What is Space-time?

Nothing is not Nothing

We cannot see space-time. We can only deduce what it is and how it works through indirect means. We have not successfully done that yet. Newton had a picture that space was in Cartesian co-ordinates of 3D space with time flowing, and things happening within that framework. This is most people’s common sense picture of the world. We have very hard time, concepually, going beyond this picture.

Figure 1, Newton’s fixed space vs. Einstein’s flexible spacetime, from the film “Testing Einstein’s Universe” by Norbert Bartel.

Einstein modified this picture with his relativity theories, and a 4D space-time. But, the picture is still very Cartesian. There is still no explanation of what space-time is made of. Einstein’s relativity is a very good top level description of how space-time works on the cosmological scales, and it is almost identical with Newtonian physics at low velocities, and low gravitational fields.

Then, there is quantum mechanics (QM) with its very precise calculations of what happens at the atomic level. There is no consideration of what space-time is. In this respect, QM is incomplete as Einstein asserted in his paper “Can the quantum-mechanical description of physical reality be considered complete?”. There are wave equations, that just work, to describe the behavior of light, atoms, quarks, gluons, and many subatomic particles and fields.