Time in physics

Prerequisites
Measurement Scientific notation Natural units

Prerequisites
differential equations partial differential equations

Prerequisites
vector notation partial differential equations

Prerequisites
algebra trigonometry

Prerequisites
physics quantum mechanics

Prerequisites
electrical engineering signal processing

Time in physics is defined by its measurement: time is what a clock reads. In classical, non-relativistic physics it is a scalar quantity and, like length, mass, and charge, is usually described as a fundamental quantity. Time can be combined mathematically with other physical quantities to derive other concepts such as motion, kinetic energy and time-dependent fields. Timekeeping is a complex of technological and scientific issues, and part of the foundation of recordkeeping.

Before there were clocks, time was measured by those physical processes which were understandable to each epoch of civilization:

Eventually, it became possible to characterize the passage of time with instrumentation, using operational definitions. Simultaneously, our conception of time has evolved, as shown below.

In the International System of Units (SI), the unit of time is the second (symbol: ${\displaystyle \mathrm {s} }$). It is a SI base unit, and it is currently defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". This definition is based on the operation of a caesium atomic clock.

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