Time in physics

In physics, time is defined by its measurement: time is what a clock reads. In classical, non-relativistic physics, it is a scalar quantity (often denoted by the symbol ) 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. is a complex of technological and scientific issues, and part of the foundation of recordkeeping. History of timekeeping devices Before there were clocks, time was measured by those physical processes which were understandable to each epoch of civilization: the first appearance (see: heliacal rising) of Sirius to mark the flooding of the Nile each year the periodic succession of night and day, seemingly eternally the position on the horizon of the first appearance of the sun at dawn the position of the sun in the sky the marking of the moment of noontime during the day the length of the shadow cast by a gnomon 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: ). It is a SI base unit, and has been defined since 1967 as "the duration of 9,192,631,770 [cycles] 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. These clocks became practical for use as primary reference standards after about 1955, and have been in use ever since. The UTC timestamp in use worldwide is an atomic time standard. The relative accuracy of such a time standard is currently on the order of 10−15 (corresponding to 1 second in approximately 30 million years). The smallest time step considered theoretically observable is called the Planck time, which is approximately 5.