1:54

the

2:00

boundary of that is called

2:02

the event horizon they even horizon is not

2:04

a physical thing it's not a wall

2:07

it's not a blob it's not a membrane

2:10

instead the black hole is really the

2:12

singularity it's the point of infinite density

2:14

in the center but that gravity from

2:16

the singularity is so strong

2:18

that at a certain distance from

2:21

that singularity nothing can escape

2:23

it's gravitational clutches

2:26

that

2:26

distance is the event horizon

2:29

so of light crosses the event horizon

2:31

it doesn't come out so it a acts

2:34

kind of like a perfectly

2:36

absorbing surface even

2:39

though it doesn't actually exist there's nothing

2:41

for you detach or taste or smell but

2:43

you can

2:44

certainly experience it those

2:46

are the surfaces of black holes

2:48

even though they technically have no service

2:50

and yes ah side of for

2:52

you piggy nerds out there yes there's hawking

2:54

radiation this exotic quantum process

2:57

where

2:57

black holes do we met a tiny

2:59

a bit of radiation but

3:02

for a typical stellar mass black hole that something

3:04

like one particle emitted per

3:06

year or so it doesn't really count

3:08

for the point i'm trying to make black

3:10

holes are perfect absorbers

3:13

every wavelength of light that encounters

3:15

the event of rise of a black hole will

3:18

not escape see

3:20

you can't ever image in

3:22

event horizon directly because a event

3:24

horizons don't really exist in the physical sense

3:26

as they're just imaginary lines in a mathematical

3:28

sand and be they don't emit

3:30

or reflect any light and

3:33

all which is kind of the whole basis

3:35

of this picture taking enterprise

3:39

so we only ever image or see

3:41

black holes based on their influence

3:44

on their surroundings like gravitational

3:47

lensing strong x

3:49

rays the oh if you get a lot material

3:51

falling into a black hole a crimes into a small

3:53

volume and it starts getting

3:55

really hot and glowing we can see that glow

3:57

even though we can't see the black hole itself we

3:59

can watch

5:58

the biggest ones

6:00

tend to be really far away

6:03

which he is also bad for astronomy

6:06

astronomers have a tough time seeing

6:08

things that are both small and

6:10

very far away in

6:12

so if we want to capture a

6:14

black hole shadow the whole

6:16

carved out from the light of it's

6:18

surroundings we have

6:21

a very very small target

6:23

in the best solution is interferometer

6:26

if you thought this is going to be petri on

6:28

pitch i hope i tricked you yeah

6:31

it's this weird thing this

6:33

massive hero in

6:35

astronomy there's

6:36

completely altered the way we do science

6:38

is as revolutionary as the telescope itself

6:40

and nobody talks about it so

6:43

we're gonna talk about it or

6:46

challenges we want to take a picture of an

6:48

incredibly tiny tiny tiny object

6:51

tiny because it's naturally small also time

6:54

because is very far away or

6:56

solution is we need resolution

6:58

we need giant honking cameras

7:00

or lenses are dishes work whatever your

7:02

instrument of choices the larger

7:05

the dish or lens the

7:07

higher the resolution you can now

7:10

take for example the supermassive black hole

7:12

at the center of the milky way which is annoyingly

7:15

an confusingly named sagittarius

7:17

a star don't

7:19

get me started secretaries a star

7:21

is huge for and a half

7:23

million solar masses in

7:25

the ring of plasma around it extends

7:27

for a light years

7:30

the problem is that it's twenty five thousand

7:32

light years away to take a picture of a you

7:34

need a telescope the size of the planet

7:36

earth and since that's

7:38

not going to happen anytime soon for a clue

7:40

as to why just listen to the dyson sphere

7:42

episode we need to

7:45

do something else enter interferometer

7:47

three

7:50

this show is brought to you by

7:52

better help one of my favorite

7:54

things about doing ask a spaceman

7:56

is how much i get to learn about

7:59

my own field is

9:57

object.

10:01

We can do this a whole bunch of times with a whole bunch

10:03

of telescopes. We do it over and over. We

10:05

can build up an image.

10:08

The upside to this technique is that you can

10:10

very quickly get huge resolution

10:12

gains just by putting your telescopes farther

10:15

apart. The downside is

10:17

that it's a really crappy telescope because most

10:19

of the light from the distant source is

10:21

hitting the ground between your

10:24

dishes and not the dishes themselves. And it's

10:26

very hard to turn photons that

10:28

hit dirt into useful information. And

10:31

so you waste a lot of time.

10:34

Usually these telescopes

10:36

or dishes,

10:37

this works best in the radio, by

10:39

the way, because of the long wavelengths involved,

10:42

are connected directly together. But we can take

10:44

this idea to its ultimate extreme

10:47

and put our dishes on the other side of

10:49

the planet, giving us well, well,

10:50

to be honest, a really garbage telescope the

10:53

size of the Earth. And

10:55

we can do this. We can separate the elements in

10:57

as long as each element has a

11:00

very accurate timekeeping device

11:02

like its own atomic clock. Then later

11:05

we can do these correlations once we haul

11:08

the data around and put it in the same

11:10

room together and get everyone to talk. The

11:12

downside is, is yeah, it's a really, really

11:14

awful telescope because you don't get to

11:16

use a dish with the entire collecting

11:19

area of the Earth. You have the resolution there,

11:22

and not the sensitivity.

11:25

And so you have to wait a really long

11:27

time. You need to run this for

11:29

a very, very long time. You need to run

11:31

your interferometer for incredibly

11:33

long periods of time. You need

11:36

to get, have as many stations

11:38

scattered around the globe. So you

11:40

get different pieces of information here

11:42

and there. And then you have to spend any

11:44

enormous amount of computational power

11:47

to put the picture back together again. We

11:50

can make up for this relative garbage

11:53

dish of the telescope with Patreon.

11:56

There it is. Patreon.com slash PMSutter

11:58

is how you...

11:59

make astronomy better or at least keep

12:02

this show going. Patreon supporters

12:04

do get early ad-free

12:06

access to episodes, direct connection

12:09

to me and other perks, shoutouts in

12:11

episodes and on show notes and all that good

12:13

stuff, and my eternal

12:15

gratitude.

12:16

And you know what, if you don't contribute to Patreon, you

12:18

still have my eternal gratitude for just listening

12:21

to this show. It really is a pleasure to

12:23

give you these episodes

12:25

year after year. I love doing it.

12:28

Patreon.com slash PM

12:30

Sutter.

12:32

How we can actually make up for the relative

12:34

garbage properties of an interferometer is

12:36

with lots of work and math. And

12:38

especially this is true when we place

12:41

our instruments on opposite sides of the globe.

12:43

This is a technique called very long baseline

12:46

interferometry.

12:47

And the ultimate expression of this is the

12:50

Event Horizon Telescope, a telescope designed

12:53

to take a picture of the shadow

12:55

of giant black holes. And that's

12:58

exactly what the Event Horizon Telescope did. It

13:00

imaged the regions around the supermassive

13:02

black holes at the center of a galaxy M87

13:05

and the one at the center of our own galaxy.

13:08

The resolution that they achieved is equivalent

13:11

to taking a picture of an apple on the surface

13:13

of the moon, which for you astronomy aficionados

13:16

out there is really stinking impressive.

13:19

And when we see these images, if you haven't seen them

13:21

already type in event horizon telescope, you'll

13:24

get images also on the show notes

13:27

at ask a spaceman.com. There's an image

13:29

right there. It's

13:32

beautiful. We see this ring of light,

13:34

this halo of fuzzy light with

13:36

a hole cut out of it. You

13:38

see the shadow of a black hole. Now

13:42

the shadow is already a little bit weird. And

13:45

this is the first taste of the strangeness

13:48

of black hole geometries. The

13:51

clump of material, the disk of material around

13:54

these black holes

13:56

looks like it's face on it. It looks like a

13:58

donut, honestly, but it's actually

13:59

edge on to us or nearly edge

14:02

on to us. There's a part in front

14:04

of the black hole and then a part behind the black hole.

14:06

But it looks like a donut to us because

14:09

of the weird geometry because a light from

14:11

behind the black hole is

14:13

actually following the curved space

14:15

time around the black hole and going around

14:17

it before hitting our eyes and that's super cool.

14:20

And here's another

14:22

weird thing. We see the shadow

14:25

in these images. I was just about

14:27

to say it's clear as day but that that seemed

14:29

wrong. We see very clearly

14:32

the shadow but the

14:34

event horizon, the actual

14:36

black hole, is much smaller

14:38

than the shadow itself. When you look at those

14:41

images which are haunting and

14:43

you look into that void, the black hole

14:46

itself is buried somewhere

14:48

inside the dark region

14:50

but does not

14:52

fill it up. Why?

14:55

Well, black holes are capable

14:57

of making even shadows not

15:00

boring and they do it through

15:02

their intense gravity. Black

15:04

holes bend light so much that

15:06

they even bend themselves. You

15:10

don't just see in front of the black hole.

15:13

You see the back side of the black hole in

15:15

the same image. The gravity is so strong

15:17

that the sphere of the black hole appears

15:20

larger than it should. It's

15:22

like the sphere of a black hole unfolds

15:25

itself,

15:26

making it seem even larger. Here's a

15:29

metaphor to picture what is

15:31

going on around black holes.

15:33

Let's say you're looking at me and

15:35

you're looking at

15:37

my head and my head is going to be a black hole and

15:40

you, well you're just going to be you,

15:42

okay? And you're going to take a laser

15:44

pointer

15:45

and you're going to stick it right in the middle of your

15:47

eyes. So this is your line of sight. This is where

15:49

your eyes look. The laser pointer

15:51

will tell you where you're looking.

15:53

Where the laser pointer lands

15:56

is what you're currently looking at. Lines

15:58

of sight, okay?

16:01

Now, let's say I'm a black

16:03

hole.

16:04

If you're looking right

16:07

directly, not an inch off,

16:10

straight down right at my nose, then that

16:13

laser pointer is going to end right at the tip

16:15

of my nose. If I were a black hole, it

16:17

would end in blackness, it would end in nothing,

16:19

and you would see a void. You would

16:22

see the event horizon.

16:24

Okay. Now look a little bit to the side. You're

16:27

going to look at my eye, just a little

16:29

bit off angle. You have your little

16:31

laser pointer and now it's shining in my eye,

16:33

making me go blind, which I'm willing to do

16:37

for the sake of you understanding this. I hope you appreciate

16:39

my sacrifice. If

16:41

I were a black hole, you'd just be looking at a different

16:44

part of the event horizon. So far, this is nothing

16:46

special. And then you look all the way to

16:48

the edge of my head

16:50

and you see just the

16:52

tip of my ear. All

16:54

right. So I'm looking off the side and the laser pointer is

16:56

coming at me and it's grazing right by

16:59

my cheek and it's just hitting my earlobe,

17:01

like right here. And you see my earlobe.

17:04

So far, this is normal.

17:06

Don't worry, it's going to get more interesting. And

17:10

then let's say you move just a little bit to the right

17:12

and the laser pointer no longer

17:14

hits me. Instead, the laser pointer grazes

17:16

right by my head and goes off

17:19

into the back wall. And now you see the back

17:21

wall.

17:22

Okay. This is basic shadow stuff.

17:24

You're just following lines of sight and

17:27

see where they go. When they land on me, you get

17:29

my shadow.

17:30

When they don't land on me, you get the background. Pretty

17:33

easy. Black holes are not easy. Black

17:36

holes have intense gravity. Black

17:39

holes bend the path

17:41

of light. They bend the path

17:43

of lines of sight.

17:47

When you're looking at me, the black hole,

17:49

if you're looking dead on, you see

17:51

the event horizon. You see blackness. If

17:53

you see the edge of the event horizon,

17:56

the outermost radius, you

17:58

still see the black hole. Still see event

18:01

horizon. Nothing is major here. But

18:03

then imagine you're looking at me and there's that

18:05

laser point grazing my cheek grazing the

18:07

edge of my ear not touching me if

18:10

I'm a black hole I have such intense gravity that

18:12

I pull

18:13

on that line of sight a pole on that laser

18:15

beam and It circles

18:18

around my head because of the intense

18:20

gravity and then it lands But it

18:23

lands somewhere like back here

18:25

my occipital lobe. Well

18:27

right back here I'm I know you can't see it, but

18:30

I'm touching it right now

18:32

And so you see that when you look past

18:35

my ear lobe You don't

18:37

see the back wall. You see the back

18:39

of my head Because of my

18:41

intense black hole gravity and then you look

18:43

a little bit further away And there's another line

18:46

of sight that gets a little bit further away

18:48

before getting captured by the black hole by

18:50

my head It wraps around and hits right dead

18:53

center

18:53

and you see You know the spot

18:56

on the back of my neck that I missed with my

18:58

razor when I shaved Right

19:00

there. You can see it and

19:03

then you look a little bit further away from that

19:06

and that light Makes it pretty

19:08

far away from my head that line of sight that laser

19:10

point before it curls in and

19:11

it loops around and It

19:14

sees the other side So

19:17

you get an image of my ear

19:20

Ear by looking past

19:22

my opposite ear and then you do it again and

19:24

you see my face again It's all

19:26

weird and

19:27

distorted. It's at the very edge

19:29

of your your field of view, but it's there

19:32

If I were a black hole, I would appear to be

19:34

about twice as large

19:37

Or my face would appear to be Twice

19:39

as large and in the middle of

19:41

the image you would see my face then you would see the edge

19:44

of my head Then you would see the back of my head then you'd see

19:46

the other opposite side of the head and then you'd see

19:48

the front again You

19:51

Can imagine doing this with a real? Black

19:54

hole

19:54

you follow this laser pointer anytime a

19:56

laser pointer lands on event horizon you see nothing

20:00

Eventually, you do see so far out

20:02

you're like looking all the way to the

20:04

side, and that light gets

20:06

bent, but it doesn't land on the black hole, and

20:09

so you end up seeing the background universe.

20:11

The shadow appears larger

20:13

than the event horizon,

20:15

because that's how powerfully

20:18

black holes bend the paths of light.

20:21

That means that if you approach a black hole, the event

20:23

horizon looks larger than it should, which to me is

20:26

way creepier fact than it should

20:28

be.

20:29

And there's more.

20:32

Hey space cadets, we need to take another

20:35

small break, because I need to mention

20:37

that this episode is brought to you

20:39

by the T-minus Space

20:42

Daily Podcast. Yes, I am advertising

20:44

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I think there are more than a few listening

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21:32

one for you. So go

21:35

check them out, give them a listen. I

21:37

would really appreciate it. Now back

21:39

to the show.

21:42

That's the shadow. What about the ring of light

21:45

that we see in these images? Some of that light is

21:47

from the accretion disk around the black hole.

21:50

It's plasma. It's hot. It's

21:52

glowing. It's emitting photons, and it's just some of those photons

21:55

are aimed towards the earth. And so we see it. But

21:58

most of the light. comes

22:00

from a ring. Something

22:05

called the photon ring. It's much brighter. The

22:08

location of the photon ring depends

22:10

on the exact geometry of the situation.

22:12

Sometimes that ring is actually embedded in

22:14

the shadow.

22:15

Sometimes it's outside of it. It depends. That's

22:18

right, a ring of light forged in darkness.

22:21

Feel free to insert your own Lord of the Rings

22:23

joke here. What

22:25

the heck is this photon ring? Well,

22:28

to understand that, we have to understand the photon

22:31

sphere. You see, black holes bend

22:33

the path of light. Cool, got it.

22:36

Check, we understand this. But at a

22:38

very special distance from the black hole, the gravity

22:40

is so strong that light gets bent so

22:42

much that it circles around the black

22:44

hole in an orbit. Yes,

22:47

black holes can force light to

22:49

orbit around them. If you were near

22:52

a black hole at this very special distance,

22:55

you could literally see the back of your own

22:57

head in front of you. Why? Because

23:00

a photon would leave the back of your head

23:02

and it would orbit around the black

23:04

hole, circle around, and then come

23:06

right into your eye. You would look in front

23:08

of you and you would see your own back of your

23:10

own head.

23:13

This special distance where

23:15

the gravity is just right to make photons

23:18

or light follow orbits is called the photon

23:21

sphere.

23:22

And for a regular plane non-rotating black

23:24

hole, it's around three halves

23:26

the radius of the event horizon. So about 50%

23:30

bigger than the event horizon.

23:32

Rotating black holes actually have two photon

23:34

spheres that counter rotate against each other because

23:37

they're awesome and complicated in a different episode.

23:40

So what's this photon sphere do? Let's imagine

23:42

a bunch of light rays coming in and encountering

23:44

the black hole. The light rays could be from the accretion

23:47

disk around the black hole, could

23:49

be from the wider universe, it doesn't matter. They're

23:51

just photons. We're surrounded

23:53

by photons all the time. That's what photons do.

23:55

Some of these photons pass near the black

23:58

hole.

23:59

And when they do that,

23:59

if they do well outside the photon sphere,

24:02

then they get bent a little, you know, like

24:04

a curve bank shot, and they go

24:06

off in some random direction.

24:08

They move on with their lives.

24:10

Maybe those photons end up aiming for the Earth. Maybe

24:12

they don't. No big deal. That's some of the

24:15

light that we see in these images. But

24:17

some of the photons cross into

24:20

the photon sphere, where they start

24:22

to loop around the black hole. But the

24:24

photon sphere itself is unstable. Photons

24:26

will not last long there. They are

24:28

guaranteed to leave.

24:31

They can't stay in orbit forever. It's

24:34

not a stable location. There'll be some

24:36

interaction, some trade and energy,

24:38

or something will force them to leave.

24:41

When they leave the photon sphere, some of them plunge

24:44

down into the event horizon, never to be seen in

24:46

this universe again. And

24:49

some escape. When

24:51

they do, they can be beamed

24:54

at us at this special

24:56

distance. There

24:59

are a lot of photons that can

25:01

loop around in this special way, because there

25:04

are a lot of photons in the universe. And

25:06

they're going in all

25:09

sorts of crazy random directions,

25:11

the usual diffuse scattering

25:14

of light. But the photon

25:15

sphere acts like

25:17

a magnifying lens. Light

25:20

from all these directions end up

25:22

getting beamed. So

25:25

what you get at the photon sphere

25:27

is an accumulation of

25:30

light. Then all this light coming

25:32

from all these different directions, instead of continuing

25:35

on in random directions, instead

25:37

get beamed away from the black hole, scattered

25:39

away from the black hole at this special

25:41

distance, the radius of the photon sphere.

25:45

The photon sphere appears to us as

25:47

a ring because all of our images are two-dimensional.

25:50

So we flatten the whole thing and we see a ring.

25:54

In which case, this source,

25:57

this magnifying lens of light gets a

25:59

new name. the photon ring, appropriately

26:01

enough, that is light

26:05

from the wider universe, from the accretion

26:07

disk, from everywhere, getting

26:09

scattered in a very special

26:12

place that accumulates

26:14

that light and sends

26:17

it on its way at a very specific

26:19

distance from the black hole. In

26:22

a perfect world, this should appear as

26:24

a super thin, bright ring,

26:26

but remember that observing black holes is really hard,

26:29

and so our pictures are really fuzzy. So

26:31

this super bright, thin ring gets

26:33

smeared out like an out-of-focus picture,

26:35

because remember, interferometers are just

26:38

really garbage telescopes.

26:40

And instead of a bright, thin ring, we

26:43

see a generally blobby, diffuse

26:46

donut of plasma around the black

26:48

hole. So when you see that light

26:51

in the event horizon telescope pictures, you're

26:53

not seeing

26:54

light from the accretion disk itself, you're

26:56

seeing a little bit of it, but most of that

26:59

light is coming from the photon sphere, this

27:01

special place

27:02

where light can orbit around a black hole.

27:06

There's more. See,

27:09

the light can take multiple loops

27:12

around the black hole. It might just take

27:14

one U-turn, you know, you imagine there's

27:16

this photon coming in from the right, it

27:18

encounters the black hole at this special distance,

27:21

and it gets looped around, and then it scatters it

27:23

just the right way and aims for us. And

27:27

then there's another bit of light coming from a completely

27:29

random direction, like from the top, and it's coming

27:31

down, and it's coming down, and

27:33

it gets bent around the photon sphere

27:36

because it had just that right radius, and

27:38

it gets beamed towards us. If

27:40

the distances are off, if it's inside the

27:42

photon sphere or outside the photon sphere, then

27:45

there's nothing special about it. The light just scatters

27:47

randomly as usual. But

27:50

because the photon sphere is there, the light gets

27:52

scattered in a very special direction. And we

27:54

see this emphasis of

27:57

light.

27:58

But photons can take... One trip around, two

28:01

trips around, three trips, four or five.

28:03

20 trips around the black hole before eventually

28:05

leaving.

28:07

Each one of these trips represents its own

28:09

ring,

28:11

its own sphere.

28:13

Takes one trip. That's the main sphere. The

28:16

main ring that we see takes two trips. There's

28:18

this tiny, subtle inner ring

28:20

to it. A third trip,

28:23

a third ring. Each ring

28:26

is fainter than the one before because the chances

28:28

of photons making multiple trips

28:31

gets lower and lower.

28:33

And this can tell us so much about black holes, these

28:36

nested photon rings, because

28:38

this is an exquisite test

28:41

of general relativity, as close

28:43

to the event horizon as we can get. Because remember

28:45

any light passing closer than

28:47

the photon radius is going to end

28:50

up in the event horizon. It's not leaving.

28:53

This is the closest we can observationally

28:55

get to a black hole

28:56

is this photon ring. Like I said, the photon

28:58

ring, depending on the exact geometry of

29:01

the disc, the source of light, the

29:04

size, whether it's spinning or not, that ring can

29:06

appear actually inside the shadow

29:08

or it can appear outside the shadow. It depends,

29:11

which is really weird to think about,

29:14

but this is the closest we can see to a

29:16

black hole. This is the greatest test we

29:18

can have of general relativity

29:21

because this is the most extreme source

29:23

of gravity that we can directly

29:26

see.

29:28

Yeah, there's Hawking radiation, but one particle

29:30

per year. Good luck.

29:32

This is the closest we can see to the actual

29:34

black hole is this photon ring.

29:39

We currently do not have the resolution needed

29:41

to see

29:43

the sub rings, let alone the main

29:45

photon ring. It's still this fuzzy blob in

29:47

all of our pictures. It's hoped that

29:49

extending the event horizon telescope into space

29:52

should allow us because we put one more element

29:54

in space. That's an even longer baseline that gives

29:56

us even more resolution. And,

29:59

but we'd have to work even harder

30:01

to reconstruct the image. It's

30:04

hoped that we can get better and

30:06

better resolution images of these photon

30:08

rings. But it's

30:10

hard. But

30:13

no matter what, this

30:15

is the best

30:18

observations we can make of a black hole, the

30:20

shadow itself, which is larger than the event

30:22

horizon because of the extreme gravity, the

30:25

photon ring, another expression of the extreme

30:27

gravity that beams light from

30:29

all sorts of random directions at this

30:31

special radius for our enjoyment.

30:35

No matter what the shadows of black holes

30:37

are not boring at

30:40

all.

30:42

Thank you to Kelsey R on email and

30:44

him and W on YouTube for the

30:46

questions that led to today's episode. Please

30:49

keep sending me questions hashtag ask

30:51

a spaceman ask a spaceman at gmail.com

30:54

or just the website ask a spaceman.com.

30:57

Thanks to all my Patreon contributors, but

30:59

especially my top ones this patreon.com

31:01

slash pm Sutter like to thank Justin

31:03

G, Chris L, Barbara K, Duncan M, Corey D, Justin

31:06

Z, Nalia Scott M, Rob H, Justin

31:08

Lewis M, John W, Alexis Gilbert M,

31:10

Joshua, John S, Thomas D, Simon G,

31:12

Aaron J and Jessica K.

31:15

Again, that's patreon.com slash pm

31:17

Sutter. Thank you so much to all the

31:20

supporters, all the people who ask questions,

31:22

all the people who write reviews of

31:24

this podcast and share it with your friends. I

31:27

really do appreciate it and I will see you next time

31:29

for more complete knowledge of time

31:31

and space.

31:53

Thank you. you