Philosophy of Physics: A Very Short Introduction (Very Short Introductions)

by David Wallace

Yet there is an apparent contradiction in the very idea of time dilation. Moving clocks run slowly, I said—but motion is relative. If you are moving rapidly relative to me, I predict that your clock runs slowly. But I am moving rapidly relative to you, and—according to the relativity principle—you predict that it is my clock that slows down. That starts to sound close to a contradiction: how can two clocks each run more slowly than the other? If A is twice as slow as B, and B in turn is twice as slow as A, doesn’t that make A four times as slow as itself? This clock paradox can be sharpened into one of the most famous thought experiments in physics, the twin paradox. Two twins fall out badly, and one of them heads off, in a rocket and in a huff, at (say) 80 per cent of light speed. Five years later (as measured on the Earth), he regrets his anger and heads back at the same speed (Figure 2).

twin paradox. interesting.