But let’s return to 1917, when Einstein tries to insert the idea of the 3-sphere into his equations. Here he encounters a problem. He is convinced that the universe is fixed and immutable, but his equations tell him that this is not possible. It isn’t difficult to understand why. Everything attracts, therefore the only way for a finite universe not to collapse on itself is to be expanding: just as the only way to prevent a football from falling to the ground is to kick it upward. It either goes up or falls down—it can’t stay still, halfway up in the air.
