The several styles of singularities

Daniel and Kelly take a tour through the various singularities that are predicted by general relativity.

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Episode Transcript

Speaker 1 (00:07):
Few words are as heavily loaded as the word singularity.
To AI futurists, it means the moment that ais can
teach themselves and might accelerate beyond human control. To physicists,
it means a divergence, a divide by zero, a point
of nonsense, a failure, a breakdown. To the broader public,
it represents a mystery, an unseen, unknown point at the

(00:29):
heart of black holes, something fascinating and mind bending, something
we are a little bit afraid of, but also entrance
by a bizarre extremum that might reveal something deep about
the nature of the universe. To you, our listeners, it
means a whole episode dedicated to demystifying, detoxifying, deconstructing the

(00:49):
several subclasses of singularities. So welcome to Daniel and Kelly's
Extraordinary Singular Universe.

Speaker 2 (01:10):
Hello. I'm Kelly Wienersmith. I studied parasites and space, and
I'm excited to talk about singularities today.

Speaker 1 (01:15):
Hi, I'm Daniel Whitson. I'm a particle physicist, but I'm
actually not a singularity. There are other Daniel Whitson's. How many,
I'm not sure I've only ever found one he's an
artist in the UK and he's quite good, if I
have to say so myself.

Speaker 2 (01:30):
Oh fantastic. It's something in the name. Maybe. I don't
think there's any other Kelly Wienersmith, which is why we
picked this name. Have I told you the story about
how I ended up with this name?

Speaker 1 (01:41):
No, tell us the story, all right.

Speaker 2 (01:43):
So I was working on my master's degree and I
published my first paper on smallmouth bass, and there's a
lot of papers on smallmouth bass, and so I looked
to see how hard was it going to be for
people to find my first paper by looking up papers
by Kay Smith, and there were literally like over one
hundred thousand, and if you did k L Smith, there

(02:04):
was like, you know, eighty thousand or there were still
so many that it was going to be really hard
to find me, Okay. And then I fell in love
with Zach, and I was like, all right, I don't
really want the last name Wiener. But with the order.

Speaker 1 (02:19):
You're telling this story makes it sound like you picked
Zach because he had an unusual last name. No.

Speaker 2 (02:24):
No, So you know, I dated some guys with some
pretty great last names.

Speaker 1 (02:28):
Wow.

Speaker 2 (02:29):
And then there was Zach, who I fell in love with.

Speaker 1 (02:34):
And despite the last name I see.

Speaker 2 (02:38):
So I looked up how many papers there were by K.
Wiener and K. L Weener and they were literally still thousands.

Speaker 1 (02:46):
Wow.

Speaker 2 (02:46):
And I was like, I don't really know if I
want to take this name if it only helps me
by like one order of magnitude. And so we smoothed
our names together and we our little family are the
only Wiener Smith's I found. There's a Wiener hyphen Smith
who's an obstetrician, which is amazing because she's helping people
Smith Wieners, which I love and.

Speaker 1 (03:08):
Uh, professionally accurate last names. I love it.

Speaker 2 (03:15):
That's right. But now Google scholar Kelly Wiener Smith, you
only get me. All right, we've gotten off track. I
am a singularity.

Speaker 1 (03:23):
You are the one and only. Congratulations.

Speaker 2 (03:26):
Oh thanks. I don't think there's a lot of vying
for this position.

Speaker 1 (03:31):
And Kelly, if you were a point particle, then there
would be an infinite density if Kelly Wiener Smith's at you.

Speaker 2 (03:37):
There are days where it feels like that's happening as
we cruise onto the holiday season, and that is.

Speaker 1 (03:46):
Exactly what we're talking about here today. You're listening to
this episode in January, after surviving the holiday season, potentially
gaining mass, potentially losing mass, however you do it. But
today we're talking about that very massive question of gravitational singularity.

Speaker 2 (04:00):
Amazing, and whenever we have a massive topic to handle,
we always go to the extraordinaries to see what they
think we do.

Speaker 1 (04:07):
This question actually was inspired by an extraordinary he wrote
to me. Julie Bud wants to learn more about singularities.
Here's her question.

Speaker 3 (04:16):
Hi, Daniel and Kelly, I'd love it if you could
do an episode about all things singularities. I have this misconception,
which I think maybe a common one, of a singularity
just being a tiny point in space where density becomes infinite.
But I recently learned it's actually kind of just a
term for where our understanding of physics breaks down and

(04:37):
we don't really know what happens. I also learned that
there are different types of singularities inside black holes, like
infalling and outflying singularities, and a BKL singularity, which I
can almost wrap my head around but also not really.
So I'd love if you could do a deep dive
into what singularities are and the different types of them.

Speaker 2 (04:56):
Thanks, fantastic question from Julie. Frankly, I don't know the
answer here, so I think I'm going to learn a
lot today.

Speaker 1 (05:01):
And I love this kind of question because one of
the things I love doing on the podcast is going
deeper than your typical pop side treatment, breaking through to
the next layer of knowledge, and really sharing with people
a lot of the subtlety and the nuance and how
people on the forefront of knowledge are actually thinking about
this stuff.

Speaker 2 (05:19):
Yeah, and before we get to how the scientists at
the forefront of knowledge you're thinking about this stuff, let's
see how the people on the street are thinking about
this kind of stuff.

Speaker 1 (05:28):
So I went out there and asked folks, what is
the best kind of gravitational singularity. Here's what people had
to say.

Speaker 4 (05:36):
Gravitational singularities weren't real, so maybe the best one is
one you can study in your particle collider kind it
would make me lose ten pounds. I like classical gravitational
singularity is the best because I don't like the idea
of space being broken up into pieces.

Speaker 1 (05:51):
As far as I can tell, it's being a grandparent,
the kind that's close enough to provide insight into the
universe and far enough away that I don't have to
worry wry about her breaking my stuff.

Speaker 4 (06:01):
I would say a black hole a naked singularity, because
you could observe it and learn from it.

Speaker 1 (06:08):
I have always had a fond spot for neutron stars.
Black holes are the best kind of gravitational singularity, the
singularity where it all started with the Big Bang, The
one that lets you view itself from afar, doesn't spaghetify
you and tells you all it's secrets. The one that
doesn't have any side effects.

Speaker 4 (06:28):
All the singularities, all the singularities, all the singularities. Some
cosmic grads up being the telescope. It looks like there's
a ring on it. That's the accretion disking. Gravity is
lensing it. We'd like to study quantum gravity inside of it,
but if we visited, we just s begetify in it
will uh oh.

Speaker 1 (06:45):
The kind that stays the heck away from me.

Speaker 3 (06:47):
Is a black hole or neutron star.

Speaker 2 (06:50):
Well, I think from now on the rule is that
everybody has to sing their answer to the tune of
a Beyonce song.

Speaker 1 (06:57):
Wow, dude, really raise the bar for everybody, didn't he?

Speaker 2 (07:01):
Thank you? Zach?

Speaker 1 (07:04):
All right, So let's be singular in our focus and
talk about what singularities are. And I want to start
by talking about singularities sort of mathematically and theoretically, because
like many things in science, this word is overused.

Speaker 2 (07:19):
I think these days I hear about it most and
as it relates to tech. But we are not at
all talking about tech singularities today.

Speaker 1 (07:26):
We are not talking about that moment when ais can
teach themselves and accelerate beyond human control, and whether that's
already happened or not. We're talking about singularities from a
mathematical and physical point of view, and unfortunately, there's not
a whole lot of agreement on what singularities mean. There's
sort of two different kinds of singularities, though there are

(07:46):
ways to unify them in your mind. The first one
is essentially what Julie mentioned, which is where there's a
breakdown in your theory because you predict something unphysical and infinite.
Like if I flip a coin and I want to
calculate the probability for the coin to come up heads,
and my answer gives me infinity, I'm like, well, that
can't be right. The probability has to be less than

(08:07):
one so infinity is wrong. Something went wrong in my
theory I divided by zero, or I just made a
dumb error, or the theory is broken. Right, that's an
unphysical prediction, something which can't happen. It's outside the bounds.
And that happens a lot when math confronts physics. Right, mathematics,
let's you do lots of things which the physical world
doesn't let you do. A simple example of that is

(08:28):
like when you throw a ball, it follows a parabola.
That parabla moves forward, but also it could move backwards
in time, and the same solution allows the ball to
like go deep into the earth behind you or whatever. Obviously,
that's not going to happen. There's a boundary there that
you haven't included in your calculations. But there's lots of
times when you're solving problems in math and there's an
unphysical solution that you don't consider. And so one idea

(08:52):
of a singularity is when your math predicts infinity but
physics rejects that.

Speaker 2 (08:57):
I guess I hadn't thought of singularity as another word
for were we're probably wrong about this, but is that
the right way to think about it?

Speaker 1 (09:04):
It's an indication that something is probably wrong with your theory.
Not all the time. It could be that the infinities
are real. We don't really know. But if your physics
is telling you that the infinities are impossible and the
mathematics is predicting infinity, then there is something wrong with
your theory. That's the idea, okay, and it doesn't have
to be gravitational. You can take, for example, an electron
and think about the electric field of the electron. How

(09:27):
does that electric field vary with distance from the electron? Well,
it goes like one over distance square, like most things
in physics. So you twice as far away, the electric
field drops by a factor of four. Well, what happens
if you get twice as close. The electric field goes
up by a factor of four. What if you get
twice as close again, up by another factor of four.
If the electron is a point particle, you can get

(09:48):
infinitely close to it. What is the electric field at
the electron itself? Well, you're dividing by zero or it's
approaching infinity, And so the electric field in classical electrodynamics
is predicted to go to infinity at the electron itself.
There's a divergence there, a singularity and infinity. We don't
believe the electric field really is infinity there. This is

(10:09):
a breakdown of the theory because the electron is not
really a point particle. There's a limit to how close
you can get to it. So that's an example of like,
you didn't build all the physics into your theory, and
then you took the theory who literally and extrapolated it
beyond where you thought it was relevant, and so you
got a singularity. It's a warning sign from the mathematics
to say, oh there, buddy, back up and think about
what you're doing.

Speaker 2 (10:30):
Interesting, So when you get a singularity like that, how
much of the theory do you throw away or do
you just say this theory is good up until this
point where we don't understand something.

Speaker 1 (10:41):
Yeah, exactly, you can just put a boundary and say, well,
this theory works up to here, beyond that it's not applicable.
And that's the case where basically every theory of physics
we have right, every theory has boundaries because they start
from some assumptions, and those assumptions are only true in
limited cases. Even like the standard model of part of
physics are best. Quantum theory that predicts things to ten

(11:02):
decimal places. We know it's not valid everywhere because in
order to do any calculations we have to ignore gravity.
And a lot of times you can ignore gravity. When
two particles interact to the Large Hadron Collider, Gravity's are relevant,
but it's there, and if those particles get really really massive,
it's no longer irrelevant. So when things get really really massive,
we're really really dense, then you can no longer ignore gravity,

(11:24):
and you can't just calculate blindly assuming gravity doesn't exist.
So they're always boundaries to your theory. And when principle,
you'd love to find a new theory that doesn't have
those boundaries, or works beyond the boundary or something. But
there's lots of different possible approaches.

Speaker 2 (11:39):
And I seem to remember infinities coming up in our
understanding of gravity when we were talking with Jonathan Oppenheim
and Thomas van reet Am. I remembering that correctly.

Speaker 1 (11:48):
Yeah, exactly. String theory is an extension of these theories
that avoids some of the infinities that appear in the
standard model and in gravity. So it's an example of
a quantum gravity theory that avoids some of those infinities
by replacing points with lines. Essentially, instead of having dots,
you have strings. And of course it's still very controversial,

(12:09):
but that's an example of how you might extend a
theory to be more broadly applicable. Or you could just say, look,
this theory works here, and we have a different theory
over there. That's what we do. For like water, we
have a theory for fluids, and we have a different
theory for crystals and different theory for vapor And we
don't say we want a general theory of water, you know,
we're like, look, Navria Stokes works here, and crystal theory

(12:30):
works there, and we're fine. And so there's lots of
different approaches, and so a singularity more generally is where
things go to infinity. And in the case of gravity,
for example, you have gravitational curvature in the heart of
the black hole. We'll talk about that more in a minute.
That goes to infinity. And that's a question is does
that really happen or is it a sign that the
theory is broken somehow? Or in the early universe, the

(12:53):
Big Bang was a singularity, and in the future if
there's a big crunch that could be a.

Speaker 2 (12:57):
Singularity, don't I don't want there to be a big crunch, Daniel, what.

Speaker 1 (13:02):
You want doesn't really matter, unfortunately, Kelly.

Speaker 2 (13:04):
Yeah, that's a little harsh. But also because we're extrapolating
an infinity, I'm going to decide that physicists are probably
just wrong about that.

Speaker 1 (13:13):
Okay, great, I hope that works for you. That's one
concept of a singularity, where things get infinite as sign
that probably your theory is breaking down. There are other
concepts of a singularity. One that I think is really
fascinating are paths that are not extendable. So imagine you
are moving through space. You can go here, you can
go there. There's always a place you can go right,

(13:35):
you can walk around, you can go downstairs, you can
go upstairs, you can take off from the Earth, you
can go to Mars, whatever. There's always somewhere to go.
So your path is infinitely extendable. But what if there
was a place where a path couldn't continue, where like
it just ran into a dead end, like a boundary
or an endpoint. A great example of this is the
heart of a black hole. Inside a black hole, space

(13:58):
is curved and there's only one direction of space. It
just goes towards the center. And so your path will
continue towards the heart of the black hole, and that's it.
It just ends there. You'll be there forever at the
heart of the black hole. So that path is not extendable.
And so that's another way to think about a singularity.
It doesn't have to be in conflict, but it starts

(14:18):
from a different definition and so sort of leads you
in a different direction conceptionally.

Speaker 2 (14:22):
So like, how generalizable is that use of the word singularity?
Like I get lost when I'm driving all the time.
So when I go down a wrong road and I
hit a dead end, can I turn around to my
passengers and be like singularity or is that? Is that
the incorrect usage of this word.

Speaker 1 (14:39):
Yeah, you can't blame math for the reason you get lost, Kelly. Sorry, Unfortunately,
you know math is not that useful. It needs to
really be a non extendable path, you know, partner space
where you just cannot continue.

Speaker 2 (14:55):
All right, So you hinted that we're going to get
into black holes again, which I think is maybe one
of the top topics that the extraordinaries ask us about.
Nobody ever gets tired of hearing about black holes. So
tell me about a black hole singularity. No, wait, first,
remind me what a black hole is.

Speaker 1 (15:11):
Yeah, So to understand what a black hole singularity is,
we have to remind ourselves how black holes work. And
remember that black holes have very powerful gravity, so powerful
that not even light can escape. They have an event
horizon beyond which nothing can return. But you need to
unshackle yourself from you intuitive sense of like Newtonian gravity,

(15:31):
where gravity is pulling on stuff, where black holes are
like huge vacuum cleaners sucking on things because force. It's
not the right way to think about gravity, because gravity
is not a force and also doesn't work for black holes.
Like black holes, we know can bend the path of
light and can trap light, but light has no mass.
And then Newtonian gravity the gravitational forces between two objects
with mass, and so photons shouldn't feel black holes in

(15:55):
Newtonian gravity, but they do in the universe, so we
know Newtonian gravity is wrong. Actually had a list are
writing last week to correct me and say, actually, photons
do have mass. We know that because they feel gravity
because they get sucked into black holes. And it was like,
interesting argument. Let's unwind this to see where you went wrong.

Speaker 2 (16:12):
Anyway, And I remember that email. They titled it a
massive correction. Yeah, that was great.

Speaker 1 (16:20):
They definitely had a lot of gravitas in their email.
I welcome it. Please if you think I made a mistake,
right to me, I'm pretty sure I'm right on this
one that photons do not have mass. And the right
way to think about black holes is not in terms
of a force, but in terms of the curvature of space. Right, So,
mass bends space, which means it changes the relationships between

(16:40):
objects their distances. This kind of gravitational curvature is very
tricky to think about. I think about it in terms
of relative distances between pairs of objects. And if you
like throw a bunch of ping pong balls into space,
and then you measure all the distances between those ping
pong balls, and you're like, this one is for me
to from that one, this one is six meters from

(17:02):
that one. Then you have all these like pair wise distances.
You have all your ping pong balls, and you know
the distances between all of them. Right now, the way
to think about curvature is to think about could you
actually like put meter sticks between all of those ping
pong balls and have it all line up. If you can,
then space is flat. You've measured those spaces and you

(17:22):
have those meter sticks, and everything sort of cooks together.
But if you can't, if you can't do that with
like straight meter sticks, then it's because the relationships between
the points have become distorted. Because space is curved. You
can't see the curvature of space directly, but you can
measure it by like putting stuff in space and measuring
the distances between them and see, like do they click together?

(17:43):
Can I describe that without gravitational curvature?

Speaker 2 (17:46):
Okay, to be honest, I always have trouble with this
idea of space being curved. But would it be similar
to say, like, all right, so an object isn't going
into a black hole because a black hole is pulling
it in. It's going into a black because space is
curved in such a way that it's directing it into
the black hole. And that that's a little bit different.

Speaker 1 (18:06):
Yes, So now we're talking about the consequences of the curvature.
So if you have the idea of a curvature in
your head, now what happens to stuff?

Speaker 3 (18:12):
Now?

Speaker 1 (18:12):
The space is curved. Is that the concept of like
moving in a straight line is different. Your intuition about
a straight line assumes flat space. You're like, I'm going
to go from here to there. I'm just going to
go from here to there straight line, a shine of flashlight.
It's going to go from here to there in a
straight line. But what is a straight line in curved
space is following the curvature of space itself. And so

(18:33):
if you just have an object in free fall, it's
moving under the influence only of the curvature of space.
It's going to follow the curvature of space. And so
if space is flat, it's just going to drift the
way you would normally into it. But if it isn't,
then it's going to bend. It's going to follow that
invisible curvature of space. And so the Earth goes around
the Sun because it's following that curvature, and things fall

(18:53):
into the black hole. Yes, Kelly, because they're following that
curvature of space.

Speaker 2 (18:58):
And it does gravity make that curvature or these two
different things.

Speaker 1 (19:02):
So gravity sort of is that curvature. Right. Gravity is
not a force, it's just the effect of that curvature.
We can't see the curvature, but we see the curvature's
effect on stuff, and we call that gravity. And we
know it's not a force because like when you jump
off a building, you are feeling no force, no acceleration.
You're moving towards the center of the Earth. We say,
under the influence of gravity. But if you like held

(19:23):
up a scale underneath you, you would feel no acceleration. There's
no force happening to you there. It's not like a
chemical rocket. If you fire a chemical rocket, then you
definitely feel that acceleration. Right, If you like hit the
brakes in your car, that's acceleration you feel. Those Those
are real forces. Gravity is not a real force because
it doesn't create those feelings of acceleration. You're just naturally

(19:44):
following the curvature of space, which means something amazing. If
you're falling into a black hole, you don't feel any gravity,
but you're dying, especially if you let Kelly do the driving.

Speaker 2 (19:58):
Oh all right, all right, you know what, I need
some time to recover from that insult. So let's take
a break and when we get back we'll talk more
about this concept of black hole singularities. All right, I've

(20:28):
recovered from the wicked burn Daniel gave me regarding my
driving skills.

Speaker 1 (20:32):
You're the one who said you're driv into singularities.

Speaker 2 (20:35):
I said, well, mate, what happened to?

Speaker 3 (20:37):
Yeah?

Speaker 2 (20:37):
Maybe I did? All right, you got me? All right,
tell me about black hole singularities. Let's stop talking about
my driving record.

Speaker 1 (20:42):
All right. So we have this concept of spatial curvature
and it explains the effect we call gravity. And so
what happens near massive objects is things move towards those
massive objects because that's where the curvature of space is.
And a black hole is an example of that curvature,
growing so powerful that life becomes trapped inside that curvature
because the direction of space is just one d inside

(21:05):
that black hole. Beyond the event horizon, space is curved
so much it only points towards the core, and at
the core, according to the calculations of general relativity, that
curvature becomes infinite. Right, So the curvature of space itself
becomes infinite. So like, what does that mean? Well, if
you have two points in the opposite sides of a
black hole, what is the distance between them? If it's

(21:25):
going to pass through the singularity. That distance is infinite.

Speaker 2 (21:28):
So is this a case where the equations are breaking
down or is this like actually what we expect will happen.

Speaker 1 (21:35):
Yeah, that's a great question. My suspicion is that the
equations are breaking down because what we've done here is
extrapolate general relativity beyond the point of its relevance. We're
doing a calculation in general relativity and we're ignoring quantum mechanics.
And most of the time you can do that. When
you're calculating how two black holes are going to fall
into each other, or how Jupiter moves around the Sun,

(21:56):
you can ignore quantum mechanics. It doesn't really matter the
same way you can or gravity when you're doing quantum
mechanical calculations on individual particles. But when you're talking about
super tiny, dense objects like the heart of a black hole,
quantum mechanics matters. Again. Quantum mechanics says that that prediction
is wrong. It can't happen. You can't have that much
mass isolated with that much energy in that small space.

(22:17):
It violates the Heisenberg and certainty principle. It definitely would
not happen. So what you've done here is take in
general relativity, ignore quantum mechanics, but done it in a
region where quant mechanics can't be ignored. So it's in
that sense kind of a nonsense prediction. It's also sort
of a useful diagnostic to say, like, well, is general
relativity valid here? I think this is the universe telling

(22:37):
us no. Quantum mechanics says it can't happen. General relativity
says it does happen. We don't have a great theory
of quantum gravity to make these calculations, and so it's
sort of a question mark.

Speaker 2 (22:47):
Is this level of nuance conveyed in like pop culture
treatments of black holes?

Speaker 1 (22:53):
Or is it?

Speaker 2 (22:54):
How is this usually conveyed in pop science?

Speaker 1 (22:57):
Most of the popular science treatments just say that there's
a single already at the hearts of the black holes,
but scientists don't understand it. I think more often we
should underline that this is a prediction of a classical theory,
and we already know that classical theory is limited and
that we shouldn't trust its predictions. So doesn't mean you
shouldn't make those predictions and use them again, like as
a diagnostic. But I think some of that nuance is missing, Yeah.

Speaker 2 (23:19):
Okay, and what do experiments tell us about whether or
not this is where things are breaking down or if
we're really describing the universe.

Speaker 1 (23:26):
Experiments have been really influential in our understanding of black holes. Originally,
this whole idea, people thought, well, it's ridiculous, there's no
way the universe is going to let a black hole form,
not to mention, is there a singularity? And it's hard,
but do black holes exist at all? For a long time,
they were just like a curiosity of the math, and
people are like, well, this is definitely a sign that
something is wrong and the universe would never let this happen.

(23:47):
But then we found things that look a lot like
black holes that are very very dense and very very
massive and have incredibly powerful gravity on the things near them.
We see them in the hearts of galaxies, We see
them where stars have colapsed. Do we know that these
are actually black holes that have an event horizon and
potentially singularity within them?

Speaker 4 (24:06):
Not.

Speaker 1 (24:06):
Technically, we have only indirect evidence that these things are
black holes, in that there's nothing else in our physics
that is so massive and so dense and so compact
all at the same time. They can't be neutron stars,
they can't be white dwarfs, they can't be anything else
in our physics. So the only thing left in the
box is a black hole. But you know, there are
other ideas out there of things that they could be.

(24:28):
String theory predicts things like fuzzballs. There are other quantum
versions of these predictions. So we have only indirect evidence
that black holes actually exist, even though they're treated as
like definitely existing and having been observed in the sort
of broad popular literature.

Speaker 2 (24:42):
Yeah, and I really enjoyed our recent conversation about Bechdel
stars and black holes are confusing, but we're we're learning
more all the time.

Speaker 1 (24:49):
Yeah, and that's not a criticism of astronomers, like it's
amazing what they've found. And you know, seeing black holes
is hard. And even the event horizon telescope, which just
image the accretion disk around the black holes, tells us
a lot, but isn't direct observation of the event horizon itself. Right.

Speaker 2 (25:04):
Okay, so we've talked about black hole singularities, right, are
there other kinds of gravitational singularities to talk about?

Speaker 1 (25:12):
Well, all the singularities we're going to talk about which
makes me think of Beyonce. The singularity are at the
hearts of various kinds of black holes, okay, and so
less catalog those the sort of vanilla singularity, and the
classic one is at the heart of a black hole
that's very simple. It's spherical, it has a lot of mass,
it has no spin or no charge. Remember that the

(25:35):
kinds of things a black hole can have, according to
general relativity, are just three. There's only three things you
can know about a black hole. It's mass, it's spin,
and it's charge. You can't know anything about how those
things are arranged within the event horizon, and you can't
measure any other physical properties. So those three things completely
determine the black hole, again according to classical general relativity.

(25:58):
So if you have the simplest kind, which use a
short style black hole, just a really massive stuff with
no spin and no electric charge, then you get an
event horizon that's spherically symmetric, and at its heart is
that point that's singularity, the location of infinite density or
infinite curvature, and that's the sort of the nilla classic singularity.
I think most people think about when they think about

(26:19):
black holes and singularities.

Speaker 2 (26:21):
Okay, sorry, I'm still absorbing. Okay, So these are all
examples of stuff at the middle of a black hole
and just differences in the way that we think about them.

Speaker 1 (26:29):
Yeah, exactly. And these characteristics of the black hole determine
what's happening and ats the core. So if you just
think about it, a lot of stuff, a star collapsing
into a very dense object, that's a short style black hole.
And that was the first solution to Einstein's equations ever
found to describe a black hole. No spin, no charge,
just mass.

Speaker 3 (26:47):
All right.

Speaker 2 (26:47):
I think it would be pretty awesome to have a
type of black hole named after yourself, but that's probably
not going to happen for me. But all right, so
we've got swar style, if I could say it, we've
got the first kind of black hole we talked about
what what? Okay, so we're going to be looking at
variations on mass, spin, in charge. Our next kind has
what spin?

Speaker 1 (27:08):
Right, It's very unlikely that something collapses into a black
hole and isn't spinning. Why because everything in the universe
is spinning. All the stars out there are spinning, all
the galaxies out there are spinning. Everything is spinning, and
The reason everything is spinning is the reason we have
any structure at all. You started with vast clouds of hydrogen,
but then you had little regions of over densities and
underd densities. With gravity took hold and gathered stuff together

(27:31):
and collapsed it and overall in the universe we think
there's probably no spin, but if you take a random
region of the universe, there's going to be, on average
a little bit of spin. It's like taking a million
coins and flipping them and saying, do you get exactly
fifty percent heads? Now you get a little bit extra
tail sometimes or a little bit extra heads another time.
To randomly chop the universe up into chunks, each one's

(27:52):
going to have a little bit of spin. They all
add up to zero, but each one's going to have
a little bit of positive a little bit of negative spin.
It's not impossible to have zero, but most likely it'll
have non zero spin. And then that collapses into stars
and gets exaggerated because as things collapse, they spin faster,
like a figure skater pulling in her arms. So if
you have a star, it's most likely spinning when it collapses.

(28:14):
Where does that spin go, Well, it can't just go away.
Spin is conserved in the universe, and so a spinning
star has to turn into a spinning black hole. So
I think that most of the black holes in the
universe are not schwartzy black holes. They're curve black holes.
They have mass and.

Speaker 2 (28:31):
Spin, and so they're called curve black holes.

Speaker 1 (28:34):
curR kerr from the New Zealand physicists who came up
with them.

Speaker 2 (28:38):
Ah, all right, curve black holes. Okay, so you wrote ringularity.
What does ringularity mean?

Speaker 1 (28:44):
Yeah? So what is at the heart of a spinning
black hole? Well, it can't be a singularity. Why not
because a singularity a point can't have spin. Oh right,
If you have something which literally has no extent, then
it can't spin. There's nothing to spin. And so what's
at the heart? How do you make something which is
infinitely dense and still can spin. The prediction is at

(29:07):
the heart of a spinning black hole is not a
singularity but a ring gularity.

Speaker 2 (29:13):
Love it? Did you cop with that term?

Speaker 1 (29:14):
I did not come up with that term, but I
love it so much I'm gonna say that at every opportunity,
imagine a ring like literally like the kind of ring
around your finger and it's spinning, so it's a circle
of infinite density, and it carries that angular momentum and
it can't collapse into a point like singularity because it
can't get rid of its angular momentum.

Speaker 2 (29:32):
Okay, so we've talked about how you almost certainly have spin,
and it feels to me like you'd almost certainly have
charge because what is the probability that all the charges
cancel out? And so we determined that a schwarz Child
black hole is probably less likely than a cur black hole.
Is a curb black hole also less likely than whatever

(29:54):
black hole we're gonna probably talk about that also has charge.

Speaker 1 (29:59):
Yes, I think that most black holes in the universe
probably have mass and spin and charge. And the effect
of having charge is very similar to the effect of
having spin. It changes where the event horizon is. So
number one, if you have spin, you have a ringularity
instead of a singularity. If you add charge, then you
get a charged ringularity. It doesn't change the structure of

(30:21):
the singularity, because even a point could carry charge. Having
charge doesn't determine the structure of the singularity.

Speaker 2 (30:28):
Sometimes you all do get naming right, good job.

Speaker 1 (30:33):
But the spin and the charge do have a really
interesting effect on the black hole. The spin forces it
into aringularity instead of singularity, but it also changes where
the event horizon is. So as you speed up a
black hole, you spin it faster and faster, the event
horizon actually shrinks. Or as you add electric charge to
the black hole, the event horizon actually shrinks, which creates

(30:56):
a maximum spin or a maximum charge to a black hole.
Because you can spin the black hole so fast, the
event horizon shrinks to zero, and then boom, you get
what's called a naked singularity, which you talk about a
little bit more in a minute, a singularity that's not
behind an event horizon, a singularity you could actually like
stare at, though it would be rude. Please don't ogle

(31:18):
our naked singularity.

Speaker 2 (31:20):
Wait, could you escape from a naked singularity if there's
no event horizon or my misunderstanding event horizons.

Speaker 1 (31:26):
No, you're exactly correctly understanding the consequences of not having
an event horizon. Absolutely.

Speaker 2 (31:31):
Whow cool.

Speaker 1 (31:32):
The other fascinating thing about spinning black holes is to
have much more complex space time than people typically think about.
You think about the space time near a short sized singularity,
there's an event horizon, things fall in. That's the end
of the story. But a spinning black hole has much
more complex space time. It doesn't just have an event horizon.
It has another horizon within the black hole called a

(31:53):
Cauchhi horizon that we'll talk about later. And then outside
the event horizon there's a region called the ergosphere where
itself is spun. Because the black hole is spinning, there's
an effect in general relativity called frame dragging. So Einstein's
general relativity is not just like Newton's gravity, but you're
replacing the force with curvature. It predicts different stuff. So,

(32:15):
for example, a spinning object in Einstein's theory has different
gravity than a non spinning object. Whereas like Newton says
it doesn't matter, like if the Earth is spinning or
not spinning, Newton predicts exactly the same force. Einstein says, no, no,
that spin matters. It contributes to the gravitational energy, and
not in a simple way. You have an object near

(32:35):
spinning black hole, it will pick up a spin right,
So the curvature doesn't just pull things in, it can
also spin things. We have these satellites around the Earth
called gravity pro b which measured these very small effects
with incredible detail using these quartz balls that are the
most spherical thing humanity has ever constructed. They have this

(32:56):
incredible process where they create these things in Germany, they
send them to our Gentina for this careful polishing and
they're like incredibly spherical and they're spinning in gyroscopes in
space to measure the effect of the Earth's spin, the
gravitational effect of the Earth spin on these walls. Really
an incredible experiment.

Speaker 2 (33:13):
I love that multiple nations were needed to make that happen.
That's wonderful.

Speaker 1 (33:19):
I'm imagining some like old lady in Argentina with like
a really smooth cloth and she's like rubbing it and
rubbing it, rubbing.

Speaker 2 (33:25):
It, and she's the best smoother in the world. So
it had to go to Argentina exactly.

Speaker 1 (33:30):
You know, there are really bespoke processes, like also for
developing the massive lenses needed for telescopes. There's like a
few people who like know how to do this and
cook it just right, and like it's still an art anyway.
Outside the event horizon is the ergosphere and you can
do cool stuff like drop stuff into the ergosphere and
you'll pick up a spin and then come back out,
and so you can extract energy from the black hole

(33:52):
this way.

Speaker 2 (33:53):
Oh oh man, So if we were closer to a
black hole, we could have like free energy.

Speaker 1 (33:58):
Yeah, this is called the Penrose process, like slowing down
the spin of a black hole by stealing its spin.

Speaker 2 (34:03):
Absolutely, something about that feels unsafe.

Speaker 1 (34:09):
Again, don't let Kelly drive.

Speaker 2 (34:11):
Nope, nope, nope, or extracts the energy from a black hole.

Speaker 1 (34:15):
But much more interesting than what's outside the black holes,
of course, is what's inside the black hole. Let's talk
about a naked singularity. To me, this is like one
of the most tantilizing ideas in physics because seeing the
singularity would tell us so much about the universe. Right
we talked earlier, like gravity tells us it should be
a point or a ring. Quantum mechanics says, no, that's impossible.

(34:35):
But doing those calculations is hard. People have tried it.
They predict x, they predict why. What's really there? If
we could just see it, If the universe could just
tell us what's going on, that would be such valuable
insight and guide us in forming a deeper theory in
physics and understanding the whole nature of reality and the universe,
so seeing the singularity would be incredibly powerful.

Speaker 2 (34:56):
And when we come back, we're going to talk about
how to take the clothes off a black hole so
that you can and see inside. All right, Daniel, you

(35:22):
and I are having a creepy afternoon talking about how
to de robe a black hole. Let's talk some more
about naked singularities. How can you see inside?

Speaker 1 (35:29):
Do you think it's inappropriate to like undress nature and
try to understand how it works?

Speaker 2 (35:33):
I mean, I think animals are walking around naked all
the time. We're very comfortable with it. It's the physicists
that are uncomfortable.

Speaker 1 (35:40):
All right, that's fair, that's fair. So how do you
take a part in event horizon? We always hear you
can't see inside an event horizon. That's true for a
schwartziled black hole, But for a black hole that's spinning,
where the event horizon is depends also on that spin,
and also on the electric charge. Remember that Einstein's gravity
is more complicated than Newton's gravity. Newton's gravity just basically

(36:02):
has charges and where they are, but Einstein has a
stress energy tensor, and different things contribute in different ways.
Remember the episode where we talked about like why potatoes
moving near the speed of light don't turn into black
holes because they have a lot of energy. Well, that
kinetic energy doesn't contribute the same way that the mass
energy does, which is why potatoes don't turn into black
holes at high speed. And so in the same way

(36:25):
like the spin of the black hole or it's electric charge,
they do contribute to the gravitational energy of the object,
but not in the same way. It's just like adding
mass to the black hole. So if you add spin
to the black hole, you do change the gravity. But remember,
like an example of gravity pro b it's complicated. And
so what happens as you spin up the black hole
is the event horizon actually shrinks.

Speaker 2 (36:46):
And is it harder to see because it's shrinking. No,
we decided that that makes it so you can see it.

Speaker 1 (36:52):
It doesn't become harder to seeze just now it's spinning.
And some of these black holes in the universe we
think are spinning super duper fast, so their event horizons
are smaller than they would be otherwise. But if you
could spin it beyond some threshold, then in principle you
would undress the black hole. Its event horizon would be
at radius zero, and.

Speaker 2 (37:10):
So if you wanted to try to spin it up,
so you could do that, could you throw a bunch
of extra mass at it that was spinning the right
way to make that happen.

Speaker 1 (37:18):
Yeah, So this is an area of much debate and confusion.
General relativity in principle allows naked singularities, but it doesn't
tell you how to make one right. And this is
the problem with general relativity. A lot of times. It's
like general relativity allows for wormholes, like if one existed
in the universe, it wouldn't be breaking the rules, but
it doesn't tell you how to go from we don't

(37:38):
have a wormhole? Do we have a wormhole? It's like
saying slew flays are possible, Okay, but what's the recipe.
I don't know how to make that arrange the particles
into a southflay, Like it's important distinction. And so people
have tried to go from like non spinning black holes
and think about adding electric charge or think about adding spin,
like keep dropping stuff into the black hole and angle
it away from the core. That when you're adding the master,

(38:01):
you're also adding spin, right, spinning up the black hole
getting faster and faster and so people have wondered about, like,
what happens when you reach the maximum spin? Is it
possible in our universe to overspin a black hole and
reveal the singularity inside of it? Roger Penrose has his
cosmic censorship hypothesis. He just predicts like, no, it can't happen.

(38:22):
There's something in the universe that's going to prevent them
from existing. But remember people also thought that about black
holes in general, and then they were like, oh my gosh,
these things are real. What In nineteen ninety one, Stephen
Hawking bet John Preskill and Kip Thorn that these things
are impossible, and this bet today still unsettled.

Speaker 2 (38:41):
Ah, I guess Hawking wouldn't pay up either way.

Speaker 1 (38:44):
Yeah, exactly. And so some people have done calculations try
to figure out like, well, what happened if you tried
to overspin a black hole? Like literally, what would prevent you?
And there's some calculations out there that suggest that it's
impossible that as you drop an object in, it loses
angular momentum due to a back reaction. The acceleration of
the object into the black hole causes gravitational emission, which

(39:06):
effectively loses that angular momentum, so it's going to radiate
away some angular momentum, just the same way that like
if you accelerate an electron, the way that happens is
by emitting photons. Like for an electron to turn left,
it has to emit a photon right. And so the
idea is if you drop stuff into a black hole
in a way that would increase its spin too close

(39:26):
to the maximum, it's going to radiate away some of
that spin as gravitational waves.

Speaker 2 (39:31):
So if we can't make it spin faster, is there
anything else that could make it spin faster or is
this just something that can never happen because nothing's going
to spin up a black hole.

Speaker 1 (39:41):
There's a lot of disagreements, and these calculations are hard,
so nobody knows is the answer, if this is possible
or not. I would love to have a black hole
and shoot a bunch of stuff into it and see
what happens. We don't know, of course, Okay, we just
don't know. So maybe naked singularities are possible in the universe,
or maybe these voculations are correct and you just can't overspin,

(40:02):
or very similar argument for overcharging a black hole, that
there's a maximum charge to a black hole. We just
don't know. But that's a really fascinating kind of singularity
and naked singularity.

Speaker 2 (40:13):
Okay. And so if I can take a step back
for a second and do a big overview. Basically, so
we're talking about what's happening at the center of black holes,
and the answers are either something completely different than everything
we've talked about because we get infinities, which means maybe
we're wrong.

Speaker 1 (40:28):
Most likely outcome yes okay, Or.

Speaker 2 (40:30):
You can have a naked black hole, a schwarz Child
black hole, and a cur black hole if those equations
are correct, and it turns out we are describing something real, okay.

Speaker 1 (40:40):
And if you have a black hole that has spin
and charge is called Akur Newman black hole. And there's
another variation where there's a black hole that's not rotating
but it has charge, and that's called a reister Nerdstrum
black hole. So you have all the varieties there in
general relativity. But those are all the classical predictions all
ignore quantum mechanics.

Speaker 2 (40:58):
So we've talked about black holes that have charge, spin,
and mass. Have we looked at every permutation of those
three traits?

Speaker 1 (41:06):
Now, yes, we talked about all those variations, but there's
still other kinds of singularities that we can explore. Like
even in those scenarios, we're assuming that the black hole
is symmetric, that as the mass falls in, it's like
a sphere, because the star is mostly a sphere, right,
But in the universe, nothing is ever perfectly a sphere.

(41:27):
So there's another kind of singularity called a BKL singularity
after three physicists whose name start with BK and L.
That describes the chaos you might get at the core
of a black hole if it's not actually symmetric.

Speaker 2 (41:40):
Okay, yeah, I guess to be honest, I'm having a
little trouble imagined. So when things spin, I always think
of them as becoming more spherical. I'm having trouble imagining
an asymmetrical black hole. But that is the limit of
my imagination. So okay, how would.

Speaker 1 (41:52):
Well look at the Sun? Is the Sun perfectly spherical?
It has features on the surface, which means that you
can tell one location on the Sun from another location,
and it's not perfectly spherical. And you might think, okay,
those are small things, but the problem with gravity is
it's very non linear, so small deviations create larger deviations.
Like the whole reason we have structure in the universe

(42:12):
is because gravity is nonlinear and it started from very
small deviations from perfectly smooth to make like stars and galaxies,
and gravity is very powerful. Another difference between new Tony
and Einsteini and gravity is that in Einstein gravity, the
gravitational energy itself has gravity. And so think about what happens.
For example, as things are collapsing, you get title forces,

(42:35):
which we've talked about a lot, right, things get stretched
out and squeezed as you approach the singularity. But if
you follow those calculations through, those title forces contribute also
to the gravity, and so actually the title forces end
up oscillating. So you get something which was compressed like
in the long way, like a football, Then those title

(42:55):
forces give a feedback effect and now the thing gets
squished and it gets compressed in another direction. So the
point is if you're not perfectly smooth, you're going to
create all sorts of crazy chaotic fluctuations. Any tiny deviation
from perfectly smooth symmetry is going to create chaos. So
this collapse is not symmetric but very chaotic. So that's

(43:16):
the idea of a BKL singularity. It's just like chaotic gravity,
and we're not used to thinking about gravity as like
having fluctuations and randomness. This is still deterministic, but it's
so hard to calculate because it's so sensitive to the
initial conditions that we call it chaos. It's not random,
it's chaotic. So that's what a BKL singularity is in
broad terms.

Speaker 2 (43:36):
So you have made me switch from thinking how could
it not be a perfect sphere? To how could it
possibly be a perfect sphere? So that and that's probably
the right way to think about it, right.

Speaker 1 (43:49):
Yeah, it's just much much harder. And so you know,
in physics, what we do is we say the universe
is too complex to model completely, so let's take a
simplified approach and get rid of some of the details
we hope are irrelevant. We can get sort of the
big picture view, which is why you start with like
spherical objects that are not spinning and have no charge,
and you do that calculation and then you're like, Okay,
I think we figured that out. Let's add this feature,

(44:09):
let's add that feature. And so we're trying to move
more gradually towards a more realistic description of the universe.

Speaker 2 (44:16):
Okay, so if it's chaotic, then I imagine it's also
like inconsistent, Like if you look at it, you know,
year to year, it probably has a slightly different shape.
Wouldn't that be fair to say?

Speaker 1 (44:26):
Yeah, but we're talking about what's happening inside the black hole,
so we can't see it. It probably also means that
the event horizon has these kind of ripples too, right,
because the event horizon is determined by the curvature. But
these effects might be subtle. And again, we've never directly
measured the extent of the event horizon, but that could
be super cool. See a fluctuating, oscillating event horizon.

Speaker 2 (44:47):
Yeah.

Speaker 1 (44:47):
Another kind of singularity people talk about a lot is
the Big Bang singularity, and this is another kind of singularity,
but it is quite different from a black hole singularity.
People really like to make these connections, be like, is
our universe inside a black hole? Because black hole is
a singularity and the Big Bang was a singularity. Dot
dot dot, we're inside a black hole. Awesome, pop science points, right,

(45:07):
But the singularity that might have existed at the beginning
of the universe is a very different kind of singularity
from the one that's at the heart of a black hole.
It's a singularity not in space, right, a black hole
singularity and principles should last forever and is at one
location in space. Right. The Big Bang singularity says the
universe is old and cold. That's where we live now.

(45:28):
But if you wind the clock back, things get hotter
and denser, right, And so the universe was denser, and
as you go back in time, things get denser and
denser and denser, and eventually you get a singularity in
density at one moment in time, but everywhere in space.
So black hole singularity one point in space, everywhere in time.

(45:49):
Big Bang singularity everywhere in space, one moment in time.
So technically a singularity in the same sense of singularities
is having infinities, but physically very very different from a
black hole singularity.

Speaker 2 (46:02):
Was it spinning.

Speaker 1 (46:05):
We don't know. That would be the whole universe spinning, right,
And we think probably the whole universe on average has
spinned zero. But that's a whole other fascinating topic. And
also remember that this prediction that the universe goes to
a singularity and density that also ignores quantum mechanics. What
really happens is you go back to a certain moment
thirteen point eight billion years ago, and beyond that we

(46:26):
can't predict with just gravity. We need to incorporate quantum mechanics,
and we don't know how to do that. So modern
theories of the Big Bang do not include a singularity.
They have a question mark before this moment when things
are so dense we can no longer calculate. But there
is a concept of a singularity, which again ignores quantum
mechanics if you want to go that far. But it's
different from a black hole singularity. Okay, this moment beyond

(46:48):
which you can no longer calculate without knowing how to
do quantum gravity sometimes called the plank temperature, like when
things get so hot we can no longer ignore gravity.
Or my favorite description of it is absolute hot. You've
heard absolute zero, like the universe can't get blue zero.
This is absolute hot. It's like the hottest possible temperature
beyond which, like temperature, doesn't make sense anymore, like we

(47:09):
don't know it can't get hotter. We just we don't
know anything about what happens beyond the plank scale.

Speaker 2 (47:14):
Do we know what temperature is absolute hot?

Speaker 1 (47:16):
Yeah, it's the plank temperature. It's a very big number.

Speaker 2 (47:19):
Oh okay, but it's not like five thousand. It's no,
it's just some very big number, got it?

Speaker 1 (47:24):
Okay, all right, So let's talk about the last two
kinds of singularities that Julie mentioned. These are in flying
and outgoing singularities. These are not standard terms for singularities.
You don't find them often in the literature in general relativity.
These are phrases made up by Kip Thorn and explained
in his book The Physics of Interstellar Kip Thrn definitely

(47:47):
knows a lot of general relativity, a lot more than
I do, due to the total expert in a badass
and a Nobel Prize winner for general relativity. So no
criticism implied in any of these comments. It's just like
he's very poetic with these words, and not all of
those words have caught on everywhere. So if you google
this stuff, you don't find like textbooks on it. But
the dude knows what he's talking about. When he refers

(48:07):
to an inflying singularity, he's talking about the complex space
time structure inside a spinning or charge black hole. So Remember,
this black hole is not just a sphere of mass
that's gotten compressed into a point. It's spinning, and so
the space time is much more complicated on the outside
and on the inside. So there's the event horizon, but
if you go past the event horizon, there's another horizon.

(48:30):
It's called the Kaushi horizon. So again these space times
have more structure than just an event horizon and a singularity.
There's this Kaushi horizon. Outside the Kaoshi horizon, you can
actually survive like you can live. People have theories that
you could have like stable orbits within the black hole,
and maybe even life could evolve, which could be amazing.
Within the Kaushi horizon, things get crazy. The Kaoshi horizon

(48:54):
essentially is the point where classical determinism breaks down, where
there's not enough information to predict what's going to happen
in the future. Remember that the equations of general relativity
are nonlinear, which means they blow up. Sometimes small deviations
in your initial conditions can lead to very large differences
in your predictions. Right You're going from infinitesimal changes in

(49:16):
your conditions to huge predictions in the future. That's also
a kind of infinity, and so this is what Kip
Thorne means by a singularity.

Speaker 2 (49:25):
Here, Okay, And so just to confirm it, it sounds
like what you're saying is this is the kind of
singularity where we don't understand it at all as physicists.
And so when you say there's not enough information to
predict the future, that's not like people living in the
Cauchy horizon can't predict the future. It's us. We can't
predict what happens in that situation.

Speaker 1 (49:46):
We can't predict it with our limited understanding of relativity
and general relativity and our current ability to do calculations.
It doesn't mean that it's impossible. Okay, got it. And
so in that sense, is this a physical thing? Is
to reflect the limit in our theory. There are some
interesting physical effects here, like in falling radiation or matter.
Blue shifts very dramatically as it approaches the Kushi horizon,

(50:08):
and so the energy density diverges, like you get these
predictions of essentially infinite energy density which don't seem to
make sense. And so this is what he means by
the inflying singularity. There's this other threshold after you come
into the black hole, as you fall in towards the ringularity,
you experience this inflying singularity of the Kushi horizon.

Speaker 2 (50:30):
Okay, but what if you were trying to get out,
because it sounds like you don't really want to be
at the middle of a black hole. So what is
an outgoing singularity?

Speaker 1 (50:39):
And outgoing singularity is related to the topic we talked
about earlier, the BKL singularities. You have a collapse of
black hole that's not symmetric, right, or maybe you have
like two black holes that collide, and so that's very
much not symmetric. You have an axis along which they collide.
So again, the nonlinearity of the Einstein equations lead to
chaotic effects, and you can get things like proping gation

(51:00):
of a curvature shock wave that's moving out from the core.
So you can get this outgoing singularity. What we mean there, again,
is a singularity in the curvature, so like huge gredients,
very nonlinear reactions, not a dot of matter that's infinitely dense,
but a deviation in the curvature that's moving in space
and time.

Speaker 2 (51:21):
Okay, and this is maybe not the most important question
that I could be asking right now, but you said
this was in a book called the physics of Interstellar.
Are these concepts part of that movie which I have
to admit I've never seen.

Speaker 1 (51:31):
You have not seen in Interstellar? Wow? Oh wow? And
married to a nerd never seen that movie.

Speaker 2 (51:37):
He's never seen it? I know.

Speaker 1 (51:39):
Oh my gosh. Is that on purpose? You guys like
boycotting that movie?

Speaker 2 (51:43):
No, just never no interest, just never got around to it.
Maybe I need to.

Speaker 1 (51:47):
Well, there's lots of fascinating and wonderful stuff about that movie.
It's also stuff I can't stand about that movie. So anyway,
a very popular movie, and a lot of real physics
went into it. Not all the plot elements are based
in sound physics, but a lot of it is. And
so Kip wrote this book, The Physics of Interstellar for
people who are interested diving more deeply into these topics.
So some of the stuff in the book the Physics

(52:08):
of Interstellar, is not covered in the movie. So we
don't have in flying or outgoing singularities in the movie
as far as I can recall.

Speaker 2 (52:15):
Okay, got it all right.

Speaker 1 (52:17):
So that's our tour of singularities. We think that the
appearance of these singularities in our predictions is a sign
that those predictions are probably nonsense, but we don't really know,
because sometimes the universe acts sort of nonsensical in a
way that seems crazy at us, but later we come
to learn actually makes more sense than anything we could
have imagined, and so we always should be ready for surprises.

(52:39):
And as usual, there's a lot more interesting, rich physics
here that goes well beyond what you typically see in
popsie coverage.

Speaker 2 (52:46):
And thank you so much to Julie for inspiring this
rousing conversation about the many kinds of singularities.

Speaker 1 (52:52):
Let's check in with Julie to see if we scratched
her singular.

Speaker 3 (52:55):
Ititch consider my singular itch scratched, or maybe my reingular
I did not know regularities were a thing. I like
how you described singularities as a warning sign from the
math and an indication that something is wrong, like the
math and the physics contradict each other. But also we
can't say for sure, so if you physicists could figure

(53:15):
out quantum gravity for the rest of us, that'd be great.
He also inspired new questions to ponder, like if there
are fuzzballs instead of black holes, or if life could
evolve inside a spinning or charge black hole. Wild and yep.
I absolutely did read the Signs of Interstellar by Kip Thorne,
which is where I got those terms, so it was
helpful to have that terminology cleared up.

Speaker 4 (53:36):
Thanks again, all.

Speaker 1 (53:37):
Right, thank you very much everybody for tuning into this
survey of several singularities. We hope this helps stretch your
mind to the limits of what humans can conceive of.

Speaker 2 (53:46):
Until next time, Daniel and Kelly's Extraordinary Universe is produced
by iHeartRadio. We would love to hear from you.

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