Not take the ticks to be the arrivals of the light signal at either end of the rod? Then the clock ticks once every second. Why, you might ask, do we use the "round trip" time of the light signal to define the ticks of the clock? Why
Thus the clock registers a tick for the user once every two seconds. The clock user is located at one end of the rod and records the successive returns of the light signal as the ticking of the clock. Since light moves at 186,000 miles per second, the signal will require one second to traverse the length of the rod on its outward journey and one second to make the return journey.
A light signal is reflected back and forth between the mirrors. Simplest design of clock imaginable, one specifically chosen to make our analysis easy.Ī light clock is an idealized clock that consists of a rod of length 186,000 miles with a mirror at each end. All we need is to demonstrate the effect for just one clock and that will be enough, as we shall see shortly, to give it to us for all clocks. That would be difficult and complicated-and unnecessarily so. To see how this comes about, we could undertake a detailed analysis of a real clock, like a wristwatch, a pendulum clock or a mechanical wind-up clock. An inertially moving clock runs more slowly than one at rest. The first change we will investigate has to do with time. They will overturn our classical presumptions about space and time. Let us begin investigating these changes. If we are to retain both of Einstein's postulates, we will have to make systematic changes throughout our physics. That is impossible according to classical Newtonian physics. An immediate consequence of the light postulate is that observers in the inertially moving spaceship will not judge the light signal to have slowed, no matter how fast they are moving past us. Imagine a light signal flying past us and an inertially moving spaceship that speeds after it. If we adopt these two principles, we already know that things cannot remain as they have been in classical Newtonian physics. Are Contracted in the Direction of their MotionĮinstein based his special theory of relativity on two postulates, the principle
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