What is special relativity?
Answered by HowStuffWorks
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  1. Devised by Albert Einstein in 1905, the theory of special relativity states that the laws of physics are the same in all inertial frames and the speed of light is the same for all observers. In other words, the idea of being at rest has no meaning. The physics are the same in a moving train as they are in a stationary train. According to special relativity, measurements of time and distance depend on travel speed. Travel speed doesn't affect time until speeds increase toward the speed of light. The speed of light is relative to the observer, as Einstein demonstrated, not to the light or sound [source: University of Virginia]. At these super-speeds, relativistic effects become more apparent. These are called dilation and length contraction.

    In basic terms, dilation is slowing of clocks, and length contraction is objects coming closer. Time dilation could be demonstrated if you set off on a long business trip to a star in the galaxy, traveling at about the speed of light, but your spouse stayed home with the kids. When you returned 10 years later (according to your spouse's Earth clock, anyway), your spouse would be a little ticked off. And it wouldn't just be because of being stuck with child care for 10 years. It's because you only aged the eight years of interstellar time, while your spouse aged 10 (or more, depending on how the kids behaved…).

    The most well-known aspect of relativity is the formula that has come to stand for Einstein: e=mc2. It's a theory that many have tested, and even tried to disprove, but that has stood the test of time [source:]. The theory of special relativity also means that the speed of light is a universal speed limit. For years, the truth has held that nothing in the universe can pass the speed of light. That part of Einstein's general relativity theory, however, might have come under legitimate attack recently. Scientists at the European Organization for Nuclear Research (CERN) reportedly made neutrinos --fundamental particles -- travel faster than light. The results are suspect, however, because neutrinos, also known as "ghost particles," are so hard to detect, and therefore to measure. Some argue that the discovery is fundamentally flawed [sources: The Ranger, International Business Times].

    circa 1925: German-born physicist Albert Einstein (1879 - 1955) in Paris (Hulton Archive/Getty Images)

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