Editing Michelson–Morley experiment (section)

=== Observer comoving with the interferometer ===
If the same situation is described from the view of an observer co-moving with the interferometer, then the effect of aether wind is similar to the effect experienced by a swimmer, who tries to move with velocity <math display="inline">c</math> against a river flowing with velocity <math display="inline">v</math>.<ref group=A name=teller />

In the longitudinal direction the swimmer first moves upstream, so his velocity is diminished due to the river flow to <math display="inline">c-v</math>. On his way back moving downstream, his velocity is increased to <math display="inline">c+v</math>. This gives the beam travel times <math display="inline">T_1</math> and  <math display="inline">T_2</math> as mentioned above.

In the transverse direction, the swimmer has to compensate for the river flow by moving at a certain angle against the flow direction, in order to sustain his exact transverse direction of motion and to reach the other side of the river at the correct location. This diminishes his speed to <math display="inline">\sqrt{c^2-v^2}</math>, and gives the beam travel time <math display="inline">T_3</math> as mentioned above.