Latest / Daniel and Kelly’s Extraordinary Universe / How do we measure the gravitational constant?

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How do we measure the gravitational constant? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Quor, Hey, have you found your first gray hair yet?
• 0:11
  Speaker 2: I have. I have quite a few of them. Yeah,
• 0:14
  Speaker 2: although I like to think of them as silver not gray.
• 0:17
  Speaker 1: Oh that sounds like a good plan. Really lean into
• 0:20
  Speaker 1: the dignity of aging.
• 0:22
  Speaker 2: I don't have a lot of dignity, but I am
• 0:24
  Speaker 2: definitely aging. How about you? How's your transition going to
• 0:29
  Speaker 2: full Einstein hairstyle mode?
• 0:32
  Speaker 1: I still have no silver hair. In fact, I'm thinking
• 0:34
  Speaker 1: about dying my.
• 0:35
  Speaker 2: Temples, dyeing them, like bleaching them to get gray hairs.
• 0:40
  Speaker 1: Yeah. So people take me a little bit more seriously.
• 0:42
  Speaker 2: Oh, I see, you want to look like one of
• 0:43
  Speaker 2: those senior established physicists gray hairs exactly.
• 0:48
  Speaker 1: I want to increase my gravitass not just my personal gravity.
• 0:53
  Speaker 2: Yeah, I may. Your problem is you're increasing in with
• 0:55
  Speaker 2: and not with them.
• 0:58
  Speaker 1: That was a weighty burn. H.
• 1:14
  Speaker 2: I am Poor hammy cartoonist and the creator of PhD comics.
• 1:17
  Speaker 3: Hi.
• 1:18
  Speaker 1: I'm Daniel. I'm a particle physicist and a professor at
• 1:20
  Speaker 1: UC Irvine. And I was once accidentally c seed on
• 1:23
  Speaker 1: an email where I was described as young ish.
• 1:27
  Speaker 2: Oh all right, yeah, that's good. But how long ago
• 1:30
  Speaker 2: was this? Was this like twenty years ago, in which
• 1:32
  Speaker 2: case it was true back then.
• 1:34
  Speaker 1: Yeah, long enough that I shouldn't be telling that story anymore.
• 1:39
  Speaker 2: Right, You're like I was in my thirties and people
• 1:43
  Speaker 2: were calling me youngish. That's a better adjective than many
• 1:46
  Speaker 2: other adjectives people can call you.
• 1:48
  Speaker 1: Absolutely, I'd rather be youngish than oldish or stinkish.
• 1:51
  Speaker 2: But anyways, welcome to our podcast, Daniel and Jorge Explain
• 1:54
  Speaker 2: the Universe, a production of iHeartRadio.
• 1:56
  Speaker 1: In which we dig into the mysteries of this oldish
• 1:59
  Speaker 1: universe and our youngish attempts to understand it. We think
• 2:03
  Speaker 1: that the universe should make sense to humans. We should
• 2:06
  Speaker 1: be able to go out there and measure things about it,
• 2:09
  Speaker 1: to figure it out, to unravel its mysteries, and to
• 2:11
  Speaker 1: explain it to each other. And that's our job on
• 2:14
  Speaker 1: this podcast to unravel as many of those mysteries as
• 2:17
  Speaker 1: possible and to explain them to you.
• 2:19
  Speaker 2: That's right, It's our job to increase the gravity of
• 2:21
  Speaker 2: your brain. Hopefully all of this amazing knowledge about the
• 2:24
  Speaker 2: universe is maybe making more connections in the neurons in
• 2:28
  Speaker 2: your brain and making your brain grow a little bit,
• 2:30
  Speaker 2: and also increasing the wisdom in there. Because I guess
• 2:32
  Speaker 2: knowing about the universe sort of increases your wisdom, right,
• 2:35
  Speaker 2: if you know how the world works, that's sort of
• 2:38
  Speaker 2: the definition of wisdom.
• 2:39
  Speaker 1: Yeah, what other kind of wisdom is there other than
• 2:41
  Speaker 1: knowing how the world works? If you lump like people
• 2:44
  Speaker 1: and animals and society and all that kind of stuff
• 2:47
  Speaker 1: into the world. And that's exactly what we're trying to do.
• 2:49
  Speaker 1: We're trying to describe the world. And the way we
• 2:52
  Speaker 1: do it is by telling these mathematical stories. We say
• 2:55
  Speaker 1: there are relationships between these things. We notice if you
• 2:58
  Speaker 1: push on this thing a certain way, goes a certain speed,
• 3:01
  Speaker 1: or they don't move if you don't push on them.
• 3:03
  Speaker 1: All these things are mathematical stories that we use to
• 3:06
  Speaker 1: describe the universe that's out there. We hope to boil
• 3:09
  Speaker 1: it down to a bunch of equations, which in the end,
• 3:12
  Speaker 1: they're just describing what we see out there in the universe.
• 3:15
  Speaker 2: It's right, we're trying to find what the wise crack
• 3:18
  Speaker 2: of the universe is. That kind of what the job
• 3:20
  Speaker 2: of a physicist is.
• 3:24
  Speaker 1: I hope there will be some humor in these stories.
• 3:26
  Speaker 1: You know, every good story has some comic relief in it,
• 3:28
  Speaker 1: even mathematical.
• 3:29
  Speaker 2: Stories, but I guess it's wisden the same as common sense.
• 3:32
  Speaker 2: Do you think the universe has common sense?
• 3:34
  Speaker 1: Absolutely not. Intuitive ideas about the universe what makes sense
• 3:38
  Speaker 1: to us from our limited experience here on Earth are
• 3:41
  Speaker 1: not always reflective of what's really happening in the universe.
• 3:44
  Speaker 1: You know, it made sense to Aristotle that things fell down,
• 3:48
  Speaker 1: but that doesn't mean that everything always falls down, or
• 3:50
  Speaker 1: that down't even means something when you're out in space
• 3:53
  Speaker 1: far away from any gravity.
• 3:55
  Speaker 2: Yeah, it is a pretty perplexing universe, and sometimes it
• 3:58
  Speaker 2: sort of seems like it does think that that don't
• 4:00
  Speaker 2: make sense, And in fact, you can sort of ask
• 4:02
  Speaker 2: the question whether humans will ever make full sense of
• 4:05
  Speaker 2: the universe or if there are just some things about
• 4:07
  Speaker 2: it that are sort of random, right, or arbitrary.
• 4:10
  Speaker 1: Yeah, there's lots of layers. There are humans smart enough
• 4:13
  Speaker 1: to describe the workings of the universe in terms of
• 4:15
  Speaker 1: our mathematics. Is our mathematics actually the language of the
• 4:19
  Speaker 1: universe itself or just our description of what we see?
• 4:23
  Speaker 1: And philosophically, we aren't even sure if there is a
• 4:27
  Speaker 1: single mathematical prescription that describes everything that happens out there
• 4:30
  Speaker 1: in the universe. A whole group of philosophers believe in
• 4:33
  Speaker 1: disunity that there might not be a single holistic description
• 4:37
  Speaker 1: of the universe. So it's pretty complicated, but we do
• 4:40
  Speaker 1: our best. We find these mathematical stories, which are equations.
• 4:43
  Speaker 1: They relate things like force and acceleration, or force and mass,
• 4:47
  Speaker 1: all sorts of things. But they're not just equations. The
• 4:49
  Speaker 1: equations also have numbers in them, constants that describe the
• 4:54
  Speaker 1: way the universe works.
• 4:55
  Speaker 2: Yeah, the universe seems to have lots of consonants, lots
• 4:58
  Speaker 2: of numbers in them, like pi, and I guess the
• 5:01
  Speaker 2: expansion of the universe is also defined by a number.
• 5:04
  Speaker 1: Yeah, that's right, the speed of light, All sorts of
• 5:06
  Speaker 1: things seem to control the way that the universe works.
• 5:09
  Speaker 1: And in lots of cases, we don't know why they
• 5:11
  Speaker 1: have this value and not some other value. Why is
• 5:14
  Speaker 1: the universe expanding at this rate? Why is the speed
• 5:16
  Speaker 1: of light not faster or slower? Why are some of
• 5:18
  Speaker 1: the forces strong and some of them are powerful. It
• 5:21
  Speaker 1: seems like there's a control panel somewhere on the universe,
• 5:24
  Speaker 1: and all these things are just parameters. They're just like
• 5:26
  Speaker 1: knobs on the control panel, and you could have twirled
• 5:28
  Speaker 1: them one way or another way and still gotten a
• 5:31
  Speaker 1: universe one very different from ours. But we don't know
• 5:34
  Speaker 1: if there's a reason why the parameters have the values
• 5:37
  Speaker 1: they do.
• 5:37
  Speaker 2: I feel like every time you say that the universe
• 5:39
  Speaker 2: has a control panel, I always imagine, for some reason,
• 5:42
  Speaker 2: the Simpsons, you know, the opening scene with Homer sitting
• 5:45
  Speaker 2: in front of like the control panel floor for the
• 5:47
  Speaker 2: nuclear plant that he works at. I always always imagine
• 5:50
  Speaker 2: that when you say the control panel of the universe, Like,
• 5:53
  Speaker 2: is there a Homer Simpson about to spill some coffee
• 5:56
  Speaker 2: or donuts onto the fabric of our universe?
• 6:00
  Speaker 1: That would explain maybe why the universe seems so crazy
• 6:02
  Speaker 1: and bonkers sometimes because maybe there's an idiot in charge.
• 6:06
  Speaker 2: Because it was designed by bad groaning exactly, or because
• 6:09
  Speaker 2: Homer symptom isn't charge.
• 6:11
  Speaker 1: Yeah, either because there's a cartoonist who's the designer of
• 6:14
  Speaker 1: the universe and we all know they can't be trusted.
• 6:18
  Speaker 2: Are you saying God is the ultimate cartoonists or cartoonists
• 6:21
  Speaker 2: are the ultimate gods?
• 6:22
  Speaker 1: I'm saying if either of those are true, then we're screwed.
• 6:26
  Speaker 2: No, wouldn't you want to live in a cartoon, Like
• 6:29
  Speaker 2: if the universe was controlled by cartoon physics, I mean,
• 6:33
  Speaker 2: wouldn't that be more fun? Wouldn't your job be more fun?
• 6:36
  Speaker 1: My job would be impossible because there is no physics
• 6:39
  Speaker 1: in cartoons. There don't seem to be any laws that
• 6:42
  Speaker 1: anybody follows. It's just like make it all up as
• 6:44
  Speaker 1: you go. So science is basically out the window.
• 6:47
  Speaker 2: Is that your goal, well, to put you out of
• 6:49
  Speaker 2: a job. meEach episode, I'm trying to embarrass us to
• 6:54
  Speaker 2: the point where we don't know what he gots anymore.
• 6:58
  Speaker 1: But it does seem like there are the laws that
• 7:00
  Speaker 1: describe what's out there. And sometimes in these laws there
• 7:03
  Speaker 1: are just numbers, like if you look at Maxwell's equations
• 7:06
  Speaker 1: or how electromagnetic radiation propagates to the universe, there are
• 7:09
  Speaker 1: a couple constants in there, the permittivity of free space,
• 7:12
  Speaker 1: for example. All those things determine the speed of light.
• 7:14
  Speaker 1: But these are just numbers that we measure in the universe.
• 7:17
  Speaker 1: We don't have like a theoretical reason to say why
• 7:20
  Speaker 1: should be this number or the other number. It's just
• 7:23
  Speaker 1: like an unknown parameter in the equations that we have
• 7:25
  Speaker 1: to go out and do experiments to discover.
• 7:28
  Speaker 2: Yeah, like you were saying, like the speed of light,
• 7:30
  Speaker 2: it is three hundred thousand meters per second, but it
• 7:33
  Speaker 2: could also be something else. And that's what you mean
• 7:35
  Speaker 2: by a control not like somehow when the universe was created,
• 7:38
  Speaker 2: somebody said that not to three hundred thousand meters per second,
• 7:41
  Speaker 2: but it could have been something else.
• 7:42
  Speaker 1: Yea, actually I think it is something else. It's three
• 7:44
  Speaker 1: hundred million meters per second.
• 7:45
  Speaker 2: Oh three, that's what I said.
• 7:47
  Speaker 1: You go, somebody fell asleep on the control panel and
• 7:49
  Speaker 1: the speed of light is slower over there in Pasadena
• 7:51
  Speaker 1: than it is down here, apparently.
• 7:53
  Speaker 2: Well there you go. Don't put me in charge of
• 7:54
  Speaker 2: the knob because obviously we've set it to a thousand
• 7:58
  Speaker 2: times the wrong aunt.
• 8:01
  Speaker 1: All right, Homer. But these constants are fascinating, and physicists
• 8:04
  Speaker 1: look at them and they go, why this number, why
• 8:07
  Speaker 1: not some other number? Especially when the numbers are weird.
• 8:10
  Speaker 1: The numbers are like one or two, people are like, yeah, cool,
• 8:13
  Speaker 1: that makes sense. But if the numbers are like seventy
• 8:15
  Speaker 1: four bajillion or ten to the negative thirty two, people
• 8:19
  Speaker 1: are like, that's really strange. It's got to be a story.
• 8:21
  Speaker 2: There, right, Because I guess if it's one, then that
• 8:24
  Speaker 2: means that something canceled out sort of right.
• 8:26
  Speaker 1: This is a really controversial way of thinking in theoretical
• 8:29
  Speaker 1: physics to say that like numbers like one are natural,
• 8:32
  Speaker 1: that they make sense. That you know, two things are
• 8:34
  Speaker 1: related by a factor or close to one, that means
• 8:37
  Speaker 1: that it's a natural relationship. And if the factor is
• 8:40
  Speaker 1: really really big, then you got to ask why what's
• 8:42
  Speaker 1: going on? So why didn't things cancel out? Why are
• 8:45
  Speaker 1: these things not in balance? It's really kind of esthetics.
• 8:48
  Speaker 1: It's not really driven by any deep principle in theoretical physics.
• 8:53
  Speaker 1: It's just like wondering why numbers are not close to one,
• 8:56
  Speaker 1: just preferring numbers close to one. There isn't even really
• 8:59
  Speaker 1: a great argument that I could take for why you
• 9:02
  Speaker 1: would prefer numbers close to one.
• 9:03
  Speaker 2: Well, they say one is the loneliest numbers, So maybe
• 9:07
  Speaker 2: you're an introvert. That sounds like the best number.
• 9:09
  Speaker 1: Maybe the secrets to the universe are actually hidden in
• 9:12
  Speaker 1: the names of eighties pop.
• 9:13
  Speaker 2: Songs, Yeah, there you go, or in the lyrics, right,
• 9:17
  Speaker 2: maybe eighties pop stars are the gods of the universe.
• 9:20
  Speaker 1: Yeah, maybe it's all just about ice. Ice baby.
• 9:23
  Speaker 2: Yeah, there wasn't there a group called Genesis back in
• 9:25
  Speaker 2: the eighties. There you go. Well, so there are all
• 9:28
  Speaker 2: these amazing numbers that seem to sort of control how
• 9:31
  Speaker 2: the universe behaves and what it does and what the
• 9:34
  Speaker 2: particles in it all do. But we don't understand some
• 9:37
  Speaker 2: of these constants, and in some cases we don't even
• 9:39
  Speaker 2: know exactly what they are, right.
• 9:41
  Speaker 1: That's right. Sometimes we can do experiments to measure them
• 9:44
  Speaker 1: very very precisely, but some of these are a little slippery.
• 9:47
  Speaker 1: Some of them are very difficult to actually nail down,
• 9:50
  Speaker 1: especially one of the most fundamental constants in the universe.
• 9:55
  Speaker 2: Yeah. So today own the program, we'll be asking the question,
• 10:02
  Speaker 2: how do we measure the gravitational constant G upper case G,
• 10:06
  Speaker 2: that's how you call it.
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  Speaker 1: In physics, we call it big G, or the universal
• 10:10
  Speaker 1: gravitational constant, because we want to distinguish it from little G,
• 10:13
  Speaker 1: which is the acceleration due to gravity here on Earth
• 10:17
  Speaker 1: nine point eight meters per second squared, which everybody uses
• 10:19
  Speaker 1: in their freshman physics class, and big G is the
• 10:23
  Speaker 1: number that appears in like Newton's equation for gravity.
• 10:26
  Speaker 2: Mmm, do you think G has anyone asked, gieve it
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  Speaker 2: minds being called big G.
• 10:33
  Speaker 1: I'm glad that you're always thinking about these things from
• 10:35
  Speaker 1: the point of view of the subject. You know, in
• 10:37
  Speaker 1: physics we tend not to anthropomorphize everything. But I'm glad
• 10:41
  Speaker 1: that somebody out there is looking out for the little
• 10:43
  Speaker 1: g's and the big g's of the world.
• 10:45
  Speaker 2: Well, that's what cartoonists are here for, to anthrop forum
• 10:48
  Speaker 2: orphize everything, even physics, I guess, But I guess it's
• 10:51
  Speaker 2: big G, like like you said, to distinguish it from
• 10:54
  Speaker 2: the little G that I think most people are familiar
• 10:56
  Speaker 2: with from like high school physics. Right, little G is
• 10:58
  Speaker 2: the one that tells you the exceleration of gravity here
• 11:01
  Speaker 2: on Earth, Like if you drop a ball, it's going
• 11:03
  Speaker 2: to accelerate at nine point eight meters per second square.
• 11:05
  Speaker 2: That's little G. Uppercase G is the more general gravitational constant.
• 11:10
  Speaker 1: That's right. Little G is only relevant on the surface
• 11:12
  Speaker 1: of the Earth. If you go up in an airplane
• 11:14
  Speaker 1: and you go deep down into the Earth, you're going
• 11:16
  Speaker 1: to feel a different acceleration due to gravity because you're
• 11:19
  Speaker 1: going to have a different amount of mass of the Earth,
• 11:21
  Speaker 1: or be a different distance from the Earth. And on
• 11:23
  Speaker 1: other planets or in other solar systems. Little g's are
• 11:27
  Speaker 1: totally irrelevant number. I mean, there's like a medium G
• 11:30
  Speaker 1: and a smallish G. There's a G junior and a
• 11:33
  Speaker 1: G the third and all sorts of gs. Men is
• 11:35
  Speaker 1: the OG you know, a G whiz. But big G
• 11:42
  Speaker 1: is universal. It's supposed to reflect something about the universe
• 11:45
  Speaker 1: itself and be independent of anything that happens on Earth
• 11:48
  Speaker 1: or the size of the Earth, or your distance from
• 11:50
  Speaker 1: the Earth. It's something about the universe, not something about
• 11:53
  Speaker 1: our neighborhood.
• 11:55
  Speaker 2: Right, So this is like the G that relates to
• 11:57
  Speaker 2: the actual force of gravity exactly.
• 11:59
  Speaker 1: It's the numbers that controls really the strength of gravity
• 12:02
  Speaker 1: in the universe.
• 12:03
  Speaker 2: Well, as usually, we were wondering how many people out
• 12:05
  Speaker 2: there had thought about the uppercase G of the universe
• 12:09
  Speaker 2: and how we might measure it.
• 12:11
  Speaker 1: So thank you very much to everybody who answers these
• 12:13
  Speaker 1: questions for this fun segment of the podcast. We love
• 12:16
  Speaker 1: hearing your thoughts, and if you would like to share
• 12:19
  Speaker 1: your thoughts for a future segment, please write to me
• 12:22
  Speaker 1: too questions at Danielandjorge dot com. Everyone wants to hear
• 12:26
  Speaker 1: your voice.
• 12:27
  Speaker 2: So think about it. For a second, how do you
• 12:29
  Speaker 2: think the universal gravitational constant G is measured? Here's what
• 12:33
  Speaker 2: people have to say.
• 12:34
  Speaker 4: I assume gravitational constant G is unique to planet Earth,
• 12:37
  Speaker 4: which exerts gravity upon us. To calculate that constant, I
• 12:41
  Speaker 4: would try to figure out how much force is necessary
• 12:44
  Speaker 4: for us to go against the gravitational force of Earth,
• 12:48
  Speaker 4: and then we know what the gravitational force itself is.
• 12:51
  Speaker 2: I think we measure the gravitational constant by measuring how
• 12:55
  Speaker 2: quickly galaxies and moving away from us and stretching the
• 12:59
  Speaker 2: space in between.
• 13:00
  Speaker 1: This is an easy one for me to answer, because
• 13:02
  Speaker 1: I don't know what the gravitational constant G is, but
• 13:05
  Speaker 1: I'm looking forward to hearing and finding out.
• 13:07
  Speaker 5: I guess it's true. The observation of planets, stars and
• 13:10
  Speaker 5: other celes your bodies, and coming to a number that
• 13:14
  Speaker 5: adjusts that motion to our other units of measuring.
• 13:19
  Speaker 3: Probably something to do at the moon. See we can
• 13:22
  Speaker 3: estimate its mass, estimate the Earth's mass, then see what's
• 13:27
  Speaker 3: going on there, and we can probably find a G.
• 13:29
  Speaker 2: All right, Well, somebody here confusedly with little G. You
• 13:32
  Speaker 2: know they do look alike.
• 13:33
  Speaker 1: I guess do theyre?
• 13:34
  Speaker 3: Though?
• 13:35
  Speaker 1: Big capital G and little G look pretty different. I
• 13:38
  Speaker 1: sent this an email, so I wrote explicitly big G. Hmmmm,
• 13:42
  Speaker 1: I'm not giving a lot of partial credit on that one.
• 13:44
  Speaker 2: Oh boy, there's pointage involved here. Do you get a grade?
• 13:48
  Speaker 1: I'm handing out degrees over here.
• 13:50
  Speaker 2: And I don't think that's going to encourage people to
• 13:52
  Speaker 2: call it.
• 13:53
  Speaker 1: Well, look, if you want to get your PhD in
• 13:54
  Speaker 1: podcast science, then you know you got to take it
• 13:56
  Speaker 1: for credit.
• 13:57
  Speaker 2: None of this past fial stuff I see. But everyone
• 14:00
  Speaker 2: gets an A though, right.
• 14:02
  Speaker 1: Yeah, I'm a softy. I'm not going to give you
• 14:04
  Speaker 1: a lot of.
• 14:05
  Speaker 2: A little A or a capital A.
• 14:08
  Speaker 1: I give everyone a big A plus. I'm a softy
• 14:10
  Speaker 1: in the end. All right.
• 14:11
  Speaker 2: But some interesting answers here, Like some people are saying
• 14:13
  Speaker 2: you can measure the gravitational constant by looking at planets
• 14:16
  Speaker 2: and stars and how things move around in space.
• 14:18
  Speaker 1: Yeah, those are interesting ideas, but fundamentally they don't work
• 14:21
  Speaker 1: because they don't let you establish what G is because
• 14:24
  Speaker 1: you don't know what the masses of those planets are,
• 14:26
  Speaker 1: and so there's too many unknowns in that equation.
• 14:30
  Speaker 2: And somebody said it has something to do with the moon,
• 14:32
  Speaker 2: like maybe you can measure G using the moon.
• 14:34
  Speaker 1: Yeah, and again, you could measure G using the Moon
• 14:37
  Speaker 1: and the Earth if you knew very very precisely the
• 14:41
  Speaker 1: mass of the Earth and the mass of the Moon.
• 14:43
  Speaker 1: But if you don't, then you can't use that to
• 14:46
  Speaker 1: measure G.
• 14:46
  Speaker 2: All right, let's dig into it. Lets first of all
• 14:48
  Speaker 2: define for our audience, what is the gravitational constant G.
• 14:52
  Speaker 1: So the gravitational constant G is the number that defines
• 14:55
  Speaker 1: the strength of gravity, and it appears in Newtonian gravity
• 14:59
  Speaker 1: in his equation for the force between two objects that
• 15:03
  Speaker 1: have mass. So Newtonian gravity says that the force is
• 15:07
  Speaker 1: big G times one mass times the other mass divided
• 15:11
  Speaker 1: by the distance squared, So gmm over R squared, and
• 15:15
  Speaker 1: that number G is the one that controls it. If
• 15:17
  Speaker 1: G was bigger, you would have a larger force between objects,
• 15:20
  Speaker 1: and if G was smaller, you would have a smaller
• 15:23
  Speaker 1: force between objects of the same mass and at the
• 15:25
  Speaker 1: same distance. So it's really just this like tunable parameter.
• 15:29
  Speaker 2: And it's kind of what determines how strong the gravity
• 15:32
  Speaker 2: is between two things. Basically, right, Like, what's the basic
• 15:35
  Speaker 2: Newtonian formula for gravity?
• 15:36
  Speaker 1: Exactly? The Newtonian formula for gravity has big G in it.
• 15:39
  Speaker 1: It's just gmm over R squared. And it's a similar
• 15:43
  Speaker 1: structure to other forces, right, Like the electrostatic repulsion between
• 15:47
  Speaker 1: two objects like two electrons or whatever, has the same structure,
• 15:51
  Speaker 1: and it also has a constant in front of it.
• 15:53
  Speaker 1: It's a different constant. So each of the forces you
• 15:55
  Speaker 1: can write using this kind of equation, and each one
• 15:57
  Speaker 1: has a constant in front of it that tells you
• 15:59
  Speaker 1: how powerful the force is.
• 16:01
  Speaker 2: So like, if you had two things floating out there
• 16:03
  Speaker 2: in space, a mass one and a mass two, you
• 16:06
  Speaker 2: can compute the force that attracts them together. We're using
• 16:09
  Speaker 2: this formula, right. You take the mass of one thing,
• 16:11
  Speaker 2: and then take the mass of the other thing. You
• 16:13
  Speaker 2: multiply together. You divide by the square of the distance
• 16:17
  Speaker 2: between them, and then you take the number and that's
• 16:19
  Speaker 2: what you multiply by G to get the force of
• 16:22
  Speaker 2: gravity between them exactly.
• 16:24
  Speaker 1: And if we lived in a universe where G was
• 16:26
  Speaker 1: twice as big, or if the cartoonist at the control
• 16:29
  Speaker 1: panel fell asleep on the knob and doubled big G,
• 16:32
  Speaker 1: then all the forces of gravity would be twice as big.
• 16:35
  Speaker 1: And if you divided G by a factor of two.
• 16:38
  Speaker 1: If you made it twice as small, then the force
• 16:40
  Speaker 1: of gravity would be twice as small.
• 16:42
  Speaker 2: Yeah, And so that's why it's called the universal gravitational constant,
• 16:45
  Speaker 2: because it's supposed to be the same all over the universe, right, Like,
• 16:48
  Speaker 2: if you measured the gravity between two things here or
• 16:51
  Speaker 2: in another planet or in another part of the galaxy,
• 16:54
  Speaker 2: you should be able to use the same constant G
• 16:56
  Speaker 2: to calculate that force exactly.
• 16:58
  Speaker 1: And what if Newton's great achievement was using this to
• 17:01
  Speaker 1: describe gravity here on Earth between fairly small masses and
• 17:04
  Speaker 1: small distances, and gravity between planets and stars and moons,
• 17:09
  Speaker 1: to show that it works in lots of different settings
• 17:12
  Speaker 1: over huge differences and masses and huge differences in distances.
• 17:15
  Speaker 1: So you sort of unified the heavens and the Earth
• 17:18
  Speaker 1: in that sense. So yeah, it's supposed to be universal.
• 17:21
  Speaker 2: Yeah, And it's pretty amazing that the formula is so
• 17:23
  Speaker 2: simple if you think about it, right, it's like one multiplication,
• 17:26
  Speaker 2: one division, and one squared and boom, you can like
• 17:30
  Speaker 2: decipher the workings of the universe, you know, Like it's
• 17:33
  Speaker 2: not like the one point seventh square root of the
• 17:36
  Speaker 2: distance between them nuts, it's like the square of the
• 17:39
  Speaker 2: distance between them. And it's just like mass one times
• 17:41
  Speaker 2: mass too. It's not like mass one plus seventeen divided
• 17:45
  Speaker 2: by five point two. You know.
• 17:47
  Speaker 1: Yeah, it is kind of cool, and the structure sort
• 17:50
  Speaker 1: of makes sense. Like, first of all, it has to
• 17:52
  Speaker 1: be symmetric. It can't be like mass one times mass
• 17:55
  Speaker 1: two squared, right, because it needs to be the same
• 17:58
  Speaker 1: force between mass one and mass two. At mass two
• 18:00
  Speaker 1: and mass one, right, it shouldn't matter which one you
• 18:02
  Speaker 1: call mass one or mass two, So it has to
• 18:05
  Speaker 1: be symmetric. It also makes sense that it's one over
• 18:07
  Speaker 1: the distance squared because as things get further and further apart,
• 18:11
  Speaker 1: the force gets diluted over a larger and larger area,
• 18:14
  Speaker 1: and the surface area of that sphere grows with the
• 18:17
  Speaker 1: distance squared, so the same way like if you have
• 18:20
  Speaker 1: a light source like a star, and you're twice as
• 18:22
  Speaker 1: far away from it, then the same number of photons
• 18:24
  Speaker 1: are now distributed over a larger sphere. That sphere has
• 18:27
  Speaker 1: four times the area, and so one of our distance square.
• 18:30
  Speaker 1: It really makes sense, and I think that's why it
• 18:32
  Speaker 1: appears in all of the force laws, not just the
• 18:34
  Speaker 1: one for gravity, the one for electromagnetism also has a
• 18:38
  Speaker 1: one over distance squared.
• 18:40
  Speaker 2: Right, it makes sense. But it didn't have to be
• 18:41
  Speaker 2: that way, right, it could have been r to the
• 18:43
  Speaker 2: one point seventy two or something like that.
• 18:46
  Speaker 1: Right, Yeah, it could have been. And there are actually
• 18:48
  Speaker 1: theories of gravity that do change that that suggest that
• 18:51
  Speaker 1: gravity changes at very small distances maybe or at different accelerations.
• 18:56
  Speaker 1: So it's not one over are squared, but one of
• 18:58
  Speaker 1: our squared is also the simpler. But you're right, the
• 19:00
  Speaker 1: universe didn't have to make sense, and it doesn't have
• 19:02
  Speaker 1: to be simple. It could be crazy complicated.
• 19:05
  Speaker 2: All right, Well, that's the universal gravitational constant uppercase G,
• 19:09
  Speaker 2: which tells you the strength of the force of gravity
• 19:11
  Speaker 2: in the universe. But as we know, gravity is not
• 19:13
  Speaker 2: quite a force, and also maybe this constant can change,
• 19:16
  Speaker 2: so let's dig into that. But first let's take a
• 19:19
  Speaker 2: quick break. All right, we're talking about the universal gravitational
• 19:34
  Speaker 2: constant uppercase G. It basically kind of tells you the
• 19:38
  Speaker 2: general strength of the force of gravity in the universe. Right, Like,
• 19:41
  Speaker 2: if G was much bigger, then the gravity would be
• 19:45
  Speaker 2: much stronger in the universe. If it was smaller, gravity
• 19:47
  Speaker 2: would be much weaker.
• 19:48
  Speaker 1: Exactly, and we don't know why it has this value.
• 19:51
  Speaker 1: There's nothing in physics that says it should be this number.
• 19:54
  Speaker 1: There's no set of equations you can use to like
• 19:56
  Speaker 1: derive it or predict this value. It's just something we
• 19:59
  Speaker 1: have to go out and measure and discover in the universe.
• 20:02
  Speaker 2: Right. So the origin of this constant is that it
• 20:05
  Speaker 2: came from Newtont's right, Newtent's laws about the force of
• 20:09
  Speaker 2: gravity between like planets and the sun and things like that.
• 20:13
  Speaker 2: But nowadays we think of gravity more like a bending
• 20:15
  Speaker 2: of space. Does the gravitational constant G still come into play?
• 20:19
  Speaker 1: Then it actually does. The same constant G also appears
• 20:23
  Speaker 1: in Einstein's equation. So we've replaced Newtonian physics that says
• 20:27
  Speaker 1: that there's a force between masses and that pulls them
• 20:30
  Speaker 1: together by saying, actually, there's no force there. It just
• 20:34
  Speaker 1: looks like a force. What's really happening is that masses
• 20:37
  Speaker 1: are bending space, and when they move through that bent space,
• 20:42
  Speaker 1: it looks like there's a force on them. And Einstein
• 20:45
  Speaker 1: gives us equations that describe how that space is bent
• 20:48
  Speaker 1: when mass is around, and those equations the Einstein field
• 20:51
  Speaker 1: equations also have constants in them, and one of those
• 20:54
  Speaker 1: constants is big G, the same exact number, And that
• 20:57
  Speaker 1: shouldn't be a surprise because einstein field equations also reproduce
• 21:02
  Speaker 1: all the predictions of Newtonian physics, like Einstein and Newton
• 21:06
  Speaker 1: agree about the force on the Earth from the sun,
• 21:08
  Speaker 1: for example, because you know, Newtonian physics got a lot
• 21:10
  Speaker 1: of stuff right, So it wouldn't make sense if they
• 21:12
  Speaker 1: had totally different constants.
• 21:14
  Speaker 2: Now that Einstein sort of derived this constant independently, or
• 21:17
  Speaker 2: did he like start with Newton's equations and kept it in.
• 21:20
  Speaker 1: You can't derive this constant, right. If we didn't have
• 21:23
  Speaker 1: Newton and we just started with Einstein, he would have
• 21:25
  Speaker 1: come up with his field equations and said, okay, but
• 21:27
  Speaker 1: there's a number in it, and I don't know what
• 21:29
  Speaker 1: that number is. Let's go out and measure it. In
• 21:32
  Speaker 1: the same way that when we first got Newton's equations.
• 21:35
  Speaker 1: There's a constant in there, and we have to go
• 21:36
  Speaker 1: out and measure it. And Newton actually suggested some ways
• 21:39
  Speaker 1: to go and measure this. So Einstein just sort of
• 21:42
  Speaker 1: inherited this constant from Newton.
• 21:45
  Speaker 2: Interesting, all right, Well, then what's the current value of
• 21:48
  Speaker 2: what we think G or uppercase G is.
• 21:50
  Speaker 1: So it's a really tiny number. It's six point six
• 21:54
  Speaker 1: seven times ten to the minus eleven, and the units
• 21:58
  Speaker 1: on it are kind of weird. It's meters cubed divided
• 22:01
  Speaker 1: by kilograms times seconds squared. It's this very small number
• 22:05
  Speaker 1: like ten to the minus eleven.
• 22:07
  Speaker 2: WHOA, So that's one of the reasons kind of why
• 22:10
  Speaker 2: gravity is so weak too, right, because the G is
• 22:12
  Speaker 2: such a.
• 22:12
  Speaker 1: Small number, that's exactly the reason why gravity is so weak.
• 22:16
  Speaker 1: If G was much much bigger, gravity would be much
• 22:19
  Speaker 1: more powerful. And so this number is the number for
• 22:22
  Speaker 1: G in our universe, and we don't know are there
• 22:24
  Speaker 1: other universes out there with different values for G that
• 22:27
  Speaker 1: have like much more powerful gravity and they all collapsed
• 22:30
  Speaker 1: into black holes to a few seconds after being birthed.
• 22:32
  Speaker 1: Are the universes out there with even weaker g's and
• 22:35
  Speaker 1: those universes still haven't even made stars because there isn't
• 22:38
  Speaker 1: a powerful enough gravity to pull that stuff together. We
• 22:41
  Speaker 1: just don't know if there are other options for this
• 22:43
  Speaker 1: thing and why we have this value. But you're right,
• 22:45
  Speaker 1: it completely controls the strength of gravity. And what's super
• 22:49
  Speaker 1: weird about it being so small is that the other
• 22:51
  Speaker 1: forces have much bigger constants, which is why gravity is
• 22:55
  Speaker 1: so much weaker than all of the other forces.
• 22:57
  Speaker 2: Well, I feel like you're blaming the gravitational constant here,
• 23:00
  Speaker 2: but it could also just be like things don't have
• 23:02
  Speaker 2: enough mass, Like maybe things were more massive, do you
• 23:05
  Speaker 2: know what I mean, and then the force of gravity
• 23:07
  Speaker 2: would be stronger.
• 23:08
  Speaker 1: Yeah, exactly. There's a subtlety there in how you compare
• 23:11
  Speaker 1: different forces, Like how do you compare electromagnetism and gravity. Well,
• 23:16
  Speaker 1: you take objects that have mass and have charge, like protons,
• 23:21
  Speaker 1: and you hold them apart at a certain distance, and
• 23:23
  Speaker 1: you calculate their relative strengths. And so, for example, if
• 23:25
  Speaker 1: you hold two protons like a centimeter apart, then you
• 23:29
  Speaker 1: discover that gravity is ten to the thirty three times
• 23:33
  Speaker 1: weaker than the electromagnetic force. But you might say, hold
• 23:36
  Speaker 1: on a second, that's just because protons have almost no
• 23:39
  Speaker 1: mass and a big charge. If we lived in a
• 23:41
  Speaker 1: universe where protons had tiny charge and huge masses, then
• 23:45
  Speaker 1: you would say gravity is stronger. And yeah, you're absolutely right,
• 23:47
  Speaker 1: But the kind of things that exist in our universe
• 23:50
  Speaker 1: tend to have a certain amount of mass per charge,
• 23:53
  Speaker 1: and that means that gravity ends up being really really
• 23:55
  Speaker 1: weak compared to electromagnetism. So yeah, blame it on the constant,
• 23:59
  Speaker 1: or blame it on the particles. But it's somebody's fault.
• 24:02
  Speaker 2: It's somebody's fault that the things that you don't weigh more,
• 24:04
  Speaker 2: or that you do weigh a lot.
• 24:07
  Speaker 1: Everything is somebody's fault, somebody else's fault. Right in this case,
• 24:11
  Speaker 1: I think it is a fair comparison to say, typical
• 24:13
  Speaker 1: particles in the universe, what is their relative gravity versus
• 24:16
  Speaker 1: their relative electromagnetic repulsion? And what you find is that
• 24:19
  Speaker 1: they're not even close. Right, Like gravity is weaker. It's
• 24:23
  Speaker 1: not even a little bit weaker or a lot weaker.
• 24:25
  Speaker 1: It's ridiculously weaker. It's ten to the thirty three times weaker.
• 24:29
  Speaker 1: It's like negligible compared to the force of electromagnetism. And
• 24:33
  Speaker 1: that's due to this constant, Like electromagnetism has its own constant,
• 24:37
  Speaker 1: and it's just a much bigger number, right.
• 24:39
  Speaker 2: I think what you mean is like if I had
• 24:40
  Speaker 2: two protons out there in space and I bring them
• 24:43
  Speaker 2: close together, like the force of electromagnetism, that's going to
• 24:46
  Speaker 2: be repelling them is like thirty two orders of magnetude
• 24:50
  Speaker 2: more than the force of gravity bringing them together exactly.
• 24:53
  Speaker 2: All right, well, let's dig into what it takes to
• 24:56
  Speaker 2: measure the gravitational constant G. It's pretty hard, right because
• 25:00
  Speaker 2: as we're as we're saying, gravity is super weak.
• 25:02
  Speaker 1: Yeah, there's like three reasons why measuring big G is
• 25:05
  Speaker 1: actually really really hard. Number one is what you said
• 25:08
  Speaker 1: that gravity is weak. You know, it's not easy to
• 25:11
  Speaker 1: measure these things because you need big masses. You can't
• 25:14
  Speaker 1: really measure the gravity between two protons. It's so small
• 25:17
  Speaker 1: that you could never really measure it. So you need
• 25:19
  Speaker 1: bigger and bigger objects. And that brings you to the
• 25:21
  Speaker 1: second reason why it's so difficult, which is that it's
• 25:24
  Speaker 1: hard to shield gravity from other things. Like you're always
• 25:28
  Speaker 1: going to be feeling the gravity of everything else around you,
• 25:31
  Speaker 1: you know, like your laboratory and the mountains and the Earth.
• 25:34
  Speaker 1: So it's hard to get like an isolated system to
• 25:37
  Speaker 1: study gravity.
• 25:38
  Speaker 2: What do you mean isolated? Like because the Earth is
• 25:40
  Speaker 2: pulling you down with gravity, but you can still maybe
• 25:42
  Speaker 2: measure gravity. It's side to side, right, like if I
• 25:45
  Speaker 2: just put two balls on my table technically, they are
• 25:47
  Speaker 2: being attracted to each other by gravity. Couldn't I measure that?
• 25:50
  Speaker 1: Yeah, you certainly could. But they're also being attracted by
• 25:53
  Speaker 1: the gravity of your wall, and the gravity of the
• 25:55
  Speaker 1: tree outside, and the gravity of the mountains nearby. And
• 25:59
  Speaker 1: that's not true for other forces. Like for electromagnetism, you
• 26:02
  Speaker 1: can you have positive and negative charges, and so you
• 26:05
  Speaker 1: can shield things. You can like balance all the forces
• 26:07
  Speaker 1: out so that electromagnetism is effectively zeroed out and study
• 26:11
  Speaker 1: it at small scales. But for gravity, there's no way
• 26:13
  Speaker 1: to shield your laboratory from the gravity of your surroundings
• 26:17
  Speaker 1: unless you get like really really far away from everything.
• 26:20
  Speaker 2: What do you mean, Like, I can't just conduct my
• 26:22
  Speaker 2: experiment on a really tall tower or you know, at
• 26:25
  Speaker 2: the top of a mountain, or maybe even on a satellite.
• 26:28
  Speaker 1: Yeah, on a satellite would be great. The further you
• 26:30
  Speaker 1: can get from other masses, the better you could do
• 26:33
  Speaker 1: this experiment. So if you did your experiment measuring the
• 26:36
  Speaker 1: gravitational strength between two objects, like out in the middle
• 26:39
  Speaker 1: of a super bubble, far away from everything, that would
• 26:41
  Speaker 1: be great. But that's one of the challenges, right we
• 26:43
  Speaker 1: don't have the way to do that experiment out in
• 26:45
  Speaker 1: the middle of space. We have to do our experiments
• 26:47
  Speaker 1: here in the vicinity of Earth, which has its own gravity.
• 26:51
  Speaker 2: Well, it sounds like maybe the difficulty is in like
• 26:53
  Speaker 2: isolating it. It's more like it's super weak, right, because
• 26:57
  Speaker 2: like you could do it through this experiment out in
• 26:58
  Speaker 2: a satellite, right and and you know the as it
• 27:01
  Speaker 2: goes around the Earth, things would cancel out anyways. Right.
• 27:04
  Speaker 1: Yeah, it's just one reason why it's difficult. The primary
• 27:07
  Speaker 1: reason I think is that gravity is just so weak.
• 27:09
  Speaker 1: You know, you're trying to measure a very very small
• 27:11
  Speaker 1: effect and it's swamped by all sorts of other effects.
• 27:15
  Speaker 1: You know, if you have, for example, two masses and
• 27:17
  Speaker 1: you want to measure their gravity between them, then you
• 27:19
  Speaker 1: have to hope that there are no other forces bigger
• 27:22
  Speaker 1: than the gravitational force also operating on those masses that
• 27:25
  Speaker 1: would just swamp your measurement. You know, if there's like
• 27:28
  Speaker 1: a tiny residual electric charge on these masses because you
• 27:31
  Speaker 1: touch them and you got static electricity on them, it
• 27:33
  Speaker 1: will be so much more powerful than the gravitational force
• 27:36
  Speaker 1: you're trying to measure that it will just swamp your measurements.
• 27:39
  Speaker 2: Well, maybe we should paint a picture here for people, like,
• 27:41
  Speaker 2: how would you even design an experiment like this? Like,
• 27:44
  Speaker 2: let's say I'm proposing to you, Daniel, that we go
• 27:46
  Speaker 2: up in the Space Shuttle or we go out in
• 27:48
  Speaker 2: a rocket out to the International Space Station. I'm going
• 27:51
  Speaker 2: to take two Billard balls, put them, you know, ten
• 27:54
  Speaker 2: centimeters apart, and then I'm going to watch how long
• 27:56
  Speaker 2: it takes them to get attracted to each other by gravity.
• 28:00
  Speaker 2: Wrong with that experiment?
• 28:01
  Speaker 1: Yeah, you could do that, that would work. Nothing that's
• 28:03
  Speaker 1: wrong with that experiment except that it requires going up
• 28:06
  Speaker 1: to space, and also you have to account for all
• 28:09
  Speaker 1: the other masses nearby, right, Like the space station is
• 28:12
  Speaker 1: also going to be tugging on these things, and the
• 28:14
  Speaker 1: space station is probably a lot more massive than the
• 28:16
  Speaker 1: balls you brought. You can't bring super duper heavy balls
• 28:20
  Speaker 1: up into space because you have limitations on the expense
• 28:23
  Speaker 1: due to the launches.
• 28:24
  Speaker 2: You mean, like I said, I have those two balls
• 28:26
  Speaker 2: leading in front of me. They're being pulled together by
• 28:28
  Speaker 2: the gravity they have with each other, but maybe they're
• 28:30
  Speaker 2: also being pulled apart a little bit by the space
• 28:33
  Speaker 2: station around them, right, Or like if my fellow astronauts
• 28:37
  Speaker 2: it's to the right of me or to the left
• 28:38
  Speaker 2: of me. It might influence how those two balls come
• 28:41
  Speaker 2: together exactly.
• 28:42
  Speaker 1: And because you're trying to measure something very very small,
• 28:45
  Speaker 1: then you need to be very very accurate about your measurements,
• 28:48
  Speaker 1: and small changes in your results can lead to large
• 28:51
  Speaker 1: changes in the results that you get.
• 28:53
  Speaker 2: What if I just do it a lot, or what
• 28:55
  Speaker 2: if I tell everyone to stay still not move in
• 29:01
  Speaker 2: the space station? Wouldn't that give me a pretty good experiment?
• 29:04
  Speaker 1: Yeah, that would measure it. I don't think that would
• 29:06
  Speaker 1: come close to the precision we have today. And also
• 29:08
  Speaker 1: it would be really expensive. Everything out in space is
• 29:11
  Speaker 1: very expensive and very complicated.
• 29:13
  Speaker 2: All right, What are some of the other reasons that
• 29:15
  Speaker 2: make it difficult?
• 29:16
  Speaker 1: I think the last reason is just that there's no
• 29:18
  Speaker 1: like relationship to the other constants, you know, the other forces.
• 29:22
  Speaker 1: We think there might be some relationship with them. Electromagnetism
• 29:25
  Speaker 1: and the weak forces can get bundled together into the
• 29:28
  Speaker 1: electroweak force, and there's some unity there. We have theories
• 29:31
  Speaker 1: about how the strong force might connect with that, and
• 29:33
  Speaker 1: so we have like a unity of the forces. But
• 29:36
  Speaker 1: gravity is by itself. We don't know how to bring
• 29:38
  Speaker 1: gravity into quantum physics, so we have no like way
• 29:41
  Speaker 1: to predict or like constrain the value of this force.
• 29:45
  Speaker 1: You really just have to go out and measure it.
• 29:47
  Speaker 1: There's no other way to analyze it, right.
• 29:49
  Speaker 2: I always thought the hard thing about measuring the gravitational
• 29:52
  Speaker 2: constant was that you know, to get a measurement of it,
• 29:54
  Speaker 2: you sort of need to know like in our biller
• 29:56
  Speaker 2: ball example, you sort of need to know exactly what
• 29:59
  Speaker 2: the masses of those biller balls are. But it's hard
• 30:02
  Speaker 2: to know what the mass of something is if you
• 30:04
  Speaker 2: don't already know the gravitational constant. Right, isn't that one
• 30:07
  Speaker 2: of the big problems. It's like a chicken and egg problem,
• 30:10
  Speaker 2: like how do you measure gravity. To measure gravity, you
• 30:12
  Speaker 2: need to know the mass of something. But to the
• 30:14
  Speaker 2: mass of something you need to weigh it, which you
• 30:16
  Speaker 2: need for which you need to know the gravitational constant.
• 30:19
  Speaker 1: Yeah, in the end, it comes down to what do
• 30:21
  Speaker 1: you know? First? If you know the masses of two objects,
• 30:23
  Speaker 1: you can measure the force between them, and then you
• 30:25
  Speaker 1: get big G. If you know big G and the forces,
• 30:28
  Speaker 1: then you can measure the masses between them. So the
• 30:31
  Speaker 1: basic story of measuring big G is finding a scenario
• 30:35
  Speaker 1: where we already for other reasons, know the masses of
• 30:39
  Speaker 1: two objects that we can use to measure big G.
• 30:42
  Speaker 1: That's the struggle. That's why we can't, for example, have
• 30:45
  Speaker 1: looked at the Earth in the sun hundreds of years
• 30:47
  Speaker 1: ago and used those to determine big G because we
• 30:50
  Speaker 1: didn't know the masses of the Earth and the Sun.
• 30:52
  Speaker 1: To derive the masses of the Earth and the Sun
• 30:54
  Speaker 1: from like their relative motion, you have to know the
• 30:56
  Speaker 1: force between them, and you'd have to know big G.
• 30:58
  Speaker 2: Well, maybe a step as through then, like what's the
• 31:01
  Speaker 2: history of trying to measure this universal constant?
• 31:03
  Speaker 1: So it goes all the way back to Newton right.
• 31:06
  Speaker 1: Newton described this relationship between stuff and there was a
• 31:08
  Speaker 1: constant there, and he suggested how you might measure this constant.
• 31:13
  Speaker 1: He said, maybe if you had, for example, like a
• 31:15
  Speaker 1: pendulum basically a heavy ball on a string, and you
• 31:18
  Speaker 1: brought it near something massive like a mountain, then you
• 31:21
  Speaker 1: might be able to measure the deflection of that ball
• 31:24
  Speaker 1: as it's like tugged on by the mountain. Interesting bit
• 31:27
  Speaker 1: of history, though, is that Newton didn't write down big G.
• 31:30
  Speaker 1: He never wrote that down doesn't appear in his works
• 31:34
  Speaker 1: because Newton was working at a time before we expressed
• 31:37
  Speaker 1: our theoretical laws in terms of algebraic expressions. Back then,
• 31:41
  Speaker 1: all of our physics was done in terms of sentences
• 31:44
  Speaker 1: rather than in terms of algebra.
• 31:46
  Speaker 2: I thought you were going to say, he did it
• 31:47
  Speaker 2: in a time before we started body shaming our letters.
• 31:52
  Speaker 1: No, So if you go back and like read the Principia,
• 31:54
  Speaker 1: you know, he expresses his law of gravity in terms
• 31:57
  Speaker 1: of a sentence. You know, he says it will be
• 31:59
  Speaker 1: mutually gravitating towards each other at a rate relative to
• 32:03
  Speaker 1: the reciprocal the square of their distances. You know, he
• 32:06
  Speaker 1: doesn't summarize at all in terms of mathematics. So he
• 32:08
  Speaker 1: never actually wrote down Big G. It wasn't until a
• 32:10
  Speaker 1: couple hundred years later that I started being called big G.
• 32:14
  Speaker 1: But Newton had the basic idea. He's like, if you
• 32:16
  Speaker 1: know the mass of a mountain and the mass of
• 32:18
  Speaker 1: a pendulum, maybe you could make this measurement.
• 32:21
  Speaker 2: Right, But again that's kind of the problem that you
• 32:23
  Speaker 2: don't really know the mass of the mountain.
• 32:25
  Speaker 1: Well, what you could do is measure the mass of
• 32:26
  Speaker 1: the mountain. You could say I know the density of rock,
• 32:29
  Speaker 1: and I could measure the volume of the mountain and
• 32:32
  Speaker 1: so from that I could estimate the mass of the mountain.
• 32:34
  Speaker 1: And this is actually what people did. The first measurements
• 32:37
  Speaker 1: of big G come from holding a pendulum near a
• 32:41
  Speaker 1: mountain in Scotland and seeing how it deflected.
• 32:44
  Speaker 2: No way, this actually.
• 32:45
  Speaker 1: Works, This actually works. Yes, it was a huge project.
• 32:49
  Speaker 1: This was done in the seventeen seventies. There's a mountain
• 32:53
  Speaker 1: in Scotland and it's a good choice because it's like
• 32:56
  Speaker 1: isolated from other mountains. It just sort of like sticks
• 32:59
  Speaker 1: up and like a nice symmetrical shape, which means that
• 33:02
  Speaker 1: it was not super complicated to describe its shape and
• 33:06
  Speaker 1: calculate its volume. And also it's like got really steep slopes,
• 33:10
  Speaker 1: so you get kind of close up to its center
• 33:12
  Speaker 1: of mass. Take the maximum effect. And there's like a
• 33:14
  Speaker 1: whole team of people that spent like years up there
• 33:17
  Speaker 1: making very precise measurements of pendula with heavy masses on
• 33:21
  Speaker 1: them and measuring their deflection and surveying the mountain to
• 33:24
  Speaker 1: try to estimate its volume as precisely as possible. It's
• 33:27
  Speaker 1: a huge project.
• 33:28
  Speaker 2: Wait, what so like if I hang a Billard ball
• 33:31
  Speaker 2: from a string right and hold it in front of me.
• 33:34
  Speaker 2: It's going to hang straight down. But as I walk
• 33:36
  Speaker 2: towards a big mountain, it's going to start to lean
• 33:39
  Speaker 2: or get pulled and actually start swinging that way.
• 33:43
  Speaker 1: Yeah, it will get pulled towards that mountain, so its
• 33:45
  Speaker 1: resting position will not be straight down. If you follow
• 33:49
  Speaker 1: the string up, it will not point perfectly towards the zenith.
• 33:52
  Speaker 1: It will be slightly deflected. And the bigger the mass
• 33:55
  Speaker 1: of the mountain, or the bigger the value of big G,
• 33:59
  Speaker 1: the stronger the deflection. So if you know the mass
• 34:01
  Speaker 1: of the mountain, then you can measure big g s.
• 34:04
  Speaker 1: This is the whole game, is knowing the masses of
• 34:06
  Speaker 1: two things and then measuring the forces between them. There's
• 34:09
  Speaker 1: a fun little wrinkle here, though. You might think that
• 34:12
  Speaker 1: you also need to know the mass of the earth,
• 34:14
  Speaker 1: because that's also pulling on your pendulum. But if you
• 34:17
  Speaker 1: know the volume of the mountain and the volume of
• 34:20
  Speaker 1: the Earth, which we do, then the angle of deflection
• 34:24
  Speaker 1: of the pendulum depends on the relative densities of the
• 34:27
  Speaker 1: Earth and the mountain, which one is denser. So really
• 34:32
  Speaker 1: the experiment measures the density of the Earth, which wasn't
• 34:36
  Speaker 1: known at the time. Of course, knowing the density of
• 34:38
  Speaker 1: the earth and the volume lets you calculate the mass
• 34:40
  Speaker 1: of the earth, and therefore let you get big g m.
• 34:44
  Speaker 2: I guess you could use this to like measure people's
• 34:46
  Speaker 2: masses too. Like if you walk around a better ball
• 34:49
  Speaker 2: on the string and just kind of walk around and
• 34:51
  Speaker 2: get it close to people, you could technically right measure
• 34:54
  Speaker 2: their mass.
• 34:55
  Speaker 1: Technically, yes, you could measure their mass.
• 34:58
  Speaker 2: And be like, hey, big Daniel, I mean uppercase Daniel.
• 35:01
  Speaker 1: If somebody had like accidentally ingested a lot of heavy metal,
• 35:04
  Speaker 1: you could detect it. Yeah, absolutely all right.
• 35:06
  Speaker 2: So that was in the seventeen hundred And they did
• 35:08
  Speaker 2: this and what did they find? What value did they
• 35:11
  Speaker 2: come up with?
• 35:11
  Speaker 1: So they made a measurement of this thing and they
• 35:13
  Speaker 1: got the number right to within about twenty percent. So
• 35:16
  Speaker 1: they measured this, which is I think pretty awesome. Like
• 35:19
  Speaker 1: this is a hard piece of work. There's one guy
• 35:21
  Speaker 1: who spent just like years calculating the volume of this
• 35:25
  Speaker 1: mountain from all these survey measurements. He turned it into
• 35:28
  Speaker 1: prisms and calculated the volume of each of those and
• 35:31
  Speaker 1: added them all up. And you know, this is before
• 35:33
  Speaker 1: computing and before any sort of modeling. This is in
• 35:35
  Speaker 1: the seventeen hundreds. He's working by like lamp light and
• 35:38
  Speaker 1: with a quill. But they got the right number within
• 35:41
  Speaker 1: twenty percent. So it's pretty impressive. And that also means
• 35:44
  Speaker 1: that they made the first real measurement of the density
• 35:47
  Speaker 1: of the Earth. They found that it was four and
• 35:50
  Speaker 1: a half times the density of water and almost twice
• 35:53
  Speaker 1: the density of that mountain in Scotland. That was a
• 35:56
  Speaker 1: bit of a surprise because we didn't know the internal
• 35:59
  Speaker 1: structure of the Earth in Newton's time. Some people thought
• 36:02
  Speaker 1: the Earth was like a huge hollow shell. So this number,
• 36:06
  Speaker 1: so much higher than the density of the mountain, was
• 36:09
  Speaker 1: a really fascinating early clue that the Earth has a
• 36:13
  Speaker 1: really very dense core. It's a really very cool result
• 36:16
  Speaker 1: with pretty basic tools.
• 36:18
  Speaker 2: Wow. Pretty cool. And so let's get maybe into some
• 36:21
  Speaker 2: of the other ways that people have measured this as
• 36:23
  Speaker 2: well as maybe the most recent measurements and see how
• 36:26
  Speaker 2: they measure up and it's weigh them together. But first
• 36:28
  Speaker 2: let's take another quick break. All right, we're talking about
• 36:43
  Speaker 2: the universal gravitational constant. We know that it's kind of
• 36:47
  Speaker 2: weak compared to the other forces, But it's super monumentally
• 36:50
  Speaker 2: important in the universe because it basically determines how stars
• 36:54
  Speaker 2: and planets form, how galaxies form, basically determines the whole
• 36:59
  Speaker 2: structure of the universe.
• 37:00
  Speaker 1: Yeah, exactly, it's one of the parameters on that control
• 37:03
  Speaker 1: panel of the universe that tells us why our universe
• 37:06
  Speaker 1: is this way and not some other way.
• 37:09
  Speaker 2: And so we're talking about how you actually measure this
• 37:11
  Speaker 2: because it's tricky because a gravity is so weak, but
• 37:14
  Speaker 2: also b you need to know the masses of things
• 37:17
  Speaker 2: before you can measure this constant. But to measure the
• 37:19
  Speaker 2: masses of things, you sort of need to know the constant.
• 37:22
  Speaker 2: And so people have tried different ways. They did it
• 37:24
  Speaker 2: first in the seventeen hundreds and they got within twenty percent.
• 37:28
  Speaker 2: What was the next step?
• 37:29
  Speaker 1: So that method holding a pendulum near a mountain worked,
• 37:32
  Speaker 1: but it was pretty imprecise because the mountain is like
• 37:35
  Speaker 1: a big fuzzy object. We don't really have a strong
• 37:37
  Speaker 1: handle on its density. You know, is it all the
• 37:40
  Speaker 1: same rock all the way through? What exactly is the
• 37:42
  Speaker 1: volume of it? And so people decided to shrink the
• 37:45
  Speaker 1: experiment down to something smaller that they could control. But
• 37:49
  Speaker 1: then you need a lot more precision because in the
• 37:51
  Speaker 1: effect it's going to be a lot lot smaller. So
• 37:53
  Speaker 1: instead of having one pendulum and a mountain, instead they
• 37:56
  Speaker 1: basically have two pendulam But if you just have like
• 37:59
  Speaker 1: two billion balls hanging near each other, the force between
• 38:02
  Speaker 1: them is so small that you're not going to be
• 38:04
  Speaker 1: able to measure any sort of deflection. So there was
• 38:07
  Speaker 1: a geologist, John Mitchell who came up with a really
• 38:10
  Speaker 1: clever way to measure a very very tiny force between
• 38:14
  Speaker 1: two billiard balls.
• 38:15
  Speaker 2: Essentially, how'd they do it?
• 38:17
  Speaker 1: So what they do is they have a pair of
• 38:19
  Speaker 1: these balls on a rod, and then they hang that
• 38:22
  Speaker 1: rod from a string, and then they bring two other
• 38:25
  Speaker 1: massive balls closer to these two balls on the rod,
• 38:28
  Speaker 1: and they measure how strongly the rod is attracted to
• 38:32
  Speaker 1: these other massive balls by measuring how far the string twists.
• 38:36
  Speaker 1: So instead of measuring like the deflection from the vertical,
• 38:39
  Speaker 1: which is a tiny, tiny amount, they can measure like
• 38:42
  Speaker 1: how far this thing has twisted this string it's hanging from.
• 38:46
  Speaker 1: So it's called a torsion balance.
• 38:49
  Speaker 2: Right, you're talking about a setup that's like what do
• 38:50
  Speaker 2: you call those like ornaments? You hang them from your ceiling.
• 38:53
  Speaker 1: It's a mobile.
• 38:54
  Speaker 2: A mobile, Yeah, that's kind of what you're talking about, right,
• 38:57
  Speaker 2: Like you make a mobile out of two biller balls
• 39:00
  Speaker 2: where you put them on a rod, and then you
• 39:02
  Speaker 2: hang the row from the center of it on a
• 39:03
  Speaker 2: string from the ceiling, and then you sort of see
• 39:06
  Speaker 2: how this mobile swings or turns when you put mass
• 39:10
  Speaker 2: bigger masses next to or near the to bigger ball exactly.
• 39:14
  Speaker 1: And so now you're measuring the angle of rotation of
• 39:17
  Speaker 1: your mobile basically as it turns towards the other balls.
• 39:21
  Speaker 1: And that's a little bit easier to measure than the
• 39:24
  Speaker 1: deflection relative to some vertical where you need to like
• 39:27
  Speaker 1: calibrate it to the stars. Here, you know how much
• 39:30
  Speaker 1: force it takes to twist this string. You can calibrate
• 39:33
  Speaker 1: that when the balls aren't around, And then you bring
• 39:36
  Speaker 1: the balls in and you see, like how much do
• 39:38
  Speaker 1: they twist the string? What is the equilibrium position between
• 39:42
  Speaker 1: the force that's trying to bring the mobile back to
• 39:44
  Speaker 1: its resting position and the force from the balls that's
• 39:47
  Speaker 1: pulling on it in the other direction.
• 39:48
  Speaker 2: The twisting of the string also tends to want to
• 39:51
  Speaker 2: bring it back to a neutral position, right.
• 39:54
  Speaker 1: Exactly If you just like twisted this thing up and
• 39:56
  Speaker 1: let it go. They would spin back eventually to its
• 39:59
  Speaker 1: resting position. And so just the way like a pendulum
• 40:02
  Speaker 1: is deflected by the mountain, here, this whole balance is
• 40:05
  Speaker 1: twisted a little bit by the presence of these other masses.
• 40:09
  Speaker 2: Interesting, and so they did this kind of at the
• 40:10
  Speaker 2: end of the seventeen hundreds, and how close did they get?
• 40:13
  Speaker 1: So Yeah, so this idea was by John Mitchell, a geologist. Unfortunately,
• 40:17
  Speaker 1: John Mitchell built the whole experiment and then died before
• 40:20
  Speaker 1: he could really use it. And it was Cavendish who
• 40:23
  Speaker 1: inherited this thing and did a bunch of really really
• 40:25
  Speaker 1: careful experiments, and he's the one for whom this experiment
• 40:28
  Speaker 1: is known. Unfortunately, Mitchell's sort.
• 40:30
  Speaker 2: Of lost line sounds very suspicious.
• 40:33
  Speaker 1: Yeah, lost to history, but you know Cavendish.
• 40:37
  Speaker 2: Seventeen hundred murder, Mystery Physics, it's a winning podcast episode, True.
• 40:41
  Speaker 1: Crime Science exactly. Anyway, he got a very precise measurement.
• 40:44
  Speaker 1: He measured it to within one percent of the true value.
• 40:48
  Speaker 1: And this was a big elaborate thing. It was like
• 40:49
  Speaker 1: a two meter wide box that this whole thing was in,
• 40:52
  Speaker 1: and he had to be enclosed in there to avoid
• 40:54
  Speaker 1: like air currents. He could only observe it through these
• 40:57
  Speaker 1: tiny little holes through which there were lenses. So it's
• 40:59
  Speaker 1: really elaborate setup, but it worked. And this is in
• 41:02
  Speaker 1: the late seventeen hundreds, and that was the most precise
• 41:05
  Speaker 1: measurement for about one hundred years.
• 41:07
  Speaker 2: Wow, it's pretty impressive what happened at one hundred years later.
• 41:10
  Speaker 1: So for the nexte of years, the folks who were
• 41:11
  Speaker 1: using the mountain method tried to beat Cavendish but failed.
• 41:14
  Speaker 1: They kept trying, like different mountains and different surveys, and
• 41:17
  Speaker 1: they spent lots of money and lots of time, and
• 41:19
  Speaker 1: sometimes they drank too much and actually like burned down
• 41:21
  Speaker 1: their whole facility. It's a very colorful history if you
• 41:24
  Speaker 1: look into it. But they never succeeded in beating Cavendish,
• 41:27
  Speaker 1: and wasn't until people improved on his torsion balance method.
• 41:30
  Speaker 1: But one hundred years later a scientist named CV Boys
• 41:35
  Speaker 1: was able to bring down the uncertainty, and people made
• 41:38
  Speaker 1: a little bit of progress over the next few decades,
• 41:40
  Speaker 1: so that like by the nineteen thirties we had a
• 41:43
  Speaker 1: measurement of it to within a tenth of one percent
• 41:45
  Speaker 1: and That sounds pretty good, right, But remember, like this
• 41:48
  Speaker 1: is a fundamental constant of the universe. Other constants we've
• 41:52
  Speaker 1: measured to like one part in a billion, so having
• 41:55
  Speaker 1: this down to like one part in one hundred or
• 41:57
  Speaker 1: one part in a thousand is not very impressive. It's
• 42:00
  Speaker 1: one of the worst measured physical constants in the universe.
• 42:04
  Speaker 2: Oh man, are you physics shaming those experimenters?
• 42:07
  Speaker 1: Now, I'm doing exactly the opposite. I'm saying, this is
• 42:10
  Speaker 1: so hard. It's a really, really difficult measurement. You know,
• 42:13
  Speaker 1: in order to do this, you have to completely isolate
• 42:15
  Speaker 1: your setup from everything else. You have to come up
• 42:18
  Speaker 1: with clever ways to account for everything to measure the
• 42:21
  Speaker 1: bias in your experiment. You know. The more recent measurements
• 42:24
  Speaker 1: people have been doing in the last few decades involve
• 42:27
  Speaker 1: clever tricks like put a mirror on the wire and
• 42:30
  Speaker 1: instead of measuring the angle of the balls, which is
• 42:33
  Speaker 1: really small, shine a laser on the mirror. Use the
• 42:35
  Speaker 1: motion of the laser spot to measure how much the
• 42:38
  Speaker 1: wire has twisted. These kind of tricks and all sorts
• 42:40
  Speaker 1: of other techniques to reduce the electrostatics on these balls.
• 42:44
  Speaker 1: It's really impressive. Amount of work.
• 42:46
  Speaker 2: I guess my main question is you're saying, like, we're
• 42:49
  Speaker 2: getting closer to the true measurement or the true value
• 42:52
  Speaker 2: of this constant. But how do you know what the
• 42:54
  Speaker 2: true value is? Like, how do you know you're only
• 42:56
  Speaker 2: tenth of a percent off or ten percent off? Like,
• 42:59
  Speaker 2: how do you know what act is supposed to be?
• 43:01
  Speaker 1: Yeah, that's a great point, and we don't know what
• 43:03
  Speaker 1: it's supposed to be. There's no prediction, right, so any
• 43:06
  Speaker 1: number could be the right number. And in the history
• 43:08
  Speaker 1: of these measurements, typically what happens is you have a
• 43:10
  Speaker 1: first measurement which is sloppy and rough, and then you
• 43:13
  Speaker 1: improve it and you get more and more precision. So
• 43:15
  Speaker 1: if you look at these things over time, they tend
• 43:17
  Speaker 1: to converge towards one value, which we say, oh, that
• 43:20
  Speaker 1: must be the true value. In reality, it doesn't always
• 43:22
  Speaker 1: work like that. We have some cases in history where
• 43:24
  Speaker 1: the value seems to converge to one number and then
• 43:26
  Speaker 1: it shifts, and that's because people know about the previous
• 43:30
  Speaker 1: results and they sort of want to reproduce the previous results.
• 43:33
  Speaker 1: So if like one of the first results was off
• 43:35
  Speaker 1: by a bit, then there's like an implicit bias in
• 43:38
  Speaker 1: people's experiments that tend to like find mistakes and bugs
• 43:41
  Speaker 1: until their number agrees with the previous number. It takes
• 43:44
  Speaker 1: a little bit more bravery and courage to disagree with
• 43:47
  Speaker 1: an established measurement. So you see those sort of like
• 43:49
  Speaker 1: jumps sometimes in the history of a measurement. This one
• 43:52
  Speaker 1: is particularly interesting because as the measurements have gotten more
• 43:55
  Speaker 1: and more precise, like in the last ten or fifteen years,
• 43:58
  Speaker 1: it's been a real cottage industry of making these measurements,
• 44:01
  Speaker 1: they've started to disagree. So now we have a bunch
• 44:04
  Speaker 1: of measurements of the gravitational constant with fairly small uncertainties
• 44:08
  Speaker 1: that disagree with each other by more than the uncertainties.
• 44:12
  Speaker 2: Interesting, So I think maybe when you say, like they
• 44:15
  Speaker 2: got within point one percent, you're not saying that that
• 44:18
  Speaker 2: it's not by point one percent. You're saying like their
• 44:20
  Speaker 2: confidence in their measurement is down towero point one percent,
• 44:24
  Speaker 2: like they think they're within that range. Right, It's more
• 44:27
  Speaker 2: like a measure of confidence.
• 44:29
  Speaker 1: Yeah, they usually quote and uncertainly they say it's this
• 44:31
  Speaker 1: number within a certain range. But we can also compare
• 44:34
  Speaker 1: their measurement to our current best understanding of what the
• 44:37
  Speaker 1: value is, so we can analyze their historical accuracy by
• 44:40
  Speaker 1: comparing it with modern measurements.
• 44:42
  Speaker 2: And so, is there no way to like derive this
• 44:45
  Speaker 2: from the equations of the universe or to you know,
• 44:48
  Speaker 2: tie back to some other more fundamental thing like the
• 44:51
  Speaker 2: mass of an electron, for example, or something.
• 44:53
  Speaker 1: No, there is not. There's no way to derive this.
• 44:56
  Speaker 1: It's totally unrelated to every other physical constant and every
• 45:00
  Speaker 1: other process in the universe. The gravitational constant doesn't just
• 45:04
  Speaker 1: control gravity. It only controls gravity. It doesn't determine anything
• 45:07
  Speaker 1: else in the universe. So there's no other way to
• 45:10
  Speaker 1: figure it out. The only way to do it is
• 45:12
  Speaker 1: to measure the force of gravity between two things. And
• 45:15
  Speaker 1: to do that you got to know their masses.
• 45:17
  Speaker 2: M we can't tell by you know, how light bends
• 45:21
  Speaker 2: around a black hole or something like that, or around
• 45:23
  Speaker 2: the sun.
• 45:24
  Speaker 1: Yes. Actually, as our calculations in general relativity get more
• 45:28
  Speaker 1: and more precise, we may be able to do things
• 45:30
  Speaker 1: like seeing how space is bent in the vicinity of
• 45:33
  Speaker 1: strong gravity, which might be able to give us a
• 45:36
  Speaker 1: new handle on how to measure this constant.
• 45:38
  Speaker 2: Pretty cool, But I guess until then that means like,
• 45:41
  Speaker 2: if our measurement of G is off by point one percent,
• 45:44
  Speaker 2: that means that any calculation that we make using G
• 45:47
  Speaker 2: is also off by at least that point one percent, right.
• 45:50
  Speaker 1: Mm hmm. Yeah. These days, we're down to about five
• 45:53
  Speaker 1: times ten to the negative five as a fractional uncertainty
• 45:56
  Speaker 1: on big G, so you know, like a few parts
• 46:00
  Speaker 1: per million, which is much more precise than historically but
• 46:03
  Speaker 1: still by far the worst measured constant. And you're right,
• 46:06
  Speaker 1: it means that we can't make very precise predictions about
• 46:09
  Speaker 1: what happens near black hole because they depend on big G.
• 46:13
  Speaker 1: So you need super precise measurements to nail big G
• 46:16
  Speaker 1: so we can then make super precise predictions.
• 46:19
  Speaker 2: It kind of sounds like maybe we'll never know the
• 46:21
  Speaker 2: true value of G.
• 46:22
  Speaker 1: It might be because the true value of G has
• 46:23
  Speaker 1: an infinite number of digits in it. In that sense,
• 46:26
  Speaker 1: will never know the true value of anything, even like
• 46:29
  Speaker 1: you know the mass of an electron or any other parameter,
• 46:32
  Speaker 1: because it has an infinite number of digits and you
• 46:34
  Speaker 1: can't have an infinite amount of experiments or an infinite
• 46:36
  Speaker 1: number of graduate students to measure them.
• 46:38
  Speaker 2: What I guess what I mean is like, at some
• 46:40
  Speaker 2: point you do need to know the masses of the
• 46:42
  Speaker 2: things involved in your experiment, and so but that for
• 46:44
  Speaker 2: that you also kind of need g and so there's
• 46:46
  Speaker 2: as maybe going to be a little uncertain because of that.
• 46:48
  Speaker 1: There's always going to be uncertainty exactly, and because we
• 46:51
  Speaker 1: don't know this one very well, it makes everything in
• 46:53
  Speaker 1: gravity more uncertain.
• 46:55
  Speaker 2: All right, Well, sounds like there's still a lot of
• 46:59
  Speaker 2: room for people to come up with some interesting experiment
• 47:01
  Speaker 2: to measure this exactly.
• 47:02
  Speaker 1: You might think it's a historical quantity, but people have
• 47:05
  Speaker 1: been measuring these things in the last five ten years.
• 47:08
  Speaker 1: It's like an area of active research understanding Newton's constant
• 47:12
  Speaker 1: for gravity.
• 47:14
  Speaker 2: So I guess the next time you weigh yourself and
• 47:16
  Speaker 2: you're like, what, I weigh this much, you can maybe
• 47:18
  Speaker 2: blame it on the uncertainty of the gravitational constant.
• 47:22
  Speaker 1: That's right, blame Newton.
• 47:24
  Speaker 2: I guess that still doesn't help you explain why you're getting.
• 47:26
  Speaker 1: Old or why you're getting more silver.
• 47:27
  Speaker 2: All right, Well, we hope you enjoyed that and maybe
• 47:30
  Speaker 2: thought a little bit more about what we know about
• 47:32
  Speaker 2: the universe and we still don't know and how we
• 47:34
  Speaker 2: still don't know very basic things about it, like how
• 47:37
  Speaker 2: much you weigh or how much how hard the Earth
• 47:40
  Speaker 2: is pulling domin you.
• 47:41
  Speaker 1: So for those of you looking to crack a deep
• 47:43
  Speaker 1: secret of the universe, this is one of those frontiers.
• 47:47
  Speaker 1: Maybe you'll find a reason why G has to be
• 47:49
  Speaker 1: a certain value, or maybe you'll come up with a
• 47:51
  Speaker 1: super clever experiment to nail it down very.
• 47:54
  Speaker 2: Precisely, and then everyone will go ge whiz, I mean
• 47:57
  Speaker 2: big g wheez I mean uppercase. All right, thanks for
• 48:01
  Speaker 2: joining us, see you next time.
• 48:11
  Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain
• 48:14
  Speaker 1: the Universe is a production of iHeartRadio. For more podcasts
• 48:18
  Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
• 48:23
  Speaker 1: you listen to your favorite shows.