The Idiots' Guide to Einstein's Special Theory of Relativity

It's the most important theory of the 20th century. It identifies that light always travels at the same speed (186,000 miles per second - the fastest velocity in the universe). Speed is relative - it only tells you how fast something is moving relative to something else. As a train goes by at right angles to us, its speed can be measured. But imagine moving towards an oncoming train at the same speed - it would seem to be moving twice as fast. Light, however, doesn't behave in the same way: its speed is constant, regardless of the speed of its source or observer. Time passes differently, depending on whether you are moving or still. The faster you move, the slower your clock ticks compared with that of someone stationary. If a clock was in a spaceship that was travelling at the speed of light (if that were possible), the clock would stop Relativity also gave us one of the world's best- known equations: E = mc2. It predicts the energy (E) released from a mass (m), or object, when it is annihilated. C is the speed of light. Einstein's theory prompted man to split the atom, ultimately resulting in the creation of the atomic bomb and the nuclear age. Einstein's thinking radically changed our understanding of the universe, causing revolutions in particle physics, the idea of black holes, and the thinking behind Big Bang. And just when you thought you were beginning to understand, last month Dr Alphonsus Kelly of University College, Dublin, challenged the very heart of Einstein's theory: he believes the speed of light is not constant. But deconstructing a genius isn't easy, and Dr Kelly has yet to convince the sceptical academic world.