0:00

God, I'm so old. Do you ever

0:02

see yourself in a Zoom and just

0:04

be absolutely shocked? Sometimes.

0:07

But you

0:09

can do the touch up my appearance thing

0:11

on Zoom. Can I do a baby filter

0:13

that turns me into a nine-year-old? No,

0:16

no. But it'll smooth you, so

0:18

you'll be an inhumanly smooth

0:21

person. That's always an option. It

0:23

just goes in one direction. Time.

0:28

Well, or maybe it doesn't actually come to think

0:30

of it. I don't know. If I've learned anything

0:32

in the last few weeks, it's that I have

0:34

no idea if time goes in one direction. If

0:37

you could see yourself from far enough away, you

0:39

could see yourself as a baby. Right.

0:43

Yeah. You would have to set

0:45

up the mirror a long time ago. Yeah.

0:49

But, you know, in principle. Welcome

0:56

back to the universe. So far, Katie

0:58

and I have been talking about the

1:00

astonishing amount of information that we have

1:02

been able to learn about our universe.

1:04

But in this episode, we're going to

1:07

focus on a mystery of our cosmos,

1:09

black holes. On

1:11

one hand, black holes are a

1:13

valuable tool. Their distinct properties or

1:16

lack thereof help us to map

1:18

the universe. But there are

1:20

also things about them that by their

1:22

nature we are unable to learn about.

1:24

So join me as I attempt to

1:26

wrap my head around these space-time objects that

1:28

are infinitely compelling, literally,

1:31

and yet also ultimately

1:34

unknowable. Here's our conversation.

1:41

So today we are going to

1:43

pause from our cosmic timeline, where

1:46

we've been going through the first seconds

1:49

and then millions of years of the

1:51

universe. And we're going to

1:53

take a quick gander at something

1:56

I have genuinely, deeply

1:59

known. understanding of,

2:02

which is black holes. I

2:04

mean, you are painting onto a blank canvas

2:06

right now, Dr. Mack. OK.

2:10

All right. So black holes are one

2:12

of these topics that, like, everybody

2:15

is just constantly fascinated by black

2:17

holes because they are genuinely one

2:19

of the weirdest objects

2:22

that we know exists in the universe. And

2:24

they're everywhere. So there are lots of them?

2:27

That's the first thing I didn't know. I don't think I

2:29

knew that there are lots of them. There are lots of

2:32

them. We know there are a lot of

2:34

black holes in our galaxy. There are somewhere

2:36

around 50 that we can identify

2:38

and point at and say we know that there is

2:40

a black hole right there. But

2:43

the estimate is that there's probably closer

2:45

to, like, 100 million in

2:47

our galaxy. 100 million? Yeah.

2:50

Wow. OK. The way

2:52

it works, some stars become black

2:54

holes when they die. It's

2:56

a process that happens often enough that

2:59

black holes just kind of are

3:01

everywhere in the universe. And

3:03

I'll talk about how they happen. But

3:05

we know that there are a lot

3:07

in our galaxy. We know that there's

3:09

a supermassive one in the center of

3:11

our galaxy. And we know that there

3:13

are supermassive black holes in the centers

3:15

of other galaxies. Pretty much every large

3:17

galaxy appears to have a supermassive

3:20

black hole in the center. So they

3:22

really are completely ubiquitous objects

3:24

in the universe. They exist all over

3:26

the place. And that's a fascinating thing

3:29

because when you get into the details

3:31

of how they work and what they

3:33

are and what they represent, they are

3:36

just an incredibly bizarre kind of thing

3:38

to be in the universe. They

3:41

really are one of the places

3:43

where our intuitive understanding of physics

3:45

and even our detailed understanding of

3:47

physics gets very, very challenged. There

3:49

are things about black holes we

3:51

still don't understand, especially in the

3:53

interiors. There are aspects of

3:55

black holes that we may never have

3:57

information about because of the universe. the

4:00

way that they sort of limit our

4:02

ability to understand them. They are truly

4:04

awesome. And to an

4:06

astrophysicist, they're incredibly useful, which sounds

4:10

strange, but that's partially

4:12

because in some ways

4:14

they are some of the brightest things

4:16

in the universe. And we'll talk more

4:19

about that when we talk about the

4:21

supermassive black holes, because supermassive black holes

4:23

tend to be some

4:25

of the brightest things that we can see in

4:28

the cosmos, and they're some of the things that

4:30

help us to map the cosmos. Now, that is

4:32

surprising from the perspective of somebody who's interested in

4:34

how these things get named. So okay, let's just

4:36

back up. So that's kind of the preview of

4:39

why astronomers are excited about black holes, but let's

4:41

go back up and like, let's talk about what

4:43

a black hole is. That would be super helpful

4:45

for me because I don't even know whether to

4:47

be anxious about all these black holes until you

4:49

tell me what they are. Okay, it may not

4:52

help when I tell you what

4:54

they are. Sure, I don't expect it to

4:56

help. I expect it to get worse, but

4:58

I'm still excited. Okay, okay, great. So a

5:00

black hole is something that

5:02

happens when a really

5:04

massive star dies. So I'll

5:06

tell you about how they form first, because that's kind of important.

5:09

Stars go through a life cycle, and

5:12

the life cycle the star goes through depends

5:14

on how much mass it starts with. The

5:16

first thing that happens is a

5:19

bunch of gas gets together, and a protostar,

5:21

it forms a star. A

5:23

star is technically born when nuclear fusion starts

5:25

happening in the center, it lights up, right?

5:27

That's the birth of the star. And stars

5:29

can start out at a lot of different

5:32

kinds of masses. The lowest mass stars

5:35

start at about sort of

5:37

8% the mass of the sun,

5:40

something like that, 0.08 times

5:42

the solar mass. That's the kind of

5:44

lowest mass star that can still be

5:46

burning hydrogen into helium in its core.

5:49

And those really low mass stars, about 0.08

5:52

to about 0.4 solar masses, those ones

5:55

just like, they burn hydrogen into helium in

5:57

their core, they kind of burn slow they

5:59

don't have a whole lot of mass to

6:01

create a bunch of pressure in the core,

6:03

so they kind of just glow for billions

6:05

of years. So those red

6:07

dwarf stars can just continue burning slowly

6:09

for billions of years. Those

6:13

are long-lived stars that don't burn very hot,

6:15

they don't do anything spectacular when they die,

6:17

they just kind of burn out eventually, but

6:19

over a very, very, very long time. Now,

6:22

before you go any further, I just want

6:24

to say that my great ambition as a

6:26

person is to

6:28

have a life that is as much

6:30

like one of those small stars as

6:32

possible. Yeah, yeah. Long, not too much

6:34

drama. Peaceful. Yeah, exactly. I

6:36

don't want to have a ton of nuclear

6:38

explosions. I want to have just a steady

6:40

diet of them. Yes,

6:44

yeah, just like a nice kind of

6:47

constant burning, but not sort

6:49

of inflamed. Yes. Yeah,

6:51

that sounds great. Right. When

6:54

you start to get a little bit bigger,

6:56

that's when things start to get more dramatic.

6:59

So our star, the sun, is kind of

7:01

an intermediate-mass star. So this is a

7:04

range from about 0.4 solar

7:06

masses to about 8. These are the

7:08

stars like our sun, where it's burning hydrogen into

7:10

helium in the core, and it's

7:12

going to keep doing that for billions of years, but

7:15

at some point, it's going to run out of hydrogen

7:17

in the core to burn. And there's going

7:19

to be a few processes that

7:21

go on. It's going to burn a little

7:23

helium for a bit. As it's getting toward

7:25

the end of its hydrogen-burning life, it's going

7:28

to be expanding and turning red. So

7:31

eventually, our sun will turn into a red

7:33

giant star. So over the course of about

7:35

a billion years, it'll get brighter and brighter,

7:37

and it'll become bright enough to burn off

7:39

the oceans of the Earth. So that's going to be it for

7:41

us. Well, the

7:43

Earth part of us. The Earth part of

7:46

us. We'll be on so

7:48

many planets by then, Dr. Mack.

7:50

Sure. I mean, we've got a

7:53

billion years to figure it out, right? Like,

7:55

we'll sort something out. We'll find some other

7:58

possibility. If we make it to the end of the

8:00

Earth, the ocean's boiling, I'm just going to say it.

8:02

It'll be a dang miracle. That's true.

8:04

That's true. Yeah, that'll be a good run. I

8:07

like our odds of making it

8:09

through the next billion

8:11

years at about zero.

8:13

Like that's, I would put it at

8:15

about zero. Yeah, I don't

8:17

blame you there. I think that

8:19

would be impressive for sure. I

8:23

think it'd be great if we're in the first quarter of

8:25

human history. I don't think we're in the first 1%, but

8:28

I've been wrong before. And one thing

8:30

about me is I will

8:32

not be around to find out. That's true.

8:34

Okay, so these are the stars that are

8:36

about the size of our sun, that

8:39

include our sun. Yes, yeah. So in

8:41

the future, you know, in

8:43

a few billion years, the sun will turn into a

8:45

red giant star. And at that

8:47

point, it'll go through changes in the

8:49

core. It'll sort of blow off the

8:51

outer layers of the star. And that'll

8:53

be really cool because that'll create this

8:55

like big nebula. We see a lot

8:57

of these in the sky. They're called

8:59

planetary nebula. They don't really have anything

9:02

to do with planets, but it's an

9:04

historical term. Anyway, so there'll be this

9:06

big colorful nebula created from the sloughed

9:08

off outer layers of the star. And

9:10

the core of the star will collapse

9:12

into an object we call a white

9:14

dwarf. So a white dwarf is

9:17

the super dense core of a star.

9:19

And what happens is that when you stop

9:22

having nuclear reactions

9:24

to kind of puff out the gas, then

9:27

that gas can collapse on itself. Right now,

9:29

you know, the sun is kind of held

9:31

up against its own gravity by the pressure

9:34

from these nuclear reactions in the

9:36

center. So there's this balance between

9:39

the outward push of those nuclear

9:41

reactions and the inward push

9:43

of the gravity of just all of the

9:45

stuff trying to fall together and collapse together

9:47

under its own gravity. And

9:50

so when you get to the point where you

9:52

stop being able to have those nuclear reactions holding

9:54

everything up, it can collapse and it can get

9:56

really, really dense. So there's a

9:58

kind of maximum density of... regular matter, where

10:00

you can put a whole lot of regular

10:02

matter together and make it really really dense.

10:04

And you know, we have materials that are

10:07

super dense, you know, really heavy metals can

10:09

be very very dense. But there's a point

10:11

where even that can't hold

10:13

up the matter anymore, where the electromagnetic

10:15

forces that hold up atoms and molecules

10:17

can't hold up that matter anymore, and

10:20

it gets condensed even more. That creates

10:22

what's called a white dwarf, where it's

10:24

called degenerate matter, electron degenerate matter, where

10:27

you kind of push everything together in

10:29

this way that's sort of denser than

10:31

ordinary matter. It's called degenerate because it

10:34

has to do with how the electrons

10:36

are kind of in each other's energy

10:38

levels in this weird way. It's

10:41

a different kind of matter that's just super super

10:43

dense. So a white dwarf

10:45

can be about the mass

10:47

of the sun in about

10:49

the volume of the earth. Oh. So

10:52

it's condensing a whole lot of matter

10:54

into a very small space. Yeah. And

10:57

creating this weird form of matter, this

10:59

electron degenerate matter, where you've got kind

11:01

of these the protons and neutrons and

11:03

electrons kind of in this sort of

11:05

strange space. And

11:08

just to do a quick thought experiment, if I

11:10

were to visit a white dwarf, I

11:13

assume it would be a big

11:15

problem for me because the gravity

11:17

would be really intense, even though

11:20

it would be an earth-sized object

11:22

potentially. It would not be

11:24

an earth-like experience for me as a

11:26

visitor. You would definitely be

11:28

crushed. You would

11:30

be sort of squished onto the surface of

11:33

a white dwarf. It would

11:35

not be a pleasant thing to be because it would

11:37

just be this super super dense object that you're very

11:39

very close to. Okay. In general,

11:42

the more mass in the smaller amount

11:44

of space, the worse the gravity is.

11:47

Those words I did not put together in the right way. But

11:50

I know what you mean. The more

11:52

extreme the gravity is because there's

11:54

still all the same mass in

11:57

a small space. In a small space. Okay. Yeah,

12:00

so it's this compactness that's the issue.

12:02

So a white dwarf is much more

12:05

compact than a star. It's

12:07

a way that you can pack in

12:10

a whole lot of matter into a small

12:12

space by kind of messing with how the

12:14

electrons interact with each other so that they're

12:16

not kind of held up in these sort

12:18

of levels the way that regular matter

12:20

is. Now, there are some stars that are

12:23

more massive than that that

12:26

have a different cycle that they go through. So

12:28

a higher mass star, more than about eight times

12:30

the mass of the sun. So when it goes

12:32

through its life cycle, it

12:34

starts out doing the same thing. It sort of

12:36

burns hydrogen in the core. But when

12:38

it finishes up all the hydrogen, it can

12:41

start to burn higher elements,

12:43

heavier elements too. So instead of just like

12:45

giving up and collapsing when it finishes its

12:47

hydrogen, it has, there's enough matter pushing everything

12:49

in. There's enough pressure and temperature in the

12:52

core that it can burn heavier elements. So

12:54

it can go through like carbon and nitrogen

12:56

and oxygen, and it can create these kinds

12:58

of shells of burning of different elements as

13:01

it goes through its life cycle. So these

13:03

really massive stars, they can burn heavier

13:05

elements. And this is part of how we

13:07

get a lot of these heavier

13:09

elements in the universe is through

13:11

burning inside really massive stars, also

13:14

through the death of stars like our

13:16

sun. When our sun sort of blows off

13:19

its outer layers, it creates some heavier elements

13:21

in that whole process too. But higher mass

13:23

stars are doing this kind of interior to

13:26

themselves. But when a

13:28

higher mass star gets to a certain point,

13:30

it's burning through heavier and higher heavier elements.

13:32

When it gets to iron, it can't

13:35

burn iron into a heavier element. There's

13:37

this thing that happens in nuclear physics

13:40

that's I guess it's a little complicated

13:42

to explain. If you put together light

13:44

elements into heavier ones, that

13:46

creates energy up to a

13:48

point. So all the elements lower than iron

13:51

if you put the lighter elements together to

13:53

make heavier elements that creates energy. But on

13:55

the heavier side on the higher side, if

13:57

you try to put together things heavier iron,

13:59

it would take energy to do that. And

14:02

so on the higher side, if you split

14:04

the nucleus apart, that's what creates energy. This

14:06

is why you can create

14:08

a bomb out of either splitting

14:11

uranium or plutonium or whatever,

14:13

splitting the big heavy elements,

14:15

or by using hydrogen and

14:17

fusing it into helium. So

14:20

those are two different kinds of big bombs that you can

14:22

do, because on the low end, fusion,

14:25

putting elements together creates energy. On the

14:27

high end, fission, pulling elements apart creates

14:29

energy. So what that means is that

14:31

when you get to iron, when you're

14:33

burning up all these heavy elements and

14:35

you get to iron, you can't fuse

14:37

beyond iron. You can't just kind of

14:39

push things together and create energy beyond

14:41

iron. So when you get to iron,

14:43

you're not creating any more energy when

14:45

you're trying to push those elements together

14:47

anymore. You're not creating new

14:50

pressure to hold up the star. And

14:52

so at that point, the star can't hold itself up

14:54

anymore, and that's when it starts to collapse. So

14:57

for these really massive stars, they start to

14:59

collapse once they get to the iron burning

15:01

stage. And the

15:03

way that collapse happens, it's a

15:06

really massive thing. The collapse is

15:08

more violent. It implodes, and then

15:10

it explodes spectacularly. There's

15:12

like a bounce off the core, and

15:14

this creates a supernova. So it's only

15:16

these high mass stars that can do

15:18

that supernova as the end of their

15:21

life. There's another kind of

15:23

supernova that can happen that involves white dwarfs

15:25

sort of gathering more mass

15:27

from neighbor stars, and that can create a

15:29

supernova too. But that's like a different process.

15:31

That's not an end of life supernova. That's

15:34

something else that can happen that can destroy

15:36

white dwarf stars. But

15:38

in terms of the end stage of a

15:40

star, it's only the high mass ones that

15:42

die by blowing up as a

15:44

supernova. And a supernova is just an explosion of

15:46

the star that creates a really bright

15:49

explosion. It outshines its own galaxy for

15:51

a short time. It can

15:53

be seen billions of light years away. It's

15:56

a spectacular thing. Now our star is not going

15:58

to do that. that, when

16:00

you say for a short period of time, sometimes

16:02

that means like a picosecond and sometimes that means

16:04

like two million years. It's days.

16:07

It's like several days. Okay, so you've

16:09

got to catch a supernova in a relatively

16:11

short frame of time to be able to

16:13

enjoy that beautiful explosion.

16:15

Yeah, yeah, that's right. Okay, and so

16:18

just to make sure I've got it,

16:21

stars that are larger than eight times the

16:23

size of our sun, which is a fairly

16:25

large number of stars. Yeah, I

16:27

mean, most stars are low mass. Most

16:29

stars are lower mass than our sun,

16:31

but some stars are heavier. So there's

16:33

a range, you know, and they can be much

16:36

more massive than our sun. So there's

16:38

a range that's kind of weighted toward the low

16:40

end, but there's quite a lot of stars that

16:42

are more massive than our sun, yeah. So, and

16:44

when those stars die, they

16:47

kind of run out of the elements that they can burn.

16:49

They get to iron. They can't burn

16:51

it. And then there's this massive implosion

16:53

followed by a massive explosion that

16:55

becomes the brightest thing in that galaxy for a

16:57

few days. Yeah, yeah,

16:59

exactly. Okay, all

17:02

right. I mean, so far it seems fine

17:04

because this seems like other galaxies' problems. Yeah,

17:09

although, I mean, we're a little overdue for one in

17:11

our galaxy. We haven't had

17:13

one in a while and we're kind

17:15

of crossing our fingers that because it would

17:17

be, I mean... Whoa, whoa, whoa, whoa, whoa,

17:20

wait, are we crossing our fingers in hopes

17:22

that it does happen or it doesn't happen?

17:25

Yeah, because we would learn a lot.

17:27

Oh, okay. And we wouldn't die

17:29

in the process. We wouldn't, no. Great.

17:32

So to give you some comfort, there

17:34

are no stars within the like lethal

17:37

range of us that we think could

17:39

go supernova at any

17:42

time in the next, like, I don't know, some

17:45

ridiculously long number of years. Great. So

17:54

you know what that means? As a planet

17:56

anyway, we're going to be here for a while.

17:59

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18:53

because there are no nearby

18:55

stars about to go supernova.

19:04

So when that supernova happens, the

19:07

remnant, the core of

19:09

that star that did the

19:12

implosion, it can go a couple of

19:14

different ways. So it

19:16

can either become a neutron star or

19:18

a black hole. And I'm going to say a little

19:20

bit about neutron stars first because

19:22

neutron stars are these really

19:25

amazing objects. So if

19:27

the star is less than something

19:29

like 20 times the mass of the sun but more

19:31

than 8, then when that

19:34

supernova happens and the core collapses, it's

19:37

too massive to be held up by

19:39

even electron degeneracy pressure, even this white

19:41

dwarf thing. It's too massive to be

19:43

a white dwarf. It compresses even further.

19:46

And the thing that holds it up when it

19:48

compresses even further is called neutron

19:50

degeneracy pressure. And basically what

19:53

happens there is the whole star, the

19:55

whole core part of the star

19:57

becomes like a giant nucleus. So

20:00

it smashes all of the protons

20:02

and neutrons together. It

20:04

turns most of the protons into neutrons. You

20:06

have this giant kind

20:08

of nucleus of matter. So it's

20:11

as dense as an atomic nucleus,

20:13

but it's about, it's like the mass

20:15

of the sun or a little bit more. So

20:18

it's this super massive nucleus

20:20

basically. So it's held up

20:23

basically by the fact

20:25

that you can't have two particles in

20:27

the same place at the same time,

20:29

just sort of quantum mechanically. There's this

20:31

weird thing that it's

20:34

just about trying to

20:36

keep the particle existing that holds it

20:38

up. So it's this very, very extreme

20:41

form of matter. And we don't understand

20:43

all of the details of what that

20:45

matter is. There's all sorts of theories

20:47

about what's going on inside. It might

20:50

be like a superfluid inside, could have

20:52

these weird vortices, could

20:54

have a kind of strange structure that's like

20:56

a sort of lasagna like structure. There are

20:58

all these models with like,

21:00

they call it nuclear pasta. Like

21:03

what the configuration of stuff inside the

21:05

neutron star is. It's a really weird

21:07

kind of stuff, but it's so dense

21:09

that you can take the mass of

21:11

the sun. And if you turn that

21:13

into a neutron star, it's now the

21:15

size of a city. Wow. It's like

21:17

a couple of kilometers or like, like,

21:19

like 10 kilometers or something. Wow. That

21:22

is extraordinarily dense matter, extraordinarily

21:25

compact. Wow. And one

21:27

cool thing about neutron stars is a lot of

21:29

times when they're formed, when they're

21:31

compressed that way, the magnetic fields of

21:34

the star are kind of compressed and

21:36

twisted around and the star is born

21:38

like spinning really rapidly. And so you

21:40

can end up with this like strong

21:42

magnetic field in the neutron star and

21:44

it's spinning like a little magnet. And

21:47

it can create these jets of radiation

21:49

from its poles, from its magnetic poles

21:52

that like throw out radiation like

21:54

positrons and gamma rays and that

21:57

that jet of radiation can like

21:59

spin. around because the magnetic

22:01

pole and the rotation pole might not be

22:03

quite lined up, so it's a

22:06

little bit off-kilter, and that means

22:08

that that jet of radiation can

22:10

sweep through the universe like a

22:12

lighthouse beam. And

22:16

if we are lined up with

22:18

one correctly, then we see this

22:20

pulsing beam of radiation coming

22:23

to us every time the neutron star spins. They

22:26

can spin at periods of milliseconds. So

22:28

it depends on the stage of the

22:30

neutron star that it's at, but it

22:32

can be these millisecond pulses. These

22:35

are called pulsars, these kinds of stars.

22:37

And we can use them kind of

22:39

like clocks because they have a very

22:41

regular pattern, because they're just rotating, and

22:43

every time they go through a rotation,

22:45

they flash because of this beam

22:48

of light. And so you can

22:50

use them kind of like lighthouses, like

22:52

clocks, and you can use that to

22:54

kind of map out a lot of

22:56

things about the galaxy by seeing these

22:58

across the galaxy, these dozens of neutron

23:00

stars, these pulsars. And I'll

23:02

talk later about how we can use pulsars

23:04

to learn about supermassive black holes. There's

23:08

a kind of cool connection by,

23:10

you know, we use these really

23:13

super extreme objects as sort

23:15

of markers throughout the universe because we can

23:17

see them really well, and they have these

23:19

very distinct properties. Wow. So that's

23:21

a neutron star. So that's what happens when

23:23

the initial star is somewhere around 20 or

23:26

fewer solar masses. If

23:29

it's bigger than that, if the star starts

23:31

out more massive than that, and I don't

23:33

know exactly where all the boundaries are on

23:35

these things, but if it's a little bit

23:37

more massive than that, then when that collapse

23:40

happens at the supernova stage, you can get

23:42

a situation where there's so much matter that

23:45

even that extreme neutron degeneracy

23:47

pressure cannot hold it up.

23:50

So even by turning the whole thing into

23:52

a nucleus, it's still,

23:54

there's too much gravitational force.

23:57

It compresses it more than that.

23:59

And then there's no force known

24:01

to nature that can hold it up. There's

24:03

nothing at all that we know

24:06

of that could provide an outward push

24:08

that can counter that much gravity. So

24:10

you get this runaway, just

24:12

runaway collapse. You get this

24:15

sort of cycle of collapse. Yeah,

24:17

so there's just nothing that can

24:19

stop it. And so all that

24:21

matter that would have formed

24:23

the core of that star just

24:26

keeps going inward, keeps

24:28

compressing and compressing and compressing.

24:31

Like infinitely? Well,

24:34

yeah, because there's nothing to stop

24:36

it, right? So this is the

24:38

theory and this is where things

24:40

get fuzzy because at some point

24:42

we cannot learn more about that kind

24:44

of matter, about that process.

24:47

Because based on the

24:49

theory, if there's nothing to

24:51

stop that collapse, it'll just,

24:53

it'll come to a point in the center,

24:56

the center of that gravity, and

24:58

it'll just become an infinitely dense point. We call

25:00

that a singularity. Mm-hmm. Sorry,

25:03

I got really anxious there. It's

25:06

okay. There's a lot of weird

25:08

stuff about black holes. So how

25:11

come they don't suck in

25:13

everything? Well, because not everything

25:15

is already falling toward the black

25:18

hole. Oh, okay. So it's only

25:20

things that are already falling toward the black hole.

25:22

It doesn't like expand

25:24

infinitely. Yeah, it

25:26

can't like reach out to distant things. But

25:28

if you get, if I got near it. If you get

25:30

near it is a problem. If I got in a spaceship

25:32

and got close to it, that would be a big problem.

25:35

Yeah. The way that gravity

25:37

works, like let's say that you're

25:39

in like, I don't know,

25:41

like a giant cloud of gas or something.

25:43

And it's, there's a whole lot of density

25:45

at the center, but you're kind of toward

25:47

the outer edges. You're going to

25:49

feel the gravity of everything within, like

25:52

closer to the center than you. Like

25:54

you can draw like a sphere and you're at the

25:56

edge of that sphere and everything

25:58

close to the center. You're going to feel. the gravity of

26:00

that. If you get closer and closer

26:02

to the middle, you're just going to feel the

26:05

gravity of the stuff interior to you. And so

26:07

if you get really far away from that

26:10

gas cloud, you're going to feel as much

26:12

gravity as if all of that whole gas

26:14

cloud was compressed to a single point in the

26:16

center. The amount of gravity that you'll feel is the

26:18

same because you're still just feeling all of the gravity

26:20

of the stuff interior to you. Okay,

26:23

okay. So what matters is how

26:25

close you are to the center

26:29

of where the matter is

26:31

concentrated. So on the

26:33

Earth, we're not that close to the

26:35

center of the Earth. We're feeling

26:37

the gravity in a sense of all of

26:39

the matter interior to us, so the whole

26:41

planet. We're feeling it

26:43

the same as though all of that mass

26:46

were concentrated at the center. The

26:48

amount of gravity we feel from it is

26:50

the same. So if we could compress all

26:52

that matter to a smaller space, we could get closer

26:55

to the center. But as it is, if we got

26:57

closer to the center of the Earth, there would be

26:59

less matter between us and the center of the Earth.

27:01

So we wouldn't feel all of the matter of

27:03

the Earth interior to us. Okay,

27:07

so if you made the Earth half the size

27:09

that it is now, but the same density... Well,

27:11

the same amount of stuff. Same overall

27:13

amount of stuff, but in half the

27:16

size, I would still walk

27:18

around the Earth feeling the way I feel

27:20

now. Let me put it this way. If

27:22

you could keep the outer layer of the Earth, but

27:24

compress all of the rest of it into a smaller

27:27

space, you would feel the same. Right,

27:29

right, right, okay. But if you

27:31

actually compressed it all and then you were down there, if you

27:33

compressed everything

27:35

and they were down there, then you would feel more gravity because

27:37

you're closer to the center of

27:40

where all that gravity is. Okay,

27:42

okay. However you redistribute the matter,

27:44

as long as it's still kind of

27:46

interior to you in that sort of

27:48

sphere, it's how far you

27:50

are from... I mean, to first approximation, there

27:52

are things that can change, but how

27:55

far away you are from the center. So a neutron

27:57

star, a white dwarf, you

27:59

know, the Sun, they can all be around

28:01

the same mass, like around a solar mass.

28:04

There's some variation. Neutron stars have to be

28:06

a little bit more massive, but they

28:08

can be in the same sort of range

28:10

of masses, but it would kill

28:12

you a lot more to be

28:14

... The gravity would kill you a lot

28:16

more to be on a white dwarf or

28:18

a neutron star than it would on

28:21

something as massive as the Sun, because you would

28:23

be closer ... All that matter is much

28:25

more compact. You'd be closer to the center of

28:27

it. It's

28:30

the compactness of matter that really

28:32

affects how much gravity you feel. And

28:35

because these black holes are

28:38

really, really, really compact ... Then if you get

28:40

close enough to them, you get really screwed up.

28:43

But if you're far away, like if

28:45

you have a black hole that has the same mass as

28:47

the Sun, if you're far enough away from

28:49

it, you don't feel any different

28:51

gravitationally than if it

28:54

were just a regular star. So

28:56

the same reason the Sun is not sucking up all

28:58

the matter around it, the black

29:00

hole won't either. Got it. Okay.

29:04

But if you get close enough, it will.

29:06

If you get close enough, then things get really weird.

29:14

Okay. So to recap, when

29:16

a high mass star reaches the end

29:18

of its life, it sort of blows

29:20

off its outer layer, creating a nebula,

29:22

basically a huge cloud of gas and

29:24

dust. And eventually, the

29:26

core of the star collapses

29:28

and then explodes. This

29:30

is a supernova, an explosion so bright

29:32

that for a few days at least,

29:35

it can be seen billions

29:37

of light years away. After

29:40

the star goes supernova, it could either become

29:42

a neutron star or a

29:44

black hole. When a high mass

29:47

star below a certain mass collapses,

29:49

it compresses into an extraordinarily dense

29:51

star and essentially becomes a giant

29:53

nucleus known as a neutron star.

29:56

When a high mass star above a certain mass

29:58

collapses, it compresses into a certain mass. is

30:00

even further past the point where

30:02

any force we know of could

30:04

hold it up and it continues

30:07

to compress, presumably infinitely, but the

30:09

details of this part are beyond what

30:12

we are currently able to learn, and

30:15

that space-time object is known as a

30:17

black hole. And it

30:19

only gets weirder from here. So

30:25

not only does the gravity get strong in a

30:27

way that becomes really super-lethal,

30:29

but also it distorts

30:31

space in a complicated

30:34

way. And that's connected. Our

30:36

understanding of gravity from Einstein, from

30:39

general relativity, is that gravity is

30:42

the result of the distortion of space. So

30:45

I don't know if you've seen these kind of demonstrations

30:47

where you take like a big rubber

30:50

sheet and you put a bowling ball in the center. And

30:53

then you can roll tennis balls or golf

30:55

balls around. Then they make

30:57

little orbits. This is like a sort

31:00

of two-dimensional representation of what gravity does

31:02

to three-dimensional space. That analogy

31:04

is pretty good. So the analogy

31:06

that you put something really heavy in the center

31:09

of this sheet, it makes a big dent.

31:11

You put something less heavy, it makes a little

31:13

dent. And because of the

31:15

way that the gravity distorts the space, that

31:17

causes the space to be

31:19

bent so that objects don't follow straight

31:21

lines that go around the more massive

31:24

object. That's more or less

31:26

how we think that orbits work in the universe,

31:28

but you have to kind of

31:30

add another dimension, which makes it really hard

31:32

to visualize. But essentially, a massive

31:35

object like the sun kind of pulls the

31:37

space in all around it. It's

31:39

kind of tucking itself in all directions, kind

31:41

of distorting space toward it in all directions.

31:43

And that creates a kind of curvature of

31:45

space so that the Earth, instead of going

31:48

just in a straight line, it's

31:50

following that curve of the space around the

31:52

sun. And that's why it makes an orbit,

31:54

because it's trying to go in a straight

31:56

line, but the space is curved. And so

31:58

it's following that curve around. the sun. If

32:02

the sun were more massive

32:04

or if you got closer to the sun,

32:06

that curvature is stronger. So for

32:09

example, Mercury has to go

32:11

around the sun a lot faster to not fall in for

32:14

the same reason that in those demonstrations,

32:17

like if you're trying to get the golf

32:19

ball to go around the bowling ball toward

32:21

the center, you have to push it really

32:23

fast or else it'll fall in.

32:25

Whereas on the outside, it can go really slowly because

32:28

the space is more curved toward the center.

32:31

And so it kind of, you

32:34

know, it has to be going more quickly. There's

32:36

more that makes you want to fall in. And

32:38

so you have to go fast to not fall

32:41

in. Yeah, you have to get more angular momentum,

32:43

like more orbital inner momentum to not fall in.

32:46

Got it. That curvature of the space

32:48

also messes with things like the orbit of Mercury

32:50

like processes. It's not, it doesn't trace the same

32:52

shape all the time. It kind of makes

32:55

these weird little sort of loopy shapes

32:57

in its orbit because I mean, it's an

32:59

ellipse, but it's an ellipse where the

33:02

longer side kind of shifts around as

33:04

it's going around. It distorts the orbit

33:06

of Mercury because the space is so

33:09

distorted that it changes the way

33:11

that that planet moves around. Now,

33:13

with something like a

33:16

neutron star or a black hole, the

33:18

space is even more distorted around

33:20

that object. And you can start to get

33:23

these other effects that get

33:25

even weirder. So for

33:28

example, near a neutron star or even a

33:30

white dwarf, there's so much gravity

33:32

there, there's so much curvature of space that

33:34

it changes the way that light moves around

33:37

in that area. So if

33:39

you, if you shine the flashlight from

33:41

the surface of a white dwarf or

33:43

a neutron star, then

33:45

because the space is so distorted, it's stretching

33:47

out the light in this, in a similar

33:50

way to how the light of, you

33:52

know, a distant star is stretched out by the

33:54

expansion of the universe. You get the same kind

33:56

of, you get this red shifting of the light.

33:59

So from the surface

34:01

of a white dwarf or a neutron star, that light

34:03

would be redshifted, so it would be redder by

34:06

the time it gets to the places going than when the

34:08

light was emitted. And

34:11

there's also an effect on time. These

34:14

are kind of connected, but it means that

34:16

time moves more slowly in

34:19

a gravitational well than outside it. It

34:22

kind of stretches time as well. So

34:25

if you were, you know, at the

34:27

surface of a neutron star or a white dwarf, you

34:29

would experience time more slowly than someone

34:31

on the outside. So you'd look out and look like,

34:33

it would look like everything outside is moving a lot

34:35

more quickly, and people looking down at

34:38

you would think that you were moving really slowly. That's

34:40

called time dilation, gravitational time dilation.

34:43

And that's something that also happens in the

34:45

presence of a strong gravitational field. And

34:48

that can be observed on Earth. Like, you

34:50

can take two clocks, you can put one at

34:52

the bottom of a tower and one at the top of the tower, the

34:54

one at the bottom of the tower will, you

34:56

know, fewer seconds will have gone by

34:58

by the end of the experiment on

35:00

the bottom clock than on the top

35:02

clock. Wow. So these are all

35:05

effects that get even more extreme with black

35:07

holes. So if you get close

35:09

to a black hole, the red shifting of light

35:11

gets really extreme, the time dilation gets really extreme,

35:14

the curvature of space is so extreme

35:16

that light gets bent around black holes

35:18

very strongly. So, for example, there's

35:20

been this effort to take images of black

35:23

holes, there have been a couple of these images of black holes

35:26

that have been produced by the Event Horizon Telescope. And

35:29

in those, you can see this extreme distortion of the

35:31

light from going around the black hole. So

35:34

you can see a black hole in that sense? Well,

35:37

that's complicated. Okay. You can

35:39

see the light of the stuff that's around the black hole. Oh,

35:42

right. Okay. So you

35:44

can't see the black hole itself, but you can see the light

35:46

that's getting bent around the black hole. Yeah.

35:50

So the black hole itself is because

35:52

the kind of definitional property

35:54

of a black hole is that

35:57

it has an event horizon. And an event horizon

35:59

is a black hole. is a kind

36:02

of region around the black hole where

36:06

anything that gets closer than the event

36:08

horizon cannot ever escape, and that includes

36:10

light. Oh. So, a black

36:12

hole itself cannot produce light. There's

36:15

one tiny caveat to that that has to do with the

36:17

end stage of a black hole and the distant-digit,

36:20

distant future Hawking radiation, and we can talk

36:22

about that later, but just talking about sort

36:24

of astrophysical black holes, regular black holes right

36:26

now. Whatever goes into the black hole cannot

36:29

come out. And light cannot

36:31

be emitted by the black

36:34

hole because once you

36:36

get to the event horizon, there's

36:38

only one direction you can go, and that's toward

36:40

the singularity. So,

36:42

essentially what's happening here is the

36:45

space gets so curved that it's

36:47

impossible for anything to leave

36:49

to go in a direction away from

36:51

the singularity anymore. So, at a certain point,

36:54

the space is curved such that all paths

36:57

point toward the singularity. And

37:00

so, if you get past that event horizon, if

37:02

you're closer to the black

37:04

hole than the event horizon, even if

37:06

you're light,

37:08

you're going in. And so, you go into

37:10

this very, very dense

37:14

point, very, very

37:16

dense area. Can you help me

37:18

understand how big these are? It was helpful for me

37:20

when you were like, the white dwarf is like the

37:22

Earth, and a neutron

37:25

star is like a city. Is

37:28

a black hole like

37:30

a town in size? So,

37:32

okay, so the event horizon, the distance

37:35

from the singularity to the event horizon, that's

37:37

called the Schwarzschild radius. And the

37:39

Schwarzschild radius for something as massive as

37:42

the sun is three kilometers. Oh.

37:44

That's small, right? That's

37:47

pretty little. But these are usually

37:49

much bigger than the sun, right? So, we're talking

37:51

about maybe like 60 kilometers.

37:54

Yeah, so it's proportional to the mass. So,

37:56

something 10 times as massive as the sun,

37:59

if that were a black hole. be 30 kilometers in

38:01

radius. Okay. So if you got closer

38:04

than 30 kilometers to the singularity

38:07

of a black hole, 10

38:09

times the mass of the sun, then the

38:11

only direction you're going to go is toward

38:13

the singularity. Proper emergency. Now that's what the

38:16

equations tell us. We can't know for sure

38:18

what's going on beyond the event horizon because

38:20

no information can escape. Oh,

38:23

and we can never know. If no information can

38:25

escape, then can we ever know? I

38:28

mean, it gets a

38:30

little tricky because there's a lot of debate

38:33

about what really happens to information that goes

38:35

into a black hole. So this is a

38:37

debate that's been going on for decades called

38:39

the black hole information paradox. And the paradox

38:41

part is that there's some principles of physics

38:43

that say that information cannot be destroyed and

38:45

then there's black holes. And

38:48

black holes seem to suggest that if you throw

38:51

a dictionary into the black hole, that dictionary is

38:53

destroyed, that information is destroyed. But

38:55

then there's other arguments to say,

38:58

well, somehow that information should be

39:00

encoded in some property of the

39:02

black hole that could be read

39:04

in some way. But as far as we

39:06

know, black holes don't really have properties. Like

39:08

they have a mass, they can have a

39:10

spin. They

39:13

can have an electric

39:15

field. They can have a charge, but

39:17

they can't have any other properties really. As far as

39:19

we can tell, they can't have like mountains. They

39:22

can't have sort of stuff coming out of

39:24

them. By the time a black hole forms,

39:27

it's really just like a kind of

39:29

a defect in space. Like it's just

39:32

a pure space time object because the

39:34

only observational thing that you have is

39:36

the curvature of space around it. And,

39:39

you know, maybe it's it could be spinning, right?

39:41

It could be distorting

39:44

the space through through spinning. It could even

39:46

have a charge, although, you

39:48

know, the the ones in in space that we

39:50

know about, they don't seem to do that. They

39:52

seem to neutralize in some way, but it can't

39:55

be it can't be anything but

39:57

a sphere, you know, except.

39:59

unless it's spinning, in which case it can

40:01

be kind of a sort of

40:03

distorted sphere, but like one that's

40:05

not spinning, say, it's just defined

40:07

by there's a singularity somewhere in

40:09

the middle, but all we can

40:11

do is we can observe that

40:14

around this event horizon, things fall

40:16

in and don't come back. Like

40:18

the light gets distorted inward at

40:20

that place. And so we can

40:22

observe some properties of the event

40:24

horizon through things like this event

40:26

horizon telescope. What I was looking at was, it

40:28

was looking at a supermassive black hole, so something

40:30

billions of times as massive as the Sun. And

40:33

I'll talk about how those are formed later because

40:35

we don't really understand that. There's

40:38

a supermassive black hole in the center of

40:40

our galaxy, it's about four million times as

40:42

massive as the Sun, and

40:45

there are supermassive black holes that

40:47

are billions of times as massive as the

40:49

Sun as in other galaxies. We got a

40:51

picture of one of these, and what the

40:53

picture looks like was like,

40:55

that black hole has an accretion

40:58

disk around it, there's matter falling into it

41:00

that lights up, there's jets of radiation coming

41:02

out from the matter and

41:04

the accretion disk and the magnetic fields

41:06

throwing things around. So there's

41:08

a whole lot of light around this object. And

41:11

what we saw was like a ring of light and

41:13

a dark hole in the center. And

41:15

the reason we saw the dark hole in

41:17

the center is because even though there's light

41:20

all around it, some of that light is

41:22

being twisted into the black hole. Like right

41:24

now, like in every moment. In

41:26

every moment. And so there are certain directions that

41:28

if you look at it from that direction, all

41:31

you see is darkness because every

41:33

bit of the light that should have come

41:35

at you from that direction instead got redirected

41:37

into the black hole. So we're

41:40

not really seeing the black hole, we're

41:42

seeing where the light

41:45

would have been if it weren't for the black hole.

41:47

Yeah, that's one of the things that really shows us

41:50

that it's black hole because if it were any other

41:52

kind of object, that light would

41:54

have been able to get to us. There would

41:56

have been light shining toward us. hole,

42:00

it pulled in the light that could have gone

42:02

from that direction. We can work out the geometry

42:04

of where all the light rays are going, and

42:07

there's going to be some direction, some vantage point

42:09

from which you look at it where in that

42:11

direction it'll always be dark because

42:13

all of the light that could have come

42:15

to you from that direction goes into the

42:17

black hole instead. Right. Okay.

42:21

And so that's called the black hole shadow. And

42:23

that's one of the ways that we can observe

42:26

the event horizon of a black hole is

42:28

through the way that it just swallows light

42:30

and kind of removes it from our universe

42:32

by taking it into the event horizon where

42:34

the only direction it can go is toward

42:37

the singularity and we can't see it anymore. This

42:40

would imply on some level that

42:44

when we say like matter cannot be

42:46

created or destroyed, maybe

42:49

not. You know,

42:51

yeah, that's a term that's gotten a

42:53

lot of popularity that matter cannot be

42:55

created or destroyed. I mean, you

42:58

can change matter into energy and vice versa. That's

43:00

something that you can do in

43:02

lots of ways. Well, information cannot be

43:04

created or destroyed. But information, yeah. So

43:08

it's possible that black holes really do

43:10

destroy information. Although there are some theories

43:12

that if you wait long enough, a

43:15

black hole will somehow release

43:17

that information in some unreadable

43:20

but technically existing kind of

43:22

way. Okay. So

43:24

now on our kind of understanding of how black

43:26

holes grow, if you put matter

43:28

or energy into a black hole, it

43:31

just gets more massive. Like

43:33

you just add to that singularity, the event

43:35

horizon gets a little larger because the event

43:37

horizon is just proportional to the mass of

43:39

the thing. So put

43:41

more energy in, that kind of increases the mass. And

43:44

that would happen because say a rogue

43:46

planet happens across the path of a

43:48

black hole, goes past the event horizon

43:50

and vroomf. Yeah, anything

43:52

that gets too close will go in. And so

43:55

black holes should just grow over time, the event

43:57

horizon should just get bigger over time. But...

44:00

There's a theory that quantum

44:02

effects that happen near the event

44:04

horizon can kind of pull energy

44:07

out of the black hole, just

44:10

by, just sort of these

44:12

complicated quantum processes that can occur toward the

44:14

edges of the black hole, toward the event

44:16

horizon. And so there's something weird that can

44:18

happen toward the edges of a black hole,

44:21

and that can over time kind of slowly

44:23

leech energy out of the black hole. This

44:26

is called Hawking radiation, because Stephen

44:28

Hawking was one of the people who came up with this

44:30

idea. And so if

44:32

you have an isolated black hole, like let's

44:34

say you just have a black hole that

44:37

you, that there's nothing falling into it, it's

44:39

just pure vacuum around it, if you wait

44:41

long enough, then that black hole will shrink.

44:43

It'll radiate a little bit of energy toward

44:46

the edges as it gets smaller, as it

44:48

gets, you know, lower

44:50

mass, it gets brighter, and it radiates

44:52

more and more energy more quickly. And

44:55

so toward the end, when it gets tiny, it

44:58

may kind of explode toward the end and

45:00

like destroy itself toward the end. But for

45:02

astrophysical black holes, for black holes of the

45:04

masses of things that we see in the

45:07

universe, this process would take, I

45:09

think I've worked out for a five solar mass

45:11

black hole, which is about the smallest black hole

45:14

we know of. There may be some that are

45:16

a little bit lower mass than that, but it's

45:18

around there. For that kind of that massive black

45:20

hole, the lifetime for this Hawking radiation thing is

45:23

something like 10 to the power of 69 years. So

45:26

it's a really long time. I

45:29

was thinking maybe on a scale of trillions, but

45:31

no, we're talking on the scale of numbers that

45:33

we don't have words for. Yeah, exactly. All

45:42

right. So to summarize, there's a lot we still

45:44

don't know about black holes, but there is also

45:46

a lot we do know. Once

45:48

formed black holes distort space, time

45:50

and light through gravity, which is

45:52

why when we look at a

45:54

black hole, we don't see the

45:56

object itself, but the light

45:59

warping around. it. And

46:01

a black hole's event horizon essentially refers to

46:03

the point of no return, the distance you

46:05

can be from the black hole before

46:08

being, like, irrevocably consumed

46:10

by it. In

46:12

simpler terms, once something passes the

46:14

event horizon, nothing can

46:17

escape it, including light and

46:19

information. Which raises

46:21

the question, does everything

46:23

that gets pulled in get destroyed?

46:26

Well, we don't know for

46:28

sure, at least not yet. Now, Katie also

46:30

offhandedly mentioned that there is a supermassive black

46:33

hole in the center of our galaxy, and

46:36

I wasn't going to let the episode end

46:38

without circling back to that. So

46:43

I have two remaining questions, Katie. The first is

46:45

that you've mentioned that

46:48

there are these supermassive black holes that can be

46:50

billions of times the mass of our sun. Yes.

46:53

And that seems improbable to me,

46:55

because I cannot imagine that

46:57

there is a star that could

47:00

go supernova that would be billions of times the

47:02

size of our sun. That's right. That's right. So

47:04

how do we get these black

47:07

holes that are so massive that, like,

47:09

the entire Milky Way galaxy is spinning

47:12

around one? This is an

47:14

excellent question, because it is something we

47:16

still don't entirely understand. What

47:18

we know is that large galaxies

47:20

all seem to have a supermassive

47:22

black hole in the center. And it's

47:25

not really that everything is orbiting the

47:27

supermassive black hole, per se. It's

47:29

more that the supermassive black hole

47:31

is in the center because it kind of falls to

47:33

the center of the galaxy or it grows up in

47:36

the center of the galaxy. The

47:38

mass of the black hole in the center of

47:40

our galaxy is about four million times as massive

47:42

as the sun. And so,

47:44

you know, it's not super important to the

47:46

whole galaxy. It's not a large fraction of

47:48

the mass of the whole galaxy. It's

47:51

a small fraction of the mass of the whole galaxy. But

47:53

it's at the center because that's where the

47:56

most massive thing would naturally be. So

47:58

we think that most. massive galaxies

48:00

seem to have a supermassive black

48:03

hole in the center. And we think that the

48:05

supermassive black holes kind of grow up with the

48:07

galaxy. There's a strong correlation between the mass of

48:09

the galaxy and the mass of the black hole.

48:11

So really massive galaxies tend to have more massive

48:13

black holes. Really low mass galaxies

48:16

tend to have low mass black holes, depending

48:18

on how you sort of measure the mass of

48:21

the galaxy. But there's a correlation. So we think

48:23

that they kind of grow up together. Like as

48:25

matter is coming into the galaxy, matter is also

48:27

going into the black hole, and they kind of,

48:29

it seems like what happens

48:32

is that when you get a whole lot of

48:34

matter together to create galaxies, black

48:36

holes form through, you know, the

48:38

end stages of stellar evolution or

48:40

something. They kind of coalesce in

48:43

the center and pull in matter

48:45

and grow through the accretion of

48:48

matter over time. Now,

48:50

when you work out the details and the time

48:52

scales, it's

48:54

kind of tough to get that to

48:56

work out. We still don't know exactly how they

48:58

grow as quickly as they do, because even really,

49:00

really early galaxies seem to have really massive black

49:06

holes. So some of those galaxies we

49:08

talked about before that JWST is seeing that are really,

49:11

really early galaxies, they seem to have supermassive

49:13

black holes that are pretty bright in the

49:16

sense that there's a lot of matter falling

49:18

into them. And so we see that brightness

49:20

of the matter that's falling

49:22

in, that's heating up as it's falling in.

49:24

There are very, very distant quasars. A quasar

49:26

is a supermassive black hole that's accreting a

49:28

lot of matter, and that matter is lighting

49:30

up and creating jets of radiation. And those

49:32

are really, really bright because there's a whole

49:34

lot of matter falling into a really massive

49:37

thing, and it gets heated up as it's

49:39

swirling around. Those quasars can be really, really

49:41

bright and really, really distant. And those

49:43

seem to have grown up very, very quickly in

49:45

the early universe. And we don't know how to

49:47

get that much matter into that small space that

49:49

quickly. So if you try and

49:52

work it out, it seems

49:55

like if you just throw a whole lot of matter

49:57

at a black hole, some of them are going to be really, really

49:59

bright. And so I think Some of it will just fall in, but

50:01

a lot of it will kind of create a disk of matter

50:04

called an accretion disk, kind of like a

50:06

whirlpool. Like if you see a whirlpool in

50:08

a lake or something, you might not see the water

50:10

so much, but you see like the splashy, like

50:13

white caps and you see stuff that's like

50:15

the leaves that are swirling around in the

50:17

whirlpool. And some of that stuff gets spit

50:19

off, right? It doesn't all fall in. Some

50:21

of it gets spit off in these jets

50:23

of radiation. Yeah, yeah. And so if you

50:25

just try and throw more matter at a

50:27

black hole that's accreting, then the intensity of

50:29

the glow of that accretion disk lights up,

50:32

you know, brightens, and there's more pressure from all

50:34

the other matter that's trying to fall in. And

50:36

so it kind of puffs out. That seems like

50:39

it should slow down the accretion of

50:41

matter. It seems like if you put in too much

50:43

at once, then it should,

50:45

it kind of puffs out and it kind of blows

50:47

itself away, right? Because you're just

50:49

trying to put it all too much in, you know, in the same

50:51

place at the same time. And we don't

50:53

know exactly how to resolve that at the

50:56

detailed level. It seems like

50:58

these things, at least in the

51:00

very early universe, grow very, very efficiently in a

51:03

way that seems like they shouldn't grow quite that

51:05

fast. There are a lot

51:07

of people working on this problem of how do you

51:09

make the black holes grow that fast? You know, one

51:11

idea is that the first stars were really, really

51:13

massive. And so they left these really massive

51:16

remnants, remnant black holes. And then so they

51:18

got a head start in growing really quickly.

51:20

Other ideas are that, you know, if you

51:22

let the matter fall in in a particular

51:24

way, it can kind of overwhelm the outward

51:26

pressure of the radiation and it

51:29

can kind of fall in any way. So

51:31

there are a couple of different ideas for

51:33

this. But whatever happens,

51:35

you know, somehow black holes can get

51:38

really supermassive and pretty much

51:40

all the large galaxies seem to have supermassive black

51:42

holes. So we know that ours is about four

51:44

million times as massive as the sun. We call

51:46

this black hole Sagittarius A

51:49

star. And that's written

51:51

like Sagittarius and then a capital A and then an

51:53

asterisk. So it's

51:55

one of the most annoying pieces

51:57

of terminology in astrophysics.

51:59

Because every time you tell

52:01

someone who's not an astronomer about it, you have

52:04

to explain the name for like five minutes. You're

52:06

not going to be able to justify this one

52:08

to me. No, it doesn't. It's just weird

52:11

historical reasons. Yeah. I mean, we

52:13

call the sun the sun, right?

52:15

Like we should call it something

52:17

like that. She just called it the big sink. The

52:20

big sink. The big sink. We did it.

52:22

It's over. The big sink. My

52:42

second question. Mm-hmm. And

52:44

this may be one of those things

52:46

where it turns out that y'all just

52:48

use like the same word for things,

52:50

regardless of whether they're the same. Again,

52:52

not to be critical. I only learned

52:54

what a black hole was about 20

52:56

minutes ago. But you keep

52:59

referring to this singularity,

53:02

that the center of a black hole

53:04

is an extraordinarily

53:07

dense singularity,

53:09

single point. And

53:12

then you said at the very beginning

53:14

that it's possible that our universe,

53:16

that the Big Bang, began in

53:19

a singularity. Are

53:22

those related terms? Yes.

53:25

Yeah. Oh, no. Is

53:28

it possible that we are just like the

53:31

what got sucked in by a black hole? No.

53:35

No. Oh, good. Great. We can

53:37

measure the curvature of space and we

53:39

can see that light can move in

53:41

lots of different directions. And so we're

53:43

not in the interior of a black

53:45

hole. So the term singularity means

53:49

in this case, a singularity is an

53:51

infinitely dense point in

53:54

space. Or in the case

53:56

of the Big Bang, singularity may

53:58

be an infinitely dense point of space. It's

54:01

a place where spacetime becomes

54:03

infinite in some way. It

54:05

becomes sort of pinched together in some way.

54:08

If you imagine spacetime as like a

54:10

grid and you pinch a

54:13

grid in a point, that's

54:15

kind of what singularity means.

54:18

So my big concern about the

54:20

idea of the universe beginning in

54:22

a singularity, which I know it

54:24

didn't necessarily, but my big concern

54:26

about that has always been, well,

54:28

it seems very unlikely to me

54:30

that there is an infinitely dense

54:32

point of space. But

54:35

what you're telling me is that there are

54:37

actually lots of infinitely dense points of space.

54:40

Well, the idea of the Big Bang singularity is

54:42

that all of space was

54:45

in that point. So

54:48

like when the Big Bang happened, it

54:50

created all of space by

54:53

that point expanding. Right. But

54:55

that seems less crazy to me

54:58

if you're telling me that infinite

55:00

density is not unprecedented. Right.

55:03

Right. But the thing is, we can't

55:05

observe any of that. We can't observe

55:07

a Big Bang singularity. If it happened

55:09

or not, we don't know. And we

55:11

can't observe it partially because we

55:13

think that this cosmic inflation happened

55:15

and that kind of obscures the

55:18

view in some way. And then

55:20

partially because if it really was

55:22

infinitely dense, you just can't

55:24

get information out of something like that. And

55:26

with black holes, there's a theorem

55:28

that says that every singularity has

55:30

to be shrouded by this event

55:32

horizon. It's called the cosmic censorship

55:34

something. I

55:37

like that one. That's good. That's funny. Yeah.

55:40

So you can't have

55:42

a naked singularity is what that is called.

55:44

And that's because if you get too close

55:46

to any singularity, then the curvature has to

55:48

be so strong that light can't escape anymore.

55:51

And that is by definition an event horizon.

55:53

Right. And you

55:55

can't see past that. Like

55:58

you literally can't because... Like if you

56:00

were just inside the event horizon and

56:02

you shine a flashlight outward, that

56:04

light will curve around and go in. Right.

56:07

So it will not leave. Right.

56:10

But we don't know for sure. So there are

56:12

lots of theories about what happens inside the black

56:14

hole that, you know, maybe there's some kind of

56:16

like stringy ball fuzz thing

56:18

that has to do with string theory

56:20

and like things get complicated and quantum

56:23

and like we just, we don't know.

56:26

There are lots of ideas for what might

56:28

be happening inside a black hole. And so

56:30

the idea of the singularity, that's what happens

56:32

if you just follow the equations of general

56:34

relativity to their conclusion, you reach this infinity.

56:37

But in general, like singularities,

56:40

like infinities in equations

56:42

and stuff are just a sign that

56:44

something's gone wrong. Like you've just, you've,

56:46

your model is broken in some way

56:48

and they're generally not a good thing

56:50

in our sort

56:52

of physical models. One of the reasons

56:54

black holes would be so interesting then

56:56

is that it's a place where at

56:59

least so far you can't get rid of the

57:01

infinity. Yeah, exactly. Exactly. So

57:04

this might be a place where there really is, there

57:07

really is just a singularity there and it

57:09

just kind of breaks space and time, you

57:11

know, but you know, we

57:13

don't know because we can't observe beyond the

57:15

event horizon. Wow.

57:18

Yeah. Wow. So

57:21

there's all sorts of wild things that happen. Like, you

57:24

know, for example, the space is so

57:26

curved that if you were to fall

57:28

toward a black hole, let's

57:30

say you're falling in feet first, right?

57:33

At some point the space is so

57:35

curved that the gravity on

57:37

your feet is way, way stronger

57:40

than the gravity on your head. And so your

57:42

feet get pulled in really fast. And

57:44

so you kind of get stretched out by

57:47

the tidal forces. It's called a tidal force when

57:50

you have that gravity gradient like that. And

57:53

so there's a technical term

57:55

for that process for getting stretched

57:57

out by the black hole and

57:59

that technical term is spaghettific. See,

58:01

again, I feel like

58:03

progress is being made. That's good. Yeah.

58:07

Yeah, you're turned into spaghetti. You get spaghettified.

58:09

I mean, you could also call it tidal disruption,

58:11

but yeah, spaghettification is a

58:13

term. Yeah, spaghettification is perfect.

58:16

Yeah, yeah. Even at my body size,

58:18

the gravity would be so different between

58:20

my head and my feet that I

58:22

would get stretched. Yes, yeah, yeah. Wow.

58:25

Everything would get stretched and disrupted. I mean, you'd probably

58:27

just, you'd be broken and destroyed. I understand that. But

58:29

like theoretically. Yeah, yeah. And we

58:31

can see things fall into supermassive black

58:33

holes sometimes. These tidal disruption

58:36

events where a star can be ripped apart

58:38

by a black hole and it creates a

58:40

big X-ray explosion, which is kind

58:42

of cool to see. And like our own

58:44

black hole, our own black hole isn't eating very much. It

58:46

doesn't have like a big accretion disk around it. It's not,

58:48

it doesn't have bright jets or anything like that, which is

58:50

good. We don't want, we don't want Sagittarius A star to

58:53

be super, like an active black

58:55

hole. That would be bad. Big sink.

58:58

We don't want the big sink. But every once in a

59:00

while, like a gas cloud gets really close to it and

59:02

gets kind of pulled in and the astronomers get really

59:04

excited that we get to see a little bit of

59:06

gas fall into the black hole. So like

59:08

a little bit of stuff can happen. But with,

59:10

you know, with really distant black holes, like black

59:12

holes in the centers of other galaxies, when they

59:14

have that strong accretion, there's a whole lot of

59:16

stuff falling in and they have these jets of

59:19

radiation. Those, that's when they're called quasars. They have

59:21

the, they're really, really bright and we can see

59:24

those across the universe in very, very distant galaxies.

59:26

And because they're so bright, we can use them

59:28

as like signposts and use them

59:30

as like markers to measure things in

59:32

the universe and measure like the distribution

59:34

of matter on really large scales in

59:36

the universe and measure other things about

59:38

like the expansion and so on. And so

59:40

they're, they're really useful, bright

59:42

objects in the universe. And in our, in our

59:45

own galaxy, we can see a lot of black

59:47

holes too, but the ones we see here, mainly

59:49

we see them when they're eating their neighbor's stars.

59:51

So it's a kind of similar thing, but like

59:53

a black hole can be in a binary system

59:55

with another star. So maybe because they're born together

59:57

and one of them explodes and becomes a black

59:59

hole. and the other one is still kind of

1:00:02

in its orbit. Sometimes they're eating, they're

1:00:04

like pulling the material off of their

1:00:06

neighbor star, and it creates an accretion

1:00:08

disk around the black hole, and that can

1:00:11

light up in X-rays, and those are called X-ray

1:00:13

binaries. And we see somewhere

1:00:15

around 50 of those in our galaxy.

1:00:17

Wow. Where we're pretty sure

1:00:19

that it's a black hole eating its neighbor

1:00:21

star, and that's what this right X-ray source

1:00:24

is. Thank God we don't have

1:00:26

a neighbor star. I'm really glad we don't have

1:00:28

a, we're not on one of those planets with

1:00:30

two suns like Luke Skywalker. Yeah, yeah,

1:00:32

that would be, that could be real unpleasant. Like

1:00:34

can you imagine like, you know, watching the other

1:00:37

sun and like it's getting close to going supernova,

1:00:39

and you're like don't do it, don't do it.

1:00:42

Ssssss. Anyway.

1:00:48

The universe is wild. Yeah.

1:00:50

Wild, Katie. Yeah, yeah. This

1:00:53

was thrilling for me. Good. It felt like

1:00:56

I was in an episode of Star Trek

1:00:58

or something. It was like, it

1:01:00

had all the drama of a proper movie. I

1:01:02

felt like, you know what I felt? I felt

1:01:04

like I was in a Christopher Nolan movie. Nice,

1:01:07

nice, good. Yeah, so the

1:01:09

black hole in Interstellar, the

1:01:12

supermassive black hole in Interstellar, they call it gargantuan.

1:01:15

The visualization of that, it includes

1:01:17

the black hole shadow, so the

1:01:20

way that the light is distorted around the

1:01:22

black hole in the imagery in that movie

1:01:24

is pretty accurate. There are a couple

1:01:26

things they change to make it more cinematically

1:01:29

engaging, but it's pretty accurate the

1:01:31

way the light is like lensed around the

1:01:33

black hole because it's distorted. And

1:01:35

there's kind of a

1:01:37

fun story where basically like Kip

1:01:39

Thorne, who's a Nobel laureate who

1:01:42

studies black holes, basic how tech.

1:01:44

So he was like the science advisor for the

1:01:46

film. I think he sort of came up

1:01:48

with the idea of the film. But anyway, he was

1:01:51

working with the filmmakers, and

1:01:53

he kind of convinced them to run

1:01:55

the code to figure out what this

1:01:57

black hole would look like as

1:02:00

as a way of, you know, kind of,

1:02:02

sure, making the movie more realistic, but it

1:02:04

also, they wrote some papers based on this.

1:02:07

And it's a great way

1:02:09

to get, you know, supercomputing time, convince

1:02:11

Hollywood that it's a good idea.

1:02:14

Then you don't have to apply for, like,

1:02:16

the supercomputing, you know, selection committee to

1:02:18

get to use the supercomputers at the university. Use

1:02:21

the Hollywood one. It's much better. That's

1:02:23

amazing. That's amazing. So, we need

1:02:25

more science fiction films about

1:02:28

black holes. Yeah. All right. I'll

1:02:32

make the case the next time I'm talking to

1:02:34

Christopher Nolan. Excellent. You know what you

1:02:36

should do? Next time you write a novel, just put a

1:02:38

black hole in there somewhere where, like, there's a

1:02:40

plot point about, you know, some property of

1:02:42

it. And so when they make it into

1:02:44

the film, they have to do that

1:02:46

calculation. One

1:02:48

thing you should probably know about the movie

1:02:51

adaptations of my books is that they are,

1:02:53

they don't have that budget. Unfortunately,

1:02:56

the whole reason Hollywood likes my

1:02:58

books is because there's no, like,

1:03:00

explosions. Oh, man. You

1:03:05

got to work with me here, John. No,

1:03:07

no, no. They just want, they're like, oh,

1:03:09

people in a room conversing? Perfect. So,

1:03:13

I will endeavor to write plotier fiction, but I

1:03:15

think you might be talking to the wrong brother.

1:03:17

I think we need to get Hank on this

1:03:19

one. OK. Considering

1:03:29

all the well-known aspects of our universe

1:03:31

that I don't understand, it's a real

1:03:34

thrill to talk about something that bumps

1:03:36

up against the edge of what proper

1:03:38

experts don't know. Also,

1:03:40

I was mildly thankful that I

1:03:42

didn't live on Tatooine before this

1:03:44

conversation, and now, like, I'm very

1:03:47

thankful. But I do

1:03:49

have some regrets. I should be incorporating more black

1:03:52

hole subplots into my novels. Next

1:03:54

episode, Katie tries to walk me through

1:03:56

something I somehow know even less about

1:03:58

than black holes. matter.

1:04:01

I'll see you then. This

1:04:11

show is hosted by me, John Green,

1:04:13

and Dr. Katie Mack. This episode was

1:04:16

produced by Hannah West, edited by Linus

1:04:18

Openhouse, with music and mix by Joseph

1:04:20

Tuna-Metish. Special thanks to the

1:04:22

Perimeter Institute for Theoretical Physics. Our

1:04:25

editorial directors are Dr. Darcy Shapiro

1:04:27

and Megan Modaferri, and our executive

1:04:29

producers are Heather DiDiego and Seth

1:04:31

Radley. This show is a

1:04:34

production of Complexly. If you

1:04:36

want to help us keep Crash Course

1:04:38

free for everyone, forever, you can join our

1:04:40

community on Patreon at

1:04:42

patreon.com/crash course.