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Thursday, April 17, 2008



I am studying neersociunce and interested in physics. Recently, I read some popular books on the special theory of relativity and have a simple question on the optical clock that many people used to explain the time dilation. I wish experts here can give me a hand.The optical clock is a beam of light emitted by a source bouncing between two mirrors arranged vertically inside of a train. Initially the train is at rest, the source is at rest and the beam bounces vertically between the two mirrors. Then the train moves to the right at a speed of v.All the books say that for an observer inside of the train, the beam still bounces vertically. This puzzles me! Since the speed of light is independent of the source, the light beam should not have a velocity v like the train and the observer. Then how could it move straight up and down to the observer inside of the train?I am sorry to take your time for this simple question. I would appreciate any help since it kind of bothers me.Thanks.

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I'm an editor in the Style section at The Washington Post, and I've written about arts, entertainment, business, and technology for the Los Angeles Times, New York Times, Slate, Los Angeles Magazine, and other publications. I was previously deputy editor at L.A. Weekly, overseeing arts and culture coverage. I've won awards for my stories about slash fiction, magicians, and television in India, among others. I've appeared on CNN and NPR and I wrote a book about Avenue Q. I've taught journalism at Loyola Marymount University and creative nonfiction at the University of Virginia Young Writers Workshop.

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