According to the dictates of Special Relativity, the rate of a clock is unaffected by motion of the frame at which it is at rest. But a puzzle arises when one considers the reciprocity of the frames in a case where a precedent determination of the temporal interval is given for each frame. Here there are two trains, each 5 light years in length from beginning to end - they are approaching each other on parallel tracks at velocity approximately c. E* ---------------train 2---->B* .............................. S* frame S frame................................B <--------train 1---------E B is the beginning of the train at rest in the S frame and E is its end, B* is the beginning of the train at rest in the S* frame and E* is its end. E and B represent clocks that are synchronized in S, E* and B* represent clocks that are synchronized in S* As beginning points BB* pass each other, each takes note of the others time - we will assume this to be “0" for convenience. This is the position shown in the Fig 1. At this time, the operator of train 1 conducts an experiment that produces a muon M, and the operator of train 2 does a similar experiment and produces a muon M* Both muons have a lifetime of 2 μsec in their own frame and M remains at rest with respect to clock B and M* remains at rest with respect to B* If frame S is considered at rest, the time required for B* to reach E will be 5 years as measured by clocks B and E. In the S* frame, the muon M* will be at rest relative to B* and if the relative velocity is sufficiently close to c, will not yet have decayed when B* reaches E. So, B* clock will read only 2 μsec when the S clock has logged 5 years. Now consider the S* frame at rest. The time that has passed in S* as measured by the synchronized clocks B* and E* when B reaches E* should be 5 years (not 2 usec) since the proper distance in S* is 5 light years. But muon M will only have aged 2 usec in its rest frame S - so clock B should read 2 usec, not 5 years. Question: When B* is opposite E and B is opposite E* Does clock B read 2 μsec or 5 years? Does clock B* read 2 μsec or 5 years?