Episode Transcript
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Speaker 1 (00:08):
Hey, Jorge, did you pack a bag? Why do I
need to back one? We're going on a field trip today.
Oh nice, Where are we going? We're going on a
physics field trip. Actually, does I mean I have to
go to a classroom or something. It means it's a
thought field trip, sort of like a thought experiment. I
don't need to bring snacks. It's a thought experiment, So
you can bring any snacks you can think of. Well,
(00:30):
I can think of a lot of snacks. I am
or handmade cartoonists and the creator of PhD comics. I'm Daniel.
(00:52):
I'm a particle of physicist and I enjoy snacks from
around the world. Do you just think you're a physicist, Daniel,
or are you an actual physicist? What's the difference? Welcome
to our philosophy podcast, folks, the philosophy of being a physicist.
That's another branch of philosophy we invented on this podcast.
Now is it a branch of philosophy or physics. It's
(01:13):
a whole new field all into itself. No. On this podcast,
we'd like to think that anybody who asked questions about
the universe is a physicist. So anyone who claims to
be a physicist is a physicist in my book, Well,
welcome to our podcast Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we all
act like physicists and ask deep questions about the universe.
(01:34):
How does it work, what does it look like? What
are those little bits over there? How did these things
even happen? And we ask those questions and try to
find the answers. Sometimes we get some answers, Sometimes we
just find more questions. But we hope to entertain you
a little bit along the way because there are a
lot of questions out there in the universe, because there
are a lot of mystery still out there unresolved in
(01:55):
the cosmos. That's right, because there are a lot of
people out there asking questions. Remember that questions don't belong
to the universe. They belong to us. There are our
personal questions about the universe. And the way that science
moves forward is people asking those questions, people thinking how
does that work? Or what does it look like in there?
Let's go find out. So the way we unravel secrets
(02:16):
to the universe is first by asking those big questions. Yeah,
I guess you're right. That is interesting to think about
that there are like the universe doesn't have mysteries on
its own. Like, I mean, the universe knows everything it
knows about itself. It's just us who don't know it.
And so we're asking questions and we make it a mystery.
That's right, except that we are part of the universe.
So maybe we are that part of the universe that's
(02:38):
asking itself questions about itself. Universe know thyself. Man, you
just give me a headache, a philosophical headache of physics tache.
But yeah, there are a lot of mysteries out there,
are a lot of pockets of unknown in the universe
that humans we just don't know what's inside of him,
that's right. And we are a curious species. We want
(02:58):
to see what's under that what's over that hill, what's
in that weird bag over there. We just want to know.
So we want to see all the cool stuff in
the universe. We use our telescopes to zoom in on
far away stuff. We pick up rocks that fell from space.
We want to see it all. Yeah, and there is
no more mysterious pocket of mystery, mysterious pocket of mystery
(03:21):
in the universe than a black hole. Black Holes are these,
you know, pockets of space time that nobody really knows
what's inside of them, And there might not be anything
interesting inside of them, but just the fact that nobody
can look inside, that nobody can go inside and tell
us what comes out, makes it mysterious. It's like if
I hold an empty pillow case in front of kids
(03:42):
and say what do you think's inside? They get excited.
Maybe this is a dinosaur were in there. It doesn't
really matter as long as it's hidden. It's mysterious and
what makes us curious. It sounds like something someone might
call the cops on you, Daniel, if you started doing
that random kids on the street. That was another thought experiment,
not recommended that in your thought arrest. Yeah, so black
(04:05):
holes are super weird. I mean they were theoretical up
until a little while ago, until we first saw one
with our own eyes, our own in tennis and cameras.
I mean that we saw them gravitationally, but nobody had
really sort of seen one until recently. That's right. There
were a theoretical curiosity because general relativity predicted that they existed,
(04:25):
but also predicted that they had something really really weird.
At their hearts, a singularity, a place where the theory
actually broke down because it predicted that space curvature went infinite.
So for a long time physicist thought, well, that probably
doesn't actually happen. The universe does something, there's something to
prevent that from really existing in reality. But then we
(04:46):
saw them. We see them at the center of our galaxy,
these super massive black holes with stars whizzing around them,
and we see stars that have collapsed into black holes.
So we know they are a real thing in the universe.
We just don't know it's in them. Yeah, well we
got a picture of them. They do indeed look like
black holes. I mean the picture that we got was
it last year, I think, or a couple of years ago.
(05:08):
It shows you a black circle basically with all the
right stuff scrolling around it, but it is indeed black
like there you can't see anything in the middle of it. Yeah,
it's simultaneously like amazing technologically and also kind of disappointing
because all you're really seeing is a sort of doughnut
of accretion disc around the black hole that's glowing in
(05:29):
just the way you'd expect, But the black hole itself
just looks black, and so it's sort of hard to
distinguish a black hole from nothing. Well, you can't see
any stars behind it, like in we look anywhere else
in space, you would see its stars behind him, right, Yeah,
that's true. And next to that really bright region from
the accretion disc, it is a pretty black shadow. So yeah,
(05:51):
you're seeing a very dark shadow from the black hole. Yeah.
So the question is what's inside of that dark region?
What's going on in there? Because you can't see in
it and there's nothing to see when you take a
picture of it. Yeah, and so the explorer and us
wonders what's in there? Is it possible to like send
a probe or a ship or even a person to
go explore? Yeah, and this is a situation that a
(06:13):
lot of people have considered, like what would it be
like to send something into the black hole? So today
on the podcast, we'll be asking the question, what's it
like to fall into a black hole? Is this something
you would wondered about? Horrid these days that have other
things to wonder about the faith of humanity without a
(06:38):
black hole. It makes me wonder like if I was
standing at the edge of a black hole, would I
be tempted to jump in because you might learn some
of the secrets of the universe. But on the other hand,
you'd probably die and at the very least would never
come back to see your family. So that'd be a
tough choice to have to make. Yeah, I think I
would wonder what it's like to jump into it, but
maybe not me jumping into it, you know what I mean?
(06:59):
Like I would love for I don't know somebody else
to do it, And for example, you'd happily send me
into a black hole if I could report back what
I mean, if you're standing at the edge of a
black hole, would I push you? Maybe don't answer that question. Well,
the good thing is the evidence would be totally gone.
(07:20):
They could never ask you if I pushed you or not.
The perfect crime. Wow, somebody has to write that science
fiction novel death by black hole black hole murder. Yeah,
the perfect crime because it technically it might not never happen.
It might even destroy the information. Wow. Well that's that.
The other next sci fi novels are criminals who use
black holes to erase their evidence. Yeah. So, as usually,
(07:42):
we were wondering how many people out there had asked
themselves this question, and how many people out there. I
think they have an answer for this. So, as usual,
Daniel went out into the wild of the Internet to
ask listeners what they think it's like to fall into
a black hole. And all of these folks answered this
question with knowing it in advance and no googling allowed.
(08:03):
If you would like the answer that questions for future episodes,
please write to us do questions at Daniel and Jorge
dot com. We will happily put your uninformed speculation on
the podcast. So I think about it for a second,
what do you think it would be like to fall
into a black hole. Here's what people had to say.
Either you fall in a really huge, immense black hole,
(08:25):
uh those super massive black holes, and you fall really
slowly and then died at some point, or you get
spaghettified in one of these smaller ones. I know that
as you fall into a black hole, you will be
experiencing everything in your own real time, so it's not
like everything will stop or anything like that for you.
(08:48):
As far as the rest, I would say, very painful,
thank you, be very good. I think it depends on
what you mean. So, like the body itself would be
what they're called spaghettifier and broken into its constituent atomic particles,
so you'd be pretty much ripped apart. But like the
gravity is so strong, but like even the difference in
(09:10):
your height would see a pretty significant shift in gravitational forces,
So the gravity on your feet would be very much stronger,
depending on restruction you're following in than the gravity you
know you'd feel. And at the top of your head, well,
I know that from the outside you'd never seen fallen
because time would slow down so much that they love frozen.
(09:32):
But you yourself, I think you'd probably be torn to shreds.
As you know, any part of you that was ahead
of some other part of you would be sucked in
much faster. And I guess we don't really know the answer,
but I assume it would be like torn to shreds.
I don't know. I think from what I know you
(09:55):
my body would be ripped apart. It would just be
and him, you know, stretched like a spaghetti uh, stretchy,
like one second you're there and the next ye're not. Obviously,
no one knows what it's actually like to fall into
a black hole, but from what I've read, the theory
(10:16):
is that you would be spaghettified because you know, if
you fell in the feet first, for example, your feet,
the gravity at your feet would be much stronger than
your head, so you get pulled apart. So I don't
think it would be much fun. All right, and not
good is the general message I get from. Yes, our
council of Advisers recommend against pushing your podcast host into
(10:39):
a black hole. Although that I heard a lot of
mentions of pasta, so you know that could be a
good snack. Yeah, exactly. I said, come prepared. You know,
we're going on a field trip, so everybody packed spaghetti
at something. All right, Well, there a lot of interesting answer,
is there. Some people think you would survive and some
people don't think you would survive. Yeah. I seem to
me like these folks have about black holes and if
(11:01):
even maybe given some thought to what it would be
like to fall into it before they heard this question,
so they were pretty well prepared. I was impressed. Good answers, listeners. Yeah,
and nobody have to push anyone into it either that
we know off and no people were tossed in the
black hole for the making of this episode. Yeah, so alright,
let's jump into it, Daniel. So today we're gonna sort
(11:22):
of paint the scenario right, like, we're gonna go next
to a black hole and then we're actually gonna go in.
We're going to talk about what it's going to be
like for somebody doing that. That's right, because a lot
of people mentioned this concern that you might not even
be able to go into a black hole because you
would get torn up before you get too close. So
I guess a question is is even possible to go
(11:42):
into a black hole and still feel that experience or
would you just die before even going in? Yeah, And
it turns out that it depends. It depends on the
size of the black hole. If you have a really
small black hole, then the edge the event horizon, the
part past which nothing thing can escape, is much closer
to the singularity at the heart, and so it's actually
(12:04):
difficult to get to the event horizon because you have
to get sort of closer to the black hole. But
for a bigger black hole, a massive one like the
one at the center of our galaxy, the event horizon
is so far from the singularity that it's safer to
approach the event horizon. Wait, what what do you mean?
So it is possible to go into the black hole.
It is actually possible to survive going over the event
(12:27):
horizon into the black hole as long as the black
hole is big enough. That's sort of the twist. All right, Well,
maybe break that down for us. What what would cause
me not to survive going into the black hole. So
the reason that a black hole pulls you apart this
spaghettification is due to the strength of its gravitational field,
but also due to how fast that gravitational field is
changing the slope of the gravitational field, if you will,
(12:51):
Because you are not a tiny point, you are an
actual physical thing. You have a front and a back
and two sides. And if the black holes pulling on
for example, you're head at a different strength than is
pulling on your feet, then effectively it's pulling your head
off of your body. It's tearing you apart. That's what
we call tidal forces. When gravitational forces are different on
(13:11):
one side of an object and on the other, that's
the same as having a force pull that thing apart.
And that's happening like right now. It's not something that
just happens near a black hole. Is like if I
stand up here on Earth, the Earth is pulling my
feet more than it's pulling my head. That's right. The
Earth is trying to pull your head off your body
right now. But the slope of the Earth's gravitation of field,
(13:35):
how much different it is at your feet and at
your head, is very, very small, and your body is
strong enough to resist that tension. You can resist the
tidal force of the Earth. So here on Earth it's weak.
Post gravity is relatively weak pasta compared to a black hole.
But in a black hole gravity is stronger, and so
the difference between the gravity and my head and my
(13:56):
legs is stronger. So that could pull me apart. That's right,
because these tidal forces, the thing that pulls you apart
depends on the forces being different at your foot and
at your head. And as you get closer to a
black hole, closer to the center of the black hole,
the curvature increases, the difference of the gravitational force at
your head and your foot increases. Now the Earth, you
(14:17):
can't get that much closer to the Earth than its surface.
If you dug into the Earth, the gravitational forces actually
start to weaken because you have a lot of the
mass on the outside of you. The cool thing about
a black hole is that it has so much mass
in a tiny little area, so you can get really
close to it without losing any of the gravitational force.
It just gets stronger and stronger and stronger. So that's
(14:37):
why for a black hole, you can get close enough
to it that it has an event horizon, and that
the tidal forces, these things pulling you apart, can be
stronger than your body can withstand. Now, I think that
usually people talk about that happening before you enter the
black hole. But you're telling me that it's not necessarily
Like I won't get spaghettified necessarily outside of the black hole.
(15:00):
I could get spaghettified inside of the black hole, in
which case I could technically go into a black hole. Yeah, exactly.
The threshold of which you get spaghettified depends on the
mass of the black hole. Now, so it is the
size of the black hole. But there's this point where
they cross. If a black hole is big enough, then
the sort of spaghettification point is inside the event horizon.
If the black hole is small enough, then the spaghetification
(15:22):
point is outside the event horizon. You get torn apart
before you go inside to a small black hole you
get torn apart after you go inside. For a really
big black hole, that's because the size of the black
hole where the edge of the event horizon is depends
differently on the mass than the spaghettification point. Right, And
when you say event horizon, you mean like the actual
like what this is considered the edge of the black hole,
(15:44):
Like that's the point where light can't escape exactly. And
that's just an arbitrary definition. We say the black hole
begins at the event horizon, the place where a ray
of light, even if pointed exactly away from the black hole,
still would not be able to escape. All right, So
then I guess then technically, if I wanted to go
into a black hole like pass the event horizon, I
(16:04):
need to pick a big black hole, right, because the
little one would turn me apart before I go into it. Yeah,
that's right. You need to pick a black hole about
a thousand times bigger than the mass of our sun
before it's safe to enter, before coming to the event
horizon will not tear you apart from the tidal forces.
That doesn't sound like a lot like I would expect to,
(16:26):
you know, decent sized black hole to be more than
a thousand times the mass. Yeah, and there are big
black holes out there that are much bigger than that.
In fact, there's a bit of a gap. There are
a few black holes out there that are like ten
or fifty or eighty times the mass of our sun.
These come from stars just collapsing into a black hole.
And then there are black holes in this sort of
million sun range, and these are the black holes at
(16:47):
the centers of galaxies, these super massive black holes. There
actually aren't a lot of black holes in that intermediate
range because there isn't a process to form them, unless there,
of course, are primordial black holes that nobody's seen those.
So your options are black holes that are sort of
small like the mass of our sun, which would be
too dangerous to get close to, or really really big
(17:07):
ones which are pretty safe to approach. There's no medium option,
like you can only buy a compact car or a
giant suv. Yeah, but the giant SUVs are safer, and
the bigger they get, the safer they are, because the
bigger they get, the further from the singularity the event
horizon is, and so the safer it is to go
across that event horizon, right, And the bigger black holes
(17:29):
are also harder to park, so there's a trade off. Yeah,
well you're going to the black hole. Nobody's gonna bring
the black hole to you. What do you mean, no
matter how important you are. Well, technically everyone near a
black hole pools on the black hole. So the technically
the black hole moves a little bit towards you tube. Yeah, exactly,
just like blowing in the wind. You know, it has
almost no impact. But you're right, You and the black
(17:50):
hole are a system, and together you're orbiting the center
of mass, which is basically the black hole. You're basically
negligible when it comes to black holes. Well, it sounds
like if you're curious to see what's inside of a
black hole and you want to get inside past the
event horizon, you need to pick a big one, otherwise
you'll be torn apart before you even get there. Right, Well,
let's get into what you would actually see as you
(18:10):
go into the black hole, and let's talk about what's
inside of it. But first let's take a quick break.
All right. I know, we are taking the probably the
worst field trip anyone can take, which is to go
inside of a black hole. He probably wouldn't come back
(18:32):
in time for races or the school bell. But we're
talking about what it would be like to go into
into it, Like what would you experience, what would you see,
what would you hear or feel as you go in? Yeah,
it would be pretty fun. I think it would be
an awesome field trip. I mean, it's not when you
could return from and so the permission slip would be
pretty crazy. But yeah, I think you might learn something
(18:54):
deep and true about the universe. You might reveal a secret,
an answer nobody knows, and so I think it'd be
pretty someome. I would definitely sign myself up for this
field trip really, like tomorrow, if somebody said, hey, Daniel,
would you like to go into a black hole and
learn the secrets of the universe? You would jump in?
You know, I might. I don't think I would take
a trip to Mars just for like planting a flag
(19:14):
on Mars, but to know the answer to one of
the deepest questions in the universe. I might, just because
remember that black holes are where general relativity breaks down,
is where we know that our theory of how space
and mass bend and dance with each other doesn't work anymore.
And something quantum, something new, something weird is going on
in there, and the only place we think general relativity
(19:36):
does break down is inside these black holes, and so
they literally hold secrets to really deep questions about the
universe that physicists are struggling over. So it's sort of
like asking, if I had an answer to a deep
question about the universe in this envelope, would you tear
it open? Yeah? Of course I would. I would shred
it instantly, right, even if the envelope tells you apart
they never come back. Well that's really interesting, because I mean,
(20:00):
you would give up knowing everything else about dear life
just to get this one secret. I think you'd be
really tempting. Yeah. I mean, fortunately, nobody's going to actually
ever offer me this, so I can bluff as much
as i'd like. Oh I see, I see, we can
have any opinion really doesn't matter, all right, So what
would it be like? So I picked the big black
hole because those are safer to approach than little black holes,
(20:21):
which is kind of weird, but I guess that's how
the math works. So now I picked a really big
one and I'm approaching it. What do I see? Well,
the first thing you see is the thing around the
doughnut around the black hole. Because most black holes are active,
they're still sucking stuff into them. But stuff doesn't fall
directly into the black hole because it still has some
angular momentum, which means it's spinning the way that we're
(20:44):
spinning around the Sun for example, without just falling in.
So the black hole has this stuff in orbit around it. Also,
that's called the accretion disc. It's the swirling mass of
gas and dust and doing all sorts of crazy stuff,
and some of it is falling in. But to all end,
it has to like lose some energy, has to give
up some angular momentum, and it does that by like
(21:05):
getting squeezed and glowing and giving off light. I guess.
In technically, a black hole is not that different than
like a sun, for example, so it would have stuff
orbiting around it. Yeah, it has hot gas orbiting around it,
and that hot gas is glowing. It's not having fusion necessarily,
but it's a very bright emitter. These are some of
the brightest things in the galaxy, these disks of gas
(21:26):
around a black hole. When they get really intense, you
call them quasars or even blaze ours, and they are
super duper crazy bright, So it's sort of ironic that
some of the brightest things in the universe are next
to some of the blackest things in the universe. Right,
But it's also not true that every black hole has
a bright accretion this like, some black holes could just
be like sitting there, right. Yeah, they certainly could be,
(21:47):
and it just depends on what's around it. If you
have a primordial black hole that was created during the
Big Bang and it's just like floating out around there
and there's nothing to eat, then it could just be
silently stealthily sitting there, totally black. The creation disc just
comes from stuff that's sort of on deck about to
fall into the black hole. But you're right, it's not
necessary that there's anything like that there. But if we're
(22:09):
choosing a supermassive black hole, the only way that happened
is if it's at the center of a galaxy, which
means it's been sucking in gas and dust and it's
surrounded by stuff. So you're pretty likely to find an
accretion disk around a supermassive black hole. Okay, right, that's right,
because we picked a big one to go into, so
it's it probably has that disk, and so as you
approach it, you probably want to avoid that disc, right,
(22:31):
and it's going to be a flat disc, so it
it's not that hard to avoid because it's pretty hot,
that's right. It's pretty hot and pretty nasty. And say
you want to avoid that, you want to come in
from the top or from the side, unless it's one
of these quasars, in which case it also has a crazy,
you know, spinning magnetic field that's beaming a huge beam
of light along the north and south axis, So you
want to come in a bit of an angle. But yeah,
(22:52):
there's an opening there. I mean, this is a black
hole field trip, but we still want to keep safety
in mount right, Yeah. And so the acretion disc is
at about three and a half times the width of
the black hole, right. Yeah. If you say the event
horizon is the radius of the black hole, then the
accretion disc sort of ends around three and a half
times that. Anything less than that and the stuff is
(23:12):
going to fall into the black hole. So everything at
that point and a little bit further out can sort
of stably orbit the black hole for a while until
it loses its energy through collision or radiation or whatever,
and then eventually falls in. But you don't find much
gas and dust less than three and a half because
that stuff has already falling in. I see, it's kind
of like the rings of Saturn. Like the rings of
(23:33):
Saturn are a ring, they don't go all the way
to the surface of the planet because stuff that's closer
to the planet falls in. So it is like it's
hanging out there in space. Yeah, exactly. So that's sort
of the inner edge of the accretion disc of the
black holes three and a half times of radius for
just that same reason. Yeah, alright, so I'm going into
the black hole, I move past the acreation disk. What
(23:54):
am I seeing now? So now what you're looking at
is the shadow of the black hole. And we have
to keep in mind as we get closer and closer
to the black hole, that space around the black hole
is bent. Remember that space is not just the backdrop,
is not just the emptiness. There's a dance between space
and mass. Mass tells space how to bend, and space
tells mass and light how to move. So that means
(24:17):
that photons don't travel in straight line. So what you're
looking at, what you see isn't necessarily what's there and
the environment of Earth where light mostly travels in straight lines.
You can assume that you're seeing things as they are,
but if there are lenses and distortion happening, then you
expect to see an image which doesn't necessarily correspond exactly
to what's literally there. But what you see in front
(24:39):
of you is this big black circle, this shadow of
the black hole that's like two and a half times
the size of the actual event horizon, and you'll see
it be all black. And why is that? Why is
the black circle that I see bigger than the actual
black circle of the black hole Because anywhere in that
black shadow there's no path for light to get to
your eye. Is like if you traced it backwards, if
(25:02):
you started from your eyeball and trace the path of light,
that light would always end up in the black hole. So,
for example, what you're looking at when you look straight
at the heart of the black hole is the black hole,
and no light comes from there, obviously, so you see black.
But if you look above the edge of the black hole,
like larger than the rays the black hole, then the
light would come from the back of the black hole
(25:23):
and then curve around the black hole to your eye.
So what you're actually seeing there is sort of the
path light would take to get to the back of
the black hole, but again, no light is emitted from
the back of the black hole. Yeah, so that means
that I'm actually seeing kind of the back of the
black hole right like above the actual black hole. I'm
actually seeing the back of it because it's sort of
(25:44):
the light gets distorted from the back around the black
hole and onto my eye. It's sort of like a
lens like it's being magnified in three D exactly. Another
way to think about it is put a star on
the other side of the black hole, like a really
bright star. Would you see it's light creeping over the
edge of the event horizon. The answer is no, because
(26:05):
those photons would get bent down and then sucked into
the black hole. Even if they don't hit the event horizon.
As they pass over it, the black hole curves their path,
It slurps them down, and they get sucked into the
black hole eventually, so you don't see that light. So
the first light that you could see, the light that
would survive being passed to your eyes is about two
(26:25):
and a half times the event horizon. That's why we
talked about seeing an image of the black hole. It's
not the literal black hole itself. It's a distortion of
the space that makes it look bigger. It's like a mirrage,
like it's the black hole is pretending to be bigger
than it is. That's right. It's like a black cat
that's gotten really mad, and it's like and it posts
up and post itself up. Yeah, there you go. Black
(26:48):
cats black holes both are bad luck if they cross
your path, that's right. And so maybe the black hole
is more scared of us than we are of it.
The black hole is full of black cats, which case,
they will tear you apart. Also, I think we just
figured out we just answered the deepest question about quantum
gravity right there. All right, So I see the shadow
and now but now I'm getting closer, even closer than
(27:12):
the shadow, or can I enter the shadow? Well, as
you get closer this shadow, this circle of black that
you see in front of you, it starts out like
two and a half times the rays of the black hole,
it just gets bigger and bigger in your field of view.
And not just because you're getting closer to it, but
because the distortion of space is magnifying it more and more. Right,
So as you get closer and closer to it, it
(27:32):
sort of grows and takes up more and more of
your field of view, and it gets bigger and bigger
and bigger. But there is awesome stuff that happens even
before you get to the event horizon. Yeah, what happened. Well,
once you get to like one and a half times
the event horizon, this is the last point where light
can orbit the black hole stably. Remember the event horizon
(27:53):
is where light pointed outwards can't escape the black hole.
But even before that, light pointed inwards won't just gave
the black hole. Right, If you're two times the size
of the event horizon and you shine a flashlight into
the black hole, boom, it's going to get slurped in. Now,
if you're one and a half times the event horizon
and you shine a flashlight sideways so that it's not
going towards or away from the black hole, and that
(28:15):
light has just enough power to bend all the way
around the black hole and come back and hit you
in the back of the head. What light can orbit
a black hole? But light has no mass. Light has
no mass. But remember space is curved, and light doesn't
move in straight lines. It moves in the shortest path.
And what gravity is is the bending of space, which
(28:38):
changes effectively what is the shortest path between two things?
Like the reason that the Earth is going around the
Sun is not because the Earth has mass, but because
the Sun has bent the shape of space. So it's
more natural for this Earth to go in this circle.
And so light also can follow these curved paths when
space is bent. We see that all the time in
gravitational lensing, etcetera. And so what how upens here is
(29:00):
light essentially is orbiting the black hole. So if you
shine a flashlight, it will illuminate the back of your
own head, or if you don't have a flashlight, you
look out and you'll see photons from the back of
your own head. Whoa I've always wanted to know at
the back of my head looks like And you can't
just ask somebody around you. You have to take a
field chip to the black hole. That's the most efficient
(29:20):
way just to find that. It's easier than putting you
a couple of mirrors. And so at that point, any
light that's pointed at all towards the black hole will
fall in. Any light that's pointed perfectly tangent to the
black hole will orbit, and any light that is pointed
at all out will still escape. It needs some angle
away from the black hole to escape, but it's still possible.
(29:43):
So this is the point of no return for almost
like for for light in a way, Yeah, as you
get closer, light can still escape if it's pointed further
and further out, And as you get closer and closer,
the event horizon is a point at which even light
pointed straight out from the black hole can no longer
get out. But the angle for light to escape gets
(30:04):
smaller and smaller as you get closer and closer to
the event horizon, until eventually it disappears. All right, So
then I've satisfied my curiosity. I know what the back
of my head looks like, finally, And then what happens
as I go in further than one and a half
times the radius of the black hole. Well, it's pretty
cool because this shadow, which has become bigger and bigger,
grows and grows and grows, and eventually it's even taking
(30:26):
up more than half of your vision. All right. If
you imagine yourself sort of at the center of a sphere,
you can look all around you. The black hole starts
out at one point of it, and as you get
closer and closer, it fills up more of that sphere,
and then it takes up even more than half of
that sphere. So you're not yet inside the event horizon,
but the distortion of light makes it look like it's
(30:47):
sort of eating you. It's like all around you, behind you,
you can still see out to the universe, but a
head of you and sort of around you you see
just the black hole shadow and it's completely pitch black, right,
like there's you're seeing the black hole, which is completely black.
There's no stars or anything that's right. It's there's no stars,
there's nothing there because everything that has fallen into the
(31:09):
black hole is ahead of you and cannot radiate towards you. Right,
Nothing that's inside the black hole can shoot anything in
your direction. The thing that remember is that black holes
change the shape of space, and inside the black hole,
space is sort of one directional. Every direction points towards
the center. And that's sort of beginning to happen now
as you approach the event horizon. So things that are
(31:31):
closer to the event horizon than you, they could still
send you a photon, like if a friend had jumped
in before you and he was sending flashlight pins back
to you. You could still see your friend, but the
black hole itself is not radiating anything else. But if
you turn around and you look at sort of the
shrinking window out to the rest of the universe, you
do see something pretty awesome. Yeah, I guess if it's
(31:52):
eating you up the blackness, it would be like going
into a tunnel and the entrance to the tunnel gets
smaller and smaller. Yes, it's exactly like going into a
tunnel and that engines gets smaller and smaller. Except when
you look at from a tunnel, you can only see
sort of light that shoots straight into the tunnel. But here,
if you look back where you came from, you can
see every direction. You can see the whole universe. Advantage
(32:13):
point from the black hole. You can see from the
other side of the black hole. Right, it'd be like
a super powerful fish eye lens that let's you see
everything in a small view. That's right, because things like
that are on the other side of the black hole.
Their light can bend around the black hole and then
come back and hit your eyeball. So the black hole
while it's about to eat you, is also still giving
(32:34):
you like a three hundred and sixty degree view of
the universe because of all this distortion. You wow. And
so it's a shrinking view two, right, Like, as I
moved towards the black hole, the actual radius of the
actual edge of the black hole, that view is going
to be shrinking and shrinking and shrinking, and then eventually
what it disappears and becomes a dot. Yeah, And as
you move further and further in your window to the
(32:55):
universe shrinks and eventually becomes a tiny dot. Right, Because
think about it. If you're at the event horizon and
you're looking out, the only way for light to escape
is to go straight out from the event horizon any
other angle and it's gone, which means if you invert that,
like coming at any other angle has to have come
from the black hole. So you're gonna see all black
(33:17):
except for one dot. And in that dot is a
picture of the entire universe compressed into a tiny little image.
Oh man, And then you're inside the black hole. And
then you're inside the black hole, and take careful notes,
because I want to hear all about it. All right,
let's talk about what is actually happening to you as
you go into the event horizon. But first let's take
(33:37):
another quick break. All right, Daniel, we are answering the
deepest questions of the universe right now, which is that
inside of the event horizon of a black hole, the
(33:59):
universe has disappeared behind me into a dot. Now I
am inside of a black hole. What's happening to me?
So we don't know. We don't know what's inside a
black hole. We don't know like the structure of matter.
Is there really a singularity at its core? Is there
something else weird going on? We're pretty sure a singularity
can't be right because it's an infinity, and we don't
(34:21):
think that those things actually exist in nature. That's what
general relativity predicts. Though general relativity also breaks down at
the singularity, can't make any real predictions. So we don't know.
But we could just sort of like assume that general
relativity is right and talk about what it would be
like to go into a general relativistic black hole. Well,
it only breaks down at the very center, right, That's right,
(34:42):
we think, So you know, up until you get to
the center, we you can sort of predict what's going
to happen. Yeah, if general relativity just needs like a
small modification, if it means like, okay, general relativity is
right about most of the black holes, just at the core.
It doesn't quite get the singularity right, and it gets
like fuzzed out by quantum mechanics. Maybe, but it could
be that we're totally wrong and that what's inside a
(35:02):
black hole is completely different and a huge surprise, and
quantum gravity is something totally unexpected. We just don't know,
we can't see, which is frustrating. But yeah, we can
imagine that there's some sort of singularity the core, and
then we could talk about what it's like then to
cross the event horizon and dive in. Right, we can
just assume e signs right. I mean a guy, he's
gotten some things, right, He had a couple of big hits.
(35:26):
He's got a pretty good track record. Yeah, so let's
go with Einstein is right and think about what would
be like to fall into the black hole. And again,
the most important thing to remember is that once you're
inside the black hole, space is now one directional. That
means it's not just like gravity is pulling you really
hard and you can't manage to escape. If you could
just go faster than the speed of light, it means
(35:47):
that every path moves towards the center of the black hole.
It's weird to think about space being one directional, but
think about the way you think about time time very
naturally think of as one directional. Outside a black hole,
time is one directional, moves forwards. Inside a black hole,
space is one directional and moves towards the center. Right,
(36:07):
But that's the ending is only because you can't go
faster than the speed of light. If you could go
faster than the speed of light, you could sort of
come out of that space. Right. Oh man, It's like
relativity says you can't escape a black hole and that
you can't go fast in the speed of light. So
what would happen if you believed relativity about one thing
but ignored it about the other thing. I'm not sure
(36:29):
that would sort of break the fundamental assumption of relativity.
You would break into thine. All right, Well, I guess
one thing that's interesting to note is what happens when
you actually cross the event horizon, Like when you step
through that point where light can escape. You know, what
actually happens to me as I go in. Yeah, And
(36:50):
the surprising thing is if you've picked a large black
hole where the tidal forces are not going to pull
you apart, the event horizon is not a special place.
It's not like there's a fire our wall, or there's
a gate, or there's dragons or anything right there. It's
just the point at which light can no longer escape.
And so your view the universe continues to shrink, and
you see more and more blackness around you. And that's
(37:12):
because every path for you, every future direction of yours,
is towards the center of the black hole, which of
course isn't emitting anything and looks black. So that's the
only thing that happens as you cross the event horizon.
No fireworks, no craziness, just sort of a ceiling of
your fate. And so then what I would experience is
just pure darkness. Right according to general relativity, I wouldn't
(37:38):
see any flashes of light or Matthew McConaughey and a
you know, premidal kind of space talking to his daughter.
Depends on if you believe in the power of love rhead, Yeah,
the fifth dimension. Yeah, to like, what according to general alativity,
what would I see if anything, would I just see
stuff continuing to fall into the black hole or what?
(38:00):
You can't see anything that's in front of you, right,
because even if stuff has fallen into the black hole
before you, it's ahead of you and it can't emit
light in your direction, so you just see blackness ahead
of you. The only light you could see while you're
inside the black hole is from things behind you. If
you're not traveling at the speed of light, then someone
outside the black hole can still be shooting a laser
(38:22):
at you that could catch up to you in that
remaining little tunnel from the outside universe, so you still
can't see a little dot. Otherwise, all around you is
just the blackness, a view of the singularity. But that's
assuming that your brain still works. Right. We talked about
spaghetification and the tidal forces tearing you apart, but we
don't even know if your mind, if your brain could work,
(38:45):
if space becomes one directional, because you know, your brain
relies on nervous signals, which go forwards and backwards and
sideways and all around, and the space is one directional,
could your blood even pump, could your nerves even fire? Right?
You couldn't have a thought that goes from the front
of your head to the back of your head, right, exactly, exactly,
Nothing can move further from the center of the black
(39:08):
hole because space is just one directional and so it's
hard to imagine how your body could be configured in
a way that you would actually even survive. What would
my body hold together? Even? Technically, yes, right, technically, yes,
the gravitational title forces near the edge of the event horizon,
a big black hole would not be enough to tear
you apart, so it wouldn't pull you into pieces. But
(39:28):
I don't think you could function very well. I mean,
your body would have to be reorganized in a way
so that things just flow in one direction. And so
you know, like if your blood is getting pumped away
from your heart, it's never returning to your heart, And
that doesn't sound like the recipe for a long life.
Well what if I was just thinking, like, could be
blood go from your heart to the back of your head?
Still if your head moves faster than the blood, like
(39:51):
you could you still have circulation that way? Like? Could
parts of my body accelerate more than the other parts,
which means they do go to the back of my head?
I think that your motion here would be ondly dominated
by the gravitational forces of the black hole, which would
swamp everything else. And so I think practically it would
be impossible to counter that and be like, you know,
taking a jump on a super supermassive planet is basically impossible.
(40:13):
And here we're talking about gravitational forces, which, even though
they're not strong enough to pull you apart yet, are
still very very strong. And you have nothing to push against, right,
so how are you going to accelerate your head relative
to the rest of your body. Well, it'd be like,
you know, in a super heavy planet, it's hard to
jump from the surface, But if I'm falling towards a
giant planet, you could still jump off of the inside
(40:34):
of your spacehip. Yeah. I suppose if you brought something
along to push against, then you could relatively change your
acceleration compared to that object. So you bring like a
huge bag of bananas and you start throwing them into
the center of the black hole, then you would change
your acceleration relative to those things. Yeah, so you could
maybe still have thoughts as you go in if each
(40:56):
of your little blood molecules throws tiny bananas and just
the right direction, then they could continue to park because
that the idea. Then you'd be glad you ate a
bunch of bananas before you went in. We've just proved it, folks.
Bananas will save you inside a black hole. They do
help you think. Alright, So, um, I would not see
a lot of interesting things, but maybe I would experience
(41:19):
something different, you know, like how would you you said
I might discover the secrets of the universe inside? Would
I see them, feel them or what I need to
like bring in an experiment with me. Yeah, well, we
don't know, because we don't know what you would see.
You can imagine, you know, what would it be like
under various scenarios, like if the singularity is real? Well, unfortunately,
(41:39):
there's no way you survive getting that close. I mean,
we talked about picking a black hole that's big enough
to cross the event horizon. But definitely, at some point
if you get close enough to a singularity, that title
forces will shred you and so you just cannot survive.
But imagine then you had some like super duper strong
probe that was strong enough to resist that sort of
crushing force and it could get closer. Well, it doesn't
(42:01):
really matter because the closer you get to singularity, there's
always more singularity ahead of you, or the singularity is
a point of infinite density, so you can't be like
in it. You can't be past some of it. There's
always space between you and it, and that space gets
smaller and smaller and smaller, sort of like Zeno's paradox,
and the gravitational force gets greater and greater and greater
until it grows to infinity. But you can't like be
(42:24):
inside of it or in part of it. It's a
tiny infinitesimal dot. You'ld only just see blackness ahead of
you unless quantum mechanics is right, and that singularity isn't
actually infinite. If it exists in some sort of finite
space it's like fuzzed over by the Heisenberg uncertainty principle,
and it's a little bit larger, then it might be
possible to like interact with that in some way, or
(42:45):
to see it like talking to other parts of the singularity. Well,
but even if it was fuzzy in the middle, it
would still tear you apart at some point, right, it
would tear you apart before you got anywhere close enough
to do any physics experiments. But you know, if you
had some really powerful probe, they could get even closer
than because the quantum singularity might have some actual spatial extent.
(43:07):
There might be as small window in there for you
to do some experiments and actually measure it. But I
think even before then, it would be fascinating just to
map out the gravity as a function of the radius.
See like how does the gravitational intensity change or the
curvature space change as you get closer and closer to
the black hole center. That could have some surprises. You
could like measure the gravity in your hands as opposed
to the gravity your feet, and that might tell you
(43:29):
whether or not general relativity is working. Yeah, they're doing
lots of really cool tests of general relativity, and right now,
from the outside of black holes, they're watching gas swirl
and they're asking, does it swirl at the velocity we expect?
And they're using that essentially to measure the gravitational force
just past the edge of the black hole, because they
can't measure it on the inside, which is what they
would actually love to do. So if you're inside the
(43:51):
black hole, you can measure those forces of gravity, and
then you could understand something about what's happening at the
center just from the gravitational twisting and the tweaking of
the gravitation forces inside the black hole, you can see like,
is its spinning? Maybe the singularity is a ring and
it's actually spinning, or maybe it's just the dot. So
you could learn something about its structure by taking more
detailed measurements of the gravitational force as a function of
(44:12):
the distance from the center. Now, Daniel, would that be
worth it for you to learn that and never come back,
because I mean it doesn't sound like you know, the
universe would reveal, so you would just sort of confirmed
this one part of general relativity. Would that be worth
it for you? Or there could be a surprise. Right,
this is just two ideas humans have had, and we
don't think either of them is right. Was waiting inside
(44:34):
a black hole could be something totally different. It could
be a portal to another place in the universe, right,
a wormhole? It could be something completely surprising that unexpected.
I mean, think about how much of a surprise relativity
was or quantum mechanics, and now we're talking about something
which fuses and changes both of them, and so that
could be a surprise on the same scale or even
grander than the intellectual leap required to absorb those and
(44:58):
so yeah, I'm thirsty for that kind of realization. Sign
me up right. Well, but you wouldn't be able to
tell anyone, right because you're inside of the black hole.
But I would know you would be the only human
not to know this information and you wouldn't be able
to tell anyone. Would you still want to know if
you couldn't tell anyone, Well, since nobody's ever going to
call my bluff, I can confidently say yes, I would
(45:19):
want to know, because for me, physics isn't about like
finding it out and writing the paper. It's about knowing.
It's about that moment when you're asking nature a question
and forcing nature to reveal the answer, to extracting some
piece of information about the universe and just knowing it
because remember, science is about people's individual curiosity and this
is mine. All right, Well, it sounds like it's possible
(45:43):
to go into a black hole and discover some secrets
of the universe and actually survive, and it would be
pretty interesting and pretty mind and light bending to do it. Yeah,
I think that's something that surprises a lot of people
is that as you fall into the black hole is
sort of like nothing there. I mean, you get torn
apart by the ay, but there's no like threshold of
the event horizons, no trumpets that play, and you just
(46:04):
sort of fall right on in. Now, it's a different
story for people who are watching you jump into a
black hole. There's all sorts of weird relativistic effects there
as they see your time slowing down, even though for
you you can just fall right into the black hole
without noticing any effects on your clock. Wow, so it
would be a pretty smooth fall until it tears your
(46:26):
part inside of it. Yes, it would be pretty smooth
fall until you get torn into shreds. Better at your
spaghetti before you get to that point, then that's right.
How do you review that on Yelp? Would have been
five stars but got shredded and got turned into dinner
for the black hole? All right? Well, um, I think
it's you know, a part of a great exercise of
(46:47):
human evolution to imagine and to wonder what would be
like to do these crazy things in the universe. Absolutely,
and these are fun mind experiments because they sharpen our ideas.
They make us think what would it actually be. Let's
do the calculation. Let's think about what would happen to
somebody who jumped into a black hole. And hey, maybe
someday somebody's bluff will be called and they will actually
(47:10):
be transported to the edge of the event horizon and
encouraged to dive in to learn the answers some of
these deep questions about physics of the universe. And by somebody,
we mean you, Daniel, You're going to be the first one.
Because he said it on the record, We're throwing you in.
That's right. This is a legal statement right here. I
am bound by these statements. All right, Well, we hope
(47:30):
you enjoyed that little field trip inside of a black hole.
I hope you packed enough snacks. Thanks for joining us,
see you next time. Thanks for listening, and remember that
Daniel and Jorge Explain the Universe is a production of
I Heart Radio. For more podcast for my Heart Radio,
(47:52):
visit the I Heart Radio app, Apple Podcasts, or wherever
you listen to your favorite shows. No