Physics of Time

 In the sciences generally, time is simply what a clock reads, but this hides a whole host of different conceptions of time used in physics

Physics is the only science that explicitly studies time, but even physicists agree that time is one of the most difficult properties of our universe to understand. Even in the most modern and complex physical models, though, time is usually considered to be an ontologically “basic” or primary concept, and not made up of, or dependent on, anything else.In the sciences generally, time is usually defined by its measurement: it is simply what a clock reads. 

Physics in particular often requires extreme levels of precision in time measurement, which has led to the requirement that time be considered an infinitely divisible linear continuum, and not quantized (i.e. composed of discrete and indivisible units). With modern atomic time standards like TAI and UTC (see the section on Time Standards) and ultra-precise atomic clocks (see the section on Clocks), time can now be measured accurate to about 10−15 seconds, which corresponds to about 1 second error in approximately 30 million years.But several different conceptions and applications of time have been explored over the centuries in different areas of physics, and we will look at some of these in this section.

In non-relativistic or classical physics, the concept of time generally used is that of absolute time (also called Newtonian time after its most famous proponent), time which is independent of any perceiver, progresses at a consistent pace for everyone everywhere throughout the universe, and is essentially imperceptible and mathematical in nature. This accords with most people’s everyday experience of how time flows.