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THE HARPER DICTIONARY OF SCIENCE IN EVERDAY LANGUAGE: SCIENTIFIC TERMS EXPLAINED SO YOU CAN REALLY UNDERSTAND THEM / Herman and Leo Schneider Relativity The relativity theories of Albert Einstein (1879-1955) deal with the most fundamental descriptions of the physical universe: the concepts of time, space, motion, mass and gravitation. To attempt to "explain" (rather than describe) them here would be an act of presumption, for the following reasons: 1.Much heavy mathematics is involved. 2.The concepts of relativity are not readily accessible within our (relatively) crude experience, where the weight of a grain of salt is considered a small amount; where a rocket's escape velocity— seven miles per second—is regarded as very high; where a clock gaining a second in ten years is labeled super-accurate. The ideas of relativity emerge only at the boundaries of such a world, in the domains of the super-small, the super-fast, the superlarge , the super-massive. And yet the proofs of relativity's seemingly wild assertions are quite evident and useful to physicists working with particle accelerators (atom smashers), astronomers studying galaxies, and mathematicians calculating the orbits of space ships. First, to help us cast loose from pre-relativity thinking, we will engage in a "thought experiment." Theoretical physicists delight in these. Resting or Moving? You're sitting in a stationary train, reading. Alongside is another train, also stationary. After a while you look up and notice that the other train is in motion—or is it your train, gliding silently—or are both trains moving at different speeds? Use your camera to pop a time exposure of the other train. You'll get a streaky picture—but will it tell you about the absolute motion of either train, or only the relative motion between them? The Missouri Review · 145 There is no absolute "stationaryness" (rest state) or absolute motion. If you choose to label yourself as the single rest-state object in the entire universe, with everything else revolving about you, why so be it (as doting grandparents say, "The sun rises and sets on him/her.") Think of it this way: (1) suppose there are only two objects in the entire universe, and (2) the distance between them is increasing steadily in a straight line. Then (3) is it possible to say which one (or both) is moving? How about three objects? Three million? There is no absolute rest, no absolute motion, with one exception, which shaU be revealed at its logical place in this discourse. Space: Absolute or Relative? Onward in our quest for absoluteness, seeking an unvarying, "unrelative" place or standard of measurement. A yardstick? Can we paraphrase Gertrude Stein and say a yard is a yard is a yard?Here's a thought experiment similar (but without heavy mathematics) to one proposed by Einstein: Parf 1: A train is waiting at a station platform. A passenger lays a yardstick against a window, fore and aft, and finds that it fits exactly; the window's width is one yard. Outside, an observer on the platform measures the same window. Both "measurers" are stationary in relation to each other; physicists say they are in the same frame of reference. Both get the same result: one yard. Part 2: What if the measurements are taken in different frames of reference, one stationary and one moving? Back up the train and send it forward. As it passes the station, the window is measured again, by the passenger from the inside and by the observer from the outside (it can be done in a split second, but needs paragraphs to describe and explain). Result from the inside: no surprise, one yard, because window, yardstick and passenger are moving together. They are in the same frame of reference, as when all were standing still. And from the platform outside? The window measures shorter than one yard. Only a tiny bit shorter—at a sixty-mile-per-hour crawl—but at 163,000 miles per second (seven-eighths the speed of light), the distance from front edge to rear edge is only half as long as during the rest measurement. Is the passenger aware of it? No, because his yard stick...


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