CLOCK STATIONARY

Suppose that we start up the ship’s engines and get moving. We accelerate with the eventual goal of reaching nearly the speed of light. Suppose that we manage to accelerate to a sizable fraction of the speed of light, and then we shut off the engines so that we are coasting through space. You ask, “Relative to what are we moving?” This, as we shall see, is an important question! For now, suppose that we measure speed with respect to the Earth.
We measure the time it takes for the laser to go across the ship and back again. We are riding along with the laser, the mirror, and all the luxuries of a small spacecraft. We find that the time lag is still exactly the same as it was when the ship was not moving relative to Earth; the oscilloscope still shows a delay of 20.0 ns. This follows directly from Einstein’s axiom. The speed of light has not changed because it cannot. The distance between the laser and the mirror has not changed either. Therefore, the round trip takes the same length of time as it did before we got the ship moving.
If we accelerate so that the ship is going 60 percent, then 70 percent, and ultimately 99 percent of the speed of light, the time lag will always be 20.0 ns as measured from a reference frame, or point of view, inside the ship.
At this point, let’s add another axiom to Einstein’s: In free space, light beams always follow the shortest possible distance between two points. Normally, this is a straight line. You ask, “How can the shortest path between two points in space be anything other than a straight line?” This is another good question. We’ll deal with it later in this chapter. For now, note that light beams appear to follow straight lines through free space as long as the observer is not accelerating relative to the light source.