Chapter 2.7 The relativity of simultaneity


Before Einstein, physicists thought that time flows at the same rate everywhere. There was supposed to be, we might say, a “universal Tuesday”: if it were Tuesday here on Earth, it was Tuesday throughout the entire universe.

That is, it was believed that one and the same instant of time occurred simultaneously throughout the universe, and was then followed by the next instant everywhere at once.

Einstein quickly realized that his theories ruled out such a universal simultaneity. This is easy to see. Suppose Jack and Jill synchronize their watches at noon and plan to speak again an hour after Jill has blasted off in her spaceship.

At 1 pm on Earth, Jack waits by his radio but Jill fails to make contact. Jack checks his watch against those of his colleagues in mission control, and finds that they all show the same time. Jill is, however, blissfully unaware of her rudeness: her hour has dilated and only a part of her stretched out hour has passed. Jill’s 1 pm is not simultaneous with Jack’s 1 pm; instead, say, Jill’s 12.45 pm is simultaneous with Jack’s 1 pm Since time flows differently for bodies moving relatively to each other, they disagree about which events are simultaneous. Thus, according to Einstein, simultaneity is relative.

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