Einstein’s special theory of relativity calls for radical renovation of common-sense ideas about time. Different observers, moving at constant velocity relative to one another, require different notions of time, since their clocks run differently. Yet each such observer can use his “time” to describe what he sees, and every description will give valid results, using the same laws of physics. In short: According to special relativity, there are many quite different but equally valid ways of assigning times to events.
Einstein himself understood the importance of breaking free from the idea that there is an objective, universal “now.” Yet, paradoxically, today’s standard formulation of quantum mechanics makes heavy use of that discredited “now.” Playing with paradoxes is part of a theoretical physicist’s vocation, as well as high-class recreation. Let’s play with this one.
First, some background. Despite special relativity’s freedom in assigning times, for each choice there is a definite ordering of events into earlier and later. In a classic metaphor, time flows like a river through all space, and the flow never reverses.