The light clock is restricted to one [color=#cc0000]vertical ping-pong[/color] of light at rest and one [color=#0000ff]observer in motion[/color].[br]According to the principle of relativity [b]switching the roles[/b] must give the [b]same results[/b]. By sliding [i][b]f[/b][/i] you can do here more than that: [i]Both[/i] may switch the roles partly, move a little bit such that the relative[b] velocity[/b] (v_R+v_B)/c = v/c remains [b]invariant[/b]. We may detect that the time indicated by t [nuts] does vary despite ct does not.