Why Is It A Colour Wheel & Not A Line?

Colour wheel or colour line? Light itself runs in a straight line from red to violet, yet every colour theory chart bends that spectrum into a circle and the reason has nothing to do with physics. Professor Hannah Fry and Michael Stevens (VSauce) dig into why human vision refuses to see colour the way light actually behaves, tracing the visible light spectrum from one end to the other and asking where all the "missing" colours, like magenta and purple, actually come from if they don't exist on that spectrum at all. It turns out the colour wheel is less a description of light and more a diagram of a workaround your brain performs constantly. Send your questions to [email protected] or find us at r/TheRestIsScience on Reddit. Find the wetter water Michael discusses at - https://winsol.com/products/wetter-water ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton + Oli Oakley + Jack Meek Animator: Sam Benson Video & Social: Bex Tyrrell Assistant Producer: Lucy Lipscombe Producer: Simona Rata Senior Producer: Lauren Armstrong-Carter Chief Digital Officer: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

• 0:01Speaker 00

  Welcome to The Rest Is Science. I'm Hannah Fry.

• 0:03Speaker 01

  And I'm Michael Stevens.

• 0:04Speaker 00

  And this is Field Notes, our weekly exposition into the mind, this week, of Michael Stevens.

• 0:10Speaker 01

  All right, so what's going on in my mind? I'll tell you. I got to apologize. I've gotten really into something brand new, once again. This time, it's Crayola crayons. So, I'm still, I'm still a mechanical pencil guy, don't get me wrong.

• 0:25Speaker 00

  Sure.

• 0:26Speaker 01

  But I visited my mom a few days ago, and she had this box of 152 crayons. She'd bought for this coffee shop. Turns out they didn't need them. She was gonna return them, and I'm like, well, I'll take them. And they don't come ordered by any sort of color theory that exists in human knowledge, right? It looks random. It isn't, but it's just based on the skew of the products. Like, they just shove together the eight-pack, the 16-pack, the 24-pack, and that. So it's a mess. And I'm like, I gotta arrange these.

• 0:58Speaker 00

  But it's 152 distinct colors.

• 1:00Speaker 01

  152 distinct colors. And I'm like, I gotta arrange these. That means I've gotta learn color theory. And that began what has now consumed my life. I've had to get special paper to do swatches.

• 1:16Speaker 00

  Oh, my gosh. This is amazing.

• 1:19Speaker 01

  So, because I'm looking at the wax going, okay, but what is the difference between scarlet and red? Okay?

• 1:25Speaker 00

  For the purpose of those of you who are joining us in audio only, first of all, what are you doing? Secondly, you are missing Michael holding up what looks like a, one of those fancy spiral-bound artist notebooks. Except, instead of it being some sketch of a building or a face, he has laid out every color crayon that he has in his pack. And each one, he has labeled with the name of that crayon. He's also done a gradient from very dark, extremely heavy crayon, all the way through to very light. But what is interesting about this also, that I already noticed, is that this is not a linear progression of colors. They appear in a grid. I think there's something going on here.

• 2:09Speaker 00

  And genuinely, and I say this without any humor whatsoever, I am thrilled that this is the subject today.

• 2:18Speaker 01

  I should show you that I've got piles of different crayons. I had to buy different packs because some of them don't give you exactly the, you can't finish the spectrum unless you get the Colors of Kindness pack. And the 96 has dandelion in it, but the 152 doesn't. Anyway, I've learned so much. And so today, I want to show off a couple of cool things about color, but especially, I want to show you some things that I can't show you.

• 2:45Speaker 00

  Ooh, I'm so excited.

• 2:55Speaker 00

  This episode is brought to you by Cancer Research UK.

• 2:57Speaker 01

  Our bodies are incredible machines, worrying away, making more and more DNA to build the proteins that keep us alive.

• 3:05Speaker 00

  In fact, in the last minute, your body has made over 200 million new cells, and enough DNA to stretch to the moon and back.

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  To the moon and back? That's so much DNA that if you compared it to the size of the cell it fits into, that would be like squeezing the London Underground into a suitcase.

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  But incredibly, Cancer Research UK scientists can spot these errors. And by finding them, they've helped double UK cancer survival over the last 50 years, and are driving even more discoveries that could tackle over 200 types of cancer.

• 3:53Speaker 00

  For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org/restisscience.

• 4:04Speaker 02

  Observe the North American dad. 50s, great polo collection. He enters his vehicle information on Carvana. An offer appears in minutes. Remarkable. He squints, reads it again, smiles, and says, dang, that's a good offer. The highest form of approval. No haggling, no hassle. No one called him boss. He's still on the couch. Astonishing. Sell your car today on carvana.com. Pickup fees may apply.

• 4:35Speaker 03

  This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required. Compatibility and availability varies. 18+..

• 5:07Speaker 01

  Okay, so we'll start with the biggest mystery. And this struck me as I was driving around yesterday. Literally, this is how fresh this whole thing is. I was like, wait a second. The electromagnetic spectrum, okay, which is how we lay out all the different energies light can have, makes sense to me. I get it. You've got radio waves, you make them more energetic. Oh, whoops, now they're microwaves. Eventually, you get into visible light that our eyes can see. And if you crank up the energies there, you go from, from red light to orange, yellow, green, blue, violet, and then ultraviolet, all the way up to X-rays, gamma rays. It's a line, okay?

• 5:46Speaker 00

  Mm, and then you scale.

• 5:47Speaker 01

  Yeah, you either have more energy, or you have a longer wavelength than, say, 700 nanometers or less. It's one, it's one way or the other along one line. And yet, artists are always telling us about color wheels. What the heck?

• 6:04Speaker 00

  Come on, you guys.

• 6:05Speaker 01

  You guys, it's a line. It doesn't connect back on itself. It's not like when you reach violet, you add more energy and it's red again. Why is it a wheel? I'm holding up a color wheel, by the way. This is just a classic artist's coloring wheel. You might see this in like, elementary school even, where the teacher wants to talk about, you know, how the colors mix and how they change, where they fall on the spectrum. And this helped me sort my crayons really well, because I just did not know, when I looked at a red, does that red have more orange in it or yellow? So this allowed me to like, add more red to yellow-orange and be like, oh, I'd hold the crayon up. And then if it was a tone, or a shade, or a tint, I can use the back to kind of determine where it should go. I can sort the grays, you know?

• 6:49Speaker 01

  Anyway, why do artists use a color wheel, but scientists use a color spectrum that's a line?

• 6:57Speaker 00

  Right. This is, genuinely, this is something I've always wondered. In addition, which is sort of the same thing, you know, if you ask a physicist what the primary colors are, they will tell you that it is red, blue, and green. But if you ask an artist, they'll tell you it's red, yellow, and blue. And I'd never understood. What, what, how, why?

• 7:16Speaker 01

  Look, it depends what artist you ask. I think if you ask an artist who has also become obsessed with this, they will say there are no primary colors. It's all made up. You can choose any group of colors to be your primaries. It might not be as helpful, but it's not like the universe at the Big Bang was like, red, blue, green. Oh, my gosh, look at all the other stuff that you can make out of this. No, there's, that's not how it works.

• 7:42Speaker 00

  Okay.

• 7:43Speaker 01

  The reason we put color in a wheel, even though physically it lies on a line, is that purple does not exist.

• 7:53Speaker 00

  Excuse me?

• 7:54Speaker 01

  Yeah. Here's the problem. So you've got a spectrum of color. That means a spectrum of energies light can have that causes our brain to experience different colors, from red all the way up to the really small wavelength and high-energy blues and violets. Now violet is a spectral color. That means it's created from light. It's a real thing that has physical properties. And our eyes, the human eye, is only sensitive to basically the short, the medium, and the longs, okay? If it, if your eye receives short and medium wavelengths, then the perception in your brain is that it must be somewhere in between, so maybe it's orange, okay? If you get a bunch of middles and a bunch of longs,

• 8:40Speaker 01

  then your brain goes, well, we'll give you the experience of blue, okay? Or maybe like a blue-green.

• 8:45Speaker 00

  Because you only have the three cones, right? That in your eyes, you have the three cones, which are like, is there any red there? Yes or no? I mean, and how much, yes, and how much, no? Is there any blue there? Is there any green there? You haven't got an orange cone, you're right. There isn't one. There isn't a purple cone.

• 9:01Speaker 01

  But we can see orange because our brains are able to process, hey, the, the, the shorts and the mediums are firing. So let's, let's call it orange. And by call, I mean, it creates the experience of orange on that object in the world for you. But here's the, the big question. What happens when your eye is receiving both short and long? It doesn't give you middle as a perception, because that would be triggering the, the middle guys. It'd be green. So instead, our brains have created a color that does not physically exist, and it is the color purple, or magenta. That is purely a psychological phenomenon, not a spectral color.

• 9:46Speaker 00

  Wow, never thought about this. This is so good. This is so good.

• 9:52Speaker 01

  And so that's why we're able to do, starting at red, increase the wavelength until all of a sudden, boom, you're at violet, and then it goes back. Because in between the violet and the red, we have a combo that we experience as a mental construct, and that is purple or magenta-type colors. Okay, so then I'm start, I'm chronicling my crayons, I'm taking photos. Uh-oh. Phone camera sensors cannot recreate every color that Crayola makes.

• 10:26Speaker 00

  Go on.

• 10:27Speaker 01

  Let me show you a color. This one.

• 10:31Speaker 00

  Okay, this is like a, it's orange, but it's like a ready orange. Sort of like the color of a tomato as it becomes ripe, but before it goes into that deep red.

• 10:45Speaker 01

  Wrong. This color does not look anything like what you just described to me, seeing it with my own eyeballs.

• 10:53Speaker 00

  What does it look like?

• 10:54Speaker 01

  You're seeing it only as the camera and your computer screen can show it to you. This is, this is a color that Crayola calls outrageous orange. And it is brighter than safety orange. It's like a high vis, shocking, hurts your eyes orange. And yet on camera, it looks like, oh, that's a tomato. Or, you know, it looks almost creamy. It's almost like a salmon to me on my screen, but in real life.

• 11:21Speaker 00

  Yeah.

• 11:22Speaker 01

  This thing is outrageous orange. That's the perfect name for it. And you cannot photograph this the way it actually appears because it's fluorescent. That's the problem. The way this works, the reason this color is so bright in real life. And if you, if you ever stumble upon a box of crayons, just open it up, pull out an outrageous orange and, and you'll, you'll see what I mean. Basically, Crayola has added special optical brightener chemicals to this that take in ultraviolet light, which is coming in from all kinds of sources, especially the sun. And those chemicals convert that invisible ultraviolet light down into visible light that we can see that's re-emitted. And so the crayon is brighter than it should be.

• 12:05Speaker 01

  It is brighter than any normal thing in the world because it's actually producing, it's reflecting more light than is landing on it that we can see.

• 12:13Speaker 00

  Hold on, hold on. Can I see, can I see some of it on the page? Can you draw some on the page?

• 12:17Speaker 01

  I can show you, uh, this corner up here. I'm going to show my swatches again. So outrageous orange is right there.

• 12:24Speaker 00

  Right.

• 12:25Speaker 01

  And it looks a lot more like a carrot. It's a little bit more brown. There are three colors here on my swatches that really do not come across on screen. The three are outrageous orange. Neon carrot. And what's this one called? Atomic tangerine. So these three look almost more like an earth tone on a screen.

• 12:47Speaker 00

  Yeah.

• 12:47Speaker 01

  Outrageous orange does look a little lighter than natural.

• 12:51Speaker 00

  It still looks ready. It doesn't look like, it doesn't look like hazard orange at all. It just looks like, hmm, a sort of a ready, ready orange smudge.

• 13:00Speaker 01

  I know. And so this is what's sort of disappointing about this episode. So if you're, if you're listening only, it doesn't matter. Um, you're, you're, the people watching aren't seeing anything different because the camera cannot capture this. Now neon carrot is really interesting because, um, neon carrot is another neon fluorescent color that has optical brighteners in it that make its color brighter than physically possible, uh, without light being literally changed in, in energy levels. But neon carrot on screen looks almost exactly like regular old orange. So can you tell which one? They clearly are different, but which one of these is the fluorescent neon carrot? And which one is just standard orange?

• 13:45Speaker 00

  Okay, so you're holding two up to, to the camera. I mean, honestly, they look almost identical. They both look sort of traffic cone orange. One of them is slightly redder than the other. I, I'm going to go for the, the, I, I think the slightly redder one is the, is normal orange.

• 14:03Speaker 01

  Okay, this one?

• 14:06Speaker 01

  This one? You're wrong. This one is neon carrot. Okay? And this one that almost looked brighter, this is just the regular orange that you would get in an eight pack. You know, at a restaurant, this is just regular orange. But to me, actually in front of these crayons, this one that you thought was regular orange, this one is bright. This is a like high vis, you would never miss it. It's, you could spot it across the room kind of orange. Side note, because all you guys are going to be wondering, what are Michael's favorite Crayola colors? These are them and they, they do appear on screen exactly the way they appear in real life.

• 14:46Speaker 00

  Okay, that was not the colors I thought you were going to go for. For those of you who are listening, uh, Michael is holding up two pens that these are, you know, apparently his favorites. One of them is a gray brown. The other one is a red brown.

• 14:59Speaker 01

  Yeah.

• 14:59Speaker 00

  I, um, look, they're muted tones.

• 15:02Speaker 01

  They're not muted tones. Their loudness is why I love them. I don't know what is going on, but the maroon crayon is like paint. It gets so thick. You put a line on a sheet of paper, you'd see it from a mile away. I don't get it. I can show you this on the swatches.

• 15:20Speaker 00

  Yes, please.

• 15:21Speaker 01

  Okay, so maroon is, uh, this one that's screaming at you right there. You see that?

• 15:28Speaker 00

  Oh, yeah. Yeah, that is a lot has come down there.

• 15:31Speaker 01

  A lot comes down. It is such a pleasure to draw with. It is like drawing with blood. I don't know. It can't just be the, the pigment unless the pigment is chemically changing the wax a little bit, but it's night and day. There's no other crayon like it. Um, the, the browner one, this one, this is called shadow.

• 15:52Speaker 00

  Right.

• 15:52Speaker 01

  And it's, it's, it's pretty fun to draw with, but it is so confusing. Try to draw with a shadow Crayola and it is, it's not a mix of brown and green. It's sometimes brown, sometimes green, depending on where you decide the look. It is so strange. I can't wrap my head around it. I've been thinking about it every day.

• 16:12Speaker 00

  What, depending on the light that's hitting it?

• 16:14Speaker 01

  No, it depends on, like, maybe it depends on the way the wax is structurally on the surface. Because I'll show you the swatch. Okay, here's shadow. Shadow is this one.

• 16:29Speaker 00

  It does look green. It looks very green on camera.

• 16:31Speaker 01

  It looks pretty green on camera, but it looks like, you know, where it's thicker, it becomes browner. But sometimes for no reason whatsoever, it looks more green in areas than others.

• 16:42Speaker 00

  Do you think this is intentional part of the design of it?

• 16:44Speaker 01

  I think someone there was like trying to make a greenish brown and then they made this. And it scared them so much, they called it shadow.

• 16:54Speaker 00

  The one Crayola they all feared.

• 16:57Speaker 01

  I've got one more thing to show you. This is, this is, this is something that, um, I'm sure you've, you've seen many times before. But phone cameras and screens can also pick up infrared colors that we cannot see. And they pick them up and then they convert them and display them as colors that we can see. The most easy way to see this is to use a television remote control or any kind of remote control that uses infrared light to communicate with a device. So if you, if you, uh, look at the emitting light bulb on your remote and you push buttons, nothing happens. It looks like it's off, but point it at your phone and look at how it shows up on your phone screen. Or I'll point it at the camera right now and you guys can see.

• 17:36Speaker 00

  We can see it. There it is.

• 17:38Speaker 01

  A little purple light. And that purple light is infrared light that the cameras can pick up on. They go, oh, there's light there. And then they render it as usually that kind of purplish color.

• 17:49Speaker 00

  The version of that that I really enjoy is, um, I have a, one of those doorbells, the, the smart doorbell things with the camera on it. And, uh, the one I really enjoy is, uh, it, when it's nighttime, uh, and the camera switches on as somebody comes home, you know, whatever. Someone's coming home from the, they're getting a taxi home or whatever. And they open their phone. If you have face ID on your phone, you, uh, it essentially does an infrared scan of your face. That's essentially how, it's like a depth measurement of your face that it's doing. And when you watch somebody on something like a door camera that's in night vision mode, where it is essentially doing the same thing as they open their phone, you can see it's flashing, it's flashing onto their face.

• 18:33Speaker 00

  This like very bright light flashes onto their face that in real life, you do not ever see that at all.

• 18:39Speaker 01

  Yes, that's another great experiment to do. It shows you how much of the world we don't see that's happening. But then when it comes to the fluorescent colors, the neon colors, there is a world that cameras still can't capture and show us, that you have to see in real life. And I've got a whole short about neon colors and using how they work to create neon brown. What would neon brown look like? And as it turns out, the definition of brown is basically. It can't be neon. Neon means more light is coming out than is even available in the environment. But brown means that there's so little light coming out that it looks dim.

• 19:22Speaker 01

  Take an orange, dim down the amount of light coming off of it, it's going to look brown. So brown is not only not neon, it's like the opposite, it's neon.

• 19:32Speaker 00

  So neon and neon off together doesn't work. It doesn't, you've got to choose one. This is a light switch situation. Going back to your color wheel, can I tell you the reason why I was so excited about this episode? Because there is one time where understanding color theory comes up on an almost daily basis for a lot of people, which is in makeup. Do you know about this? Okay, so I know that like makeup feels like quite a girly subject, but I'm just going to go with it for just a moment because I think this is absolutely, I firstly think makeup is very, very interesting and exciting for these exact reasons, for these exact reasons that you are using the color wheel the entire time and you are essentially changing, the whole point of makeup is that you are changing

• 20:16Speaker 00

  lightness and darkness on your face in order to correct or give the illusion that your face is doing something different than it actually is.

• 20:25Speaker 01

  Yeah.

• 20:26Speaker 00

  So, so for instance, I have like these little blue, my skin is like basically translucent. So you can kind of see the veins just underneath my eyes. And so every morning I wake up in the morning and because I know the color theory, I get a tiny bit of orange, like a tiny, tiny little bit of orange and I paint a bit of orange on top of the blue because they are opposite on the color wheel. And then sometimes I get a little bit of red around here, so I get some green and I put some little bit of green around here. But there are now these people on TikTok, on Instagram, who call themselves color theory witches and they, I don't know if you've ever seen this, Michael, it's so amazing. But they will start off with their completely clear face and they will chuck on like some

• 21:08Speaker 00

  vivid green and then like a blodge of purple and then like a tiny little bit of yellow and they'll just mix it in and it will perfectly match, like perfectly match the color of their face.

• 21:18Speaker 01

  Yeah.

• 21:18Speaker 00

  I like, I find this so endlessly phenomenal and impressive.

• 21:24Speaker 01

  It's impressive and it's impressive, but it's, it's, it's reality.

• 21:28Speaker 00

  Yeah.

• 21:29Speaker 01

  Because your skin is not one color. It's not like a pure peach or whatever was like put on you. Instead, all the colors are there. And if there's a lot of them, then you wind up with something that's very light in color because it's approaching whiteness, right? The white light containing all the wavelengths. And so you can combine green and orange and all these colors and make something that looks like it doesn't have any green in it at all to a naive perspective.

• 22:01Speaker 00

  But it goes back to what you were saying earlier, that you've got, you know, your brain is only like, is it short, medium or long? And how much short, medium and long photons essentially are there that are hitting the back of my retina at this exact moment? That's it, yeah, that's all there is to it. Yeah, that's great.

• 22:18Speaker 01

  Yeah, so I've got a lot more to learn about color. One of these days I will have my crayons arranged in, it's going to have to be a 3D shape because we've got three things going on. We've got how much gray, how much black, how much white? That's one spectrum and then what the hue is. But I'm excited to show that off once I've got it.

• 22:37Speaker 00

  Mm hmm. Okay, this is good. I'm, I'm looking forward to this. Also, can I just say, what an absolute delight to get a little insight into the way you spend your time, Michael. This is really genuinely, I'm endlessly fascinated by you.

• 22:51Speaker 01

  Well, Hannah, I'm glad, I'm glad, I'm glad that I have someone to share it with, not just you, but the listeners out there because otherwise I'm just sitting alone, talking to myself at night, being like, ah, oh, okay. So is the atomic tangerine more red or orange? And then I try to like use my camera phone, realize that doesn't work at all because it's a neon and boom, podcast episode.

• 23:15Speaker 00

  I'll tell you who else I'm endlessly fascinated by, Michael, our listeners and their questions, which we will come to after the break.

• 23:34Speaker 01

  This episode is brought to you by Cancer Research UK.

• 23:37Speaker 00

  Dentist appointments, MOTs, routine checkups, help us to spot faults early before we need a root canal or breakdown at the side of the road. And cancer screening is the same idea.

• 23:48Speaker 01

  It's about aiming to spot cancer early before symptoms show and when treatment works best. The UK's breast, bowel and cervical cancer screening programs developed with Cancer Research UK save thousands of lives every year.

• 24:02Speaker 00

  Today, Cancer Research UK is backing innovations that could transform the future of screening. Picture a smart bra that could one day detect breast changes as it's worn, making screening more accessible.

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  And AI trained on a database of over 10 million mammograms that helps doctors spot early signs of cancer.

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  Research like this saves and improves lives.

• 24:24Speaker 01

  For more information about Cancer Research UK, their research and breakthroughs and how you can support them, visit cancerresearchuk.org/restisscience.

• 24:36Speaker 03

  This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required. Compatibility and availability varies, 18 plus.

• 25:08Speaker 01

  All right, welcome back. It is now time for us to dive into questions from you all. And you were ready to just jump in, Hannah?

• 25:16Speaker 00

  You bet I am because the first one's about fluid dynamics, Michael.

• 25:18Speaker 01

  I know, I know, I know, I love this one. So this comes from Patrick Farrell. He emailed this question into us. By the way, if you want to do that, you can email us at the rest is science at goalhanger.com. Please do so, just like Patrick did. Patrick asks, when I stir a cup of tea, the leaves collect in the center of the bottom rather than being flung to the edges, which to me feels backwards since everything spinning should be pushed outward. I've read that Einstein once wrote about this and that the same effect explains why rivers erode their banks the way they do. What's actually going on in the cup?

• 25:50Speaker 00

  Patrick, I don't believe for a second that you don't know the answer to this, but I think that you're, you're here to amuse me because you've given yourself away in the question that you know so much about this already. But I love it and I'm very, very excited to get to talk about this. Okay, this is called the tea leaf paradox. Have you come across this before, Michael?

• 26:10Speaker 01

  Yeah, I have, but I don't remember enough about it. So I'm just like Patrick, well, no, I'm not like Patrick. Patrick probably knows the answer. He's like, well, Einstein wrote about it, but I think he wants to hear, he wants to hear how you explain it and so do I.

• 26:26Speaker 00

  Right, okay, so he's absolutely right. If you stir a cup of tea, I mean, frankly, you, you should be straining the cup of tea before it ends up in your cup, but, but fine. But if you end up with leaves in your tea and you stir it, then the leaves, you expect they should get pushed out to the sides because the, the, the whole of the fluid is, is sort of rotating in your cup and you should expect they get kicked out to the sides, but they don't, they end up clustering in the middle. It doesn't make any sense. People have been like, what the hell is going on for a really long time, since the 1850s, essentially. And the key thing, the key reason why this happens is because the top of your cup is not the same as the bottom of your cup.

• 27:05Speaker 00

  Because the bottom of your cup has a bottom and the top of your cup is free to the air. The, the, the level of the fluid is free to the air. And so essentially what's going on here is friction is causing this great big kerfuffle because as you have friction at the bottom, you get something called a boundary layer. This, by the way, was quite literally what my PhD was in boundary layer and fluid dynamics. So you, you, you get like right next to the, to the physical surface of your cup, it's completely jagged, it's like a mountain range if you zoom in. So the fluid cannot be moving at that point. And yet, a little bit further up, where your spoon is spinning around, it's going really fast.

• 27:44Speaker 00

  So what happens is you have this like, this range of flow speed from zero to very fast at the bottom. And it's sort of like it drags along the bottom, right? It's sort of like, it kind of, it goes slowest along the bottom and then sort of slower, slower, slower, and then, and then kind of speeds up as it goes up, up towards the top. So all around the sides of the cup and all around the bottom, you've got this, this boundary layer, this friction is kind of like dragging fluid. So what that ends up meaning is you have this like rotating fluid, it ends up, there's a secondary flow. So the dominant flow is round in a circle, but the secondary flow is that it kind of goes up and then it has to go back down and round the sides.

• 28:26Speaker 00

  You know, your, your, your spinning tea is pushing outwards, but near the bottom of the cup, it's slower, so you end up with this hidden circular movement. And the thing is, is that once it gets down to the bottom, that is like so much more dominant than the spinning that's going on at the top. And so you end up with these leaves being pushed along by that, that secondary circular circulation. So this is, it is the same thing as goes on in rivers. Thank you so much, Patrick. But as rivers go round corners, you end up with essentially the same thing. You have like fast flow on one side, then you have all of the, the like friction along the bottom. And so you end up with this secondary circulation that ends up dumping loads of sediment on the inside, is that right?

• 29:10Speaker 00

  Yes, on the inside of the, the rivers. So you should be able to tell, if I just take a photograph of a river, you should be able to tell based on the riverbank, which is the sort of inside and the outside bend, even if you can't fully see the bend. If I show you a cross section of a river, you can, you can tell just from that because of this effect essentially. Anyway, fluid dynamics is just really great and thank you for letting me have, thank you for letting me have three or four minutes on it. It was worth all the training.

• 29:40Speaker 01

  That's really cool. Yeah, I didn't know the answer was so sort of complicated.

• 29:44Speaker 00

  Oh, okay, maybe I explained it complicatedly.

• 29:47Speaker 01

  No, no, no, what I mean is I thought that there was some like, oh, it's because of, you know, gravity.

• 29:52Speaker 00

  Well, it's all because of friction. It's all because of friction. It sort of slows it down.

• 29:56Speaker 01

  So when, when did we figure out the tea leaf paradox?

• 30:00Speaker 00

  Uh, 1857. It's, uh, yeah, 1850. Like, all of this stuff was going on, was, you know, the, the, all of this boundary layer stuff was going on around this time, late 1800s, early 1900s. It's like this explosion in like how fluids flow. I should tell you one other thing, actually, when it comes to a cup of tea. If you are stirring your tea in a circle because you're like, oh, I want to get, you know, I really want to get this, this sugar stirred in really quickly. Terrible idea. Don't do it that way. Because actually, the sugar will dissolve much better in a turbulent, turbulent fluid, right?

• 30:33Speaker 01

  Oh, okay. Hold on. Let me, let me, let me guess. Because this is what I do when I need to like get salt to, to dissolve in pasta water.

• 30:40Speaker 00

  As quickly as possible, yeah. Don't do a circle. Don't do that.

• 30:43Speaker 01

  Well, hold on. I'll do a circle this way, and then I'll do a circle the other way really quickly. So it gets all like, you know, turbulent. And it feels like, at least that fixes the problem of salt congregated in, in the bottom, just like tea leaves.

• 30:56Speaker 00

  Absolutely. You can also, I stir tea like this. Like, for people who are watching and who are just listening, it's sort of an erratic, just, just do an erratic shape with your teaspoon. You look completely insane.

• 31:11Speaker 01

  Like you're just bouncing back and forth off the walls randomly. Yeah.

• 31:14Speaker 00

  It's like, oh, let's just, you know, wiggle it around a bit. It's much quicker. It's much quicker. Also, I should probably give up having sugar in my tea. But, but all the same. I'll tell you what, we should definitely do an episode on the effect of sugar on your brain. And, uh, because it's not good. It's not good.

• 31:28Speaker 01

  Oh, I would love to. Yeah.

• 31:30Speaker 00

  Fun for another time. But for now, sugar fans, until then, um, you can, you carry on enjoying the fluid dynamics of what's going on behind the scenes. Actually, I've just been reliably informed by Neil, who was watching the World Porridge Making Championships. That's the kind of caliber of human who works, who works on our show, Michael.

• 31:48Speaker 01

  Correct.

• 31:49Speaker 00

  And, uh, he's, uh, he's told us that actually, when it comes to stirring porridge, you need to do it with your right hand, because that keeps the devil out.

• 31:56Speaker 01

  Oh, we forgot about the devil. That's right. Whoops.

• 32:00Speaker 00

  Whoop.

• 32:01Speaker 01

  Continue stirring with your right hand, probably clockwise, because, well, I mean, what do you want? You want, you want, you want Satan to like come bother you? Or do you want sugar that's not quite dissolved? Okay? Your choice. All right. Well, you know what? Our next question is actually a little bit about, uh, water as well. This one comes from our subreddit, The Rest Is Science, where JessXX underscore says, "So, my boyfriend has this really weird theory proposing that water can be differently wet. He doesn't mean in terms of hardness. He means sometimes tap water is more wet than other days. Honestly, I just want to prove him wrong, but apparently no one has ever asked a question like this,

• 32:41Speaker 01

  so I can't find any studies on it. Can water get more wet?" I loved that. I didn't know how to answer this, but I had to look into it. And I had to go all the way back to 1946.

• 32:55Speaker 00

  Go on.

• 32:56Speaker 01

  And as it turns out, do you know where this is going?

• 32:59Speaker 00

  No.

• 33:00Speaker 01

  So I had to go all the way back to 1946 to Fire Engineering Magazine. They wrote about this problem that firefighters have had for a long time, which is that sometimes water isn't wet enough.

• 33:13Speaker 00

  What?

• 33:14Speaker 01

  So before I explain what that means, let's look at like what causes fire. Okay, fire is a chemical reaction, and you need three, maybe four things to really have a fire start. You need the fuel, but you also need oxygen, and you also need heat. It's that third leg, the heat, that water fixes. Water absorbs heat so well that you, it cools stuff down to the point at which the chemical reaction of combustion can no longer happen. The water isn't like washing away the elemental particles of fire. It's just cooling down the fuel to the point where it no longer combines with oxygen and combusts.

• 33:53Speaker 00

  Mm hmm.

• 33:53Speaker 01

  Okay. Here's the problem. A bale of cotton on fire is very hard to put out, because as soon as you put that water on, and you need that water to seep into the cotton and cool it down, the cotton on the top, the surface, absorbs the water, and it stops the water from penetrating deep into the cotton to cool down the entire bale, especially in the middle. And this 1946 article is like, honestly, it's been shown that kerosene can put out a cotton bale fire faster than water.

• 34:26Speaker 00

  Yeah. Because it slips through.

• 34:30Speaker 01

  Because it slips through. Kerosene being this hydrocarbon, it just slips right into the cotton. And if you have enough kerosene, yeah, it's flammable. But if its temperature is low enough, it'll drop the temperature of the cotton all the way through. So quickly, combustion stops, the fire is out long before water could do it. And so, they had to come up with ways to make water wetter, to reduce its surface tension, to reduce the extent to which it, it sticks to things and itself. And they do this with all kinds of chemicals nowadays. They do it with, uh, like glycol's and, um, surfactants, obviously, basically soaps. And these make the water wetter. Yeah. And so, uh, let me send you, this is a hilarious

• 35:14Speaker 01

  link where you can buy like, um, firefighting products. This is a product called wetter water. The water extender. And it is water that's more wet. It wets things, it clings to things and seeps through them faster than regular water.

• 35:34Speaker 00

  So this here says, "Lower surface tension, cooled faster, super concentrated, safer pumps and seals." Wow. I had no idea about this.

• 35:44Speaker 01

  So as I dove into this, I saw a lot of people saying, oh, yeah, you know, I'm, I'm a firefighter and will sometimes put soap into the water, because it's going to soak through especially fabric materials, absorbent materials faster and cool them down faster. So, yeah, they use, they use wetter water.

• 36:02Speaker 00

  I am so blown away by this. I'm also thinking, aren't there surfactants in your lungs? If I remembered that correctly?

• 36:09Speaker 01

  Ah, to, to help in what way? To help?

• 36:13Speaker 00

  So the inside of your lungs is basically like a wet balloon, and surface tension would mean that they would collapse. They're sort of covered in this very thin layer of water. And because the water molecules are attracted to one another, it continues to pull this little sack into a very tight little droplet. So if your lungs just had pure water in there, then the surface tension would be so strong that, that every time you, you breathed out, they would collapse flat and they wouldn't open again.

• 36:41Speaker 01

  Right.

• 36:42Speaker 00

  So you've got this wetter water.

• 36:45Speaker 01

  In your lungs.

• 36:46Speaker 00

  In your lungs, to make it so that you can actually breathe.

• 36:50Speaker 01

  So the tiny parts of your lungs don't just get stuck together by the water's surface tension. The water is wetter, it's thinner, it flows even more easily.

• 37:00Speaker 00

  Slipperier. Slipperier, less sticky.

• 37:02Speaker 01

  Slipperier.

• 37:04Speaker 00

  I've loved that so much. That is such a good question. I had no idea. Do they put it in, in the, like, when they're sort of there with the hose, are they pumping out wetter water?

• 37:14Speaker 01

  Sometimes. Not, not all the time. It, it depends on the type of fire, the type of fuel being burned. Um, but, uh, it, it is, it's a real thing. And so, JessXX, I, I think, I, I wouldn't have learned any of this. It wasn't for your question. I feel like, honestly, I haven't really answered your question, because your boyfriend is talking about tap water. Um, and I don't think any municipalities add surfactants and glycol's to their drinking water in the tap. Um, but that could go up and down day to day. Or even what's already on your boyfriend's skin might be mixing with that water and creating a more or less wet feel day to day. So that's, that's my guess.

• 37:59Speaker 00

  But wouldn't it be the other way around, though? Because I think like, feeling wet is about water sticking to you, no? Whereas we're talking about water slipping off.

• 38:09Speaker 01

  Well, I don't know.

• 38:10Speaker 00

  Hmm.

• 38:12Speaker 01

  I would say that you're, actually, you're right. Things get wet when water sticks to them. So a hydrophobic substance, uh, like, you know, glass on cars coated with special things, so the water droplets just come right off. Um, they don't get wet by the water, where, you know, wet is this verb, where wetting means to stick to. Um, some, some metals like gallium can wet glass, meaning they stick to it. And so, the feeling of water being wet, does wetter water feel less wet?

• 38:42Speaker 00

  I think it would. I think we need to buy some, and we need to try this out.

• 38:46Speaker 01

  And we need to feel, and then you need to drink it.

• 38:50Speaker 01

  And tell me what it's like. No, but this is, this is sort of two different definitions of wet being used here. One is how quickly does it penetrate materials? And the other is how much does it do the opposite, stick to them? And you're right, wetting is more about sticking to things and staying on them through surface tension.

• 39:07Speaker 00

  We, we need more information from, from Jess at sex. We, we, we need more. I think we need, um, what does he actually mean?

• 39:14Speaker 01

  Yeah. Does he mean that it feels thicker? Uh, cause you know, what does your boyfriend do? He might be like coming into contact with a hydrophobic substances, like fine dusts that then cause the water to just slough right off his skin and not wet his skin as much. That could be going on.

• 39:34Speaker 00

  Hmm. Look, I feel like there's, um, there's a follow-up episode on this coming. Do you?

• 39:40Speaker 01

  Yeah, I think there is. Okay. So yeah, keep us, keep us up to date and we will be following along on the subreddit. Here's a question that came from T. Senary. How does space agencies such as NASA communicate with satellites, space probes and rovers in a way where they cannot be hacked? And has this changed over time such that older space probes may be more vulnerable to hacking? Are there any known instances of people hacking satellites?

• 40:07Speaker 00

  Okay. So I had to look into this, right? And I think you would imagine that the older ones would be easier to hack. So like Voyager two, for example, because that was launched in 1977, which is, you know, before anyone was worried about cybersecurity, before all of this, like modern encryption stuff was put out. And, uh, I, you know, you, you probably could. The only slight problem is that it's now 20 billion kilometers away.

• 40:33Speaker 01

  Yeah. You're going to need a really powerful antenna to like contact. I never thought about that though. Like clearly NASA communicates it with probably radio waves. Couldn't I spoof that and tell Voyager to like come back or something?

• 40:47Speaker 00

  I mean, yes, but you're, I don't know if you could tell it to come back. I'm not sure if it's got enough fuel on board, but, um, you could, you, you, you would need to get into NASA's giant deep space network antennas that they have dotted around the world.

• 40:59Speaker 01

  That's the problem. Yeah.

• 41:00Speaker 00

  I mean, they are capable of transmitting these incredibly powerful beams, radio waves, exactly as you described, with extraordinary precision. But, um, you're, I mean, no, you're just not gonna, you're not gonna have one of them sitting around in your backyard. You're not gonna be able to build one of them without anyone noticing. And even if you did, you would also need to know Voyager's, uh, original language, programming language, which would be, I mean, who even knows what it was. Um, but it'd be decades old. It would be really, I'm sure there would be details on it out there, but, but don't bother. Basically what I'm saying is the, the, the startup costs are prohibitive and, and the output that you would get would be also quite limited. Ironically though, the modern satellites, which are much closer to earth, that sort of makes them slightly easier targets. The

• 41:44Speaker 00

  thing is these do have much more standard technology, this sort of encryption that, um, that people have, uh, you know, auth- authentication, because they know that they are easy to intercept. In fact, you know, um, I can't remember if I've told this story on this podcast before, but you know Sputnik? When Sputnik originally was launched, it was a group of students in a bedroom who worked out where Sputnik was because Sputnik was giving out this beep. Have I told you this story before?

• 42:11Speaker 01

  Yeah, yeah, yeah, yeah. But I guess Sputnik wouldn't have the ability to do anything. You couldn't like tell it to, you know, fall or change its orbit. It just kind of was there and was gonna not be there eventually.

• 42:23Speaker 00

  Exactly right. But, but it really, I think demonstrates how these, these young students were able to tune into it using their radios, um, and, and to hear the beep that it was giving out. These things are overhead the entire time.

• 42:37Speaker 01

  Yeah.

• 42:37Speaker 00

  It's really not that hard. It really wouldn't be impossible to create an antenna that could tune into what they are putting out and have the original coded message. The problem is, is unencrypted it is going to be way, way harder because they're using modern techniques. Um, there have been a couple of mishaps though.

• 42:56Speaker 01

  Oh.

• 42:58Speaker 00

  There was a really famous one in 2022, 24th of February. This, by the way, is about an hour before Russian tanks crossed the border into Ukraine. So kind of a really critical moment.

• 43:09Speaker 01

  Oh.

• 43:09Speaker 00

  But attackers managed to reach the, the management network of, uh, ViaSat's KASAT system, right? Uh, which is basically, it was a misconfigured VPN. And what they did is they, they pushed a virus effectively, a, a, a wiper called Acid Rain, um, out to tens of thousands of, of modern consumer broadband modems and just bricked them.

• 43:35Speaker 01

  It rained down. So it was like just a, just a vector for this virus.

• 43:41Speaker 00

  Exactly right. Exactly right. You send it up, it sends it down. All of this thing goes. And so what it did was it took out the Ukrainian military satellite communications right at the beginning of the war.

• 43:54Speaker 01

  Wow.

• 43:55Speaker 00

  Um, it also instantly knocked out 5,800 wind turbines in Germany.

• 44:01Speaker 01

  I didn't even know about this.

• 44:03Speaker 00

  ViaSat, who are the provider, they had to ship out 30,000 new modems. There was no recovering these things. Um, and also the satellite itself was completely fine. You know, the attack sort of, it just, it just was a little IT system that kind of went up and then, and then disseminated to all of the terminals. But, um, I mean, no one knows who managed to do that. No one knows who, who, who carried out that attack. Um, I don't have any suspects. Don't know about you? Not sure.

• 44:29Speaker 01

  No, no, no one's told me.

• 44:31Speaker 00

  Complete mystery. Complete, complete coincidence and complete mystery. Um, but yeah, it does happen. It's just, just hard to do. And you have to be, I think, quite motivated. Tell you what though, quantum computing is coming and, um, encryption's looking a bit ropeier.

• 44:46Speaker 01

  Yeah, man, you, I never even thought about hacking a satellite. Like, could I get, is Hubble still up there? Could I get Hubble to like point down at my house?

• 44:55Speaker 00

  Send you, send you pictures. Yeah.

• 44:57Speaker 01

  Take some family photos. I don't think Hubble does well with short distances, but you know what I mean? There, there, there are other cameras up there that are pointing at earth. Um, I could commandeer them and set up some fun photo shoots. That, with quantum computing, that could become a much bigger problem. I, I, I guess you have to hope that encryption keeps up with the computing.

• 45:18Speaker 00

  Yeah, there are ways to do it. There are ways to do it. I'll be honest with you, Michael, you're starting to sound suspiciously like Mark Zuckerberg wanting to point a mirror to his house.

• 45:25Speaker 01

  For to, to give himself day during the night. Um, yeah, look, I was kidding. I don't really want, um, to do a photo shoot from space.

• 45:36Speaker 00

  I knew you were. Um, that was a joy. I really enjoyed those questions. That was, that was a lot of fun.

• 45:41Speaker 01

  They were really good. You guys bring us not just fun questions to answer, but the, the learning behind how we kind of approach them is just so fun. So thank you. And please send in more. We've got our subreddit, uh, the rest is science and the rest is science at goalhanger.com.

• 45:57Speaker 00

  Do you, do you worry though, that we research for these every single week? Do you worry that your brain is like, gonna get to a point where it's like, no, I'm actually, I'm actually full now?

• 46:05Speaker 01

  No, I don't think so. Uh, but I think that it might get so full that like, imagine that no one can stump us. They're just like, hey, what if, uh, human hair was made of spaghetti? And, and we're like, oh my gosh, this one again. Yeah. Look, you know, every week you'd get 2,000 calories of hair and blah, blah, blah, blah, blah, blah, blah. And we would just become know-it-alls to a like boring extent.

• 46:29Speaker 00

  I, I strongly suspect we might already be there, Michael, but, but hey, I think there's still, there's still mileage in this. Um, thank you so much for joining us. We will be back with another few episodes next week. Bye-bye.

• 46:40Speaker 01

  See you next week.

• 46:53Speaker 04

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• 47:24Speaker 05

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