Kind: captions
Language: en
Black holes curve space and time around
them in the way that we've been
describing. Things follow along the
curves in space. If the black holes move
around, the curves have to follow them,
right? But they can't travel faster than
the speed of light either. So what
happens is black holes, let's say, move
around. Maybe I've got two black holes
in orbit around each other. That can
happen. It takes a while. A wave is
created in the actual shape of space.
And that wave follows the black holes.
Those black holes are undulating.
Eventually, those two black holes will
merge. And as we were talking about, it
doesn't take an infinite time, even
though there's time dilation because
they're both so big. They're really
deforming spaceime a lot. I don't have a
little tiny marble falling across an
event horizon. I have two event
horizons. And in the simulations, you
can see it bobble and they merge
together. They make one bigger black
hole. And then it radiates in the
gravitational waves. It radiates away
all those imperfections and it settles
down to one quscent perfectly silent
black hole that's spinning. Beautiful
stuff. And it emits E= MC² energy. So
the mass of the final black hole will be
less than the sum of the two starter
black holes. And that energy is radiated
away in this ringing of spaceime. It's
really important to emphasize that it's
not light. None of this has to do
literally with light that we can detect
with normal things that detect light.
X-rays form of light. Gamma rays are a
form of light. Infrared, optical, all
this whole electromagnetic spectrum.
None of it is emitted as light. It's
completely dark. It's only emitted in
the rippling of the shape of space. A
lot of times it's likened closer to
sound. Technically, we've kind of
argued. I mean, I haven't done an
anatomical calculation, but if you're
near enough to two colliding black
holes, they actually ring spaceime in
the human auditory range. The frequency
is actually in the human auditory range
that the shape of space could squeeze
and stretch your eardrum even in vacuum.
And you could hear literally hear these
waves ringing.
The following is a conversation with
Jenna Levan, a theoretical physicist and
cosmologist specializing in black holes,
cosmology of extra dimensions, topology
of the universe, and gravitational waves
in spaceime. She has also written some
incredible books including how the
universe got its spots on the topic of
the shape and the size of the universe,
a mad man dreams of touring machines on
the topic of genius madness and the
limits of knowledge. Black hole blues
and other songs from outer space on the
topic of LIGO and the detection of
gravitational waves and black hole
survival guide all about black holes.
This was a fun and fascinating
conversation. This is a Lexman podcast.
To support it, please check out our
sponsors in the description. And now,
dear friends, here's Jenna 11. I should
say that you sent me a message about not
starting early in the morning, and that
made me feel like we're kindred spirits.
You wrote to me, "When the great
physicist Sydney Coleman was asked to
attend a 9:00 a.m. meeting, his reply
was, "I can't stay up that late." Yeah.
So, classic. Sydney was beloved. I think
all the best thoughts, honestly, maybe
the worst thoughts, too, are all come at
night. There's something There's
something about the night. Maybe it's
the silence. Maybe it's the peace all
around. Maybe it's the darkness and you
just
you could be with yourself and you could
think deeply. I feel like there's stolen
hours in the middle of the night because
it's not busy. Your gadgets aren't
pinging. There's really no pressure to
do anything but I'm off and awake in the
middle of the night and so it's sort of
like these extra hours of the day. I
think we were exchanging messages at 4
in the morning. Okay. So in that way,
many other ways were kindred spirits. M.
So, let's go in the one of the coolest
objects in the universe, black holes.
What are they? And maybe even a good way
to start is to talk about how are they
formed?
Yeah. In a way, people often confuse how
they're formed with the concept of the
black hole in the first place. So when
black holes were first proposed,
Einstein was very surprised that such a
solution could be found so quickly but
really thought nature would protect us
from their formation. And then nature
thinks of a way nature thinks of a way
to make these crazy objects which is to
kill off a few stars. But then I think
that there's a confusion that dead
stars, these very very massive stars
that die are synonymous with the
phenomenon of black hole. And it's
really not the case. Black holes are
more general and more fundamental than
just the death state of a star. But even
the history of how people realize that
stars could form black holes is is is
quite fascinating because the entire
idea really just started as a thought
experiment. And if you think of it's
1915 1916 when Einstein fully describes
relativity in a way that's the canonical
formulation. It was a lot of changing
back and forth before then. And it's
World War I and he gets a message from
the Eastern Front from a friend of his,
Carl Shortfield, who's who solved
Einstein's equations, you know, between
sitting in the trenches and like cannon
fire. Um, it was joked that he was
calculating ballistic trajectories. He's
also perusing the proceedings of the
Prussian Academy of Sciences as you do.
and he was an astronomer um who had
enlisted in his 40s and he finds this
really remarkable solution to Einstein's
equations and it's the first exact
solution. He doesn't call it a black
hole. It's not called a black hole for
decades. But what I love about what
Schwarz shield did is it's a thought
experiment. It's not about observations.
It's not about making these things in
nature. Um it's really just about the
idea. He sets up this completely
untenable situation. and he says,
"Imagine I crush all the mass of a star
to a point." Don't ask how that's done
because that's really absurd. Um, but
let's just pretend and let's just
imagine that that that's a scenario. And
then he wants to decide what happens to
spacetime if I set up this confounding
but somehow very simple scenario. And
really what Einstein's equations were
were telling everybody at the time was
that matter and energy curve space and
time and then curved spacetime tells
matter and energy how to fall once the
spacetime shaped. So he finds this
beautiful solution and the most amazing
thing about a solution is he finds this
demarcation which is the event horizon
which is the region beyond which not
even light can escape. And if you were
to ask me today all these
decass crushed to a point. The black
hole is the event horizon. The event
horizon is really just a point in
spaceime or or a region in spaceime.
It's actually in this case a surface in
spaceime. And it marks uh a separation
in events which is why it's called an
event horizon. Everything outside is
causally separated from the inside in so
far as what's inside the event horizon
can't affect events outside. What's
outside can affect events inside. I can
throw a probe into a black hole and
cause something to happen on the inside.
But the opposite isn't true. Somebody
who fell in can't send a probe out. And
this oneway aspect really is what's
profound about the black hole.
Um, sometimes we talk about the black
holes being nothing because at the event
horizon there's really nothing there.
Uh, sometimes when we when we think
about black holes, we want to imagine a
really dense dead star. But if you go up
to the event horizon, it's an empty
region of spaceime. It's it's more of a
place than it is a thing. And Einstein
found this fascinating. He helped get
the work published, but he really didn't
think these would form in nature. I
doubt Carl Schwarz shield did either. Um
I think they thought they were uh
solving theoretical mathematical
problems. Um but not describing this
what turned out to be the end state of
gravitational collapse. And maybe the
purpose of the thought experiment was to
find the limitations of the theory. So
you you find the most extreme versions
in order to understand where it breaks
down. Yeah. And it just so happens in
this case that might actually predict
these extreme kinds of objects. It does
both. So it also describes the sun from
far away. So the same solution does a
great job helping us understand the
Earth's orbit around the sun. It's
incredible. Does a great job. It's
almost overkill. You don't really need
to be that precise as relativity. Um,
and yes, it predicts the phenomenon of
black holes, but doesn't really explain
how nature would form them. But then it
also on top of that does signal the
breakdown of the theory. I mean, you're
quite right about that. It actually
says, "Oh, man." But you you go all the
way towards the center and yeah, this
doesn't sound right anymore. Um,
sometimes I liken it to, you know, it's
like a dying man marking in the dirt
that something's gone wrong here, right?
it it it's signaling that that there's
some culprit there's something wrong in
the theory and um and even Roger Penrose
who did this general work trying to
understand
uh the formation of black holes from
gravitational collapse he thought oh
yeah there's a singularity that's
inevitable it's in every there's no way
around it once you form a black hole but
he said this is probably just a
shortcoming of the fact that we've
forgotten to include quantum mechanics
and that when we do we'll understand
this um differently. So according to him
the closer you get to the singularity
the more quantum mechanics comes into
play and therefore there is no
singularity there's something else. I
think everybody would say that. I think
everybody would say the closer you get
to the singularity for sure you have to
include quantum mechanics. You just
can't consistently talk about magnifying
such small scales, having such enormous
uh ruptures and and curvatures and
energy scales and not include quantum
mechanics that that's just inconsistent
with the world as we understand it. So
you've described the brainbreaking idea
that a black hole is
uh not so much a super dense matter as
it's sometimes described, but it's more
akin to, you know, a region of space
time, but even more so just nothing.
Yeah, it's nothing. That that's a thing
you seem to like to say. I do I do like
to say that black holes are no thing.
They're nothing. Okay. So what what what
does that mean? That's that's what I
mean. That's the more profound aspect of
the black hole. So you asked originally
um how do they form? And I think that
that that even when you try to form them
in messy astrophysical systems, there's
still nothing at the end of the day left
behind. And um this was a very big
surprise. Even though Einstein accepted
that this was a true prediction, he
didn't think that that they'd be made.
And it was quite astounding that that
people like Oppenheimer actually it's
probably Oenheimer's most important
theoretical work um who are thinking
about nuclear physics and quantum
mechanics but in the context of these
kind of utopian questions why do stars
shine um why is the sun radiant and hot
and this amazing source of light and it
was people like Oenheimer who began to
ask the question well could stars
collapse to form black holes Could they
become so dense that uh eventually not
even light would escape? And that's why
I think people think that black holes
are these dense objects. That's often
how it's described. But actually what
happens these very massive stars,
they're burning thermonuclear fuel. You
know, they're earthfuls of thermonuclear
fuel. They're burning um and emitting
energy in E= MC² energy. So it's fusing.
It's a fusion bomb. It's a constantly
going thermonuclear bomb. And um
eventually it's going to run out of
fuel. It's going to run out of hydrogen,
helium, stuff to fuse. It hits an iron
core. Iron to go past iron with fusion
is actually energetically expensive. So
it's no longer going to do that so
easily. So suddenly it's run out of
fuel. And if the star is very very very
massive, much more massive than our sun,
maybe 20, 30 times the mass of our sun,
it'll collapse under its own weight. And
that collapse is incredibly fast and
dramatic and it creates a shock wave. So
that's the supernova explosion. So a lot
of these they rebound because once they
crunch they've reached a new critical uh
capacity where they can reignite to
higher elements, heavier elements and
that sets off a bomb essentially. So,
the star explodes helpfully because
that's why you and I are here because
stars send their material back out into
space and you and I get to be made of
carbon and oxygen and all this good
stuff. We're not just
hydrogen. So, the suns do that for us.
And then what's left sometimes ends at a
neutron star, which is a very cool
object, very fascinating object, super
dense, uh, but bigger than a black hole,
meaning it's it's it's not compact
enough to become a black hole. It's an
actual thing. A neutron star is a real
thing. It's like a giant neutron.
Literally, electrons get jammed into the
protons and make this giant nucleus and
this superconducting matter. Very
strange, amazing objects. But if it's
heavier than that the core and that's
you know heavier than twice the mass of
the sun um it will become a black hole
and Oenheimer was wrote this beautiful
paper in
1939 with his student uh saying that
they believed that the end state of
gravitational collapse is actually a
black hole. This is stunning and really
um a visionary conclusion. Now, the
paper is published the same day the
Nazis advance on Poland and so it does
not get a lot of fanfare in the
newspapers. Yeah, we think there's a lot
of drama today on social media. Imagine
that. Like here's a guy who predicts how
actually in nature would be the
formation of this most radical of object
that broke even Einstein's brain while
one of the most evil if not the most
evil humans in history starting a uh the
first steps of a global war. What I also
love about that lesson is how agnostic
science is because he was asking these
utopian questions as were other people
of the time about the nuclear physics
and stars. You might know this play
Copenhagen by Michael Fra. There's this
line that he attributes to Boore and
Boore was the great thinker of early
foundations of quantum mechanics, Danish
physicist, where Boore says to his wife,
"Nobody's thought of a way to kill
people using quantum mechanics." Now, of
course, then there's the nuclear bomb.
And what I love about this was the
pressure scientists were under to do
something with this nuclear physics and
and to enter this race over um a nuclear
weapon. But really at the same time,
1939 really uh Oenheimer's thinking
about black holes. There's a there's
even a small line in Chris Nolan's film.
It's very hard to catch. There's a
reference to it in the film where he
they're sort of joking well I guess
nobody's going to pay attention to your
paper now you know because uh because of
the Nazi advance on Poland that's the
other remarkable thing about Oppenheimer
is he's also a central figure in the
construction of the bomb right so it's
theory and experiment clashing together
with the geopolitics exactly so of
course Oppenheimer now known as the
father of the atomic bomb um he talks
about destroyers of worlds um But it's
the same technology and that's what I
mean by science is agnostic, right? It's
the same technology overcoming a
critical mass um igniting thermonuclear
fusion. Eventually there was a fision
the original bomb was a fision bomb and
fision was first shown by Le Mitner who
showed that a certain uranium when you
bombarded it with protons broke into
smaller pieces that were less than the
uranium. Right? So some of that mass
that E= MC² energy had escaped and it
was the first kind of concrete
demonstration of this Einstein's most
famous equation. So all of this comes
together but the story of um they still
weren't called black holes. This is
1939 and they had these very long-winded
ways of describing the end state the
catastrophic end state of gravitational
collapse. But what you have to imagine
is as this star collapses. So now, so
what's the sun? The sun's a million and
a half kilometers across. So imagine a
star much bigger than the sun. Much
bigger radius. And it's so heavy it
collapses. It supernovas. What's left is
still maybe 10 times the mass of the
sun. Just what's left in that core. And
it continues to collapse. And when that
reaches about 60 kilometers across, like
just imagine 10 times the mass of the
sun citys sized. That is a really dense
object. And now the black hole
essentially has begun to form. Meaning
the curve in spaceime is so tremendous
that not even light can escape. The
event horizon forms. But the event
horizon is almost imprinted on the
spacetime because the star can't sit
there in that dense state any more than
it can race outward at the speed of
light because even light is forced to
rain inwards. So the star continues to
fall and that's the magic part. The star
leaves the event horizon behind and it
continues to fall and it falls into the
interior of the black hole. Where it
goes, nobody really knows. But it's gone
from sight. It goes
dark. There's this quote by John Wheeler
who's like granddaddy of American
relativity and he has a line that's
something to the effect. Um, the star
like the Cheshire cat fades from view.
One leaves behind only its grin, the
other only its gravitational attraction.
And he was giving a lecture. It's
actually above Tom's restaurant, you
know, from Seinfeld near Colombia in New
York. Nice. There was a a place or there
still is a place there where people were
giving lectures about astrophysics. And
it's 1967.
Wheeler is exhaustively saying this
loaded term, the end state of
catastrophic gravitational collapse. And
rumor is that someone shouts from the
back row, well, how about black hole?
And um apparently he then foists this
term on the world. Wheelerhead way of
doing that. Well, I love terms like
that. Big bang, black hole. There's some
I mean, it's just pointing out the
elephant in the room and calling it an
elephant. It is a black hole. That's a
pretty uh accurate and deep description.
I just wanted to point out that the just
looking for the first time at a 1939
paper from Oppenheimer. It's like two
page. It's like three pages. Oh yeah,
it's gorgeous. The simplicity of some of
these that's so gangster. Just
revolutionize all of physics with this
with you know Einstein did that multiple
times in a single year. Mhm. When all
thermonuclear sources of energy are
exhausted, a sufficiently heavy star
will collapse. That's an opener. Mhm.
Unless fision due to rotation, the
radiation of mass or the blowing off of
mass by radiation reduce the stars mass
to orders of that of the sun, this
contraction will continue indefinitely.
And it goes on that way. Yeah. Now, I
have to say that Wheeler, who actually
coins the term black hole, uh gives
Oenheimer quite a terrible time about
this. He thinks he's wrong. and they
entered what has sometimes been
described as kind of a bitter I don't
know if you would actually say feud but
there were bad feelings and um Wheeler
actually spent decades uh saying
Oenheimer was wrong and eventually with
his computer work that early work that
Wheeler was doing with computers when he
was also trying to understand nuclear
weapons and in peace time world found
themselves returning again to these
astrophysical questions
uh decided that actually Oenheimer had
been right. He thought it was too
simplistic, too idealized a setup that
they had used and that if you you looked
at something that was more realistic and
more complicated that it it just simply
it just would go away. And in fact, he
he draws the opposite conclusion.
There's a story that Oppenheimer was
sitting outside of the auditorium when
Wheeler was coming forth with his
declaration that in fact black holes
were the likely end state of
gravitational collapse for very very
heavy stars and um when asked about it
Oppenheimer sort of said well I've moved
on to other things because you've
written in many places about the human
beings behind the science I have to ask
you about this about nuclear weapons
where is the greatest of physicists
coming together to create this most
terrifying and powerful of a technology.
And now I get to talk to world leaders
for whom this technology is part of the
tools that is used perhaps implicitly on
the chessboard of geopolitics. What what
can you say as a person who's a
physicist and who have studied the
physicists and written about the
physicists the humans behind this about
this moment in human history when
physicists came together and created
this weapon that's powerful enough to
destroy all of human civilization. I
think it's an
excruciating moment in in the history of
science and
um people talk about Heisenberg who
stayed in Germany and and uh worked for
the Nazis in their own attempt to build
the bomb. There was this kind of hopeful
talk that maybe Heisenberg had
intentionally derailed the nuclear
weapons program. But I think that's been
largely discredited that he would have
made the bomb could he had he not made
some really kind of simple errors in his
original estimates about how much
material would be required or how they
would get over the energy barriers. And
that's a terrifying thought.
Um, I I don't know that any of us can
really put ourselves in that position of
imagining that we're faced with that
quandry, having to take the initiative
to participate in thinking of a way that
quantum mechanics can kill people and
then making the bomb. I think
overwhelmingly physicists today feel we
should not continue in the proliferation
of nuclear weapons. Very few um
theoretical physicists want to see this
continue. that moment in history, the
Soviet Union had incredible scientists.
Nazi Germany had incredible scientists
and the United States had incredible
scientists. And it's very easy to
imagine that one of those three would
have created the bomb first, not the
United States. And how different would
the world be? The game theory of that I
think say the probability is 33% that it
was the United States. If the Soviet
Union had the
bomb, I think I think they would have
used it in a much more terrifying way in
the in the European theater and maybe
turn on the United States. And obviously
with Hitler, he would have used it. I
think there's no question he would have
used it to to to kill hundreds of
millions of people. In the game theory
version, this was the least harmful
outcome. Yes. Yes. But there is no
outcome with no bomb that that any game
theorist would uh I think would play.
But I I think if we just remove the
geopolitics and the ideology and the
evil dictators,
all of those people are just
scientists. I think they don't
necessarily even think about the
ideology. And it's a it's a it's a deep
lesson about the connection between
great science and the annoying sometimes
evil politicians that use that science
for means that are either
good or bad. Mhm. And the scientists
perhaps don't boy do they even have
control of how that science is used.
It's hard. They don't have control.
Right. once it's once it's made, it's no
longer scientific reasoning that
dictates the use or um it's restraint.
But I will say that I do believe that it
wasn't a 30 one-third down the line
because America was different and I
think that's something we have to think
about right now in this particular
climate. So many scientists fled here.
They fled to here.
Americans weren't fleeing to Nazi
Germany. they came here and and they
were motivated um by uh it's more than a
patriotism, you know, it was um I mean
it was a patriotism obviously, but it
was sort of more than that. It was
really understanding the threat of
Europe, uh what was going on in Europe
and um and what that life, how quickly
it turned, how quickly this freespirited
Berlin culture, you know, was suddenly
in this repressive and terrifying uh
regime. So, I think that it was a much
higher chance that it happened here in
America. Yeah. And there's something
about the American system, the you know
it's cliche to say but the freedom all
the different individual freedoms that
enable a very vibrant at its best a very
vibrant scientific community and that's
really exciting absolutely to scientists
and it's very valuable to ma maintain
that right the the vibrancy of the
debate of the funding those mechanisms
absolutely the world flocked here and
that won't be the case if we no longer
have intellectual freedom yeah there's
there's something interesting to think
about the tension the cold war between
China and the United States in the 21st
century you know some of those same
questions some of those ideas will rise
up again and we want to make sure that
um there's a vibrant free exchange of
scientific ideas I believe most Nobel
prizes come from the United States right
oh yeah I don't have the number but I
disproportionately so disproportionately
so in fact a lot of them from particle
physics came from the Bronx
[Laughter]
and they were European immigrants. How
do you explain this? Fled Europe um
precisely because of the geopolitics
we're describing. Yeah. And so instead
of being Nobel Prize winners from the
Soviet Union or from the Eastern Block,
they were from the Bronx.
And that's the thing you write about and
we'll return to time and time again that
you know science is done by humans. And
some of those humans are fascinating.
There's tensions. There's battles.
There's some are loners. Some are great
collaborators. Some are tormented, some
are easygoing, all this kind of stuff.
And that's the beautiful thing about it.
We forget sometimes is it's humans and
humans are messy and complicated and
beautiful and all of that. Yeah. Uh so
what were we talking about? Oh, the star
is collapsing.
Okay. So can we just return to the
collapse of a star that forms a black
hole? At which point does the super
dense thing become nothing? if we can
just like linger on this concept. Yeah.
So if I were falling into a black hole
and I I I tried really fast right as I
crossed this empty region but this
demarcation I happened to know where it
was. I calculated because there's no
line there. There's no sign that it's
there. There's no signpost. Um I could
emit a little light pulse and try to
send it outward exactly at the event
horizon. So it's racing outward at the
speed of light. It can hover there
because from my perspective, it's very
strange. The spaceime is like a
waterfall raining in and I'm being
dragged in with that waterfall. I can't
stop at the event horizon. It comes, it
goes. It's behind me really quickly.
That light beam can try to sit there
because it's like it's like a fish
swimming against the Niagara, you know,
swimming against the waterfall. It's
like stuck there. But it's like stuck
there. Um, and so that's one way you
could have a little signpost. You know,
if you fly by, you think it's moving at
the speed of light. It flies past you at
the speed of light, but it's sitting
right there at the event horizon. So,
you're falling back, cross the event
horizon. Right at that point, you shoot
outwards a photon. Yes. And it's just
stuck there. It just gets stuck there.
Now, it's very unstable. So, the star
can't sit there is the point. It It just
can't. So, it rains inward with this
waterfall. But from the outside, all we
should ever really care about is the
event horizon because I can't know what
happens to it. It could be pure matter
and antimatter thrown together which
annihilates into photons on the inside
and loses all its mass into the energy
of light. Won't matter to me because I
can't know anything about what happened
on the inside. Okay. Can we just like
linger on this? So what models do we
have about what happens on the inside of
the black hole at that moment? So I
guess that one of the intuitions, one of
the big reminders that you're giving to
us is like, hey, we know very
little about what can happen on the
inside of a black hole. And that's why
we have to be careful about making it's
better to think about the black hole as
an event horizon. But what can we know
and what do we know about the physics of
of space time inside the black hole? I
don't mind being incautious about
thinking about what the math tells us.
So I'm not such
a an observer. I'm very theoretical in
my work. It's really pen on paper a lot.
Um these are thought experiments that I
think we we can perform and contemplate.
Um whether or not we'll ever know is
another question. And um so one of the
most beautiful things that we suspect
happens on the inside of a black hole is
that space and time in some sense swap
places. So while I'm on the outside of
the black hole, let's say I'm in a nice
comfortable space station. This black
hole is maybe 10 times the mass of the
sun, 60 kilometers across. I could be a
100 kilometers out. That's very, very
close. Orbiting quite safely. No big
deal. You know, hanging out. Uh I don't
bug the black hole. Black hole doesn't
bug me. It won't suck me up like a
vacuum or anything crazy. But uh some my
my astronaut friend jumps in.
Um, as they cross the event horizon,
what I'm calling space, I'm looking on
the outside at this spherical shadow of
the black hole cast by maybe light
around it. It's a shadow because
everything gets too close, falls in.
It's just this um uh just contrast
against a bright sky. I think, oh,
there's a center of a sphere and in the
center of the sphere is the singularity.
It's a point in space from my
perspective, but from the perspective of
the astronaut who falls in, it's
actually a point in
time. So their notions of space and time
have rotated so completely that what I'm
calling a direction in space towards the
center of the black hole, like the
center of a physical sphere, they're
going to tell me, well, they can't tell
me, but they're going to come to the
conclusion, oh no, that's not a location
in space. That's a location in time. In
other words, the singularity ends up in
their future and they can no more avoid
the singularity than they can avoid time
coming their way. So there's no
shenanigans you can do once you're
inside the black hole to try to skirt it
the singularity. You can't set yourself
up in orbit around it. You can't try to
fire rockets and stay away from it
because it's in your future and there's
an inevitable moment when you will hit
it. Usually for a stellar mass black
hole, we think it's micros secondsonds.
Micros secondsonds to get from the event
horizon to the to the singularity. To
the singularity. Oh boy. Oh boy. So
that's describing from the your
astronaut friend's perspective. Yes.
From their perspective, the
singularities in their future. But from
your perspective, what do you see when
your friend falls into the black hole
and you're chilling outside and
watching? So, one way to think about
this um is to is to think that as you're
approaching the black hole, the
astronaut's spaceime is rotating
relative to your spacetime. So, let's
say right now my left is your right.
We're not shocked by the fact that
there's this relativity in left and
right. It's completely understood. And I
can perform a spatial rotation to align
my left with your left. Right now I've
completely rotated left out. Right. Um
if I just want to draw a a a kind of uh
compass diagram, not a compass diagram,
but you know at the top of maps there's
a northsoutheast west. But now time is
up down and one direction of space is
let's say east west. As you approach the
black hole it's as though you're
rotating in spaceime is one way of
thinking about it. So what is the effect
of that? The effect of that is as this
astronaut gets closer and closer to the
event
horizon, part of their space is rotated
into my time and part of their time is
rotated into my space. So in other
words, their clocks seem to be less
aligned with my time. And the overall
effect is that their time seems to
dilate. the spacing between ticks on the
clock of their watch, let's say, um on
the on the face of their watch, uh is is
elongated, dilated relative to mine. And
it seems to me that their watches are
running slowly, even though they were
made in the same factory as mine. They
were both synchronized beautifully and
they're excellent Swiss watches. Um, it
seems as though time is elapsing more
slowly for my
companion and uh likewise for them it
seems like mine's going really
fast. So years could elapse in my space
station. My plants come and go. They
die. I age faster. I've got gray hair.
Um, and they're falling in and it's been
minutes in their frame of reference.
Um, flowers in their little rocket ship
haven't rotted. They don't have gray
hair. Their biological clocks have
slowown down relative to ours.
Eventually at the event horizon, it's so
extreme. It's so slow. It's as though
their clocks have stopped altogether
from my point of view. And that's to say
that it's as though their time is
completely rotated into my space. And
this is connected with the idea that
inside the black hole space and time
have switched places.
Um, so I might see them hover there for
millennia. Other astronauts could be
born on my space station. Generations
could be populated there watching this
poor astronaut never fall in. So
basically the time almost comes to a
standstill, but we still they do fall
in, right? They do fall in eventually.
Now that's because they have some mass
of their own. Yeah. So they're not a
perfectly light particle and so they
deform the event horizon a little bit.
You'll actually see the event horizon
bobble and absorb the astronaut. So in
some finite time the astronaut will
actually fall in. So it's a it's like
this weird space-time bubble that we
have around us. Mhm. And then there's a
very big space-time curvature bubble
thing from the black hole and they
there's a nice swirly type situation
going on. That's how you get sucked up.
Yeah. So if you're a perfect like uh
infinitely small particle, you would
just be take longer and longer and
probably just be stuck there or
something. But no, there's quantum
mechanics. Mhm. Eventually you'll fall
in there. Any perturbation will only go
one way. It's unstable in one direction.
In one direction only. Um, but it's it's
really important to remember that from
the point of view of the astronaut, not
much time has passed at all. You just
sail right across as far as you're
concerned and nothing dramatic happens
here. You might not even realize you've
come to the event horizon. You you might
not even realize you've crossed the
event horizon because it's there's
nothing there. Right? This is an empty
region of spaceime. There's no marker to
tell you you've reached this very
dangerous point of no return. You can
fire your rockets like hell when you're
on the outside and maybe even escape,
right? But once you get to that point,
there's no amount of energy. All the
energy in the universe will not save you
from uh this demise. You know, there's
different size black holes. Mhm. And
maybe can we talk about the experience
that you have falling into a black hole
depending on what the size of the black
hole is? Yeah. cuz um as I understand if
the the the bigger it
is, the less drastic the experience of
falling into it. Yeah, that might
surprise people. The bigger it is, the
less noticeable it is that you've you've
crossed the event horizon. One way to
think about it is um curvature is less
noticeable the bigger it is. So, if I'm
standing on a basketball, I'm very aware
I'm I'm balancing on a curved surface. I
my two feet are in different locations
and I really notice. But on the Earth,
you actually have to be kind of clever
to deduce that the Earth is curved. The
bigger the planet, the less you're going
to notice the curvature. Um the the
global curvature. And it's the same
thing with a black hole, a huge huge
black hole. It just is kind of feels
like just flat. You don't really notice.
I'm trying to figure out how the phys
because if you don't notice and there's
nothing there but the physics is weird
in your frame of reference.
No. Well, so another cool thing. So I'd
like to dispel myths.
Yeah. Do you need a
minute? You're holding your head.
There's a sense like you you should be
able to know when you're inside of a
black hole when you've crossed the event
horizon. But no, from your frame of
reference, you might not be able to
know. Yeah, at first at least, you might
not realize what's happened. There are
some hints. For instance, black holes
are dark from the outside, but they're
not necessarily dark on the inside. So
this is uh a kind of fascinating that
your experience could be that it's quite
bright inside the black hole because all
the light from the galaxy can be shining
in behind you and it's focusing down
because you're all approaching this
really focused region in the interior.
And so you actually see a bright white
flash of light as you approach the
singularity. Um, you know, I kind of uh
I joke that it's a, you know, it's like
a near-death experience. You see the
light at the end of the tunnel. So, you
would see millennia pass on Earth. You
could see the evolution of um the entire
galaxy, you know, one big bright flash
of light. So, it's like a near-death
experience, but it's a definitely a
total death experience. It goes pretty
fast. But you looking out, you looking
out, everything's going super fast.
Yeah. the
clocks um on the earth on the space
station seem to be progressing very
rapidly relative to yours. The light can
catch up to you and you get this bright
beam of light as you see the evolution
of the galaxy unfold and
um I mean it sort of depends on the size
of the black hole and how long you have
to hang around. The bigger the black
hole the longer it takes you to expire
in the center. Obviously the human uh
sensory system we're not able to process
that information correctly right it
would be a microcond in a right that
would be too fast. Yeah but it would be
wow it' be so cool to get that
information but a big black hole you
could actually you know hang around for
some months. So yeah what's uh how are
small black holes versus super massive
uh black holes formed just so people can
kind of load that in. Are they are they
all is it always a star? No. So this is
also why it's important to think of
black holes more
abstractly. They are something very
profound in the universe and there are
probably multiple ways to make black
holes. Um making them with stars is most
plentiful. There could be hundreds of
millions maybe even a billion black
holes in our Milky Way galaxy alone.
that many stars. It's only about 1% of
stars that will um end their lives in in
in a death state that is a black hole.
But we now see and this was really quite
a surprise that there are super massive
black holes. They're billions or even
hundreds of billions of times the mass
of the sun and um uh millions to to tens
of billions maybe even hundreds of
billions. So extremely massive. We don't
think that the universe has had enough
time to make them from stars that just
merge. We know that two black holes can
merge and make a bigger black hole and
then those can merge and make a bigger
black hole. We don't think there's been
enough time for that. So, it's suspected
that they're formed very early, maybe
even a hundred few hundred million years
after the big bang and that they're
formed directly by collapsing out of
primordial stuff. Mhm. that there's a
direct collapse right into the black
hole. So like in the in the very early
universe, these are primordial black
holes from the stars. Not quite Wait,
how how do you get from that soup black
holes right away, right? So it's odd,
but it's weirdly easier to make a big
black hole out of something that's just
the density of air if it's really really
as big as what we're talking about. So,
in some sense, if they're just allowed
to directly collapse very early in the
universe's history, they can do that
more easily. Um, and it's so much so
that we think that there's one of these
super massive black holes in the center
of every galaxy. So, they're not rare
and we know where they are. They're in
the nuclei of galaxies. So, they're
bound to the very early formation of
entire galaxies in um in a really
surprising and deeply connected way. I
wonder if the like the chicken or the
egg is it uh like how critical how
essential are the super massive black
holes to the formation of galaxies?
Yeah, I mean it's ongoing, right? It's
ongoing. Which came first, the black
hole or the galaxy? Um probably um big
early stars which were just made out of
hydrogen and helium from the big bang.
Um there wasn't anything else, not much
of anything else. um those early stars
were forming and then maybe the black
holes and kind of the galaxies were like
these gassy clouds around them. Um but
there's probably a deep relationship
between the black hole powering jets,
these jets blowing material out of the
galaxy that that shaped galaxies maybe
kind of curbed their growth. Um and so I
think the mechanisms are still are still
ongoing attempts to understand exactly
the ordering of these things. Can we get
back to spacetime? Just going back to
the beginning of the 20th century. How
do you imagine spacetime? How do we as
human beings supposed to visualize and
think about spacetime where you know
time is just another dimension in this
4D space that combines space and time?
Because we've been talking about
morphing in all kinds of different ways.
is a curvature of spacetime like how do
you how are we supposed to conceive of
it? How do you think of it? Yeah, time
is just another dimension. There are
different ways we can think about it. We
can
imagine drawing a map of space and
treating time as another direction in
that
map. But we're limited because as
three-dimensional beings, we can't
really draw four dimensions, which is
what I'd require. three spatial because
I'm pretty sure there's at least three.
I think there's probably more, but um
I'm happy just talking about the large
dimensions, the three we see up, down,
right, east, west, uh north, south,
three spatial dimensions and time is the
fourth. Nobody can really visualize it.
Um but we know mathematically how to
unpack it on paper. I can mathematically
suppress one of the spatial dimensions
and then I can draw it pretty well. Now
the problem is that we'd call it a
ukitian spacetime. A uklitian spacetime
is when all the dimensions are
orthogonal and are treated equally. Time
is not another ukitian dimension. It's
actually a manowskian spacetime. But it
means that the spacetime, we're
misrepresenting it when we draw it, but
we're misrepresenting it in a way that
we deeply understand. I can give you an
example. The Earth, I can project onto a
flat sheet of paper. I am now
misrepresenting a map of the Earth. And
I know that, but I understand the rules
for how to add distances on this
misrepresentation because the Earth is
not a flat sheet of paper. It's a
sphere. And um and as long as I
understand the rules for how I get from
the north pole to the south pole that
I'm moving along really a great arc and
I understand that the distance is not
the distance I would measure on a flat
sheet of paper then I can do a really
great job with a map and understanding
the rules of addition multiplication and
the geometry is not the geometry of a
flat sheet of paper. I can do the same
thing with spacetime. I can draw it on a
flat sheet of paper but I know that it's
not actually a flat uklidian space. And
so my rules for measuring distances are
different than the rules I would use
that for instance cartisian rules of
geometry. I I would know to use the
correct rules for manovski spacetime and
and that will allow me to to to to
calculate how long uh time has elapsed
which is now a kind of a length a
space-time length on my map um between
two relative observers. and I will get
the correct answer. Um but only if I use
these different rules. So then what does
according to general relativity does
uh objects with mass due to the
spacetime? Right. Exactly. So Einstein
struggled for this completely general
theory not a specific solution like a
black hole or an expanding spaceime or
galaxies make lenses or those are all
solutions. That's why what he did was so
enormous. It's an entire paradigm that
says over here is matter and energy. I'm
going to call that the right hand side
of the equation. Everything on the right
hand side of Einstein's equations is how
matter and energy are distributed in
spaceime. On the left hand side tells
you how space and time deform in
response to that matter and energy. And
it can be impossible to solve some of
those equations. What was so amazing
about what Shell did is he found this
very elegant simple solution within like
a month of reading um this final
formulation. But Einstein didn't go
through and try to find all the
solutions. He sort of gave it to us,
right? He shared this and then lots of
people since have been scrambling to try
to ah I can predict the curvature of the
spaceime if I tell you how the matter
and energy is laid out. If it's all
compact in a spherical system like a sun
or even a black hole, I can understand
the curves in the spaceime around it. I
can solve for the for the shape of the
spacetime. I can also say, well, what if
the universe is full of gas or light and
it's all kind of uniform everywhere and
I'll find a different and equally
surprising solution, which is that the
universe would
expand. In response to that, that it's
not static, that the distances between
galaxies would grow. This was a huge
surprise to Einstein. Um, so all of
these consequences of his theory, you
know, came with
revelations that were not at all obvious
when he first wrote down um the general
theory and he was afraid to take the
consequences of that theory seriously,
which is aen the theory itself in its
scope and grandeur and power
is scary. So I can understand. Then
there's, you know, the the edges of the
theory where it falls apart. The
consequences of the theory that are
extreme, it's hard to take seriously. So
you can sort of empathize. Yeah. He very
much resisted the expansion. So if you
think about 1905 when he's writing these
sequence of unbelievable papers as a
25year-old who can't get a job, you
know, as a physicist and he writes all
of these remarkable papers on relativity
and quantum mechanics. Um and then even
in
191516 he does not know that there are
other galaxies out there. This this was
not known. People had mused about it. Um
there were these kind of smudges on the
sky that
people contemplated what if there are
other island universes. You know going
back to Kant thought about this. But it
wasn't until Hubble it really wasn't
until the late 20s um that it's
confirmed that there are other galaxies.
Wow. Yeah. He didn't
obviously there's so much we think of
now that he didn't think of. So there's
no big bang
static universe. But these are all
connected. Wow. Yeah. So he's operating
on very little information. Very little
information. That's absolutely true.
Actually, one of the things I like to
point out is the idea of relativity was
foisted on people in this kind of
cultural way. But there's many ways in
which you could call it a theory of
absolutism. And um the way Einstein got
there with so little information um is
by adhering to certain very strict
absolutes like the
absolute limit of the speed of light and
the absolute constancy of the speed of
light which was completely bizarre when
it was first uh discovered. really that
was observed through experiments trying
to figure out
um you know what would the relative
speed of light be? It's the only really
only massless particles have this
property that they have an absolute
speed and if you think about it it's
incredibly strange. Yeah, it's really
strange. Incredibly strange. And so so
from from a theoretical perspective he
he's he takes that seriously. He takes
it very seriously and everyone else is
trying to come up with models to make it
go away. Um to make uh the speed of
light be a little bit more reasonable
like everything else in the universe. Um
you know if I run at a car, two cars
coming at each other, they're coming at
each other faster than if one of them
stops. It's really a basic observation
of reality right here. This is saying
that if I'm racing at a light beam um
and you're standing still relative to
the source, uh we'll measure the same
exact speed of light. Very strange. And
he gets to relativity by saying, well,
what's speed? Speed is distance. It's
space over time. It's how far you
travel. Um it's the space you travel in
a certain duration of time. And he said,
"Well, I bet something must be wrong
then with space and time." So this is an
enormous leap. He's willing to give up
the absolute character of space and time
in favor of keeping the speed of light
constant. How was he able to
intuitit a world of curved spaceime?
Like I think it's like one of the most
special leaps in human history, right?
Cuz you're it's amazing. like it's very
very very difficult to make that kind of
leap. I I'll tell you it took me I think
a long time to I can't say this is how
he got there exactly. It's not as though
I studied the historical accounts of or
his description of his internal states.
This is more having learned the subject
how I try to tell people how to get
there in a few short steps. Um, one is
to start with the equivalence principle
which he called the happiest thought of
his
life. And the equivalence principle
comes pretty early on in his thinking.
And and um it starts with something like
this. Like right now I think I'm feeling
gravity because I'm sitting in this
chair and I feel the pressure of the
chair and it's stopping me from falling
and um lie down in a bed and I feel
h