Time Is Relative

Einstein’s special theory of relativity, published in June 1905, disagreed with a fundamental belief of scientists such as Isaac Newton​—that the measurement of time is a constant throughout the universe. The implications of Einstein’s now generally accepted theory seem quite bizarre.

For example, imagine that you and a friend perfectly synchronize your watches. Your friend then flies around the world, while you stay at home. When he returns, the time displayed by his watch will lag a fraction behind the time shown on your watch. From your perspective, time slowed down for your traveling friend. The difference is, of course, infinitesimal at human speeds. However, when approaching the speed of light, not only does time slow down significantly but objects also become smaller and their mass increases. Einstein’s theory maintained that the speed of light, not time, is constant across the universe.

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The faster you travel, the slower time moves

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Clocks aboard satellites of the Global Positioning System (GPS) do not tick at the same speed as clocks on earth. Without correcting for this effect of relativity, the GPS signal would be rendered useless