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My book states :

The scope of physics is truly vast. At one end, it studies phenomena at the very small scale of length (10^-14 m or even less) involving electrons, protons, etc.; at the other end, it deals with astronomical phenomena at the scale of galaxies or even the entire universe whose extent is of the order of 10^26 m. The range of time scales can be obtained by dividing the length scales by the speed of light : 10^–22 s to 10^18 s.

I cannot understand why we have divided length scales by speed of light to get time scales. Is it because light takes shortest time to travel a distance.

Please explain me the reason. Thank you in advance.

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Light speed is used as a conversion factor between space and time length because is (in vacuum) a universal constant on which all observers agree. The convention of expressing space length as the time that takes light to travel it, and vice versa, is widely used in physics together with the one of setting light speed $c = 1$; in this so called natural units, time and space length exactly coincide without having to care about conversion factors.

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