... Lesson plan (frames) . ... 22b. Relativity . ... By Newton's laws, two frames of reference moving with constant velocity relative to each other observe exactly the same physical behavior. There exists no way to tell, which of them is moving and which one is at rest: there is no "absolute rest frame" and no "absolute (constant) velocity. ... But for a while it seemed that by certain subtle effects, electromagnetic forces could distinguish whether a frame was in motion of not. ...