Science › Space, time & gravity
"Now" is relative
1905· Idea
ProvenTested many times; part of working physics.
Two events that happen at the same moment for one observer can happen at different moments for another moving observer.
See it on the Science map →How it works
Einstein's 1905 paper used light signals to sync clocks; his 1916 popular book used a train. Picture lightning striking both ends of a moving train. Someone standing beside the track, halfway between, sees both flashes together. A passenger at the train's midpoint is moving toward one flash, so it reaches her first, and since light speed is the same for her, she concludes that strike happened first. Both are right. There is no single universal "now" across space.
Why it matters
It ended the idea of one master clock for the universe. Time becomes a matter of viewpoint, which leads directly to spacetime.
How we know
It is a direct logical result of the constant speed of light, which is tested to great precision. Clock-synchronization in GPS and particle accelerators must account for it.
Still a mystery
Philosophers still debate whether only the present exists or the past and future are equally real, given that "now" differs between observers.
Connections (2)
▸Special relativity predicts "Now" is relativeProven
The relationship
To decide whether two distant events happen at the same time, you compare light signals. Because light moves at the same speed for everyone, observers moving differently compute different times for distant events. So "at the same time" depends on who is asking.
How the link was found
Einstein, 1905, the first section of his paper on synchronizing clocks with light.
Why it's proven
It follows directly from tested postulates; precision timing systems such as GPS must account for it.
Open questions and mysteries
What it means for the reality of past and future is a philosophical debate.
▸"Now" is relative can reorder only unconnected events, protecting Causality and light conesProven
The relationship
Different observers can disagree about the order of two events only if the events are too far apart for light to connect them. For events that could cause one another, everyone agrees on which came first. So relativity's flexible "now" never puts an effect before its cause.
How the link was found
Follows from Einstein's 1905 theory; clarified by Minkowski's 1908 light-cone geometry.
Why it's proven
It is a mathematical consequence of the Lorentz transformations, which are experimentally confirmed.
Open questions and mysteries
If faster-than-light signals existed, this protection would fail; that is why they are believed impossible.