The fundamental theory of physics that deals with time is Einstein’s theory of special relativity.

Every particle traces out a worldline in space-time.  If its velocity is constant, its worldline would be straight.

The worldline of a particle lies within the lightcone, since no particle can travel faster than light.

o Time is the fourth dimension
o Time exists as a block, quite unlike our psychological perception of time.
o Time is relative–time slows down for moving particles.
o Events simultaneous to one observer need not be simultaneous to another.
o Causality is preserved.
o The cosmic speed limit is the speed of light.
​o Time moves forward in a straight line (worldline).