Time - Wikipedia

The concept of time can be complex. Multiple notions exist, and defining time in a manner applicable to all fields without circularity has consistently eluded scholars.[7][10][11] Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.[12][13][14] Traditional definitions of time involved the observation of periodic motion such as the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the Global Positioning System, other satellite systems, Coordinated Universal Time and mean solar time. Although these systems differ from one another, with careful measurements they can be synchronized.

In physics, time is a fundamental concept to define other quantities, such as velocity. To avoid a circular definition,[15] time in physics is operationally defined as "what a clock reads", specifically a count of repeating events such as the SI second.[6][16][17] Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the spacetime continuum, where events are assigned four coordinates: three for space and one for time. Events like particle collisions, supernovas, or rocket launches have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In general relativity, these coordinates do not directly correspond to the causal structure of events. Instead, the spacetime interval is calculated and classified as either space-like or time-like, depending on whether an observer exists that would say the events are separated by space or by time.[18] Since the time required for light to travel a specific distance is the same for all observers—a fact first publicly demonstrated by the Michelson–Morley experiment—all observers will consistently agree on this definition of time as a causal relation.[19]

General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. In quantum mechanics, time is treated as a universal and absolute parameter, differing from general relativity's notion of independent clocks. The problem of time consists of reconciling these two theories.[20] As of 2025, there is no generally accepted theory of quantum general relativity.[21]