What are land tides?

00:00:08
Speaker 1: Hey, Daniel, when you're at the beach, are you able to relax and forget about physics for a minute.

00:00:13
Speaker 2: You know, I love the beach. It's beautiful, but it's actually kind of hard to forget about physics at the beach. There's just so much physics happening right there in front of your eyes. The surf, the tides, the shimmering sunlight.

00:00:26
Speaker 1: It sounds like physics is burnt into your brain.

00:00:29
Speaker 2: No, it's just the physics is all around us.

00:00:31
Speaker 3: You know.

00:00:32
Speaker 2: The moon is up there in the sky literally squeezing the planet with its gravity.

00:00:36
Speaker 1: Yeah, it sounds like you need some physics sunblock there for your brain.

00:00:40
Speaker 2: I think you might have to just extract my whole brain.

00:00:43
Speaker 1: Feed it to the fish.

00:00:43
Speaker 2: It's good for something.

00:00:44
Speaker 1: I hope maybe the fish will get smarter. There'll be fishes. Him form a cartoonist and the author of Oliver's Great Big Universe.

00:01:07
Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor. You see irvine down here on the coast. But I hope not to end up as fish food eventually.

00:01:15
Speaker 1: Aren't we all fish food or warm food? Which would you prefer to be eaten by worms? Or fish or aliens aliens, But you'll be you'll be food. You won't really have a preference by them.

00:01:31
Speaker 2: Yeah, but you know, as long as they send me some secrets to the universe before they fry me up for dinner, we're all cool. Nice.

00:01:36
Speaker 1: It's all about the legacy and what do you know right before you become dinner.

00:01:41
Speaker 2: Yes, it's a huge difference to die knowing the answers to the secrets of the universe and not.

00:01:47
Speaker 1: I wonder, though, if your brain is full of all his knowledge, would you cause the aliens indigestion?

00:01:52
Speaker 2: No? I think I taste better. That's why they should tell me.

00:01:55
Speaker 1: Sor you'll be juicier. You'll be juicier with knowledge.

00:01:59
Speaker 2: I mean, I'll be happier, which probably makes me taste better. I don't know.

00:02:03
Speaker 1: Maybe happiness tastes bitter to the aliens. Maybe I prefer people who don't know a lot of things.

00:02:07
Speaker 2: In that case, physics is my survival strategy for when the aliens arrive.

00:02:11
Speaker 1: Taste like a nerd, so nobody wants to eat you.

00:02:13
Speaker 2: Don't eat me, I'm dorky.

00:02:15
Speaker 1: Taste don't eat me. I'm a geek. But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.

00:02:22
Speaker 2: In which we do our best to make you filled with knowledge so that you are less tasty to the aliens, or whichever direction that goes. We think that everything out there in the universe can and should be understood, and that everybody out there on Earth is capable of understanding it. And we want to share with you that joyous moment when the idea is click into your head and you go, Aha, that's how it all works.

00:02:46
Speaker 1: That's right. We like to serve up the entire universe to you on a plate and fill you up with amazing facts and knowledge about how the universe works, what makes it all tick, and what moves the Earth and the oceans out there around us.

00:03:00
Speaker 2: Some of the deepest questions in physics have to do with what's happening and the objects really far away at the hearts of neutron stars or black holes. Some of them have to do with particles between our toes and how they wiggle and dance and to and fro to make our reality. Sometimes these questions are connected, and what's happening up there in the cosmos affects things down here on Earth.

00:03:21
Speaker 1: Yeah, it's right. We are surrounded by invisible forces that move the things around us, that cost the weather, the earthquakes, the magnetic field around us, and also the tides.

00:03:32
Speaker 2: The deep history of physics is just people trying to understand everything they see. Why does it rain? Why does the sun rise? Why does the water rise and fall? Is it possible to understand everything that's happening to us in terms of some like clockwork mechanistic system. If we make our understanding complex enough, can we predict even the rise and fall of the ocean?

00:03:53
Speaker 1: And so today on the podcast we'll be tackling the question what are land tides now, Daniel, We're not talking about like the closed laundry detergent, are we like the kind of gets dirt out of your clothes tide pods?

00:04:12
Speaker 2: I think given the poor history of physics naming things, we should be pretty careful drawing conclusions just from the name of this effect.

00:04:20
Speaker 1: Oh so it is possible to a laundry episode.

00:04:24
Speaker 2: This episode might be like pouring detergent on your brain.

00:04:27
Speaker 1: Yes, we're airing out the dirty laundry of physics here.

00:04:31
Speaker 2: No, In fact, I think this is a topic which is actually probably pretty well named.

00:04:35
Speaker 1: Oh yes, sometimes it's well named. Sometimes it's not well named.

00:04:38
Speaker 2: Sometimes the quality of the naming ebbs and.

00:04:40
Speaker 1: Flows, that's right, And sometimes we like to serve that wave hopefully get to a place where we understand more about the universe and our whole planet. And so this is I guess something that is happening here on Earth, Daniel right.

00:04:52
Speaker 2: Land tides are happening here on Earth. They're happening on the Moon, They're happening on every planet we.

00:04:56
Speaker 1: Know of whoa interesting, even the gas planets.

00:05:00
Speaker 2: I guess they're probably having gas tides. I think there's medication you can take for that.

00:05:05
Speaker 1: All right, Well, it's an interesting question, and so, as usually, we were wondering how many people out there had thought about the idea of tides on land.

00:05:12
Speaker 2: Thanks to everybody who volunteers for this audience participation segment. We'd love to hear your voice on our podcast, so please write to me two questions at Daniel and Jorge dot com if you'd like to play.

00:05:23
Speaker 1: So think about it for a second. What do you think of when you hear the words land tides? Here's what people had to say.

00:05:29
Speaker 2: Well, we all know what sea tides are I guess it's probably the same effect, but not pulling water, pulling land.

00:05:40
Speaker 1: Maybe some land that behaves a little bit like water. I don't know.

00:05:43
Speaker 2: Maybe there are tides in the Sahara Desert that.

00:05:47
Speaker 4: Makes me think of the ocean tide. So the water being pulled by the Moon. I know, the Earth is kind of being squished a bit from it's not perfectly round, from the gravitational forces and centrifugal force and things like that. So I'm going to guess that that's part of all that together, Especially gravity due to the Moon, the Sun, other you know, astrological bodies all contribute to the land of moving.

00:06:14
Speaker 3: I haven't heard of land tides before, but I would imagine that they are similar to ocean tides, where the Moon's gravity affects the surface of the Earth, just on land instead of on the ocean.

00:06:29
Speaker 2: If I had to guess, I would say the effects of tidal force from the Moon non plate tectonics.

00:06:36
Speaker 1: Seems a little bit straightforward sounding. Perhaps tides related to the land.

00:06:42
Speaker 2: Yeah, exactly. The name seems to convey basically.

00:06:44
Speaker 3: What it is.

00:06:45
Speaker 1: Sounds like I'm the only one who thought about laundry detergent. Might be because it's time for me to do laundry. The tidy my laundry basket is getting a little high.

00:06:53
Speaker 2: You have a big stack of pajamas.

00:06:55
Speaker 1: That's right, my daily uniform.

00:06:59
Speaker 2: It's growing so long that it has its own gravitational pull, and so it's got some tide inside your house.

00:07:04
Speaker 3: That's right.

00:07:05
Speaker 1: It's causing the tide of bananas to evan flow just from the sheer gravity of its size.

00:07:11
Speaker 2: It might cause some marital tides if you're arguing about who's going to do that laundry.

00:07:14
Speaker 1: Yeah, yeah, yeah, but then it all tides over, so it's all good. Well, let's talk about tides in general, and we'll get to land tides, but first and maybe let's talk about the regular tides most people are familiar with, which are ocean tides or the tides in the sea.

00:07:29
Speaker 2: Yeah, because it turns out that land tides and ocean tides are pretty intimately connected. So you've got to understand ocean tides first. Ocean tides come from tidal forces. That's how they get their name. And tidal forces are just an effective gravity and of objects not being point masses, of things actually being like extended spheres, so the gravity varies. I mean that gravity gets weaker or stronger as you get further or closer to something. And so if you're in different places on the Earth, for example, you're going to feel the Sun's gravity or the Mund's gravity with a different strength.

00:08:02
Speaker 1: Well, let's maybe take a step back here and talk about what tides are. So like, if you're at the beach, the tide just means that the general, the average level of the ocean goes up and down.

00:08:11
Speaker 2: Yeah, when we talk about tides, we usually mean the ocean tides, and yeah, the ocean goes up and it goes down. Like if you're at the beach, sometimes the water rises up higher than it does other times. And you might notice when you're sitting there on the sand, you take a nap and you wake up, Oops, you're underwater. And that's not because of a rogue wave. It's because the overall level of the ocean has risen.

00:08:32
Speaker 1: When do you think humans first noticed this? Probably the first time they reached the beach, right.

00:08:36
Speaker 2: Yeah, I think people have always noticed this. I mean people have been living on the beach in southern California for tens of thousands of years and so pretty sure tides were part of their life.

00:08:45
Speaker 1: Right, But I imagine they started in the beaches of Africa perhaps.

00:08:49
Speaker 2: Oh yeah, absolutely, there's tides on every shore around the world.

00:08:52
Speaker 1: And now does it depend on the size of the body of water, Like is it bigger in some oceans or lakes? Does it happen in a lake.

00:08:59
Speaker 2: It turns out to be quite complicated the size of the tides. It depends on not just the depth of the ocean and the size of the body of water, but also like the shape of the harbor and the shape of the land nearby. There's all sorts of complicated effects that change how the ocean water goes up and down at a particular point on the shore.

00:09:20
Speaker 1: All right, Well, let's dig into the physics. You said it's sort of related to the gravity of the moon.

00:09:25
Speaker 2: The reason the ocean goes up and down is mostly because of the moon, also because of the Sun. The physics that underlies it is what we call tidal forces. If the moon is pulling on you, then it pulls on you harder. If you're on a spot on the Earth it's closer to the Moon than if you're on the other side of the Earth.

00:09:41
Speaker 1: And that's because that's just how gravity works, right.

00:09:43
Speaker 2: Yeah, that's how gravity works. The closer you are the heart of the force. So physicists tend to like to think about planets as point particles or Earth is just like a dot that has the mass of the Earth at its center of mass, and we think about the motion of the Earth using fat But in actuality, every place on the Earth feels a different tug from the Moon because they're all at different distances and different angles.

00:10:06
Speaker 1: Right Like, if you were just out there in space, if you were closer to the Moon, you would feel a stronger pull towards the Moon than if you were farther away from the Moon. And so the same thing happens to all the points around the Earth.

00:10:17
Speaker 2: The same thing happens to all the points around the Earth. And this isn't just connected to the Moon or to the Sun. It happens for everything that's gravitational. We talk about it in the context of black holes because black holes are extremely strong gravity. If you get too close to them, then the difference between the gravity on your feet and on your head might be strong enough to pull you apart. That's the origin of spaghetification. It's just tidal forces. The fact that you're not a point particle. If you're just a point particle, there'd be one force of gravity on you. But because you're an extended object with different distances from the black hole or from the Moon or from the Sun, you feel different forces of gravity at different points on your body.

00:10:54
Speaker 1: Interesting. So then where does the word title come in?

00:10:57
Speaker 2: Oh, the word title comes in because this is base an explanation for the ocean tides. So that's why we call them tidal forces, all.

00:11:04
Speaker 1: Right, So then how does the moon cause the tides in the ocean?

00:11:08
Speaker 2: So the way to think about tidal forces is to think about how the force of gravity on you is different than if you were at the center of the Earth. The center of the Earth is sort of like the reference gravity. If you're at the center of the Earth, what would be the gravity of the moon. And then as you move around the surface of the Earth, you can ask how is my gravity different than if I was at the center of the Earth. And so if you're on a point on the Earth that's closer to the Moon than the center, then you're going to feel stronger gravity towards the Moon. If you're on the other side of the Earth. Now you're farther from the Moon, you're going to feel weaker gravity. So the tidal force is the difference. So what that means is that on the closer side of the Earth, there's basically a tidal force towards the Moon, and on the distant side of the Earth is a tidal force away from the moon.

00:11:52
Speaker 1: Oh why is it away? And I still being pulled towards the moon even if I'm on the far side of the Earth.

00:12:00
Speaker 2: Are still being pulled towards the moon. The force of gravity is towards the Moon, but it's less than the force of gravity you would feel at the center of the Earth. So relative to the force of the center of the Earth, the tidal force is away from the Moon.

00:12:12
Speaker 1: I see, you're being pulled less than the center. So in a way, you're being pulled away from the center of the Earth exactly.

00:12:17
Speaker 2: And that's all we usually talk about, how one part is being pulled towards and one part is effectively being pulled away in a relative sense. But this also sort of sideways tidal forces. If you're at a point along a circle that's between the point that's closest to the Moon and the point that's furthest from the Moon. Then the effective tidal forces are towards the center of the Earth because the Moon is pulling you not just towards the Moon, but also basically towards the center of the Earth.

00:12:43
Speaker 1: So, for example, like the moon sort of rotates around the Earth sort of along the equator line more or less, right, Like that's the kind of the circle that goes around and A. So you're saying, like the north and south poles of the Earth are being pulled kind of towards the equator basically.

00:13:00
Speaker 2: Yeah, that's right. And so the overall effect is just try to squeeze the Earth like into a football shape. The closest part gets pulled closer, the furthest part gets pulled less, and the center gets squeezed towards the center of the Earth.

00:13:14
Speaker 1: All right, So then the effect of the Moon is to squish the Earth. It's not just like pulling at the center of the Earth. It's like kind of pulling out the Earth, but also kind of squishing it or kind of trying to pull into a football shape.

00:13:25
Speaker 2: Yeah, exactly, And this is all relative to how the Moon would be pulling on the Earth. If it's just like a point particle at the center of mass, because the Earth isn't. The point particle has all these different gravitational forces on it, and the effect is basically to try to squeeze it into a football.

00:13:40
Speaker 1: And the weird thing is that the Moon is moving relative to the Earth, so it's like it's squeezing in one moment, it's squeezing it in one direction to make it look like football. But then as the Moon rotates, then now all those forces changes, so it's almost like you're massaging the Earth exactly.

00:13:54
Speaker 2: You're massaging the Earth, and you're running circles around the Earth at the same time, so you're always massaging different spots.

00:14:00
Speaker 1: Yeah, it's a three sixty massage.

00:14:03
Speaker 2: And that's the origin of the tidal patterns, right the tides go up and the tides go down. Basically, the water on the surface of the Earth gets pulled into that football shape because water is much more flexible than the earth is, So the water is deeper the conical parts of that football and shallower at the edges of the football.

00:14:21
Speaker 1: What do you mean, it's more flexible.

00:14:22
Speaker 2: It's easier to squeeze water than it is to squeeze granite.

00:14:26
Speaker 1: Or in this case, you're saying it's easier to stretch water.

00:14:29
Speaker 2: Yeah, exactly, to stretch water.

00:14:31
Speaker 1: So like the parts of the the equator there are closest to the Moon, the Earth is being squeezed and stretched, but the water on the surface on that side is getting stretched even more, and that's what causes the tides.

00:14:42
Speaker 2: And that's what causes the tides. Not just the Moon though, right the Sun also plays a role because there's tidal forces from the Sun. The Sun also has gravity on the Earth, and as the Earth moves around the Sun, different parts of the Earth feel gravity differently from the Sun because there are different distances from the Sun.

00:15:00
Speaker 1: Says maybe as a kid, I thought that maybe I knew that the tides were caused by the Moon, but I thought maybe it was like it was pulling the water. And so it's basically like sucking all the water to one side of the Earth, like it's bringing water from the far side of the Earth to the near side of the Earth facing the Moon.

00:15:15
Speaker 2: It is doing that, but on the other side, it's also sort of pulling the Earth harder than it's pulling the water, and so it's leaving some of the water behind, so you get deeper water on the spot closest to the Moon and on the other side as well.

00:15:28
Speaker 1: Whoa wait, wait, wait, but I thought the water was more squishable.

00:15:32
Speaker 2: Water is more squishable, so it responds more quickly to these tidal forces. So the tidle forces on the spot closest to the Moon are pulling it towards the Moon, making that water deeper. The tidal forces on the spot opposite side are pushing things away from the center of the Earth, and so that's sort of pushing that water up and then around the circumference it's squeezing it. So squeezes that water and pushes it towards the sort of cones of the football.

00:15:56
Speaker 1: Mmmm. And so like if you look at a tide schedule for a bee, which you're seeing like the dynamics of the Moon and the Sun and the effects they have on the Earth. Yeah, exactly, It's like this dance between all three things is what causes those weird tidal charts.

00:16:10
Speaker 2: Yeah, the dance between those three things is part of the explanation. The Moon dominates. Of course, the Sun is much more massive and has more gravity, but we're much further from the Sun, and on balance, the Moon's tidal forces are like two times as powerful as the Sun's. Overall, this is a pretty tiny effect relative to Earth's gravity. It's like one part per million or less variation in the Earth's gravity, but it can cause big effects. Like there's places in Canada where there's a sixteen meter difference between the low tide and the high tide.

00:16:40
Speaker 1: Whoa wait, sixteen meter difference in the height of the water, or like the shoreline in the height of the water. Oh wow, why is it more there than in other places?

00:16:50
Speaker 2: So there's a bunch of different things going on. Number one is it's not just the simple model of the Earth and the Moon. Also, the Moon is sort of declined relative to the Earth. It's not actually orbiting around the equator, which means that like sometimes it's closer to the northern hemisphere and sometimes it's closer to the southern hemisphere. So you get this cycle they call a fortnightly cycle fortnite meeting every two weeks, because the Moon's orbit is about a month obviously, and so every two weeks you're in one half of the cycle and every two weeks you're in the other half of the cycle. So there's lots of these little effects that are all gravitational that tug on the water differently. And then there's the response of the water. So it depends on like the shape or the harbor and how the deep ocean tides are affected and all sorts of like resonance effects in the water because you're pulling on the water, but it's not responding instantaneously that it's sloshing around, So which is why you get different tides in different places.

00:17:42
Speaker 1: Right, Like I think in the Caribbean Ocean the tide changes are much less than in the Pacific Ocean, for example.

00:17:49
Speaker 2: Mm hmm, yeah, exactly. So it depends on the depth of the water, depends on the shape of the shore, it depends on how the water is flowing. It's very complicated, and actually out there in the deep ocean, we don't know very much about the effective tides and the depths of them. We've measured them on the shore because they're very important for boats coming in and boats leaving, but out there in the deep ocean it's much murkier.

00:18:10
Speaker 1: Well, I wonder if it's a bigger effect, Like the deeper the ocean is, the more it's going to basically get stretched by the moon. So like maybe in the middle of the ocean, the height of the ocean is changing a lot.

00:18:21
Speaker 2: Yeah, exactly. We just have fewer sensors out there and fewer like immediate reasons to know. We're curious from a sort of like geological perspective, you know, for people who really want to understand the tides, but it's not as important for understanding like when your tunic crawler is going to come back.

00:18:36
Speaker 1: All right, well, let's dig into what land tides are now and how those affect the shape of the Earth and maybe how these forces can also affect our whole galaxy. So let's dig into that. But first let's take a quick break. All right, we're talking about tides now, Daniel, what's the history of tides and our understanding of them.

00:19:07
Speaker 2: Yeah, we've known about tides forever, right, as long as we've known about lightning and rain and all sorts of stuff, and people have been wondering for a long time about the cause of it and trying to understand it. And it's obviously connected to the moon, right, the cycles of the moon are connected to the tides, and so as far back as the Greeks, we have records of people speculating that somehow the moon caused it. The Greeks had no understanding of gravity as we do, of course, so they definitely didn't understand tidal forces and couldn't put together like a mechanistic explanation.

00:19:39
Speaker 1: But somehow they knew is related to the moon.

00:19:41
Speaker 2: They suspected it was related to the moon, just because it's linked to lunar cycles, right, is it?

00:19:46
Speaker 1: I don't know, like it the tide goes up and down twice a day, I think for most beaches, at least in California, But I don't see the moon go up and down twice a day.

00:19:56
Speaker 2: I think if you keep close track of where the Moon and the Sun and the Earth are, and the Greeks definitely did this, and we have also evidence of Islamic astronomers doing this. You notice the connection between where the moon is and where the tides are, right, because remember it's the location of the moon that's dictating. It's where the moon is closest to the Earth is where there are going to be high tides, and then also on the other side of it. So the location of the Moon is definitely connected to the height of the tides. You don't need to know gravity just to see this connection.

00:20:24
Speaker 1: But is our wonder if there's a delay, like do you see a delay between when the moon is closest to the Earth and when the tides are highest.

00:20:31
Speaker 2: There's definitely a delay, right, because water doesn't respond instantaneously and there are all these other effects. But people have been debating what the actual mechanism was. Galileo, for example, suggested that it just had to do with the sloshing of water as the Earth moon around the Sun. He thought that if you had an object covered in water and it was orbiting the Sun, that the water was going to slosh around it the way like a bucket would slash as you walk with.

00:20:53
Speaker 1: It, or whonderver. They thought like maybe there's a like a giant tick in a bath in the ocean, Like did they think maybe, you know, like the archimittees of fact, like if you get an ocean the water rises, Did they think maybe it was related to something.

00:21:07
Speaker 2: Like that, that'd be a very regular giant. Yeah. The Greeks probably did have some mythology related to the tides. Poseidon probably controlled them. I'm not an expert in that, but the first real explanation for the tides in terms of a mechanism comes of course from Isaac Newton in his groundbreaking where Principia, where he lays out the theory of gravity. He's also at the same time able to explain the tides and why the tides happen twice a day because the water piles up at the spot closest to the moon and the spot furthest.

00:21:35
Speaker 1: Interesting because the Moon goes around the Earth once a day, or I guess technically the Earth spins once a day.

00:21:41
Speaker 2: Yeah, the motion of the Moon relative to the Earth is roughly once per day. Yeah, that's what a day is really.

00:21:46
Speaker 1: And then later, I guess we got fancier with the models of this and we're able to predict more accurately what the tides are going to be.

00:21:53
Speaker 2: Yeah, exactly. Newton's calculations allow you to predict the forcing, like how much force there is from the Moon on the water, but the actual motion of the water requires you to understand fluid dynamics and ocean depth and take ato account like the Earth's rotation and the shape of the coast, and so Over the last few hundred years people have added all that understanding to a model of the tides, and now we can predict the tides like years in advance at any point on the coast.

00:22:19
Speaker 1: Wow, just from the position of the moon where we know it's going to be in the future, and the.

00:22:23
Speaker 2: Sun exactly, and the models of the ocean and the Earth's rotation taking all of this into account.

00:22:28
Speaker 1: M all right, Well, let's get to the question of the episode, which is what are land tides?

00:22:33
Speaker 3: Now?

00:22:34
Speaker 1: Daniel, I don't see the land going up and down at the beach or anywhere, So what does this term mean?

00:22:41
Speaker 2: So the term means exactly what you think. It means that the Earth's land goes up and down the same way that water in the ocean does. It's just that the Earth doesn't respond the same way the ocean does because the Earth is not made of water.

00:22:54
Speaker 1: All right, So what do you mean by the land tide? Am I going to see this effect? Or am I just gonna have to think about the idea that maybe I'm further away from the center of the Earth sometimes than other times.

00:23:05
Speaker 2: It's not something you need to factor into your daily life. It's not really gonna change your drive to work, or even if you work on like a shrimp trawler or any sort of ocean vessel. It's not really gonna affect you, but it is cool, I think to understand that the whole earth is being squeezed. The taidle forces don't just affect the water. The earth is also there feeling them. And the earth is not perfectly rigid. Right, Yes, it's made of rock, which is definitely stiffer than the water, but it's not perfectly rigid, and so it responds.

00:23:33
Speaker 1: Meaning like you can technically squeeze and stretch rock. Right, It won't stretch or squeeze a lot, but technically it's gonna move a little bit, tiny bit.

00:23:40
Speaker 2: Yeah, exactly, and it's not even a tiny little bit. I was reading a calculation that says that New York City rises by more than a foot as these land tides progress.

00:23:50
Speaker 1: Whoa, but relative to what I guess, yeah.

00:23:54
Speaker 2: Relative to the center of the earth. So it's not something you notice because it's not like one block in Manhattan moves up a foot relative to another block. The whole city moves together as the crust of the earth is flexing.

00:24:08
Speaker 1: Basically, you're sort of treating the earth like a bouncy ball, right, or like one of those stress balls.

00:24:12
Speaker 2: Yeah. And part of the history here is really cool because it goes back to when people were trying to figure out what is the Earth made out of? Like is the Earth just a big ball of solid rock or is it totally molten and fluid? Like is the Earth a water balloon or is it a marble or is it a bouncy ball? And so people were trying to figure this out hundreds of years ago, before they could like dig into the earth, or before we had modern seismology that reveals what's going on inside of the Earth by how earthquakes like bounce around on all the internal layers.

00:24:41
Speaker 1: You mean more like a lava balloon.

00:24:43
Speaker 2: Yeah, a lava balloon exactly. In the late eighteen hundreds, the prevailing theory of what was inside the Earth was that it was molten. Right, people saw lava, and oh, well, maybe the Earth is just filled with lava, right, maybe it's fluid. So then scientists try to calculate like, all right, well then, and what should be the earth tides? If the Earth is just a balloon filled with lava, then Lord Kelvin actually calculated that we should have huge earth tides because if the Earth is as flexible as the ocean is. Right, if it's just basically an ocean of lava underneath a thin crust, then we should be seeing huge tides as big as we see in the ocean.

00:25:21
Speaker 1: But would lava be as quishable as water, Isn't it the thicker and more denser.

00:25:26
Speaker 2: Yeah, it's definitely thicker and denser. So it's not exactly like water. But if you take all that into account, you predict huge earth tides, you predict that the land was should change by meters and meters, and we definitely didn't see that. And so that's one very early sign they had that the earth is more rigid than water internally. It's just like a way to see what's inside the earth without even looking inside.

00:25:49
Speaker 1: So if the earth was made out of all a lava inside, we would we would basically see New York City rise and fall by a lot more than a food Yeah, exactly would Like would you would you? I guess you would see it as a wave, like New York City would rise a bunch and then go down, and then you know New Jersey or Pennsylvania would then rise and fall, right, that's kind of the idea.

00:26:11
Speaker 2: Yeah, And you would have much more dramatic like earthquakes, because land tides can cause a lot of pressure on the boundaries between the plates and cause all sorts of earthquakes, So it would make life on Earth much more dramatic.

00:26:23
Speaker 1: But thankfully we're not a giant love of ball. We're fairly rigid. We're mostly rock up until the core of the earth, right or the inner mantle.

00:26:33
Speaker 2: Yeah, it's quite dense, but it is actually pretty fluid. But the outer edge is pretty hard and definitely much less fluid than water. So original calculations by Lord Kelvin, the fact that he couldn't see any earth tides himself, basically let him estimate that the internal part of the Earth was more rigid than glass, but less rigid than iron for example, so very very rough. And then around one hundred years ago people started trying to actually measure this thing. So people built these tilt meters to try to see, like, can you tell if the Earth is tilting, Like if you have essentially a bathtub filled with water, then the surface of the water tells you like where gravity is all equal, and then if the surface of the Earth tilts on which the bathtub is sitting, right, there's like a wave that passes through the ground, it'll tilt the bathtub and you'll see waves in the bathtub. So people were using these like long tubes of water looking for waves in them to see if they could measure the earth tilting, looking basically for waves in the ground.

00:27:33
Speaker 1: Interesting, So, like if you wanted to see New York City when up a foot or not or more than a foot, you would maybe put one of these tiltmeters on the outskirts of New York City, so that when New York City goes up, this thing would tilt and would tell you, hey, new York City went up.

00:27:47
Speaker 2: Yeah, exactly. These days, you can measure land ties most precisely with gravitometers. Things are basically accelerometers. You can measure the strength of gravity because you're measuring the acceleration of the Earth Earth pushing up on you to overcome your natural freefall. So essentially every accelerometer really is a gravitometer. So the way they measure land tides now is they measure how the gravity is varying, because as you get further from the center of the Earth, your gravity decreases.

00:28:15
Speaker 1: But doesn't the moon effect that gravity.

00:28:17
Speaker 2: As well, it absolutely does. Yeah, So what you're measuring is you decrease on your gravity from being further or closer to the center of the Earth. And you're also measuring the Moon pulling on you directly and the sun. So you got to disentangle all of this stuff to get the land tide effect out. But that's how they measure it most precisely.

00:28:34
Speaker 3: Now.

00:28:35
Speaker 1: So like you're saying, like, my weight here on Earth varies depending on where the moon is, not on what I had for lunch. Yeah, all of those things.

00:28:44
Speaker 2: All of those things absolutely. Yeah, the moon is pulling on you differently based on where it is, so it's like the tidal force on a joorghe directly. But then also the Moon is changing the shape of the Earth, which changes your distance from the center of the Earth, which changes the earth gravity on you. And at the same time, the Moon is pulling on the oceans, which makes some of the oceans deeper or less deep, and that also has an effect on the shape of the Earth. Right, there's this like ocean pressure, this irregular loading on the Earth also affects the shape of the Earth. And then back and forth because as Earth itself gets squeezed by the moon that affects the ocean tides, So a whole thing is coupled in a really complicated way. Yeah, it seems complicated. So then if you measure just the gravity, how do you know what's doing what? You have to have a model of all these things, and then you can disentangle the various pieces as you measure how it changes over time.

00:29:35
Speaker 1: So then if the moon is directly over me, then I'm going to weigh a little bit less? Am I going to be able to feel that? Like, what's the effect of the moon on me?

00:29:43
Speaker 2: You could measure it if you had a very sensitive gravitometer. The size the effect relative to the acceleration due to Earth's gravity again is like one part in a million or one part in ten million, So you need a pretty precise gravitometer to detect this effect.

00:29:57
Speaker 1: Well, I wonder you know, there's so much of biology on Earth that is tied to the moon cycle, or at least sort of tied to the moon cycle. Do you think all of this biology is measuring the gravity somehow or is it just going by moonlight?

00:30:10
Speaker 2: I think you're absolutely right that a lot of biology is tied to the tidal cycle and the cycles of the moon. But I think that's because the local effects, you know, the tide going up, the tide going down, or there being more moonlight or less moonlight. I think all these things are dependent on those secondary effects, not directly on the gravity. But hey, I could be wrong. You know, we recently discovered that birds have little quantum mechanical sensors in their eyeballs that detect the magnetic field of the Earth and help them navigate. So biology continues to surprise us.

00:30:38
Speaker 1: Yeah, we might have sensors within us that can detect a millionth of a grammar or something of gravity changes.

00:30:45
Speaker 2: Biology is amazing. I'll never understand it.

00:30:47
Speaker 1: All right, well, those are land tides. Now let's get into even bigger tides, bigger waves. Let's talk about galactic tides what that means about our search for other planets. But first, let's take an o quick break. All right, we're talking about the tides, why they're high weather low, And we talked about how the tides here on Earth a fact, not just the oceans, but the Earth itself. The Moon is squeezing all the rocks on Earth, changing the shape of the Earth, making its rise and fall and now Daniel, you're saying this also happens on a galactic level.

00:31:34
Speaker 2: Yeah, the motion of the Sun through the galaxy that depends on gravity. It's being pulled by the gravitational center of the galaxy, this huge black hole and all those stars. It's also being pulled by lots of other stars. And so the motion of the Sun through the galaxy is gravitational. But our solar system itself is not a point object, right, So the galaxy pulls differently on different parts of the Solar system. The parts that are closer to the center feel a stronger gravitational force than things that are further from the center. And so the same way that like the Moon is massaging the Earth, and of course the Earth is massaging the Moon, and the Sun is massaging everybody, the center of the galaxy is massaging our whole solar system.

00:32:15
Speaker 1: WHOA, because I guess technically we're going around the galaxy center sort of like the Earth is going around the Sun, right exactly, Like we're trapped in the gravitational pol or pull of everything in the galaxy, of.

00:32:28
Speaker 2: Everything in the galaxy. Yeah, it takes about a quarter of a billion years for the Sun to go all the way around the galaxy, and it's like a thirty million year cycle for it to go like above the plane and below the plane. It's got this really funky orbit around the center of the galaxy. But yeah, the Sun is locked in gravitationally around the center of the galaxy, and then gravity is massaging the Solar System. Remember the Solar System is not just the Sun, right, it's also the planets. So the Earth's or it gets tweaked a little bit by this gravitational massage, which is one reason we talked about, like the stability of the Earth's orbit is important because there's all sorts of things like perturbing it slightly. But on the outside of the Solar System is this huge cluster of icy rocks called the ort cloud, billions or maybe trillions of blocks of ice, which we think are probably the source of long period comets. Commets with periods like hundreds of years. The galaxies squeezing them can perturb them, so they might be like in a stable orbit around the Sun, but then they get pushed to tying the little bit and they get tugged on by other stuff and boom, now they're falling in towards the center of the Solar system.

00:33:32
Speaker 1: M I guess what you mean is like as the Earth goes around the Sun in the Solar system, sometimes it's closer to the center of the galaxy than it is in other times in our orbit. And so that you're saying the galaxies then is shaping our orbit here of the Earth.

00:33:47
Speaker 2: The galaxy definitely plays a role in the orbit of the Earth and in its eccentricity. The Earth is pretty stable in its orbit, as we talked about, but other things are much less stable. They're lower mass, they're much further from the Sun. It's unlikely that the galactic tides are going to cause the Earth to plummet into the Sun. Some little distant rock that's barely being held by the Sun's gravity is much more likely to get its orbit disturbed and then to fall in towards the center of the Solar system.

00:34:14
Speaker 1: Doesn't it depend on like the orientation of our Solar system relative to the plane of the galaxy. Like at the galaxy and our Solar system we're in the same plane, then you would see a huge effect. Right, Like as you go around the Sun, then sometimes you're really far away from the galaxy the center of the galaxy. Sometimes you're really closer. But let's say like our solar system was more like a bicycle wheel going around the center of the galaxy, then would it matter where in the orbit you are, you would be almost the same distance from the center of the galaxy.

00:34:43
Speaker 2: Yeah, you're right. For things that orbit in the plane of the Solar system, that is very important. And our solar system is tilted relative to the plane of the galaxy. How tilted, so there's like a sixty degree angle between the plane and the galaxy and the planetary orbital plane, so it's really quite tilted. It's not fully a bicycle wheel, but there's like a sixty degree tilt there. But the stuff in the orc cloud is not mostly in the plane of the planets. The oor cloud is like a sphere hasn't collapsed into a disk or into the plane, and so that stuff is always feeling tidal forces. There's always something closer to the center of the galaxy and something further interesting.

00:35:19
Speaker 1: So then and as you're saying, these rocks in these asteroid belt are more sensitive because they're smaller, and so then the galactic tidal forces can really make a difference.

00:35:29
Speaker 2: There yeah, they're smaller, but also they're further from the Sun and so the Sun just has less powerful hold on them. And this is a big deal for life, right, because commets plummeting towards the center of the Solar system can be pretty deadly. One of them impacts Earth than boom, right, mass extinction.

00:35:45
Speaker 1: But also commets might bring water and things like that, Right, those might be important then for life.

00:35:51
Speaker 2: That's exactly right. We think that huge fraction of our oceans may come from commets doing just that. So in terms of like life evolving, you want a bunch of galactic tides squeezing your work cloud so that you rain down frozen comets onto newly formed planets so they get oceans and form life. But then you don't want a whole lot later on while that life is being incubated and it's still fragile.

00:36:16
Speaker 1: Well, speaking of life, also, aren't tidal forces in other moons, for example, around the moons of Jupiter. Don't those effect like the temperature of those moons, which might affect whether there's life or not.

00:36:28
Speaker 2: Absolutely we think that some of the internals of moons around Jupiter are warm precisely because of these tidal forces, and you're squeezing the entire planet, which causes friction inside of it. Because you're massaging, you're changing its shape. It's not just like it becomes a football and now it moves like a football, which part of it is the cone of the football is changing. So you're constantly squeezing it. It's like you take cold dough out of the fridge and you need it. It warms up, right, you're applying energy to it. Internal friction.

00:36:57
Speaker 1: Yeah, like if you massage a ball of clay, it's going to warm up eventually.

00:37:01
Speaker 2: Exactly. So some of these moons are warm on the inside. We think they have liquid water oceans underneath a frozen crust because of tidal forces, not because of internal heat like the Earth, but because of tidal forces. Could be keeping those moons warm and providing a place for life here in our solar system or in other solar systems whoa so.

00:37:21
Speaker 1: In for example like Europa, right, which is the moon of Jupiter. If there's life down there, basically their only source of energy might be the tidal forces of Jupiter.

00:37:30
Speaker 2: Yeah, exactly, and those are moon tides, I guess you'd call them. Not sure, you're squeezing the whole Moon, and that's where the energy comes from. Basically, you're extracting it from the gradient of gravity of Jupiter, right.

00:37:43
Speaker 1: Yeah, and they wouldn't see the Sun from underneath those oceans in the ISO, like basically, Jupiter gravitationally would be like their Sun.

00:37:52
Speaker 3: Yeah.

00:37:52
Speaker 2: I was just thinking about whether they'd also be able to detect the gravity of the Sun. Imagine your life that's formed under that ocean. You've never seen the star, but if you're very sensitive to gravity, you might detect the presence of Jupiter, and you probably eventually would detect the presence of the Sun as well, perturbing effectively the gravity of Jupiter. That'd be much more difficult than it is here on Earth because Jupiter's gravity would be stronger and the Sun's gravity would be weaker, so it'd be a much more subtle effect than the Sun's gravity here. But eventually those underwater scientists would figure it out.

00:38:22
Speaker 1: Interesting, you mean those fishieses.

00:38:26
Speaker 2: And that would make them tastier because of that knowledge.

00:38:30
Speaker 1: Now, wonder if your fish that studies the stars, are you astrophisies?

00:38:35
Speaker 2: I think I'm finishist with this.

00:38:39
Speaker 1: I'm just fishing for ideas here, Daniel, It's.

00:38:41
Speaker 2: True that tides may power life underneath the frozen crust of a moon. It's also possible the tides had a really important role in the development of life here on Earth and the development of life on other exoplanets.

00:38:52
Speaker 1: Yeah, because I think scientists think that the tides had a lot to do with how life evolved here on Earth, right, or at least maybe how life got out of the oceans.

00:39:01
Speaker 2: I hear a lot of different ideas, and all of it's very speculative, all the way back to like the actual formation of life itself from non life. That the tides and the sloshing of all that water at the shore helps like mix together primitive molecules that can come together to make DNA or RNA or whatever, self replicating molecule. Basically that the turbulence itself there was crucial biochemically that you know, if water was just like static and sitting there, it would take much longer for self replicating processes to get going.

00:39:32
Speaker 1: Meaning like the moon and the tides are a way to steer the ocean kind of in a way which in which you maybe kind of need in order to just get life going. Makes it makes the right chemical.

00:39:42
Speaker 2: I mean, nobody likes to be called a pot stir but yeah, the moon is basically stirring the pot down here. The Yeah, there's lots of different speculations. It might just be the tub stirring. Some people think that having the tides is like a way to concentrate solutions. Like you throw water up on hot rocks that were sitting in the sun, and then the water evaporates and you like have concentrated all your chemicals so you can get like more reactions in them. Lots of speculative ideas about what it might have happened billions of years ago to get life started, and then that speculation extends all the way to like actually getting life out of the ocean. Some people think that tide pools are a place that fish learned to walk. Basically, being stranded in a tide pool meant you better evolve some legs.

00:40:27
Speaker 1: Well, you mean, like some fish we're hanging out by the beach by the shore during high type, but then when low tie came, they're like, oh, we're stuck here in the land, and maybe we should learn to walk.

00:40:36
Speaker 2: You know, that makes me wonder I never thought about this do fish enjoy the beach the same way land animals do. Do they go near the shore and they're like, ah, this is so beautiful and relaxing.

00:40:45
Speaker 1: Interesting, Like do they take vacations at the shore?

00:40:47
Speaker 2: Exactly? Yeah, do fish take vacations at the shore? Deep questions were uncovering today.

00:40:53
Speaker 1: I think they kind of do, right, because in the deep ocean they're more vulnerable to getting eaten, and there's I think more food for them near the shore, So it is kind of a kind of a result. It's kind of a vacation spot for them.

00:41:07
Speaker 2: Here a physicist and a cartoonist speculate, but marine biology. But I think the actual idea speculative as it is, is that land walking animals could have started as fish stranded in tidepools. Not that like an individual fish of course is going to sprout legs, but the fish that have this ability to like scramble over a rock or two are more likely to survive getting stranded in type pool m.

00:41:30
Speaker 1: Like maybe stranded one hundred fish or something. Most of them died, but the ones that somehow had some weird gene that let them flop around a little bit better with their fins. Maybe those eventually evolved into the animals that left the ocean and became us exactly.

00:41:45
Speaker 2: And so having tithes on a planet really changes what it's like for life to evolve there, to even start from pre life organic molecules, and to develop. And so when we look out into other solar systems for planets that might have life on them, also very interested in whether they have moons, stirring the pot down there on the surface.

00:42:05
Speaker 1: Well, like, maybe you're saying the only reason we have life here on Earth is because we had moon. Like, if we hadn't had a moon, maybe life wouldn't have started.

00:42:12
Speaker 2: We don't know, it's just an equals one, but that's a really interesting speculation. And if we go out there and we search all these solar systems we find a bunch of planets with moons that have life on them, and a bunch of planets very similar but without a moon and no life, then that would be a strong argument that you need a moon to get life started. But hey, most likely we go out there and find something even weirder than we expected.

00:42:32
Speaker 1: Mm, Like, maybe we're all moonies.

00:42:35
Speaker 2: I don't know what that means, but it sounds good.

00:42:37
Speaker 1: I mean, some parts of the Internet will be over the moon to learn this truth. All right, Well, that's another reminder that there are all these hidden forces all around us, pushing and pulling us, and sometimes we're not aware of the effects that they might have. But if you look at the physics of it, if you look at the details, maybe they have the ultimate effect, and maybe they're responsible for life itself.

00:42:57
Speaker 2: And the simple physics model of the Solar system of just points orbiting each other with gravity isn't enough to describe the reality of our lives. And in the origin of life, everything is not just pushing and pulling on each other. It's squeezing. It's tugging, it's massaging itself. It's not just squeezing the ocean. It's actually squeezing our entire planet, causing the crust of flex and stimulating earthquakes. But also maybe the reason we're here.

00:43:21
Speaker 1: Well, not just the Earth, it's also squeezing us technically.

00:43:23
Speaker 2: Right, Yeah, that's true. Yeah, you have tides as well.

00:43:27
Speaker 1: And does that mean the moon is actively chewing.

00:43:29
Speaker 2: Me, It's trying to spaghettify it, turning.

00:43:33
Speaker 1: Into pasta di slurt me up or to feed to the fishes. Is all right, well, we hope you enjoyed that. Thanks for joining us, See you next time.

00:43:46
Speaker 2: For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, This Earth, Instant and now TikTok. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows