Speaker 1: I just think it's fascinating that it's such a fundamental
Speaker 1: force in the universe, right, Like, it's basically the thing
Speaker 1: that builds galaxies and keeps planets moving right, and gives
Speaker 1: structure to the entire cosmos.
Speaker 2: That's right. On the largest scale, it's actually the most
Speaker 2: important force. It's the reason why things look the way
Speaker 2: they do. It's the reason why our planet is round,
Speaker 2: it's the reason why we're on the planet.
Speaker 1: It's pretty important, and yet we don't know a lot
Speaker 1: about it, right Like, there's some really deep and strange
Speaker 1: mysteries about it.
Speaker 2: On one hand, we have a theory which works really
Speaker 2: really well. On the other hand, we have questions about
Speaker 2: it which seems really really basic.
Speaker 1: And not only that, it's very different than all the
Speaker 1: other forces of nature.
Speaker 2: That's right, one of these things is not like the
Speaker 2: other ones.
Speaker 1: Hi, I'm Hohe, I'm a cartoonist.
Speaker 2: And I'm Daniel. I'm a particle physicist.
Speaker 1: And this is our podcast Daniel and Jorge explain the universe.
Speaker 2: In which a cartoonist and a physicists try to figure
Speaker 2: out how to make the universe understandable to anybody.
Speaker 1: Yeah, and today on the podcast we are examining a
Speaker 1: very heavy topic, gravity and specifically why is gravity so.
Speaker 2: Weak and strange. Gravity, as we said earlier, is something
Speaker 2: which controls the structure of the universe. I mean, the
Speaker 2: reason the Solar system looks the way it does is
Speaker 2: because of gravity. The reason the Earth is round is
Speaker 2: because of gravity. The reason we have galaxies is because
Speaker 2: of gravity.
Speaker 1: The reason we weigh so much, it is because of gravity. Right,
Speaker 1: it's not my belt.
Speaker 2: No, that's because of late night cake eating.
Speaker 1: But it's such a fundamental force of nature, right, Like
Speaker 1: it's present in our everyday life. We spend a lot
Speaker 1: of time thinking about gravity, right, how not to fall down,
Speaker 1: how not to drop things, how to go up buildings,
Speaker 1: how to go down buildings.
Speaker 2: Right, that's right. It seems like one of the most
Speaker 2: important forces. I mean, if you ask people, you know,
Speaker 2: to name a force or what kind of forces they
Speaker 2: experience in their life, gravity is the one that's present
Speaker 2: in their lives. Right, you're climbing upstairs, you're fighting gravity,
Speaker 2: your trip, you fall down, you're feeling gravity. You look
Speaker 2: around you. The shape of things is controlled by gravity,
Speaker 2: and that's why it's particularly strange that gravity is the
Speaker 2: weakest force of all the forces we've discovered, it's by
Speaker 2: far the weakest.
Speaker 1: Yeah, it's really strange to hear you say that, Like,
Speaker 1: how can gravity be weak? Like you know, like it's
Speaker 1: it's keeping the whole Earth together, it's making the entire
Speaker 1: higher planet swing around going a circle basically, right. Without gravity,
Speaker 1: we would just shoot off into space.
Speaker 2: That's right. It's a really strange situation. And there's other
Speaker 2: things about gravity we don't understand as well. It's really strange.
Speaker 2: It doesn't play well with the other forces. It's very,
Speaker 2: very weak. It's a total mystery to science, except that
Speaker 2: we have a theory which works beautifully right. We can
Speaker 2: calculate exactly how mercury orbits the Sun. We can send
Speaker 2: things into outer space and know with to millimeter precision
Speaker 2: exactly where they're going to land. We have a working
Speaker 2: theory that we can use, right, but we don't understand
Speaker 2: it on a conceptual level. We have these basic, deep
Speaker 2: questions about what gravity is and how the universe works
Speaker 2: because of it.
Speaker 1: So it's a weird question, and maybe one of the
Speaker 1: people hadn't thought about before. So Daniel went out as
Speaker 1: usual and asked people on the street, why do you
Speaker 1: think gravity is so weak.
Speaker 2: Here's what a random selection of folks who were willing
Speaker 2: to talk to me on a Tuesday morning had to
Speaker 2: say about gravity. I don't know. I should don't know
Speaker 2: about that, all right. I thought it was a pretty
Speaker 2: strong force, so I don't know.
Speaker 1: But yeah, because it depends on the distance, and it's
Speaker 1: a long range one, so that's why we feel it
Speaker 1: very weak most of the time. Cool, No, okay, I
Speaker 1: have no I'm sorry, it's not very fruitful.
Speaker 2: Hmmm, I have no idea, but I'd be interested in
Speaker 2: finding out why.
Speaker 1: All right, that that was pretty good. Most people weren't
Speaker 1: surprised when you said gravity's weak.
Speaker 2: I don't know. I feel like, of all the questions
Speaker 2: I've asked people, this is the one that flumms them
Speaker 2: the most. You know. People were like, what, I have
Speaker 2: no idea, or they had crazy ideas why gravity must
Speaker 2: be weak. I feel like usually we get one person
Speaker 2: who knows what the answer is or has a good
Speaker 2: clue about what's going on. This time, I feel like
Speaker 2: almost everybody was pretty clueless. I mean, one person said
Speaker 2: I always thought gravity was pretty strong, right, which kind
Speaker 2: of sums up the situation. Right, Gravity's omnipresent in our lives.
Speaker 2: It dominates our experience, and yet it's so weak compared
Speaker 2: to the other really powerful forces we've discovered.
Speaker 1: Well, some people a couple of answers were that it
Speaker 1: had to do with distance, Like gravity gets really weak
Speaker 1: with distance.
Speaker 2: That's right. And the problem there is that all the
Speaker 2: forces get weak with distance, like electromagnetism also falls as
Speaker 2: a distance grows. Right, So all of these forces follow
Speaker 2: this one over R squared rule or are as your
Speaker 2: distance from the things that's giving.
Speaker 1: You the force, right, maybe maybe right.
Speaker 2: Maybe, yeah, mostly we think, And so that can't be
Speaker 2: the answer, right, because all the other forces have that
Speaker 2: same feature.
Speaker 1: So when you say it's the weak is it's not
Speaker 1: that it changes over distances differently than the other forces.
Speaker 2: That's right. So maybe we should talk about what the
Speaker 2: forces are and compare them to each other so folks
Speaker 2: can get understanding of how crazy weak gravity is. Right.
Speaker 1: So, Daniel, what are the forces of nature besides a
Speaker 1: bad movie with Ben Affleck than center bullet.
Speaker 2: Well, I think comedy. Comedy is definitely a force of nature.
Speaker 2: You know, it solves big problems around the world. Now,
Speaker 2: the fundamental force are electromagnetism, right, that's the one that
Speaker 2: controls electricity and magnetism obviously, and is responsible for the
Speaker 2: cool things like light and lightning and all that cool stuff.
Speaker 2: And then there's the weak nuclear force, which is a
Speaker 2: force which is responsible for radioactive decay of a nuclei. Right.
Speaker 2: And the cool thing about electricity and magnetism and the
Speaker 2: weak nuclear force is that we actually have shown that
Speaker 2: there are two sides of the same coin. As a
Speaker 2: particle physicist, we refer to them as one force. We
Speaker 2: call it the electro week. So sort of magnetism lost
Speaker 2: out there in the name merger, right, it should be
Speaker 2: electromagnetic week. But nobody voted to keep magnetism in the
Speaker 2: sort of the name of the partners in the law firm.
Speaker 1: Nobody lobbied for weak electro.
Speaker 2: Or magneto weak force. Yeah. Yeah, again, we are suffering
Speaker 2: the fate of some anonymous committee of scientists that get
Speaker 2: to name these things. Right, Who are these people?
Speaker 1: Probably some grad student, right or some you know, like
Speaker 1: this is really weird, we'll call it this.
Speaker 2: Yeah. So we have electricity and magnetism, which is a
Speaker 2: single force. We have the weak nuclear force, which is
Speaker 2: really should be combined with electricity and magnetism. And then
Speaker 2: there's the strong nuclear force, and this is the one
Speaker 2: that holds the nucleus together. You know, the nucleus is
Speaker 2: of just a bunch of positively charged protons and neutral neutrons. Right,
Speaker 2: it says only positively charged particles in the nucleus. So
Speaker 2: you might think, what it even holds the nucleus together? Right,
Speaker 2: you have all this positively charged stuff should be repelling themselves. Well,
Speaker 2: it's the strong nuclear force, and it does so by
Speaker 2: exchanging these crazy little particles we call gluons, and that
Speaker 2: holds the nucleus together, and it's pretty strong. It's even
Speaker 2: stronger than electromagnetism.
Speaker 1: Well, let's take a step back. So in the universe
Speaker 1: there's stuff.
Speaker 2: There's like, yes, firm that there is stuff in the university. Yes,
Speaker 2: without reservation, there is stuff.
Speaker 1: I'm glad we saw that question. But I mean it's
Speaker 1: like there's stuff that has substance to it, that has
Speaker 1: mass to it, or you know that it sort of exists.
Speaker 1: And then there's also besides the at how these things
Speaker 1: interact with each other, like how they affect each other.
Speaker 2: That's right, there's the matter and then there's the forces, right,
Speaker 2: and the forces affect how they interact with each other.
Speaker 1: And that's pretty much the universe. That's like, it's matter
Speaker 1: and forces.
Speaker 2: Yeah. One way to look at the universe is that
Speaker 2: it's particles, right, or you would say matter and their forces.
Speaker 2: In modern particle physics, we think about one level deeper,
Speaker 2: which is we think of quantum fields, and quantum fields
Speaker 2: are responsible both for matter and for forces. So we
Speaker 2: can talk about that maybe in another podcast. What is
Speaker 2: a quantum field? And how can I get one? You know,
Speaker 2: for lease or rent? What can they do for me?
Speaker 2: But yeah, I think it's fair still to think about
Speaker 2: the universe in terms of particles and forces.
Speaker 1: On that note, let's take a quick break. There are
Speaker 1: only four kinds of forces.
Speaker 2: Yeah, there are four kinds of forces, so electromagnetism, weak
Speaker 2: nuclear force, strong nuclear force, and then of course gravity. Right,
Speaker 2: that's the fourth force that we've discovered.
Speaker 1: Okay.
Speaker 2: The fascinating thing is that different particles feel different forces, right, Like,
Speaker 2: some particles feel this set of forces, some particles feel
Speaker 2: those set of forces. For example, right, particles with electric
Speaker 2: charge feel electromagnetism. Right. The electron, for example, is negatively charged.
Speaker 2: The proton is positively charged. You bring them close together,
Speaker 2: they're gonna pull on each other, They're gonna suck each
Speaker 2: other together, right, because they have opposite charges. We all
Speaker 2: know that. But you bring a neutral particle nearby, it
Speaker 2: just totally ignores it, right, It doesn't feel it at all. Right, Right,
Speaker 2: It's like it's like somebody's walking through a crowd of
Speaker 2: people shouting, but they have headphones on, so they can't
Speaker 2: hear anything, and they're totally oblivious to it.
Speaker 1: It's kind of like how we talked about in a
Speaker 1: previous podcast. They're almost like languages or like social media platforms.
Speaker 1: Like some people are on Twitter, some people are on Facebook,
Speaker 1: but some people are not on this. And so if somebody,
Speaker 1: if you're not on Twitter and somebody sends you a tweet,
Speaker 1: you're not gonna get it. And so it's just different
Speaker 1: ways that particles interact.
Speaker 2: That's right. Gravity is the Google plus of social media,
Speaker 2: right because nobody uses the trends. It's ancient but powerless. Yeah,
Speaker 2: And so different particles feel different forces. And for example,
Speaker 2: an electron, while it feels electromagnetism because it has a
Speaker 2: negative charge, it doesn't feel the strong force at all.
Speaker 2: It will pass right by a bunch of particles that
Speaker 2: are really tugging on each other with a strong force
Speaker 2: and not be affected at all. Whereas quarks, quarks feel
Speaker 2: all the forces. They feel a strong force, which is
Speaker 2: how they get pulled together in the nucleus. Remember, protons
Speaker 2: and neutrons are made of quarks. Quarks feel electromagnetism because
Speaker 2: they have electric charge. They feel the weak force. They
Speaker 2: also feel gravity, of course, because they have masks. So
Speaker 2: quarks get their fingers in everything feel.
Speaker 1: They get the feels for everything.
Speaker 2: They feel everything that's right, They got the strong feels.
Speaker 2: They're really deeply emotional.
Speaker 1: Part of the.
Speaker 2: And the other side of the spectrum you got things
Speaker 2: like neutrinos. The trinos don't have electric charge, so they
Speaker 2: ignore all electricity and magnetism. Right, They don't interact with light.
Speaker 2: They're invisible. They pass right through anything that They ignore
Speaker 2: electromagnetic bonds, so they pass through most materials. They don't
Speaker 2: feel the strong force. The only way they interact is
Speaker 2: with the weak force, and the weak force is pretty weak,
Speaker 2: which is why neutrinos can mostly just pass through matter unaffected.
Speaker 1: So we have four fundamental forces right, And gravity is
Speaker 1: one of these forces, and so when you say that
Speaker 1: gravity is weak, you actually mean it's weak compared to
Speaker 1: these other three forces.
Speaker 2: That's right. And so the ranking is the strong force
Speaker 2: is the strongest, so that one is actually well named.
Speaker 2: Congratulations anonymous group of scientists. Yeah, we should be called
Speaker 2: the as of twenty eighteen, currently known to be the
Speaker 2: strongest force force. After that comes electromagnetism. And you know,
Speaker 2: we know that force is pretty powerful. Stick your finger
Speaker 2: in a socket, you're gonna feel the wrath of electromagnetism. Right,
Speaker 2: It's not an unfamiliar feeling, right.
Speaker 1: Try to stick your finger in anything. You feel it, right,
Speaker 1: because it's Electromagnetism is the force that keeps you from
Speaker 1: basically passing through the table or passing through your car.
Speaker 2: Right, that's right. Because electromagnetism is the basis of chemical bonds, right,
Speaker 2: And chemical bonds are really the thing that form the
Speaker 2: structure of your body. Right. You think of your body
Speaker 2: is like a bunch of particles, but it's held together
Speaker 2: by all these forces. It's like a chain link fence
Speaker 2: binding together these little particles that prevents you from passing
Speaker 2: through something else. Yeah, So we got the strong force,
Speaker 2: and then electromagnetism, and then actually the weak nuclear force. Right,
Speaker 2: this is the force that like powers neutrinos and radioactive decay.
Speaker 2: It's much weaker than electromagnetism and much weaker than the
Speaker 2: strong force.
Speaker 1: Even weaker than the weak is gravity.
Speaker 2: That's right. If you make a list like strong force, electromagnetism,
Speaker 2: in the weak force, then you should leave like one
Speaker 2: hundred blank pages and then you get to grab because
Speaker 2: when we compare these forces, we put things like an
Speaker 2: equal distance apart, and we compare the strength of the forces.
Speaker 2: Gravity is ten to the thirty six times weaker than
Speaker 2: the weak force. That's ten with thirty six zeros in
Speaker 2: front of it.
Speaker 1: But isn't that sort of a matter of units or scale?
Speaker 1: Do you know what I mean? Like, it's much weaker,
Speaker 1: but only if you compare apples to apples, right, orange oranges?
Speaker 2: That's right. But put two protons next to each other, right,
Speaker 2: h two protons have a certain amount of mass and
Speaker 2: a certain amount of electric charge, and the force of
Speaker 2: their charges is going to be much much stronger than
Speaker 2: the force from their masses. Oh I see, So yeah,
Speaker 2: if everything was much much more massive, then there would
Speaker 2: be stronger gravity. But you can compare these things apples
Speaker 2: to apples by comparing them at you know, the same
Speaker 2: distance and the same basic unit of interaction.
Speaker 1: Right, But what if you take an apple put it
Speaker 1: next to another apple?
Speaker 2: Well, I think you can do that experiment. Nothing's going
Speaker 2: to happen because gravity is so weak. Right. You don't
Speaker 2: see two apples like pulling themselves together on the counter, right,
Speaker 2: built in apple collider. You know, the apples are not
Speaker 2: drawn to each other. Gravity is a super weak force,
Speaker 2: and you can see this yourself. Right, you can do
Speaker 2: an experiment where you counter the entire gravitational force of
Speaker 2: an enormous celestial body like the Earth. Right, take a
Speaker 2: small kitchen magnet and use it to hold up a nail,
Speaker 2: and think about what's happening there. Right, you have the
Speaker 2: nail is being pulled down by every single rock in
Speaker 2: the Earth. It's pulling with all of its gravity. But
Speaker 2: a tiny little kitchen magnet totally overcomes that.
Speaker 1: It can lift a nail even though it's pulling. It's
Speaker 1: being pulled down by the whole entire planet Earth.
Speaker 2: Right exactly. And now imagine a magnet the size of
Speaker 2: the Earth, right, I mean that would be that would
Speaker 2: be extraordinarily powerful. And so you have basically like a
Speaker 2: gravitational blob the size of the Earth. Still pretty ineffective
Speaker 2: compared to electromagnetism.
Speaker 1: Mmmm, so it's weak if you sort of compare it
Speaker 1: by object, Like you said, if you take a proton
Speaker 1: and put a nice proton, the force are going to
Speaker 1: feel from electromagnetism is so much bigger than the force
Speaker 1: of gravity. They're going to feel towards each other, the
Speaker 1: same with like two electrons or two quarks and things.
Speaker 1: So in the scale of like the particles that we know,
Speaker 1: it's a really weak.
Speaker 2: Force, that's right, exactly, And yet and yet it seems
Speaker 2: to dominate, right. That's a bit of a puzzle, Like,
Speaker 2: on one hand, it's super duper weak, and we're telling
Speaker 2: you that it hardly counts for anything. On the other hand,
Speaker 2: it's responsible for the structure of the Solar System, man
Speaker 2: for the galaxy, and it's the reason the universe looks
Speaker 2: the way it is, right, right, And so that can
Speaker 2: be confusing to people, Like how do you reconcile those
Speaker 2: two things in your head?
Speaker 1: Yeah, Like, why doesn't the Earth feel an electromagnetic force
Speaker 1: with the Sun, which it would be so much bigger
Speaker 1: than the force of gravity.
Speaker 2: Yeah, well it would be pretty shocking. And that's actually
Speaker 2: the reason is gravity is different from the other forces
Speaker 2: and that it can't be canceled out right, If there
Speaker 2: was some huge electrostatic difference between the Sun and the Earth,
Speaker 2: like a bunch of positive charges there and a bunch
Speaker 2: of negative charges, it would create such an enormous force
Speaker 2: that it would be very quickly balanced. Like that's what
Speaker 2: lightning is. Right. When there's a charge differential between clouds
Speaker 2: and the grounds, it doesn't take that much before those
Speaker 2: charges want to rearrange themselves to a lower energy configuration.
Speaker 2: They rush down to the ground, or they rush up
Speaker 2: to the clouds, or they jump from clouded cloud to
Speaker 2: balance themselves out. Because you have two kinds of charges,
Speaker 2: you have positive and you have negative, so you can
Speaker 2: find an arrangement where basically everybody's happy. It's an equilibrium, right,
Speaker 2: But that's not true for gravity.
Speaker 1: Okay, I get it. So for example, if the Earth
Speaker 1: was every particle on Earth had a positive electromagnetic charge,
Speaker 1: and every particle in the Sun had a negative electromagnetic charge,
Speaker 1: there would be a humongous pull from electromagnetism pulling the
Speaker 1: Earth into the Sun.
Speaker 2: Yeah, we'd be toast pretty quick. Yeah, very long lived experment.
Speaker 1: Yeah, it would be huge. Even the opposite, if we
Speaker 1: were all positive and the Sun was all positive, we
Speaker 1: would get shot out of the sort of system very quickly.
Speaker 2: That's right. And that's why you know, early days of
Speaker 2: the Solar system being formed, you have these gases and
Speaker 2: the gas and dust coalescing, and very rapidly things neutralize, right,
Speaker 2: because anything that feels an electrostatic force to something else
Speaker 2: is going to find the opposite charge and they're going
Speaker 2: to coalesce and they're going to make something neutral. Right.
Speaker 2: That's why most of the things around you are neutral, right,
Speaker 2: Most of the elements are neutral because any deviation from
Speaker 2: neutral results in a powerful force to neutralize it.
Speaker 1: So, thankfully the Earth is made out of both like
Speaker 1: equal amounts of positive and negative particles, right, that's right.
Speaker 1: Thankfully we're sort of balanced electromagnetically, and so even if
Speaker 1: the Sun was all positive, we would look like neutral,
Speaker 1: like a neutral ball to the Sun.
Speaker 2: Yeah, that's right. Where on large scales the Earth is neutral, right,
Speaker 2: I mean there might be some residual positive or negative
Speaker 2: charge depending on the solar wind, et cetera. But basically
Speaker 2: the Earth is neutral, and so the largest force of
Speaker 2: the Earth feels is the gravity from the Sun. Even
Speaker 2: though gravity is super duper weak, right, it doesn't take
Speaker 2: a lot to counteract gravity. But it's the only player
Speaker 2: left because everybody else is sort of pair it up
Speaker 2: and danced off for the night, and gravity's just there left,
Speaker 2: hold in the bag. And gravity can't be balanced, right.
Speaker 2: You feel gravity if you have any mass, right, there's
Speaker 2: only positive masses. There's no such thing as a negative
Speaker 2: mass to give anti gravity.
Speaker 1: Wow, well, let's keep going, but first let's take a
Speaker 1: quick break. Okay, So that's how gravity is so much
Speaker 1: weaker than the other forces. So how's it different than
Speaker 1: the other three forces of nature?
Speaker 2: There's like no end to waste. The gravity is weird,
Speaker 2: you know, there's no end to like the puzzles of
Speaker 2: gravity is fascinating.
Speaker 1: Bottomless pit.
Speaker 2: That's right, it's a black hole of questions. And one
Speaker 2: of my favorites is just that we have no way
Speaker 2: to sort of fit gravity in with the way the
Speaker 2: universe works according to everything else. You know, we talked
Speaker 2: earlier about how we have particles and we have forces
Speaker 2: or quantum fields equivalently, and that's a really successful way
Speaker 2: to describe the universe. You know, we have the Large
Speaker 2: Hadron Collider to explore these things really high energies, and
Speaker 2: we've understood all sorts of things using this theory. But
Speaker 2: that theory is used as quantum mechanics. So the way
Speaker 2: we describe interactions, you know, the way we talk about
Speaker 2: two electrons repelling each other, or the way lightning is
Speaker 2: formed or anything, involves passing quantum particles back and forth.
Speaker 2: And that's just not true for gravity.
Speaker 1: What does that mean? Passing particles back and forth? Like
Speaker 1: when like if I have two magnets and they're attracted
Speaker 1: to each other, they're not. They're not It's not like
Speaker 1: an invisible telekinesis pulling on each other. They're actually swapping
Speaker 1: particles and I can't see that is that kind of
Speaker 1: what do you mean?
Speaker 2: That's exactly what I mean. That the way two things
Speaker 2: interact via some four is by exchanging particles. And so
Speaker 2: for example, electromagnetism, right, is the force behind a magnet,
Speaker 2: and the way electromagnetism works, we think at a sort
Speaker 2: of microscopic particle level is that there's a particle that
Speaker 2: transmits that force, that sends sort of the information back
Speaker 2: and forth between two things that are feeling it. And
Speaker 2: in the case of electromagnetism, that particle is the photon.
Speaker 2: The particle is also a packet of light. So each
Speaker 2: of the quantum forces that we talked about before electromagnetism,
Speaker 2: the weak force and the strong force, each of them
Speaker 2: have a particle we associate with it. And that's not
Speaker 2: just like some name tag we put on and say, hey,
Speaker 2: you get this one, you get this one. We think
Speaker 2: that that's the particle that's responsible for making the force work.
Speaker 2: So when two electrons come near each other, how do
Speaker 2: they repel each other? How does that actually happen? What
Speaker 2: we think that they send photons out right, the electric
Speaker 2: field of a moving electron, Right, an accelerating electron generates photons,
Speaker 2: and those photons interact with the others, and so basically
Speaker 2: the passing messages back and forth using these quantum particles.
Speaker 1: So gravity is weird because we don't know that there
Speaker 1: is a quantum particle being exchanged when two things get
Speaker 1: attracted gravitationally.
Speaker 2: That's right. So we have this great framework. We say, oh,
Speaker 2: maybe all forces are quantum mechanical fields interacting with each other. Right,
Speaker 2: Let's apply that to the electromagnetic field. Yeah it works.
Speaker 2: Let's apply that to the weak force. Yeah it works.
Speaker 2: Let's apply to the strong force. Ooh, cool, it works.
Speaker 2: Maybe this is something deep about the way the universe works.
Speaker 2: Let's apply it to gravity. Uh oh, it doesn't work, right,
Speaker 2: So what does that mean? What does it mean when
Speaker 2: I say it doesn't work? Well? For a theory to work,
Speaker 2: it has to provide predictions for experiments. You have to
Speaker 2: be able to say, okay, theory, what would happen in
Speaker 2: this configuration if I shot a proton and another particle,
Speaker 2: predict what would happen, and then you can go off
Speaker 2: and do the experiments and compare it. Right. Well, when
Speaker 2: you do that for gravity, you try to form a
Speaker 2: quantum theory of gravity, it doesn't work. You get nonsense answers.
Speaker 2: You get answers like infinity right, or things disappear, or
Speaker 2: it's just it doesn't mathematically function. Like, there's no way
Speaker 2: to build a theory of gravity that we've discovered so
Speaker 2: far that works, that actually explains the way these things happen.
Speaker 2: There are a few candidates out there there pretty far
Speaker 2: from being a functional theory of quantum gravity, things like
Speaker 2: loop quantum gravity or string theory. But the basic problem
Speaker 2: is that quantum mechanics and general relativity, which is our
Speaker 2: best theory of gravity, do not play well together, and
Speaker 2: we have no functioning quantum theory of gravity.
Speaker 1: So does that mean that we don't have the right
Speaker 1: theory or is that gravity is just not quantum in nature?
Speaker 2: That's exactly the question we don't know the answer to.
Speaker 2: Right in one hundred years from now, somebody will know
Speaker 2: the answer to that, I hope, and they'll look back
Speaker 2: and they'll wonder, you know, why did those guys see
Speaker 2: the clues? But we don't know. It could be that
Speaker 2: there is a quantum theory of gravity. We're just not
Speaker 2: smart enough to think it up yet, right, Like the
Speaker 2: right person hasn't been born yet to put the math together.
Speaker 2: Or maybe it requires a different kind of math that
Speaker 2: we're using. Right, there's some assumption we're making that's a mistake.
Speaker 1: Or maybe just giving it a wrong name, like maybe
Speaker 1: it should be gravi tunis or gravitinos gravitas. Wait, that's
Speaker 1: taken exactly.
Speaker 2: That's definitely the problem. That's step number one, right. We
Speaker 2: made a mistake in step number one when we could
Speaker 2: define the particle. The other option, of course, is that
Speaker 2: maybe gravity is not a quantum force the way the
Speaker 2: other forces are. Right. The other forces we call them
Speaker 2: quantum forces because they're well described by quantum mechanics. But
Speaker 2: gravity is kind of different. I mean, the current theory
Speaker 2: we have a gravity general relativity. It doesn't like to
Speaker 2: describe gravity as a force, right, describes gravity instead as
Speaker 2: a bending of space. It says that when you have
Speaker 2: mass somewhere in space, space no longer becomes straight, becomes bent. Right,
Speaker 2: So the things move in curves and circles.
Speaker 1: And it's not like an actual just a mathematical nuance
Speaker 1: or a mathematical persons afect of What really confirms it
Speaker 1: is the idea that gravity can affect things that don't
Speaker 1: have mass. Right. That's how we know it's more than
Speaker 1: just a force between things that have mass. It actually
Speaker 1: like affects space for things that don't have mass.
Speaker 2: Right, that's exactly right. So if you shoot a photon
Speaker 2: through space that has mass nearby, the photon will not
Speaker 2: move in what we consider to be a straight line, right,
Speaker 2: it'll find a path through this bent space that involves
Speaker 2: basically curving. And this is what Einstein predicted with his theory,
Speaker 2: and they saw it, you know. And you can see
Speaker 2: in space it's called gravitational lensing. You can see photons
Speaker 2: get bent by heavy objects. And it's because, as you say,
Speaker 2: the heavy objects are bending space itself.
Speaker 1: Right, It's not like gravity is pulling the photon because
Speaker 1: the photon doesn't have any mass.
Speaker 2: Right, that's right, the photon doesn't have any mass.
Speaker 1: Yeah, So that's how it's different. Like gravity seems to
Speaker 1: affect things that don't have sort of its fundamental property,
Speaker 1: you know, like electromanicnetic forces can affect something that does
Speaker 1: not have an electric charge.
Speaker 2: That's true.
Speaker 1: Gravity can affect thing everything else, right.
Speaker 2: Yeah, that's a pretty deep insight there.
Speaker 1: Well, not bad for a cartoonist, not at all.
Speaker 2: Yeah, that's a fascinating way to think about it. I
Speaker 2: think that's totally correct. Yeah. And so if gravity is
Speaker 2: instead of being a force, if it's a way we
Speaker 2: change the shape of space itself, then maybe that's why
Speaker 2: we don't have a quantum theory of it. Right, And
Speaker 2: that's amazing and it's fantastic and it's exciting. And another
Speaker 2: reason why we have a hard time bringing these two
Speaker 2: things together is that quantum mechanics, the theory we've developed
Speaker 2: only works so far in flat space. That is, if
Speaker 2: there's really heavy stuff nearby, we don't know how to
Speaker 2: do those quantum calculations. We can basically only do quantum
Speaker 2: mechanics in places where there isn't really strong gravity.
Speaker 1: So wait, it's a quantum physics doesn't work in reality basically?
Speaker 1: Is that what you're saying, Like, it doesn't work in
Speaker 1: the space that we actually live.
Speaker 2: In, Well, it works basically everywhere except for close to
Speaker 2: black holes. Mmm, right, you need basically a black hole.
Speaker 2: I have enough gravity to break down quantum mechanics because
Speaker 2: it's when when space gets really distorted that you start
Speaker 2: to see the effects of gravity on space, and then
Speaker 2: it becomes comparable to the strength of other stuff, and
Speaker 2: that's when that's when quantum mechanics breaks down. Yeah, quantum
Speaker 2: quantum field theory works basically what we call flat space,
Speaker 2: whereas gravity bends space.
Speaker 1: Wow, so earlier when we categorize gravity as part of
Speaker 1: these four fundamental forces, maybe that's just the wrong approach.
Speaker 1: Maybe you know, do you know what I mean? Like,
Speaker 1: maybe we shouldn't be categorizing these four things as one
Speaker 1: category of quote forces.
Speaker 2: That's right. It could be that it could be that
Speaker 2: there is no quantum theory of gravity as a fundamental
Speaker 2: force because it isn't one. Yeah, and it's just a
Speaker 2: feature of space, right, Absolutely, that's one possible explanation. But
Speaker 2: then we still need a way to make quantum mechanics
Speaker 2: work in bend space, right, And we still need to
Speaker 2: understand how to make our theory of general relativity play
Speaker 2: well with quantum mechanics, because we think quantum mechanics describes
Speaker 2: the universe, right, and general relativity is not a quantized theory.
Speaker 2: It's it's continuous, right. It treats space and everything as
Speaker 2: if it's infinitely divisible. Right, it's not a quantum theory
Speaker 2: at all. In fact that it came about before quantum
Speaker 2: mechanics was even invented. Right. And so while the basic
Speaker 2: tenets of it and how it distorted space are probably correct,
Speaker 2: i mean, been verified to zillion degrees of accuracy, it
Speaker 2: doesn't feel like it can be a fundamental description of
Speaker 2: nature because it's not quantum mechanical.
Speaker 1: So like we want to call it a force because
Speaker 1: it seems to move things like all the other forces,
Speaker 1: but it's maybe it's not a force. Maybe it's just
Speaker 1: kind of like some other weird property of space.
Speaker 2: Yeah, exactly. You know, maybe we've been trying to put
Speaker 2: a round peg into a square hole all these years.
Speaker 1: A gravity peg and a quantum hole.
Speaker 2: That's right, that's right. And there are other ways that
Speaker 2: people are trying to solve this problem. Like one way
Speaker 2: is things that maybe gravity is a fundamental force, but
Speaker 2: it just works a little bit differently from the other forces.
Speaker 2: For example, people think about how the universe might have
Speaker 2: additional spatial dimensions, you know, like instead of just being
Speaker 2: able to move in three directions, maybe there's like four
Speaker 2: or five six dimensions that you can move in. And
Speaker 2: folks who are interested in that should listen to our
Speaker 2: podcast on extra dimensions.
Speaker 1: No, yeah, we did a whole episode on extra dimensions,
Speaker 1: but we didn't sort of get into this particular topic.
Speaker 1: So tell us how extra dimensions might explain why gravity
Speaker 1: is so weak.
Speaker 2: Yeah, The idea is that maybe gravity isn't so weak.
Speaker 2: Maybe gravity is just as strong as all the other forces.
Speaker 2: But if there's a whole other set of dimensions out there.
Speaker 2: There's ways directions that think can move. It might be
Speaker 2: that gravity is the only thing that feels those dimensions, right.
Speaker 2: It might be that those dimensions are invisible to electromagnetism
Speaker 2: and to the weak force into the strong force, but
Speaker 2: visible to gravity. And what that means is that gravity
Speaker 2: might be basically leaking out into those other dimensions. You know,
Speaker 2: we talked about how the farther away you get from something,
Speaker 2: the weaker the forces. So like Mercury feels the force
Speaker 2: of the Sun's gravity much more strongly than Pluto does. Right,
Speaker 2: irrelevant of whether or not you call it a planet,
Speaker 2: it doesn't feel gravity very strongly. And that's because it's
Speaker 2: further from the Sun, right. I mean that goes like
Speaker 2: one over are squared or are as the distance it's
Speaker 2: one of our squared because we have three dimensions. If
Speaker 2: we had six dimensions, it would be one over R five, right,
Speaker 2: which falls much more rapidly. So if there are additional
Speaker 2: dimensions out there, okay, and only gravity feels them, then
Speaker 2: that might be the reason why gravitational force falls so quickly.
Speaker 2: Maybe gravity's actually just as strong as everything else. When
Speaker 2: you get really really close. But then these extra dimensions exist,
Speaker 2: and most of gravity leaks out into those other dimensions.
Speaker 1: Oh, sort of like between you and me, there's not
Speaker 1: just the three dimensions between you and me. May there
Speaker 1: are other secret hidden spaces kind of between you and me?
Speaker 1: Are these other dimensions exactly?
Speaker 2: Other ways for gravity to spread out, all right?
Speaker 1: And so gravity would be like just as strong as
Speaker 1: all the other forces, but it's just fleshing its muscles
Speaker 1: in these other spaces that we can't see or feel exactly.
Speaker 2: It's like, you know, if somebody's at the center of
Speaker 2: a crowd and they let go a really stinky far right.
Speaker 2: The people next to them they smell it strongly, and
Speaker 2: the people further away they smell it much more weakly,
Speaker 2: and people outside don't smell it at all.
Speaker 1: All right, now, imagine the farts really suddenly. But let's
Speaker 1: let's let's go with it.
Speaker 2: Hey, I'm trying to make this successible. You know, this
Speaker 2: is something everybody canna.
Speaker 1: Appreciate it trying to make way I get it cut it.
Speaker 2: But if there was somewhere else for that far to go,
Speaker 2: you know, if it could move not just sideways, but
Speaker 2: also could float up right so you had a really
Speaker 2: tall room in the far floated up, then people wouldn't
Speaker 2: feel it as much because most of them far would
Speaker 2: dissipate into the upper corners of the room. And so
Speaker 2: gravity might be the same way. It might be that
Speaker 2: you know, for the first millimeters, so the first centimeters,
Speaker 2: so gravity gets very weak, very quickly. It falls off
Speaker 2: really rapidly, and that then you know, at normal distances
Speaker 2: like a meter or ten meters or whatever, you don't
Speaker 2: feel those other dimensions anymore because the other other dimensions
Speaker 2: only activate it really really short distances. This is the
Speaker 2: theory people came up with, and we don't know if
Speaker 2: it's real. You know, we've tested it so far. It
Speaker 2: seems like gravity works the same way for galactic scales
Speaker 2: and for Earth scales, and for microscopic scales. It seems
Speaker 2: to always fall off at the same rate as a
Speaker 2: function of distance. So nobody's ever seen any evidence of
Speaker 2: these extra dimensions. But it's a fascinating theory and it's
Speaker 2: a you know, it's one that would give kind of
Speaker 2: a natural explanation for why gravity would fall off so
Speaker 2: quickly and why gravity is so weak. It wouldn't explain
Speaker 2: all these other things, But.
Speaker 1: In fact people sort of try to use gravity to
Speaker 1: see if there are other dimensions.
Speaker 2: Right, Yeah, that's right. It would be a really cool clue,
Speaker 2: right if And that's a fascinating way that science has done.
Speaker 2: You know, you try to look at everything around you
Speaker 2: and see if you can fit it all into one framework,
Speaker 2: like can I use this one set of ideas to
Speaker 2: describe everything? Right? Can irge edit onto one part of concepts? Yeah,
Speaker 2: that's right. In my fart theory of you in the universe.
Speaker 2: The best possible way I think to unravel this is
Speaker 2: to actually go visit a black hole, because quantum mechanics
Speaker 2: and general relativity tell you very different things about what's
Speaker 2: happening inside a black hole. Right, As we said before,
Speaker 2: general relativity tells you it's an infinite tesimal dot of
Speaker 2: almost infinite density. Quantum mechanics says, you know, the universe
Speaker 2: is quantized first of all, so you can't have infinite
Speaker 2: testimal dots. And also there's sort of a minimum size
Speaker 2: to stuff, right, and you can't have all that stuff
Speaker 2: compressed in such a tiny little area. And so if
Speaker 2: you could see inside a black hole, you would learn
Speaker 2: a lot about gravity.
Speaker 1: So what would be the plan. You would go into
Speaker 1: a black hole. You would observe and discover how the
Speaker 1: universe works, and then and then you'd be stuck there.
Speaker 2: That's right. They would have to send you a Nobel
Speaker 2: Prize into the black hole after that, which is assume
Speaker 2: you'd figure it out and pause the Nobel Prize into
Speaker 2: space into the black hole. Anybod who's listening, please do
Speaker 2: not go into a black hole. Please please do not
Speaker 2: go into a black hole. But you know, we don't
Speaker 2: need to visit black holes. We could try to create
Speaker 2: them here on Earth.
Speaker 1: That sounds like a great idea.
Speaker 2: Yeah, doesn't that sound like a great idea? I mean
Speaker 2: I'm excited make it. So, Yeah, let's create a black
Speaker 2: hole and study it. Right, if gravity gets really really
Speaker 2: powerful when you get to really short distances because of
Speaker 2: this extra dimension theory, then it might be that if
Speaker 2: you shoot two protons together really really hard and they
Speaker 2: get really really close to each other, that you can
Speaker 2: create a super duper mini extra cute, little fuzzy black hole. Right,
Speaker 2: I'm trying to make it sound like a cozy thing.
Speaker 1: Yeah, you're trying to sell it, right.
Speaker 2: And so before we turned on the large Hadron collide.
Speaker 2: About ten years ago, people thought maybe by smashing these
Speaker 2: protons together we could actually create black holes and we
Speaker 2: could study them. We can reveal the deep secrets of gravity.
Speaker 1: Right, hmmm, So then the idea would be to try
Speaker 1: to make them at the Large Headron Collider and just
Speaker 1: can of see what happens, Like, does it tell us
Speaker 1: something about gravity or quantum physics at the same.
Speaker 2: Time, Yeah, exactly. By seeing how often they're made and
Speaker 2: how strong they are and what they turned into, whether
Speaker 2: they decay, we could understand something about the way black
Speaker 2: holes work, and that would have been really powerful. But
Speaker 2: unfortunately or fortunately, depending on how you feel above black holes.
Speaker 2: We haven't made any black holes at the Large Hadron
Speaker 2: Collider that we've discovered.
Speaker 1: But maybe isn't it true that maybe you've made them
Speaker 1: but they evaporate.
Speaker 2: Yes, these black holes would be very short lived, but
Speaker 2: you know, everything we make at the Large Hadron Collider
Speaker 2: is really short lived. These things last for like ten
Speaker 2: to the negative thirty seconds or ten to the negative
Speaker 2: twenty three seconds. We're pretty good at seeing short lived
Speaker 2: stuff because it usually blows up into other things, and
Speaker 2: a black hole would have a really unusual signature in
Speaker 2: our detector, it would be pretty clear to see if
Speaker 2: we had made them.
Speaker 1: Okay, but short of going into a black hole or
Speaker 1: detecting farts in extra dimensions, we may not know in
Speaker 1: the near future what what makes gravity so different?
Speaker 2: That's right, take some work. I mean. The other direction,
Speaker 2: is theoretical, is to build up a theory of quantum
Speaker 2: gravity sort of from the bottom up.
Speaker 1: Like start from the beauty of math and physics, and
Speaker 1: then try to build it up to our level exactly.
Speaker 2: And that's that's a wonderful way to do, is to say, like,
Speaker 2: maybe the universe works in this way its most basic
Speaker 2: fundamental nature, and build it up from there and see
Speaker 2: if you can describe the universe that we see around us. Wow.
Speaker 1: All right, well, that's pretty shocking to think gravity is
Speaker 1: such a place, such a big role in our lives,
Speaker 1: and yet it's like the weakling in the universe, right.
Speaker 1: It's like, imagine if if gravity was stronger, life would
Speaker 1: be a lot more chaotic, right and crazy?
Speaker 2: Yeah, exactly. We would be closer to the Sun and
Speaker 2: everything would feel more intense. It's fascinating to me that
Speaker 2: gravity has been a mystery to physics for hundreds of years.
Speaker 2: I mean it was the focus of Isaac Newton's studies,
Speaker 2: you know, like hundreds of years ago, people working on gravity.
Speaker 2: And still today, even though we've made so much progress
Speaker 2: in terms of gravity, we still have so much, so
Speaker 2: many basic questions about it that we don't know the
Speaker 2: answers to, not even the really beginning of how to
Speaker 2: answer them to meet. That's fascinating. Gravity such a rich
Speaker 2: source of mystery for physics and for everybody.
Speaker 1: Wow, all right, cool, I think it's maybe time to
Speaker 1: push down this question. Thanks for joining us.
Speaker 2: If you still have a question after listening to all
Speaker 2: these explanations, please drop us a line. We'd love to
Speaker 2: hear from you. You can find us at Facebook, Twitter,
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Speaker 2: email us at Feedback at Daniel Andorge dot com.