Temporal measurement has occupied scientists and technologists and was a prime motivation in navigation and astronomy. Through advances in both theoretical and experimental investigations of spacetime, it has been shown that time can be distorted and dilated, particularly at the edges of black holes. General relativity is the primary framework for understanding how spacetime works. Investigations into the relationship between space and time led physicists to define the spacetime continuum. It does not address why events can happen forward and backward in space, whereas events only happen in the forward progress of time. The operational definition of time does not address what the fundamental nature of time is. To describe observations of an event, a location (position in space) and time are typically noted. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. Time is used to define other quantities, such as velocity, so defining time in terms of such quantities would result in circularity of definition. The SI base unit of time is the second, which is defined by measuring the electronic transition frequency of caesium atoms. Time is one of the seven fundamental physical quantities in both the International System of Units (SI) and International System of Quantities. As of 2023, there is no generally accepted theory of quantum general relativity. General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. Distance and time are intimately related, and the time required for light to travel a specific distance is the same for all observers, as first publicly demonstrated by Michelson and Morley. In general relativity, the question of what time it is now only has meaning relative to a particular observer.
However, the numerical values are different for different observers. Every event can be assigned four numbers representing its time and position (the event's coordinates). Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a rocket ship. The physical nature of time is addressed by general relativity with respect to events in spacetime.
Time in physics is operationally defined as "what a clock reads". 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. Time has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Time is often referred to as a fourth dimension, along with three spatial dimensions. It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future.
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