REMARKABLE RELATIVITY
    The theory of relativity - published in two parts in 1905 and 1915 by the German-born physicist Albert Einstein - rests on a remarkable realisation: that at very high speeds, commonsense assumptions break down.
    Commonsense dictates, for example, that two cars travelling towards each other at 100 km/h will pass at a relative speed of 200 km/h. So they do. But if one of the cars is replaced by a beam of light, the ordinary assumption does not work. No matter how fast the remaining car travels towards or away from the beam, the light will always hit at exactly the same speed: approximately 300,000 km/sec (186,000 mps).
Einstein's starting point
    From this starting point, which had been established by experiment in the 1880s, Einstein came to the realisation that the speed of light was the Universe's only constant, and that size, mass and even time were all relative, measurements of any of them depend on the position and relative speed of the observer.
    Einstein also deduced that mass and energy are interchangeable, and that they are related by the equation E = mc2, where E is the energy, and m the mass of a particle, and c is the speed of light.
    Modern experiments have confirmed the existence of many of the effects predicted by Einstein's theory. In 1972, for instance, sensitive atomic clocks carried on spacecraft and then compared with identical clocks on the ground after the flight were found to have slowed down - meaning that time itself had passed more slowly aboard the spacecraft.
    Particle accelerators, used in the study of sub-atomic particles, have to be specially designed to take account of the increased mass of particles boosted to significant proportions of light speed. And E = mc2, which shows that a small amount of mass can be converted into a huge amount of energy; has been put to work in nuclear reactors and atomic bombs.
How Time Slows Down
    As the speed of any object increases, its properties, as measured by an observer at rest, change. Its mass increases, its length in the direction of travel decreases and time slows down. At ordinary Earthbound speeds, even those of a jet plane, the changes are infinitesimal.
    At very high speeds, however, the changes become extremely important. An astronaut travelling at 90 per cent of the speed of light, for instance, would not feel any different from his twin on Earth. But the mass of his spacecraft would be more than double, its length would be less than half and a clock on board would take an hour to record 25 minutes because time had slowed - and he would therefore be ageing at less than half the speed of his brother.
    At the speed of light, the mass of his spacecraft would become infinite, its length would shrink to nothing and time aboard it would slow to a complete stop. Since this is impossible, nothing, other than light, can travel as fast as, nor faster than light.

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