We covered SR last semester in Modern Physics I and also covered early Quantum Theory and started Modern Quantum Theory. I've been speculating a bit on SR recently since I started reading The Universe and Dr. Einstein a guy at work gave me. If we have the Earth as a stationary reference frame and a rocket ship as a moving frame at some appreciable value of c with respect to Earth, according to Earth's observations, humans on the rocket ship will age more slowly, the physiological processes will slow down and so forth. However, it's equally as valid to reverse the situation, i.e. the stationary frame is the rocket ship and the moving frame is Earth. The people on Earth from the rocket ship's observations will appear to age more slowly and the proper time will then be on the rocket ship. Which is actually physically true? Because both cannot simultaneously be true. They cannot both age more slowly and age more rapidly. I'm wondering if this is a conventionally-valid paradox I conjured up. I'm sure my textbook covered this situation, but I just haven't opened it up to refresh my memory on such an event.