## Physics Question #284

Arpa, a 18 year old female from Ubon asks on November 19, 2001,

What is "time"? Does the word "time" in Physics have the same meaning as the word time in the phrase, "What time is it?" and how do scientists know whether time runs slower or not. I don't think a clock can measure it because if you place the clock in different places such as on the mountain or at the very low land, there might be some factors that effect the clock.

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### Barry Shell answered on November 19, 2001

I went to www.google.com and typed in "time physics definition" and found many discussions that answer your question. You could try it yourself. Any library will also have introductory physics texts and they will all have discussions of the nature of time. I will type out for you the definition in Oxford University Press Dictionary of Physics:

"Time: A dimension that enables two otherwise identical events that occur at the same point in space to be distinguished. The interval between two such events forms the basis of time measurement."

Based on your reference to clocks in two locations, if you are worried that environmental factors might affect the time on two different clocks, scientists have developed ways to synchronize separated clocks perfectly. This often relies on the use of electromagnetic vibrational frequencies of very stable atoms.

If however, you are asking about Einstein's theory of special relativity in which he deduced that intervals of time are not absolute but are relative to the motion of the observers. Again from the Oxford Dictionary of Physics: "If two identical clocks are synchronized and placed side by side in an inertial frame of reference they will read the same time for as long as they both remain side by side. However, if one of the clocks has a velocity relative to the other, which remains beside a stationary observer, the travelling clock will show to the observer that less time has elapsed than the stationary clock."

In practice, on earth, we are all pretty much in the same inertial frame of reference, hence all our clocks give the same time when you ask "What time is it?" (this assumes that you adjust for the shift in time due to time-zones which has nothing to do with relativity and is just something that people have invented for convenience). In fact you can calculate that airplane pilots who spend a lot of time flying at nearly 1000Km/hr (i.e. in a frame of reference moving at a greater velocity relative to the rest of us) actually age a second or so less than the rest of us over a lifetime. In other words if they could carry a clock with them everywhere, and if it was a perfect clock, it would be a second or two slower than an identical clock that was stationary if you or I observed it. Of course, if the pilot observed it, the clock would not appear to have lost any time at all.

You can read Einstein's own analysis of time in his original book on Relativity written in 1916.

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