Let's return to the question about what
happens inside a black hole.
So the two figures here are Stephen Hawking, of course,
and Roger Penrose.
Who, in the '60s and early '70s, really pioneered and laid
the foundations for understanding
the true properties of black holes.
And in particular, what happens deep inside the black hole.
If you cross into the event horizon, as I've said before,
you're never coming out past the event horizon.
In fact, if you crossed big enough black hole,
you wouldn't even notice that anything was different.
You would feel exactly the same as if I stood on this table
and jumped off the table.
So I'd be in freefall for a moment.
That sense of freefall would be exactly what
you would feel, if the black hole was big enough,
as you crossed the event horizon.
As time went on, you would then find, actually,
you would be squeezed and stretched.
It's called spaghettification.
You'd be squeezed and stretched.
And eventually, you would confront
what's inside the black hole.
The space time singularity.
And the space-time singularity is inevitable.
And the picture I've drawn doesn't capture a key feature
of space-time singularities.
And the key feature of space-time singularities
inside black holes, is that the they're singularities in time
and not in space.
If a singularity was a point in space,
let's imagine it was in the centre of this room,
you have this strong sense that you should be able to avoid it.
If you find your rockets, surely I
could just move away from the space-time singularity.
Why do I have to hit the space-time singularity?
That's true.
And that's because space-time singularities and black holes
are not of that form.
Space-time singularity is a singularity in time,
and you should think of a moment in time, in the future,
that you can't avoid.
We can't avoid next Tuesday.
And that's exactly, not roughly, exactly how you
can't avoid the space-time singularity in a black hole.
And it's this change of grammar that's actually important.
A space-time singularity happens.
You go inside the event horizon and the space-time singularity
will inevitably happen.
For the biggest black holes that we know of,
it's of the order of 100 hours.
Inside the black hole in the centre of our galaxy,
it's about three hours.
I think.
Or maybe it's one hour.
It's roughly that.
Interestingly, the longest you have,
before the singularity happens, is just to relax.
You just freefall.
And that's the longest you've got.
Any other motion you do, if you switch on your rockets,
you'll make sure the singularity happens quicker.
I think there's a life lesson there somewhere.
But I'm not sure what it is.
But it's interesting, I think it's not
emphasised enough, that--
one more level of that analogy with Tuesday,
is firing your rockets, trying to avoid
the space-time singularity, would
be exactly analogous to saying, I'm going to hop in my car
and I'm not sure which direction you were driving,
but you just try and drive away from next Tuesday.
It's meaningless.
And similarly, that's exactly what's
happening inside the black hole.
So what is a space-time singularity?
So a space-time singularity is where
the laws of general relativity break down.
So general relativity is no longer
valid at the space-time singularity.
And at first, hearing this, that sounds like a very bad thing.
It sounds like general relativity is no longer--
I mean, it's somehow a flawed theory.
It can't tell us what we need to know about what would happen
deep inside a black hole.
But in fact, that's completely the wrong way of looking at it.
What the space-time singularities are telling us,
is that there is an opportunity for deeper
insight into the fundamental laws of physics.
We know that general relativity is not
the theory of everything.
After all, it doesn't describe any
of the quantum forces, which we'll
be discussing in a moment.
So there must be a bigger theoretical framework,
deeper fundamental physics, beyond general relativity.
And what the result of Stephen Hawking and Roger Penrose says,
is that black holes are a great place
to think about where you should go beyond our existing
knowledge.