Latest / Daniel and Kelly’s Extraordinary Universe / If the Universe is infinite, why is the night sky dark?

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If the Universe is infinite, why is the night sky dark? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: A, Kelly, Is it getting warm where you guys are?
0:11
Speaker 1: Oh my gosh. Yeah, it's Virginia and the Virginia summer
0:14
Speaker 1: is definitely here. So does that mean sunscreen for the
0:17
Speaker 1: kids every single day? Yeah? They hate it, but I'm
0:20
Speaker 1: a pretty big believer in that. And what about at
0:23
Speaker 1: night time? Do you guys put sunscreen on at night?
0:25
Speaker 1: Why would we put sunscreen on at night? What do
0:27
Speaker 1: you know, physicists? But I don't know. Well, you know,
0:29
Speaker 1: stars are actually just other suns, right, So it's still
0:33
Speaker 1: sunlight even at night. So you're trying to tell me
0:35
Speaker 1: that the stars are going to give us skin cancer. Look,
0:37
Speaker 1: I'm not trying to make astronomy scary. I'm just looking
0:40
Speaker 1: out for your kids. Uh huh. Well, I guess I
0:43
Speaker 1: gotta order a gallon to star screen. Somebody out there
0:46
Speaker 1: is going to get star cancer, oh boy. Or someone's
0:48
Speaker 1: gonna make a lot of money off of star screen. Hi.
1:06
Speaker 1: I'm Daniel. I'm a particle of physicist and a professor
1:09
Speaker 1: at UC Irvine, and it was the night sky that
1:12
Speaker 1: got me into science. I'm Kelly Weener Smith. I'm an
1:15
Speaker 1: adjunct assistant professor at Rice University. And while I prefer parasites.
1:19
Speaker 1: I do love looking at the night sky. What about
1:22
Speaker 1: parasites at night? I mean, you know, the parasites are
1:25
Speaker 1: always out there, and they're just as good at night
1:27
Speaker 1: as they are during the day. They're always fascinating. And
1:31
Speaker 1: then the super meta question, do parasites look at the stars?
1:34
Speaker 1: Can you enjoy the stars with your parasites? Oh, you know,
1:38
Speaker 1: ecto parasites can probably look at the stars. But the
1:42
Speaker 1: endo parasites, the ones like in your guts, they probably
1:44
Speaker 1: don't have as good of you. Exo parasites are those
1:48
Speaker 1: like exo planets and exo moons. Are you talking about
1:51
Speaker 1: astro parasites parasites on other planets? I said ecto E, C,
1:57
Speaker 1: T O M. But you know there could be exo parasites. Uh,
2:01
Speaker 1: it wouldn't surprise me to find out that, you know,
2:03
Speaker 1: the second organism on another planet was a parasite of
2:06
Speaker 1: the first. That would not be too surprising to be Well.
2:09
Speaker 1: Welcome to the podcast Daniel and Hoordey explain the universe
2:12
Speaker 1: where we have just invented the field of exo parasitology. Yes,
2:19
Speaker 1: job security. On this podcast we do delve deep into
2:24
Speaker 1: the mysteries of the universe, What is out there? How
2:27
Speaker 1: does it work? What squirmy little beings are living on
2:30
Speaker 1: the backs of other squirmy little beings on weird planets
2:33
Speaker 1: around other stars. We don't shy away from the biggest, deepest,
2:37
Speaker 1: squishiest questions in the universe, and sometimes we like to
2:41
Speaker 1: ask really basic, fundamental questions about the nature of the universe.
2:45
Speaker 1: We find ourselves in because we think that our tiny, little,
2:48
Speaker 1: squishy human brains are weirdly, somehow capable of understanding the
2:53
Speaker 1: deepest cosmic mysteries, of looking out into the night sky
2:57
Speaker 1: and from that information gleaning something big and deep and
3:00
Speaker 1: true about the nature of the cosmos. My friend and
3:04
Speaker 1: co host Orgy can't be with us today, but we
3:06
Speaker 1: are very happy to have our regular guest host, Kelly Smith. Kelly,
3:10
Speaker 1: thanks again for joining us today. Thanks for having me.
3:12
Speaker 1: I'm excited to be here again, and today's topic is
3:14
Speaker 1: super interesting. I can't wait to talk about it. Awesome. Well,
3:17
Speaker 1: one reason I love thinking about today's topic is because
3:20
Speaker 1: it really was the night sky that got me into science.
3:22
Speaker 1: That wasn't just a sound bite when I was a kid.
3:24
Speaker 1: I was just like all those other undergrads who write
3:27
Speaker 1: cheesy essays about staring up in the night sky and
3:30
Speaker 1: wondering about the universe. You know, it really is a
3:32
Speaker 1: very attractive and easy way to get into asking questions
3:36
Speaker 1: about the universe. Were you also a star watcher as
3:38
Speaker 1: a kid, you know, I was more looking down at
3:41
Speaker 1: the ground, and you know, as an ecologist, all of
3:43
Speaker 1: our essays start off with I always liked playing in
3:46
Speaker 1: the woods, so I mostly was looking down, but I
3:49
Speaker 1: did like looking up when it got too dark to
3:51
Speaker 1: find salamanders and stuff. And as an adult, I really
3:54
Speaker 1: like looking at the night sky. Well, the funny thing
3:56
Speaker 1: about the night sky in terms of science is that
3:58
Speaker 1: it's very attractive. It's like easy if your people to
4:01
Speaker 1: hear about, and astronomy seems like a very accessible topic.
4:04
Speaker 1: But when it gets down to it, you know, like
4:06
Speaker 1: doing science of the stars is mostly like standing around
4:10
Speaker 1: in the cold looking through a telescope. And so while
4:13
Speaker 1: I was very excited about astronomy as a young kid,
4:16
Speaker 1: when I got older and tried to get into it
4:18
Speaker 1: a little bit, I found that it wasn't really actually
4:20
Speaker 1: for me. You know, I didn't really like staying up
4:23
Speaker 1: late at night or staring through a telescope or dealing
4:25
Speaker 1: with optics, So sometimes the day to day grind of
4:28
Speaker 1: science doesn't quite match with your like fantastical aspirations. And
4:32
Speaker 1: you know, I think one thing that draws astronomy students
4:35
Speaker 1: and ecology students together is that both of those groups
4:38
Speaker 1: of students think that it's going to be one thing,
4:40
Speaker 1: like playing outside a lot, and then it ends up
4:42
Speaker 1: being a lot of math, for example, and that is
4:45
Speaker 1: where we lose a bunch of the outside enthusiasts. Is
4:48
Speaker 1: there a lot of math in ecology and then you're
4:50
Speaker 1: like putting down squares and counting how many slugs or something. Well,
4:53
Speaker 1: I mean there's counting, but then there's also trying to like,
4:56
Speaker 1: you know, write mathematical models to describe host predator dynamics
4:59
Speaker 1: or whatever. Actually I took ecology because I thought it
5:02
Speaker 1: was going to be an easy a because it's like
5:03
Speaker 1: hippies playing in the woods. But then when I found
5:06
Speaker 1: the equations, I fell in love. So for me, that's
5:08
Speaker 1: what hooked me, which I'm sure is what hooked some
5:11
Speaker 1: students on astronomy as well. Yeah. Absolutely, it is amazing
5:14
Speaker 1: to think that we can describe what's going on over
5:17
Speaker 1: there and it's fascinating to me that we can learn
5:19
Speaker 1: about the rest of the universe just from looking at
5:21
Speaker 1: our local neighborhood. You know that we have an example
5:23
Speaker 1: star right here in our solar system, and we can
5:26
Speaker 1: study it and think about and understand it, and then
5:28
Speaker 1: we can apply that knowledge to the stars to saying, oh,
5:31
Speaker 1: those are just other sons. And you know, I have
5:33
Speaker 1: to embarrass my daughter because it was only six months
5:36
Speaker 1: ago or so. I was talking to her and I
5:38
Speaker 1: said something like, you know, the stars, they're just other sons, right,
5:41
Speaker 1: And she was like, what are you serious? And then
5:44
Speaker 1: I was like, what are you serious? You didn't know
5:47
Speaker 1: that already. She was like no, I had no idea.
5:50
Speaker 1: And I was so embarrassed, Like, here I am, you know,
5:52
Speaker 1: a physicist, a science communicator, and my own daughter doesn't
5:56
Speaker 1: know this very basic thing about the universe. Yeah. I
5:58
Speaker 1: was gonna ask if you're a little embarrassed admitting that,
6:01
Speaker 1: and I, yeah, I guess you are. But you know,
6:03
Speaker 1: we all have gaps things that we just assume our
6:05
Speaker 1: kids intu it in, but they didn't, and it's you know,
6:07
Speaker 1: surprising sometimes. Yeah, she thought that they were sort of
6:10
Speaker 1: like other stars, but they were much closer and much
6:12
Speaker 1: dimmer that we were surrounded by like an ocean of
6:15
Speaker 1: not very bright suns or something. I don't think she
6:18
Speaker 1: had actually thought about it very much, and so you're right,
6:20
Speaker 1: it's just something we never actually came to the point
6:22
Speaker 1: of talking about. And now we have a fun running joke,
6:25
Speaker 1: which is, at any time something obvious comes up about
6:27
Speaker 1: the universe, I'm like, you are aware, of course that
6:30
Speaker 1: the Earth is round, right, She's like, Dad, it's important
6:35
Speaker 1: you never let kids forget if they got something wrong.
6:38
Speaker 1: You know, you need to remind them of that all
6:40
Speaker 1: the time. No, trying not to make her feel bad. Now,
6:43
Speaker 1: it's just my opportunity to get to like insert a
6:45
Speaker 1: little bit of science into any random conversation because now
6:47
Speaker 1: I have an excuse because maybe this is another thing
6:50
Speaker 1: we just never got around to talking about. That's right,
6:53
Speaker 1: And I'm sure she doesn't find it frustrating that you
6:55
Speaker 1: do that at all. Oh, she teasings me back. She
6:59
Speaker 1: is quite capable of that. I'm glad. But this question
7:03
Speaker 1: of the stars and the night sky really has lots
7:05
Speaker 1: of fascinating layers to it. A lot of people are
7:08
Speaker 1: surprised when they learn, for example, that the stars are
7:11
Speaker 1: there during the daytime, like you could see them in
7:14
Speaker 1: principle they are shooting starlight at you and it's hitting
7:17
Speaker 1: the earth. It's just sort of like overwhelmed by the sun.
7:20
Speaker 1: I feel like that's probably something I didn't figure out
7:23
Speaker 1: until I was much older, Like that's another one of
7:25
Speaker 1: those obvious things that you should have known. But I
7:27
Speaker 1: feel like I remember, maybe like as a senior in
7:29
Speaker 1: high school, having a like, oh, they're still there moment,
7:34
Speaker 1: which I should be particularly embarrassed about. But as I said,
7:36
Speaker 1: I wasn't looking up that much. I was mostly looking down. Yeah,
7:39
Speaker 1: And there's those shame in coming to understanding of the universe.
7:42
Speaker 1: At any age. We should just be rewarding people for
7:44
Speaker 1: their curiosity and their enthusiasm. And one of the things
7:48
Speaker 1: for me that I love about the night sky and
7:50
Speaker 1: just staring at it is trying to get a mental
7:53
Speaker 1: image or conception, grappling with the idea of how ridiculously
7:58
Speaker 1: far away they are. Know, I like standing on the
8:00
Speaker 1: top of a mountain and seeing eighty miles hundred miles
8:04
Speaker 1: on a clear day even further, but looking up in
8:06
Speaker 1: the night sky is like standing on the top of
8:09
Speaker 1: a mountain. The side of the Earth and staring across
8:12
Speaker 1: billions of light years, right, those photons left those stars
8:16
Speaker 1: millions or billions of years ago, just now arriving on Earth.
8:19
Speaker 1: It's like it's the most spectacular view in the universe.
8:23
Speaker 1: My brain has so much trouble wrapping its head around
8:26
Speaker 1: distances in space. Like even just knowing that it takes like,
8:29
Speaker 1: what six months to get to Mars, that seems crazy
8:31
Speaker 1: because you know, it's like two days of driving to
8:33
Speaker 1: get across the US. So it feels to me like
8:35
Speaker 1: anything should be accessible within forty eight hours. I should
8:39
Speaker 1: be able to drive to Mars in two days, right,
8:41
Speaker 1: stopping at gas stations and getting like space snacks. I mean,
8:46
Speaker 1: I understand you're going to need a different vehicle, but
8:48
Speaker 1: it seems like it should be reachable pretty quickly. And
8:51
Speaker 1: one of the real challenges with understanding these distances in
8:54
Speaker 1: particular is that you have two things you have to
8:56
Speaker 1: grapple with. Not just are the stars super far away,
8:59
Speaker 1: but they are enormous, Like the size of a star
9:03
Speaker 1: is just flabberghastingly huge, Like the Sun for example, right,
9:07
Speaker 1: so much bigger than the Earth. Jupiter is like a
9:10
Speaker 1: thousand times the volume of the Earth, and the Sun
9:14
Speaker 1: is like a million times the volume of the Earth. Right,
9:16
Speaker 1: that's flabbergasting. It's hard to hold that many Earth's in
9:20
Speaker 1: your head. And there are stars out there there are
9:22
Speaker 1: much much bigger, And yet you look up at the
9:24
Speaker 1: night sky and they are these tiny little pin pricks, right,
9:27
Speaker 1: And so you have to have both of these things
9:29
Speaker 1: in your head. That these objects are enormous fireballs really
9:33
Speaker 1: just like bigger than you can even imagine, and yet
9:36
Speaker 1: so distant that these enormous fireballs now look super duper tiny. Right,
9:41
Speaker 1: So two things in your head you have to sort
9:43
Speaker 1: of like divide by each other, both huge numbers. How
9:46
Speaker 1: big is our son relative to some of the others
9:48
Speaker 1: you said, some of the others were much much bigger.
9:50
Speaker 1: Is our son sort of a tiny sun? Or is
9:52
Speaker 1: always like about average? Our son is not unusual. But
9:55
Speaker 1: there are many stars out there that are much bigger.
9:59
Speaker 1: For example, Beetles Juice, if you put it in our
10:01
Speaker 1: solar system, it's radius would extend out into the asteroid
10:05
Speaker 1: belt like Mars would be inside Middle Juice. We did
10:08
Speaker 1: an episode about the biggest stars in the universe. Some
10:11
Speaker 1: of them are even larger. Some of them, their radii
10:13
Speaker 1: would extend out, you know, into the outer solar system,
10:16
Speaker 1: you know. So there are monsters out there, absolute gargantuan monsters,
10:22
Speaker 1: balls of burning plasma. That's really hard for me to imagine.
10:26
Speaker 1: That's right. They're huge, they're enormous, and yet they only
10:29
Speaker 1: appear as pin pricks in our sky. And the reason,
10:32
Speaker 1: of course is math, because as those photons shoot out
10:35
Speaker 1: from that star, they go in all directions, and we
10:38
Speaker 1: only see a tiny fraction of those photons. Right. If
10:41
Speaker 1: you're standing like a meter away from a light bulb
10:44
Speaker 1: and you have a solar panel gathering some of that light,
10:47
Speaker 1: you're not gathering all of the light, right, You're gathering
10:49
Speaker 1: some fraction of the photons that come off of that
10:51
Speaker 1: light bulb. Right, it's shooting in all directions, and the
10:54
Speaker 1: fraction that you get like the area of your solar
10:56
Speaker 1: panel divided by the area of that sphere that's at
11:00
Speaker 1: your distance, right, So that's the fraction of the photons
11:03
Speaker 1: you're getting. If you take two steps away, for example,
11:05
Speaker 1: now you're twice as far. Then the area of that
11:08
Speaker 1: sphere that the photons are getting spread over is now
11:10
Speaker 1: four times as big. Because the surface of a sphere
11:14
Speaker 1: with twice the radius is four times as big. So
11:17
Speaker 1: the same solar panels now capturing one fourth of what
11:20
Speaker 1: it did before. So the luminosity of a star, the
11:23
Speaker 1: brightness of its appearance, goes like one over the distance square.
11:27
Speaker 1: And these distances we're talking about are huge, right, Other
11:30
Speaker 1: stars are not just super bright, they're really really far away,
11:34
Speaker 1: So one over their distance square becomes a really big number.
11:37
Speaker 1: But if you do that over and over and over
11:39
Speaker 1: and over again, like billions or trillions of times, shouldn't
11:43
Speaker 1: it be much brighter out there? Yeah, it's a really
11:45
Speaker 1: good question. So we're balancing lots of big numbers here.
11:48
Speaker 1: We have huge stars, so they should be admitting a
11:50
Speaker 1: lot of light, but they're really far away, so that
11:53
Speaker 1: cuts down their light a lot. But then there's a
11:55
Speaker 1: lot of them. Right, three have like three almost infinite numbers,
11:58
Speaker 1: and we're wondering, like, how how do those get fit together?
12:01
Speaker 1: You know, if the universe is in fact infinite, wouldn't
12:04
Speaker 1: all those numbers add up to be like a crazy
12:07
Speaker 1: amount of light. Wouldn't Kelly's kids get like star burns
12:10
Speaker 1: if they go out at night looking at the stars. Right,
12:13
Speaker 1: this is a question that people have asked sort of
12:15
Speaker 1: since antiquity, like why isn't the night sky like catastrophically
12:19
Speaker 1: blazing full of starlight? Thank goodness for star screen. And
12:22
Speaker 1: so today on the podcast we'll be answering why is
12:31
Speaker 1: the night sky dark? And I love this kind of
12:33
Speaker 1: question because it's such a simple, basic question, and it's
12:36
Speaker 1: the kind of thing that people can think about and
12:38
Speaker 1: have been thinking about for a long time. Right, you
12:40
Speaker 1: don't need special tools or apparatus. Is just to sit
12:43
Speaker 1: here on the surface of this rock and to ask
12:46
Speaker 1: this question, like, if I have an understanding of how
12:48
Speaker 1: the universe works, my mental model is that there are
12:50
Speaker 1: stars everywhere, then what should I expect to see in
12:53
Speaker 1: my night sky? Right? This is a perfect example of
12:56
Speaker 1: what physics really is at its core. Build a model
13:00
Speaker 1: to love the universe in your head. We make a
13:01
Speaker 1: prediction for what you should see, and then you ask,
13:04
Speaker 1: like does that agree with how the universe actually works?
13:07
Speaker 1: And if it doesn't, then you get an important clue
13:10
Speaker 1: about how the universe might be different from your mental model. Well,
13:13
Speaker 1: you forgot about the part where you spend like a
13:15
Speaker 1: decade writing to grant so that you can get the
13:17
Speaker 1: money to try to test the model. But but yes,
13:19
Speaker 1: essentially that's how physics works. We're trying to make science
13:22
Speaker 1: sound glamourus here talking all the emails and the meetings.
13:28
Speaker 1: I'm sorry, I'm sorry, didn't mean to take away all
13:30
Speaker 1: the beauty of physics. Al Right, Well, let's tap into
13:33
Speaker 1: the apparent beauty of physics by asking our listeners if
13:36
Speaker 1: they know why the night sky is dark. So I
13:39
Speaker 1: wrote to all of them, and I asked them, if
13:41
Speaker 1: the universe is infinite, why is the night sky dark?
13:45
Speaker 1: If this sounds like a lot of fun to you,
13:46
Speaker 1: and you would be willing to participate for future episodes,
13:50
Speaker 1: please don't be shy right to us, to Daniel and
13:53
Speaker 1: Jorge dot com, and we'll send you our recent questions.
13:57
Speaker 1: So think about it for a minute. Do you know
13:59
Speaker 1: why in an infinite universe the night sky is not
14:03
Speaker 1: blazingly bright? Here's what our listeners had to say. Well,
14:06
Speaker 1: I guess that's because from the electromagnetic spectrum that's the
14:11
Speaker 1: light emitting of all stars and everything in the universe,
14:14
Speaker 1: we are only able to see just this tiny fraction
14:16
Speaker 1: that's the visible the visible spectrum. So I guess that
14:20
Speaker 1: there are photons all around the universe, but they are
14:24
Speaker 1: in different wavelength so we are not able to see them.
14:27
Speaker 1: That's why the night sky looks dark for us, because
14:30
Speaker 1: we cannot perceive it. Okay, so, first of all, the
14:32
Speaker 1: universe is not in finite. It's just insanely huge, and
14:37
Speaker 1: it expanding faster than the spade of light. So it
14:40
Speaker 1: may seem infinite, but it's not infinite. As for the
14:44
Speaker 1: question itself, I can come up with two different explanations.
14:48
Speaker 1: One is that the light coming from really distant stars
14:51
Speaker 1: and galaxies that they get that eventually they get really
14:54
Speaker 1: spread out. They're not concentrated enough for us to spot,
14:58
Speaker 1: so like in our able to see them in the
15:00
Speaker 1: night sky. The other reason that I could come up
15:03
Speaker 1: with was that although the speed of flight is fast,
15:06
Speaker 1: it's not fast enough compared to the size of the universe.
15:09
Speaker 1: So I imagine this bubble of visibility around Earth that
15:13
Speaker 1: keeps growing every second as new light reaches us from
15:17
Speaker 1: further places. But there is still a lot of the
15:20
Speaker 1: universe out there from which the light has simply not
15:23
Speaker 1: had enough time to reach us. It's because all those
15:26
Speaker 1: stars are just too spread out and they're too far
15:28
Speaker 1: away so that they can really make a strong light.
15:30
Speaker 1: And I also believe that inflation plays a role, right,
15:33
Speaker 1: because I think that in a few million or billion
15:37
Speaker 1: years there will be rarely any stars to be seen
15:40
Speaker 1: in the night sky. So maybe that's the point. Well,
15:42
Speaker 1: first of all, proved to me that we live in
15:43
Speaker 1: an infinite universe. But supposing we do, it's not just
15:48
Speaker 1: jam packed with stars all the way out, and the
15:53
Speaker 1: intensity of light fairies with the square of your distance
15:57
Speaker 1: from it, so the really far away stuff is extremely
16:00
Speaker 1: him um. And I'm sure there's some kind of math
16:03
Speaker 1: you can do that shows a curve that approaches an
16:07
Speaker 1: awsome tote or one of the axes and the zero
16:11
Speaker 1: pans out there. It's kind of like, um, if you
16:13
Speaker 1: keep going, you go one ft, and then you go
16:16
Speaker 1: half a foot, and then you go a quarter of foot,
16:18
Speaker 1: you never quite get there two feet away. And I'm
16:22
Speaker 1: glad it's not effinitely late, because um, it's nice to
16:26
Speaker 1: have some darkness every once in a while. It is
16:28
Speaker 1: not dark at all frequencies. If we shift to the
16:31
Speaker 1: right frequency, there's usually something coming from every point in
16:34
Speaker 1: the sky we look at, however, for the visual range
16:39
Speaker 1: of humans, even though at any given point in the sky.
16:43
Speaker 1: If we look deep enough in there probably is a
16:45
Speaker 1: source of light. There are dust clouds and gas clouds
16:49
Speaker 1: and lots of other things that obstruct the light before
16:52
Speaker 1: it gets to us, so within human visual range it
16:57
Speaker 1: can look very dark, but if we shift to a
17:00
Speaker 1: different frequency, there's usually some form of light coming in.
17:03
Speaker 1: So I know this one. The answer is dust. The
17:06
Speaker 1: interstellar gases are just thick enough to absorbs enough of
17:12
Speaker 1: the radiation sent by the more distant stars that we
17:16
Speaker 1: can't with our naked eye see things all the way
17:20
Speaker 1: back to the dawn of time. It's what we need
17:21
Speaker 1: things like Hubble for. So those more distant and fainter
17:25
Speaker 1: lights do get filtered out by the dust, and so
17:29
Speaker 1: the night sky appears dark to us instead of appearing light.
17:33
Speaker 1: Light from different stars takes time to travel from whatever
17:38
Speaker 1: the star is to where we are, so depending on
17:41
Speaker 1: how far the star is, the light from it might
17:43
Speaker 1: not have reached us yet, and so the space in
17:47
Speaker 1: the interim for the duration for which the light hasn't
17:49
Speaker 1: gotten here yet will be dark. Well, that is debatable
17:52
Speaker 1: if that's even true, that we live in an under
17:54
Speaker 1: infinite universe, but I guess the question is why wouldn't
17:59
Speaker 1: there be as star in every direction we look? And
18:02
Speaker 1: this would be due to um that there would be
18:05
Speaker 1: still a couple of photons arriving from any direction. I guess, um.
18:10
Speaker 1: But the brightness dissipates um over long distances UM in
18:16
Speaker 1: a sphere, so we are not getting a lot of
18:19
Speaker 1: photons um. So I guess if our eyes would be
18:22
Speaker 1: good enough, we would be able to see a totally
18:25
Speaker 1: lit up universe at night. You know, for this set
18:29
Speaker 1: of answers, I was particularly impressed with your audience. Now,
18:32
Speaker 1: of course your audience is brilliant and they always give
18:35
Speaker 1: good answers, but for this set in particular, they really
18:39
Speaker 1: had some clever answers. And so so, first of all,
18:41
Speaker 1: like when you ask this question to the listeners, do
18:43
Speaker 1: they have to answer like immediately or do they get
18:45
Speaker 1: the question, think about it for a day and then
18:47
Speaker 1: call in. That's a good question. Well, the rules are
18:50
Speaker 1: no preparation, no googling, and you're supposed to give immediate
18:54
Speaker 1: off the cuff answers. Now, I can't police them, so
18:57
Speaker 1: I don't know if they've googled or they've gone and
18:59
Speaker 1: looked up their star on the professor from college and
19:01
Speaker 1: then trying to give an intelligent answer. But I'm trust
19:04
Speaker 1: in them that these are what they immediately think that
19:07
Speaker 1: these are. They're off the cuff responses. Yeah, yeah, I
19:09
Speaker 1: think they're not just smart, but they're trustworthy and good
19:12
Speaker 1: looking and good looking, that's right, many many good features.
19:17
Speaker 1: You know, you'd have to be a great person to
19:18
Speaker 1: be listening to this podcast. I think that's called the
19:21
Speaker 1: halo effect. If anyway, getting off topic, but yeah, there
19:25
Speaker 1: were like some good skeptical answers. You know, we don't
19:27
Speaker 1: know if the universe is infinite. You've clearly taught them
19:30
Speaker 1: about what we do and don't know. And Dust came
19:32
Speaker 1: up to be honest, Dust I hadn't thought of as
19:34
Speaker 1: a potential explanation until reading the answers, and so I
19:38
Speaker 1: was I was pretty impressed with this set of answers.
19:40
Speaker 1: What did you think? Yeah, they really run the game,
19:41
Speaker 1: and there's a lot of different things to think about.
19:43
Speaker 1: And that's why this is a great question because it
19:46
Speaker 1: forces you to sort of clarify your understanding for what
19:49
Speaker 1: really is going on, what is interfering with the light
19:51
Speaker 1: that's coming to earth, how much should be arriving. There's
19:54
Speaker 1: a lot of good stuff here. There is a lot
19:56
Speaker 1: of good stuff there. And so one of the listeners said,
19:58
Speaker 1: we don't know if the universe is infinite. Is that right?
20:01
Speaker 1: What do we know about the size of the universe?
20:03
Speaker 1: That is correct, We do not know if the universe
20:07
Speaker 1: is infinite. You know. We know that our patch of
20:09
Speaker 1: the universe, what we can see, what we call the
20:11
Speaker 1: observable universe, no part of it seems to be different
20:15
Speaker 1: from any other part of it. That it's the universe
20:16
Speaker 1: seems to be sort of homogeneous, and so that suggests
20:20
Speaker 1: that no part of the universe should be special, and
20:23
Speaker 1: it implies that perhaps the universe is infinite. We don't
20:27
Speaker 1: know that. We've tried to measure things like we can
20:29
Speaker 1: measure the curvature of space, how much space itself is bent.
20:34
Speaker 1: Though we talked about in a recent episode about whether
20:36
Speaker 1: the universe is shaped like a doughnut or a sphere
20:39
Speaker 1: or an infinitely flat plane, we can't actually tell. We've
20:42
Speaker 1: measured the universe is curvature to be consistent with zero curvature,
20:46
Speaker 1: like as if it was flat. But the universe could
20:49
Speaker 1: be flat and also not be infinite. It could like
20:51
Speaker 1: tie together at the edges sort of like the screen
20:54
Speaker 1: and pact Man or asteroids. So there's lots of things
20:57
Speaker 1: we don't know, but so far it's consistent with us
21:00
Speaker 1: seeing a finite patch of an infinite universe. But absolutely
21:04
Speaker 1: we don't know. What do you mean by homogeneous like that?
21:07
Speaker 1: Because you know, when I think about galaxies, they sort
21:09
Speaker 1: of seem like clumps. But I guess, just like in general,
21:12
Speaker 1: it's everything somewhat evenly distributed. Is that what that means
21:16
Speaker 1: by homogeneous? We don't mean literally exactly homogeneous, because there
21:20
Speaker 1: are parts of the universe where there are galaxies in
21:22
Speaker 1: parts where there are not. We mean that if you
21:23
Speaker 1: zoom out far enough, like sort of on the biggest scales,
21:26
Speaker 1: everything seems basically smooth. There's no clusters and superclusters, but
21:31
Speaker 1: there's no real structure beyond that, and the rules of
21:33
Speaker 1: the universe seem to be the same everywhere. There don't
21:36
Speaker 1: seem to be any special locations. There's no center to
21:39
Speaker 1: the universe. As far as we can tell, everyone in
21:41
Speaker 1: the universe seems to be similar to everywhere else in
21:43
Speaker 1: the universe. That's what we mean. And so I thought
21:46
Speaker 1: that everything was moving away from a central point, But
21:52
Speaker 1: is that not true? There's no like point that we
21:54
Speaker 1: know that everything is moving away from, like expanding out.
21:56
Speaker 1: Everything is moving away from everything else. So the expansion
22:00
Speaker 1: is also homogeneous, like every point in the universe is
22:03
Speaker 1: expanding just like every other point. No, no matter where
22:06
Speaker 1: you are in the universe, it will look like everything
22:08
Speaker 1: is moving away from you. So either you are at
22:12
Speaker 1: the center of the universe and there's just expansion away
22:15
Speaker 1: from that one point, or everywhere in the universe is expanding,
22:18
Speaker 1: which is much more likely. I am probably the center
22:21
Speaker 1: of the universe, but maybe I'm not. I can accept that. Okay,
22:24
Speaker 1: you're the center of your kids universe, I'm sure. But
22:26
Speaker 1: this question about why the night sky is dark is
22:29
Speaker 1: actually a really old question and really predates any modern
22:32
Speaker 1: understanding of sort of like the size and shape of
22:34
Speaker 1: the universe, and it helped spark some of these questions
22:37
Speaker 1: like is the universe infinite? Could we possibly tell? And
22:40
Speaker 1: a long time ago people thought it was obvious that
22:42
Speaker 1: the universe was infinite. One of my favorite things about
22:44
Speaker 1: the history of astronomy is seeing how like conceptions of
22:48
Speaker 1: what is obvious or natural change with time. Right, like
22:51
Speaker 1: a long time ago, people thought the universe was obviously infinite,
22:55
Speaker 1: it should just go on forever. And it was just
22:57
Speaker 1: sort of sprinkled with stars. There were no galaxies at all.
22:59
Speaker 1: They just thought, here's a star, there's a star. The
23:02
Speaker 1: whole universe is just like a vast sea of stars.
23:06
Speaker 1: So when did we start learning enough to start having
23:09
Speaker 1: a good answer to these questions. Well, it was like
23:11
Speaker 1: a hundred years ago with Hubble and lots of others
23:13
Speaker 1: who discovered that there are galaxies and that those galaxies
23:17
Speaker 1: are moving away from us and the universe is expanding.
23:19
Speaker 1: And then it became later much more natural to imagine
23:22
Speaker 1: that the universe might be finite. In age, it seemed
23:25
Speaker 1: like obvious that the universe should have a beginning, whereas
23:28
Speaker 1: before that people that I was obvious that the universe
23:30
Speaker 1: had existed forever and had no beginning and went on forever. Right,
23:34
Speaker 1: So what seems to be like obvious and natural changes
23:36
Speaker 1: with time, which I think it's really fascinating, an important lesson,
23:40
Speaker 1: you know, for like making assumptions about the universe. Anyway,
23:43
Speaker 1: thank yourself. Back to like h fifty years ago, when
23:45
Speaker 1: people thought, Okay, the universe is infinite, there are stars everywhere.
23:49
Speaker 1: There's sort of like dotted in this vast ocean. So
23:52
Speaker 1: pretend you're an astronomer two fifty years ago, and that's
23:55
Speaker 1: the sort of understanding. People were asking themselves a question
23:58
Speaker 1: like how much starlight should expect to fall on the
24:02
Speaker 1: Earth in that situation, in an infinite universe filled with
24:05
Speaker 1: equally spaced stars, And so, I mean, of course you
24:08
Speaker 1: look out at the night sky and you sort of
24:10
Speaker 1: know what you get. What did they think that the
24:12
Speaker 1: answer was right? So obviously they knew that the night
24:15
Speaker 1: sky was dark, right, That wasn't the mystery. The question
24:18
Speaker 1: was like, why is the night sky dark? Because when
24:20
Speaker 1: they sat down to do the calculation, they were quite surprised.
24:23
Speaker 1: They couldn't explain it right. Their calculation suggested that the
24:28
Speaker 1: night sky should be infinitely bright if there are an
24:31
Speaker 1: infinite number of stars out there. Okay, well, I am
24:34
Speaker 1: dying to know what explanation they came up with to
24:37
Speaker 1: explain the discrepancy between what they thought they should see
24:39
Speaker 1: and what they were seeing. But first, I think we
24:41
Speaker 1: need to take a break, all right, and we're back.
24:56
Speaker 1: So before we left, you were telling us that in
24:59
Speaker 1: the past there was this big question about like, well,
25:03
Speaker 1: how much light should we be seeing, and why aren't
25:05
Speaker 1: we seeing a lot more of it? And and what
25:07
Speaker 1: sort of answers did they come up with? Hopefully, hopefully
25:10
Speaker 1: the answers we have now are better. Well. The interesting
25:13
Speaker 1: thing is they were trying to figure out whether the
25:16
Speaker 1: nice guy should be dark or not. They knew something
25:19
Speaker 1: about how light was transmitted that they didn't really understand
25:21
Speaker 1: it on a microscopic scale, but they knew, for example,
25:24
Speaker 1: that as you got further away from something, it's light
25:27
Speaker 1: dimmed by a power of one over the distance squared.
25:30
Speaker 1: And so you know, you apply that math to for example,
25:32
Speaker 1: like Proxima Centauri. That's the nearest star to Earth, but
25:36
Speaker 1: it's really still very very far away. It's only a
25:38
Speaker 1: few light years. But in terms of like distances in
25:41
Speaker 1: our Solar system and au, for example, is the distance
25:45
Speaker 1: between the Earth and the Sun, proximate Centauri is two
25:48
Speaker 1: hundred and seventy thousand a U. Right, so it's two
25:52
Speaker 1: d seventy thousand times further from the Earth than the
25:54
Speaker 1: Sun is. So that means that proximates Centauri is light
25:57
Speaker 1: gets reduced by one. Were tend to the eleven relative
26:02
Speaker 1: to the Sun, right, So our distance to the Sun
26:04
Speaker 1: means the light from the Sun is reduced by a
26:06
Speaker 1: certain amount, but proximate centauri is one over ten to
26:09
Speaker 1: the eleven times dimmer than the light from the Sun
26:13
Speaker 1: because the distance between here and there is so large. Okay,
26:16
Speaker 1: so the equation where they were trying to figure out
26:19
Speaker 1: that like light dims at one over the distant square.
26:21
Speaker 1: That must have been done on Earth when they were
26:24
Speaker 1: looking out at space, where they assuming that like space
26:27
Speaker 1: sort of had the same atmosphere and the same sort
26:30
Speaker 1: of dynamics, or like, how did they deal with that
26:33
Speaker 1: sort of uncertainty? Oh yeah, great question. It wasn't always
26:36
Speaker 1: obvious to people that the rules here on Earth also
26:39
Speaker 1: applied to space, that you could say, like things were
26:42
Speaker 1: fundamental and physics was universal. Newton was one of the
26:45
Speaker 1: first people to do that, to say gravity here on
26:47
Speaker 1: Earth also applies in space and can describe the motions
26:50
Speaker 1: of the planets. So you're right, that is kind of
26:52
Speaker 1: a big leap. Plus, they're assuming that light can travel
26:55
Speaker 1: through those distances, that there isn't like dust or gas
26:58
Speaker 1: blocking it. We'll get into the details of whether or
27:00
Speaker 1: not gas and dust can explain it in just a minute,
27:02
Speaker 1: but first let's start with sort of the simpler vision,
27:04
Speaker 1: like let's just imagine that light is not blocked by anything.
27:07
Speaker 1: It's just a question of distance and the number of stars.
27:11
Speaker 1: And that's sort of like simpler model. How much light
27:13
Speaker 1: do we expect to fall on Earth? Okay, so they
27:16
Speaker 1: had an understanding then about how light should fall off
27:19
Speaker 1: with distance. Did they have like an estimate for how
27:22
Speaker 1: many stars were out there? So they didn't know how
27:25
Speaker 1: many stars were out there, but they were assuming that
27:27
Speaker 1: the stars were uniform right there, like every chunk of
27:30
Speaker 1: space had on average the same number of stars. And
27:33
Speaker 1: so you can take the fact that stars get dimmer
27:36
Speaker 1: as they get further by one over distance squared and
27:39
Speaker 1: the fact that there are an infinite number of stars,
27:41
Speaker 1: and you can do the math and you can ask,
27:43
Speaker 1: like which one wins out? Does the infinity of the
27:46
Speaker 1: number of stars win out? Or does the growing distance
27:49
Speaker 1: to those stars suppress the light faster than like more
27:53
Speaker 1: stars get added, And so then because the night sky
27:56
Speaker 1: is dark, that means the distance must win. Is that
27:58
Speaker 1: the conclusion they came to, not the conclusion they came to. Actually,
28:01
Speaker 1: they did the math and they discovered that the number
28:04
Speaker 1: of stars should win. It's actually not that hard to
28:06
Speaker 1: do the math yourself. Brightness of a star goes like
28:08
Speaker 1: one over distance squared, and at a given distance, the
28:11
Speaker 1: area of a sphere around the Earth also goes like
28:13
Speaker 1: distance squared. So that means that, like any shell of
28:17
Speaker 1: universe at an arbitrary distance from the Earth, the stars
28:20
Speaker 1: from that shell get suppressed by one over distance squared,
28:24
Speaker 1: but the number of stars in that shell goes like
28:26
Speaker 1: distance squared, So those two numbers both canceled. So that
28:30
Speaker 1: means that, like any shell of universe an arbitrary distance
28:34
Speaker 1: away from you should have a constant brightness, like every
28:37
Speaker 1: shell should have the same amount of brightness. Because you have,
28:40
Speaker 1: like a shell of universe, further and further away, each
28:43
Speaker 1: star in it gets dimmer, but the number of stars
28:46
Speaker 1: grows perfectly to match that. So that means that every
28:49
Speaker 1: shell of universe around us should be equally bright. Further
28:53
Speaker 1: ones have many many more stars, each of which are dimmed,
28:57
Speaker 1: and the closer ones have fewer stars, but they're not
28:59
Speaker 1: dim as much. Okay, but his wifind physics so frustrating.
29:04
Speaker 1: But but but the night sky is dark? So what
29:08
Speaker 1: what what were they missing? What? What was not in
29:10
Speaker 1: the equation that should have been there. You're absolutely right,
29:13
Speaker 1: the night sky is dark, and so something is wrong
29:15
Speaker 1: with this calculation. But just to underline it for the listeners,
29:17
Speaker 1: you know, that means that every shell of the universe
29:20
Speaker 1: is a fixed brightness. But there's an infinite number of shells, right,
29:23
Speaker 1: so all those shells should add up to be an
29:26
Speaker 1: infinite amount of light. And so this is like Heinrich Olbers.
29:29
Speaker 1: Here's a guy in the seventeen hundreds. He's doing this calculation,
29:32
Speaker 1: and you know he gets this number. He's like, hold
29:34
Speaker 1: on a second, something must be wrong, as you say,
29:37
Speaker 1: because if there's an infinite number of stars, even if
29:41
Speaker 1: distance dims them like one over distance squared, the night
29:44
Speaker 1: sky should be infinitely bright. You know. His calculations suggests
29:48
Speaker 1: that if you go out at night you should be
29:50
Speaker 1: brighter than the daytime. You should get like infinitely crisped up,
29:54
Speaker 1: you know, by all of these stars. Right. So did
29:57
Speaker 1: he start like the Olbers Prize to be like I
30:00
Speaker 1: try to solve this famous problem or like how many
30:02
Speaker 1: people cared about about this problem? At the time. A
30:05
Speaker 1: lot of people were thinking about this, and Olbers thought
30:08
Speaker 1: that this was proof that the universe wasn't infinite, right, thought, oh, well,
30:12
Speaker 1: something must be wrong. One of the assumptions that go
30:15
Speaker 1: into this calculation, right, that stars go like one of
30:17
Speaker 1: a distant squared and that the universe is infinite and
30:20
Speaker 1: uniformly filled with stars. One of those assumptions must be wrong.
30:24
Speaker 1: And he thought it was clear that the universe couldn't
30:27
Speaker 1: be infinite because I would solve the problem. You can't
30:29
Speaker 1: get infinite light on Earth if there aren't infinite stars
30:32
Speaker 1: out there. So he was convinced that that means that
30:34
Speaker 1: the universe is not infinite. Okay, that makes sense. But apparently,
30:38
Speaker 1: well you've told us that that is a question that
30:40
Speaker 1: we're still trying to figure out the answer to, so
30:43
Speaker 1: it must be more complicated. Yet it is in fact
30:46
Speaker 1: more complicated. It's possible to live in an infinite universe
30:50
Speaker 1: without an infinitely bright night sky. Right. There are other
30:55
Speaker 1: solutions out there to this problem, but first let's tackle
30:57
Speaker 1: one of the ones that the listeners brought up that
31:00
Speaker 1: you mentioned earlier, which is gas and dust. Another suggestion
31:03
Speaker 1: people had, not just our listeners, but people over the centuries,
31:07
Speaker 1: was imagining that not all the light is getting to
31:09
Speaker 1: the Earth, that maybe some of it is getting absorbed
31:12
Speaker 1: by gas and dust. Basically, it's being blocked before it
31:15
Speaker 1: gets here. And so I mean, I guess when I
31:17
Speaker 1: think about space, I don't think about there being a
31:19
Speaker 1: lot of gas and dust. But of course the International
31:22
Speaker 1: Space Station needs to have what is it called a
31:24
Speaker 1: whiffle barrier because it gets hit by all of this
31:27
Speaker 1: space dust that you want to make sure it doesn't
31:28
Speaker 1: get into the International Space Station. So there must be
31:31
Speaker 1: dust up there. But is there enough dust up there
31:34
Speaker 1: when you get out past like low Earth orbit to
31:37
Speaker 1: be blocking the light? Absolutely, most of the mass of
31:40
Speaker 1: the galaxy is in terms of gas and dust. Right,
31:43
Speaker 1: Stars do not make up most of the stuff in
31:46
Speaker 1: the galaxy. Even if we're just talking about barrionic matter,
31:49
Speaker 1: we're not even talking about dark matter. They're vast clouds
31:52
Speaker 1: of gas out there which might in the future become stars,
31:55
Speaker 1: and they're huge expanses of dust from old solar systems
31:59
Speaker 1: that blew up up And so there's a lot of
32:01
Speaker 1: gas and dust out there, and people who study the
32:03
Speaker 1: universe have to map that gas and dust. If they're
32:05
Speaker 1: looking at a star and they're trying to figure out
32:07
Speaker 1: how bright it should be, they have to know how
32:09
Speaker 1: much stuff is between us and that star. There are
32:12
Speaker 1: people whose entire theses are like maps of dust in
32:16
Speaker 1: the galaxy because it's so important and it's one reason,
32:19
Speaker 1: for example, that we can't see through the galaxy. One
32:22
Speaker 1: of the big mysteries in astronomy these days is the
32:25
Speaker 1: Great Attractor, this weird source of gravitational attraction that seems
32:29
Speaker 1: to be on the other side of the Milky Way,
32:32
Speaker 1: and we can't see what's there because the Milky Way
32:34
Speaker 1: is so gassy and dusty that is basically blocking our view.
32:38
Speaker 1: So it's definitely something that people have to take into
32:40
Speaker 1: account when they're figuring out how bright and individual star
32:43
Speaker 1: should be when seen from Earth. Are the people who
32:45
Speaker 1: study that like not invited to the parties of people
32:49
Speaker 1: who are excited about space stuff because to me, when
32:51
Speaker 1: I think about space dust, I think you can't travel
32:54
Speaker 1: to another galaxy because that space dust is going to
32:56
Speaker 1: be like bullets going through your capsule or whatever. On
32:59
Speaker 1: the wage with other galaxies where these like not popular
33:02
Speaker 1: people because they kill dreams. No, they're super popular because
33:06
Speaker 1: they enable the rest of astronomy. You know, there's so
33:08
Speaker 1: many people who need to know how much gas and
33:11
Speaker 1: dust is there between this object I'm studying for my thesis.
33:14
Speaker 1: So I think people are very grateful that somebody has
33:17
Speaker 1: gone out there and like devoted five years their life
33:19
Speaker 1: to mapping dust in the galaxy. And you know, they
33:22
Speaker 1: also get zillions of citations. I see these These sometimes
33:25
Speaker 1: have like five thousand citations because it's such vital work
33:29
Speaker 1: just to understand like what's out there between us and
33:32
Speaker 1: other stuff. Well, I'm happy for them, But unfortunately this
33:35
Speaker 1: idea that gas and dust is dimming the otherwise infinite
33:39
Speaker 1: light from stars that can't explain why the night sky
33:42
Speaker 1: is dark. The problem is that gas and dust also
33:45
Speaker 1: absorbs energy. Right, so if the universe was filled with
33:48
Speaker 1: effectively infinite light from infinite stars had infinite time to
33:53
Speaker 1: get here, then that gas and dust would be super
33:55
Speaker 1: duper hot. Right. If it's blocking infinite light, then it's
33:59
Speaker 1: absorbed being infinite light and basically gets to be the
34:02
Speaker 1: same temperature as the stars, and it should also be glowing.
34:06
Speaker 1: So you can't explain the non infinitely bright night sky
34:10
Speaker 1: with gas and dust, and that would make those theses
34:12
Speaker 1: much easier if mapping it just meant you had to
34:15
Speaker 1: look for the glowing penches exactly, and yet we know
34:18
Speaker 1: that the night sky is not dark, and so we
34:20
Speaker 1: have to think more deeply about what those assumptions are
34:23
Speaker 1: and which one might be wrong. And one of my
34:26
Speaker 1: favorite bits of history is that an early thinker on
34:28
Speaker 1: this question is somebody you might not imagine, somebody you
34:31
Speaker 1: all know very well but probably don't think of in
34:34
Speaker 1: terms of astronomy, and that's Edgar Allan Poe. What's no
34:38
Speaker 1: way he was at UVA for a little while. You
34:40
Speaker 1: can go to the dorm room that he was in
34:42
Speaker 1: briefly before he left. But I don't know if I
34:44
Speaker 1: had highlight the fact that he was only here briefly
34:46
Speaker 1: so much during the tour. But anyway, so did he
34:49
Speaker 1: generally think about astronomy questions or is this like the
34:52
Speaker 1: one area of astronomy that really drew him in. No,
34:54
Speaker 1: he was really prolific, and he wrote about all sorts
34:56
Speaker 1: of stuff. But you know, a guy who writes horror
34:58
Speaker 1: movies spends a lot of time up it at night,
35:00
Speaker 1: and so I think he spent a lot of time
35:02
Speaker 1: thinking about the stars and going for long crisp walks,
35:06
Speaker 1: you know, in the evening. So let me redo this
35:08
Speaker 1: quote from Edgar Allan Poe. He says, we're the succession
35:11
Speaker 1: of stars endless. Then the background of the sky would
35:14
Speaker 1: present us a uniform luminosity like that displayed by the galaxy,
35:19
Speaker 1: since there could be absolutely no point in all that
35:22
Speaker 1: background at which would not exist a star. So he's saying, basically,
35:26
Speaker 1: in any direction you look, you should be able to
35:29
Speaker 1: see a star. If the universe is infinite, then every
35:31
Speaker 1: line you make you should hit a star, and so
35:34
Speaker 1: every point in the sky should basically look like a star.
35:37
Speaker 1: Then he goes on to say, the only mode, therefore,
35:40
Speaker 1: in which under such a state affairs, we could comprehend
35:44
Speaker 1: the voids which our telescopes find in innumerable directions, would
35:48
Speaker 1: be by supposing the distance of the invisible background so
35:51
Speaker 1: immense that no ray from it has yet been able
35:55
Speaker 1: to reach us at all. Oh, so he's saying, everything
35:58
Speaker 1: is so far away that the light hasn't gotten here yet,
36:01
Speaker 1: gotten here yet exactly. And that's a really vital point.
36:04
Speaker 1: He's thinking about the time it takes light to get
36:07
Speaker 1: here from those stars, and he's assuming that it hasn't
36:12
Speaker 1: had an infinite amount of time, because in Olber's calculation,
36:16
Speaker 1: he not only assumes that the universe is infinite in
36:18
Speaker 1: size but also infinite in age, And that's necessary because
36:23
Speaker 1: if a star is super duper far away from us,
36:26
Speaker 1: it's light will take a long time to get here.
36:28
Speaker 1: If the universe is infinite in age, then it will
36:31
Speaker 1: get here, and it will already be here because that
36:34
Speaker 1: stars existed infinitely in the past and it's had plenty
36:36
Speaker 1: of time to get here. But if the universe is
36:39
Speaker 1: not infinitely old, if it started a certain number of
36:42
Speaker 1: years ago, that means that there are some stars who
36:45
Speaker 1: are so far away that the light from them has
36:48
Speaker 1: never reached us, that not a single photon from that
36:50
Speaker 1: star has arrived here on Earth. It's always surprising to
36:53
Speaker 1: me how quick physics goes from math to like existential questions.
36:58
Speaker 1: So was Poe sort of on track with what other
37:02
Speaker 1: physicists at the time we're thinking, or was he the
37:05
Speaker 1: first one to sort of think about it this way?
37:08
Speaker 1: Or should we wait until after a break and then
37:11
Speaker 1: find out if Poe was right. Let's leave our listeners
37:14
Speaker 1: in deep Dark Edgar Allan Poe suspense and come back
37:17
Speaker 1: after the break. All right, I've been dying to know.
37:33
Speaker 1: Was Edgar Allan Poe right. Edgar Allan Poe was totally correct,
37:36
Speaker 1: and he was also sort of a vanguard of thinking
37:38
Speaker 1: at the time. Remember that until Hubble saw that the
37:41
Speaker 1: universe was expanding, this is the beginning of the nineteen hundreds,
37:45
Speaker 1: people thought it was very likely that the universe was
37:47
Speaker 1: infinitely old, that it didn't have a beginning. The natural
37:51
Speaker 1: concept was that everything is just hanging in space. Nothing
37:55
Speaker 1: is changing. I mean, you don't see stars blinking out,
37:57
Speaker 1: our new ones appearing. They thought the universe was a
37:59
Speaker 1: turn and static, and so it wasn't until we saw
38:02
Speaker 1: that the universe was expanding and that we could sort
38:05
Speaker 1: of wind that history backwards to some time when the
38:08
Speaker 1: universe was infinitely dense, before which it didn't even make
38:12
Speaker 1: any sense to talk about time. So this whole idea
38:14
Speaker 1: of the age of the universe being finite, being about
38:17
Speaker 1: fourteen billion years old, that's only about a hundred years old.
38:21
Speaker 1: Before that, even mostly thought the universe was static and
38:24
Speaker 1: infinitely old. So I was sort of ahead of his
38:27
Speaker 1: time here imagining this as an explanation for why the
38:30
Speaker 1: night sky was dark. He's so great. He has a
38:34
Speaker 1: story the title of which I forget, where essentially one
38:36
Speaker 1: guy locks or says like, oh, let's go find this
38:40
Speaker 1: wine which is in this basement, thank you, yes, and
38:44
Speaker 1: he locks the other guy in the basement and then
38:48
Speaker 1: he leaves. And a collaborator and I were talking about
38:51
Speaker 1: how that's sort of the way, sort of like how
38:53
Speaker 1: parasitoids lock their hosts inside the goals that they make. Anyway,
38:58
Speaker 1: you know, we we love connecting literature who are wasps
39:00
Speaker 1: whenever we can, and that was one of our connections.
39:03
Speaker 1: But anyway, I digress. That means that Edgar Allan Poe
39:07
Speaker 1: is sort of like our two degrees of separation, right.
39:09
Speaker 1: He wrote about parasitology and he wrote about cosmology, so
39:13
Speaker 1: he connects us well, except he didn't think he was
39:15
Speaker 1: writing about parasitology, whereas he did think he was writing
39:18
Speaker 1: about cosmology. But sure, yes, let's go with it. He
39:21
Speaker 1: was just so far ahead of his time he didn't
39:23
Speaker 1: realize he was breaking new ground in every field. Okay,
39:26
Speaker 1: so we are back to an area where my brain
39:28
Speaker 1: sort of has trouble keeping up with the distances and
39:31
Speaker 1: the speeds and the sizes. So okay, so the universe
39:34
Speaker 1: is expanding, and there we're looking at like the speed
39:38
Speaker 1: of expansion relative to the speed of light. So does
39:41
Speaker 1: this mean that, like, at some point in the future,
39:44
Speaker 1: the light is gonna catch up to us and the
39:46
Speaker 1: night sky is going to be bright? Or am I
39:49
Speaker 1: missing something fundamental? I think I'm missing something fundamental. No,
39:52
Speaker 1: you're absolutely not. That's a great question. So let's take
39:55
Speaker 1: it one step at a time. So first we say
39:57
Speaker 1: the universe is not infinitely old, which means that light
40:01
Speaker 1: from some places hasn't reached us yet. So you can
40:03
Speaker 1: imagine maybe the universe is infinite, maybe it goes on forever,
40:07
Speaker 1: but we can only see a part of it. The
40:09
Speaker 1: part of it that we can see is determined by
40:11
Speaker 1: the speed of light and the age of the universe.
40:14
Speaker 1: So there are things out there that have sent us
40:16
Speaker 1: photons that are on their way screaming at the speed
40:19
Speaker 1: of light across billions of light years, but have not
40:22
Speaker 1: yet arrived, but will in the future arrive. Right, We'll
40:25
Speaker 1: get here at some point. While that explains why the
40:27
Speaker 1: night sky is not infinitely bright right now, it makes
40:30
Speaker 1: you wonder, like, does that mean it will be much
40:32
Speaker 1: brighter in the future. Right in that sort of scenario,
40:35
Speaker 1: you can imagine the nights guy getting brighter and brighter
40:38
Speaker 1: every year is sort of like more stars appear in
40:41
Speaker 1: the night sky, so that eventually it's sort of like
40:43
Speaker 1: fills in all the gaps, right, and then you really
40:47
Speaker 1: need Star Screen. I feel like we've got to start
40:51
Speaker 1: this company before this episode goes out, because otherwise somebody
40:53
Speaker 1: else is going to pick up on this idea and
40:55
Speaker 1: we're gonna be really grumpy that they make a billion
40:57
Speaker 1: dollars off of star Screen. I agree. I think this
41:00
Speaker 1: is going to be the thing that makes us forget
41:02
Speaker 1: our books and everything else. Star Screen start exactly endorsed
41:07
Speaker 1: by astronomers. We need like seven out of a astronomers
41:09
Speaker 1: agree or something. Yeah, that's right, that's right. Let's convince
41:12
Speaker 1: people that are have to worry about medical doctors, they
41:15
Speaker 1: should listen to uh any old specialist in anything, as
41:19
Speaker 1: you might worry about that the future of the infinitely
41:21
Speaker 1: bright night sky. And while I might want you to
41:23
Speaker 1: believe in that, so you buy a big tub of
41:26
Speaker 1: star screen and smear on your kids, it's not actually
41:28
Speaker 1: something we think is going to happen. We actually think
41:30
Speaker 1: the opposite is happening. That as time goes on, the nights,
41:34
Speaker 1: guy gets darker and darker and that's for the reason
41:36
Speaker 1: that you mentioned earlier, that the universe is expanding, Right,
41:40
Speaker 1: So even in the universe that wasn't expanding, where the
41:42
Speaker 1: stars are just sort of like hanging there, but the
41:44
Speaker 1: universe is not infinitely old. You wouldn't expect the nights
41:47
Speaker 1: guy to be infinitely bright, but it would be getting
41:49
Speaker 1: brighter and brighter as time goes on, and the size
41:52
Speaker 1: of the observable universe, the fraction of it that we
41:55
Speaker 1: can see got larger and larger. But what's happening is
41:58
Speaker 1: that the universe is expand ending, and so things are
42:01
Speaker 1: rushing away from us, and they're rushing away from us
42:04
Speaker 1: faster and faster every year. So it's this sort of
42:06
Speaker 1: weird effect where we can see a larger portion of space,
42:10
Speaker 1: but things are sort of moving out of that space
42:12
Speaker 1: faster than light itself. Right, So imagine like a sphere
42:17
Speaker 1: that's growing. That's our observable universe. We can see anything
42:20
Speaker 1: in that sphere, but things are rushing out of that
42:23
Speaker 1: sphere faster than the sphere itself is growing. So the
42:26
Speaker 1: expansion of the universe, the creation of new space between
42:30
Speaker 1: us and other galaxies, is decreasing the number of things
42:34
Speaker 1: in the observable universe, increasing the darkness of the night sky.
42:38
Speaker 1: So is it that as it moves away super fast,
42:42
Speaker 1: that our square distance is increasing so much that it
42:45
Speaker 1: dissipates before it gets to us, or the light never
42:47
Speaker 1: gets to us period because it's moving away so quickly.
42:51
Speaker 1: Both effects actually are happening. There's some things that we
42:53
Speaker 1: can see now. So their photons have reached Earth, and
42:55
Speaker 1: some of the photons they have sent in the meantime
42:58
Speaker 1: will reach Earth. But there are some photons that are
43:01
Speaker 1: traveling through space, and that space is expanding faster than
43:04
Speaker 1: the speed of lights, so those photons will never reach us.
43:08
Speaker 1: It's like if Hussain Bolt is running at you, but
43:11
Speaker 1: somebody's laying new track between you and him faster than
43:14
Speaker 1: he's running, He's never gonna get it to you, no
43:16
Speaker 1: matter how fast he is. Right, you have to be
43:18
Speaker 1: laying pretty fast. And so there are some things out
43:23
Speaker 1: there that we will eventually see that we have not
43:25
Speaker 1: seen yet, things that are about sixty two billion light
43:28
Speaker 1: years away. There are some photons they admitted that will
43:31
Speaker 1: get here and we will be able to see them,
43:34
Speaker 1: But anything past sixty two billion light years we will
43:37
Speaker 1: never see it because the space between us and it
43:40
Speaker 1: is expanding so fast that all of its photons are
43:44
Speaker 1: basically moving away from us. So they're moving locally through
43:47
Speaker 1: space to the speed of light, but globally, because that
43:50
Speaker 1: space is expanding between us and them, they're actually moving
43:53
Speaker 1: away from us, which is really weird because like, photons
43:56
Speaker 1: pointing towards us are actually getting further away from us.
44:00
Speaker 1: For a year, nothing about physics is intuitive, or plenty
44:04
Speaker 1: of it is, I guess, but plenty of it's not okay,
44:06
Speaker 1: And so then that is the final answer, Like is
44:08
Speaker 1: that an answer that we definitely know to be true
44:11
Speaker 1: or is this sort of on the Like we revisit
44:14
Speaker 1: this in a couple of years and people will be
44:15
Speaker 1: talking about how cute it was that Daniel and Kelly
44:17
Speaker 1: thought this was the answer, Like, how confident are we
44:20
Speaker 1: that this is the truth? Oh, you're definitely trying to
44:22
Speaker 1: trap me here. You want to sound by so in
44:24
Speaker 1: five years you can go like, man, Daniel, you were
44:26
Speaker 1: so overconfident. We should definitely always be taking these things
44:30
Speaker 1: with a grain of salt, because our understanding of them
44:32
Speaker 1: is fairly recent. Our measurement of the expansion of the
44:35
Speaker 1: universe and its acceleration is only twenty years old, and
44:38
Speaker 1: that's a blink of an eye in terms of cosmology
44:40
Speaker 1: and science, right, And so while we're fairly certain that
44:44
Speaker 1: the universe is expanding and that that expansion is accelerating,
44:47
Speaker 1: we have no idea what's causing that. We call this
44:51
Speaker 1: dark energy, but that's just like a name we give
44:53
Speaker 1: it because we have no concept of what could be
44:56
Speaker 1: causing it. Is nothing in our equations that can't explain it.
44:59
Speaker 1: You can put a number into Einstein's theory of general
45:01
Speaker 1: relativity to describe this happening, but that doesn't explain why
45:06
Speaker 1: it's happening, Like what part of the universe is doing this?
45:09
Speaker 1: Why does it have to happen? Could it stop and
45:11
Speaker 1: turn around and do something else in the future. We
45:13
Speaker 1: just don't know. So there's a heavy dose of like,
45:17
Speaker 1: oh that should be added to this, But it's sort
45:19
Speaker 1: of like our current understanding of what might be going
45:21
Speaker 1: on with the universe. And so does this answer help
45:24
Speaker 1: us answer the question about the universe being infinite or no?
45:27
Speaker 1: Because you it can be infinite well expanding, because infinity
45:32
Speaker 1: is also complicated exactly. Infinity is though complicated. Unfortunately, what
45:36
Speaker 1: this tells us is that the question of whether the
45:38
Speaker 1: night sky is dark can't tell us whether the universe
45:42
Speaker 1: is infinite, because an infinite universe and a non infinite
45:47
Speaker 1: universe both can give you dark night skies, so you
45:50
Speaker 1: can't tell the difference. Sorry, olders, but that's not even
45:54
Speaker 1: the full answer. There's another reason why the night sky
45:57
Speaker 1: is dark in an expanding universe, and I think you
46:00
Speaker 1: i'd have mentioned earlier, which is while space is expanding,
46:03
Speaker 1: it doesn't just affect the space between us and other galaxies.
46:07
Speaker 1: It also stretches that light. So if space is expanding,
46:11
Speaker 1: if it's getting larger, that means like new spaces being created,
46:15
Speaker 1: and so photons that are flying through that space, they
46:18
Speaker 1: get stretched, their wavelengths get longer, they get red shifted.
46:23
Speaker 1: So your photon, for example, that was emitted from the
46:25
Speaker 1: cosmic microwave background, this super hot plasma that existed just
46:29
Speaker 1: after the Big Bang. That plasma was really really hot,
46:32
Speaker 1: and the light that it generated was very very high frequency,
46:35
Speaker 1: short wavelength. But it's been flying through the universe ever
46:39
Speaker 1: since and it's been getting stretched out to very long wavelengths.
46:43
Speaker 1: So does that mean that the night sky could be
46:47
Speaker 1: bright but just at a wavelength that we don't see. So, yeah,
46:51
Speaker 1: what does that mean. It means the night sky actually
46:54
Speaker 1: is kind of bright, but just in wavelengths we cannot see.
46:57
Speaker 1: So some of the starlight that's been emitted has been
47:00
Speaker 1: red shifted outside of the visible spectrum. Like, there are
47:04
Speaker 1: stars out there that are shining that are bright, but
47:07
Speaker 1: your eyeballs cannot see them in optical telescopes cannot see
47:11
Speaker 1: them because their wavelengths have been shifted into the infrared.
47:15
Speaker 1: So the night sky is actually brighter than you see, right,
47:18
Speaker 1: because there are all these invisible stars out there that
47:21
Speaker 1: are shining at us in the infrared that our eyeballs
47:24
Speaker 1: cannot see. Don't some species see in the infrared? So like,
47:27
Speaker 1: are there some species out there for which the night
47:29
Speaker 1: sky is bright? That's a great question. I should ask
47:33
Speaker 1: my biology friend about that, because I don't know not
47:37
Speaker 1: this one. But we did an episode recently about eyeballs
47:41
Speaker 1: and how they work, and we do know that different
47:43
Speaker 1: species are sensitive to different wavelengths of light. But you know,
47:47
Speaker 1: we are very ingenious as a species, and we're capable
47:49
Speaker 1: of building other kinds of eyeballs, and so we can
47:53
Speaker 1: build telescopes that can see in the infrared and can
47:55
Speaker 1: look up at the night sky in the very very
47:58
Speaker 1: long wavelengths. And something that's really fun to realize as
48:01
Speaker 1: we think about this question is that if you look
48:03
Speaker 1: deep enough into long enough wavelengths, the sky actually is
48:07
Speaker 1: very very bright cosmic micro background radiation that's everywhere, that's
48:12
Speaker 1: in every direction. No matter what direction you look at
48:16
Speaker 1: in the sky, you will see some of it. So
48:18
Speaker 1: if your eyeballs could see the cosmic microwave background radiation
48:22
Speaker 1: is light from the very very early universe plasma plasma
48:25
Speaker 1: that filled the whole universe, then the night sky would
48:29
Speaker 1: be bright, all right, So we gotta find a star
48:31
Speaker 1: screen angle to like, we gotta make sure everybody knows
48:34
Speaker 1: this and then explain to them that the star screen
48:36
Speaker 1: protects against these other parts of the spectrum, and that's
48:38
Speaker 1: where the millions are going to come from. Right, and
48:41
Speaker 1: not to confuse the listeners, the nice guy is not
48:43
Speaker 1: infinitely bright in the cosmic microwave background radiation. Right. Ober's paradox,
48:48
Speaker 1: this question of why we weren't seeing light in every
48:50
Speaker 1: directions really suggested that the nice guy should be infinitely bright.
48:54
Speaker 1: The cosmic microwave background is almost uniformly bright because it
48:58
Speaker 1: comes not from like little lots of stars, the way
49:01
Speaker 1: optical visible light does, but because it came from this
49:04
Speaker 1: plasma which filled the whole universe back before the universe
49:08
Speaker 1: was itself transparent. And so we can still see those photons,
49:11
Speaker 1: and we see them in every direction because that plasma
49:14
Speaker 1: used to be filling all of space in every direction.
49:17
Speaker 1: So if you were a little creature like a parasite
49:19
Speaker 1: that looked up by the night sky, you could only
49:21
Speaker 1: see the microwave, then at night the sky would be
49:24
Speaker 1: uniformly bright. Wouldn't be infinitely bright, right, but it would
49:27
Speaker 1: be uniform It would look very different from our night sky.
49:30
Speaker 1: It's important to know how the parasites see things. So
49:33
Speaker 1: I've had like three four, maybe five existential crises during
49:37
Speaker 1: the span of our talk here. So what what does
49:40
Speaker 1: this all mean then? So what it means is that
49:43
Speaker 1: while we can think about the size of the universe
49:46
Speaker 1: and what it might be made out of, and we
49:47
Speaker 1: can play these games trying to figure out how the
49:49
Speaker 1: universe actually looks, we can't actually tell what's out there
49:53
Speaker 1: past our observable universe. You know, this felt like maybe
49:56
Speaker 1: a trick, like maybe a way to figure out what
49:59
Speaker 1: was in the deep dark ancient depths of the universe,
50:02
Speaker 1: because it should be contributing to our night sky. Really,
50:05
Speaker 1: what this tells us is that we're forever in a bubble.
50:08
Speaker 1: We can't ever see what's past that bubble, so we'll
50:10
Speaker 1: never really know what's out past the observable universe. And
50:14
Speaker 1: even worse than that, our bubble is sort of shrinking,
50:17
Speaker 1: while like physically in terms of length, it's getting larger
50:20
Speaker 1: and larger spaces expanding faster than that bubble is expanding.
50:23
Speaker 1: So it's sort of like our bubble of the universe
50:26
Speaker 1: is shrinking and shrinking, and as time goes on it
50:28
Speaker 1: gets smaller and smaller. It's like a fraction of the universe.
50:32
Speaker 1: So our children in the far far future will see
50:34
Speaker 1: an even smaller fraction of the universe than we are
50:37
Speaker 1: seeing today. So if we want to answer those questions,
50:40
Speaker 1: we better get on it. Like sat, well, I'm feeling
50:42
Speaker 1: slightly depressed now, but I guess I'm going to go
50:44
Speaker 1: ahead and hope that this is one of those like
50:46
Speaker 1: a hundred years from now, they'll be like Daniel, didn't
50:48
Speaker 1: think we could ever study everything, but now we know
50:50
Speaker 1: how to study everything. So I suppose in this particular area,
50:53
Speaker 1: I'm hoping that you're wrong, but you're probably not, and
50:56
Speaker 1: I agree we should be. We should be studying as
50:59
Speaker 1: much of this stuff we can. Now. Well, our great
51:01
Speaker 1: grandchildren will be sitting in their mansions paid for by
51:04
Speaker 1: the Star Screen Empire, and they'll be laughing all the
51:07
Speaker 1: way to the bank. That's they'll be thanking us, and
51:10
Speaker 1: it'll be great. So we still don't know if the
51:14
Speaker 1: full universe is infinite or if it's finite and wraps
51:18
Speaker 1: around on itself forever. It's a question people are still
51:21
Speaker 1: thinking about. People will still be thinking about. Maybe someday
51:24
Speaker 1: in the future people will find a way to probe
51:26
Speaker 1: this question, to sort of like extend our minds past
51:29
Speaker 1: the edge of the observable universe. Currently it feels like
51:32
Speaker 1: that's impossible according to the current laws of physics. But hey,
51:35
Speaker 1: let's hope future Daniel proves today Daniel wrong, and that's
51:39
Speaker 1: how you'll get your Nobel Prize. That's my plan, a
51:45
Speaker 1: good plan. I wish you luck. In case the Star
51:48
Speaker 1: Screen doesn't work out, you can, you know, make all
51:50
Speaker 1: the money that the Nobel Prize winners make. But I
51:52
Speaker 1: think The larger lesson aside from this particular question of
51:55
Speaker 1: why isn't the night sky dark, is that the larger
51:58
Speaker 1: strategy physics is employing a building a model in your head,
52:01
Speaker 1: thinking about the consequences of it, and then asking yourself like, well,
52:05
Speaker 1: why doesn't that work? What does that mean that the
52:08
Speaker 1: ideas I have don't describe the universe I'm seeing? That's
52:11
Speaker 1: generally the best way to win a Nobel prize is
52:14
Speaker 1: to try to find some discrepancy there, to discover some
52:17
Speaker 1: thread to pull on to unravel some deep mystery of
52:20
Speaker 1: the universe. It definitely is beautiful the way these models
52:23
Speaker 1: that you create in physics can have these amazing existential
52:26
Speaker 1: implications the more you think about them, and that in
52:29
Speaker 1: our minds we can imagine what would happen in an
52:31
Speaker 1: infinitely old universe with an infinite number of stars beaming
52:35
Speaker 1: an infinite number of photons to us. Somehow the math
52:39
Speaker 1: all hangs together. It's incredible. Well, thanks everyone for going
52:42
Speaker 1: on that journey with us to answer the question why
52:45
Speaker 1: is the night sky dark? After all? Why don't you
52:48
Speaker 1: need to buy a big tub of star scream to
52:50
Speaker 1: protect your children from star cancer? Don't tell them that
52:55
Speaker 1: in the end we believe in honesty and advertising. So
52:59
Speaker 1: thanks everyone for join us, and thanks again Kelly for
53:01
Speaker 1: coming along, asking great questions and contributing a lot of laughs.
53:04
Speaker 1: Thank you for having me a head fun as always.
53:06
Speaker 1: Thanks everybody until next time. Alright, tune in next time.
53:17
Speaker 1: Thanks for listening, and remember that Daniel and Jorge explained.
53:20
Speaker 1: The Universe is a production of I Heart Radio. For
53:23
Speaker 1: more podcast For my heart Radio, visit the i heart
53:26
Speaker 1: Radio app, Apple Podcasts, or wherever you listen to your
53:30
Speaker 1: favorite shows.