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Does the Universe need dark matter? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, Daniel, do you ever wonder if physics might be
0:11
Speaker 1: like really wrong? Are you talking about me or like
0:13
Speaker 1: the entire community? Well, I know you're never wrong, Daniel,
0:18
Speaker 1: that's unthinkable, but you know, like, has physics ever gotten
0:22
Speaker 1: something about the very nature of the universe? Kind of? Uh?
0:27
Speaker 1: Not right? You mean, like what's the best snack food?
0:30
Speaker 1: Or what colored lab coat should we wear? Hey? Yeah,
0:33
Speaker 1: you know, like can I eat antimatter or not? Maybe
0:36
Speaker 1: it's delicious. Physics could definitely be wrong about snacks for you. Well,
0:40
Speaker 1: I mean think bigger, like, is it possible that maybe
0:42
Speaker 1: things are not quite what they seem? Well, it wouldn't
0:44
Speaker 1: be the first time physics is wrong, and I hope
0:47
Speaker 1: it's not the last time. I am organ made cartoonists
1:05
Speaker 1: and the creator of PhD comments, Hi, I'm Daniel. I'm
1:08
Speaker 1: a particle physicist, and I've never been wrong about snack foods,
1:12
Speaker 1: and I've never been wrong about being wrong. So I
1:15
Speaker 1: think that means that I'm always wrong. I don't know,
1:18
Speaker 1: but welcome to our podcast. I'm definitely right about that.
1:21
Speaker 1: This is our podcast, Daniel and Jorge Explain the Universe,
1:24
Speaker 1: a production of I Heart Radio. That's right. Our podcast
1:27
Speaker 1: in which we take a mental tour of all the
1:29
Speaker 1: crazy stuff that's out there in the universe and try
1:32
Speaker 1: to bring it into your head. We try to wrap
1:34
Speaker 1: up the entire universe, all those trillions and trillions of
1:38
Speaker 1: stars and weird blobs of gas and dust and invisible stuff,
1:42
Speaker 1: and inserted through a little hole in your ear into
1:45
Speaker 1: your brain. Yeah, all of the amazing and incredible stuff
1:47
Speaker 1: out there in the universe, and also all of the
1:50
Speaker 1: stuff that's normal. You know, And sometimes I think when
1:53
Speaker 1: you examine what's spears to be normal in your everyday lives,
1:56
Speaker 1: it turns out to have all kinds of wonders and
1:58
Speaker 1: all kinds of small miracles in it. Are there small
2:01
Speaker 1: miracles in your snack foods? Is that what you're talking about? Like, Oh,
2:03
Speaker 1: look this chocolate chips in this trail mix. Every snack
2:06
Speaker 1: is a miracle, Daniel, especially banana's net. But in this podcast,
2:11
Speaker 1: we go beyond the miracles of banana based snack foods
2:15
Speaker 1: and talk about the incredible things that scientists are trying
2:18
Speaker 1: to figure out, Because, contrary to popular perception, scientists don't
2:23
Speaker 1: have it all figured out. There are lots of really
2:25
Speaker 1: big mysteries out there and wondering about the universe is
2:29
Speaker 1: something that belongs to everybody, including you. Yeah, so they're
2:32
Speaker 1: one of the biggest mysteries out there in the universe,
2:36
Speaker 1: and literally sort of like not just like because we
2:38
Speaker 1: don't know it, but also because it's a huge part
2:40
Speaker 1: of the universe. Is dark matter? Dark matter is a
2:44
Speaker 1: little bit crazy, right, It's like twenty of the universe,
2:48
Speaker 1: but we have no idea what it is. That's right
2:50
Speaker 1: of the energy budget of the universe. It's a pretty
2:53
Speaker 1: big slice. It's like a little bit more than a
2:55
Speaker 1: quarter of all the energy in the universe. Is this
2:58
Speaker 1: weird stuff and we've never seen it directly. We're pretty
3:01
Speaker 1: sure it's there, but we don't really know what is it.
3:05
Speaker 1: Is it made out of particles? Isn't made out of
3:07
Speaker 1: black holes? Isn't made out of lost socks? Two really
3:10
Speaker 1: huge mystery in modern physics. In fact, I would say
3:14
Speaker 1: it's one of the biggest open questions in science. I
3:17
Speaker 1: think it's just dark socks actually, but didn't that makes sense?
3:20
Speaker 1: I always lose those. How many dark socks have you
3:23
Speaker 1: lost for her? I mean we're talking a lot of songs. Well,
3:26
Speaker 1: to be out of time a cartoonists I don't have
3:28
Speaker 1: to wear dark socks very often or socks at all,
3:32
Speaker 1: because I also lived in California. But yeah, dark matter,
3:34
Speaker 1: I mean, it's a big deal. There's five times more
3:36
Speaker 1: dark matter than there is regular matter, like planets and
3:39
Speaker 1: stars and gas and dust and black holes, there's all
3:43
Speaker 1: that stuff. There's actually it's only of the stuff in
3:47
Speaker 1: the universe, which means the normal matter, the regular matter,
3:50
Speaker 1: the stuff that you're familiar with, is not actually regular
3:53
Speaker 1: or normal. It's the unusual stuff. If you just took
3:56
Speaker 1: like a survey of stuff in the universe, most of
3:59
Speaker 1: the stuff is dark matter. The things that make up
4:02
Speaker 1: stars and planets and galaxies and hamsters and me and
4:05
Speaker 1: you and bananas and snack foods. That's unusual in the universe.
4:09
Speaker 1: It's a minority of what's out there in the universe.
4:12
Speaker 1: And yet we still don't know what this dark matter is.
4:16
Speaker 1: It's a big mystery and a lot of times, you know,
4:18
Speaker 1: whenever people talk about dark matter, I feel like a
4:20
Speaker 1: common question we get, at least and in talks and
4:23
Speaker 1: appearances is that people ask us like what if dark
4:26
Speaker 1: matter doesn't exist? Like what if it's just an error
4:29
Speaker 1: in the equations that you have about physics and the universe,
4:32
Speaker 1: Like what if we just maybe like misunderstood gravity, or
4:36
Speaker 1: haven't counted all the stars in the galaxy, or you
4:39
Speaker 1: know what if there's something else that is maybe normal
4:42
Speaker 1: but we just haven't thought about. And it's a great
4:44
Speaker 1: question because most of the evidence we have for dark
4:47
Speaker 1: matter is a little bit indirect, like because dark matter
4:51
Speaker 1: is so dark and hard to interact with, we don't
4:54
Speaker 1: have clear pictures of it, right, we haven't seen what
4:56
Speaker 1: it's made out of. We've only sort of seen its
4:59
Speaker 1: effects and sometimes second hand, and so it's tempting to
5:02
Speaker 1: wonder if it's really there. You know. It's like if
5:05
Speaker 1: you've only seen the footprints of an animal, are you
5:07
Speaker 1: really sure it exists? Or could it be something else
5:10
Speaker 1: spoofing you Until you really capture one or see one
5:13
Speaker 1: in the wild, you don't really believe it exists. And
5:16
Speaker 1: dark matter has been eluding our searches for decades and
5:19
Speaker 1: makes people wonder like, well, maybe you people have it wrong. Yeah,
5:22
Speaker 1: I mean it's kind of a crazy idea, right to
5:24
Speaker 1: think that there's that much stuff out there, and conveniently
5:28
Speaker 1: it's invisible and you can't see it. You know what,
5:31
Speaker 1: I mean like I would be like, maybe you should
5:34
Speaker 1: take your math or you know, maybe you should have
5:36
Speaker 1: another grad student do the calculation. So, which do you
5:39
Speaker 1: think is the conspiracy theory dark matter like there's so
5:42
Speaker 1: much of it and you can't see it because it's
5:44
Speaker 1: so dark and that proves that it exists, or the
5:47
Speaker 1: anti dark matter conspiracy theory. Wait, anti dark matter? That's
5:52
Speaker 1: another episode right there? Can you have anti dark matter Danny? Yeah,
5:56
Speaker 1: that's another great question. We think probably not because then
5:59
Speaker 1: they would annihilate a dark matter and turn into photons,
6:01
Speaker 1: which we would see, what if it turns into dark photons? Yeah,
6:05
Speaker 1: that's actually a thing, dark photon. Really? Yeah, there goes
6:09
Speaker 1: my double price. You don't get credit for that one.
6:12
Speaker 1: But you know which idea sounds more bonkers, right, that
6:15
Speaker 1: the universe has filled with an incredible amount of invisible
6:18
Speaker 1: matter nobody had detected until recently, or that it's not. Yeah.
6:22
Speaker 1: So today on the podcast, we'll be asking the question
6:30
Speaker 1: does the universe need dark matter? Is it an essential
6:34
Speaker 1: part of the unit, Like could you have a universe
6:36
Speaker 1: without dark manner? Does that make sense? Or is it
6:39
Speaker 1: maybe that the universe doesn't need it? And maybe it
6:41
Speaker 1: is kind of an error that we have in our
6:43
Speaker 1: calculations and observations about the universe. And this kind of
6:46
Speaker 1: skepticism is very healthy. When you have a crazy idea
6:50
Speaker 1: you're trying to accommodate when you see results in your
6:52
Speaker 1: experiments you don't understand. You need to be flexible about
6:56
Speaker 1: this sort of theoretical framework with which you come at
6:58
Speaker 1: the problem. You need to be open to crazy new ideas,
7:02
Speaker 1: but you also need to be open to the fact
7:04
Speaker 1: that maybe you got your measurements wrong. There always can
7:06
Speaker 1: be an alternative explanation. So before you go big and say, wow,
7:11
Speaker 1: we're going to revolutionize our understanding of the universe, you've
7:14
Speaker 1: got to rule out all the more prosaic, basic, simpler explanations.
7:18
Speaker 1: And so it's always a good idea to keep those
7:20
Speaker 1: ideas in mind. YEA, So how sure are we that
7:23
Speaker 1: dark matter exists in the universe? And could it be
7:26
Speaker 1: something else? So I went out there into the internet
7:28
Speaker 1: and I asked people, can we explain what we see
7:31
Speaker 1: in the universe without dark matter? Are there good alternative
7:35
Speaker 1: theories that don't require a new particle or a new
7:38
Speaker 1: blob of stuff. Before you listen to these answers from
7:41
Speaker 1: the internet. Think about it for a second. Do you
7:42
Speaker 1: think dark matter is necessary in the universe or do
7:46
Speaker 1: you think the universe could ignore it or live without it.
7:49
Speaker 1: Here's what people had to say. I would think we
7:51
Speaker 1: would be much more baffled if we didn't have dark
7:56
Speaker 1: matter to explain the expansion of the universe. Oh man,
8:00
Speaker 1: And this is tough because I still don't have a
8:02
Speaker 1: good handle on what dark matter really is. But I
8:06
Speaker 1: think I think we really don't know what dark matter is,
8:09
Speaker 1: So I'm going to say, yeah, we could definitely explain
8:12
Speaker 1: what we see without it, because we're kind of just
8:14
Speaker 1: making up what it is. Yes, dark matter is just
8:17
Speaker 1: a made a term for the stuff that's there that
8:21
Speaker 1: we can't explain. So truly, any theory is under the
8:27
Speaker 1: umbrella of dark matter. I have a strong suspicion that
8:31
Speaker 1: the thing could be explained without dark matter, recalled Daniel,
8:37
Speaker 1: saying that dark matter is just the name that we've
8:40
Speaker 1: come up with for the phenomenon that we can't explain.
8:44
Speaker 1: I don't know what we would see without dark matter.
8:47
Speaker 1: My understanding is that we do know that there's dark
8:51
Speaker 1: matter because the math doesn't work out. At some point,
8:56
Speaker 1: we will have to explain the universe without dark matter,
8:59
Speaker 1: because like the dark in the matter sort of implies
9:03
Speaker 1: that we don't know what it is. I believe that
9:06
Speaker 1: dark matter is a theory that we came up with
9:09
Speaker 1: to help explain why we couldn't account for all of
9:12
Speaker 1: the gravity that we see in the universe. I think
9:16
Speaker 1: it's a fairly recent discovery. I don't believe Einstein knew
9:21
Speaker 1: about it, so he must have had some other way
9:25
Speaker 1: to account for all of the gravity. All Right, some
9:28
Speaker 1: pretty good answers there, Yeah. I like the people who
9:31
Speaker 1: treat dark matters just sort of like as an umbrella
9:33
Speaker 1: idea for all the things we don't understand. And so
9:36
Speaker 1: whatever we find out there, we just call that dark matter.
9:39
Speaker 1: And I think that really touches on the sense people
9:41
Speaker 1: have that we don't really have a clue what's out there.
9:43
Speaker 1: We just sort of labeled it dark matter, and we're
9:46
Speaker 1: talking about it as if it's a thing, but it's
9:48
Speaker 1: really just a name we applied to our cluelessness. Interesting,
9:51
Speaker 1: like if you put an S at d end, it
9:53
Speaker 1: becomes dark matters, and then then it's sort of like
9:55
Speaker 1: an umbrella term for things that are dark. That's true,
10:00
Speaker 1: and you know, that is definitely true of dark energy.
10:03
Speaker 1: Dark energy is another piece of the universe pie. Right,
10:06
Speaker 1: the universe pie is five percent normal matter, twenty dark matter,
10:12
Speaker 1: dark energy. Dark energy definitely in the category of just
10:15
Speaker 1: stuff we don't understand. We gave a fancy sounding name
10:19
Speaker 1: dark energy, this stuff that's making the universe expand right.
10:22
Speaker 1: Dark matter, on the other hand, is much better understood,
10:26
Speaker 1: is much more concrete and idea much more detailed observation.
10:30
Speaker 1: So they're both called dark both things we don't understand.
10:33
Speaker 1: But dark energy definitely a label for our cluelessness, while
10:37
Speaker 1: dark matter is a much better founded, well described theory.
10:40
Speaker 1: It's less dark. I guess we're less in the dark
10:44
Speaker 1: about exactly. We are less in the dark. Our minds
10:47
Speaker 1: are not quite so filled with dark shadows. All right.
10:49
Speaker 1: Well that's the question for today, And that's the question is,
10:51
Speaker 1: you know, do we need dark matter, like is it
10:54
Speaker 1: a concept that is totally necessary for the universe to
10:57
Speaker 1: make sense or is it just something weird that exists
11:01
Speaker 1: out there and that maybe we could have a totally
11:03
Speaker 1: wrong idea about, you know, And are there other theories
11:06
Speaker 1: that are being worked on in the scientific community that
11:09
Speaker 1: might explain it without needing to add some new kind
11:12
Speaker 1: of stuff to the universe, darker matter, the darkest matter.
11:16
Speaker 1: And you'll find that in science there are always competing voices.
11:19
Speaker 1: You know, there's often like a mainstream most people think
11:22
Speaker 1: the answer is X, but there's always somebody out there
11:24
Speaker 1: who thinks it's why, somebody who thinks it's Z. And
11:27
Speaker 1: you've got to give these people room because sometimes they're right,
11:30
Speaker 1: and sometimes their ideas of the ones that turn into
11:33
Speaker 1: the mainstream. That's how the mainstream became mainstream. It used
11:36
Speaker 1: to once be lunatic fringe or their alternative physicists, you know,
11:41
Speaker 1: French physicists. There definitely are there are people out there,
11:44
Speaker 1: once they get tenure, start working on crazy Bonker's theories
11:48
Speaker 1: and sometimes for decades that nobody pays attention, nobody really
11:51
Speaker 1: reads their papers, that people even laugh behind their hands.
11:54
Speaker 1: But sometimes they're right, you know. Literally, the history of
11:57
Speaker 1: physics is filled with revolutions that start as crazy ideas,
12:02
Speaker 1: and so we definitely got to pour water on some
12:04
Speaker 1: of those seedlings because they could sprout into huge new
12:07
Speaker 1: intellectual trees, awesome dark physics trees. I'm picturing all right, Well,
12:12
Speaker 1: step us through um like what's the main argument for
12:15
Speaker 1: dark matter, Like, what's the main evidence about it, and
12:19
Speaker 1: what makes this think that, you know, it's something new
12:22
Speaker 1: and different as opposed to maybe it's just more stuff
12:24
Speaker 1: out there that we can't see. Yes, So if you're
12:26
Speaker 1: going to come up with another theory of the universe,
12:28
Speaker 1: another way to explain the way the universe works, you
12:31
Speaker 1: have to explain what we do see, right, I mean
12:34
Speaker 1: that's the whole idea behind making a theory of physics.
12:37
Speaker 1: So if you're gonna come up with your theory, you
12:39
Speaker 1: have to understand what are the observations, what are the
12:41
Speaker 1: experiments reveal that need to be explained that you know,
12:45
Speaker 1: what's the motivation for creating this idea of dark matter?
12:48
Speaker 1: And the short version is it's all gravity. Like everything
12:52
Speaker 1: we see out there in the universe that we need
12:54
Speaker 1: dark matter to explain our weird gravitational effects that we
12:58
Speaker 1: can't explain with all the other stuff, just the gas
13:01
Speaker 1: and the dust and the stars right with the universe
13:04
Speaker 1: feels dark matter in terms of gravity, like it's they're
13:08
Speaker 1: affecting the gravity of other things, but you can't see it.
13:11
Speaker 1: That's the main evidence for it. Yeah, basically there's unexplained gravity,
13:14
Speaker 1: like we thought we knew where all the stuff was
13:16
Speaker 1: in the universe from the stars and the gas and
13:18
Speaker 1: the dust, and from that you can calculate how much
13:21
Speaker 1: gravity there should be, and we can see the effects
13:23
Speaker 1: of gravity, and will go through in a list of
13:25
Speaker 1: how we see the effects of gravity. But there's more
13:28
Speaker 1: gravity than we expected. So either there's more stuff e
13:32
Speaker 1: dark matter, or gravity is weird and different, and so
13:35
Speaker 1: it's all about the gravity then, right, because that's kind
13:38
Speaker 1: of at the basis of the theory about dark matter,
13:41
Speaker 1: is that it feels gravity but not anything else exactly,
13:43
Speaker 1: and that's why it's dark. Because if it felt electromagnetism,
13:47
Speaker 1: it would reflect light, or it would give off light
13:49
Speaker 1: like everything else does than the universe, it glows, or
13:53
Speaker 1: if it felt the strong force, it would bind with
13:55
Speaker 1: corks and form nucleons and interact with us. So it
13:58
Speaker 1: doesn't interact with us in any way that we know
14:00
Speaker 1: of other than gravitational And that's why we call it
14:03
Speaker 1: matter because we think it's something that has new gravity.
14:07
Speaker 1: But you know, it could also just be a tweak
14:08
Speaker 1: to the way we understand gravity. But at its core,
14:11
Speaker 1: it's really an observation that our theory of gravity doesn't
14:15
Speaker 1: work either because there's missing mass or the theory is wrong. Right,
14:20
Speaker 1: So those are two sides of the coin, right, Like,
14:22
Speaker 1: according to what we know about how gravity works, there's
14:25
Speaker 1: a lot of gravity missing, or there's too much gravity
14:29
Speaker 1: in the universe. Almost. Yeah, there's gravity out there and
14:32
Speaker 1: we don't have mass to explain it, and so we
14:35
Speaker 1: have to sort of fill in those gaps. And that's
14:37
Speaker 1: an uncomfortable feeling, right, You're like, well, you can't just
14:39
Speaker 1: like fill in the gaps and assume that your theory
14:42
Speaker 1: is right and add extra stuff to make it work out.
14:45
Speaker 1: You know, that feels uncomfortable, Like there's snack food missing
14:49
Speaker 1: from my fridge. Surely it was my son who cut
14:52
Speaker 1: up in the middle of the night to eat it.
14:54
Speaker 1: But you can't you can't just assume that. You can't
14:56
Speaker 1: just assume that could have been your daughter. Maybe you
14:59
Speaker 1: sleepwalked and aided or or And that's when you install
15:01
Speaker 1: a camera in front of your fridge and you get
15:03
Speaker 1: those weird videos of yourself at four am stuffing cake
15:06
Speaker 1: in your face. Experience the hypothetically hypothetically hypathetically right, Alright,
15:13
Speaker 1: So we think dark matter is there because of gravity.
15:16
Speaker 1: And there are several ways that we have seen this
15:19
Speaker 1: kind of missing gravity, right, that's in the galaxy and rotations.
15:22
Speaker 1: The way galaxies rotade is kind of one of the
15:25
Speaker 1: first one, maybe even Yeah, one of the most dramatic
15:28
Speaker 1: and earliest pieces of evidence that there was more gravity
15:31
Speaker 1: in the universe than we expected was looking at how
15:35
Speaker 1: galaxies rotate. And we can add up all the mass
15:38
Speaker 1: of the stars and the stuff in the galaxy and say, okay,
15:41
Speaker 1: we know how much gravity there should be, and then
15:44
Speaker 1: we can calculate how fast those galaxies are rotating. And
15:48
Speaker 1: you know, for a galaxy, when it's rotating, it's trying
15:50
Speaker 1: to push stuff off. It's trying to throw stuff off
15:53
Speaker 1: the edge. Like if you put ping pong balls on
15:55
Speaker 1: a merry go round and spin it, the ping pong
15:57
Speaker 1: balls fly out. The thing that keeps the galaxy from
16:00
Speaker 1: tearing itself apart from throwing those ping pong balls out
16:03
Speaker 1: into inter galactic space, is the gravity. So you can
16:07
Speaker 1: ask how fast is the galaxy spinning and is there
16:11
Speaker 1: enough gravity to hold it together, because the faster spins,
16:14
Speaker 1: the more gravity unied. Right. Yeah, It's kind of like
16:16
Speaker 1: our solar system, right, like you know, the mass of
16:18
Speaker 1: the Sun is what keeps all the planets kind of
16:20
Speaker 1: spinning around it. But what if, like the planets for
16:22
Speaker 1: spinning faster than what you could explain by the mass
16:25
Speaker 1: of the Sun. You would need some other explanations, that's right,
16:27
Speaker 1: You need more force to hold them into their orbits.
16:30
Speaker 1: And so you say, well, maybe there's extra gravity or
16:32
Speaker 1: some other force or something. And so we know there's
16:35
Speaker 1: something else holding the galaxy together. And so the first explanations,
16:40
Speaker 1: oh well, what if there's more invisible mass. It's just
16:43
Speaker 1: some stuff in the galaxy that's providing gravity and we
16:46
Speaker 1: just can't see it. And you can explain these rotation
16:50
Speaker 1: curves the way the stars move around centers of galaxies.
16:53
Speaker 1: If you distribute a bunch of mass sort of smoothly,
16:56
Speaker 1: it's like a big clump in the middle, and then
16:58
Speaker 1: sort of smoothly outpass the edge of the galaxy into
17:01
Speaker 1: a big halo that's even actually bigger than the galaxy.
17:04
Speaker 1: That explains to this as that would give you the
17:06
Speaker 1: kind of gravity that we are seeing, right, because if
17:09
Speaker 1: you look at a galaxy doesn't have that enough mass, right,
17:12
Speaker 1: like there aren't enough stars or planets in it that
17:14
Speaker 1: you can see. That's right. We count up all the
17:16
Speaker 1: visible stuff and we say how much mass does it have?
17:19
Speaker 1: And that just doesn't give us enough gravity to explain
17:22
Speaker 1: how those galaxies are holding themselves together. And that was
17:25
Speaker 1: the sort of genesis of it. I mean, for a
17:27
Speaker 1: long time people were like, well, wow, that's must be
17:30
Speaker 1: a mistake, you know, because coming to the idea that
17:32
Speaker 1: there's five times as much mass as you can see
17:36
Speaker 1: is a really big idea. It's not something people just
17:38
Speaker 1: came to an afternoon and then accepted, Yeah, it doesn't
17:41
Speaker 1: seem likely. It does not seem likely. It seems more
17:44
Speaker 1: likely that you mismeasured something that you got the velocity
17:47
Speaker 1: is wrong, or you know your grad student is pranking
17:50
Speaker 1: you or something. It's a big idea, right, It's like
17:52
Speaker 1: jumping to the conclusion right away that maybe there are
17:55
Speaker 1: five ghosts in my house eating my snacks. You know,
17:57
Speaker 1: that's like a big that's a big leap from like, hey,
18:00
Speaker 1: maybe it's just your u. And so it took other
18:04
Speaker 1: pieces of evidence before dark matter became mainstream. Right, all right,
18:08
Speaker 1: let's get into those other ways that we know dark
18:10
Speaker 1: matter is there, and let's get into whether or not
18:14
Speaker 1: it is there, or me we just have gravity wrong.
18:16
Speaker 1: But first, let's take a quick break, all right, Daniel,
18:31
Speaker 1: we're talking about whether dark matter is necessary in the
18:35
Speaker 1: universe or you know, maybe it's just the hanger on
18:37
Speaker 1: and the universe could care less about dark matter. But
18:40
Speaker 1: we we know it's definitely there because we see it
18:42
Speaker 1: from the rotation of galaxies and also we've set off
18:46
Speaker 1: kind of can see it, right, We can see it
18:48
Speaker 1: in the way that distorts the light from other far
18:51
Speaker 1: away stars. That's right. These days, we have a good
18:53
Speaker 1: handful of ways to indirectly detect dark matter, and one
18:58
Speaker 1: of the coolest is seeing it act like a lens
19:01
Speaker 1: in the sky because dark matter only has gravitational forces,
19:05
Speaker 1: but gravity can bend space, right, It's a bending of
19:09
Speaker 1: space and time. So if you have a big blob
19:11
Speaker 1: of invisible stuff in the sky, it will curve the
19:15
Speaker 1: space it's in, so that photon is traveling through it
19:18
Speaker 1: will get bent as if they're moving through a huge lens. Right,
19:21
Speaker 1: So if there's a big blob of dark matter between
19:24
Speaker 1: you and some really far away galaxy, it will distort
19:27
Speaker 1: that galaxy, creating duplicates of it, stretching it just as
19:31
Speaker 1: if there was a huge lens in the sky, right,
19:33
Speaker 1: and then we can see that, like if you look
19:35
Speaker 1: at pictures as a sky, you see these kind of distortions,
19:37
Speaker 1: these ripples, these lensing effects. You can sort of see
19:40
Speaker 1: dark matter almost, you can definitely see it. And there's
19:42
Speaker 1: sort of two categories. There's strong lensing, like there's a
19:45
Speaker 1: big dense blob and it's distorting the galaxies and it's
19:49
Speaker 1: pretty hard to explain that kind of lensing in any
19:51
Speaker 1: other way. And then there's weak lensing. We just sort
19:54
Speaker 1: of like look at all the galaxies out there to see, like,
19:58
Speaker 1: are any of them sort of just a little distorted?
20:00
Speaker 1: They're just looking a little tweaked. And from that we
20:02
Speaker 1: can get sort of like a map of where in
20:04
Speaker 1: the sky we think the dark matter is just by
20:07
Speaker 1: looking at small distortions. And that's how we've gotten a
20:10
Speaker 1: pretty good map for where we think this missing mass is.
20:13
Speaker 1: How we know that it's mostly in the center of
20:15
Speaker 1: the galaxy and how far out past the edge of
20:18
Speaker 1: the visible galaxy the dark matter halo might go. So
20:21
Speaker 1: it's a really powerful technique, right, And so that's kind
20:23
Speaker 1: of how dark matter entered in kind of our view
20:26
Speaker 1: of the universe was these first initial ways. But since
20:30
Speaker 1: then and we sort of have put more nails into
20:32
Speaker 1: sort of the coffin of whether or not dark matter
20:35
Speaker 1: is out there, right, I mean, it's now sort of
20:38
Speaker 1: shows up in pictures of the universe, the early universe,
20:41
Speaker 1: it kind of shows up in our calculations about all
20:44
Speaker 1: of the energy in the universe. Right, It's gotten more
20:46
Speaker 1: and more convincing that there's something there that's right, it
20:49
Speaker 1: becomes very difficult to explain the universe you see without
20:52
Speaker 1: dark matter. For example, we see the influence of dark
20:55
Speaker 1: matter on the formation of structures in the universe. Like
20:59
Speaker 1: the universe be in as a sort of diffuse cloud,
21:02
Speaker 1: you know, just of gas, mostly hydrogen, and then it
21:05
Speaker 1: started clumping together, and that clumping comes from gravity. Right,
21:08
Speaker 1: So gravity is the thing that draws these things together
21:11
Speaker 1: and eventually gives you stars and galaxies and planets and
21:14
Speaker 1: all that cool stuff. If you run a stimulation of
21:16
Speaker 1: the universe without any dark matter, then you don't get
21:19
Speaker 1: galaxies in the first ten billion years. It takes like
21:22
Speaker 1: another ten or twenty billion years. So it's dark matter
21:25
Speaker 1: that's that's created these like gravitational wells for stuff to
21:29
Speaker 1: fall into to make the stars in the galaxies and us.
21:33
Speaker 1: So just like the whole structure of the universe would
21:35
Speaker 1: look very different without some kind of gravitational stuff out there,
21:40
Speaker 1: and we should have just called them dark clumps or
21:43
Speaker 1: dark do it. And even earlier in the universe, like
21:47
Speaker 1: we've talked on this program about the cosmic microwave background radiation.
21:51
Speaker 1: Those are photons from the very very early universe, three
21:54
Speaker 1: hundred thousand years after the universe was created, the first
21:58
Speaker 1: moment when the universe became Rand's parent to light. Before
22:01
Speaker 1: that it was thick and soupy and light got reabsorbed,
22:04
Speaker 1: and after that it was cool enough that light could
22:06
Speaker 1: travel through the universe without being absorbed. And the shape
22:10
Speaker 1: of that plasma that gave off that light was affected
22:13
Speaker 1: by dark matter and how much dark matter there was
22:16
Speaker 1: and much normal matter there was, and that stuff like
22:18
Speaker 1: bounced off each other and oscillated and like squeezed and
22:22
Speaker 1: squished that plasma. So the amount of dark matter in
22:25
Speaker 1: that moment of the universe affects the shape of that light,
22:28
Speaker 1: the currents, the sort of patterns we see in that
22:31
Speaker 1: light in a very very precise way that's hard to
22:35
Speaker 1: describe in any other way other than there's some other
22:38
Speaker 1: kind of stuff out there. So that's giving us this graphic. Yeah,
22:42
Speaker 1: you can see like it's imprint in the light from
22:44
Speaker 1: the early universe, Like it's visually and like tangibly and like,
22:49
Speaker 1: you know, you can calculate it the storts that light.
22:51
Speaker 1: That's right. And so if you just take the universe
22:53
Speaker 1: and you add to it a new particle, a particle
22:56
Speaker 1: that doesn't move really fast, we call it cold and
22:59
Speaker 1: doesn't feel anything but gravity, it explains all of this stuff.
23:03
Speaker 1: It explains why galaxies rotate the way they do, why
23:06
Speaker 1: the cosmic microwave background rediation looks the way it does,
23:10
Speaker 1: why the structure of the universe has this structure at
23:13
Speaker 1: this time in the universe. And also there are very specific,
23:17
Speaker 1: awesome experiments that the universe has done to sort of
23:20
Speaker 1: demonstrate dark matter to us. Yeah, I guess, you know,
23:24
Speaker 1: I think maybe a question that a lot of people have,
23:27
Speaker 1: and I'm sure if physicists had at the beginning, was,
23:29
Speaker 1: you know, why does it need to be something special?
23:32
Speaker 1: Like why does it need to be a new kind
23:35
Speaker 1: of particle or matter? Couldn't it also just be something regular?
23:39
Speaker 1: But that you just can't see, Like, you know, what
23:41
Speaker 1: if there's a whole bunch of black asteroids out there
23:44
Speaker 1: that are hard to see, or you know, a lot
23:46
Speaker 1: of small dust that we can see, or maybe even
23:49
Speaker 1: like a whole bunch of little black holes kind of
23:51
Speaker 1: spread around the universe. Yeah, that's a great question, like
23:53
Speaker 1: why do we know it's a new kind of thing.
23:56
Speaker 1: Why can't it just be more of the same but
23:58
Speaker 1: just kind of dark, right? Yeah, yeah, Well we don't
24:01
Speaker 1: really know anything in our current set of particles that
24:04
Speaker 1: is that dark. I mean, other than neutrinos. Everything else
24:08
Speaker 1: has some kind of interaction, like if it's a black
24:11
Speaker 1: rock or something, well that does reflect light, it does
24:14
Speaker 1: give off light. So if it's made out of the
24:16
Speaker 1: normal kind of matter, it's going to have the kind
24:18
Speaker 1: of interactions we have, and therefore we would be able
24:22
Speaker 1: to see it, except of course, neutrinos. Neutrinos were candidate
24:26
Speaker 1: for dark matter for a long time. The problem is
24:28
Speaker 1: neutrinos move away too fast, they zoom around the universe
24:31
Speaker 1: because they're so light, so they don't give the same
24:34
Speaker 1: sort of structure to the universe that dark matter does.
24:37
Speaker 1: You need this thing to be sort of slow moving
24:39
Speaker 1: and cold in order to stick around long enough to
24:42
Speaker 1: give the structure of galaxies. Could you have cold neutrinos
24:46
Speaker 1: like slow moving neutrinos. Then we talk about that the
24:48
Speaker 1: other in another episode. Could you have cold neutrinos, Yes,
24:52
Speaker 1: but we don't think that neutrinos are that cold. We
24:55
Speaker 1: haven't seen cold neutrinos. Neutrinos are so light they have
24:58
Speaker 1: almost no mass that they essentially almost always travel near
25:02
Speaker 1: the speed of light. They're always in a rush, They're
25:05
Speaker 1: always in a hurry. Yes, neutrinos are hot, as we
25:08
Speaker 1: call them in particle physics. And it literally and figuratively
25:12
Speaker 1: is it's it's a pretty big trend right now, that's right,
25:15
Speaker 1: all right? Well, I guess if it's not maybe something
25:18
Speaker 1: we know about, could it be that just maybe we
25:20
Speaker 1: have our theories wrong about how things work, Like, you know,
25:23
Speaker 1: maybe it's not a new particle or a new kind
25:25
Speaker 1: of matter, but maybe we just have gravity wrong. Like
25:28
Speaker 1: maybe gravity doesn't work the way we think it is.
25:31
Speaker 1: And maybe these larger universe size scales could gravity, you know,
25:36
Speaker 1: could there be a different theory of gravity that would
25:38
Speaker 1: maybe account for what we think is dark matter. It's
25:40
Speaker 1: definitely something to consider, right, because all these observations are
25:44
Speaker 1: just observations of gravity. And what we're doing is we're saying,
25:48
Speaker 1: we assume there's a certain amount of mass out there,
25:50
Speaker 1: we assume we know how gravity works. We estimate how
25:53
Speaker 1: much gravity there should be based on that mass. Right,
25:56
Speaker 1: But there are two steps in there. There's figure out
25:58
Speaker 1: where the mass is and in calculate how much gravity
26:01
Speaker 1: there is from that mass. If that second step is wrong, right,
26:05
Speaker 1: if the theory of gravity works differently from what we expected,
26:08
Speaker 1: then yeah, that could possibly explain it. Because remember, gravity
26:11
Speaker 1: is very very weak, which makes it very very hard
26:14
Speaker 1: to test, Like it's difficult to measure the force of
26:18
Speaker 1: gravity at the scale of one centimeter between two pebbles
26:22
Speaker 1: because there's force there's almost zero, Like you need to
26:26
Speaker 1: build a very sensitive instrument to measure the gravitational force
26:29
Speaker 1: between anything that's smaller than you know, planets and moons. Yeah,
26:33
Speaker 1: because you know, like our current theory of gravity says that,
26:36
Speaker 1: you know, gravity changes by one of our squared, Like
26:40
Speaker 1: the force of gravity kind of depends on the distance
26:43
Speaker 1: between two things squared. And then you put down in
26:45
Speaker 1: the denominator and that always seems kind of almost too
26:49
Speaker 1: simple to me, Like, what are the chances at the
26:51
Speaker 1: universe would pick such a simple little formula to calculate gravity.
26:55
Speaker 1: You know, why isn't it like one over our squared
26:58
Speaker 1: times are to the zero point seven five? You know,
27:01
Speaker 1: I mean, like it seems so simple. Maybe what if
27:04
Speaker 1: it's wrong, Like what if gravity isn't one of our
27:06
Speaker 1: our square but maybe changes over distances in a way
27:09
Speaker 1: that could maybe explain dark matter. Yeah, that's a totally
27:11
Speaker 1: realistic thing to think. Although you know, there's a lot
27:14
Speaker 1: of these patterns, these one of our our squared patterns
27:16
Speaker 1: in physics and enforces and there is a good reason
27:19
Speaker 1: for it and a fairly simple way to understand it.
27:21
Speaker 1: If you imagine like the surface of a sphere surrounding
27:24
Speaker 1: a point. Think about like all the gravitational energy coming
27:27
Speaker 1: out of a point, the surface of a sphere surrounding it,
27:30
Speaker 1: all the gravitational energy passes through that sphere, and then
27:33
Speaker 1: as the radius of that sphere gets larger, what's the
27:36
Speaker 1: surface of that sphere? Will it goes like are squared,
27:39
Speaker 1: and so the power at any point should go like
27:42
Speaker 1: one over our squared. It sort of makes sense geometrically
27:46
Speaker 1: but what if geometry is wrong. What if geometry is wrong? Right?
27:50
Speaker 1: What is wrong? What if podcasting is wrong? What if
27:54
Speaker 1: one plus one is wrong? Well, it kind of, I mean,
27:57
Speaker 1: you know, we talk about sometimes about how space is
28:00
Speaker 1: not you know, this nice and neat, orderly thing, and
28:02
Speaker 1: that sometimes you can even like measure triangles in real
28:06
Speaker 1: space that have angles that are bigger than a hundred
28:09
Speaker 1: and eighty degrees, could maybe, like space be weird in
28:13
Speaker 1: such a way that it's not really one over our squares? Yeah? Absolutely,
28:16
Speaker 1: And there are forces that don't go like one of
28:18
Speaker 1: our our squared, Like the strong force doesn't go like
28:20
Speaker 1: one of our squared at small distances. It gets even
28:23
Speaker 1: stronger as you get further away. So you definitely got
28:26
Speaker 1: to be open to weirdness. And so around the time
28:29
Speaker 1: when dark matter was sort of coming up in the
28:31
Speaker 1: world as an idea, when it was based mostly on
28:34
Speaker 1: galaxy rotations, people thought, well, how can I tweak gravity
28:38
Speaker 1: to explain what I'm seeing without dark matter? What would
28:41
Speaker 1: I need to change? How what would you have to
28:42
Speaker 1: be like one over our to the third or one
28:44
Speaker 1: of our at one point five or whatever in order
28:47
Speaker 1: to explain these galaxy rotations without dark matter, and so
28:51
Speaker 1: they came up with a different theory. It's called mond
28:54
Speaker 1: m O n D for modified Newtonian dynamics. Know, I
29:01
Speaker 1: would have just called the dark bath. I wish you
29:04
Speaker 1: could get in a time machine and go back and
29:05
Speaker 1: tell them because I hate moms. Would that be a
29:08
Speaker 1: lot catch here and fun? Dark bath? Well, mon, I
29:13
Speaker 1: guess if you're French, then it's like, oh, yeah, it
29:15
Speaker 1: means the world. I suppose it's missing the E and
29:19
Speaker 1: it commits the terrible acronym crime of taking two letters
29:21
Speaker 1: from one of the words, you know, modified. Yeah, all right,
29:26
Speaker 1: so there is a kind of a theory in physics.
29:28
Speaker 1: It says like maybe we do have gravity wrong, right,
29:31
Speaker 1: is like an idea you guys take seriously, like maybe
29:34
Speaker 1: there is no dark matter, it's just dark bath. It's
29:36
Speaker 1: definitely an idea that physics should take seriously in the
29:39
Speaker 1: sense that we should think about alternatives. This one theory
29:42
Speaker 1: in particular doesn't have a lot of supporters, and we'll
29:44
Speaker 1: get into exactly why, but you know, it's an interesting
29:47
Speaker 1: idea and it says like maybe gravity works differently at
29:52
Speaker 1: very very large distances, like you know, we've tested gravity
29:55
Speaker 1: here on Earth. We know how it works. We've tested
29:57
Speaker 1: gravity in the Solar System pretty well. But you know,
30:00
Speaker 1: maybe the first time we're looking at gravity on galaxy scales,
30:03
Speaker 1: maybe gravity just works differently over like, you know, fifty
30:07
Speaker 1: thou light years, right, we haven't done that experiment before.
30:10
Speaker 1: Maybe it gets going, like, maybe it gets stronger in
30:13
Speaker 1: galaxy size scales. Yeah, And so the idea is actually
30:16
Speaker 1: even weirder than that. It says, maybe gravity works differently
30:21
Speaker 1: when you have a very very small acceleration, things are
30:25
Speaker 1: not being pulled very hard. Maybe gravity works a little
30:30
Speaker 1: bit different. Oh my god, you just blew my mind.
30:32
Speaker 1: All right, let's let's get into this crazy idea that
30:35
Speaker 1: gravity depends on how you're moving maybe, and whether or
30:39
Speaker 1: not that could work. But first let's take another quick break, right, Daniel,
30:55
Speaker 1: we're talking about dark math to maybe explain dark matter.
30:58
Speaker 1: And so there's an idea that maybe our formulation of
31:01
Speaker 1: gravity or how we think gravity works could be wrong.
31:04
Speaker 1: Maybe and it could maybe act very differently over larger scales,
31:08
Speaker 1: like maybe it gets stronger the galaxy scale or when
31:11
Speaker 1: you're telling me it's actually when there's lower accelerations. And
31:14
Speaker 1: this is just an idea again, right, this is just
31:16
Speaker 1: an idea. But hey, everything is just an idea, right,
31:18
Speaker 1: even lunch lunch is just an idea. Remember, Oh no, Daniel,
31:22
Speaker 1: lunch is very real to me. Well, the idea is,
31:25
Speaker 1: you know, somehow you have to get gravity to be
31:27
Speaker 1: stronger without breaking things we already know. So what if
31:30
Speaker 1: you know affecting things on the edge of the galaxy,
31:33
Speaker 1: you could have gravity instead of going like one over
31:36
Speaker 1: our squared, which you make it very very weak as
31:38
Speaker 1: our gets large, you can make it go like one
31:40
Speaker 1: over R. It doesn't fall off as quickly, doesn't get
31:43
Speaker 1: weak as quickly with large distances. Interesting, So like the
31:47
Speaker 1: gravity I feel with another planet on the other side
31:50
Speaker 1: of the Milky Way, maybe it's stronger than I think. Yeah,
31:54
Speaker 1: maybe it's not like one over our squares, more like
31:56
Speaker 1: one over R and R is still really really large.
31:59
Speaker 1: So you know, those planets don't affect you because the
32:01
Speaker 1: gravity from them is still really tiny. But if you're
32:04
Speaker 1: adding up lots of planets and you're trying to calculate,
32:07
Speaker 1: like how a whole galaxy spins, then it really does
32:10
Speaker 1: make a big difference to go like one of our
32:11
Speaker 1: are instead of one of are squared. But it seems
32:15
Speaker 1: a bit hacky as opposed to dark matter. Dark matter
32:18
Speaker 1: doesn't seem hacky. Well, dark matter, you know, you can
32:21
Speaker 1: explain a lot of really different things adding just one
32:24
Speaker 1: simple idea, like hey, what if there's a new particle
32:27
Speaker 1: that's not hacky, Like we don't know why there's a
32:29
Speaker 1: certain number of particles in the universe and not more,
32:32
Speaker 1: or if there are more, so adding one new particle
32:34
Speaker 1: doesn't feel as hacky. Is like let's have gravity have
32:37
Speaker 1: a knob on it, or like a distance above which
32:40
Speaker 1: it starts behaving different rules, or you know, small accelerations
32:44
Speaker 1: below which it starts behaving differently. Really, what why does
32:46
Speaker 1: it need to be low accelerations? How does that work? In?
32:49
Speaker 1: It doesn't really work as I think I heard you
32:52
Speaker 1: react because like the acceleration with respect to what right,
32:55
Speaker 1: all of a sudden, that gravity you feel depends on
32:57
Speaker 1: your reference frame, and it brings all sorts of symmetries.
33:00
Speaker 1: But there is that one idea that accelerations like one
33:04
Speaker 1: ten trillions of the gravity on Earth. If you feel
33:07
Speaker 1: an acceleration less than that, then the gravity on you
33:10
Speaker 1: behaves differently than it does for larger accelerations. Oh, I see.
33:14
Speaker 1: It's not you're not saying when you're going at a
33:16
Speaker 1: low acceleration, it's just you mean the when the effects
33:19
Speaker 1: of gravity are small, maybe it doesn't behave as one
33:21
Speaker 1: over our square. Yeah, and the effects of gravity at
33:24
Speaker 1: very very far distances are really small. Like what is
33:27
Speaker 1: the acceleration on some star at the edge of the
33:30
Speaker 1: galaxy due to the black hole the center of the galaxy. Well,
33:33
Speaker 1: the distance is fifty thou light years, so the acceleration
33:37
Speaker 1: is very small, also because the masses are large, and
33:40
Speaker 1: so it's just a way to say, let's have it
33:42
Speaker 1: have an effect where we see this weirdness and not
33:46
Speaker 1: have an effect anywhere else. Let's try not to break
33:48
Speaker 1: what we already know and understand and only have this
33:51
Speaker 1: theory turn on in special cases. All right, so that's
33:54
Speaker 1: a possibility. But where did this idea come from? Like
33:57
Speaker 1: why would we think that maybe gravity is wrong? So
33:59
Speaker 1: for a long time there wasn't really a good idea
34:01
Speaker 1: that was just like, well, I don't know, but if
34:02
Speaker 1: I put in this other formula, I can explain the
34:05
Speaker 1: galaxy rotation curves without dark matter. Without dark matter. Yeah,
34:09
Speaker 1: So like all right, so either there's missing masks or
34:11
Speaker 1: gravity works this other weird way, and then people started thinking, well,
34:15
Speaker 1: why would gravity work that way? Is there any reason
34:18
Speaker 1: for it? And that's a totally valid line of inquiry, right,
34:20
Speaker 1: like what theory do we have to have to explain
34:22
Speaker 1: the data? And does that theory make sense? Can we
34:25
Speaker 1: come up with a reason why? Maybe that theory is right?
34:28
Speaker 1: A theory about a theory underpinnings of the theory, right.
34:32
Speaker 1: We always in physics want to have a microscopic understanding.
34:35
Speaker 1: We don't want to just say gravity just has this
34:37
Speaker 1: number on it. We want to know why does it?
34:39
Speaker 1: Where does that come from? Why isn't it something else?
34:41
Speaker 1: Just like you were saying earlier, why is it one
34:43
Speaker 1: of our square not one over our two point one
34:45
Speaker 1: or whatever? That's the next version upgraded gravity. And so
34:51
Speaker 1: recently there is an idea from a guy in Holland
34:54
Speaker 1: to Eric Verlinda, and he has this crazy idea of
34:57
Speaker 1: gravity called entropic gravity that i'd be able to explain
35:02
Speaker 1: why gravity works differently at these distance scales. I feel
35:06
Speaker 1: like maybe he just grabbed two cool sounding things and
35:08
Speaker 1: put them together. Tropic answer like dynamic gravity or something.
35:14
Speaker 1: But it comes from a cool place. You know, it
35:15
Speaker 1: was Stephen Hawking who first connected sort of thermodynamics and gravity.
35:21
Speaker 1: He started thinking about the temperature of black holes and thinking,
35:25
Speaker 1: you know, if black holes have temperatures, that means they
35:28
Speaker 1: should radiate. Oh wait, and that's Hawking radiation. So in
35:32
Speaker 1: the last sort of forty years, people have been thinking
35:34
Speaker 1: about gravity and through our dynamics as a connection. They're
35:37
Speaker 1: trying to understand like, you know, actually, maybe gravity isn't
35:40
Speaker 1: a fundamental force, and maybe, you know Einstein's idea that
35:44
Speaker 1: gravity is just a curvature spacetime, maybe that's also wrong.
35:49
Speaker 1: Maybe instead, gravity is just like an emergent phenomena of thermodynamics.
35:54
Speaker 1: Maybe it's just like comes out of the manipulation and
35:58
Speaker 1: interaction of some tiny little bits of space in a
36:02
Speaker 1: way that feels like gravity to us. That's a little
36:05
Speaker 1: bit mind blowing. Yeah, it's a little bit mind blowing.
36:08
Speaker 1: This whole idea of emergent phenomena can be hard to
36:11
Speaker 1: get your mind around, but it's actually very familiar. You know,
36:13
Speaker 1: Like we can talk about wind, right, Wind is an
36:16
Speaker 1: emergent phenomena. It's not a fundamental force in the universe.
36:19
Speaker 1: You know, two particles interacting don't feel wind. Wind is
36:23
Speaker 1: a combination of lots of other things we do understand
36:26
Speaker 1: on a microscopical level that has a macroscopic effect, or
36:30
Speaker 1: like economics. You know, there are laws of economics that
36:33
Speaker 1: come from you know, supply and demand or whatever. It's
36:35
Speaker 1: not a fundamental force in the universe, but still you
36:38
Speaker 1: could have an understanding of it. And so they think
36:41
Speaker 1: maybe gravity is just like a macroscopic effect of something microscopic,
36:45
Speaker 1: maybe the thermodynamics of space time. Like there there is
36:49
Speaker 1: no gravity, it's just kind of like how space itself
36:53
Speaker 1: kind of arranges itself. Yeah, because we know that space
36:56
Speaker 1: likes to increase entropy, likes to increase disorder, and so
37:00
Speaker 1: we think entropy always increases in the universe, and so
37:03
Speaker 1: maybe gravity is just an effect of that. You know.
37:06
Speaker 1: The argument goes something like when things fall into a
37:08
Speaker 1: black hole, for example, that increases the entropy of the
37:12
Speaker 1: black hole, because otherwise it would violate the second law
37:14
Speaker 1: of thermodynamics. Things can't just disappear into the black hole.
37:18
Speaker 1: But maybe it's the opposite. Maybe it's not that gravity
37:21
Speaker 1: pulls things into the black hole and then increases the entropy.
37:24
Speaker 1: Maybe it's entropy that's pushing things into black holes, and
37:28
Speaker 1: that's actually what gravity is. That's just like you know,
37:31
Speaker 1: the way gas diffuses in a box. You put a
37:34
Speaker 1: blob of gas in the corner of a box and
37:36
Speaker 1: it spreads out into the box. That's entropy the same way.
37:40
Speaker 1: Maybe entropy is stuff falling into itself, like you know,
37:44
Speaker 1: having more stuff near itself dense blobs of mass increases
37:48
Speaker 1: the temperature, which changes the entropy of the situation. I mean,
37:51
Speaker 1: it's a complicated argument involving like quantum entangled space time,
37:55
Speaker 1: but I think that's the gist of it, and all
37:57
Speaker 1: of that kind of crazy idea is just explain how
38:00
Speaker 1: maybe gravity could not be one over our square. Yeah,
38:03
Speaker 1: if you have in tropic gravity, so maybe if gravity
38:06
Speaker 1: is not a fundamental force in the universe but just
38:09
Speaker 1: like an emergent property of quantum space time, and you
38:13
Speaker 1: have dark energy, then Virlindis theory predicts that space curs
38:17
Speaker 1: in this way, that gravity, the effective force of gravity,
38:21
Speaker 1: goes the way mind needs it to explain the galactic
38:24
Speaker 1: rotation curves. So that's kind of cool thing. That's an
38:28
Speaker 1: alternative to dark matter. Is one alternative to dark matter
38:31
Speaker 1: to say we have this weird gravitational thermodynamics idea that
38:36
Speaker 1: changes the way gravity works. That explains why we thought
38:39
Speaker 1: that was missing stuff. Instead, it's just the gravity works
38:42
Speaker 1: differently than we expect it. Interesting alright, So then I
38:44
Speaker 1: guess the big question is could that work? Is that
38:47
Speaker 1: a valid or plausible theory that maybe explains gravity in
38:52
Speaker 1: a different way that then explains dark matter and and
38:56
Speaker 1: so that it's not just another particle or kind of stuff.
38:59
Speaker 1: Could that war? Well, it's a cool idea, and it
39:02
Speaker 1: does explain galaxy rotation curves, and it actually explains galaxy
39:06
Speaker 1: rotation curves better than dark matter. There's some galaxies out
39:10
Speaker 1: there that we still can't explain using dark matter, Like
39:12
Speaker 1: they rotate in weird ways and we don't understand it.
39:15
Speaker 1: And you know, but hey, galaxies are weird. They they
39:20
Speaker 1: have their own history. No prize, sir. My theory, that's right,
39:26
Speaker 1: My theory that the universe is just weird explains everything,
39:29
Speaker 1: oh man, the whites and theory of everything, the whites
39:32
Speaker 1: and theory of weirdness. But it explains the gaxy rotation
39:36
Speaker 1: curves really nicely. But there's a big caveat there, which
39:40
Speaker 1: is it was sort of invented to explain those curves,
39:43
Speaker 1: like you came up with a theory to fit that data. Really,
39:47
Speaker 1: a good test of a theory is does it explain
39:50
Speaker 1: other data? Is that a real general principle about the universe,
39:54
Speaker 1: something which is really deeply true? Or is it just
39:57
Speaker 1: a mathematical tweak to this one plot, this one fig
40:00
Speaker 1: or to make. Because if it only works to explain
40:02
Speaker 1: one thing, it's it's kind of suspicious, right exactly, And
40:04
Speaker 1: if it only works to explain the one thing that
40:07
Speaker 1: motivated it, and then probably it's not a deep truth
40:10
Speaker 1: of the universe, right a right, So maybe it doesn't work.
40:14
Speaker 1: Can it explain some of the other things that we
40:16
Speaker 1: know about dark matter, like the lensing and the cosmic
40:20
Speaker 1: microwave background and the structure of the universe? A tweak
40:24
Speaker 1: to our theory of gravity also explain all of these things?
40:26
Speaker 1: In a word, No, it just doesn't work. No, galax
40:31
Speaker 1: is are just weird. No galax is just weird. No,
40:33
Speaker 1: it does not explain the cosmic microwave background, Like, it's
40:36
Speaker 1: very difficult to explain that using anything else other than
40:40
Speaker 1: some new kind of particle or promodial black hole or
40:43
Speaker 1: something some new kind of stuff. You just can't explain
40:47
Speaker 1: it using deviation and gravity because it was on a
40:49
Speaker 1: really small scale. This is like, you know, things were
40:51
Speaker 1: near by each other. This wasn't galactic distances interesting, And
40:55
Speaker 1: it's very difficult to explain the structure of the universe.
40:58
Speaker 1: And then there is one really awesome example of dark
41:01
Speaker 1: matter that's sort of like a smoking gun that makes
41:03
Speaker 1: it almost impossible to explain using anything but some new
41:07
Speaker 1: kind of matter stuff, some new kind of stuff. Yeah,
41:10
Speaker 1: and that's this Bullet cluster. There was this collision millions
41:14
Speaker 1: and millions of years ago, far far away between two
41:17
Speaker 1: clusters of galaxy that passed through each other. And we
41:20
Speaker 1: thought that those clusters of galaxies they had normal matter
41:23
Speaker 1: like stars and planets, etcetera, and then also clumps of
41:26
Speaker 1: dark matter. So what happened is they passed through each other,
41:30
Speaker 1: and the gas and the dust it all collided amid
41:33
Speaker 1: big collisions and slowed down and all that stuff. But
41:36
Speaker 1: the dark matter, it doesn't interact with the normal matter,
41:38
Speaker 1: and it doesn't interact with itself very much at all
41:40
Speaker 1: either because gravity is so weak, so the dark matter
41:43
Speaker 1: is sort of passed through. So what we see when
41:45
Speaker 1: we look up in the skies, we see like a
41:47
Speaker 1: big blob in the center while the gas and dust interacted,
41:51
Speaker 1: and then on either side we see the dark matter
41:53
Speaker 1: that passed through, and we can see that because of
41:56
Speaker 1: the gravitational lensing. Right, But doesn't it all I know,
41:58
Speaker 1: we talked about the bullet cluster it before, but doesn't
42:01
Speaker 1: it still just come down to gravitational lensing, Like what
42:05
Speaker 1: if gravity can explain gravitational lensing, could that also explain
42:09
Speaker 1: the bullet cluster that we see. It's very difficult to
42:12
Speaker 1: explain the bullet cluster in any other way because you
42:14
Speaker 1: need to have gravitational lensing in exactly those right spots
42:17
Speaker 1: on opposite sides of this very obvious collision. So it's
42:21
Speaker 1: a very nice explanation to think, oh, there's some invisible
42:24
Speaker 1: matter that passed through and it's now sitting there distorting
42:27
Speaker 1: the background galaxy. See, the bullet cluster is like proved
42:30
Speaker 1: that whatever it's causing these gravitational things is mobile, like
42:35
Speaker 1: it can move like stuff. But if it was just
42:37
Speaker 1: a gravitational theory tweak, that wouldn't like keep going or
42:41
Speaker 1: move or change from here to there exactly, and it
42:44
Speaker 1: can be separated from the normal matter. It's not just
42:47
Speaker 1: a gravitational tweak from the normal matter, right, the normal
42:49
Speaker 1: matter was left behind in the middle. It's its own
42:52
Speaker 1: kind of stuff. It has its own gravity. And so
42:55
Speaker 1: that's sort of like a bullet in the brain of
42:58
Speaker 1: the you know, non dark matter theories. The bullet cluster
43:01
Speaker 1: is a bullet suspicious. People really took Moms sort of
43:05
Speaker 1: seriously until then. And then when the bullet cluster was discovered,
43:09
Speaker 1: people are like, oh, well, that's it. Dark matter is real,
43:12
Speaker 1: and Mond is dead and never Mond. And at that
43:16
Speaker 1: point people really didn't take months seriously. So since then,
43:20
Speaker 1: Mond has been much more of a fringe theory. I mean,
43:23
Speaker 1: Virlin's idea is more recent, and it's quote of cool,
43:26
Speaker 1: but it sort of explains something that nobody really takes
43:29
Speaker 1: seriously anymore. So it's very fringe in a way. The
43:32
Speaker 1: bullet cluster shows you that dark matter or whatever it's
43:35
Speaker 1: causing these gravitational distortions and footprints, is mobile like it's
43:39
Speaker 1: it can move like stuff. It moves like a stuff
43:41
Speaker 1: can move. Yeah, And so again it's unsatisfying because we
43:45
Speaker 1: don't know what it is and and we've been looking
43:48
Speaker 1: for for a while it's not like we're just like, oh,
43:50
Speaker 1: it's some invisible stuff, let's move on. We've built dedicated
43:54
Speaker 1: experiments to look for it. We thought several times that
43:57
Speaker 1: we would definitely find it and then haven't seen it.
44:00
Speaker 1: So there's definitely some tension there. It's not like dark
44:02
Speaker 1: matter is a beautiful theory that's all wrapped up, like
44:05
Speaker 1: we don't understand why we haven't been able to find
44:08
Speaker 1: the dark matter yet. There's definitely something weird going on there,
44:11
Speaker 1: and it's very healthy to think about new ideas, but
44:14
Speaker 1: mond doesn't quite work. There is no other idea out
44:18
Speaker 1: there that explains what we see nearly as well as
44:21
Speaker 1: some new cold particles. I have an idea is it's
44:24
Speaker 1: snack based. It's called Hey, maybe dark matter is weird.
44:29
Speaker 1: It's weird, yeah, And that's why people are sort of
44:32
Speaker 1: digging further and further into the barrel of ideas for
44:35
Speaker 1: dark matter. People thought for a long time, Okay, dark
44:37
Speaker 1: matter must just be some weakly interacting massive particle, some
44:41
Speaker 1: new lava stuff, but we haven't seen it, and so
44:44
Speaker 1: then people are well, maybe it's axions, or maybe it's
44:47
Speaker 1: primordial black holes, or and there are some other even
44:50
Speaker 1: crazier ideas like squirmyons, what like they're uncomfortable and social situations.
44:55
Speaker 1: What do you mean introvertons? Yeah, no, squirmy on are
45:00
Speaker 1: this crazy idea that you know how particles are like
45:03
Speaker 1: excitations of quantum fields are like little bundles of energy
45:06
Speaker 1: in a quantum field. People think, well, maybe not all
45:10
Speaker 1: energy and quantum fields are particles. Maybe some of them
45:12
Speaker 1: are like more distributed or spread out in weird ways.
45:15
Speaker 1: And they found that you can like tangle up quantum
45:18
Speaker 1: fields in these weird ways, like make nots of them,
45:21
Speaker 1: and that's what they call squirmyons. And it's not could
45:24
Speaker 1: like it could create gravity. Yeah, because any energy density
45:27
Speaker 1: creates gravity. Well, we'll have to dig into that for
45:32
Speaker 1: another episode. Sounds pretty for sure. So there's lots of
45:36
Speaker 1: ideas the whole spectrum, right, dark matter is not just
45:38
Speaker 1: one idea, the sort of spectrum of ideas all satisfied
45:42
Speaker 1: the condition that it's some new kind of cold object.
45:46
Speaker 1: But what that object is. It could be one particle,
45:48
Speaker 1: could be many kinds of particles. It could be black holes,
45:51
Speaker 1: it could be squirmyons, it could be something else we
45:54
Speaker 1: haven't even thought of yet. Whatever, it is. We're pretty
45:57
Speaker 1: sure it's stuff. It's something that has gravity stuff when
46:00
Speaker 1: it's there, that's right. But I mean it's really interesting
46:02
Speaker 1: because I feel like we've known dark matters is there
46:04
Speaker 1: for a while now, and we just can't seem to
46:06
Speaker 1: crack it. We can't seem to find it keeps um
46:08
Speaker 1: eluding us, you know, like it just keeps on hiding
46:11
Speaker 1: there and not letting us know what it is. That's right,
46:13
Speaker 1: But we don't know how long the story is. You know,
46:15
Speaker 1: we were looking for the Higgs boson for fifty years
46:18
Speaker 1: and then we found it. We were looking for the
46:20
Speaker 1: top cork for twenty years before we found it. A
46:23
Speaker 1: lot of those things. People thought, oh, we'll find this
46:25
Speaker 1: in the next year two and then they were confused
46:27
Speaker 1: and disappointed to not discover it soon. But you know,
46:29
Speaker 1: eventually we got there and we we figured it out.
46:32
Speaker 1: So maybe we're just five years away from discovering dark matter,
46:35
Speaker 1: or maybe it's going to be another hundred. Maybe we
46:37
Speaker 1: need a new crazy idea for what dark matter is.
46:40
Speaker 1: But dark matter, I'm pretty sure is so all those
46:43
Speaker 1: physicists squirming around relax. Maybe dark matter is in our future.
46:47
Speaker 1: That's right. I certainly hope it is. It would be fascinating.
46:50
Speaker 1: And remember that it's most of the stuff out there
46:52
Speaker 1: in the universe. Like, what a crazy opportunity to learn
46:56
Speaker 1: about the way the universe works, you know, to know
46:59
Speaker 1: that of the stuff out there in the universe has
47:01
Speaker 1: been hidden from us. The day we crack that open
47:03
Speaker 1: and get to learn about it, like, it could have
47:05
Speaker 1: incredibly complex structure, it could have interactions and biology and
47:09
Speaker 1: chemistry and all sorts of crazy stuff. Movest of the
47:12
Speaker 1: stuff out there in the universe we haven't yet gotten
47:14
Speaker 1: to play with, and so we're eager, we're desperate to
47:17
Speaker 1: figure out what this stuff is. Yeah, because if you're
47:19
Speaker 1: the scientist that discovers what dark matter is, I mean,
47:22
Speaker 1: that's like a lot of three cred you know. It's
47:25
Speaker 1: like you could say that you discovered eight eight percent
47:29
Speaker 1: of everything in the universe. I think they would have
47:31
Speaker 1: to give you five Nobel Prizes, you know, just for
47:33
Speaker 1: that one discovery. They have to give you eighty percent
47:35
Speaker 1: of all the Nobel prizes. I think, Yeah, yeah, there
47:38
Speaker 1: you go. You get all the Nobel Prizes for the
47:40
Speaker 1: next four hundred years, just to balance it out. Yeah,
47:43
Speaker 1: So it's a big question, and who knows, maybe one
47:45
Speaker 1: of our listeners will be the one who discovers it.
47:48
Speaker 1: That's right, It could be you out there, it could
47:49
Speaker 1: be your kids out there. What we definitely need to
47:51
Speaker 1: do or keep our minds open and come up with
47:53
Speaker 1: new ideas for what dark matter is really cool? All right, Well,
47:57
Speaker 1: we hope you enjoyed that view into this story is
48:00
Speaker 1: dark matter and how we know it's there and how
48:02
Speaker 1: we know it's not just a weird fluke of gravitational equations.
48:06
Speaker 1: Thanks for tuning in, See you next time. Thanks for listening,
48:17
Speaker 1: and remember that Daniel and Jorge Explain the Universe is
48:20
Speaker 1: a production of I Heart Radio. For more podcast For
48:23
Speaker 1: my heart Radio, visit the i heart Radio app, Apple Podcasts,
48:27
Speaker 1: or wherever you listen to your favorite shows.