Daniel and Jorge Explain the UniverseDaniel and Jorge talk about a speculative theory called "rainbow gravity" which might help us understand the origins of the Universe.
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Hey, Orge, you know what still amazes me That.
They gave too introverts like us a podcast.
That blue my mind every week, but also rainbows.
Yes, are awesome, but what about them amazes you? Everything is amazing about them.
They are amazingly beautiful. But I'm amazed that they exist, 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're read about this in a science fiction book, you think it's pretty far fetched.
Well, depends on what kind of science fiction you're reading. Does it also involve in unicorns?
Unicorns? Like a podcast with two introverts?
Wait, who's the unicorn in this case? Because you can only have one. It's in the name unicorn.
Collectively, we are one unicorn.
Him Jorham, a cartoonist and the creator of PhD Comics.
Hi. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I call DIBs on being the front two legs of the unicorn costume.
Oh good, who's going to be the back end?
Not me?
Maybe you can have a guest hostko.
That's why we have guest hosts exactly.
But anyways, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio, the.
Podcast in 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 to explain to you what scientists are thinking about, what they are puzzling over, and what ideas they are bouncing around their unicorn brains.
It'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 pot of gold or some other element. You know, Physicists don't really discriminate between elements, do they.
We don't, But I would like to accept my grant funding in pots of gold, if that was possible, you know, rather than just like dollars in a bank account.
Yeah, but then your grant funding is depending on gold currency market.
What is the exchange rate between gold and science?
Anyway, I don't keep track, but you made me think of another question, Daniel. In a multiverse, right, technically, in a multiverse, or maybe even in an infinite universe, unicorns probably exists, right, I mean, it is possible, so therefore it must be true. There must be horses out there in the multiverse or in infinite universe with horns in 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 we're doing science instead of.
Them humans were real, Like, maybe humans are the unicorns in the unicorn world.
If you're already unicorns, why would you even think up humans? Right, how 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.
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 extrapolating that model out to the very far edges of the universe, far forward in 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 that 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 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, 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 Frae says, the way the universe works affects our daily lives because it tells us about the 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 made a lot of progress and understanding 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 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 rainbows now.
But the arc of history is an interesting one, 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 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, 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? 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 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? Now, Daniel, is this like how much does a rainbow wag? Like? Is a rainbow heavy? Can you measure the happiness in the 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 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.
Now, 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 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 jj Abram 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, 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 is better while you're writing a unicorn. Surely. I mean, I wouldn't know.
But ice cream is better when you're riding a unicorn, for.
Example, Absolutely well depends can this unicorn fly just gallop along because it might be hard to lick an ice cream cone.
While you're yeah, you know, I am not an expert on the categories of unicorns. Do unicorns come with wings or is that a pegasust 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 rainbow, right, is that what happens? Like you're galloping and then like a rainbow bridge pops up and then you're flying. It's like the bifrost and thor.
And Welcome to the Science of Unicorns, a sub episode of rainbow Gravity.
Well, let's get back you right, let's get back on topic here. We're talking about rainbow gravity or gravitational rainbow Oh, what's the right way to call this? Is there such a thing as rainbow gravity.
There is really a theory out there in the community called rainbow gravity, and that's what we're talking about today. We're going to 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 sequels called unicorn gravity.
Flying 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 to get an ex Thanks for imaginary.
In some version of the multiverse that earns me a huge pot of grain funding, which I get in delivery of gold coins.
Well, I'm going to one up you and put wings on that unicorn. So my theory is going to be the rainbow unicorn Pegasu's gravity theory.
All right, Well then I'm gonna put horns on the wings on that unicorn.
Okay, this is getting a little it should be Lovecraft in here, but it is an interesting theory, this idea of rainbow gravity or gravitational rainbows and can gravity me 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 and unicorns who volunteer to answer these questions. We greatly appreciate it and enjoy hearing your thoughts. If you would like to participate for future episodes, please don't be shy. Write to me to questions at Danielandhorge dot com.
Think about it for a second, what do you think is rainbow gravity. Here's what people have 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 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 them, 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 light moist that split the light based on frequency. Gravity is split the thing based off your density, and you call the outcome rainbow gravity.
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 affects light.
Maybe it's a property of gravitational lenthing.
Oh geez, 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 A right, We've got a nice spectrum of answers here, very colorful orful. Yeah that was low hanging fruit. But yeah, a lot of some people had never heard of it, and some people had 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 stopped physicists from naming things in counterintuitive ways. Let's see how this goes, Daniel, I mean rainbow gravity. But is gravity actually a rainbow? I don't know.
Yeah, 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, and along the way predicts beautiful events like rainbows popping out of black holes.
Wait out of black holes? That would be impossible. Or maybe that gets it the rainbow, so maybe it's magical.
Perhaps I should have said at the edge of black holes.
Well, let's stick into it. You said it's a new theory or a potential theory that's out there in the physics community. Then maybe track 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, 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, and you can use quantum mechanics to 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 have 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 didn't know the physics the Radical Committee was so dysfunctional and headed for a potential split in the future.
It's not all rainbows and unicorns, man.
Well, maybe help us understand this a little bit. Where is that incompatibility. Is it just that you physicis haven't been able to make it work in the mathematical way, or is there something fundamental about how they view the universe that is just totally incompatible.
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 talk about in the podcast. A lot of space being filled with quantum fields, and particles are like ripples in those fields that propagate through space and interact 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. It seems like a very accurate description of what happens two 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 they're pushing against each other with powerful forces, you don't really care about the gravitational effects onto 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 space universe, but general relativity 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 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 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, 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, that make totally different, incompatible assumptions about the world, and they make predictions about different parts of the world, and they're both correct in their own regimes.
Right, Although I guess, you know, coming at this from the outside, not having been too immerse indies, they don't sound that incompatible to me. I guess maybe that's one of that's 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 words glasses like those are not necessarily incompatible. Things like couldn't quantum fields exist 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 is possible to describe both because 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. 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 quantis is 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 a specific location that have probabilities to be in different locations, Like an electron could be on the other side of the room, or it 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's 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 gravity 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 right now because we don't have a theory that does gravity for quantum objects.
Right, But I guess you know, particle has someone certain 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 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 the 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 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 test it either. We can't just like 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, calculate the force between two electrons to like the electric magnetic 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 you just 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, 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 now you're trying to turn gravity into a quantum field theory, which is fine and that's cool, and a lot of 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 negati of infinity, which makes 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. Something has to change when particles have a really really high energy in order for those infinities to go away.
All right, So it's hard getting from you, 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 the details of that. But first let's take a quick break.
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All right, we're talking about rainbow gravity, which sounds great. Who doesn't want gravity to have its own spectrum of awesomeness?
Yeah, exactly. 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?
I know.
The answer actually is alacorn, and alcorn is a unicorn with wings or a pegasus with a horn. There's a name for this theingh.
So somebody already did that theory.
I think it's the my little universe that might have coined this phrase.
What if I cross it with a lie in that and then like a griffin, unicorn pegasus.
I think we better trademark that and start selling plush dolls.
Of those Lionala corn, maybe griffic corn. We are talking about rainbow gravity, which sounds great, and it's maybe a theory that tries to unify quantum mechanics and general relativity, which are the two big theories that try to explain rus 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 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 pivs. Right, if there is a singularity at the heart of a black hole, then 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 holes, 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 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 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 an example, for example, of a quantized thing following gravity?
Technically I suppose, 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. 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 is moving through curved space, and so light, like 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 what I mean, Like you can you have the mask to describe a single particle, single quantum particle moving through bent space. Is that possible or is that just 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 know how to calculate is the gravity of those particles and how that contributes to bend space.
Right, But doesn't the bending 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 the bending of space depends on the thing that's moving. So if you 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 throw a baseball or a photon near the Moon, You're not taking into account the bending of space due to the baseball or due to the photon. You're just calculating the trajectory of a test particle through the bend space due to the Moon or due to the black hole. You're not taking into account the gravity of the object itself.
Don't you need that to calculate how it's going to curve around how it's going to bend around the moon.
No, you just need to know it's inertial mass. You don't need to know what the effect of it on space time itself. You just need to understand what it's inertia is and so that you can understand how it moves through bent space.
So the real problem is not how to calculate how quantum particles move through bend space, but more about how quantum particles give off gravity or know how they attract other particles through gravity.
Yes, do they bend space exactly? 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 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 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 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 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 at very high energy the rules change a little bit and gravity for those really high energies is a little bit different.
What so, 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 a 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 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. 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 of gravity as you move up in energy. Now, currently we think that all particles feel gravity the same way. 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 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 these particles. We're ignoring the gravitational effect of those particles and so rainbow gravity says, what if that's not true? What if as you move through space, how much curvature you see to 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 on your energy, and so as the photon's 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 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 made gravity is not constant throughout all situations, like somehow gravity is you know, dependent 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 typically that metric is constant, doesn't depend on what your energy is. But now they say, well, let's take that METAe 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 more or less mass, I'm going to see space bent differently.
I think that's probably true. Very very massive particles would see space bend differently than lower mass particles.
Okay, so the idea is 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 have. We think about it now, right like right now, general relativity says that gravity is the same everywhere.
Exactly. It says that gravity is the same everywhere. And you 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 science as a 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, 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 an electron has uncertainty and it's bending space time, then maybe space time itself has an uncertainty, a quantum uncertainty, like maybe we live in this universe with this bent space time, or maybe we live in that universe with that bent space time. 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 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 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. But maybe if gravity is different in the two situations, then it's like maybe has half gravity here half everyady there, which kind of makes a full one gravity.
Yeah, exactly, And we don't know, right We do not know if 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 through space that's uncertain, how do they bend? It turns out that particles that different energies interact with that space different that see a different slice of that uncertainty. So 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 its actually part of our universe. For example, it gives you a very strong hint. It tells you that space time itself is uncertain. One possibility is that gravity is quantum mechanical, right, that space time has uncertainty to it. That we live in the universe where space time 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, and then electron has a location here and 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 that themselves, right, just like a planet does.
Exactly, And like if an electron is fifty percent over here and fifty percent over there, then space time is also fifty percent bent over here and fifty percent bent over there.
All right, Yeah, then that's how you avoid the infinities that you were talking about earlier.
Yeah, because it changes how particles move at very very high energies, which 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 if you change the behavior of the particles a very very high energy, you can basically delete those infinities because you change the rules and 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? Would we see rainbows around black holes? Would we see unicorns? And would that 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 half unicorn.
That's a good point.
All right, well's to get into that, but first, let's take another quick break.
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I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford, and I've spent my career exploring the three pound universe in our heads. We're looking at a whole new series of episodes this season to understand why and how our lives look the way they do.
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All right, we're talking about rainbow gravity and we talk 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 everywhere, maybe it's different or different energy particles, which would sort of help explain how gravity works at the quantum level. Now, if this theory was true, Daniel, does that mean we would see rainbows around black holes?
It does exactly mean that we would see rainbows around black holes, because this is the 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 very small effect until you get to 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, then white light near the event horizon would be split into all the colors and you would see rainbows before you get squished.
Meaning like I shoot a beam of light of white light at a black hole at the very edge, and as this beam of light gets very close to the black hole, photons 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 would be 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, is that it bends light based on its wavelength. Different wavelengths of light. It 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, then black holes are basically prisms, and they would change a beam of white light into a spread based on the colors.
Yeah, they're prisms. And they're prisms because I guess you know, white light is 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 white light is. It's not like the photons are white.
Yeah, there is no individual photon that has the white. Right. White is not a single color. It's a combination of many photons of different colors.
Right. And so around a black hole, 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 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, do we 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'd 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 distorted, you have to know what it looked like before it went around the black hole, and so we don't have any nice, clear, 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. The way gravity varies depending on the energy, Yeah.
And this theory, gravity does depend on the energy, but it's basically unobservable until you get to super duper high energies. In the same 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 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 involve 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 in the podcast a few times called gamma ray bursts, where something out there in the universe sends a huge spray of very high energy gamma rays all about the same time. These things can last like seconds or minutes. We don't really understand the source of them. Check out our whole podcast episode on that top 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 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 gets split. Or are you saying they might arrive at different times.
So if for example, they whizzer in a black hole, they would get bent differently. But the universe has some curvature, and as you move through it, you're slowed down by that curvature, so that 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 sent like a green or red and a blue photon across the universe to us, then if this is true, those photons would arrive at different times here on Earth because they would see different amounts of curvature. And you know, curvature affects the passage of time 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 red shifted more than others. That's kind of what you're saying, because they have to arrive at the same time, don't they If they're moving at the speed of light.
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 camera rays, and we can also create you know, like ultraviolet or here anorder that can create high energy light and super low energy light. Couldn't we do some experiment where we should 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 energy as 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. They fly through 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, but just 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.
But wouldn't we see that with 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 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 are all contributing to that. The vanilla version of general relativity predicts that wouldn't happen, right, 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 it happens differently. So for very very long distances, these would accumulated it all the dark 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 just 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 would get spread out across the universe, but we 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 tried 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 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 lens 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 were 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, linked contraction is happening all the time. You just can't tell because it's so negligible.
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 if it is true that rainbow gravity exists, or that 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 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 where the universe is basically compressed into incredible density. So that's the prediction of general relativity, and you know, we think that's kind of bonkers. We think that those kinds 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. And so you put this into the calculations instead of general relativity, and now particles that different energy are affected 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 reaches sort of a minimum plateau. It's like, intuitively, it can't just like squeeze all the particles down to the same place because the particles are all now bent differently by the space.
M What would that mean for black holes too? Does that mean there's no singularity at the center of black holes?
Yes, This basically erases 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 tells us maybe 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.
Mmm, 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, matte, black, jet black.
Yeah. So maybe now this sounds amazing and sounds like it would solve a big problem, 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 fly basically because it violates a central principle something that we all think probably is true, which is that observers can all agree about the physical laws 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 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 invariance.
M right, because I guess if gravity depends on how much energy you have, energy 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 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, so 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 that's pretty weird. There's a lot of really strong constraints on measurements of a 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 quantum mechanics have completely incompatible assumptions at their foundation, and so to unify them, we are going to have to get rid of something that we hold true, something that we cherish. Maybe it's lorents and variance, 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 physicist that don't like things to change.
We love things to change. Actually, our dream is to find some new theory which blows everything up. But yeah, it does 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 aguesst that because it would mean slop your math, or because you haven't seen any experimental proof of that.
We 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 and those laws are the same for everybody. It doesn't have to be true in 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 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 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 the rainbow gravity theory might be a 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 a continual process that evolves and changes our understanding of the universe out there and how it works and where it came from. And as we learn 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 and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy hackling greenhouse gases? Many farms use anaerobic digesters to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
There are children, friends, and families walking, riding on passing the roads every day. Remember they're real people with loved ones who need them to get home safely. Protect our cyclists and pedestrians because they're people too. Go safely. California. From the California Office of Traffic Safety and Caltrans.
We're just days away from our twenty twenty four iHeartRadio Music Festival, presented by Capitol on.
The biggest headliners in live music will be taking over to Mobile Arena.
Las Vegas lost some special surprises and moments you are not going to want to miss.
Stream only on Hulu.
IHeartRadio Music Festival.
And listen on iHeartRadio.
The most anticipated live music events
Of the year this Friday and Saturday, starting at ten thirty pm Eastern, seven thirty Pacific,
