The Rest Is Science - The Threshold of Human Sensation

Episode Date: September 6, 2026

⁠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

Hello and welcome to the rest is science. I'm Michael Stevens. And I'm Hannah-Fri. And I was just really shaking my camera by movie. Look at that. Whoa. I'm 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 in 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. Let's clarify here. It's the more as thing you can feel, we're talking about physical sensation, not an emotion, right? We're talking about every sense of the word feel.

Ooh. 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 mold 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? Look at that, Michael. We've got the title already. This episode is brought to you by Cancer Research UK. Do you remember when we discussed why feet are so weird? Well, one particular footbone holds an even stranger surprise. It's helping shape our understanding of cancer timelines. 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. footbone. Researchers have identified a tumour in it in the oldest known example of cancer in people. Which really shows that cancer is far from a modern disease. Beating a disease so deeply rooted in our biology won't happen overnight. But today, Cancer Research UK scientists are discovering incredible ways to turn our biology against cancer. 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. For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visitcanterresearchuk.org

forward slash rested science. New from Nespresso. Blend wellness into your coffee routine with a coffee plus range, infused with functional benefits. Choose the coffee you love with added be vitamins. Like coffee plus B12 to help support immune function, and coffee plus B6 to keep your day moving. Or go with the flow and choose ginseng delight. Our new double espresso with ginseng extract. Whatever lies ahead, don't change your morning. Let your morning change you.

Discover coffee plus on espresso.com. You gotta try breakfast at A.W. You gotta try for A.W. And what you? better way than with a delicious pre-organic coffee starting with just one dollar all day every day now until December 31st. You gotta try breakfast at A&W at participating A&W locations in Ontario. 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 a 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, perception, threshold, anything. 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. It'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. I'm there. I'm there. A watch ticking 20 feet away. What kind of watch they don't tell us. Well, it's not your one that just says now on it. I'm not even wearing a watch today, but the one I've been wearing, 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. 30 miles away. 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. 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. 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. Yeah, it'll accelerate. I mean, eventually you'll be like, wow, it's flying. But initially it's kind of like, this is pretty good, pretty level. I mean, we're talking about, again, look at 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 the size of the size of the,

of three rooms. I've also seen this written a volume of six rooms. And I'm like, what kind of perfume? These also sound far too poetic, you know? They're too poetic, right? Maybe we shouldn't even start here. Like, what's the point? 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 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 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. Is that one of them? That's one of them. No, come on. And 7.6 sounds like really, really precise. Scientified. But they're just converting three inches. to metric. Okay. Yeah, that's... 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. Yes. 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 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 to say, you know what, there's a single atom of carbon shifted over here is the difference between boredom and on we. Mm-hmm. Yeah, agree, agree. I mean, I think even trying to define what boredom and non-we are

is going to be, well, it proves to be extremely, extremely difficult. This is an amazing book on this by Lisa Feldman Barrett. Have you come across it? No. Oh, it's so good. It's one of my absolute phase, 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. Oh, I'm familiar with that book, yeah. 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 apples. They have a stroop. And there's like, oh, a hippo that's really cross when they're in labour. And 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-Barrar 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 nuanced. 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 happiness. and this is joy. We understand that there's a difference of strength in these words,

but, you know, it's just too soft, I think, for the purposes of today's episode. So we're not doing the smallest happiness you can feel? No, we're not, because I think that's also super subjective. I mean, you could look at something like coordinate maps 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 aroused are we? Like how off of... Intense. 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 kind of a chart, you get something like happy is 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. But if you go down, staying with the positive valence, you run into comfort, relaxation, and calm. Okay. I think that's a fun way to think about emotions. 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 let's move on to hearing, because I think that our sense of hearing needs the, the most matter of all of our major senses.

Because I was thinking, what's the smallest thing we can hear? 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. Well, even if it was like, you know, up to the speed of light,

even then, even if I had an enormous amount of energy. Even then, I don't think the way it interacts with the eardrum is going to cause a vibration. It's going to go right through. It's also, here's the other problem. We don't hear 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? 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 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, couldn't I hear a sound from something small that was making a noise? Like rather than looking at the actual pressure wave, 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? No. Really?

Yeah. A single proton turning into energy. All of its mass converts into energy. It's only 1.5 nanodules of energy. Really? We need a million times more than that for there to be a sound we can hear.

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? It's silent. I mean, they're shooting like a proton beam. So we're talking about more than just one. If you're doing one. If you're doing one, it's like nothing. And if you've got millions, okay, you're going to 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 got to release the energy more slowly. It can't be quote unquote instantaneous. But if you slow down that release, now you're in the frequency range of human hearing, but the energy is too low. So you will need, again,

I don't know if I calculated the exact number, but you're going to need probably on the order of trillions of protons turning into energy slowly enough that you can actually hear something. A trillion protons is smaller than a trillion molecules. Yeah, yeah, much cooler. So we're getting closer, but we can do better. But we're still talking sound.

All right, okay. 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 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 mouths are a pretty neutral place. It's got a pH of, like, seven. But that means that there are, at any one moment, about 6 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. You're going in the wrong direction here. I'm going in the wrong direction. You need 2.39 quadrillion more protons free in your mouth before you're like, oh, yeah, something sour's in there. Really? Yeah.

And 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 sounds. That's not, that's not like, whoa, Warhead. You know, that's just straight up like, hmm, okay, yeah, with that many free hydrogen ions, 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 tastes we are the most sensitive to, one of the best candidates I found is this, kind of new thing, lugdename.

Lugdename is a molecule that is sweeter than sugar by like hundreds and hundreds of thousands of times. It blows aspartame and saccharine way out of the water. Where does it come from? 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 lugdename attaches to all available receptors on 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. Literally, an industrial silo would need like a few nanograms of it. What? This is like the fentanyl of the sugar world. 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 unbelievable. It's, um, Detection threshold is 0.6 parts per billion.

Right. So if I put a quarter teaspoon in an Olympic-sized swimming pool, you'd get in that swimming pool and go, this is all sweet. No, really? You could detect. What was it? A quarter, a quarter of a teaspoon.

In the sugar example, the poetic one that you started with was one teaspoon for two gallons. Exactly. Whereas I'm saying, lug the name, a quarter teaspoon. in 660,000 gallons. It gives you a sense of how much sweeter lugdename is than sugar. Yeah. Wow.

Wow, that's crazy. That's crazy. And so... So that is sort of your... You are detecting. Wait, how many molecules could you detect then? How many molecules of the stuff would it take? 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 lugdename in that water. I taste that it's sweet. How many molecules of lugdename 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. Right.

That's quite a big improvement. 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 lugdename is not the world's smallest molecule. But I do like that it's only 11 billion things. Yeah, me too.

And we're already detecting them. Okay, so we're getting better. Let's move on to smell. Smell, I thought, would be a lot worse than taste because, I don't know, 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. They're 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.

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 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've look at these skulls all the time. I don't know the names of all their parts.

I've learned now, here's a skull. 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 pot-marked with holes. Is that where the little tendrils, as you described, them come down?

Yeah, that's where the tendrils come down. So I'm going to get a brain model and show you. We're talking about here, basically. We're talking about right between your eyes, like between your eyebrows even, is even better because it's a bit above your eyes. Okay. And through that part of the bone, here's what's happening. You've got at the base of your brain, here's your brain, here's how it sits 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... Oh, tell me it's got a Latin name that means little hole.

They do, they do. And they're called the crib reform, foramina. So a foramina is a tiny hole. And crib reform means shaped like a sieve. Because we have essentially a sieve in our skull. You say sieve rather than sieve. Yeah, I've heard it both ways. A sieve, a colander.

Okay. Basically, it's a colander of holes in your skull. And I can show you 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, multi-colored 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, and that's where your olfactory nerves sit,

ready to detect any kind of odor molecule that comes into your nose. So this is the thing that makes it so much more sensitive because you're basically, your brain is, directly sensing the molecules as they come up through your nose, rather than using a sort of go-between. 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 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 going to go crazy just because, you know, one free proton hit you, right? So our sense of smell is so much more sensitive. In fact, there's... How much more sensitive, though? Okay, I'll tell you about this molecule. It's called isopropyl methoxypirazine. Okay. It's got a long name.

But we can detect this. We can smell this at a level of 0.3 parts per trillion. How does that compare to the sweet tasting thing? Okay, here's how it compares. I'm going to be using a lot of like American units where you just start talking about swimming pools and football fields and stuff. But remember that lugdename was detectable in an Olympic-sized swimming pool if you put one quarter teaspoon in.

But an equivalent detection threshold for isopropyl methoxy pyrazine 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. Whoa. And this is you can smell. You're able to smell it. Yeah, you can smell it. So you can't actually get in the pool and taste it. But if you imagine that those 20 Olympic high swimming pools were air,

you could smell one single drop. See, this is the thing. People go on about sharks being like, oh, they one drop of blood in da-da-da-da-da-da. No, no, no. We can do it the same. We can just only do it with really complicated sounding molecules. 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 smell it from a mile away. It's more like, yeah, there's this thing that you're never going to smell. but if you had some around, you'd be like, wow, I can smell such a small amount of it. What does it actually smell like? 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. Oh.

And it's responsible for causing an off-taste called potato taste in East African coffee. Interesting. Yeah. Okay, there you go. You know when you were saying, I thought you were going to go in a different place with this, when you were talking about smell, I thought you were going to talk about carbon. Do you know about this?

No. 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. And natuilar's. Right, exactly. One is like the left. hand and the right hand. So the same atoms, same molecules, same bonds, but they can be constructed

in a way that they are mirror images of one another. The thing is, is that 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. Yes, you're smelling the difference between not even one atom and another, but just the positions the atoms are in. Exactly. And admittedly, you need more than one molecule of carbon to be able to do this,

but you can detect the difference in the structure of the atoms with your nose alone. 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 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 a one proton difference. Well, yeah, across quadrillions of them in a pool. but we should definitely do an episode on mirror images and an adiomers because there's a lot of cases.

There's cases of molecules that like smell like oranges, but then the mirror image of it is poisonous. Okay. Wow. Okay. Yeah, all right. I accept it's cheating, though. I accept it's cheating. 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 methoxypirazine 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. 0-0-0-0-0-0-0-0-0-2 grams of isopropyl methoxy pyrozyme. And you could smell it. Which is how many molecules?

It's two picograms, which is only a few million molecules. about 8 million molecules. Look, we are, we're gaining here. We're doing good. 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. That should be on the Wikipedia page. All of a sudden, potato tastes in East African coffee feels more like we're back to where we started. 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 yeah, smell is just beating taste out of the water. We're down to 8 million molecules that we're able to detect and say there's something there.

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? You know what? We've only got touch and sight. And wobbliness. So after the break, after the break, we're going to start feeling even smaller things. This episode is brought to you by Cancer Restore.

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For more information about Cancer Research UK, their research and breakthroughs and how you can support them, visit cancerresearchuk.org slash rest is science. New from Nespresso. Blend wellness into your coffee routine with a coffee plus range, infused with functional benefits. Choose the coffee you love with added B vitamins, like coffee plus B12 to help support immune function, and coffee plus B6 to keep your day moving. Or go with the flow and choose ginseng delight. Our new double espresso with ginseng extract. Whatever lies ahead, don't change your morning.

Let your morning change you. Discover coffee plus on nispresso.com. You gotta try breakfast at A&W. You gotta try. And what better way than with a delicious Pratt organic coffee? Starting with just $1 all day, every day now until December 31st. You gotta try breakfast. I'm participating in Wobbuncations in Ontario.

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 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, that one turns out to be true. I was able to find papers that backed it up, but not everyone can. In the sense that they would notice it, they'd be like, why is the floor suddenly tilted? Exactly. That 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. It does. It does.

Like, is that a small thing? It's a small change. But I'm looking at the smallest amount of matter. So what we're left with them? We're left with physical touch. We're left with touch and we're left with sight. Of course. 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 really weird one. I mean, both of the senses we have left are weird because they're reflexive. Like I cannot smell myself smelling. I can't take. 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 move them back and forth, you are both 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 to. with that fact for a long time, and I've tried to read so much phenomenology about what this tells us about identity 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 I just said. I do like it, though. I do like how you can, I think the point that you make there about meditating on it is really key. It really demonstrates how you're brain's attention mechanism interacts with your senses, that you can direct your focus to one hand or the other as you're going. Yeah, yeah. A lot of molecules going on there. A lot of

molecules going on here, though, Michael. There's a lot of molecules involved there. So, like, 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. This works better if the year is 2004. Imagine it's 2004. I don't know. 2004 was that, you'd be pretty advanced to have Blu-ray in 2004, wouldn't you? I'm going to look this up.

I don't remember. Early 2000s. Yeah. It was released worldwide in 2006. Okay. You'd be really advanced then. You'd be really, 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. Right. A DVD surface has wider tracks between 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 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. This is, I mean, look, we've come down quite a long way here. 3000 feels like we're getting somewhere. 3,000 is really small now. But this goes back to that 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 repeats. So I don't think it really counts. So I looked up 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 10 to 40 micrometers. So 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 indomboats.

on your skin if you were to touch it. And this isn't you're rubbing your finger over the top of it? 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, mm-hmm, I'm being indented, something sticking into me. 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. Yeah. Yeah. I agree. That's very small. But we can go smaller. We got to move to the eyeballs. Okay. Now, some of you may have already been thinking about this because we talked about this effect many, many, many,

episodes ago, many months ago, we talked about astronauts up in orbit or traveling between the Earth and the moon, actually experiencing flashes of light. Of course. Oh my gosh. Cosmic rays. Of course. Cosmic rays. Because the thing is, I'd sort of written off-site 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 light, it sort of feels like 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, it didn't really feel fair. But you're right. If you're in space, there is a work around here, isn't there?

There is a feeling that can be produced by a single cosmic ray particle. Yes. Which means, like, a proton and a neutron or a lone proton flying through space They 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. Wow. Space shrapnel, basically.

Space shrapnel. Where you get sort of supernovae and like neutron stars colliding. They're sending out all of this spray. 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. That's right.

It can be a lot of things because 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, I don't know, a 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 99.999% the speed of light, and yet still, it does something.

They're not hearing them. They're not hearing them. 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 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. Okay. 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. 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, no, no, no, we're not doing any of that. We're getting straight to the exact neuron in the brain. Yeah. That is doing the registering of sensation, not sensation, and we're hijacking that. I think this is a bit backdoor, you know? It's a bit more like, I believe that that should still count. Okay, because you do feel it. 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. Ooh, go on. Okay. So here's the thing. How many photons does it take for us to see something?

thing. Well, so apparently it's one single candle 30 miles away, Michael. Yeah, whatever that means. Like, I want something that's a little more concrete. And if we can see phot, I mean, the fact that we can see photons at all kind of is going to 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 were 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, 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 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. Right. 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 firing all the time.

But when light falls on them, they become depolarized and they go quiet. Huh, that's interesting. And that is what happens when your brain detects that that cell saw something. So it's upside down, basically. It's upside down from what you would expect. So we see in silence, what we see is the cells stopping their conversation. So actually, does it take more energy to look at blackness, essentially? 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 after images 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 don't know I think that recovering from an encounter with light

is probably more expensive to the body than just the regular background firing 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. No, I think 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. Yeah, right. Go outside on a sunny day and just look at a white wall and you'll burn more calories because your cells will need to be re-energized more per second. Yeah, wow. Okay. Take that one, beauty influences. I did not expect some weight loss advice to come from this. Okay, so one photon, one photon is enough.

But then do you register it? Maybe it's enough for yourselves. 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 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 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. 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.

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

something happened. You did something. What did you do? I want to try this. Where can we go, Michael? Where can we go that they can do this to us? CERN? Can they shoot photons?

I mean, I don't know how you do it. How do you? I know how to shoot photons. Just get in the way of the beam, be like, and go. And go. How do I get a discrete number of photons fired at my eye? Maybe just get a laser and turn it on and off really fast. 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. The thing is that they do, 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? Yeah. So it's possible. Look, if you in the comments are someone who can throw a photon at our eyes, just one. Less than 10. Less than 10. I'll accept less than 10. Yeah.

Ideally, every integer number of photons between zero and 100 and 10, sorry. Yes, please. Would be good, would be good. I'd love to try this. And I want to know the threshold at which you can see them as well. I want to know that threshold. Yeah, I don't know the threshold for seeing something. Presumably they're sitting in the dark when this is happening.

Yeah, they're sitting in the dark. That's right. Not losing weight. They're not losing weight. they are gaining while we gain knowledge about human perception. So thank you, experiment subjects. Yes, indeed. So my answer is one photon.

I accept. I accept your answer. And now have a new life's mission to have that experience. The most minimal experience. 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. Sorry, you're watching a movie. You're having how many photons shot? Yeah, exactly. So I don't think there's any way we're going to beat an individual photon.

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, that's what we do here. We move trillions upon quadrillions upon bazillions of molecules around for your enjoyment and pleasure. You can move more. Leave us a comment. Electrons flying around all over the place. You have that power.

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, one is enough. Five to nine is more than enough. Yeah. So you guys are welcome. I've got an idea for a new podcast, Michael. Yes.

But until then, we will continue going over the top for you, our listeners. Okay, I guess that wraps us up. We'll see you next time. See you next time.