Einstein noted that the above equation seemed to imply something absurd: Given that the transmission of light from S 2 to S 1 is continuous, how could the number of periods emitted per second from S 2 be different from that received at S 1 ? It is impossible for wave crests to appear on the way down from S 2 to S 1 . The simple answer is that this question presupposes an absolute nature of time, when in fact there is nothing that compels us to assume that clocks situated at different gravitational potentials must be conceived of as going at the same rate. The principle of equivalence implies gravitational time dilation.