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.
