Time
A fundamental physical quantity used to sequence events, quantify durations, and measure rates of change in the universe.
Governing formula
Δt = t2 - t1
SI unit
s
In depth
Time is defined in the SI system by the hyperfine transition frequency of cesium-133 atoms. In relativity, time is not absolute but forms a four-dimensional spacetime continuum where rate of passage depends on gravity and relative velocity.
Examples in the real world
Using atomic clocks to synchronize GPS satellites, timing heartbeat intervals (~0.8 s), or measuring radioactive decay half-lives.