Does gravity make rainbows?

Daniel and Jorge talk about a speculative theory called "rainbow gravity" which might help us understand the origins of the Universe.

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Episode Transcript

Speaker 1 (00:08):
Hey, or hey. You know what's still amazes me that
they give to introverts like us a podcast that blows
my mind every week. But also, rainbows are awesome, But
what about them amazes you? Everything is amazing about them.
They are amazingly beautiful, But I'm amazed that they like exist,

(00:29):
that they happen in the universe. You know that there's
a science behind them, right, They're not just like magical.
I get that there's science there, but I just think
it's incredible that we live in the universe where this
kind of thing actually happens. I mean, if you read
about this in the science fiction book, you would think
it's pretty farfetched. Well, it depends what what kind of
science fiction you're reading. Does it also involve unicorns? Unicorns

(00:51):
like a podcast with two introverts? Wait, who is the
unicorn in this case? Because you can only have one,
it's in the name unicorn. Collectively, we are one unicorn.

(01:16):
Hi am Jorhemmack, cartoonists and the creator of PhD comics. Hi,
I'm Daniel. I'm a particle physicist and a professor at
U c Irvine, and I called dibbs on being the
front two legs of the unicorn costume. Oh good, it's
going to be the back end. Maybe you can have
a guest host go. That's why we have guest hosts exactly.
But anyways, welcome to our podcast, Daniel and Jorge Explain

(01:37):
the Universe, a production of I Heart Radio, the podcasting
which we talk about everything that's out there in the
universe and try to explain all of it to you.
We explore the complete spectrum of possible physical phenomena, including rainbows.
If unicorns were real, we would talk about them as well.
But we don't shy away from digging deep into the
fabric of reality, understanding how it all works, and trying

(02:00):
to explain to you what scientists are thinking about, what
they are puzzling over, and what ideas they are bouncing
around their unicorn brains. That's right, because it is a
very colorful universe. Well have amazing sites like rainbows, and
we like to follow that arc of science to see
what is at the end of it if there is
indeed a big part of gold or some other element,

(02:22):
you know, because it just don't really discriminate between elements,
do they We don't. But I would like to accept
my grand funding in pots of gold if that was possible,
you know, rather than just like dollars in a bank account. Yeah,
but then then your grand funding is depending on gold
currency markets. What is the exchange rate between gold and science? Anyway,

(02:42):
I don't keep track, but he's made me think of
another question, Daniel, in a multiverse, right, technically in a
multiverse or maybe even in an infinite universe. Uh, unicorns
probably exists, right, I mean it is possible, so therefore
it must be true. It must be horses out there
in the multiverse or an infinite universe with horns in

(03:02):
their foreheads. Probably out there somewhere in the multiverse there
are unicorn physicists being paid to do their science in
pots of gold. Maybe even two unicorns on a podcast
talking about what it would be like if humans were
doing science instead of them humans were real, Like, maybe
humans are the unicorns in the unicorn world. If you're

(03:23):
already unicorns, why would you even think up humans? Right?
Are do you think of something so boring and ugly
compared to unicorns? Or maybe like regular horses are the
unicorns for them? They're like can you imagine a horse
without a horn? Or maybe it's the other direction, Maybe
they're imagining two horned horses, right, Yeah, that would be
like wild to them. That would be an imaginary and magic.

(03:43):
And on this podcast, we do like to use our
imagination to consider the ways that the universe might be.
After all, we are trapped on this tiny little rock
in a little corner of space, trying to understand the
entire cosmos, which requires somehow developing a model for how
it all works, and extrappling in that model out to
the very far edges of the universe, far forward in

(04:05):
time and far backwards in time. Because we want to
do more than just tell stories about unicorns and rainbows.
We want a mathematical story that explains what we see
in the universe and tells us what has already happened
and hopefully why. Yeah, because the nature of the universe
doesn't just affect the cosmos out there in space and
other galaxies. It affects us here and on Earth and

(04:26):
in our everyday lives. Every time you look up after
a nice rainfall and see a rainbow, that's physics kind
of affecting how you see the world. And I don't
want to talk more about rainbows and unicorns, but I
do think rainbows are amazing. It's incredible that this physical
effect happens, and it shimmers in the air, and it
happens so often, and it's so beautiful. It's just like,

(04:46):
how lucky are we to live in the universe where
such beauty occurs? Makes me wonder about the whole nature
of beauty. But that's a whole philosophical rainbow that we
definitely don't want to walk down today. What we do
want to wonder about is why the universe works this
way and what it means about the history of the
universe as well. He says, the way the universe works
affects our daily lives because it tells us about the

(05:07):
context of our lives. It tells us how the universe
came to be and what it means that we are here. Yeah,
and the history of humanity has been about making theories
that hopefully explain what's going on and gives us an
understanding about why things are the way they are and
how we can maybe affect them or change them, or
at least dream up of fantastical things. And while we've

(05:30):
made a lot of progress and understand in the nature
of the universe and its history, how it got to
look the way that it is, and wondering about how
it started, or at least the very first few moments
of it. There are still a lot of question marks
there about what happened early on, a lot of things
about our theories that don't quite make sense, leaving lots
of room for creative people to imagine all sorts of

(05:50):
theoretical rainbows and unicorns to fill in the gap. I
thought you didn't want to talk about rainbows anymore, Daniel,
keep bringing it up. Not literal rainbows. These are figurative
rain us now. But the arc of history is an
interesting wine, but it's not a necessarily straight line. Sometimes
we come up with theories that explain what we can
see and what we can experiment with out there, but

(06:10):
then later they turn out to be wrong, and that's
okay because that's part of science and it's an evolving process.
It's a really interesting distinction, for example, between math and science,
Like the science that we had two hundred years ago
is now evolved to the science we have today, and
we expect in the future we will have even crisper
ideas of how the universe works. But mathematical proofs that
were developed thousands of years ago. Those are still correct,

(06:31):
and we expect those to still be correct in a
few thousand years. So it's fascinating how science and math
develop sort of differently, even though science is built on math. Yeah,
and so we have theories that currently describe everything we
can see around those quantum mechanics and gravity or I
guess special relativity. But the question is are those actually right?

(06:51):
Do those theories actually describe the universe in all instances
or do they break down at some point? And what
does that mean about our understanding of the universe. One
of the most frustrating things about general relativity and quantum
mechanics is that they don't agree about what happens, and
maybe the most interesting moment of the universe, that is
the very first few moments when things are very high

(07:13):
energy and very dense, and we need both gravity and
quantum mechanics to understand what happened. Was there singularity, was
there something else? What was going on at the very
beginning of the universe. We're pretty sure that our current
theories can't be right, and so we're on the hunt
for new ideas. So today on the podcast, we'll be
tackling the question what is rainbow gravity, Daniel, is this like,

(07:41):
how much does a rainbow way like it is a
rainbow heavy? Can you measure the happiness in a rainbow?
How many pots of gold are generated by general relativity?
Maybe we have a new theory called golden relativity. Hey,
that sounds good. No, this is literally a theory of
the universe that predicts that gravity could make rainbows the

(08:05):
same way that like prisms or drops of water make
rainbows in your eyeball, Like gravity itself could bend white
light and turn it into rainbows. M I guess the
question is would those be regular rainbows or would you
need to call them like gravitational rainbows. Yeah, those would
be gravitational rainbows because you definitely wouldn't see them in
the atmosphere on Earth. This would be like something you

(08:27):
see in your spaceship as you're falling towards the edge
of a black hole. But this wouldn't just be like
a lens flare, like a J. J. Abrahm style lens
flare on your camera or your glasses or your spaceship window.
This would be like real black hole rainbows, real black
hole rainbows. That's right, and you don't need to ride
a unicorn across the sky to see it. If this
theory is actually true, these rainbows exist in the universe,

(08:49):
and they might have existed very early on, and they
could completely change the way we think about the very
first moments of the cosmos. Well, you don't need to
be writing a unicorn, but obviously anything better while you're
writing a unicorn, surely, I mean, I wouldn't know, but
ice cream is better when you're writing a unicorn, for example? Absolutely,
Well depends. Can this unicorn fly just gallop along? Because

(09:12):
it might be hard to eat it a lick and
ice cream cone while you're gelping. You know, I am
not an expert on the categories of unicorns. Do unicorns
come with wings or is that a pegasus? Or pegasus
is just a horse with wings? What do you call
it if it's a unicorn with wings? I think unicorns
just fly on a on a rainbow, right? Is that
what happens? Like you're galloping and then like a rainbow

(09:33):
bridge pops up, and then then you're flying. It's like
the by Frost and Thor and Welcome to the Science
of Unicorns, a sub episode of rainbow gravity. Well, let's
get that. You're right, let's get back on topic here.
We're talking about rainbow gravity or gravitational rainbow or what's
the right way to call this. Is there such a
thing as rainbow gravity. There is really a theory out

(09:55):
there in the community called rainbow gravity, and that's what
we're talking about today. We're gonna try to avoid talking
about unicorns, but I suspect the gravitational attraction of their
gorgeousness is going to pull us back in anyway. You're
saying this is sequels called unicorn gravity, lying unicorn gravity,
that's the title of my next paper on this topic. Well,
you're gonna take one theory combined with an imaginary theory

(10:15):
to get an X ray imaginary in some version of
the mole diverse that earns me a huge pot of
grand funding, which I get in delivery of gold coins. Well,
I'm gonna have one up. Even put wings on that unicorn.
So my theory is gonna be the rainbow unicorn pegass
gravity theory. All right, Well, then I'm gonna put horns
on the wings on that unicorn. Okay, this is kidding
a little HP Lovecraft in here, but it is an

(10:38):
interesting theory. This idea of rainbow gravity or gravitational rainbows?
And can gravity rainbows? So, as usual, we were wondering
how many people out there had thought about this or
maybe dreamt it in one of their dreams. So thank
you to all the people in Unicorns who volunteered to
answer these questions. We greatly appreciate it and enjoy hearing
your thoughts. If you would like to participate for a

(10:59):
future your episodes, please don't be shy. Right to me
two questions at Daniel and Jorge dot com. Think about
it for a second. What do you think is rainbow gravity?
He was what people had to say. Okay, So rainbows
are created by the reflection refraction of light, and the
wavelength of that light depends on what we see in

(11:19):
terms of its color. So rainbow gravity. If I think
of gravitational waves, maybe as those waves are passing through
or having light passed through. Um, maybe it's the effect
that the light and the different wavelengths of the light
has on those gravitational waves. Maybe I don't know. Maybe

(11:40):
like moist that split the light based on frequency, gravity
split the think based off their density, and you call
the outcome rainbow gravity. And well, I've definitely never heard
of rainbow gravity. So my best guess is that it
is something that has to do with the way gravity
effects light. Um, maybe it's a property of gravitational lensing.

(12:03):
Oh jeez, I do not know. I would imagine that
it has to do with the spectrum of light and
how gravity could affect light, but I'm not sure. Maybe
how gravity could split light. All right, we got a
nice spectrum of answers here, very colorful, powerful. Yeah that
was low hanging fruit. But yeah, a lot of some
people have never heard of it, and some people had

(12:24):
some pretty good ideas, like it's maybe gravity causing lensing,
which might create a rainbow effect. Yeah, exactly. This seems
to be a well named theory because it inspired some
good ideas and listeners. Well, I would obviously disagree, but
that's never stop physicists from naming things in counterintuitive ways.
Let's see how this goes, Daniel, I mean rainbow gravity,

(12:45):
but it is gravity actually a rainbow? I don't know.
It's a really fun theory that tries to solve a
problem at the heart of physics, the connection between general
relativity and quantum mechanics, or more specifically, the lack of
the connection along the way predicts beautiful events like rain
those popping out of black holes, wait out of black holes,
that would be impossible, or maybe that gets it's the rainbow,

(13:06):
so maybe it's magical. Perhaps I should have said at
the edge of black holes, Well, let's dig into it.
Um you said, it's a new theory or a potential
theory that's out there in the physics community that maybe
try to explain the intersection between quantum mechanics and gravity,
because those two things are not quite compatible with each other, right, Yeah,
all of modern physics is built on these two ideas,

(13:27):
quantum mechanics and general relativity. But at their hearts, these
two theories are incompatible. They have completely different views of
the world. Even though they're in philosophical contradiction with each other,
they've survived together as twin pillars of our theory of
physics because they're never actually relevant at the same time.
So you can use gravity talk about really big, massive things,

(13:49):
and you can use quantum mechanics talk about really small things,
and you never really need to use both at the
same time. So they've sort of survived without having to
talk to each other. They're like a married couple that
turned into roommates. Well that's kind of said, Well, you know,
if they do try to talk to each other, it's
gonna be a big argument, so they just try to
avoid it. Why did another physics theoretical committee was um

(14:10):
dysfunctional and ahead of for our potential split in the future.
It's not all rainbows and unicorns. Man, all right, well,
maybe I can help us understand this a little bit.
Where is that incompatibility? Is it just that you do
is this haven't been able to make it work in
the mathematical way, or is it is there something fundamental
about how they view the universe that is just totally incompatible.

(14:31):
So there's something fundamental about the way they view the
universe that is just incompatible. We'll talk about exactly what
that is in a minute. And all attempts so far
to unify them have failed. Mathematically just do not work.
So that's essentially what the struggle is. And so let's
start with quantum mechanics. Quantum mechanics specifically quantum field theory,
which is this description we talked about in the podcast.

(14:52):
A lot of space being filled with quantum fields, and
particles are like ripples in those fields that propagate through
space and to with each other. That's very, very successful, right.
It's an amazing theory. It describes all of the particle
experiments that we've done. It's made very specific predictions of
numbers like ten decimal places that have all been verified experimentally.

(15:13):
It seems like a very accurate description of what happens
to particles. But it assumes that space is not curved,
that space is flat, that the shortest distance between two
points is always what appears to us to be a
straight line, and operating in flat space essentially means that
we're ignoring gravity. Quantum field theory is very successful because
basically gravity is irrelevant. If you have two particles and

(15:35):
they're pushing against each other with powerful forces, you don't
really care about the gravitational effects on two electrons because
gravity is so weak compared to the other forces. So
quantum field theory assumes that gravity is irrelevant and does
a great job of describing what happens to quantum particles. Right.
Quantum mechanics assumes a flat, spased universe, but general relativity

(15:56):
kind of assumes the opposite. Yeah, general relativity tells us
that the reason we have gravity is not because there's
some force out there tugging on masses, but that space
bends that when you put mass into a space, it
curves space. And this is not curvature like relative to
some external ruler. This is intrinsic curvature, which means that

(16:17):
it changes the relative distances between points, and effectively, it
makes the shortest distance between two points seem to us
like a curve because we can't see this curvature directly,
and so it appears to us like there's a force
because things are moving along these curves. In reality, it's
just the curvature of space. But general relativity assumes that
everything has a specific location and velocity, and that can

(16:40):
be perfectly well known. We call it a classical theory
because it ignores the quantum mechanical nature of the world,
the fact that particles, for example, can't be pinned down
to have a specific location and velocity at all times.
They don't have a trajectory like that. But general relativity
assumes that everything does. Fortunately, because gravity is so weak,

(17:01):
we've only tested general relativity in scenarios where you have
a huge mass like a planet or a star or
a black hole, where you can basically ignore quantum mechanics,
so they sort of have each their own domains. They
view the world very very differently. They make totally different,
incompatible assumptions about the world, and they make predictions about
different parts of the world, and they're both correct in

(17:22):
their own regimes, right, Although I guess, you know, coming
at this from the outside, not having been too in mercendies,
they don't sound that incompatible to me. I guess it's
maybe that's one of that something other people struggle with.
I mean, it's sort of like one of them says
that Daniel is tall, and the other one says Daniel
wards glasses like those are not necessarily incompatible. Things like

(17:44):
couldn't quantum fields exists in space that is also bending,
And couldn't bending space exists in also in the universe
where particles have a minimum size and are uncertain. Yeah,
we think that probably it's possible to describe both because
the universe exists and it seems to be self consistent.
So we think it probably is possible to reconcile gravity
and quantum mechanics because both things are happening in our world.

(18:08):
We haven't been able to do that, Like light is
being bent around the Sun and it's being bent around
black holes. And we know light is quantized as well.
Right well, let me give you an example of something
that we can't do because we can't unify gravity and
quantum mechanics, is that we don't know how to calculate
the gravity of particles. Quantum particles, for example, don't have

(18:28):
a specific location that have probabilities to be in different locations,
Like electron could be on the other side of the
room or could be right next to you. What is
the gravity of that electron whose position is uncertain? General
relativity doesn't allow for any uncertainty in the position. It
says your gravity depends on where you are. If where
you are is uncertain, then where is your gravity? Is

(18:48):
your gravity also uncertain or is it shared between the
two different locations? Does gravity collapse that wave function forcing
the electron to be in one spot from where it's
gravity emanates, or is have any quantum mechanical and it
allows the superposition of different gravitational attractions. That's something we
don't know the answer to. That's something we can't calculate

(19:09):
right now because we don't have a theory that does
gravity for quantum objects. Right, But I guess you know,
particle has some uncertain to it to it in terms
of where it is and where it's going. But if
you step back far enough, it does have sort of
a location, right. I mean, you step back far enough
from a particle, you can calculate what its gravity is. Yeah,
if you step back far enough or you love enough

(19:29):
particles together, then they start to act like classical objects
like a baseball. Baseball effectively has no uncertainty on where
it is and it's velocity because it acts like a
classical object, which is ten to twenty six quantum objects
all moving together. And so general relativity assumes that that
is still true the baseball description of the world when
you get down to one electron. But we know that

(19:50):
one electron doesn't move like a baseball. It doesn't have
a whole path, and so we don't know how to
talk about like how two electrons interact with each other gravitationally,
And we can't usted either. We can't just go and
look and watch two electrons pushing against each other gravitationally
because the gravitational force is so weak that we could
never measure that. And so it's a big question. Right.
But you, for example, you are able to, for example,

(20:12):
calculate the force between two electrons to like the electromagnetic
force between two electrons, right, Like, you can do that,
I guess the question is why can't you do it
with gravity, Like why can't you juice kind of like
average it out or use probability to figure out what
the most probable gravity is. So we can calculate the
gravitational force between two electrons if their positions are known,

(20:35):
but because they're quantum objects, we don't know their positions.
So what you're proposing, basically, is a theory of quantum
gravity that says gravity is a quantum force and interacts
not with the location of the objects but with their probabilities,
that it actually interacts with the wave functions of these guys.
So they're trying to turn gravity into a quantum field theory,
which is fine and that's cool, and a lot of

(20:55):
people are working on that. But when you do that,
you run into a lot of mathematical problems. Basically, you
get lots of infinities when you try to do these calculations,
and we get infinities when we do all quantum field theories.
Like we talked on the podcast about something called renormalization
where we tuck infinities under the rug. For example, the
true charge of the electron, if you look at it
really really close, seems to be negative infinity, which makes

(21:17):
no sense. But you can sort of like wrap it
in a cloud of particles and renormalize it and subtract
away that infinity and say that's all fine. You can't
do that for gravity, because you get an infinite number
of those infinities. Gravity couples to itself and it couples
to everything, and so it just sort of goes crazy,
and we just don't know how to do those calculations.
We don't have the mathematical framework that can successfully do that.

(21:39):
Something has to change when particles have a really really
high energy in order for those infinities to go away,
all right, So that it's hard from you, uh, and
nobody has been able to do it. But there is
maybe a new theory or a theory out there that
does seem to have maybe a magical solution to this problem,
gravitational rainbows or rainbow gravity, and so let's dig into

(22:01):
the details of that. But first let's take a quick break. Alright,
we're talking about rainbow gravity, which sounds great. Who doesn't
want gravity to have its own, you know, spectrum of awesomeness. Yeah, exactly.

(22:25):
It's a very colorful theory. And you know, during the break,
I went off and did a little bit of research,
and I discovered the answer to one of the open
questions we left dangling before. Is the answer unicorns? You know,
the answer actually is alcorn, and alcorn is a unicorn
with wings or a pegasus with a horn. There's a
name for this thing. So somebody already did that theory.

(22:45):
I think it's my little pony universe that might have
coined this phrase. What if I crossed it with a
lion that and then like a griffin unicorn pegasus. I
think we better trademark that and start selling plush dollars
of those lion ala corn maybe griffin corn. But we
are talking about rainbow gravity, which sounds great, and it's
maybe a theory that tries to unify quantum mechanics and

(23:08):
general relativity, which are the two big theories that try
to explain us, which don't quite match up when you
get to certain situations. Daniel, we talked about how quantum
mechanics is not good at describing gravity, and we talked
about how general relativity is not good at describing things
at the microscopic level exactly, and most of the time
they don't conflict because quantum mechanics is king of the

(23:28):
microscopic particles and gravity is king of the big massive stuff.
But there are scenarios we think when gravity and quantum
mechanics are both important. One of those is inside black holes,
where things are obviously very powerful gravitationally, but they're also
compressed to super tiny little divots. Right, if there is
a singularity at the hart of a black hole, then

(23:50):
that's small enough to be a quantum mechanical object. So
general relativity and quantum mechanics both have something to say
about what happens there. We can't see inside black hole,
and so we're left only just wondering about what's going
on inside. But there are scenarios outside of black holes
where we think both general relativity and quantum mechanics have
something to say, and that's the case of very very

(24:11):
high energy particles. Yeah, but I guess I wonder if
you need to go that extreme just to kind of
see where the two theories take effect, right, because I
think Einstein kind of famously proved general relativity, or at
least the bending of space by looking at how light
bends around solar eclipse, for example. We know that light
is quantized, it follows quantum mechanical rules. So isn't that

(24:34):
an example for example of a quantized thing following gravity
technically as opposed but technically everything is a quantized thing.
You are a quantized thing. I am a quantized thing.
We are affected by the Earth's gravity. You know, in
that case, they're not relying on the quantum nature of light.
Light could have just been classical electromagnetic waves and the
same thing would have happened, because in that scenario, light

(24:57):
is moving through curved space, and so light everything else
would move in a curve. So you're not relying on
the quantum nature of the object there. So like a
quantized particle like light or an electron, it can move
through bent space, but due to gravity, you know, I
mean you can you have the mass to describe a
single particle, single quantum particle moving through bent space. That

(25:20):
possible or is that is not possible? That's certainly possible,
But there you're not relying on the gravity of the
quantum particle. You have something else something really big and
massive like the Earth or the Sun or the Moon
or a black hole that's doing the bending. And so
we do know how to talk about quantum particles moving
through bent space that we can calculate or we don't

(25:40):
know how to calculate is the gravity of those particles
and how that contributes to bent space? Right? But doesn't
the bending a space depend on the gravity on the particle? Right,
Like technically you're talking about space time, right, Like if
I throw a baseball at the Sun, it's not going
to curve the same way that a photon is going
to curve. Right, So, like eat, the bend thing of
space depends on the thing that's moving. So if you

(26:03):
can calculate the bending of space for a particle, what's
worth the contradiction there? Well, because there you're ignoring the
gravity of the object, like you're through a baseball or
a photon near the Moon. You're not taking to account
the bending of space due to the baseball or due
to the photon. You're just calculate the trajectory of a
test particle through the bent space due to the Moon

(26:26):
or due to the black hole. You're not taking to
account the gravity of the object itself. Don't you need
that to calculate how it's going to curve around how
it's gonna bend around the Moon. Now, you just need
to know it's inertial mass. You don't need to know
the effect of it on space time itself. You just
need to understand what it's inertia is and so that
you're can understand how it moves through bent space. M

(26:46):
M I see. So the real problem is not in
like how to calculate how quantum particles move through bent space,
but more about how quantum particles give off gravity or
you know, how they attract other particles through gravity. Yes,
do they bend space exactly? And and do they bend
space where they are or where they might be. That's
really the question, all right, So then talk to us

(27:08):
about this new theory rainbow gravity. So the problem comes
up at very very high energies. If you have particles
with super duper crazy high energies, energies like the plant
energy or energies that particles had at the very beginning
of the universe, they have so much energy that their
gravity starts to not be ignorable. Usually when you talk
about like two protons bouncing against each other. You can

(27:28):
ignore the gravitational effects, but if those two protons have
enough energy, then their gravity becomes really really powerful. Because remember,
gravity is the bending of space time in response not
just to mass, but in response to energy. And so
take for example, two particles and collide them at super
duper high energy, much higher energy than we've done before,
you might get, for example, a black hole, or at

(27:51):
least you have to take into account the gravitational interaction.
So we think that maybe the solution to this problem
lies in understanding what happens to particles at very very
high energy, and rainbow gravity says, maybe a very high energy,
the rules change a little bit, and gravity for those
really high energies is a little bit different. What so,

(28:11):
as a particle gets moving faster and faster, it accumulates energy, right,
that's what you're saying. And for when you have that
much energy in the small package, then you kind of
have to think about how that affects gravity because gravity,
I mean, we always talk about gravity being a function
of mass, like the more mass you have, the more
gravity you have, but it's really just about how much

(28:32):
energy you have, right, Like the bending of space is
due to the energy that you have, yes, due to
energy density, not literally mass. Mass is a component of
the energy density, but it's not the only contribution. Right. So,
now you have like a quantum particle. It's moving really fast,
but it has a lot of energy in a small place.
But it's quantum, so there are some uncertainties to it.

(28:52):
And so now the question is like, can you explain
gravity in that situation. Yeah, So rainbow gravity theory says,
let's change the rules gravity as you move up in energy.
That currently we think that all particles feel gravity the
same way. If, for example, you have a big massive
objects and it's bending space, and you shoot a red
photon through it or a green photon through it, those

(29:13):
things will bend around that object the same way. They'll
end up at the same place. Right that all different
energies of photons all get bent the same way because
they all see the same curvature of space. They're like
running along the same track. But wait, don't each of
those photons have a different amount of energy. Yeah, but
remember we're not considering the gravitational effect of the particle,
just of the other object that's curving space, that's guiding

(29:35):
these particles. We're ignoring the gravitational effect of those particles,
and the rainbow gravity says, what if that's not true?
What if as you move through space, how much curvature
you see depends on your energy. So for example, maybe
a red photon and a green photon see a different
amount of curvature. That somehow, the curvature you see depends

(29:56):
on your energy, and so as the photons wavelength or
its color, or its energy, these are all equivalent changes
you see different curvature. If that happens, then if you take,
for example, a beam of white light and you bend
it around a black hole, each different wavelength would get
bent a different amount, resulting in a rainbow. So the
idea of rainbow gravity is to say, maybe gravity depends

(30:19):
on the energy of the particle in this way, so
they see a different amount of curvature, which would change
how things happen at very very high energies. You're saying,
like many, gravity is not constant throughout all situations, like somehow,
gravity is you know, depending on how fast you're going. Technically,
this is called an energy dependent metric. Metric is like
the curvature of space in general relativity theory, and so

(30:42):
typically that metric is constant, doesn't depend on what your
energy is. But now they say, well, let's take that
metric and make it energy dependent. Let's say that if
you're moving through the universe, you might see different curvature
depending on the energy you have. But then isn't like
mass the same as energy? Right? So are you saying
that if I have or or less mass, I'm going
to see space bend differently. I think that's probably true.

(31:05):
Very very massive particles would see space meant differently than
lower mass particles. Okay, so the ideas that gravity is
different depending on how fast you're moving or how much
energy or mass you have, which is very different than
what we we how we think about it now? Right
right now, general relativity says that gravity is the same everywhere. Exactly.
It says that gravity is the same everywhere. And you

(31:27):
might be wondering, like, well, how does this solve the
problem of general relativity and quantum mechanics. It's not exactly
a solution. It's just sort of like maybe in this
direction a solution lies. It's sort of like exploratory. Remember,
you know we talked about sciences and developing process We
don't always have like the final answer all at once.
Sometimes what we do is we say, what's in this direction,

(31:48):
and what happens if we try this kind of thing?
Does this lead to a solution? And so it's not
clear yet whether this might lead to a solution, but
there's a little bit of a sketch of an argument
for why it might. People have this idea for how
to solve the problem of like what's the gravity of
an uncertain quantum particle, of saying maybe the curvature itself
has uncertainty, like I think you were saying earlier, if

(32:08):
an electron has uncertainty and it's bending space time, then
maybe spacetime itself has an uncertainty, a quantum uncertainty, like
maybe we live in this universe with this bent spacetime,
or maybe we live in that universe with that bent spacetime.
So people have done a bunch of calculations and shown
that if the curvature itself has some uncertainty, it would
lead to an energy dependence of that curvature, Basically that

(32:30):
particles moving through the universe with different energies would see
different aspects of this uncertainty. So this is like saying,
if space itself has some uncertainty, then you might get
this kind of effect for very high energy particles I see,
because you sort of maybe need gravity to be not
constant throughout the universe, because you know, if these particles

(32:51):
are quantum, that means they're here and they're there. And
if they're here and there, then that can't mean that
they have gravity here and they have gravity there. Or
but maybe if gravity is different in the two situations,
then it's like maybe has half gravity here half every there,
which kind of makes a full one gravity. Yeah, exactly,
And we don't know, right We do not know if

(33:11):
gravity operates that way, if it really can be probabilistic
or not. This is an attempt to incorporate that quantum
uncertainty into the theory of gravity. And if you do
the calculations and say what happens to particles moving to
space that's uncertain, how do they bend? It turns out
that particles at different energies interact with that space different
that that's see a different slice of that uncertainty. So

(33:32):
particles with really high energy would bend differently than particles
with low energy as they move through that uncertain space,
and that gives you rainbows and again, how does that
help bring together quantum mechanics and gravity. If this is
true and you see it and you confirm it's actually
part of our universe, for example, it gives you a
very strong hint. It tells you that spacetime itself is uncertain.

(33:52):
One possibility is that gravity is quantum mechanical, right, that
spacetime has uncertainty to it, that we live in a
universe where spacetime can have two different possibilities and they
get collapsed when you test them. Right. The other is
that gravity is not quantum mechanical, and that gravity itself
collapses those wave functions that when two electrons interact gravitationally,
their wave functions don't interact. They collapse each other's wave functions.

(34:15):
And then electron has a location here in another location there,
and you have very specific sort of classical gravity. So
if we see rainbow gravity, that tells us that gravity
can accommodate uncertain space times, and that single particles can
have gravity themselves, right, just like a planet does exactly,
And I have been electron over here and over there,

(34:35):
then space time is also fifty percent bend over here
and fifty bent over there, all right, Yeah, then that's
how you avoid the infinities that you're talking about earlier. Yeah,
because it changes how particles move at very very high energies,
which is exactly where these infinities crop up. Thinking about
like what happens to particles with really high energies which
bend space time, which create more gravitons, and you get
this infinite pile of gravitons and a runaway energy. So

(34:58):
if you change the behavior of the articles are very
very high energy, you can basically delete those infinities because
you change the rules when you get near infinity, so
the infinities basically go away. Cool. All right, Well that's
the theory of rainbow gravity, and so let's talk about
what would happen if it is true. What kinds of
things would we see out there where we see rainbows
around black holes? Would we see unicorns? And would that

(35:21):
mean that unicorns are also quantized? I hope? So I
don't ever want to see one and a half unicorns. Well,
if nobody wants to be your back end, then you
might have to be a halfy unicorn. That's a good
All right, we'll get into that, but first let's take
another quick break. All right, we're talking about rainbow gravity, uh,

(35:50):
and we're talking about what it is. And it's the
new theory that kind of tries to bring together quantum
mechanics and general relativity by saying that maybe gravity is
not the same aim everywhere, maybe it's different for different
energy particles, which would sort of help explain how gravity
works at the quantum level. Now this theory was true, Daniel,
does that mean we would see rainbows around black holes?

(36:12):
It does exactly mean that we would see rainbows around
black holes. Because this is a theory that tries to
unify quantum mechanics and gravity, which mostly ignore each other
and live in different regimes. It's a hard kind of
thing to test. You need special circumstances. So this doesn't
mean that we should expect to see gravitational rainbows all
over the place, you know, through the moon or the sun.
It's only in extreme conditions because this is a very

(36:35):
very small effect until you get two very very high
energies or very very strong curvature of space, like around
a black hole. So you're in a spaceship and you're
falling towards a black hole, and then white light near
the event horizon would be split into all of the colors,
and you would see rainbows before you get squished. Meaning
like I shoot a beam of light of white light

(36:56):
at a black hole at the very edge, and as
this beam of light gets very close to the black hole,
the photos that are more red would get pulled one way,
and the photons that are more blue would get pulled
another way, and the purple ones a bit pulled a
little bit differently, And so that would actually kind of
prism or split the beam of white light. Yeah, prism
exactly is the right analogy. That's what a prism does,

(37:18):
is it it bends light based on its wavelength. Different
wavelengths of light get bent differently as you go from
air to glass and back to air, and so they
spread out white light into a rainbow. And because now
we're introducing an energy dependent effect or a wavelength dependent
effect on gravity, than black holes are basically prisms, and
they would change a beam of white light into a

(37:41):
spread based on the colors. Yeah, their prisms and their prisons.
Because I guess you know, white light, it's not like
the each photon is white. Is that you have a
combination of photons, some of them are white, some of
them are higher and lower energy. Right, that's what what
white light is. It's not like the photons are white. Yeah,
there is no individual photon that has the color white. Right.

(38:02):
White is not a single color. It's a combination of
many photons of different colors. Right. And so around a
black hole in the gravity, we would be so strong
that it would actually start to affect each of those
photons differently, which is sort of like a prism or
or a lens. I guess, which makes me wonder, like,
why have we seen this effect? Have we seen? I mean,
now we now have pictures of black holes that we

(38:24):
see any rainbows around it. We do not see any
rainbows around black holes yet. This would be a very
very slight effect. And you need to look very carefully
right at the edge of a black hole. You need
a well calibrated source also to know whether you're seeing
any distortion. To know whether it's a story, you have
to know what it looked like before it went around
the black hole. And so we don't have any nice, clear,

(38:45):
crisp examples of that. Even around black holes. This would
be a very small effect. But what do you mean
it would be a small effect? Like this effect is
not very strong. It's very small that the way gravity
varies depending on the energy. Yeah, and this theory gravity
does pen on the energy, but it's basically unobservable until
you get too super duper high energies. In the same

(39:05):
way that like effects of special relativity, you can't really
observe them when you're throwing a baseball around. You don't
notice clocks going slow. Your baseball doesn't seem to shrink
when you throw it even though it's going faster. You
don't notice the effects of special relativity here on Earth
because they're negligible. In the same way, this energy dependent
effect of gravity is negligible except in extreme circumstances. And

(39:26):
so you need like a very crisp, clear setup of
a beam of white light right next to a black
hole and basically nothing else around so that you can
observe it. But there are some other ways we might
be able to test this theory. Yeah. They involved gamma rays. Yeah.
So gamma rays are basically just a fancy name for
super high energy photons. And there are these strange phenomena
that we've talked about the podcast a few times called
gamma ray bursts, where something out there in the universe

(39:49):
sends a huge spray of very high energy gamma raise
all about the same time. These things can laster like
seconds or minutes. We don't really understand the source of them.
Check out our whole podcast episode on that topic for
a deeper dive into it. But the cool thing is
that it's a nice test of this theory because it
sends us a big packet of photons, some of which
have huge energy, like crazy high energy photons, and they

(40:13):
all come to us about the same time, so we
can sort of use them as a probe of how
they've responded to the gravity of the universe that they
have flown through. Right, they might like arrive at different
places in our sensor or you know, like a rainbow
kind of get split, or are you saying they might
arrive at different times? So if for example, they whizz
around a black hole, they would get bent differently. But

(40:35):
the universe has some curvature, and as you move through it,
you're slowed down by that curvature, so they would be
differently time dilated as they move through the universe if
they see different curvature, so they would effectively arrive at
different times. If you send like a green or red
and blue photon across the universe to us. Then if
this is true, those photons would arrive at different times

(40:56):
here on Earth because they would see different amounts of curvature.
Curvature affects the passage of time and effectively how long
it takes light to traverse from the source. So like
some of the photons would get blue shifted more than others,
or wretch shifted more than others. That's kind of what
you're saying, because they have to arrive at the same time,
don't they Because they're moving at the speed of light,

(41:18):
these would actually arrive at different times. I mean from
their point of view, they would see different distances between
the source and the destination because they're seeing different amounts
of curvature of the universe. Well, that's pretty interesting, But
I guess couldn't we do that experiment here on Earth?
Like you know, we can create gamma rays, and we
can also create you know, like ultra violet or here
in other can create high energy light and super low

(41:40):
energy light. Couldn't we do some experiment where we shoot
both and see what happens. Yes, but we can't create
very strong gravitational curvature right, So we can create pretty
high energy photons, but not that high energy, not as
high energies exist out there in the universe. Also, we
don't have very strong curvature. The value of this test
is that the photons are super duper high energy because
they come from some astrophysical source and they fly through

(42:02):
a huge amount of curvature. So even the small effects
of curvature add up over very very long distances. What
do you mean, like curvature not due to any particular
thing like a black hole, which is like the general
curvature of space from having stuff in it, from having
stuff in it. Yeah, exactly, as you fly through the galaxy,
for example, there's a small gravitational well that the whole
galaxy sits in. That's the curvature of our local space.

(42:26):
But wouldn't we see that regular light because I know
there's something called gravitational lensing out there, where like a
planet can sort of lens and bend light to give
us a better view of another galaxy or another star,
or you know, a dark matter can also kind of
lens light. Wouldn't we see rainbows caused by dark matter? Too?
So I think that's exactly what this is suggesting to

(42:48):
probe right. Send a bunch of really high energy photons
through space, and all the matter that's in space creates
some curvature, and those photons would respond to that curvature
differently based on their energies. And so you would see
that then on Earth, and so yes, the galaxy and
all the dark matter and are all contributing to that.
The vanilla version of general relativity predicts that that wouldn't happen, right,

(43:10):
that they all get bent the same way. And you're
right that that's something that is predicted. As photons fall
into a gravitational well or dig themselves out of a
gravitational well, they do get shifted in frequency, for example.
But we think that happens equally for photons of all energy.
Rainbow gravity says that happens differently. So for very very
long distances, these would accumulate, do it, all the dark

(43:30):
matter and the other stuff in the galaxy and create
a small difference in the arrival time of those photons,
but not in their location, like they would all arrive
at the same spot. They would use maybe be colored
a little bit different, or would they actually split like
a rainbow. They would actually split like a rainbow, so
we wouldn't see the whole thing, right, So the whole
thing we get spread out across the universe, but we

(43:50):
would get a slice of it, and in theory we
might get ones of different color that we could also
test their time of arrival. So we've actually done this,
and we've looked at gamma ray bursts and we've i
to see if we see effects for it. There's no
evidence for it so far. Even these gamma ray bursts.
The evidence would be pretty subtle, and we don't have
that many examples of it. So the jury is still
sort of out on this theory. We don't have any

(44:12):
evidence for it, but we also can't yet rule it out.
It sounds like it's not an effect that you would
see in our everyday lives, like when we see galaxies
being lensed by dark matter out there, even that is
not strong enough to split light due to rainbow gravity. Yeah,
it requires a huge amount of energy or integration over
very very long distances. I guess what maybe we are

(44:32):
just saying is that if rainbow gravity is true, it
is happening even around us, all around us, you know,
like if I look at my hand, or if we
look at galaxies through a dark matter gravitational lens. It
is happening, but maybe just at a level that we
can't discern. In the same way that like time dilation
is happening all the time, length contraction is happening all
the time, you just can't tell because it's so negligible.

(44:53):
All right, well, let's talk about now what would happen
if this was true. We don't have evidence for it
either way, whether it's true or not, but what would
it mean if it If it is true that rainbow
gravity exists, or like gravity is not the same everywhere
for everyone except unicorns, it has really interesting consequences if
you run the clock of the universe backwards. Currently we

(45:13):
see the universes expanding, and we run the clock backwards
and we do the calculations in general relativity, and they
predict something really weird. They predict a singularity that as
the universe got denser and denser, there's a sort of
runaway gravitational effect in reverse where you end up with
a singularity, but with the universe is basically compressed into
incredible density. So that's the prediction of general relativity, and

(45:35):
you know, we think that's kind of bonkers. We think
that those kind of infinities probably don't exist in our universe.
And this, to most physicists is a sign that general
relativity needs some work right where this is a problem,
and so this is where we look for alternatives, and
so rainbow gravity. If you modify gravity in this way,
it says well, in very high energies, things actually do change.

(45:55):
And so you put this into the calculations instead of
general relativity, and now particles that different energy are affected
slightly differently by that curvature, and you don't end up
with a singularity. You end up with something which like
smoothly gets denser and denser. But we just sort of
a minimum plateau. It's like, intuitively, you can't just like
squeeze all the particles down to the same place because

(46:16):
the particles are all now bent differently by space. M M.
What would that mean for black holes? Two? Does that
mean there's no singularity at the center of black holes? Yes?
This basically a racist singularities in the universe, both singularities
in time like what might have happened before the Big
Bang and singularities in space like what's inside a black hole?
Doesn't mean black holes shouldn't exist. It tails us maybe

(46:38):
that the structure of matter inside the black hole isn't
a singularity, it's something else, weirder. It's controlled by rainbow gravity.
It's going to be something with a non zero size
to it. It'll have a fuzzy core to it, not
like a single point. Yeah, a fuzzy, colorful core probably. Yeah.
There are many colors of black, absolutely glossy black, matt

(46:58):
black right at black. Yeah. So maybe this sounds amazing,
and it sounds like it would solve a big problem
and maybe the biggest problem in physics right now theoretically
at least, But we haven't seen evidence for it experimentally,
and are there any reservations about just the theory of it.
Most mainstream physicists think that this is crazy, right. They
think that this is a monker's idea and just wouldn't

(47:21):
fly basically because it violates as a central principle something
that we all think probably is true, which is that
observers can all agree about the physical laws. Like in
our universe, we think that observers all see different things happening.
But one of the foundational principles of special relativity is
that the universe always follows the same laws. You might

(47:41):
tell a different story about what happened, but everybody's story
follows the same laws. This breaks that because now everything
is like energy dependent, and so the amount of curvature
you see depends on the energy you have. It breaks
this thing we call Lorentz invariants, right, because I guess
if gravity the anson how much energy you have, energy

(48:02):
can be a relative concept, right, energy depends on how
fast you're moving your kinetic energy. Then that can look
differently to different people, right, Like, if you're moving super
duper duper duper fast to somebody outside of yourself, you
would have a lot of energy, but to you yourself,
you wouldn't have a lot of energy. Is that what
you mean? Yeah, there's that aspect to it, but it

(48:22):
goes a little bit deeper than that. You know. Right now,
we think that space is relative in an important way,
but that there are some invariants that some things hold fast,
Like the speed of light is the same for all observers.
But if this theory is true, then the speed of
light sort of depends on the energy of the photons. Right,
These photons traveling through space effectively have different speeds and

(48:45):
that's pretty weird. There's a lot of really strong constraints
on measurements of variable speed of light, energy dependent speed
of light, and so it would require, you know, a
real revolution in the way we think about the nature
of the universe. But maybe that's what's required, right. Our
current principles general relativity and quantum mechanics have completely incompatible
assumptions at their foundation, and so to unify them, we

(49:07):
are going to have to get rid of something that
we hold, choose something that we cherish. Maybe it's Lorentz
and variants, maybe it's not. But before this theory is accepted,
it would require us to really rebuild a lot of
physics from the ground up. Yeah, I get a Physicists
don't like things to change. We love things to change. Actually,
our dream is to find some new theory which blows

(49:29):
everything up. But yeah, it doesn't mean a lot of work.
But that also means, you know, hey, more grand funding,
more piles of gold. But I guess what you mean
is like, you don't like the laws of physics to
be different, and depending on where you are in the universe, yes,
that's true. It would be very nice if the laws
of physics were the same for everybody, and you're a
guest that because it would mean slop your math, or
because you haven't seen any experimental proof of that. We

(49:50):
haven't seen any experimental proof of it. And also it's
kind of a nice principle if philosophically, it just sort
of like makes sense to imagine that the universe runs
on a certain set of laws. Those laws are the
same for everybody. It doesn't have to be true, you know,
the same sense, Like, we don't even know why the
universe has laws and why those laws don't change with time.
There's a lot of just basic assumptions at the foundation

(50:12):
of fundamental physics that we make and seem to work,
but we don't really understand why, and we might have
to revisit. Cool. Well, I guess once again, it's a
stay tuned. It might be true. It might be that
special thing that solves a lot of problems and feels
magical to everyone. What does that term people use for
something like that, a unicorn? A unicorn, right, right, So

(50:36):
the rainbow gravity theory might be at unicorn in itself,
according to a physicist right here on the podcast, Yeah,
it might fly in on rainbows and solve all the
problems that we have, and that will let us see
the full spectrum of the universe, see all of its
beautiful colors, and also let us see how it changes
in space and in time. So remember that science is

(50:57):
a continual process that involves and changes. Are understanding of
the universe out there, how it works, and where it
came from. And as we learned more and more about
the nature of the universe, we understand more and more
about how it began and what it means to be here.
We hope you enjoyed that. Thanks for joining us, See
you next time. Thanks for listening, and remember that Daniel

(51:23):
and Jorge explained The Universe is a production of I
Heart Radio. Or more podcast from my heart Radio visit
the I heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows.