Clock Stationary:

Assume that we begin up the ship's engines and acquire moving. We accelerate with the ultimate aim of reaching almost the speed of light. Assume that we control to accelerate to a sizable fraction of the speed of light, and afterward we shut off the engines therefore we are coasting via space. You ask, "Relative to what is we moving?" This, as we shall perceive, is an extensive question! For now, assume that we measure speed in respect to the Earth.

We measure the time which it takes for the laser to go across the ship and return again. We are riding all along with the laser, the mirror, and all the comforts of a small spacecraft. We find that the time lag is yet precisely similar as it was whenever the ship was not moving associative to Earth; the oscilloscope still displays a delay of 20.0 ns. This obeys directly from Einstein's axiom. The speed of light has not changed since it cannot. The distance among the laser and the mirror has not altered either. Hence, the round trip takes similar length of time as it did prior to us have the ship moving.

When we accelerate therefore the ship is going 60 %, then 70 %, and eventually 99 % of the speed of light, then the time lag will forever be 20.0 ns as measured from the reference frame, or point of view, within the ship.

At this point, let us add the other axiom to Einstein's: In free space, light beams constantly follow the shortest feasible distance among two points. Usually, this is a straight line. You inquire, "How can the shortest path among two points in space be everything other than a straight line?" This is the other very well question. Note that the light beams emerge to follow straight lines via free space as long as the observer is not accelerating associative to the light source.