Einstein says we’re all moving at the speed of light all the time – it’s just that if we stay still in space, we move at the speed of light through time. Things that move in space travel more slowly through time. Things that move at the speed of light (i.e., light) don’t travel through time at all; for a beam of light, emission and absorbtion are simultaneous. You can only move at the speed of light if you’re massless; massy things always move through time.

Far, far more strange is that the measuring sticks we use to define time and space (rulers and ticking clocks) change shape depending on who’s doing the observing, and how much mass there is around. In fact they change in just such a way as to prevent us from defining what ‘staying still in space’ actually means. Relativity is observer-centric, just like quantum mechanics. Which brings us back to the question: what IS an observer?

I have a suspicion it depends on your definition of ‘IS’…