How fast would a spaceship have to travel in order to experience Einstein Relativity effects of exactly order 2? That is, the time slowed exactly by 1/2, the mass increased by exactly doubling? The solution, expressed as a percent of the speed of light, would come from this equation: .5 = sqr(1-v^2) and solve for v. lessee (mumble mumble) that means the velocity would be .866 or roughly 87 per cent of the speed of light. Hmmm. Kind of reminds me of the recent riddle I saw posted on this forum. What a coincidence. Well, that makes for an interesting question. Physicists are fond of saying Einstein's relativity theory has always proven right when tested, but what does this mean? How can it be? Consider that Einstein's theory is reciprocal. In our earlier riddle, the planets stayed still while the space ship moved. But we could, instead, suppose the space ship was actually still (it would seem to be still to those inside coasting along) and the planets moving in relation to the ship. That would mean, in particular, that the planets would appear to no longer be spheres but squashed ovals! That the time on the planets would appear to have slowed, so that, as they send out annual new year's celebrations, it takes them two years to get each one out! Is it possible that under these conditions, the same events would occur? That is, would the spaceship from the first riddle STILL be expected to receive 21 annual new year celebrations on the way to planet Beta? This is a serious test of Einstein's theory. Because if considering the relative point of view of the spaceship causes us to predict different events from the relative point of view of the planets, then we have definately uncovered a contradiction within Einstein's theory! What do you think? (a) Einstein's theory is safe. The same 21 annual new year celebrations would be expected to be received, because . . . . (insert reasoning/calculating here) (b) There's no way to reconcile these views! Einstein was wrong!