Welcome back everybody. So now we want to

see what would happen if we got close to a black hole.

We want to take an imaginary journey to a black hole which,

you know, nobody's able to do right now because we don't have any nearby black holes.

But in the future, perhaps some day when we have interstellar travel,

we will be able to get close to black holes and study them.

And what would it look like for someone taking a journey towards a black hole?

Well, imagine first that if we could take the sun right now and suddenly,

magically shrink it down to a black hole,

which would be a singularity with a 3 kilometer event horizon surrounding it.

So some things change and some things don't change and this tells us something

important about Einstein's view of gravity or Einstein's theory of gravity.

The important thing is that if you are a long way away from the event horizon,

say at the distance the earth orbits the sun,

not much would really change.

In the sense, of course there'd be no sun there anymore,

there'd be no light and there'd be no solar winds coming out.

But you would still orbit this object.

Gravity would still be operating and you'd still orbit

the black hole as if nothing had changed in terms of least the orbit.

So people often think that black holes are these monsters that suck

everything in the universe into them and that's absolutely not true.

As long as you're far enough away from the black hole, you know,

you don't really feel any strange effects.

So the earth would still pretty much be in the same orbit.

But now it's all about getting close to the black hole.

That's really where the problem is.

So if you were able to take a journey and head towards the black hole, what would change?

Well, so as you were to get closer,

what would happen you'd eventually reach a point at

around three times the radius of the event horizon.

We consider the event horizon to be sort of the surface of the black hole.

At that point, once you were closer than three times that radius,

you would find that there were no more stable orbits.

You couldn't hang out in a nice stable orbit and go around the black hole.

You'd have to either be continually firing your rocket motors to keep yourself

from being drawn in or really have to fire your rocket motors very hard to escape.

You still could escape, but there's no longer any stable orbits.

And of course anything passing inside,

any time when you cross the event horizon,

which is not a physical structure, it's just a location.

It's just a place where the escape speed changes.

Once you were inside the event horizon, then of course that was it.

You'd never be able to escape again.

And space-time is so much bent that if you were - when you're close to the event horizon,

if you're looking towards the black hole,

the structure of space-time is so strongly

distorted that you could actually see objects directly behind the black hole.

So you'd see sort of a beautiful array of stars behind the black hole,

all collected around the surface of the event horizon.

So now let's actually imagine us taking a journey towards a black hole.

So we begin very far from the black hole and there's

you and your friend and your friend gets into

another spaceship and your friend is going to stay at

this large distance and you're going to communicate via radio.

And you also have a clock.

You'll synchronize your clocks and you're going

to carry a clock and they're going to carry a clock.

And your friend is going to track your progress as you descend,

as you slowly spiral and you're going to fire your retro rockets to

allow yourself to slowly spiral in towards the black hole.

Now the first thing that's going to happen as you get towards

the black hole is your friend is going to notice what's

called a gravitational time dilation.

Your friend is going to see your clock moving more slowly than their clock.

As you get closer to the black hole,

as you get deeper into the gravitational well, as we call it,

where the gravity is more or space-time is more strongly distorted,

your clock will appear to go slower than theirs.

There's also going to be a gravitational redshift.

And what we mean by this is that signals that you're sending to

your friend via radio or any kind of images,

that these signals will become stretched out because they have to

travel through this distorted space-time and everything will be shifted towards the red.

So if you were to send a picture to your friend that was all in blue tones,

the picture would all end up looking more red.

As you reach closer to the star,

you're also going to find that tidal forces,

the differential force from say,

your head to your feet,

if your head was pointed towards the - I'm sorry if your feet were

pointed towards the black hole that the tidal forces

were so strong that your feet would start becoming very

strongly pulled towards the black hole where your head was not so strongly pulled.

And this is a real, this is one of

the hallmarks of very strong distortions of space-time,

that the difference between the gravitational force in one place

that's closer to the central object is much

stronger than the force a little bit further away.

So basically what we call,

what physicists call spaghettification would occur.

You would be drawn out into a long piece of spaghetti,

which would be very painful.

And eventually your body wouldn't be able handle it and you'd be torn apart,

which would be sort of a painful, pretty painful death.

So these extremely strong tidal forces

are part of the experience of going towards a black hole.

These tidal forces get so strong that even eventually atoms,

from one side of an atom to another,

the gravitational force is so strong that the side of the atom that's pointed,

that is closer to the black hole will actually be pulled apart.

But if you imagine that you had some super spacesuit or

some super kind of physics machine that allowed you to

keep from becoming spaghettified then you would continue to fall

inward and eventually pass right through the event horizon.

But for your friend,

the gravitational time dilation would become so strong that

he would see you forever perched - time would stand still.

Your time would appear to stand still for

your distant observing friend and he would see

you forever perched at the edge of the black hole.

You, of course, would have passed through the event horizon.

And the interesting thing is that inside the event horizon,

physics shifts such that space becomes time in a sense and time becomes

space where you have - just as we are propelled into the future,

we can't go into the past.

Once you're inside the event horizon,

that you have no choice but to move towards the singularity.

So eventually you would make it to the singularity.

And if you somehow have managed not to become spaghettified

then the mystery of the singularity would be revealed to you.

Perhaps you'd pass on to,

pass through a wormhole or something like this,

some distortion in space-time or you actually went to another universe.

Or perhaps you would end up at another place in our own universe because

the interesting thing about gravity and

Einstein's view of gravity is that since gravity is a distortion of space-time,

it's possible to take two distant points and

fold space-time together and allow something to jump across.

So people have talked about the possibility of black holes acting as wormholes,

essentially from one place in the universe to another.

But we just don't know because we just don't understand the singularity.

What we do know is that essentially you would now be completely cut off from your friend.

You would never see the outside world again and it would essentially be a one-way trip

Journey into a Black Hole

Confronting The Big Questions: Highlights of Modern Astronomy

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