Cracking the SAT Physics Subject Test

15. As stated in Postulate 2 of the Theory of Special Relativity, the speed of
    light through empty space is always measured to be c, regardless of the motion
    of the source (or observer).

2) The Relativity of Time

Let’s go back to me on my spaceship, passing by your planet at a speed of c. Let’s
say, as I’m standing in the ship, I sneeze, and then, 1 second later, I sneeze again. If
you were holding a stopwatch and measured the time between my sneezes, you
would not get 1 second. Time is relative. The time as measured by you would not
agree with the time as measured by me.

To see how these time intervals would be related, imagine the following. In my
reference frame, let me call the time between my sneezes ∆T 1. If my velocity past

you is v, then the time that you’d measure between my sneezes would be

∆T 2 = γ • ∆T 1

where γ is the relativistic factor

Proper Time and

Dilated Time

∆T 1 can be referred to as

the proper time, which

is the time measure in