Kind: captions
Language: en
So before we go further, let's
>> define what a black hole is. Let's do
black hole 101,
>> right?
>> Tell the audience
>> how you define a black hole.
>> Yeah. I think there's many different
ways to approach it. Most people will
say um an object so dense that not even
light can escape. Yeah.
>> And there's good things about that
description. There's really bad things
about that description. So a good thing
about the description is that it
captures one of the most fundamental
attributes of the black hole and that is
that it goes completely dark.
>> Yeah.
>> Nothing will ever escape not even light.
And so that part is the crux in some
sense of what a black hole is. Okay.
>> We we say that around the black hole we
circumscribe it with the region around
which light can no longer escape. We
call that the event horizon.
>> Yeah.
>> What's bad about that description is
that there's nothing there. There's no
dense object
>> at the event
>> at the event horizon, right? So, I don't
like the image of a thing.
>> Yeah.
>> You know, a thing, right? That's just
really, really dense. Now, it's true
often black holes are made by dense
things on their way out,
>> right? Yeah. Yeah.
>> But at the event horizon where the light
is forced to fall in, there's just empty
space.
>> So, the black hole is just a spaceime,
>> a volume of space,
>> right? It's just uh it's more of a place
in some sense than a thing. Yeah.
>> So, I can go up to this event horizon
and if I'm trying to shine a flashlight
and I'm shining light, the light as I
get closer will be trapped in orbit
>> at some point around the black hole. And
then eventually as I get closer, it
won't even be able to do that anymore.
>> It's just going straight in.
>> And it's just going straight in as am I.
>> Right.
>> But there's no I don't bump into any
matter. I don't smack into any surface.
I just sail across an empty space.
>> Yeah. So, what are black holes?
>> Disappointing.
>> Well, I mean, honestly, you might not
even notice anything bad was happening
to you. Yeah.
>> It would be no more dramatic in some
sense than stepping into the shadow of a
tree. It's just a shadow.
>> So, suddenly,
>> Yeah. But for cut off Yeah. So, so if
you say, "What is a black hole?" Well, I
would say the black hole is really the
event horizon.
>> Okay.
>> It's this horizon beyond which
>> light cannot escape.
>> If you're looking out, you can see light
coming in though, right?
>> That's right. So, if you fall in, it can
be bright on the inside. Uh because the
light can fall in from all the stars
shining, all the galaxies, all of that
can fall in behind you and you can see
all of this happening. Um so, it can be
right on the inside. It's just dark on
the outside. So, let's do it. It's a one
way. It's a one-way uh transition.
>> So, suppose the light's coming in toward
you and you hold up a mirror
>> to reflect the light back out. What
happens?
>> It's tricky because we're standing on
the outside of a black hole, let's say.
>> Yeah.
>> And we're throwing that light in and
your companion has fallen in right
>> ahead of you. We are imagining that
interior to that event horizon that that
is a spatial direction. I mean, all our
intuition says it. It says it says once
you cross inside there's a spatial
direction pointing towards the center of
a point in space
>> but space and time are relative
>> okay
>> and to the observer who has fallen
inside they're are very rotated relative
to your space and time
>> what they're calling space and what
you're calling space are now very
misaligned
>> and so that direction for the observer
who fell in that points towards what we
sometimes call the singularity
>> that is a direction in time now,
>> right?
>> So the person falling in can no more
bounce the light that comes in behind
them back out than they can bounce the
light backward in time.
>> Wow.
>> So there is no such option to do that.
Nor would they imagine such an option
because to them they and the light are
continuing to fall forward in time
>> and that is driving them in the same
direction towards the center.
>> So let me let me create it's tricky. I'm
not saying
>> is it like refraction like where you see
the spoon in the water and so this was
going in that direction but now it looks
like it's
>> somewhere else disconnected but it's
more extreme.
>> Yeah. I mean
>> so the event horizon would be like the
surface of the water.
>> Yeah. I I would say um there is you can
bounce the light in different spatial
directions just that direction is no
longer space at all. So, uh, you know,
it really is,
>> it really is, um, in your past,
>> the event horizon. So, there's no even
turning around anymore. And, you know,
the light can fall in behind you. You
can see things, but you can't claw your
way back. Nor can you send anything back
that way. Now, if you could travel back
in time, we could get tricky and start
to talk about things like that. But then
you're doing stuff on the outside that's
pretty crazy, too.