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What is Time?

Picture of a Clock
Time

Clocks are based on seconds, minutes, and hours. While the basis for these units has changed throughout history, they trace their roots back to ancient Sumeria. The modern international unit of time, the second, is defined by the electronic transition of the cesium atom. But what, exactly, is time?

Physicists define time as the progression of events from the past to the present into the future. Basically, if a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of reality, used to describe events in three-dimensional space. It is not something we can see, touch, or taste, but we can measure its passage.

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 hematical in nature. This accords with most people’s everyday experience of how time flows.

However, since the advent of relativity in the early 20th Century, relativistic time has become the norm within physics. This takes into account phenomena such as time dilation for fast-moving objects, gravitational time dilation for objects caught in extreme gravitational fields, and the important idea that time is really just one element of four-dimensional space-time.

Relativity also allows for, at least in theory, the prospect of time travel, and there are several scenarios which allow for the theoretical basis of travel in time. There are even theoretical faster-than-light time-travelling particles like tachyons and neutrinos. However, the concept of time travel also brings with it a number of paradoxes, and its likelihood and physical practicality is questioned by many physicists.

Quantum mechanics revolutionized physics in the first half of the 20th Century and it still represents the most complete and accurate model of the universe we have. Time is perhaps not as central a concept in quantum theory as it is in classical physics, and there is really no such thing as “quantum time” as such. For example, time does not appear to be divided up into discrete quanta as are most other aspects of reality. However, the different interpretations of quantum theory (e.g. the Copenhagen interpretation, the many worlds interpretation, etc) do have some potentially important implications for our understanding of time.

http://www.exactlywhatistime.com/physics-of-time/

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