JACK and the STARSTALK (spacetime and gravity)

Abstract

“So now I have told you something of the nature and scale of the Universe,’’ remarked the Storyteller, carefully straightening his cravat “I have described stars and galaxies, galactic clusters and the great filaments and voids that fill the Universe out to the very limit of vision.

“I admit that I may not have told you everything of how the Universe is now, but I have attempted to describe how you may see it now, which is a quite different thing. Light from its various distant parts finally arrives at your eyes only after long delays. The vast reaches of space are terrifyingly empty, occupied for as far as you may see by just a few blobs of matter-by stars and galaxies. However, as it is on one of these isolated blobs of matter that we live, they are of some interest to us.

“Matter makes up the stars and planets, together with any people that live on them. Matter, on the large scale, is held together by gravity, and gravity plays a vital role in controlling its distribution and its motion. It was gravity that made a newborn Universe of gas condense and curdle into stars and galaxies. Gravity determines the shape of the whole Universe.

“Gravity tells matter how to move, and it also tells space how to bend. Gravity makes SpaceTime curve in long billowing folds that hold the stars and planets.”

“How can space bend?” asked Rachel. “There is nothing in empty space. You cannot bend nothing!”

“There may be no matter present, but that does not mean that space is nothing, “ replied the Storyteller. “Space and time form the framework of our experience. It is not space alone that we need consider. It is through the combined framework of SpaceTime that a thrown ball travels. If the framework of SpaceTime were straight then the ball would travel in a straight line, but the frame is bent, SpaceTime is curved, and your ball will curve round and fall back to Earth.

“That is what you see. A ball will fall back to Earth; a planet will orbit in a curved path around the Sun. They are all following the most direct path available to them in a curved SpaceTime. Looked at from the viewpoint of space alone it seems to you that a force is acting on them to deflect them from a straight path, and this force you know as gravity.

“Sit quietly and I shall tell you the story of ‘Jack and the Starstalk:’“