Daniel and Kelly’s Extraordinary Universe - Can the human eye see a single photon?

Episode Date: May 5, 2022

Daniel and Katie talk about the physics of color and whether the eyeball is a quantum measurement device. See omnystudio.com/listener for privacy information.

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so you can work, create, and boost productivity all on one device. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then everything changed. There's been a bombing at the TWA. Terminal, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged. Terrorism.

Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back-to-school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast on the Iheart Radio app, Apple Podcasts, or wherever you get your podcasts. Culture eats strategy for breakfast, right? On a recent episode of Culture Raises Us, I was joined by Belisha Butterfield, Media Founder, Political, Political Strategy, and tech powerhouse for a powerful conversation on storytelling, impact, and the intersections of culture and leadership. I am a free black woman. From the Obama White House to Google to the Grammys, Valicia's journey is a masterclass in

shifting culture and using your voice to spark change. Listen to Culture raises us on the iHeart radio app, Apple Podcasts, or wherever you get your podcasts. Hey, Katie, what is your favorite color? I'm going to say blue. Well, you can't just say blue. There's like a million different shades of blue. Okay, I guess say more blue green than a navy blue.

Oh, you can do better than that. Pretend you're like writing copy for a paint catalog. Give us a really descriptive name. Okay, something like seafo mermaid tears or radiant peacock party. Now we're talking. But for this podcast, can you give us like a really good physics-y name for your shade of blue? How about sparsely hydrogenated blue dwarf? That sounds awesome.

For me, I'm personally looking forward to the day when somebody launches physics paint colors, and I can go to the store and ask for a gallon of sultry supernova scarlet. That might blow up your nerve face. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I've never met a color I didn't like. And I'm Katie. I'm stepping in for Jorge this week.

I am the host of Creature Feature. And I like all colors all the time simultaneously. It makes me hesitate to ask what your closet looks like, Katie. Brown. I remember wondering as a kid if there was a possibility for you to imagine a color you hadn't seen before. You know, like if you thought hard enough, could you invent a color in your mind that didn't exist out there in the world? What do you think, Katie? Do you think that's possible?

Yeah, I mean, I used to have that same exact thought thinking of, well, couldn't there be other colors than. what we know on the spectrum and what would that be like? And I would imagine that that experience of this new color might be something beyond just a vision, maybe accompanied by a feeling like a tingling or something. I like the connection of the senses there. Maybe you see a new kind of fruit and it has a new kind of taste and it opens up a whole new branch of philosophy. Now we're getting into synesthesia territory. Exactly. But this question of color is it really deep and fascinating one because it extends all the way from philosophy, like what is it like to experience a color down to biology? How does the human eye respond to photons? And finally, to

physics. What exactly is a red photon and a blue photon and a green photon? And that's exactly the kind of topic we love to dive into on this podcast. So welcome to the podcast, Daniel and Jorge explain the universe, in which we ask the deepest, the reddest, the bluest, the whitest, the darkest, questions about the nature of the universe. We ask questions that go all the way from your eyeball, deep, deep down through your brain and all the way into your soul, whether or not it exists. I'm so excited to smash biology and physics together and create some kind of new particle. And we're very excited to have you with us today, Katie. Thanks very much for filling in for Jorge. We always love talking about the biological side of physics with you. Yes, the squishy side of

physics, as I call it. And today we are going to be talking about. talking about all the things that we can see because vision plays such an important role in how we perceive the world and how we imagine it. I think if you closed your eyes and tried to imagine the room around you, probably the image in your mind would be an image. It would be something built out of your visual perception of the world around you. That might be different, of course, for blind listeners or other people who don't have strong vision. There's also people with Afantasia who can see. but they actually don't think in pictures. They don't imagine pictures.

That blows my mind that you can see things but that you can't have images in your head. Yeah, there's just so many different types of ways that humans can perceive the world. Exactly. And that means that there are so many layers to these kinds of questions. Like what is happening at which stage?

How is your eyeball seeing a photon? How is it sending a message? How is that message interpreted? How is that message experienced by your brain? How is that used to make decisions and think about the world? But it's clear that eyeballs are an important part of understanding the world around us. And not just the eyeballs that we have in our skulls,

but the other kinds of eyeballs that we build to look at the universe, our X-ray telescopes and our infrared telescopes and our gamma-ray telescopes, they can see photons that are well out of our visual spectrum. And so understanding how photons work and how we see them is pretty core to understanding the universe. See, not just with the eyes in our skulls, but the eyes in our hearts and also the eyes attached to a giant telescope. It would be really weird if we built the telescope that was literally made out of human eyeballs

like strung together. That sounds pretty good. I'm into it. That's a real biology, physics collaboration right there. But the eyeball does touch on a lot of deep questions. There are questions there are philosophy. There are questions of biology. And there are also questions of quantum physics.

And so today on the podcast, we're going to be peering into all of these questions and asking, Can the human eye see a single photon? So what do you think, Katie? Is the human eyeball a quantum device? I mean, this is a really interesting question, and I really love how people are kind of thinking about this in terms as like,

well, it seems really tiny. Wouldn't you need to magnify it? Wouldn't you need to make it bigger? how can you see one wavelength? And when you think about the eyeball, it certainly seems like the eye is too big, right, to see just a little tiny particle. But I think once you examine inside the eye and see how teeny, tiny and delicate some of these cells that actually allow us to see makes me a little more convinced that maybe we could see something even as small as a single photon. on. Yeah, I think when people think about quantum mechanics, they think physics and then they think about mechanical devices that humans have built. Optics and semiconductors and specialized materials, things that you don't typically associate with biology. But of course, our bodies are built out of molecules and atoms. The same building blocks as what's in that crazy quantum lab in the basement

of your building. And so in principle, it's possible for the human body or anybody to have quantum effects and I think there's a whole burgeoning field now of people studying quantum biology things that happen in bodies that rely on quantum mechanics and so in principle as you say it's possible for biological cells to develop capabilities which rely fundamentally on quantum mechanics I mean the fact that we can step outside and get burned by UV rays from the sun is pretty compelling evidence to me of the direct impact of physics even at the very very small scale on our bodies. And I deal with that all the time. Exactly. And so I went outside and walked around the campus of UC Irvine and asked a bunch of random students I ran to and one chemistry

professor if they thought that the human eye could see a single photon. That is, if you were in a dark room and I shot one photon at your eyeball, would you see a flash of light or not? So think about it for a minute before you hear these answers, do you think the human eye could see a single photon? Here's what folks on the UC Irvine campus had to say. There's a neurological sense and also a physical sense. I mean, if physical sense doesn't happen, then you will neurologically, you cannot sense it. So it's like, I think it's physically, it makes sense. Yeah, I mean, and photon is absorbing quantum in a quantum sense.

I think a single is possible. I just don't know the threshold. I don't think so, no. I think so, why not? Probably because it's too small. I don't think so. Just because the size of it, like even the human eye, it's only capable of seeing so much. So you'd even need to magnify it or make it a little larger.

No, I don't think so. Why not? I just think it's too minute for the eye to like distinguish. Yes. Why is that? I'm just guessing, honestly. I feel like it's, you can't see one single wavelength, right? So how many photons?

do you think it takes before your eyeball response? I don't know if it's like an amount of photons. Maybe it's like a unit of energy. One, probably not. Isn't light just photons? So yes, you can see it? So you think you could see a single photon? Oh, a single one.

Yeah. No? No. No, why not? I'm not sure. I don't feel like you could see it. Well, I don't know if my brain could register it, but I feel like a rotter cone could pick it up.

But it might just like consider it some kind of like random, you know, burst of something or some neuron misfire or something. So there's a lot of nose there. What do you think, Katie? I understand the skepticism because it really does seem like that would be too small of a stimulus for us to really pick up on why would we need to see a single photon. Humans aren't necessarily known for being the most sensitive of animals, not to be mean about it, but we don't have the best sense of smell.

We don't certainly don't have the best sense of vision or the best hearing. So why would we be able to see a single photon? And I really like people kind of questioning whether even if we could pick it up at the cellular level, whether the brain could even process that. So I understand the skepticism, but I do think that the fact that the inner workings of the eyeball are so incredibly small and precise. I'm somewhat leaning towards, yes, we might be able to see a single photon. Yeah, I was a little disappointed that these folks didn't have more confidence in our eyeballs. Though I think you're right. I don't understand why we would need to be sensitive to

a single photon. It's not like we typically go hunting in a dark for rodents, right? Like if an owl could see a single photon or an eagle could see a single photon, I get it. But a human like We're pretty much napping at night. Anyway, I was interested in learning more about the fundamental science at the heart of this process where photons are absorbed by cells in your eye and converted into signals. So I reached out to an expert we have here at UC Irvine. Rachel Martin is a professor of chemistry and she studies vision. And I asked her to share some thoughts with us about why vision is cool.

I'm Rachel Martin and I'm a professor of chemistry and molecular biology and biochemistry at UC Irvine. Great. And I understand that one of the focuses of your research is vision in the eye. So tell me, what do you love about eyeballs? Why devote your career to the study of vision? What entrances you about vision? Vision is amazing. I mean, for one thing, humans are very visual animals.

This is mostly how we perceive our world. It's one of the most important senses for us, which is not true for a lot of mammals. A lot of mammals experience the world through smell. But for us, a lot of it is about seeing things, and that's kind of our main detection system for the world. And I think that's really neat. And I also think it's pretty amazing that vision works at all. There are a lot of kind of happy accidents of physics that have to be the way they are to enable vision. I mean, so for instance, the human visual range overlaps almost exactly with the kind of the hole in the absorption spectrum of water.

So, you know, water absorbs at a lot of frequencies, but there's just this tiny little window that where it doesn't. And that's where we're able to see. And I think that's really neat. All right. So Rachel is clearly very excited about the eyeball. She has devoted her entire research career to studying the human eye. And I love that about science. That every time we're making progress, it's because one person has decided this is the most important question in the universe.

and I'm going to devote my entire research career to studying the lens in the human eyeball. I love it. I mean, the eyeball is so bizarre that it even works. It has caused things like people to be skeptical of evolution thinking, well, the eye is such a complex device. How could it possibly naturally evolve, which I'm sure we're going to talk about? but I am not surprised by the fascination with eyeballs because they're so clever in how they work and it is not straightforward at all.

I totally agree. And so let's dig into it a little bit. Since you're the biologist of the pair of us, why don't you give us a rundown on how the eye works? What are the essential elements of it? What comes into play when you are seeing a photon? Right. I mean, the eye, weirdly enough, is basically like a little camera.

So if you've ever played with like a big lens or magnifying glass or something, you know how you can like focus the sun's light into a little dot, burn some ants with it if you're evil. So the eye actually has lenses in it that can have light come in. It refracts and focuses on a spot. So first outside of your eye, you have the cornea. So that's the thing that you may directly place a contact lens on or if you're unlucky, you can scratch the cornea and that's really bad. But that's basically this, it's a convex shape. And so light comes in and the light is refracted in this cornea.

And then it goes through the pupil. And the iris can constrict or expand, allowing more or less light in and just like the aperture of a camera and after that you actually have the lens which is like the focus on the camera you have these two little muscles that attach to the linch and can like pull it

or kind of relax and let it contract and that allows you to focus the light so if you've ever tried to focus on something and then it's blurry at first and then it comes into focus that's your lens actually squashing and stretching in order for you to focus on something and that will focus the light onto the retina. So that is the back of your eye

and on the retina are these photosensitive receptors. So there are rods and cones. There are three kinds of cones and only one type of rod and it is through the way that these rods and cones detect this light that you can see everything from colors to shapes to distances. It's really incredible.

And then we haven't even talked about how it gets to the brain. You have a bundle of nerves. Basically, do you do good cable management, Daniel? And my cable management is a disaster, actually. Yeah, I'm no one to preach about cable management. But our eyes bundle all of these basically cables that run, all of these photosensitive cells and then into the optic nerve at the back of the eye. It actually creates a blind spot in our eye because there's this cluster of nerves that can't

perceive any light, but they're just transmitting the signals from these cells. And that runs all the way back to the back of our brain and the occipital lobe. So it is not a straightforward system. it works really well. It's really amazing. I like the way you've made an analogy to a camera. So we have, just a review, the cornea bends the light onto the lens. The iris decides how much light goes into the eye.

The lens itself focuses light on the retina and the retina is sort of like the film or the digital sensors that form of the image and then translates it into these neurological signals. And that step to me is super fascinating because the thing that you observe, the thing that you experience are just the messages along the nerve itself, right? Your brain, your subjective experience, you don't observe the photons themselves. You just get these messages in your brain plays that in your head sort of as an experience of color. But we'll dig into that in a minute. I think that's really interesting. And it's fascinating to me that the way we constructed a camera is so similar to the way

the eyeball worked. Do you think that we stole, we cribbed from the eyeball? We're like, hmm, this is a good design. Let's do it like that. Or do you think it's an example of like, virgin evolution of technology and biology? You know, that's a really good question. I don't know, but I could believe either because we have been studying the eye for many years, even long before our sort of modern understanding of biology. So I could definitely see there being some inspiration from the eye, but it could also have just been from coincidence because, you know, the very earliest cameras, the camera

obscure where it's just basically a little pinhole where light comes in and it's such a small pinhole. You have this refraction of light that turns up an image that's upside down in the wall behind it. I could see that having been just discovered kind of by coincidence or accident. And then we essentially reverse engineered the eyeball only in a mechanical sense. And again, what's really cool is that with the camera, it's the same thing. The image is upside down in the camera. It's the same way in the eye, actually, the image as it's projected onto the retina, that area where all of the photoreceptors are, is upside down. And it flips because our brain is able to flip that image right side up.

So there's so many middlemen happening in our brain to interpret what we're seeing. You can't always trust your eyes to be exact reporters of reality. And there's another layer of similarity there because the evolution of the technology of the camera, as you say, we started with pinhole cameras and then we got fancy lenses. Also mirrors, doesn't it, the evolution of the eyeball in biology, right? Like, we think that early eyes actually were more like pinhole cameras. Can you take us through roughly, like, how do the human eye evolve? Because this is an argument, some creationist use some time to say that evolution can't possibly be reality because how could you evolve the eyeball. Can you

take us through sort of the rough picture of how the eyeball evolved? Yeah, so first of all, it's funny to me about people who are so skeptical of the eyeball being able to evolve. It's like not only has it evolved, it's evolved multiple times. So actually cephalopods, including octopuses, completely independently evolved their eye from almost every other animal on the planet. And so this is not just some kind of ridiculous luck. it does make it seem a little bit more inevitable. So when we were basically flatworms, we would have kind of just like a cluster of photosensitive cells

that could detect light or dark, not really images, just kind of like, hey, that's light, I go towards it. Or this is dark, I go towards it. So at this very basic level, something like a flatworm and the, you know, very early ocean could go up towards the sunlight or recede back down into the darkness or go towards a spot obscured by the sunlight, maybe something to eat or something, but it couldn't form like an actual image.

But already, that's a huge step forward. That's recognizing that the universe around you is filled with useful information, information you were literally blind to before you developed this capacity. And now that you can sense the fact that there are, oh, a bunch of photons over here and not a bunch of photons over there is useful. That always makes me fantasize that we might be able to like develop some new kind of cell that's sensitive to dark matter or neutrinos or something. You know, the form of the basis of seeing the universe in another way, right? Like, because we've done that before. We've developed the capacity to see a previously invisible part of the universe.

Anyway, that just gives me hope, you know, for future evolution. If we live a few more million years, I think it's definitely possible. All right. So how do you go from having cells that can sense the existence of light to, you know, forming images and watching TV shows? Well, first, there's something important that's missing with just this cluster of photoreceptors. It's that you don't necessarily understand what direction something is or what's up and what's down. And so if you actually recede those cells into like a little cave, like a little socket,

it matters which way the light is shining because you've limited the entry point of the light. And now your brain can tell whether the light is coming from up or from down. And then from there, now that you've got basically an empty eye socket with receptors at the back of the socket, Once you start to close that opening to the socket, now you're getting that pinhole. So we're getting to the camera obscure part where you will get the light. Not only are you able to better tell what direction the light is, not just up and down, but maybe side to side, this 360 kind of understanding of where the light's coming from. You could also start to form very simple images because now you're able to actually bend the light.

such that it can become refracted and hit the back of your eye in this image. I think that's a little counterintuitive that you go from, here's a slab where I can see photons. And the step forward is to hide that, is to like bury that inside you so that it only can be hit by a few photons that happen to pass through like a little hole you make. That's counterintuitive because you're getting less information, it seems like, because you're getting fewer photons.

But you're right, it's more information because you're restricted. the photons. So now you can tell this photon must have come from up or must have come from down and based on where it hits on the inside of this cavity, as you say, you can form an image. That's a pretty cool technology. I don't know if I would have thought of that myself. I mean, that is one of the fascinating things about the senses. So much of it isn't just the ability to sense something, but the ability to prune out information, to restrict the information you're getting so that your brain can make sense of what is happening. Because if you're just getting all the information all the time, you can't differentiate it.

And so you're not actually going to form a clear picture of what is happening around you. And I think you're right. It's that differentiation that's key. If you're seeing photons on one side of the cavity and not the other, the relative intensity there is what is telling you, oh, this source of light is only coming from that direction and not from the other. So it's about like comparative processing of those signals, not just are there photons or not, but like looking at where you're getting the signals and where you're not getting the signals and using that to form like a mental model of what's going on out there in the world.

Right. And so as we're tracing this evolution of the eye, now we've got this cave, right? This eyeball-shaped cave with this little pinhole now allowing us to see better direction and maybe start to form these very blurry images. and then you can seal that off, right, to protect the photoreceptors. And however, you need to make sure that that inside of the eye is still fluid filled. We are, remember at this point, we are marine animals, probably some kind of very early predecessor to some species of fish. And you can't just have an air pocket as a fish.

You typically want to have your organs filled with fluid. Since your eye has been evolving to be able to refract light in water, if you have this sudden air pocket, that's not going to allow you to see. So you actually have this fluid-filled eye now of this vitreous humor, and we actually still have that in the human eye. It is not just an empty balloon filled with air. It is filled with fluid. And then all you need to start to get to,

to the eye of a more complex animal that can see more detailed images is a lens and a cornea. So once you've started to get this lens and that cornea, you're able to actually focus and make these specific images. And then from there, you have so many possibilities opened up to you. Even though animals sense of vision can be really different, the thing that they all share is they're really useful in terms of their specific evolutionary niche. It's amazing that we can reconstruct this story and that along with this story gives us a clue about our own history, seeing things underwater from our great, great, great, great, great, great, great, great, great, great, great, great, great, great ancestors. And it also helps explain why some eyeballs are different from other eyeballs.

Here's a nice little story from Rachel Martin explaining why fish eyeballs look different from ours. When a photon hits your eyeball, you know, first it has to go through the, the corny and the lens. And those things are really important because that's where we get a lot of the focusing power. And particularly in terrestrial organisms, you know, a lot of the focusing power of your lens actually comes from that air water interface at the cornea. And it's something that we don't necessarily think about. And I know when I was a little kid, you know, I really liked swimming.

And I was really sure that if I practiced enough, I would be able to see underwater without goggles. And so, you know, I would try and try and try. And I thought it was just, you know, I have to keep trying and keep practicing. But, you know, my eyes are not optimized to work underwater because I'm a terrestrial organism. And so, you know, so no matter how much you practice, you need that air layer in front of your eye for the, you know, for the light to be properly refracted to make an image. Our eyes are optimized that way because, you know, the, the lens is kind of flattened and also, you know, the distribution of proteins, structural proteins inside the lens is optimized to work with that airwater interface at the cornea. Whereas if you

look at a fish eye lens, that's usually very spherical. And that's because in a fish lens, there's no air, of course, the fish is underwater. And so the proteins in that lens have to do all the work. And this is a really interesting thing to notice next time you're at like a public aquarium, you know, find the biggest fish you can and look at their eyes. You can usually see the lens. It's, you know, it's often, you know, pretty easy to see in the fish eye. So you can see the shapes. All right. So I think now we have an understanding of what the eyeball is and the basic mechanism and geometry of it. I want to dig into the physics of color, what color means and how our eyeballs see these photons. But first, let's take a quick break.

Ah, come on, why is this taking so long? This thing is ancient. Still using yesterday's tech, upgrade to the ThinkPad X1 Carbon, ultra-light, ultra-powerful, and built for serious productivity, with Intel core ultra-processors, blazing speed, and AI-powered performance. It keeps up with your business, not the other way around. Whoa, this thing moves. Stop hitting snooze on new tech.

Win the tech search at Lenovo.com. Lenovo, Lenovo. Unlock AI experiences with the ThinkPad X1 Carbon, powered by Intel Core Alter processors so you can work, create, and boost productivity all on one device. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA.

Apparently, the explosion actually impelled metal glass. The injured were being loaded into ambulances, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and order, criminal justice system is back. In season two, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System

on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but, I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor, and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet.

So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the Iheartedly. Radio app, Apple Podcasts, or wherever you get your podcast. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Whitmer, Jody Sweeten.

Monica Patton. Elaine Welterah. I'm Jessica Voss. And that's when I was like, I got to go. I don't know how, but that kicked off the pivot of how to make the transition. Learn how to get comfortable pivoting because your life is going to be full of them. Every episode gets real about the why behind these changes and gives you the inspiration and maybe the push to make your next pivot. Listen to these women and more on She Pivots now on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

All right, we're back and we're talking about eye. We're not eating eyeballs. We're talking about shooting photons into eyeballs in thinking about how they respond and what that means and what your brain tells you about the signals that crawl up the optic nerve. So Katie, what color are you surrounded by right now? What color are you looking at as we talk about this? I'm actually looking at a very lovely shade of teal because this apartment has painted in beautiful colors. The owner of this apartment, really loves teal and so almost all the walls are painted teal it's it's very nice i'm not complaining it's just you can really tell he loves he loved teal the the guy who painted this apartment my daughter hazel loves teal also and she loves a very particular shade of it and for me i'm like well it's all just sort of this blue and she's like oh my god it's not just all blue like this is blue and that's teal and this is something else and you monster it's interesting to me that we do have this very

rich experience of color you know that we have a very different reaction to photons of one frequency and another so i think it's important that we dig into what that means what the physics is of it why it is that some photons give us a different reaction from other colors when i was a kid and i learned there were only three different wavelengths of light it was really bewildering because It's like, okay, how do we see so many different colors than if there are only these three wavelengths of light? You would think we would only see like red or green or blue. Yeah, that's really interesting. And of course, there are more than just three wavelengths of light, right?

There are an infinite number. So in the end, what is light? Light are these little packets of energy, these photon, these quantized little wiggles in the electromagnetic field. So the electromagnetic field is something that fills all of space. The whole universe has the field in it. And when there's energy in the field, it can travel through that field. It can sort of like wiggle. Imagine like a guitar string and you're sending a pulse along it.

That's a photon. It's a wiggle in the electromagnetic field. And it's a very special kind of wiggle because it's self-perpetuating. You know, it can exist and then it can zoom across the universe and exist somewhere else. It doesn't like diffuse or spread out. It's like persistent and discrete. And that's in the end what we call a particle. All particles that we see and experience in the universe are these little discretized

wiggles in quantum fields. Now, people think about photons as photons, as quantum particles, as discrete, and it's true that they are quantized, like you can have one photon or two photons or 74 photons. You can't have one and a half photons or 1.72 photons. You can only have integer numbers of photons. For those of you curious about the physics of it and how it was discovered, we have a whole podcast episode about the photoelectric effect and how Einstein realized that photons came in these little numbered packets.

But there's another really important fact about each photon, and that's its energy. And that's something that is not quantized. So a single photon can have any energy, meaning it can have any wavelength. So typical visible light is like 400 to 650 nanometers, but there's an infinite number of different wavelengths between 400 and 650. Just because the photon is quantized doesn't mean its energy levels are quantized. So if there's an infinite number of frequencies, would that mean there are an infinite number of colors? Great question and a really deep philosophy question.

And the way I think about it is that photons don't have colors. Color is your brain's interpretation of the signals that it's getting. There are an infinite number of different photons with an infinite number of different frequencies, right? But those frequencies don't necessarily all have a color assigned to them. The color is something that your brain does, assigning it to the response of the optical nerve. But in theory, if you get enough living organisms that have some kind of eyeball and some kind of brain that can detect some kind of frequencies, we get enough, including all life on Earth and probably a bunch of aliens,

could we all perceive infinite colors? It's a great question of neurology and philosophy that I don't know the answer to. And, you know, we don't know the answer to even more basic questions like, is the red that you perceive the same red as I perceive? It's a really old question that people have been asking since they've been smoking banana peels around the campfire and one that we still don't have an answer to. And I think it's a great kind of question because it shows us sort of the limitations of science. People think about science as maybe all-powerful ways to reveal the nature of the universe.

And science is very powerful, but not every question is necessarily a scientific question. If you can't conduct an experiment or come up with a theory that helps you probe it, then science isn't necessarily the best tool. And so the question of like, is your red the same as my red relies purely on our subjective experiences, which we can't translate from my brain to your brain. So we have no way of knowing. I can't, like, measure the redness of your experience. So some of these really deep and fascinating questions are frustratingly just past the fingertips of science.

Well, I have a really easy answer to that. And that my red is a fire truck red, so question solved. Exactly. And when we point to a fire truck and we say that's red, we, of course, don't know what you're seeing and what I'm seeing. If that actually looks like teal to my daughter, we'll see. Let's go through a little bit, the mechanism of how your eye can tell. which photons have hit it, like which frequencies of light are hitting it. But before we do that, let's just remind ourselves, like, why are there different frequencies? You know, why is it that

the fire truck is red and blueberries are blue and that leaves are green and the sky is blue? Why are there these different colors in our world? And the fundamental level, the reason from physics is two. One is that sometimes things get hot and hot things glow at different temperatures. Like the reason that we get light from the sun is because the sun is super duper hot. And as things get hotter, they tend to emit light in higher frequency photons. So things that are cold emit light in the infrared, things that are warmer emit light in the visible, like the sun. Things that are super duper hot, like accretion disks around black holes, emit light in the ultraviolet or even up to the x-rays and gamma rays. These are all just different frequencies along the

electromagnetic spectrum. Is this why when you have a fire, you have sort of a really, really hot fire, it's going to be like white hot versus orange hot and blue hot. Exactly. And if you were an amateur at home blacksmith, you know that white hot and red hot are different temperatures of your steel. That's something people have known for thousands of years before we understood the physics involved. Right? That's like folk physics. It's pretty cool. And the other way that color comes into play physically isn't how materials absorb and reflect color. Like if you're looking at an object and it's red, it's because you are getting red photons. You're getting photons of a frequency that your brain assigned to the color red to your eyeball from that object. Now say it's being

hit by white light. Light from the sun tends to cover the whole visible spectrum. The reason you see it as red is not because it's absorbing red, right? It doesn't get red from the red photons. It's because it's absorbing everything but red and only the red light is getting reflected to your eyeball. Right. A general thing with our senses is anything you sense from hearing to vision has to physically hit you. Like your eyes have to be physically smacked by photons for you to see them. Your ears have to be physically smacked by sound waves. It's like, you know, our sense of touch, we understand you have to physically touch something to feel it. But it's the same thing with our eyes and our ears, even if it doesn't feel that way because you don't have to press a leaf

to your eye to see that it's green. But you are technically feeling that leaf with your eyeball because the photons that are bouncing from the leaf are hitting your eyeball physically. And that's exactly why things that look green are things that reflect green, not things that absorb green. Like if you eat a bunch of blueberries, it makes you blue on the inside. You might think that's the same way it works for light. But the reason blueberries are blue is because they reflect blue. They don't absorb blue. Isn't that why black things like a black shirt gets really hot in the sun because it's not that it's absorbing black, which you can't really absorb black because black is just the absence of light. It's absorbing that white light, which is a really high

energy light, which makes it hot. Yeah, and that's why in the desert people wear white, because white is more reflective. It doesn't absorb as many photons as black does. And so that's why snow, for example, doesn't melt unless it gets dirty. When snow gets dirty, it absorbs more light and then it melts. But pure crystal white snow can reflect a lot of sunlight and doesn't melt as quickly. And you can actually get sunburned from the reflections of snow because it's reflecting a lot of light at you.

And I learned this while skiing. You can get sunburnt on the bottom of your nose. Yes, it's fun. Exactly. So now let's trace those photons into your eyes. You say they go through the cornea and the iris and the lens and they hit the retina. And what's going on on the retina that allows you to see these things? Well, essentially, you have a huge number of sensors, millions and millions of these sensors of two kinds.

You have the rods and you have the cones. Rods are really good at seeing small numbers of photons. They're good at seeing in the dark. But you only have one kind of them. And so they basically just say, like, yes or no. The cones, however, you have three kinds of those. And you referred to them earlier as blue, green, and red. And the reason we call them that sometimes is because they're good at seeing different

frequencies of light. It's not like they can only see one frequency. There's sort of like a width to it. If you look at a graph of like how likely one of these cones is to see a photon of a different frequency, you see that the ones we call S or for short wavelength, the ones that see blue light peak at about 420, but they can still see 450 nanometer photons. They're less likely to see 500 and almost impossible for them to see 600, but you know, it never goes to zero. And the red ones, the ones we call long cones, those peak at about 560 nanometers, but they could still see a photon at

450. It's just less likely. So they're like good at different kinds of things. They're more likely or less likely to see photons at different frequencies. And that's what your brain is pulling together to say, oh, we're seeing something that's red because it tends to be lighting up the long cones and not the short cones. Or, oh, we're looking at something that's teal because it tends to be lighting up the short cones and not the medium or long cones as much. So there's a lot of interpretation there. Just like back when you were talking about how the eye evolved, you're explaining how we have a cave and we're deducing where the light came from based on where we're seeing, light and where we're not. Now we're deducing the color of the source of that light based on

which cones are lighting up and which cones are not. And it's the relative excitement of the cones. Like if something's getting really lit up versus a little bit lit up, that allows us to tell the difference between a bluish green and a greenish blue. Exactly. And there's a lot of analysis there that's going on in your head. And so I was curious like how this actually works. Why is it that this cone is better at that frequency of light and this other cone is better at the other frequency of light? What is the real mechanism of it? So I asked the professor Martin to explain to us what is actually happening when photons hit these sensitive cells and here's her explanation. The rods have redopsin and then the cones have they're just called opsons. And these

are proteins that undergo a conformational change when a photon hits them. And so the photon hits the protein and then its confirmation changes. And that sends a signal inside the cell that is what transmits the signal for that detection to our brains. I have an understanding of a protein is like a little molecular machine. And you're saying it can absorb a single photon and it like chunks over mechanically from like one physical shape to another physical shape and that reflects having absorbed that energy. And then something detects that the protein has shifted. It's like a little lever has been pushed by the photo. Yeah. Isn't that amazing? So it undergoes a conformational change and then something on the other side of the cell

transmits a signal and then that's how your brain detects that you saw that photon. And the rods and cones are different in the proteins that can absorb the photons? So they're going to absorb photons at different frequencies because it's a different protein? They absorb photons at different frequencies. And the reason that they have that difference in absorption is all about the protein. So it's about the specifics of the, you know, the amino acid sequence and the structure of the protein. And so why is it that the cones can't respond to a single photon? Like do they need five or seven? Is it because that lever on the protein is harder to push? So yeah, the rhodopsin is the most sensitive one, but like that's kind of a weak answer,

right? Like why is it the most sensitive one? I'm not actually sure. Well, speaking of mysteries of the eye, what are some sort of frontiers of research? Is there any unknown physics that's happening in the eye? Are there any processes that go on that we don't understand? Tons. So for one thing, a really active area of research is just how do the the rhodopsins work because this is one of the fastest known processes in biology, like the early steps of what happens when a photon hits that redopsin. So I mentioned that it undergoes a conformational change. So it actually has a small molecule that's bound inside the protein. It's a retinol, and it changes confirmation. So, you know, it goes from like a bent confirmation to a straight

confirmation. That's what makes the protein undergo the conformational change. But the first step of that happen really, really fast. And so you need a fast laser to be able to study it. And you also need to do the whole thing in the dark. All right. So I thought that was super cool that basically we have these proteins inside these rods and cones that change configuration when a photon hits them. My mental picture is like we have a little machine there. I think a proteins is little machines. And like the photon like shifts a lever. It like, you know, flips a switch almost physically. That's incredible. Do you like to watch videos of Rube Goldberg machines?

I do. I do. It's like when you have a marble hit a thing and then that releases a domino and then soon you've got a teapot boiling and then that pops a balloon and then a hammer lands on a lever. That's how I like to visualize

a lot of these complex cellular processes and that is very true of how these rods and cones work. So like you have a photon literally hit like a little lever, it's changing the molecule's shape which triggers a cascade of responses inside of the cell, but you can visualize like a Rube Goldberg machine

or like a domino effect. And it's not just something that happens once and then somebody spends like three hours setting it up again. It not just responds to a photon and then unresponse and gets ready for another photon sometimes like milliseconds later, right? Rachel was saying, This is one of the fastest processes we know of in biology.

Yeah, and it is interesting because as fast as your cells can be in responding to this, you can also overload your cells if they're too sensitive, which is also happening, it's not just happening on the cellular level. This is something that happens inside the brain. So as you're getting this information, right, because these sensors are sensing it and sending that information to the brain, but it doesn't just go directly to brain and say, hey, look at this color, it has to activate your neurons.

And then that's a whole other Rube Goldberg machine that happens at the neural level and you may have some threshold of activation for these neurons. And so you get some really weird things that happen with this interplay between the sensitivity of your photoreceptors in your eye and the sensitivity of your neurons

in your occipital lobe, the vision center of the brain. And that's why if you like stare at a bright color and then you look at the wall, you see this like weird after image. So it's a really interesting, I guess, just thinking about these little tiny machines working really hard. But it's happening so quickly. It's the kind of thing that makes you amazed that it ever works and suddenly doubt that it will continue to. You know, whenever I learned how delicate these things are inside my body, I'm immediately terrified.

Like, oh my gosh, how has this thing been going for so long? Isn't it about to just fall apart? But it's amazing. It really works. Something was really interesting to me was thinking about, why the cones are sensitive to different frequencies. And it's because they have different proteins inside them, which are better or worse at absorbing photons of different frequencies.

And why the rods are more sensitive than the cones? Why make cones less sensitive than rods? And why make rods more sensitive than cones? But I think the answer is just diversification. Like you want to do two different things. One is you want to be able to see in low light conditions. And the other is you want to be able to see color. So you can spot that fruit or spot that predator.

And being able to see in low light conditions is actually somewhat mutually exclusive to being able to see in higher light conditions because that requires your rods to be really, really sensitive to light. And the more sensitive they are to light, the more hyperactive they'll be when you have too much lights. So your rods aren't that useful in high light conditions, but they're really useful in low light conditions. That's why if you've ever stepped from a bright theater lobby into a dark movie theater, you can't see where you're going and you fall over and you land in a puddle of Coke and popcorn, it's because your eyes haven't adjusted to the dark. But then as those rods kind of like calm down, they adjust to that low light of the dark room. And you can actually see in the dark.

And so, yeah, the rods being hypersensitive. is useful for us to be able to see in low light conditions, but then we have the cones that allow us to see in brighter light and not only that, but differentiate color. And the cones are in the very center of your eyeballs. When you're just looking straight at something, you're good at seeing colors. And the rods tend to be distributed in the other parts of the eyeball,

which means that like your peripheral vision is better at seeing in low light conditions than your central vision. So, for example, if you're an amateur astronomer and you like looking at the night sky, Sometimes you might notice a star out of the corner of your eye. If you then turn your eyeball to it, it can disappear. And that's why, because the center of your eyeball doesn't have a lot of rods. So if you want to observe faint things in the sky, don't look straight at them or they disappear.

It seems almost magical. Which is interesting evolutionarily, right? Because you think of when we would need this low light vision, it probably would be at night when we're trying to avoid getting eaten by a nocturnal predator. So being able to see movement out of the corner of our eye and flee or hide is really important. Whereas in lower light, being able to like focus on something specifically, well, this is when we're sleeping. We don't really need to hunt at night. We're not nocturnal predators.

So our eyes are optimized for being able to see somewhat in low light so we can protect ourselves, but not optimized for being predators, nocturnal predators. That's right. And some of us have a different kind of eyeball than the rest of us. Most of us have three kinds of cones. So we can see light that peaks in three different places. But some people have special eyes. They are called tetrachromats. And they have a fourth kind of cone, which means they have another kind of thing in their eye that peaks at a different place. It peaks bluer than blue on the spectrum. And this to me was fascinating when I first learned about. I thought, wow, maybe they can see a. another kind of color. Maybe this is like my childhood fantasy. There's like a super blue that they can see in their minds that we can't see. But unfortunately, it's not like that. What it means

is that they are better at distinguishing between shades of blue. They're like my daughter who can tell teal from slightly not teal because they get more information about the relative intensities of those photons. And so they're better at distinguishing colors. They can't necessarily see or experience any new colors. And I mean on the opposite end of the visual spectrum, you have people who are colorblind or partially colorblind. And it's not that people necessarily only see in gray scale, but they can be like blue-green colorblind where they have trouble distinguishing between blues and greens. So yeah, there's this whole range of human vision. And it makes me wonder, like your earlier question, whether there's a lot of difference

in vision amongst people who are not necessarily tetachromats or people who people who have colorblindness, but just like whether vision comes in a spectrum like many other human experiences. Right, because people experience chocolate differently, right? Like some people actually enjoy white chocolate and aren't just pretending. You know, it's a mystery to me. They're just wrong. Thank you. Thank you. People respond subjectively, very differently to stimuli. And so it would make sense if people responded differently to different kinds of photons. And we know, that out there in the animal kingdom, there's also an incredible wide variety of eyeballs, right? Different kinds of critters have evolved to different scenarios where they need more or less

kinds of vision. For example, we know that owls have incredible vision, especially in low light because they have an enormous number of these rods. And geckos have really good actually night color vision so that they can survive. Plus, they have a tongue that lets them clean their own eyeballs, which I think is pretty crazy. It's adorable. Yeah. No, it is one of the most fascinating things in evolutionary biology, the difference in vision. It's mysterious because we cannot objectively measure what an animal's experience of vision is.

We can only guess basically based on their eye structure and based on their brain. So when you research it, it can come to some really surprising results that are hard to conceptualize. So for instance, rats actually have double vision and can somewhat move their eyes independently of each other. And we discovered this by putting little teeny tiny high-speed cameras attached to little hats on rats and looking at their eye gaze. And they found that their eye gaze indicates that they prioritize keeping a view of the sky as well as their surroundings so that they can avoid that owl who has that great night vision.

So rats are running along the ground with one eye up and one eye down. That's crazy. Basically, yeah. And what's so weird is that researchers think that these fields of vision in either eye are too different to be fused into a single image. So there's a possibility that rats basically have like two TV monitors in their head that they're both keeping track of. And unlike us, where we have basically one. combined image. And I mean, just the ingenuity that animals have when it comes to eyeballs can be completely baffling. Like, this is a funny named one, but it's a brown-nosed spook fish,

which is a species of barrel-eye fish. It's this very weird-looking fish. It's not very big. It's like about seven inches, maybe like the length of your hand. It's got a transparent body. It looks kind of creepy like some kind of ghost of a fish. And it is actually only vertebrate that is known to, in addition to a lens, have an actual mirror in their eye. So the way that animals can have mirrors is they use guanine crystals to form a mirror because a guanine crystal is a protein that an organic animal can produce, but its structure is reflective. And so this is actually also used in scallops.

Scallops have eyes. In fact, they have hundreds of eyes, which is a fun thought next time you enjoy your scallop. But, yeah, so this brown snout spook fish, it has a normal eye that looks upwards. And then, in addition to that, it actually has an annex eye, like just stuck to the side of it, like a side view mirror on a car.

It has a mirror that allows it to look downwards. So this fish can look both upwards and downwards at the same time. Well, that's a very useful kind of side eye. One of my favorite kind of eyeballs out there in the world is the mantis shrimp. The mantis shrimp doesn't just have three or four kinds of cones. They estimate it might have 15 or 16 different kinds of cones in its eye. And when you first read about that, you think, wow, the mantis shrimp. strip must have like a really vibrant visual experience. It's like Mardi Gras every day for the

mantis shrimp, right? Well, it turns out the mantis shrimp actually isn't any better than humans at distinguishing colors. They do these experiments where they train the shrimp, like go to food if you can see the difference between the colors. And they aren't any better at distinguishing colors than we are. We only have three kinds of cones. And the reason is that the mantis shrimp basically has much more specific hardware, but it doesn't have the processing power to really take advantage of that. Their brains are really simple. So we have like simpler hardware, only three sensors, but very complicated software to interpret and analyze that. And the mantis shrimp has made like a different optimization, like go all in on the hardware and have really simple software to

interpret it. Pretty fascinating. Right. And the reason for that is that the mantis shrimp is optimizing the speed of its site rather than sort of the detail of its perception. So it's thought that with less software and more hardware, you can actually sense something much more quickly, because the speed of light is quite fast. And then if you can have these photoreceptors pick up on that photon really quickly because you just have so many,

then even if your software is relatively simple of just like, hey, there's a thing, if it's quick enough, it will have incredible reflexes. That makes a lot of sense. The speed of light is faster than the speed of brains. I definitely don't think at the speed of light. Is that a shark or is that dinner? Oops, too late.

I'm being eaten already. Well, one of my favorite stories about color and animals comes again from Rachel Martin who told us a story about how birds that seem sort of boring and blue turned out to be actually ultraviolet and spectacular. So here's Rachel talking about one of her favorite studies. One of my favorite papers in this area is one where these scientists, These scientists were looking at blue tits, so little birds, and they thought that these birds didn't really have a strong sexual selection system because they all kind of look the same.

And you know, for a lot of birds, like the males are really pretty and showy and they have markings that the females are choosing. And for these birds, they didn't seem to. They seemed to all look the same. And then somebody finally thought of doing some experiments with whether they had pigments in the UV. So they put Vaseline on the bird's heads. So they discovered that they had some markings in the UV, like on their heads. And so if they put Vaseline over it so that that blocked the UV, the females, you know, didn't, you weren't able to see those markings. And so then there was a big

difference in the sexual selection. So the males that were really popular before because they had these beautiful UV markings on their heads. When you put Vaseline on them, then they don't get any attention because the females can't see this. And so it led to, you know, one of my favorite paper titles of all time, which was blue tits or ultraviolet tits. I love that story, and I wish I'd get to write a paper using the phrase ultraviolet tits. I wish I'd gotten to rub Vaseline on a bird's head. So many adventures in science.

You know, the more animals we stick under black light, the more we're finding have is like biofluorescence. Exactly. So we see a little slice of the universe that's out there. And a lot of animals can see further into that spectrum and are advertising to each other in that spectrum. And so it's like we're not seeing what's going on out there. Maybe there are some animals that can see neutrinos and are sending neutrino messages to

each other. You know, they have like pigments in their feathers that glow in neutrinos. No, I'm sure they don't. But that sounds like a fun science fiction. story. But let's get back to the actual science of the universe. And I want to answer our question about whether a human eye can respond to a single photon, the human eyeball as a quantum device. But first, let's take a second break. Ah, come on. Why is this taking so long? This thing is ancient. Still using yesterday's tech,

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on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Oh, wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professional. and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him?

Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Whitmer, Jody Sweeten, Monica Patton, Elaine Welteroff.

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or wherever you get your podcasts. All right, we're back, and we have explained to ourselves how the eyeball works and how it receives photon, how it triggers this ridiculous Rube Goldberg machine of flipping levers and rolling balls and cascading, so that you can experience the reddest red that there is.

Yeah, I'm really excited about this because I would love to see a single photon. They're responsible for so much stuff that happens in the universe, and I'd like to personally thank it for being there. And we could finally answer one of the most ancient questions in philosophy, which is, what does a photon look like? But it's interesting and irrelevant to physics

because if you look up at the night sky, you see some things that are very bright, the moon or nearby planets and stars but you also see some things that are very very dim just at the edge of your perception and i've often wondered when looking at the night sky how many photons am i seeing you know imagine some incredibly huge bright star that's billions and billions of miles away shooting 10 to the 50 photons per second out into the cosmos all those photons just a few have managed to cross that enormous ocean of dark and hit your eyeball but it makes me wonder how many photons have to make it before I can see that star.

What happens to those photons on the way from the star to our eyeball? Did they get knocked around by other particles? They have a great adventure along the way. They make friends. They complete quests. No, it's incredible. Mostly those photons just fly unimpeded through the universe because space is mostly transparent to those photons.

There are things out there, the solar wind and particles, that will interact with really high energy photons, but lower energy photons like the ones in the visible spectrum can mostly fly untouched through space. The last thing they interacted with was the surface of the sun, and the next thing they interact with is your eyeball. The reason, of course, you don't see 10 to the 50 photons from that star is just because the photons are going in every single direction.

So if you have 10 to the 50 covering the surface of the star, and they all shoot out, then a year later, now that same number of photons is painting the inside of a sphere that has a radius of one light ear. And so like per area, there are many fewer photons. By the time it gets to you, billions of miles away, there's just very few photons per square meter. That's why the star feels distant. And that's why the intensity of light falls like one over the radius squared, because that's the surface area of the inside of a sphere of that radius. Well, I'm glad it's a little bit diffused because otherwise I think it might just kind of,

instantly vaporize our eyeballs. And it's a question that scientists have been asking since we understood what photons were. Like, could we see an individual photon? Is that even possible? And first, we have to separate it in two questions. One is, could you see a photon which hits the outside of your eye? And the second is, could you see a photon which hits the actual receptors on the back of your eye? Because it turns out something that's pretty easy to measure is the efficiency for photons to get to the back of your eyeball. And only like one in 10 photons that hits the surface of your eye actually makes it to the retina.

Is it getting sort of lost on the way? Is it not reaching the pupil or is it bouncing off something on your eye before reaching the back of your eye? Yeah, they get scattered and they get absorbed. Remember that as light travel through materials, if you change the reflectivity or basically the index of refraction of the material, you're going to get some reflection at that surface. Even when photons go from air, which is transparent, to glass, which is transparent, there's always some reflection. And so as you go through the vitreous humor and go through the lens and go through the cornea, there's little bits of reflection here and there and scattering and absorption.

It's not 100% transparent. So only one in 10 photons actually makes it to the back of the eyeball, which is crazy. Yeah, it seems like we're losing a lot of photons on the way over there. We've got to put some like signs up on our eyeballs. Like photons, enter here. Don't get lost. So people started doing experiments to see how sensitive the eye was back in like the 1940s. They didn't experiment at Columbia University where they shot very, very low intensity light into the eyeball to try to understand what the threshold was.

But back then, they didn't have like a great understanding of quantum mechanics and quantum optics. It was not easy for them to manipulate the light to really get a handle on having a single photon. So what they could tell was that the human eye was very, very sensitive to small numbers of photons, but they couldn't conclusively pin it down because it's very difficult to provide a single photon source. One individually wrapped photon. Yeah. And to know that that's when the photon was there, right? The basic experiment you want is to shoot a photon at somebody's eyeball to know that you shot the photon there and when you did it.

And then have them press a button and say, I saw a photon. And that way you can correlate the button. button presses with when the photons arrive, you could say, yeah, there are reliable indicators of when the photon arrives. And if they're always just pressing the button, right, then you can tell, oh, this person is crazy. This data is useless. And so to prove that somebody could see a single photon, you need to know when that single photon is hitting the eye. That's the crucial thing. And if all you have are light sources that are sort of classical, like hot things glowing, like a light bulb, you know, which has a tungsten filament in it, which glows because it gets

hot and shoots out photons, it's difficult to manage because you can turn it down. You can make it very low intensity, but you don't have control over when the photons are emitted. So you never know, for example, like, was that one photon that came out or two? Did Katie press the button because that was the one time when a couple of photons actually hit her eyeball and not the time when a single photon hit the eyeball. And so the breakthrough in these experiments actually wasn't until about 10 years ago when people developed really crazy nonlinear quantum optics to separate. individual photons. So is it like a little gun that shoots a photon? It would be pretty cool to have

like a button you could press to shoot a single photon. The key idea actually is to split a photon. So you have a very low intensity source that shoots out photons at you. And what you do is you pass it through a special kind of crystal. It's called a down converter. And it takes a single photon and it splits it into two photons of less energy. Now one you can use for your experiment and the other one you can use is a tag that tells you like, oh, a photon just came through. You can measure those using very high precision optics so you know when the experimentee is observing photons and when they're not. And so you can tell, oh, there are two photons in the experiment right now. Let's disregard this. And you can tell when a single photon has arrived and no other photons have come

through. So it's more about being able to count the photons than actually manipulating the photons themselves. Oh, that's so interesting. Yeah, because like if you want to detect a photon, you kind of have to have whatever thing detects the photon interact with the photon, which would not necessarily allow it to also reach the eye, right? Exactly. You don't want to interfere with that photon which is headed toward the eyeball. So this crazy quantum optics, these strange crystals, split it and give you one photon for your accounting and another photon for your experiment.

And they're, of course, entangled together. And so you know that when you see a photon and your little detector, that the eyeball should also have seen one. So then they could have people sitting there pressing a button and answering like, oh, I saw one. Oh, no, I didn't see one. It's like one of those, those like heart necklaces you'd get at Chucky Cheeses with your best friend forever. It's like split in half and each one of you got one piece of the heart necklace. Yeah.

It's just like that. In fact, it's called the Chucky Cheese experiment just for that reason. And so it's only in 2016 that they finally put all of this together, this crazy experimental apparatus and, confirmed that the human eye can see a single photon. If it makes it to the retina, it will be able to detect it. So if you're sitting in a dark room and a single photon from a distant star hits your retina, you will see a flash of light. So with people in this experiment, they're pressing this button when they're seeing a flash

of light. Did they describe like what that experience was like? Yeah, they see like a tiny little pinprick of light, like the smallest little flashlight possible. That's amazing. It is amazing. And it tells you like sort of the limit of your ability, like lights up a single pixel in your brain. So now you can tell like how big is one of your brain pixels. I think it's super cool. And people of course have gone beyond that and started to ask questions about the quantum mechanics of it. Now we know the human eyeball is basically a quantum optics device. It can interact with single photons. So now we can ask interesting questions like

what happens if we send photons to the eyeball that are in an undetermined state. You know, that quantum mechanical objects can have like the possibility of being in two different locations at once. But the strange thing about us is that we don't observe things quantum mechanically. When you look at something, it's either here or it's there. And this is one of the deepest questions in quantum physics is why we can't observe things to be in superpositions. Why, if quantum particles can have like probabilities to be in two different states, we only ever observe them to be in one. So people are doing experiments to see, can the eyeball see photons that are in a superposition?

So they take like this single photon and they pass it through a half-silvered mirror. This is a mirror which sometimes sends the photon to the left and sometimes sends it to the right. And it's a quantum mechanical thing. It's random. So now what happens when the photon passes through it is that because it's quantum mechanical, it doesn't like actually go left or actually go right every time. it has a probability to go left and a probability to go right. This is sort of like the double slit experiment.

So then what happens when it hits your eyeball? Is that when the measurement collapses and the universe says, okay, we have to decide which way the photon went? Or can your eye somehow see a quantum mechanically superimposed photon? Do you get like two little flashes, one on the left and one on the right? These are the kind of experiments people are doing right now. That's incredible. I love that so much.

There's something about these kinds of experiments where you are seeing how we're perceiving things in the world, especially like on the quantum level, that it's like it kind of gives me chills that we can actually have that direct human observation of quantum physics. It is really amazing. And there are some fun theories of quantum mechanics that we might actually be able to test using this kind of scenario. People wonder like, when does the wave function collapse? When does the universe decide, oh, the photon went left or went right? With lots of theories we talked about with experts on this podcast, some ideas being that the universe splits and one goes left and one goes right, others that it's actually dependent on sensitive details of the initial conditions. But there are some theories called spontaneous collapse that says that the collapse to decide left or right depends in some way on like the size of the object that it's interacting with, which is a little weird.

And in that scenario, different size parts of the eye might be more or less likely to induce this collapse. You could actually test this theory by doing this experiment. So this is the kind of thing people are working on right now, quantum eyeball experiments. This is great. So we're turning humans into sort of like an actual quantum detection instrument. Yeah, because the critical question in all of these experiments is, when does the wave function collapse? And if you're interacting with a quantum object using a classical device,

like a finger or an eyeball, then it has to collapse at some point. But the device you're interacting with, of course, is made of little quantum bits. And so if your eyeball can stay a quantum object and interact with a quantum photon in a quantum way and maintain its superposition, then maybe your eyeball can be in a superposition of quantum states. Your eyeball can be in two probabilities, like it saw it on the left and it saw it on the right. And then how does your brain interpret that, right? Does it collapse when it gets to the optical nerve? We don't know.

These are super fun questions that we'll be digging into probably for hundreds of years. Well, you just made me go cross-eyed. All right. Well, I think we dove deep into supernova scarlet and crazy blue on this podcast. And we do know now that the human eye can actually see a single photon. And that's going to allow us to probe the frontiers of quantum mechanics and understand crazy things about superpositions. and what it's like to be a mantis shrimp.

I just love that when I see a star, I'm directly kissing the protons from that star with my eyeballs. Fortunately, you don't have to put the whole star against your eyeball in order to see it. All right, well, thank you very much, Katie, for joining us on this examination of the physics of the human eyeball. And thanks to everybody out there for listening and coming with us on this journey of curiosity and discovery. Thanks for having me. Tune in next time, everyone. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio.

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