Time is relatively well-described in current physical theories – it’s just that those descriptions are perplexingly at odds both with each other and with our perception of what time should be.
Time is not absolute
Einstein’s general theory of relativity established time as a physical thing: it is part of space-time, the gravitational field produced by massive objects. The presence of mass warps space-time, with the result that time passes more slowly close to a massive body such as Earth.
This effect, although tiny in our own neighbourhood, has been confirmed in experiments. Clocks, for instance, run faster on mountain peaks than they do at sea level, and more slowly at our feet than they do by our heads. While we may think of time as a constant, metronomic beat against which the events of the universe play out, that is an illusion borne out of our own imprecise perception of time. Relativity says there is no single beat to which the cosmos moves.
Time has no direction
The irony of relativity’s abolition of absolute time is that this sort of time keeps our other basic physical theories ticking – from Newton’s laws of dynamics to the equations governing the evolution of the quantum world.
But these equations come with their own wrinkle: they are all fully reversible in time, running backwards just as well as forwards. Relativity gives no direction to time, either: time just “is”. That is at odds with our own perception, in which time determines our direction of travel, propelling …