The nature of Space – Boundaries of the Knowable (6/10)
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 spacestation. 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, soin 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 himthan 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 forceand instead thinks of gravitycurving the space around it. Okay, this is something I’ve rigged up. This surface represents normal, empty spaceand if we have an object going through that space, perhaps a spacecraft,then the natural path to follow is a straightline, like that. But look what happensonce I put a heavy object into that space, the earth. Now you see what it’s done to the surface, it’scurved the surface so now the natural pathfor 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 herebut for the spacecraft above theatmosphere, there’s no friction to speakof so we get a steady, repeated orbit. Let me just show you how it comes about that the astronaut and the spacecraft go in exactlythe same orbit at the same sort of speed. As you can see I happen to have got a curvedsurface here and now we’ll make the heavy ball represent the spacecraftand the lighter ball is the astronaut who’s just stepped outsidethe spacecraft to go for a spacewalk. And this is what happens. So the reason why the astronautand the spacecraft follow the same orbitis 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 curvedbecause of the action of a force. This way of viewing gravity we owe toEinstein. 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 intothe 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 galaxyis 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 becausethe rubber in between the galaxies has expandedand carried them along with it. Okay, that’swhat’s happening out there. Space – so-called ’empty space’- is pushing the galaxy clusters apart. This is a diagram of the various stagesof the expansion of the universe. The Big Bang occurred here. Followed by a brief period of exceedinglyrapid 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 themutual 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 whereasmatter attracts through its gravity,empty space,through its dark energy, repels. Now I reckon that’s weird. Space, so-called empty spaceis 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 sequenceagain, I’ve just had a brilliant idea. Okay, I get this far blowing it upbut then if I blow some moreand it burst okay we’ve got all the bits flying around,that could like the acceleratedexpansion 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 andsafety, sorry.