If you love big thoughts, this is the episode for you. We’ll ponder what would happen if a fly flew into the window of a moving bus. Would it have to keep flying forward to avoid crashing into the back window? Or could it just hover inside the bus? Plus we’ll unpack one of the most important ideas in science, the Theory of General Relativity. You’ll learn how creative thinking helped Albert Einstein change the world and get a little creative yourself guessing an all-new Mystery Sound!

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

PRESENTER: Brains On! is supported in part by a grant from the National Science Foundation.

BUS DRIVER: This is the bus to Worcester. If you're going to Leicester, that's the next bus. And if you're going to Bermuda, take me with you, please!

FLIES: Salutations! The one human ticket please for me, a human, named Henry J. Flyman.

BUS DRIVER: Nice try, pal. Read the sign.

FLIES: Hold your farts till you step outside?

BUS DRIVER: What? No. The other side.

FLIES: Honk if you're Gaga for Goudag.

BUS DRIVER: No, this sign.

FLIES: No flies allowed. Oh, how did you--

BUS DRIVER: How did I know you were hundreds of flies in a trench coat? Well, maybe the fact that you were all floating or that buzz or your face is literally a ball of flies spinning around in circles.

FLIES: But why, why can't flies step onto the bus, flying to Worcester takes so long.

BUS DRIVER: Because if we let you on, you'll be flying there mid-air not touching a seat or the ground. And when I speed up the bus, we'll go forward, and you won't! Spot, it's flagged up around me all over the back of the buzz! No, I mean bus!

FLIES: Wait, that's not how physics works.

BUS DRIVER: Yeah, pretty sure it is, according to TikTok, I vaguely remember.

FLIES: No, I may be hundreds of flies in a trench coat, but I can assure you that is not how physics works. If that were the case, then if I hovered here right now, this is not touching the ground, then you would move away from me. This is because you are touching the Earth and the Earth is always spinning. But we're both staying still.

BUS DRIVER: Well, I'll be. I think you may be right.

FLIES: So will you sell me a ticket now?

BUS DRIVER: No way I'm selling you a ticket. Read this sign.

FLIES: Oh, the no flies is the sign?

BUS DRIVER: No, the other, other, other sign. The one that says anyone who teaches me something cool gets to ride the bus for free! Come on up here, pal.

FLIES: This is superb. Thank you.

MOLLY BLOOM: You're listening to Brains On! from APM Studios I'm Molly Bloom. And my co-host today is Bruce from Wellesley, Ontario. Hi, Bruce.

BRUCE: Hi, Molly.

MOLLY BLOOM: One of the best parts of working on this show is reading the many emails and letters we receive from our listeners every day. They're brimming with curiosity and fun and emojis, lots of emojis. We got two emails that were so similar and so intriguing, we had no choice but to investigate.

BRUCE: The first one was from Uma.

UMA: The other day, I was riding the bus to school and there's a fly flying round. I was wondering that if the fly was just hovering there in the middle of the bus, shouldn't it crash? The bus is moving fast and the fly is not connected to it or sitting down like everyone else. If it did, would it crash in the back or the front?

MOLLY BLOOM: And the second was this one from Hazel and Eleanor.

HAZEL: If a bee flies into a moving car, does it need to fly as fast as the moving car while it's inside or can it just hover?

MOLLY BLOOM: These are the kind of questions that can send your brain in circles. Right. Bruce, have you ever wondered something like this?

BRUCE: Yeah. I certainly have.

MOLLY BLOOM: Do you like thinking about these kind of brain-bending ideas?

BRUCE: Yeah.

MOLLY BLOOM: What do you daydream about?

BRUCE: Life, the universe, everything, dolphins.

MOLLY BLOOM: Dolphins. Excellent. What kind of questions do you have about those big universe questions?

BRUCE: If the universe is really infinite and there's only a finite amount of worlds with life. Because finite is such a small amount of infinite, it could be seen as zero. So does that technically mean there's no life in the universe?

MOLLY BLOOM: Whoa, that is a big question, Bruce?

BRUCE: I think that's our good brain bender.

MOLLY BLOOM: Very good brain bender. Well, to answer this question about flies in vehicles, we talk to someone very comfortable with mind-bending questions.

SUNG KYU KIM: Sung Kyu Kim. I've taught physics at Macalester College for the last 50 years. So I'm a long-time teacher.

MOLLY BLOOM: So what's the fate of a hitchhiking flying friend? The short answer is this.

SUNG KYU KIM: The fly is simply hovering in the air and the air is carried by the bus.

BRUCE: That means the bus is carrying the air and the air is carrying the fly. So if the bus is moving, the air is also moving.

MOLLY BLOOM: The fly can just keep hovering in that air and be carried along like the rest of the passengers. So if the fly is in the bus, and the bus is going down the street at 30 miles per hour, the fly itself is not flying 30 miles an hour to keep up. It's just hovering.

BRUCE: But also, the fly, the bus, and the people on the bus, they're all moving at 30 miles per hour because that's how fast the bus is moving through space.

MOLLY BLOOM: But also, maybe the fly isn't moving at all. That's because it's all relative. We'll explain more about that in a second. These questions from Uma, Hazel, and Eleanor, and their answers are very complicated.

SUNG KYU KIM: Actually, this is the kind of question Einstein thought about. So it's a profound question. I want them to know that. They're asking me very profound questions

MOLLY BLOOM: Albert Einstein was a scientist, and when he asked these kinds of questions, the same kind that Uma and Eleanor and Hazel are asking, he came up with something called the Theory of General Relativity, which was really a brand new way of thinking of time, space, gravity, and the universe itself

SUNG KYU KIM: Essentially, what Einstein said is motion and rest are relative concepts.

BRUCE: That means whether you are moving or staying still, all depends on how you are looking at it.

SUNG KYU KIM: Relative to the bus. If you're on the bus, you are at rest. But if you are on the ground, then the bus is moving.

MOLLY BLOOM: So you're sitting on the bus that's driving down the road. In relation to the bus, you're not moving. You're sitting still. You look around at the floor or the seat in front of you, and it's all staying put. But let's say I'm standing on the sidewalk. And I see you sitting on the bus through the bust window as you go zooming by. I would definitely say you are moving, right? That's because relative to me, you were moving.

BRUCE: Whoa, brain bending here's another example from Sung Kyu Kim.

SUNG KYU KIM: When you're on an airplane, especially if the flight is very smooth, and you are flying over the ocean, and you don't see anything underneath except water, then you look out the window and you feel like the plane is not moving. So there's no sensation of motion.

BRUCE: It's actually moving really fast, over 500 miles per hour.

MOLLY BLOOM: So it turns out you can't always tell the difference between whether you're moving or at rest.

BRUCE: That's called Einstein's Principle of Relativity it's the idea that motion and rest are relative concepts it depends on who's looking and how they are looking.

MOLLY BLOOM: Right. But all of that is based on a car or bus going in a straight line at a constant speed. Not speeding up not slowing down. No turns or bumps in the road.

BRUCE: If the bus or car were to quickly stop or turn, the fly and the people inside would feel the effects. They'd all slide forward to the side, for example. Then they'd suddenly feel like they were moving.

MOLLY BLOOM: Now that you're starting to grasp relative motion in cars and planes, ponder this. Even when you're sitting on the floor, no vehicle in sight, you're still moving relative to something.

BRUCE: After all, the Earth is constantly spinning. You just don't feel it because everything around you is moving just as fast as you are.

MOLLY BLOOM: Not only that, but this spinning planet is also making large circles around the sun. So it's flying 67,000 miles per hour relative to the sun.

BRUCE: And the galaxy, our whole Solar System is moving relative to other galaxies.

MOLLY BLOOM: So depending on how you look at it, nothing is actually still anywhere in the universe. Try explaining that to someone next time they say you need to sit still. Mind bending enough for you?

BRUCE: It's mind-bending all right, but I could go even bend here. I was doing brain yoga before this taping. So my mind is super limber today.

MOLLY BLOOM: Well, just you wait. We're going to turn your brain into a pretzel in a minute. But first, this idea of general relativity is a very important. So important that we wanted to really explore it a bit.

BRUCE: We asked some scientists to help explain it in different ways.

MOLLY BLOOM: Right. You heard our bus example. So here are some other explanations using different ways of getting around.

BRUCE: Take it away, physics friends.

ANDREA BRYANT: My name is Andrea Bryant. I'm a PhD candidate at University of Chicago. So let's imagine we have a little green alien on Saturn, and also, we have a Brains On! spaceship right outside of Saturn, looking at the little green alien. And let's say that the spaceship is, more or less-- it's not moving. The little green alien would look and see this spaceship moving because, from his point of view, he's just glued to Saturn as it's rotating. But the Brains On! spaceship, since it's stationary, would see the little green alien moving because he's rotating with Saturn.

CARLOS: My name is Carlos I'm a PhD student in the physics department here at the University of Chicago in Chicago, Illinois. And I study cosmology, which is basically the study of the entire 14-billion-year history of our universe. So the classical example that we're taught in school is that-- imagine you were a sailor on a ship, below deck, and the water is really calm, and so the ship is moving steadily along. It's not rocking back and forth or anything. And because you're below deck, you wouldn't be able to tell if the ship was moving or standing still. But then as soon as the ships, say, start speeding up or waves start crashing against the ship, then you start feeling these forces of acceleration, and you become aware that you are moving.

JAMI VALENTINE MILLER: My name is Jami Valentine Miller. I'm a patent examiner for the US Patent and Trademark Office. I'm also the founder of African-American Women in Physics, Inc, which is a nonprofit that works to help support diversity in STEM. There's an experiment you can try when you're on a roller coaster or one of those drop rides. You can put the penny on your knee. And as you drop, you can see what happens to the penny. For a bit, the penny will stay stationary if your eyes are open-- [LAUGHS]-- and if you're able to really focus on it. But that's another way to think about general relativity because, even though you are moving, the penny, for a bit, stays still until gravity starts to act on it.


MOLLY BLOOM: OK, let's give the deep-thinking part of the brain a break and work out another set of brain cells. It's time for the--


GIRL 1: (WHISPERING) Mystery sound.

MOLLY BLOOM: Bruce, are you ready for the mystery sound?

BRUCE: Yep, I'm ready.

MOLLY BLOOM: All right, here it is.


BRUCE: Oh, I think-- I think that is raindrops pattering on a metal roof.

MOLLY BLOOM: Mm, I like that idea. It was noisy. That's for sure. I don't know what it is either. So we will have another chance to hear it and guess and hear the answer right after the credits.

BRUCE: So keep listening.


MOLLY BLOOM: We're working on an episode about how creatures would evolve on other planets. So we want you to do a little dreaming with us. Imagine you find life on another planet. How would that life greet you? What would it sound like, in their language, to say, hi? Would they even have language? Or maybe they would greet you another way. Bruce, if you found a creature on another planet, how do you imagine they would say, hi?

BRUCE: I don't know. It really depends on the planet. If it were Jupiter, I think they wouldn't have evolved communications yet. I think they'd be really basic because the force of gravity is so high that it would be really hard for them to move muscles without training them for Jupiter years on end.

MOLLY BLOOM: Mm, very good answer.

BRUCE: On the other hand, if they were on Mars, then I think they'd have a very deep voice because of their large lungs, which I think they would have because of the very thin atmosphere on Mars. So they'd say, hi, like, (DEEP VOICE), hello, but in alien voice.

MOLLY BLOOM: Yeah, what do you think the word for, hello, would be in their language?

BRUCE: Moom.

MOLLY BLOOM: Oh, very nice. Can I hear that in your low voice?


MOLLY BLOOM: Oh, very good. Very well-reasoned responses, Bruce. Listeners, we want to hear how you imagine an alien creature would say, hi. Record yourself and send it to us our BrainsOn.org/contact. While you're there, you can also send us mystery sounds, drawings, high fives, and questions.

BRUCE: Like this one.

GIRL 2: My question is, why do snails have shells and slugs do not?

MOLLY BLOOM: You can find an answer to that question on our Moment of Um podcast. It's a short, daily dose of facts and curiosity you can find wherever you listen to Brains On! Again, that's BrainsOn.org/contact.

BRUCE: And keep listening.

This is Brains On! I'm Bruce.


BRUCE: And we're talking about one of the most important ideas in physics-- relativity.

MOLLY BLOOM: It's one of many big ideas thought up by Albert Einstein. Today the word Einstein is shorthand for genius. Someone'll say, wow, he's a real Einstein. She's a real Einstein. But Einstein, himself, wasn't always seen that way.

BRUCE: As a kid, he said he was slow to talk, and he didn't get great grades in school, especially in subjects that bored him.

MOLLY BLOOM: But when he was interested in something, he'd often spend a lot of time thinking about it.

BRUCE: And when he was thinking, he'd often make up thought experiments.

MOLLY BLOOM: A thought experiment is when you try to imagine something as a way of seeing how it might work.

BRUCE: One of his famous thought experiments involved a person in an elevator.

MOLLY BLOOM: For our purposes, let's use Bob.

BOB: Hi, there.

BRUCE: Let's stick Bob in an empty elevator.


BOB: Oh, an elevator-- so fun. I love how it has four walls. Classic.

MOLLY BLOOM: Now, if Bob was just standing in this elevator on Earth and the elevator wasn't going up or down, he would simply feel the weight of gravity holding him in place.

BRUCE: Gravity is the force that holds us down on the planet. Without it, we'd float away.

BOB: Good, old gravity, like a hog that keeps me grounded. Shouldn't there be buttons in this thing? Where am I going, anyway?

BRUCE: But what if we put Bob and the elevator in space, where there is no gravity.


BOB: Oh, boy. OK. Am I supposed to be floating? This is a strange elevator. I should call someone to come fix it.


Oops, I dropped my cell phone, and now it's floating away. Get back here.

MOLLY BLOOM: Next, let's imagine what would happen if there were rocket boosters attached to the bottom of the elevator. When the boosters fired up, the elevator would start flying fast, and the force of that would push Bob back down.

BRUCE: Suddenly, Bob and the cell phone and anything else in that elevator would suddenly fall back to the floor, as if there was gravity again.


BOB: Ooh-hoo. Ow, what happened. Oh, and now I can grab my cell phone. I should get a leash for this thing. Well, glad they fixed the gravity.

MOLLY BLOOM: But Bob isn't actually experiencing gravity.

BRUCE: He's still in space, far from any planet. But to him, firing up those rocket boosters creates a feeling like gravity. He doesn't know what he's experiencing isn't actually the real thing.

MOLLY BLOOM: For Bob, that feeling is relative. Einstein's thought experiment was a lot like our example, and it helped him come up with some of the ideas found in his theory of general relativity.

BRUCE: He also said-- when he was a teenager, he would imagine what it would be like to fly along a beam of light.

MOLLY BLOOM: And he had other famous thought experiments that involved people falling from roofs and moving trains.

BRUCE: These thought experiments helped him work out his ideas. They helped other people understand his ideas, too.

MOLLY BLOOM: And it's exactly what Uma, Hazel, and Elinor were doing when they asked us those questions about flying bugs. Here's Sung Kyu Kim again.

SUNG KYU KIM: Einstein thought pure thought was so powerful, and he kept saying you can understand the universe by pure thought, curiosity. That's a starting point. Like, these girls-- they are curious. They observe something and want to know. They want to be able to explain that.


GIRL 3: Brains On!

MOLLY BLOOM: So once you can wrap your mind around this idea that everything is relative, that things change depending on who is observing and from where, Sung Kyu Kim says you're ready to start tackling some really big questions about the nature of the universe itself.

SUNG KYU KIM: When you push that principle, relativity, deeply, it eventually shows you that time is relative.

BRUCE: Wait, time is relative? So how time works can be different for different people?


BRUCE: OK, now my brain is feeling pretty stretched. Tell me everything.

MOLLY BLOOM: Einstein figured out that not only is motion relative, but time and space are relative, too. Part of this idea is that if you travel very, very, very fast, time will move slower for you. So imagine you have two clocks set to the exact same time.

AUTOMATED VOICE: Ready? Synchronize.

MOLLY BLOOM: Now imagine you took one of those clocks and sent it up into space on a super fast rocket, and the other clock is sitting on Earth. On that ship, time flows normally, but the rocket is circling the planet at near light speeds. But after, say, a year of this, when that rocket lands, the clocks will be different.

AUTOMATED VOICE: Oh, look! The clocks! They don't match up anymore. They're out of sync.

MOLLY BLOOM: So even though only one year passed on the rocket, five years may have passed here on Earth.

BRUCE: Whoa. Time was different, depending on where you were and how fast you were moving. That is bananas, and so cool.

MOLLY BLOOM: Yes, and that was just a made-up example, a hypothetical one. But get this-- we actually see this happen with satellites. They zoom around the planet at pretty fast speeds. They also have clocks on them. And after a while, they'll get out of sync with Earth clocks.

BRUCE: Meaning time on the satellites was moving at a different speed relative to time here on the ground.

MOLLY BLOOM: Exactly. As they circle the planet, they slowly fall out of sync with clocks on Earth and need to be reset every once in a while, just as Einstein's work predicted.

BRUCE: Mind bent and kneaded like sourdough.

MOLLY BLOOM: Right? I know. So twisted. So next time you see a fly hovering on the bus--

BRUCE: Or a bee flies through the window of a car--

MOLLY BLOOM: --or something else catches your eye and your imagination, don't just forget it. Follow the thought, and see where it takes you.


Movement is relative. That means it depends on whose point of view you're talking about.

BRUCE: A person on a bus might feel like they are sitting still in their seat.

MOLLY BLOOM: But someone on the sidewalk would say they are moving really fast.

BRUCE: If we stand in one place, we feel like we're still.

MOLLY BLOOM: But if you zoom way out, we're on a planet that is spinning and orbiting the sun.

BRUCE: These are some of the ideas in Albert Einstein's theory of relativity.

MOLLY BLOOM: He came up with these thoughts and many more using his imagination. That's it for--

BOB: Hey, anyone there? I've been stuck in this elevator for a while now, and I am not really sure what's going on.

BRUCE: Oh, no we forgot about Bob. He's stuck in our thought experiment.

MOLLY BLOOM: Quick, let's imagine him somewhere nicer.

BRUCE: A beach?

MOLLY BLOOM: Too sunny for Bob.

BRUCE: A theme park?

MOLLY BLOOM: He gets sick on coasters.

BRUCE: The movies?

MOLLY BLOOM: Too exciting.

BRUCE: The zoo?

MOLLY BLOOM: Too exciting.

BRUCE: A museum?

MOLLY BLOOM: Too exciting. Bob likes to keep things real low key.

BRUCE: [GASPS]. I got it-- a museum before they hang up any paintings.

MOLLY BLOOM: [GASPS], perfect.

BOB: [GASPS]. Hey, now, is this a museum before they hang up any paintings? Don't mind if I do. Well, well, well, look at this beautiful wall-- so white, so empty. A blank wall is the real art, I always say. Oh, is that an electrical outlet? I wonder if it has three holes or just two. I better go inspect. Oh, three holes. This is so exciting. I love how the outlets look like little faces. Hello, there, outlet face. Did you see these beautiful walls?

MOLLY BLOOM: That's it for this episode of Brains On!

BRUCE: This episode was produced by Sanden Totten, Molly Bloom, Rosie DuPont, Anna Goldfield, Anna Weggel, Nico Gonzalez Wisler, Aron Woldeslassie, Molly Quinlan, Ruby Guthrie and Mark Sanchez.

MOLLY BLOOM: This episode was edited by Shahla Farzan and sound design by Rachel Brees. Beth Perlman is our executive producer. The executives in charge of APM Studios are Chandra Kavati, Alex Schaffert, and Joanne Griffith. Special thanks to Chris Colvin, Melissa Winkler, and Eric Ringham.

BRUCE: And my class, 6C, at Wellesley Public School, and my brother Clyde. Brains On! Is a Nonprofit Public Radio program.

MOLLY BLOOM: There are a ton of ways you can support the show. Head to BrainsOn.org.

BRUCE: While you're there, you can send your fan art, your questions, even your mystery sounds.

MOLLY BLOOM: And you can also subscribe to our Smarty Pass.

BRUCE: Add-free episodes and bonus stuff, just for you.

MOLLY BLOOM: OK, Bruce, are you ready to listen to the mystery sound again?

BRUCE: Oh, yeah.

MOLLY BLOOM: All right, here it is.


There is a lot happening there. What did you hear this time?

BRUCE: I still-- hmm, I actually think it's the classic raindrops falling into a bucket out of an ewe's trough.

MOLLY BLOOM: Classic. Should we hear the answer?

BRUCE: Yep, also, I'd like to say, thanks, to whoever sent that mystery sound to Brains On!

MOLLY BLOOM: [LAUGHS]. Well, we're about to find out. You ready?

BRUCE: Yeah.


CHARLIE: Hi, my name is Charlie, and this is the sound of my mom flipping over puzzle pieces.


MOLLY BLOOM: Puzzle pieces.

BRUCE: Yeah. Well, thanks, Charlie.

MOLLY BLOOM: Very good mystery sound, Charlie. That was a tough one.

BRUCE: Mm-hmm. It makes sense, though. I also thought it was drumsticks falling out of a bag. That would make a--

MOLLY BLOOM: Yes, I could hear that.

BRUCE: --clattering sound, you know?

MOLLY BLOOM: Definitely. Mystery sounds are tough.


Now it's time for the Brains honor roll. These are the kids who keep the show going with their questions, ideas, mystery sounds, drawings, and high fives.


We'll be back next week with more answers to your questions. Thanks for listening.

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