Question

what is time ? physics

Answer 1

Well , here is my experimentalist answer:

Time is a necessary parameter in order to describe observed changes in three dimensional space, dx/dt, dy/dt, dz/dt. If there were no observable changes in the (x,y,z) contour map of the world (including us as contours also) there would be no time parametrization needed.

These changes are an experimental fact and to start with the day and night clock was used to define the parameter. Clocks can be anything that consistently reproduces periodically the same (x,y,z) for a specific location/point.

This is classical time. Special relativity and even more General Relativity are a different story with much more sophisticated mathematical modeling.

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Answer 2

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.

In the International System of Units (SI), the unit of time is the second $(symbol: {\displaystyle \mathrm {s} } \mathrm {s} )$. It is a SI base unit, and it has been defined since 1967 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".[12] 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.

Galileo, Newton, and most people up until the 20th century thought that time was the same for everyone everywhere. This is the basis for timelines, where time is a parameter. The modern conception of time is based on Einstein's theory of relativity, in which rates of time run differently depending on relative motion, and space and time are merged into spacetime, where we live on a world linerather than a timeline. In this view time is a coordinate. According to the prevailing cosmological model of the Big Bang theory time itself began as part of the entire Universeabout 13.8 billion years ago.

In 1583, Galileo Galilei (1564–1642) discovered that a pendulum's harmonic motion has a constant period, which he learned by timing the motion of a swaying lamp in harmonic motion at mass at the cathedral of Pisa, with his pulse.[18]

In his Two New Sciences (1638), Galileo used a water clock to measure the time taken for a bronze ball to roll a known distance down an inclined plane; this clock was

"a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent, whether for the whole length of the channel or for a part of its length; the water thus collected was weighed, after each descent, on a very accurate balance; the differences and ratios of these weights gave us the differences and ratios of the times, and this with such accuracy that although the operation was repeated many, many times, there was no appreciable discrepancy in the results