Time

Time is a dimension in physics and a metric for ordering events. In classical mechanics, time is treated as a universal parameter that flows identically for all observers. In Einstein's theory of relativity, time and space are interwoven into spacetime, and time intervals can vary with motion and gravitational strength.

Time is measured with clocks and calendars. The SI unit of time is the second, defined by transitions between hyperfine levels of the cesium-133 atom. Prior devices included sundials, water clocks, hourglasses, and mechanical pendulum clocks. Modern clocks use quartz oscillators or atomic transitions, and time scales such as UT, TAI, and UTC coordinate local timekeeping, with leap seconds occasionally added to keep UTC aligned with Earth's rotation.

Time has psychological and cultural dimensions. Humans perceive duration and sequence, influenced by attention, aging, and circadian rhythms. Cultures organize time through calendars, weeks, and time zones, and have developed various timekeeping systems and observances that structure daily life and historical memory.

The arrow of time refers to the apparent one-way direction of time from past to future, linked to increasing entropy in thermodynamics and to causality. In cosmology, the universe's expansion provides a large-scale temporal direction, though the fundamental laws may be time-symmetric.

The age of the universe is about 13.8 billion years. Time is a central parameter in cosmology and physics, but its fundamental nature—whether time is emergent, fundamental, or a feature of a deeper theory such as quantum gravity—remains an open area of inquiry.