................ C1................. C2-> P-> In the above illustration, P is a particle, C1 and C2 are detectors able to measure the energy/mass of a light pulse. C1 is at rest, P and C2 move horizontally to right with the same velocity. Let's say that P is actually a matter-antimatter pair that annihilates to two light pulses, one of which heads horizontally towards the detectors and other heads in the opposite direction. The rest mass of P is 2 mass units and the produced pulses have 1 mass unit each. From C1's frame, P is moving towards it and has kinetic energy, so the produced light pulse is expected to have mass more than 1 unit, when measured at C1. From C2's frame, P is at rest, so the pulse is expected to have mass 1 at C2. I find it a bit strange that the mass can be measured to be different (isn't it the same pulse after all?), but let's continue. Let's put a filter in C1. The filter checks the pulse mass (perhaps comparing it to a weight of rest mass 1) and lets the pulse through if its mass is above 1. If mass is 1 or below, the pulse is trapped in filter. From C1's own frame it seems clear that the pulse must be passed through the filter. From C2's frame, it seems clear that the pulse must be trapped by the filter in C1. Trapping/passing the pulse sounds like an event that both frames must agree. What have I missed, or does this though experiment make sense at all?. Thanks for help.