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Speaker 2: Hi.

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Speaker 3: I'm David Ego from the podcast Inner Cosmos, which recently

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Speaker 3: hit the number one science podcast in America. I mean

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Speaker 3: neuroscientists at Stanford, and I've spent my career exploring the

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Speaker 3: three pound universe in our heads.

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Speaker 4: Join me weekly to explore the relationship.

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Speaker 3: Between your brain and your life, because the more we

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Speaker 3: know about what's running under the hood, better we can

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Speaker 3: steer our lives. Listen to Inner Cosmos with David Eagleman

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Speaker 3: on the iHeartRadio app, Apple Podcasts or wherever you get

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Speaker 3: your podcasts.

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Speaker 2: Hey, Daniel, what's the best question anyone has ever asked

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Speaker 2: you about dark matter?

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Speaker 5: Well?

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Speaker 1: It's tough to pick. There's so many good ones. But

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Speaker 1: I remember a nine year old who read our book

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Speaker 1: once and emailed me to ask is dark matter bad matter?

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Speaker 2: It must be a Star Wars fan, you know it's

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Speaker 2: the dark side. It doesn't sound very friendly.

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Speaker 1: Yeah, dark matter sounds sort of unfriendly and mean doesn't

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Speaker 1: have a really good pr team.

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Speaker 2: Well, you know, if there are dark matter physicists out

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Speaker 2: there that we can't see or touch, what do you

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Speaker 2: think they would call us?

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Speaker 1: M I guess they'll probably call us dork matter.

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Speaker 2: We wouldn't seem dark to them and insidious.

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Speaker 1: No, actually we would. I mean, if they have their

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Speaker 1: own dark photons. They wouldn't see ours, so we would

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Speaker 1: be dark to them.

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Speaker 2: Oh man, I can just imagine the conferences. You know,

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Speaker 2: just spent the whole day saying you're a dark matter. Oh,

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Speaker 2: you're a dark matter.

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Speaker 1: That's probably actually true. That's exactly what would happen.

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Speaker 2: I am four Hm, a cartoonist and the creator of

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Speaker 2: PhD Comms.

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Speaker 1: I'm Daniel Whitson. I'm a particle physicist, and I'm mostly

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Speaker 1: made out of dork particles.

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Speaker 2: Or particles, dark dork particles. That's the worst.

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Speaker 1: Well, mostly in the summer. Right now, I'm not going

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Speaker 1: outside very much, so oh, I see you're pale dark.

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Speaker 1: My dark particles are not very dark.

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Speaker 2: But welcome to our podcast, Daniel and Jorge Explain the Universe,

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Speaker 2: a production of iHeartRadio.

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Speaker 1: In which we take you on a mental tour in

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Speaker 1: the universe, zooming down to see what the universe is

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Speaker 1: made out of, what it looks like at its largest scale,

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Speaker 1: what crazy weird stuff we encounter in it, and how

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Speaker 1: we can all understand it because humanity is on a journey,

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Speaker 1: a journey to understand this universe. We started out really

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Speaker 1: kind of clueless about the way things work, and we

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Speaker 1: slowly put together little pieces of the picture. But we're

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Speaker 1: here to give you a tour of what we have

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Speaker 1: figured out and what still remains unknown.

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Speaker 2: Yeah, and it turns out we haven't even left the garage,

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Speaker 2: it seems, Daniel, in our journey of discovery, because a

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Speaker 2: huge percentage of the universe is made out of stuff

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Speaker 2: that we have no idea about. You know, twenty five

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Speaker 2: percent of the universe is made out of something we

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Speaker 2: call dark matter, which is a huge mystery.

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Speaker 1: Right, that's right. And that might seem a bit deflating

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Speaker 1: to you, like, wow, we haven't figured anything out, But

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Speaker 1: to me that's exciting, Like, wow, we haven't figured anything out,

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Speaker 1: which means more discoveries are to come, which means mind

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Speaker 1: blowing revelations about the true nature of the universe are

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Speaker 1: in our future.

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Speaker 2: Yeah. You know, if you haven't left the garage, that

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Speaker 2: means you can still go back inside. If you forget

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Speaker 2: something that's a positive.

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Speaker 1: That's right, Like you forgot to pack your banana. How

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Speaker 1: can you even take a journey of discovery without banana?

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Speaker 2: Yeah? About these days? Who can go anywhere?

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Speaker 6: Right?

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Speaker 2: Yeah, we're talking about dark matter today and it's a

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Speaker 2: huge mystery and it's out there, but a big question

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Speaker 2: is what is it made out of? What is it?

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Speaker 2: Is it a fluid, is it a an energy, or

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Speaker 2: is it made out of particles or something totally different.

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Speaker 1: That's right, Everything we've ever experienced, everything you've ever eaten

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Speaker 1: or touched or tasted, are made out of quarks and

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Speaker 1: electrons and stuff like that, and so we wonder does

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Speaker 1: the same thing hold for dark matter. But it's a

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Speaker 1: pretty big extrapolation. I mean, the kind of matter that

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Speaker 1: we're made out of, it that's very familiar, is unusual

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Speaker 1: in the universe, and so it seems kind of odd

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Speaker 1: to extrapolate from this tiny little corner to a huge

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Speaker 1: fraction of the universe. And so particle physicists think maybe

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Speaker 1: it's made of particles, but I like to think, hey,

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Speaker 1: maybe it's made of something totally weird and new.

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Speaker 2: Yeah, we really have no idea. And it's there's five

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Speaker 2: times more dark matter in the universe than there is

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Speaker 2: of the stuff that we are made out of. And

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Speaker 2: it really could be anything like it could be something

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Speaker 2: that's not even a particle.

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Speaker 1: That's right. We've never seen matter that's not made of particles.

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Speaker 1: Everything we've ever experienced can be broken down into these

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Speaker 1: little units. But that doesn't mean that everything is like that, right.

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Speaker 1: We have to be very careful about generalizing. We've seen

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Speaker 1: so many times in physics when we've learned something we thought, oh,

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Speaker 1: this must be the way everything works, and then we

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Speaker 1: discover it's not actually, and there are bigger stories, is

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Speaker 1: a larger context, and that's the goal of science, is

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Speaker 1: to reveal the actual truth, not just confirm our expectations.

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Speaker 2: Yeah, and so one thing we know about dark matter

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Speaker 2: is that we know that it feels gravity, because that's

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Speaker 2: how we know that it's there. I mean, we can't

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Speaker 2: see it, it's dark. We know it's there because of gravity.

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Speaker 2: It's keeping the galaxies together, it's pulling things together, and

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Speaker 2: it's you know, affecting the trajectories of galaxies and stars.

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Speaker 2: So we know it it feels gravity.

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Speaker 1: We certainly do. It's basically an invention to explain missing gravity.

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Speaker 2: It's like a gravity fudge factor.

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Speaker 5: Yeah.

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Speaker 1: We see things in the universe that gravity has done,

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Speaker 1: and we don't see the source of that gravity. We're like,

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Speaker 1: what's this invisible thing that's making this gravity? And so

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Speaker 1: dark matter is basically an explanation for this gravity that

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Speaker 1: we do observe, and so it has to feel gravity

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Speaker 1: otherwise it doesn't fit the bill at all. But then

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Speaker 1: the question is what else does it do? Does it

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Speaker 1: feel anything else?

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Speaker 2: Does it feel lonely? Does it feel rescentful that we

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Speaker 2: call it dark matter? Yeah, we know that it feels gravity,

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Speaker 2: and so the question is does it feel any other forces?

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Speaker 2: And I think more interestingly about this question is does

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Speaker 2: it feel itself?

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Speaker 7: Like?

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Speaker 2: Can it bump into itself?

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Speaker 1: Yeah? And we got a really interesting question from a listener.

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Speaker 1: Here's an email I got from Joe from China, who

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Speaker 1: I think really put his finger on the question. He says,

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Speaker 1: if you were to take two particles of dark matter

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Speaker 1: and let them go in a vacuum, the gravity would

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Speaker 1: attract the two particles together. But then would they pass

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Speaker 1: right through each other and keep moving? Eventually would they

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Speaker 1: settle down and be able to share the same point

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Speaker 1: in space like overlapping?

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Speaker 2: Can two dark matter particles but I guess, bump into

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Speaker 2: each other or occupy the same space?

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Speaker 1: Yeah? Do they knock into each other and push back?

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Speaker 1: Do they pass right through each other? Can they sit

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Speaker 1: right on top of each other? It's a really fascinating question.

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Speaker 1: And it goes right to the heart of the sort

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Speaker 1: of the science matter of whether dark matter feels itself?

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Speaker 2: This question blow your mind a little bit.

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Speaker 1: Yeah, it did blow my mind a little bit. I

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Speaker 1: just love when somebody asks a new question, a question

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Speaker 1: I haven't heard before, or asks it in a new way.

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Speaker 1: So I haven't really thought about whether dark matter feels itself?

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Speaker 2: Okay, so it's kind of a funny question. Does dark

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Speaker 2: matter feel itself? That means that you know, it feels

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Speaker 2: itself gravitationally. Right, we have one bit of dark matter

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Speaker 2: here and a bit of dark matter there. They're going

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Speaker 2: to be They're going to feel each other. They're going

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Speaker 2: to be attracted to each other by gravity. But the

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Speaker 2: question is, you know what happens after they feel attracted

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Speaker 2: by gravity. You know, they they're moving towards each other.

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Speaker 2: Are they gonna bump against and repel themselves or are

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Speaker 2: they going to pass through each other? Or are they

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Speaker 2: going to stick together?

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Speaker 1: Yeah, it's a great question. I mean, if you and

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Speaker 1: I have gravitational attraction, if we were out in the

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Speaker 1: middle of space, then gravity would very slowly pull us

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Speaker 1: together and we would bump into each other, or we

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Speaker 1: wouldn't phase through each other.

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Speaker 2: It would get uncomfortable really fast, is what you're saying.

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Speaker 2: More awkward than no one can in space. No one

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Speaker 2: can hear you.

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Speaker 1: Say, Hey, I need more space, Daniel, get off me.

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Speaker 2: But you couldn't, right, you'd be stuck together.

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Speaker 1: That's right. We'd need some other kind of force. But

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Speaker 1: you and I do have other kind of forces, right,

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Speaker 1: The electromagnetic force would keep us from passing through each other.

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Speaker 1: So it's a good way to ask the question.

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Speaker 2: Yeah, so the question is does dark matter feel itself?

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Speaker 2: And so, as usual, we went out into the world

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Speaker 2: old and in this case, the Internet, to see how

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Speaker 2: many people felt that they knew the answer to this question.

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Speaker 1: That's right, So thank you to everybody who volunteered to

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Speaker 1: answer random Internet physics questions. If you'd like to volunteer,

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Speaker 1: write to us at questions at Daniel and Jorge dot com.

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Speaker 2: Yeah, you can ask us questions or get questions from us.

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Speaker 1: Questions in and questions out. It's an eternal flux of questions, right.

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Speaker 2: But think about it for a second. If you were

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Speaker 2: to try to answer the question does dark matter feel itself?

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Speaker 2: What would you answer? And no googling, No, googling what

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Speaker 2: how about? Wikipedia also not allowed about calling my physicist friend.

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Speaker 1: Totally allowed total.

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Speaker 2: All right, Well, here's what people on the internet had

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Speaker 2: to say.

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Speaker 1: I guess I'm really not sure.

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Speaker 3: I know that some scientists hypothesize that dark matter is

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Speaker 3: comprised of filaments, and if there were these filaments in

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Speaker 3: the dark matter, one would think that they would interact.

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Speaker 1: In some way.

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Speaker 8: I really don't think so.

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Speaker 2: I remember it interacting with something. That's a good one.

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Speaker 2: The interaction between light matter and dark matter is for

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Speaker 2: obviously still figuring that out, but I would think that

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Speaker 2: it does interact with itself.

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Speaker 5: I don't think that dark matter interacts with itself. Where

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Speaker 5: they discussed two galaxies colliding with each other, but there

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Speaker 5: wasn't any evidence sort of the details. So no, that'self.

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Speaker 8: I would guess that dark matter would interact with itself

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Speaker 8: because I know that one guess of what dark matter

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Speaker 8: is is the dark matter particle axion. And it's kind

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Speaker 8: of hard to visualize how a particle can pass through

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Speaker 8: another particle.

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Speaker 1: I don't know.

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Speaker 9: I've never thought about it. Okay, and dark matter and

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Speaker 9: dark energy confuse me a little bit. I thought that

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Speaker 9: dark matter was just a theory, and so it's like

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Speaker 9: we've we've seen something out there, and the theory is

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Speaker 9: that this dark matter, but we don't really know. I

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Speaker 9: don't think it can interact with normal matter. I surely

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Speaker 9: if it could pass through itself, we would be able

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Speaker 9: to see it.

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Speaker 5: There might be a dark force which shows them intact.

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Speaker 6: I know it doesn't interact with regular matter.

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Speaker 4: I think we don't know whether how it interacts with itself.

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Speaker 1: It is matter, so it has maths, so it must

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Speaker 1: interact gravitationally.

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Speaker 10: I don't know.

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Speaker 2: Well, I think it does on some level.

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Speaker 6: I don't know whether it would be quantum or molecular,

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Speaker 6: but I think it's got to interact with itself enough

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Speaker 6: to come together and blobs big enough to cause gravitational

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Speaker 6: lensing that we can see here from Earth.

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Speaker 1: I guess it interact with yourself since it's gravity.

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Speaker 2: All right, pretty thoughtful answers, you know, people start of

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Speaker 2: thought through it.

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Speaker 1: Yeah, there's some good stuff in here. People are really

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Speaker 1: thinking about what it means to interact and whether gravity counts,

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Speaker 1: and whether gravity would make something bump or not?

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Speaker 2: Does gravity count? Does the Earth feel us?

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Speaker 8: Oh?

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Speaker 1: Yeah? But also some people getting confused about dark energy

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Speaker 1: versus dark matter.

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Speaker 2: Boy, that's another rabbit hole there. Does dark energy feel?

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Speaker 1: What is dark energy? Anyway? We have no idea, all.

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Speaker 2: Right, So the question is does dark matter feel? And

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Speaker 2: so Daniel maybe step us through. What do we know

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Speaker 2: about dark matter and spin?

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Speaker 1: It's not very sensitive, unfortunately, you know, we wish it

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Speaker 1: would react more.

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Speaker 2: Oh good, that's a good thing, right as a thick skin.

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Speaker 1: Well perhaps you know if you want to get along

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Speaker 1: with your dark matter roommate, but if you want to

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Speaker 1: do experiments to reveal the nature of dark matter, you'd

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Speaker 1: likely to respond sometimes. And dark matter turns out to

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Speaker 1: be very difficult to probe because the only way we've

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Speaker 1: been able to interact with dark matter so far at

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Speaker 1: all is with gravity. Like we said, we've discovered dark

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Speaker 1: matter because we looked at how galaxy spin and we

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Speaker 1: notice that to spin as fast as they do and

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Speaker 1: hold all the stars in place, you need a lot

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Speaker 1: more gravity than is generated by the stars we see,

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Speaker 1: which means there must be something else there make that gravity.

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Speaker 1: And we see blobs of dark matter in the sky.

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Speaker 1: We don't see them. We deduce blobs of dark matter

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Speaker 1: in the sky because they're bending the light that passes

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Speaker 1: through them. That's a gravitational effect. So we have very

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Speaker 1: concrete evidence that dark matter feels gravity. That's why we

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Speaker 1: call it matter, not just like dark mysterious thing in

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Speaker 1: the space, right, because it has gravity.

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Speaker 7: I see.

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Speaker 2: It's sort of like the properhial elephant in the room.

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Speaker 2: You know it's there, everyone knows it's there, but it's

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Speaker 2: it's nobody's talking about it sort of.

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Speaker 1: It's also sort of like the opposite of that, because

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Speaker 1: the elf in the room, everybody's ignoring it, right though

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Speaker 1: you can all see it. In this case, we're all

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Speaker 1: desperately searching for it, but nobody can find it.

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Speaker 2: It's like you're right orra except you're not.

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Speaker 1: Well, you know, sometimes being exactly the opposite direction is

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Speaker 1: also a way of being right.

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Speaker 2: Yeah, yeah, you know, beans.

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Speaker 1: Down, Yes, time's minus one.

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Speaker 2: All right. So we know it feels gravity, But there

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Speaker 2: are other forces that we know about in the universe,

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Speaker 2: and as far as we know, dark matter doesn't feel them.

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Speaker 1: That's right. And so for example, electromagnetism is a very

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Speaker 1: powerful force and one that we experience every day. The

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Speaker 1: reason that your hand doesn't go through the wall when

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Speaker 1: you try to touch it is that the wall has

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Speaker 1: particles that are bound together with electromagnetic forces, and so

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Speaker 1: does your hand, and so you think of them like

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Speaker 1: a chain link fence trying to pass through another chain

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Speaker 1: link fence. That's what prevents you from passing through another object.

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Speaker 1: It's electromagnetism, right, And it's also responsible for why things glow, right,

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Speaker 1: why we can see things.

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Speaker 2: Or why they reflect light too, right, Like this light

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Speaker 2: hits it and then it interacts and it comes back out.

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Speaker 1: That's right. Even blobs of rock that don't glow on

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Speaker 1: their own, we can see them in the sky because

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Speaker 1: they reflect the Sun's light, like the moon, or like asteroids.

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Speaker 1: And so we know dark matter isn't like that because

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Speaker 1: it doesn't give off light, doesn't reflect light. Light passes

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Speaker 1: right through it. So you write through it, you can

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Speaker 1: see through it. Yeah, exactly. And I see this all

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Speaker 1: the time in science fiction television that they encounter dark

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Speaker 1: matter and it obscures their You right, Oh, we can't

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Speaker 1: get out of this cloud of dark matter. We can't

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Speaker 1: see where we are.

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Speaker 2: Oh, what show are you watching? I haven't seen that one.

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Speaker 1: I don't remember the name of it right now. I

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Speaker 1: watch too much science fiction. But you know that. To me,

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Speaker 1: that sounds like a fantastic situation, like, oh my gosh,

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Speaker 1: look we found dark matter. That's right home. We win

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Speaker 1: the Nobel Prize.

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Speaker 2: It's not like, get out of my picture, my photo.

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Speaker 2: It's like I got a photo of dark matter exactly.

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Speaker 2: We've been trying to spot dark matter forever and you

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Speaker 2: just figured it out and now seems like a hassle

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Speaker 2: to you. But if you were in a cloud of

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Speaker 2: dark matter, you couldn't tell because it's invisible.

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Speaker 6: Right.

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Speaker 1: In fact, we think we are in a cloud of

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Speaker 1: dark matter. We think the Earth is surrounded by dark matter,

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Speaker 1: but we can't see it because it doesn't reflect light,

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Speaker 1: it doesn't feel any electromagnetic forces at all.

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Speaker 2: Yeah, we're swimming in dark matter right Like everyone who's

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Speaker 2: listening to this podcast right now is probably swimming in

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Speaker 2: dark matter like you right in for another eyes, is

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Speaker 2: dark matter passing through or sitting there?

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Speaker 1: That's right. Dark matter, we think is spread out through

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Speaker 1: the galaxy. It's fairly dilute, so it's there's a lot

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Speaker 1: of it. There's a lot more dark matter than normal matter,

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Speaker 1: but it's much more spread out, we think than normal matter,

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Speaker 1: which tends to cluster into planets and stars and stuff.

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Speaker 1: So we think it's like one squirrels worth of dark

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Speaker 1: matter in the volume of the Earth. But it's there,

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Speaker 1: it's out there.

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Speaker 2: Wait, what one squirrels worth? Is that what you say?

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Speaker 1: One squirrel? Yeah, well it's a standard international unit of mass,

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Speaker 1: one squirrel. Are you not familiar with squirrels? I don't

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Speaker 1: usually bake with squirrels, Oh all. My favorite recipes are

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Speaker 1: in units of squirrels. You know, a third of a

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Speaker 1: squirrel of oats, and you know half a squirrel of sugar.

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Speaker 2: That's the pandemic for you. It's forcing us into a

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Speaker 2: more primitive.

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Speaker 1: Yeah, come over to my house sometime. We'll have a

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Speaker 1: squirrel roast.

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Speaker 2: So wait, the Earth you're saying is in a pool

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Speaker 2: of dark matter that has the same amount of mass

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Speaker 2: as a squirrel. That's what it means.

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Speaker 1: That's right, And there's more dark matter than this normal matter,

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Speaker 1: but the dark matter is more diffuse, it's more spread out,

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Speaker 1: and so out there in empty space, that's about as

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Speaker 1: much dark matter as there is here in our solar system.

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Speaker 2: You just made me think of a giant squirrel the

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Speaker 2: size of the Earth. That dark matter.

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Speaker 1: Well, you're a very visual person, so I'd love to

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Speaker 1: see a drawing of that of a squirrel attacking the Earth,

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Speaker 1: an Earth size squirrel attacking us. Finally, the aliens have

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Speaker 1: come and they're just giant space squirrels.

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Speaker 2: You did it again, just because it's a dark matter squirrel,

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Speaker 2: you assumed there was bad Daniel.

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Speaker 1: Yeah, maybe it's a friendly planet sized space squirrel. Yeah,

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Speaker 1: you're right.

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Speaker 8: Yeah.

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Speaker 1: No, So we know that it doesn't feel electromagnetism, and

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Speaker 1: that's principally because it's dark, we can't see it. But

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Speaker 1: there are other forces out there, right. We talk about

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Speaker 1: the weak nuclear force. That's a very, very non powerful force.

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Speaker 1: It's a weak force between particles. Neutrinos feel this force,

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Speaker 1: Leptons feel this force. All the particles we know feel

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Speaker 1: this force, but it's not very powerful. Like neutrinos pass

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Speaker 1: through a light year of lead and have a fifty

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Speaker 1: percent chance of feeling anything using the weak force?

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Speaker 2: Did our particles also feeling like the particles in my

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Speaker 2: body they feel the weak force.

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Speaker 1: Yeah, all the particles in your body, of the electrons

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Speaker 1: and the quarks, they all feel this force. It's responsible,

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Speaker 1: for example, for beta decay. So one of the quarks

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Speaker 1: in a neutron can turn into another kind of quark,

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Speaker 1: which turns your neutron into a proton. That happens through

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Speaker 1: the weak force, and that's why a little neutrinos emitted

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Speaker 1: along with the electron that's created. So all the particles

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Speaker 1: in your body are sensitive to the weak force. Neutrinos

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Speaker 1: that pass through you can interact with you. It's just

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Speaker 1: very very unlikely, Like there's one hundred billion neutrinos passing

400
00:18:32,200 --> 00:18:35,640
Speaker 1: through your fingernail every second and you don't feel them.

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00:18:35,680 --> 00:18:37,919
Speaker 2: Wow, And we know that dark matter doesn't feel it.

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00:18:37,960 --> 00:18:40,120
Speaker 1: We suspect that dark matter doesn't feel it because we've

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Speaker 1: looked very very carefully. We've set up these big underground

404
00:18:43,560 --> 00:18:47,359
Speaker 1: experiments filled with very quiet, heavy liquids, and we have

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Speaker 1: a lot of those atoms in there, and we hope

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Speaker 1: that a dark matter particle would come by and bump

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Speaker 1: into one of those xenon atoms, for example, and we

408
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Speaker 1: could spot it. And this is similar to how neutrinos

409
00:18:57,520 --> 00:19:01,760
Speaker 1: were first discovered with vast tanks underground liquid waiting for

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Speaker 1: a little recoil from one of those atoms. So we

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Speaker 1: have a similar in spirit kind of experiment done underground

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Speaker 1: to look for passing dark.

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Speaker 2: Matter particles, all right. And there's also the strong force,

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Speaker 2: right which that one's pretty important. But dark matter doesn't

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Speaker 2: feel that one either.

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Speaker 1: That's right. The strong force is very powerful and so

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00:19:19,040 --> 00:19:21,400
Speaker 1: unlike the weak force, which you know, if dark matter

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Speaker 1: felt it, it would be difficult but possible to detect

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Speaker 1: in large underground experiments. If dark matter felt the strong force,

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Speaker 1: we would notice that right away. I mean, it's very powerful.

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Speaker 1: It wouldn't take a very big experiment. If dark matter

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Speaker 1: felt the strong force, then it would react to almost

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Speaker 1: every particle in your body and every particle in the Earth.

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Speaker 1: It wouldn't be hard to notice. It would break up

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Speaker 1: particles all the time. It would be a very powerful.

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Speaker 2: Force, all right. So then those are the four main

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Speaker 2: forces that we know about, and as far as we know,

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Speaker 2: dark matter only feels gravity.

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Speaker 1: That's right.

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Speaker 2: And so the question is what happens to dark matter then?

431
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Speaker 2: Does it bump into itself or does it ignore even itself?

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Speaker 2: That's right, And so let's get into the question of

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Speaker 2: does dark feel itself? But first let's take a quick break.

434
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Speaker 2: Right there, we're talking about DarkMatter and whether it can

484
00:22:49,359 --> 00:22:53,119
Speaker 2: feel itself. And so we know it feels gravity, and

485
00:22:53,160 --> 00:22:55,520
Speaker 2: we know it doesn't feel the other forces. So I

486
00:22:55,520 --> 00:22:58,120
Speaker 2: guess the question is, you know what happens when two

487
00:22:58,160 --> 00:23:01,920
Speaker 2: DarkMatter particles, if they are particles, get attracted to each other.

488
00:23:02,080 --> 00:23:03,920
Speaker 2: What's gonna happen? And are they going to bump into

489
00:23:03,920 --> 00:23:05,840
Speaker 2: each other or just totally ignore each other.

490
00:23:05,920 --> 00:23:08,080
Speaker 1: It's a really fun sort of thought experiment. This is

491
00:23:08,119 --> 00:23:11,120
Speaker 1: exactly what Joe is asking. And so, you know, let's

492
00:23:11,160 --> 00:23:14,280
Speaker 1: assume first that dark matter only feels gravity that have

493
00:23:14,359 --> 00:23:17,320
Speaker 1: no other forces we don't know about that are involved.

494
00:23:17,320 --> 00:23:19,840
Speaker 1: That it's just a gravitational thing, and you know that

495
00:23:20,040 --> 00:23:23,040
Speaker 1: is totally possible. It's possible for a particle to have

496
00:23:23,240 --> 00:23:26,199
Speaker 1: mass and no other interactions. It's sort of incredible, but

497
00:23:26,520 --> 00:23:28,920
Speaker 1: it would be something that we'd never seen before. But hey,

498
00:23:29,200 --> 00:23:30,800
Speaker 1: this physics is full of surprises.

499
00:23:31,119 --> 00:23:34,119
Speaker 2: Well, because we know about neutrinos, right, Neutrinos are also

500
00:23:34,160 --> 00:23:36,320
Speaker 2: sort of selective about which forces it feels.

501
00:23:36,400 --> 00:23:38,359
Speaker 1: Yeah, we don't understand that at all, Like why do

502
00:23:38,480 --> 00:23:41,800
Speaker 1: neutrinos only feel the weak force? You know, why don't

503
00:23:41,800 --> 00:23:44,000
Speaker 1: they feel the strong force? Why don't electrons feel the

504
00:23:44,040 --> 00:23:46,960
Speaker 1: strong force. It's just sort of like a thing we observe.

505
00:23:47,080 --> 00:23:50,080
Speaker 1: We notice these things. All the quarks feel the strong force.

506
00:23:50,440 --> 00:23:52,920
Speaker 1: None of the electrons or neutrinos or muons or twels,

507
00:23:53,040 --> 00:23:56,919
Speaker 1: those particles feel it. Why not We don't know. We

508
00:23:57,040 --> 00:24:00,879
Speaker 1: know that neutrinos don't feel electromagnetism because they don't have

509
00:24:00,920 --> 00:24:03,520
Speaker 1: a charge. Only things with electric charge can feel that.

510
00:24:04,240 --> 00:24:06,719
Speaker 1: But we don't understand this. It's just it's something we observe,

511
00:24:06,800 --> 00:24:09,280
Speaker 1: something that we hope one day to explain from the

512
00:24:09,359 --> 00:24:12,400
Speaker 1: bottoms up principle, but at this point we're pretty clueless.

513
00:24:13,320 --> 00:24:16,159
Speaker 2: All right, So then what happens, Like Joe put it,

514
00:24:16,240 --> 00:24:18,600
Speaker 2: what if I take two particles of dark miier in

515
00:24:18,640 --> 00:24:21,439
Speaker 2: an empty universe, set them a meter apart, and then

516
00:24:21,480 --> 00:24:21,959
Speaker 2: I let go.

517
00:24:22,320 --> 00:24:24,960
Speaker 1: Well, they attract each other, right, gravity pulls them together.

518
00:24:25,400 --> 00:24:27,560
Speaker 1: But then now, imagine what happens when sort of their

519
00:24:27,760 --> 00:24:30,359
Speaker 1: edges meet, right, what happens in one blog versus the

520
00:24:30,400 --> 00:24:33,480
Speaker 1: other blob? Now, if this was normal matter, when their

521
00:24:33,640 --> 00:24:37,040
Speaker 1: edges meet, then they would repel each other. They would bump, literally,

522
00:24:37,200 --> 00:24:39,720
Speaker 1: because they have forces to push against each other with.

523
00:24:40,480 --> 00:24:44,399
Speaker 1: But if all you have is gravity, gravity can't push.

524
00:24:44,840 --> 00:24:49,359
Speaker 1: Gravity only pulls. Right, you have two objects with positive mass,

525
00:24:49,560 --> 00:24:52,320
Speaker 1: they only pull on each other unless you have negative

526
00:24:52,359 --> 00:24:55,080
Speaker 1: mass particles. And we have a whole other podcast episode

527
00:24:55,119 --> 00:24:58,160
Speaker 1: about that. Gravity can only attract. So there's no way

528
00:24:58,160 --> 00:25:01,080
Speaker 1: for gravity to push, which means means these particles just

529
00:25:01,119 --> 00:25:05,160
Speaker 1: get closer and closer. There's no repelling force, and eventually

530
00:25:05,240 --> 00:25:06,399
Speaker 1: they pass through each other.

531
00:25:06,480 --> 00:25:10,080
Speaker 2: Right, because they are I guess point particles.

532
00:25:09,560 --> 00:25:11,680
Speaker 1: Right, Well, we don't know if they're made of particles, right,

533
00:25:11,760 --> 00:25:13,800
Speaker 1: but imagine they are made of particles. So I think

534
00:25:13,800 --> 00:25:16,280
Speaker 1: these arguments hold even if they're made of some weird

535
00:25:16,640 --> 00:25:18,679
Speaker 1: other kind of thing that you know isn't divided up

536
00:25:18,720 --> 00:25:23,400
Speaker 1: into little particles. But they can basically cohabitate the same space. Right.

537
00:25:23,440 --> 00:25:27,240
Speaker 1: The only thing that prevents things from overlapping are the forces. Right.

538
00:25:27,280 --> 00:25:30,160
Speaker 1: The reason that one proton doesn't want to be next

539
00:25:30,160 --> 00:25:33,240
Speaker 1: to another proton isn't because the stuff in them is

540
00:25:33,320 --> 00:25:36,600
Speaker 1: like you're crowding me or something. It's all about the forces.

541
00:25:36,840 --> 00:25:38,720
Speaker 1: As you said, If you imagine that they're point particles,

542
00:25:38,760 --> 00:25:42,040
Speaker 1: they have zero volume. So the only thing that gives

543
00:25:42,040 --> 00:25:45,400
Speaker 1: distance to things are forces that repel, and gravity can't

544
00:25:45,400 --> 00:25:45,679
Speaker 1: do that.

545
00:25:45,760 --> 00:25:45,879
Speaker 8: Well.

546
00:25:45,920 --> 00:25:48,480
Speaker 2: I think what's interesting to think about is that you know,

547
00:25:48,560 --> 00:25:51,280
Speaker 2: they wouldn't share the same space for very long, right,

548
00:25:51,400 --> 00:25:53,600
Speaker 2: because if you set them a meter apart, they're going

549
00:25:53,640 --> 00:25:55,399
Speaker 2: to ask some velocity by the time they get to

550
00:25:55,440 --> 00:25:56,920
Speaker 2: the center, and so they'll just keep going.

551
00:25:57,000 --> 00:25:58,959
Speaker 1: Yeah, they'll pass through each other, but then gravity will

552
00:25:59,000 --> 00:26:01,840
Speaker 1: slow them down, stop them, turn them around, and they'll

553
00:26:01,840 --> 00:26:04,520
Speaker 1: come through each other again. So they like slash back

554
00:26:04,520 --> 00:26:05,520
Speaker 1: and forth forever.

555
00:26:05,440 --> 00:26:09,359
Speaker 2: Right, forever? Right, because there's nothing nothing would be making

556
00:26:09,359 --> 00:26:10,160
Speaker 2: you lose energy.

557
00:26:10,359 --> 00:26:13,720
Speaker 1: Yeah, in your hypothetical universe where there is nothing else,

558
00:26:13,920 --> 00:26:16,439
Speaker 1: then they would slash back and forth forever. If they

559
00:26:16,480 --> 00:26:19,679
Speaker 1: had some other way to lose energy. There's like larger

560
00:26:19,800 --> 00:26:21,959
Speaker 1: blobs of dark matter that are sort of you know,

561
00:26:22,000 --> 00:26:25,000
Speaker 1: equilibrating them or something. Then eventually they would slow down

562
00:26:25,080 --> 00:26:27,280
Speaker 1: and then stop and sit right in the same place.

563
00:26:27,960 --> 00:26:31,000
Speaker 2: But maybe not dark matter. Wait, you're saying in the

564
00:26:31,040 --> 00:26:34,200
Speaker 2: real universe it is possible out there that dark matter

565
00:26:34,320 --> 00:26:34,879
Speaker 2: has settled.

566
00:26:34,960 --> 00:26:37,080
Speaker 1: Yeah, it is possible. In fact, we think that dark

567
00:26:37,080 --> 00:26:39,679
Speaker 1: matter is not moving very much. There's two theories of

568
00:26:39,720 --> 00:26:42,600
Speaker 1: dark matter. There's warm dark matter and cold dark matter.

569
00:26:43,040 --> 00:26:44,880
Speaker 1: And warm dark matter just means that the dark matter

570
00:26:44,920 --> 00:26:48,040
Speaker 1: is sort of zipping around everywhere. It's like has high velocity,

571
00:26:48,359 --> 00:26:51,840
Speaker 1: and cold dark matter means it's mostly sitting around. It's

572
00:26:51,840 --> 00:26:55,080
Speaker 1: like sluggish, chilling and not moving very fast. It's chilling

573
00:26:55,160 --> 00:26:59,920
Speaker 1: matter exactly, called chill dark matter. That's for a statement

574
00:27:00,040 --> 00:27:02,960
Speaker 1: of its feelings. Man. Yeah, so we think that dark

575
00:27:03,000 --> 00:27:05,560
Speaker 1: matter is cold. It's cold dark matter, and so it's

576
00:27:05,600 --> 00:27:07,760
Speaker 1: not moving very much. But yeah, if you imagine, so,

577
00:27:07,920 --> 00:27:11,120
Speaker 1: like a gravitational well constructed by something else, some larger

578
00:27:11,280 --> 00:27:14,159
Speaker 1: object than two pieces of dark matter in that well

579
00:27:14,359 --> 00:27:16,359
Speaker 1: would fall together to the bottom of it and then

580
00:27:16,480 --> 00:27:17,920
Speaker 1: they could just sit on top of each other.

581
00:27:18,320 --> 00:27:20,359
Speaker 2: And I guess the same would happen with dark matter

582
00:27:20,400 --> 00:27:24,320
Speaker 2: and regular matter, right, they would technically, like we only

583
00:27:24,359 --> 00:27:27,280
Speaker 2: interact with gravity with dark matter, and so therefore we

584
00:27:27,520 --> 00:27:29,359
Speaker 2: would just pass through us back and forth.

585
00:27:29,359 --> 00:27:31,159
Speaker 1: Oh that's a good point. Yeah, And I think you're right.

586
00:27:31,200 --> 00:27:33,920
Speaker 1: And I think dark matter is passing through us right now,

587
00:27:34,080 --> 00:27:36,760
Speaker 1: or we're passing through dark matter in the same way

588
00:27:36,760 --> 00:27:39,480
Speaker 1: that like Neutrino's pass through us. But if there literally

589
00:27:39,640 --> 00:27:43,120
Speaker 1: is no interaction other than gravity, then you're right. Dark

590
00:27:43,160 --> 00:27:45,159
Speaker 1: matter can be in you and you can be in

591
00:27:45,280 --> 00:27:45,760
Speaker 1: dark matter.

592
00:27:45,960 --> 00:27:48,680
Speaker 2: Oh man, I feel like now we're getting a serial

593
00:27:48,720 --> 00:27:52,600
Speaker 2: here about dark matter and spiritual It's in you, it's.

594
00:27:52,560 --> 00:27:55,520
Speaker 1: In all of us, and it's not just a hypothetical question.

595
00:27:55,600 --> 00:27:59,959
Speaker 1: We sort of did this experiment on a massive cosmological scale.

596
00:28:00,000 --> 00:28:00,879
Speaker 2: Oh yeah, we did it.

597
00:28:01,000 --> 00:28:02,560
Speaker 1: Oh we didn't sort of do it. I mean, as

598
00:28:02,600 --> 00:28:05,160
Speaker 1: much as astronomers do anything, they just sort of watch

599
00:28:05,280 --> 00:28:06,399
Speaker 1: the universe do stuff.

600
00:28:07,560 --> 00:28:09,200
Speaker 2: You say, astronomers don't do anything.

601
00:28:09,320 --> 00:28:11,800
Speaker 1: Well, they don't construct experiments, right. They don't go like,

602
00:28:12,160 --> 00:28:15,639
Speaker 1: let's build a sun that has these properties and distribute

603
00:28:15,640 --> 00:28:17,479
Speaker 1: these planets around it and let it go right. They

604
00:28:17,520 --> 00:28:19,920
Speaker 1: don't do that except in simulations. But they just look

605
00:28:20,000 --> 00:28:23,720
Speaker 1: around and find that situation. Because the universe is so

606
00:28:23,880 --> 00:28:26,760
Speaker 1: vast and so varied that they can usually find whatever

607
00:28:26,760 --> 00:28:27,440
Speaker 1: they're looking for.

608
00:28:27,520 --> 00:28:28,760
Speaker 2: Their shoppers not doers.

609
00:28:30,240 --> 00:28:33,200
Speaker 1: I love astronomers. Nothing I say should be construed as

610
00:28:33,240 --> 00:28:38,680
Speaker 1: negative towards astronomy or astronomers. But yeah, they're mostly observers, right,

611
00:28:39,160 --> 00:28:43,720
Speaker 1: And what they found was these two clusters of galaxies

612
00:28:44,000 --> 00:28:45,400
Speaker 1: that had slammed into each other.

613
00:28:45,480 --> 00:28:47,120
Speaker 2: Yeah, it's called the bullet cluster.

614
00:28:47,840 --> 00:28:49,959
Speaker 1: It's called the bullet cluster. And this is a collision

615
00:28:49,960 --> 00:28:53,480
Speaker 1: that happened a long long time ago. And by galaxy cluster,

616
00:28:53,560 --> 00:28:56,640
Speaker 1: we just mean literally a group of galaxies, because galaxies

617
00:28:57,000 --> 00:28:59,360
Speaker 1: orbit each other and are sort of grouped together the

618
00:28:59,400 --> 00:29:02,640
Speaker 1: same way like stars or grouped together into a galaxy.

619
00:29:02,880 --> 00:29:06,680
Speaker 1: Galaxies themselves are grouped together into clusters of galaxies, and

620
00:29:06,720 --> 00:29:10,360
Speaker 1: sometimes these things collide. Right. Dark energy is pushing the

621
00:29:10,360 --> 00:29:14,040
Speaker 1: whole universe apart, creating spaces between things, but there are

622
00:29:14,040 --> 00:29:17,200
Speaker 1: still forces and still directions that can pull things together.

623
00:29:17,520 --> 00:29:20,840
Speaker 1: And so gravity pulled these two clusters together, and they

624
00:29:20,920 --> 00:29:23,920
Speaker 1: slammed into each other, and so we basically got to

625
00:29:23,920 --> 00:29:25,080
Speaker 1: see this experiment happen.

626
00:29:25,160 --> 00:29:26,680
Speaker 2: Yeah, you can look it up online if you a

627
00:29:26,760 --> 00:29:30,720
Speaker 2: Google bullet cluster, and they're pretty spectacular images, like imagine

628
00:29:30,720 --> 00:29:34,240
Speaker 2: a cluster of galaxies slamming into another cluster of galaxies.

629
00:29:34,280 --> 00:29:35,960
Speaker 2: It's like a special effects of bonanza.

630
00:29:36,040 --> 00:29:39,800
Speaker 1: It's like a large hadron collider on the biggest scale possible, right,

631
00:29:39,880 --> 00:29:45,080
Speaker 1: the largest, the large galaxy collider. Yeah, And of course

632
00:29:45,280 --> 00:29:47,720
Speaker 1: you can't see it happen in real time. The whole

633
00:29:47,720 --> 00:29:50,479
Speaker 1: collision took, you know, hundreds of millions of years. What

634
00:29:50,480 --> 00:29:52,120
Speaker 1: we're looking at when we see it in the sky

635
00:29:52,640 --> 00:29:55,080
Speaker 1: is just the after effects, and we can reconstruct what

636
00:29:55,120 --> 00:29:57,840
Speaker 1: we think happened from what we're seeing now. And so,

637
00:29:58,280 --> 00:30:00,160
Speaker 1: you know, the clusters are made of sort of three

638
00:30:00,200 --> 00:30:02,680
Speaker 1: basic constituents. You have the stars, of course, are the

639
00:30:02,760 --> 00:30:05,400
Speaker 1: things that are visible in the galaxies. And then between

640
00:30:05,440 --> 00:30:08,200
Speaker 1: the stars you have gas, the things that make the stars.

641
00:30:08,400 --> 00:30:11,040
Speaker 1: And then of course each one has dark matter, these

642
00:30:11,160 --> 00:30:13,880
Speaker 1: huge blobs of dark matter. And I remember that most

643
00:30:13,880 --> 00:30:15,280
Speaker 1: of the things that are out there in the sky,

644
00:30:15,360 --> 00:30:19,480
Speaker 1: the visible galaxies, have these halos of dark matter around them.

645
00:30:19,880 --> 00:30:22,520
Speaker 1: So wherever you see stuff in the sky is probably

646
00:30:22,520 --> 00:30:24,440
Speaker 1: where the dark matter is. They sort of tend to

647
00:30:24,480 --> 00:30:25,120
Speaker 1: follow each other.

648
00:30:25,200 --> 00:30:27,880
Speaker 2: We've never seen, or have we seen dark matter by itself.

649
00:30:28,160 --> 00:30:30,840
Speaker 1: We have noticed some clumps like that's what gravitational lensing

650
00:30:30,960 --> 00:30:34,000
Speaker 1: is so it is distributed, it goes out further past

651
00:30:34,000 --> 00:30:36,320
Speaker 1: the edge of the galaxy. Like each galaxy has a

652
00:30:36,360 --> 00:30:40,000
Speaker 1: halo that's longer and wider and larger than the galaxy itself.

653
00:30:40,240 --> 00:30:42,560
Speaker 1: And we have seen blobs of dark matter in the

654
00:30:42,600 --> 00:30:45,920
Speaker 1: universe by themselves. They do this gravitational lensing, but for

655
00:30:46,000 --> 00:30:48,760
Speaker 1: the most part, the structure they follow each other. And

656
00:30:48,760 --> 00:30:51,600
Speaker 1: that's not an accident. That's because of gravity. The structure

657
00:30:51,640 --> 00:30:54,760
Speaker 1: of dark matter created the structure of normal matter, like

658
00:30:54,880 --> 00:30:58,240
Speaker 1: created these gravitational wells for normal matter to fall into.

659
00:30:58,400 --> 00:31:02,120
Speaker 1: That's why we have galaxies and stars and planets and squirrels.

660
00:31:02,200 --> 00:31:04,560
Speaker 2: It's like which came first, the dark matter or the galaxies,

661
00:31:04,600 --> 00:31:06,040
Speaker 2: And the answer is to dark matter.

662
00:31:06,160 --> 00:31:08,000
Speaker 1: Yeah, well, all the matter was created, i think, at

663
00:31:08,040 --> 00:31:10,160
Speaker 1: the same time, but dark matter there's more of it,

664
00:31:10,440 --> 00:31:13,600
Speaker 1: and so it has a huge influence on the shape

665
00:31:13,640 --> 00:31:16,760
Speaker 1: of the universe. So we saw this collision happen and

666
00:31:16,800 --> 00:31:18,920
Speaker 1: we looked at it, and what we noticed is that

667
00:31:19,400 --> 00:31:22,800
Speaker 1: when these two galaxy clusters passed through each other, the

668
00:31:22,880 --> 00:31:25,680
Speaker 1: gas reacted like you would expect what to happen when

669
00:31:25,880 --> 00:31:28,320
Speaker 1: two huge piles of stuff hit each other, right big

670
00:31:28,360 --> 00:31:31,480
Speaker 1: special effects explosions, lots of friction and radiation at the

671
00:31:31,640 --> 00:31:32,920
Speaker 1: edges and all that kind of stuff.

672
00:31:33,120 --> 00:31:35,080
Speaker 2: Oh, and so you could see the gas not just

673
00:31:35,160 --> 00:31:38,440
Speaker 2: kind of mixing with itself but actually like violently reacting.

674
00:31:38,600 --> 00:31:41,840
Speaker 1: Oh yeah, these things are moving really fast, absolutely.

675
00:31:41,360 --> 00:31:44,000
Speaker 2: Like if you shoot gas really fast at another cloud

676
00:31:44,000 --> 00:31:45,760
Speaker 2: of gas, then things would happen.

677
00:31:45,800 --> 00:31:47,560
Speaker 1: Things will happen. And they slowed down, right, and so

678
00:31:47,600 --> 00:31:50,200
Speaker 1: that energy had to go somewhere, and that energy went

679
00:31:50,280 --> 00:31:52,160
Speaker 1: to heating up the gas and then it radiates and

680
00:31:52,160 --> 00:31:54,840
Speaker 1: you have all this friction between these two colliding clouds

681
00:31:54,840 --> 00:31:58,520
Speaker 1: of gas. But the dark matter in these clusters got

682
00:31:58,560 --> 00:32:01,600
Speaker 1: stripped away. It passed right through. It went right through

683
00:32:01,640 --> 00:32:04,840
Speaker 1: the clouds of gas and right through the clouds of

684
00:32:04,920 --> 00:32:07,960
Speaker 1: dark matter from the other galaxy and passed through to

685
00:32:08,000 --> 00:32:08,960
Speaker 1: the other side.

686
00:32:09,000 --> 00:32:10,920
Speaker 2: Like it went through the windshield. Or you know, I'm

687
00:32:10,920 --> 00:32:13,160
Speaker 2: thinking maybe like in the matrix, remember when they're finding

688
00:32:13,160 --> 00:32:15,640
Speaker 2: those ghosts and they're in the car and they slam

689
00:32:15,760 --> 00:32:17,520
Speaker 2: the brakes and the ghosts just keep going.

690
00:32:17,720 --> 00:32:19,840
Speaker 1: Yeah, kind of like that, or like you're walking down

691
00:32:19,880 --> 00:32:22,720
Speaker 1: the sidewalk with your kid and they notice something interesting

692
00:32:22,760 --> 00:32:24,920
Speaker 1: and they stop and you just on your phone and

693
00:32:24,920 --> 00:32:26,480
Speaker 1: so you don't even pay attention to a block later,

694
00:32:26,480 --> 00:32:30,760
Speaker 1: you're like, wait a second, where's my kid? I mean,

695
00:32:30,840 --> 00:32:31,640
Speaker 1: that's never happened to me.

696
00:32:31,760 --> 00:32:34,080
Speaker 2: Like they're reacting with some gas or is it.

697
00:32:34,040 --> 00:32:36,560
Speaker 1: Running to another kid that you didn't spot? Right, there's

698
00:32:36,560 --> 00:32:37,360
Speaker 1: the analogy phone.

699
00:32:37,360 --> 00:32:39,160
Speaker 2: Oh I see, and the two parents just kept going

700
00:32:39,200 --> 00:32:43,040
Speaker 2: looking at their phone. Ooh boy, we're really modeling responsible parent.

701
00:32:43,080 --> 00:32:45,640
Speaker 1: I know, really talking about our feelings. And so we

702
00:32:45,680 --> 00:32:47,760
Speaker 1: can see the dark matter came out the other side.

703
00:32:47,920 --> 00:32:50,840
Speaker 1: Because we use gravitational lensing, we can see it distorting

704
00:32:50,880 --> 00:32:55,040
Speaker 1: the light from background galaxies. So dark matter passed right

705
00:32:55,040 --> 00:32:58,840
Speaker 1: through normal matter gas, et cetera, collided and so the

706
00:32:58,880 --> 00:33:01,640
Speaker 1: two got separated. So we really can see in that case,

707
00:33:01,680 --> 00:33:03,840
Speaker 1: this is a collide a blob of dark matter with

708
00:33:03,880 --> 00:33:06,600
Speaker 1: another blob of dark matter. What happens, Well, they've passed

709
00:33:06,680 --> 00:33:08,080
Speaker 1: right through nothing.

710
00:33:07,960 --> 00:33:10,600
Speaker 2: Exactly, nothing happens it. It just passes right.

711
00:33:10,520 --> 00:33:12,560
Speaker 1: Through least exciting collision ever.

712
00:33:13,760 --> 00:33:15,840
Speaker 2: Well, I'm sure it's exciting for them to lose their

713
00:33:15,920 --> 00:33:19,200
Speaker 2: kid a few blocks back, all right. So we know

714
00:33:19,440 --> 00:33:23,400
Speaker 2: then for sure through observational experiments that dark matter doesn't

715
00:33:23,440 --> 00:33:25,280
Speaker 2: seem to feel itself at least not in the way

716
00:33:25,280 --> 00:33:27,880
Speaker 2: that we know about or with any forces that we

717
00:33:27,960 --> 00:33:30,560
Speaker 2: know about. And so the question is, is there maybe

718
00:33:31,080 --> 00:33:33,400
Speaker 2: another way that dark matter can feel itself that we

719
00:33:33,600 --> 00:33:35,800
Speaker 2: don't know about, And so let's get into that. But

720
00:33:35,880 --> 00:33:37,560
Speaker 2: first let's take a quick break.

721
00:33:42,080 --> 00:33:43,840
Speaker 1: When you pop a piece of cheese into your mouth,

722
00:33:43,960 --> 00:33:47,120
Speaker 1: or enjoy a rich spoonful of Greek yogurt, you're probably

723
00:33:47,160 --> 00:33:51,200
Speaker 1: not thinking about the environmental impact of each and every bite.

724
00:33:51,240 --> 00:33:53,840
Speaker 1: But the people in the dairy industry are. US Dairy

725
00:33:53,880 --> 00:33:58,200
Speaker 1: has set themselves some ambitious sustainability goals, including being greenhouse

726
00:33:58,200 --> 00:34:00,800
Speaker 1: gas neutral by twenty to fifty That's why they're working

727
00:34:00,800 --> 00:34:03,160
Speaker 1: hard every day to find new ways to reduce waste,

728
00:34:03,240 --> 00:34:07,440
Speaker 1: conserve natural resources, and drive down greenhouse gas emissions. Take water,

729
00:34:07,480 --> 00:34:10,560
Speaker 1: for example, most dairy farms reuse water up to four

730
00:34:10,640 --> 00:34:14,080
Speaker 1: times the same water cools the milk, cleans equipment, washes

731
00:34:14,120 --> 00:34:16,920
Speaker 1: the barn, and irrigates the crops. How is US dairy

732
00:34:16,960 --> 00:34:20,719
Speaker 1: tackling greenhouse gases? Many farms use anaerobic digestors that turn

733
00:34:20,760 --> 00:34:24,680
Speaker 1: the methane from maneuver into renewable energy that can power farms, towns,

734
00:34:24,680 --> 00:34:26,759
Speaker 1: and electric cars. So the next time you grab a

735
00:34:26,800 --> 00:34:28,840
Speaker 1: slice of pizza or lick an ice cream cone. Know

736
00:34:28,880 --> 00:34:31,560
Speaker 1: that dairy farmers and processors around the country are using

737
00:34:31,600 --> 00:34:35,120
Speaker 1: the latest practices and innovations to provide the nutrient dense

738
00:34:35,239 --> 00:34:37,919
Speaker 1: dairy products we love with less of an impact. Visit

739
00:34:38,040 --> 00:34:40,840
Speaker 1: usdairy dot com slash sustainability to learn more.

740
00:34:41,480 --> 00:34:45,040
Speaker 3: Hi, I'm David Eagleman from the podcast Inner Cosmos, which

741
00:34:45,080 --> 00:34:48,320
Speaker 3: recently hit the number one science podcast in America. I

742
00:34:48,400 --> 00:34:52,200
Speaker 3: mean neuroscientists at Stanford, and I've spent my career exploring

743
00:34:52,280 --> 00:34:55,279
Speaker 3: the three pound universe in our heads. We're looking at

744
00:34:55,280 --> 00:34:58,520
Speaker 3: a whole new series of episodes this season to understand

745
00:34:58,640 --> 00:35:01,719
Speaker 3: why and how our lives look the way they do.

746
00:35:02,200 --> 00:35:05,319
Speaker 3: Why does your memory drift so much? Why is it

747
00:35:05,480 --> 00:35:09,040
Speaker 3: so hard to keep a secret, When should you not

748
00:35:09,200 --> 00:35:13,160
Speaker 3: trust your intuition? Why do brains so easily fall for

749
00:35:13,320 --> 00:35:16,400
Speaker 3: magic tricks? And why do they love conspiracy theories.

750
00:35:17,080 --> 00:35:19,680
Speaker 2: I'm hitting these questions and hundreds.

751
00:35:19,200 --> 00:35:22,560
Speaker 3: More because the more we know about what's running under

752
00:35:22,600 --> 00:35:25,560
Speaker 3: the hood, better we can steer our lives.

753
00:35:26,200 --> 00:35:29,320
Speaker 4: Join me weekly to explore the relationship.

754
00:35:28,600 --> 00:35:33,319
Speaker 3: Between your brain and your life by digging into unexpected questions.

755
00:35:33,920 --> 00:35:37,200
Speaker 3: Listen to Inner Cosmos with David Eagleman on the iHeartRadio app,

756
00:35:37,280 --> 00:35:40,200
Speaker 3: Apple Podcasts, or wherever you get your podcasts.

757
00:35:42,000 --> 00:35:45,200
Speaker 10: Parents, are you looking for a screen free, engaging way

758
00:35:45,200 --> 00:35:47,200
Speaker 10: to teach your kids the Bible, one that's easy to

759
00:35:47,280 --> 00:35:51,680
Speaker 10: understand and enjoyable for multiple ages? Kids Bible Stories podcast

760
00:35:51,760 --> 00:35:54,720
Speaker 10: is here to help. I created this for my own children,

761
00:35:54,800 --> 00:35:58,040
Speaker 10: and it's now a favorite among thousands of families. Kids

762
00:35:58,200 --> 00:36:01,600
Speaker 10: love the vivid imagery, scriptures, and sound effects, while parents

763
00:36:01,600 --> 00:36:05,440
Speaker 10: appreciate the apply section for meaningful conversations. We have hundreds

764
00:36:05,440 --> 00:36:08,680
Speaker 10: and hundreds of beautiful episodes that bring the Bible to

765
00:36:08,760 --> 00:36:12,400
Speaker 10: life when you simply press play. It's a sound and

766
00:36:12,560 --> 00:36:16,200
Speaker 10: practical resource that walks alongside you as you teach your kids.

767
00:36:16,840 --> 00:36:19,839
Speaker 10: We want kids to see how incredible God's word is

768
00:36:20,000 --> 00:36:24,320
Speaker 10: in an engaging and memorable way with Kids' Bible Stories Podcast.

769
00:36:25,560 --> 00:36:28,359
Speaker 10: Listen to Kid's Bible Stories Podcast on the iHeartRadio app,

770
00:36:28,440 --> 00:36:31,359
Speaker 10: Apple podcasts, or wherever you get your podcasts.

771
00:36:39,719 --> 00:36:41,680
Speaker 2: All right, Daniel, have you have you found your child yet?

772
00:36:42,719 --> 00:36:44,520
Speaker 1: I've find your child. I'm still looking for mind.

773
00:36:45,280 --> 00:36:48,759
Speaker 2: Wait, this one looks like, oh man, what a galactic

774
00:36:48,880 --> 00:36:49,480
Speaker 2: mishap here?

775
00:36:49,560 --> 00:36:51,400
Speaker 1: Yeah, it's like parent trap galaxy style.

776
00:36:51,440 --> 00:36:53,319
Speaker 2: All right, So we're talking about dark matter and whether

777
00:36:53,400 --> 00:36:55,440
Speaker 2: or not it can feel itself, and we know you

778
00:36:55,480 --> 00:36:58,680
Speaker 2: can feel itself with gravity, but gravity doesn't help you really,

779
00:36:59,000 --> 00:37:02,520
Speaker 2: you know, permanently feel or clump or things like that.

780
00:37:02,600 --> 00:37:04,399
Speaker 2: And so the question is it could there be maybe

781
00:37:04,440 --> 00:37:07,000
Speaker 2: another way that dark matter interacts with itself that we

782
00:37:07,080 --> 00:37:07,640
Speaker 2: don't know about.

783
00:37:07,640 --> 00:37:10,399
Speaker 1: There absolutely could be, I mean, the existence of dark

784
00:37:10,440 --> 00:37:13,120
Speaker 1: matter and the existence of dark energy, and our sheer

785
00:37:13,280 --> 00:37:16,480
Speaker 1: ignorance about how the universe works, how it begins and

786
00:37:16,520 --> 00:37:19,400
Speaker 1: what controls its largest dynamics tells us that there's a

787
00:37:19,440 --> 00:37:22,160
Speaker 1: lot to learn about the universe. And so there certainly

788
00:37:22,200 --> 00:37:25,319
Speaker 1: could be a whole other force out there, some force

789
00:37:25,400 --> 00:37:28,760
Speaker 1: we've never discovered, some force that only dark matter feels

790
00:37:28,800 --> 00:37:31,759
Speaker 1: and feels with other dark matter, and that might be

791
00:37:31,800 --> 00:37:32,600
Speaker 1: the reason we've.

792
00:37:32,440 --> 00:37:34,920
Speaker 2: Never seen it, the dark side of the force.

793
00:37:35,239 --> 00:37:38,840
Speaker 1: Yeah, there's nothing preventing another kind of force from existing.

794
00:37:39,040 --> 00:37:41,560
Speaker 1: It's not like we have any idea as to the

795
00:37:41,680 --> 00:37:44,880
Speaker 1: number of forces that there could be, or any limit

796
00:37:45,000 --> 00:37:46,640
Speaker 1: to how many forces there are. It's just sort of

797
00:37:46,680 --> 00:37:49,640
Speaker 1: like a list of stuff we've seen so far and

798
00:37:49,719 --> 00:37:53,440
Speaker 1: our attempt to understand it. And so that's it. That

799
00:37:53,520 --> 00:37:56,680
Speaker 1: list is almost certainly incomplete, the way our list of

800
00:37:56,800 --> 00:38:01,480
Speaker 1: particles is incomplete, or the lists of b is incomplete. Right,

801
00:38:01,560 --> 00:38:03,799
Speaker 1: Like every year we discovered like a thousand new kinds

802
00:38:03,840 --> 00:38:04,880
Speaker 1: of beetle. It's amazing.

803
00:38:05,160 --> 00:38:12,680
Speaker 2: Yeah, Ringo Paul Bob Robert popping up. That was such

804
00:38:12,719 --> 00:38:13,960
Speaker 2: a bad joke. I'm sorry.

805
00:38:14,400 --> 00:38:16,760
Speaker 1: No, I did learn about a new Beetle last week. Actually,

806
00:38:16,840 --> 00:38:19,560
Speaker 1: I discovered that they brought Ringo in. That they fired

807
00:38:19,600 --> 00:38:21,200
Speaker 1: the original drummer and he's still kind.

808
00:38:21,040 --> 00:38:23,360
Speaker 2: Of out the fifth Beatle. One of the fifth Beatles

809
00:38:23,400 --> 00:38:26,720
Speaker 2: is usually refers to their producer, but yeah, the Ringo

810
00:38:26,800 --> 00:38:27,800
Speaker 2: was not the original drummer.

811
00:38:27,840 --> 00:38:30,640
Speaker 1: Yeah, not a music podcast, So let's stick to our strengths.

812
00:38:30,680 --> 00:38:36,200
Speaker 2: Here's stick to dark matters. Yeah, well this is a

813
00:38:36,200 --> 00:38:39,120
Speaker 2: really interesting idea. I think that the you know, because

814
00:38:39,239 --> 00:38:41,279
Speaker 2: it's it's like totally possible for there to be a

815
00:38:41,280 --> 00:38:43,879
Speaker 2: whole other universe on top of us, right, Like there

816
00:38:43,880 --> 00:38:47,040
Speaker 2: could be a whole set of other particles to feel

817
00:38:47,080 --> 00:38:49,440
Speaker 2: a whole other set of forces, but we just can't

818
00:38:49,520 --> 00:38:51,080
Speaker 2: see them or feel them or touch them.

819
00:38:51,160 --> 00:38:53,640
Speaker 1: Yeah, there's a whole party going on and we weren't invited.

820
00:38:53,680 --> 00:38:55,919
Speaker 1: And even if we had been invited, we couldn't even

821
00:38:55,960 --> 00:38:59,640
Speaker 1: really attend because it just ignores us completely, right, And

822
00:38:59,680 --> 00:39:01,840
Speaker 1: weh that there's a lot going out there that we

823
00:39:01,920 --> 00:39:04,640
Speaker 1: can't see. Right, what we see in the universe is

824
00:39:04,680 --> 00:39:07,879
Speaker 1: a tiny fraction of what's actually happening, even just here

825
00:39:07,920 --> 00:39:11,400
Speaker 1: on Earth with your eyeballs. Your eyeballs see a little

826
00:39:11,480 --> 00:39:15,080
Speaker 1: sliver of all the electromagnetic radiation that's out there. There's

827
00:39:15,080 --> 00:39:17,239
Speaker 1: a lot of stuff going on you can't see. I mean,

828
00:39:17,280 --> 00:39:20,400
Speaker 1: imagine if you could see you know, cell phone signals

829
00:39:20,719 --> 00:39:23,839
Speaker 1: and television signals and radio waves. The world would be

830
00:39:24,000 --> 00:39:26,799
Speaker 1: a much different looking place. And so we see a

831
00:39:26,840 --> 00:39:28,960
Speaker 1: little fraction of what's actually happening.

832
00:39:29,200 --> 00:39:32,560
Speaker 2: Wow, and good thing too, right, like it'd be overwhelmed.

833
00:39:33,719 --> 00:39:36,160
Speaker 2: I mean, you can get Netflix in Hulu for free.

834
00:39:36,480 --> 00:39:40,560
Speaker 1: Good, you know, cloud based delivery would be a different

835
00:39:40,600 --> 00:39:41,440
Speaker 1: kind of thing, all right.

836
00:39:41,480 --> 00:39:43,640
Speaker 2: So there could be another force out there? And is

837
00:39:43,680 --> 00:39:46,719
Speaker 2: it possible that dark matter feels it? Then? When don't

838
00:39:46,719 --> 00:39:49,120
Speaker 2: we see it in these collisions that it was interacting

839
00:39:49,120 --> 00:39:50,359
Speaker 2: with itself in another way?

840
00:39:50,480 --> 00:39:53,080
Speaker 1: Yeah, So there's sort of two different categories of ideas.

841
00:39:53,360 --> 00:39:56,759
Speaker 1: One is, could there be another force that dark matter

842
00:39:56,840 --> 00:39:59,879
Speaker 1: feels and uses to interact with our kind of man.

843
00:40:00,560 --> 00:40:03,080
Speaker 1: Some new kind of force we've never seen before that

844
00:40:03,239 --> 00:40:06,600
Speaker 1: mediates the interaction between those two kinds of particles and

845
00:40:06,640 --> 00:40:09,239
Speaker 1: that we're looking for and we haven't ever seen, and

846
00:40:09,280 --> 00:40:11,280
Speaker 1: we're looking for that in the same way we're looking

847
00:40:11,320 --> 00:40:13,600
Speaker 1: for like its interaction via the weak force. We have

848
00:40:13,680 --> 00:40:17,160
Speaker 1: these big tanks of underground normal matter particles and we

849
00:40:17,200 --> 00:40:19,160
Speaker 1: wait for dark matter to bump into it. It doesn't

850
00:40:19,560 --> 00:40:22,240
Speaker 1: matter if it's the weak force or some other new kind.

851
00:40:22,080 --> 00:40:25,320
Speaker 2: Of fruation Like wouldn't be really hard, for example, detect

852
00:40:25,400 --> 00:40:27,960
Speaker 2: one squirrel that's flying through the Earth.

853
00:40:28,160 --> 00:40:32,120
Speaker 1: Yes, and now you're talking about one squirrel particle and

854
00:40:32,160 --> 00:40:35,480
Speaker 1: it's flying through like a ton of liquid xenon. These

855
00:40:35,520 --> 00:40:39,520
Speaker 1: are very difficult experiments, and they're done underground, like a

856
00:40:39,600 --> 00:40:43,040
Speaker 1: kilometer underground to keep them cold and isolated from all

857
00:40:43,120 --> 00:40:46,480
Speaker 1: the sources of background. They're very hard experiments, and they

858
00:40:46,480 --> 00:40:47,600
Speaker 1: haven't seen anything so far.

859
00:40:47,760 --> 00:40:50,480
Speaker 2: Right, But can you say it conclusively, like there's a

860
00:40:50,520 --> 00:40:53,799
Speaker 2: pink squirrel out there with purple Polko dods, but we

861
00:40:53,880 --> 00:40:56,360
Speaker 2: haven't found it on Earth, so therefore we must not exist.

862
00:40:56,440 --> 00:40:58,319
Speaker 1: You're right, We can't say it does not exist. We

863
00:40:58,320 --> 00:41:02,880
Speaker 1: can say it's less common than one squirrel per cubic

864
00:41:02,960 --> 00:41:06,239
Speaker 1: volume per year. Right, you can just say how likely

865
00:41:06,239 --> 00:41:08,680
Speaker 1: it is to exist at a certain level, and so

866
00:41:09,160 --> 00:41:11,000
Speaker 1: it could still be there, and people are still running

867
00:41:11,040 --> 00:41:13,799
Speaker 1: these experiments, and the longer we run them, the more

868
00:41:13,840 --> 00:41:16,160
Speaker 1: we can push those limits. We can say, if this

869
00:41:16,320 --> 00:41:20,439
Speaker 1: force does exist between standard model matter and dark matter,

870
00:41:20,640 --> 00:41:23,759
Speaker 1: then it must be even weaker than this limit, than

871
00:41:23,800 --> 00:41:25,840
Speaker 1: this limit, than this limit. We keep pushing that down.

872
00:41:26,320 --> 00:41:28,719
Speaker 1: But there's a whole other category, which is like, maybe

873
00:41:28,840 --> 00:41:32,279
Speaker 1: dark matter just feels stuff with itself, not with our experiments,

874
00:41:32,280 --> 00:41:35,480
Speaker 1: not with our bodies, but just between itself, Like, is

875
00:41:35,520 --> 00:41:38,879
Speaker 1: it possible that dark matter bumps into itself and feels something.

876
00:41:38,640 --> 00:41:43,359
Speaker 2: Right or repels each other with Because you were telling

877
00:41:43,400 --> 00:41:46,800
Speaker 2: me that this idea that it only feels gravity is

878
00:41:46,840 --> 00:41:50,960
Speaker 2: sort of on on thin ice, Like maybe there's evidence

879
00:41:50,960 --> 00:41:52,399
Speaker 2: that maybe it does feel something else.

880
00:41:52,480 --> 00:41:54,360
Speaker 1: Yeah, And this is really hard to probe because you

881
00:41:54,400 --> 00:41:59,879
Speaker 1: can't do experiments using normal matter material to probe dark

882
00:42:00,080 --> 00:42:03,040
Speaker 1: matters interactions with itself. You need to build your experiment

883
00:42:03,160 --> 00:42:05,680
Speaker 1: out of dark matter. Right, So all we can do

884
00:42:05,800 --> 00:42:08,160
Speaker 1: is look at how the dark matter is laid out

885
00:42:08,200 --> 00:42:10,640
Speaker 1: in the universe, like where did it end up? And

886
00:42:10,680 --> 00:42:13,440
Speaker 1: then try to see is that consistent with how it

887
00:42:13,480 --> 00:42:16,480
Speaker 1: should end up if it only feels gravity, because if

888
00:42:16,480 --> 00:42:19,239
Speaker 1: it is bumping into itself, that will change the way

889
00:42:19,239 --> 00:42:21,680
Speaker 1: it's laid out, just like if it's hot or cold,

890
00:42:21,719 --> 00:42:24,839
Speaker 1: if it's fast or slow, will also change the distribution

891
00:42:24,920 --> 00:42:27,239
Speaker 1: of dark matter. So we look at the distribution and

892
00:42:27,280 --> 00:42:30,239
Speaker 1: try to ask is it consistent with gravity only or

893
00:42:30,239 --> 00:42:32,080
Speaker 1: do we need another piece of the puzzle?

894
00:42:32,440 --> 00:42:34,200
Speaker 2: And what do we see do we see out there

895
00:42:34,200 --> 00:42:37,400
Speaker 2: in the universe that it maybe does feel another force?

896
00:42:37,560 --> 00:42:40,160
Speaker 1: We see that mostly gets things right sort of on

897
00:42:40,239 --> 00:42:44,239
Speaker 1: the scale of galaxies and larger like the distribution of

898
00:42:44,280 --> 00:42:47,680
Speaker 1: galaxies and how they cluster together and superclusters. That all

899
00:42:47,719 --> 00:42:49,719
Speaker 1: seems right. And if you run a simulation of the

900
00:42:49,800 --> 00:42:52,759
Speaker 1: universe with dark matter normal matter and dark matter is

901
00:42:52,800 --> 00:42:55,440
Speaker 1: only gravity, it mostly gets that right. But then if

902
00:42:55,480 --> 00:42:58,040
Speaker 1: you zoom in, like on the smaller scale here, smaller

903
00:42:58,040 --> 00:43:02,920
Speaker 1: scales like less than a galaxies then sub galaxy, then

904
00:43:03,000 --> 00:43:06,319
Speaker 1: it's not quite right. It doesn't predict things the way

905
00:43:06,360 --> 00:43:09,759
Speaker 1: we see them. For example, we know that we have

906
00:43:09,800 --> 00:43:12,239
Speaker 1: a big blob of dark matter in our galaxy, and

907
00:43:12,280 --> 00:43:14,560
Speaker 1: we know that it's densest at the center just because

908
00:43:14,600 --> 00:43:18,120
Speaker 1: of gravity. But the exact shape of that blob is

909
00:43:18,120 --> 00:43:21,239
Speaker 1: not something that we understand, like dark matter with only

910
00:43:21,280 --> 00:43:23,840
Speaker 1: gravity predicts it sort of like peaked right at the

911
00:43:23,880 --> 00:43:26,080
Speaker 1: center that the center is like a hot spot for

912
00:43:26,160 --> 00:43:28,200
Speaker 1: dark matter. But we go out and measure it, we

913
00:43:28,239 --> 00:43:30,080
Speaker 1: see it's much more spread out than that. It's like

914
00:43:30,360 --> 00:43:32,880
Speaker 1: much more of a rod core, not so much like

915
00:43:32,960 --> 00:43:36,920
Speaker 1: peaked like a cusp. And so that's not something we understand.

916
00:43:37,080 --> 00:43:39,799
Speaker 2: Sort of fuzzier than what you would expect. You would

917
00:43:39,800 --> 00:43:43,560
Speaker 2: expect it to sort of cluster more, but it's actually

918
00:43:43,640 --> 00:43:45,920
Speaker 2: kind of fuzzy. It's like a big fuzzy squirrel, not

919
00:43:46,000 --> 00:43:47,680
Speaker 2: a little tiny intense squirrel.

920
00:43:48,080 --> 00:43:51,719
Speaker 1: Yeah, it's like a hedgehog or something like that, rather

921
00:43:51,719 --> 00:43:54,560
Speaker 1: than a tiny little squirrel. And if you change your

922
00:43:54,560 --> 00:43:57,600
Speaker 1: simulations and you add a little bit of bumping this,

923
00:43:57,800 --> 00:43:59,960
Speaker 1: like a little bit like maybe sometimes dark matter does

924
00:44:00,040 --> 00:44:03,239
Speaker 1: bump into itself, that describes what we see better.

925
00:44:03,080 --> 00:44:05,600
Speaker 2: Meaning like a little bit of a repulsive force with itself.

926
00:44:05,760 --> 00:44:08,640
Speaker 1: M you just add in a made up force. You're like,

927
00:44:08,719 --> 00:44:11,320
Speaker 1: we don't know, but let's see if it describes the situation,

928
00:44:11,840 --> 00:44:14,880
Speaker 1: just like we made up dark matter to describe this

929
00:44:15,000 --> 00:44:17,359
Speaker 1: gravity we were seeing. We're like, all right, well, now

930
00:44:17,440 --> 00:44:20,280
Speaker 1: let's give another force to these particles that are creating

931
00:44:20,280 --> 00:44:23,200
Speaker 1: the gravity, so that the distribution of gravity is the

932
00:44:23,239 --> 00:44:24,080
Speaker 1: way we are seeing it.

933
00:44:24,200 --> 00:44:26,239
Speaker 2: Let's make something up about something that we've made up.

934
00:44:28,080 --> 00:44:29,720
Speaker 1: You're talking about all the physics.

935
00:44:29,719 --> 00:44:33,880
Speaker 2: Now, man, we welcome to science. What is that thing

936
00:44:33,920 --> 00:44:36,160
Speaker 2: we made up for the thing we made up as

937
00:44:36,160 --> 00:44:37,160
Speaker 2: something else we make up?

938
00:44:38,440 --> 00:44:40,200
Speaker 1: Yeah, and so this is sort of the state of

939
00:44:40,200 --> 00:44:42,759
Speaker 1: the art. Now we're like looking really carefully trying to

940
00:44:42,800 --> 00:44:45,960
Speaker 1: understand where is the dark matter? Does it agree with

941
00:44:46,080 --> 00:44:48,920
Speaker 1: what we predict? And it turns out it doesn't quite agree.

942
00:44:49,239 --> 00:44:52,760
Speaker 1: And there are other little details that the gravity only

943
00:44:52,880 --> 00:44:54,560
Speaker 1: theory is getting wrong.

944
00:44:54,719 --> 00:44:57,640
Speaker 2: See, so they're sort of strong maybe or at least

945
00:44:57,640 --> 00:45:01,240
Speaker 2: a hint of an indication that dark matter does feel

946
00:45:01,400 --> 00:45:03,759
Speaker 2: a whole new force or could it be one of

947
00:45:03,800 --> 00:45:04,279
Speaker 2: the old force.

948
00:45:04,320 --> 00:45:05,960
Speaker 1: It couldn't be one of the old forces. It would

949
00:45:06,000 --> 00:45:08,319
Speaker 1: have to be some sort of self interaction. So this

950
00:45:08,560 --> 00:45:12,200
Speaker 1: theory is called self interacting dark matter, and it solves

951
00:45:12,200 --> 00:45:15,040
Speaker 1: some of these nagging problems that we've had in the

952
00:45:15,080 --> 00:45:18,400
Speaker 1: gravity only theory of dark matter. So that makes it compelling.

953
00:45:18,440 --> 00:45:21,239
Speaker 1: It's not proof at all that this is happening, but

954
00:45:21,320 --> 00:45:23,640
Speaker 1: it's like, huh, we've had this problem. We add this

955
00:45:23,680 --> 00:45:27,040
Speaker 1: new idea. It gives a better explanation. That's cool. And

956
00:45:27,200 --> 00:45:29,640
Speaker 1: you always have to be careful when you're adding more

957
00:45:29,680 --> 00:45:33,319
Speaker 1: stuff to your theory, Right, you'd like the simplest possible explanation,

958
00:45:33,800 --> 00:45:35,600
Speaker 1: but if that doesn't work, then you have to add

959
00:45:35,640 --> 00:45:37,160
Speaker 1: a little widget to your theory.

960
00:45:37,440 --> 00:45:39,320
Speaker 2: Like maybe what if there are two kinds of squirrels

961
00:45:39,719 --> 00:45:40,359
Speaker 2: in dark matter?

962
00:45:40,440 --> 00:45:43,120
Speaker 1: Yeah, and that's a whole other idea, right, Like if

963
00:45:43,200 --> 00:45:45,600
Speaker 1: dark matter made of particles, is it one kind of particles?

964
00:45:45,680 --> 00:45:47,960
Speaker 1: Is it two? Or is it ten? Do they decay

965
00:45:48,040 --> 00:45:50,239
Speaker 1: from one kind of particle to the other? Do only

966
00:45:50,320 --> 00:45:52,640
Speaker 1: some of them have self interactions? I mean, look at

967
00:45:52,640 --> 00:45:54,560
Speaker 1: the particles that make up our kind of matter. This

968
00:45:54,640 --> 00:45:57,920
Speaker 1: is all sorts of complicated interactions with weird rules. We

969
00:45:57,920 --> 00:46:01,040
Speaker 1: don't understand. Why would we imagine dark matter would be

970
00:46:01,120 --> 00:46:01,760
Speaker 1: like simpler.

971
00:46:01,800 --> 00:46:02,920
Speaker 2: There could be different shades of.

972
00:46:02,960 --> 00:46:05,960
Speaker 1: Darkness, the darkest matters.

973
00:46:07,239 --> 00:46:08,760
Speaker 2: Squirrels and cats and dogs.

974
00:46:08,800 --> 00:46:11,200
Speaker 1: And then then you wonder like, all right, well, maybe

975
00:46:11,320 --> 00:46:13,920
Speaker 1: dark matter could feel itself and that would explain like

976
00:46:14,120 --> 00:46:16,719
Speaker 1: the weird distribution of gravity that we're seeing. But what

977
00:46:16,880 --> 00:46:20,040
Speaker 1: about the bullet cluster? Right, didn't we do this experiment?

978
00:46:20,160 --> 00:46:23,879
Speaker 2: Yeah, because the two clouds of dark matter didn't repel

979
00:46:23,920 --> 00:46:24,239
Speaker 2: each other.

980
00:46:24,280 --> 00:46:27,520
Speaker 1: They didn't repel each other. But people went back and

981
00:46:27,520 --> 00:46:29,359
Speaker 1: looked at that again and they thought, all right, well,

982
00:46:29,719 --> 00:46:33,480
Speaker 1: can we explain this and have some amount of interaction

983
00:46:33,600 --> 00:46:36,680
Speaker 1: in there, because it's not easy to tell exactly where

984
00:46:36,760 --> 00:46:39,879
Speaker 1: all the dark matter is very precisely, like maybe some

985
00:46:39,920 --> 00:46:42,279
Speaker 1: of it did interact and got sort of stuck in

986
00:46:42,320 --> 00:46:44,840
Speaker 1: the middle. Most of it didn't, but maybe some of

987
00:46:44,880 --> 00:46:45,279
Speaker 1: it did.

988
00:46:46,040 --> 00:46:47,960
Speaker 2: You know, it's like we made it up in the

989
00:46:47,960 --> 00:46:48,439
Speaker 2: first place.

990
00:46:48,480 --> 00:46:50,920
Speaker 1: Why don't we just you know, it's like there's a

991
00:46:50,960 --> 00:46:53,200
Speaker 1: first pass and then there's a more precise test, right,

992
00:46:53,680 --> 00:46:56,000
Speaker 1: you know, first you try to understand the broad strokes.

993
00:46:56,080 --> 00:46:57,719
Speaker 1: Then you really dig in and see if it gets

994
00:46:57,719 --> 00:47:00,200
Speaker 1: the details right. And the other thing is that turns

995
00:47:00,200 --> 00:47:03,000
Speaker 1: out that this is not a great dark matter collider.

996
00:47:03,280 --> 00:47:06,880
Speaker 1: It's too big dark matter. Well, dark matter is very dilute,

997
00:47:07,239 --> 00:47:10,920
Speaker 1: and so if you throw a huge, very dilute cloud

998
00:47:10,960 --> 00:47:13,000
Speaker 1: of dark matter at another one, most of it's just

999
00:47:13,040 --> 00:47:15,560
Speaker 1: going to pass through anyway, Like dark matter is not

1000
00:47:15,640 --> 00:47:18,520
Speaker 1: as dense as these gas clouds. It's sort of like

1001
00:47:19,120 --> 00:47:22,839
Speaker 1: the stars. Like if you have two galaxies that pass

1002
00:47:22,920 --> 00:47:25,960
Speaker 1: through each other, mostly the stars don't collide because the

1003
00:47:25,960 --> 00:47:30,359
Speaker 1: stars are embedded in huge, vast empty spaces, and so

1004
00:47:30,440 --> 00:47:32,759
Speaker 1: they don't really collide with each other head to head.

1005
00:47:32,880 --> 00:47:34,600
Speaker 2: So you were saying, like, if I take two dark

1006
00:47:34,600 --> 00:47:36,600
Speaker 2: matter squirrels and I throw them at each other, they

1007
00:47:36,719 --> 00:47:38,840
Speaker 2: might repel the one against the other. But if I

1008
00:47:38,960 --> 00:47:42,560
Speaker 2: take a whole cloud of squirrels that are really far apart,

1009
00:47:42,640 --> 00:47:45,200
Speaker 2: they might just pass through each other, like we've seen

1010
00:47:45,320 --> 00:47:45,880
Speaker 2: in the cluster.

1011
00:47:46,040 --> 00:47:48,600
Speaker 1: Yeah, and they would mostly miss, just like in the

1012
00:47:48,680 --> 00:47:51,040
Speaker 1: large Hadron collider. What we do is we collide two

1013
00:47:51,600 --> 00:47:55,200
Speaker 1: clouds of protons against each other, because one proton will

1014
00:47:55,200 --> 00:47:57,719
Speaker 1: never hit another proton they're too small. So we have

1015
00:47:57,800 --> 00:48:00,359
Speaker 1: like ten to the ten protons in one cloud ten

1016
00:48:00,400 --> 00:48:02,359
Speaker 1: to the ten to the other, and we get you know,

1017
00:48:02,719 --> 00:48:06,400
Speaker 1: ten or twenty or fifty collisions. And so it's not

1018
00:48:06,520 --> 00:48:10,360
Speaker 1: that easy to make things collide. And so it turns

1019
00:48:10,360 --> 00:48:13,640
Speaker 1: out the Bullet cluster is actually also consistent with dark

1020
00:48:13,680 --> 00:48:15,400
Speaker 1: matter feeling itself at sort of.

1021
00:48:15,400 --> 00:48:17,480
Speaker 2: A low level, you sort of tweak how you look

1022
00:48:17,520 --> 00:48:17,719
Speaker 2: at it.

1023
00:48:17,800 --> 00:48:20,080
Speaker 1: Yeah, and so you know, the game here is come

1024
00:48:20,160 --> 00:48:23,279
Speaker 1: up with a theory a model that explains everything that

1025
00:48:23,320 --> 00:48:26,239
Speaker 1: we do see all the same time. And so you

1026
00:48:26,280 --> 00:48:29,400
Speaker 1: have to add some of this stuff to explain the

1027
00:48:29,760 --> 00:48:32,920
Speaker 1: distribution of dark matter and the core versus cuff effect

1028
00:48:32,960 --> 00:48:34,959
Speaker 1: to the center of the galaxy, but not too much

1029
00:48:35,000 --> 00:48:36,959
Speaker 1: that you mess up the description of the bullet cluster.

1030
00:48:37,080 --> 00:48:39,239
Speaker 1: So you sort of hemmed in on both sides there.

1031
00:48:39,960 --> 00:48:42,560
Speaker 2: Interesting, but I guess maybe the takeaway is that the

1032
00:48:42,640 --> 00:48:46,279
Speaker 2: answer is, yes, maybe dark matter does feel itself. It's

1033
00:48:46,320 --> 00:48:49,239
Speaker 2: totally possible, and there might be some indication out there

1034
00:48:49,320 --> 00:48:50,200
Speaker 2: that it does.

1035
00:48:50,400 --> 00:48:53,319
Speaker 1: Yeah, it's a new idea and it's gaining traction in

1036
00:48:53,360 --> 00:48:57,200
Speaker 1: the community self interacting dark matter. So it's possible that, yes,

1037
00:48:57,400 --> 00:48:59,959
Speaker 1: dark matter could feel itself and there could be multiple forces.

1038
00:49:00,160 --> 00:49:02,520
Speaker 2: Is that it feels all right, So maybe dark matter

1039
00:49:02,560 --> 00:49:04,120
Speaker 2: does have feelings, Daniel.

1040
00:49:04,440 --> 00:49:06,319
Speaker 1: It might have a lot more feelings than we could

1041
00:49:06,360 --> 00:49:09,160
Speaker 1: ever understand. Shades and shades of nuance.

1042
00:49:09,600 --> 00:49:11,640
Speaker 2: Well, I think it's all again. All of this is

1043
00:49:11,680 --> 00:49:14,480
Speaker 2: just an interesting example of how there are things in

1044
00:49:14,480 --> 00:49:18,400
Speaker 2: the universe happening that we don't really know about or

1045
00:49:18,480 --> 00:49:20,160
Speaker 2: see or feel or affect This.

1046
00:49:20,200 --> 00:49:23,120
Speaker 1: We're only seeing a tiny fraction of this grand play

1047
00:49:23,160 --> 00:49:26,640
Speaker 1: of the universe, and we're making huge, broad conclusions about

1048
00:49:26,680 --> 00:49:29,360
Speaker 1: how the universe works from this tiny little corner of

1049
00:49:29,400 --> 00:49:31,719
Speaker 1: the stage. And there's a lot going on that we

1050
00:49:31,800 --> 00:49:35,040
Speaker 1: haven't ever observed. And at least we know now. We

1051
00:49:35,080 --> 00:49:37,800
Speaker 1: know that we're ignorant, so we know to be humble,

1052
00:49:37,840 --> 00:49:41,480
Speaker 1: and we know not to overgeneralize our ideas. But it

1053
00:49:41,520 --> 00:49:44,040
Speaker 1: also is tantalizing because you hope that one day we

1054
00:49:44,080 --> 00:49:45,839
Speaker 1: will see the rest of the action, we will get

1055
00:49:45,880 --> 00:49:47,320
Speaker 1: a glimpse for what's really happening.

1056
00:49:47,760 --> 00:49:51,200
Speaker 2: We might all be swimming in a giant squirrel of mystery.

1057
00:49:53,200 --> 00:49:55,200
Speaker 1: I like to think of myself as a physics raccoon

1058
00:49:56,680 --> 00:49:59,240
Speaker 1: sneak in late at night and steal other people's garbage.

1059
00:49:58,840 --> 00:50:02,520
Speaker 2: With dark circles on your That makes sense, eating garbage,

1060
00:50:02,560 --> 00:50:07,640
Speaker 2: Oh peace. All right, Well that's pretty interesting and cool

1061
00:50:07,680 --> 00:50:11,480
Speaker 2: and or chill, and so we hope you enjoyed that discussion.

1062
00:50:11,560 --> 00:50:13,680
Speaker 1: Thanks Joe for writing with your question, and if you

1063
00:50:13,800 --> 00:50:16,240
Speaker 1: have a question you'd like us to explore on the podcast,

1064
00:50:16,480 --> 00:50:20,000
Speaker 1: please write to us. We love your questions to questions

1065
00:50:20,040 --> 00:50:21,919
Speaker 1: at Danielanjorge dot com.

1066
00:50:22,000 --> 00:50:24,399
Speaker 2: A bit mysterious See you next time.

1067
00:50:32,600 --> 00:50:35,400
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain

1068
00:50:35,480 --> 00:50:39,480
Speaker 1: the Universe is a production of iHeartRadio. For more podcasts

1069
00:50:39,480 --> 00:50:43,360
Speaker 1: from iHeart Radio, visit the I Heart Radio app, Apple Podcasts,

1070
00:50:43,560 --> 00:50:54,640
Speaker 1: or wherever you listen to your favorite shows. When you

1071
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Speaker 1: pop a piece of cheese into your mouth, you're probably

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1080
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Speaker 3: Hi, I'm David Eagleman from the podcast Inner Cosmos, which

1081
00:51:28,200 --> 00:51:30,960
Speaker 3: recently hit the number one science podcast in America.

1082
00:51:31,080 --> 00:51:32,440
Speaker 4: I mean neuroscientists at.

1083
00:51:32,440 --> 00:51:35,800
Speaker 3: Stanford and I've spent my career exploring the three pound

1084
00:51:35,920 --> 00:51:37,200
Speaker 3: universe in our heads.

1085
00:51:37,520 --> 00:51:40,880
Speaker 4: Join me weekly to explore the relationship between your brain

1086
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Speaker 4: and your life.

1087
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Speaker 3: Because the more we know about what's running under the

1088
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Speaker 3: hood that or we can steer our lives. Listen to

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Speaker 3: Inner Cosmos with Savid Eagleman on the iHeartRadio app, Apple podcasts,

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00:51:52,160 --> 00:51:53,879
Speaker 3: or wherever you get your podcasts.

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Speaker 10: Parents looking for a screen free, fun and engaging way

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00:51:57,880 --> 00:52:00,160
Speaker 10: to teach your kids the Bible. As a mom, I

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00:52:00,200 --> 00:52:03,160
Speaker 10: was looking for the same thing, so I created Kids'

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Speaker 10: Bible Stories podcast. Thousands of families are raving about it,

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00:52:06,600 --> 00:52:10,960
Speaker 10: and kids actually request to listen. With captivating sound effects, voices,

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00:52:11,000 --> 00:52:13,960
Speaker 10: and an apply section at the end to spark meaningful conversations,

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00:52:14,239 --> 00:52:17,160
Speaker 10: it's a hit with both kids and parents. Listen to

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Speaker 10: Kids Bible storye podcast on the iHeartRadio app, Apple podcasts,

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Speaker 10: or wherever you get your podcasts,