Speaker 1: Hey, or hey, do you think that our podcast exists
Speaker 1: out there in the multiverse?
Speaker 2: Like if I think there are other versions of us
Speaker 2: doing this podcast?
Speaker 1: Yeah? You know. Do you think in other universes there
Speaker 1: are people puzzling over how the universe works and podcasts
Speaker 1: trying to explain it to them.
Speaker 2: I guess if they live in a universe, then there
Speaker 2: are probably people wondering about the universe. And there might
Speaker 2: be a cartoonist and a physicist also wondering about deniers,
Speaker 2: and they might have also met and started a podcast.
Speaker 2: That's technically possible.
Speaker 1: I wonder if they also make the same chocolate and
Speaker 1: banana jokes.
Speaker 2: Ooh, maybe they make chocolate covered banana jokes.
Speaker 1: Ooh, I think we're in danger of crossing over into
Speaker 1: the multiverse.
Speaker 2: What if they're making white chocolate and strawberry jokes, which
Speaker 2: I'm allergic to? That'd be like the diversion of this podcast.
Speaker 1: Then we would annihilate if we ever met.
Speaker 2: Well, it's like the anti Daniel Horra explains the universe.
Speaker 2: But would they still explain the universe if it's an
Speaker 2: anti podcast.
Speaker 1: Hmm, maybe they'd be explaining the anti universe.
Speaker 2: They'll be not explaining the universe, So they're like, how
Speaker 2: we do here most weeks?
Speaker 1: I just want to taste their exotic chocolate.
Speaker 2: What if we're the anti version of their podcast?
Speaker 1: It's all relative, man, Does.
Speaker 2: That mean our listeners are anti listeners.
Speaker 1: As long as they're not anti listening?
Speaker 2: Hi am hoe May, cartoonist and the author of Oliver's
Speaker 2: Great Big Universe.
Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor
Speaker 1: at UC Irvine, and I definitely listen rather than anti
Speaker 1: listen to all the questions from our listeners.
Speaker 2: But do you anti answer them or do you actually
Speaker 2: answer them?
Speaker 1: I think of my answers are sort of like anti
Speaker 1: questions because they annihilate the question when they receive them.
Speaker 2: Wait what oh I see? And then they become pure
Speaker 2: energy and they disappear into the air. So really there's
Speaker 2: no subsustenance or matter to them. Is that what you're saying?
Speaker 2: The whole interaction just goes up in a flash of light.
Speaker 1: I'm hoping that my answer turns their question into pure
Speaker 1: mental energy illuminating the inside of their mind.
Speaker 2: WHOA sounds a little dangerous and hippieb a little bit cunchy. Yeah,
Speaker 2: but anyways, Welcome to our podcast, Daniel and Jorge Explain
Speaker 2: the Universe, a black Chin of iHeartRadio.
Speaker 1: Where we try to light up the inside of your
Speaker 1: mind with our understanding of the universe. We tackle the
Speaker 1: biggest questions out there, from the smallest particles to the
Speaker 1: most vast astrophysical objects that humanity has ever observed. We
Speaker 1: don't stop at the deep questions. We go for the
Speaker 1: deepest questions. We don't stop at the big questions. We
Speaker 1: try to answer the biggest questions because we think all
Speaker 1: of those questions deserve our best answers.
Speaker 2: That's right. This is the podcast where we're anti itching
Speaker 2: and pro asking questions about the universe and learning how
Speaker 2: everything works and why we are here specifically in this
Speaker 2: part of the universe.
Speaker 1: That's right. And we are pros some kinds of chocolate
Speaker 1: and some kinds of bananas at certain times of the day.
Speaker 2: Actually, you can't be pro against other kinds of bananas
Speaker 2: because most of the bananas out there are the same kind.
Speaker 2: Did you do that?
Speaker 1: That's true? Isn't it like a monoculture?
Speaker 2: Yeah? Yeah, Like they found one banana that tasted really
Speaker 2: good and then they're like, everybody make the same banana.
Speaker 2: So they've just been cloning the same banana for all
Speaker 2: over the world basically.
Speaker 1: But I've had like little red bananas sometimes in Mexico.
Speaker 1: Aren't those different?
Speaker 2: Yeah? Yeah, there are other kinds of bananas, but most
Speaker 2: of the ones that you see, like the yellow ones
Speaker 2: that you see in most supermarkets everywhere pretty much around
Speaker 2: the world, they're the same bananas.
Speaker 1: And welcome to our podcast, The Science of Bananas.
Speaker 2: Ooh, hey, we did a podcast about chocolate. I challenged you,
Speaker 2: Daniel uh huh, to do a podcast about the physics
Speaker 2: of bananas.
Speaker 1: We're doing.
Speaker 2: It's a slippery subject.
Speaker 1: I think it's quite appealing.
Speaker 2: Actually yeah, I'll put a big yellow smile on your face.
Speaker 1: All right. If you're a professor of banana physics, please
Speaker 1: write to me. I want to talk to you about
Speaker 1: the physics of bananas. If you are the chief banana
Speaker 1: Officer of Chikita, also please reach out to me.
Speaker 2: Aren't all physicists technically banana physicists.
Speaker 1: We're all a little bit bananas, that's true.
Speaker 2: Yeah, yeah, yeah, yeah. I mean you're discovering banana facts
Speaker 2: about the universe.
Speaker 1: The universe is bananas, that's for sure.
Speaker 2: But Anyways, we do like to try to answer questions
Speaker 2: here on the podcast because everybody has questions.
Speaker 1: That's right, and we think that everybody's questions deserve answers,
Speaker 1: not just professional physicists who've been spending their whole lives
Speaker 1: working their ways to the forefront of human knowledge. Turns out,
Speaker 1: everybody has questions that are tricky to answer that bring
Speaker 1: us right into the abyss of human ignorance.
Speaker 2: Yeah, because as much as we've learned about the universe
Speaker 2: and discovered its laws and how it all works, there
Speaker 2: are still a whole lot that we don't know about
Speaker 2: how it's all put together, and there are still lots
Speaker 2: of mysteries out there.
Speaker 1: And one of our goals on this podcast is to
Speaker 1: teach you to think about the universe like a physicist,
Speaker 1: to try to make in your mind a minu the
Speaker 1: model for how it all works, and then look for inconsistencies.
Speaker 1: Try to figure out, if you really understand how it works,
Speaker 1: does that mental model work in this scenario or that scenario,
Speaker 1: How does it explain this thing or that thing, or
Speaker 1: do you have a new idea for how we can
Speaker 1: understand something. We want you to flex your brain in
Speaker 1: that direction. And when it doesn't quite work, reach out
Speaker 1: to us and ask us about it. You might be
Speaker 1: surprised that lots of other people out there have the
Speaker 1: same questions you do.
Speaker 2: Does that mean you're trying to convert people to dark chocolate?
Speaker 2: This is starting to feel like a little cult here,
Speaker 2: like a physics cult.
Speaker 1: I think if becoming educated and thinking clearly about the
Speaker 1: universe leads people to eating dark chocolate, that tells you
Speaker 1: something about chocolate.
Speaker 2: Yeah, you're saying that's like a win win.
Speaker 1: I think the universe has a dark chocolate bias.
Speaker 2: Yes, well, we do like to answer questions. Answer Today
Speaker 2: on the podcast, we'll be tackling listener questions number fifty. Well,
Speaker 2: should this be kind of a celebration the fiftieth question episode?
Speaker 1: Yeah? Congratulations us, and congratulations are listeners for asking so
Speaker 1: many awesome questions?
Speaker 2: Now? Did you have a rule that only people over
Speaker 2: fifty can ask questions in this episode?
Speaker 1: Oh? Definitely not. I think one of our listeners is
Speaker 1: quite young, but I try not to ask them because
Speaker 1: some people are sensitive.
Speaker 2: But yeah, fifty is kind of a big number. Are
Speaker 2: we supposed to have a midlife crisis here at some point?
Speaker 1: No, a mid life celebration. I'm turning fifteen a couple
Speaker 1: of years and I'm looking forward to it. I'm embracing it.
Speaker 2: Nice. Do you have any big plans Sit in the
Speaker 2: couch and eat dark chocolate. It's the big five zero
Speaker 2: party for you.
Speaker 1: Yeah. I was going to say that, but that sounds
Speaker 1: like every day around here, So I'm not sure how
Speaker 1: I'm going to distinguish that one day from the other days.
Speaker 2: Oh well, you have to spend it the right way.
Speaker 2: It means you you celebrate life every day, Danna. There
Speaker 2: you go by sitting in your couch and eating dark
Speaker 2: to embrace it.
Speaker 1: And if you celebrate life and celebrate understanding the universe
Speaker 1: and you have questions about how it all works, you
Speaker 1: could have your question answered here on the podcast. Just
Speaker 1: to write to us questions at Danielandjorge dot com. Definitely
Speaker 1: get an answer to your question and you might even
Speaker 1: have it played on the podcast.
Speaker 2: Yes, so we like to answer listener questions here on
Speaker 2: the podcast, And so today we have three great questions
Speaker 2: from people all over the world, and they are about
Speaker 2: dark matter, about rainbows and the wavelength of light. Daniels
Speaker 2: your a theme to these questions.
Speaker 1: These are all questions that arrived on the same day.
Speaker 2: Are they like in the top fifty questions we've ever gotten?
Speaker 1: They're in the top one hundred and fifty questions. Yes,
Speaker 1: because we answer three purposes.
Speaker 2: See, I see there's a special selection process here because
Speaker 2: straight from the inbox into the microphone.
Speaker 1: That's right, that's the pipeline right now.
Speaker 2: But we do have some pretty cool questions here today,
Speaker 2: and so we'll start with this one from Johnny from Sweden.
Speaker 3: Hi, Daniel and Jorge, this is Johnny from Sweden. I
Speaker 3: have a question about dark matter and that I have
Speaker 3: thought about for a long time and haven't found an
Speaker 3: answer to. If you can shed some light on this question,
Speaker 3: I would be very grateful. Could dark matter actually be
Speaker 3: the gravity from matter that exists in a higher dimension.
Speaker 3: Thank you for the best podcast in the whole universe.
Speaker 2: All right, great question from Johnny here, kind of a
Speaker 2: trippy question. There's a lot going on here higher dimensions,
Speaker 2: dark matter gravity hmmmm hmm.
Speaker 1: I think Johnny is trying to find an out of
Speaker 1: the box solution to the question of dark matter.
Speaker 2: It's a heavy question.
Speaker 1: It is a heavy question, one that physicists have been
Speaker 1: struggling with I know that there's a lot of dissatisfaction
Speaker 1: out there with the concept of dark matter being this
Speaker 1: weird invisible matter that nobody ever noticed before but actually
Speaker 1: dominates the universe. Sounds like a conspiracy theory, but we
Speaker 1: think it's real. Of course, we're open to other ideas
Speaker 1: and other theories and other explanations because in the history
Speaker 1: of physics, sometimes the craziest ideas turn out to be true.
Speaker 2: Yeah, even the banana ones, right.
Speaker 1: Dark matter could just be bananas?
Speaker 2: Yeah, okay, So the question here is kind of interesting.
Speaker 2: So dark matter is this thing out there in the
Speaker 2: universe that we don't know much about, but we feel
Speaker 2: its presence, but we can't see it, and it's pretty big.
Speaker 2: It's like a quarter of the mass and energy of
Speaker 2: the universe, right.
Speaker 1: That's right, it's like eighty percent of the stuff in
Speaker 1: the universe and like a quarter of all the energy
Speaker 1: in the universe. Is this invisible source of gravity.
Speaker 2: And it's super mysterious because we know it's there from
Speaker 2: its gravity, like we can feel its pull, or at
Speaker 2: least we can see the effects of its gravitational pull,
Speaker 2: but we've never actually seen it or have any idea
Speaker 2: what it could be like it's it's super weird, right,
Speaker 2: Like it's there, but we can't see it or touch it.
Speaker 1: Yeah, that's right. What we actually know, what we really observe,
Speaker 1: is gravity. So in the most pure sense, the mystery
Speaker 1: is what's the source of this unexplained gravity. There's no
Speaker 1: matter out there in the universe we could see that
Speaker 1: explains all the gravity that we measure, that's holding the
Speaker 1: universe together, that's keeping galaxies from spinning apart, that shape
Speaker 1: the structure of the whole universe. So there's unexplained gravity
Speaker 1: out there, and dark matter is our best explanation for it,
Speaker 1: to say, there must be some new kind of matter
Speaker 1: that's invisible and probably intangible, and that's creating all of
Speaker 1: this gravity, which seems.
Speaker 2: Kind of bonkers if you really think about it, because
Speaker 2: most of the matter that we know about that we've
Speaker 2: known for one, hundreds or thousands of years. You can
Speaker 2: see it and touch it. But this kind of matter,
Speaker 2: it's like you're almost inventing a new kind of matter
Speaker 2: to explain this gravitational phenomenon. But we don't really know
Speaker 2: if that's true or not.
Speaker 1: Right, it's true that it's kind of bonkers, but maybe
Speaker 1: it's less bonkers than extrapolating from our kind of matter
Speaker 1: to the whole universe. It's true that everything we've ever
Speaker 1: experienced is made of atoms, but that doesn't mean that
Speaker 1: everything has to be made out of atoms. The history
Speaker 1: of physics tells us we should be really careful about
Speaker 1: extrapolating from our little corner of the universe and our
Speaker 1: kind of experience to the whole universe. The revolutions of
Speaker 1: quantum mechanics and of relativity were moments when we realize
Speaker 1: we've been looking too narrowly at only our own experience.
Speaker 1: So to say that the rest of the universe should
Speaker 1: also be made out of atoms because we are and
Speaker 1: the Earth is seems like a pretty big leap. What
Speaker 1: we're doing here is saying, well, maybe there are other
Speaker 1: kinds of matter out there, ones that are not made
Speaker 1: out of atoms and so don't interact in the same
Speaker 1: way that atoms do.
Speaker 2: Well, that's one possible explanation, and that seems to be
Speaker 2: where most of physics is leaning. But Johnny here has
Speaker 2: an alternate theory for what causes dark matter exactly.
Speaker 1: He's asking me if this gravity could instead be coming
Speaker 1: from matter that exists in a higher dimension rather than
Speaker 1: like a new invisible kind of matter in our three
Speaker 1: D space.
Speaker 2: M Yeah, fascinating idea. If he's right, he would get
Speaker 2: like all the Nobel prizes. Right.
Speaker 1: If he's right and we can test it and prove it,
Speaker 1: then yeah, Johnny has changed the world and history.
Speaker 2: All right, Well, let's dig into his idea. He's saying
Speaker 2: that maybe a dark matter what we experience or feel
Speaker 2: is dark matter, is maybe actually matter that is not
Speaker 2: in the dimensions we're in, but they're in other dimensions.
Speaker 1: Yeah. I wasn't exactly sure what he meant by matter
Speaker 1: that exists in a higher dimension, because the word dimension
Speaker 1: is a little bit fuzzy, like in physics is a
Speaker 1: crisp definition of it, But in popular science and in
Speaker 1: science fiction and a normal everyday conversation, I think dimension
Speaker 1: means something else. Like often I think people say dimension
Speaker 1: when they mean like a parallel universe, you know, like
Speaker 1: people talk about interdimensional aliens visiting the Earth, Right, I
Speaker 1: think what they mean there are aliens from like a
Speaker 1: parallel universe, from like another copy of the universe, like
Speaker 1: a different set of space.
Speaker 2: Right. And we've talked about this in our books and
Speaker 2: in previous episodes. And I know that you're sort of
Speaker 2: against this sci fi idea of the dimension as being
Speaker 2: like in a separate universe that you go into, but
Speaker 2: it sort of kind of is right. It is sort
Speaker 2: of a the idea of a separate part of space
Speaker 2: than the one we're sitting on.
Speaker 1: I'm not against it. I mean, the idea of a
Speaker 1: parallel universe is certainly possible. This concept of the multiverse,
Speaker 1: that we could have multiple universes, maybe with different starting
Speaker 1: conditions or different even laws of physics. Who knows, right,
Speaker 1: I think that's totally plausible scientifically. I'm just pointing out
Speaker 1: that that's not what we mean by dimension when we
Speaker 1: talk talk about mathematically or physically. But you know, that's
Speaker 1: just semantics. So let's assume first that this is what
Speaker 1: he means by dimension, Like, is dark matter actually gravity
Speaker 1: from a parallel universe? Like maybe in another universe there's
Speaker 1: a huge blob of normal matter and we're feeling it's
Speaker 1: gravity somehow leak out into our universe, and that's what
Speaker 1: we're calling dark matter.
Speaker 2: That's how you're interpreting his question, his scenario.
Speaker 1: Yeah, that's one possibility. We could also talk about the
Speaker 1: other definition, Well, let's do it one aount of time.
Speaker 1: So in this scenario where a dimension means a parallel
Speaker 1: universe and you're wondering if dark matter is actually like
Speaker 1: some blob of stuff in another universe that's creating gravity
Speaker 1: in our universe, I don't think that can work because
Speaker 1: you know, gravity is the bending of space and time,
Speaker 1: and so if matter is bending our space and time,
Speaker 1: then it's in our space time. Like that's sort of
Speaker 1: what it means to be in our space time. If
Speaker 1: we have a shared space time like that matter can
Speaker 1: bend our space time, then it's part of our universe.
Speaker 1: So you can't both be in another universe and affect
Speaker 1: things in this universe.
Speaker 2: Well, well, I think you're sort of arguing semantics now.
Speaker 2: It's like you're saying, like, if there is sort of
Speaker 2: this whole part of this whole universe basically that you
Speaker 2: can exist in but that we don't have a lot
Speaker 2: of access to, but if it's somehow connected to our universe,
Speaker 2: you're saying that means both the universes are really just
Speaker 2: one universe exactly.
Speaker 1: I'm saying it's one universe, and then it's really the
Speaker 1: same idea right, then it's just some kind of matter
Speaker 1: that we can't interact with for some reason, but is
Speaker 1: able to bend our space time therefore cause gravity. That's
Speaker 1: just the same idea as dark matter.
Speaker 2: Well, you sort of sound like my daughter because she's
Speaker 2: always saying like, if the multiverse exists, wouldn't it all
Speaker 2: just be the universe? So I mean, like physicists do
Speaker 2: have a name for like a universe, and something bigger
Speaker 2: than a universe, which is a multiverse.
Speaker 1: Yeah, I think this is a tiny bit different from
Speaker 1: your daughter's point, and though she has a good one.
Speaker 1: You know, the idea of a multiverse is that these
Speaker 1: universes don't interact. They really are separate universes. But if
Speaker 1: they do interact, why would you call them separate universe
Speaker 1: versus how are these higher dimensions? If we're able to
Speaker 1: interact with this kind of matter, and if we can
Speaker 1: interact with it, why can't we see it? Well, the
Speaker 1: answer has to be you with some other new kind
Speaker 1: of matter, which basically we're back at dark matter, right right.
Speaker 2: Well, I feel like it's kind of like, you know,
Speaker 2: if I dig a tunnel from my house to your house,
Speaker 2: I mean a pretty long tunnel, but if we do that,
Speaker 2: would that mean that we all live in one house
Speaker 2: or could you still say that we live in separate houses,
Speaker 2: which is the tunnel connecting?
Speaker 1: You know, I think if I can go from my
Speaker 1: house to your house without going outside, then yeah, it's
Speaker 1: probably just one house.
Speaker 2: What if I put a door in the tunnel and
Speaker 2: I lock it?
Speaker 1: Interesting? I'm sure there's some California legal statute that defines
Speaker 1: what is a residence and what is not, And I'm
Speaker 1: not qualified to a pine on that question.
Speaker 2: All right, all right, all right, So you're shooting down
Speaker 2: this idea that his question actually means that dark matter
Speaker 2: is matter in a separate universe, because then it would
Speaker 2: just be the same.
Speaker 1: Yeah, exactly, Then it would just be dark matter.
Speaker 2: I think maybe Johnny's trying to get at the idea
Speaker 2: that we can sort of feel it, but sort of
Speaker 2: not feel dark matter. So like, maybe this is matter
Speaker 2: that exists in another universe or another huge chunk of
Speaker 2: the universe, but that is not fully connected to our universe.
Speaker 2: Like it maybe it's only connected to tunnel, which in
Speaker 2: this case would be gravity.
Speaker 1: Yeah, potentially, But then you have to ask, like why
Speaker 1: we can't interact with that other kind of matter, and
Speaker 1: then you have to make it another kind of matter.
Speaker 1: If it's sharing our space, bending our space time, it
Speaker 1: doesn't interact with us, then you have to ask, you know,
Speaker 1: why not what makes our matter our matter and that
Speaker 1: matter that matter, and so you have to make it
Speaker 1: another kind of matter, and then it becomes dark matter
Speaker 1: all over again.
Speaker 2: All right, now, say that quickly seven times in a row.
Speaker 1: But there is another way to interpret his question, which
Speaker 1: I think is much more interesting, which is the physics
Speaker 1: definition of dimension. Because he doesn't say another dimension. He
Speaker 1: says a higher dimension, which I think might be a
Speaker 1: useful clue because in physics we think of dimensions not
Speaker 1: as you know it came from another dimension, but as
Speaker 1: directions of motion, like we describe our space as three
Speaker 1: dimensional space or four dimensional space time because we say
Speaker 1: there are three direct of motion, Like if you draw
Speaker 1: axis out in space, you get x, y, and z.
Speaker 1: There's three directions you can move. You don't need four
Speaker 1: directions to describe our space. And so I think he
Speaker 1: maybe he's asking, like, what if our space actually is
Speaker 1: higher dimensional? What if there are four or five, six,
Speaker 1: ten dimensions of space and we're only seeing a slice
Speaker 1: of it and dark matter somehow exists in those higher
Speaker 1: dimensions and is influencing us.
Speaker 2: Interesting, Okay, So and I think maybe this is what
Speaker 2: he means because he uses the word higher dimension, right,
Speaker 2: Like if he meant like an alternate dimension, he wouldn't
Speaker 2: say the word higher.
Speaker 1: Yeah, I think so, although maybe he's smoking something and
Speaker 1: he's higher you never.
Speaker 2: Know, yeah, or maybe he's more elevated or enlightened exactly.
Speaker 1: And this can be hard to visualize what is higher
Speaker 1: dimensional space because three D space sort of fills up
Speaker 1: your mind because we're so used to thinking in three
Speaker 1: D space because we live in three D space, So
Speaker 1: it's easier to take a step from two D space
Speaker 1: to three D space to like take a step back
Speaker 1: and imagine what if our universe was two D. What
Speaker 1: would it be like to have a three dimensional universe
Speaker 1: rather than trying to go from three to four. So imagine,
Speaker 1: for example, we're living on the surface of the ocean.
Speaker 1: You know, our universe is just the two D surface
Speaker 1: of the ocean. But what if the universe actually has
Speaker 1: another dimension that can influence us, like the depth of
Speaker 1: the ocean.
Speaker 2: And so then Johnny's ideas and maybe there's matter in
Speaker 2: our universe that maybe only exists in these other dimensions
Speaker 2: that are not our regular three D dimensions. Mm hmm,
Speaker 2: Now is that possible? Like can matter exists in some
Speaker 2: dimensions and not others? If there is maybe a fourth
Speaker 2: or fifth dimension to our universe? Am I moving around
Speaker 2: in it? Or do I have to be moving around
Speaker 2: in it? Or do you know what I mean?
Speaker 1: I do know what you mean, and I think there's
Speaker 1: an important but maybe subtle distinction here. Matter has to
Speaker 1: exist in all of the dimensions of space time. You
Speaker 1: have to have a location in all those directions. But
Speaker 1: it could be that there's matter that has a different
Speaker 1: location in this new dimension. Like say, for example, our
Speaker 1: universe is the two D surface of the ocean, so
Speaker 1: we have a location in the depth dimension. But you
Speaker 1: could imagine having matter that exists deeper in the ocean,
Speaker 1: so it doesn't intersect our two D surface at all.
Speaker 1: It's in the universe. It exists in all three dimensions
Speaker 1: of that three D universe, but now part of it
Speaker 1: overlaps in our two D slice of the universe. But
Speaker 1: it could still influence us.
Speaker 2: It could still influence it because it's connected to us
Speaker 2: three dimension exactly.
Speaker 1: It's part of our space time, right, And so then
Speaker 1: Johnny's question is, like, well, what if there's matter that's
Speaker 1: part of a four dimensional space, but its location in
Speaker 1: that four D space means we can't see it. It's
Speaker 1: like in a different location along that new fourth dimension
Speaker 1: than we are, Like we're on the three D surface
Speaker 1: of that four D ocean, and it's like deeper down
Speaker 1: along that fourth dimension. If gravity is the curvature of
Speaker 1: space time, and there's matter in that part of the universe,
Speaker 1: the deeper ocean of the universe, wouldn't it also affect
Speaker 1: our space time and therefore look like dark matter?
Speaker 2: Right? It might affect us gravitationally, because let's say, like
Speaker 2: we're on the surface of the ocean, as you're saying,
Speaker 2: and this other matter is right below us. We might
Speaker 2: feel its gravity might pull us in a certain direction,
Speaker 2: but when we try to look around and feel for
Speaker 2: it and touch it and see it, we can't because
Speaker 2: it's actually underneath this exactly.
Speaker 1: And this is a really compelling idea. There's sort of
Speaker 1: two problems with it. One is you need an explanation
Speaker 1: for why we can't touch it, Like why are we
Speaker 1: restricted to the surface of this four dimensional ocean. Why
Speaker 1: can't this kind of matter exist and move in four
Speaker 1: dimensions and we're stuck at this one location in that
Speaker 1: fourth dimension? Like why are we on the surface of
Speaker 1: this ocean and this other matter can like sink or
Speaker 1: swim or whatever.
Speaker 2: You mean, Like, why are we stuck in two dimensions?
Speaker 2: Why can't we go down into the ocean?
Speaker 1: Exactly? Yeah? And why does that matter never surface? Right?
Speaker 1: How can we never see it? But the real problem
Speaker 1: is mathematical, which is that we know how many dimensions
Speaker 1: there are to space time. We can measure how many
Speaker 1: dimensions there are to space time because the dimensionality of
Speaker 1: space time, like how many ways you can move changes
Speaker 1: how gravity works.
Speaker 2: But isn't higher dimensions or other dimensions part of things
Speaker 2: like string theory, Like isn't this an active possibility that
Speaker 2: physicists are considering?
Speaker 1: Yeah, you're exactly right. People do look for extra dimensions.
Speaker 1: But one of the big challenges to those ideas like
Speaker 1: string theory that requires the universe has ten or twenty
Speaker 1: six dimensions or other crazy theories that look for additional
Speaker 1: dimensions of space time is that we're not seeing them.
Speaker 1: We can go out in the universe and we can
Speaker 1: pretty easily look for extra dimensions and we don't see them.
Speaker 1: You know, for example, the number of dimensions of space
Speaker 1: time changes how gravity varies with distance. We know in
Speaker 1: the Newtonian description that gravity gets weaker as you get
Speaker 1: further away, But it doesn't just fall off linearly. It's
Speaker 1: not like twice as far away gravity as twice as weak.
Speaker 1: Twice as far away gravity is four times this week.
Speaker 1: It goes like one over the distance squared. And that
Speaker 1: number of the where tells you the dimensionality of space time.
Speaker 1: If we lived in four dimensional universe, that would be
Speaker 1: one over the distance cubed. If we lived in a
Speaker 1: nineteen dimensional universe, it would be one over the distance
Speaker 1: to the eighteenth power.
Speaker 2: But isn't that only if you assume that gravity works
Speaker 2: a certain way? Like what if the gravity doesn't work
Speaker 2: the way you think it does.
Speaker 1: Yeah, if you want to also overthrow general relativity, then
Speaker 1: you've got a lot more work ahead of you.
Speaker 2: Yeah, I'm not afraid of work. I got on there.
Speaker 1: You and Johnny should get to that higher dimension and
Speaker 1: get to work on it. But you know, if we're
Speaker 1: not changing gravity, if we're not changing how gravity works,
Speaker 1: we're just asking does the universe have additional dimensions? And
Speaker 1: could there be matter hiding those dimensions that creates gravity
Speaker 1: that we see that could be explaining what we're seeing
Speaker 1: instead of dark matter. Then if we're sticking with our
Speaker 1: theory of gravity, that's pretty hard to make it happen.
Speaker 2: Isn't your assumption here that this theory of gravity that
Speaker 2: you're saying that you're sticking by, isn't that a theory
Speaker 2: of gravity that we've gotten by doing experiments then the
Speaker 2: dimensions that we have.
Speaker 1: Access to hmm, yeah, absolutely, could.
Speaker 2: Maybe gravity be actually more powerful than you think and
Speaker 2: those so therefore when you try to measure it, there
Speaker 2: are other dimensions, but it drops off as one over
Speaker 2: our square.
Speaker 1: As you're saying, there are some really cool theories that
Speaker 1: there are these other dimensions, but those dimensions are not
Speaker 1: like our dimensions. They don't go out to negative and
Speaker 1: positive infinity like ours, but they're like curled up little rings,
Speaker 1: so that you can really have like two centimeters or
Speaker 1: one centimeter, and that would solve this problem with gravity
Speaker 1: getting stronger or weaker. It actually would solve the problem
Speaker 1: in a really fascinating way, because if you get really
Speaker 1: close to something, then gravity would get very very powerful,
Speaker 1: and that would actually help us understand why gravity is
Speaker 1: so weirdly weak. But go check out our podcast on
Speaker 1: extra dimensions. But you got to assume something, right, And
Speaker 1: so if you're assuming our theory of gravity that I
Speaker 1: don't think this quite works because if those extra dimensions exist,
Speaker 1: they would have to be super duper tiny and they
Speaker 1: wouldn't be mass in them. But if we're also getting
Speaker 1: rid of our theory of gravity, than they anything's possible.
Speaker 2: Well, it sounds like you kind of have to have
Speaker 2: that open mind right when you explore.
Speaker 1: Yeah, absolutely you do, And we do think that there
Speaker 1: are problems with our theory of gravity. We think that
Speaker 1: general relativity is probably not the final answer to how
Speaker 1: space time works because it ignores the quantum nature of it.
Speaker 1: So we're definitely open to modification of those ideas.
Speaker 2: Yeah, all right, well, then what's the final answer for
Speaker 2: Johnny Here?
Speaker 1: I would say that it's possible that there are additional
Speaker 1: dimensions with matter in them that doesn't appear in our
Speaker 1: three D slice of space, But it would be pretty
Speaker 1: hard to explain all of the dark matter using that
Speaker 1: because those dimensions would be tiny and would be curled up.
Speaker 1: I think it's much more likely that dark matter really
Speaker 1: is a new kind of matter that exists in our.
Speaker 2: Universe, unless, of course, our whole theory of gravity is.
Speaker 1: Wrong, right, yeah, absolutely, all.
Speaker 2: Right, Well, great question, Johnny, Thanks for sending that in.
Speaker 2: Now let's get to our other questions. We have a
Speaker 2: question here about rainbows and one about the wavelength of light,
Speaker 2: so let's dig into those. But first let's take a
Speaker 2: quick break.
Speaker 1: All right.
Speaker 2: We're answering listener questions here today, and our next question
Speaker 2: comes from Sonia. Hi, Daniel, and Jorge. My name is Sonia,
Speaker 2: and I was wondering if there was a minimum and
Speaker 2: or maximum wavelength for light. Thank you, all right, great question, Sonya.
Speaker 2: I wonder what made her think about the nature of light.
Speaker 1: I get this question all the time, which is one
Speaker 1: of the reasons why I wanted to answer it on
Speaker 1: the podcast. It's something that a lot of people seem
Speaker 1: to wonder about because you learn that light is described
Speaker 1: by this wavelength, and the wavelength is connected to the color,
Speaker 1: and then I think it just makes people wonder about
Speaker 1: the extremes, Like we talk about the extreme nature of
Speaker 1: the universe all the time. What is the beginning what
Speaker 1: is the end, what is the biggest, what is the smallest.
Speaker 1: People's curiosity make them want to push to the extreme
Speaker 1: edges of knowledge.
Speaker 2: All right, Well, her question is whether light has a
Speaker 2: minimum or maximum wavelength. So, Daniel, how do you describe
Speaker 2: what the wavelength of light is?
Speaker 1: So we understand light to be electromagnetic oscillations. That just
Speaker 1: means waves in electrical and magnetic fields. Like electrical fields
Speaker 1: are things that we're very familiar with. You have charged particles,
Speaker 1: they make electric fields. Magnetic fields something we're familiar with.
Speaker 1: You have magnets moving electric charges can make magnetic fields.
Speaker 1: And so light is this particular configuration of electromagnetic fields
Speaker 1: where the electric field creates a magnetic field, which creates
Speaker 1: an electric field slashes back and forth. But it's fundamentally
Speaker 1: a wave. It's a wave in these fields that fill space,
Speaker 1: and those waves are described, like all waves, by a
Speaker 1: wave length, which is like how long it takes in
Speaker 1: distance as the light travels before it repeats the same configuration.
Speaker 1: So the electric field's going up and down, the magnetic
Speaker 1: fields going up and down. How far does the light
Speaker 1: travel before the electric field is back into its original configuration.
Speaker 1: That's the wavelength of the.
Speaker 2: Light sort of like a wave in the ocean, right.
Speaker 1: Yeah, exactly. Something that I understand about these waves is
Speaker 1: people see visualizations of light traveling and they see it
Speaker 1: as like a sinusoidal wave moving through space, and they
Speaker 1: get the impression that the light is like wiggling sideways
Speaker 1: that has like a width to it. But light moves
Speaker 1: along a line, and what's changing is the magnitude of
Speaker 1: the electric and magnetic field to draw like little arrows
Speaker 1: that are pegged to that line. Sometimes those arrows are
Speaker 1: drawn like with their base pointing in some direction and
Speaker 1: their tip away from the line. That's just telling you
Speaker 1: the magnitude of the electric field. The electric field doesn't
Speaker 1: like reach out sideways. It's a vector field, and so
Speaker 1: it can point in different directions.
Speaker 2: Okay, So it's an oscillation of these fields. And so
Speaker 2: things can have a wavelength, which, as far as we know,
Speaker 2: can vary, right Like, that's how we get different colors
Speaker 2: when we look at things.
Speaker 1: Exactly. Light comes in all kinds of different wavelengths, and
Speaker 1: different wavelengths are interpreted by our eye as different colors.
Speaker 1: So there's a very narrow set of those wavelengths that
Speaker 1: we can actually see. You call that the visible spectrum.
Speaker 1: It goes, you know, from the reddest colors we can
Speaker 1: see up to the most violet colors we can see.
Speaker 1: But light extends its spectrum well above that and well
Speaker 1: below that the ultraviolet, the infrared, X rays, gamma rays,
Speaker 1: radio waves, all these are just light with different wavelengths.
Speaker 2: Yeah, I think that's basically Sonia's question is, like, light
Speaker 2: can go up in frequency from what we can see,
Speaker 2: and can go down in frequency, and their question is
Speaker 2: how far can that go?
Speaker 1: Yeah, exactly, And there's a few different concepts there. Right now,
Speaker 1: we're just talking about wavelength, right, long wavelengths or like
Speaker 1: radio waves short wavelengths, or like ultraviolet gamma rays. You
Speaker 1: can also talk about frequency. Frequency is the inverse of
Speaker 1: wavelength because light always has the same velocity. So short
Speaker 1: wavelengths means high frequency radio waves that have long wavelengths
Speaker 1: have very low frequency. It's all the same thing. You
Speaker 1: can just express it in different ways. But the frequency
Speaker 1: is useful because it's connected to the energy, like the
Speaker 1: energy of light is connected to its frequency. Higher frequency
Speaker 1: light ultraviolet X rays have more energy than lower frequency
Speaker 1: light like radio waves.
Speaker 2: But it's all the same thing, right, Like for light,
Speaker 2: it's all tied to the speed of light, and so
Speaker 2: you can also say that the energy of a light
Speaker 2: wave is tied to its wavelength as well.
Speaker 1: Yeah, absolutely has an inverse relationship to its wavelength, and
Speaker 1: it's linearly proportional to its frequency.
Speaker 2: Right, it's the same thing. So you can also say
Speaker 2: that the energy or something is tied to its wavelength.
Speaker 2: The higher the wavelength, the lower the energy of light.
Speaker 1: Yeah, that's exactly right.
Speaker 2: Okay, So her question is how far up and down
Speaker 2: the frequency spectrum or the wavelength spectrum can you go
Speaker 2: with light?
Speaker 1: Yeah, and so in theory there's no limit, Like, as
Speaker 1: you say, it's just controlled by the energy. Is there
Speaker 1: any limit to how much energy a photon can have?
Speaker 1: Is there any minimum to how much energy a photon
Speaker 1: can have? The only limit there is zero. Theoretically, it
Speaker 1: can't have zero energy, but it could have any tiny
Speaker 1: amount of energy, any number you want, zero points zero zero,
Speaker 1: zero zero zero zero, one hundred zeros and then a
Speaker 1: one uels or whatever. You could have a photon with
Speaker 1: that energy.
Speaker 2: Could you, though? Are you sure about that? Like what
Speaker 2: if the electromagnetic fields are quantized?
Speaker 1: So if we're still just talking theoretically, then electromagnetic fields
Speaker 1: are quantized, but they're quantized in number of photons, not
Speaker 1: in energy of those photons. And so if we're just
Speaker 1: talking about like photons out in free space, they're not
Speaker 1: confined or anything, then they can have any wavelength and
Speaker 1: therefore any energy. This is just in theory. There are
Speaker 1: some maybe practical limits to how long a wavelength of
Speaker 1: photon might have or how short a wavelength of photon
Speaker 1: might have, But according to like theoretical quantum field theory
Speaker 1: of electromagnetism, there are no limits there except for zero.
Speaker 1: You can't have a photon with zero energy.
Speaker 2: According to the theory.
Speaker 1: Right, Yeah, according to the theory.
Speaker 2: Is it possible that maybe they'll turn out later that
Speaker 2: these magnetic fields are quantized or to have a minimum
Speaker 2: wavelength to them.
Speaker 1: Yeah, it's possible, Like we might extend our theory to
Speaker 1: understand the universe differently. For example, this theory assumes that
Speaker 1: that space is continuous, that it's smooth, that there's no
Speaker 1: shortest possible distance in space, and therefore no minimum wavelength,
Speaker 1: that you can keep getting smaller and smaller and smaller
Speaker 1: distances than those things are meaningful but that might be wrong.
Speaker 1: It might be that space is not smooth and continuous,
Speaker 1: but it's pixelated, that there's a minimum meaningful distance to space,
Speaker 1: in which case there would be a minimal possible wavelength
Speaker 1: to light.
Speaker 2: Or at least one meaningful minimum wavelength, right like, maybe
Speaker 2: you could still have minimum wavelengths, smaller wavelengths which you
Speaker 2: just wouldn't be able to see them, or are you
Speaker 2: saying they might not exist at all?
Speaker 1: In this scenario, they wouldn't exist at all because remember
Speaker 1: that light is an oscillation of the electromagnetic field, which
Speaker 1: is part of space. And so if you have like
Speaker 1: a fundamental unit of space, like a space pixel, then
Speaker 1: you can't have anything very inside that pixel. The electromagnetic
Speaker 1: field can only have like one value in that pixel,
Speaker 1: which means you couldn't have an oscillation with a wavelength
Speaker 1: smaller than a pixel, sort of like on your screen,
Speaker 1: each pixel can only have one color, right, It's like
Speaker 1: it's green, or it's blue, or it's pumpkin or whatever
Speaker 1: the color is, but it can't be two colors at
Speaker 1: the same time. That's what a pixel is. And so
Speaker 1: if space is quantized, then every unit of space can
Speaker 1: only have one value of the electromagnetic field, and therefore
Speaker 1: you couldn't have oscillations smaller than the size of that pixel.
Speaker 2: I see, all right, Well that's only if space is pixelated.
Speaker 2: Can you imagine other scenarios, like maybe inside of a
Speaker 2: black hole at this singularity, could maybe the wavelength of light?
Speaker 2: Good is zero? There?
Speaker 1: Yeah, that's a really cool question. But singularities are a
Speaker 1: feature of general relativity, and general relativity assumes that space
Speaker 1: is smooth and continuous. The whole idea of having a
Speaker 1: singularity is having a bunch of mass in an infinitely
Speaker 1: small space. So you can't both have a bunch of
Speaker 1: stuff in an infinitely small space and a minimum wavelength
Speaker 1: of light because having stuff in an infinitely small space
Speaker 1: assumes that there's no limit to how small stuff can get,
Speaker 1: so near singularities or near the event horizon of black holes,
Speaker 1: light can go to very extremely long or short wavelengths.
Speaker 2: I see, But what about the singularity? Then? Would light
Speaker 2: have a zero wavelength if a singularity exists?
Speaker 1: What is the wavelength of light inside a singularity? Yeah,
Speaker 1: that's a cool question.
Speaker 2: That's why they pay me the big books.
Speaker 1: Then that's a really cool question. I don't think it
Speaker 1: could have zero wavelength, because that would imply infinite energy,
Speaker 1: and a singularity is infinite density. But it's not infinite energy,
Speaker 1: it's finite energy. And so in principle, you can like
Speaker 1: make a black hole and singularities just piling up a
Speaker 1: bunch of high energy photons in a small space. You
Speaker 1: don't need infinite energy to do that.
Speaker 2: All right, Well, that's the minimum of light for a wavelength.
Speaker 2: What about the maximum? Can light have a maximum wavelength.
Speaker 1: In order to generate a photon? In order to generate light,
Speaker 1: you usually need an antenna. You need something to oscillate
Speaker 1: to make these electromagnetic fields. Like you want to make
Speaker 1: a radio waves, you need to have an antenna that's
Speaker 1: pretty big where the electrons can zoom up and down
Speaker 1: that antenna to make wiggles in the electromagnetic field. So,
Speaker 1: in principle, if you want to make a photon that's
Speaker 1: like a galaxy wide, you need an antenna that's like
Speaker 1: a galaxy wide. So, for example, if the universe is
Speaker 1: not infinitely big, if it's a finite size, that limits
Speaker 1: the practical size of a photon's wavelength because you couldn't
Speaker 1: make an antenna bigger than the universe, and therefore you
Speaker 1: couldn't make a photon with a wavelength bigger than the universe.
Speaker 2: I feel like now you're sort of mixing practicality with theory.
Speaker 2: Oh yeah, so if we stick with theory, is there
Speaker 2: a theoretical maximum wavelength of light?
Speaker 1: I don't think there is theoretically, even if the universe
Speaker 1: is finite, because you could just have like a part
Speaker 1: of the photon's wave. So theoretically, I don't think there's
Speaker 1: any limit to the wavelength of a photon. You're talking
Speaker 1: really really low energy photons with really long wavelengths, so
Speaker 1: long that even if the universe is finite size, you
Speaker 1: might only be able to fit a part of that
Speaker 1: wave in it. Still technically would have that very low wavelength.
Speaker 1: So I think there's only a practical limit, not a
Speaker 1: theoretical limit.
Speaker 2: So like, theoretically you could have a photon that's bigger
Speaker 2: than the universe, Well.
Speaker 1: You would have a photon whose wavelength is bigger than
Speaker 1: the universe. You wouldn't have the full period of that
Speaker 1: photon's oscillation inside the universe. Yeah.
Speaker 2: Well, I mean, like, let's say the universe is much
Speaker 2: bigger than what we can see than the observable universe. Technically,
Speaker 2: the whole observable universe could right now be sitting inside
Speaker 2: of a photon.
Speaker 1: Yeah, it sounds like the cool title of a book,
Speaker 1: the photon bigger than the universe, the.
Speaker 2: Light that contains our universe. Oh my gosh, bestseller.
Speaker 1: Boom. Let's get Anthony Door to write that one. All
Speaker 1: the light we cannot fit in the universe.
Speaker 2: Yeah, there you go, all the light It Waitedn't somebody
Speaker 2: already write that?
Speaker 1: I think you want a pultzer for it, but I'm
Speaker 1: sure we can just copy it.
Speaker 2: Yeah, we're changing a few words. You know. That counts
Speaker 2: as fair game, fair use. Well, so you talked about
Speaker 2: the theory of it. Now now you're sort of getting
Speaker 2: into the practicality of it. And you said, there's maybe
Speaker 2: a practical limitation to making a photon bigger than the
Speaker 2: observable universe. What about is there a practical limitation to
Speaker 2: making a minimum or smallest photon? Because isn't all lights
Speaker 2: sort of generated by the motions or interactions of particles
Speaker 2: which are quantized.
Speaker 1: Which are quantized, but you only need one electron, like
Speaker 1: a single electron wiggling can make a photon The only
Speaker 1: practical limit to making a photon with a really short
Speaker 1: wavelength is that it's really high energy. So you need
Speaker 1: a lot of energy. So if you took like, for example,
Speaker 1: all the light the Sun emitted in a year and
Speaker 1: poured that into a single, like really powerful photon, that
Speaker 1: would be very very short wavelength.
Speaker 2: Well, there might be a limitation to like how much
Speaker 2: energy there is in the universe, in which case that
Speaker 2: would give you maybe limitation to how small of a
Speaker 2: photon you can make.
Speaker 1: Yeah, if you annihilated all of the particles in the
Speaker 1: universe and converted all of their energy into a single photon,
Speaker 1: then number one, you would no longer exist to observe
Speaker 1: this amazing photon. But even then it would be finite,
Speaker 1: because the amount of energy in the universe, the observable
Speaker 1: universe at least, is finite. So, yeah, you're right, that's
Speaker 1: a practical limit to how short wavelength photon you can make.
Speaker 2: Yeah, that would be the sequel novel all the light
Speaker 2: we cannot make in the universe.
Speaker 1: The other limitation there is the black hole. Right, you
Speaker 1: make a very short wavelength photon, you're going to make
Speaker 1: a black hole.
Speaker 2: Oh. Interesting, And so if you use all the energy
Speaker 2: in the universe to make a photon, it would just
Speaker 2: instantly turn into a giant black hole, or would it.
Speaker 1: I don't know, No, it definitely would. I mean you're
Speaker 1: talking about putting all the energy in the universe in
Speaker 1: one spot. That's definitely a black hole for sure.
Speaker 2: Oh so maybe that is maybe a limitation on the
Speaker 2: minimum size of a photon, Like at some point a
Speaker 2: photon would be so small it would turn into a
Speaker 2: black hole.
Speaker 1: Yeah, a photon with energy greater than the Plank energy
Speaker 1: would turn into a black hole. That's basically how you
Speaker 1: calculate the Plank energy.
Speaker 2: So there is a limitation to a minimum wavelength of light.
Speaker 1: Then, yeah, that's the practical limitation.
Speaker 2: Because after that you get a black hole.
Speaker 1: Unless that's what you wanted, right, that's sort of what
Speaker 1: you were going for, like I want a black hole
Speaker 1: singularity photon.
Speaker 2: So there is a theoretical and practical minimum wavelength of light.
Speaker 1: Yeah, the asterisk there is. We don't really know what
Speaker 1: happens in that scenario. Like if you make a photon
Speaker 1: with energy greater than the Plank energy, so it collapses
Speaker 1: into a black hole, is it still a photon? Does
Speaker 1: it turn into like singularity stuff. We don't know what
Speaker 1: goes on inside a black hole, so I don't know
Speaker 1: if that really is technically a limit. It might just
Speaker 1: be like, Okay, you made a photon, but now it's
Speaker 1: also a black hole at the same time, is it
Speaker 1: still a photon? I don't know.
Speaker 2: All Right, Well, I guess that's all the answer that
Speaker 2: we can fit in this universe podcast.
Speaker 1: That was both the longest and shortest answer we could find.
Speaker 2: Yeah, we reached the maximum in both directions. Here. We
Speaker 2: need another dimension just to fully explore this question. That
Speaker 2: was a great question. Thank you, Sonia. And so let's
Speaker 2: get into our last question of the day, which is
Speaker 2: about rainbows. Let's jump into that rainbow, but first let's
Speaker 2: take another quick break. All right, we're answering listener questions
Speaker 2: here today, and our last question comes from Clark, who
Speaker 2: is seven years old.
Speaker 1: And I love getting questions from kids. I love when
Speaker 1: kids listen to our podcast. I love when kids listen
Speaker 1: with our parents and talk about it. Thanks to all
Speaker 1: of you parents who are raising the next generation of scientists.
Speaker 2: Why do we always see rainbows bend the same way?
Speaker 2: Why can't they bend sideways or upside down. All right,
Speaker 2: great question from Clark. I guess basically the question is
Speaker 2: why does a rainbow look like a rainbow? Like, why
Speaker 2: does it look like an arc in an S shape
Speaker 2: or any other shape? And specifically, why is always an
Speaker 2: arch bending down? From what we can see? Could you
Speaker 2: have a smiley face rainbow or a S shade rainbow
Speaker 2: without turning your head?
Speaker 1: Yeah, it is very cool. Rainbows actually do bend sideways
Speaker 1: and upside down. Rainbows are actually circles, so you're only
Speaker 1: seeing part of it because the earth is blocking the
Speaker 1: rest of the view.
Speaker 2: They're not actually circles, right, because they're not actually things.
Speaker 1: Yeah, so rainbows are not physical objects. There are image
Speaker 1: that you are seeing. Your brain reconstructs what it thinks
Speaker 1: is going on out there based on the directions of
Speaker 1: light that hit your eye, and so you can see
Speaker 1: things that are like not there, the same way you
Speaker 1: can see yourself in the mirror, but you're not actually
Speaker 1: behind the mirror. It's just an image of yourself in
Speaker 1: the mirror.
Speaker 2: Right. A rainbow is like a mirage or like a
Speaker 2: hologram basically, right.
Speaker 1: Yeah, it's an image your brain has reconstructed. It says
Speaker 1: all this light is coming in in this direction, so
Speaker 1: it looks like there is something they are giving off
Speaker 1: this light. But it doesn't have to be like just
Speaker 1: like with a mirror, you don't have to have somebody
Speaker 1: standing through a window to create that image. You can
Speaker 1: just use a mirror and it looks exactly as if
Speaker 1: there's an inverted copy of you standing on the other
Speaker 1: side of the wall.
Speaker 2: Right, Although I guess philosophical you could say that the
Speaker 2: rainbow is a physical object, right, because it's the water
Speaker 2: droplets that make up the rainbow that it is the rainbow.
Speaker 1: Yeah, in the same way that the mirror is a
Speaker 1: physical object and it's creating this image in your mind.
Speaker 1: And you're right about the physical mechanism. Also, the reason
Speaker 1: you see a rainbow is that the sunlight is hitting
Speaker 1: the water droplets, and those water droplets are spreading out
Speaker 1: the light. When light passes from air to water, different
Speaker 1: colors of light bend a different amount as that happens,
Speaker 1: and so when they go into the water droplet and
Speaker 1: then back out of the water droplet, different colors green, red, blue, yellow,
Speaker 1: all take a slightly different path, And that's why you
Speaker 1: see a rainbow. It spreads the light out into those
Speaker 1: different colors, right.
Speaker 2: I wonder if it would help to sort of paint
Speaker 2: the situation that leads to a rainbow, right, because we
Speaker 2: don't see rainbows every day. You also don't see them
Speaker 2: anywhere in the sky. They have to mean a certain
Speaker 2: position in the sky relative to the sun for you
Speaker 2: to see a rainbow, and there also has to be
Speaker 2: a lot of rain or water in the air, right.
Speaker 1: Yeah, exactly. So in order to make a rainbow, you
Speaker 1: need to distribute a bunch of water droplets in the
Speaker 1: air and have the sun behind you. Then the sun
Speaker 1: will pass like over your head, it will enter those
Speaker 1: water droplets, bounce off the back of the water droplet,
Speaker 1: come back out, and then hit your eye. And so
Speaker 1: if you imagine just like a single water droplet, it's
Speaker 1: going to be spreading the light out in different colors green, red, orange, purple,
Speaker 1: or whatever. And you will see one color of light
Speaker 1: from each droplet. But if the droplet is a little
Speaker 1: bit higher, you're going to be seeing red light from it.
Speaker 1: If a droplet is a little bit lower, you're going
Speaker 1: to be seeing purple light from it. Because those different
Speaker 1: colors come out of the droplets at slightly different angles.
Speaker 2: Right, you can have to imagine like a wall of
Speaker 2: water drops floating in the air in front of you,
Speaker 2: and the light the sun is right behind you. It's
Speaker 2: shining in light sort of like a projector or a flashlight.
Speaker 2: It's shining the light behind you over your head, and
Speaker 2: it's hitting this giant wall of water drops. Some of
Speaker 2: the light will just go through, right, Yeah, and you'll
Speaker 2: never see that light again. But over the arch of
Speaker 2: the rainbow, light actually gets reflected back to you in
Speaker 2: different colors.
Speaker 1: That's right, And every droplet there is sending every color
Speaker 1: of light out, so it's like a big screen. Every
Speaker 1: droplet is sending out a whole spectrum purple, blue, green, orange, red,
Speaker 1: all in different directions. Where you're standing, You're seeing a
Speaker 1: ring of droplets that are all sending you red light,
Speaker 1: and then you see another ring of droplets that are
Speaker 1: all sending you green light, and then another ones that
Speaker 1: are all standing you purple light. And you see them
Speaker 1: in a ring because they all have the same angle
Speaker 1: relative to you.
Speaker 2: Yeah. I think this is maybe where it gets kind
Speaker 2: of hairy to describe in an audio podcast, But basically
Speaker 2: the answer to Clark's question is geometry, right, Basically, the
Speaker 2: reason it looks like an arch is that the physics,
Speaker 2: the phenomenon that creates the illusion of a rainbow basically
Speaker 2: depends on the geometry of you and your eyeball and
Speaker 2: the sun, and that's why it looks like it's a semicircle.
Speaker 1: Yeah, exactly. And if the ground wasn't in the way
Speaker 1: and there was water down there as well, like if
Speaker 1: you were floating up in the atmosphere and you're looking
Speaker 1: at a sheet of water, you would see the rainbow
Speaker 1: as a complete.
Speaker 2: Circle, right, And so the reason it's not like an
Speaker 2: arch bending upwards or to the side has to do
Speaker 2: with the fact that the sun is a circle, or
Speaker 2: because your eyeball is a circle or is it? Oh,
Speaker 2: because the water drops are a circular, right, they're little spheres.
Speaker 1: That's the real reason, right, Well, the water drop is
Speaker 1: our little spheres, but they don't have to be spheres
Speaker 1: for this to work. The reason that the rainbow is
Speaker 1: an arch is because actually it's a circle, and you're
Speaker 1: only seeing part of it circle, because any drop along
Speaker 1: that circle has the same angle relative to your eye
Speaker 1: of the same colored light coming out of it.
Speaker 2: Now, but I think if the water drops for a
Speaker 2: different shape, the rainbow would be a different shapeouldn't it.
Speaker 2: Like let's say if the all the rain drops were
Speaker 2: shaped like a cylinder vertically, then you wouldn't see maybe
Speaker 2: an arch, or maybe they were all football shade pointing
Speaker 2: in the same direction. Then the arch would be kind
Speaker 2: of distorted.
Speaker 1: Oh yeah, I see what you're saying. That's definitely true.
Speaker 1: The reason it's a circle is that there's a symmetry
Speaker 1: right that the water droplets can hit you from any
Speaker 1: angle because there's spheres. Yeah, I get your point. So
Speaker 1: if they all were distorted, that would definitely would distort
Speaker 1: the rainbow. It wouldn't be a circle anymore. There's other
Speaker 1: interesting things going on there with the geometry, like sometimes
Speaker 1: you can see a second rainbow, and that happens because
Speaker 1: the light bounces twice within the rain drop. Like it
Speaker 1: goes into the rain drop, it bounces off the back
Speaker 1: of the rain drop, and then bounces off again from
Speaker 1: the back of the rain drop and then comes out
Speaker 1: of the rain drop. So it comes out at a
Speaker 1: different angle than the like primary beam, which is why
Speaker 1: you see it in a different location. And because it
Speaker 1: bounces twice, it actually flips the colors. So the normal
Speaker 1: rainbow has like red on the top and purple on
Speaker 1: the bottom. The double bow, the second one has purple
Speaker 1: on the top and red on the bottom.
Speaker 2: So maybe the real answer for Clark here is that
Speaker 2: the reason it bends the same way is because you're
Speaker 2: always looking at it the same way, and because the
Speaker 2: ground is always beneath you.
Speaker 1: Mm hmm. It's generated by the angle between the sun
Speaker 1: and your eyeball and the rain drops, and so you're
Speaker 1: going to see it wherever that relationship exists, which is
Speaker 1: why it always looks the same, right.
Speaker 2: But I think as you were saying, like, so, let's
Speaker 2: say we were out there in space and there was
Speaker 2: a huge cloud of water drops in front of me
Speaker 2: and the sun was behind me, I would see a
Speaker 2: rainbow as a circle, yes, And if there was, you know,
Speaker 2: a planet blocking some of that water on top, then
Speaker 2: I would see the rainbow as an up pointing arch, yeah, exactly. Yeah,
Speaker 2: Or somehow the water drops that ended on the right
Speaker 2: side of this cloud, or there was a planet on
Speaker 2: my right, then the rainbow would look like a sea
Speaker 2: exactly exactly.
Speaker 1: And if you're an alien or a different species that
Speaker 1: could see different wavelengths of light. Then you would see
Speaker 1: the rainbow extend. Because rainbows have not just visible light
Speaker 1: but also U V and in for red light in them.
Speaker 1: They are invisible rings to the rainbow, so it would
Speaker 1: look like a thicker rainbow. There'd be more stripes to it,
Speaker 1: depending how your alien or manti shrimp brain interpreted it. Interesting.
Speaker 2: Okay, so here's the answer for Clark. The reason rainbows
Speaker 2: are circular is because the water drops our little spheres.
Speaker 2: There are also little circles, little balls of water. And
Speaker 2: the reason you only see the top arch of that
Speaker 2: circle is because usually the ground is blocking the bottom half.
Speaker 2: But if the ground was blocking the side half or
Speaker 2: the top half, then we would see rainbows as a
Speaker 2: different shape. Or if it just tilt your head.
Speaker 1: Yeah, exactly, if you stood next to a cliff, you
Speaker 1: could see a rainbow sideways.
Speaker 2: All right, Well, great question, Klark. Thanks for sending that in,
Speaker 2: and thanks to everyone who send the question in today,
Speaker 2: and thanks.
Speaker 1: To everybody who asks questions and thinks about the universe
Speaker 1: and joins this community of curious people wanting to understand.
Speaker 2: Yeah, and thank you for listening to this program and
Speaker 2: telling all your friends about it, and posting it on
Speaker 2: social media and reviewing us on the podcast apps, laying
Speaker 2: it for your children, and liking banana jokes. All right, well,
Speaker 2: we hope you enjoyed that. Thanks for joining us. See
Speaker 2: you next time.
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