Time passes identically everywhere, and all observers can agree on one universal present.

Newtonian mechanics treated time as a universal background. Every ideal clock in the universe could, in principle, agree on the same elapsed time and the same definition of “now.”

That assumption becomes incompatible with two experimentally supported ideas: the laws of physics are the same for inertial observers, and the speed of light in vacuum is invariant.

Einstein’s special relativity resolved the conflict by changing time itself. Two observers moving relative to one another can compare clocks and measure different elapsed times between events. They can also disagree about whether two distant events happened at the same time. This relativity of simultaneity is not a clock error; each measurement is consistent within the observer’s reference frame.

General relativity extended the revision. Clock rates also depend on gravity: clocks deeper in a gravitational field run differently from clocks higher up.

These effects are not philosophical abstractions. They are measured with precision clocks and must be accounted for in technologies such as satellite navigation.

Relativity does not mean that time is arbitrary or that every description is equally valid. Reference frames are connected by exact mathematical transformations.

At ordinary speeds and weak gravitational fields, Newtonian absolute time remains an excellent approximation—which is why it feels universal in everyday life.