We may think that it is easy to imagine empty space. We look around us and remove every item that we can see and that leaves us with empty space. We tend to treat empty space as a cavity of some kind, some region from which all matter and radiation has been removed. And that is fine as long as matter and/or radiation exists in some other part of the universe, so that we can envisage the space between things. But what if there was no matter or radiation anywhere in the universe? Would space still exist?
Albert Einstein in a letter to Karl Schwarzschild on 9 January, 1916 said no.
“The essence of my theory is precisely that no independent properties are attributed to space on its own. It can be put jokingly this way. If I allow all things to vanish from the world, then following Newton, the Galilean inertial space remains; following my interpretation, however, nothing remains.”
The idea that something must exist for space itself to exist is hard to digest. To some, this may sound similar to the Zen-like question of whether a tree falling in a forest makes a sound if no one is there to hear it. Don’t things have an objective existence irrespective of the existence of an observer? Whatever the answer you give for sound, Einstein felt that the existence of space required matter.
The analog to the tree question is to imagine spinning around on your own axis, like ice skaters do. You will find your arms will want to get tugged outward and you will start to feel dizzy because of the way that the fluid in your ear gets sloshed around. Now imagine that you are all alone in the universe. According to Einstein, you would not be able to tell if you were spinning or not because there is no reference frame that you can use to tell the difference.
If you are finding this hard to digest, welcome to the club. Many of Einstein’s contemporaries also found it hard to accept. I have encountered this when trying to explain the Big Bang theory and the expanding universe to nonscientists. People tend to think of the Big Bang as similar to the explosions they are familiar with, where particles fly off in all directions. So they tend to think that the universe is expanding into an already existing space and naturally ask what exists beyond the boundary of that space. It is hard to grasp the idea that there is no boundary, the universe is all there is. That implies that if we run the clock backwards, space itself will shrink until we get to the Big Bang, when presumably there will be no space either.
You can read more about this here.