The Audio Illusion That Proves We Don't Experience Reality

Your ears are lying to you right now. In this episode of The Rest Is Science, Professor Hannah Fry and Michael Stevens (Vsauce) dive into the bizarre world of audio illusions where your brain hears sounds that don't exist, fills in frequencies that were never there, and constructs a version of reality that simply isn't real. From the impossible endless ascending tone to the battle between your eyes and ears, explore how auditory illusions reveal the cracks in human perception. Along the way, we uncover how hearing works at a neurological level, why your brain edits sounds before you consciously register them, and what sensory perception research tells us about whether any of us truly experience reality as it is. Could the gap between what's real and what you perceive be wider than you think? The answer might change how you listen to the world. ------------------- 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 Head Of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Michael Stevens?

  Hello and welcome to The Rest Is Science. I'm Michael Stevens.

• 0:04Hannah Fry?

  And I'm Hannah Fry.

• 0:05Michael Stevens?

  And Hannah, today, I want to talk about some illusions. And how, as I've grown older, I'm starting to feel like maybe they aren't illusions.

• 0:14Hannah Fry?

  Oh, go on.

• 0:16Michael Stevens?

  Maybe. You know how people will often say, like, "Everything's an illusion." I'm starting to wonder if nothing is an illusion.

• 0:24Hannah Fry?

  Or is it?

• 0:26Michael Stevens?

  Or... All right, look, here's, I want to start right out with an audio. I was gonna say "audio illusion," but no, it's just, it's some audio. I want you to listen to this, okay? I've got a little Hertz generator here that gives me precise audio tones. These are sawtooth waves. It's called a sawtooth wave because if you graph it out over time, amplitude over time, you get this gradual rise that looks almost like the side of a triangle, and then it suddenly shuts off, and then rises again and shuts off. So it's like a series of right triangles, like the teeth of a saw. I'm gonna start it at just 1 Hz, okay? I'm gonna turn my microphone to my computer so you can hear it really clearly. I hope this works.

• 1:10Michael Stevens?

  Can you hear that?

• 1:14Hannah Fry?

  Yeah.

• 1:15Michael Stevens?

  Okay, that's 1 Hz. That's one sawtooth wave every second. I'm gonna raise this up to 5.

• 1:24Hannah Fry?

  So it goes up and decays, up and decays, up and decays.

• 1:28Michael Stevens?

  Yeah, it goes up and then it very quickly is cut off.

• 1:31Hannah Fry?

  Right.

• 1:31Michael Stevens?

  Unlike a sine wave, which is gonna go up and then down, or a square wave, which suddenly is on and suddenly is off. This one comes up and is off. All right. So here it is, this is 5 Hz. Now we're at a rave. Now we're talking about 300 bpm.

• 1:49Hannah Fry?

  That's, um, this is reminding me of my youth.

• 1:52Michael Stevens?

  It's a little bit stressful. I've never been to a rave. I just have not had certain experiences that I should have had when I was young.

• 1:59Hannah Fry?

  I've never been to an illegal rave. I've only been to ticketed, well-organized...

• 2:05Michael Stevens?

  Poser raves, okay.

• 2:07Hannah Fry?

  With sensible health and safety restrictions.

• 2:10Michael Stevens?

  That sounds smart. That sounds smart. Now what I'm gonna do is I'm gonna turn this beat up to 20 Hz.

• 2:22Michael Stevens?

  This episode is brought to you by Cancer Research UK.

• 2:25Hannah Fry?

  Scientists have found that cancer risks usually increase with age and size, but some species defy the odds.

• 2:32Michael Stevens?

  For example, deep sea Greenland sharks. They can grow over six metres long, weigh more than a small car, and yet live for up to 400 years.

• 2:43Hannah Fry?

  Now, understanding how Greenland sharks' cellular repair and immune systems seem to have managed to keep them cancer-free for centuries, that could open up exciting research pathways.

• 2:54Michael Stevens?

  Essentially, over millions of years, evolution has been running the world's most successful cancer prevention trial. And sometimes breakthroughs can be found in unusual places.

• 3:04Hannah Fry?

  So by exploring the unexpected, Cancer Research UK scientists are uncovering new ways to tackle over 200 types of cancer. Their work has helped to double survival in the UK over the last 50 years and continues to save and improve lives around the world.

• 3:20Michael Stevens?

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

• 3:32Speaker 02

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• 5:36Michael Stevens?

  Now what I'm gonna do is I'm gonna turn this beat up to 20 hertz.

• 5:46Michael Stevens?

  Okay, what are you hearing now?

• 5:49Hannah Fry?

  It's no longer stressful, just irritating.

• 5:53Michael Stevens?

  Just irritating? Okay, good. Now I want you to pull it all the way up to like 80 or 100.

• 5:58Hannah Fry?

  Oh, it's actually making my brain go a bit funny. Hang on.

• 6:01Michael Stevens?

  Good.

• 6:05Hannah Fry?

  I don't like it.

• 6:07Michael Stevens?

  Where are you, where are you at now?

• 6:08Hannah Fry?

  100.

• 6:09Michael Stevens?

  Okay, at 100. Now pull it up to like 400.

• 6:14Hannah Fry?

  Okay.

• 6:16Hannah Fry?

  Okay, now it's, it's less stressful now. It's more like someone is playing a kazoo.

• 6:23Michael Stevens?

  Yeah.

• 6:23Hannah Fry?

  A bit too close to my ear.

• 6:25Michael Stevens?

  It sounds like a tone. But what you're actually hearing are 400 beats every second. If your ear was better, it wouldn't sound like, "Ooh." It would sound like, "Brrrr."

• 6:42Hannah Fry?

  Because it's still the same thing. It sounds continuous, but it's just because it's too fast for my ear to hear.

• 6:47Michael Stevens?

  Yes, at a certain point, our brains give up. They actually fail. They just cannot account for every detail in each second. And so they just go, you know what? Make it a pitch, okay? And so what I'm getting at here is that there is no music. Everything is just percussion. Every oboe, every trumpet, every singing voice is just a combination of drums that are going so fast, we give up and we hear it as a pitch, not a boom, boom, boom, boom, boom, boom. But there could be animals or aliens who listen to our music and think, man, these people never progressed above percussion.

• 7:29Hannah Fry?

  Hold on a second, though. Is that really true? There's no such thing as a continuous noise?

• 7:33Michael Stevens?

  It depends on what we mean by is. That's the famous Clintonian philosophical doctrine. Do we really know what is is? We know, using tools that we can measure, that all noises are just motions of air. They're pressure waves that are either coming faster or slower, and they're either louder, involving more motion or less. But scientifically, there's nothing that happens magically in the air when we go from five hertz to a thousand hertz, where the air changes. And it goes from being just a pressure wave to becoming a beautiful spirit of pitch. No, it's all just waves, always has been.

• 8:18Hannah Fry?

  It's all just pressure waves wobbling backwards and forwards.

• 8:21Michael Stevens?

  That's right.

• 8:22Hannah Fry?

  Before you get all down on hearing for a second here, it took till 400 hertz there before it became a single tone. The ears are way better at this than the eyes, because we're doing this, this recording remotely, I'm watching you on something that's probably 24 frames per second, and you look like you're moving continuously.

• 8:41Michael Stevens?

  I know, right? Yeah, it totally makes sense, though, because in nature, where we evolved, there are a lot of percussive sounds that can be pretty quick. Even like a buzzing fly. However, there aren't a lot of strobing visuals. There aren't a lot of objects that are somewhere and then magically disappear and then reappear somewhere else. And so, of course, we evolved to just be much more ready to assume that visual staccato is actually smooth movement. But with sound, the same thing happens. And it's biomechanical. I mean, when sounds hit our ears, we can differentiate individual beats only up to a point. At a certain point, instead of our ears experiencing this succession of waves, a standing wave

• 9:26Michael Stevens?

  occurs in our brains and in our ears. And our neurons can only fire so quickly. They can't say, another one happened, another one happened, another one happened. Eventually have to just be like, okay, a lot are happening.

• 9:39Hannah Fry?

  It's happening.

• 9:41Michael Stevens?

  It's happening. It's all happening, guys. The really deep, hard problem of consciousness asks, why does it become a pitch? This sounds really wild, but why isn't it that once you cross that threshold of hearing, around 16 or 20 hertz or higher, why doesn't the percussive beat sequence become the smell of peanut butter? If you're just gonna have it become an entirely new phenomenon, a whole new experience, why did "oo" get chosen and not, oh, gosh, I'm seeing orange?

• 10:15Hannah Fry?

  Yeah, that's definitely one way to frame the hard problem of consciousness. When does the switch happen there? When did you say it happens?

• 10:23Michael Stevens?

  It depends on a lot of factors, not just you and your body, but also what kind of wave is being used. We're using a sawtooth wave here. But the threshold for human hearing is down around like 16 hertz, maybe 20. But it goes up very, very high into the thousands. But it's very easy to hear 5 hertz, that means 5 beats a second, as individual beats.

• 10:46Hannah Fry?

  Tell you what I'm gonna do, I just wanna hear, okay, so this now is sawtooth wave at 24 hertz. So the same number of beats per second as you get when you're watching footage generally is 24 frames per second. I think that still sounds... Like distinct moments rather than a single tone.

• 11:05Michael Stevens?

  Yeah, so again, it really depends on what it is that's making the sound and how it's physically moving the air. For example, the world record for fastest drumming was set in 2013 by Tom Grosse, and he was able to hit his drum 1,280 times in a minute. Now, that is more than 20 times a second, but it still sounds like someone hitting a drum really quickly.

• 11:33Hannah Fry?

  Hang on, let's do, let's play, can we play 20 hertz just to, just to, that's essentially how quickly he was playing the drum.

• 11:39Michael Stevens?

  Yeah.

• 11:43Hannah Fry?

  I mean, that's wild.

• 11:44Michael Stevens?

  But if those drummers were able to, you know, drum 10, 20 times faster, we'd start being like, oh, what instrument is that? Is that a bass clarinet? Not really, because of course, the timbre of an instrument depends on a lot of frequencies all working together. But what I'm wondering is, is this an illusion? Because there isn't a pitch there in reality in and of itself. Or maybe there is. I mean, this is all really unknown. We're not going to spend a whole hour just on this one question, but that's what I want everyone to keep in mind.

• 12:20Hannah Fry?

  Is pitch just completely constructed in the human mind?

• 12:24Michael Stevens?

  That's right. Are we really just decorating the air with percussion and then our brains go, no, no, no, no, that, that is Pink Floyd's Dark Side of the Moon. You know, that is beautiful harmonizing tones. I mean, pressure waves can harmonize physically, we know how to describe that. But the qualia, the experience of the sound, the subjective thing that we hear, the thing that, you know, the famous Mary's Room experiment, which asks about a girl named Mary who lives her whole life inside a little house where there's no color. She's not allowed to see her body. Let's put her in a black and white bodysuit. She has no mirrors. Everything is just black or white. And she's got a lot of books, though.

• 13:09Michael Stevens?

  And so she can read about the color red. She can read about what wavelength is and electromagnetic radiation. And she can read that roses are red. And she could read about how the retina works and the different vision parts of the brain. She knows everything there is to know about red. Here's the question. If she's ever allowed to leave that room and she sees a red rose, has she learned something new about red? It's that redness that I'm getting at. Pitchness. Is it an illusion properly?

• 13:39Hannah Fry?

  Actually experiencing it is different to understanding what it is and where it comes from. I've always wondered, you know that thing about Mary's room? I've always wondered, A, why does it have to be a girl? B, why is she called Mary? I've never really understood those two things. But this, of course, is something that has come back around to the sort of forefront of the scientific mind with the advent of artificial intelligence. Because this whole question of experiencing something as opposed to understanding it, or even understanding it as opposed to having loads of knowledge about it. What does it mean to conceptually grasp something if you can't really experience it? All of these questions are now incredibly prevalent in the scientific literature.

• 14:23Michael Stevens?

  Yeah, what's it like to be a bat? What's it like to be an AI model? Well, you can play a note for AI and it could tell you exactly what note it was or what its frequency was, and it could play that frequency back. But maybe it's just its microphone is vibrating at a certain speed. It takes that speed, gives you the answer, and then it plays it back for you by moving its speaker at the same frequency. How do we know inside, in its subjectiveness, wherever that would be, if it's there, that they're experiencing

• 14:56Michael Stevens?

  and not just whatever's really there? Let's look at some other audio illusions. Some of these are incredibly famous and have been for a long time. So we don't need to spend forever on them or pretend like, what? Yanni or Laurel? But I didn't know the stories behind them. So Yanni Laurel, I'm sure many listeners have heard of it before. We'll play it for you. Here it comes.

• 15:20Michael Stevens?

  Okay, did you hear Laurel or Yanni there?

• 15:23Hannah Fry?

  I can only hear Laurel.

• 15:27Hannah Fry?

  And if I try really hard.

• 15:34Hannah Fry?

  I can't hear Yanni at all.

• 15:36Michael Stevens?

  I can't hear Yanni at all either.

• 15:38Hannah Fry?

  Are we sure it's in there?

• 15:40Michael Stevens?

  I'm sure it's in there. I really do believe the people who say they hear it because if you cut out all of the lower frequencies in the sound, it sounds just like Yanni. Listen.

• 15:54Michael Stevens?

  Okay, that's without like the lower half of the frequencies. Now let's take out the upper half. Listen to this.

• 16:02Michael Stevens?

  Clearly Laurel. So they're both in there. Which one you hear depends a lot on how your ears work and also what kind of sounds you've grown up around. In fact, it's been shown that geographically, people in LA tend to hear Laurel, but people in the southern England, apparently, hear Yanni more often. That's why I thought maybe because you're from the same country.

• 16:25Hannah Fry?

  But wait, why? Why? Why?

• 16:28Michael Stevens?

  We don't totally know, but the hypothesis is that the types of sounds, like literally the speaking voice registers that you grow up around, train you to focus on certain parts of noises. And so if you're focusing on lower noises, you're going to hear Laurel. But if in your daily life, you're experiencing higher pitched noises, that's what's relevant to your fitness and survival in the world, you're going to hear Yanni.

• 16:53Hannah Fry?

  Is that the reason why people in LA hear Laurel? Because there's so much around them of people going, oh my gosh, hi, that they just learn to tune it out?

• 17:02Michael Stevens?

  Look, it could be. It literally could be. And I think it's important to emphasize, we do not understand this illusion well enough that we can just write it up, make a little two-minute YouTube video and say, here's how it works. We can do that for some illusions. A similar illusion to Yanni Laurel is the famous blue and gold dress, right? This is one where it's also very hard for people to see it one way or the other. There are other illusions where it's very easy to flip, like the necker cube. That's the little two-dimensional line drawing of a cube that can either be seen inside out or right side out, depending on how you look. The brainstorm green needle one, have you heard that one?

• 17:43Hannah Fry?

  No. Hang on, the brainstorm. Lib, can I ask you about the dress though while I'm looking this up?

• 17:49Hannah Fry?

  Which one do you see?

• 17:50Michael Stevens?

  I see blue and black.

• 17:51Hannah Fry?

  Do you?

• 17:52Michael Stevens?

  I always have, but you don't?

• 17:54Hannah Fry?

  No, I see green and green and white, uh, green? What was I talking about?

• 17:57Michael Stevens?

  Green, whoa, okay, there's a third option, folks.

• 18:00Hannah Fry?

  I see gold and white. However, that one I can switch.

• 18:04Michael Stevens?

  You can switch?

• 18:06Hannah Fry?

  With difficulty, but I have seen both. If you really look at it, especially if you sort of squint at it, then I can switch. But that one, in the same way, the sort of research on it demonstrates that people who have grown up around artificial lighting tend to interpret it differently to people who have spent their life outdoors, right? So like farming communities, for example, will see it one way and people who grew up in cities will see it another.

• 18:29Michael Stevens?

  Exactly. It's like very, I don't, I don't want to say hardwired, but it's like over easy wired into our brains based on where we live. My wife saw gold and yellow.

• 18:40Hannah Fry?

  Right.

• 18:41Michael Stevens?

  Heck, she saw gold and white first. And I think it, it has to do with, yeah, the, the world around you, the kind of lighting you are used to. That affects how you assume the ambient lighting in a photograph is. And that changes how you perceive the colors of the dress.

• 18:58Hannah Fry?

  Because the question is, is this a blue and black dress, which is just overexposed in the background? And so it kind of looks a bit washed out? Or is it something that is a green? Oh, my gosh. What is wrong in this? We found a new illusion.

• 19:15Michael Stevens?

  We found a new one. It's impossible for us to talk about the dress.

• 19:20Hannah Fry?

  Yeah. Or is it a gold and white dress that has been overexposed by artificial lighting?

• 19:25Michael Stevens?

  If you, listeners out there haven't looked at it for a while, go take a look. I mean, it's still a phenomenal illusion. And the teams that I worked with at the University of Chicago, when I was a psychology student, they were emailing me about this illusion because it really did lead to new questions and progress in vision research.

• 19:41Hannah Fry?

  Oh, yeah.

• 19:42Michael Stevens?

  The illusion is so strong and we don't know exactly how it works.

• 19:47Hannah Fry?

  Have you seen the socks and crocs version of this?

• 19:50Michael Stevens?

  No, no, tell me about it.

• 19:52Hannah Fry?

  Okay, so this is a direct result of the of the dress and some researchers working on vision perception. But I think in particular, it comes back to this idea that it's like your lived experience ends up changing the way that you interpret the world around you. And what they were trying to do was to recreate the illusion that was there in the dress, but also, I mean, in a different way in Yanni Laurel. And they were looking for things that has ambiguity over their color that would allow them to sort of tap into people's lived experiences. And they came up with crocs and socks that they took in sort of ambiguous lighting.

• 20:32Michael Stevens?

  Okay, so to me, it looks like seafoam green socks and like beige brownish crocs.

• 20:40Hannah Fry?

  Okay, so to me, it looks like I can see that those are actually white socks. I can see they're white socks and the lighting's a bit off. And then there's sort of a pinky tone to the crocs.

• 20:52Michael Stevens?

  So if you assume that there's just like a more electric green light on the scene, you've got white socks and pink crocs. Whereas I'm coming at this thinking, oh, the white is just a white, it's got all the colors in it. So the socks themselves are electric, seafoamy green. And those shoes are just kind of like a drab olive color.

• 21:13Hannah Fry?

  Right, but I think that what this managed to do was it really demonstrated this this hypothesis that actually in any situation where there's ambiguity and requires your brain to make a sort of binary interpretation of a scene, right? Is this a particular lighting that is imposing and changing color of this? And should I interpret it in the original way? Or do I take this image as as fact? It's really your lived experience and how much you have witnessed those kind of environments of artificial lighting, say, in the past that changes your perception, changes your actual experience of the world. Going back to what you were saying at the beginning about qualia, right? About the hard problem of consciousness. Is your green my green? Is your experience of the world the same as mine? And this is like a tiny little crack in the sort of great.

• 21:57Hannah Fry?

  Wall that we've been unable to break through to answer this question.

• 22:00Michael Stevens?

  It's a tiny crack. It doesn't demolish the whole thing because we still know that we're disagreeing. You know, we both know that, wait, you think that it's pink? I know what pink is and that's not pink. Like the word still means something to both of us. By the way, look at the Crocs and socks picture and look at the caption below. This image is from Pascal Wallish. That's exactly the guy I worked with at U Chicago.

• 22:25Hannah Fry?

  Amazing.

• 22:25Michael Stevens?

  This became a huge thing. Yeah, he was one of my teachers. And we talked on Facebook about the illusion. So I don't know, I'm just trying to, um, Berg, I'm trying to Berg here, bask in reflected glory. I knew that guy.

• 22:39Hannah Fry?

  Berg's a good name for it. Berg's a good name for it because it's like, hey, look, you guys, you're seeing, you're seeing the tip of the iceberg, but underneath is me.

• 22:46Michael Stevens?

  Yeah, but really Berg, B I R G, basking in reflected glory. It's a thing people do a lot. The opposite is called corphing, cutting off reflected failure. Oh, I was never really for this, or I wasn't really there that long.

• 22:59Hannah Fry?

  Mistakes were made.

• 23:01Michael Stevens?

  Yeah, yeah. So back to illusions, there are other illusions that are multi-stable. It's very easy for everyone to flip between one perception and the other. So this is the famous brainstorm, green needle illusion. What you're about to watch is a toy that says brainstorm. That's the Ben 10 character. It's saying brainstorm. And yet, just like the stroop color illusion, where it's really hard to, to say the name of a color. When you're reading a word printed in that color, here, if text green needle is put on the screen, you hear green needle.

• 23:37Hannah Fry?

  That is absolutely wild. I almost don't believe it. I almost sort of want to rewind and it's just, it's like, have I just by fluke? Uh, managed to match. Is the actual audio changing? And by fluke, I've sort of been, been concentrating on the correct word at the correct time. But no, it really is. You listen to the, if you rewind and, and play the precise same piece of audio, but concentrate on the other word, you hear something different.

• 24:03Michael Stevens?

  Let's set this up so that the listeners can have their own experience. Say, so what you're going to hear is a sound and it will either sound like brainstorm or green needle. Just as we loop it, decide which one you're going to hear, you will hear that one.

• 24:22Michael Stevens?

  But it was the same sound every single time. So here, again, we've got a like kind of small, low-quality speaker producing a sound with a lot of noise in it. However, here, different modalities can get involved to shift the brain to perceive it one way or the other. So if you see green needle or brainstorm written out whenever you hear it, your brain goes, oh, that's what I'm expecting. Even if you watch the video of the original toy and you see that the light blinks three times, that makes you go, oh, there must be three syllables and you're more likely to hear green needle. But green needle and brainstorm sound like such different words. How could there be ambiguity between those?

• 25:05Michael Stevens?

  And yet there is, because the stimulus is very complex and the way we take what's out there and turn it into a little show in our brain is dramatic.

• 25:15Hannah Fry?

  What is going on there? Is it about attention? Is it about something particular in the way that sound is constructed? Like, what is going on? Why is that possible? Can I make my own version? Can I do one that says Michael and Hannah at the same time?

• 25:30Michael Stevens?

  You could, you could definitely. It might take some work. You might need to, you know, test it on a lot of people. But yeah, you can always add in noise and change around frequencies and overlay things to the point at which it becomes ambiguous. And maybe only children would hear one word and adults hear the other. And then maybe even like elderly people hear a whole third thing. It's not completely drag and drop, because we still don't know all the mechanisms behind it. You can watch every video on these illusions and find audiologists describing different aspects of the waveform. Though, at the end of the day, it all has to do with either what mindset you're in, what other modalities are there. If you're using your eyes to see the word brainstorm.

• 26:15Michael Stevens?

  Or you're thinking the word green needle. That's what the audio sensation will be perceived as. However, with Yanni Laurel, that's like a deeper mechanism in the brain at fault. Assuming these are illusions, which we'll get to later. Where your lived experience is affecting your ability or disability to hear one thing or the other.

• 26:38Hannah Fry?

  This is called the brainstorm one is the McGurk effect, right?

• 26:42Michael Stevens?

  Yes, exactly. The McGurk effect is another example of how different modalities work together to create a different reality.

• 26:50Hannah Fry?

  Tell me if this is the wrong thing, because I don't know very much about this, right? But like, when you mouth the word colorful, but the person thinks you're saying, I love you.

• 26:58Michael Stevens?

  Yeah.

• 26:59Hannah Fry?

  Is that an example?

• 27:00Michael Stevens?

  Let's do a little experiment.

• 27:01Hannah Fry?

  Okay.

• 27:02Michael Stevens?

  I'm going to say either I love you or colorful. Those of you listening, you'll know the answer. But those of you who are watching the video, hit mute when I say so. And then write in the comments below what you think I said. Okay, here we go. And I'll put my hand up when you should turn it back on. Okay, mute your audio in three, two, one. Colorful.

• 27:27Michael Stevens?

  Unmute, come back, come back. If you were only watching me, write down below what you thought. So, so yeah, we, we don't have like a completely isolated hearing system in our brains. We are taking clues from everywhere and putting them together. And ultimately, that's what we should be doing. It doesn't matter what the truth is. Here's the truth, okay? Is it Laurel or Yanni? Truthfully, it's Laurel. Because, here's the true story behind it. A man named Jay Aubrey Jones recorded the word Laurel. That's what you're hearing his voice when you hear that sound. He recorded the word Laurel for vocabulary.com. He recorded thousands of words so that people could hear the pronunciations.

• 28:11Michael Stevens?

  Well, then in May of 2018, there was a high school student named Katie Hetzel, who either recorded a computer playing back his sound, or she downloaded a low quality version of that file. And another classmate of hers was like, that sounds like Yanni. And she's like, no, no, this is a recording of someone saying Laurel. And her classmate put it up on social media and was like, Yanni or Laurel, it's the new dress. The dress, by the way, I want to tell a story about the dress. Because I've never told this story before, and this might be where I finally get some help. So I almost played a role in the dress.

• 28:49Hannah Fry?

  Go on.

• 28:50Michael Stevens?

  Here's what happened.

• 28:51Hannah Fry?

  Burgas, Michael.

• 28:52Michael Stevens?

  The, the, the, the famous dress illusion started back in 2015, in February of 2015. Two people were getting married, Grace and Keir Johnson of Colonsay, Scotland? Colon say?

• 29:07Hannah Fry?

  Either or.

• 29:08Michael Stevens?

  Either or?

• 29:09Hannah Fry?

  Say it twice, we'll lay them both over each other. We can ask the audience which one they hear correctly.

• 29:14Michael Stevens?

  What does the colon say? That's all I can think of. But anyway, that's where they lived and they were getting married. And the brides, so Grace's mother went to, and I got the exact location here. She went to the Cheshire Oaks Designer Outlet, north of Chester.

• 29:30Hannah Fry?

  That's Cheshire, but yeah.

• 29:32Michael Stevens?

  Cheshire.

• 29:33Hannah Fry?

  Cheshire.

• 29:34Michael Stevens?

  Cheshire.

• 29:35Hannah Fry?

  Cheshire.

• 29:36Michael Stevens?

  Cheddar.

• 29:38Hannah Fry?

  You Americans with your glow dust of Shire. Anyway, go on.

• 29:42Michael Stevens?

  She went to this outlet and she saw the dress, took, took the photo, the photo that we now, it has gone viral and put it up on Facebook. And all of Facebook friends were like, whoa, is, is that, you know, gold and white or blue and black? And people were like, what? How can you see it differently than me? But it didn't go viral until the day of the wedding, when a friend of the bride and grooms, whose name was Caitlin McNeil, saw it in person, being worn by the, the now mother-in-law, the mother of the bride, and said, holy cow, it really is blue and black. But the photo didn't look that way. And she shared it to her Tumblr on the 26th of February, 2015. And then it got really popular, BuzzFeed wound up covering it.

• 30:26Michael Stevens?

  But at four in the afternoon, on the 26th of February, 2015, I received an email.

• 30:34Michael Stevens?

  From a girl named Lizzie Rhodes. I'm giving out her name because I need your help, Lizzie. Who are you? She wrote, "Michael, my friends and I were stumped when we began to argue about the color of a certain dress taken by one of us. Some see the colors as white and gold, others see blue and black. I see blue and black. One of my friends posted it and we were getting all different comments about it. Are we crazy? Is there any way to explain this? What do you see?" So, I remember getting this email a little after midnight, eight minutes after midnight, in fact, if you want to be specific, on that day. And I was like bleary and sleepy and about to go to bed. And I'm like, this is one of the craziest fan emails I've ever received because it's

• 31:19Michael Stevens?

  obviously blue and black. But she says people see it as gold and white? Ugh. I get a lot of emails that are just like, "Hey, how come my nose is floating?" And I'm like, "No, you don't need a Vsauce video, you need help." So I just didn't think anything of it. The next day, I went into work and everyone's talking about this thing. And I'm like, "What? Oh, my gosh. If I had recognized the potential here, it wouldn't have been BuzzFeed that scooped it, it would have been me."

• 31:45Hannah Fry?

  It would have been you. But to be fair, you didn't have very long, though, right? You didn't have very much time?

• 31:50Michael Stevens?

  No.

• 31:51Hannah Fry?

  So wait, who was the person that emailed you?

• 31:53Michael Stevens?

  I was emailed by a person named Lizzie Rhodes. And a few times since then, which this was 11 years ago, I have like done a deep dive through Facebook to figure out, is she friends with the Caitlyn McNeil who originally posted it, or friends with Grace and Keir, the bride and groom? I've never replied to her email because of my social anxiety and because it feels almost like rude to be like, "Yo, what up? I thought 11 years was long enough to finally get to this email. Thanks for reaching out, thanks for watching." But yeah, I just realized this podcast is my chance to say, "Here's what happened. Lizzie, if you're out there, reply to the thread." I still have it. You emailed me three times, actually. I have all three of them and I never replied, but I've got them all.

• 32:38Michael Stevens?

  Yeah, she emailed me some other visual illusions that might be related. It was really great and I just... I think at first I didn't believe that there was any illusion in this dress. And then the next day I felt like I'd missed out. I think ASAP Science made a video about it immediately and I was just, it was just sour grapes. I just felt like, you know what? I'm not going to talk about it because I missed the boat. So now I have to act like it's not cool. So that's a huge mystery.

• 33:02Hannah Fry?

  Wow, what a great story, but I also think though, Michael. I mean, I think that that really demonstrates how well regarded you are, right?

• 33:13Michael Stevens?

  I know.

• 33:14Hannah Fry?

  Someone would come across something that was a bit strange and unusual and their first thought is to email you about it. There's something really lovely about that.

• 33:21Michael Stevens?

  I know, you're right. That really is the first primary emotion. It's a kind of honor. It's also a kind of responsibility. And I fell short. I was reached out to and I just went to bed.

• 33:34Hannah Fry?

  Right, here's, here's the question. We, you've mentioned this before, but like, I also get a lot of like, uh, I mean, some of them frankly unhinged. Emails and letters from people.

• 33:44Michael Stevens?

  Yeah, I can imagine.

• 33:45Hannah Fry?

  Like lots of them are really, really lovely. Lots of them are so gorgeous. I've got like a little feel good folder. In my house where I like put everything that's just like really lovely stuff. But some of them, you're right, you filter the ambiguity through your own experience. And my experience of reading some of these letters is that there's some people who sort of claim to have solved, you know, the Riemann hypothesis. And it's like, it's kind of a bit, a bit kooky. And so my expectation when there's ambiguity is that like, this is not going to turn out to be anything. And so it does genuinely play on my mind sometimes that I might have missed something really remarkable. Because while this dress story turned out to be like this global sensation that we're still talking about 11 years later.

• 34:26Hannah Fry?

  I can completely imagine in 20, 30, 40 years that people will still occasionally mention this dress. It's sort of become like a phenomenon that's, that's, that's kind of evergreen. I wonder about all of the emails in your inbox that just didn't get picked up by BuzzFeed. They just didn't get posted to Tumblr, but they are just as valuable and just as interesting.

• 34:46Michael Stevens?

  I gotta take them all seriously.

• 34:48Hannah Fry?

  You'll never know.

• 34:48Michael Stevens?

  And taking them seriously is all we can do. We can't take them literally. I'll talk about what I mean after the break. We're going to come back, I'm going to show you another audio illusion and we're going to talk about perception, reality and whether anything is an illusion.

• 35:09Speaker 04

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• 35:43Michael Stevens?

  All right, welcome back. We are back and I have a correction to make. I mentioned that Yanni Laurel perception can be geographically distributed. That I don't think has actually been found. I was thinking of what's called the tritone paradox. That's the one where where someone is from correlates with how they hear the sound. So Hannah, I'm going to play for you two notes, ba, ba. And I want you to tell me if they're going down or up in pitch.

• 36:14Hannah Fry?

  All right, here we go.

• 36:19Hannah Fry?

  Up.

• 36:20Michael Stevens?

  No kidding. That's what I expected because you are British. I hear them going down. To me, it sounds like ba, ba.

• 36:34Michael Stevens?

  Really?

• 36:35Hannah Fry?

  Yeah.

• 36:36Michael Stevens?

  So this was discovered by Dana Deutsch and she calls it the tritone paradox. It involves tritones, which are sounds that are a half octave apart and they're kind of sandwiched together. So you hear two of them and then one pair goes down and one goes up. Depending on what frequency you in your lived experience have had to learn to prioritize, you will either hear the ascending notes or the descending notes as being more operative in what's happening. When you answer the question, is this going up or down? I hear that as ba, ba.

• 37:11Hannah Fry?

  So hold on. Okay, this is south of England versus?

• 37:15Michael Stevens?

  Southern California.

• 37:16Hannah Fry?

  Southern California. Why? What, what, why? Like, what possible thing could we be hearing that would be so different?

• 37:23Michael Stevens?

  I think it's the different vocal registers. This is not my guess. This is from researchers who say that apparently people in Southern California talk with a lower pitch.

• 37:32Hannah Fry?

  Oh.

• 37:34Michael Stevens?

  So they're all like, yo, you know, it was gnarly. I guess even, even like, no matter what gender you are, you're, you're lower in general. But in, in southern England, they're all like, hey, governor, oh, give me some brown sauce. Right?

• 37:53Hannah Fry?

  The most British thing you could think of, governor and brown sauce.

• 37:56Michael Stevens?

  Oh, my gosh. I, I'm glad that was so bad because I did just do that off the top of my head. No prep.

• 38:03Hannah Fry?

  I enjoyed it.

• 38:03Michael Stevens?

  And I nailed it.

• 38:04Hannah Fry?

  That's so strange. Wait, are those the only two? Have they tried for other countries around the world? Or is, are those just the only two?

• 38:10Michael Stevens?

  Those are the only two that I've heard of in my limited research on the subject. But it's very cool because tritones also have a long history of being fascinating to people. They, because of these weird effects, they were even seen as possibly demonic back in the Middle Ages or medieval, medieval period, I believe. I know at some point in history, people were like, don't use them. It's the devil's voice, right?

• 38:32Hannah Fry?

  In the comments, can you tell us which way you hear it and where you're from?

• 38:37Michael Stevens?

  Yeah. And maybe also tell us your age because I think that as we age, we hear different frequencies, better or worse, usually worse. So yeah, give us that info. Here are the two notes we're talking about. Okay, right down below, what you heard, were they going up in pitch or down in pitch?

• 38:56Hannah Fry?

  This is really interesting though. I mean, it's coming back to a similar thing, right? Which is that like your perception of reality is sort of constructed in your own mind as a basis of your experience.

• 39:06Michael Stevens?

  Yeah, it's not just, all of our brains do this one funny trick. No, they all do tricks in their own way, based on what environment they have been raised in and learn to navigate. Um, there are a lot of other really good videos online of audio illusions. Veritasium did one a year ago that's really good. I'll tell you what not to do though. I watched the video last night with no headphones on at one and a half speed. Sorry, Derek. And if you have no headphones on and you've sped it up, instead of being a video of illusions, it is a video of no illusions. Oh, none of them work, right? I, I just took his word for it. But I also could not figure out what the opposite of an illusion was.

• 39:51Michael Stevens?

  What do you call that? So last night, I didn't want to look it up. I just tried to like ideate some things. And I came up with, it's not an illusion, it's an illaburare.

• 40:03Michael Stevens?

  Because laborare means labor, which is the opposite of ludere, which is where we get illusion from. Ludere means to play.

• 40:12Hannah Fry?

  Oh, really?

• 40:13Michael Stevens?

  An illusion is in play. Yeah, illusions are playful. And if it's not playful, then it's labor. It's work.

• 40:20Hannah Fry?

  I like that a lot. That was me just thinking you were saying God would go. But actually, it was it was well researched. A lifetime of studying words.

• 40:28Michael Stevens?

  I did do the research. I don't know Latin off the top of my head, but I wanted to come up with my own antonym of illusion. So like, an illusion is cool, but a in laborare would be, look at this circle. It really is a circle. It's an anti-illusion. Then I thought maybe un-illusion, but that sounded like an illusion that you've undone.

• 40:51Hannah Fry?

  Yeah.

• 40:51Michael Stevens?

  Then I thought, well, even though it's etymologically not correct to do this, you could say it's not an illusion. This is a illusion. It's just the way it is. Because ill sounds like wrong. Ill, sick. However, there is a real word scientists use, psychologists use for something that's just true.

• 41:10Hannah Fry?

  Are yours frankly better?

• 41:12Michael Stevens?

  No, mine are worse. The word they use is veridical.

• 41:15Hannah Fry?

  As in like truthful?

• 41:16Michael Stevens?

  As in true. Yeah. Like veridically, these lines are the same length. But there's an illusion where one appears to be longer than the other.

• 41:24Hannah Fry?

  Nah, I'm going for illusion. Illusion for the rest of turn. There are some places that are called something sound on this?

• 41:31Michael Stevens?

  Yeah, like Puget Sound.

• 41:33Hannah Fry?

  Yeah.

• 41:33Michael Stevens?

  Interestingly, this is completely unrelated, but let's just do it. The Mariana Trench was discovered like purely out of luck. It was discovered in 1875 by a research vessel called the Challenger that was just measuring the depth of the ocean in different places. One of the places they tested happened to be what we now know is the southern end of the Mariana Trench. And they just dropped like a weight tied to a rope off the boat. They were like, this isn't, this isn't stopping. We haven't reached the bottom. What the heck? And it went on and on and on. And they were like, dude, this is, this is unbelievable. And they wrote it down and everyone's like, whoa, the ocean is a lot deeper than we thought.

• 42:17Michael Stevens?

  So they named it, it became a feature. They knew that it was a trench. But that was in 1875, just by chance. That was one of the spots they checked. We didn't discover the second deepest trench in the ocean until 1952. That's the Tonga Trench. So the fact that like almost a hundred years prior, someone had just checked.

• 42:39Hannah Fry?

  Yeah.

• 42:39Michael Stevens?

  Right at the Mariana Trench is like total, total coincidence that we got the deepest one. We just tried that right off the bat.

• 42:48Hannah Fry?

  The chances of that have seemed pretty, pretty insignificant.

• 42:51Michael Stevens?

  The ocean is a big place.

• 42:53Hannah Fry?

  The ocean is a very big place.

• 42:55Michael Stevens?

  There was one other really neat thing in Veritasium's video that I'm just gonna share. You should go watch the whole video, but he talks about why our ear shapes are so weird. Why do we need to have this funky thing? Microphones don't look like these. Microphones are just, look at this. It's just like a cylinder or a ball or something.

• 43:15Hannah Fry?

  Mm hmm.

• 43:16Michael Stevens?

  And it has to do with helping us localize the position, the location, the origin of a sound when it's not off to the side. Cause when a sound is off to the side, we have a lot of clues about where it's coming from. Because it gets attenuated when it passes through our head. So one ear hears it louder than the other. One ear hears it before the other. But if a sound is right in front of us, along this line, either below or up above me, even behind me, it's symmetric about my ears. They both get the same stimulus.

• 43:49Hannah Fry?

  Yeah.

• 43:50Michael Stevens?

  But they don't because my ears are so unsymmetrical along that axis.

• 43:55Hannah Fry?

  Oh.

• 43:56Michael Stevens?

  So sounds coming from above me come in like this and they're like going past my ear lobe, down past the top of my ear. And they're gonna reverberate and be reflected differently than frequencies that come from below or, or from right in front of me. Those are gonna hit, you know, I don't know how they hit, but they're hitting the ear in different ways. And different frequencies are better or worse reflected by the different weird spiral folds of the cartilage of our Ear and over time from when we're little babies to when we're a bit older, we learn how to connect the perception we get because of what our ear does to the sound to where it is. And researchers have done experiments where they've put little like molds on top of people's outer ears to make them differently spiraled. I could have

• 44:40Michael Stevens?

  someone make ears that look like yours, stick them on top of mine. And now I'm hearing with your outer ear. By the way, this, the flap, the thing that we call an ear, like when you, when you say someone, oh, you didn't draw the ears yet. And they draw this outside thing that we can see. This is called the oracle or the penna. Penna is usually used in zoology, like a penna is a non-human animal ear, outer ear. But for humans, we call this an oracle. Anyway, they put prosthetic oracles on people that were different shapes. And the people suddenly could not tell where a sound was, if it was symmetrically situated to their ears.

• 45:16Hannah Fry?

  Did the ear look like it could conceivably be another human person's ear?

• 45:20Michael Stevens?

  Yes.

• 45:21Hannah Fry?

  So it's just that you hadn't had the training of tuning your, I mean, because you sort of get to verify it, right? Like, you think a sound is coming from over there and you get to verify. You haven't had that, that training process essentially for your entire life with that particular ear shape.

• 45:36Michael Stevens?

  That's right. And it only takes a few days, like a week at most, for people to learn how to use their new ear shapes. And suddenly they're really good at picking out where a sound is that's right above them, right in front of them, right behind them. And then when they removed the prosthetics, people were able to go back to their normal ears very quickly. Yeah.

• 45:55Hannah Fry?

  Okay, I want to go and grab something that I have downstairs because there are people who study ear shapes to an unbelievable degree. Just hold on a second.

• 46:03Michael Stevens?

  Oh, I'm gonna get something too.

• 46:08Hannah Fry?

  So this idea about people really caring about the shape of the ears, of course, if you design headphones,

• 46:18Michael Stevens?

  Hmm.

• 46:19Hannah Fry?

  Right? And particularly if you are designing headphones that are noise cancelling, where you want to play a sound into the ear that exactly cancels out any external sounds, then the way that an ear is shaped becomes really, really integral

• 46:39Michael Stevens?

  Yeah.

• 46:40Hannah Fry?

  to, to what you're doing. So anyway, a little while ago, I made this program where I got to go and visit Bose in Boston. And literally all over the place. I'm just opening this as I go. Literally all over.

• 46:53Michael Stevens?

  Opening like some Japanese puzzle box?

• 46:57Hannah Fry?

  I know, it's like I've had it framed, so I'm opening frame.

• 46:59Michael Stevens?

  Oh.

• 47:00Hannah Fry?

  Okay, so anyway, in Bose, literally everywhere are these really weird little ears.

• 47:06Michael Stevens?

  Oh, cool. Is that your ear?

• 47:08Hannah Fry?

  No, they didn't do one of my ear, but they have like genuinely hundreds and hundreds and hundreds of these that they are continually testing for how sound changes, for how the different headphones work, for how it reflects sounds, for how it understands like, I mean, exactly what you were describing about how sound changes depending on the shape of your ear. And they attach them to these models that have microphones inside them essentially, and run these like insane scientific experiments, like really, really sophisticated, in order to make sure that they're making headphones that can actually be used by everybody rather than just, um, just a few people. I want to show you it because it's really fun.

• 47:48Michael Stevens?

  Okay, while she's opening it, I'm going to show off my new mechanical pencils. And editors, I dare you to leave this in. If you don't, I'll be so disappointed. So, look, I, I hate long pencils. This is an unsharpened Ticonderoga. So long, I feel embarrassed to use it. But look at this. Look at this winner. This is the Kohinoor 5228. Look how much shorter it is.

• 48:10Hannah Fry?

  Kohinoor? Is it like diamond?

• 48:12Michael Stevens?

  It's named after the diamond, the Queen's diamond. There you go. Versatile. And they make an even thinner one. That's actually too thin, but it's metal. So this was, this is wood. This feels good. But let me show you something else. Okay, here's the Kohinoor 5228. Look at this one. Oto WN02. It's a needle.

• 48:35Hannah Fry?

  Dinky.

• 48:36Michael Stevens?

  Look at that dinkster.

• 48:38Hannah Fry?

  Okay, I'm finally in.

• 48:39Michael Stevens?

  It's in. Okay, so I won't show off.

• 48:41Hannah Fry?

  Once. Okay, here we go.

• 48:43Michael Stevens?

  I have one more small one and you guys will have to just subscribe and I'll show it off later.

• 48:48Hannah Fry?

  When we do the club, that's what the club can be.

• 48:50Michael Stevens?

  Yeah, ooh, members only, whatever's hiding behind my hand.

• 48:55Hannah Fry?

  Okay, so here, here's the little ear. It's the most.

• 49:00Michael Stevens?

  Oh, goodness, that is.

• 49:01Hannah Fry?

  Some people have, some people do have really flippy floppy ears.

• 49:04Michael Stevens?

  Whose ear is that floppy?

• 49:06Hannah Fry?

  Children's.

• 49:07Michael Stevens?

  Really?

• 49:08Hannah Fry?

  I reckon. I mean, they do get harder, sort of, the cartilage solidifies as you age. But anyway, when you are in Bose, I realized that they had just hundreds and hundreds and hundreds of these lying around all over the place, which are essentially silicon molds of different ear shapes. This one, it's, it's so much fun to play with. It's like, it's really floppy. You know, you can kind of like tickle it and dangle it.

• 49:33Michael Stevens?

  Yeah.

• 49:33Hannah Fry?

  It's a mold of a human ear, right? So it looks exactly like a human ear. But you realize when you're in this company, just how much variation there is in human ear shape, because I mean, the general structure is, is kind of the same. You have like the outer part, you have, this is the helix, is it? You've got the, uh, the dath here. I think this is the conch. Oh no, hang on, that's the tragus, isn't it? Sorry. I forget on which bit's which. Look how big your dath is. Oh my gosh. You could get a monster piercing on that if you wanted.

• 50:05Michael Stevens?

  Uh, maybe I will. Talk about monster dates. Here's a monster ear.

• 50:10Hannah Fry?

  Look.

• 50:12Michael Stevens?

  My ear, my ear is not floppy.

• 50:15Hannah Fry?

  No, yours is not floppy.

• 50:17Michael Stevens?

  But it has a middle and inner ear, so I'm holding up, by the way, a very oversized plastic model for, for like doctors' offices and medical schools of the ear in, in the inside parts of the skull, the, the ossicles, cochlea.

• 50:30Hannah Fry?

  But you know what? You can see the S shape of the ear canal in that, which I always think is really fascinating, that it's not just this straight tube. I'm a bit obsessed with the ear, you know? I'm gonna bring up ears again, but, but not today.

• 50:44Michael Stevens?

  We're not done with ears. No, today we're talking about illusions or unillusions or inlabrarays.

• 50:53Michael Stevens?

  Maybe, maybe. So, look, we're not going to come to any answers about the true nature of reality today, but you know, on this show, we bring this up a lot. It usually falls under the hard problem of consciousness. I've said a lot of times that I think illusions show us that being right doesn't really matter. What matters when it comes to existing is survival. It's fitness. Just because you're right, doesn't mean you're alive. It's better to be wrong and alive than right and dead.

• 51:25Hannah Fry?

  And it's better to make your decisions quickly than wait until you have complete information. It's better to update your guesses based on the evidence that's in front of you and have them as guesses rather than only proceed when you're certain.

• 51:41Michael Stevens?

  That's right. That's right. I mean, and this has been mathematically shown to be true when constructing little environments. So like, of course, we don't get everything right. Of course, we listen to fast percussive beats and we perceive a tone. It's more helpful in some way, probably, because natural selection chose that to happen. And if instead, we had the machinery in our brains and we had some kind of special nerve cell that could fire 20,000 times a second so that we could tell where each individual beat was, that would be a waste of resources. So instead, we're wrong. We, as a species, or as, as

• 52:21Michael Stevens?

  life, invented, completely invented, a completely made up thing called pitch that we experience in our own heads alone, because I think it helps us survive. It helps us make decisions faster and process more efficiently.

• 52:34Hannah Fry?

  Well, thank goodness, because it doesn't just help us survive. It also turns out to be one of the great sources of joy.

• 52:41Michael Stevens?

  Exactly. Exactly. It's beautiful as well. And I don't know if it was always beautiful, if we just learned to think it's beautiful. But Donald Hoffman talked about this in The Case, The Case Against Reality, which is a fantastic book. He actually gave this a name, and I forgot about it until I was researching this morning, but he calls it the fitness beats truth theorem.

• 53:02Michael Stevens?

  He defines an illusion as a perception that fails to guide adaptive behavior. So he's like, look, let's look at insects that eat feces, okay? To them, it's amazing. They love it. When they smell it, they go to it and they consume it. So what's going on there? Are they under some kind of taste illusion where they think it's good? Or maybe it does taste good and we all have an illusory experience of, ew, that's gross. Who's right? Which one's the illusion? And he's like, neither. If it's adaptive, if it helps your species survive, then it just is what it is. And the truth out there will never be accessed.

• 53:42Hannah Fry?

  Because there is no grand truth, because it is only via experience.

• 53:46Michael Stevens?

  All we have access to is a user interface, basically. What I call my office is actually the user interface my brain makes that I can then navigate without needing to worry about the complexities of all the electromagnetic wavelengths and all of the noisy signals that are coming at me right now. And so he has a name for that as well, and he calls that the interface theory of perception. Where he's like, to ask whether my perception of the moon is veridical. Like, is this an illusion? Is it, does it really appear that large? It looks bigger at the horizon. But he's like, just give it up. Because whether I see the true color, shape, or position of a moon that exists, even when no one looks,

• 54:27Michael Stevens?

  is like asking whether the paintbrush icon in my graphics app reveals the true color, shape, and position of a paintbrush inside my computer.

• 54:36Hannah Fry?

  Right.

• 54:37Michael Stevens?

  Reality is the computer. The user interface is on a screen inside here, and we're just perceiving that. And it's very helpful, and it has been built by natural selection to help us live, survive, make decisions as quickly as we should.

• 54:52Hannah Fry?

  I like that a lot. I like that a lot. I also just like anything that ends in, give up.

• 54:58Michael Stevens?

  Just give up, just embrace it. Yeah. We'll never know what veridical reality is. What we do have access to is even better. It's what's kept us alive. It's what's kept us safe and sound.

• 55:15Hannah Fry?

  And, and in play. Thank you very much.

• 55:18Michael Stevens?

  And in play. Ludair, lu- illusion, ludicrous, they all come from to play. You know, sound, sound meaning healthy, like of sound mind. To be safe and sound, that's not redundant. Safe means there's no risk of injury. Sound means there are no current injuries. And that just comes from a totally different etymological root. That comes from like old Germanic languages, gezund, like health.

• 55:46Hannah Fry?

  Like Gesundheit.

• 55:47Michael Stevens?

  Gesundheit, right? Sound in the sense of healthy comes from there. Whereas sound that you hear comes from the proto Indo-European like Sven or something, which just meant a sound. It's where we get resonate and sonic. But then there's a third meaning of sound, like Puget sound. Sound can also be a body of water. And that comes also from Germanic. It comes from sund, meaning swim. And that is why when we check how deep a body of water is, a sund, a channel of water, we're sounding the water. We have a sounding line. So sounding can mean making a noise.

• 56:29Michael Stevens?

  It can mean being healthy, but it can also mean measuring the depth of a water body.

• 56:34Hannah Fry?

  That brings this episode to a resounding end.

• 56:39Michael Stevens?

  Yeah, it sure, a resounding. Oh, it sure does. And I'm, I hope you all are safe and sound who are out there listening. We appreciate you listening to our sounds. If you have questions, please send them in to [email protected].

• 56:54Hannah Fry?

  And as ever, leave us comments, like and subscribe. I mean, I feel slightly sick saying that, but it genuinely does make a massive difference. And we will see you next time. Bye.

• 57:27Speaker 05

  The Great Game: Strategic Struggle Between Britain and Russia.

• 57:34Speaker 05

  We assess, Prime Minister,

• 57:38Speaker 05

  with high confidence it was Russian.

• 57:41Speaker 04

  Holding the Russians accountable is World War III. They took down a British passenger jet containing 350 people. Man in the Kremlin is playing, Alex. Got to ask yourself, you want to play or not? You're the Prime Minister. You've done nothing!

• 57:59Speaker 04

  We would need to understand the policy context.

• 58:01Speaker 02

  All the relevant context will be structured as a framework today, and disseminated into an operation order, or FRAGO.

• 58:10Speaker 02

  A FRAGO that says to kill the President of Russia? Yes.

• 58:17Speaker 02

  New from Goldhanger. Who is going to be able to pull this off? You're my soldier, are you? Sergeant Tom McDuff, Duffy! An audio drama journey like no other. Men like Duffy, they are our most valuable weapons. Devized as a technically rigorous geopolitical action thriller. We have two tight friendly KIA. Where Britain plays its strongest hand He's the best. Against its strongest foe. Don't do it, Duffy! Bear Hunt. Available now wherever you get your podcasts.