As the pulse is received, the clock automatically emits another, and the time between emission and reception is the unit of time. We can do some algebra and compare the times marked off by the light clock fastened in the train and a light clock fastened on the platform. Let us begin by looking at the light clock fastened in the train. It sends a beam vertically upward a distance D and then back down. Well, the denominator is always less than 1 so t is never less than t o .
Some people say that moving clocks run fast .
This is another way of saying that more ticks occur between events. But, you say, that light clock is really a very special clock. What about heartbeats such as Galileo used in establishing the pendulum properties, or my Rolex, or Big Ben, or Br. Andrew s clock tower. These must be in agreement with the light clock else the first postulate would be violated.