The Threshold of Human Sensation

⁠What is the smallest thing you can possibly feel? ⁠ ⁠ Michael Stevens and Professor Hannah Fry journey through varying levels of perception, from the faintest touch on skin to the faintest trace of flavour and odour, and on to the remarkable question of whether the eye can register a single particle of light. ⁠ ⁠ This is an exploration of human senses at their most sensitive, blending physics and neuroscience with everyday experience to reveal where our biology draws the line between nothing and something, and what about emotion... how do we feel those?⁠ ⁠What is the smallest thing you can possibly feel? ⁠ ⁠ Michael Stevens and Professor Hannah Fry journey through varying levels of perception, from the faintest touch on skin to the faintest trace of flavour and odour, and on to the remarkable question of whether the eye can register a single particle of light. ⁠ This is an exploration of human senses at their most sensitive, blending physics and neuroscience with everyday experience to reveal where our biology draws the line between nothing and something, and what about emotion... how do we feel those?⁠ Darwin's The Expression Of The Emotions In Man And Animals. Lisa Feldman Barrett - How Emotions Are Made: The Secret Life of the Brain. ------------------- 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 MeekAnimator: Sam BensonVideo & Social: Bex TyrrellAssistant Producer: Lucy LipscombeProducer: Simona RataSenior Producer: Lauren Armstrong-CarterChief Digital Officer: Samuel OakleyExec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

• 0:01Speaker 00

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

• 0:04Speaker 01

  And I'm Hannah Fry.

• 0:05Speaker 00

  And I was just really shaking my camera by moving. Look at that.

• 0:08Speaker 01

  Whoa.

• 0:09Speaker 00

  Michael. Speaking of feeling things, today, we're going to be asking, what's the smallest thing you can feel? And whether or not we arrive at an answer, I don't know. I've got a bunch of notes here. I did a bunch of research and this is going to be a look into my mind, just as much as it's a look into human sensation and perception.

• 0:30Speaker 01

  Let's clarify here. Smallest thing you can feel. We're talking about physical sensation, not an emotion, right?

• 0:36Speaker 00

  We're talking about every sense of the word feel.

• 0:40Speaker 01

  Ooh.

• 0:40Speaker 00

  Because I can't just leave it at, what I mean is of the classic five senses. It's like, no, we're going to go all over. We're going to really mull over what this question might mean. What does it mean to feel? What does it mean to touch yourself? That is a huge question in phenomenology. I still don't understand their approach to it, but we'll get to what I mean by that later. Okay?

• 1:03Speaker 01

  And look at that. Look at that, Michael. We've got the title already.

• 1:10Speaker 01

  This episode is brought to you by Cancer Research UK.

• 1:13Speaker 00

  Do you remember when we discussed why feet are so weird? Well, one particular foot bone holds an even stranger surprise. It's helping shape our understanding of cancer timelines.

• 1:24Speaker 01

  And for that, we're going to need to go all the way back before Neanderthals even existed, to a 1.7 million year old foot bone. Researchers have identified a tumor in it in the oldest known example of cancer in people.

• 1:39Speaker 00

  Which really shows that cancer is far from a modern disease. Beating a disease so deeply rooted in our biology won't happen overnight.

• 1:48Speaker 01

  But today, Cancer Research UK scientists are discovering incredible ways to turn our biology against cancer.

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  In fact, Cancer Research UK has helped double UK cancer survival over the past 50 years. And their world-class research is driving even more discoveries to tackle over 200 types of cancer.

• 2:09Speaker 01

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

• 2:19Speaker 02

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• 2:50Speaker 03

  This episode is brought to you by Michelob Ultra. Good times mean competitions between you and your friends, like seeing who can throw some pigskin the farthest, or rush the field the fastest. Winner gets an ultra. Because it's not just a superior like beer, it's also giving fans superior access to their favorite sporting events. Superior is worth playing for. Visit michelobultra.com/superioraccess to enter. Enjoy responsibly. Copyright 2026 Anheuser-Busch InBev Michelob Ultra Beer, St. Louis, Missouri.

• 3:24Speaker 00

  I think the first place I want to start in is about me. I've been trying to learn an answer to this question for at least six years. My daughter was a little newborn baby when I first started trying to write a script about this. And I found a whole lot of answers with like no source. Okay? If you go to the Wikipedia page for sense, they've got a human sensation section with the whole like chart that shows the absolute threshold of human sensation for different senses with absolutely no sourcing, except for one article about humans' ability to make decisions with probability. It doesn't mention sensation,

• 4:06Speaker 00

  perception, thresholds, anything. Like, I don't even know where these numbers came from. But that's been copied all over the internet. There are all these images that you can find that do this. I'll read to you what it says. And then we'll dive into what the actual science tells us. The absolute threshold of our sense of hearing. Is this just straight from Wikipedia or literally dozens of other websites that have turned this into their own little infographic? The quietest thing we can hear, they say, is the ticking of a watch 20 feet away in an otherwise silent environment. That's not like an amount of decibels or an amount of energy in the pressure wave. It's just like a very intuitive, physical object thing.

• 4:49Speaker 01

  I'm there. I'm there.

• 4:51Speaker 00

  A watch ticking 20 feet away. What kind of watch? They don't tell us.

• 4:55Speaker 01

  Well, it's not, not your one that just says now on it.

• 4:58Speaker 00

  I'm not even wearing a watch today, but my, the one I've been wearing, it, it is loud enough that if it gets really quiet here in my office, I hear it. I have to take it off because I just can't stop hearing it. For vision, they say a candle, a single candle flame, 30 miles away on a dark and clear night.

• 5:16Speaker 01

  30 miles away?

• 5:18Speaker 00

  30 miles away, on a clear dark night, a single candle flame can be seen. And that's like the threshold. That's the bottom. Any dimmer than that, and we don't see it.

• 5:27Speaker 01

  That seems crazy.

• 5:28Speaker 00

  That's all it says. And can I tell you the proof? No. But we will be visiting the truth of these later. Okay, your vestibular sense. This is your sense of your orientation in space. This says that you can detect a tilt of three degrees, which is half of one minute graduation on a clock's minute hand. Okay, so people can tell how tilted the floor is. There's no source here. I have found some sources, though.

• 5:55Speaker 01

  Three degrees is very, very, very slight. I mean, if you had a table and you put a ball on a table that was tilted three degrees, it's not going to like fly off. It might just gradually pick up a little bit of a roll.

• 6:09Speaker 00

  Yeah, it'll accelerate. I mean, eventually, you'll be like, wow, it's flying. But initially, it's kind of like, it's pretty good, pretty level. Yeah. I mean, we're talking about, again, look at the, the difference between one minute mark on a clock and the next. We're talking about half of that distance, tilting just that much off of level. For smell, they say a drop of perfume in a volume the size of three rooms. I've also seen this written, a volume of six rooms. And I'm like, what kind of perfume?

• 6:37Speaker 01

  These also, these also sound far too poetic, you know?

• 6:40Speaker 00

  They're too poetic, right? Maybe we shouldn't even start here. Like, what's the point? It's, I don't think they're far off, but I will tell you that six years ago, I spent months looking into this. And the actual scientific papers all said there is no simple poetic absolute threshold for the, the smallest thing we can sense in all these different modalities, because it depends on the person. It depends on what they've been eating that day. It depends on what room they're in. It depends on how they're feeling. Are they sad? Are they agitated? There's no such thing as touch. The, the smallest touch you can feel is a wing of a fly falling on your cheek from a height of 7.6 centimeters. Give me a break.

• 7:21Speaker 01

  Is that one of them?

• 7:22Speaker 00

  That's one of them.

• 7:23Speaker 01

  No, come on.

• 7:25Speaker 00

  And 7.6 sounds like really, really precise.

• 7:29Speaker 01

  Scientific.

• 7:30Speaker 00

  But it's, they're just converting three inches to metric.

• 7:32Speaker 01

  Okay. Yeah.

• 7:34Speaker 00

  Finally, with taste, they say a teaspoon of sugar in two gallons of water. That's seven and a half liters. There you go. I don't know what we're supposed to do with that information, except dig in and really pin down the smallest thing we can feel, we can sense. By that, I mean the smallest amount of matter. Don't give me the poetic, like, oh, a drop of perfume in three rooms. Tell me how many molecules.

• 8:02Speaker 01

  Yes.

• 8:03Speaker 00

  I need to know that something's there. Okay? So I guess like the first place to start is with emotions, because I think that they involve the most matter. Like, the number of neurons involved, the number of neurotransmitter molecules involved in emotions as we understand them today is gargantuan. If you further say, but how many, how many atoms are involved? It's trillions. Trillions of atoms are needed for us to feel sad, interested, bored. Like, we barely can describe what's happening in the mind or in the brain when these things happen. But I think that we can just write that one off. There's no way we're going

• 8:48Speaker 00

  to say, you know what, there's a single. A single atom of carbon shifted over here is the difference between boredom and ennui.

• 8:56Speaker 01

  Mm hmm, mm hmm, yeah, agree, agree. I mean, I think even trying to define what boredom and ennui are is gonna be, uh, well, it proves to be extremely, extremely difficult. There's an amazing book on this by Lisa Feldman Barrett. Have you come across it?

• 9:16Speaker 00

  No.

• 9:16Speaker 01

  Oh, it's so good. It's one of my absolute faves, where she looks at the research into categorizing emotions, which started with Darwin, incidentally. It's about the emotions of animals and man, I think.

• 9:29Speaker 00

  Oh, I am familiar with that book, yeah.

• 9:31Speaker 01

  And what he does is he goes around and he meets all these different animals and then writes about how they feel. So there's like, there's one orangutan who's really annoyed because they want an apple and they're not given an apple, so they have a strop. And there's like, oh, a hippo that's really cross when they're in labor. And it's like, there's a sweaty horse who's like a bit annoyed. And he's trying to come up with like, if there are fundamental emotions in each creature that are universal, and he comes up, I think, with a definition of six, essentially. And we should do a full episode on this, because I genuinely fascinated. But Lisa Feldman Barrett, essentially, the very modern research says you cannot draw distinct lines that say happiness is here and that is where it changes over to sadness. It's like, actually, these whole things are so much more complex, so much more.

• 10:16Speaker 00

  It's so much more complicated. It is a blur and we have drawn little lines around them to be like, oh, okay, this is, this is happiness and this is joy. We understand that there's a difference of, of strength in these words, but, you know, it's, it's just too soft, I think, for the purposes of today's episode.

• 10:40Speaker 01

  So we're not doing, we're not doing the smallest happiness you can feel?

• 10:43Speaker 00

  No, we're not, because I think that's also super subjective. I mean, you could look at something like coordinate maps of, of emotions. My favorite is an axis for the valence of the emotion, meaning is it good or bad? Do we like it or not? And then the other axis is arousal, like how, how aroused are we? Like, how, how, how off of?

• 11:04Speaker 01

  Intense.

• 11:05Speaker 00

  How else would you describe arousal? Because let me put it this way, if neutrality is in the middle, like literally no emotion is in the middle, you go up, you get into alertness, and you go down, you get into boredness and calmness. Okay, so neutral is not calm. Neutral is in between calm and alert. That's what the arousal scale is telling us. And so on that, on that kind of a chart, you get something like happy is a little, a little more aroused than neutral, and it's on the positive valence side. It's a good state of a higher arousal. Even higher but good is excited, right? And then you've got like joy and overflowing joy.

• 11:49Speaker 00

  But if you go down, staying with, with the positive valence, you run into comfort, relaxation and calm. Okay? I think that's a fun way to think about emotions. It, I think it helps like therapeutically to just think about yourself, but I don't think it's going to give us, it's not going to spit out a number of molecules involved in each, which is what I'm going for. So, so let's move on to hearing, because I think that our sense of hearing needs the most matter of all of our major senses. Because I was thinking, what's the smallest thing we can hear?

• 12:29Speaker 00

  Like, could I hear a single air molecule that was like shot into my eardrum at some abnormally high speed? And you just can't, you just can't, as it turns out.

• 12:42Speaker 01

  Well, even if it, even if it was like, you know, up to the speed of light, even then, even if it had an enormous amount of energy.

• 12:48Speaker 00

  Even then, I don't think the way it interacts with the eardrum is going to cause a vibration, it's gonna go right through. It's, also, here's the other problem, we don't hear because, simply because the eardrum is vibrating. It has to vibrate in a way that is at a frequency we can detect and that goes over the threshold that our brain imposes on what's going to be brought to the awareness. So even thousands of air molecules, all moving together, still doesn't get over that hump of just regular thermal activity in the air. You need trillions of air molecules moving for a sound to be heard. Molecules are small, okay?

• 13:32Speaker 00

  They're so small that trillions sounds huge, but we're talking about a tiny, a tiny packet, like just, I couldn't even really show it by pinching my fingers together, right? That's how small they are. But, but a trillion is kind of like where we are now. Let's see if we can sense fewer than trillions, all right? For that, well, okay, let me say this, things got a bit more complicated. I wasn't done with hearing yet, actually. I was like, well, couldn't I hear a sound from, from like something small that was making a noise? Like, rather than looking at the actual pressure wave.

• 14:06Speaker 01

  Like the watch?

• 14:07Speaker 00

  Let's look at an object's ability to create a pressure wave. So I said, what if a single proton converted instantaneously into energy, pure energy? All right, so we're doing equals mc squared to this, like that's the most energy we can get out of this tiny thing, a single proton. Could I hear that happen?

• 14:28Speaker 00

  No.

• 14:28Speaker 01

  Really?

• 14:29Speaker 00

  Yeah, a single proton turning into energy, all of its mass converts into energy. It's only 1.5 nanojoules of energy.

• 14:39Speaker 01

  Really?

• 14:39Speaker 00

  We need a million times more than that for there to be a sound we can hear.

• 14:43Speaker 01

  So does that mean in CERN, where they are taking two protons and smashing them together to convert them into energy, at that point, it's silent? Is that what you're saying?

• 14:53Speaker 00

  It's silent. I mean, they're shooting like a proton beam, so we're talking about more than just one.

• 14:59Speaker 01

  If you're doing one?

• 15:00Speaker 00

  If you're doing one, it's like nothing. And if you've got millions, okay, you're gonna have enough energy to produce a sound that you could hear. Here's the problem, though, the frequency will be too high. So you've gotta, you gotta release the energy more slowly. It can't be quote unquote instantaneous. But if you slow down that release, now you're in the, the frequency range of human hearing, but the energy is too low. So you will need, again, um, I don't know if I calculated the exact number, but you're going to need probably on the order of trillions of, of, of protons turning into energy slowly

• 15:43Speaker 00

  enough that you can actually hear something. A trillion protons is smaller than a trillion molecules.

• 15:49Speaker 01

  Yeah, yeah.

• 15:51Speaker 00

  So we're getting closer, but we can do better.

• 15:54Speaker 01

  But we're still talking sound, right, okay.

• 15:56Speaker 00

  Yeah, we can do better if we move on to our sense of taste. What's the fewest number of molecules I need to taste something? Like, right off the bat, I'm thinking, well, surely I could taste a single proton, right? Like, that's what an acid is, really, an acid. There's like three different ways to define an acid, but one way is that it's a liquid with some like free protons in there. I mean, not really, they're hydrogen ions and they'll, they'll connect up with, you know, say, water molecules to make hydronium ions, and that's what we're tasting. But as it turns out, our tongues are already bathed in these hydrogen ions, just with our own saliva. Our, our mouths are a, are pretty neutral place, it's got a pH of like seven, but

• 16:40Speaker 00

  that means that there are, at any one moment, about six trillion free protons in our mouths that contribute to how we sense acid tastes, sour tastes. But the threshold of human sour tasting happens at a pH of about 4.4. That requires 2.4 quadrillion protons.

• 17:06Speaker 01

  You're going in the wrong direction here.

• 17:07Speaker 00

  I'm going in the wrong direction. You need 2.39 quadrillion more protons free in your mouth before you're like, ooh, yeah, something sours in there.

• 17:17Speaker 01

  Really?

• 17:18Speaker 00

  Yeah.

• 17:18Speaker 01

  And that's the, that's the like, the threshold of like, is that, is that, can you taste something sour? Is that, that's what we're talking about here? That's not like, whoa, that's sour.

• 17:26Speaker 00

  That's not, that's not like, whoa, warhead, no, that's just straight up like, mm, okay, yeah, with that many free hydrogen ions, you would, you would start to, many of us would start to say, mm hmm, yep, yep, that's sour. As opposed to, is there anything in my mouth? I can't tell. So we're going in the wrong direction. But if we start looking at molecules, things get a little bit better. I mean, the number gets smaller, but the objects we're talking about get bigger, but it's fine, because taste is not where this story is going to end. So when it comes to the most like pungent tastes, the, the tastes we are the most sensitive to, one of the best candidates I found is this kind of new thing, Lugdaname.

• 18:10Speaker 00

  Lugdaname is a molecule that is sweeter than sugar by like hundreds and hundreds of thousands of times. It blows aspartame and saccharin way out of the water.

• 18:24Speaker 01

  Where does it come from?

• 18:25Speaker 00

  It was engineered by chemists to basically bear hug our sweet receptors. And by that, I mean that all the other artificial sweeteners that we have, they mimic sugar molecules and they attach in, you know, one, maybe two places onto our receptors for sweetness. But Lugdaname attaches to all available receptors on a, a, a taste, on a sweet, sensitive receptor cell. It basically just covers it. And it does so, so effectively, that this stuff is like so sweet, we can't use it in food production, because it'd be hard to measure such a small amount.

• 19:09Speaker 00

  Literally, a, a, in an industrial silo would need like a few nanograms of it.

• 19:16Speaker 01

  What? This is, this is like the fentanyl of the sugar world.

• 19:20Speaker 00

  Exactly. Now, there's another reason we don't use it, and it's that it contains toxic byproducts after it's digested in the body, so it would probably hurt us a lot. However, it's really neat, it's really neat stuff and you could taste it, you could taste it, this is, this is unbelievable. It's, um, detection threshold is 0.6 parts per billion.

• 19:43Speaker 01

  Right.

• 19:43Speaker 00

  So if I put a quarter teaspoon in an Olympic-sized swimming pool, you'd get in that swimming pool and go, ugh, this is all sweet.

• 19:51Speaker 01

  No, really?

• 19:53Speaker 00

  You could detect.

• 19:53Speaker 01

  One, what was it?

• 19:54Speaker 00

  A quarter, a quarter of a teaspoon.

• 19:56Speaker 01

  One teaspoon to two gallons, in the, the sugar example, the poetic one that you started with was one teaspoon for two gallons.

• 20:02Speaker 00

  Exactly. Whereas I'm saying Lugdaname, a quarter teaspoon in 660,000 gallons.

• 20:12Speaker 00

  It gives you a sense of how much sweeter luggedaname is than sugar.

• 20:17Speaker 01

  Yeah. Wow. Wow, that's crazy.

• 20:21Speaker 00

  That's crazy. And so

• 20:23Speaker 01

  That is sort of your scent, your, your, you are detecting. Wait, how many molecules could you detect then? How many molecules of this stuff could it take?

• 20:30Speaker 00

  Here's how I calculated that. I said, okay, so I'm in this Olympic sized swimming pool. I'm in 660,000 gallons of water. And there's a quarter teaspoon of luggedaname in that water. I tasted it sweet. How many molecules of luggedaname are on the surface of my tongue at that moment? And again, I'm probably off by a magnitude of 10 here, but the answer is about 11 billion.

• 20:54Speaker 01

  Right. Well, that's quite a big improvement.

• 20:57Speaker 00

  So, yes, we are away from the trillion numbers. We're away from the quadrillion numbers. We're down to billion. We are talking about molecules, though, and luggedaname is not the world's smallest molecule. But I do like that it's only 11 billion things.

• 21:12Speaker 01

  Yeah, me too.

• 21:13Speaker 00

  And we're already detecting them. Okay, so we're getting better. Let's move on to smell.

• 21:21Speaker 00

  Smell, I thought, would be a lot worse than taste. Because, I don't know, we, we've done episodes about smell before. And like, oh, humans are so bad at smelling. Like, if only we could be like a dog. Like, every other animal has a better sense of smell than us. And that's, you know, as we've discussed, not entirely true. Our sense of smell is pretty, pretty incredible. And one of the reasons why it can be so much more sensitive than our sense of taste is that our taste buds are secondary receptors. There are receptors on our tongue. We've got like a million of them. When they get triggered by some chemical compound, they then release a neurotransmitter onto another nerve that then sends a signal to the brain.

• 22:06Speaker 00

  So they're not directly connected to the brain. They have to be like, oh, I think I might be detecting something here. I'll send a message through this, this neuronal gap. And then maybe it'll get to the brain. But the nose, the nose is freaky. All right? You've got, at the top of your sinuses, these olfactory nerves. And they go right into the brain. Like, your nose, or at least the olfactory nerves up inside your nose, are basically little tendrils that come out of your brain to sample reality. And I've got these skulls back here to show you how that happens. Because I, I, I, I've looked at these skulls all the time. I don't know the names of all their parts. I've learned now, here's a skull.

• 22:51Speaker 00

  I'm holding up a skull. It's just a little, it's not a real skull. It's a plastic teaching model. But here's where the brain sits. And if you look just right between the eyes and a little bit above, you can see tiny valleys here that are pockmarked with holes.

• 23:11Speaker 01

  Is that where the little tendrils, as you described them, come down?

• 23:13Speaker 00

  Yeah, that's where the tendrils come down. So I'm gonna get a brain model and show you.

• 23:18Speaker 01

  We're talking about here, basically.

• 23:19Speaker 00

  We're talking about right between your eyes, like between your eyebrows even, is even better. Because it's a bit above your eyes. And through that part of the bone, here's what's happening. You've got, at the, at the base of your brain, here's, here's your brain. Here's how it sits in your, in your head. You've got these yellow things on my model. I'm holding up a model of a brain now. And these yellow things, this one's not painted on very well. But this is the olfactory tract. And that's the olfactory bulb, a little fleshy bulb just at the tip of the bottom of the brain. And that's where the olfactory nerves come down out of the brain. And they go through the bone of your skull, through tiny little holes in your skull. And those holes are called your

• 24:03Speaker 01

  Oh, tell me it's got a Latin name that means little hole.

• 24:06Speaker 00

  Wait, they do, they do. And they, they're called the cribra form foramina. So a foramina is a tiny hole. And cribra form means shaped like a sieve. Because we have essentially a sieve in our skull.

• 24:23Speaker 01

  You say sieve rather than sieve.

• 24:24Speaker 00

  Yeah, I, I've heard it both ways. A sieve, a colander.

• 24:28Speaker 01

  But I am now.

• 24:28Speaker 00

  Basically, it's a, it's a colander of holes in your skull. And I can show you, this, this is an even more detailed model of the skull. This one has all the bones separate and they're colored differently. So it's a bright, um, multicolored skull. I'll rip off the front of the skull. And then you see this bone, that's actually bone colored on my model. This bone is called the ethmoid bone. And it contains, if I show it to the camera, you can see once again that you've got these grooves. Boy, it's kind of hard to see with this color. But there's little tiny holes in there, right? And those holes are actually holes that go down from your brain into your sinuses.

• 25:13Speaker 00

  And that's where your olfactory nerves sit, ready to detect any kind of odor molecule that comes into your nose.

• 25:21Speaker 01

  So this is the thing that makes it so much more sensitive. Because you, you're basically, your, your brain is directly sensing the molecules as they come up through your nose, rather than using a sort of go-between.

• 25:34Speaker 00

  I mean, it kind of freaks me out that I've got little tendrils coming from my brain out into the air around me. I mean, they're inside my nose. They're much more sensitive because they, they go directly to the brain. And because the way olfaction works, there's a lot more like cascades that are created where if one olfactory neuron is triggered, all the others around it go, okay, fine, I'll do it too. But your taste receptors don't work the same way. They're more like, hey, you mind your own business, all mind mine. We're not all gonna go crazy just because, you know, one free proton hit you, right? So, so our sense of smell is so much more sensitive. In fact, there's

• 26:15Speaker 01

  How much more sensitive, though? Tell me.

• 26:16Speaker 00

  I'll tell you about this molecule. It's called isopropyl methoxypyrazine.

• 26:21Speaker 01

  Okay.

• 26:22Speaker 00

  It's got a long name, but we can detect this. We can smell this at a level of 0.3 parts per trillion.

• 26:33Speaker 01

  How does that compare to the, the sweet tasting thing?

• 26:37Speaker 00

  Okay, here's how it compares. I'm gonna be using a lot of like, American units, where you just start talking about swimming pools and football fields and stuff. But remember that luggedaname was detectable in an Olympic sized swimming pool if you put one quarter teaspoon in. But an equivalent detection threshold for isopropyl methoxypyrazine is one drop, which is like a 30th of a single teaspoon. So less than a quarter teaspoon. One drop in 20 Olympic sized swimming pools.

• 27:13Speaker 01

  Whoa. And this is, you can smell, you're able to smell it?

• 27:17Speaker 00

  Yeah, you can smell it. So you can't actually get in the pool and taste it. But you, if you imagine that those 20 Olympic sized swimming pools were air, you could smell one single drop.

• 27:28Speaker 01

  See, this is the thing. People go on about sharks being like, oh, they, one drop of blood in da, da, da, da, da. No, no, no. We can do it the same. We can just, we can just only do it with really complicated sounding molecules.

• 27:39Speaker 00

  Yeah, exactly. So it's kind of a bummer. It's not like, go to the store and buy this readily available thing. And you can, you can smell it from a mile away. It's more like, yeah, there's this thing that you're never gonna smell. But if, if you had some around, you'd be like, wow, I can smell such a small amount of it.

• 27:56Speaker 01

  What does it actually smell like?

• 27:58Speaker 00

  You know what? I don't know. Let me see. The odor is rather undesirable and is produced by the Asian lady beetle. It can be detected by human taste at concentrations of two nanograms per liter. The odor tends to be undesirable in the case of certain wines. It's an important flavor compound in coffee. And it's responsible for causing an off-taste called potato taste in East African coffee.

• 28:24Speaker 01

  Interesting.

• 28:25Speaker 00

  Yeah. Okay, there you go. So

• 28:28Speaker 01

  You know, when you were saying, I thought you were gonna go in a different place with this. When you were talking about smell. I thought you were gonna talk about carbone. Do you know about this?

• 28:36Speaker 00

  No.

• 28:37Speaker 01

  Okay, so this is a molecule. There are essentially two different ways that it can be built. And they are mirror images of one another, sort of this

• 28:46Speaker 00

  And natriomers.

• 28:47Speaker 01

  Right, exactly. One, one is like, uh, the left hand and the right hand. So the same atoms, same molecule, same bonds. But they can be constructed in a way that they are mirror images of one another. Thing is, is that you can't, you can't superimpose them, basically. One version smells really like spearmint. And the other version smells of caraway seeds. So there is no real difference to the chemistry. But your brain can tell the structure down at the molecular level.

• 29:21Speaker 00

  Yes, you're smelling the difference between not even one atom and another, but just the positions the atoms are in.

• 29:31Speaker 01

  Exactly. And admittedly, you need more than one molecule of carbone to be able to do this. But you can detect the difference in the structure of the atoms with your nose alone.

• 29:43Speaker 00

  Yes. And I ran into a lot of examples like that in this research. But yeah, ultimately, if it involves a small change done to a lot of things, it's not as cool. Like, yeah, I could take one single proton to every atom of mer, of, of gold in a sample. And you could immediately tell the difference. Because it would go from being a solid gold colored metal to being a silvery liquid metal called mercury. Like, you could detect, you can detect a one proton difference? Well, yeah, across quadrillions of them in, in a pool. But we should definitely do an episode on mirror images and, um, natriomers. Because there's a lot of cases, there's cases of, of molecules that like smell like oranges.

• 30:28Speaker 00

  But then the mirror image of it is poisonous.

• 30:30Speaker 01

  Right, okay. Wow. Okay. Yeah, all right. I, I accept it's cheating, though. I accept it's cheating.

• 30:36Speaker 00

  Before you ask if it's the best we can do, let me kind of give you a number here. So you can smell isopropyl methoxypyrazine.

• 30:45Speaker 00

  In tiny amounts, so tiny, in fact, that a one-second whiff at the detection threshold would mean that you got into your body, onto your olfactory receptors, point zero, zero, zero, zero, zero, zero, zero, zero, zero, zero, two grams of isopropyl methoxypyrazine. And you could smell it.

• 31:10Speaker 01

  Which is how many molecules? Which is how many molecules?

• 31:12Speaker 00

  It's two picograms, which is only a few million molecules, about eight million molecules.

• 31:19Speaker 01

  Look, we are, we're gaining here. We're doing good.

• 31:22Speaker 00

  We're gaining here. We've gone from quadrillions to trillions to billions to now just millions. Eight million molecules is enough for your body to be like, yep, yep, we're smelling something. It smells a little bit like potato taste in East African coffee.

• 31:36Speaker 01

  That should be on the, that should be on the Wikipedia page. All of a sudden, potato taste in East African coffee feels more, more like we're back to where we started.

• 31:45Speaker 00

  I want to also say that your sense of smell is so much more sensitive than your sense of taste because of the way chemicals interact with and are introduced to our receptors. In the mouth, they like wash over them in a liquid, but in the nose, we slam them into our olfactory receptors at high speed with air. You know, they like literally just get sucked in and smacked into it. It's not like, oh, I hope that I diffuse towards that receptor. So, so yeah, smell is just beating taste out of the water. We're down to eight million molecules that we're able to detect and say there's something there.

• 32:25Speaker 01

  Okay, so what have we done then? We've done smell, we've done taste, and we've done hearing. What have we got left here?

• 32:32Speaker 00

  You know what? We've only got touch and sight.

• 32:36Speaker 01

  And wobbliness.

• 32:38Speaker 00

  So after the break, after the break, we're going to start feeling even smaller things.

• 32:54Speaker 00

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• 33:57Speaker 03

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• 35:01Speaker 00

  Wobbliness, let's talk about it. There have been studies done that had people come in to do psychological experiments, like, oh, match these or rate your opinion on these facial expressions. But all the while, there was a little secret going on where they would suddenly tilt the floor. And they would tilt it and see if the person said anything, if the person reacted. They didn't tell them, like, tell me if you can, can, can feel a difference now. Instead, they just surprised them with it. And people really could tell when something changed by like three degrees. I mean, it's, that, that one turns out to be true. I was able to find papers that backed it up, but not everyone can.

• 35:42Speaker 01

  In the sense that they would notice it, they'd be like, why is the floor suddenly tilted?

• 35:46Speaker 00

  Exactly. That they would, they would hesitate, they would pause, they would ask what just happened. I kind of skipped over that because a tilt of the floor involves an enormous amount of molecules?

• 35:57Speaker 01

  It does, it does.

• 35:58Speaker 00

  Like, is that a small thing? It's a small change, but I'm looking at the smallest amount of matter.

• 36:06Speaker 01

  So what we left with then? We're left with physical touch?

• 36:09Speaker 00

  We're, we're left with touch and we're left with sight.

• 36:12Speaker 01

  Hmm, of course.

• 36:13Speaker 00

  So which one are we going to talk about next? We're going to talk about touch. Touch is an incredible sense. It's a, it's a really weird one. I mean, both of the, the senses we have left are weird because they're reflexive. Like, I cannot smell myself smelling. I can't taste myself tasting, but I can see myself seeing in the sense that I can look in a mirror. But even more mind-blowingly, I can feel myself feeling. I can touch myself. If you touch one hand to the other and you like move them back and forth, you are both

• 36:57Speaker 00

  an object and a subject simultaneously. I'm feeling what my left hand feels like as I stroke it with my right, but I'm also feeling what it feels like for my right hand to be touching something. And if you do this and you really meditate on it, you can switch between which hand is the toucher and which hand is the touched. You can go, ah, yes, I'm feeling the fingers on my left hand. And then just with a little mental flip, I can suddenly be feeling instead my right hand on my left. I don't know what that has to do with what's the smallest thing you can feel, but I've just been obsessed with that fact for a long time. And I've tried to read so much phenomenology about like what this tells us about identity

• 37:42Speaker 00

  and perception, and it's all just kind of opaque to me. But maybe someone in the comments can say, yes, here's why that's significant. Or maybe it's not. Maybe it's just a weird thing that, that I just said.

• 37:54Speaker 01

  I do like it, though. I do like how you can, I think, I think the point that you make there about meditating on it is really key. It really demonstrates how your brain's attention mechanism interacts with your senses, that you can direct your focus to one hand or the other as you're going.

• 38:15Speaker 00

  Yeah, yeah.

• 38:16Speaker 01

  Lot of molecules going on there. Lot of molecules going on here, though, Michael.

• 38:19Speaker 00

  There's a lot of molecules involved there. So like, let's try to, let's try to get back on track. Okay, here's an experiment you can do at home. Grab a DVD and a Blu-ray.

• 38:34Speaker 01

  This works better if the year is 2004.

• 38:37Speaker 00

  Imagine it's 2004. I don't know, 2004 was that, that'd be, you'd be pretty advanced to have Blu-ray in 2004, wouldn't you? I'm going to look this up.

• 38:45Speaker 01

  I don't remember.

• 38:46Speaker 00

  Early 2000s, yeah. It was released worldwide in 2006.

• 38:50Speaker 01

  Okay, you'd be really advanced then.

• 38:52Speaker 00

  You'd be real, yeah, you'd be one of the like designers of Blu-ray. The point is, imagine, hopefully some of you are still living in 2007, okay? And you've got Blu-rays and DVDs at hand. Grab a couple and then rub your fingers on the optical sides, the sides that contain the data. It's okay, you can clean them, but touch them and you can feel the difference between their textures.

• 39:20Speaker 01

  Right.

• 39:21Speaker 00

  A DVD's surface has wider tracks between the, the information lines than a Blu-ray does. On the Blu-ray, the information's packed in tighter, like physically tighter. And the difference is about 420 nanometers. The, the DVD tracks are 420 nanometers wider across than the tracks on the Blu-ray. And if you rub your finger on them, you can feel that they are different. You can feel that difference. 420 nanometers is literally the width of like 3,000 carbon atoms.

• 39:57Speaker 01

  This is, I mean, look, we've come down quite a long way here. 3,000 feels like we're getting somewhere.

• 40:02Speaker 00

  3,000 is really small now, but this goes back to that like problem that we had earlier where we were like, yeah, but you're not feeling 3,000 atoms. You're feeling a difference of 3,000 atoms over and over and over again. So like, you're literally feeling trillions of them in order to detect this smaller difference that's, that repeats. So I don't think it really counts. So I looked up the, the static indentation threshold, like what's the smallest thing I can tell is there even when I'm not moving my hand? And that's about, that's about 10 to 40 micrometers.

• 40:41Speaker 00

  So let's, let's use the lower end there, 10 microns, 10 millionths of a meter. That's something like 70,000 carbon atoms lined up. You could feel that as a little bump, an indentation on your skin if you were to touch it.

• 41:01Speaker 01

  And this isn't you rubbing your finger over the top of it?

• 41:03Speaker 00

  This is the static stimuli threshold. So you're not moving your hand, you're not causing vibrations across all the molecules on your skin for help. You are just sitting there resting going, mhm, I'm being indented, something's sticking into me. Or we're talking about a size, could be made of anything, but a size that's only about as large as 70,000 carbon atoms in a line. That's it. You can feel that. You can rest your finger on that and say, it's there. And when I took it away, you could say, it's not there anymore. 70,000 atoms wide. That's pretty small. Like, I'm feeling pretty happy about this.

• 41:45Speaker 01

  Yeah, yeah, I agree. That's very small.

• 41:48Speaker 00

  But we can go smaller. We got to move to the eyeballs.

• 41:53Speaker 01

  Okay.

• 41:53Speaker 00

  Now, some of you may have already been thinking about this because we talked about this effect many, many, many, many episodes ago, many months ago. We talked about astronauts up in orbit or traveling between the Earth and the Moon.

• 42:08Speaker 00

  Actually experiencing flashes of

• 42:12Speaker 01

  Of course. Oh, my gosh. Cosmic rays.

• 42:15Speaker 00

  Cosmic rays.

• 42:16Speaker 01

  Because the thing is, I'd sort of written offsite in a way here, because if we were going for atoms or particles, I was thinking, well, you know what, like photons, okay, I guess you could sort of claim that photons are particles, but really you're talking about like, this feels a bit cheaty. You know, like, what's the smallest amount of light you can see? Okay, sure, but translating that to 70,000 carbon atoms just didn't really feel fair. But you're right, if you're in space, there is a workaround here, isn't there?

• 42:43Speaker 00

  There is a feeling that can be produced by a single cosmic ray particle.

• 42:49Speaker 01

  Yes.

• 42:50Speaker 00

  Which means like, a proton and a neutron or a lone proton flying through space, that came from some star light years away, it's traveling near the speed of light, it flies through your brain and ionizes things in just the right way, that boom, you experience the sensation of a flash of light, caused by one helium nuclei.

• 43:14Speaker 01

  Wow. Space shrapnel, basically.

• 43:17Speaker 00

  Space shrapnel.

• 43:18Speaker 01

  Where you get sort of supernovae and like neutron stars colliding, they're sending out all of this, spraying out this stuff all over the universe. And that is enough for you to be able to detect it. Because it appears as a flash of light, right? Or can sometimes be a feeling of tasting metal.

• 43:37Speaker 00

  That's right. It can be a lot of things, because it's not, it's not being detected by a dedicated sensory receptor cell that we have. Instead, it's literally messing with the neurons deeper in our brains. And our brain is going, ugh, I don't know, flash of light, maybe the taste of metal, what the heck is this? It's like a very strange signal that can sometimes be interpreted as an actual feeling, as the sensation of something. And again, this can be accomplished not with 70,000 atoms in a line, but with a single atomic nuclei, traveling at 99.99999% the speed of light, and yet still, it does something.

• 44:18Speaker 01

  They're not hearing them, though, are they? They're not hearing them.

• 44:20Speaker 00

  You're not hearing them. I don't know if it could cause like a weird click perception. We just haven't shot enough cosmic rays at people's brains to know. I also read on the topic of cosmic rays, and I don't think this counts towards the answer of this video's purpose, but I found a paper about how a cosmic ray shower, like when a cosmic ray hits our atmosphere and causes this cascade of ions to like, all be created and smack into each other, those can hit the ground, they can hit like a body of water, and then interact with the molecules of water in that body of water, to create an audible clicking sound that a person underwater could hear.

• 45:01Speaker 01

  Okay.

• 45:02Speaker 00

  I just thought that was really cool. It doesn't count, though, because you aren't hearing a single, you're not hearing the sound of a single cosmic ray particle, you're hearing its effect across probably quadrillions of particles.

• 45:17Speaker 01

  I mean, the thing is, I also think, I also sort of think, if I'm honest, the cosmic ray thing in general is cheating, because you're bypassing your senses here, you know? It's not like it's going into your eye, or into your nose, into your ear, and then you're sensing it, and then you're like, you're like, no, no, we're not doing any of that, we're getting straight to the exact neuron in the brain.

• 45:38Speaker 00

  Yeah.

• 45:38Speaker 01

  That is doing the registering of sensation, not sensation, we're hijacking that. I think this is a bit backdoor, you know?

• 45:47Speaker 00

  It's backdoor. I, I believe that that should still count.

• 45:51Speaker 01

  Okay, because you do feel it.

• 45:52Speaker 00

  Because I didn't say, what's the smallest thing you can sense with one of your major sense organs? I'm saying, the smallest thing you can feel. Where feeling is your ability better than chance to say something happened. Even if you can't describe it, I think that should still count. But that's okay, because there is a final step in our journey. And it does involve photons.

• 46:15Speaker 01

  Ooh, go on.

• 46:16Speaker 00

  Okay, so here's the thing. How many photons does it take for us to see something?

• 46:23Speaker 01

  Well, so apparently, it's one single candle 30 miles away, Michael.

• 46:26Speaker 00

  Yeah, whatever that means. Like, I want, I want something that's a little more concrete. And if we can see photon, I mean, the fact that we can see photons at all, kind of is gonna have to win, because photons, to what extent they have mass, is basically, you know, that doesn't count. So no matter how few helium nuclei we're sensing, the weight of all the photons we see is smaller. But it still feels like, it's kind of unfair to be like, well, then, but then a million photons have a mass of like, you know, nothing, they have momentum, though. How exactly do we compare? What's more, a million photons or one helium

• 47:07Speaker 00

  nuclei? Basically, I'll let you guys decide how to rank this stuff. But I find this really cool. So, experiments have been done where a single photon is shot at an eyeball. And in these experiments, we know for a fact that a single photon is all it takes for a rod cell in our retina to change its state.

• 47:35Speaker 01

  Right.

• 47:36Speaker 00

  Which, by the way, here's a cool fact about the receptor cells in our eyes, the rods and the cones. Intuitively, I thought, oh, surely, when light hits them, they send a signal to the brain that like, hey, I noticed something. And the brain goes, oh, you're a cone cell for long wavelengths, guess it's like red light. No, it's a little bit different than that. Instead, in the darkness, your vision receptor cells, your light receptor cells are constantly screaming. They are constantly sending signals to the brain. They are just like, like firing all the time. But when light falls on them, they become depolarized and they go quiet.

• 48:16Speaker 01

  Huh, that's interesting.

• 48:17Speaker 00

  And that is what happens when your brain detects that that cell saw something.

• 48:24Speaker 01

  So it's upside down, basically?

• 48:25Speaker 00

  It's upside down.

• 48:26Speaker 01

  From what you would expect.

• 48:27Speaker 00

  So we see in silence. What we see is the cells stopping their conversation.

• 48:34Speaker 01

  So actually, does it take more energy to look at blackness, essentially?

• 48:40Speaker 00

  That's a good question. I guess the receptor cells fight against the depolarization. They try to get back to a normal level, and that's ultimately what causes afterimages. You know, like if there's a bright flash, and you look away, you still see this flash there, that's the cell being like, whoa, whoa. So I, I don't know, I think that recovering from an encounter with light is probably more expensive to the body than, than just the regular background firing that you, that happens when you're in darkness. But that's a great question to look at. Like, can you imagine it? Hey, here's a weight loss tip, to burn some calories, keep your eyes closed.

• 49:19Speaker 00

  No, I think, I think even, even when they work their way back to a baseline, that baseline is still lower than it is in darkness. So yeah, I think, if you want to lose weight, keep your eyes open.

• 49:30Speaker 01

  Yeah, right.

• 49:31Speaker 00

  Go outside on a sunny day, and just look at a, at a white wall, and you'll burn more calories, because your cells will need to be re-energized.

• 49:40Speaker 01

  Mm-hmm, mm-hmm.

• 49:41Speaker 00

  More per second. Yeah, wow. Okay.

• 49:45Speaker 01

  Take that one, beauty influencers.

• 49:47Speaker 00

  I did not expect some weight loss advice to come from this, but.

• 49:51Speaker 01

  Okay, so one photon, one photon is enough. But then do you register it? Maybe it's enough for your cells.

• 49:57Speaker 00

  That's, that's the right question. So here's the really trippy thing. We do not, and we have never, in fact, found a person who, when a single photon is shot at their eye, says, I saw something. They don't see anything. However, if you ask them to say, did anything happen? They have a feeling that something happened or didn't, but it's not a visual experience. And they get this right above chance. So if they were just, if they really had no way of knowing whether a photon had hit their eye or not, and they just guessed, they should be right half the time. But they're right more than half the

• 50:39Speaker 00

  time in these experiments. So I feel like that's evidence that at some non-conscious level, the brain is going, okay, that cell changed, not, it's not enough for us to actually become aware of it. I'm not going to tell Michael's awareness, but it happened. And my awareness may be able to tap into my non-conscious registering of that photon in a different way, not as, oh, yeah, I saw something, but something's different. You did, did you do something? I don't know what it feels like. I haven't been part of these experiments. But better than chance, people can say, something happened. And when that something is a single photon hitting their eyeball.

• 51:20Speaker 01

  Well, they did feel something, which I guess was the original definition that you were going for.

• 51:24Speaker 00

  Yeah, there has been a change, maybe at a non-conscious level that we're just a little bit aware of, that makes us better at guessing. It's almost like blind sight, where people do not know what just happened, but they know something happened. And they can't explain why, and they can't fabulate reasons. But if you get up to five or nine photons, that's the threshold where people really, they get it right all the time. They say something happened. Yeah. But again, it's still not a visual sensation, it's not like a flash of light or a pin prick of light. It's just that they, they, they go, they say, they report, something happened, you did something. What did you do?

• 52:00Speaker 01

  I want to try this. Where can we go, Michael? Where can we go that they can do this to us?

• 52:04Speaker 00

  CERN? Can they shoot photons? I mean, I don't know how you do it. How do you, I know how to shoot photons.

• 52:09Speaker 01

  Get in the way of the beam, be like, and go.

• 52:11Speaker 00

  And go. How do I, how do I get a discreet number of photons fired at my eye?

• 52:19Speaker 01

  Maybe just get a laser and turn it on and off really fast.

• 52:21Speaker 00

  Really, really quickly. Or if I passed it, you know, if I passed it through like, enough filters, at a certain point, maybe only one's getting through every minute, I don't know. That would be cool. I would love to sit, I'm imagining that I get to sit in a chair, and a technician is like, okay, here's one photon. Now here's two. And they just keep going until I'm like, whoa, whoa, turn down the lights.

• 52:44Speaker 01

  The thing is, is that they, I mean, they definitely, this is one of the ways that quantum encryption works. I think we talked about this once before, where they are essentially throwing down single photons from space, and then like catching them on earth, right?

• 52:59Speaker 00

  Yeah.

• 52:59Speaker 01

  So it's possible. Look, if you, in the comments, someone who can throw a photon at our eyes, just one. Less than 10.

• 53:09Speaker 00

  Less than 10.

• 53:10Speaker 01

  I'll accept less than 10.

• 53:11Speaker 00

  Yeah, ideally, every integer number of photons between 0 and 100, and 10, sorry.

• 53:17Speaker 01

  Yes, please.

• 53:18Speaker 00

  Would be good, would be good. I'd love to try this.

• 53:20Speaker 01

  And I want to know the threshold at which you can see them as well. I want to know that threshold.

• 53:24Speaker 00

  Yeah, I don't know the threshold for seeing something.

• 53:27Speaker 01

  Presumably, they're sitting in the dark when this is happening.

• 53:29Speaker 00

  Yeah, they're sitting in the dark. That's right.

• 53:31Speaker 01

  Not losing weight.

• 53:33Speaker 00

  They're not losing, they're not losing weight. They are gaining while we gain knowledge about human perception. So, thank you, experiment subjects.

• 53:43Speaker 01

  Yes, indeed.

• 53:44Speaker 00

  So, my answer is 1 photon.

• 53:47Speaker 01

  I accept. I accept your answer and now have a new life's mission to have that experience, the most minimal experience possible.

• 53:56Speaker 00

  Exactly. That's like embracing extreme minimalism. Movies? I don't watch movies. No, that's maximalism. For me, just a single photon shot at my eye, more than half the time I'll know it happened. That's enough for me.

• 54:11Speaker 01

  Sorry, you're, you're watching a movie. You're having how many photons shot at you?

• 54:16Speaker 00

  Yeah, exactly. So, I don't think there's any way we're going to beat an individual photon.

• 54:22Speaker 01

  Yeah, I mean, the irony that we're doing all of this through a video format, video and audio format, and hopefully some emotion as well. Look, we, that's what we do here. We, we move trillions upon quadrillions upon bazillions of molecules around for your enjoyment and pleasure. You can move more. Leave us a comment. That's electrons flying around all over the place. You have that power.

• 54:48Speaker 00

  Yeah, we are. I mean, yeah, when you put it that way, we are really going over the top. Like, there's no reason for us to be moving this many photons around or this many air molecules into the microphone. Just, I mean, 1 is enough. 5 to 9 is more than enough.

• 55:05Speaker 01

  Yeah.

• 55:05Speaker 00

  So, you guys are welcome.

• 55:07Speaker 01

  I've got an idea for a new podcast, Michael.

• 55:10Speaker 00

  Yes.

• 55:11Speaker 01

  But until then, we will continue going over the top for you, our listeners.

• 55:17Speaker 01

  Okay, I guess that wraps us up. We'll see you next time.

• 55:20Speaker 00

  See you next time.

• 55:32Speaker 06

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  This week, it pays extra to be a Milo's Rewards member. Through October 14, members get $10 Milo's money when you spend $200 or more on an eligible purchase during Milo's Money Days. Also, using your Milo's Rewards credit card, even better, that's an additional $20 Milo's money. Use it to save on your next Lowe's haul. Exclusions, restrictions, and more terms apply. See lowe's.com/milosmoneydays for details. Milo's money expires 30 days after issuance. Subject to change. Credit offer subject to credit approval.

• 56:31Speaker 08

  Your heart can tell you a lot about your health. Apple Watch Series 12 measures your heart rate every 5 seconds with the most accurate heart rate sensing in a wearable. So, your vitals app, now with heart rate variability, can tell you when something is off. And your readiness score can let you know when to rest and when to push. Hear the story in every heartbeat with Apple Watch Series 12. The features described are for wellness purposes only and not for medical use. iPhone 11 or later required. Based on Apple conducted study of heart rate accuracy, August 2026. Visit apple.com slash apple watch series 12.