This post was inspired by Chandru Iyer’s post *here*.

Consider a light ray sent a certain *distance* *s* that is immediately reflected back. According to Newtonian mechanics if a light ray travels at speed *c*, then for a body moving at speed *v *relative to the stationary frame, the light ray should travel at the speed *c − v* one way and at speed *c + v* the other way.

The *total distance* of the light ray is 2*s*. The *total time* of the light ray is

Then the mean speed is

However, according to Einstein’s relativity theory, the mean speed of light is a constant, *c*. So the above speed needs to be multiplied by the gamma factor squared, *γ*². As Iyer notes, this is accomplished by contracting the moving rulers by the factor (1/*γ*) and dilating the moving clocks by the factor *γ*.

**But that is not the correct approach.**