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Can Dark Matter feel itself · Daniel and Kelly’s Extraordinary Universe

0:09
Speaker 1: Hey, Daniel, what's the best question anyone has ever asked
0:12
Speaker 1: you about dark matter? Well, it's tough to pick. There's
0:15
Speaker 1: so many good ones. But I remember a nine year
0:18
Speaker 1: old who read our book once and emailed me to
0:20
Speaker 1: ask is dark matter bad matter? It must be a
0:24
Speaker 1: Star Wars fan. You know the dark side doesn't sound
0:27
Speaker 1: very friendly. Yeah, dark matter sounds world of unfriendly and mean.
0:31
Speaker 1: It's doesn't never a really good pr team. Well, you know,
0:33
Speaker 1: if there are dark matter physicists out there that we
0:36
Speaker 1: can't see or touch, what do you think they would
0:39
Speaker 1: call us? M I guess they'll probably call us dork matter.
0:42
Speaker 1: We wouldn't seem dark to them and insidious. No, actually
0:46
Speaker 1: we would. I mean if they have their own dark photons,
0:48
Speaker 1: they wouldn't see ours, so we would be dark to them. Man,
0:52
Speaker 1: I can just imagine the conferences. You know, you just
0:54
Speaker 1: spent the whole day saying you're a dark matter. No,
0:56
Speaker 1: you're a dark matter. That's probably actually true. That's exactly
0:59
Speaker 1: what would happen. I am, forhand, a cartoonist and the
1:17
Speaker 1: creator of PhD comments. Hi, I'm Daniel Whitson, I'm a
1:20
Speaker 1: particle physicist, and I'm mostly made out of dork particles. Particles,
1:26
Speaker 1: dark dork particles. That's the worst. Well, mostly in the
1:30
Speaker 1: summer right now. I'm not going outside very much, so
1:33
Speaker 1: pale dark. My dork particles are not very dark. But
1:35
Speaker 1: welcome to our podcast Daniel and Jorge Explain the Universe,
1:38
Speaker 1: a production of I Heart Radio, in which we take
1:41
Speaker 1: you on a mental tour of the universe, zooming down
1:44
Speaker 1: to see what the universe is made out of, what
1:46
Speaker 1: it looks like at its largest scale, what crazy weird
1:49
Speaker 1: stuff we encounter in it, and how we can all
1:52
Speaker 1: understand it because humanity is on a journey, a journey
1:55
Speaker 1: to understand this universe. We started out really kind of
1:59
Speaker 1: clueless about the way things work, and we slowly put
2:01
Speaker 1: together little pieces of the picture. But we're here to
2:04
Speaker 1: give you a tour of what we have figured out
2:06
Speaker 1: and what still remains unknown. Yeah, and it turns out
2:08
Speaker 1: we haven't even left the garage, it seems, Daniel, in
2:11
Speaker 1: our journey of discovery, because a huge percentage of the
2:15
Speaker 1: universe is made out of stuff that we have no
2:17
Speaker 1: idea about. You know, twenty five percent of the universe
2:20
Speaker 1: is made out of something we call dark matter, which
2:24
Speaker 1: is a huge mystery. Right, that's right, And that might
2:26
Speaker 1: seem a bit deflating to you, like, wow, we haven't
2:28
Speaker 1: figured anything out, But to me that's exciting, Like, wow,
2:31
Speaker 1: we haven't figured anything out, which means more discoveries are
2:35
Speaker 1: to come, which means mind blowing revelations about the true
2:39
Speaker 1: nature of the universe are in our future. You know,
2:41
Speaker 1: if you haven't left the garage, that means you can
2:43
Speaker 1: still go back inside. If you forget something that's positive,
2:46
Speaker 1: that's right, Like you forgot to pack your banana. How
2:48
Speaker 1: can you even take a journey of discovery without banana?
2:52
Speaker 1: About these days, who can go anywhere? Yeah, we're talking
2:55
Speaker 1: about dark matter today and it's a huge mystery and
2:58
Speaker 1: it's out there. But a big question is what is
3:00
Speaker 1: it made out of? What is it? Is it a fluid,
3:03
Speaker 1: is it a an energy? Or is it made out
3:06
Speaker 1: of particles or something totally different. That's right, Everything we've
3:10
Speaker 1: ever experienced, everything you've ever eaten or touched or tasted,
3:13
Speaker 1: are made out of quarks and electrons and stuff like that,
3:17
Speaker 1: and so we wonder does the same thing hold for
3:20
Speaker 1: dark matter. But it's a pretty big extrapolation. I mean,
3:22
Speaker 1: the kind of matter that we're made out of that's
3:25
Speaker 1: very familiar, is unusual in the universe, and so it
3:28
Speaker 1: seems kind of odd to extrapolate from this tiny little
3:31
Speaker 1: corner to a huge fraction of the universe. And so
3:35
Speaker 1: particle physicists think maybe it's made of particles, but I
3:37
Speaker 1: like to think, hey, maybe it's made of something totally
3:40
Speaker 1: weird and new. Yeah, we really have no idea. And
3:43
Speaker 1: it's there's five times more dark matter in the universe
3:46
Speaker 1: than there is of the stuff that we are made
3:49
Speaker 1: out of. And it really could be anything like it
3:51
Speaker 1: could be something that's not even a particle. That's right,
3:54
Speaker 1: we've never seen matter that's not made of particles. Everything
3:57
Speaker 1: we've ever experienced can be broken down into these units.
4:00
Speaker 1: But that doesn't mean that everything is like that, Right.
4:03
Speaker 1: We have to be very careful about generalizing. We've seen
4:06
Speaker 1: so many times in physics when we've learned something we thought, oh,
4:10
Speaker 1: this must be the way everything works, and then we
4:12
Speaker 1: discover it's not. Actually and there are bigger stories, is
4:15
Speaker 1: a larger context, and that's the goal of science, is
4:18
Speaker 1: to reveal the actual truth, not just confirm our expectations. Yeah,
4:22
Speaker 1: and so one thing we know about dark matter is
4:25
Speaker 1: that we know that it feels gravity, because that's how
4:28
Speaker 1: we know that it's there. I mean, we can't see it,
4:30
Speaker 1: it's dark, but we know it's there because of gravity.
4:32
Speaker 1: It's keeping the gaxies together, it's pulling things together, and
4:36
Speaker 1: it's you know, affecting the trajectories of galaxies and stars.
4:41
Speaker 1: So we know it feels gravity. We certainly do. It's
4:43
Speaker 1: basically an invention to explain missing gravity. It's like a
4:48
Speaker 1: gravity fudge factor. Yeah. We see things in the universe
4:50
Speaker 1: that gravity has done, and we don't see the source
4:53
Speaker 1: of that gravity. We're like, what's this invisible thing that's
4:57
Speaker 1: making this gravity? And so dark matter is basically an
5:00
Speaker 1: explanation for this gravity that we do observe, and so
5:04
Speaker 1: it has to feel gravity otherwise it doesn't fit the
5:06
Speaker 1: bill at all. But then the question is what else
5:09
Speaker 1: does it do? Does it feel anything else? Does it
5:11
Speaker 1: feel lonely? Does it feel resentful that we call it
5:14
Speaker 1: dark matter? Yeah, we know that it feels gravity, and
5:18
Speaker 1: so the question is does it feel any other forces?
5:20
Speaker 1: And I think more interestingly about this question is does
5:24
Speaker 1: it feel itself like canna bump into itself? Yeah, And
5:27
Speaker 1: we got a really interesting question from a listener. Here's
5:30
Speaker 1: an email I god from Joe from China, who I
5:32
Speaker 1: think really put his finger on the question. He says,
5:35
Speaker 1: if you were to take two particles of dark matter
5:38
Speaker 1: and let them go in a vacuum, the gravity would
5:41
Speaker 1: attract the two particles together. But then would they pass
5:44
Speaker 1: right through each other and keep moving? Eventually would they
5:47
Speaker 1: settle down and be able to share the same point
5:50
Speaker 1: in space like overlapping? Can too dark matter particles? But
5:55
Speaker 1: I guess bump into each other or occupy the same space? Yeah?
5:59
Speaker 1: Do they not into each other and push back? Do
6:01
Speaker 1: they pass right through each other? Can they sit right
6:03
Speaker 1: on top of each other. It's a really fascinating question,
6:06
Speaker 1: and it goes right to the heart of the sort
6:07
Speaker 1: of the science matter of whether dark matter feels itself?
6:11
Speaker 1: This question blow your mind a little bit? Yeah, I
6:13
Speaker 1: did blow my mind a little bit. I just love
6:14
Speaker 1: when somebody asks a new question, a question I haven't
6:17
Speaker 1: heard before, or asks it in a new way. So
6:20
Speaker 1: I haven't really thought about whether dark matter feels itself? Okay,
6:23
Speaker 1: so it's kind of a funny question. Does dark matter
6:26
Speaker 1: feel itself? That means that you know it feels itself. Gravitationally, right,
6:31
Speaker 1: if you have one bit of dark matter here and
6:33
Speaker 1: a bit of dark matter, they're they're gonna be They're
6:35
Speaker 1: going to feel each other. They're gonna be attracted to
6:37
Speaker 1: each other by gravity. But the question is, you know
6:40
Speaker 1: what happens after they feel attracted by gravity. You know,
6:43
Speaker 1: they they're moving towards each other. Are they gonna bump
6:46
Speaker 1: against and and repel it themselves or are they going
6:49
Speaker 1: to pass through each other? Or are they going to
6:51
Speaker 1: stick together? Yeah, it's a great question. I mean, if
6:53
Speaker 1: you and I have gravitational attraction, if we were out
6:56
Speaker 1: in the middle of space, then gravity would very slowly
6:58
Speaker 1: pulls together and we would bump into each other. I
7:00
Speaker 1: wouldn't phase through each other. You woul get uncomfortable really fast.
7:04
Speaker 1: What you're saying more awkward than in space. No one
7:08
Speaker 1: can hear you say, hey, I need more space, Daniel,
7:11
Speaker 1: get off me. But you couldn't write. You'd be stuck together.
7:17
Speaker 1: That's right, we need some other kind of force. But
7:20
Speaker 1: you and I do have other kind of forces, right,
7:22
Speaker 1: the electromagnetic force who keep us from passing through each other.
7:24
Speaker 1: So it's a good way to ask the questions. Yeah,
7:26
Speaker 1: so the question is does dark matter feel itself? And so,
7:29
Speaker 1: as usual, we went out into the world, and in
7:31
Speaker 1: this case, the Internet, to see how many people felt
7:35
Speaker 1: that they knew the answer to this question. That's right,
7:37
Speaker 1: So thank you to everybody who volunteered to answer random
7:40
Speaker 1: Internet physics questions. If you'd like to volunteer right to
7:44
Speaker 1: us at questions at Daniel and Jorge dot com. Yeah,
7:47
Speaker 1: you can ask us questions or get questions from us.
7:51
Speaker 1: Questions in and questions out. It's an eternal flux of questions, right.
7:55
Speaker 1: But think about it for a second. If you were
7:56
Speaker 1: to try to answer the question does dark matter feel itself?
8:00
Speaker 1: What would you answer? And no googling, no googling? What?
8:03
Speaker 1: How about? Wikipedia? Also not allowed? About calling my physicist friend?
8:09
Speaker 1: Totally allowed, totally allowed. All right, Well, here's what people
8:13
Speaker 1: on the Internet had to say. I guess I'm really
8:15
Speaker 1: not sure. I know that some scientists hypothesize the dark
8:19
Speaker 1: matter is comprised of filaments, and if there were these
8:22
Speaker 1: filaments in the dark matter, one would think that they
8:24
Speaker 1: would interact in some way. I really don't think so.
8:28
Speaker 1: I remember it interacting with something visible. That's a good one.
8:33
Speaker 1: The interaction between light matter and dark matter is for
8:37
Speaker 1: obviously still figuring that out, but I would think that
8:39
Speaker 1: it does interact with itself. I don't think that turn
8:43
Speaker 1: dark matter interacts with itself. Um. I think whether discussed
8:47
Speaker 1: two galaxies colliding with each other, but there wasn't an
8:50
Speaker 1: evidence or the details, so not itself. I would guess
8:56
Speaker 1: that dark matter would interact with itself because I know
9:00
Speaker 1: that one guess of what dark matter is is the
9:04
Speaker 1: dark matter particle axion. And it's kind of hard to
9:10
Speaker 1: visualize how a particle can pass through another particle. I
9:14
Speaker 1: don't know. I've never thought about it. Okay, Um, dark
9:17
Speaker 1: matter and dark energy confuse me a little bit. I
9:20
Speaker 1: thought that dark matter was just a theory, and so
9:25
Speaker 1: it's like we've we've seen something out there, and the
9:28
Speaker 1: theory is that this dark matter, but we don't really know.
9:31
Speaker 1: I don't think it can interact with normal matter. I
9:35
Speaker 1: surely if it could pass through itself, we would be
9:39
Speaker 1: able to see it. It might be a dark force,
9:42
Speaker 1: which helps them incorrect. I know it doesn't interact with
9:45
Speaker 1: regular matter. I think we don't know whether how it
9:48
Speaker 1: interacts with itself. It is matter, so it has maths
9:51
Speaker 1: al must interact gravitationally. I don't know. Well, I think
9:55
Speaker 1: it does on some level. I don't know whether there
9:58
Speaker 1: would be quantum or molecular, but I think it's got
10:02
Speaker 1: to interact with itself enough to come together and blobs
10:06
Speaker 1: big enough to cause gravitational lensing that we can see
10:09
Speaker 1: here from Earth. I guess it's interact with yourself, since
10:13
Speaker 1: it's i've gravity. All right, pretty thoughtful answers, you know,
10:18
Speaker 1: people sort of thoughts through it. Yeah, there's some good
10:19
Speaker 1: stuff in here. People are really thinking about what it
10:21
Speaker 1: means to interact and whether gravity counts, and whether gravity
10:25
Speaker 1: would make something bump or not? Does gravity count? Does
10:31
Speaker 1: the Earth feel USh? Yeah? But also some people getting
10:34
Speaker 1: confused about dark energy versus dark matter. That's another rabbit
10:38
Speaker 1: hole there. Does dark energy feel? What is dark energy? Anyway?
10:43
Speaker 1: We have no idea, all right, So the question is
10:45
Speaker 1: that does dark matter feel? And so Daniel Um maybe
10:48
Speaker 1: step us through what we know about dark matter and
10:50
Speaker 1: it's it's not very sensitive. Unfortunately, you know, we wish
10:54
Speaker 1: you would react more. Oh good, that's a good thing,
10:57
Speaker 1: right as a thick skin. Well, perhaps know if you
11:00
Speaker 1: want to get along with your dark matter roommate. But
11:01
Speaker 1: if you want to do experiments to reveal the nature
11:04
Speaker 1: of dark matter, you'd like it to respond sometimes, and
11:07
Speaker 1: dark matter turns out to be very difficult to probe
11:09
Speaker 1: because the only way we've been able to interact with
11:11
Speaker 1: dark matter so far at all is with gravity. Like
11:15
Speaker 1: we said, we've discovered dark matter because we looked at
11:18
Speaker 1: how galaxies spin, and we notice that to spin as
11:21
Speaker 1: fast as they do and hold all the stars in place,
11:24
Speaker 1: you need a lot more gravity than is generated by
11:27
Speaker 1: the stars we see, which means there must be something
11:30
Speaker 1: else they're making that gravity. And we see blobs of
11:34
Speaker 1: dark matter in the sky. We don't see them. We
11:36
Speaker 1: deduce blobs of dark matter in the sky because they're
11:39
Speaker 1: bending the light that passes through them. That's a gravitational effect.
11:43
Speaker 1: So we have very concrete evidence that dark matter feels gravity.
11:47
Speaker 1: That's why we call it matter, not just like dark
11:50
Speaker 1: mysterious thing in the space right because it has gravity.
11:54
Speaker 1: I see. It's sort of like the probervial elephant in
11:57
Speaker 1: the room. You know it's there, everyone knows it's there,
12:00
Speaker 1: but it's, uh, it's nobody's talking about. Sort of. It's
12:02
Speaker 1: also sort of like the opposite of that, because the
12:05
Speaker 1: elf in the room. Everybody is ignoring it, right though
12:07
Speaker 1: you can all see it. In this case, we're all
12:09
Speaker 1: desperately searching for it, but nobody can find it. It's
12:13
Speaker 1: like you're right or right, except you're not. Well. You know,
12:16
Speaker 1: sometimes being exactly the opposite direction is also a way
12:19
Speaker 1: of being right. Yeah, yeah, you know up bean staff
12:24
Speaker 1: yes times minus one. Alright, So we know it feels gravity,
12:27
Speaker 1: but there are other forces that we know about in
12:29
Speaker 1: the universe, and as far as we know, the dark
12:32
Speaker 1: matter doesn't feel them. That's right. And so for example,
12:34
Speaker 1: electromagnetism is a very powerful force and when that we
12:37
Speaker 1: experience every day. The reason that your hand doesn't go
12:40
Speaker 1: through the wall when you're trying to touch it is
12:42
Speaker 1: that the wall has particles that are bound together with
12:45
Speaker 1: electromagnetic forces, and so it does your hand and so
12:48
Speaker 1: you think of them like a chain link fence trying
12:50
Speaker 1: to pass through another chain link fence. That's what prevents
12:53
Speaker 1: you from passing through another object. It's Electromagnetism is also
12:57
Speaker 1: responsible for why things glow right, why we can see things,
13:01
Speaker 1: or why they reflect light. To write like, it's light
13:03
Speaker 1: hits it and then it interacts and it comes back out.
13:06
Speaker 1: That's right. Even blobs of rock that don't glow on
13:08
Speaker 1: their own, we can see them in the sky because
13:11
Speaker 1: they reflect the sun's light, like the moon or like asteroids.
13:14
Speaker 1: And so we know dark matter isn't like that because
13:17
Speaker 1: it doesn't give off light, doesn't reflect light. Light passes
13:20
Speaker 1: right through it. So right through it, yeah, exactly. And
13:25
Speaker 1: I see this all the time in science fiction television,
13:28
Speaker 1: that they encounter dark matter and it obscures their view. Right. Oh,
13:32
Speaker 1: we can't get out of this cloud of dark matter
13:34
Speaker 1: and we can't see where we are. What show are
13:36
Speaker 1: you watching and seen that one? I don't remember the
13:40
Speaker 1: name of it right now. I watched too much science fiction,
13:42
Speaker 1: But you know that, to me, that sounds like a
13:44
Speaker 1: fantastic situation, like, oh my gosh, look we found dark matter.
13:47
Speaker 1: That's right home. We win the Nobel Prize. It's not like,
13:50
Speaker 1: get out of my picture, my photo. It's like I
13:52
Speaker 1: got a photo of dark matter. Exactly. We've been trying
13:56
Speaker 1: to spot dark matter forever and you just figured it
13:58
Speaker 1: out and now it seems like a hassled you. But
14:00
Speaker 1: if you were in a cloud of dark matter, you
14:02
Speaker 1: couldn't tell because it's invisible right. In fact, we think
14:05
Speaker 1: we are in a cloud of dark matter. We think
14:07
Speaker 1: the Earth is surrounded by dark matter, but we can't
14:10
Speaker 1: see it because it doesn't reflect light. It doesn't feel
14:12
Speaker 1: any electromagnetic forces at all. Yeah, we're swimming in dark
14:15
Speaker 1: matter right, Like everyone who's listening to this podcast right
14:19
Speaker 1: now is probably swimming in dark matter. Like right in
14:22
Speaker 1: front of their eyes, is dark matter passing through or
14:25
Speaker 1: sitting there? That's right. Dark matter, we think is spread
14:28
Speaker 1: out through the galaxy. It's fairly dilute, so it's there's
14:31
Speaker 1: a lot of it. There's a lot more dark matter
14:33
Speaker 1: than normal matter, but it's much more spread out we
14:35
Speaker 1: think the normal matter, which tends to cluster into planets
14:38
Speaker 1: and stars and stuff. So we think there's like one
14:41
Speaker 1: squirrel's worth of dark matter in the volume of the Earth.
14:44
Speaker 1: But it's there, it's out there. Wait, what one squirrels worth?
14:51
Speaker 1: One squirrel? Yeah, well it's a standard international unit of mass.
14:55
Speaker 1: One squirrel. Are you not familiar with squirrels? I don't.
14:59
Speaker 1: I didn't usually baked with squirrels. All my favorite recipes
15:03
Speaker 1: are in units of squirrels. You know, a third of
15:06
Speaker 1: a squirrel of oats and you know, half a squirrel
15:08
Speaker 1: of sugar man. That's that's the pandemic for you. It's
15:12
Speaker 1: forcing us into a more primitive Yeah, come over to
15:15
Speaker 1: my house sometime. We'll have a squirrel roast. So wait,
15:18
Speaker 1: the Earth you're saying is in a pool of dark
15:20
Speaker 1: matter that has the same amount of mass as a squirrel.
15:23
Speaker 1: That's what it means. That's right, And there's more dark
15:25
Speaker 1: matter than this normal matter, but the dark matter is
15:27
Speaker 1: more diffuse, it's more spread out, and so out there
15:30
Speaker 1: in empty space is about as much dark matter as
15:33
Speaker 1: there is here in our solar system. You just made
15:35
Speaker 1: me think of a giant squirrel the size of the
15:37
Speaker 1: Earth that a dark matter. You're a very visual person,
15:41
Speaker 1: so I'd love to see a drawing of that of
15:43
Speaker 1: a squirrel attacking the Earth and earth size squirrel attacking us. Finally,
15:47
Speaker 1: the aliens have come and they're just giants space squirrels.
15:51
Speaker 1: You did it again. Just because it's a dark matter squirrel,
15:53
Speaker 1: you assume it was bad, Daniel. Yeah, maybe it's a
15:55
Speaker 1: friendly planet sized space squirrel. Yeah, you're right. Now, so
15:59
Speaker 1: we we know that it doesn't feel electromagnetism, and that's
16:02
Speaker 1: principally because it's dark, we can't see it. But there
16:05
Speaker 1: are other forces out there, right, we talk about the
16:07
Speaker 1: weak nuclear force. That's a very very non powerful force.
16:11
Speaker 1: It's a weak force between particles. Neutrinos feel this force,
16:15
Speaker 1: Leptons feel this force. All the particles we know feel
16:18
Speaker 1: this force, but it's not very powerful. Like neutrinos pass
16:21
Speaker 1: through a light year of lead and have a fifty
16:25
Speaker 1: percent chance of feeling anything using the weak force. Our
16:28
Speaker 1: particles also feeling, like the particles in my body that
16:31
Speaker 1: they feel the weak force. Yeah, all the particles in
16:33
Speaker 1: your body, on the electrons and the corks, they all
16:35
Speaker 1: feel this force. It's responsible, for example, for a beta decay.
16:39
Speaker 1: So one of the corks in a neutron can turn
16:42
Speaker 1: into another kind of quirk which turns your neutron into
16:45
Speaker 1: a proton. That happens through the weak force, and that's
16:48
Speaker 1: why a little neutrinos emitted along with the electron that's created.
16:52
Speaker 1: So all the particles in your body are sensitive to
16:55
Speaker 1: the weak force. Neutrinos that pass through you can interact
16:58
Speaker 1: with you. It's just very very un likely, like there's
17:00
Speaker 1: a hundred billion newtrinos passing through your fingernail every second,
17:05
Speaker 1: and you don't feel them well, and we know that
17:07
Speaker 1: the dark matter doesn't feel it. We suspect that dark
17:10
Speaker 1: matter doesn't feel it because we've looked very very carefully.
17:13
Speaker 1: We've set up these big underground experiments filled with very quiet,
17:16
Speaker 1: heavy liquids, and we have a lot of those atoms
17:20
Speaker 1: in there, and we hope that a dark matter particle
17:21
Speaker 1: would come by and bump into one of those xenon atoms,
17:25
Speaker 1: for example, and we could spot it. And this is
17:27
Speaker 1: similar to how newtrinos were first discovered with vast tanks
17:31
Speaker 1: of underground liquid waiting for a little recoil from one
17:34
Speaker 1: of those atoms. So we have a similar in spirit
17:37
Speaker 1: kind of experiment done underground to look for passing dark
17:41
Speaker 1: matter particles. All right, So, and there's also the strong force,
17:44
Speaker 1: right which that one is pretty important, but dark matter
17:46
Speaker 1: doesn't feel that one either. That's right. The strong force
17:48
Speaker 1: is very powerful and so unlike the weak force, which
17:51
Speaker 1: you know, if dark matter felt it, it would be
17:53
Speaker 1: difficult but possible to detect in large underground experiments. If
17:57
Speaker 1: dark matter felt the strong force, we would notice that
17:59
Speaker 1: right way. I mean, it's very powerful. It wouldn't take
18:02
Speaker 1: a very big experiment. If dark matter felt the strong force,
18:04
Speaker 1: then it would react to almost every particle in your
18:07
Speaker 1: body and every particle in the Earth. It wouldn't be
18:09
Speaker 1: hard to notice. It would break up particles all the time.
18:12
Speaker 1: It would be a very powerful force. All right. So
18:14
Speaker 1: then those are the four main forces that we know about,
18:17
Speaker 1: and as far as we know, dark matter only feels gravity.
18:21
Speaker 1: And so the question is what what happens to dark matter? Then?
18:24
Speaker 1: Does it bump into itself or does it ignore it
18:27
Speaker 1: even itself? And so let's get into the question of
18:30
Speaker 1: does dark matter feel itself? But first let's take a
18:33
Speaker 1: quick break right there. We're talking about dark matter and
18:49
Speaker 1: whether it can feel itself, and so we know it
18:52
Speaker 1: feels gravity, and we know it doesn't feel the other forces.
18:56
Speaker 1: So I guess the question is, you know what happens
18:58
Speaker 1: when too dark matter? Part of goals if they are
19:01
Speaker 1: particles get attracted to each other, what's going to happen
19:03
Speaker 1: and are they gonna bump into each other or just
19:05
Speaker 1: totally ignore each other. It's a really fun sort of
19:07
Speaker 1: thought experiment. This is exactly what Joe is asking. And
19:11
Speaker 1: so you know, let's assume for the dark matter only
19:14
Speaker 1: feels gravity, that no other forces we don't know about
19:17
Speaker 1: that are involved, that it's just a gravitational thing. And
19:20
Speaker 1: you know, that is totally possible. It's possible for a
19:23
Speaker 1: particle to have mass and no other interactions. It's sort
19:26
Speaker 1: of incredible, but it would be something that we've never
19:28
Speaker 1: seen before. But hey, this physics is full of surprises. Well,
19:32
Speaker 1: because we know about neutrinos, right, Neutrinos are also sort
19:35
Speaker 1: of selective about which forces it feels. Yeah, we don't
19:37
Speaker 1: understand that at all, Like why do neutrinos only feel
19:40
Speaker 1: the weak force? You know, why don't they feel the
19:43
Speaker 1: strong force? Why don't electrons feel the strong force. It's
19:46
Speaker 1: just sort of like a thing we observe. We notice
19:48
Speaker 1: these things. All the corks feel the strong force. None
19:51
Speaker 1: of the electrons or neutrinos or muans or towels, those
19:54
Speaker 1: particles feel it. Why not? I mean, we don't know.
19:57
Speaker 1: We know that neutrinos don't feel elect a magnetism because
20:01
Speaker 1: they don't have a charge. Only things with electric charge
20:03
Speaker 1: can feel that. But we don't understand this. It's just
20:06
Speaker 1: it's something we observe, something that we hope one day
20:09
Speaker 1: to explain From the bottoms of principle. But at this
20:12
Speaker 1: point we're pretty clueless. All right, So then what happens,
20:15
Speaker 1: Like Joe put it, What if I take two particles
20:18
Speaker 1: of dark matter in an empty universe, said I'm a
20:21
Speaker 1: meter apart, and then I let go. Well, they attract
20:24
Speaker 1: each other, right, gravity pulls them together. But then now
20:26
Speaker 1: imagine what happens when sort of their edges meet, right,
20:29
Speaker 1: what happens that one blob versus the other blob. Now,
20:32
Speaker 1: if this was normal matter, when their edges meet, then
20:35
Speaker 1: they would repel each other. They would bump, literally, because
20:38
Speaker 1: they have forces to push against each other with. But
20:42
Speaker 1: if all you have is gravity, gravity can't push. Gravity
20:46
Speaker 1: only pulls. Right, you have two objects with positive mass,
20:50
Speaker 1: they only pull on each other unless you have negative
20:53
Speaker 1: mass particles. And we have a whole other podcast episode
20:56
Speaker 1: about that. Gravity can only attract. So there's no way
20:59
Speaker 1: for gravity to put ish, which means these particles just
21:02
Speaker 1: get closer and closer. There's no repelling force, and eventually
21:06
Speaker 1: they pass through each other. Right, Because there I guess
21:09
Speaker 1: point particles right, Well, we don't know if they're made
21:11
Speaker 1: of particles, right, but imagine they are mative particles. So
21:14
Speaker 1: I think these arguments hold even if they're made of
21:16
Speaker 1: some weird other kind of thing that you know isn't
21:19
Speaker 1: divided up into little particles. But they can basically cohabitate
21:23
Speaker 1: the same space. Right. The only thing that prevents things
21:25
Speaker 1: from overlapping are the forces. Right. The reason that one
21:29
Speaker 1: proton doesn't want to be next to another proton isn't
21:32
Speaker 1: because the stuff in them is like you're crowding me
21:35
Speaker 1: or something. It's all about the forces. As you said,
21:38
Speaker 1: if you imagined that their point particles, they have zero volume.
21:41
Speaker 1: So the only thing that gives distance to things are
21:44
Speaker 1: forces that repel, and gravity can't do that. Well. I
21:46
Speaker 1: think what's interesting to think about is that you know
21:49
Speaker 1: they wouldn't share the same space for very long, right,
21:52
Speaker 1: because if you set them a meter apart, they're gonna
21:54
Speaker 1: ask some velocity by the time that you get to
21:56
Speaker 1: the center, and so they'll just keep going. Yeah, they
21:58
Speaker 1: pass through each other, but then gravity will slow them down,
22:01
Speaker 1: stop them, turn them around, and they'll come through each
22:03
Speaker 1: other again. So like slash back and forth forever and
22:06
Speaker 1: right forever, right, because there's nothing, nothing would be making
22:10
Speaker 1: you lose energy. Yeah, in your hypothetical universe where there
22:13
Speaker 1: is nothing else, then they would slash back and forth forever.
22:17
Speaker 1: If they had some other way to lose energy. There's
22:19
Speaker 1: like larger blobs of dark matter that are sort of
22:22
Speaker 1: you know, equilibrating them or something. Then eventually they would
22:25
Speaker 1: slow down and then stop and sit right in the
22:27
Speaker 1: same place. But maybe not dark matter what you're saying.
22:31
Speaker 1: In in the real universe, that would it is possible
22:34
Speaker 1: out there that dark matter has settled. Yeah, it is possible.
22:36
Speaker 1: In fact, we think that dark matter is not moving
22:39
Speaker 1: very much. There's two theories of dark matter. There's warm
22:41
Speaker 1: dark matter and cold dark matter. And warm dark matter
22:44
Speaker 1: just means that the dark matter sort of zipping around everywhere.
22:47
Speaker 1: It's like has high velocity, and cold dark matter means
22:50
Speaker 1: it's mostly sitting around. It's like sluggist chilling and not
22:54
Speaker 1: moving very fast. It's chilling matter exactly, chill dark matter.
23:00
Speaker 1: That's for a statement of its feelings. Man. Yeah, so
23:03
Speaker 1: we think the dark matter is cold. It's cold dark matter,
23:06
Speaker 1: and so it's not moving very much. But yeah, if
23:08
Speaker 1: you imagine so, like a gravitational well constructed by something else,
23:11
Speaker 1: some larger object than two pieces of dark matter in
23:14
Speaker 1: that well would fall together to the bottom of it,
23:17
Speaker 1: and then they could just sit on top of each other.
23:19
Speaker 1: And I guess the same would happen with dark matter
23:21
Speaker 1: and regular matter, right, they would technically, like we only
23:25
Speaker 1: interact with gravity with dark matter, and so therefore we
23:28
Speaker 1: would just pass through us back and forth. Oh that's
23:30
Speaker 1: a good point. Yeah, And I think you're right, And
23:32
Speaker 1: I think dark matter is passing through us right now,
23:35
Speaker 1: or we're passing through dark matter in the same way
23:37
Speaker 1: that like neutrinos passed through us. But if there literally
23:40
Speaker 1: is no interaction other than gravity, then you're right. Dark
23:44
Speaker 1: matter can be in you and you can be in
23:46
Speaker 1: dark matter. Oh man, I feel like now we're getting
23:49
Speaker 1: ethereal here about dark matter and spiritual It's in you,
23:53
Speaker 1: in all of us, and it's not just a hypothetical question.
23:56
Speaker 1: We sort of did this experiment on a massive cause,
24:00
Speaker 1: a logical scale. Oh yeah, we did it. Oh we
24:02
Speaker 1: didn't sort of do what I mean, as much as
24:03
Speaker 1: astronomers do anything. They just sort of watched the universe
24:06
Speaker 1: do stuff. You say, astronomers don't do anything, Well, they
24:10
Speaker 1: don't construct experiments, right, They don't go like, let's build
24:13
Speaker 1: a sun that has these properties and distribute these planets
24:17
Speaker 1: around it and let it go. Right. They don't do
24:18
Speaker 1: that except in simulations, but they just look around and
24:22
Speaker 1: find that situation because the universe is so vast and
24:25
Speaker 1: so varied that they can usually find whatever they're looking for.
24:28
Speaker 1: The shoppers, not doers. I love astronomers. Nothing I say
24:33
Speaker 1: should be construed as negative towards astronomy or astronomers. But yeah,
24:38
Speaker 1: they're mostly observers, right, And what they found was these
24:42
Speaker 1: two clusters of galaxies that had slammed into each other. Yeah,
24:46
Speaker 1: it's called the Bullet cluster. Right, it's called the Bullet cluster.
24:50
Speaker 1: And this is a collision that happened a long long
24:52
Speaker 1: time ago. And by galaxy cluster, we just mean literally
24:55
Speaker 1: a group of galaxies, because galaxies orbit each other and
24:59
Speaker 1: are sort of grouped to either the same way that
25:01
Speaker 1: like stars are grouped together into a galaxy, galaxies themselves
25:04
Speaker 1: are grouped together into clusters of galaxies, and sometimes these
25:08
Speaker 1: things collide. Right. Dark energy is pushing the whole universe apart,
25:12
Speaker 1: creating spaces between things, but there are still forces and
25:15
Speaker 1: still directions that can pull things together. And so gravity
25:19
Speaker 1: pulled these two clusters together and they slammed into each other,
25:23
Speaker 1: and so we basically got to see this experiment happened. Yeah,
25:26
Speaker 1: you can look it up online if you google bullet cluster,
25:29
Speaker 1: and they're pretty spectacular images. Like, imagine a cluster of
25:32
Speaker 1: galaxies slamming into another cluster of galaxies. It's like a
25:35
Speaker 1: special effects bonanza. It's like a large hadron collider on
25:39
Speaker 1: the biggest scale possible, the largest, the large gaxy collider collider. Yeah,
25:45
Speaker 1: and of course you can't see it happen in real time.
25:48
Speaker 1: The whole collision took, you know, hundreds of millions of years.
25:51
Speaker 1: What we're looking at when we see it in the sky,
25:53
Speaker 1: it's just the after effects, and we can reconstruct what
25:56
Speaker 1: we think happened from what we're seeing now. And so,
25:59
Speaker 1: you know, the clusters are made of sort of three
26:01
Speaker 1: basic constituents. You have the stars, of course, so the
26:03
Speaker 1: things that are visible in the galaxy, and then between
26:06
Speaker 1: the stars you have gas, the things that make the stars.
26:09
Speaker 1: And then of course each one has dark matter, these
26:12
Speaker 1: huge blobs of dark matter. Remember that most of the
26:15
Speaker 1: things that are out there in the sky, the visible galaxies,
26:17
Speaker 1: have these halos of dark matter around them. So wherever
26:21
Speaker 1: you see stuff in the sky is probably where the
26:23
Speaker 1: dark matter is. They tend to follow each other. We've
26:26
Speaker 1: never seen or have we seen dark matter by itself.
26:29
Speaker 1: We have noticed some clumps, like that's what gravitational lensing is,
26:32
Speaker 1: so it is distributed. It goes out further past the
26:35
Speaker 1: edge of the galaxy, Like each galaxy has a halo
26:37
Speaker 1: that's longer and wider and larger than the galaxy itself.
26:41
Speaker 1: And we have seen blobs of dark matter in the
26:43
Speaker 1: universe by themselves. They do this gravitational lensing, but for
26:46
Speaker 1: the most part, the structure they follow each other. And
26:49
Speaker 1: that's not an accident. That's because of gravity. The structure
26:52
Speaker 1: of dark matter created the structure of normal matter, like
26:55
Speaker 1: created these gravitational wells for normal matter to fall into.
26:59
Speaker 1: That's why we have galaxies and stars and planets and squirrels.
27:03
Speaker 1: It's like, which came first, the dark matter or the galaxies,
27:05
Speaker 1: And the answer is the dark matter. Yeah, well, all
27:07
Speaker 1: the matter was created, i think at the same time,
27:09
Speaker 1: but dark matter there's more of it, and so it
27:11
Speaker 1: has a huge influence on the shape of the universe.
27:15
Speaker 1: So we saw this collision happen and we looked at it,
27:18
Speaker 1: and what we noticed is that when these two galaxy
27:21
Speaker 1: clusters passed through each other, the gas reacted like you
27:24
Speaker 1: would expect what to happen when two huge piles of
27:28
Speaker 1: stuff hit each other, right, big special effects, explosions, lots
27:30
Speaker 1: of friction and radiation at the edges and all that
27:33
Speaker 1: kind of stuff. And so you could see the gas
27:35
Speaker 1: not just kind of mixing with itself but actually like
27:38
Speaker 1: violently reacting. Oh yeah, these things are moving really fast. Absolutely,
27:42
Speaker 1: like if you shoot gas really fast and another cloud
27:44
Speaker 1: of gas, then things would happen. Things will happen, and
27:47
Speaker 1: they slow down, right, And so that energy had to
27:49
Speaker 1: go somewhere, and that energy went to heating up the
27:51
Speaker 1: gas and then it radiates, and you have all this
27:53
Speaker 1: friction between these two colliding clouds of gas. But the
27:57
Speaker 1: dark matter in these clusters got stripped way. It passed
28:00
Speaker 1: right through. It went right through the clouds of gas
28:04
Speaker 1: and right through the clouds of dark matter from the
28:06
Speaker 1: other galaxy and passed through to the other side like
28:10
Speaker 1: it went through the windshield or you know, thinking maybe
28:12
Speaker 1: like in the matrix, remember when they're fighting those ghosts
28:15
Speaker 1: and they're in the car and they slammed the brakes
28:17
Speaker 1: and the ghosts just keep going. Yeah, kind of like that,
28:19
Speaker 1: or like you're walking down the sidewalk with your kid
28:22
Speaker 1: and they notice something interesting and they stop and you
28:24
Speaker 1: just on your phone, and so you don't even pay
28:26
Speaker 1: attention to a block later, you're like, wait a second,
28:28
Speaker 1: where's my kid? I mean, that's never happened to me.
28:32
Speaker 1: They're reacting with some gas or is it running to
28:35
Speaker 1: another kid that you didn't spot? Right, there's the analogy,
28:38
Speaker 1: and the two parents just keep going looking at their phone.
28:41
Speaker 1: We're really modeling responsible parenting. I know, we're really talking
28:44
Speaker 1: about our feelings. And so we can see the dark
28:47
Speaker 1: matter came at the other side because we use gravitational lensing,
28:50
Speaker 1: we can see it distorting the light from background galaxies.
28:54
Speaker 1: So dark matter passed right through normal matter, gas, etcetera.
28:58
Speaker 1: Collided and so they got separated. So we really can't
29:01
Speaker 1: see in that case, this is a you know, collidab
29:03
Speaker 1: lab of dark matter with another lav of dark matter.
29:06
Speaker 1: What happens, Well, they massed right through nothing exactly, nothing happens.
29:10
Speaker 1: It just it just passes right through at least exciting collision. Ever, Well,
29:14
Speaker 1: I'm sure if it's exciting for them to lose their
29:16
Speaker 1: kids A few blocks back. All right. So we know
29:20
Speaker 1: then for sure through observational experiments that dark matter doesn't
29:24
Speaker 1: seem to feel itself, at least not in the way
29:26
Speaker 1: that we know about or with any forces that we
29:28
Speaker 1: know about. And so the question is, is there maybe
29:32
Speaker 1: another way that dark matter can feel itself that we
29:34
Speaker 1: don't know about? And so let's get into that. But
29:36
Speaker 1: first let's take a quick break. All right, Daniel, have
29:52
Speaker 1: you have you find your child yet? I've got your
29:54
Speaker 1: child and still looking for mine. Oh well no, wait,
29:56
Speaker 1: this one looks like, oh man, what take mishap here? Yeah,
30:01
Speaker 1: it's like parent trapped galaxy style. All right. So we're
30:03
Speaker 1: talking about dark matter and whether or not it can
30:05
Speaker 1: feel itself. And we know you can feel itself with gravity,
30:08
Speaker 1: but gravity doesn't help you really, you know, permanently feel
30:12
Speaker 1: or clump or things like that. And so the question
30:14
Speaker 1: is it could there be maybe another way that dark
30:16
Speaker 1: matter interacts with itself that we don't know? There absolutely
30:19
Speaker 1: could be, I mean, the existence of dark matter and
30:22
Speaker 1: the existence of dark energy, and our sheer ignorance about
30:26
Speaker 1: how the universe works, how it begins, and what controls
30:28
Speaker 1: its largest dynamics tells us that there's a lot to
30:31
Speaker 1: learn about the universe, and so there's certainly could be
30:34
Speaker 1: a whole other force out there, some force we've never discovered,
30:38
Speaker 1: some force that only dark matter feels and feels with
30:41
Speaker 1: other dark matter, and that might be the reason we've
30:43
Speaker 1: never seen it, the dark side of the force. Yeah,
30:47
Speaker 1: there's nothing preventing another kind of force from existing. It's
30:50
Speaker 1: not like we have any idea as to the number
30:53
Speaker 1: of forces that there could be, or is any limit
30:56
Speaker 1: to how many forces there are. It's just sort of
30:58
Speaker 1: like a list of stuff we've seen so far in
31:01
Speaker 1: our attempt to understand it. And so that's it. That
31:05
Speaker 1: list is almost certainly incomplete, the way our list of
31:08
Speaker 1: particles is incomplete, or the list of Beatles is incomplete. Right,
31:13
Speaker 1: Like every year he's scovered like a a thousand new kinds
31:15
Speaker 1: of Beatle. It's amazing. Yeah, Ringo, Paul Bob Robert And
31:23
Speaker 1: that was such a bad joke. I'm sorry. I did
31:26
Speaker 1: learn about a new Beatle last week. Actually, I discovered
31:28
Speaker 1: that they brought Ringo in, that they fired the original drummer,
31:31
Speaker 1: and he's still kind of better about the fifth One
31:34
Speaker 1: of the fifth Beatles is usually refers to their producer.
31:37
Speaker 1: But yeah, that the Ringo was not the original drummer. Yeah,
31:39
Speaker 1: not a music podcast, So let's stick to our strengths
31:42
Speaker 1: here matters. Yeah, well, this is a really interesting idea.
31:49
Speaker 1: I think that the you know, because it's it's like
31:51
Speaker 1: totally possible for there to be a whole another universe
31:53
Speaker 1: on top of us, right, Like there could be a
31:56
Speaker 1: whole set of other particles to feel, a whole other
31:59
Speaker 1: set of forces, but we just can't see them or
32:01
Speaker 1: feeling or touch them. Yeah, there's a whole party going on,
32:04
Speaker 1: and we weren't invited, and even if we had been invited,
32:06
Speaker 1: we couldn't even really attend because it just ignores us completely, right,
32:11
Speaker 1: And we know that there's a lot going out there
32:13
Speaker 1: that we can't see. Right, what we see in the
32:15
Speaker 1: universe is a tiny fraction what's actually happening, even just
32:19
Speaker 1: here on Earth with your eyeballs. Your eyeballs see a
32:22
Speaker 1: little sliver of all the electromagnetic radiation that's out there.
32:26
Speaker 1: There's a lot of stuff going on you can't see.
32:28
Speaker 1: And imagine if you could see, you know, cell phone
32:31
Speaker 1: signals and television signals and radio waves, the world would
32:35
Speaker 1: be a much different looking place. And so we see
32:38
Speaker 1: a little fraction of what's actually happening. Wow, and good
32:41
Speaker 1: thing to write like it be overwhelmed me. I mean,
32:45
Speaker 1: you can get Netflix in the Hulu for free, but
32:49
Speaker 1: you know, cloud based delivery would be a different kind
32:52
Speaker 1: of thing. All right, So there could be another force
32:54
Speaker 1: out there, and it's im possible the dark matter feels
32:57
Speaker 1: it then when we see it in these collisions, that
33:00
Speaker 1: was interacting with itself in another way. Yeah. So there's
33:02
Speaker 1: sort of two different categories of ideas. One is, could
33:06
Speaker 1: there be another force that dark matter feels and uses
33:09
Speaker 1: to interact with our kind of matter, some new kind
33:12
Speaker 1: of force we've never seen before that mediates the interaction
33:15
Speaker 1: between those two kinds of particles and that we're looking
33:18
Speaker 1: for and we haven't ever seen, and we're looking for
33:21
Speaker 1: that in the same way we're looking for like its
33:23
Speaker 1: interaction via the weak force. We have these big tanks
33:26
Speaker 1: of underground normal matter particles and we wait for dark
33:29
Speaker 1: matter to bump into It doesn't matter if it's the
33:31
Speaker 1: weak force or some other new kind of force. Like
33:34
Speaker 1: would it be really hard to, for example, to take
33:36
Speaker 1: one squirrel that's flying through the earth. Yes, And now
33:40
Speaker 1: you're talking about one squirrel particle and it's flying through
33:44
Speaker 1: like a ton of liquid zenon. These are very difficult experiments,
33:49
Speaker 1: and they're done underground, like a kilometer underground to keep
33:52
Speaker 1: them cold and isolated from all of the sources of background.
33:56
Speaker 1: They're very hard experiments, and they haven't seen anything so far. Right,
33:59
Speaker 1: But can you say it conclusively, like there's a pink
34:02
Speaker 1: squirrel out there with the purple polka dots, but we
34:05
Speaker 1: haven't found it on Earth, so therefore it must not exist.
34:07
Speaker 1: You're right. We can't say it does not exist. We
34:09
Speaker 1: can say it's less common than one squirrel per cubic
34:14
Speaker 1: volume per year. Right, We can just say how likely
34:17
Speaker 1: it is to exist at a certain level, and so
34:20
Speaker 1: it could still be there. And people are still running
34:22
Speaker 1: these experiments, and the longer we run them, the more
34:25
Speaker 1: we can push those limits. We can say, if this
34:27
Speaker 1: force does exist between standard model A matter and dark matter,
34:32
Speaker 1: that it must be even weaker than this limit, than
34:35
Speaker 1: this limit, than this lem We keep pushing that down.
34:37
Speaker 1: But there's a whole other category, which is like maybe
34:40
Speaker 1: dark matter just feel stuff with itself, not with our experiments,
34:43
Speaker 1: not with our bodies, but just between itself, like is
34:47
Speaker 1: it possible that dark matter bumps into itself and feels
34:49
Speaker 1: something right or repels each other with because you're telling
34:54
Speaker 1: me that this idea that it only feels gravity is
34:58
Speaker 1: sort of on thin nice like maybe there's evidence that
35:02
Speaker 1: maybe it does feel something else. Yeah, And this is
35:04
Speaker 1: really hard to probe because you can't do experiments using
35:08
Speaker 1: normal matter material to probe dark matters interactions with itself.
35:13
Speaker 1: You need to like build your experiment out of dark matter, right,
35:16
Speaker 1: So all we can do is look at how the
35:18
Speaker 1: dark matter is laid out in the universe, like where
35:20
Speaker 1: did it end up, and then try to see is
35:23
Speaker 1: that consistent with how it should end up if it
35:26
Speaker 1: only feels gravity, because if it is bumping into itself,
35:29
Speaker 1: that will change the way it's laid out, just like
35:32
Speaker 1: if it's hot or cold, if it's fast or slow,
35:34
Speaker 1: will also change the distribution of dark matter. So we
35:37
Speaker 1: look at the distribution and try to ask is it
35:39
Speaker 1: consistent with gravity only or do we need another piece
35:43
Speaker 1: of the puzzle aid And what do we see do
35:44
Speaker 1: we see out there in the universe that maybe it
35:47
Speaker 1: does feel another force? We see that mostly gets things
35:51
Speaker 1: right sort of on the scale of galaxies, and larger
35:54
Speaker 1: like the distribution of galaxies and how they cluster together
35:57
Speaker 1: and superclusters. That all seems right, and you run a
36:00
Speaker 1: simulation of the universe with dark matter and normal matter
36:03
Speaker 1: and dark matter is only gravity, it mostly gets that right.
36:06
Speaker 1: But then if you zoom in, like on the smaller
36:08
Speaker 1: scale here, smaller scales like less than a galaxy siza,
36:12
Speaker 1: then sub galaxy, then it's not quite right. It doesn't
36:16
Speaker 1: predict things the way we see them. For example, we
36:20
Speaker 1: know that we have a big blob of dark matter
36:22
Speaker 1: in our galaxy, and we know it's the densest at
36:24
Speaker 1: the center just because of gravity, But the exact shape
36:28
Speaker 1: of that blob is not something that we understand, like
36:31
Speaker 1: dark matter with only gravity predicts it sort of like
36:34
Speaker 1: peaked right at the center, that the center is like
36:36
Speaker 1: a hot spot for dark matter. But we go out
36:39
Speaker 1: and measure it, we see it's much more spread out
36:41
Speaker 1: than that. It's like much more of a rod core,
36:43
Speaker 1: not so much like peaked like a cusp, and so
36:47
Speaker 1: that's not something we understand. Fuzzier than what you would expect.
36:51
Speaker 1: You would expect it to sort of cluster more, but
36:54
Speaker 1: it's actually kind of fuzzy it's like a big fuzzy squirrel,
36:57
Speaker 1: not a little tiny intense squirrel. Yeah, it's a hedgehog
37:01
Speaker 1: or or something like that rather than a tiny little squirrel.
37:04
Speaker 1: And if you change your simulations and you add a
37:07
Speaker 1: little bit of bumping this like a little bit like
37:10
Speaker 1: maybe sometimes dark matter does bump into itself, that describes
37:13
Speaker 1: what we see better, meaning like a little bit of
37:15
Speaker 1: a repulsive force with itself. You just add in a
37:19
Speaker 1: made up force. You're like, we don't know, but let's
37:21
Speaker 1: see if it describes a situation, just like we made
37:24
Speaker 1: up dark matter to describe this gravity we were seeing.
37:27
Speaker 1: We're like, all right, well, now let's give another force
37:30
Speaker 1: to these particles that are creating the gravity, so that
37:32
Speaker 1: the distribution of gravity is the way we are seeing it.
37:35
Speaker 1: Let's make something up about something that we've made up.
37:39
Speaker 1: You're talking about all the physics, now, man, we'll come
37:42
Speaker 1: to science. What if that thing we made up for
37:46
Speaker 1: the thing we made up, that's something else we make up. Yeah,
37:50
Speaker 1: and so this is sort of the state of the art.
37:52
Speaker 1: Now we're like looking really carefully trying to understand where
37:55
Speaker 1: is the dark matter? Does it agree with what we
37:57
Speaker 1: predict and it turns out it doesn't quite agree, and
38:00
Speaker 1: there are other little details that the gravity only theory
38:05
Speaker 1: is getting wrong, so that they're sort of strong maybe
38:08
Speaker 1: or at least a hint of an indication that maybe
38:11
Speaker 1: dark matter does feel a whole new force or could
38:14
Speaker 1: it be one of the old force. It couldn't be
38:16
Speaker 1: one of the old forces. It would have to be
38:17
Speaker 1: some sort of self interaction. So this theory is called
38:21
Speaker 1: self interacting dark matter, and it solves some of these
38:24
Speaker 1: nagging problems that we've had in the gravity only theory
38:27
Speaker 1: of dark matter. So that makes it compelling. It's not
38:30
Speaker 1: proof at all that this is happening, but it's like, huh,
38:33
Speaker 1: we've had this problem. We add this new idea. It
38:36
Speaker 1: gives a better explanation. That's cool. And you always have
38:39
Speaker 1: to be careful when you're adding more stuff to your theory. Right,
38:42
Speaker 1: you'd like the simplest possible explanation, but if that doesn't work,
38:46
Speaker 1: then you have to add a little widget to your theory,
38:48
Speaker 1: like maybe what if there are two kinds of squirrels matter? Yeah,
38:52
Speaker 1: and and that's a whole other idea, right, Like if
38:54
Speaker 1: dark matter made of particles, is it one kind of particles?
38:57
Speaker 1: Is it two or is it ten? Didn't they decay
38:59
Speaker 1: from one kind particle to the other? Do only some
39:02
Speaker 1: of them have self interactions? I mean, look at the
39:04
Speaker 1: particles that make up our kind of matter. There's all
39:06
Speaker 1: sorts of complicated interactions with weird rules we don't understand.
39:10
Speaker 1: Why would we imagine dark matter would be like simpler.
39:13
Speaker 1: It could be different shades of dark matter, the darkest
39:16
Speaker 1: matters squirrels and cats. And and then then you wonder, like,
39:21
Speaker 1: all right, well, maybe dark matter could feel itself and
39:24
Speaker 1: that would explain like the weird distribution of gravity that
39:27
Speaker 1: we're seeing. But what about the bullet cluster? Right? Didn't
39:30
Speaker 1: we do this experiment because the two clouds of dark
39:34
Speaker 1: matter didn't repel each other. They didn't repel each other.
39:37
Speaker 1: But people went back and looked at that again, and
39:39
Speaker 1: they said, all right, well, can we explain this and
39:43
Speaker 1: have some amount of interaction in there, because it's not
39:46
Speaker 1: easy to tell exactly where all the dark matter is
39:49
Speaker 1: very precisely, like maybe some of it did interact and
39:52
Speaker 1: got sort of stuck in the middle. Most of it didn't,
39:55
Speaker 1: but maybe some of it did. You know. It's like
39:58
Speaker 1: we made it up in the first place when we
40:00
Speaker 1: just you know, it's like there's a first pass and
40:03
Speaker 1: there's a more precise test, right, you know, first you
40:06
Speaker 1: try to understand the broad strokes. Then you really dig
40:08
Speaker 1: in and see if it gets the details right. And
40:10
Speaker 1: the other thing is it turns out that this is
40:12
Speaker 1: not a great dark matter collider. It's too big dark matter. Well,
40:17
Speaker 1: dark matter is very dilute, and so if you throw
40:20
Speaker 1: a huge, very dilute cloud of dark matter at another one,
40:23
Speaker 1: most of it's just going to pass through anyway. Like,
40:26
Speaker 1: dark matter is not as dense as these gas clouds.
40:29
Speaker 1: It's sort of like like the stars. Like if you
40:31
Speaker 1: have two galaxies that pass through each other, mostly the
40:35
Speaker 1: stars don't collide because the stars are embedded in huge,
40:39
Speaker 1: vast empty spaces, and so they don't really collide with
40:43
Speaker 1: each other head to head. So you were saying, like,
40:45
Speaker 1: if I take two dark matter squirrels and I throw
40:47
Speaker 1: them at each other, they might repel one against the other.
40:50
Speaker 1: But if I take a whole cloud of squirrels that
40:53
Speaker 1: are really far apart, they might just pass through each
40:55
Speaker 1: other like we've seen in the cluster. Yeah, and they
40:58
Speaker 1: would mostly miss, just like the large Hadron collider what
41:01
Speaker 1: we do is we collide to clouds of protons against
41:04
Speaker 1: each other, because one proton will never hit another proton,
41:07
Speaker 1: they're too small. So we have like ten to the
41:10
Speaker 1: ten protons and one cloud and ten to the ten
41:12
Speaker 1: to the other, and we get you know, ten or
41:14
Speaker 1: twenty or fifty collisions. And so it's not that easy
41:18
Speaker 1: to make things collide. And so it turns out the
41:22
Speaker 1: bullet cluster is actually also consistent with dark matter feeling
41:25
Speaker 1: itself at sort of a low level of tweak how
41:28
Speaker 1: you look at it. Yeah, and so you know, the
41:30
Speaker 1: game here is come up with a theory um model
41:33
Speaker 1: that explains everything that we do see all the same time.
41:37
Speaker 1: And so you have to add some of this stuff
41:40
Speaker 1: to explain the distribution of dark matter and the core
41:43
Speaker 1: versus cup effect to the center of the galaxy, but
41:45
Speaker 1: not too much that you mess up the description of
41:47
Speaker 1: the bullet cluster. So you sort of hemmed in on
41:49
Speaker 1: both sides there. Interesting, But I guess maybe the takeaways
41:53
Speaker 1: that the answer is, yes, maybe dark matter does feel itself.
41:57
Speaker 1: It's totally possible, and there might be some indication out
42:00
Speaker 1: there that it does. Yeah, it's a new idea and
42:03
Speaker 1: it's gaining traction in the community self interacting dark matter.
42:07
Speaker 1: So it's possible that yes, dark matter could feel itself
42:10
Speaker 1: and there could be multiple forces that it feels all right,
42:13
Speaker 1: So maybe dark matter does have feelings, Daniel, It might
42:16
Speaker 1: have a lot more feelings than we could ever understand.
42:19
Speaker 1: Shades and shades of nuance. Well, I think it's all again,
42:22
Speaker 1: just all of this is just an interesting example of
42:24
Speaker 1: how there are things in the universe happening that we
42:27
Speaker 1: don't really know about or see or feel or effective.
42:31
Speaker 1: We're only seeing a tiny fraction of this grand play
42:34
Speaker 1: of the universe, and we're making huge, broad conclusions about
42:38
Speaker 1: how the universe works from this tiny little corner of
42:40
Speaker 1: the stage. And there's a lot going on that we
42:43
Speaker 1: haven't ever observed. And at least we know now. We
42:46
Speaker 1: know that we're ignorant, so we know to be humble
42:49
Speaker 1: and we know not to over generalize our ideas. But
42:52
Speaker 1: it also is tantalizing because you hope that one day
42:55
Speaker 1: we will see the rest of the action, We will
42:57
Speaker 1: get a glimpse for what's really happening. We might all
42:59
Speaker 1: be swimming in a giant squirrel up mr. I like
43:04
Speaker 1: to think of myself as a physics raccoon, because it's
43:08
Speaker 1: sneaking late at night to steal other people's garbage with
43:10
Speaker 1: dark circles on your eyes. That makes sense eating garbage.
43:16
Speaker 1: All right, Well that's pretty interesting and cool and or
43:19
Speaker 1: chill um, and so we hope you enjoyed that discussion.
43:23
Speaker 1: Thanks Joe for writing with your question. And if you
43:25
Speaker 1: have a question you'd like us to explore on the podcast,
43:27
Speaker 1: please write to us. We love your questions. Two questions
43:31
Speaker 1: at Daniel and Jorge dot com. A bit mysterious. See
43:35
Speaker 1: you next time. Thanks for listening, and remember that Daniel
43:46
Speaker 1: and Jorge Explain the Universe is a production of I
43:48
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