Daniel and Jorge Explain the UniverseDaniel and Katie talk about the amazing physics, chemistry and biology that determines how we see the Sun.
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Hey, Katie, I have a funny question. What color are your eyes?
Daniel? How do you not know this?
How would I possibly know? We've never actually met in person. I don't even know that you actually have eyes.
Well, I could tell you any color in you would believe me, right.
I suppose I mean any color within reason.
Well, my husband says that they're like two sunsets on Mars.
All right, Now, I'm not very sure I should believe you for your husband.
Well, my parents say that they are infinite pools of starlight.
All right. And when you look in the mirror, what do you see? Darren back at you?
Really good podcasting eyes.
And we'll just leave you all with that mystery. Hi. I'm Daniel. I'm a particle physicist and a professor at uc Erine, and I have brown eyes, but both of my kids have blue or green eyes.
My name is Katie Golden. I host an evolutionary biology podcast called Creature Feature, and I have lots of eyes of many different colors. One could say their compact eyes.
The eyes have it. It sounds like, well, I have a evolutionary biologist question for you, which is that my wife is Scandinavian. She has blonde hair and blue eyes, and I have dark hair and dark eyes, and both of our kids have blonde hair and light eyes, and I thought that dark colors were dominant. Is there a nefarious explanation here or is there biology?
No? No, this is what's so interesting is yes, dark hair is dominant, but actually that makes it likely that you can pass on recessive traits to your kids, especially because your wife seems to have double recessives. So if you have brown eyes, right, and brown eyes are a dominant trait, so you could have just one dominant allele of brown eyes. So you can absolutely, as a brown eye, brown haired person, have a child who has blue eyes, blonde hair, red hair, whatever.
So you're saying, I must have some blue eyes and blonde hair jeans, they just don't show up in me.
Yeah, exactly. I think a lot of marriages would be saved if people understood genetics.
Well, not for this reason, but we did do DNA testing for our kid, and so welcome to the podcast Daniel and Jorge Explain the Universe, a production of iHeart Radio in which we talk about all of the mysteries in the universe, not just Daniel's kids DNA.
It's not Mari.
We're not going to do any live DNA testing on the show today. We speculate and wonder about everything in the universe. We scratch our heads and wonder how things work, why things look the way they do, and how the whole universe came to be. From the tiniest quantum particles to those colors in your eyeballs, to the very size and age of the universe. We think all of these questions are fascinating. My usual co host and friend, Jorge, can't be here today, but we're very happy to have the spectacularly eyeballed Katie along for our ride today.
And maybe Jorge's eyeballs are along for the ride. I didn't steal them.
I don't have his eyeballs in like a glass jar over here.
It's so he can see what's going on when he can't be here for the recording.
One of my favorite things about asking questions about the universe is that it's so easy to quickly get into deep territory. You know, even if you are not an academic physicist, you wonder about the nature of the universe, how things work, And that comes just from looking at the world around you and wondering why is it this way and not some other way? Why do my kids have this color eyeballs? Why is water transparent? Why is the sky blue? Really basic questions of the very nature of our environment can lead often to fascinating science puzzles that help you understand the way the world works.
I love all kinds of science, and I think that it is interesting because you can go from a relatively simple question something that doesn't seem that deep on the surface. In fact, the simpler the question, usually the more complex the answer, the deeper you have to go. Many other questions branch off from some of the more simple questions, and I love how many rabbit holes you can go down.
Absolutely, And when I'm giving a public talk like I did in Aspen last week, my favorite questions are the ones that start with, well, this is probably a really easy question, but and then they ask a really deep, very simple sounding question, but one that has huge ramifications and requires you to understand all sorts of fascinating things. And the thing to know about science is that it's all just people being curious. The whole reason we're trying to understand the cosmos is that the Sumerians and the Mayans and the Chinese and the Greeks, we're looking up at the sky and wondering like, hmm, why do the planets sometimes go the other way? And why do things rotate around the pole stars? And all these things come from our very human, very ancient curiosity about the nature of the world around us. It drives everything we do in science, from people who study blind mole rats to those people trying to understand why supernova explode.
I think one of the interesting kind of misconceptions people have is that a basic question is one that's not worth asking, or it's a dumb question, whereas like you should be asking somebody like, oh, what is this specific coefficient for this specific formula, But when you ask that kind of question, the answer could be as simple as like a decimal. So when you're asking a really basic question, you may think it's basic, but the fact is, like so many of the more complex science, the things that Daniel studies and takes years and years to understand, are things that have come from these really basic questions that we may still not even really know the objective answer to exactly.
And those questions come from within us. You know, they are not objective questions. There's no way to like rank what are the most important questions in the universe. They just come from our burning desire to know. And everybody's different that way. The questions that I ask are different from the questions other people ask. And hey, that's cool. And before we dig into the topic of today's podcast, I think we need to do a little bit of house cleaning from the last time you and I met, because we were talking about the fantastic and amazing Greek ancient aftronomical computer, the anti Kittherra mechanism, and we got into a little tangent and made some jokes about chemistry. Maybe you remember Katie comparing chemistry to torture. I think.
I haven't slept a wink since that episode. I've been filled with remorse. Chemistry is not torture. I was merely joking at my sort of own expense in terms of my limited ability to do chemistry or figure out chemistry equations, because it takes a very specialized knowledge and it can be quite complex, but I mean without chemistry, my two favorite sciences, astrophysics and evolutionary biology, would not exist exactly.
And I was commenting about how my son, who's in tenth grade and taking tenth grade chemistry, was asking me questions that I couldn't answer, and that was complaining. Chemistry didn't seem like to me the funnest science man. We got a bunch of feedback from chemistry loving folks about our comments, and so I just want to address them really quickly. You know. On one hand, I think it's good that people understand that science is personal. The kind of science that I do, which involves a lot of programming, would bore a lot of other people. And there's lots of science I think is valuable, but I could never spend my days like pipetting or working at an optical table. The point is that we all have different tastes, and that works out because we don't want everyone to be doing the same kind of science. But I do think that we went too far and hurt joking, because while it's good to have different preferences, we shouldn't be negative about the science that gets other people excited. I'm very glad that chemistry is a thing and that people love it because it helps us all understand the world better. So chemists and fans of chemistry with respect, my apologies, and please keep chemistrying.
Yes, I think chemistry is amazing. For an example, I one summer was helping out with some oceanographic research just by calibrating this instrument that would look at diatoms, these very very teeny tiny microscopic organisms, and so I spent a lot of time on a microscope just counting diatoms. And every time I would tell people about it, they're like, oh, that sounds awful, And I actually really liked it because it was, I don't know, it was fun to see the little guys going and also really fun to try to calibrate this machine to do the work that I was doing counting these little microscopic organisms. And so for one person's you know, kind of tedious activity, is another person's fascinating fun activities. So yes, I really do not have any native attitudes towards chemistry. Perhaps one might even say I'm jealous of people who understand chemistry better than I do.
That's what it is. And if you're curious how people ended up in their particular subfield. Everybody's got a fascinating story about what they tried and what they were excited about, how they ended up having their passion stimulated. It's a really fun podcast called The People Behind the Science, where you can go and hear people tell their stories about how they got to do the kind of science they're doing. There's even one featuring me if you're curious. But let's move on and talk about the topic of the day, because today we are talking about a really fun question, one that sounds really simple but reveals a lot of fascinating physics and might even teach you something about the way we see the world.
I am so excited, Daniel hit me with it. What are we exploring?
So today in the podcast we are asking the very simple, the very amazing, the very revealing question, what color is the sun yellow?
Podcast over roll credits. No, No, it can't be that easy.
It in fact isn't that easy. And in finding the answer, we're going to learn a lot about what it means to be color, where color comes from, why different things emit different colors, and why we see things in different colors, and how most of the popular descriptions about the color of the sun aren't exactly quite right. But before we do that, let's hear what our listeners had to say about the color of the sun. I love hearing what people out there are thinking about a topic before we dig into it, and I have a cadre of volunteers who answer our questions so that you can understand what people out there are thinking. And if you would like to participate for future episodes of the podcast, please everybody's welcome right to me to questions at Danielandjorge dot com. So before you hear these answers, think about it for a moment yourself. What color do you think the sun is? Here's what people had to say in space.
I think it's really really, really bright yellow.
I think because of the fusion inside the sun, it gives off all type of photons and the reason it looks a little bit more yellow is maybe how much fusion is going on inside.
My guess is that the Sun is emitting radiation on every wavelength and also in the visible spectrum. But when we look at the Sun, we are seeing it's light filtered by the atmosphere, and I believe that the atmosphere scatters the blue wavelengths, So when we look at the sun, we see it yellow because it's missing that blue wavelength.
Yes, maybe there's like a wavelength that it's you know, spin out more light of that wavelength. I feel like it's got to be some sort of yellowish white, or maybe it's like all the colors that'd be cool.
I'm surprised nobody said it's the color of ouch, because when I look at the sun it hurts, and I have heard that one is not supposed to do that.
Yes, we probably should have put a disclaimer like, please do not pause the podcast and go out and stare at the sun. Not a good idea.
No, the sun is very powerful, lots of energy coming from it. It's why it feels so warm, and you might not want to stare at it.
Well, most of these answers basically say that it's yellow, and some even specifically comment about where you are looking at it from. Somebody says in space, it's really bright yellow, and that's a really insightful comment, because things do look different down here on the surface of the Earth than they do in space.
So why is that. I know we've got this atmosphere around us, but what exactly does the atmosphere do to make things look different here versus space, Because I know, like down here, you look at the sky and it can be blue, it can be gray, it can be pink. I mean at night, it can be black. But when you're in space, usually the sky is sort of black. I guess it's not even a sky at that point. It's sort of a void and then stuff is scattered in it.
Yeah, that's really fun. If you are standing on the Moon, what color is the sky? The sky on the Moon is black because there's no atmosphere. We did a podcast episode actually about the atmosphere of the Moon, which is really fun. But the reason the sky is blue here on Earth is that not all wavelengths, not all frequencies, not all colors of light move through the atmosphere equally. Blue light gets bounced around more, and so if you're on the surface of the Earth, some of the light that otherwise wouldn't have hit your eye gets bounced down to your eye, and it's mostly the blue light that gets bounced around and ends up at your eyeball. So on the Earth during the day, we see the sun, of course very brightly, and all around it we see blue light from the Sun that's been scattered off the atmosphere to our eyeballs. Now, if you're on the Moon during the daytime, you see a very bright sun and you see a black sky. So even during the day, the sky is black on the Moon because there's almost no atmosphere there to scatter the light down to your eyeballs. So that tells you that when you're on Earth, you're not seeing all the frequencies of light from the Sun the same way because the atmosphere doesn't treat them all equally.
So why is blue so special? Why isn't our sky always red or green?
Yeah, it depends on the composition of our atmosphere, And in fact, on Mars, the sky is red mostly because the dust in the atmosphere which reflects that red light. So on Mars during the daytime, the sky is red and sunsets are actually blue.
Wow, that sounds pretty although I associate red sky with wildfires, so I'd be mildly panicking when I'm on Mars. I'd actually probably be mildly panicking on Mars for many different reasons.
But before we talk about why exactly that is, I think we need to get down to the basics and understand what it is we're talking about when we say color and frequency and wavelength and why that even exists.
Color comes from paint cans, right, podcast over roll credits.
Exactly did somebody paint you?
They must have when I was a baby.
Light in color is really actually super fascinating to understand from a physics point of view. Fundamentally, you see color when photons hit your eyeball. Right, so we're talking about light hitting your eyeball somehow carrying that information about color. So to understand color, you really have to understand what is light and light we can think about either classically as like wiggling electromagnetic field, or we can think about it from a quantum mechanical point of view, where there are individual photons moving through a quantum field. But in either way it's wiggling fields. And this can be hard for people to visualize sometimes, but you know, just imagine like an electron in space. Around that electron is an electric field, and that's what the electron uses to like push on other electrons. If there's another electron somewhere far away, the electric field from the first one pushes on the other electron. That's how two electrons talk to each other. Now, if you take that first electron and you wiggle it, then the field wiggles with it. Right, It's sort of like if you put your hand on the bathtub surface and you tap it, you generate a series of waves. Right, the whole surface of the bathtub doesn't move down with you instantly. In the same way, when you wiggle an electron, the electric field doesn't move instantly. The part in nearby moves down first, and then the next part, and the next part in the next part. So the motion of the field moves like a wave through the field as you wiggle the electron.
So you're saying that light waves are sort of like lazy rivers with a bunch of electrons and inner tubes wiggling the water around while they affect other electrons and inner two.
Yeah, exactly. It's a little bit more energetic than that, but exactly so, you can think about electrons generating static electric fields that don't change, and then when the electric field needs to change. That's what a photon is. A photon is a wiggle in the electromagnetic field. That's what we mean when we say that that's what a photon is. Anyway, light is oscillating electric fields, and you can oscillate it different frequencies. You can move that electron really slowly and have long wavelengths and low frequencies, or you can wiggle it super duper fast and you can get high frequencies and short wavelengths. It just depends on how fast you're wiggling the source of the field to generate those photons. And this is like how antennas work. They have a bunch of electrons in them and they wiggle them up and down. They generate radio waves or cell phone towers or whatever. The antenna on your phone receives that electromagnetic radiation because it has electrons in it which get wiggled by that radiation those oscillating fields, and you receive those messages. So in the end, light is just wiggling electromagnetic fields, and every wiggle has a different frequency.
I think most of our senses are wiggly things that end up hitting some kind of sensory cell like hearing, site. Maybe not touch, but sometimes I don't know. Electrons are still wiggly, even like if I'm touching my table. Those electrons aren't completely static, are they.
Yeah, that's a really interesting and insightful point that sight and sound are both just oscillations in something. There are vibrations, right, and that actually makes it easier to do things like digitize them. You can take any song and break it down into all the different frequencies that are involved in it, and then you can record those frequencies and recreate it later. You can do the same thing with images. You can say, oh, what frequencies of light arrived, record those and regenerate them. So it's sort of like a recipe for breaking down your experience and recreating it. Taste and smell, as you well know, Katie, are not like that. You have like so many thousands of different receptors to different molecules, and that's one reason why we haven't been able to like digitize smell. You can't like email a smell to somebody or a taste to somebody, or have like a digital smell creator or taste creator on the other side for you to receive it.
I mean, someday, maybe if we can figure out how to like have a sort of tongue VR experience where you use small electrical impulses to sort of trigger the receptors on your tongue maybe, but we're not there yet.
We're not there yet, and maybe the chemistry folks can help us with that. Right, that seems like definitely a chemistry based experience rather than a physics based sensation.
I will phosphate myself before the chemists if they can bring us smell a vision.
But back to normal vision. We have all these different frequencies of light. So when you get a beam of light, for example, like a laser pulse, it has photons of a specific frequency in it, right, and so one photon can have that frequency and another photon can have the other frequency. Here's where we start to talk about the quantum mechanical nature of light. So far, we've just been saying it's ripples in the electromagnetic field. That's like the classical view. That's what Maxwell thought one hundred and fifty years ago. Then we realize that you have to actually quantize you and break it into pieces. You don't just get like a whole massive wave in the field. It's broken into these little packets, and each one has a different frequency, but you can still think of them as like little ripples. It's just ripples in a quantum field instead of a classical field.
So these different frequencies will basically be different colors that we see, right exactly.
Each frequency means a different energy in the photon, which we perceive as different colors. And it's important to understand that photons are quantized in the number of photons you can receive. You can receive one photon or seven photons, but not three point two photons. But they're not quantized in the energy. There's like an infinite spect possible energies. Any number you can imagine for a wavelength of light, you could have a photon of that wavelength. There's no frequencies that are not allowed. There's an infinite number of frequencies between like five hundred and six hundred nanometers, right, So they're quantized only in the number of photons you can receive, not in the frequencies, which means is an infinite number of different kinds of photons that you can see. But that doesn't mean that there's an infinite number of colors that you can see. Right. While color is connected to frequency, it's actually subtly different. Concept is a little bit more biological and philosophical.
Yeah, it is interesting because when you look even at just life on Earth right, So many different animals have so many different ways to perceive color, and they will perceive the world differently just in terms of color. And that's because the eye and the brain is very different for many different species. And so humans we see color a certain way or brains interpret it a certain way. But even among humans we have variation in how people perceive color. But before we dive too deep inside of the eyeball, let's take a quick break, rest our eyes, and when we get back, we'll talk about how color is a product of both biology, are psychology, and philosophy. And don't forget chemistry and chemistry. Don't you dare forget chemistry.
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And we are back and I have been closing my eyes and trying to think of a new color that I've never seen before, and I can't do it.
That's a really fascinating sort of philosophy question, and it really gets to the heart of like what color is, because a lot of people imagine that photons are colored. They're like a photon flying through space is either a red photon or a green photon or a blue photon. And in shorthand, that's fine. We can attach labels to photons and say they're a color that's connected to their frequency, But the photons themselves don't have color, right, Like different animals will see the same photon and have a different experience. The color is actually your experience of the photon, right the way, like your experience of sugar or your experience of fats are not a characteristic of that molecule, They're characteristic of your experience of your brain chemistry giving you a sensation.
Yeah, two people can taste the same dish and one person can love it and the other person can think it tastes disgusting, And it's the same food, and you know, we were essentially quite similar in terms of our biology. We have tongues and eyes generally, but both the structure of the eye and the structure of the brain can be different, as can things like our sort of lived experience and how that influences how we see things or taste things. So even among humans, when you look at green, Daniel, and I look at green, we may have different experiences of what green is, and it's so hard to know whether it's the same green or a different green because if I'm like, well, yeah, it's green like an apple, and You're like, yeah, we haven't gotten any closer to answering the question of whether it's the same green because we may just have different perception of what an apple looks like.
Right, And before we dig into like the deep brain philosophy, let's take one step back and trace the path of the information. Like the photon hits my eyeball and it has a certain frequency, do you know the biology of how it then generates like a signal in the nerve, Like how do you go from the photon to the signal of the optical nerve to your brain?
Yeah, so we have sensory cells at the back of our eyes called rods and cones, and the rods generally help us see light or not light, so they really help us see at night in low levels of light. But the cones are responsive to those light photons. And so what actually happens on the very very teeny tiny microscopic level is a protein structure in the cone will actually kind of snap when it is attacked with a sort of certain level of energy from this photon, and it will trigger a neural cell that is connected to that photoreceptive cell, and then there is a whole sort of chain reaction to the bundle of nerves that go from the back of the eye and then go to your brain. So that is sort of how each individual cone works, Like you have that like snapping action of that protein once it is activated by specific frequencies of these photons, and then the color you see isn't from that one individual sensory cell, it is from sort of the average of these the level of activations these cells have in combination, and all of that information goes through your nerve from the eye to your occipital lobe and it's interpreted in your occipital lobe, and so you have this interface between the kind of structure of your eye and the cones and your brain to then interpret what that color is. And that's why we don't just see like blue or red or green. We see a spectrum of color. We see different shades, different darknesses, different depths and qualities of color.
Yeah, I think it's really interesting. I used to think about it working like a TV screen or a computer screen. We had like RGB and you had a certain number of photons in each one and you could mix them to get any color. But it is a little bit more sub than that. If you look at like the wavelength of visible wavelength, it starts at like four hundred and goes up to like seven hundred nanometers. Each kind of cone responds better to a different frequency, Like there's one kind of cone that peaks at like four point fifty, but it can still see photons up to like five hundred.
Yeah, there's overlap, a good amount of overlap.
Yeah, they do overlap, and all of them have a width to them, And so what your brain is doing is some kind of interpolation. It's like, well, I saw this kind respond really brightly and that kind very dimly, So it must be a lower frequency photon, or it must be a higher frequency photon. And as you say, it does some in there and like then gives you an experience. And that experience comes from its calculation, its interpretation, right, that color is in your mind, it's not in the photon.
Yeah, it's really interesting because difference in color perception and humans, say, for people who have color blindness, it can be due to basically the range of frequencies that those cones pick up can either overlap more which makes it harder to distinguish between two different colors because you have more overlap of these cones, or they can kind of be shifted on the spectrum of where you are picking it up, so you don't pick up certain frequencies as well. So it's not so simple as like an operator switchboard for turning selecting colors. It is a spectrum experience, but it's also one that differs from person to person and is very observable in cases of say like color blindness.
And there are even people out there there who have a fourth kind of cone. They're called tetrachromats, so instead of having three cones in their eye, they have four. When I first learned about this, I thought, oh, my gosh, that's amazing. Is there like a fourth primary color that they can see that we can't, some color out there that they experience that we never do. It's actually a little bit more complicated and less exciting than that. They have four kinds of cones, so they sort of sample the spectrum four ways instead of three, and that just lets them interpolate better. There's not like a new primary color that they can see that we can't. It's just like they have like more resolution on their ruler, and so they're better able to distinguish colors. Like you put two very close shades of green in front of people, some people can't tell the difference and some people can. And tetrachromats, these people with four cones are especially good at that.
Yeah, it's more granular. All throughout the animal kingdom we have monochromats, die chromats, trichromats, which is generally humans, and then tetrachromats, which is animals and some humans too. And so it is really interesting because we think a vision like in this sense of like, oh, a monochromat must have a very boring kind of visual experience because they're really only seeing, you know, this very limited range. They're not seeing color like we do. But that's not necessarily true. So a monochromat has only one kind of photoreceptive cell to generally like a rod, something like an octopus is actually a monochromat. And so we think that, oh, well, it just kind of seas in black and white. It must be, you know, not exciting to be an octopus and have octopus vision. But there's also some speculation that the octopus is unique eye shape. Have you noticed how that like pupil is sort of stretched and elongated. It might be that way so that it actually measures the focal length of an object as it hits the eye. It creates this sort of blurriness or an out of foe from different sort of like wavelengths of light. And so it could be that with only rods, they are still able to have a brain's response to color because they are actually seeing the wavelengths hitting at different sort of angles on the eye based on its shape, could actually be interpreted as colors. We don't know. We have never been able to figure out how to ask an octopus whether it's season color. I'm sure there will be studies to see whether octopuses can detect color, but it's very complicated. The power of the brain is in being able to interpret sensory information. So as long as you're getting that information to the brain somehow, even if it is a different way than say, like humans do, you could still be perceiving color, but just using very different hardware.
Yeah, and the hardware and software is sort of important. I was reading. The manta strip has an incredible number of different kinds of cones, like maybe even up to twenty, but it has no like processing behind it. The brain behind it is very simple, and so even though it has so many different kinds of cones, its experience of color might just be that it can only see one color per cone. It's just like an on off because it doesn't have the interpolation power that our brain does.
And this may work for the mantistrimp because it gives it really really lightning fast reflexes and the ability to detect something that is in its perphery really easily and really quickly. So it's going for speed over say the definition of its vision. But that works for the mantastrimp because that's what it needs.
Whereas we humans are slow and we like to think, hmmm, what is that really? Is that a red or is that a maroon? I have all these arguments with my daughter about like what is blue and what is green? And I think we see colors differently at least to really fascinating philosophy questions about how people's experiences might be different. And this is famous thought experiment about the color red. You know, if you put somebody in a box and never give them the experience of red, could you then describe read to them in a way that when they came out of the box they would see a rose and be like, oh, that's what you meant. You know, is there a way to capture redness other than the actual experience of red? Anyway, it's a whole fun question in philosophy that we don't have time to dig into. Today. We're thinking about the color of the sun, and now we understand that color is an experience in our mind, but it's linked to this physical property that photons have different frequencies and we experience those frequencies in different ways. So really we need to think about, like why photons have different frequencies so that we can understand what frequency of light the sun generates.
Yeah, Like this kind of gets back to my question earlier of like why is blue so special when we see like why does that interact with our atmosphere not say red or green? Why do they have these different wavelengths? What is going on?
Yeah, you might wonder like why are some things blue and other things green? And there's really a couple of different reasons, and they're both actually due to quantum mechanics, which is super awesome. And the first is that there are certain and energy levels allowed to electrons, Like electrons flying through space. We call them free particles. They can have any energy they want, they can have super high energy, they can have no energy. They can do whatever they like. They are not bound by anything. But when you put an electron into an atom, there are only certain solutions to the Schroeninger equation, to the quantum mechanical equation that tells an electron what it can and cannot do. And because you are binding the electron, you're like putting it in a little well, only certain solutions work, and that's where quantization comes from. It comes from confining the electron to the atom, and you end up with this like ladder of possible energy levels. So if you've studied some chemistry, for example, an extremely important and valuable science, then you've understood that, like around an atom, an electron can have a certain energy level or the next one or the next one, but it can't be in between. So there's this ladder of energy levels for an electron.
Maybe this is why I'm so jealous of people who can understand chemistry well, because I've always been confused why there are these different ladder stages for the electron to sort of jump into, and why can't it go in between.
The technical answer is that those are the only solutions to the shorting air equation. The intuitive answer is like, why are those the only solutions to the shorting air equation? And there's like sort of a cartoon picture which is helpful those not technically accurate, which is like, imagine electrons wave wiggling around the central proton. It like wiggles up and wiggles down, and wiggles up and wiggles down. Energy levels the electron have are those where it completes an integer number of half wavelengths, so that when it gets back to where it was, it sort of matches up to its previous wiggle. It's sort of like a standing wave, it reinforces itself anything else it ends up canceling itself out. And so you can sort of like fit a certain number of half wavelengths sort of the same way when you pluck a guitar string, right, it can oscillate in different modes, and like the whole thing can move up and down. Or you can have a node in the middle where one side is moving up and the other that is moving down. Or you can have two nodes. So you have like three half wave lengths. There's into your number of ways to fit them into your fixed length guitar string. And that's actually a pretty good example because you're confining the wiggle of the string because you've nailed the string down on the two sides of the guitar. And here we're confining the electron to the atom and restricting it to certain states, and that has important consequences for the color of that atom, because the light generated by that atom comes from electrons moving down energy levels. An electron gets excited and it absorbs some energy and it jumps up a few energy levels. A moment later, it's going to relax and come back down to the lowest energy level, because that's what nature likes to do, is relax down to lower energy levels and spread things out. When it does that, it gives up some of that energy in the form of a photon and emits light. So that light has a certain energy, which is due to the difference in its higher energy level and the lower energy level jumps down the ladder and it gets a certain amount of energy, and that's always going to be the same frequency. Every time it makes a certain jump from like three to two or from three to one, it's always going to have exactly the same energy and exactly the same frequency of photons.
So you'll have photons that are sort of imbued with different energy based on the way that this electron has jumped down from the ladder exactly.
And every atom has a different ladder, So the spacings of the ladder for hydrogen are different than the spacings of the ladder for zinc, for example, or for copper, So copper will emit photons of different frequency than zinc will, or hydrogen will, or oxygen will. And this is actually super valuable and important because when you're looking, for example, at light from a distant star and you're wondering, hmm, what's that star made at of, what's in its atmosphere, what is burning in its core? You can tell without ever going to that star, just by looking at the light that it emits. There's a fingerprint for hydrogen, and a fingerprint for zinc, and fingerprint for oxygen, and you can see those fingerprints in its spectrum. If you count up the number of photons you get at different frequencies, you'll be like, oh, to get these peaks, to get light at these frequencies, you have to have copper. And so we can tell what distant stars and distant planetary atmospheres are made out of just by looking at the color of the light, the frequency of the light that they emit.
Is this kind of how classic neon lights work. You can fill a glass tube with sort of different chemicals or different atoms, and then once you put some energy into it, it produces a different color.
Exactly. A neon is an element that tends to glow at certain colors, and so that's exactly right. You're filling it with a hot plasma, a gas that's become excited and it glows at certain energies. And to get back to your question of like why are something's blue and something's red. Like your T shirt is not blue because it's glowing. It's actually the opposite process happening there. It's absorbing. Like when light hits your T shirt, the atoms in that T shirt can absorb photons of certain energy. It's like the reverse process of electrons jumping down the ladder and giving off photons. The reverse process is them absorbing photons of certain energy and jumping up the ladder. So when light hits your T shirt, it can absorb only certain frequencies of light, only ones that match the energy levels of the electrons in your T shirt, and so those things get absorbed. Anything else is not absorbed, and so it can be reflected.
So my shirt is basically every color except blue.
Well, what it means to be blue is to reflect blue light. Right, It's absorbing all the other photons and reflecting blue light, and so you see it as blue. When we call something blue, what we really mean is that thing reflects blue light. It's counterintuitive. You'd like to think, oh, it absorbs blue light and becomes blue, but then you wouldn't see any blueness from it. You only see blue. If it's reflecting the blue light.
I mean, this is in nature, this is how we see different colors. Usually you have some kind of like pigment that absorbs every color except for the color that you see, which is reflected back at you. But you can also have things like tiny microscopic prisms that actually scatter light structural coloration, and instead of absorbing the light, it scatters it, and then that scattered light hits your eye.
Yeah. Really fun. So that's quantum mechanics process number one. That's like energy levels of the atom determine what colors it glows in and what colors it absorbs. There's a second process, which is also due to quantum mechanics, which is just that hot things tend to glow, and they tend to glow at different frequencies. So, like the sun is really really hot and it's glowing, and part of the light that it emits is due to these atoms jumping up and down the energy levels, but part of it is also just due to having like a big soup of electrons up there that are all zipping around at certain temperatures and giving off photons. And the hotter the thing is, the more these electrons that are not bound to atoms are going to be wiggling around and giving off photons. So things that are really really hot, how their freer electrons sort of wiggling around a lot in giving off high frequency photons. Things that are colder have their electrons not moving this fast and giving off lower energy, lower frequency photons. So for example, Katie, you glow, and not just in a sort of intellectual way or because of your brilliant personality, you are literally emitting photons right now. They're just not photons in our visible range. So when your husband looks at you, he can't see that you are glowing in the infrared. You are giving off light like in a dark room when you imagine nobody could see me, you are glowing in the infrared. And that's what for example, night vision goggles can see I see.
Is that also called black body radiation?
Yeah, that's the physics name for it, black body radiation. So we imagine if you have an object which does not reflect any light, which absorbs everything and only emits light through this process, or your electrons inside your sort of zipping around and giving off photons. These are the electrons that are again not totally bound to an atom, and so they're freer to emit in sort of a broader spectrum than we call this thing a black body. Now, nothing in the universe is actually a black body. Everything in the universe is sort of partially a black body and partially emitting due to the energy levels of the bound atoms. And so, for example, our sun is like that. Our sun can be described as a black body object because it does emit at a certain frequency based on its temperature. But also you can tell what's in the sun because the atoms in it glow. So the Sun is a complicated mixture of mostly black body radiation due to its temperature, and also a lot of peaks and dips because of the other quantum mechanical effect where electrons around atoms are absorbing and emitting only at specific frequencies.
Well, it sounds like the Sun contains multitudes, kind of literally and maybe very violently. So we are not going to look at the Sun with our eyeballs, but with our minds when we get back.
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So we are back and we are going to take a safe look at the sun, not by staring at it, but by thinking about it.
Exactly. And so first we will think about the sun just as a black body object, or as something which glows due to its temperature. And as we said before, the theory of black body radiation tells you that the hotter you are, the shorter the wavelength of light, or the higher the frequency of light, the higher the energy of photons that you emit. So if you are cooler like you and me, then we glow in long wavelengths, low frequencies and if you are hotter, then you tend to glow in higher energies or higher frequencies or shorter wavelengths. But that simplifies it a little bit there. We're really just describing the peak where most of the energy is emitted. If you're an object like the sun at five thousand degrees kelvin, you tend to emit mostly in the visible spectrum, right, but you also emit at lower and higher frequencies. It's a very broad range there. So the Sun isn't limited to only emitting light at one frequency, but it does tend to peak at exactly the kind of frequency that our eyes are good at seeing.
It's kind of like how in our eyes our cones can see a whole range of frequencies. Things can also emit a whole range of frequencies.
That's exactly right, And so the sun emits a very very broad range. We can actually only see a little slice of it. Like if you develop infrared telescopes or UV telescopes, things that can see frequencies below and above what our eyes can, you can still see the sun, right. The sun is not black and higher frequencies, and it's also not a coincidence that the sun peaks in its emission at exactly the frequency range that we can see.
The sun was made for us, clearly, Can I stop you right there?
I think it might be the opposite, right, Our eyes clearly evolved in an environment that's dominated by certain frequencies, so those frequencies are more useful. And I think if the sun had been hotter or colder and admitted at different frequencies, that very likely we would see at different frequencies. And like, I don't know what that experience would be like, but you know, the reason that our eyeballs can see exactly in the range where the sun peaks probably not a coincidence.
Well, that's why certain animals, especially invertebrates and pollinators can often see UV light is because it is something that they've evolved to see that helps them pollinate, because they can see light that is given off by flowers, and the flowers of covolved with them to emit even more UV lights that they can see, and this helps them, you know, I identify basically their target to get nectar from flowers, and so birds and insects can see UV light and we generally can't. And that's not a failure of ours. That's because it does not generally have an evolutionary benefit for us to do.
So. Yeah, and it's a fascinating story. We told on the podcast once about a kind of bird, kind of bird called the blue tit, which actually looks spectacular in the ultraviolet. But let's get back to the Sun and start in outer space before we think about the atmosphere. If you're in outer space and you look at the Sun with of course protection, what would you see? And you often hear science communicators and popular science articles describing the Sun as green, saying the Sun is actually green. The reason is that if you just treat the Sun as a black body object, and you know it's temperature, its peak is in the green, it would emit more green photons than orange photons or red photons or blue photons. So that's why people say, sometimes, oh, the Sun is actually green. What does that really mean?
Though?
I mean, if you're in outer space and you look at the Sun, you do not see it as green. You see it as white, because it does peak in the green. But as we said before, it emits in a very broad spectrum, and it emits plenty of red photons and blue photons and orange photons. So if you're in outer space, you're an astronaut, and you look at the Sun, you do not see it as green. You see it as white because you see a huge mixture of these colors. In order for you to see it as green, it would have to be like overwhelmingly only green photons and very very few other photons.
This is a trick you kind of can do with lights here on Earth. If you mix light, not pigment, but light together, you get white. When you mix pigments together, you often get brown, because that's a different sort of thing going on with pigments. But yeah, when you mix a bunch of different lights, you'll get white light.
You get white light exactly. The second problem with saying that the Sun is green is that that's only true if the Sun was a perfect black body, if all of its radiation only came from its temperature. But the Sun is not a black body. It emits light not just because it's hot, but because the atoms in it are doing other quantum mechanical things. Electrons are jumping up and down energy levels. It emits in many complicated ways. As you say, it contains multitudes.
It's got electron shoots and electron ladders.
So if you actually go out into space and you measure the photons coming from the Sun, you find that it doesn't peak in the green the way it would if it was a perfect black body object. It actually peaks in the violet, right, And so the most common frequency of light coming from the Sun if you're in outer space is not green or it's not yellow, it's violet.
That must be a very interesting experience for animals on Earth who can see more in the ultraviolet sort of frequencies. They must see the Sun differently, at least from space.
Yeah, yeah, well, I hope they're also not looking up at the sun, So any animal or insectlictenors be careful.
But if we put birds in space and give them eye protection and then ask them put a pairrot up there so it can talk to us, I'm glad.
That we're thinking about these details now, that's definitely the most important thing. So, if you're in outer space, the photons you get from the Sun are most often violet. Green is also very common, but you still see the Sun as white. If you look at pictures of the Sun from space, then it's definitely white. But here on Earth. If you look up at the sun safely, of course you don't see it white, right, You do not see a white sun. And the reason is the one that we talked about before, that the atmosphere is not democratic when it comes to letting light through that the very high frequencies of light, the blue gets scattered, and so all the photons that are zooming towards your eyeball when they come from the Sun through the atmosphere, they don't all make it, so the blue ones get scattered sideways and make somebody else's blue sky. If you're standing next to me, Katie, then those blue photons which originally were pointed towards my eyeball might end up at your eyeball, and so you will see the sky as blue, whereas I will see the sun as having less blue than it actually does. And that's why we see the sun as yellow. It's like originally white but then distorted by the atmosph to look yellow.
And is that why the sun changes color as it rises and as it sets, because it is hitting the atmosphere at a different angle relative to our eyes.
Exactly during sunrise and during sunset, you're going through more atmosphere than usual, and so the higher frequencies of light get scattered more and you get even more biased towards the low frequency red light. So the scattering the atmosphere depends on the frequency to the fourth power. Actually, so high frequency light like blue, gets scattered much more than red light, and if you go through more atmosphere, you get more scattering, and that's why sun rises and sunsets are red.
This might be a topic for a different episode, but I do have a question. Do you have an opinion on whether the green flash exists? Have you heard of that in terms of like as the sunsets, people claim that there's a green flash just as it goes beyond the horizon, there.
Is a green flash, and I've seen it.
I feel like I've seen it too, but I didn't know if it was just sort of psychosomatic.
But yes, that is a topic for another day. But it's also fascinating to think about the colors of everything else out there in the Solar System. You probably have mental images of what various planets look like from pictures you've seen online, but they're not always representative. Mars usually looks red because of the red rocks on Mars. But Venus, for example, looks different than you might imagine. The images we have of Venus are mostly from UV telescopes, where the pictures they've taken have been shifted down into the visible spectrum so that we can see them. If you were to fly by Venus, it would look just kind of like a big gray blob with almost no features to it at all in the visible spectrum.
Well, that's too bad, because I was really planning a road trip to go sight see Venus.
Bring your acid proof goggles because I think you're gonna need them on the surface of Venus, all right. And so to recap Photons are wiggles in the electromagnetic spectrum that come at all different frequencies, and objects can emit and absorb photons based on their energy levels or also just based on their temperature. The Sun out in space is white, even though it peaks in the violet. You would see it as white with your eyeballs, but here on Earth we see it as yellow because of atmospheric effects.
And all of that depends on our eyeballs, biology and our brains.
And the chemistry that makes it all work.
We owe everything to chemistry, and I am extremely remorseful to make it sound like anything else.
And in the end, everybody should be out there following their personal curiosity. You can love a kind of science that I would find super boring, and I can find it super interesting even if it makes you yawn. That diversity of interest is why we have such a beautiful diversity of scientists and such a beautiful diversity of discoveries about the universe. It's that widely varying curiosity and enjoyment of different kinds of puzzles that powers our broad spectrum human curiosity about the universe.
It's not a competition except for the universal scientist decathlon.
That's right. So thanks very much everybody for going on this journey with us, following photons from the sun all the way to your eyeball, and following these phonons from our mouths all the way to your earballs. And thank you very much Katie for joining us today. Yeah, of course, thanks for listening. Tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app Apple Podcasts or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digesters to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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