What is space
- empty space?
I'm not just thinking of outer space. I mean any sort of space
- the space here -
the space where you're sitting.
Well, the answer seems obvious you know.
Empty space?
It's just another name for Nothing.
But that's not how the modern physicist views it.
Take for example a space station.
It's orbiting the earth.
Why?
Why doesn't it just go drifting off into outer space in a straight line?
Gravity
- the earth's gravitational force acting on it.
Well fair enough.
But what if an astronaut steps outside the spacecraft?
He too is orbiting the earth and in more or less the same orbit.
Why? Earth's gravity again.
But the astronaut is much lighter than the space station, so
in order to get him to go in the same orbit,
the gravity force on him has to be less than on the craft.
So how does gravity know how much less it has to pull on him
than on the spacecraft?
And in any case, why would gravity want them to go in the same orbit?
The puzzle is solved once one sets aside the idea of the earth exerting a gravitational force
and instead thinks of gravity
curving the space around it.
Okay, this is something I’ve rigged up.
This surface represents normal, empty space
and if we have an object going through that space, perhaps a spacecraft,
then the natural path to follow is a straight line, like that.
But look what happens
once I put a heavy object into that space, the earth.
Now you see what it’s done to the surface, it’s
curved the surface so now the natural path
for an object passing through it like a spacecraft is no longer a straight line,
it’s a curved path a bit like this.
Okay well forget about the spiraling in that’s just due to the friction we’ve got here
but for the spacecraft above the
atmosphere, there’s no friction to speak of so we get a steady, repeated orbit.
Let me just show you how it comes about that the astronaut and the spacecraft go in exactly
the same orbit at the same sort of speed.
As you can see I happen to have got a curved
surface here and now we’ll make the heavy ball represent the spacecraft
and the lighter ball is the astronaut who’s just stepped outside
the spacecraft to go for a spacewalk.
And this is what happens.
So the reason why the astronaut and the spacecraft follow the same orbit
is that that is the natural path to follow.
The natural path is no longer a straight line,
a straight line which has then got to be curved
because of the action of a force.
This way of viewing gravity we owe to
Einstein.
It's part of his Theory of Relativity.
So, space is not nothing.
Space is a something
- a mysterious something that can be curved.
And that's not all.
The universe is expanding
- distant galaxy clusters are receding into the distance.
This is all in the aftermath of the Big Bang.
But that's not to say the galaxies are moving through space,
into an outer space where previously there had been nothing.
No. They are moving because it’s, it’s space itself that is expanding.
The space between us and the distant galaxy
is carrying the galaxy along on a tide of expanding space.
Okay, here I've got some galaxies.
The galaxies have moved apart, not because they have slid over the rubber, but because
the rubber in between the galaxies has expanded
and carried them along with it.
Okay, that's what's happening out there.
Space - so-called 'empty space'
- is pushing the galaxy clusters apart.
This is a diagram of the various stages
of the expansion of the universe.
The Big Bang occurred here.
Followed by a brief period of exceedingly rapid expansion called inflation.
And that's followed by the more gentle, normal kind of expansion - like what I showed you with the balloon.
The rate of expansion at this stage further slows down somewhat because of the
mutual gravity between the galaxies.
But look what's happening here.
The rate of expansion is now increasing.
This is due to what is called dark energy.
Space
- empty space -
has energy.
And whereas
matter attracts through its gravity,
empty space,
through its dark energy, repels.
Now I reckon that's weird.
Space, so-called empty space
is not only something that can be curved by gravity,
but it can also push around heavy things such as galaxies.
No, let's face it, the simplest thing we can think of - so-called empty space,
is not simple at all.
It's a deeply mysterious something
- but what?
Tony.
Can we do the balloon sequence again, I’ve just had a brilliant idea.
Okay, I get this far blowing it up
but then if I blow some more
and it burst okay we’ve got all the bits flying around,
that could like the accelerated expansion due to dark energy yeah?
Shall I try it yeah?
No, no, Russell, Russell, no, no.
Well, well why not?
Sorry Russell, blowing up universes is not allowed,
it’s against health and safety, sorry.