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Speaker 1: Quor, Hey, have you found your first gray hair yet?

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Speaker 2: I have. I have quite a few of them. Yeah,

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Speaker 2: although I like to think of them as silver not gray.

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Speaker 1: Oh that sounds like a good plan. Really lean into

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Speaker 1: the dignity of aging.

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Speaker 2: I don't have a lot of dignity, but I am

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Speaker 2: definitely aging. How about you? How's your transition going to

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Speaker 2: full Einstein hairstyle mode?

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Speaker 1: I still have no silver hair. In fact, I'm thinking

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Speaker 1: about dying my.

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Speaker 2: Temples, dyeing them, like bleaching them to get gray hairs.

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Speaker 1: Yeah. So people take me a little bit more seriously.

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Speaker 2: Oh, I see, you want to look like one of

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Speaker 2: those senior established physicists gray hairs exactly.

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Speaker 1: I want to increase my gravitass not just my personal gravity.

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Speaker 2: Yeah, I may. Your problem is you're increasing in with

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Speaker 2: and not with them.

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Speaker 1: That was a weighty burn. H.

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Speaker 2: I am Poor hammy cartoonist and the creator of PhD comics.

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Speaker 3: Hi.

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Speaker 1: I'm Daniel. I'm a particle physicist and a professor at

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Speaker 1: UC Irvine. And I was once accidentally c seed on

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Speaker 1: an email where I was described as young ish.

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Speaker 2: Oh all right, yeah, that's good. But how long ago

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Speaker 2: was this? Was this like twenty years ago, in which

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Speaker 2: case it was true back then.

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Speaker 1: Yeah, long enough that I shouldn't be telling that story anymore.

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Speaker 2: Right, You're like I was in my thirties and people

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Speaker 2: were calling me youngish. That's a better adjective than many

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Speaker 2: other adjectives people can call you.

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Speaker 1: Absolutely, I'd rather be youngish than oldish or stinkish.

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Speaker 2: But anyways, welcome to our podcast, Daniel and Jorge Explain

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Speaker 2: the Universe, a production of iHeartRadio.

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Speaker 1: In which we dig into the mysteries of this oldish

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Speaker 1: universe and our youngish attempts to understand it. We think

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Speaker 1: that the universe should make sense to humans. We should

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Speaker 1: be able to go out there and measure things about it,

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Speaker 1: to figure it out, to unravel its mysteries, and to

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Speaker 1: explain it to each other. And that's our job on

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Speaker 1: this podcast to unravel as many of those mysteries as

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Speaker 1: possible and to explain them to you.

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Speaker 2: That's right, It's our job to increase the gravity of

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Speaker 2: your brain. Hopefully all of this amazing knowledge about the

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Speaker 2: universe is maybe making more connections in the neurons in

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Speaker 2: your brain and making your brain grow a little bit,

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Speaker 2: and also increasing the wisdom in there. Because I guess

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Speaker 2: knowing about the universe sort of increases your wisdom, right,

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Speaker 2: if you know how the world works, that's sort of

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Speaker 2: the definition of wisdom.

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Speaker 1: Yeah, what other kind of wisdom is there other than

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Speaker 1: knowing how the world works? If you lump like people

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Speaker 1: and animals and society and all that kind of stuff

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Speaker 1: into the world. And that's exactly what we're trying to do.

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Speaker 1: We're trying to describe the world. And the way we

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Speaker 1: do it is by telling these mathematical stories. We say

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Speaker 1: there are relationships between these things. We notice if you

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Speaker 1: push on this thing a certain way, goes a certain speed,

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Speaker 1: or they don't move if you don't push on them.

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Speaker 1: All these things are mathematical stories that we use to

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Speaker 1: describe the universe that's out there. We hope to boil

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Speaker 1: it down to a bunch of equations, which in the end,

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Speaker 1: they're just describing what we see out there in the universe.

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Speaker 2: It's right, we're trying to find what the wise crack

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Speaker 2: of the universe is. That kind of what the job

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Speaker 2: of a physicist is.

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Speaker 1: I hope there will be some humor in these stories.

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Speaker 1: You know, every good story has some comic relief in it,

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Speaker 1: even mathematical.

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Speaker 2: Stories, but I guess it's wisden the same as common sense.

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Speaker 2: Do you think the universe has common sense?

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Speaker 1: Absolutely not. Intuitive ideas about the universe what makes sense

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Speaker 1: to us from our limited experience here on Earth are

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Speaker 1: not always reflective of what's really happening in the universe.

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Speaker 1: You know, it made sense to Aristotle that things fell down,

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Speaker 1: but that doesn't mean that everything always falls down, or

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Speaker 1: that down't even means something when you're out in space

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Speaker 1: far away from any gravity.

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Speaker 2: Yeah, it is a pretty perplexing universe, and sometimes it

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Speaker 2: sort of seems like it does think that that don't

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Speaker 2: make sense, And in fact, you can sort of ask

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Speaker 2: the question whether humans will ever make full sense of

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Speaker 2: the universe or if there are just some things about

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Speaker 2: it that are sort of random, right, or arbitrary.

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Speaker 1: Yeah, there's lots of layers. There are humans smart enough

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Speaker 1: to describe the workings of the universe in terms of

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Speaker 1: our mathematics. Is our mathematics actually the language of the

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Speaker 1: universe itself or just our description of what we see?

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Speaker 1: And philosophically, we aren't even sure if there is a

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Speaker 1: single mathematical prescription that describes everything that happens out there

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Speaker 1: in the universe. A whole group of philosophers believe in

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Speaker 1: disunity that there might not be a single holistic description

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Speaker 1: of the universe. So it's pretty complicated, but we do

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Speaker 1: our best. We find these mathematical stories, which are equations.

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Speaker 1: They relate things like force and acceleration, or force and mass,

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Speaker 1: all sorts of things. But they're not just equations. The

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Speaker 1: equations also have numbers in them, constants that describe the

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Speaker 1: way the universe works.

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Speaker 2: Yeah, the universe seems to have lots of consonants, lots

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Speaker 2: of numbers in them, like pi, and I guess the

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Speaker 2: expansion of the universe is also defined by a number.

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Speaker 1: Yeah, that's right, the speed of light, All sorts of

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Speaker 1: things seem to control the way that the universe works.

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Speaker 1: And in lots of cases, we don't know why they

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Speaker 1: have this value and not some other value. Why is

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Speaker 1: the universe expanding at this rate? Why is the speed

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Speaker 1: of light not faster or slower? Why are some of

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Speaker 1: the forces strong and some of them are powerful. It

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Speaker 1: seems like there's a control panel somewhere on the universe,

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Speaker 1: and all these things are just parameters. They're just like

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Speaker 1: knobs on the control panel, and you could have twirled

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Speaker 1: them one way or another way and still gotten a

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Speaker 1: universe one very different from ours. But we don't know

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Speaker 1: if there's a reason why the parameters have the values

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Speaker 1: they do.

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Speaker 2: I feel like every time you say that the universe

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Speaker 2: has a control panel, I always imagine, for some reason,

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Speaker 2: the Simpsons, you know, the opening scene with Homer sitting

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Speaker 2: in front of like the control panel floor for the

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Speaker 2: nuclear plant that he works at. I always always imagine

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Speaker 2: that when you say the control panel of the universe, Like,

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Speaker 2: is there a Homer Simpson about to spill some coffee

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Speaker 2: or donuts onto the fabric of our universe?

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Speaker 1: That would explain maybe why the universe seems so crazy

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Speaker 1: and bonkers sometimes because maybe there's an idiot in charge.

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Speaker 2: Because it was designed by bad groaning exactly, or because

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Speaker 2: Homer symptom isn't charge.

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Speaker 1: Yeah, either because there's a cartoonist who's the designer of

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Speaker 1: the universe and we all know they can't be trusted.

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Speaker 2: Are you saying God is the ultimate cartoonists or cartoonists

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Speaker 2: are the ultimate gods?

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Speaker 1: I'm saying if either of those are true, then we're screwed.

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Speaker 2: No, wouldn't you want to live in a cartoon, Like

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Speaker 2: if the universe was controlled by cartoon physics, I mean,

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Speaker 2: wouldn't that be more fun? Wouldn't your job be more fun?

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Speaker 1: My job would be impossible because there is no physics

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Speaker 1: in cartoons. There don't seem to be any laws that

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Speaker 1: anybody follows. It's just like make it all up as

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Speaker 1: you go. So science is basically out the window.

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Speaker 2: Is that your goal, well, to put you out of

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Speaker 2: a job. meEach episode, I'm trying to embarrass us to

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Speaker 2: the point where we don't know what he gots anymore.

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Speaker 1: But it does seem like there are the laws that

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Speaker 1: describe what's out there. And sometimes in these laws there

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Speaker 1: are just numbers, like if you look at Maxwell's equations

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Speaker 1: or how electromagnetic radiation propagates to the universe, there are

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Speaker 1: a couple constants in there, the permittivity of free space,

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Speaker 1: for example. All those things determine the speed of light.

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Speaker 1: But these are just numbers that we measure in the universe.

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Speaker 1: We don't have like a theoretical reason to say why

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Speaker 1: should be this number or the other number. It's just

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Speaker 1: like an unknown parameter in the equations that we have

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Speaker 1: to go out and do experiments to discover.

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Speaker 2: Yeah, like you were saying, like the speed of light,

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Speaker 2: it is three hundred thousand meters per second, but it

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Speaker 2: could also be something else. And that's what you mean

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Speaker 2: by a control not like somehow when the universe was created,

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Speaker 2: somebody said that not to three hundred thousand meters per second,

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Speaker 2: but it could have been something else.

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Speaker 1: Yea, actually I think it is something else. It's three

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Speaker 1: hundred million meters per second.

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Speaker 2: Oh three, that's what I said.

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Speaker 1: You go, somebody fell asleep on the control panel and

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Speaker 1: the speed of light is slower over there in Pasadena

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Speaker 1: than it is down here, apparently.

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Speaker 2: Well there you go. Don't put me in charge of

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Speaker 2: the knob because obviously we've set it to a thousand

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Speaker 2: times the wrong aunt.

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Speaker 1: All right, Homer. But these constants are fascinating, and physicists

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Speaker 1: look at them and they go, why this number, why

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Speaker 1: not some other number? Especially when the numbers are weird.

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Speaker 1: The numbers are like one or two, people are like, yeah, cool,

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Speaker 1: that makes sense. But if the numbers are like seventy

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Speaker 1: four bajillion or ten to the negative thirty two, people

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Speaker 1: are like, that's really strange. It's got to be a story.

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Speaker 2: There, right, Because I guess if it's one, then that

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Speaker 2: means that something canceled out sort of right.

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Speaker 1: This is a really controversial way of thinking in theoretical

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Speaker 1: physics to say that like numbers like one are natural,

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Speaker 1: that they make sense. That you know, two things are

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Speaker 1: related by a factor or close to one, that means

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Speaker 1: that it's a natural relationship. And if the factor is

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Speaker 1: really really big, then you got to ask why what's

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Speaker 1: going on? So why didn't things cancel out? Why are

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Speaker 1: these things not in balance? It's really kind of esthetics.

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Speaker 1: It's not really driven by any deep principle in theoretical physics.

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Speaker 1: It's just like wondering why numbers are not close to one,

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Speaker 1: just preferring numbers close to one. There isn't even really

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Speaker 1: a great argument that I could take for why you

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Speaker 1: would prefer numbers close to one.

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Speaker 2: Well, they say one is the loneliest numbers, So maybe

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Speaker 2: you're an introvert. That sounds like the best number.

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Speaker 1: Maybe the secrets to the universe are actually hidden in

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Speaker 1: the names of eighties pop.

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Speaker 2: Songs, Yeah, there you go, or in the lyrics, right,

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Speaker 2: maybe eighties pop stars are the gods of the universe.

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Speaker 1: Yeah, maybe it's all just about ice. Ice baby.

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Speaker 2: Yeah, there wasn't there a group called Genesis back in

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Speaker 2: the eighties. There you go. Well, so there are all

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Speaker 2: these amazing numbers that seem to sort of control how

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Speaker 2: the universe behaves and what it does and what the

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Speaker 2: particles in it all do. But we don't understand some

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Speaker 2: of these constants, and in some cases we don't even

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Speaker 2: know exactly what they are, right.

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Speaker 1: That's right. Sometimes we can do experiments to measure them

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Speaker 1: very very precisely, but some of these are a little slippery.

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Speaker 1: Some of them are very difficult to actually nail down,

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Speaker 1: especially one of the most fundamental constants in the universe.

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Speaker 2: Yeah. So today own the program, we'll be asking the question,

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Speaker 2: how do we measure the gravitational constant G upper case G,

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Speaker 2: that's how you call it.

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Speaker 1: In physics, we call it big G, or the universal

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Speaker 1: gravitational constant, because we want to distinguish it from little G,

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Speaker 1: which is the acceleration due to gravity here on Earth

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Speaker 1: nine point eight meters per second squared, which everybody uses

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Speaker 1: in their freshman physics class, and big G is the

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Speaker 1: number that appears in like Newton's equation for gravity.

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Speaker 2: Mmm, do you think G has anyone asked, gieve it

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Speaker 2: minds being called big G.

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Speaker 1: I'm glad that you're always thinking about these things from

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Speaker 1: the point of view of the subject. You know, in

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Speaker 1: physics we tend not to anthropomorphize everything. But I'm glad

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Speaker 1: that somebody out there is looking out for the little

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Speaker 1: g's and the big g's of the world.

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Speaker 2: Well, that's what cartoonists are here for, to anthrop forum

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Speaker 2: orphize everything, even physics, I guess, But I guess it's

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Speaker 2: big G, like like you said, to distinguish it from

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Speaker 2: the little G that I think most people are familiar

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Speaker 2: with from like high school physics. Right, little G is

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Speaker 2: the one that tells you the exceleration of gravity here

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Speaker 2: on Earth, Like if you drop a ball, it's going

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Speaker 2: to accelerate at nine point eight meters per second square.

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Speaker 2: That's little G. Uppercase G is the more general gravitational constant.

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Speaker 1: That's right. Little G is only relevant on the surface

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Speaker 1: of the Earth. If you go up in an airplane

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Speaker 1: and you go deep down into the Earth, you're going

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Speaker 1: to feel a different acceleration due to gravity because you're

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Speaker 1: going to have a different amount of mass of the Earth,

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Speaker 1: or be a different distance from the Earth. And on

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Speaker 1: other planets or in other solar systems. Little g's are

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Speaker 1: totally irrelevant number. I mean, there's like a medium G

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Speaker 1: and a smallish G. There's a G junior and a

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Speaker 1: G the third and all sorts of gs. Men is

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Speaker 1: the OG you know, a G whiz. But big G

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Speaker 1: is universal. It's supposed to reflect something about the universe

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Speaker 1: itself and be independent of anything that happens on Earth

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Speaker 1: or the size of the Earth, or your distance from

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Speaker 1: the Earth. It's something about the universe, not something about

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Speaker 1: our neighborhood.

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Speaker 2: Right, So this is like the G that relates to

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Speaker 2: the actual force of gravity exactly.

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Speaker 1: It's the numbers that controls really the strength of gravity

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Speaker 1: in the universe.

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Speaker 2: Well, as usually, we were wondering how many people out

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Speaker 2: there had thought about the uppercase G of the universe

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Speaker 2: and how we might measure it.

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Speaker 1: So thank you very much to everybody who answers these

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Speaker 1: questions for this fun segment of the podcast. We love

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Speaker 1: hearing your thoughts, and if you would like to share

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Speaker 1: your thoughts for a future segment, please write to me

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Speaker 1: too questions at Danielandjorge dot com. Everyone wants to hear

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Speaker 1: your voice.

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Speaker 2: So think about it. For a second, how do you

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Speaker 2: think the universal gravitational constant G is measured? Here's what

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Speaker 2: people have to say.

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Speaker 4: I assume gravitational constant G is unique to planet Earth,

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Speaker 4: which exerts gravity upon us. To calculate that constant, I

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Speaker 4: would try to figure out how much force is necessary

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Speaker 4: for us to go against the gravitational force of Earth,

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Speaker 4: and then we know what the gravitational force itself is.

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Speaker 2: I think we measure the gravitational constant by measuring how

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Speaker 2: quickly galaxies and moving away from us and stretching the

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Speaker 2: space in between.

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Speaker 1: This is an easy one for me to answer, because

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Speaker 1: I don't know what the gravitational constant G is, but

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Speaker 1: I'm looking forward to hearing and finding out.

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Speaker 5: I guess it's true. The observation of planets, stars and

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Speaker 5: other celes your bodies, and coming to a number that

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Speaker 5: adjusts that motion to our other units of measuring.

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Speaker 3: Probably something to do at the moon. See we can

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Speaker 3: estimate its mass, estimate the Earth's mass, then see what's

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Speaker 3: going on there, and we can probably find a G.

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Speaker 2: All right, Well, somebody here confusedly with little G. You

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Speaker 2: know they do look alike.

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Speaker 1: I guess do theyre?

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Speaker 3: Though?

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Speaker 1: Big capital G and little G look pretty different. I

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Speaker 1: sent this an email, so I wrote explicitly big G. Hmmmm,

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Speaker 1: I'm not giving a lot of partial credit on that one.

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Speaker 2: Oh boy, there's pointage involved here. Do you get a grade?

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Speaker 1: I'm handing out degrees over here.

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Speaker 2: And I don't think that's going to encourage people to

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Speaker 2: call it.

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Speaker 1: Well, look, if you want to get your PhD in

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Speaker 1: podcast science, then you know you got to take it

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Speaker 1: for credit.

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Speaker 2: None of this past fial stuff I see. But everyone

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Speaker 2: gets an A though, right.

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Speaker 1: Yeah, I'm a softy. I'm not going to give you

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Speaker 1: a lot of.

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Speaker 2: A little A or a capital A.

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Speaker 1: I give everyone a big A plus. I'm a softy

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Speaker 1: in the end. All right.

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Speaker 2: But some interesting answers here, Like some people are saying

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Speaker 2: you can measure the gravitational constant by looking at planets

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Speaker 2: and stars and how things move around in space.

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Speaker 1: Yeah, those are interesting ideas, but fundamentally they don't work

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Speaker 1: because they don't let you establish what G is because

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Speaker 1: you don't know what the masses of those planets are,

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Speaker 1: and so there's too many unknowns in that equation.

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Speaker 2: And somebody said it has something to do with the moon,

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Speaker 2: like maybe you can measure G using the moon.

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Speaker 1: Yeah, and again, you could measure G using the Moon

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Speaker 1: and the Earth if you knew very very precisely the

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Speaker 1: mass of the Earth and the mass of the Moon.

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Speaker 1: But if you don't, then you can't use that to

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Speaker 1: measure G.

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Speaker 2: All right, let's dig into it. Lets first of all

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Speaker 2: define for our audience, what is the gravitational constant G.

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Speaker 1: So the gravitational constant G is the number that defines

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Speaker 1: the strength of gravity, and it appears in Newtonian gravity

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Speaker 1: in his equation for the force between two objects that

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Speaker 1: have mass. So Newtonian gravity says that the force is

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Speaker 1: big G times one mass times the other mass divided

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Speaker 1: by the distance squared, So gmm over R squared, and

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Speaker 1: that number G is the one that controls it. If

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Speaker 1: G was bigger, you would have a larger force between objects,

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Speaker 1: and if G was smaller, you would have a smaller

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Speaker 1: force between objects of the same mass and at the

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Speaker 1: same distance. So it's really just this like tunable parameter.

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Speaker 2: And it's kind of what determines how strong the gravity

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Speaker 2: is between two things. Basically, right, Like, what's the basic

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Speaker 2: Newtonian formula for gravity?

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Speaker 1: Exactly? The Newtonian formula for gravity has big G in it.

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Speaker 1: It's just gmm over R squared. And it's a similar

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Speaker 1: structure to other forces, right, Like the electrostatic repulsion between

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Speaker 1: two objects like two electrons or whatever, has the same structure,

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Speaker 1: and it also has a constant in front of it.

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Speaker 1: It's a different constant. So each of the forces you

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Speaker 1: can write using this kind of equation, and each one

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Speaker 1: has a constant in front of it that tells you

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Speaker 1: how powerful the force is.

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Speaker 2: So like, if you had two things floating out there

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Speaker 2: in space, a mass one and a mass two, you

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Speaker 2: can compute the force that attracts them together. We're using

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Speaker 2: this formula, right. You take the mass of one thing,

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Speaker 2: and then take the mass of the other thing. You

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Speaker 2: multiply together. You divide by the square of the distance

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Speaker 2: between them, and then you take the number and that's

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Speaker 2: what you multiply by G to get the force of

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Speaker 2: gravity between them exactly.

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Speaker 1: And if we lived in a universe where G was

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Speaker 1: twice as big, or if the cartoonist at the control

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Speaker 1: panel fell asleep on the knob and doubled big G,

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Speaker 1: then all the forces of gravity would be twice as big.

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Speaker 1: And if you divided G by a factor of two.

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Speaker 1: If you made it twice as small, then the force

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Speaker 1: of gravity would be twice as small.

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Speaker 2: Yeah, And so that's why it's called the universal gravitational constant,

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Speaker 2: because it's supposed to be the same all over the universe, right, Like,

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Speaker 2: if you measured the gravity between two things here or

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Speaker 2: in another planet or in another part of the galaxy,

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Speaker 2: you should be able to use the same constant G

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Speaker 2: to calculate that force exactly.

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Speaker 1: And what if Newton's great achievement was using this to

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Speaker 1: describe gravity here on Earth between fairly small masses and

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Speaker 1: small distances, and gravity between planets and stars and moons,

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Speaker 1: to show that it works in lots of different settings

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Speaker 1: over huge differences and masses and huge differences in distances.

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Speaker 1: So you sort of unified the heavens and the Earth

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Speaker 1: in that sense. So yeah, it's supposed to be universal.

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Speaker 2: Yeah, And it's pretty amazing that the formula is so

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Speaker 2: simple if you think about it, right, it's like one multiplication,

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Speaker 2: one division, and one squared and boom, you can like

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Speaker 2: decipher the workings of the universe, you know, Like it's

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Speaker 2: not like the one point seventh square root of the

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Speaker 2: distance between them nuts, it's like the square of the

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Speaker 2: distance between them. And it's just like mass one times

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Speaker 2: mass too. It's not like mass one plus seventeen divided

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Speaker 2: by five point two. You know.

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Speaker 1: Yeah, it is kind of cool, and the structure sort

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Speaker 1: of makes sense. Like, first of all, it has to

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Speaker 1: be symmetric. It can't be like mass one times mass

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Speaker 1: two squared, right, because it needs to be the same

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Speaker 1: force between mass one and mass two. At mass two

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Speaker 1: and mass one, right, it shouldn't matter which one you

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Speaker 1: call mass one or mass two, So it has to

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Speaker 1: be symmetric. It also makes sense that it's one over

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Speaker 1: the distance squared because as things get further and further apart,

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Speaker 1: the force gets diluted over a larger and larger area,

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Speaker 1: and the surface area of that sphere grows with the

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Speaker 1: distance squared, so the same way like if you have

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Speaker 1: a light source like a star, and you're twice as

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Speaker 1: far away from it, then the same number of photons

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Speaker 1: are now distributed over a larger sphere. That sphere has

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Speaker 1: four times the area, and so one of our distance square.

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Speaker 1: It really makes sense, and I think that's why it

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Speaker 1: appears in all of the force laws, not just the

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Speaker 1: one for gravity, the one for electromagnetism also has a

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Speaker 1: one over distance squared.

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Speaker 2: Right, it makes sense. But it didn't have to be

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Speaker 2: that way, right, it could have been r to the

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Speaker 2: one point seventy two or something like that.

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Speaker 1: Right, Yeah, it could have been. And there are actually

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Speaker 1: theories of gravity that do change that that suggest that

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Speaker 1: gravity changes at very small distances maybe or at different accelerations.

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Speaker 1: So it's not one over are squared, but one of

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Speaker 1: our squared is also the simpler. But you're right, the

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Speaker 1: universe didn't have to make sense, and it doesn't have

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Speaker 1: to be simple. It could be crazy complicated.

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Speaker 2: All right, Well, that's the universal gravitational constant uppercase G,

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Speaker 2: which tells you the strength of the force of gravity

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Speaker 2: in the universe. But as we know, gravity is not

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Speaker 2: quite a force, and also maybe this constant can change,

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Speaker 2: so let's dig into that. But first let's take a

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Speaker 2: quick break. All right, we're talking about the universal gravitational

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Speaker 2: constant uppercase G. It basically kind of tells you the

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Speaker 2: general strength of the force of gravity in the universe. Right, Like,

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Speaker 2: if G was much bigger, then the gravity would be

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Speaker 2: much stronger in the universe. If it was smaller, gravity

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Speaker 2: would be much weaker.

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Speaker 1: Exactly, and we don't know why it has this value.

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Speaker 1: There's nothing in physics that says it should be this number.

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Speaker 1: There's no set of equations you can use to like

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Speaker 1: derive it or predict this value. It's just something we

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Speaker 1: have to go out and measure and discover in the universe.

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Speaker 2: Right. So the origin of this constant is that it

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Speaker 2: came from Newtont's right, Newtent's laws about the force of

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Speaker 2: gravity between like planets and the sun and things like that.

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Speaker 2: But nowadays we think of gravity more like a bending

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Speaker 2: of space. Does the gravitational constant G still come into play?

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Speaker 1: Then it actually does. The same constant G also appears

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Speaker 1: in Einstein's equation. So we've replaced Newtonian physics that says

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Speaker 1: that there's a force between masses and that pulls them

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Speaker 1: together by saying, actually, there's no force there. It just

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Speaker 1: looks like a force. What's really happening is that masses

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Speaker 1: are bending space, and when they move through that bent space,

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Speaker 1: it looks like there's a force on them. And Einstein

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Speaker 1: gives us equations that describe how that space is bent

439
00:20:48,440 --> 00:20:51,800
Speaker 1: when mass is around, and those equations the Einstein field

440
00:20:51,840 --> 00:20:54,840
Speaker 1: equations also have constants in them, and one of those

441
00:20:54,840 --> 00:20:57,920
Speaker 1: constants is big G, the same exact number, And that

442
00:20:57,960 --> 00:21:02,080
Speaker 1: shouldn't be a surprise because einstein field equations also reproduce

443
00:21:02,160 --> 00:21:06,040
Speaker 1: all the predictions of Newtonian physics, like Einstein and Newton

444
00:21:06,080 --> 00:21:08,359
Speaker 1: agree about the force on the Earth from the sun,

445
00:21:08,440 --> 00:21:10,800
Speaker 1: for example, because you know, Newtonian physics got a lot

446
00:21:10,840 --> 00:21:12,600
Speaker 1: of stuff right, So it wouldn't make sense if they

447
00:21:12,600 --> 00:21:14,000
Speaker 1: had totally different constants.

448
00:21:14,480 --> 00:21:17,840
Speaker 2: Now that Einstein sort of derived this constant independently, or

449
00:21:17,880 --> 00:21:20,480
Speaker 2: did he like start with Newton's equations and kept it in.

450
00:21:20,440 --> 00:21:23,360
Speaker 1: You can't derive this constant, right. If we didn't have

451
00:21:23,480 --> 00:21:25,840
Speaker 1: Newton and we just started with Einstein, he would have

452
00:21:25,840 --> 00:21:27,840
Speaker 1: come up with his field equations and said, okay, but

453
00:21:27,880 --> 00:21:29,439
Speaker 1: there's a number in it, and I don't know what

454
00:21:29,520 --> 00:21:32,040
Speaker 1: that number is. Let's go out and measure it. In

455
00:21:32,080 --> 00:21:34,879
Speaker 1: the same way that when we first got Newton's equations.

456
00:21:35,119 --> 00:21:36,679
Speaker 1: There's a constant in there, and we have to go

457
00:21:36,720 --> 00:21:39,720
Speaker 1: out and measure it. And Newton actually suggested some ways

458
00:21:39,760 --> 00:21:42,520
Speaker 1: to go and measure this. So Einstein just sort of

459
00:21:42,560 --> 00:21:44,320
Speaker 1: inherited this constant from Newton.

460
00:21:45,240 --> 00:21:48,000
Speaker 2: Interesting, all right, Well, then what's the current value of

461
00:21:48,040 --> 00:21:50,480
Speaker 2: what we think G or uppercase G is.

462
00:21:50,680 --> 00:21:54,040
Speaker 1: So it's a really tiny number. It's six point six

463
00:21:54,200 --> 00:21:58,119
Speaker 1: seven times ten to the minus eleven, and the units

464
00:21:58,119 --> 00:22:01,160
Speaker 1: on it are kind of weird. It's meters cubed divided

465
00:22:01,160 --> 00:22:05,120
Speaker 1: by kilograms times seconds squared. It's this very small number

466
00:22:05,240 --> 00:22:06,800
Speaker 1: like ten to the minus eleven.

467
00:22:07,160 --> 00:22:09,960
Speaker 2: WHOA, So that's one of the reasons kind of why

468
00:22:10,040 --> 00:22:12,439
Speaker 2: gravity is so weak too, right, because the G is

469
00:22:12,440 --> 00:22:12,800
Speaker 2: such a.

470
00:22:12,720 --> 00:22:16,040
Speaker 1: Small number, that's exactly the reason why gravity is so weak.

471
00:22:16,320 --> 00:22:19,240
Speaker 1: If G was much much bigger, gravity would be much

472
00:22:19,280 --> 00:22:22,119
Speaker 1: more powerful. And so this number is the number for

473
00:22:22,240 --> 00:22:24,240
Speaker 1: G in our universe, and we don't know are there

474
00:22:24,240 --> 00:22:27,280
Speaker 1: other universes out there with different values for G that

475
00:22:27,359 --> 00:22:30,040
Speaker 1: have like much more powerful gravity and they all collapsed

476
00:22:30,040 --> 00:22:32,879
Speaker 1: into black holes to a few seconds after being birthed.

477
00:22:32,880 --> 00:22:35,560
Speaker 1: Are the universes out there with even weaker g's and

478
00:22:35,600 --> 00:22:38,520
Speaker 1: those universes still haven't even made stars because there isn't

479
00:22:38,520 --> 00:22:41,159
Speaker 1: a powerful enough gravity to pull that stuff together. We

480
00:22:41,280 --> 00:22:43,439
Speaker 1: just don't know if there are other options for this

481
00:22:43,480 --> 00:22:45,640
Speaker 1: thing and why we have this value. But you're right,

482
00:22:45,760 --> 00:22:49,480
Speaker 1: it completely controls the strength of gravity. And what's super

483
00:22:49,520 --> 00:22:51,720
Speaker 1: weird about it being so small is that the other

484
00:22:51,800 --> 00:22:55,280
Speaker 1: forces have much bigger constants, which is why gravity is

485
00:22:55,280 --> 00:22:57,600
Speaker 1: so much weaker than all of the other forces.

486
00:22:57,760 --> 00:23:00,600
Speaker 2: Well, I feel like you're blaming the gravitational constant here,

487
00:23:00,640 --> 00:23:02,560
Speaker 2: but it could also just be like things don't have

488
00:23:02,680 --> 00:23:05,119
Speaker 2: enough mass, Like maybe things were more massive, do you

489
00:23:05,160 --> 00:23:07,879
Speaker 2: know what I mean, and then the force of gravity

490
00:23:07,880 --> 00:23:08,640
Speaker 2: would be stronger.

491
00:23:08,840 --> 00:23:11,879
Speaker 1: Yeah, exactly. There's a subtlety there in how you compare

492
00:23:11,960 --> 00:23:16,680
Speaker 1: different forces, Like how do you compare electromagnetism and gravity. Well,

493
00:23:16,720 --> 00:23:20,600
Speaker 1: you take objects that have mass and have charge, like protons,

494
00:23:21,119 --> 00:23:23,040
Speaker 1: and you hold them apart at a certain distance, and

495
00:23:23,040 --> 00:23:25,680
Speaker 1: you calculate their relative strengths. And so, for example, if

496
00:23:25,680 --> 00:23:29,119
Speaker 1: you hold two protons like a centimeter apart, then you

497
00:23:29,200 --> 00:23:32,920
Speaker 1: discover that gravity is ten to the thirty three times

498
00:23:33,160 --> 00:23:36,800
Speaker 1: weaker than the electromagnetic force. But you might say, hold

499
00:23:36,840 --> 00:23:39,359
Speaker 1: on a second, that's just because protons have almost no

500
00:23:39,520 --> 00:23:41,800
Speaker 1: mass and a big charge. If we lived in a

501
00:23:41,880 --> 00:23:45,119
Speaker 1: universe where protons had tiny charge and huge masses, then

502
00:23:45,160 --> 00:23:47,800
Speaker 1: you would say gravity is stronger. And yeah, you're absolutely right,

503
00:23:47,840 --> 00:23:50,280
Speaker 1: But the kind of things that exist in our universe

504
00:23:50,359 --> 00:23:52,840
Speaker 1: tend to have a certain amount of mass per charge,

505
00:23:53,320 --> 00:23:55,920
Speaker 1: and that means that gravity ends up being really really

506
00:23:55,920 --> 00:23:59,440
Speaker 1: weak compared to electromagnetism. So yeah, blame it on the constant,

507
00:23:59,600 --> 00:24:02,159
Speaker 1: or blame it on the particles. But it's somebody's fault.

508
00:24:02,280 --> 00:24:04,560
Speaker 2: It's somebody's fault that the things that you don't weigh more,

509
00:24:04,680 --> 00:24:06,080
Speaker 2: or that you do weigh a lot.

510
00:24:07,920 --> 00:24:11,200
Speaker 1: Everything is somebody's fault, somebody else's fault. Right in this case,

511
00:24:11,240 --> 00:24:13,600
Speaker 1: I think it is a fair comparison to say, typical

512
00:24:13,600 --> 00:24:16,679
Speaker 1: particles in the universe, what is their relative gravity versus

513
00:24:16,680 --> 00:24:19,840
Speaker 1: their relative electromagnetic repulsion? And what you find is that

514
00:24:19,840 --> 00:24:23,120
Speaker 1: they're not even close. Right, Like gravity is weaker. It's

515
00:24:23,160 --> 00:24:25,400
Speaker 1: not even a little bit weaker or a lot weaker.

516
00:24:25,520 --> 00:24:29,760
Speaker 1: It's ridiculously weaker. It's ten to the thirty three times weaker.

517
00:24:29,760 --> 00:24:33,600
Speaker 1: It's like negligible compared to the force of electromagnetism. And

518
00:24:33,640 --> 00:24:37,280
Speaker 1: that's due to this constant, Like electromagnetism has its own constant,

519
00:24:37,320 --> 00:24:39,160
Speaker 1: and it's just a much bigger number, right.

520
00:24:39,160 --> 00:24:40,439
Speaker 2: I think what you mean is like if I had

521
00:24:40,440 --> 00:24:42,960
Speaker 2: two protons out there in space and I bring them

522
00:24:43,000 --> 00:24:46,720
Speaker 2: close together, like the force of electromagnetism, that's going to

523
00:24:46,760 --> 00:24:50,160
Speaker 2: be repelling them is like thirty two orders of magnetude

524
00:24:50,200 --> 00:24:53,400
Speaker 2: more than the force of gravity bringing them together exactly.

525
00:24:53,560 --> 00:24:56,159
Speaker 2: All right, well, let's dig into what it takes to

526
00:24:56,280 --> 00:25:00,439
Speaker 2: measure the gravitational constant G. It's pretty hard, right because

527
00:25:00,480 --> 00:25:02,520
Speaker 2: as we're as we're saying, gravity is super weak.

528
00:25:02,640 --> 00:25:05,520
Speaker 1: Yeah, there's like three reasons why measuring big G is

529
00:25:05,560 --> 00:25:08,000
Speaker 1: actually really really hard. Number one is what you said

530
00:25:08,000 --> 00:25:11,080
Speaker 1: that gravity is weak. You know, it's not easy to

531
00:25:11,359 --> 00:25:14,040
Speaker 1: measure these things because you need big masses. You can't

532
00:25:14,080 --> 00:25:17,480
Speaker 1: really measure the gravity between two protons. It's so small

533
00:25:17,640 --> 00:25:19,440
Speaker 1: that you could never really measure it. So you need

534
00:25:19,480 --> 00:25:21,639
Speaker 1: bigger and bigger objects. And that brings you to the

535
00:25:21,680 --> 00:25:24,360
Speaker 1: second reason why it's so difficult, which is that it's

536
00:25:24,440 --> 00:25:28,080
Speaker 1: hard to shield gravity from other things. Like you're always

537
00:25:28,080 --> 00:25:31,600
Speaker 1: going to be feeling the gravity of everything else around you,

538
00:25:31,600 --> 00:25:34,280
Speaker 1: you know, like your laboratory and the mountains and the Earth.

539
00:25:34,760 --> 00:25:37,000
Speaker 1: So it's hard to get like an isolated system to

540
00:25:37,160 --> 00:25:38,080
Speaker 1: study gravity.

541
00:25:38,200 --> 00:25:40,639
Speaker 2: What do you mean isolated? Like because the Earth is

542
00:25:40,680 --> 00:25:42,720
Speaker 2: pulling you down with gravity, but you can still maybe

543
00:25:42,760 --> 00:25:45,119
Speaker 2: measure gravity. It's side to side, right, like if I

544
00:25:45,240 --> 00:25:47,480
Speaker 2: just put two balls on my table technically, they are

545
00:25:47,720 --> 00:25:50,439
Speaker 2: being attracted to each other by gravity. Couldn't I measure that?

546
00:25:50,640 --> 00:25:53,320
Speaker 1: Yeah, you certainly could. But they're also being attracted by

547
00:25:53,359 --> 00:25:55,840
Speaker 1: the gravity of your wall, and the gravity of the

548
00:25:55,880 --> 00:25:58,960
Speaker 1: tree outside, and the gravity of the mountains nearby. And

549
00:25:59,000 --> 00:26:02,320
Speaker 1: that's not true for other forces. Like for electromagnetism, you

550
00:26:02,320 --> 00:26:05,040
Speaker 1: can you have positive and negative charges, and so you

551
00:26:05,080 --> 00:26:07,800
Speaker 1: can shield things. You can like balance all the forces

552
00:26:07,840 --> 00:26:11,080
Speaker 1: out so that electromagnetism is effectively zeroed out and study

553
00:26:11,119 --> 00:26:13,840
Speaker 1: it at small scales. But for gravity, there's no way

554
00:26:13,880 --> 00:26:17,360
Speaker 1: to shield your laboratory from the gravity of your surroundings

555
00:26:17,520 --> 00:26:20,240
Speaker 1: unless you get like really really far away from everything.

556
00:26:20,800 --> 00:26:22,520
Speaker 2: What do you mean, Like, I can't just conduct my

557
00:26:22,560 --> 00:26:25,560
Speaker 2: experiment on a really tall tower or you know, at

558
00:26:25,560 --> 00:26:28,000
Speaker 2: the top of a mountain, or maybe even on a satellite.

559
00:26:28,080 --> 00:26:30,119
Speaker 1: Yeah, on a satellite would be great. The further you

560
00:26:30,160 --> 00:26:33,080
Speaker 1: can get from other masses, the better you could do

561
00:26:33,160 --> 00:26:35,919
Speaker 1: this experiment. So if you did your experiment measuring the

562
00:26:36,000 --> 00:26:39,200
Speaker 1: gravitational strength between two objects, like out in the middle

563
00:26:39,280 --> 00:26:41,760
Speaker 1: of a super bubble, far away from everything, that would

564
00:26:41,800 --> 00:26:43,560
Speaker 1: be great. But that's one of the challenges, right we

565
00:26:43,560 --> 00:26:45,560
Speaker 1: don't have the way to do that experiment out in

566
00:26:45,600 --> 00:26:47,800
Speaker 1: the middle of space. We have to do our experiments

567
00:26:47,840 --> 00:26:50,880
Speaker 1: here in the vicinity of Earth, which has its own gravity.

568
00:26:51,080 --> 00:26:53,560
Speaker 2: Well, it sounds like maybe the difficulty is in like

569
00:26:53,680 --> 00:26:56,480
Speaker 2: isolating it. It's more like it's super weak, right, because

570
00:26:57,000 --> 00:26:58,800
Speaker 2: like you could do it through this experiment out in

571
00:26:58,840 --> 00:27:01,119
Speaker 2: a satellite, right and and you know the as it

572
00:27:01,119 --> 00:27:04,520
Speaker 2: goes around the Earth, things would cancel out anyways. Right.

573
00:27:04,720 --> 00:27:07,200
Speaker 1: Yeah, it's just one reason why it's difficult. The primary

574
00:27:07,240 --> 00:27:09,480
Speaker 1: reason I think is that gravity is just so weak.

575
00:27:09,640 --> 00:27:11,760
Speaker 1: You know, you're trying to measure a very very small

576
00:27:11,760 --> 00:27:15,000
Speaker 1: effect and it's swamped by all sorts of other effects.

577
00:27:15,359 --> 00:27:17,320
Speaker 1: You know, if you have, for example, two masses and

578
00:27:17,359 --> 00:27:19,800
Speaker 1: you want to measure their gravity between them, then you

579
00:27:19,880 --> 00:27:22,400
Speaker 1: have to hope that there are no other forces bigger

580
00:27:22,440 --> 00:27:25,480
Speaker 1: than the gravitational force also operating on those masses that

581
00:27:25,520 --> 00:27:28,000
Speaker 1: would just swamp your measurement. You know, if there's like

582
00:27:28,000 --> 00:27:31,639
Speaker 1: a tiny residual electric charge on these masses because you

583
00:27:31,760 --> 00:27:33,720
Speaker 1: touch them and you got static electricity on them, it

584
00:27:33,760 --> 00:27:36,440
Speaker 1: will be so much more powerful than the gravitational force

585
00:27:36,480 --> 00:27:39,119
Speaker 1: you're trying to measure that it will just swamp your measurements.

586
00:27:39,880 --> 00:27:41,800
Speaker 2: Well, maybe we should paint a picture here for people, like,

587
00:27:41,840 --> 00:27:44,119
Speaker 2: how would you even design an experiment like this? Like,

588
00:27:44,200 --> 00:27:46,560
Speaker 2: let's say I'm proposing to you, Daniel, that we go

589
00:27:46,840 --> 00:27:48,919
Speaker 2: up in the Space Shuttle or we go out in

590
00:27:48,960 --> 00:27:51,719
Speaker 2: a rocket out to the International Space Station. I'm going

591
00:27:51,760 --> 00:27:54,520
Speaker 2: to take two Billard balls, put them, you know, ten

592
00:27:54,560 --> 00:27:56,680
Speaker 2: centimeters apart, and then I'm going to watch how long

593
00:27:56,720 --> 00:27:59,480
Speaker 2: it takes them to get attracted to each other by gravity.

594
00:28:00,040 --> 00:28:00,920
Speaker 2: Wrong with that experiment?

595
00:28:01,080 --> 00:28:03,640
Speaker 1: Yeah, you could do that, that would work. Nothing that's

596
00:28:03,680 --> 00:28:06,360
Speaker 1: wrong with that experiment except that it requires going up

597
00:28:06,400 --> 00:28:09,400
Speaker 1: to space, and also you have to account for all

598
00:28:09,440 --> 00:28:12,119
Speaker 1: the other masses nearby, right, Like the space station is

599
00:28:12,160 --> 00:28:14,439
Speaker 1: also going to be tugging on these things, and the

600
00:28:14,440 --> 00:28:16,960
Speaker 1: space station is probably a lot more massive than the

601
00:28:16,960 --> 00:28:19,840
Speaker 1: balls you brought. You can't bring super duper heavy balls

602
00:28:20,160 --> 00:28:23,040
Speaker 1: up into space because you have limitations on the expense

603
00:28:23,440 --> 00:28:24,280
Speaker 1: due to the launches.

604
00:28:24,440 --> 00:28:26,119
Speaker 2: You mean, like I said, I have those two balls

605
00:28:26,160 --> 00:28:28,760
Speaker 2: leading in front of me. They're being pulled together by

606
00:28:28,760 --> 00:28:30,840
Speaker 2: the gravity they have with each other, but maybe they're

607
00:28:30,880 --> 00:28:33,359
Speaker 2: also being pulled apart a little bit by the space

608
00:28:33,359 --> 00:28:37,240
Speaker 2: station around them, right, Or like if my fellow astronauts

609
00:28:37,240 --> 00:28:38,680
Speaker 2: it's to the right of me or to the left

610
00:28:38,720 --> 00:28:41,120
Speaker 2: of me. It might influence how those two balls come

611
00:28:41,160 --> 00:28:42,160
Speaker 2: together exactly.

612
00:28:42,160 --> 00:28:44,880
Speaker 1: And because you're trying to measure something very very small,

613
00:28:45,520 --> 00:28:48,560
Speaker 1: then you need to be very very accurate about your measurements,

614
00:28:48,560 --> 00:28:51,520
Speaker 1: and small changes in your results can lead to large

615
00:28:51,640 --> 00:28:53,520
Speaker 1: changes in the results that you get.

616
00:28:53,600 --> 00:28:55,440
Speaker 2: What if I just do it a lot, or what

617
00:28:55,520 --> 00:29:00,840
Speaker 2: if I tell everyone to stay still not move in

618
00:29:01,280 --> 00:29:04,360
Speaker 2: the space station? Wouldn't that give me a pretty good experiment?

619
00:29:04,560 --> 00:29:06,320
Speaker 1: Yeah, that would measure it. I don't think that would

620
00:29:06,320 --> 00:29:08,160
Speaker 1: come close to the precision we have today. And also

621
00:29:08,160 --> 00:29:11,600
Speaker 1: it would be really expensive. Everything out in space is

622
00:29:11,760 --> 00:29:13,360
Speaker 1: very expensive and very complicated.

623
00:29:13,680 --> 00:29:15,440
Speaker 2: All right, What are some of the other reasons that

624
00:29:15,480 --> 00:29:16,240
Speaker 2: make it difficult?

625
00:29:16,400 --> 00:29:18,240
Speaker 1: I think the last reason is just that there's no

626
00:29:18,360 --> 00:29:22,440
Speaker 1: like relationship to the other constants, you know, the other forces.

627
00:29:22,480 --> 00:29:25,720
Speaker 1: We think there might be some relationship with them. Electromagnetism

628
00:29:25,720 --> 00:29:28,400
Speaker 1: and the weak forces can get bundled together into the

629
00:29:28,440 --> 00:29:31,680
Speaker 1: electroweak force, and there's some unity there. We have theories

630
00:29:31,720 --> 00:29:33,880
Speaker 1: about how the strong force might connect with that, and

631
00:29:33,880 --> 00:29:36,120
Speaker 1: so we have like a unity of the forces. But

632
00:29:36,160 --> 00:29:38,720
Speaker 1: gravity is by itself. We don't know how to bring

633
00:29:38,760 --> 00:29:41,800
Speaker 1: gravity into quantum physics, so we have no like way

634
00:29:41,840 --> 00:29:44,840
Speaker 1: to predict or like constrain the value of this force.

635
00:29:45,040 --> 00:29:47,120
Speaker 1: You really just have to go out and measure it.

636
00:29:47,160 --> 00:29:49,760
Speaker 1: There's no other way to analyze it, right.

637
00:29:49,800 --> 00:29:52,360
Speaker 2: I always thought the hard thing about measuring the gravitational

638
00:29:52,360 --> 00:29:54,480
Speaker 2: constant was that you know, to get a measurement of it,

639
00:29:54,560 --> 00:29:56,560
Speaker 2: you sort of need to know like in our biller

640
00:29:56,600 --> 00:29:59,120
Speaker 2: ball example, you sort of need to know exactly what

641
00:29:59,160 --> 00:30:02,840
Speaker 2: the masses of those biller balls are. But it's hard

642
00:30:02,840 --> 00:30:04,440
Speaker 2: to know what the mass of something is if you

643
00:30:04,440 --> 00:30:07,520
Speaker 2: don't already know the gravitational constant. Right, isn't that one

644
00:30:07,840 --> 00:30:09,920
Speaker 2: of the big problems. It's like a chicken and egg problem,

645
00:30:10,120 --> 00:30:12,840
Speaker 2: like how do you measure gravity. To measure gravity, you

646
00:30:12,880 --> 00:30:14,400
Speaker 2: need to know the mass of something. But to the

647
00:30:14,440 --> 00:30:16,000
Speaker 2: mass of something you need to weigh it, which you

648
00:30:16,080 --> 00:30:19,480
Speaker 2: need for which you need to know the gravitational constant.

649
00:30:19,640 --> 00:30:21,400
Speaker 1: Yeah, in the end, it comes down to what do

650
00:30:21,440 --> 00:30:23,720
Speaker 1: you know? First? If you know the masses of two objects,

651
00:30:23,920 --> 00:30:25,720
Speaker 1: you can measure the force between them, and then you

652
00:30:25,760 --> 00:30:28,000
Speaker 1: get big G. If you know big G and the forces,

653
00:30:28,040 --> 00:30:30,960
Speaker 1: then you can measure the masses between them. So the

654
00:30:31,600 --> 00:30:35,360
Speaker 1: basic story of measuring big G is finding a scenario

655
00:30:35,520 --> 00:30:39,240
Speaker 1: where we already for other reasons, know the masses of

656
00:30:39,320 --> 00:30:41,959
Speaker 1: two objects that we can use to measure big G.

657
00:30:42,520 --> 00:30:45,160
Speaker 1: That's the struggle. That's why we can't, for example, have

658
00:30:45,280 --> 00:30:47,240
Speaker 1: looked at the Earth in the sun hundreds of years

659
00:30:47,240 --> 00:30:50,160
Speaker 1: ago and used those to determine big G because we

660
00:30:50,160 --> 00:30:52,320
Speaker 1: didn't know the masses of the Earth and the Sun.

661
00:30:52,480 --> 00:30:54,160
Speaker 1: To derive the masses of the Earth and the Sun

662
00:30:54,200 --> 00:30:56,520
Speaker 1: from like their relative motion, you have to know the

663
00:30:56,520 --> 00:30:58,560
Speaker 1: force between them, and you'd have to know big G.

664
00:30:58,840 --> 00:31:00,960
Speaker 2: Well, maybe a step as through then, like what's the

665
00:31:01,080 --> 00:31:03,560
Speaker 2: history of trying to measure this universal constant?

666
00:31:03,680 --> 00:31:05,880
Speaker 1: So it goes all the way back to Newton right.

667
00:31:06,000 --> 00:31:08,800
Speaker 1: Newton described this relationship between stuff and there was a

668
00:31:08,840 --> 00:31:12,760
Speaker 1: constant there, and he suggested how you might measure this constant.

669
00:31:13,120 --> 00:31:15,120
Speaker 1: He said, maybe if you had, for example, like a

670
00:31:15,160 --> 00:31:18,320
Speaker 1: pendulum basically a heavy ball on a string, and you

671
00:31:18,360 --> 00:31:21,520
Speaker 1: brought it near something massive like a mountain, then you

672
00:31:21,600 --> 00:31:24,080
Speaker 1: might be able to measure the deflection of that ball

673
00:31:24,120 --> 00:31:27,320
Speaker 1: as it's like tugged on by the mountain. Interesting bit

674
00:31:27,360 --> 00:31:30,600
Speaker 1: of history, though, is that Newton didn't write down big G.

675
00:31:30,960 --> 00:31:34,000
Speaker 1: He never wrote that down doesn't appear in his works

676
00:31:34,080 --> 00:31:36,920
Speaker 1: because Newton was working at a time before we expressed

677
00:31:37,000 --> 00:31:41,120
Speaker 1: our theoretical laws in terms of algebraic expressions. Back then,

678
00:31:41,280 --> 00:31:44,200
Speaker 1: all of our physics was done in terms of sentences

679
00:31:44,560 --> 00:31:45,960
Speaker 1: rather than in terms of algebra.

680
00:31:46,480 --> 00:31:47,760
Speaker 2: I thought you were going to say, he did it

681
00:31:47,800 --> 00:31:50,560
Speaker 2: in a time before we started body shaming our letters.

682
00:31:52,560 --> 00:31:54,840
Speaker 1: No, So if you go back and like read the Principia,

683
00:31:54,960 --> 00:31:57,560
Speaker 1: you know, he expresses his law of gravity in terms

684
00:31:57,600 --> 00:31:59,720
Speaker 1: of a sentence. You know, he says it will be

685
00:31:59,800 --> 00:32:03,680
Speaker 1: mutually gravitating towards each other at a rate relative to

686
00:32:03,760 --> 00:32:06,200
Speaker 1: the reciprocal the square of their distances. You know, he

687
00:32:06,240 --> 00:32:08,479
Speaker 1: doesn't summarize at all in terms of mathematics. So he

688
00:32:08,520 --> 00:32:10,760
Speaker 1: never actually wrote down Big G. It wasn't until a

689
00:32:10,760 --> 00:32:13,800
Speaker 1: couple hundred years later that I started being called big G.

690
00:32:14,080 --> 00:32:16,480
Speaker 1: But Newton had the basic idea. He's like, if you

691
00:32:16,520 --> 00:32:18,440
Speaker 1: know the mass of a mountain and the mass of

692
00:32:18,440 --> 00:32:21,160
Speaker 1: a pendulum, maybe you could make this measurement.

693
00:32:21,440 --> 00:32:23,640
Speaker 2: Right, But again that's kind of the problem that you

694
00:32:23,680 --> 00:32:25,240
Speaker 2: don't really know the mass of the mountain.

695
00:32:25,360 --> 00:32:26,800
Speaker 1: Well, what you could do is measure the mass of

696
00:32:26,800 --> 00:32:29,280
Speaker 1: the mountain. You could say I know the density of rock,

697
00:32:29,480 --> 00:32:32,000
Speaker 1: and I could measure the volume of the mountain and

698
00:32:32,080 --> 00:32:34,480
Speaker 1: so from that I could estimate the mass of the mountain.

699
00:32:34,600 --> 00:32:37,240
Speaker 1: And this is actually what people did. The first measurements

700
00:32:37,280 --> 00:32:41,440
Speaker 1: of big G come from holding a pendulum near a

701
00:32:41,480 --> 00:32:44,360
Speaker 1: mountain in Scotland and seeing how it deflected.

702
00:32:44,560 --> 00:32:45,720
Speaker 2: No way, this actually.

703
00:32:45,520 --> 00:32:49,720
Speaker 1: Works, This actually works. Yes, it was a huge project.

704
00:32:49,800 --> 00:32:53,240
Speaker 1: This was done in the seventeen seventies. There's a mountain

705
00:32:53,360 --> 00:32:56,480
Speaker 1: in Scotland and it's a good choice because it's like

706
00:32:56,960 --> 00:32:59,320
Speaker 1: isolated from other mountains. It just sort of like sticks

707
00:32:59,400 --> 00:33:02,440
Speaker 1: up and like a nice symmetrical shape, which means that

708
00:33:02,480 --> 00:33:06,239
Speaker 1: it was not super complicated to describe its shape and

709
00:33:06,400 --> 00:33:09,960
Speaker 1: calculate its volume. And also it's like got really steep slopes,

710
00:33:10,040 --> 00:33:12,160
Speaker 1: so you get kind of close up to its center

711
00:33:12,240 --> 00:33:14,440
Speaker 1: of mass. Take the maximum effect. And there's like a

712
00:33:14,440 --> 00:33:17,280
Speaker 1: whole team of people that spent like years up there

713
00:33:17,360 --> 00:33:21,120
Speaker 1: making very precise measurements of pendula with heavy masses on

714
00:33:21,160 --> 00:33:24,560
Speaker 1: them and measuring their deflection and surveying the mountain to

715
00:33:24,600 --> 00:33:27,560
Speaker 1: try to estimate its volume as precisely as possible. It's

716
00:33:27,600 --> 00:33:28,560
Speaker 1: a huge project.

717
00:33:28,680 --> 00:33:30,960
Speaker 2: Wait, what so like if I hang a Billard ball

718
00:33:31,000 --> 00:33:33,920
Speaker 2: from a string right and hold it in front of me.

719
00:33:34,000 --> 00:33:36,560
Speaker 2: It's going to hang straight down. But as I walk

720
00:33:36,640 --> 00:33:39,400
Speaker 2: towards a big mountain, it's going to start to lean

721
00:33:39,640 --> 00:33:42,880
Speaker 2: or get pulled and actually start swinging that way.

722
00:33:43,000 --> 00:33:45,800
Speaker 1: Yeah, it will get pulled towards that mountain, so its

723
00:33:45,880 --> 00:33:49,320
Speaker 1: resting position will not be straight down. If you follow

724
00:33:49,320 --> 00:33:52,720
Speaker 1: the string up, it will not point perfectly towards the zenith.

725
00:33:52,920 --> 00:33:55,800
Speaker 1: It will be slightly deflected. And the bigger the mass

726
00:33:55,800 --> 00:33:58,600
Speaker 1: of the mountain, or the bigger the value of big G,

727
00:33:59,240 --> 00:34:01,680
Speaker 1: the stronger the deflection. So if you know the mass

728
00:34:01,680 --> 00:34:04,200
Speaker 1: of the mountain, then you can measure big g s.

729
00:34:04,320 --> 00:34:06,760
Speaker 1: This is the whole game, is knowing the masses of

730
00:34:06,760 --> 00:34:09,719
Speaker 1: two things and then measuring the forces between them. There's

731
00:34:09,760 --> 00:34:12,200
Speaker 1: a fun little wrinkle here, though. You might think that

732
00:34:12,280 --> 00:34:14,760
Speaker 1: you also need to know the mass of the earth,

733
00:34:14,920 --> 00:34:17,560
Speaker 1: because that's also pulling on your pendulum. But if you

734
00:34:17,600 --> 00:34:20,640
Speaker 1: know the volume of the mountain and the volume of

735
00:34:20,680 --> 00:34:24,080
Speaker 1: the Earth, which we do, then the angle of deflection

736
00:34:24,280 --> 00:34:27,920
Speaker 1: of the pendulum depends on the relative densities of the

737
00:34:27,920 --> 00:34:32,360
Speaker 1: Earth and the mountain, which one is denser. So really

738
00:34:32,520 --> 00:34:35,960
Speaker 1: the experiment measures the density of the Earth, which wasn't

739
00:34:36,000 --> 00:34:38,440
Speaker 1: known at the time. Of course, knowing the density of

740
00:34:38,480 --> 00:34:40,919
Speaker 1: the earth and the volume lets you calculate the mass

741
00:34:40,960 --> 00:34:43,800
Speaker 1: of the earth, and therefore let you get big g m.

742
00:34:44,520 --> 00:34:46,440
Speaker 2: I guess you could use this to like measure people's

743
00:34:46,440 --> 00:34:49,160
Speaker 2: masses too. Like if you walk around a better ball

744
00:34:49,239 --> 00:34:51,440
Speaker 2: on the string and just kind of walk around and

745
00:34:51,480 --> 00:34:54,240
Speaker 2: get it close to people, you could technically right measure

746
00:34:54,360 --> 00:34:54,960
Speaker 2: their mass.

747
00:34:55,880 --> 00:34:58,439
Speaker 1: Technically, yes, you could measure their mass.

748
00:34:58,080 --> 00:35:01,400
Speaker 2: And be like, hey, big Daniel, I mean uppercase Daniel.

749
00:35:01,120 --> 00:35:03,880
Speaker 1: If somebody had like accidentally ingested a lot of heavy metal,

750
00:35:04,080 --> 00:35:06,440
Speaker 1: you could detect it. Yeah, absolutely all right.

751
00:35:06,480 --> 00:35:08,719
Speaker 2: So that was in the seventeen hundred And they did

752
00:35:08,760 --> 00:35:10,799
Speaker 2: this and what did they find? What value did they

753
00:35:11,080 --> 00:35:11,520
Speaker 2: come up with?

754
00:35:11,880 --> 00:35:13,719
Speaker 1: So they made a measurement of this thing and they

755
00:35:13,760 --> 00:35:16,680
Speaker 1: got the number right to within about twenty percent. So

756
00:35:16,719 --> 00:35:19,360
Speaker 1: they measured this, which is I think pretty awesome. Like

757
00:35:19,400 --> 00:35:21,879
Speaker 1: this is a hard piece of work. There's one guy

758
00:35:21,920 --> 00:35:25,120
Speaker 1: who spent just like years calculating the volume of this

759
00:35:25,200 --> 00:35:28,399
Speaker 1: mountain from all these survey measurements. He turned it into

760
00:35:28,440 --> 00:35:31,239
Speaker 1: prisms and calculated the volume of each of those and

761
00:35:31,280 --> 00:35:33,280
Speaker 1: added them all up. And you know, this is before

762
00:35:33,320 --> 00:35:35,759
Speaker 1: computing and before any sort of modeling. This is in

763
00:35:35,760 --> 00:35:38,759
Speaker 1: the seventeen hundreds. He's working by like lamp light and

764
00:35:38,800 --> 00:35:41,000
Speaker 1: with a quill. But they got the right number within

765
00:35:41,120 --> 00:35:44,760
Speaker 1: twenty percent. So it's pretty impressive. And that also means

766
00:35:44,840 --> 00:35:47,600
Speaker 1: that they made the first real measurement of the density

767
00:35:47,719 --> 00:35:50,600
Speaker 1: of the Earth. They found that it was four and

768
00:35:50,640 --> 00:35:53,640
Speaker 1: a half times the density of water and almost twice

769
00:35:53,760 --> 00:35:56,480
Speaker 1: the density of that mountain in Scotland. That was a

770
00:35:56,480 --> 00:35:59,120
Speaker 1: bit of a surprise because we didn't know the internal

771
00:35:59,160 --> 00:36:02,560
Speaker 1: structure of the Earth in Newton's time. Some people thought

772
00:36:02,560 --> 00:36:06,400
Speaker 1: the Earth was like a huge hollow shell. So this number,

773
00:36:06,680 --> 00:36:09,520
Speaker 1: so much higher than the density of the mountain, was

774
00:36:09,560 --> 00:36:12,880
Speaker 1: a really fascinating early clue that the Earth has a

775
00:36:13,000 --> 00:36:16,680
Speaker 1: really very dense core. It's a really very cool result

776
00:36:16,760 --> 00:36:18,120
Speaker 1: with pretty basic tools.

777
00:36:18,560 --> 00:36:21,680
Speaker 2: Wow. Pretty cool. And so let's get maybe into some

778
00:36:21,719 --> 00:36:23,759
Speaker 2: of the other ways that people have measured this as

779
00:36:23,760 --> 00:36:26,319
Speaker 2: well as maybe the most recent measurements and see how

780
00:36:26,360 --> 00:36:28,880
Speaker 2: they measure up and it's weigh them together. But first

781
00:36:28,920 --> 00:36:43,719
Speaker 2: let's take another quick break. All right, we're talking about

782
00:36:43,719 --> 00:36:47,000
Speaker 2: the universal gravitational constant. We know that it's kind of

783
00:36:47,040 --> 00:36:50,480
Speaker 2: weak compared to the other forces, But it's super monumentally

784
00:36:50,480 --> 00:36:54,280
Speaker 2: important in the universe because it basically determines how stars

785
00:36:54,320 --> 00:36:59,000
Speaker 2: and planets form, how galaxies form, basically determines the whole

786
00:36:59,040 --> 00:36:59,959
Speaker 2: structure of the universe.

787
00:37:00,320 --> 00:37:03,680
Speaker 1: Yeah, exactly, it's one of the parameters on that control

788
00:37:03,760 --> 00:37:06,560
Speaker 1: panel of the universe that tells us why our universe

789
00:37:06,680 --> 00:37:09,200
Speaker 1: is this way and not some other way.

790
00:37:09,320 --> 00:37:11,400
Speaker 2: And so we're talking about how you actually measure this

791
00:37:11,480 --> 00:37:14,040
Speaker 2: because it's tricky because a gravity is so weak, but

792
00:37:14,160 --> 00:37:17,400
Speaker 2: also b you need to know the masses of things

793
00:37:17,480 --> 00:37:19,640
Speaker 2: before you can measure this constant. But to measure the

794
00:37:19,640 --> 00:37:21,600
Speaker 2: masses of things, you sort of need to know the constant.

795
00:37:22,080 --> 00:37:24,600
Speaker 2: And so people have tried different ways. They did it

796
00:37:24,640 --> 00:37:27,920
Speaker 2: first in the seventeen hundreds and they got within twenty percent.

797
00:37:28,000 --> 00:37:28,880
Speaker 2: What was the next step?

798
00:37:29,080 --> 00:37:32,600
Speaker 1: So that method holding a pendulum near a mountain worked,

799
00:37:32,600 --> 00:37:35,279
Speaker 1: but it was pretty imprecise because the mountain is like

800
00:37:35,320 --> 00:37:37,440
Speaker 1: a big fuzzy object. We don't really have a strong

801
00:37:37,480 --> 00:37:40,120
Speaker 1: handle on its density. You know, is it all the

802
00:37:40,120 --> 00:37:42,800
Speaker 1: same rock all the way through? What exactly is the

803
00:37:42,880 --> 00:37:45,360
Speaker 1: volume of it? And so people decided to shrink the

804
00:37:45,440 --> 00:37:49,000
Speaker 1: experiment down to something smaller that they could control. But

805
00:37:49,040 --> 00:37:51,279
Speaker 1: then you need a lot more precision because in the

806
00:37:51,280 --> 00:37:53,799
Speaker 1: effect it's going to be a lot lot smaller. So

807
00:37:53,920 --> 00:37:56,960
Speaker 1: instead of having one pendulum and a mountain, instead they

808
00:37:56,960 --> 00:37:59,239
Speaker 1: basically have two pendulam But if you just have like

809
00:37:59,280 --> 00:38:02,680
Speaker 1: two billion balls hanging near each other, the force between

810
00:38:02,680 --> 00:38:04,840
Speaker 1: them is so small that you're not going to be

811
00:38:04,880 --> 00:38:07,360
Speaker 1: able to measure any sort of deflection. So there was

812
00:38:07,400 --> 00:38:10,200
Speaker 1: a geologist, John Mitchell who came up with a really

813
00:38:10,360 --> 00:38:14,480
Speaker 1: clever way to measure a very very tiny force between

814
00:38:14,640 --> 00:38:15,680
Speaker 1: two billiard balls.

815
00:38:15,760 --> 00:38:17,680
Speaker 2: Essentially, how'd they do it?

816
00:38:17,920 --> 00:38:19,520
Speaker 1: So what they do is they have a pair of

817
00:38:19,560 --> 00:38:22,680
Speaker 1: these balls on a rod, and then they hang that

818
00:38:22,800 --> 00:38:25,439
Speaker 1: rod from a string, and then they bring two other

819
00:38:25,560 --> 00:38:28,520
Speaker 1: massive balls closer to these two balls on the rod,

820
00:38:28,880 --> 00:38:32,319
Speaker 1: and they measure how strongly the rod is attracted to

821
00:38:32,360 --> 00:38:36,320
Speaker 1: these other massive balls by measuring how far the string twists.

822
00:38:36,760 --> 00:38:39,520
Speaker 1: So instead of measuring like the deflection from the vertical,

823
00:38:39,560 --> 00:38:42,239
Speaker 1: which is a tiny, tiny amount, they can measure like

824
00:38:42,520 --> 00:38:46,440
Speaker 1: how far this thing has twisted this string it's hanging from.

825
00:38:46,560 --> 00:38:49,040
Speaker 1: So it's called a torsion balance.

826
00:38:49,040 --> 00:38:50,920
Speaker 2: Right, you're talking about a setup that's like what do

827
00:38:50,960 --> 00:38:53,280
Speaker 2: you call those like ornaments? You hang them from your ceiling.

828
00:38:53,600 --> 00:38:54,320
Speaker 1: It's a mobile.

829
00:38:54,520 --> 00:38:57,080
Speaker 2: A mobile, Yeah, that's kind of what you're talking about, right,

830
00:38:57,080 --> 00:39:00,000
Speaker 2: Like you make a mobile out of two biller balls

831
00:39:00,520 --> 00:39:02,200
Speaker 2: where you put them on a rod, and then you

832
00:39:02,280 --> 00:39:03,920
Speaker 2: hang the row from the center of it on a

833
00:39:03,960 --> 00:39:06,040
Speaker 2: string from the ceiling, and then you sort of see

834
00:39:06,120 --> 00:39:10,480
Speaker 2: how this mobile swings or turns when you put mass

835
00:39:10,520 --> 00:39:14,440
Speaker 2: bigger masses next to or near the to bigger ball exactly.

836
00:39:14,640 --> 00:39:17,560
Speaker 1: And so now you're measuring the angle of rotation of

837
00:39:17,600 --> 00:39:21,040
Speaker 1: your mobile basically as it turns towards the other balls.

838
00:39:21,600 --> 00:39:24,040
Speaker 1: And that's a little bit easier to measure than the

839
00:39:24,120 --> 00:39:27,360
Speaker 1: deflection relative to some vertical where you need to like

840
00:39:27,520 --> 00:39:30,399
Speaker 1: calibrate it to the stars. Here, you know how much

841
00:39:30,440 --> 00:39:33,920
Speaker 1: force it takes to twist this string. You can calibrate

842
00:39:33,960 --> 00:39:36,480
Speaker 1: that when the balls aren't around, And then you bring

843
00:39:36,520 --> 00:39:38,520
Speaker 1: the balls in and you see, like how much do

844
00:39:38,680 --> 00:39:42,200
Speaker 1: they twist the string? What is the equilibrium position between

845
00:39:42,239 --> 00:39:44,480
Speaker 1: the force that's trying to bring the mobile back to

846
00:39:44,520 --> 00:39:47,319
Speaker 1: its resting position and the force from the balls that's

847
00:39:47,320 --> 00:39:48,680
Speaker 1: pulling on it in the other direction.

848
00:39:48,960 --> 00:39:51,160
Speaker 2: The twisting of the string also tends to want to

849
00:39:51,280 --> 00:39:54,040
Speaker 2: bring it back to a neutral position, right.

850
00:39:54,400 --> 00:39:56,279
Speaker 1: Exactly If you just like twisted this thing up and

851
00:39:56,360 --> 00:39:59,200
Speaker 1: let it go. They would spin back eventually to its

852
00:39:59,239 --> 00:40:01,960
Speaker 1: resting position. And so just the way like a pendulum

853
00:40:02,000 --> 00:40:05,200
Speaker 1: is deflected by the mountain, here, this whole balance is

854
00:40:05,320 --> 00:40:08,240
Speaker 1: twisted a little bit by the presence of these other masses.

855
00:40:09,000 --> 00:40:10,640
Speaker 2: Interesting, and so they did this kind of at the

856
00:40:10,760 --> 00:40:13,839
Speaker 2: end of the seventeen hundreds, and how close did they get?

857
00:40:13,920 --> 00:40:17,680
Speaker 1: So Yeah, so this idea was by John Mitchell, a geologist. Unfortunately,

858
00:40:17,800 --> 00:40:20,840
Speaker 1: John Mitchell built the whole experiment and then died before

859
00:40:20,920 --> 00:40:23,200
Speaker 1: he could really use it. And it was Cavendish who

860
00:40:23,239 --> 00:40:25,480
Speaker 1: inherited this thing and did a bunch of really really

861
00:40:25,520 --> 00:40:28,719
Speaker 1: careful experiments, and he's the one for whom this experiment

862
00:40:28,800 --> 00:40:30,920
Speaker 1: is known. Unfortunately, Mitchell's sort.

863
00:40:30,760 --> 00:40:33,440
Speaker 2: Of lost line sounds very suspicious.

864
00:40:33,800 --> 00:40:36,640
Speaker 1: Yeah, lost to history, but you know Cavendish.

865
00:40:37,080 --> 00:40:41,480
Speaker 2: Seventeen hundred murder, Mystery Physics, it's a winning podcast episode, True.

866
00:40:41,320 --> 00:40:44,680
Speaker 1: Crime Science exactly. Anyway, he got a very precise measurement.

867
00:40:44,760 --> 00:40:48,080
Speaker 1: He measured it to within one percent of the true value.

868
00:40:48,239 --> 00:40:49,840
Speaker 1: And this was a big elaborate thing. It was like

869
00:40:49,840 --> 00:40:52,560
Speaker 1: a two meter wide box that this whole thing was in,

870
00:40:52,800 --> 00:40:54,680
Speaker 1: and he had to be enclosed in there to avoid

871
00:40:54,719 --> 00:40:56,960
Speaker 1: like air currents. He could only observe it through these

872
00:40:57,000 --> 00:40:59,640
Speaker 1: tiny little holes through which there were lenses. So it's

873
00:40:59,719 --> 00:41:02,680
Speaker 1: really elaborate setup, but it worked. And this is in

874
00:41:02,719 --> 00:41:04,960
Speaker 1: the late seventeen hundreds, and that was the most precise

875
00:41:05,000 --> 00:41:06,720
Speaker 1: measurement for about one hundred years.

876
00:41:07,080 --> 00:41:10,080
Speaker 2: Wow, it's pretty impressive what happened at one hundred years later.

877
00:41:10,200 --> 00:41:11,759
Speaker 1: So for the nexte of years, the folks who were

878
00:41:11,800 --> 00:41:14,840
Speaker 1: using the mountain method tried to beat Cavendish but failed.

879
00:41:14,880 --> 00:41:17,600
Speaker 1: They kept trying, like different mountains and different surveys, and

880
00:41:17,600 --> 00:41:19,640
Speaker 1: they spent lots of money and lots of time, and

881
00:41:19,680 --> 00:41:21,839
Speaker 1: sometimes they drank too much and actually like burned down

882
00:41:21,880 --> 00:41:24,640
Speaker 1: their whole facility. It's a very colorful history if you

883
00:41:24,719 --> 00:41:27,520
Speaker 1: look into it. But they never succeeded in beating Cavendish,

884
00:41:27,600 --> 00:41:30,800
Speaker 1: and wasn't until people improved on his torsion balance method.

885
00:41:30,880 --> 00:41:34,720
Speaker 1: But one hundred years later a scientist named CV Boys

886
00:41:35,160 --> 00:41:38,080
Speaker 1: was able to bring down the uncertainty, and people made

887
00:41:38,120 --> 00:41:40,600
Speaker 1: a little bit of progress over the next few decades,

888
00:41:40,640 --> 00:41:43,080
Speaker 1: so that like by the nineteen thirties we had a

889
00:41:43,120 --> 00:41:45,720
Speaker 1: measurement of it to within a tenth of one percent

890
00:41:45,880 --> 00:41:48,719
Speaker 1: and That sounds pretty good, right, But remember, like this

891
00:41:48,760 --> 00:41:52,279
Speaker 1: is a fundamental constant of the universe. Other constants we've

892
00:41:52,320 --> 00:41:55,760
Speaker 1: measured to like one part in a billion, so having

893
00:41:55,800 --> 00:41:57,880
Speaker 1: this down to like one part in one hundred or

894
00:41:57,880 --> 00:42:00,319
Speaker 1: one part in a thousand is not very impressive. It's

895
00:42:00,320 --> 00:42:03,800
Speaker 1: one of the worst measured physical constants in the universe.

896
00:42:04,000 --> 00:42:06,680
Speaker 2: Oh man, are you physics shaming those experimenters?

897
00:42:07,520 --> 00:42:10,160
Speaker 1: Now, I'm doing exactly the opposite. I'm saying, this is

898
00:42:10,200 --> 00:42:13,400
Speaker 1: so hard. It's a really, really difficult measurement. You know,

899
00:42:13,440 --> 00:42:15,719
Speaker 1: in order to do this, you have to completely isolate

900
00:42:15,760 --> 00:42:18,319
Speaker 1: your setup from everything else. You have to come up

901
00:42:18,400 --> 00:42:21,560
Speaker 1: with clever ways to account for everything to measure the

902
00:42:21,560 --> 00:42:24,320
Speaker 1: bias in your experiment. You know. The more recent measurements

903
00:42:24,360 --> 00:42:26,920
Speaker 1: people have been doing in the last few decades involve

904
00:42:27,000 --> 00:42:30,400
Speaker 1: clever tricks like put a mirror on the wire and

905
00:42:30,440 --> 00:42:32,960
Speaker 1: instead of measuring the angle of the balls, which is

906
00:42:33,000 --> 00:42:35,799
Speaker 1: really small, shine a laser on the mirror. Use the

907
00:42:35,800 --> 00:42:38,040
Speaker 1: motion of the laser spot to measure how much the

908
00:42:38,080 --> 00:42:40,400
Speaker 1: wire has twisted. These kind of tricks and all sorts

909
00:42:40,400 --> 00:42:44,280
Speaker 1: of other techniques to reduce the electrostatics on these balls.

910
00:42:44,600 --> 00:42:46,239
Speaker 1: It's really impressive. Amount of work.

911
00:42:46,800 --> 00:42:49,440
Speaker 2: I guess my main question is you're saying, like, we're

912
00:42:49,480 --> 00:42:52,600
Speaker 2: getting closer to the true measurement or the true value

913
00:42:52,640 --> 00:42:54,520
Speaker 2: of this constant. But how do you know what the

914
00:42:54,560 --> 00:42:56,600
Speaker 2: true value is? Like, how do you know you're only

915
00:42:56,840 --> 00:42:59,160
Speaker 2: tenth of a percent off or ten percent off? Like,

916
00:42:59,160 --> 00:43:01,239
Speaker 2: how do you know what act is supposed to be?

917
00:43:01,440 --> 00:43:03,319
Speaker 1: Yeah, that's a great point, and we don't know what

918
00:43:03,320 --> 00:43:06,000
Speaker 1: it's supposed to be. There's no prediction, right, so any

919
00:43:06,080 --> 00:43:08,680
Speaker 1: number could be the right number. And in the history

920
00:43:08,680 --> 00:43:10,960
Speaker 1: of these measurements, typically what happens is you have a

921
00:43:10,960 --> 00:43:13,160
Speaker 1: first measurement which is sloppy and rough, and then you

922
00:43:13,200 --> 00:43:15,200
Speaker 1: improve it and you get more and more precision. So

923
00:43:15,239 --> 00:43:17,319
Speaker 1: if you look at these things over time, they tend

924
00:43:17,320 --> 00:43:19,920
Speaker 1: to converge towards one value, which we say, oh, that

925
00:43:20,000 --> 00:43:22,560
Speaker 1: must be the true value. In reality, it doesn't always

926
00:43:22,600 --> 00:43:24,640
Speaker 1: work like that. We have some cases in history where

927
00:43:24,640 --> 00:43:26,920
Speaker 1: the value seems to converge to one number and then

928
00:43:26,960 --> 00:43:29,960
Speaker 1: it shifts, and that's because people know about the previous

929
00:43:30,000 --> 00:43:33,320
Speaker 1: results and they sort of want to reproduce the previous results.

930
00:43:33,440 --> 00:43:35,520
Speaker 1: So if like one of the first results was off

931
00:43:35,560 --> 00:43:38,080
Speaker 1: by a bit, then there's like an implicit bias in

932
00:43:38,160 --> 00:43:41,640
Speaker 1: people's experiments that tend to like find mistakes and bugs

933
00:43:41,719 --> 00:43:43,960
Speaker 1: until their number agrees with the previous number. It takes

934
00:43:44,000 --> 00:43:47,160
Speaker 1: a little bit more bravery and courage to disagree with

935
00:43:47,200 --> 00:43:49,360
Speaker 1: an established measurement. So you see those sort of like

936
00:43:49,440 --> 00:43:52,479
Speaker 1: jumps sometimes in the history of a measurement. This one

937
00:43:52,520 --> 00:43:55,640
Speaker 1: is particularly interesting because as the measurements have gotten more

938
00:43:55,680 --> 00:43:58,600
Speaker 1: and more precise, like in the last ten or fifteen years,

939
00:43:58,640 --> 00:44:01,200
Speaker 1: it's been a real cottage industry of making these measurements,

940
00:44:01,239 --> 00:44:04,280
Speaker 1: they've started to disagree. So now we have a bunch

941
00:44:04,360 --> 00:44:08,360
Speaker 1: of measurements of the gravitational constant with fairly small uncertainties

942
00:44:08,640 --> 00:44:12,280
Speaker 1: that disagree with each other by more than the uncertainties.

943
00:44:12,920 --> 00:44:15,520
Speaker 2: Interesting, So I think maybe when you say, like they

944
00:44:15,520 --> 00:44:18,600
Speaker 2: got within point one percent, you're not saying that that

945
00:44:18,600 --> 00:44:20,839
Speaker 2: it's not by point one percent. You're saying like their

946
00:44:20,880 --> 00:44:24,520
Speaker 2: confidence in their measurement is down towero point one percent,

947
00:44:24,560 --> 00:44:27,839
Speaker 2: like they think they're within that range. Right, It's more

948
00:44:27,920 --> 00:44:29,279
Speaker 2: like a measure of confidence.

949
00:44:29,400 --> 00:44:31,560
Speaker 1: Yeah, they usually quote and uncertainly they say it's this

950
00:44:31,680 --> 00:44:34,400
Speaker 1: number within a certain range. But we can also compare

951
00:44:34,520 --> 00:44:37,560
Speaker 1: their measurement to our current best understanding of what the

952
00:44:37,600 --> 00:44:40,520
Speaker 1: value is, so we can analyze their historical accuracy by

953
00:44:40,520 --> 00:44:42,120
Speaker 1: comparing it with modern measurements.

954
00:44:42,840 --> 00:44:45,799
Speaker 2: And so, is there no way to like derive this

955
00:44:45,920 --> 00:44:48,480
Speaker 2: from the equations of the universe or to you know,

956
00:44:48,560 --> 00:44:51,239
Speaker 2: tie back to some other more fundamental thing like the

957
00:44:51,280 --> 00:44:53,280
Speaker 2: mass of an electron, for example, or something.

958
00:44:53,480 --> 00:44:55,840
Speaker 1: No, there is not. There's no way to derive this.

959
00:44:56,239 --> 00:45:00,319
Speaker 1: It's totally unrelated to every other physical constant and every

960
00:45:00,320 --> 00:45:04,000
Speaker 1: other process in the universe. The gravitational constant doesn't just

961
00:45:04,040 --> 00:45:07,840
Speaker 1: control gravity. It only controls gravity. It doesn't determine anything

962
00:45:07,920 --> 00:45:10,440
Speaker 1: else in the universe. So there's no other way to

963
00:45:10,560 --> 00:45:12,520
Speaker 1: figure it out. The only way to do it is

964
00:45:12,560 --> 00:45:15,400
Speaker 1: to measure the force of gravity between two things. And

965
00:45:15,440 --> 00:45:17,480
Speaker 1: to do that you got to know their masses.

966
00:45:17,680 --> 00:45:21,480
Speaker 2: M we can't tell by you know, how light bends

967
00:45:21,520 --> 00:45:23,960
Speaker 2: around a black hole or something like that, or around

968
00:45:23,960 --> 00:45:24,319
Speaker 2: the sun.

969
00:45:24,560 --> 00:45:28,200
Speaker 1: Yes. Actually, as our calculations in general relativity get more

970
00:45:28,280 --> 00:45:30,800
Speaker 1: and more precise, we may be able to do things

971
00:45:30,880 --> 00:45:33,880
Speaker 1: like seeing how space is bent in the vicinity of

972
00:45:33,920 --> 00:45:36,200
Speaker 1: strong gravity, which might be able to give us a

973
00:45:36,239 --> 00:45:38,480
Speaker 1: new handle on how to measure this constant.

974
00:45:38,800 --> 00:45:41,359
Speaker 2: Pretty cool, But I guess until then that means like,

975
00:45:41,400 --> 00:45:44,120
Speaker 2: if our measurement of G is off by point one percent,

976
00:45:44,160 --> 00:45:47,359
Speaker 2: that means that any calculation that we make using G

977
00:45:47,719 --> 00:45:50,399
Speaker 2: is also off by at least that point one percent, right.

978
00:45:50,560 --> 00:45:53,720
Speaker 1: Mm hmm. Yeah. These days, we're down to about five

979
00:45:53,840 --> 00:45:56,839
Speaker 1: times ten to the negative five as a fractional uncertainty

980
00:45:56,880 --> 00:46:00,560
Speaker 1: on big G, so you know, like a few parts

981
00:46:00,600 --> 00:46:03,640
Speaker 1: per million, which is much more precise than historically but

982
00:46:03,719 --> 00:46:06,719
Speaker 1: still by far the worst measured constant. And you're right,

983
00:46:06,760 --> 00:46:09,919
Speaker 1: it means that we can't make very precise predictions about

984
00:46:09,920 --> 00:46:13,440
Speaker 1: what happens near black hole because they depend on big G.

985
00:46:13,680 --> 00:46:16,040
Speaker 1: So you need super precise measurements to nail big G

986
00:46:16,360 --> 00:46:18,719
Speaker 1: so we can then make super precise predictions.

987
00:46:19,239 --> 00:46:21,279
Speaker 2: It kind of sounds like maybe we'll never know the

988
00:46:21,280 --> 00:46:22,080
Speaker 2: true value of G.

989
00:46:22,400 --> 00:46:23,920
Speaker 1: It might be because the true value of G has

990
00:46:23,960 --> 00:46:26,479
Speaker 1: an infinite number of digits in it. In that sense,

991
00:46:26,520 --> 00:46:28,799
Speaker 1: will never know the true value of anything, even like

992
00:46:29,000 --> 00:46:31,680
Speaker 1: you know the mass of an electron or any other parameter,

993
00:46:32,239 --> 00:46:34,759
Speaker 1: because it has an infinite number of digits and you

994
00:46:34,800 --> 00:46:36,920
Speaker 1: can't have an infinite amount of experiments or an infinite

995
00:46:36,960 --> 00:46:38,680
Speaker 1: number of graduate students to measure them.

996
00:46:38,880 --> 00:46:40,480
Speaker 2: What I guess what I mean is like, at some

997
00:46:40,520 --> 00:46:42,239
Speaker 2: point you do need to know the masses of the

998
00:46:42,239 --> 00:46:44,560
Speaker 2: things involved in your experiment, and so but that for

999
00:46:44,600 --> 00:46:46,640
Speaker 2: that you also kind of need g and so there's

1000
00:46:46,640 --> 00:46:48,839
Speaker 2: as maybe going to be a little uncertain because of that.

1001
00:46:48,960 --> 00:46:51,360
Speaker 1: There's always going to be uncertainty exactly, and because we

1002
00:46:51,400 --> 00:46:53,840
Speaker 1: don't know this one very well, it makes everything in

1003
00:46:53,880 --> 00:46:55,240
Speaker 1: gravity more uncertain.

1004
00:46:55,560 --> 00:46:59,080
Speaker 2: All right, Well, sounds like there's still a lot of

1005
00:46:59,120 --> 00:47:01,400
Speaker 2: room for people to come up with some interesting experiment

1006
00:47:01,480 --> 00:47:02,760
Speaker 2: to measure this exactly.

1007
00:47:02,800 --> 00:47:05,279
Speaker 1: You might think it's a historical quantity, but people have

1008
00:47:05,320 --> 00:47:08,280
Speaker 1: been measuring these things in the last five ten years.

1009
00:47:08,280 --> 00:47:12,719
Speaker 1: It's like an area of active research understanding Newton's constant

1010
00:47:12,840 --> 00:47:13,480
Speaker 1: for gravity.

1011
00:47:14,400 --> 00:47:16,080
Speaker 2: So I guess the next time you weigh yourself and

1012
00:47:16,120 --> 00:47:18,520
Speaker 2: you're like, what, I weigh this much, you can maybe

1013
00:47:18,520 --> 00:47:22,200
Speaker 2: blame it on the uncertainty of the gravitational constant.

1014
00:47:22,560 --> 00:47:23,880
Speaker 1: That's right, blame Newton.

1015
00:47:24,080 --> 00:47:26,520
Speaker 2: I guess that still doesn't help you explain why you're getting.

1016
00:47:26,239 --> 00:47:27,720
Speaker 1: Old or why you're getting more silver.

1017
00:47:27,960 --> 00:47:30,040
Speaker 2: All right, Well, we hope you enjoyed that and maybe

1018
00:47:30,160 --> 00:47:32,160
Speaker 2: thought a little bit more about what we know about

1019
00:47:32,160 --> 00:47:34,520
Speaker 2: the universe and we still don't know and how we

1020
00:47:34,560 --> 00:47:37,200
Speaker 2: still don't know very basic things about it, like how

1021
00:47:37,280 --> 00:47:40,440
Speaker 2: much you weigh or how much how hard the Earth

1022
00:47:40,480 --> 00:47:41,279
Speaker 2: is pulling domin you.

1023
00:47:41,560 --> 00:47:43,600
Speaker 1: So for those of you looking to crack a deep

1024
00:47:43,640 --> 00:47:47,000
Speaker 1: secret of the universe, this is one of those frontiers.

1025
00:47:47,000 --> 00:47:49,360
Speaker 1: Maybe you'll find a reason why G has to be

1026
00:47:49,400 --> 00:47:51,360
Speaker 1: a certain value, or maybe you'll come up with a

1027
00:47:51,400 --> 00:47:54,160
Speaker 1: super clever experiment to nail it down very.

1028
00:47:54,040 --> 00:47:57,839
Speaker 2: Precisely, and then everyone will go ge whiz, I mean

1029
00:47:57,920 --> 00:48:01,880
Speaker 2: big g wheez I mean uppercase. All right, thanks for

1030
00:48:01,960 --> 00:48:03,560
Speaker 2: joining us, see you next time.

1031
00:48:11,440 --> 00:48:14,280
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain

1032
00:48:14,320 --> 00:48:18,319
Speaker 1: the Universe is a production of iHeartRadio. For more podcasts

1033
00:48:18,320 --> 00:48:23,000
Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

1034
00:48:23,040 --> 00:48:24,760
Speaker 1: you listen to your favorite shows.