The Threshold of Human Sensation

14:53Michael Stevens: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, you 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 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 protons turning into energy slowly enough that you can actually hear something a trillion protons is smaller than a trillion molecules yeah yeah so we're getting closer, but we can do better.

15:54Michael Stevens:But we're still talking sound. 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.

16:36Michael Stevens: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.

17:05Hannah Fry:You're going in the wrong direction here.

17:07Michael Stevens: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 sours in there.

17:17Hannah Fry:Really? Yeah. And that's the threshold of like, is that – can you taste something sour? That's what we're talking about here. That's not like, whoa, that's sour.

17:26Michael Stevens:That's not like, whoa, warhead. No, that's just straight up like, okay, yeah. With that many free hydrogen ions, you would start to, many of us would start to say, 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, lugdaname.

18:11Michael Stevens: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.

18:24Hannah Fry:Where does it come from?

18:25Michael Stevens: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 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.

19:17Hannah Fry:What? This is like the fentanyl of the sugar world.

19:20Michael Stevens: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. Its detection threshold is 0.6 parts per billion.

19:43Hannah Fry:Right.

19:43Michael Stevens: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?

19:52Hannah Fry:You could detect a quarter of a teaspoon. In the sugar example, the poetic one that you started with was one teaspoon for two gallons. Exactly.

20:03Michael Stevens:Whereas I'm saying Lugdename, a quarter teaspoon in 660 ,000 gallons. That gives you a sense of how much sweeter Lugdename is than sugar.

20:17Hannah Fry:Yeah.

20:18Michael Stevens:Wow. Wow, that's crazy. That's crazy.

20:22Hannah Fry:So that is sort of your, you are detecting, how many molecules could you detect then? How many molecules of this stuff would it take?

20:30Michael Stevens: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.

20:55Hannah Fry:Right. That's quite a big improvement.

20:57Michael Stevens: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. Every other animal has a better sense of smell than us.

21:35Michael Stevens:And that's, as we've discussed, not entirely true. Our sense of smell is 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.

22:19Michael Stevens: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 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.

22:59Michael Stevens: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.

23:11Hannah Fry:Is that where the little tendrils, as you described them, come down?

23:14Michael Stevens:Yeah, that's where the tendrils come down. So I'm going to get a brain model and show you.

23:18Hannah Fry:We're talking about here, basically.

23:20Michael Stevens: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 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.

23:55Michael Stevens: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 cribriform foramina. So a foramina is a tiny hole. And cribriform means shaped like a sieve because we have essentially a sieve in our skull.

24:22Hannah Fry:You say sieve rather than sieve.

24:25Michael Stevens: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. 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 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?

25:06Michael Stevens: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.

25:58Michael Stevens:But your taste receptors don't work the same way. They're more like, hey, you mind your own business, I'll 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. How much more sensitive though? Tell me. Okay, I'll tell you about this molecule. It's called isopropyl methoxypyrazine. 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.

26:33Hannah Fry:How does that compare to the sweet tasting thing?

26:37Michael Stevens: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 lugdaname 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 thirtieth of a single teaspoon, so less than a quarter teaspoon, one drop in 20 Olympic-sized swimming pools.

27:13Hannah Fry:Whoa. And this is you can smell. You're able to smell it.

27:17Michael Stevens:Yeah, you can smell it. Wow. So you can't actually get in the pool and taste it. But if you imagine that those 20 Olympic-sized swimming pools were air, you could smell one single drop.

27:28Hannah Fry: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 only do it with really complicated sounding molecules.

27:39Michael Stevens: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.

27:56Hannah Fry:What does it actually smell like?

27:58Michael Stevens: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.

28:25Hannah Fry:Interesting.

28:25Michael Stevens:Yeah. Okay, there you go. So...

28:28Hannah Fry: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 carvone. 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. Anatomers. 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. Thing is, is that you can't superimpose them, basically. One version smells really like spearmint, and the other version smells of caraway seeds.

29:10Hannah Fry:So there is no real difference to the chemistry, but your brain can tell the structure down at the molecular level.

29:21Michael Stevens:Yes. You're smelling the difference between not even one atom and another, but just the positions the atoms are in.

29:31Hannah Fry: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.

29:43Michael Stevens: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 um anatomers because there's a lot of cases there's cases of of molecules that like smell like oranges but then the mirror image of

30:29Hannah Fry:it is poisonous okay wow okay yeah all right i accept it's cheating though i accept it's cheating

30:36Michael Stevens: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 in tiny amounts. It's 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, 0.0000000002 grams of isopropyl methoxypyrazine. And you could smell it.

31:10Hannah Fry:Which is how many molecules? which is how many molecules?

31:13Michael Stevens:It's two picograms, which is only a few million molecules, about eight million molecules. Look, we are, we're gaining here.

31:22Hannah Fry:We're doing good.

31:23Michael Stevens: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.

31:36Hannah Fry: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.

31:44Michael Stevens: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. 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, yes, 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.

32:26Hannah Fry: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?

32:32Michael Stevens:You know what? We've only got touch and sight.

32:36Hannah Fry:And wobbliness.

32:38Michael Stevens:So after the break, after the break, we're going to start feeling even smaller things.

32:54Michael Stevens:This episode is brought to you by Cancer Research UK.

32:57Hannah Fry:When we talk about beating cancer, we often focus a lot on survival. And that can mean overlooking impacts that last long after treatment ends.

33:06Michael Stevens:Yeah, for example, take cancers in children and young people. The treatments themselves can be incredibly harsh. they can cause lifelong side effects like infertility or hearing loss. And Cancer Research UK is working to change that because young people,

33:21Hannah Fry:they should be able to grow up hearing the voices of the people that they love and living their lives to the fullest.

33:26Michael Stevens:That's right. And one clinical trial led by Cancer Research UK showed that giving another drug alongside chemotherapy nearly halved the number of children losing their hearing. And today, the treatment combination is being used by doctors across the world. For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org slash restisscience. This episode is brought to you by Facebook. So you were scrolling on Marketplace, and there it was, the bike you'd been searching for.

34:00Hannah Fry:You sent a message, and it turned out the seller was super chatty, kind of funny, and an avid cyclist. The next thing you know, you're in a cycling crew.

34:10Michael Stevens:well, a community cycling group. The thing about Facebook, you might find more than what you're looking for.

34:16Hannah Fry:From a browse to a bike ride, this summer, find more on Facebook. Hey Chicago, class it up with Crocs. You know back to school is coming in fast. So why wait to find your new fave footwear? Step into a local Crocs store and step into your new look. Try it, style it, make it yours. because the right pair doesn't just show up, it shows off. First day fits, handled.

34:46Michael Stevens:Walk out ready for whatever's next. Visit your nearest Croc store today.

34:57Michael Stevens: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.

35:37Michael Stevens:but not everyone can.

35:38Hannah Fry:In the sense that they would notice it, they'd be like, why is the floor suddenly tilted?

35:42Michael Stevens: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 are we left with then?

36:03Hannah Fry:We're left with physical touch.

36:05Michael Stevens: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 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 an object and a subject simultaneously.

36:56Michael Stevens: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 and perception.

37:39Michael Stevens: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.

37:50Hannah Fry: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 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.

38:11Michael Stevens:Yeah. Yeah.

38:12Hannah Fry:A lot of molecules going on there. A lot of molecules going on here though, Michael.

38:15Michael Stevens: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. This works better if the year is 2004. Imagine it's 2004. I don't know, 2004, you'd be pretty advanced to have Blu-ray in 2004, wouldn't you? I'm going to look this up.

38:41Hannah Fry:I don't remember.

38:42Michael Stevens:Early 2000s, yeah, it was released worldwide in 2006.

38:47Hannah Fry:Okay, you'd be really advanced then.

38:49Michael Stevens: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 is 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.

39:39Michael Stevens: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.

39:53Hannah Fry:This is, I mean, look, we've come down quite a long way here. 3 ,000 feels like we're getting somewhere.

39:58Michael Stevens: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 3000 atoms. You're feeling a difference of 3000 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.

40:46Michael Stevens: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.

40:57Hannah Fry:And this isn't you rubbing your finger over the top of it?

41:00Michael Stevens: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, I'm being indented, something's 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.

41:43Michael Stevens:That's very small. But we can go smaller. Go on. 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, 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.

42:08Hannah Fry:Of course. Oh my gosh. Cosmic rays. Of course.

42:12Michael Stevens:Cosmic rays.

42:12Hannah Fry: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 just didn't really feel fair. But you're right. If you're in space, there is a workaround here, isn't there?

42:39Michael Stevens: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 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.

43:10Hannah Fry:Wow. Space shrapnel, basically.

43:13Michael Stevens:Space shrapnel.

43:14Hannah Fry: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.

43:33Michael Stevens: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, 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. They're not hearing them though, are they?

44:15Michael Stevens: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 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.

44:57Michael Stevens: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.

45:13Hannah Fry: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 that is doing the registering of sensation, not sensation. We're hijacking that. I think this is a bit backdoor, you know? It's a little bit like –

45:44Michael Stevens: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 majors, 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.

46:11Hannah Fry:Ooh, go on.

46:12Michael Stevens:Okay, so here's the thing. How many photons does it take for us to see something?

46:19Hannah Fry:Well, so apparently it's one single candle 30 miles away, Michael.

46:23Michael Stevens:Yeah, whatever that means. I want something that's a little more concrete. And if we can see – 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 – it 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, 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.

47:09Michael Stevens: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.

47:32Hannah Fry:Right.

47:32Michael Stevens: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.

48:12Hannah Fry:Huh, that's interesting.

48:14Michael Stevens:And that is what happens when your brain detects that that cell saw something.

48:20Hannah Fry:So it's upside down, basically. It's upside down. Upside down from what you would expect.

48:23Michael Stevens: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.

49:05Michael Stevens: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 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 yeah right go outside on a sunny day and just look at a at a white wall and you'll burn more calories

49:33Hannah Fry:because your cells will need to be re-energized more per second yeah wow yeah okay take that one

49:42Michael Stevens:beauty influencers. I did not expect some weight loss advice to come from this.

49:46Hannah Fry:But okay, so one photon, one photon is enough. But then do you register it? Maybe it's enough for your cells.

49:53Michael Stevens: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 time in these experiments.

50:37Michael Stevens: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.

51:17Hannah Fry:Well, they did feel something, which I guess was the original definition that you were going for.

51:21Michael Stevens: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 blindsight 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 pinprick of light. It's just that they They go, they say, they report, something happened.

51:55Michael Stevens:You did something. What did you do?

51:57Hannah Fry:I want to try this. Where can we go, Michael? Where can we go that they can do this to us?

52:01Michael Stevens: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 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.

52:35Michael Stevens:And they just keep going until I'm like, whoa, whoa, turn down the lights.

52:40Hannah Fry:The thing is, 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.

53:07Michael Stevens:Yeah. Ideally, every integer number of photons between zero and 100 and 10, sorry, would be good. Would be good. I'd love to try this.

53:16Hannah Fry:And I want to know the threshold at which you can see them as well. I want to know that threshold.

53:20Michael Stevens:Yeah, I don't know the threshold for seeing something.

53:23Hannah Fry:Presumably they're sitting in the dark when this is happening.

53:25Michael Stevens: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.

53:39Hannah Fry:Yes, indeed.

53:40Michael Stevens:So my answer is one photon.

53:43Hannah Fry:I accept. I accept your answer. and now have a new life's mission to have that experience, the most minimal experience possible. Exactly.

53:53Michael Stevens: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.

54:07Hannah Fry:Sorry, you're watching a movie. You're having how many photons shot at you?

54:12Michael Stevens:Yeah, exactly. So I don't think there's any way we're going to beat an individual photon.

54:18Hannah Fry: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. That's electrons flying around all over the place. You have that power.

54:44Michael Stevens: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.

55:06Hannah Fry: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.

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