Before and after

As we have seen, the only way to reconcile the experimentally observed fact that the speed of light does not depend on the inertial frame with our belief in an objective reality, was to abandon the notion that “at the same time” meant the same thing for all observers. Two events that are “at the same time” in one frame may not be “at the same time” in another frame, and vice versa.

Now, some of you may have already realized that this could lead to a problem with the notion of causality, namely, the notion of cause and effect. If an event A is the cause of another event B, then A must happen before B, and B must happen after A. But according to Einstein, the chronological order in which two events happen can depend on the frame of reference!

To make our discussion concrete, consider again the tree planted firmly in the ground, the car moving to the right with speed ½c in the tree-frame, and the ball moving to the right with speed ⅘c in the tree-frame (which corresponds to ½c in the car-frame).