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Dec21-09, 11:44 AM
DaveC426913's Avatar
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Quote Quote by croghan27 View Post
If it is all relative - who is to say that one twin is going faster than the other - they both are traveling at the same speed relative to each other. Why would one age and not the other, or conversely, what decides which clock goes faster: they both are traveling the same speed relatively.
Because one of them accelerated and the other didn't. That breaks the symmetry of relativity.