravity doesn't exist. To quantum mechanics,
and general relativity assumes that things are not fuzzy at
any level exactly, which works for a lot of situations,
but in some situations you have to take them into

(32:41):
account both at the same time.

Speaker 2 (32:42):
Exactly, and it's amazing that both of them work. They
tell very different stories about what's really happening in the universe,
and in almost every scenario you can ignore one.

Speaker 1 (32:51):
Right.

Speaker 2 (32:52):
It's like you have two friends that are like different
kinds of movies or something, and most of the time
you can just ignore one friend and listen to the
other friend. But sometimes they have opinions about the same
kind of movie, and you're like, well, is this movie
good or bad? I don't know who to listen to.
And for general relativity and quantum mechanics, currently they only
overlap in places we cannot see inside black holes, so

(33:13):
we don't know who's fundamentally right, or if either of
them are right.

Speaker 1 (33:17):
I see. It's like having a friend who only watches
sci fi movies and then having another friend who only
watches romantic comedies. And usually you can have perfectly good
conversations with either of them. But let's say science fiction
wrap the comedy comes out. Now there's true.

Speaker 2 (33:31):
Exactly exactly right. Now you can't hang out with your
friends anymore, right, boom the universe.

Speaker 1 (33:36):
You gotta stop watching movies. You can stage your couch
and just do physics and math all day. Has this
already happened to you, Ben?

Speaker 2 (33:47):
I think Passengers was not a sci fi romantic comedy.

Speaker 1 (33:50):
Oh there you go, And that was very controversial.

Speaker 2 (33:53):
Man, exactly.

Speaker 1 (33:54):
Nobody likes that we don't have the math tools or
framework or theories that let us tackle these two things
at the same time. What are some other ways that
make it hard to unify these two things?

Speaker 2 (34:05):
So a really popular approach is to trying to make
a theory of quantum gravity that has a graviton. Say,
you know, Einstein, that was cute. We like your idea
of Kurt space time. It's pretty, but maybe it's just
fundamentally the wrong direction. Maybe if you zoom in, what
really is happening is that gravity is a force and
it's exchanging gravitons, and I mean, then we can describe

(34:26):
the whole theory of gravity back sort of as like
a quantized version of Newton's theory. And people have tried
to do this because that'd be pretty right if we
could just like add gravity to the standard model and
have another particle and ignore this whole curvature business. That
would be cool.

Speaker 1 (34:40):
So that would be going back to the idea that
gravity is a force, not a bending of space and
time exactly.

Speaker 2 (34:46):
So to tell philosophically a very different story from what
Einstein is telling us about how the universe works, they
would say there is no curvature. There are these tiny,
little invisible gravitons being passed back and forth. The same
way that like electromagnetism you think about in terms of
electric fields that are sort of like virtual photons being
passed back and forth, you can think of gravity in

(35:07):
terms of gravitons being passed back and forth. So that's
one direction. The problem is nobody can make that math
work either. When you do the calculations there and you
ask like, well, what happens if you try to collide
two black holes together, or even two protons together, you
get nonsense answers. You get answers like, well, the probability
of this happening is one hundred and fifty percent, and

(35:28):
the probability of that happening is one thousand percent, Just
like numbers that do not make sense you can't have
probabilities greater than one. But that's what these calculations spit out.

Speaker 1 (35:38):
Like if you assume a gravitin exists, then you get
these weird answers, but more fundamentally, like you're ignoring general relativity, right,
You're ignoring things that a lot of experiments have verified
that Eisin was right, and so you're sort of not
really solving the problem, right, are you.

Speaker 2 (35:53):
Well, you'd have to think about it as an upgrade
to general relativity. You'd have to reproduce all the predictions
of general relativity. So you need to develop a theory
of gravitons, which when you zoom out, looks a lot
like general relativity, and that people actually can do. There
are theories of quantum gravity that involve graviton exchange that
when you zoom out, look a lot like general relativity.

Speaker 1 (36:13):
So space time is not being bent.

Speaker 2 (36:15):
Yeah, they tell a different story, but they make the
same predictions about like the motions of objects.

Speaker 1 (36:20):
Including things like gravitational ways and frame dragging and all that, all.

Speaker 2 (36:24):
That kind of stuff. The problem is what happens at
the small scale when you try to think about like
when two particles scatter against each other, or when two
black holes are being eaten, then this theory breaks down.

Speaker 1 (36:35):
But generativity still works for those.

Speaker 2 (36:37):
General relativity still works for those. But we think it's wrong, right.
General relativity, we think is giving the wrong answer for
what happens when two protons collide or what's at the
heart of a black hole, because it's ignoring the quantum
mechanical effects. You try to build a theory of quantum
gravity that has gravitons and explains all of general relativity
and gives you a gravitational quantum mechanical prediction for what

(36:59):
happens when two particles collider, two black holes collide, Then
you get all sorts of nonsense. You get all sorts
of infinities that we don't know how to wrangle.

Speaker 1 (37:07):
M I see, So graviton not a great ivid you
or one that hasn't worked so far?

Speaker 2 (37:12):
Yeah, exactly. There are also sort of fundamental problems we
just don't know how to solve for quantum gravity, like
deep inconsistencies between the picture of the universe we get
from general relativity and the picture of the universe we
get from quantum mechanics about how physics should work. That
we just don't know how to reconcile. Well, what do
you mean, Like what like a deep principle in physics

(37:32):
is this idea of locality that things should be near
each other to affect each other, that you shouldn't have
like an electron over here affecting something really really far
away in the universe, especially in quantum mechanics, and in
quantum mechanics, we have this deep connection between the distances
between things and their energies. Like the reason that we

(37:53):
use the Large Hadron Collider to study really really really
tiny things is you need really high energy to study
really small distance scales. Like things that have a lot
of energy interact with each other very very.

Speaker 1 (38:06):
Closely, because if things have low energy, then they don't
interact with the things around them.

Speaker 2 (38:11):
If things have low energy, then they can interact with
stuff that's further away. Another way to think about it
is in terms of the wavelength of stuff. You know
that you need like really high energy photons to see
really really small stuff. With lower energy photons they have
a longer wavelength you can't like resolve small details. That's why,
for example, when you want pictures of really really tiny stuff,

(38:32):
you use high frequency photons you go beyond that to
use like electrons to take pictures of atoms, for example.
So you want to see the universe on a really
small scale, you need to use really high.

Speaker 1 (38:43):
Energy probes, maybe to say high frequency instead of high energy.

Speaker 2 (38:47):
Yeah, energy and frequency very closely connected. In quantum mechanics,
you need very high frequency stuff to see really short distances.

Speaker 1 (38:55):
Okay, okay.

Speaker 2 (38:55):
In general relativity they have the opposite relationship. As you
add energy to something in general relativity, then its influence
grows to longer distances. So quantum mechanics, higher energy means
shorter distances. In general relativity, higher energy means larger distances.
Like think about what happens to a black hole's radius.

(39:16):
As you add energy to a black hole, black hole
gets bigger, you add more energy, black hole keeps getting bigger.
The short styled radius, the distance from the singularity to
the event horizon, just keeps growing as the black hole
gets more massive. So, somehow general relativity doesn't have the
same relationship between energy and frequency and the distances involved.

(39:37):
They have like this deeply opposite relationship. This might sound
like sort of weirdly philosophically handwavy to you, but it
sort of tells.

Speaker 1 (39:45):
You about it not at all. I don't know what
you mean.

Speaker 2 (39:50):
But the reason it's important is that it tells us
that these two theories have like a fundamentally different sort
of philosophical foundation. Like one of them is very very local,
the other one is very non local. So when we
go to make a theory of quantum gravity, we're like, hmm,
these two things are kind of like very different. How
do we bring them together? It's like, how do you
get your science fiction fan friend and your rom com

(40:10):
fan friend together into a single movie If they have
just like really opposing needs for pacing and jokes and
whatever in the movie, it might be fundamentally impossible. If
these two things are so deeply in conflict.

Speaker 1 (40:23):
It sounds like you're just kind of maybe saying the
same thing we talked about before, which is like, you know,
quantum gravity is good for things that are small, and
general relativity is good for things that are really big,
but there is a certain overlap between them, and that's
where you get into trouble.

Speaker 2 (40:39):
Yeah, exactly, But I think there's one more layer there, Like,
imagine you have a singularity, so at the tiny little
spot with a huge amount of mass. Right, so it's
quantum mechanically important but also has a huge amount of mass.
General relativity says it affects things really really far away
because it creates a black hole. Who's event horizon can
be really really far away. Quantum mechanics says, no, it

(41:02):
can only interact with stuff really really nearby, because that's
really really tiny frequency.

Speaker 1 (41:06):
Well, I would say that it can only interact quantum
mechanically with things that are close by, but then it
can interact through gravity or generativity for things that are
far away, like way back to the same spot.

Speaker 2 (41:19):
Yeah, exactly, we're back to the same spot that if
you have quantum gravity, then you don't know can it
only interact nearby in its vicinity or can it interact
far away as well? Gravity says far away, Quantum mechanics
says nearby. We don't know what quantum gravity says. It
seems like maybe impossible to come up with a theory
that satisfies both m.

Speaker 1 (41:38):
Just like it's impossible to come up with a good
sci fi rom com.

Speaker 2 (41:42):
Exactly, they're fundamentally opposed.

Speaker 1 (41:45):
All right, Well, let's dig into some of the other
ways that make it hard to unify generalativity and quantum mechanics.
But first, let's take another quick break. All right, we're

(42:06):
talking about quantum gravity. Can we unite quantum mechanics and
general relativity which has gravity in it? So far, Daniel,
you're saying it's really hard.

Speaker 2 (42:16):
It's really hard. Some of the smartest people in the
universe have tried for decades and failed.

Speaker 1 (42:22):
In the universe. That's a big claim.

Speaker 2 (42:25):
Well, the smartest people in the universe, it could be
aliens out there much smarter than us. I don't know.
Are they people though? Are aliens people?

Speaker 1 (42:33):
Well, you're assuming that the smartest people on Earth have
become physicists.

Speaker 2 (42:36):
Oh that's a good point. Yeah, I know that people
out there who are like hedge fund bros.

Speaker 1 (42:41):
Yeah, or cartoonist maybe you know. I'm just saying you're
sort of making a general assumption.

Speaker 2 (42:46):
Here are you saying you're not a physicist? I think
years in basically you're a physicist by now.

Speaker 1 (42:52):
Oh, if that's true, then I have a diploma for
you to sign. Yeah.

Speaker 2 (42:57):
I think we gave you that podcast diploma physics.

Speaker 1 (43:00):
Right, I'm so waiting for that in the mail. It
must have gotten lost. I guess you sent it, right.

Speaker 2 (43:05):
But we sent it. Yeah, you should definitely get a
physical copy. You do like a discount at dollar.

Speaker 1 (43:09):
Stories like this is an imaginary diploma again for an
imaginary field of study. All right, So it's hard to
combine these two big theories. We talked about how the
math makes it really difficult. The philosophy of them make
it really difficult. What are some other ways that make
it hard?

Speaker 2 (43:27):
People argue about the fundamental story of space and time
that quantum gravity will have to tell us, because quantum
mechanics and general relativity really do tell very very different stories.
Here in quantum mechanics, we have kind of like a
Newtonian view of space and time. We say space and time,
they're backdrops, they're absolute, they're fixed, and we put quantum

(43:47):
fields on top of that space and time. We assume
that space and time already exists somehow, and we say
that this quantum fields in that space and time, and
those fields operate and they're part of space. But we
don't ask about where space comes from or what it is.
That's kind of stuff. It's like a fixed background, we
call it. But in general relativity, space time is dynamical.

(44:09):
It's not like fixed can bend and twist, and it
doesn't like exist inside some other kind of space. We
have this way to like calculate the relative distance between points,
but space itself is not like some new field that's
sitting inside some sort of meta space or superspace or
subspace or some other kind of space. It tells us

(44:30):
that there is no fixed background. So people describe general
relativity as background independent, like the universe. Space itself is
not sitting inside some sort of deeper box, and which
is kind of weird because it makes you feel a
little like unmoored from the foundations of reality.

Speaker 1 (44:48):
But I guess maybe couldn't you just have quantum mechanics
sit on top of a bendy stretch hey space time,
you know, sort of like you know, the planet Earth
of the Sun or moving through space time and they're
getting they're flowing through the curvature and all the bending.
Couldn't you have quantum fields also kind of right on

(45:09):
top of the curves of space time. Mm hmm.

Speaker 2 (45:11):
And I love how you just like casually suggesting these solutions,
which are like the whole areas of research.

Speaker 1 (45:18):
Working for twenty years, Daniel, come on, I mean I
just spent five to twenty minutes talking about this, and
I already know how to come up with the answer.

Speaker 2 (45:27):
You're basically a business so long. No, I don't mean
to make it at all. I mean that these are
actually really clever ways forward, and these are exactly the
things that people are working on at the cutting edge.
It turns out to be complicated. You can do quantum
mechanics on curved space, you can put these fields on
curved space time, and mostly it works as long as

(45:48):
the curvature is small, So like if they're a little
bit curved, things are fine. As soon as the curvature
becomes large, you get back to all those crazy infinities
that we can't wrangle in all sorts of nonsense predictions.
So the quantum mechanical theory breaks down if the curvature
is really really high. So we just don't know how
to do those kinds of calculations.

Speaker 3 (46:08):
Mm.

Speaker 1 (46:09):
And that includes time, right because like in general relativity,
time can slow down and time can speed up. And
so you're saying you don't know how to do that
in quantum mechanics.

Speaker 2 (46:18):
Exactly, we don't know how to do that in quantum mechanics. Again,
when the curvature is high, for what they call weak
field gravity, where it's like a very small effect gravity,
then we can do those calculations and even feel the
influence of gravity, not just negligible gravity, just weak gravity.
But when gravity gets strong, which is what it does
near a black hole, where we think these two things

(46:38):
are both important, we don't know how to do those calculations.
That's when quantum mechanics on curve space breaks down because
of all these infinities we can't grapple with all.

Speaker 1 (46:47):
Right, Well, as we mentioned, we're not quite experts in
this topic, but we don't Maybe we will talk to
one of the smartest people in the universe who is
tackling this problem directly.

Speaker 2 (46:59):
That's right down to Nathan, whose father is a listener
of the podcast and listen to the podcast mostly so
you can have a hope of understanding his son's research.
Nathan is a physics brad student at Arizona State University,
a well known department with experts in cosmology and theories
of quantum gravity, and he was kind enough to spend
five minutes talking to me about his research and so.

Speaker 1 (47:21):
Here at Daniel's interview with Nathan Berwick, Quantum Gravity researcher.

Speaker 2 (47:26):
So then it's my pleasure to welcome Nathan to the podcast. Nathan,
please introduce yourself and tell the listeners who you are.

Speaker 3 (47:33):
Yes, I'm Nathan Berwick. I'm currently a first year PhD
student at Arizona State University studying physics and cosmology and
hoping to continue doing so and move into ideally stuff
along the lines of quantum gravity.

Speaker 2 (47:52):
Awesome. Well, we often talk on the podcast about how
science is just a bunch of people following their curiosity
and pushing forward the forefront of knowledge. So tell us
what is it about physics and gr that drew you in,
that made you decide you want to spend your life
on this question.

Speaker 3 (48:10):
That's a really good question. So I think part of
it is just I always kind of grew up as
a curious kid, and you know, when I was younger,
I always knew I wanted to be like a scientist
in air quotes. But eventually I sort of narrowed in
on physics, and I think as soon as I learned
about like the most basic concepts in general relativity, sort

(48:32):
of the idea that space time is one great big
thing and gravity is just the curvature of that it
was just immediately alluring, especially when you start to consider
how gravity is, like are most interacted with force at
least sort of perceptually. You know, we talk about it
a lot. You don't really talk about how electromagnet doesn't

(48:53):
really plays a role in your life very often. But
it's also very poorly understood in general, and there's still
like a lot of really big open questions to be
answered for general relativity. And I think that sort of
openness is very much an invitation to explore, which I
really like.

Speaker 2 (49:12):
So you're not going to give your dad any credit
for encouraging you to study physics.

Speaker 5 (49:17):
I think, I think I definitely should.

Speaker 3 (49:20):
Yeah, No, he definitely like fostered my curiosity in physics
growing up. Both my parents did, but my dad was
always fascinated in physics, and so from just like really
small things, you know, we'd always find questions on the internet,
you know, whether it be a bad string theory, which
is always like a very big, you know, media topic
and whatnot. But he definitely played a very big role

(49:40):
in me wanting to do physics.

Speaker 2 (49:42):
So a lot of people say that general relativity, once
you fully understand it, is deep and beautiful. Do you
have that kind of esthetic reaction to it?

Speaker 3 (49:51):
Yeah, I think a lot of the beauty of it
comes from the idea alone that it's all just curvat true,
that gravity is in essence just curvature of this big
manifold which you may or may not well, which you
can't really visualize. But that, for me is the part
of the esthetic that general relativity has is just like

(50:13):
a breakdown into a more fundamental concept.

Speaker 2 (50:15):
So if gr is so gorgeous and it works so well,
all these experiments confirming Einstein all the time, how can
it be wrong? I mean, is it wrong in the
way that like Newton's gravity is wrong where it was
telling fundamentally the wrong story about what was happening but
we didn't really notice until we dug into the details.
Or is it mostly telling the right story just needs

(50:37):
some like corrections and band aids here and there.

Speaker 5 (50:40):
Yeah.

Speaker 3 (50:40):
So that's a really good question, and I think it
really depends on how much of the last one hundred
years you're willing to disregard, and you know that that
has certain consequences but also benefits. But I think the
most generally accepted perspective right now is that it needs
because general relativity, as this idea of gravity being caused

(51:05):
by space time curvature, is so so rigorously tested at
the moment, you know, we keep trying to knock Einstein
down and find errors in his theory somewhere, but every
single time like he just was right. And so that's
a difficult thing to try and find errors in. But
there's obviously still big questions. So quantum gravity is probably

(51:30):
the largest area of questions regarding general relativity, and a
lot of that has to do with just quantum mechanics
not playing nicely when you try and think about how
gravitational fields work with it. General relativity doesn't have any
fundamental understanding of what probability distribution looks like. There's no
like wave function understanding when you start getting into the

(51:54):
smaller regimes for general relativity.

Speaker 2 (51:56):
So that's a really big topic, like general relativity and
quantm mechanics. What are you actually researching right now? Like
what are you working on today today?

Speaker 3 (52:04):
So throughout the last semester, I've been working on what
are called scalar tensor theories, which I actually think you
guys did an episode fairly recently on no hair theorems.
For black holes, and that's very much what I'm working
on at the moment, just showing that there's no sort
of scalar hairs for certain types of theories.

Speaker 2 (52:29):
Cool. So then I'm gonna ask you to speculate unscientifically.
What do you think is inside a black hole?

Speaker 5 (52:37):
Oh? Boy?

Speaker 3 (52:38):
What do I think is inside a black hole? Speculative
answer is, well, if it's a big black hole, there's
probably stuff sort of floating around in there being eaten up.
But I like to think that there's this sort of
taboo in physics of natural infinities. They aren't there're usually
when there's an infinity, it's something horribly horribly wrong. And

(53:00):
that was sort of the big hub up when they
figured out that black holes could be a thing that
actually existed, was that it just felt wrong that there
could be such a dense object that's just so infinitely packed.
I like to think that it is just, you know,
a perfectly oh infinite density singularity, just because I think

(53:20):
it'd be very wild and wacky and cool to have
this very strange yet natural like divergence or infinity just
existing in the universe.

Speaker 2 (53:32):
Do you think the singularity inside a black hole is
as dense as boba, you know, these chewy blobs of
death that people inexplicably like shooting up their straws as
they enjoy and otherwise relaxing beverage. I happen to know
your father agrees with me on this.

Speaker 5 (53:49):
He sent me a clip of this just the other day.

Speaker 3 (53:54):
I certainly hope that boba is it not at all
comparable to singularities. But I suppose you never know what
that would be, the hell that my dad would die on.

Speaker 2 (54:05):
I think, well, I think maybe your dad should open
up a black hole boba shop that sells black hole boba,
super dense, super dense little chunks of destruction. All right, Nathan,
thanks very much for taking some time to talk to
us about your work on the forefront of understanding of
general relativity, and good luck figuring it all out.

Speaker 5 (54:24):
Thank you very much. Thank you for having me.

Speaker 1 (54:27):
All right, interesting chat there, pretty controversial about both boba
and black holes.

Speaker 2 (54:34):
You know, this is the cutting edge of food science
and quantum gravity.

Speaker 1 (54:38):
That's right, and sugary drinks, yes, But it's interesting that
he seems to follow pretty strongly on the general latuty side.
He thinks inside of black holes are singularities.

Speaker 2 (54:50):
Yeah, I've seen this in a lot of quantum gravity theorists,
that they are intoxicated by the beauty of general relativity.
It's sort of hard to overstate the this has on people.
When they are able to import Einstein's equations into their
mind and they can see the universe in terms of
this differential manifold, they feel like the scales have fallen
from their eyes and they're seeing the universe the way

(55:12):
it really is, and it has to be true. So
they want to preserve this vision of the universe as
having curved space time. It's not exactly religious, but it's
almost like a spiritual experience.

Speaker 1 (55:23):
Oh my goodness. This is coming from, of course, a
quantum mechanics particle researcher. Right, you're trying to say that
the other side are a bunch of religious zillids because
your side is obviously right.

Speaker 2 (55:35):
I mean, they like boba, so so you can't trust
them at all.

Speaker 1 (55:40):
Well, boba are like particles, so I would have thought
they're fuzzy particles, so I would have value you be
a big fan.

Speaker 2 (55:45):
No, they're macroscopic, man, They're dominated by gravity.

Speaker 1 (55:48):
Mm. Well, I wonder if maybe there is a singularity,
you know, and maybe it's also fuzzy at the center
of it. It's just that maybe in a singularity, space
is so compressed or so squish together, that maybe the
quantum certainties also get squished down.

Speaker 2 (56:04):
Maybe we're gonna solve quantum gravity right here on this
podcast today.

Speaker 1 (56:08):
Yeah, yeah, let's do it. Why wait one hundred years,
let's come up with that romantic comedy sci fi movie
right now?

Speaker 2 (56:14):
Why is quantum gravity so hard? Because nobody asked a cartoonist.
There you go, boom, question answered, boom.

Speaker 1 (56:23):
Done. Let's go tackle something harder, like cancer or democracy.
All right, Well, you're on one side, people like Nathan
are on the other side. Sounds like we still have
a long way to go.

Speaker 2 (56:37):
We do have a long way to go, and there
are deep philosophical and mathematical hurdles to overcome. We want
to have a single theory of the universe, one that
gives us a complete picture of what's really happening the
source code for the universe. But so far none of
our attempts actually compute.

Speaker 1 (56:53):
And so it's an active area of research, maybe waiting
for the next person to tackle this and possibly solve it,
and that could be you out there asking these questions,
listening to this podcast, wondering how the universe works.

Speaker 2 (57:06):
I hope that future genius doesn't suffer an unfortunate boba accident,
choking to death before they can share their insights with humanity.

Speaker 1 (57:14):
Oh my gosh, it's a little bit dark there, a
little bit dark. I think you need a pretty large
bubba ball. There's choking.

Speaker 2 (57:22):
Then it's boba relativity.

Speaker 1 (57:25):
That's right, the answer, the choking hazard of a boba
is really all right. Well, stay tuned. You hope you
enjoyed that. Thanks for joining us, See you next night.

Speaker 2 (57:39):
For more science and curiosity, come find us on social
media where we answer questions and post videos. We're on Twitter, Discord, Instant,
and now TikTok. Thanks for listening, and remember that Daniel
and Jorge Explain the Universe is a production of iHeart Radio.
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