The concept of relativity of time is one of the important by-products of Einstein's special theory of relativity. This principle claims that when two observers moving relative to each other measure an interval of time, they may not get the same result. If two events occur at the same point in space in a particular frame of reference, and if the time interval between them, as measured by an observer who is at rest within this frame, is Δt0, then an observer in the second frame of reference moving with constant velocity u relative to the first frame, will measure the time interval to be Δt, where Δt = (Δt0)/((1 ? u2/c2)1/2) = γΔt0. (Here, c is the speed of light.) The interval Δt is always larger than the interval Δt0. This effect is called time dilation. The concept of time dilation is a logical outcome of the postulate about the speed of light (that speed of light is the same in different frames of reference.) The time interval Δt0 between two events that occur at the same point in space is called the proper time.