What's behind the waves and tides?

AUTOMATED VOICE: You're listening to Brains On, where we're serious about being curious.

[WAVES SOUND]

MOLLY BLOOM: There's just something about this sound. Flora, what do you think of this sound?

FLORA SALDANA: It makes me want to have fun at the beach.

MOLLY BLOOM: I actually find this sound relaxing and soothing. It's like taking a big calming breath in and out.

FLORA SALDANA: Sure. But where are the waves that make the sound come from, and what about the tides? Is it true that they're controlled by the moon?

MOLLY BLOOM: Those are great questions. We'll get to them eventually. Right now, I think I'm just a little too relaxed to keep taping this episode.

FLORA SALDANA: Molly? Hang on. Let me fix this. Is that better?

MOLLY BLOOM: Oh, yeah. I'm ready. We're going to answer those questions right now.

FLORA SALDANA: That's more like it. Keep listening.

MOLLY BLOOM: You're listening to Brains On. I'm Molly Bloom, and my co-host today is Flora Saldana from Los Angeles. Hello, Flora.

FLORA SALDANA: Hi, Molly.

MOLLY BLOOM: This episode was sparked by a question that you sent to us.

FLORA SALDANA: How does the moon control the tides?

MOLLY BLOOM: So what made you think of this question?

FLORA SALDANA: I was at a summer camp, and a girl said that the moon controlled the tides, and I wondered how that was possible.

MOLLY BLOOM: So in Los Angeles, you don't live too far from the ocean. Do you like to play in the ocean?

FLORA SALDANA: I like jumping over the waves. And I like it when my dad picks me up, and we measure the waves by seeing how high it goes up on me because he picked me up, so I'm taller.

MOLLY BLOOM: Nice. So like what do you think is the tallest wave you've ever seen?

FLORA SALDANA: When he was picking me up, it went all the way up to my shoulders once.

MOLLY BLOOM: Oh. That's crazy. And is it more fun to play with the wet sand or the dry sand?

FLORA SALDANA: When you're wet, wet sand. When you're dry, dry sand. Because the dry sand will get stuck to you if you're wet.

MOLLY BLOOM: And you use the wet sand or the dry sand for your sandcastles?

FLORA SALDANA: The way I actually do my sandcastles is I just put sand on the bottom, then water, then sand, then water, then sand, then water.

MOLLY BLOOM: That is very good technique. It must be pretty strong.

FLORA SALDANA: Yep.

MOLLY BLOOM: When I built sandcastles in the past, they usually get swallowed up by the waves eventually because the tide is coming in. So let's get to that original question you sent us. The moon does indeed control the tides.

FLORA SALDANA: Using?

MOLLY BLOOM: Tan-ta-da-da.

AUTOMATED VOICE: Gravity.

MOLLY BLOOM: All of this can be understood by looking at the universal law of gravitation, first postulated by the scientist Isaac Newton in the 1600s.

ISAAC NEWTON: The gravitational force between two bodies is proportional to the mass of each body and inversely proportional to the square of the distance between their centers. And the force is directed along the line between the centers of the object.

MOLLY BLOOM: This means the gravitational pull between two objects is stronger the closer they are--

FLORA SALDANA: And the heavier they are.

MOLLY BLOOM: The gravitational force of the moon is constantly pulling on the Earth. So what does that mean for the moon and the Earth?

FLORA SALDANA: It means the gravitational force between the moon and the Earth can be felt all over the Earth.

MOLLY BLOOM: We don't notice the effect the moon's gravity has on land since large landmasses are solid and not easily moved.

FLORA SALDANA: But water is a different story.

MOLLY BLOOM: Water moves easily, and the gravitational forces created by the moon cause water on the surface of the Earth to bulge. Sir Isaac Newton?

ISAAC NEWTON: Yes?

MOLLY BLOOM: Can you repeat the end of that universal law again?

ISAAC NEWTON: Surely. The force is directed along the line between the centers of the objects.

MOLLY BLOOM: So in the case of the moon and Earth, water moves to positions where all forces on it are balanced. This actually causes two bulges of water.

FLORA SALDANA: One is on the side of the Earth facing the moon.

MOLLY BLOOM: The other is on the opposite side of the Earth from the moon. It's easy to wrap your mind around why water would bulge toward the moon on the side facing it, but why would it bulge on the opposite TOO?

FLORA SALDANA: Let's have our pal Isaac Newton explain.

ISAAC NEWTON: Sir Isaac Newton.

FLORA SALDANA: Oh, of course. Sir Isaac Newton.

ISAAC NEWTON: The strength of the force due to gravity depends on distance. So over the volume of any extended mass of material, the strength of the gravitational force varies. It is this difference. Call it a force gradient that is responsible for the motion of ocean water into the tidal bulges, OK? This force gradient is nearly the same size on the near and far sides of the Earth from the moon. And do you know how I was able to discover this a very surprising result?

FLORA SALDANA: Lay it on us.

ISAAC NEWTON: Calculus. A powerful tool of mathematics that I invented just to solve tricky problems like this.

MOLLY BLOOM: Impressive, Sir Isaac.

ISAAC NEWTON: Indubitably.

FLORA SALDANA: So calculate the force gradient--

MOLLY BLOOM: Find out where these bulges are and--

FLORA SALDANA: Ta-da. That's where high tide is.

MOLLY BLOOM: That bulge keeps following the moon and moves around the planet as Earth rotates under it.

FLORA SALDANA: As the bulge moves around on different parts of the Earth, so does high tide.

MOLLY BLOOM: The sun also has gravitational pull over the Earth, but it's less than the moon. Even though the sun is much more massive than the moon, it's also much, much farther away. It's more than 90 million miles from Earth. That is so far that the gravitational force just isn't that powerful.

FLORA SALDANA: The moon, on the other hand, is about 250,000 miles away. Not millions.

MOLLY BLOOM: So its effects are much stronger on our planet.

FLORA SALDANA: But the sun does have an impact.

MOLLY BLOOM: Like during the new moon, when the moon's completely dark--

FLORA SALDANA: The tides are more extreme.

MOLLY BLOOM: High tides are higher, and low tides are lower. This is because the weaker force of the sun is added to the stronger force of the moon.

FLORA SALDANA: And when this happens, when the sun and moon are lined up, it's called spring tide.

MOLLY BLOOM: When the sun and the moon are at a right angle--

FLORA SALDANA: Like during half moon--

MOLLY BLOOM: The solar tide partially cancels out the lunar tide, meaning the difference between high and low tide is smaller.

FLORA SALDANA: This is called the neap tide.

MOLLY BLOOM: It's also important to remember that the Earth is not just covered in water. Different landmasses create ocean basins.

FLORA SALDANA: So the motion of this bulge of water is affected by the continents.

MOLLY BLOOM: Think about when you're in the bathtub. You move around a lot, and the water sloshes from side to side.

FLORA SALDANA: When that water sloshes, you can see the water by the sides of the tub going up and down in height.

MOLLY BLOOM: But there's a point in the middle where the height doesn't change at all. See for yourself next time you're in the tub.

FLORA SALDANA: The same thing happens in these ocean basins too.

MOLLY BLOOM: These points where there are no high or low tide are called amphidromic points.

FLORA SALDANA: There are several around the world, including one near Perth, Australia and one between Mexico and Hawaii.

MOLLY BLOOM: There are also spots where the opposite is true, where their topography makes the difference between high and low tide more extreme than other areas.

FLORA SALDANA: The highest tides in the world can be found in the Bay of Fundy between Nova Scotia and New Brunswick.

MOLLY BLOOM: In one part of this Canadian Bay, the difference between high and low tide is 53 feet or about five stories. This means boats on the wharf there end up sitting on dry land during low tide.

Tides are caused by the gravitational force of the moon and to a lesser, extent the sun.

FLORA SALDANA: But what about waves? Are these the same as tides?

MOLLY BLOOM: We've gotten a lot of questions about waves, like this one from Riley.

RILEY: Hi. My name is Riley, and I live in Boca Raton, Florida. And my question is what keeps the ocean waves up until they crash onto the shore? Why are some ocean waves bigger than others?

FLORA SALDANA: Jessica Carelli is an oceanographer and a surfer. She is here to answer all of our wave questions.

MOLLY BLOOM: Hello, Jessica.

JESSICA CARELLI: Hello. Thank you so much for having me.

MOLLY BLOOM: What causes waves?

JESSICA CARELLI: Well, most of the waves in the world are caused by wind blowing across the ocean. Just like when you blow on a drink that you have and you can make little ripples, when the wind blows over the ocean, it creates waves. The stronger and the longer and the farther the wind blows over the ocean, the bigger the waves.

FLORA SALDANA: That's really cool.

JESSICA CARELLI: Yeah. We can also get waves very rarely if a meteor strikes in the ocean or if there's an underwater landslide or an earthquake, and those waves are called tsunami.

MOLLY BLOOM: Why are waves that shape? That sort of curving, crashing shape?

JESSICA CARELLI: The shape of the waves is controlled very strongly by the shape of the bottom of the ocean. So when you're out in the deep part of the ocean and you go underwater. Let's say you go 20 feet underwater or something. You tend to typically, you won't feel the waves. But when you're near the shore, when the ocean gets really shallow, that means that the energy of the waves starts to interact with the bottom, and it sort of drags in a way. It starts to slow down. But the top of the wave doesn't feel the bottom. And so it keeps going faster. And that is when a wave breaks. Where the top of the wave is going faster, and the bottom of the wave is dragging on the bottom, slowing down, and it crashes over in a sort of a barrel or a tube as we like to call it when we go surfing.

FLORA SALDANA: Why are the waves so strong?

JESSICA CARELLI: The strength of the wave is really related to how large the waves are. And the size of the waves is controlled by the strength of the wind. If you have a really big storm out at sea, you'll end up with really big waves. And when those waves travel across the ocean, they don't really lose very much energy. So in different places in the world or different times of the year, you can have really, really calm conditions, very, very small waves, or you can also have very, very large waves, just depending on where that wave energy is coming from and how strong the storms out at sea have been.

FLORA SALDANA: How come when you drop something in the bottom of the ocean it gets pulled out to sea instead of pushed to shore?

JESSICA CARELLI: The bottom of the ocean close to shore where the waves are breaking, the motion of energy is actually going back and forth. So it comes in up the beach, and then it pulls back out. And a little bit further out, the energy of the waves is actually going in a circular motion with the top of the water sort of pushing towards the shore, and then the bottom pulling back away.

So sometimes, you can feel like things are getting pulled out to sea, like your feet when you're standing in the shallow water feel like they're getting pulled out to sea. But if you were to just wait there and float for a bit, you may get pushed back in. It's a little bit chaotic in the surf zone. So the direction of motion kind of changes pretty rapidly through time.

So sometimes, you'll find beaches where things are getting pushed onto the shore, like you'll find shells that have been deposited up on the sand. And sometimes, you go to beaches, and there's really nothing that has been deposited because most things are getting pulled out to sea. And the best thing to do when you notice that you've been pulled out into the ocean deeper than you'd like to be is to swim sideways. So instead of swimming directly back towards the beach against that outgoing current, if you swim sideways, you'll get back into the area where the waves are pushing on to shore, and that will help you get back into shallower water.

FLORA SALDANA: Why are waves white when they break?

JESSICA CARELLI: That is a great question. And it's actually also related to the reason that we hear the waves breaking. So when a wave breaks, it gets air trapped in it, and we have a bunch of bubbles. And so what you can see is just the little tiny bubbles in that white water that we call it. And the sound of the waves breaking is actually those bubbles bursting.

FLORA SALDANA: Cool.

MOLLY BLOOM: Yeah. Thank you so much for talking to us today.

JESSICA CARELLI: Thank you.

FLORA SALDANA: Bye, and thanks for letting me ask the questions.

JESSICA CARELLI: Well, thank you for your interest. It's really great to talk to kids who care about nature and science.

MOLLY BLOOM: I know your brain is floating in a sea of wave information, but we have another important task to take care of. It's time for the Mystery Sound. Here it is.

Any guesses?

FLORA SALDANA: No.

MOLLY BLOOM: You have to come up with a guess.

FLORA SALDANA: Maybe some sort of ocean animal doing something.

MOLLY BLOOM: That is a really good guest. We'll be back with the answer right after this. We love hearing from you. Every episode of the show is based off of questions from our listeners. So if there's a topic you want to hear us cover on Brains On or something you're super duper curious about, let us know. Send us your questions, ideas, and mystery sounds, and drawings at brainson.org/contact. That's where we got this listener question.

WYATT: My name is Wyatt from Dahlonega, Georgia. My question is why don't ears have bones?

MOLLY BLOOM: We'll be back with the answer to that question and read the most recent group of listeners to be added to the Brain's Honor Roll at the end of the show. So keep listening.

FLORA SALDANA: You're listening to Brains On. I'm Flora Saldana.

MOLLY BLOOM: And I'm Molly Bloom. Now let's go back to that mystery sound. Let's hear it one more time.

Any new guesses?

FLORA SALDANA: No. I still think it's an animal in the ocean.

MOLLY BLOOM: Here is the answer.

ALEX BROST: That was the sound of a waxing up a surfboard. You use the wax because it helps your feet stick to the surfboard. My name is Alex Brost. I make surfboards.

MOLLY BLOOM: Alex is a surfer.

ALEX BROST: When you apply the wax, it develops these little knobby bumps, and that's kind of nice because you can really feel those under your feet while you're riding.

MOLLY BLOOM: And he lives in Minnesota. Now, you might think, huh? Minnesota is in the middle of the country. Why on Earth would a surfer live there? Well, Alex has surfed all over the world, but he's also able to surf right here in the Midwest.

ALEX BROST: In 2008, I discovered that you could surf up in Lake Superior. The biggest difference in surfing between the lakes and the ocean, there's two things. There's the density of the water, which we're actually surfing in freshwater, which is about a third less dense than saltwater. The other thing is we're surfing shorter fetch breaks or waves that are traveling a shorter distance from when they originate to when they hit shore. So that means they don't develop as much power as they travel across as they would in the ocean where they travel thousands of miles. Here, we're only doing hundreds of miles. The waves might be very tall, but it's a shorter interval. That means the distance between waves.

MOLLY BLOOM: So the waves are more frequent but less powerful. The other thing you should know about surfing on the Great Lakes is that it's basically the opposite of a tropical pastime. It's most likely to happen when there's snow on the ground.

ALEX BROST: You have to really follow the winds to be a Great Lake surfer. So you watch storm systems develop. You look for low pressure systems to come through because they draw unpredictable winds. And usually, the more powerful, stronger storms come in the winter. So it's more of a winter sport actually up there in Minnesota. Basically, we're surfing the lakes until they freeze. So the water gets down to 32 degrees. There'll be icebergs floating around sometimes in the winter. And the water can be anywhere in the winter swells. It could be a 40-degree day, but it's more likely to be in the minus degrees. Usually, once your feet start going a little numb is when it's time to get out of the water and warm up in the car again.

MOLLY BLOOM: Surfing is not just for people on the coast, it's also possible for us landlocked folks too.

ALEX BROST: There's river surfing. There's wake surfing. There's Great Lakes surfing. There's kiteboarding. There's a lot of ways to get into that feeling of surfing, even if you're not by the ocean.

MOLLY BLOOM: We've talked about how the moon's gravitational pull causes high and low tides. So the tide is continually ebbing and flowing.

FLORA SALDANA: Going in and out.

MOLLY BLOOM: That place between the lowest and the highest tide is called the intertidal zone.

FLORA SALDANA: And it's teeming with super cool marine plants and animals.

MOLLY BLOOM: Ambar Espinoza covers the environment for Rhode Island Public Radio, and she is here to tell us all about these fascinating tide pools.

FLORA SALDANA: Hi, Ambar.

AMBAR ESPINOZA: Hi, Flora. I'm so delighted to join you, and thanks for inviting me.

MOLLY BLOOM: Now Amber, you've spent some time exploring tide pools in the Pacific Northwest and now in the Northeast. So how did you get interested in tide pooling?

AMBAR ESPINOZA: Well, I grew up in Los Angeles, super close to the beach. And the beach was one of my primary playgrounds. But back home, my local beaches are very sandy. And I never ventured into beaches with rocky features that make really cool tide pools. And to be honest with you, I wasn't paying close attention to the natural world around me.

And it wasn't until I was studying in the state of Washington that my ocean world totally opened up. The schools and nature camps where I was working as an outdoor science teacher frequently took their kids tide pooling. And I couldn't believe the amazing creatures I was seeing up close. I'm going to play a little bit of audio I recorded with my students describing the cool creatures they found.

STUDENT 1: We saw this jellyfish bigger than our heads. It was so colorful. It had almost all the different colors you could think of. And it was bigger than even your hand.

STUDENT 2: I saw some anemones. Did you know that anemones put out their little tentacles to eat?

STUDENT 3: And we also saw a great blue heron. And great blue herons, they use their beaks to stab fish. And their long necks, they keep a little bit of it above the water.

MOLLY BLOOM: So now, you live on the other side of the country in Rhode Island. Have you found similar critters in the Northeast?

AMBAR ESPINOZA: You know, the tide pools I've explored in Rhode Island and Massachusetts have been so different than the tide pools in the Pacific Northwest. They certainly have some similarities, but I have yet to spot any sea stars or sea cucumbers. Very different, but fun nonetheless.

FLORA SALDANA: Why is it so different on the different coast?

AMBAR ESPINOZA: You know, I wanted to find out too. So I reached out to Dr. Stan Roman, the research director at an organization called the Earthwatch Institute. He says several factors influence what we may or may not find a tide pool. One of them is the temperature of the water. In the Pacific, the water is very cold. In Southern New England, they're much warmer.

DR. STAN ROMAN: Warm water doesn't hold as much oxygen. And so for a lot of the creatures in this intertidal zone, as the tide goes out, oxygen becomes a little more limiting.

FLORA SALDANA: Marine plants and animals need oxygen, just like we do.

AMBAR ESPINOZA: On top of that, oxygen decreases when the sun warms the tide pools. If the water becomes hot, then that becomes a problem for marine plants and animals.

DR. STAN ROMAN: It also depends on who happens to be trapped in that tide pool with you. So there could be predator and prey that may not see each other frequently in other scenarios may be kind of trapped in that same tide pool. It's kind of fun to think about. Every time the tide comes in and goes back out, the community within that tide pool is different.

MOLLY BLOOM: Gosh. It's like you're moving to a new house twice a day.

AMBAR ESPINOZA: And you might not always get along with your neighbors, like they may want to eat you.

DR. STAN ROMAN: You can see sometimes sea stars wrapped around a mussel or on a barnacle, and they're eating it. They're eating it. They do that in a really interesting way. And if they're feeding on a clam or a mussel, what they do is they use those arms, and they wrap around it. They're so strong. They're like these suction cups or tube feet that they used to attach to the shells. And they slowly open that up. And then through that little gap, they stick their stomach in there. Their stomach turns inside out. And they digest their food outside of their body, and they absorb the nutrients. And then when they're done, they pull their stomach back into their mouth and it goes back into their body. Isn't that crazy?

MOLLY BLOOM: Wow. Super, super cool.

AMBAR ESPINOZA: And remember how I told you that the local beaches I enjoyed in LA are sandy and didn't have any tide pools? Roman reminded me that there's a whole world of marine creatures burrowed beneath the sand too that we can't see.

DR. STAN ROMAN: This magical interface between the sea and the land and especially in these tide pools, they give us a window into that world that, well, it almost defines our world. Most of the planet is covered with oceans. And this is where we're able to really get a better feel for what that world is like through these tide pools.

FLORA SALDANA: That was Dr. Stan Roman.

MOLLY BLOOM: He's research director of the Earthwatch Institute, and that was him tide pooling at a beach in Massachusetts.

FLORA SALDANA: Pretty cool job.

MOLLY BLOOM: And thank you, Ambar, for introducing us to him and giving us a glimpse of what makes tide pools such special places.

AMBAR ESPINOZA: My pleasure. Thanks for having me.

FLORA SALDANA: The gravitational pull of the moon causes water on the surface of the Earth to bulge.

MOLLY BLOOM: The Earth rotates under these bulges, causing high and low tides.

FLORA SALDANA: And waves are caused by wind.

MOLLY BLOOM: And tide pools offer a glimpse at some of the very cool marine life living all over the world. That's it for this episode of Brains On.

FLORA SALDANA: This episode was produced by Marc Sanchez, Sanden Totten, and Molly Bloom.

MOLLY BLOOM: Many thanks to Ellie Saldana, Jennifer Macey, Sarah Camparood, Chris Mondello, Donald Samanek, and Eric Wrangham. We also had production help from Huma Ali and engineering help from Eric Romani. Now before we go, it's time for our Moment of Um.

WYATT: Why don't ears have bones?

OLIVIA BIRMINGHAM MCDONOUGH: My name is Olivia Birmingham McDonough. My pronouns or she and her, and my job title is professor at the University of Washington. So the question was why don't the ears have bones? And the ears actually do have bones. But what we think of and what we look at is a thing called our outer ear. And you can divide our whole ear into three parts.

So our outer ear is this piece that you see that we always point to and say, there's our ear, and it is. And it's really flexible and bends. So you know there's no bone in there because you can bend it towards you. You can twist it many ways. OK. So what bones are in the ear? Well, there are three little bones in the ear, and what's really interesting about them is they are the smallest bones in the body. And the middle ear is where these little bones live.

So the sound comes in, and it hits the eardrum, which is exactly what you think of. It's like a drum. And it pushes against these little bones. And then that actually puts the signal into the inner ear. Now in the inner ear, you actually detect if someone is speaking to you, if there's a car going by, if it's a noisy room or it's really a quiet room. And as humans, like us, we can actually hear over a very wide range of sounds. So you can hear someone whisper to you, and you could also hear someone shout at you. So that's an interesting fact of humans.

Now, the inner ear, this really small structure, if you took it out of a human being, if you put it on a dime, it would fit. That's how small it is. So we have to take care of our hearing because generally, over our lifespan, as we get older, we lose hearing. So there are bones in the ear, and they're in this space called the middle ear. So there are three little bones in there, and then they link to the inner ear. And that's where we hear.

MOLLY BLOOM: Can you hear me? Because it's time for the Brain's Honor Roll these are the incredible listeners who send us their questions, ideas, mysteries sounds, drawings, and high fives. [LISTING HONOR ROLL]

Now if you need me, I'll just be here relaxing to these waves sounds.

FLORA SALDANA: Thanks for listening.