Do Black Holes Have Hair?

Daniel and Jorge wrestle with the hairy question of what's happening inside and on the surface of black holes. 

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey or he have you started to go gray yet?

Speaker 2 (00:11):
I'm not sure I ask you to answer that question.
You know, I want to preserve my air of mystery.
How about you.

Speaker 1 (00:17):
I found a silver hair too in my beard, but
so far I'm still all brown up top. But you know,
I'm getting a little.

Speaker 2 (00:24):
Impatient, getting impatient. What do you mean you want to
go gray?

Speaker 1 (00:28):
I wouldn't mind that gravitas that comes from having a
little bit of gray.

Speaker 2 (00:32):
You mean you don't have enough gravity right now?

Speaker 1 (00:36):
My gravity has actually been increasing.

Speaker 2 (00:38):
There you go. It sounds like your body's doing it
for you. But are you trying to counteract the you know,
whole physicist look with the shorts and the sandals.

Speaker 1 (00:46):
Yeah, exactly, I'm trying to look a little bit more
grown up.

Speaker 2 (00:48):
I'm not sure gray hair is going to help you there.

Speaker 1 (00:50):
Maybe I should just shave my head.

Speaker 2 (00:52):
There you go. That's one way a little older, more distinguished.
And after you shave it, you could paint it gray.

Speaker 1 (00:59):
I'm not sure that's how gravitas is accumulated.

Speaker 2 (01:02):
Sounds like physicists don't know how gravity or gravitas works.
Both big mysteries. Hi, I am Horehammick cartoonists and the

(01:24):
author of Oliver's Great Big Universe.

Speaker 1 (01:26):
Hi, I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I never really gone after gravitas.

Speaker 2 (01:34):
What do you mean you just inherently have it? Can
you say you're a physicist?

Speaker 1 (01:38):
People go ooh, No, exactly the opposite, trying to break
down those barriers, you know. That's why I wear socks
and sandals every day. Don't try to create any distance
between myself and my students.

Speaker 2 (01:49):
Or apparently your feet and the exterior world.

Speaker 1 (01:53):
And everyone knows that's another reason to live in southern California, right,
Socks optional.

Speaker 2 (01:59):
I guess that's what physics is. It's all about, you know,
breaking barriers, connecting with the universe. Anyways, Welcome to our podcast,
Daniel and Jorge Explain the Universe, a production of our
Heart Radio.

Speaker 1 (02:09):
In which we try to break down the barriers between
ourselves and nature, between you and a complete understanding of
everything we do and don't know about the universe. We
think the universe is the greatest mystery and we are
here to crack it.

Speaker 2 (02:23):
That's right. The universe is full of barriers of things
we can't see things we can't understand and things that
we may never understand, including barriers about fashion and dress
codes at universities.

Speaker 1 (02:35):
You know, people say, focus on what you're good at,
and so that's why I just don't pay attention to fashion.

Speaker 2 (02:40):
So you're good at wearing sandals, it sucks.

Speaker 1 (02:42):
I'm good at ignoring fashion. I just do my own thing.

Speaker 2 (02:45):
Well, I am totally with you. I think the most
comfortable thing in the world to wear is socks and sandals.
If anyone does not try that out there, I highly
recommend it, or or socks with flip flops even better.

Speaker 1 (02:57):
I'm so glad this is an audio only medium.

Speaker 2 (03:00):
We don't have smell of vision, thankfully, though.

Speaker 1 (03:04):
I'm terrified at the mental images you are creating in
the minds of our listeners.

Speaker 2 (03:08):
Actually have socks that have the little notch for your
flip flop strap.

Speaker 1 (03:14):
You're like one step away from those horrendous toe shoes.

Speaker 2 (03:17):
Oh no, no, no, that's where I draw the line.

Speaker 1 (03:19):
I see one notch is fine. Four notches two too many.

Speaker 2 (03:25):
I don't need each of my individual toes wrapped in
its own little pocket.

Speaker 1 (03:29):
Again with the troubling mental images.

Speaker 2 (03:31):
Here, well, but toes are a big part of the
universe theories of everything, and so physicists have been on
the lookout and on the search for one SU's theory
that can explain everything in the universe, including all the
mysterious bits of it.

Speaker 1 (03:43):
And some of the most mysterious bits of the universe
are those pieces that we cannot see, things hidden behind
the event horizon of black holes. What's going on in there?
Are there singularities or is there something else? How does
gravity work for quantum particles? All the answers are waiting
for us behind these barriers.

Speaker 2 (04:03):
Yeah, black holes seem to be the sort of the
epitome of mystery in the universe. It's almost like a
little pocket that takes you out of the universe.

Speaker 1 (04:10):
Right, Yeah, it's sort of like they are their own
little pocket universes because they are cut off from us.
And maybe the most tantalizing thing is that we can
only know a few things about what's gone past that barrier.

Speaker 2 (04:21):
Yeah, they're almost like cosmic sensors, you know, they're like
hiding information, blacking it out for everyone to uh see
or a Nazi.

Speaker 1 (04:29):
And physicists hate when they lose information. When some knowledge
is hidden from them, and so we spend a lot
of time wondering about what we can know about the
things that have fallen into a black hole. Is it
possible to show up to a black hole and know
what has been tossed into it?

Speaker 2 (04:46):
Yeah, we have a lot of questions about black holes,
but maybe not as important as the one we're asking today.
So today on the podcast, we'll be asking the question
do black holes have hair?

Speaker 1 (05:03):
And do black holes wear socks with their sandals?

Speaker 2 (05:06):
I think the real question is black holes have black
hairs or gray hairs? Or does it depend on the
age of the black hole or it's gravitas exactly?

Speaker 1 (05:15):
Maybe the really big super black holes are like the
silver backs of the universe.

Speaker 2 (05:19):
I guess. Don't all black holes by definition have gravitas?
I mean you kind of have to take them seriously, right,
I guess so if gravity is the bending of space time,
then gravitas is like the bending of the social structure. Like,
are there lightweight black holes out there or you know,
frivolous black holes? Probably not right.

Speaker 1 (05:36):
They probably aren't like teenage black holes that the parent
black holes thing should be taking their life more seriously?

Speaker 2 (05:42):
Where did that come from?

Speaker 1 (05:44):
Frivolous. You're like, what are these black holes doing with
their lives anyway? Or maybe it's just because I have
teenagers in high school.

Speaker 2 (05:51):
I think you might be bringing us the home issues
here on the Physics podcast.

Speaker 1 (05:57):
Well, in the end, we are trying to understand the
universe through the lens of our own minds, so it's
impossible to separate personal issues from physics issues.

Speaker 2 (06:05):
You could be a family physicist. You know there's family physicians. Yeah,
there are a lot of dynamics in the household.

Speaker 1 (06:11):
Yeah exactly. Maybe black hole therapy can solve some problems.

Speaker 2 (06:15):
Yeah, just throw the whole family to black hole and
they have to get along.

Speaker 1 (06:21):
It might get kind of hairy.

Speaker 2 (06:23):
But Harry, apparently black holes can be, or might be
or could be. And so that's the question we're asking
here today, which is kind of a weird question. I mean,
who thinks of black holes having hair?

Speaker 1 (06:32):
Physicists? Physicists use hair as a metaphor for all sorts
of crazy stuff.

Speaker 2 (06:37):
Well, it's a fun question, and so as usual we
were wondering calming people out there, I thought about this,
Harry question.

Speaker 1 (06:43):
So thanks very much to everybody who answers these questions.
For this really fun segment of the podcast, and we
are always looking for more volunteers and we want to
hear from you, So join the group right to me
to questions at Danielandjorge dot com.

Speaker 2 (06:56):
So think about it for a second. Do you think
black holes have hair? Here's what people had to say.

Speaker 3 (07:01):
I suppose you'd have to ask yourself what exactly you're
talking about when you mean hair. My assumption is that
you would be referring to a biological process, and since
black holes are not a biological entity, I would struggle
to believe that they create hair in it of themselves.

Speaker 4 (07:15):
Well, I don't know about actual hair, but I do
know that when things fall into a black hole, they
get spaghettified. So maybe black holes are surrounded by a
whole bunch of spaghettified ragg doll hair.

Speaker 5 (07:26):
I'm assuming by hair you're asking about something that I
have never heard of, and not asking about like hair
on your head.

Speaker 1 (07:35):
But since that's the only.

Speaker 5 (07:36):
Definition that I know, I'm going to say, no, black
holes do not have hair on them.

Speaker 4 (07:40):
The only thing I can really equate hairs to would
be like cosmic strings of some sort. I really don't know.
I don't know.

Speaker 1 (07:50):
Well, if black holes have hair, it's yet another thing
that tests more hair than I do.

Speaker 2 (07:55):
All Right, some great answers here, pretty creative talking about
like is a cosmic a hair?

Speaker 1 (08:02):
That's right? Cosmic strings? Are these cracks in space time
we talked about once, which do seem kind of hairy,
but are completely separate from what we mean when we
say black hole hair.

Speaker 2 (08:11):
I also wonder if them you can take this question literally, like,
if the Earth falls into a black hole, it would
have a lot of hair in it literally, right, or
would it? I don't know.

Speaker 1 (08:20):
Yeah, well that sort of goes to the heart of
the question. Really, are black holes what they eat? Or
does it not matter what you put into a black hole?
Can you not tell the history of a black hole
in any way?

Speaker 2 (08:31):
I see these are internal hairs, hairs you don't really
want to see.

Speaker 1 (08:37):
Maybe somehow we ended up talking about ingrown hairs on
the podcast.

Speaker 2 (08:42):
Yes, do black holes have ingrown hairs?

Speaker 1 (08:46):
Next week, the comparison between black holes and black heads?

Speaker 2 (08:50):
Yeah, oh boy, those are very popular on the TikTok.

Speaker 1 (08:55):
And we're back to teenage issues.

Speaker 2 (08:57):
Here you go, black hole popping right here on the podcast,
it's the sound of a black hole popping. But anyways,
let's get down to it, Daniel, and let's maybe recap
for people what exactly is a black hole and what
do we know about them?

Speaker 1 (09:09):
So we don't really know what a black hole is.
We have a theoretical concept from general relativity that tells
us that if you have enough matter or enough energy
density in a small region of space, that that space
will be so curved that no information can escape. No photons,
no particles, no gravitational waves, nothing from beyond this event

(09:30):
horizon can ever propagate out and tell you anything about
what's hidden behind that curtain. That's the sort of general
relativity idea of a black hole. And we've seen some
things out there in the universe that really closely resemble
a black hole. There's something at the center of our
Milky Way, there's a bunch of collapsed stars, there's something
at the center of other galaxies that really resembles a

(09:50):
black hole. But you know, because you can't see inside
the event horizon, we're not exactly sure that what we've
seen out there in the universe aligns with Einstein's idea
of what a black hole would look like.

Speaker 2 (10:01):
Yeah, it's basically like an actual hole in space. Right, Like,
once you have enough mass into one spot and it's
tense enough, that region of space becomes a hole. Right
like anything you throw in there, it's going to stay
in there.

Speaker 1 (10:13):
It's a hole in that sense. But a hole sort
of gives you an idea of like a discontinuity, like
there's a gap or something. But remember that space is smooth,
it's continuous. So what we're talking about is space being bent,
space being curved, you know the way that like the
Earth doesn't move in what looks like a straight line
to us because it's following the curvature of space. When
you put mass inside space, it bends it. It changes

(10:34):
the relative distances between things, which changes how things flow.
And so near a black hole, space is curved and
curved very very intensely, and as you get closer, that
curvature gets stronger and stronger. But it is continuous, right,
there's not like a sharp cut off. But there's a
point on this smooth curvature past which nothing can escape,
and that's the radius we call the event horizon. And

(10:54):
so you could say that's sort of like a threshold.
It's like a hole if you fall into you'll never escape.

Speaker 2 (11:00):
Yeah, I think you're saying that there's no cank in space,
but there is sort of a point, that sort of
a discontinuity where light can no longer escape.

Speaker 1 (11:07):
Yeah, there's a discontinuity sort of in your fates. Like
if you have photons on one side of it and
photons on the other side of this radius, the ones
past the radius can escape and fly through the universe,
and the ones inside the radius will never escape. So
there's a sort of discontinuity in the outcomes of particles.
But remember the event horizon. It's not a physical barrier.
It's just this difference in the outcomes of particles.

Speaker 2 (11:28):
It's sort of like a real hole, right, Like there's
no barrier, you just follow. At some point you're on
the hole, and some point you're not on the hole. Yeah,
that's right, all right. So then what do we know
about black holes, if anything at all.

Speaker 1 (11:40):
So the frustrating thing is that we have this theoretical
concept of what might be inside a black hole. In
Einstein's pictures, famelessly, this infinitely dense dot, a singularity where
gravity has had this runaway effect compressing and compressing and
compressing forever with nothing to resist it and creating this
infinitely dense dot at the heart of a black hole.
But of course we can't see inside a black hole.

(12:01):
All we can do is observe them from the outside.
No information escapes from the inside of a black hole,
according to Einstein's theory. But we can know a few
things about the black hole, like we're on the outside
of it. But we can still measure some things about
the black hole without going inside. For example, we can
know the mass of the black hole. We can measure

(12:22):
the black hole's impact on space time even past the
event horizon, So we can know something about the black
hole without even seeing past the event horizon.

Speaker 2 (12:31):
You mean, like we can know how much gravity it
exerts onto the things around it.

Speaker 1 (12:36):
Exactly. The curvature of space time continues past the event horizon, right,
there's no kink, Like we said, it's smooth, so past
the event horizon, it's still exerting an influence on space time,
and we can use that measurement of the curvature to
tell how much mass there is inside the event horizon,
in the same way that like you can measure the
mass of the Earth just by seeing its gravitational effect

(12:58):
on the Moon or on a satellite. Right, all that
gravity adds up and has the same effect on the point.
Doesn't tell you anything about like the configuration of the
Earth or whatever. And you could replace the Earth with
a point particle to have the same gravitational effect. But
from a distance you can measure the overall gravitational force.
And we can do that for a black hole obviously.

Speaker 2 (13:18):
And the idea is that like, the more gravity of
black hole exerts and of the things around it, the
more massive it is.

Speaker 1 (13:24):
Exactly, we can do the calculation to say how much
space time gets bent by black holes of a certain mass,
and so we can back that up and say, well,
we measure this amount of curvature, and so therefore black
hole must have this mass.

Speaker 2 (13:36):
I wonder if like mass is the actual right term
for it, right, Like, we don't even know if what's
inside of a black hole is mass or just a
whole bunch of pure energy.

Speaker 1 (13:45):
Right, that's exactly right. We use mass as a way
to sort of measure the energy that's inside the black hole,
because remember that space time is really curved by energy density,
not necessarily just by mass, like a proton bends space time,
even though it's just made of a bunch of quarks
which are very very low mass. But there's a lot
of energy stored inside the proton energy in the bonds
of those quarks, which contributes to its mass and contributes

(14:08):
to the curvature of space time. So on one hand
you could say, well, it's really just the stored energy
of the black hole. On the other hand, that's kind
of what mass is. Mass is the internal stored energy
of an object, and so on one hand, yeah, it's energy.
On the other hand, that's what we call mass.

Speaker 2 (14:24):
Wait, so it's like it's inertial mass or gravitational mass.
Does that mean a black hole can have kinetic energy
as well well?

Speaker 1 (14:31):
In general relativity, inertial and gravitational masses are the same thing.
So yes, this is its inertial mass. This is like
how hard it is to get it moving and how
hard it is to slow it down, et cetera, et cetera.
And yes, black holes can have kinetic energy. They can
move right, and you can move past a black hole,
and velocity is all relative. So if you're flying past

(14:51):
a black hole at half the speed of light, you
see it moving towards you at half the speed of light.
So yes, black holes can definitely move, they can have
kinetic energy.

Speaker 2 (15:00):
All right. So then how would you measure the mass
of a black hole. You would sort of like put
a scale near it or throw a pebble to see
how it swings around it. How would you do it?

Speaker 1 (15:09):
Unfortunately, we're not near enough any black holes to do
any experiments, but fortunately in astrophysics we can watch these
experiments happen. So the best way to measure the mass
with black hole is to see the motion of stuff
near it, like the mass of the black hole the
center the Milky Way. We measure by looking at stars
that whiz past it and seeing the gravitational force on
those objects and knowing how much mass has to be

(15:31):
to create that gravitational force to make the stars bend
the way they do. And for distant black holes and
other galaxies, we can see like the sort of swirl
of stars around the center of the galaxy and use
that to measure the gravitational force from the black hole.

Speaker 2 (15:45):
Yeah, that's what I meant, you know, a pebble star.

Speaker 1 (15:48):
Yeah, basically, yes, same thing, the pebble.

Speaker 2 (15:50):
Method, all right, So then what else can we know
about a black hole?

Speaker 1 (15:54):
Another thing we can know about a black hole is
it's electric charge. If you put electrons into a black hole, Well,
charge is concerned in the universe. So if an electron
falls into the black hole, the black hole now has
an overall negative charge. At another electron, get another negative charge.
So you can know the overall charge of a black hole.

Speaker 2 (16:13):
Well, you mean like a black hole can have a voltage.

Speaker 1 (16:15):
Yeah, a black hole can have an overall charge and
it can have electric fields. Right in the same way
that black hole has mass, and that makes for effectively
a gravitational field or bending of space time past the
event horizon. The black hole can have a charge and
that creates an electric field which can also be pasted
the event horizon.

Speaker 2 (16:34):
Interesting, but I guess if I wonder if a black
hole has a negative charge, can you use it as
a battery? Probably not, right, Like you can't get electrons
to flow out of it, can you?

Speaker 1 (16:44):
You can use a charged black hole the way you
can use any other kind of charged particle. Right, You
can use it to create electric fields. You can use
it to repel stuff or attract stuff. But I think
batteries involved like the flow of electrons through materials and
I don't know that's chemistry.

Speaker 2 (17:03):
That's too hairy for you.

Speaker 1 (17:04):
Yeah, But essentially, you can think of a black hole
as just like an enormous particle, right the way like
an electron sort of has a charge attached to it,
but you don't really think about like where is the charge.
It's just like a property of the electron. You could
think the black hole as just sort of like enormous particle.
You don't have any details about, like where is the
electric charge. It's just sort of like assigned to the
entire event horizon, the same way you assign the mass

(17:27):
to the entire event horizon. You don't really know what's
going on inside, how it's that mass arranged? Is it
still in bananas? Is it squished into something else? Is
it nuclear pasta? You don't know anything about the internals.
You just assigned it to the exterior, and then you
can treat it like anything else that has a charge.

Speaker 2 (17:42):
I see, sort of like maybe the Earth. You could
do that with the Earth too, right, Like, I'm sure
the Earth has an overall.

Speaker 1 (17:46):
Charge exactly, any extended object with an overall charge from
a distance, you can treat like a point particle with
a charge, the math is exactly the same. And the
case of the Earth, of course, we can know it,
and it's not hidden beyond an advent horizon, So we
could learn about this stuff in other ways. In the
case of a black hole, you can't. And there's something
sort of similar there about a black hole and a particle,
the way that like two electrons are identical. You know,

(18:09):
you can't tell the difference between this electron and that electron.
They have all the same properties. That's what defines them.
Two black holes with the same mass and the same charge,
and it'll talk about it in the same spin, are
really two identical objects?

Speaker 2 (18:22):
All right? Well, talk about spin? What a spin for
a black hole?

Speaker 1 (18:25):
So the same way that you can toss stuff into
a black hole that has charge, you can also toss
stuff into a black hole that makes it spin. You
throw something exactly towards the center of the black hole,
it'll make it grow and make it more massive. But
if you throw something into the event horizon but a
little bit off center, then it's sort of like you're
giving it a push. The way if you're like holding
a bicycle wheel on an axle and you hit the rim,
it'll start to spin. Or if you push on a

(18:46):
merry go round, it'll start to spin. If you throw
particles into the edge of the black hole, you can
get it to spin because angular momentum, like charge, doesn't
disappear in our universe. So that black hole then has
to accumulate that angular momentum, which means it has to spin.

Speaker 2 (19:02):
But I guess the angular momentum in the universe is
not like a fundamental thing. Is It's not like energy
or regular momentum. Is it like? It's really just like
the difference between two points in your body and how
they're moving relative to each other, isn't.

Speaker 1 (19:15):
It Now, angular momentum is just as fundamental as linear momentum,
and maybe even more fundamental than energy number. Energy not
actually conserved in our universe because conservation of energy requires
space to be static, and our universe space is expanding,
so energy is not conserved. But linear momentum and angular
momentum are both conserved in our universe. It comes from

(19:36):
deep symmetries. Linear momentum comes from the fact that space
is the same everywhere, that like translation from here to there,
shouldn't change your experiment, and angular momentum comes from the
fact that there's no preferred direction in space, that every
direction is equivalent, and so angular momentum has to be
conserved and that makes angular momentum something very fundamental to
the universe.

Speaker 2 (19:55):
All right, Well, so then a black hole can have
angular momentum, and that is that what it's been exactly.

Speaker 1 (20:00):
The spin of the black hole is how it stores
angular momentum. And there's all sorts of fascinating consequences there
in general relativity, because like a singularity can't spin. So
the idea is that inside a spinning black hole is
maybe not a singularity but a ringularity, like a whole
circle of singularities, and those singularities are all spinning together,

(20:21):
which is how they store the angular momentum. But nobody
really knows what's actually going on inside.

Speaker 2 (20:27):
All right, So the things we can know about a
black hole are its mass, it's spin, and it's charge.
Now the big question is can we know how hairy
it is? So let's dig into that question. But first
let's take out a quick break. All right, we're talking

(20:53):
about the hairs of a black hole. Can a whole
half hairs.

Speaker 1 (20:58):
That's exactly the question. And when physicists say hair, they
don't really mean hair. They're just taking a word that
exists that has another meaning and giving it a meaning.
In physics, which is, you know, has a great tradition
of being very confusing what.

Speaker 2 (21:12):
You mean, like in this case it actually is like
a cosmic string or a particle or something, or is
it just a metaphor, it's.

Speaker 1 (21:19):
A metaphor for anything else. Essentially, general relativity tells us
that we can know three things about a black hole.
Its mass, it's been its charge, and nothing else. Nothing
about its history, no interesting little details, no texture, no hair.

Speaker 2 (21:34):
Is that because of the event horizon. Basically, like if
you wrap the Earth around an impenetrable, you know, black shield,
you could also not tell anything about the Earth, whether
it's t had hairs or not.

Speaker 1 (21:46):
I think that's true, but maybe only because an impenetrable
black shield would basically have to be an event horizon
of a black hole. There's no other way to make
something truly impenetrable. If you just build something really solid
and black, it would like read it it's some temperature,
and that would tell you about what's going on inside
of it. The only way to really be impenetrabled to
give up no information would be to make a black hole.

Speaker 2 (22:08):
I guess that's what I mean. It's like, the reason
we can't know anything else about a black hole is
because it has an event horizon which keeps all the
information inside. It's not something like that information is destroyed, right.

Speaker 1 (22:20):
We don't know what's going on exactly. And that's the
deepest question, is like is the information destroyed? Is it
actually contained within black hole somehow or is it actually destroyed?
Like is the internal state of a black hole the same,
no matter what you've put into it. Make a black
hole out of bananas and you make another one out
of apples. Do you really get exactly the identical black

(22:41):
hole out of those two things if you put in
the same amount of mass? Or is there some history
there some way to tell a banana from an apple
black hole?

Speaker 2 (22:51):
I guess I'm trying to, you know, relate it to
the earth, and like you could make an earth out
of bananas, and if you put it behind your impenetrable shield,
you could also not tell the Earth what's made out
of bananas or apples.

Speaker 1 (23:03):
That's exactly right, but it conflicts with sort of other things.
We've seen in the universe and our quantum intuition, because
quantum mechanics tells us that information is not destroyed, that
the present is uniquely determined by the past, and that
therefore from the present you can always derive the past,
that like the past, has left a permanent imprint on
the present. Quantum mechanics tells us that we can run

(23:24):
the laws of physics forwards or backwards in time, so
we could use the present to predict the future in
the same way we could use the present to reveal
the past. And this is in conflict with that. This says, no, no, no,
there's no information here. Once you create a black hole,
you can't tell what its history is at all. There's
no texture to grab onto, no little details if you
zoom in, there are no hairs there to reveal the

(23:46):
history of the black hole.

Speaker 2 (23:47):
I wonder if maybe the real question is not whether
a black hole has hair. The question seems to be
more like, if I toss a hair into a black hole,
does the hair get destroyed forever? Or is it just
going to stay inside the black hole and it's going
to be there, but we can't tell if it's there.

Speaker 1 (24:03):
I think those are both really interesting questions, but I
think there are separate questions. In general relativity, we can't
know anything that passes the event horizon. But maybe general
relativity black holes are not the black holes we see
in our universe. Maybe there are little ripples and hairs
on the event horizon that we could use to learn
about what has fallen in. And then the second question
is like what happens to stuff that falls in does

(24:24):
form some new state of matter? Is that information still there?
Because we think that black holes eventually evaporate, that they
radiate away all of their energy, they lose their mass,
and if that information has been destroyed within them, then
that information is lost forever, which would be very confusing
from the point of view of quantum mechanics.

Speaker 2 (24:41):
Well to the second question, I know we've talked about
in the podcast before, how it is possible to go
into a black hole and survive. Right, Like, for a
big enough black hole, the event horizon happens. You don't
have to get spaghetified or shredded apart to go into
technically the event horizon of a super massive black hole, right,
That's right.

Speaker 1 (24:58):
And if for really large black hole holes, if they
do have significant charge and or spin. There are even
stable orbits within the event horizon of the black hole.

Speaker 2 (25:08):
So you could like fall into a super massive black
hole and stay in there but still be hole, still
be there.

Speaker 1 (25:14):
And still be inside. According to general relativity, yes that
is possible. Again, we don't really know right what is
going on inside there, so we don't really know the
fate of any of this stuff until you know, you
jump into a black hole and figure it out.

Speaker 2 (25:26):
Well, not me, Maybe it should be the physinessist. I mean,
I think you need the cartoonist outside to draw what
happens and then figure out if it's a good idea.
But I guess what I mean is you can throw
a hair into a black hole and it can survive,
Like you can have a hair inside of a black hole.
It's she said, if somebody else comes by, they won't
be able to know whether you threw a hair in
there or not.

Speaker 1 (25:46):
Exactly that's true. According to general relativity. You could build
a black hole out of hairs and nobody else could
tell the difference between that and a black hole made
out of toenails or something else.

Speaker 2 (25:56):
But you would know there was a hair in there.

Speaker 1 (25:57):
You would know because you saw it falling in.

Speaker 2 (25:59):
You mean, yeah, or because I tossed it into the hole.

Speaker 1 (26:02):
That's true, you would know the history, but there'd be
no way to measure it from the objects itself. There'll
be no record on the outside that would tell you. Again,
according to general relativity.

Speaker 2 (26:11):
Okay, so then what does that mean? Does that mean?
Quantum mechanics says something differently.

Speaker 1 (26:16):
Quantum mechanics definitely says something different, And quantum mechanics tells
us that this whole picture of a black hole according
to general relativity is very very likely wrong. Remember that
general relativity is not built on the foundation of quantum mechanics.
It makes very different assumptions about how the universe works.
It assumes that space and time are continuous and smooth,
that you can have like infinitely small distances and infinitely

(26:39):
small masses. Quantum mechanics gives us a very different picture.
It says everything is discreet, it's quantized, it's chunked up
into pieces, and there's a limited amount of information we
can know about the universe. General relativity says that you
can have tiny little objects moving in smooth paths, so
you can perfectly know how they move. Quantum mechanics says
that's not possible, and they come directly into inflict at

(27:00):
the heart of a black hole, where general relativity says
you have this point of infinite density and quantum mechanics says, no,
that's not possible. And so what we need is some
sort of merging of the two, a theory of quantum
gravity that tells us what happens when quantum objects feel
very strong gravity.

Speaker 2 (27:16):
I guess maybe the question is what does quantum mechanics
say about the black hole hair question?

Speaker 1 (27:22):
So quantum mechanics as we have it now doesn't know
how to deal with gravity for particles. Right, particles are
very different from things like sand, or rocks, or base
balls or even pieces of hair. Those things are basically
classical objects, so we know where they are and we
can talk about them as if they always have a
specified location. But quantum particles are different. They have like
probabilities of being here and probabilities of being there, So

(27:44):
we don't know how to do gravity for particles. Like
if a particle has a probability to be here and there,
does it have half the gravity here and half the
gravity there, or is there a gravity probability we don't know.
What we need is a theory of gravity for particles,
and nobody has one, and so until you have that theory,
you can't actually know what quantum mechanics even says about

(28:06):
what's going on inside a black hole, or whether there
are ripples in texture on the surface of the black
hole that you can use to figure out what's inside.

Speaker 2 (28:13):
I see, I think this is what I'm trying to
get to, is like, what do you mean like quantum
mechanics says there are ripples on the surface that might
be able to tell you if a black hole is
made out of hairs or bananas.

Speaker 1 (28:23):
Unfortunately, we have no perfect theory of quantum gravity, but
people are doing some calculations to try to figure out
is it possible for a quantum black hole to have hair? Wait?

Speaker 2 (28:31):
Wait, wait, what's a quantum black hole?

Speaker 1 (28:33):
So quantum black hole would just be a black hole
as described by a theory of quantum gravity, instead of
a classical black hole as described by general relativity. Right,
Einstein's classical black holes are a singularity with an event
horizon around them, with no hair whatsoever, a perfectly smooth surface,
and you can't tell anything about what's inside or what
was used to build it. Quantum black hole is a
description of a different theoretical object, one that follows rules

(28:57):
of quantum gravity.

Speaker 2 (28:58):
All right, let's dig into that. Is a quantum black hole,
hole that's both black and white at the same time.

Speaker 1 (29:03):
No, no, but that sounds awesome. I wish that were true.
Carlo Rovelli has this fun theory that black holes might
be collapsing stars that are slowed down by gravitational time dilation,
and eventually they turn around and become white holes. And
we had a fun conversation with him and some of
his colleagues about exactly that how a black hole can

(29:24):
be in a superposition of a state maybe being a
black hole, maybe being a white hole. So basically, yeah,
your theory is one of the contenders.

Speaker 2 (29:31):
There you go. So, then how is a quantum black
hole different than a regular relativity black hole.

Speaker 1 (29:37):
So there's a few different answers to that, because there
are a few different theories of quantum gravity. Right, we
have no perfect theory. We don't even have a theory
that's complete and it works on paper, not to mention
a theory which has been tested against what's actually happening
out there in the universe. So people are working in
different directions in quantum gravity, and some of them are
working specifically on this problem and have ideas for the

(29:58):
consequences of their particular quantum gravity on the hairiness of
a black hole. So in some theories of quantum gravity,
black holes have different kinds of hairs.

Speaker 2 (30:07):
Like long hair, curly hairs, luster's hairs. What did to
say about what black holes could be like?

Speaker 1 (30:13):
Yeah, so let's go through some of the options. The
first real progress in quantum gravity was Stephen Hawking. He said, actually,
let's figure out whether black holes radiate a weigh any information.
And he has his famous theory of Hawking radiation that
says that black holes generate particles. You know, if you're
near the surface of a black hole, it'll actually be
shooting off particles at you. This is Hawking radiation. The

(30:35):
thing about Hawking radiation from black holes is that it's
supposed to contain zero information that just depends on the
mass of the black hole and nothing else about the
internal configuration, about whether it's apples or bananas.

Speaker 2 (30:47):
And that's because it's like generating particles out of the vacuum,
right out of nothingness, out of pure energy that's inside
the black hole. Like, it doesn't depend on what kind
of energy, whether it has hairs or been in it's inside.
It just kind of creates stuff. As I think, I
understand that it just creates stuff and then radiates it out.

Speaker 1 (31:07):
That's right, that it creates stuff based just on the
energy the mass of the black hole. Actually it's formulated
fascinatingly in terms of like black hole thermodynamics. So you
can think about the temperature of the black hole. But
you have to be very careful about applying Hawking's arguments
to like a microscopic particle picture of what's happening about
creating these particles, because Hawking doesn't have again, the full

(31:27):
theory of quantum gravity. He did this sort of semi
classical calculation where he thought about quantum fields and near
event horizons, and he figured out that these quantum fields
have to radied. He doesn't have a microscopic picture. And
you'll often hear this story about Hawking radiation, about particles
and antip particles created near the event horizon. One falls
in and one escapes, et cetera, et cetera. That's all

(31:48):
hand wavy storytelling. That's not accurate it doesn't actually hold
together in terms of what's happening with Hawking radiation.

Speaker 2 (31:54):
Wait, it doesn't hold together, or we don't know if
it's true.

Speaker 1 (31:57):
If you start from that microscopic picture and try to
build up a prediction of Hawking radiation, it doesn't work.

Speaker 2 (32:02):
What do you mean, Like, the radiation we measure from
a black hole doesn't match what you would predict from that.

Speaker 1 (32:07):
Well, number one, we've never measured the radiation from a
black hole. This is still just theoretical.

Speaker 2 (32:11):
So then what do you mean.

Speaker 1 (32:12):
As soon as you make yourself this microscopic picture and
then you have these quantum particles, you have all sorts
of questions you can't answer, like about fuzziness and probability,
and whether the particles can fluctuate past the event horizon
or not. And so the quantum mechanical picture isn't really
complete about what's happening to these little particles. Nobody can
start from those particles and calculate up and predict Hawking radiation.
That's not what's happening. Hawking has started from sort of

(32:34):
a bigger, broader picture, just from understanding like the energy
flow the quantum fields near the event horizon, and predicted
that the emit radiation. But again there's no microscopic picture
that really holds together. But you want to know more
details about how that works. We have a whole episode
about Hawking radiation where we can review that in detail.

Speaker 2 (32:51):
But I think the main picture you're saying is that
we do know that black holes radiate Hawking radiation. We
just don't know like the specific details of what's happening
at the border of a black hole to make that happen.

Speaker 1 (33:01):
That's right, Hawking didn't come up with a complete theory
of quantum gravity, so he doesn't have a microscopic picture,
and he predicts this Hawking radiation, which again should contain
no information. So a Hawking black hole basically has no hair,
even though it's kind of a quantum black hole. But
there are other theories of quantum gravity people who've been
working on, and some of these do predict black hole hair.

Speaker 2 (33:21):
All right, let's get into the hairy details of these
theories and whether or not black holes have hair or
quantum hairs or no hairs at all, and what it
could mean about our understanding of the laws of physics.
But first, let's take another quick break, all right, we

(33:47):
are braiding our knowledge of the universe here talking about
black hole hairs and whether they have it or not.
And so according to Einstein, who is famous for his hair,
by the way, his gray hair, even as black holes
don't have hair, which seems kind of mean given how
much hair he had. But it seems like quantum mechanics
at first said they didn't have hairs, but now there
were some theories about quantum gravity that say, maybe they

(34:09):
do have hairs.

Speaker 1 (34:10):
That's right, And all these calculations are approximate. You know,
nobody has a full theory of quantum gravity that they
can start from and predict these things from first principles.
Everybody is doing sort of approximations. They're saying, well, we
don't have the full theory, but maybe it looks a
little bit like this, And if it looked like this,
then what would be the answer to this question? But
a lot of is approximate and handwavy and inconsistent with

(34:31):
other things. But you know, this is how progress is made.
We don't always just have a flash of insight with
the whole answer that we can work from. We put
things together and try to patch them together, and eventually
maybe it comes together into a bigger picture of everything, all.

Speaker 2 (34:44):
Right, Well, so then how do these fuzzy quantum gravity
theories say that black holes have hair?

Speaker 1 (34:50):
So there's a paper about ten years ago that try
to study the configuration of gravitons far away from a
black hole, so outside the event horizon. Really interesting question
to think about gravity as a quantum force because if
gravity isn't the curvature spaced time, if quantum gravity lies
in the direction of like figuring out how to express

(35:11):
gravity in the same kind of language that we express
other forces like the weak force and electricity, magnetism and
the strong force as forces mediated by particles the way
they like the photon mediates electromagnetism. Then gravity would have
a particle, the graviton that you use to mediate gravity.
And so in this picture this is like graviton based

(35:32):
quantum gravity. People thought about, like, what's happening with all
the gravitons in the vicinity of a black hole?

Speaker 2 (35:39):
Like what they get sucked in too? Right?

Speaker 1 (35:41):
Yeah? Would they get sucked in? Is there information in
the flow of the gravitons. People are often writing in
and asking me if the mass of the black hole
is contained within the event horizon and gravity is quantum mechanical,
wouldn't it need to like shoot out gravitons in order
to mediate gravity, and wouldn't that break the area of
the black hole? So you can see how it's tricky

(36:03):
to think about gravity as a quantum force. If there
really is an event horizon there.

Speaker 2 (36:08):
Whoa Okay, So let me see if I got this right,
and a quantum theory that has gravitons in it, which
we don't know if they exist or not, like the
gravity of a black hole, you would feel it because
it's shooting gravitons at me or am I shooting gravitons
at the black hole? Couldn't both be the case?

Speaker 1 (36:25):
Yeah, both would be the case. And one question is
whether those gravitons contain any information about what's inside the
black hole or not. On one hand, you can imagine
the event horizon itself just generating a gravitational field and
shooting off gravitons from the event horizon. No information about
what's going on inside, only information about the total mass. Right,

(36:46):
Like if you could change the configuration inside, if you
build it out of bananas instead of apples, you get
the same exact distribution of gravitons in the outside, or
is there information in the graviton ripples that tells you
about what's happening inside the event horizon. So there's this
paperback ten years ago that deduced some small changes in
the graviton field far away from the event horizon based

(37:09):
on the mass that was inside. Essentially like a little
bit of a history imprinted on the space around the
black hole that could tell you what had fallen in.
You could like show up to a black hole a
million years later and say, oh, there was a banana
and then an apple, and then three granola bars thrown
into this black hole.

Speaker 2 (37:26):
What do you mean, Like, how would the banana versus
the apple affect the graviton that the black hole shoots out, So.

Speaker 1 (37:32):
It affects the gravitational field outside. The gravitons themselves don't
pass the event horizon, but the event horizon itself has
some shape to it, because remember, in Einstein's general relativity,
a black hole is a perfectly spherical object, right like,
as you zoom into it, you never see any bumps,
any ripples, any change. It's perfectly spherical. But quantum mechanics

(37:53):
says that can't be the case, right, that there's always
a discritizations like a pixelization inherent to the universe, which
means the event horizon has to have some ripples to
it's some texture to it, and those tiny little deviations
can change the ripples of the gravitons on the outside,
and they're caused by the history of stuff that you've
thrown into the black hole. So you make a very

(38:14):
slightly different black hole if you put in bananas than
if you put in apples. That information is not lost,
it's somehow contained on the event horizon of the black hole.

Speaker 2 (38:23):
Like if a black hole has a banana at its
center or an apple at its center, you're saying that
it might cause a different texture of the event horizon
at the border of it.

Speaker 1 (38:33):
Exactly. Yeah, And you could detect this if you could
measure gravitons in the vicinity of the black hole.

Speaker 2 (38:39):
How would the banana or the apple affect the texture
all the way at the edge of the black hole?
Like if I put a banana or an apple on
the center of the Earth, would it I guess it
would affect the surface of the Earth in some microscopic way.

Speaker 1 (38:50):
You have to think about this quantum mechanically right, we're
talking about the quantum states of all these objects, which
contain the history of everything that's happened to them. Remember,
in a quantum world, the present is uniquely determined by
the past, which means that every possible past has a
different present, which means you can invert it. You can say, well,
because we're in this present, we can tell what the

(39:11):
past was. So all the tiny ripples, the details of
the quantum states of everything around you could be used
in principle to like rewind time and tell you about
what's happened in the past. For folks who watch that
show Devs, that's the whole basic principle. And so the
idea is that you throw a banana or an apple
into the black hole, it creates a slightly different black

(39:31):
hole in a way that imprints that information. I don't
know the details. I can't tell you, like a banana
black hole looks like this, or an apple black hole
looks like that. But in quantum mechanics, there's a lot
of potential information on the surface because the surface is
not totally smooth, it can't be whereas a gr black
hole has to be perfectly smooth with no information.

Speaker 2 (39:51):
Well, I think you're saying maybe it's not specific to
quantum black holes. Like in a relativity black hole, you
could also throw a banana and everything inside of the
black hole could remember the banana. You just couldn't tell
from the outside because it's perfectly smooth. According to Einstein,
you're saying that in a quantum black hole, the black
hole is not perfectly smooth. You could maybe read from
its surface what you threw it.

Speaker 1 (40:12):
Yes, exactly. That was one theory in a paper about
ten years ago that effectively the gravitons could tell you
about the texture of the surface, which would in turn
tell you about what's going on inside. The problem is
that gravitons may be impossible to ever detect. Remember, gravitons
are not gravitational waves. Gravitational waves are huge waves in
the gravitational field that we can detect. Gravitons would be

(40:35):
like drops in the ocean compared to waves in the ocean.
So if they exist, they're super duper tiny and it
may be impossible to ever see.

Speaker 2 (40:43):
What does it have to be gravitons? I wonder if
there is a texture to a quantum black hole, wouldn't
you also maybe detect other particles coming from the Hawking
radiation off of it, and maybe could you could tell
its texture from that.

Speaker 1 (40:56):
You could, And there was a theory a couple of
years ago that suggested, maybe you remember we talked about
on the podcast, that there might be wormholes that connect
the interior and exterior of the black hole, and that
Hawking radiation could in fact give you information about what's
going on inside. That it's not the way Hawking described it,
totally information free, but maybe actually has information about what's

(41:18):
going on inside, because these wormholes are like bridges that
are connecting quantum entangled particles, some of which on the
inside and some of which on the outside of the
black hole. So there are versions of quantum gravity in
which Hawking radiation does have information in it.

Speaker 2 (41:32):
All right, So then what are some of the other
theories about quantum black holes?

Speaker 1 (41:35):
So all these would be very very hard to detect
because you're talking about detecting Hawking radiation and differences in
it which we've never even seen, or detecting gravitons, so
we don't even know exist, and if they do, would
be very hard to see. Recently, there was an idea
to look for hair on extremal black holes. We've talked
about this on the podcast recently. Black holes have maximum
spin or maximum charge that they can maintain, beyond which

(41:58):
a vent horizon disappear and things go crazy. But when
black holes are near this extremal state, things that fall
into them might leave instabilities on the event horizon in
some theories of quantum gravity, and if so, it would
generate ripples in their gravitational waves, not in their gravitons,
and gravitational waves are things we can detect that we

(42:19):
might be able to see sometime in the future.

Speaker 2 (42:22):
What do you mean instabilities on the event horizon? What
does that mean?

Speaker 1 (42:25):
Well, think about what happens when something falls into a
black hole. It's going to fall into a specific spot.
Right you toss a banana into a black hole, You're
tossing it in one side of this sphere, not on
the other side, and so immediately what's going to happen
is that the event horizon is going to grow out
to meet you. So the event horizon is not going
to be spherical for a moment. Then it slurps in
this banana, it falls into the singularity, and the black

(42:48):
hole rings back down to a perfect sphere that's in
general relativity at least.

Speaker 2 (42:53):
Wait, what do you mean if it's not perfectly spherical?
What shape is it? Like a blong? Does it go
oblong for a second?

Speaker 1 (43:00):
Yeah, it goes oblong for a second. And if you remember,
we talked about merging black holes. When black holes merge,
their event horizons grow together, and in some moments they
look like a dumbbell or they look like something else.
And I commented on that podcast. So that's exciting because
it tells you something about the history of the black
holes in a way that you can't otherwise know. In
gr but there's no hair theorem applies to the stable

(43:22):
state of the black hole. Like you put a banana
in a black hole, you expect it it's event horizon
and have a little bit of a funky shape as
it sort of settles back down to a new stable object.
But in this theory of quantum gravity, it says that
those instabilities persist that you create like waves on the
surface of the black hole which ripple basically forever, they
don't ever.

Speaker 2 (43:40):
Go away, sort of like a gravitational wave.

Speaker 1 (43:44):
Almost yes, exactly, and these would generate gravitational waves, which,
according to this paper, we might be able to detect,
probably not with our current observatories like LEGO, but with
future gravitational wave observatories like LISA or some of the
other projects, we might be able to actually see these
gravitational waves from these hairy black holes.

Speaker 2 (44:04):
So wait, you're saying, like, even if I drop a
hair or a banana into a black hole maybe a
billion years ago, and it happens to be one of
these extreme black holes even today, it might be generating
gravitational waves that tell me what was dropped in a
billion years ago.

Speaker 1 (44:19):
That's the theory, Yes, exactly. So if you're planning to
commit crimes and drop your evidence into a black hole,
this might be a loophole.

Speaker 2 (44:27):
Wouldn't all that information just be radiated away or it
would stay there for a billion years.

Speaker 1 (44:32):
It would fade with time for sure. So if you
want to catch a killer using clues dropped into a
black hole, your best bet is to use your gravitational
wave observatory immediately afterwards, because it would fade away. Even
these ripples would eventually fade away. As you say, energy
is being lost, right, it's being radiated away, but in
principle it still persists. Forever. Technically, it just gets fainter
and fainter.

Speaker 2 (44:52):
I see, like the ripple stays on the surface of
the black.

Speaker 1 (44:55):
Hole somehow, mm hmm, exactly.

Speaker 2 (44:57):
All right, Well, what does this all mean about our
understanding black holes? Like, there's still a big mystery. It
seems we don't really even understand what black holes are.
We've seen these things out there in space. We didn't
even know if they actually are black holes, if they
are gr black holes, if there are quantum black holes.

Speaker 1 (45:13):
And this is really at the heart of the question,
what's going on out there? I suspect that what's really
happening in the universe is something different from any of
these theories. You know, it's going to be a big surprise.
And the exciting thing is if black holes do have hair,
we might be able to measure it. We might be
able to get these subtle signatures from the ripples on
the surface of the event horizon to tell us what's

(45:35):
going on inside and maybe give us some clues about
what kind of universe we live in.

Speaker 2 (45:40):
Would that be the holy grail of black holes or
the hairy banana.

Speaker 1 (45:44):
Exactly the way Einstein's hair is the holy grail for
all physicists.

Speaker 2 (45:49):
Is it think I think like Brian Cox, is it
popularized and you kind of physicists here?

Speaker 1 (45:55):
Yeah?

Speaker 2 (45:56):
The whole like emo kind of hipster aging pop star. Yeah, yeah,
there you go.

Speaker 1 (46:01):
I don't think I'm ever going to be an aging
pop star. But you know, there are all these really
deep questions about the nature of the universe we live in.
Is information destroyed or not? It might be the black
holes are like cosmic toilet, it's just flushing away this
information to be destroyed. Or it could be that they're
preserving that information. They're radiating it back out into the
universe somehow, and then information is not destroyed in our universe.

(46:25):
It's a pretty deep question about the nature of reality.

Speaker 2 (46:28):
Yeah, and the nature of information itself, Like could you
ever destroy information? Or does the universe always remember everything?
Is there always a paper trail?

Speaker 1 (46:38):
Yes, exactly. If you committed a crime a billion years ago,
justice might still be coming.

Speaker 2 (46:43):
All right, Well, more deep questions about the universe and
a big reminder that there's still a lot for us
to discover and to understand and to learn about and
potentially to comb over as well. We hope you enjoyed that.
Thanks for joining us see you next time.

Speaker 1 (47:06):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. Or more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.