I've come across an objection to relativity theory a few times, for which I've not seen a good answer, so I'm looking for clarification. A key result of relativity theory is that all speed is relative. So while we can measure a distant quasar moving in relative terms away from us at almost the speed of light, to an alien rotating the quasar, the quasar could be considered stationary, just as we can consider our own sun stationary. However, in relativity theory, E=mc^2, so energy increases dramatically as relative speed increases toward the speed of light. The energy of a spaceship moving at .99 the speed of light is far higher, to an outside observer, than a stationary spaceship. But the same spaceship can of course be considered moving at .99 the speed of light or stationary, depending on one's perspective. If energy is a real thing, however, which is inter-convertible with matter (as Einstein's famous equation establishes), then where does this energy "go"? Merely by flipping perspectives, we can create or eliminate massive amounts of energy, and thus of mass. What gives?