How does gravity escape a black hole?

Daniel and Katie warp their minds around the question of what gravitational information really is and what we know about a black hole.

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Katie, have you ever been to Las Vegas? I
have once, and so what did you think about America's
favorite adult playground?

Speaker 2 (00:18):
Well, I can't really tell you.

Speaker 1 (00:21):
Is that because it was so much fun you remember nothing?

Speaker 2 (00:24):
I mean, it is a bit of an information black hole.

Speaker 1 (00:28):
You mean, like you don't want to talk about it.
Like what happens in Vegas stays in Vegas.

Speaker 2 (00:32):
It is a singular experience for sure.

Speaker 1 (00:36):
All right, we'll leave that past. The Katie event arrived.

Speaker 2 (00:41):
Like they're just handing out margarite?

Speaker 3 (00:43):
Is there? What am I supposed to say?

Speaker 4 (01:00):
Hi?

Speaker 1 (01:01):
I'm Daniel. I'm a particle physicist and a professor at
UC Irvine, and I have definitely been to Vegas.

Speaker 2 (01:08):
I am Katie Golden, I am made out of particles.
I host a podcast about animals, and I mostly liked
the food from Las Vegas, not so much the casinos.

Speaker 1 (01:23):
I recently went to Vegas with my thirteen year old daughter,
which was quite an experience to see it through her eyes.
Definitely a place to go at least once.

Speaker 2 (01:33):
It is a fascinating city. I regret that I have
not been there since they have put the giant orban.

Speaker 1 (01:41):
Yeah, the orb is cool. There's definitely a lot of
stuff you can do even if you're not eighteen or
twenty one plus, though some of the stuff she wanted
to do, like drive tanks through the desert and shoot
automatic weapons were definitely age restricted.

Speaker 2 (01:55):
Yeah, I have a friend who I think shot guns
from a helicopter in Vegas. There's a lot of things
you can do.

Speaker 1 (02:02):
There's all sorts of crazy stuff you can do in Vegas,
stuff that helps you understand who you are and who
you aren't. And Welcome to the podcast Daniel and Jorge
Explain the Universe, in which we try to explain to
you what the universe is and what the universe isn't.
We try to pull back the curtain and help you
understand what's going on out there in the deepest, darkest

(02:24):
regions of.

Speaker 2 (02:25):
Space, such as Las Vegas.

Speaker 1 (02:30):
There is no Vegas casino yet called the black Hole,
but I am assuming one it will eventually open up.

Speaker 2 (02:35):
That is a missed opportunity, or like a mini golf
course called the black.

Speaker 1 (02:40):
Holes that just sucks in all the balls. Well, on
this podcast, we do try to explain to you black
holes and also how everything works on the smallest scales
because we don't just want to describe the universe to you.
We want to explain it. We want you to get
an understanding, a microscopic picture of how everything around you
actually works. We want to dive deep into the very

(03:02):
nature of reality and come out with a bottom up
understanding of how the universe actually works. My friend Jorge
can't be here today, but I'm very glad to be
joined by one of our regular co hosts. Thanks Katie
for joining us on this black Hole of an episode.

Speaker 2 (03:17):
I am excited because no matter how many times you
explain to me what a black hole is, I still
feel like I cannot wrap my head around it. So surely,
surely this will be the episode where I understand black holes.

Speaker 1 (03:32):
Well, I would say there's probably nobody out there who
fully deeply understands black holes, because to fully deeply understand something,
I think we need a particle level explanation for it.
You know, we need to be able to moot up
from the tiniest little bits and say, here's what's really
happening at the smallest scales when you get sucked into
a black hole or when photons try to escape. And

(03:53):
the reason I say that nobody understands them is that
we do not have a particle level understanding of black
hole holes. The way, for example, we can understand what
happens when photons go through glass, They interact with the atoms,
the electrons and all that stuff in there, and it
bends their path, and we have some way to tell
a story microscopically if what's happening to those photons and

(04:13):
related to what we see actually macroscopically, we can't do
that for black holes because we don't understand particles and gravity.
We have no theory of quantum gravity that tells us
what happens to particles when they feel gravity, and that's
what black holes are all about. Gravity. So we don't
have this sort of deep down, microscopic understanding of black holes.

(04:35):
So probably nobody understands, but that doesn't stop us from
asking questions and from trying to grapple with what we
have learned about black holes. Then, on this podcast, we
encourage everybody to think deeply about the universe and to
try to fit these ideas into their heads, to ask
themselves questions to see does this make sense to you?
Is it possible for you to grapple with what we
do and do not know about black holes.

Speaker 2 (04:58):
The problem, Daniel is I have a finite number of neurons,
a finite sort of number of synaptic pathways in my brain,
and it is very difficult for those little guys to
understand all we know about black holes and what we
don't know about black holes. If we were somehow able

(05:19):
to measure everything about a black hole, do you think
we could even fully understand it? As humans?

Speaker 1 (05:25):
It's a deep question in philosophy, like are we even
capable of describing the universe in our minds? Can we
understand the universe? And not a question I know the
answer to or I think anybody knows the answer to. Amazingly,
so far we have been able to do it, to
write these little mathematical models, tell ourselves these little stories

(05:46):
in our minds, and use them to explain everything that
we see and everything we experience, everything our experiments tell us.
Is it possible for us to understand them? The smartest
human I don't know. I think there probably is a
limit to how smart the smartest human is, and so
it might be that the universe it's just more complex
than that, and that no human, regardless of how smart

(06:08):
they are, could ever understand it. But you know, we've
been making a lot of progress in recent years asking
and answering questions about the deepest nature of the universe
and about black holes. So I'm kind of bullish on
the possibility for some humans someday to understand it. And
that's not just limited to like the Albert Einsteins of
the world. I think it's possible for basically everybody out

(06:29):
there to get some kind of intuitive grasp or how
black holes work. One thing I love about hearing from
listeners to the podcast is that it's been teaching them
to think like a physicist, you know, like, what does
that mean? To think like a physicist? It means to
try to understand the world around you, to tell yourself stories,
to ask yourself questions, to say, do I understand how

(06:52):
this works? To put these models in your head and
like turn them around and say, well, if that's true,
doesn't it mean this or how does this connect with
this other thing I do understand? And that's especially important
for weird things like black holes. So we get lots
of questions from listeners who're doing just that. Wh we're
trying to think like a physicist about black holes? To
try to incorporate it into the mathematical stories in their minds.

Speaker 2 (07:15):
This podcast is Daniel's secret weapon of trying to convert
everyone into physicists. We are onto you, Daniel. You are
trying to create an army of physicists. There are so
many questions that I have about black holes. It's hard
for me to even think about how to phrase my

(07:36):
confusion about black holes into a question. So just being
able to ask a distinct, coherent question about black holes,
I think is impressive.

Speaker 1 (07:48):
I mean, your question about black holes is like black holes,
what's the deal with that?

Speaker 2 (07:52):
Yeah? My question is like, huh what.

Speaker 1 (07:57):
That's the first step, right, is to be confused, and
then the second second step is to try to weave
together a few bits of information and say, like, what
parts specifically of the story don't make sense to me?
Which parts do I need to understand better in order
to have something in my mind that does work where
the understanding does click. Because what I'd love is for
people to have that moment where they're like, oh, I
get it. This connects with that thing, and now it

(08:19):
kind of makes sense to me, even though it's pretty
weird and a lot of people out there are trying
to do that, and many of them have run into
the same stumbling block. And so today on the podcast,
I want to answer a very common question that we
get about black holes. Can gravity escape a black hole?

Speaker 2 (08:42):
I feel like I'm going crossside just trying to think
about this question.

Speaker 1 (08:46):
I love this question because it reveals that people are
being physicists. They're thinking about what a black hole is,
how everything gets sucked into it, how nothing can escape.
But they're also thinking, hold on a second, we feel
gravity from a black hole. How's that possible? How can
the gravity get out of the black hole? And right there,
that's being a physicist, that's saying I was told this,

(09:07):
But there's also that how do I make this and
that work together? How do I put it together in
my head to tell a story that makes sense.

Speaker 2 (09:15):
So what you're asking us to do is to listen
to what you're saying and then go hold on, Daniel,
you just said this. Are you lying to us?

Speaker 1 (09:27):
Everybody's doing their best, nobody's lying to anybody. But in
the end, learning is a very personal experience, and an
explanation that makes sense to one person doesn't work for
somebody else. So that's why in this podcast, we often
try two or three different analogies or explanations or ways
to communicate an idea, and that's what you're here for, Katie,
also to make sure that what I'm saying makes sense

(09:47):
to you.

Speaker 2 (09:48):
My method of learning is definitely food based metaphors. I'm
very food motivated.

Speaker 1 (09:53):
Is your appetite something like a black hole?

Speaker 2 (09:55):
It's something like that. Yeah, one could say I'm a
bit of a black hole, but yeah, I mean, it's
a very interesting question because it's hard for me to
think of gravity as like a thing, right, like a
thing that can escape something. Gravity to me is like, well,
I don't even know exactly what gravity is. I remember

(10:17):
from previous podcasts the things that you've taught me about gravity,
but it is so hard for me to conceptualize gravity
because it feels like it is like a kind of
thing woven into like the universe, that is not necessarily like,
you know, a physical matter thing, but it has everything

(10:37):
to do with physics. So it's it's a very confusing
kind of concept. And then you add to that the
black hole, which itself is quite confusing, and so this
question feels no pun intended, but very heavy.

Speaker 1 (10:52):
Exactly well, you put your finger on. Really the concepts
here that are entertangled. You know, what do we know
about gravity? How is gravitation information communicated? Like? How is
it that you are feeling gravity from the sun even
though the Sun is super dup far away and not
touching you. Does that count as information you're getting from
the sun? What kind of stuff is captured by a

(11:14):
black hole? And what isn't captured by a black hole?
Is that information? How does that all work? You know?
Do you need to pass little particles back and forth
in order to feel gravity or what. So. One of
the reasons I love this question is that it combines
all of these fun, interesting, fascinating and difficult questions together,
and going through it I think is really helpful to
clarify for people what a black hole is and what

(11:36):
a black hole isn't and how physicists think about it.
But before we hear about how physicists think about it,
I wanted to hear what everybody else out there was
thinking about it. So we have a nice little team
of volunteers who answer questions for the podcast before we
dig into them. Helps me understand what people are thinking
and helps you calibrate your thoughts against the other listeners.

(11:57):
So thanks very much to everybody who participates. You want
to jump in for a future episode, we would love, love,
love to have your voice on the team. Right to me.
Two questions at Danielandjorge dot com, So think about it
for a moment before you hear these answers. Do you
think gravity can escape a black hole? Here's what people

(12:17):
had to say.

Speaker 5 (12:18):
I feel like, since it kind of depends on the
mass of an object, it just moves with that object,
so it would be dependent on the object being able
to escape the black hole.

Speaker 6 (12:31):
It really matters what we think of as gravity. If
you think it's gravito gravitons, like with the quantum theories,
then I guess it escapes. I think, yeah, gravity can
escape black holes, otherwise I wouldn't be stucking everything in.

Speaker 7 (12:45):
Gravity is a distortion in space time, So I would
say that gravity itself cannot escape. Actually it's being created
by the black hole itself. But if we are talking
about gravitational waves, I believe it can escape a black
hole if it is outside the event horizon. I even

(13:07):
think that gravitational waves were discovered by watching collisions of
black holes and stuff like that. So my guess is yes,
under the right circumstances, gravitational waves should be able to
escape a black hole.

Speaker 4 (13:22):
I can't even wrap my head around the question.

Speaker 5 (13:25):
I didn't even know those words could go together.

Speaker 4 (13:27):
I guess it can escape, because isn't that kind of
what gravitational waves are.

Speaker 2 (13:33):
I love all of the references to gravitational waves because
we did an episode on that recently.

Speaker 1 (13:40):
We did absolutely an episode on gravitational waves, and that's
going to turn out to be crucial in understanding what
is gravitational information and what is not, what is trapped
within the black hole, and what is not trapped within
the black hole. Absolutely, so, gravitational waves are a really
helpful way to think about that.

Speaker 2 (13:59):
I get the root of this question. Is this seeming paradox, right,
the idea that nothing can escape from a black hole,
and yet we do know about black holes, We get
information about black holes, But how can we receive any
information about a black hole if it is something that

(14:21):
is all consuming and nothing can escape it?

Speaker 1 (14:24):
Boom Exactly, Adie, you were officially a physicist because you
are putting together those two ideas that you understand and saying,
how can these two things be compatible. How can we
weave them together into a singular understanding of black holes?
And what it's going to take is a little bit
of a refinement of the idea of what a black
hole is and what a black hole isn't to make

(14:45):
this all work together in your head.

Speaker 2 (14:47):
Well, now that you have proclaimed I am a physicist,
I am just gonna waltz into like the hard drung
collider and be like physicists coming through. Let's smash some particles.

Speaker 3 (14:56):
Guys.

Speaker 1 (14:57):
That's right, you have officially a PhD in podcast physics
from the Daniel and Jorge University.

Speaker 2 (15:04):
Worth its weight in gold. So yes, I would love
some clarification on what a black hole is. You know,
I know it's a big, sucky thing. I understand that
it has something to do with just an incredible density

(15:24):
of matter, and furthermore that gravity is very much a
defining characteristic of a black hole.

Speaker 1 (15:33):
Then let's make sure we know what it is we're
talking about. So a black hole is a region of
space where the curvature of space is so powerful that
nothing that's inside that region can escape to outside, and
that region is defined by this threshold, not really like
a barrier, not like a physical wall. There's not got

(15:54):
a dotted line somebody draws in space that says beyond
this point, if you pass, then you will ever escape.
You'll be trapped within that region forever.

Speaker 2 (16:03):
Someone's got to put a sign up, a warning, you know, warning,
do not pass this region. You will be spaghettified into
a black hole. But I mean, when you say space
is curved, you know, this is definitely a concept I've
been introduced to before, but it's still something that I
struggle to understand because when I think of curvature, right,

(16:24):
I think of like a physical object that is warped,
or like say, I think of fabric and I feel like,
you know, I think of pulling down on the fabric
and then they're being sort of a curvature and things
falling into it, and then that's how I think about gravity.
But I believe that is not really exactly the correct
understanding of it. Space is not like a piece of

(16:46):
fabric that you pull on and things fall into that
sort of like dip. But what exactly does curvature mean
in terms of space?

Speaker 1 (16:55):
Yeah? Good, It's important to think about in terms of
curvature because I think a lot of people think about
black holes in sort of a Newtonian way, where gravity
is a force and the force of gravity is so
strong that it's sucking stuff up right, And that cartoon
model of a black hole breaks down very quickly because
it can't explain to you, like, well, why does light
get trapped by a black hole? Why can't a photon

(17:16):
not leave a black hole? Because the Newtonian picture of gravity,
gravity is a force between objects with mass, and photons
have no mass, so they should feel no gravity, and
so they should be able to escape. In order to
have an understanding of what black holes are, you really
have to move your thinking of gravity is from this
idea of forces between objects with mass to an apparent

(17:40):
force something that actually results from, as you say, the
curvature of space itself. So what do we mean by
the curvature of space itself? Means that space has a
characteristic to it that's invisible, something you can't see when
you look at it. You look at some chunk of
space and you can't tell whether or not it's curved.
But if you shine a laser beam through it, it

(18:01):
will either go in a straight line or it will not.
It will curve, it will bend. This way, or will
bend that way, because space has this additional, weird property
that Newton never imagined. And on the smaller scale, what
it means is that the relative distances between two points
can get changed. So when we say that like space
is curved or space is bent, it means that you

(18:22):
can take two locations in space, you can effectively make
them closer together, or you can make them further apart.
And what that does is it changes the path that
light will take through that space, because light always takes
the shortest path between two points. Now you're changing what
the relative distances are between two points, and so you're
changing the path that light will take. But I think

(18:43):
the most intuitive way to think about it is that
like space has this additional bit to it, this characteristic
so when a photon is passing through it, basically the
space tells the photon where to go, doesn't just pass
through blindly.

Speaker 2 (18:55):
It's so interesting because I think the reason it's such
a difficult concept is that as humans we deal with
physics all the time, but we can observe Newtonian physics.
We can observe that type of physics, So it's I
think easier to think of particles when even like particles

(19:16):
that you can't observe with the naked eye when they
are behaving according to forces, because we can on the
more macro level observe forces. You can see a ball
knocking into a ball, you can see something fall down
a hole. But with gravity, even though yes, it's true,
we can observe gravity. We see it all the time,
right because you know, you jump up, you fall down.

(19:37):
It's one of the main problems that we have to
encounter as a physical being in the world, and yet
we can't actually see or kind of experience this like
secret framework behind gravity, which is that what you described,
like the relative distance between two things like becoming shorter

(19:59):
and like the actual like you know, space like reality
sort of warping in a way. It's like so easy
to fall into that trap of thinking of it in
terms of the type of physics that we see and
we observe.

Speaker 1 (20:14):
It's confusing precisely because it's invisible. You can't see this
curvature of space. You can only see its effect, and
the effect of the curvature of space looks in almost
every case as if there was a force there. It
looks like there is this force we call gravity that's
pulling on things, when really things are just following the
curvature of space. The shape of space itself is changing

(20:37):
the direction in which things move. And if you can't
see space doing it, it's like a bunch of stage
hands wearing black nudging stuff. Then you imagine that there's
a force there. And so that's why we call gravity
and a parent force. It's not an actual force the
way like electromagnetism is, or the weak force or the
strong force. It's just the effect of the invisible curvature

(20:58):
of space.

Speaker 2 (20:59):
So I hop down from let's say a safe height
and land on the ground. I am not really getting
sucked into the ground, because this is an effect of gravity, right,
The gravity of Earth is affecting me, and so I'm
not really getting sucked to the ground, but I am

(21:19):
like sort of just following where space is curving.

Speaker 1 (21:24):
Yeah, that's a great example. Let's walk through it in
the sort of Newtonian intuitive picture, and then let's move
over to the Einstein space curvature picture. You'll find that
it's a very different story about exactly the same thing.
So in the Newtonian way of thinking, where gravity is
a force, you are standing on a chair, and you
imagine that there's a force of gravity pulling you down,

(21:44):
and there's a force of the chair pulling you up,
and when you stand on the chair, all forces are balanced,
so you're not going anywhere. You jump off the chair
and then you fall down because all you have is
the force of gravity pulling you down. There's nothing pushing
you up until you hit the surface of the Earth,
right in the surace of the Earth, and it's now
pushing back up on you, and so the forces are
now balanced and you're not moving anymore. Cool. So that's

(22:05):
the Newtonian picture. Einstein says, actually that's all wrong, that jerk.
There is no force of gravity, and that when you're falling,
when you jump off that chair, what's happening is you're
following the curvature of the Earth. You're in free fall.
There is no force on you at all. And in fact,
if you are carrying a little accelerometer with you, something

(22:26):
which can measure whether you're accelerating, essentially if there's any
force on you. Example of an accelerometer is just like
a ball in a box, Like is the ball pushed
towards one side or the other? Where Like a bowling
ball in a truck will tell you whether you're breaking
or accelerating. If you jump off that chair and you're
holding an accelerometer, you can do this experiment. You will
not measure any acceleration. You will not measure anything because

(22:48):
in that case, you are not accelerating. Einstein says that
you're actually accelerating when you're standing on the chair. That
what you're doing there is you're accelerating against the curvature
of the Earth. And also when you're standing on the earth,
you're accelerating upwards against your natural path, which would be
following the curvature of gravity towards the center of the Earth.

Speaker 2 (23:08):
The chair is essentially blocking where you should go naturally
go by following where space is telling you to go.

Speaker 1 (23:17):
Exactly, if you jump off the chair, you're in free fall.
You're not accelerating at all. You see people who are
still on their chairs as accelerating upwards against the natural
direction gravity wants to take you.

Speaker 2 (23:27):
So, if I'm holding something like a feather, say, and
I jump off a chair, the feather goes slower than me.
Is the only reason that happens because of wind resistance.

Speaker 1 (23:40):
Yeah, in that case, it's the air is accelerating the
feather up. It's preventing the feather from following the curvature
of space. The air itself is pushing on the feather.

Speaker 2 (23:50):
And that's why an ant can survive really high falls,
whereas a person could not. Even though the ant seems
much more structurally delicate than a human because it's so small,
The air is basically like this huge you know resistance,
this huge force that is, you know, blocking the ant

(24:11):
from I guess following the curvature of space.

Speaker 7 (24:15):
Yeah.

Speaker 1 (24:16):
I think there's also something there about the structure of
ants when they land, but I'm not a biologist.

Speaker 2 (24:21):
They just stick the dismount amazingly. If you zoom in
on an ant, it does a little summrsalt, lands, and
then takes a little bow.

Speaker 1 (24:29):
So this picture of gravity is this curvature in space
rather than a force, gives us a new way to
think about a black hole. It's not a very dense
mass that has very strong sucking gravity because of all
the mass in it. It's a region of space where
the curvature is such that there is no path outwards,
where every direction is towards the center of the black hole.

(24:50):
Anything that's flying along is going to follow the curvature
of space, including photons. Photons in Einstein's gravity can be
bent because they follow the curvature of space, not because
of this Newtonian force between masses. So if Hodan that
goes past the event horizon is trapped by the structure
of space itself, because every path forward now takes it
towards the center of the black hole. And that's where

(25:12):
this apparent mental paradox comes from that you mentioned earlier.
If a black hole traps everything, if everything is encapsulated
inside the black hole because of the structure of space,
then how is it possible for us to even know
about it? How's that gravitational information leaving this trash can
of space time.

Speaker 2 (25:29):
I do want to hear about sort of this idea
of information not being able to escape a black hole,
because that just it sounds weird, right to like say
like information is trapped somewhere, because when we think about information,
you know, it's like, wait, so I can't use Google
in a black hole? What does that mean? But maybe

(25:51):
we should take a quick break before we do that,
digest the curvature of the space. See, I need food
metaphors for me to learn and then when we get back,
I would love to learn more about what it really
means to say that information cannot escape. All right, So

(26:22):
I think I'm starting to get a grasp on this
idea of gravity being sort of a curvature of space,
not a force. But I do want to hear more
about what it means when information can't escape a black hole, because, like, again,
when I think of information, I'm like, well, I'm looking

(26:45):
on Wikipedia about how ants can stick the dismount. What
does it mean when information cannot escape a black hole?

Speaker 1 (26:52):
Well, fortunately, if you fall into a black hole, you
can still access Wikipedia because information can go into Wikipedia.
You can't request specific page because we can't hear from you,
but we can send you information. We could just like
randomly send you Wikipedia pages even after you've.

Speaker 2 (27:07):
Fallen in I would like to get some Netflix and
stuff if I'm in a black hole, because it seems
if I'm still alive, it seems really kind of like
a lot of time I'm going to spend in there.
I need some entertainment, you know what.

Speaker 1 (27:21):
I think maybe nobody does. What everybody should do is
prepare their Like, if I fall into a black hole,
what Netflix shows. Should you beam to me the way
people like you know, prepare a will other end of
life information? Like, I'd like to know this about people
in my life, so I know what to beam to them, right.

Speaker 2 (27:37):
Yeah, exactly, I need to start working on a list.
But yeah, so information can't escape a black hole? What
is going on there? And what do we mean by information?

Speaker 1 (27:48):
Really? Yeah? Great? And here's where we need to zoom
back down again to the particle level when we talk
about information, Like, imagine that you are inside the black
hole and you want to request a Wikipedia page about something,
make a Sioux fla, and you want a recipe or something.
In order to request that information, you have to send
something physical outside of the black hole. The way information

(28:09):
works is that it's transmitted physically. The way you are
hearing this podcast right now is wiggles in electrons or
photons being beamed across the Earth. All information is transmitted
as particles or some kind of a wave. And so
if particles cannot escape a black hole, then you cannot
send information outside the black hole because to send that
information would require sending something physical, a photon, an electron,

(28:34):
something outside of the black hole. So that's where this
idea comes from that no information can leave a black
hole because no thing can leave a black hole, and
you need things to transmit information. Information in the end
is physical.

Speaker 2 (28:48):
So I cannot order a pizza from a black hole,
which seems a little scary.

Speaker 1 (28:53):
So get your pizza order in before you fall into
the black hole. That's the key. It's just a little
bit of planning.

Speaker 2 (28:58):
I am writing it in sort of my black hole
instruction list, that please send pizza into the black hole.
Should I go in there.

Speaker 1 (29:07):
I think you should ask for spaghetti, actually, because everything
we send in is getting spaghettified anyway, so you might
as well start with spaghetti.

Speaker 2 (29:13):
You are what you eat, especially in a black hole,
if you're eating spaghetti.

Speaker 1 (29:17):
But let's get specific about what we mean by information,
because this is the crux of the issue. This is
how we're going to understand later why you can feel
gravity from outside a black hole, but you can't order
pizza from within the black hole. And what we mean
by information is sort of like an update, a change
of state, so you're communicating something is different, Like if

(29:39):
Katie and I are across the room from each other,
she can send me information by like shooting photons at
me right. For example, if she doesn't shoot any photons
at me, I can assume like, Okay, Katie's still there.
I haven't heard from her in a while. But if
I want any new information from Katie, if you want
any updates on her situation, she wants to warn me
about something, or tell me about the pizza she wants,

(30:00):
or change her list of Netflix shows she wants to watch,
then she would need to send me some information, some
electrons or some photons. So information is sort of like
an update. It's like something new that.

Speaker 2 (30:10):
You're learning, right, So it kind of makes me think
of like, uh, you know, the basis behind things like computers.
You have to have little on off things and you know,
this is a system inside of a computer like a
zero one an on or and off, and that change

(30:30):
from off to on is a sort of bit of
information that can be expanded into of course, the complexity
of computer. It's the same thing I think with you know,
in terms of the brain, where you have either a
synapse has fired or it has not, there's a little
more complexity because it's it's a large biological process. So

(30:51):
you have, you know, some states that are somewhat in between,
you know. But essentially it's like this on off state.
You need to you be able to clearly have an
off position and an on position and have that reach
from one you know, like be able you know, even
with like say a neuron, you are shooting neurotransmitters from

(31:14):
one end of a neuron to the other end of
another neuron. So it's like it's very much like in
our day to day life we experience this concept of
information exactly.

Speaker 1 (31:26):
And so you could also think about information from the
sense of like affecting the future. Information is something that
somebody in the future can use to make a decision.
Like if I could tell you what the Powerball winning
numbers are, you can be a billionaire tomorrow because you
would know exactly what numbers to put in. So if
I have information, I can send it to somebody and

(31:46):
they can use that to make a decision. It can
like change the future if I am sending them information.
This is closely connected with another concept we've talked about
on the podcast a lot, which is this causal link, right,
like causes and effects how they can be connected. Because
there's already a limit to how you can affect the
future in the universe. Like, for example, I can't change

(32:09):
the future in Andromeda tomorrow. Andromeda is so many light
years away that any information I send to Andromeda will
not arrive for millions of years, and so nothing I
do today can get any information to Andromeda by tomorrow.

Speaker 2 (32:24):
So we cannot get a pizza from or to Andromeda
anytime soon.

Speaker 1 (32:31):
That's right. We can't even tell Andromeda about what kind
of pizzas they should order, which in principle could travel
at the maximum speed of information, which is light speed.
And so we have this concept we call a light cone,
which tells you where in the universe your information can reach.
You can send signals, you can turn on flashing lights,

(32:51):
but people outside your light cone will not see it,
and so the light cone expands as time goes on.
You send a flare. Now that information propagates through the
universe and eventually will reach the edges of the universe.
Your light cone is expanding in time from every moment.
But there is a limitation there, right, There are already
parts of the universe that your information cannot reach at

(33:15):
an arbitrary time, So there's a limit to the information.
You can't send any information to people outside your light cone.

Speaker 2 (33:21):
I don't know if this is a pretty rudimentary question,
but from Earth, right, we can't see every star in existence,
can we? Or are we able to see, like get
light information from every star in existence? From Earth?

Speaker 1 (33:37):
We can't because the universe is not old enough. Right,
The universe is finite age, So there are some stars
that are so distant that light from them has not
had time to arrive here. Plus you've got a factor
in the expansion or the universe. It gets much more complicated.
But no, there definitely starts who have sent photons towards
us but have not yet arrived.

Speaker 2 (33:55):
So it is a matter of timing where because like
all this stars that we see right now, like that
information is really old, that light information that we're receiving now,
that's exactly right.

Speaker 1 (34:07):
Yeah, that information is very old, and it's a matter
of timing. And if space was simple, space was always flat,
if there was no curvature, photons always traveled in a
straight direction and at the speed of light, then the
only limit to information would be timing. But the curvature
of space changes things. And in curved space, your light
cone is not a simple cone. It gets bent, it
gets distorted. Near a black hole or anywhere where space

(34:30):
is curved. Your light cone gets twisted because it changes
where photons go. Right, you send out a flash of light.
Now they're going in some directions, not in other directions.
They get twisted, they get bent. Once you pass over
the event horizon, your light cone is now just pointed
towards the center of the singularity. Any flare you set off,
all photons are going to end up going towards the singularity.

(34:51):
So your light cone is now sort of trapped behind
this barrier. And that's what we mean when we say
information cannot escape a black hole. I mean that nobody
passed that threshold can ever send you a photon that
arrives to you.

Speaker 2 (35:05):
If a photon approaches a black hole but doesn't pass
the event horizon, can its light cones still get bent,
but not fall into the black hole.

Speaker 1 (35:14):
Oh yeah. In fact, there's a region near black holes
where photons can orbit. They can get trapped and forever
loop around a black hole, which is pretty cool, or
they can do other weird things like pass around the
backside of a black hole and then come back. So
you can shoot it like a laser beam just above
a black hole and kind of come around the backside
and zap you in the eyeball from below the black hole.

Speaker 2 (35:36):
It's like a laser boomerang.

Speaker 1 (35:37):
So the key thing to understand there is that information
is about changes of state, how things change, getting updates
about what's going on, decisions that are made, new facts
that arise. Information is about changes of state.

Speaker 2 (35:51):
So with gravity, Now, I thought that we had talked
about a previous episode about how with gravity there is
a phenomenon known as gravitational waves, which would indicate that
gravity can change. Right, you can have changes in gravity.
So why would it be considered that gravity is not

(36:13):
a form of information.

Speaker 1 (36:15):
Yes, great example, So gravitational wave is a great example
of gravity changing. Right. We talked about gravity in terms
of the curvature of space. So imagine space has some
curvature because of the arrangements of masses in it. So
space has that curvature, and that curvature is just fixed,
it's constant. Right. Now, take one of those masses, the
Sun maybe, and wiggle it. You put your finger on it,

(36:37):
you shake it back and forth. If you wiggle the Sun,
you're changing where the mass is. You're changing how space
is bent. So what's happening to there is the curvature
is changing. You're moving the Sun to the left, and
you're moving it to the right, and you're moving it
to the left again, you're moving it to the right,
and that's information and that information propagates out at the
speed of light. And so if you're far away from
the Sun when somebody's wiggling it, you don't feel those

(36:58):
changes instantly. It takes time for that information to get
to you. So Katie wiggles the Sun. Eight minutes later,
I feel the gravity on Earth shaking a little bit.
It gets stronger, it gets weaker, it gets stronger, it
gets weaker. That's a gravitational wave. That's information moving through
the gravitational field. So I think that's sort of a
long answer to your question, which is can you send

(37:21):
information through gravity? And the answer is yes, And that's
what gravitational waves are. There's information sent through the gravitational field.
The crucial thing is that that's all about a change
in the gravitational field. There's no information being propagated when
the sun is static. The field is not changing. There's
no information there. When you move the sun, when you've
wiggled it, that's sending information.

Speaker 2 (37:43):
So Earth has gravity, obviously, but when the Earth moves,
you're getting some change in gravity, and so you're getting
gravitational waves. And so that change of the Earth moving
is sending out information in terms of the change in gravity.

Speaker 1 (38:03):
Absolutely, And when the Earth orbits the Sun, because it's
effectively moving through space and changing the Earth's gravitational field,
we are generating gravitational waves. Our orbit around the Sun
generates gravitational waves, and it actually contributes to the decay
of Earth's orbit because gravitational waves contain energy, so we're
radiating away some of our energy every time we go

(38:26):
around the Sun. You just did an episode about this
about how stable is the Earth's orbit, and it turns
out that's a very very small effect in terms of
the stability of the Earth's orbit. But in principle, yes,
we are generating gravitational waves which contain information, and if
you had a sensitive enough gravitational wave detector, you could
pick up those wiggles in space time that are telling
you about how the Earth is moving.

Speaker 2 (38:47):
So gravity can convey information. It can be considered a
form of information if it is a change in the gravity.
So with a black hole, why aren't we seeing a
chain in gravity, because I would assume if we're not
getting gravitational information from a black hole, that would mean
that there is not a change in the gravity of

(39:10):
a black hole.

Speaker 1 (39:11):
Yes, exactly, and so now I think we have all
the pieces in place to try to answer this question,
which is a black hole does of course have gravity,
and you can feel its gravity from a distance. You
are a light year from a black hole, you will
feel its gravity. But what you can't feel are changes
to its gravity. So if there's something going on inside

(39:32):
the event horizon, Katie's in there and she's doing a dance,
or she's rearranging her living room or whatever, you can't
get that kind of information. If you change the configuration
of the masses inside the black hole, in principle, that
generates gravitational waves, but those are trapped within the event horizon.
So nothing that's happening within the event horizon is generating

(39:54):
any gravitational information that's escaping. All you're feeling is the
static field of the black hole, the one that was
created when it was formed. So you have a black
hole and it has a static gravitational field. Nothing is
changing and anything that happens inside the event horizon is
going to generate gravitational waves, but they don't leave the

(40:14):
black hole. So you can have gravity outside the black
hole from its original formation without knowing what's going on
later inside the event horizon, without getting any information from
inside the event horizon. The only information you have is
the existence of the black hole from when it was created,
which is not information from inside the event horizon. Another

(40:35):
way to think about it is you create the black hole,
you're then freezing that gravitational field. Nothing that happens pas
the event horizon can then change the gravitational field outside
the black hole. Now, that's all if the black hole
is not moving and so it has a static, unchanging
gravitational field. If you wiggle a black hole or if
black holes merge, that motion creates ripples in the gravitational field,

(40:59):
just like a d when the Sun or the Earth moves.
But that's motion of the event horizon. It's not information
from inside the event horizon about what's going on within it.
Another way to say it is that from the outside
you can see if the black hole is wiggling, but
that's not information from inside the event horizon. You can't
tell if things inside are wiggling, only if the whole

(41:20):
thing is moving.

Speaker 2 (41:22):
I mean that's interesting because it's like, I suspect this
might be the wrong way to conceptualize it, but in
my head, I'm thinking of sort of a Russian nesting doll,
where it's like the black hole is the main one,
the main sort of gravitational field, and inside you could
have like the little tiny doll dancing around doing stuff,

(41:42):
but none of that is reaching outside of this nesting
doll because like the bigger one is essentially this like
static large gravitational field, and nothing is getting outside of it.

Speaker 1 (41:56):
Yeah, I think that's a great way to think about it.
Another useful thing to think through is how a black
hole gets formed, the moment in which a black hole
is created. I think that can really help people crystallize
how that information is propagated through the universe and why
no more information is ever sent out from that object.

Speaker 2 (42:14):
All right, let's take a quick break as I mentally
prepare my mind to be blown. I'll lay out a tarp,
and then when we get back, please tell me how
a black hole is created. All right, So I'm officially

(42:40):
ready for my brain to explode from the information of
how a black hole is formed.

Speaker 1 (42:48):
So all you have to do to form a black
hole is just order enough pizzas. Really, that's all it takes. Okay,
So we know that there are black holes out there
in the universe and that they are created, right, they're
a black holes that have not existed since the beginning
of the universe. So there has to be some moment
when there wasn't a black hole and when there is
a black hole. And I think the thinking through that

(43:09):
moment really can help people understand what we're talking about
when we say a black hole, what it means for
information to be trapped within it. And so there's lots
of ways that a black hole can be made in
the universe, But let's try to think about like the
simplest possible configuration and in general relativity, one very frequently
used example is like a sphere of rocks. Imagine you
have like tiny little bits of dust or pebbles, and

(43:30):
you have a perfectly symmetrical sphere of it. Okay, and
now gravity is pulling it and so it's collapsing. It's
going to pull towards itself and get denser and denser
and denser. Now, how do you make a black hole.
You make a black hole by having enough stuff in
a small enough space. It's all about the density of matter.
You could take the Earth and squeeze it into a
peanut to get a black hole. You could take the
Sun and squeeze it into like a three kilometer wide object.

(43:53):
To get a black hole. You could take a huge
blob of stuff and squeeze it. The point is to
get high enough density. So now you have this hollow
fear of pebbles. Right, everything's like the same distance from
the center, and it's collapsing slowly. At some point it's
going to be dense enough to pass this threshold and
become a black hole. It's going to be a moment
before it passes that threshold where it's just like a

(44:14):
sphere of stuff that has gravity, and then a moment
after passes that threshold, now it's a black hole.

Speaker 2 (44:20):
How does the incredible density cause Essentially, what I'm imagining
is like pinching the curvature of space such that everything
is going towards that point, Like, how does density cause
that to happen?

Speaker 1 (44:35):
We talked about how gravity is actually just the curvature
of space. We didn't talk about how you make that
curvature why bits of space are curved here or not
curved there. So the missing piece is density energy density.
If you have a chunk of space with a lot
of energy in it, be it mass or photons or
any kind of energy, then that space gets curved. So

(44:56):
the simplest version of general relativity is mass tells space
how to bend. Space tells mass how to move, and
so it's that first bit that mass tells space how
to bend. The more energy density, usually in the form
of mass, that you pile into a little chunk of space,
the more curvature you get. When you cross past some threshold,
then you have so much curvature that nothing can escape anymore.

(45:19):
So that's the moment you become a black hole. So
you take this sphere of dust again, it's a hollow sphere.
It's not filled on the center. If it's really really big,
then you have a lot of empty space. Also, you're
including density is very very low, you don't have a
black hole. As you collapse it, the radius gets smaller
and smaller. All the bits get closer and closer together.
Eventually you pass over the short siled radius, the radius

(45:40):
of an event horizon for that amount of stuff, and
then you have enough density that there's enough curvature that
you've got a black hole.

Speaker 2 (45:48):
So mass which we can observe is sort of tied
to this hidden world of gravity which we cannot see.
We can feel the effects of, but we cannot see
the curvature of space. But that curvature of space has
direct connections to all the mass that we can directly observe.

Speaker 1 (46:09):
Yeah, that's exactly right. And remember that's not just mass.
Right in Newton's physics, it's just mass, but in Einstein's
relativity it's energy density. That's why photons and other massless
things can actually contribute to the curvature of space. You
can actually just make a black hole by overlapping powerful
laser beams. Pure photons can make a black hole in theory.

Speaker 2 (46:31):
Okay, how do you know that, Daniel, or are you
working on that? Are you doing that?

Speaker 1 (46:35):
I can either deny nor confirm those experiments.

Speaker 2 (46:38):
That's incredible that just through light, if you cram enough
light into a small enough area, not only are you
going to like blow out your retinas, you're going to
create a black hole.

Speaker 1 (46:52):
So now let's go back to our collapsing sphere of
pebbles or dust or rocks or whatever. What happens when
it creates a black hole? What is changing in space
the moment before it creates a black hole? What do
you have? What you have gravity?

Speaker 2 (47:06):
Right?

Speaker 1 (47:06):
If somebody is going to shoot a photon or a
particle near it, it's going to be bending space and
it's going to affect the path of that stuff. Right,
You're going to feel the effective force of gravity from
this sphere of rocks, right, because it has mass, it's
curving space. It's giving you that effect. No big deal,
no surprise there. What happens when it crosses that threshold
and becomes a black hole? Well, nothing changes from that

(47:29):
point of view, right, because the mass hasn't changed, the
overall energy hasn't changed. So if you're in the outside,
when this sphere becomes a black hole, nothing changes gravitationally,
the gravitational field is still the same. And that makes sense, right.
It's not like the gravity disappears when the thing becomes
a black hole. It changes from a big sphere of

(47:49):
dust to a black hole. But if it has the
same mass, it has the same gravity as before it
became a black hole.

Speaker 2 (47:55):
Okay, so you have this amount of gravity that just
before sure the black hole is created, and then once
the black hole happens, that gravity is the same. So
can you not add any more gravity to a black hole?
Like you cannot, Like say, you put more stuff into
a black hole, you put more particles into that black hole,

(48:16):
you shoot some lasers. There's no more gravity that you
can add to that.

Speaker 1 (48:20):
That's a great question, actually really illustrates this point very nicely.
Absolutely you can. Right, if you shoot a laser into
the black hole, or you throw a more mast in
the black hole, you are going to increase its gravity,
and that information will propagate outwards. Right, you add to
a black hole, you add to its gravity. You can
feel more gravity now somewhere else. That information will propagate
outwards as a gravitational waves, a change in the gravitational

(48:43):
field of the black hole. But that's external, right, that's
not information coming from within the black hole. That stuff
you're adding from the outside. If you have just a
black hole and it's collapsed and you're not changing its
gravitational field, then what does it mean for information to
not be able to aesc if you're still feeling its gravity?
What that means, is that anything that's happening to that

(49:04):
stuff that fell into the black hole, you can't tell. Basically,
the gravitation field is just frozen from that stuff. If
that stuff goes in and does a funny dance or
becomes a singularity, or becomes a quantum fuzzball, or does whatever,
you can't ever see. It can't ever tell you anything
about it. You can't ever learn anything about the configuration
of the mass within the event horizon. The gravitational field

(49:27):
of that stuff still exists, but it's now frozen in time.
You will get no more updates. If you take that
stuff inside the black hole and wiggle it around, generating
a bunch of gravitational waves within the event horizon, those
waves are trapped within the event horizon. Outside you will
still see the same exact gravitational field. And that's why
we made the distinction earlier about information being about updates,

(49:51):
because that gravitational field can be static. There's no information
escaping from within the black hole. It's just sort of
like think about the event horizon itself as having that
gravity or the last information you got about this stuff
was just before it falls into the black hole. It's
that gravitational field. You're feeling no updates, no information means

(50:11):
that gravitational field is effectively frozen.

Speaker 2 (50:13):
So would it be fair to say that we can
get information from the existence of a black hole, and
we can get information by shooting information into a black
hole and then that sort of changing the characteristics of
the black hole. But we cannot get any information from
anything that is already inside of the black hole.

Speaker 1 (50:37):
Exactly, because that would require communicating something physically. Right, if
you wanted to send information from within the black hole,
you'd need to like take a bowling ball and wiggle it,
which would generate gravitational waves. But I can't feel that
from the outside. What I feel is the gravitational field
of the stuff the moment it became a black hole.
That's when it got frozen in time. All the gravitational

(50:58):
waves generated inside of it, an updates to that gravitational
field are now trapped the same way that like if
you shoot photons or if you shoot electrons. Right, I remember,
information is physical. None of that physical information.

Speaker 2 (51:10):
Can leave all right, But now hear me out. If
you had a really long string and you were sitting
here on Earth with a dixie cup attached to your end,
and I'm all the way in a black hole with
that string and a dixie cup on my end. No,
I cannot communicate any information to you, even if somehow

(51:30):
that string was strong enough to survive the black hole.

Speaker 7 (51:34):
Right.

Speaker 1 (51:34):
Well, there, you're creating a new paradox because there is
no string strong enough to survive the black hole. Right
Because as you zoom in microscopically to that string, it's
really just a bunch of particles transmitting information to each
other using electrons or using photons, and those photons cannot
escape the black hole. But you know, electrons and photons

(51:55):
are a great way to think about this information. Also,
like say you take an electron and you just have
an electron in space. Electron has an electric field, right,
a static field all through space. It gets stronger as
you're closer to it, and weaker as you get further away. Now,
what happens if you shake that electron. You shake that electron,
you're changing the electric field. And that change is a photon.

(52:16):
It's physical information. It's changing the field of the electron
that is transmitting the information. The static, frozen constant existence
of the field is not information, which is why stationary
electrons don't generate photons in the same way that like,
the existence of the black hole and its gravitational field
is sort of like old information from before it became

(52:37):
a black hole when it was created. Any new information
created within it by moving the stuff inside is not
going to be transmitted. So information gravitational waves or photons
really are about changes in the field, not about the
existence of the field itself.

Speaker 2 (52:52):
So in my string and cup analogy, of course the
string would not survive. But you know, with a hypothetical string,
I could be shaken it, I could be talking into
the Dixie cup. But as soon as that information, you know,
is basically the part of the hypothetical string outside of
the event horizon is getting nothing from the string inside.

(53:15):
From within the event horizon.

Speaker 1 (53:17):
Exactly, your string is going to be dangling towards the
center of the black hole. All the information you create
is going to get funneled towards the center of the
black hole.

Speaker 2 (53:26):
Wow. Well, I'm gonna really emphasize that if I am
in a black hole. Pepperoni pizza is good. Also, do
a nice carbonara, just keep them common for eternity. I
suppose that sounds good.

Speaker 1 (53:42):
It's good to get that information now before you fall
into the black hole.

Speaker 2 (53:45):
Right could happen any day. We are sponsored by black
Hole Insurance. Make sure to set up your affares before
you get sucked into a black hole, or should I say,
not sucked into a black hole, but follow the curvature
of space into a black hole.

Speaker 1 (54:01):
So you should think about black holes as barriers to information.
But I hope this podcast has helped you understand a
little bit more what that means, what information really is,
because what's inside a black hole can affect what's outside
a black hole. The creation of the black hole itself
affects space time near the black hole. But sort of

(54:21):
in a simplified way. We can only know a few
things about the black hole. We can know it's mass,
we can know it's charge, we can know its spin.
Anything else that happens within the black hole can't change
what's happening on the outside. The changes behind the barrier,
wiggles and rearrangements of the stuff within the black hole
can't influence things on the outside. So I think the

(54:42):
crucial concept there is that information really is about changing
the internal arrangements. That's what you're prevented from knowing not
the existence of the black hole, but what's going on
inside of it.

Speaker 2 (54:52):
So I could be inside of the black hole, drunk
off of Martini's, standing on the black hole jack table
and screaming out the lyrics to a Katie Perry song,
and no one would ever know.

Speaker 1 (55:07):
Only people within the black hole and closer to the center.
But hopefully what happens inside a black hole stays inside
a black hole.

Speaker 2 (55:14):
Yeah, well that's good information to know, because I know
now where I'm going to do my next Vegas trip
and it's not in Vegas.

Speaker 1 (55:23):
All right. Well, I want everybody out there to keep
thinking like a physicist, trying to bring pieces of understanding
together and weave them into a deeper, more profound model
in your head about how the universe works, how these
things all interact, and when things don't jive together, when
they don't make sense to you, or when you feel
like you have hit upon a contradition right to us,
we will help you figure it out. Questions at Danielanjorge

(55:46):
dot com. Our goal is to turn everybody into a physicist.

Speaker 2 (55:50):
You and your army of physicists. What are you planning?

Speaker 1 (55:53):
I can neither confirm nor deny our plan. Thanks very
much Katie for joining me on this path towards the
center of understanding following the curvature of explanations.

Speaker 2 (56:03):
Thank you for all the information. I'm glad we're not
broadcasting from a black hole where nobody would.

Speaker 1 (56:08):
Know, and thanks everybody for listening. Tune in next time.

Speaker 4 (56:12):
Hey, this is Joege from the podcast and I'm super
excited to announce that my new book, Oliver's Great Big Universe,
is out now. It has humor, cartoons, a fun story,
and lots of awesome signs, and it's perfect for kids
that are into science, but also kids who are not
into science yet. There are chapters about black holes the
planet's dark energy, but at the heart of it is
a story about friendship and figuring out your place in

(56:33):
the world, so please check it out. You can get
it in stores, online, and at Great Big Universe dot net.

Speaker 1 (56:46):
Thanks for listening, and remember that Daniel and Jorge explain
the universe is a productive of iHeartRadio. For more podcasts
from my Heart Radio, visit the iHeartRadio app, Apple Podcasts,
or wherever you listen to your favorite