Classic episode - Why is gravity so weird?

22:51The same with like two electrons or two quarks. So in the scale of the particles that we know, it's a really weak force. That's right, exactly. And yet, it seems to dominate, right? That's a bit of a puzzle. On one hand, it's super-duper weak, and we're telling you that it hardly counts for anything. On the other hand, it's responsible for the structure of the solar system and for the galaxy. And it's the reason the universe looks the way it is, right? And so that can be confusing to people. How do you reconcile those two things in your head? Yeah. Like, why doesn't the Earth feel an electromagnetic force with the sun, which would be so much bigger than the force of gravity?

23:28Yeah, well, it would be pretty shocking. And that's actually the reason is gravity is different from the other forces in that it can't be canceled out, right? If there was some huge electrostatic difference between the sun and the Earth, like a bunch of positive charges there and a bunch of negative charges here, it would create such an enormous force that it would be very quickly balanced. Like, that's what lightning is, right? When there's a charge differential between clouds and the ground, it doesn't take that much before those charges want to rearrange themselves to a lower energy configuration.

23:59They rush down to the ground or they rush up to the clouds or they jump from cloud to cloud to balance themselves out because you have two kinds of charges. You have positive and you have negative. So you can find an arrangement where basically everybody's happy. It's an equilibrium, right? But that's not true for gravity. Okay, I get it. So for example, if the Earth was, every particle on Earth had a positive electromagnetic charge, and every particle in the sun had a negative electromagnetic charge, there would be a humongous pull from electromagnetism pulling the Earth into the sun. Yeah, we'd be toast pretty quick.

24:33Yeah, right. That would not be a very long-lived experiment. Yeah, it would be huge. Even the opposite, if we were all positive and the sun was all positive, we would get shot out of the solar system very quickly. That's right. And that's why early days of the solar system being formed, you have these gases and the gas and dust coalescing and very rapidly things neutralize. Because anything that feels like an electrostatic force to something else is going to find the opposite charge and they're going to coalesce and they're going to make something neutral. That's why most of the things around you are neutral.

25:05Most of the elements are neutral because any deviation from neutral results in a powerful force to neutralize it. So thankfully, the Earth is made out of both like equal amounts of positive and negative particles, right? That's right. Thankfully, we're sort of balanced electromagnetically. And so even if the sun was all positive, we would look like neutral, like a neutral ball to the sun. Yeah, that's right. We're on large scales. The Earth is neutral, right? I mean, there might be some residual positive or negative charge depending on the solar wind, etc. But basically, the Earth is neutral. And so the largest force that the Earth feels is the gravity from the sun, even though gravity is super duper weak, right?

25:46It doesn't take a lot to counteract gravity, but it's the only player left because everybody else has sort of paired up and danced off for the night. And gravity is just there left holding the bag. And gravity can't be balanced, right? You feel gravity if you have any mass, right? There's only positive masses. There's no such thing as a negative mass to give anti-gravity. Wow. Well, let's keep going. But first, let's take a quick break.

26:22Okay, so that's how gravity is so much weaker than the other forces. So how is it different than the other three forces of nature? There's like no end to ways that gravity is weird. You know, there's no end to like the puzzles of gravity. It's fascinating. It's a bottomless pit. That's right. It's a black hole of questions. And one of my favorites is just that we have no way to sort of fit gravity in with the way the universe works according to everything else. You know, we talked earlier about how we have particles and we have forces or quantum fields equivalently. And that's a really successful way to describe the universe.

27:00You know, we have the Large Hadron Collider to explore these things with really high energies. And we've understood all sorts of things using this theory. But that theory is used as quantum mechanics. So the way we describe interactions, you know, the way we talk about two electrons repelling each other, or the way lightning is formed or anything, involves passing quantum particles back and forth. And that's just not true for gravity. What does that mean, passing particles back and forth? Like if I have two magnets and they're attracted to each other, it's not like an invisible telekinesis pulling on each other.

27:34They're actually swapping particles and I can see that. Is that kind of what you mean? That's exactly what I mean. That the way two things interact via some force is by exchanging particles. And so, for example, electromagnetism, right, is the force behind a magnet. And the way electromagnetism works, we think at a sort of microscopic particle level, is that there's a particle that transmits that force, that sends sort of the information back and forth between two things that are feeling it. And in the case of electromagnetism, that particle is the photon, right? The particle is also a packet of light.

28:07So each of the quantum forces that we talked about before, electromagnetism, the weak force, and the strong force, each of them have a particle we associate with it. And that's not just like some name tag we put on and say, hey, you get this one, you get this one. We think that that's the particle that's responsible for making the force work. So when two electrons come near each other, how do they repel each other? How does that actually happen? Well, we think that they send photons out, right? The electric field of a moving electron, an accelerating electron, generates photons. And those photons interact with the other electrons.

28:43And so basically they're passing messages back and forth using these quantum particles. So gravity is weird because we don't know that there is a quantum particle being exchanged when two things get attracted gravitationally. That's right. So we have this great framework. We say, oh, maybe all forces are quantum mechanical fields interacting with each other, right? Let's apply that to the electromagnetic field. Yeah, it works. Let's apply that to the weak force. Yeah, it works. Let's apply that to the strong force. Ooh, cool, it works. Maybe this is something deep about the way the universe works.

29:14Let's apply it to gravity. Uh-oh, it doesn't work. So what does that mean? What does it mean when you say it doesn't work? Well, for a theory to work, it has to provide predictions for experiments. You have to be able to say, okay, theory, what would happen in this configuration? If I shot a proton at another particle, predict what would happen. And then you can go off and do the experiments and compare it, right? Well, when you do that for gravity, try to form a quantum theory of gravity, it doesn't work. You get nonsense answers. You get answers like infinity, right? Or things disappear. Or it just doesn't mathematically function.

29:51Like there's no way to build a theory of gravity that we've discovered so far that works, that actually explains the way these things happen. There are a few candidates out there that are pretty far from being a functional theory of quantum gravity, things like loop quantum gravity or string theory. But the basic problem is that quantum mechanics and general relativity, which is our best theory of gravity, do not play well together. We have no functioning quantum theory of gravity. so does that mean that we don't have the right theory or is that gravity is just not quantum in nature that's exactly the question we don't know the answer to right in a hundred years from now somebody will know the answer that i hope and they'll look back and they'll wonder you know oh why didn't those guys see the clues but we don't know it could be that there is a quantum theory gravity we're just not smart enough to think it up yet right like the right person hasn't been born yet to put the math together or maybe it requires a different kind of math that we're using right There's some assumption we're making that's a mistake.

30:49Or maybe just giving it a wrong name. Like maybe it should be Gravitunis or... Gravitinos, yeah. Gravitas. The way that's taken. Exactly. That's definitely the problem. That's step number one. We made a mistake in step number one when we could define the particle. The other option, of course, is that maybe gravity is not a quantum force the way the other forces are, right? The other forces, we call them quantum forces because they're well described by quantum mechanics. But gravity is kind of different. I mean, the current theory we have of gravity, general relativity, it doesn't like to describe gravity as a force, right?

31:26It describes gravity instead as a bending of space. It says that when you have mass somewhere in space, space no longer becomes straight, it becomes bent. Right. So the things move in curves and circles. And it's not like an actual, just a mathematical nuance or a mathematical perspective. What really confirms it is the idea that gravity can affect things that don't have mass, right? That's how we know it's more than just a force between things that have mass. It actually affects space for things that don't have mass, right? That's exactly right. So if you shoot a photon through space that has mass nearby, the photon will not move in what we consider to be a straight line, right?

32:06It'll find a path through this bent space that involves basically curving. And this is what Einstein predicted with his theory, and they saw it. You can see in space, it's called gravitational lensing. You can see photons get bent by heavy objects. And it's because, as you say, the heavy objects are bending space itself. Right. It's not like gravity is pulling the photon, because the photon doesn't have any mass, right? That's right. The photon doesn't have any mass. So that's how it's different. Gravity seems to affect things that don't have its fundamental property. You know, like electromagnetic forces can affect something that does not have an electric charge.

32:45That's true. Gravity can affect everything else, right? Yeah, that's a pretty deep insight there. Not bad for a cartoonist. Not bad at all. Yeah, that's a fascinating way to think about it. I think that's totally correct. Yeah, and so if gravity is instead of being a force, if it's a way we change the shape of space itself, then maybe that's why we don't have a quantum theory of it, right? And that's amazing. and it's fantastic and it's exciting. And another reason why we have a hard time bringing these two things together is that quantum mechanics, a theory we've developed, only works so far in flat space.

33:20That is, if there's really heavy stuff nearby, we don't know how to do those quantum calculations. We can basically only do quantum mechanics in places where there isn't really strong gravity. So wait, so quantum physics doesn't work in reality, basically. Is that what you're saying? Like it doesn't work in the space that we actually live in. Well, it works basically everywhere except for close to black holes. Right. You need basically a black hole to have enough gravity to break down quantum mechanics. Because it's when space gets really distorted that you start to see the effects of gravity on space.

33:55And then it becomes comparable to the strength of other stuff. And that's when quantum mechanics breaks down. Quantum field theory works basically in what we call flat space. whereas gravity bends space. Wow.

34:11So earlier when we categorized gravity as part of these four fundamental forces, maybe that's just the wrong approach. Do you know what I mean? Like maybe we shouldn't be categorizing these four things as one category of, quote, forces, unquote. That's right. It could be that there is no quantum theory of gravity as a fundamental force because it isn't one. And it's just a feature of space, right? Absolutely, that's one possible explanation. But then we still need a way to make quantum mechanics work in bent space, right? And we still need to understand how to make our theory of general relativity play well with quantum mechanics.

34:48Because we think quantum mechanics describes the universe, right? And general relativity is not a quantized theory. It's continuous, right? It treats space and everything as if it's infinitely divisible, right? It's not a quantum theory at all. In fact, it came about before quantum mechanics was even invented. And so while the basic tenets of it, how it distorts space, are probably correct. I mean, it's been verified to a zillion degrees of accuracy. It doesn't feel like it can be a fundamental description of nature because it's not quantum mechanical. So like we want to call it a force because it seems to move things like all the other forces.

35:22But maybe it's not a force. Maybe it's just kind of like some other weird property of space. Yeah, exactly. You know, maybe we've been trying to put a round peg into a square hole all these years. a gravity peg in a quantum hole. That's right. That's right. And there are other ways that people are trying to solve this problem. Like one way is thinking that maybe gravity is a fundamental force, but it just works a little bit differently from the other forces. For example, people think about how the universe might have additional spatial dimensions, you know, like instead of just being able to move in three directions, maybe there's like four or five, six dimensions that you can move in.

35:59And folks who are interested in that should listen to our podcast on extra dimensions. No, yeah, we did a whole episode on extra dimensions, but we didn't sort of get into this particular topic. So tell us how extra dimensions might explain why gravity is so weak. Yeah, the idea is that maybe gravity isn't so weak. Maybe gravity is just as strong as all the other forces. But if there's a whole other set of dimensions out there, there's ways, directions that thing can move, it might be that gravity is the only thing that feels those dimensions, right? It might be that those dimensions are invisible to electromagnetism and to the weak force and to the strong force, but visible to gravity.

36:37And what that means is that gravity might be basically leaking out into those other dimensions. You know, we talked about how the farther away you get from something, the weaker the force is. So like Mercury feels the force of the sun's gravity much more strongly than Pluto does, right? Irrelevant of whether or not you call it a planet, it doesn't feel gravity very strongly. And that's because it's further from the sun, right? I mean, that goes like one over r squared, or r is the distance. It's one over R squared because we have three dimensions. If we had six dimensions, it would be one over R5, right?

37:08Which falls much more rapidly. So if there are additional dimensions out there, okay, and only gravity feels them, then that might be the reason why gravitational force falls so quickly. Maybe gravity is actually just as strong as everything else when you get really, really close, but then these extra dimensions exist and most of gravity leaks out into those other dimensions. Sort of like between you and me, there's not just the three dimensions between you and me. May there are other secret hidden spaces kind of between you and me or these other dimensions. Exactly. Other ways for gravity to spread out.

37:45And so gravity would be like just as strong as all the other forces, but it's just flexing its muscles in these other spaces that we can't see or feel. Exactly. It's like, you know, if somebody is at the center of a crowd and they let go of a really stinky fart, right? The people next to them, they smell it strongly. And the people further away, they smell it much more weakly. And people outside don't smell it at all. Okay, yeah. Now imagine... We jump into farts really suddenly, but let's go with it. Hey, I'm trying to make this accessible, you know. This is something everybody's going to appreciate.

38:14You're trying to make wind. I get it. Got it. But if there was somewhere else for that fart to go, you know, if it could move not just sideways, but also could float up, right? Say you had a really tall room and the fart floated up, then people wouldn't feel it as much. because most of the part would dissipate into the upper corners of the room. And so gravity might be the same way. It might be that for the first millimeter or so, the first centimeter or so, gravity gets very weak very quickly. It falls off really rapidly. And then at normal distances, like a meter or 10 meters or whatever, you don't feel those other dimensions anymore because those other dimensions only activate at really, really short distances.

38:52This is a theory people came up with, and we don't know if it's real. We've tested it. So far, it seems like gravity works the same way for galactic scales and for Earth scales and for microscopic scales. It seems to always fall off at the same rate as a function of distance. So nobody's ever seen any evidence of these extra dimensions. But it's a fascinating theory, and it's one that would give kind of a natural explanation for why gravity would fall off so quickly and why gravity is so weak. It wouldn't explain all these other things. In fact, people sort of try to use gravity to see if there are other dimensions, right?

39:25Yeah, that's right. It would be a really cool clue. And that's a fascinating way that science is done. You try to look at everything around you and see if you can fit it all into one framework. Like, can I use this one set of ideas to describe everything? Can I merge everything into one set of concepts? Yeah. That's right. Yeah, my fart theory of the universe.

39:51The best possible way, I think, to unravel this is to actually go visit a black hole because quantum mechanics and general relativity tell you very different things about what's happening inside a black hole, right? As we said before, general relativity tells you it's an infinitesimal dot of almost infinite density. Quantum mechanics says, you know, the universe is quantized, first of all, so you can't have infinitesimal dots. Right. And also, there's sort of a minimum size to stuff, right? And you can't have all that stuff compressed in such a tiny little area. And so if you could see inside a black hole, you would learn a lot about gravity.

40:24So what would be the plan? You would go into a black hole, you would observe and discover how the universe works, and then you'd be stuck there. That's right. They would have to send you a Nobel Prize into the black hole after you. But just assume you'd figured it out and shoot the Nobel Prize into space, into the black hole. Congratulations. Of course, anybody who's listening, please do not go into a black hole. Please, please do not go into a black hole. But, you know, we don't need to visit black holes. We could try to create them here on Earth. That sounds like a great idea. Yeah, doesn't that sound like a great idea?

40:55I mean, wow, I'm excited. Make a black hole on Earth? So sell me. Yeah, let's create a black hole and study it, right? if gravity gets really really powerful when you get to really short distances because of this extra dimension theory then it might be that if you shoot two protons together really really hard and they get really really close to each other that you can create a super duper mini extra cute little fuzzy black hole right? I'm trying to make it sound like a cozy thing not a dangerous thing yeah you're trying to sell it sell the merchandising rights and so before we turned on the Large Hadron Collide about 10 years ago people thought maybe by smashing these protons together, we could actually create black holes and we could study them.

41:34We could reveal the deep secrets of gravity, right? So then the idea would be to try to make them at the Large Hadron Collider and just kind of see what happens. Like, does it tell us something about gravity or quantum physics at the same time? Yeah, exactly. By seeing how often they're made and how strong they are and what they turn into and what they decay, we could understand something about the way black holes work. And that would have been really powerful. But unfortunately, or fortunately, depending on how you feel about black holes, we haven't made any black holes at the Large Hadron Collider that we've discovered.

42:08But maybe, isn't it true that maybe you've made them, but they evaporate? Yes, these black holes would be very short-lived. But you know, everything we make at the Large Hadron Collider is really short-lived. These things last for like 10 to the negative 30 seconds or 10 to the negative 23 seconds. We're pretty good at seeing short-lived stuff because it usually blows up into other things. And a black hole would have a really unusual signature in our detectors. It would be pretty clear to see if we had made them. Okay. But short of going into a black hole or detecting farts in extra dimensions, we may not know in the near future what makes gravity so different.

42:42That's right. It's going to take some work. I mean, the other direction is theoretical, is to build up a theory of quantum gravity sort of from the bottom up. Like start from the beauty of math and physics and then try to build it up to our level. Exactly. And that's That's a wonderful way to do it, is to say, like, maybe the universe works in this way, this most basic fundamental nature, and build it up from there and see if you can describe the universe that we see around us. Wow. All right. Well, that's pretty shocking to think gravity plays such a big role in our lives. And yet, it's like the weakling in the universe, right?

43:19It's like, imagine if gravity was stronger. Life would be a lot more chaotic, right? And crazy. Yeah, exactly. we would be closer to the sun and everything would feel more intense. It's fascinating to me that gravity has been a mystery to physics for hundreds of years. I mean, it was the focus of Isaac Newton's studies, you know, like hundreds of years ago, people working on gravity. And still today, even though we've made so much progress in terms of gravity, we still have so many basic questions about it that we don't know the answers to, not even the really beginning of how to answer them. To me, that's fascinating.

43:51Gravity is such a rich source of mystery for physics and for everybody. Wow. All right, cool. I think it's maybe time to push down this question. Thanks for joining us.

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