Are black holes hot or not?

Daniel and Jorge talk about whether black holes have a temperature and if we can ever know! 

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:02):
Yeah, or hey, I've heard that you are a fan
of campfires. Yeah, you know, I'm always up for some more. Well,
do you worry about, like far future generations of humanity
space settlers living under domes or folks living on colony

(00:23):
ships getting to have that same experience? Yeah, it doesn't
sound like a good idea to start a campfire on
a spaceship. So how are they going to capture that
essential human experience or is it going to be lost
forever in the endless depths of time. I'm sure they
can have a VR campfires. Maybe only a VR extends
to having smells. They might in the future. You know,

(00:46):
you plug something into your brain and it gives you
the experience of being in front of a campfire smelling fumes.
Or you can just fly too close to the sun. Yeah,
the sun is sort of like a campfire, right, it's burning,
and you can toast marshmallows with it if you get
close enough. That's the way of the future, plasma toasted
marshmallows s'mores. Yeah, and you can do your star gazing
right there, looking at the flames, seeing stars up clothes. Yeah,

(01:09):
just don't get too close or we'll have Plasma Toasted
podcast listeners. Hi am or Hey, I'm a cartoonist and
the co author of Frequently Asked Questions about the Universe. Hi.

(01:33):
I'm Daniel. I'm a particle physicist and a professor at
u C Irvine, and I've just realized that neither of
us on this podcast speak English as their first language.
And you can probably tell. I'm sure on every episode
we speak science instead. That's right. Science was our first language.

(01:53):
I mean we were writing papers, I'm sure, out of
our cribs, right. Yeah. My first words were F equals M,
a question mark. My fair words were P value of
oh five nature paper, those words. But anyways, welcome to
our podcast, Daniel and Jorge Explain the Universe, a production
of our Heart Radio in which we try to translate

(02:14):
the incredible mysteries of the cosmos into your native language.
We think it might just be possible to decode the
incredible froth and quantum insanity of our universe into a
language that humans can understand, and we can build some
sort of mental model in our heads that describes the incredible,
vastness and mysteries of the universe that we can pour

(02:36):
all of the incredible knowledge and ignorance of humanity into
your brain through your ears. That's right. I think of
this podcast as the dual lingo of physics and science,
translating all those amazing discoveries and things we've learned about
the universe into you know, bad jokes and short sound bites.
Are we going to start a signing homework? I think
duo lingo has homework? Oh? Does it? I think the

(02:57):
whole point is that it doesn't have homework. Oh, I
thought that exercises people are like doing their dual lingo
challenges and stuff. Oh yeah, it has like challenges and rewards,
but I'm not sure that's homework. Should we give people
rewards for listening to the podcast? Yes, we should definitely
give people rewards. We'll mail them bananas, We'll mail them
son toasted marshmallows. But that is interesting that neither of

(03:19):
our first languages were English, and look at us today
we have a podcast. I mean, how many people get
to have a podcast these days? Nobody? That's right, That's right.
We are super duper rare in the universe. But it
is amazing how you can learn different languages as a kid,
and even if English isn't your first language. If you
learn it when you're young enough, you can become totally

(03:40):
fluent in it. Yeah, and that's because you were born
in Israel, right, and so you spoke Heybrew that was
your first language. Yeah. I was born in Israel and
Spokekeybrew is my first language. And I didn't really start
speaking English till we moved to the US and it
turned out you needed to use English to communicate, and
that's when I started to pick it up around age five.
And it's interesting to me to think about like how
people think what language is are intuitive to them, because

(04:01):
we also think about physics sort of the same way.
There's certain ideas and physics that are intuitive to you
because you learned them as a young kid, temperature, velocity,
this kind of stuff. And then as we grow up
it's harder and harder to learn crazy new concepts and
to really feel fluent in them. You always feel like
in some sense you're translating. Yeah, I guess things like
quantum physics and the multiverse, those are all very sort

(04:24):
of alien to our intuition because you know, as as
when you're kids, you're used to like bouncing a ball
and holding a ball and those things aren't quite true
down to some molecular level. And it's fascinating the look
at the history of physics because when these ideas first
came out, the old guard were very against them. It's
very difficult for the gray haired physicists to accept some
of these ideas. Even folks like Einstein were resistant to

(04:47):
some of the crazier concepts. But now it's the kind
of thing that we teach in college, and they're even
books like, you know, quantum Physics for babies. So I
feel like one way that we make progress is by
introducing these crazy ideas younger and younger so that physicists
confluent in them, and then it can become like virtuosos
in these ideas rather than always feeling like they're working
in some translated alien language of the universe. Interesting. So

(05:11):
I guess if you're pregnant and you're carrying a baby,
you should maybe play our podcast to your baby the womb,
just to get am used to these ideas, right, So
it's kind of shock to them later on when they're
that are on about it in grad school exactly get
them hooked early. I think there's some ethical questions, hey
about pushing our podcast on kids, you know, getting them

(05:32):
addicted when they're young. I'm not sure how I feel
about that. Yeah, I think there might be some government
regulations about children's content, but maybe not fetus is content.
You know, maybe that's different way to find the loophole there.
But yeah, I do think it's good to teach children
to think about the questions of the universe in a
different way. But I think fundamentally we're always going to

(05:54):
struggle to really understand the nature of the universe in
a way that's intuitive to us because it is just weird,
and we don't have the immediate experience of some of that,
like quantum weirdness, for example, or relativistic weirdness, unless we
are living among Bose Einstein condensates and in the vicinity
of a black hole. We may never find that stuff intuitive. Well,
I guess it's kind of hard, because you know, you

(06:15):
need sort of Newtonian, regular old school physics intuition to
just like get around the world right and play baseball
and play basketball with your friends, and that's sort of
how the immediate world around his works. But if you
train a kid to be a quantum physicist from the beginning,
would they be really sort of like awkward or how
to touch with the world around them. You know, are
you setting this kid up for being very unpopular? Well,

(06:39):
I can't speak to the social aspects of it, but
it is an interesting question. If somebody had intuition for relativity,
could they still operate in everyday in life. The amazing
thing about relativity, though, is that it does reproduce our
intuition in our scenarios, right, Like, yes, you can use
Newtonian physics to describe motion and gravity, but if you
use relativity you get the same answers. It's just a

(07:00):
lot more complicated and some Newtony physics is like a
lot faster. So like somebody will like correctly predict where
the baseball is going to go, but it's gonna be
like three hours ago, you know, Like, Okay, I can
tell you exactly where that curveball went three hours ago,
but I missed the pitch. Yeah, I just don't put
that kid in the right field or left field, or

(07:20):
center field or any part of the field exactly, Like
still doing the calculations. Hold on, I'll get back to you.
Games over like two hours ago. Dude, physic that that
kid that quantum kid. It sounds like a fun, interesting
plot for a novel maybe or a movie. Like you know,
you train a whole generation of children to be quantum
thinkers because of some special science project or something where

(07:43):
we're sending people to the quantum world. Yeah, maybe, and
then they can be in like two emotional states at once.
I hate this movie and I love this movie at
the same time. I laughed, I cried. Wait, maybe that's
what movie criticism is. It's all quantum mechanical. Yeah, there
you go. People will love it and hate it. And
this is just about Jorge coming up with plots for novels.
This is about asking questions about the nature of reality

(08:05):
and trying to understand it. Are trying to apply our
intuition are limited toolbox of ways to think about the universe,
to probe deep dark questions about the nature of space
and time. Yeah, because there are still big questions that
are unanswered out there in the universe and huge pockets
of the unknown that we have yet to figure out
how it works. And sometimes we like to think about

(08:26):
what it might be like to touch those things. I
don't know about you, but I've looked at the sun
and I've wondered, like, how close could you get to
the sun? How hot is it really? You know, if
I put my finger on it, just for like a
micro second, when I get burned. Maybe I'm the only
one who's everyone. Yeah, I think walking around wanting to
touch everything is what gets in trouble as a kid.

(08:48):
It maybe also turns you into a scientist. You're like,
I wonder what happens when I press this button? Sounds
like a very dark take a way to get rid
of a lot of future scientists. That's right. Those of
us who survive learned a valuable lesson about what not
to touch. But it is interesting that there are pockets
of the unknown out there in the universe and that
we may want to touch them just to see what

(09:08):
they're like. And there is probably not a bigger hole
in our knowledge of the universe as big as black holes.
That's right. The deepest, darkest mystery of the universe is
what's inside a black hole? What's really going on over there?
And so of course the physicist in me wants to
reach out and touch one. Yeah, And so there are
many questions we can ask about black holes, but today

(09:31):
we're going to focus on one. I guess that it
is all about the black holes appearance, right in some sense,
I guess you could say it's appearance. I guess I
would say more like it's physical description. It's more to
me about black hole temperature than about black holes ability
to find meats for example. So today on the podcast,
we'll be tackling the question are black holes hot or not?

(09:57):
Isn't that a subjective judgment there, Daniel, like our and
are you thinking about attracted the black holes? Isn't everybody
attracted to black holes? Black holes are very attractive, even
to other black holes. Yeah. You know they say opposites attract,
but not in physics, right, In physics, massive attract. But
if it's something that doesn't have an opposite anyways, I

(10:18):
like big masses, and I cannot lie that might be
taking asn't to a hole we don't want to go into.
But it is an interesting question to think about whether
black holes are hot or not. And I'm guessing you
mean temperature, not like hotness, that's right. Yeah, I make
no evaluation of whether black holes are attractive or not,
but I am interested in the question of whether or
not black holes are hot or black holes are cold.

(10:40):
It's really fascinating because black holes certainly do contain a
lot of energy. Yeah, yeah, they have a lot of energy,
for sure, but I feel like it's energy that's like
trapped somewhere that you can access, so therefore it should
feel really cold. You know. That's good intuition. Sometimes, though,
our intuition for these things breaks down. Like it's true
that the Sun looks hot, and it is hot, but

(11:02):
there's other stuff out there in space, like the interstellar medium,
which can be like millions of degrees kelvin, but if
I dropped you inside of it, you would freeze to death.
So sometimes these things can be a little bit counterintuitive. Interesting.
So something can be hot but also cold at the
same time. Is that like quantum temperature. In the case

(11:23):
of the interstellar medium, it's because it's very dilute, so
the individual particles are moving at very very high speed.
So technically it's hot, but there's not a lot of it,
so it wouldn't be enough to keep you warm. You
would freeze to death even as you're being smashed into
by these high speed particles. I see. So you're saying,
like a black hole could be hot and actually hot

(11:43):
or hot and actually cold or something in between. Yeah,
it's very weird. Think about what happens when you shoot
a laser beam at a black hole? In theory it grows,
does it also get hotter? Is it possible to shoot
a laser beam at something and cool it down? Or
what if you throw out like a campfire into black hole?
Does the temperature go up or down? A lazy team
is basically just like a high tech physics version of

(12:05):
a campfire. Yeah. Or what if you throw Brad Pitt
into a black hole? He's pretty hot? I don't know.
Is he still hot though? Like as time goes onto
is he getting hotter or colder? What do you think? Um?
Definitely some of those movies started to stay pretty hot
into their old age, for sure. Helen Mirren, you know
you could throw her into a black hole. I'm sure

(12:27):
we'll get hotter. Helen Mirren is so hot she breaks
the laws of physics ceriently, Let's not throw her into
a black hole, please, But this is an interesting question
our black holes or not? And what would happen if
you touch him? Would your hand get burned or would
it freeze or get sucked in? I guess would it
file restraining order against you. So, as usually, we were
wondering how many people out there had thought about this

(12:48):
question and whether they think black holes will burn you
or freeze you. So thank you very much to all
those who participate in answering these random questions. If you'd
like to hear your voice speculating business, leave for the podcast.
Please don't be shy, it's a lot of fun. Right
to us. Two questions at Daniel and Jorge dot com.
So think about it for a second. Would use why

(13:08):
right or left on a picture of a black hole?
There's some people had to say. I guess that would
depend on how how you're defining temperature. So temperature generally
is defined as the amount of movement of a particular
set of matter. So maybe the spinning of a black

(13:31):
hole would mean that it has lots of energy, which
would mean that it might be hot, And then if
the black hole wasn't, spinning might be called I guess
if they were hot, they would be giving out lots
of high energy radiation, and I guess that would make
them easier to spot. I know that they give away

(13:53):
hawking radiation, but that seems to me to be low
energy and low density radiations. So my guests would be
black holes are cold, as the name subjects suggests. So
there's a lot of matter, it gets compressed down into
a black hole and it gets all smashed at one point.

(14:14):
The other thing that's kind of like that's like a star, right,
that's really hot because there's all this like energy, and
maybe if it's like hot as an temperature, which is
kind of standard for energy, we're talking about a lot
of matter in a small part, I'm going to call
that hot. And this sounds very hot to me. I
would say black holes have to be cold because when
they stuck in the start of the star is obviously
circulated around the black hole until they disappear into it.

(14:36):
You can kind of see their light on the horizon.
And I feel like if they're hot, then the star
would have to then get hotter as it approached it,
and I feel like that would then cause more of
a supernova or just some kind of explosion happening to
the star as it entered the black hole. Definitely, inside
a black hole, like the object itself, the pressure death,

(14:58):
it has be had that energy that it's right there,
but like outside, when you go close by by the
black hole, a black hole that doesn't interact with anything.
And what I want to say is like, no start
close to it to feed on that start nothing, no

(15:21):
object close to the black hole to interact with it
to make it bigger. So in this case, if the
black hole doesn't let the light go out, probably doesn't
let the heat go out either. So if you're right there,
close to it, you wouldn't be had but inside right

(15:42):
there when the pressure is might be hard there. I
believe black holes must be really hot because the eating
energy in form of cooking nation, so they must be
really hot, at least on the surface. That's what I think.
My intuitions is called old, So I'm going to go
up with hot. All right. A lot of pretty um

(16:05):
hot takes on this question. I think my favorite one
is my intuitions has cold, So I'm going to guess hot.
That person has learned some lessons about how the universe works.
Right there. Yeah, a person must have been listening to
a podcast in the womb. Is every physics question really
a trick question? Is that what they've learned? Yeah? I

(16:27):
feel like the universe is a true question. Why it's
not really a universe? Is actually just a simulation. That's
the trick. Yeah, it's something else right that that seems
to be the lesson everything you thought. It is not
really the way it is, even your thoughts. Maybe there
is no truth, man, Maybe there is the truth. But
I think there are a lot of really interesting ideas here.
There's ideas about how we can think about black holes

(16:49):
because they radiate energy, and also thinking about how black
holes absorb energy so they should be hot, or the
high pressure inside a black hole that might make it hot.
It's a lot of really good ideas. Yeah, And I
guess it's all sort of depends on what you mean
by hotness, right, Like is it related to something like
pressure or how much energy it admits or what happens
if you touch it? Right? Sort of maybe depends on

(17:10):
what you mean by the question and what it means
to be hot. Yeah, And as we learn, the whole
concept of temperature is a bit of a slippery topic. Yeah,
also whether or not you slip on it. Also. Well,
let's start at the beginning, and let's recap for listeners, Daniel,
what is the black hole? Like, how do physicists think
about what a black hole is? So we have several
pictures of what a black hole might be. But the

(17:30):
truth is that we're fundamentally not really sure what a
black hole is, and that's why this question is really interesting.
A black hole temperature really goes to the heart of
what we do and do not know about black holes.
So we have a couple of ideas about black holes.
One is what we call a classical picture of a
black hole, which means that it comes from general relativity.

(17:51):
If you just follow Einstein's rules about how mass bend
space time, and space time tells mass how to move,
then you end up in this really puzzling, amazing, sort
of weird prediction, which is that gravity has this strange
runaway effect. Right, mass pulls on mass, which means it's
more massive, and then it has more gravity, which means
it pulls harder on other masses, which means it has

(18:13):
more gravity, and that just keeps going, and eventually you
get this weird thing, a singularity, where space has curved
so much that you have created an event horizon in
a region past which if something gets too close, it
can never leave. No information can leak outside this event horizon.
That's sort of the classical view of what a black
hole is. A singularity a point of infinite density surrounded

(18:36):
by a threshold called the event horizon, like a marker.
If you go past it, you can never escape, will
be trapped forever. Your future is that singularity. Yeah, I
like classic black holes, feel like the classics are always
the best. But I think what you mean is like
a classic black hole was basically physicists thinking about like
what happens at the extreme levels, Like what if I
take a whole bunch of mass and I cram it

(18:58):
into the smallest possible space, or even like a tiny thought,
like what happens if a thought has almost infinite mass? Right,
Because gravity is very strange compared to the other forces,
as you are alluding to earlier. There's no negative gravity,
there's no repulsive gravity. I mean, there's dark energy and
is the cosmological constant due to potential energy and fields perhaps,

(19:19):
But when you're talking about just masses and objects, you
only have attraction, and so there's no balance. Right, just
keeps going, and so you have enough mass somewhere, then
it's just going to keep getting denser and denser and
stronger and stronger until eventually you get infinity. Right, this
thing diverges that actually goes to infinite density if you
give it enough time, right, Because I think one of

(19:39):
the things about gravity is that it gets stronger the
closer you get to it. Right, Like the formula for
it has the distance to it in the denominator in
the bottom, So like if you can cram a lot
of mass in a small amount of space, then you
can get really close to it, which means that your
denominator kind of goes to zero, which means that your
force sort of like goes almost to infinity. Yeah, and

(20:00):
so gravity, left to its own devices, will always generate
black holes. The reason that you're not a black hole,
and I'm not a black hole, and the Earth isn't
the black hole is that there are other forces out
there that can oppose gravity, right. The electromagnetic force and
the strong force give us structures that balance out gravity,
at least for a time. And then eventually if those
things fail, then you know, you collapse into a black hole.

(20:21):
But that's the prediction of classical general relativity. This incredibly
strange phenomena of an infinitely dense point. You said the
word sort of inevitably, and I wonder if that's how
you think about it, Like Is it inevitable that a
bunch of mass, if you leave it out in space,
will become a black hole if you have enough of it,
like things will sort of crunch down down to an
infinite point. Or is it possible even in the classical sense,
that things won't crunch down forever, like the Earth is

(20:44):
not crunching down to a black hole? Right? Cool question.
If there was only gravity, then it would be inevitable.
If you have a bunch of particles floating in space
and the only force in the universe is gravity, then
it will suck them together into a black hole. The
only thing that could resist that if there are no
other forces is angularm entem. You can imagine like some
blob of stuff coalescing sort of like the Earth and

(21:04):
then orbiting a central black hole, but not falling in. Right,
So you might imagine maybe that's stable. Maybe a blob
of stuff orbiting a black hole could avoid it forever.
But you know, even if you're orbiting, even in classical
general relativity, you are radiating gravitational waves, so you're losing energy,
so eventually you will spiral into the black hole. So

(21:24):
in the universe with only gravity, then eventually, yes, black
holes are totally inevitable. But as you say, the Earth
is not a black hole and probably will not become
a black hole because the other forces are stronger than
gravity and can oppose it for a while. All right,
So that's the sort of classical view of a black hole. Um.
But then what's the other view the new black holes?
The other view is that we don't know what a

(21:45):
black hole is, but it can't be that, you know.
It's basically it says, this can't be right. Oh my gosh,
it's ridiculous. Infinities are not predictions of physical theories. They
are failures of physical theories. It says something is wrong
here and it's got to be replaced by some other idea,
some quantum version of general relativity. We call that quantum gravity.

(22:07):
We don't have that theory, but we can criticize the
current theory general relativity for not being consistent with quantum
mechanical ideas. And that's because the idea of a singularity.
You can't have that in quantum mechanics. Well, in general,
we don't think you can have singularities or infinities in
physics at all. Like whenever you get infinity is a
prediction of a physical theory, it's usually a failure. I mean,

(22:28):
look back the history. For example, the whole discovery of
quantum mechanics was trying to unravel the source of the
ultra violet catastrophe. This prediction from classical physics that objects,
as they got hotter would radiate infinite temperatures. Like it
was nonsense, right, It was a signed to us that
there was something wrong, something missing in our calculations that
gave this crazy prediction. And so anytime you get an infinity,

(22:50):
it's a sign that probably you're doing something wrong. Like
we don't see infinities anywhere in the universe. Maybe the
universe itself is infinite, but we never measure or observe
infinity directly. It seems like a mathematical failure rather than
a physical prediction. But more than that, a singularity is
a really strange thing in the universe. Right. We talked
about singularities on the podcast several times. But in some sense,

(23:12):
they're like a path that ends, you know. Quantum mechanics
says that time goes from the infinite past to the
infinite future, and you could predict what happens to an object,
you know, it's deterministic in the sense of like evolving
its wave function. But a singularity is where like a
point ends, something falls into the singularity and you can
no longer predict what's going to happen predictability of physics,

(23:33):
which is like the foundation of physics. What we try
to do is predict the future given the present. That
fails at a singularity, you can't predict what happens after.
A singularity is like paths out. You know, everything that
falls in, no matter how it started, has the same fate.
It's just like is there in the singularity? And so
that's inconsistent with like quantum information theory and basically everything

(23:55):
we know about how physics works. It just seems like
a breakdown more than a prediction. I guess that's why
they call it a a singularity. It's singular. It's sense
the coolness. All right, Well, I ask some questions about that,
and so let's get into more of this idea of
a quantum black hole, and then let's get to the
question whether it's hot or not. But first let's take
a quick break. All right, we are talking about black

(24:29):
holes and whether or not they are hot or not.
I guess whether or not you would go on a
date with a black hole because you're definitely attracted to
a black hole. Yeah, On the other hand, I also
think black holes are pretty cool, so I'm not sure
how to feel about that. Oh boy, now we're mixing
all kinds of metaphors here. So you're saying there's a
classical view of black holes means that there's sort of

(24:50):
a singularity in space because it's sort of the natural
extension of what happens if you just have gravity in
the universe. But then there's a quantum mechanical view which
says that's a singular it's not possible in a quantum world.
But I guess my question is does it have to
be possible, Like, isn't the point of a singularity that
you never actually get there? That's interesting. I mean, we
are interested in what's going on in the universe, even

(25:13):
if we can't observe it directly, right, So we definitely
want like a physical model for what's happening microscopically inside
a black hole, because that allow us to understand, you know,
the nature of space and time. It might be that
we can ever see inside them, and so that you
know the observe ability is a question. It's sort of
like thinking about the multiverse. It's useful to think about

(25:34):
the multiverse, even if you might never observe or go
to the multiverse. It's still a question we wanted to
answer to. Right, Well, you say that a singularity is
inconsistent with quantum mechanics, maybe gives the sense of why
that is, Like, you know, maybe the singularity is not consistent,
but maybe getting we're approaching the singularity is consistent all
the way to infinity. Some of the problems with quantum

(25:55):
mechanics and black holes we laid out a minute ago,
but there are other ones. As you say, you know,
quantum mechanics says that things can be localized in momentum
and in location perfectly well, right, But a singularity violates that.
It's an infinitely well localized point in space. And so
quantum mechanics says, if particles are in an infinitely well
localized point, they should have an infinite uncertainty in their energy,

(26:17):
which would mean some of them would have basically infinite energy, right,
And so it's sort of like physically impossible. There's a
minimum quantum fuzz to the universe, which has singularity violates,
and so we just don't think it's possible. That doesn't
mean that black holes aren't real, right, It might mean
that black holes are there, but that at their heart
there isn't a singularity. There's some like weird quantum fuzz happening,

(26:38):
and you might not be able to tell the difference
from the outside because from the outside it's all just
strong gravity. But you know, we want to know what's
going on inside that black hole, and it does have
consequences for whether the black hole is hot or not.
You're saying, like a black hole is still a whole.
It's still it could still look like a hole to us,
but at the very center of the black hole, like
what's going on at the very very center it doesn't

(26:59):
actually get to a singularity? Or is it kind of
fuzzy down there? And I guess, just to be contrarian,
could it be both? Like could there be a singularity?
And as you get closer to the singularity, you know,
spaces crunched down so much that you're you're still fuzzy
but also kind of a singularity, do you know what
I mean? No, but I like to hear more about
that idea. How can you be a singularity and fuzzy. Well,

(27:20):
gravity bend space right in some ways that you're sort
of compressing space and in the middle of a black hole.
So as you're a protesting cularity, maybe you're still fuzzy
in a quantum sense, but spaces crunched down so much
that you're kind of fuzzy in a very small space.
That's basically a singularity. I see. I mean it could
be something you could never approach, or something that sort
of remains at a certain observable size no matter how

(27:42):
close you get to it, without actually being a point. Uh.
And that's true, that's interesting and maybe a possibility for
quantum mechanical description, but I don't think that would qualify
as a singularity. It's definitely not what general relativity predicts,
for example, And you might think, well, why do we
care what general relativity predicts? And we care because general
relative he seems to be accurate about everything else in

(28:03):
our universe. You know, the way that space expands, the
way the space ripples, the way that objects move through
space and time, very very precisely. It seems to get
everything else exactly right, and the math of it is beautiful.
It's gorgeous. I mean, theoretical physicist who learned it, They
like fall in love with the equations, and so it
seems to like speak to something deeply true about the universe.

(28:24):
So to discover that it's not that there's something that
it fails to describe is an opportunity to learn something
really fundamental about the nature of space. Yeah, I guess
kind of. It's kind of like Newtonian physics. We thought
was great because it predicted the like the paths of
the planets and the orbits and whether or not you
can catch a baseball, but it sort of breaks down
to a different scales kind of, right, you're saying that

(28:46):
relativity also sort of works for everything else that we
see around us, but when you get down to the
smallest levels, it doesn't have quite a solution that works exactly.
And we would like to know what the fundamental nature
of space is. You know, at the smallest level, is
gravity actually a quantum force mediated by gravitons or is
space itself quantized? Did this little foam It's more than

(29:08):
just an academic question of what's that the invisible heart
of a black hole. Understanding the nature of space and
time might give us great power over it, you know,
let us develop warp drives and wormholes and explore the universe,
and so definitely a question worth asking. Yeah, well, okay,
let's get back to our main question of this episode,
which is whether black holes are hot or not? And

(29:28):
I guess we mean in the sense of temperature. Do
black holes have a high temperature or do they have
a very low temperature? And so let's start maybe by
talking about what temperature means in this context. Yeah, the
temperature is really slippery and tricky. Sometimes when we talk
about temperature, we mean like how much energy is in
the particles inside something. So you have a banana in
front of you, you can ask is this banana hot

(29:50):
or not? What you really mean is how much internal
energy is there? Are the particles inside the banana wiggling
a lot? Are they frozen in place not wiggling a lot?
And you can actually do cool calculations to connect like
how much those guys are wiggling to the apparent temperature
that you would experience if you like touch the banana.
That's so if your intuitive sense of temperature, and the
way you measure temperature usually is by like sticking a

(30:13):
thermometer in something, and the energy transfers from the banana
to the thermometer and you read it out on the thermometer.
That's your intuitive sense of temperature. But that doesn't really
work with something that you can't touch. So if you
want to measure the temperature of something you can't touch,
there are other ways to do it, Like we can
measure the temperature of the Sun, even though no human
object has ever touched the Sun and then come back

(30:34):
out again, right, or like even now they have contact
list thermometers, right, the kind of is just kind of
like cover over your forehead and it somehow measures the
temperature without touching it exactly. So the way they do
that is by measuring the radiation that you generate, not
like you know, you're shooting off alpha particles and creating
hulks everywhere you go or spider Man, but you generate

(30:55):
energy like you radiate photons everywhere you go, because everything
in the universe that has a temperature does radiate energy,
Like the Sun radiates energy in the visible spectrum because
it's pretty hot, and the Earth radiates energy in a
much much longer wavelength because it's much colder than the Sun.
But it still does, and you generate energy at a
higher wavelength than the Earth does, which is why if

(31:17):
you put on like infrared goggles, you can see if
there's somebody hanging out in your backyard at night, because
they're generating a different kind of light, a light that's
invisible to your eyes but can be picked up by
infrared goggles, and that's it's a pretty cool burger or trespasser.
You can't see them. But I guess maybe the basic
question is why why does that happen? Like why do
hot things amid infrared light? Yet has to do with

(31:41):
energy just liking to get spread out, you know, a microscopically,
imagine like a blob of stuff, and that blaba stuff
isn't inactive, right, it's constantly radiating stuff and reabsorbing it.
So electrons within your banana are shooting off photons and
then other atoms are reabsorbing that. But near the edges
that doesn't always get reabsorbed. Some of it just shoots out.
So that energy is just like getting spread out through

(32:02):
the universe, partially by radiating stuff. And what's really interesting
is that the spectrum you emit is controlled by your temperature,
so if you have a really high temperature, you tend
to emit at shorter wavelengths. At higher frequencies, you have
a really low temperature, you tend to emit lower frequencies
or longer wavelengths. There's, for example, really really hot gas
emits in the X ray or the ultra violet, and

(32:24):
really really cold objects like asteroids floating in space emit
in the infrared. Right. Yeah, because we've talked about in
this podcast about how temperature is related to the kinetic
energy or like how fast the particles or something are
moving within you know, the volume that you're measuring the
temperature of. But then I guess that question is how
does that relate to the wavelength of light that's emitted? Like,

(32:44):
first of all, why does they emit light at all?
And the second why do they emit light at a
higher frequency. So we've been talking about them emitting light,
and by not we just mean photons. Right, Remember that
by light we don't always mean things that you can see.
So we're talking about something emitting light, you shouldn't expect
it to be glowing in a way that your eyeballs
could pick it up. You sit in a dark room
with a banana and stare at it. You're still not

(33:05):
going to see that banana glow because while these are
photons and they are hitting your eyeballs, your eyeballs can't
see them. So it's emitting light because it's made out
of charged particles, you know, electrons and atoms, and when
those things move and accelerate, they always emit light, you know.
That's what electrons do. From an electron to accelerate, for
like change direction, for it to slow down or speed up,

(33:26):
it has to admit a photon because that's the only
process that allows it to do it right. It needs
to like push off of something. The same way, if
you're in a rocket ship and you want to accelerate,
you've got to throw something out the back, right, you
need some mass, some rocket fuel, or some propellant to
toss out the back. That's the only thing electron can
do to change its direction, to accelerate or to decelerate

(33:48):
is to emit a photon. So because the banana is
made out of charged particles, it tends to emit photons.
It also emits other stuff, you know, emits neutrinos and
emits other kinds of radiation from the other kinds of
forces that it interacts with. But that's why it emits light,
and that's why emits photons, because it's made of charged particles.
And why does it emit different frequencies based on its temperature. Well,
you know, higher energy stuff emits higher frequency photons. You

(34:12):
have like electrons whizzing around your atom with more energy,
they have more energy, They will jump down more energy
levels and they will admit higher energy photons, which have
higher frequencies and shorter wavelengths. So it's just sort of
like more energy available because everything moving around has more energy.
You get on average higher energy photons, not purely. It's
not like a spectral line, right, There's like a distribution,

(34:33):
so you get some of the high frequencies some of
the low frequencies. But the shape shifts as you get
to higher temperatures. The peak moves and the most likely
frequency gets higher and higher as the temperature gets higher
and higher. Interesting, it's almost like you have a bunch
of atoms and they have some charge to them, and
it had they have electrons and protons and set them

(34:53):
and they're all sort of trapped together by the electromagnetic
forces that are holding them together. Kind of right, or
something to bumping to each other. And you're saying every time,
you know, Adam gets pulled into the banana, or a
gas particle bumps into another gas particle. You know, stuff
happens in the electromagnetic force, and so that causes photons
to shoot off. Yeah, exactly. Every time two electrons talk

(35:16):
to each other, that's a photon. So if you have
a blob of charge stuff, there's photons everywhere. It's just
like a huge mass of photons and particles constantly interacting
with each other, and some of that just bleeds out
the signs. It's like if you're in an apartment next
to a party, right and everybody shouting at each other,
then you're gonna be hearing some of those conversations, right right. Yeah,
unless you're in the party, then then it's a good time.

(35:37):
And as they drink more and more, they tend to
get higher and higher energy, right, and then this analogy
breaks down, Yeah, physics, as the parties put the main point.
So in terms of black holes, I think what you're
saying is that temperature is normally defined as like the
kinetic energy or how fast the particles of something are moving.
But it's sort of related to different things in different
ways that we measured, Like we can measure it by

(35:59):
touching in and some of kinetic energy moves to you.
Or you can measure the light that comes from it
because of this radiation, so that also sort of tells
you the temperature. And so it kind of sort of
depends on what do you mean by temperature of a
black hole? Do you mean like the energy it radiates,
or do you mean like what would actually happen if
you touch it? Yeah, Well, the fascinating question about black
holes is that we don't know what's going on inside

(36:20):
the microscopically, but we can ask questions about their radiation.
Do they give off any radiation? And if we use
radiation as a way to measure temperature, then we can
talk about the temperature something without having any idea about
what's going on inside it microscopically, like we can measure
the temperature of the sun without knowing like how do
those plasma tubes work inside the sun? What is going

(36:41):
on at the heart of it. We can still measure
its temperature the same way they measure your temperature on
your forehead without touching you, without knowing, like you know,
are you sick or not, what's going on inside your brain?
They don't know any of that, but they can still
measure your temperature. So it's a bit of an extrapolation
to say we don't know what's going on inside something,
we measure radiation and then we talk about what temperature

(37:02):
it has, right. I think what you mean is like
if you take a step back from the black hole
and you know, not worry about what's going on at
the very center, you know, how hot does it look
from afar? Yeah, we use this concept of temperature a
lot in physics in a way that might be confusing otherwise.
Like we talked about the temperature of the cosmic microwave
background radiation. We say that's two point seven three kelvin.

(37:23):
What does that mean? Doesn't really have a temperature. What
it means is that the cosmic microwave background radiation is
radiation at a wavelength that would be emitted by an
object at two point seven three kelvin. That's the black
body radiation for an object at that temperature would be
the frequency of the CMB. In the same way, we
can look for radiation coming off of black holes and

(37:43):
use that to measure its temperature and then trying to
deduce what might be going on inside of it from that. Well,
I guess it's kind of weird because you know, I
imagine that a black hole has particles inside of it
moving and a certain speed, and it does have sort
of like a kinetic internal kinetic energy. But you're saying, like,
let's not even worry about that because we don't really
know what happens beyond like the black hole's event horizon.

(38:05):
So let's just take a step back and see how
hot it looks. Yeah, well, let's start there because that's
something we can do from the outside, and then let's
turn that around and try to understand based on its
temperature what might be going on inside. So let's use
that as a probe, right, because temperature is like a message.
It tells us something that's going on inside from afar,

(38:26):
So without touching something, you can get a little bit
of information about what might be going on inside. And
in the end, that's the goal, is to try to
understand what's going on inside the black hole. Right. But
I guess it's messed up because you know, the definition
of a black hole is that nothing can escape of it,
not even light. What does it mean to measure the
light coming off of a black hole? If light cannot
escape a black hole. Yeah, that's true. Nothing can escape

(38:47):
the event horizon, nothing from inside can ever come out.
But that doesn't mean that black holes don't radiate. But
there's a difference in how you describe the radiation from
black holes. In classical gr like Einstein's pure singularity black
holes and more moderate attempts like Hawking's view of what
a black hole is do they give us very different
senses for what might be going on inside the black hole. Interesting?

(39:09):
All right, Well, let's get into whether Einstein and Stephen
Hawking are also hot or not and whether or not
they're right about the temperature of black holes. Will dive
into that and what it all means, but first let's
take another quick break. All right, we're talking about whether

(39:36):
black holes are hot or not. And it's kind of
weird to think of black holes as hot because they
are hols, Like, how can a hole be hot? And
I was saying earlier, Daniel, how it's weird to think
of the heat radiated like the light hit radiated by
a hot black hole because black holes are are black,
they're by definition things that don't radiate light or trap light.

(39:57):
How can something be both radiating heat and also trapping
heat forever? Well, you know, Einstein would agree with you.
Einstein's general relativity says that black holes are perfect absorbers.
They do not radiate any light. Anything that hits them
falls in, and they radiate nothing. And so from the
general relativistic point of view, black holes are at absolute zero.

(40:20):
They have zero temperature. What is the expected radiation from
an object that absolute zero? It's nothing. And so if
black holes radiate nothing, then therefore they must have zero temperature.
That's the g R version. The classical view of a
black hole. All you can know is it's mass, its charge,
and it's spin, not its temperature, because that would be

(40:41):
like information about what's going on inside the black hole. So,
according to Einstein, black holes are cold. They're like infinitely
cold or perfectly cold. That's right. According to Einstein, you
can shoot a laser beam at a black hole and
never heat it up. It will just keep eating that
laser but never get hotter. All right, well then, but
what does Stephen Hawkinson? So Stephen Hawking says, actually, if

(41:04):
we live in a quantum mechanical universe, then that cannot
be true. So Hawking did his famous calculation where he thought,
let's think about a black hole, and now, instead of
thinking about what's going on inside the black hole, because
we can't and we have no idea how quantum gravity works,
let's just try to put the black hole in a
quantum universe and say, how do you do quantum mechanics

(41:24):
when you have an event horizon if there's something there
which eats all the information. And so he did a
bunch of fancy calculations and quantum field theory and he
discovered that the only way this works, the only way
you could have like a boundary condition like a black hole,
is if you have radiation coming out of the black hole.
So he found like solutions to the quantum field theory
that require outgoing waves from the black hole. And that's

(41:47):
really interesting. That says that, like, if you have black
holes and the universe is quantum mechanical, then they must
be emitting something. And he went a step further, he said, well,
you know, how does this radiation work, what does it
look like? And what he discovered, and this is sort
of the fascinating moment, is that the radiation spectrum that
you expect from the vicinity of a black hole follows
exactly the spectrum you get from black bodies. Black bodies,

(42:09):
like we talked about earlier, don't just emit at one number, right,
It's not like the sun emits at only one frequency.
There's this shape to their spectrum. What he discovered is
that this radiation that he was predicting black holes emit
also has a shape, and that shape matches exactly the
shape you expect from black bodies at a certain temperature,
at a non zero temperature. So he concluded there must

(42:31):
be some radiation from black holes, and black holes must
have a non zero temperature. Well, I feel like maybe
that's not quite what he said, right, Like he's I
think what you're saying that he's saying is that, um,
black holes kind of have to leak stuff out, like
they can't trap stuff in. Thereforever stuff leaks out. And
the stuff that the energy that leaks out sort of

(42:52):
looks like something that would have a temperature, But that
doesn't necessarily mean it's the temperature of the black hole. Right,
You're exactly right. And in the paper he even said,
like be careful about interpreting this literally as a temperature.
It's more like an effective temperature. It's an attempt to
describe what might be going on inside the black hole.
But we don't know microscopically, we have no idea, you know,

(43:13):
is everything inside the black hole totally frozen? Is this
like quantum space wiggling? We don't understand what's generating this radiation.
But it's a way to describe the black hole sort
of thermodynamically and statistical point of view, right, And so
he says, you're right, it seems to like it should
generate a spectrum as if it had a temperature. What
that temperature means in terms of like the microscopic wiggles inside.

(43:35):
Hawking didn't say, couldn't say, and we still don't know,
but I guess can you step us through a little
bit of how he did it or how he reached
this conclusion, Like what does it mean that you can't
have a perfect hole in the universe? Like why does
it have to leak because of quantum mechanics or what? Yeah,
because of quantum mechanics. And so you take a black
hole and you put it in the universe. The universe
also has fields, and you know, fields for fermions fields

(43:57):
for bosons, you know, the electromagnetic field, the tron field,
and now you want to quantize those fields. Is what
we do in quantum mechanics to get to quantum field theory.
We say the universe is filled with space, and the
space has fields in it, and those fields are quantized.
They can only have certain solutions, right, not any arbitrary
continuous set of solutions, but only specific solutions like a ladder,
just like electrons whizzing around an atom have a ladder

(44:20):
of solutions. And so the thing that he ran up
against is that you add a black hole to that.
That changes how you can quantize these fields or for
that quantization to work. The various consistency conditions on this
quantization you need, like unitarity, so you're not predicting things
would have probability more than one and things that not
contradict themselves. Various internal consistency conditions on the quantization of

(44:42):
the fields in the presence of a black hole requires
this radiation to exist, right. I wonder if what it
means is that you know, he's saying that like you
can have a perfect event horizon, like you can't have
like this perfect perfectly smooth boundary where if you cross one,
you know, tiny little bit, you're in the black hole,

(45:04):
and where you step out a little bit, you're out
of the black hole. Maybe what he's saying is that,
you know, we live in a quantum universe. You can
have that kind of like stark boundary or certainty, and
so as a result, you can have a fuzzy event
horizon and which will tend to leak. I think that's
one way to interpret it, but I think it's dangerous
to try to come up with a microscopic interpretation. I mean,

(45:25):
I love that your brain immediately goes to, like, what's
happening right there at the edge, What does that really
mean for an individual particle? The truth is to understand
that we need a theory of quantum gravity. We have
to understand how gravity affects quantum particles, and we just
don't you know, quantum particles can do these weird things
like have a probability be in two different places at
the same time. How does gravity affect that does a

(45:45):
tug halfway on both of the places where the particle
is Like, we just don't understand the microscopic picture of
quantum mechanics and gravity at all, So we can't really
tell a story about what happens right at the edge
of the event horizon for quanty of objects, but we
can tell a story about the temperature of these black
holes sort of in this indirect way. Well, I guess

(46:06):
I feel like you're saying there are two answers. One
is any saying from afar a black hole will seem
like it has zero temperature, and from a far Stephen
Hawkins says, the black hole should have a little bit
of a temperature and almost like it was kind of hot.
But neither of those really tell you what the real
temperature is inside of the black hole. Right, Like, I'm
thinking of the black hole as this like perfect cooler

(46:28):
that doesn't let any heat out, And say, if you
stick of, you know something hot or something cold inside
of the cooler, Like it has a temperature inside of there,
but from the outside it's gonna look totally different. Right
from the outside of perfect cooler will seems super cold,
or if you're Stephen Hawkins, the cooler will leak out
or look a little bit warm, But that doesn't tell
you like how hot or cold it is on the

(46:48):
inside of the cooler. No you're totally right. Neither of
these tell us what's going on inside and the program.
The way to make progress is to try to build
up from the ground up a theory of quantum gravity
that would describe the kind of radiation we would see
from afar, and so people are working on that. They're
trying to do that, you know, the loop quantum gravity
folks are trying to do that. They're trying to describe

(47:10):
this radiation is like the shaking of the vibrations of
the quanta of space, you know, this film. In the
other direction, people in string theory are trying to describe
what's going on at the heart of the black hole
in terms of like vibrating strings, and they've actually had
a lot of success. These supersymmetric black holes have been
able to predict very well this expected distribution of radiation

(47:32):
from a black hole. It's like one of the biggest
successes of string theory as a theory for quantum gravity. So,
you know, if we can make careful measurements of the
black hole from the outside and then come from the
other direction and try to predict those measurements, we might
be able to get some inside as to what's going
on inside, right, because it is that we've talked about before.
It is possible to go into a black hole, right, Like,

(47:52):
for certain black holes, you can go past the event
horizon without getting shredded to bits, and so it is
sort of possible to go into a black hole and
measure the temperature inside of a black hole. I mean,
you wouldn't be able to tell anybody on the outside,
but it is technically to like no possible to know
that the temperature of a black hole inside of one. Right, Yeah,
that's true. You can fall inside a black hole, and

(48:14):
for large enough black holes you can survive the title
forces just past the event horizon. Eventually you will get shredded,
but yeah, you can fall past the event horizon and
do experiments. You know, if it's a classical black hole
and you're not really going to learn anything until you
get to the singularity. Anyway, if it's a quantum black hole,
then they might still be in the extended region, and
so you could do some experiments there, you like, not

(48:35):
quite at the core of the black hole, but like
in the rind of the black hole, the crust, the
cross there you go the crust. But I think the
calculations are also super fascinating because the numbers themselves are
really weird. If you ask, all, right, what is the
temperature of a black hole that has the mass of
the sun, for example, you know, Einstein says, oh, it's zero,

(48:57):
Hawking says, no, it's not zero, but it's point zero
zero zero zero zero zero zero six kelvin. So like,
black holes are not zero, but they're definitely not hot.
It's still a pretty cold object, you mean, from afar,
like outside of the cooler. For outside of the cooler,
the coolers are pretty cool. Yeah, we're always talking about
from afar because we don't know what's going on inside this.

(49:18):
So these black hole temperature is always what you measure
from the radiation that might be coming off the black holes,
the leaking, you mean, the leaking. The thing that's really
weird is that this temperature is inversely proportional to mass.
We've talked about this before that larger black holes radiate
less and smaller black holes radiate more, which means smaller
black holes are warmer. So as black holes get larger,

(49:40):
they get colder. Einstein says, you shoot a laser beam
at a black hole, it just stays at zero. Kelvin
Hawking says, you shoot a laser beam at a black hole,
it gets colder, whereas if I shoot a laser of you,
you would get hotter and and more annoyed kind of, yeah,
I would get louder. Yeah, Oh that's interesting. So it's
almost like you're putting more energy into the black hole,

(50:03):
but from the outside it's looked looking like it's losing energy. Yeah,
it looks like it's getting colder and colder, which is
hard to understand, but think about like what might be
going on inside the black hole. It's gravity is getting
stronger and stronger, and so time is slowing down inside
the black hole, and so things are moving more slowly,
which you can kind of understand is maybe having lower temperature. Right, So,

(50:24):
again we don't know anything about what's going on inside
a black hole, but trying to put together like a
rough intuition for how black holes could be getting colder
as they get larger. Time dilation kind of helps that picture.
Oh I see, or could it be that you know,
as you add more mass or energy to the black hole,
like that gets stronger, so it pull stuff in more
and so there's less leakage and so it looks colder,

(50:47):
you know what I mean, Like, the more mass the
black hole has, the better the coda that is keeping
it hidden from us, And so maybe it just looks
colder to us. Yeah, in which case, maybe black holes
don't follow these laws of black body radiation that every
anything else in the universe does, right, because they have
this event horizon. Black body radiation can describe everything in
the universe that has charged particles. So, for example, dark

(51:09):
matter also doesn't follow that rule because dark matter doesn't
have electric charge and so it doesn't radiate photons based
on its temperature. Maybe the event horizon of a black
hole also prevents this kind of radiation, and so maybe
there's a mismatch them between the temperature we observe from
AFAR and the actual temperature inside a black hole. In
a more intuitive sense, I called the cooler theory. It's

(51:29):
definitely cooler than any theory I've heard before. Yeah, should
we come up with a great acronym for it, better
than the j CT or haze cooler theory? There you go. Well,
I feel like what you're saying is that to answer
the question whether a black hole is hot or not,
the the answer is, um, we don't know, you know,
it's sort of like asking is that dress blue with

(51:50):
black stripes or what with gold stars? It's like it
depends on who you ask. Yeah, because we don't know
what's going on inside a black hole microscopically, We don't
know what it's true temperature. We have this weird clue
that black holes do radiat a little bit, and that
tells us something about what might be happening inside of them,
but we don't know if it really is a clue
about the microscopic nature of the black hole. Were just

(52:12):
the black holes leak in a weird different way than
anything else, right, Because even this leakage theory, this hawkings
radiation is still theoretical, Like, we haven't seen that hawking
radiation in actual life. That's right, We have never ever
seen any hawking radiation from anything. We've looked for black
holes evaporating rapidly and giving bright flashes at the end
of their life, but we've never seen If we could

(52:33):
create black holes of a large hadron collider and then
we could see them evaporate on short time skills, that
would be exciting, but nobody has ever seen hawking radiation,
So it's a black hole hot or not. I feel
like you can't really swipe right or left here, you
have to just close the app. I don't know, what
do you? What do you do? I think black holes
are either zero or very very very very very cold.
There's no chance they're actually hot. I see you're saying,

(52:56):
doesn't matter they're hot or not. They're they're pretty cool. Yeah, exactly,
they're still pretty cute. You would still date them, is
I think what's is what you're saying. I definitely want
to hang out with black holes, yes, forever. Alright, Well,
it's an interesting question to tackle, and I think it
tells you a lot about what we still don't know

(53:16):
about the universe. You know, there are still places that
we know exists and our out when we have pictures
of but where our theories about the universe breakdown and
we need to come up with new theories that maybe
people out there are working on or that will work on.
In our efforts to understand the universe are usually through
the lens of concepts that we understand motion and temperature,
and sometimes those very concepts breakdown in the weird contexts

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that are our universe. Alright, Well, we hope you enjoyed that.
Thanks for joining us, see you next time. Thanks for listening,
and remember that Daniel and Jorge explained. The Universe is
a production of I heart Radio from More podcast from

(54:01):
my heart Radio, visit the i heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. H