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Speaker 5: I just think it's fascinating that it's such a fundamental
Speaker 5: force in the universe, right, Like it's basically the thing
Speaker 5: that builds galaxies and keeps planets moving right and gives
Speaker 5: structure to the entire cosmos.
Speaker 1: That's right. On the largest scale, it's actually the most
Speaker 1: important force. It's the reason why things look the way
Speaker 1: they do. It's the reason why our planet is round.
Speaker 1: It's the reason why we're on the planet.
Speaker 5: It's pretty important, and yet we don't know a lot
Speaker 5: about it, right, Like, there's some really deep and strange
Speaker 5: mysteries about it.
Speaker 1: On one hand, we have a theory which works really
Speaker 1: really well. On the other hand, we have questions about
Speaker 1: it which seems really really basic.
Speaker 5: And not only that, it's very different than all the
Speaker 5: other forces of nature.
Speaker 1: That's right, one of these things is not like the
Speaker 1: other ones.
Speaker 5: Hi am morhee.
Speaker 1: I'm a cartoonist and I'm Daniel. I'm a particle physicist.
Speaker 5: And this is our podcast Daniel and Jorge explain the universe.
Speaker 1: In which a cartoonist and a physicists try to figure
Speaker 1: out how to make the universe understandable to anybody.
Speaker 5: Yeah, and today on the podcast we are examining a
Speaker 5: very heavy topic, gravity and specifically why is gravity so.
Speaker 1: Weak and strange. Gravity, as we said earlier, is something
Speaker 1: which controls the structure of the universe. I mean, the
Speaker 1: reason the Solar System looks the way it does is
Speaker 1: because of gravity. The reason the Earth is round is
Speaker 1: because of gravity. The reason we have galaxies is because
Speaker 1: of gravity.
Speaker 5: The reason we weigh so much is because of gravity. Right,
Speaker 5: it's not my.
Speaker 1: Belt, No, that's because of late night cake eating.
Speaker 5: But it's such a fundamental force of nature, right, Like
Speaker 5: it's present in our everyday life. We spend a lot
Speaker 5: of time thinking about gravity, right, how not to fall down,
Speaker 5: how not to drop things, how to go up buildings,
Speaker 5: how to go down buildings.
Speaker 1: Right, that's right. It seems like one of the most
Speaker 1: important forces. I mean, if you ask people, you know,
Speaker 1: to name a force or what kind of forces the
Speaker 1: experience in their life, gravity is the one that's present
Speaker 1: in their lives.
Speaker 6: Right.
Speaker 1: You're climbing upstairs, you're fighting gravity, you trip, you fall down,
Speaker 1: you're feeling gravity. You look around you. The shape of
Speaker 1: things is controlled by gravity. And that's why it's particularly
Speaker 1: strange that gravity is the weakest force of all the
Speaker 1: forces we've discovered. It's by far the weakest.
Speaker 5: Yeah, it's really strange to hear you say that, Like,
Speaker 5: how can gravity be weak? Like, you know, like it's
Speaker 5: keeping the whole Earth together, it's making the entire planet
Speaker 5: swing around, go in a circle, basically. Right. Without gravity,
Speaker 5: we would just shoot off into space.
Speaker 1: That's right. It's a really strange situation. And there's other
Speaker 1: things about gravity we don't understand as well. It's really strange.
Speaker 1: It doesn't play well with the other forces. It's very,
Speaker 1: very weak. It's a total mystery to science, except that
Speaker 1: we have a theory which works beautifully right. We can
Speaker 1: calculate exactly how mercury orbits the sun. We can send
Speaker 1: things in outer space and know with to millimeter precision
Speaker 1: exactly where they're going to land. We have a working
Speaker 1: theory that we can use, right, but we don't understand
Speaker 1: it on a conceptual level. We have these basic, deep
Speaker 1: questions about what gravity is and how the universe works
Speaker 1: because of it.
Speaker 5: So it's a weird question, and maybe one of the
Speaker 5: people hadn't thought about before. So Daniel went out as
Speaker 5: usual and asked people on the street, why do you
Speaker 5: think gravity is so weak?
Speaker 1: Here's what a random selection of folks who were willing
Speaker 1: to talk to me on a Tuesday morning had to
Speaker 1: say about gravity.
Speaker 6: I don't know.
Speaker 1: I should don't know about that, all right. I thought
Speaker 1: it was a pretty strong force. I don't know, but yeah, because.
Speaker 5: It depends on the distance and it's long range one.
Speaker 5: So that's why we feel it very weak most of
Speaker 5: the time. Cool, No, Okay, I have no I'm sorry,
Speaker 5: it's not very fruitful.
Speaker 3: Hmmm, I have no idea, but I'd be interested in
Speaker 3: finding out why.
Speaker 5: All right, that was that was pretty good. Most people
Speaker 5: weren't surprised when you said gravity's weak.
Speaker 1: I don't know. I feel like if all the questions
Speaker 1: I've asked people, this is the one that flumms them
Speaker 1: the most. You know, people were like, what, I have
Speaker 1: no idea, or they had crazy ideas why gravity must
Speaker 1: be weak. I feel like usually we get one person
Speaker 1: who knows what the answer is or has a good
Speaker 1: clue about what's going on. This time, I feel like
Speaker 1: almost everybody was pretty clueless. I mean, one person said
Speaker 1: I always thought gravity was pretty strong, right, which kind
Speaker 1: of sums up the situation, right. Gravity's omnipresent in our lives.
Speaker 1: It dominates our experience, and yet it's so weak compared
Speaker 1: to the other really powerful forces we've discovered.
Speaker 5: Well, some people a couple of answers were that it
Speaker 5: had to do with distance, like gravity gets really weak
Speaker 5: with distance.
Speaker 1: That's right, And the problem there is that all the
Speaker 1: forces get weak with distance, like electromagnetism also falls as
Speaker 1: a distance grows. Right, So all of these forces follow
Speaker 1: this one over r squared rule or are as your
Speaker 1: distance from the thing that's giving.
Speaker 5: You the force, right, maybe maybe right?
Speaker 1: Maybe yeah, mostly we think, And so that can't be
Speaker 1: the answer, right, because all the other forces have that
Speaker 1: same feature.
Speaker 5: So when you say it's the weak is it's not
Speaker 5: that it changes over distances differently than the other forces.
Speaker 1: That's right. So maybe we should talk about what the
Speaker 1: forces are and compare them to each other so folks
Speaker 1: can get an understanding of how crazy weak gravity is.
Speaker 5: Right. So, Daniel, what are the forces of nature besides
Speaker 5: a bad movie with Ben Affleck than Center Bullet.
Speaker 1: Well, I think comedy. Comedy is definitely a force of nature.
Speaker 1: You know, it solves big problems around the world. Now,
Speaker 1: the fundamental forces are electromagnetism, right, that's the one that
Speaker 1: controls electricity and magnetism obviously, and his responsible for the
Speaker 1: cool things like light and lightning and all that cool stuff.
Speaker 1: And then there's the weak nuclear force, which is a
Speaker 1: force which is responsible for radioactive decay of a nuclei, right,
Speaker 1: And the cool thing about electricity and magnetism and the
Speaker 1: weak nuclear force. Is that we actually have shown that
Speaker 1: there are two sides of the same coin. As a
Speaker 1: particle physicist, we refer to them as one force. We
Speaker 1: call it the electro week. So sort of magnetism lost
Speaker 1: out there in the name merger, right, it should be
Speaker 1: electromagnetic week. But nobody voted to keep magnetism in the
Speaker 1: sort of the name of the partners in a law firm.
Speaker 5: Nobody lobbied for weak electro.
Speaker 1: Or magneto weak force. Yeah, yeah, again, we are suffering
Speaker 1: the fate of some anonymous committee of scientists that get
Speaker 1: to name these things. Right, Who are these people?
Speaker 5: Probably some grad student, right or some you know, like
Speaker 5: this is really weird, we'll call it this.
Speaker 1: Yeah. So we have electricity and magnetism, which is a
Speaker 1: single force. We have the weak nuclear force, which is
Speaker 1: really should be combined with electricity magnetism. And then there's
Speaker 1: the strong nuclear force. And this is the one that
Speaker 1: holds the nucleus together. You know, the nucleus is of
Speaker 1: just a bunch of positively charged protons and neutral neutrons. Right,
Speaker 1: it says only positively charged particles in the nucleus. So
Speaker 1: you might think, what it even holds the nucleus together. Right,
Speaker 1: you have all this positively charged stuff should be repelling themselves. Well,
Speaker 1: it's the strong nuclear force, and it does so by
Speaker 1: exchanging these crazy little particles we call gluons, and that
Speaker 1: holds the nucleus together, and it's pretty strong. It's even
Speaker 1: stronger than electromagnetism.
Speaker 5: Well, let's take a step back. So in the universe
Speaker 5: there's stuff.
Speaker 1: There's like, yes, firm that there is stuff in the universe. Yes,
Speaker 1: without reservation, there is stuff.
Speaker 5: I'm glad we saw that question. But I mean it's
Speaker 5: like there's stuff that has substance to it, that has
Speaker 5: mass to it, or you know that it sort of exists.
Speaker 5: And then there's also besides that, how these things interact
Speaker 5: with each other, like how they affect each other.
Speaker 1: That's right. There's the matter and then there's the forces. Right,
Speaker 1: the forces affect how they interact with each other.
Speaker 5: And that's pretty much the universe. That's like, it's matter
Speaker 5: and forces.
Speaker 1: Yeah, one way to look at the universe is that
Speaker 1: it's particles, right, or you would say matter and their forces.
Speaker 1: In modern particle physics, we think about one level deeper,
Speaker 1: which is we think of quantum fields and quantum fields
Speaker 1: are responsible both for matter and for forces. So we
Speaker 1: can talk about that maybe in another podcast. What is
Speaker 1: a quantum field? And how can I get one? You know,
Speaker 1: for lease or rent? What can they do for me?
Speaker 1: But yeah, I think it's fair still to think about
Speaker 1: the universe in terms of particles and forces.
Speaker 5: On that note, let's take a quick break.
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Speaker 5: There are only four kinds of forces.
Speaker 1: Yeah, there are four kinds of forces. So electromagnetism, weak
Speaker 1: nuclear force, strong nuclear force, and then of course gravity. Right,
Speaker 1: that's the fourth force that we've discovered.
Speaker 5: Okay.
Speaker 1: The fascinating thing is that different particles feel different forces, right, Like,
Speaker 1: some particles feel this set of forces. Some particles feel
Speaker 1: those set of force for example, right, particles with electric
Speaker 1: charge feel electromagnetism. Right. The electron, for example, is negatively charged,
Speaker 1: the proton is positively charged. You bring them close together,
Speaker 1: they're gonna pull on each other. They're gonna stuck each
Speaker 1: other together, right, because they have opposite charges. We all
Speaker 1: know that, but you bring a neutral particle nearby, it
Speaker 1: just totally ignores it, right, It doesn't feel it at all. Right, Right,
Speaker 1: It's like it's like somebody's walking through a crowd of
Speaker 1: people shouting, but they have headphones on so they can't
Speaker 1: hear anything. They're totally oblivious to it.
Speaker 5: It's kind of like how we talked about in a
Speaker 5: previous podcast. They're almost like languages or like social media platforms.
Speaker 5: Like some people are on Twitter, some people are on Facebook,
Speaker 5: but some people are not on this. And so if somebody,
Speaker 5: if you're not on Twitter and somebody sends you a tweet,
Speaker 5: you're not going to get it. And so it's just
Speaker 5: different ways that particles interact.
Speaker 1: That's right. Gravity is the Google plus social media, right
Speaker 1: because nobody uses the trends. It's ancient but powerless. Yeah,
Speaker 1: And so different particles feel different forces. And for example,
Speaker 1: an electron, while it feels electromagnetism because it has a
Speaker 1: negative charge, it doesn't feel the strong force at all.
Speaker 1: It will pass right by a bunch of particles that
Speaker 1: are really tugging on each other with a strong force
Speaker 1: and not be affected at all. Whereas quarks. Quarks feel
Speaker 1: all the forces. They feel a strong force, which is
Speaker 1: how they get pulled together in the nucleus. Remember, protons
Speaker 1: and neutrons are made of quarks. Quarks feel electromagnetism because
Speaker 1: they have electric charge, they feel the weak force. They
Speaker 1: also feel gravity, of course, because they have masks. So
Speaker 1: quarks get their fingers in everything.
Speaker 5: They get the feels for everything, They feel everything erect.
Speaker 1: They got the strong feels. Of course, they're really deeply
Speaker 1: emotional part of that. And on the other side of
Speaker 1: the spectrum, you've got things like neutrinos and trinos don't
Speaker 1: have electric charge, so they ignore all electricity and magnetism. Right,
Speaker 1: they don't interact with light. They're invisible. They pass right
Speaker 1: through anything that They ignore electromagnetic bonds, so they pass
Speaker 1: through most materials. They don't feel the strong force. The
Speaker 1: only way they interact is with the weak force. And
Speaker 1: the weak force is pretty weak, which is why neutrinos
Speaker 1: can mostly just pass through matter unaffected.
Speaker 5: So we have four fundamental forces, right, and gravity is
Speaker 5: one of these forces. And so when you say that
Speaker 5: gravity is weak, you actually mean it's weak compared to
Speaker 5: these other three forces.
Speaker 1: That's right. And so the ranking is the strong force
Speaker 1: is the strongest, right, so that one is actually well named. Congratulations,
Speaker 1: you know anonymous group of scientists. Yeah, we should be
Speaker 1: called the as of twenty eighteen, currently known to be
Speaker 1: the strongest force force. Right after that comes electromagnetism, and
Speaker 1: you know, we know that force's pretty powerful. You stick
Speaker 1: your finger in the socket, you're gonna feel the wrath
Speaker 1: of electromagnetism. Right, It's not an unfamiliar feeling.
Speaker 5: Right, try to stick your finger in anything, you feel it, right,
Speaker 5: because it's electromagnetism is the force that keeps you from
Speaker 5: basically passing through the table or passing through your car.
Speaker 1: Right, that's right, because electromagnetism is the basis of chemical bonds, right,
Speaker 1: and chemical bonds are really the thing that form the
Speaker 1: structure of your body. Right. You think of your body
Speaker 1: is like a bunch of particles, but it's held together
Speaker 1: by all these forces. It's like a chain link fence
Speaker 1: binding together these little particles that prevents you from passing
Speaker 1: through something else. Yeah, So we got the strong force,
Speaker 1: and then electromagnetism and then actually the weak nuclear force. Right,
Speaker 1: this is the force that like powers neutrinos and radioactive decay.
Speaker 1: It's much weaker than electromagnetism and much weaker than the
Speaker 1: strong force.
Speaker 5: Even weaker than the weak is gravity.
Speaker 1: That's right. If you make a list like strong force, electromagnetism,
Speaker 1: in the weak force, then you should leave like one
Speaker 1: hundred blank pages, and then you get to gravity. Because
Speaker 1: when we compare these forces, we put things like an
Speaker 1: equal distance apart, and we compare the strength of the forces.
Speaker 1: Gravity is ten to the thirty six times weaker than
Speaker 1: the weak force. That's ten with thirty six zeros in
Speaker 1: front of it.
Speaker 5: But isn't that sort of a matter of units or scale?
Speaker 5: Do you know what I mean? Like, it's much weaker,
Speaker 5: but only if you compare apples to apples, right, or
Speaker 5: just oranges.
Speaker 1: That's right, But put two protons next to each other, right,
Speaker 1: h Two protons have a certain amount of mass and
Speaker 1: a certain amount of electric charge, and the force of
Speaker 1: their charges is going to be much much stronger than
Speaker 1: the force from their masses.
Speaker 5: Oh I see.
Speaker 1: So yeah, if everything was much much more massive, then
Speaker 1: there would be stronger gravity. But you can compare these
Speaker 1: things apples to apples by comparing them at you know
Speaker 1: the same distance and the same basic unit of interaction.
Speaker 5: Right, But what if you take an apple put it
Speaker 5: next to another apple?
Speaker 1: Well, I think you can do that experiment. Nothing's going
Speaker 1: to happen because gravity is so weak. Right. You don't
Speaker 1: see two apples like pulling themselves together on the counter, right,
Speaker 1: And though built in apple collider, you know the apples
Speaker 1: are not drawn to each other. Gravity is a super
Speaker 1: weak force, and you can see this yourself. Right, you
Speaker 1: can do an experiment where you counter the entire gravitational
Speaker 1: force of an enormous celestial body like the Earth. Right,
Speaker 1: take a small kitchen magnet and use it to hold
Speaker 1: up a nail, and think about what's happening there, right,
Speaker 1: You have the nail is being pulled down by every
Speaker 1: single rock in the Earth. It's pulling with all of
Speaker 1: its gravity. But a tiny little kitchen magnet totally overcomes that.
Speaker 5: It can lift a nail even though it's pulling, it's
Speaker 5: being pulled down by the whole entire planet Earth.
Speaker 1: Right exactly. Now, imagine a magnet the size of the Earth, right,
Speaker 1: I mean that would be that would be extraordinarily powerful.
Speaker 1: And so you have basically like a gravitational blob the
Speaker 1: size of the Earth still pretty ineffective compared to electromagnetism.
Speaker 5: Mmmm, so it's weak if you sort of compare it
Speaker 5: by object, Like you said, if you take a proton
Speaker 5: and put an extra proton, the force are going to
Speaker 5: feel from electromagnetism is so much bigger than the force
Speaker 5: of gravity. They're going to feel towards each other the
Speaker 5: same with like two electrons or two quarks and things.
Speaker 5: So in the scale of like the particles that we know, it's.
Speaker 1: A really weak force, that's right, exactly, And yet and
Speaker 1: yet it seems to dominate, right. That's a bit of
Speaker 1: a puzzle, Like, on one hand, it's super duper weak,
Speaker 1: and we're telling you that it hardly counts for anything.
Speaker 1: On the other hand, it's responsible for the structure of
Speaker 1: the Solar System, man for the galaxy, and it's the
Speaker 1: reason the universe looks the way it is, right, right,
Speaker 1: And so that can be confusing to people, Like how
Speaker 1: do you reconcile those two things in your head?
Speaker 5: Yeah, Like, why doesn't the Earth feel an electromagnetic force
Speaker 5: with the Sun, which it would be so much bigger
Speaker 5: than the force of gravity.
Speaker 1: Yeah, Well, it would be pretty shocking, And that's actually
Speaker 1: the reason is gravity is different from the other forces
Speaker 1: and that it can't be canceled out. Right, if there
Speaker 1: was some huge electrostatic difference between the Sun and the Earth,
Speaker 1: like a bunch of positive charges there and a bunch
Speaker 1: of negative charges here, it would create such an enormous
Speaker 1: force that it would be very quickly balanced. Like that's
Speaker 1: what lightning is. Right. When there's a charge differential between
Speaker 1: clouds and the grounds, it doesn't take that much before
Speaker 1: those charges want to rearrange themselves to a lower energy configuration.
Speaker 1: They rush down to the ground, or they'd rush up
Speaker 1: to the clouds, or they jump the cloud cloud to
Speaker 1: balance themselves out. Because you have two kinds of charges,
Speaker 1: you have positive and you have negative, so you can
Speaker 1: find an arrangement where basically everybody's happy. It's an equilibrium, right,
Speaker 1: But that's not true for gravity.
Speaker 5: Okay, I get it. So for example, if the Earth
Speaker 5: was every particle on Earth had a positive electromagnetic charge,
Speaker 5: and every particle in the Sun had a negative electromagnetic charge,
Speaker 5: there would be a humongous pull from electromagnetism, pulling the
Speaker 5: Earth into the Sun.
Speaker 1: Yeah, we'd be toased pretty quick Yeah.
Speaker 5: Yeah, would be huge. Even the opposite. If we were
Speaker 5: all positive and the Sun was all positive, we would
Speaker 5: get shot out of the Solar system very quickly.
Speaker 1: That's right. And that's why you know, early days of
Speaker 1: the Solar system being formed, you have these gases and
Speaker 1: the gas and dust coalescing and very rapidly things neutralize, right,
Speaker 1: because anything that feels an electrostatic force to something else
Speaker 1: is going to find the opposite charge and they're going
Speaker 1: to coalesce and they're going to make something neutral. Right.
Speaker 1: That's why most of the things around you are neutral, right,
Speaker 1: Most of the elements are neutral, because any deviation from
Speaker 1: neutral results in a powerful force to neutralize it.
Speaker 5: So, thankfully the Earth is made out of both like
Speaker 5: equal amounts of positive and negative particles, right, that's right.
Speaker 5: Thankfully we're sort of balanced electromagnetically, and so even if
Speaker 5: the Sun was all positive, we would look like neutral,
Speaker 5: like a neutral ball to the Sun.
Speaker 1: Yeah, that's right. Where on large scales the Earth is neutral, right,
Speaker 1: I mean, there might be some residual positive or negative
Speaker 1: charge depending on the solar wind, et cetera. But basically
Speaker 1: the Earth is neutral, and so the largest force of
Speaker 1: the Earth feels is the gravity from the sun, even
Speaker 1: though gravity is super duper weak, right, it doesn't take
Speaker 1: a lot to counteract gravity. But it's the only player
Speaker 1: left because everybody else is sort of pair it up
Speaker 1: and danced off for the night, and gravity's just there left,
Speaker 1: hold in the bag. And gravity can't be balanced, right.
Speaker 1: You feel gravity if you have any mass. Right, there's
Speaker 1: only positive masses, no such thing as a negative mass
Speaker 1: to give anti gravity.
Speaker 5: Wow, well, let's keep going, but first let's take a
Speaker 5: quick break. Okay, So that's how gravity is so much
Speaker 5: weaker than the other forces. So how's it different than
Speaker 5: the other three forces of nature?
Speaker 1: There's like no end to waste. The gravity is weird,
Speaker 1: you know, there's no end to like the puzzles of
Speaker 1: gravity is fascinating.
Speaker 5: Bottomless pit.
Speaker 1: That's right, it's a black hole of questions. And one
Speaker 1: of my favorites is just that we have no way
Speaker 1: to sort of fit gravity in with the way the
Speaker 1: universe works according to everything else. You know, we talked
Speaker 1: earlier about how we have particles and we have forces
Speaker 1: or quantum fields equivalently, and that's a really successful way
Speaker 1: to describe the universe. You know, we have the Large
Speaker 1: Hadron Collider to explore these things really high energies. And
Speaker 1: we've understood all sorts of things using this theory, but
Speaker 1: that theory is used as quantum mechanics. So the way
Speaker 1: we describe interactions, you know, the way we talk about
Speaker 1: two electrons repelling each other, or the way lightning is
Speaker 1: formed or anything, involves passing quantum particles back and forth.
Speaker 1: And that's just not true for gravity.
Speaker 5: What does that mean? Passing particles back and forth? Like
Speaker 5: when like if I have two magnets and they're attracted
Speaker 5: to each other, they're not. They're not It's not like
Speaker 5: an invisible telekinesis pulling on each other. They're actually swapping particles,
Speaker 5: and I can see that is that kind of what
Speaker 5: do you mean?
Speaker 1: That's exactly what I mean. That the way two things
Speaker 1: interact via some force is by exchanging particles. And so
Speaker 1: for example, electromagnetism, right, is the force behind a magnet.
Speaker 1: And the way electromagnetism works, we think at a sort
Speaker 1: of microscopic particle level, is that there's a particle that
Speaker 1: transmits that force, that sends sort of the information back
Speaker 1: and forth between two things that are feeling it and
Speaker 1: in the case of electromagnetism, that particle is the photon. Right,
Speaker 1: the particle is also a packet of light. So each
Speaker 1: of the quantum forces that we talked about before, electromagnetism,
Speaker 1: the weak force and the strong force, each of them
Speaker 1: have a particle we associate with it. And that's not
Speaker 1: just like some name tag we put on and say, hey,
Speaker 1: you get this one, you get this one. We think
Speaker 1: that that's the particle that's responsible for making the force work.
Speaker 1: So when two electrons come near each other, how do
Speaker 1: they repel each other? How does that actually happen? Well,
Speaker 1: we think that they send photons out right, the electric
Speaker 1: field of a moving electron, right, and accelerating electron generates photons,
Speaker 1: and those photons interact with the other electrons, and so
Speaker 1: basically the passing messages back and forth using these quantum particles.
Speaker 5: So gravity is weird because we don't know that there
Speaker 5: is a quantum particle being exchanged when two things get
Speaker 5: attracted gravitationally.
Speaker 1: That's right, So we have this great framework. We say, oh,
Speaker 1: maybe all forces are quantum mechanical fields interacting with each other. Right,
Speaker 1: Let's apply that to the electromagnetic field. Yeah, it works.
Speaker 1: Let's apply that to the weak force. Yeah it works.
Speaker 1: Let's apply to the strong force. Ooh, cool, it works.
Speaker 1: Maybe this is something deep about the way the universe works.
Speaker 1: Let's apply it to gravity. Uh. Oh, it doesn't work right,
Speaker 1: So what does that mean? What does it mean when
Speaker 1: I say it doesn't work? Well? For a theory to work,
Speaker 1: it has to provide predictions for experiments. You have to
Speaker 1: be able to say, okay, theory, what would happen in
Speaker 1: this configuration if I shot a proton and another particle.
Speaker 1: Predict what would happen, and then you can often do
Speaker 1: the experiments and compare it right. Well, when you do
Speaker 1: that for gravity, you try to form a quantum theory
Speaker 1: of gravity, it doesn't work. You get nonsense answers. You
Speaker 1: get answers like infinity right, or things disappear, or it
Speaker 1: just it doesn't mathematically function Like, there's no way to
Speaker 1: build a theory of gravity that we've discovered so far
Speaker 1: that works, that actually explains the way these things happen.
Speaker 1: There are a few candidates out there there pretty far
Speaker 1: from being a functional theory of quantum gravity, things like
Speaker 1: loop quantm gravity or string theory, but the basic problem
Speaker 1: is that quantum mechanics and general relativity, which is our
Speaker 1: best theory of gravity, do not play well together and
Speaker 1: we have no functioning quantum theory of gravity.
Speaker 5: So does that mean that we don't have the right
Speaker 5: theory or is that gravity is just not quantum in nature?
Speaker 1: That's exactly the question we don't know the answer to.
Speaker 1: Right In one hundred years from now, somebody will know
Speaker 1: the answer to that, I hope, and they'll look back
Speaker 1: and they'll wonder, you know, why did those guys see
Speaker 1: the clues? But we don't know. It could be that
Speaker 1: there is a quantum theory gravity, we're just not smart
Speaker 1: enough to think it up yet, right, Like the right
Speaker 1: person hasn't been born yet to put the math together,
Speaker 1: or maybe it requires a different kind of math that
Speaker 1: we're using. Right, there's some assumption we're making that's a mistake.
Speaker 5: Or maybe just giving it a wrong name, like maybe
Speaker 5: it should be gravit tunis or gravitinos gravitas. That's taken exactly.
Speaker 1: That's definitely the problem. That's step number one, when we
Speaker 1: made a mistake in step number one, when we could
Speaker 1: define the particle. The other option, of course, is that
Speaker 1: maybe gravity is not a quantum force the way the
Speaker 1: other forces are. Right. The other forces we call them
Speaker 1: quantum forces because they're well described by quantum mechanics. But
Speaker 1: gravity is kind of different. I mean, the current theory
Speaker 1: we have a gravity general relativity. It doesn't like to
Speaker 1: describe gravity as a force, right, describes gravity instead as
Speaker 1: a bending of space. It says that when you have
Speaker 1: mass somewhere in space, space no longer becomes straight, becomes bent. Right,
Speaker 1: things move in curves and circles.
Speaker 5: And it's not like an actual just a mathematical nuance
Speaker 5: or a mathematical perspective. What really confirms is it is
Speaker 5: the idea that gravity can affect things that don't have mass. Right,
Speaker 5: That's how we know it's more than just a force
Speaker 5: between things that have mass. It actually like affects space
Speaker 5: for things that don't have mass.
Speaker 1: Right, that's exactly right. So if you shoot a photon
Speaker 1: through space that has mass nearby, the photon will not
Speaker 1: move in what we consider to be a straight line, right,
Speaker 1: It'll find a path through this bent space that involves
Speaker 1: basically curving. And this is what Einstein predicted with his theory,
Speaker 1: and they saw it, you know, and you can see
Speaker 1: in space. It's called gravitational lensing. You can see photons
Speaker 1: get bent by heavy objects, and it's because, as you say,
Speaker 1: the heavy objects are bending space itself.
Speaker 5: Right, It's not like gravity is pulling the photon because
Speaker 5: the photon doesn't have any mass.
Speaker 1: Right, that's right, the photon doesn't have any mass.
Speaker 5: Yeah, So that's how it's different. Like gravity seems to
Speaker 5: affect things that don't have sort of its fundamental property,
Speaker 5: you know, like electrominetic forces can affect something that does
Speaker 5: not have an electric charge.
Speaker 1: That's true.
Speaker 5: Gravity can affect thing everything else, right.
Speaker 1: Yeah, that's a pretty deep insight there, Not that for
Speaker 1: a cartoonist, not at all. Yeah, that's a fascinating way
Speaker 1: to think about it. I think that's totally correct. Yeah.
Speaker 1: And so if gravity is instead of being a force,
Speaker 1: if it's a way we change the shape of space itself,
Speaker 1: then maybe that's why we don't have a quantum theory
Speaker 1: of it. Right. And that's amazing and it's fantastic and
Speaker 1: it's exciting. And another reason why we have a hard
Speaker 1: time bringing these two things together is that quantum mechanics,
Speaker 1: the theory we've developed only works so far in flat space.
Speaker 1: That is if there's really heavy stuff nearby, we don't
Speaker 1: know how to do those quantum calculations. We can basically
Speaker 1: only do quantum mechanics in places where there isn't really
Speaker 1: strong gravity.
Speaker 5: So wait, it's a quantum physics doesn't work in reality basically?
Speaker 5: Is that what you're saying? Like, it doesn't work in
Speaker 5: the space that we actually live in.
Speaker 1: Well, it works basically everywhere except for close to black holes.
Speaker 1: M Right. You need basically a black hole have enough
Speaker 1: gravity to break down quantum mechanics because it's when when
Speaker 1: space gets really distorted that you start to see the
Speaker 1: effects of gravity on space, and then it becomes comparable
Speaker 1: to the strength of other stuff, and that's when that's
Speaker 1: when quantum mechanics breaks down. Yeah, quantum quantum field theory
Speaker 1: works basically what we call flat space, whereas gravity bends space.
Speaker 5: Wow, so earlier when we categorize gravity as part of
Speaker 5: these four fundamental forces, maybe that's just the wrong approach.
Speaker 5: Maybe you know, do you know what I mean? Like,
Speaker 5: maybe we shouldn't be categorizing these four things as one
Speaker 5: category of quote forces.
Speaker 1: That's right. It could be could be that there is
Speaker 1: no quantum theory of gravity as a fundamental force because
Speaker 1: it isn't one. Yeah, and it's just a feature of space, right, Absolutely,
Speaker 1: it's one possible explanation. But then we still need a
Speaker 1: way to make quantum mechanics work in bent space, right,
Speaker 1: And we still need to understand how to make our
Speaker 1: theory of general relativity play well with quantum mechanics, because
Speaker 1: we think quantum mechanics describes the universe, right, and general
Speaker 1: relativity is not a quantized theory. It's it's continuous, right.
Speaker 1: It treats space and everything as if it's infinitely divisible, right,
Speaker 1: it's not a quantum theory at all, in fact that
Speaker 1: it came about before quantum mechanics was even invented. And
Speaker 1: so while the basic tenets of it how it distorted
Speaker 1: space are probably correct, i mean, been verified to zillion
Speaker 1: degrees of accuracy, it doesn't feel like it can be
Speaker 1: a fundamental description of nature because it's not quantum mechanical.
Speaker 5: So, like we want to call it a force because
Speaker 5: it seems to move things like all the other forces,
Speaker 5: but it's maybe it's not a force. Maybe it's just
Speaker 5: kind of like some other weird property of space.
Speaker 1: Yeah, exactly. You know, maybe we've been trying to put
Speaker 1: a round peg into a square hole all these years.
Speaker 5: A gravity peg in a quantum hole.
Speaker 1: That's right, that's right. And there are other ways that
Speaker 1: people are trying to solve this problem. Like one way
Speaker 1: is thinking that maybe gravity is a fundamental force, but
Speaker 1: it just works a little bit differently from the other forces.
Speaker 1: For example, people think about how the universe might have
Speaker 1: additional spatial dimensions, you know, like instead of just being
Speaker 1: able to move in three directions, maybe there's like four
Speaker 1: or five six dimensions that you can move in. And
Speaker 1: folks who are interested in that should listen to our
Speaker 1: podcast on extra dimensions.
Speaker 6: No.
Speaker 5: Yeah, we did a whole episode on extra dimensions, but
Speaker 5: we didn't sort of get into this particular topic. So
Speaker 5: tell us how extra dimensions might explain why gravity is
Speaker 5: so weak.
Speaker 1: Yeah. The idea is that maybe gravity isn't so weak.
Speaker 1: Maybe gravity is just as strong as all the other forces.
Speaker 1: But if there's a whole other set of dimensions out
Speaker 1: there that's ways directions that think can move, it might
Speaker 1: be that gravity is the only thing that feels those dimensions, right.
Speaker 1: It might be that those dimensions are invisible to electromagnetism
Speaker 1: and to the weak force into the strong force, but
Speaker 1: to gravity, and what that means is that gravity might
Speaker 1: be basically leaking out into those other dimensions. You know,
Speaker 1: we talked about how the farther away you get from something,
Speaker 1: the weaker the forces. So like Mercury feels the force
Speaker 1: of the Sun's gravity much more strongly than Pluto does. Right,
Speaker 1: irrelevanti of whether or not you call it a planet,
Speaker 1: it doesn't feel gravity very strongly. And that's because it's
Speaker 1: further from the Sun, right. I mean that goes like
Speaker 1: one over are squared or are is the distance it's
Speaker 1: one of our squared because we have three dimensions. If
Speaker 1: we had six dimensions, it would be one over R five, right,
Speaker 1: which falls much more rapidly. So if there are additional
Speaker 1: dimensions out there, okay, and only gravity feels them, then
Speaker 1: that might be the reason why gravitational force falls so quickly.
Speaker 1: Maybe gravity's actually just as strong as everything else when
Speaker 1: you get really really close, But then these extra dimensions exist,
Speaker 1: and most of gravity leaks out into those other dimensions.
Speaker 5: Oh, sort of like between you and me, there's not
Speaker 5: just the three dimensions between you and me Ei, there
Speaker 5: are other secret hidden spaces kind of between you and me.
Speaker 5: Are these other dimensions.
Speaker 1: Exactly other ways for gravity to spread out, all right.
Speaker 5: And so gravity would be like just as strong as
Speaker 5: all the other forces. But it's just flessing its muscles
Speaker 5: in these other spaces that we can't see or feel exactly.
Speaker 1: It's like, you know, if somebody's at the center of
Speaker 1: a crowd and they let go a really stinky far right,
Speaker 1: the people next to them they smell it strongly, and
Speaker 1: the people further away they smell it much more weakly,
Speaker 1: and people outside don't smell it at all.
Speaker 5: All right, now imagine the farts really suddenly. But let's
Speaker 5: let's let's go with it.
Speaker 1: Hey, I'm trying to make this successible. You know, this
Speaker 1: is something everybody canna.
Speaker 5: Appreciate trying to make way, I get it cut it.
Speaker 1: But if there was somewhere else for that far to go,
Speaker 1: you know, if it could move not just sideways, but
Speaker 1: also could float up right, so you had a really
Speaker 1: tall room in the far floated up, then people wouldn't
Speaker 1: feel it as much because most of the far would
Speaker 1: dissipate into the upper corners of the room. And so
Speaker 1: gravity might be the same way. It might be that
Speaker 1: you know, for the first millimeters, so the first centimeters,
Speaker 1: so gravity gets very weak, very quickly. It falls off
Speaker 1: really rapidly, and that then you know, at normal distances
Speaker 1: like a meter or ten meters or whatever, you don't
Speaker 1: feel those other dimensions anymore because the other dimensions only
Speaker 1: activate it really really short distances. This is the theory
Speaker 1: people came up with, and we don't know if it's real.
Speaker 1: You know, we tested it so far. It seems like
Speaker 1: gravity works the same way for galactic scales and for
Speaker 1: earth scales, and for microscopic scales. It seems to always
Speaker 1: fall off at the same rate as a function of distance.
Speaker 1: So nobody's ever seen any evidence of these extra dimensions.
Speaker 1: But it's a fascinating theory and it's you know, it's
Speaker 1: one that would give kind of a natural explanation for
Speaker 1: why gravity would fall off so quickly and why gravity
Speaker 1: is so weak. It wouldn't explain all these other things.
Speaker 5: But in fact, people sort of try to use gravity
Speaker 5: to see if there are other dimensions, right.
Speaker 1: Yeah, that's right. It would be a really cool clue,
Speaker 1: right if And that's a fascinating way that science has done.
Speaker 1: You know, you try to look at everything around you
Speaker 1: and see if you can fit it all into one framework.
Speaker 1: Can I use this one set of ideas to describe
Speaker 1: everything right onto one part of concepts? Yep, that's right
Speaker 1: in my fart theory of the universe. The best possible
Speaker 1: way I think to unravel this is to actually go
Speaker 1: visit a black hole. Because quantum mechanics and general relativity
Speaker 1: tell you very different things about what's happening inside a
Speaker 1: black hole. Right, as we said before, general relativity tells
Speaker 1: you it's an infinite testimal dot of almost infinite density.
Speaker 1: Quantum mechanics says, you know, the universe is quantized first
Speaker 1: of all, so you can't have infinite testimal dots. And
Speaker 1: also this sort of a minimum size to stuff, right,
Speaker 1: and you can't have all that stuff compressed in such
Speaker 1: a tiny little area. And so if you could see
Speaker 1: inside a black hole, you would learn a lot about gravity.
Speaker 5: So what would be the plan. You would go into
Speaker 5: a black hole, you would observe and discover how the
Speaker 5: universe works, and then and then you'd be stuck there.
Speaker 1: That's right. They would have to send you a Nobel
Speaker 1: prize into the black hole after just assume you'd figured
Speaker 1: it out and cause the l prize into space into
Speaker 1: the black hole. Anybod who's listening, please do not go
Speaker 1: into a black hole. Please please do not go into
Speaker 1: a black hole. But you know, we don't need to
Speaker 1: visit black holes. We could try to create them here
Speaker 1: on Earth.
Speaker 5: That sounds like a great idea.
Speaker 1: Yeah, doesn't that sound like a great idea. I mean,
Speaker 1: I'm excited make it. Yeah, let's create a black hole
Speaker 1: and study it. Right, if gravity gets really really powerful
Speaker 1: when you get to really short distances because of this
Speaker 1: extra dimension theory, then it might be that if you
Speaker 1: shoot two protons together really really hard and they get
Speaker 1: really really close to each other, that you can create
Speaker 1: a super duper mini extra cute, little fuzzy black hole. Right,
Speaker 1: I'm trying to make it sound like a cozy thing, not.
Speaker 5: A yeah, you're trying to sell it, right.
Speaker 1: And so before we turned on the Large Hadron Collide
Speaker 1: about ten years ago, people thought maybe by smashing these
Speaker 1: protons together, we could actually create black holes and we
Speaker 1: could study them. We can reveal the deep secrets of gravity. Right, hmmm.
Speaker 5: So then the idea would be to try to make
Speaker 5: them at the Large Hadron Collider and just can of
Speaker 5: see what happens mm hm, like, does it tell us
Speaker 5: something about gravity or quantum physics at the same time.
Speaker 1: Yeah, exactly. By seeing how often they're made and how
Speaker 1: strong they are and what they turned into when they decay,
Speaker 1: we could understand something about the way black holes work,
Speaker 1: and that would have been really powerful. But unfortunately or fortunately,
Speaker 1: depending on how feel above black holes. We haven't made
Speaker 1: any black holes at the Large Hadron Collider that we've discovered.
Speaker 5: But maybe isn't it true that maybe you've made them
Speaker 5: but they evaporate.
Speaker 1: Yes, these black holes would be very short lived. But
Speaker 1: you know, everything we make at the Large Hadron Collider
Speaker 1: is really short lived. These things last for like ten
Speaker 1: to the negative thirty seconds or ten to the negative
Speaker 1: twenty three seconds. We're pretty good at seeing short lived
Speaker 1: stuff because it usually blows up into other things, and
Speaker 1: a black hole would have a really unusual signature in
Speaker 1: our detectors. It would be pretty clear to see if
Speaker 1: we had made them.
Speaker 5: Okay, but short of going into the black hole or
Speaker 5: detecting farts in dimensions, we may not know in the
Speaker 5: near future what what makes gravity so different?
Speaker 1: That's right, it's going to take some work. I mean
Speaker 1: the other direction, is theoretical, is to build up a
Speaker 1: theory of quantum gravity sort of from the bottom up.
Speaker 5: Like start from the beauty of math and physics and
Speaker 5: then try to build it up to our level exactly.
Speaker 1: And that's that's a wonderful way to do, is to say, like,
Speaker 1: maybe the universe works in this way, this most basic
Speaker 1: fundamental nature, and build it up from there and see
Speaker 1: if you can describe the universe that we see around us.
Speaker 5: Wow, all right, well that's pretty shocking to think gravity
Speaker 5: is such a place, such a big role in our lives,
Speaker 5: and yet it's it's like the weakling in the universe, right,
Speaker 5: It's like, imagine, imagine if if gravity was stronger, life
Speaker 5: would be a lot more chaotic, right and crazy?
Speaker 1: Yeah, exactly. We would be closer to the Sun and
Speaker 1: everything would feel more intense. It's fascinating to me that
Speaker 1: gravity has been a mystery to physics for hundreds of years.
Speaker 1: I mean, it was the focus of Isaac Newton's you know,
Speaker 1: like hundreds of years ago people working on gravity. And
Speaker 1: still today, even though we've made so much progress in
Speaker 1: terms of gravity, we still have so much, so many
Speaker 1: basic questions about it that we don't know the answers
Speaker 1: to not even the really beginning of how to answer them.
Speaker 1: To me, that's fascinating. Gravity is such a rich source
Speaker 1: of mystery for physics and for everybody.
Speaker 5: Wow, all right, cool, I think it's maybe time to
Speaker 5: push down this question. Thanks for joining us.
Speaker 1: If you still have a question after listening to all
Speaker 1: these explanations, please drop us a line. We'd love to
Speaker 1: hear from you. You can find us at Facebook, Twitter,
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