## Verifiable Predictions

The theory of relativity was developed at a time when there was no means of experimental verification. The concepts are so foreign to the 'everyday world' that it is easy to be sceptical. Perhaps the basic postulates are wrong, or maybe there is a flaw somewhere in the logic of the argument?

Einstein was convinced that his theory was not simply a mathematical fantasy, but a true description of the basic laws of the universe. When asked by someone, 'Professor Einstein, how do you know that you theories are true?', he replied: 'All I can say is, if they are not true, then the Lord has missed a wonderful opportunity.'

The great achievement of Einstein's work was that it did not end just with a beautiful theory, too good to be overlooked by God. There were concrete predictions, which many years later were to be tested by experiment. In order to experience time dilation space travellers would have to travel near the speed of light. The example of the astronaut's cigar is certainly science fiction but there are elementary particles in cosmic radiation which reach us with speeds in the range of about 0.9 c to 0.999—c, where the time dilation factor becomes significant. Studies of cosmic rays in the 1950s provided evidence that time dilation was a very real phenomenon.

Einstein's best-known prediction is the equivalence of mass and energy according to the equation E = mc2. Confirmation of this equivalence came in 1942 with the production of the first nuclear chain reaction in an experiment by Enrico Fermi's team in a squash court in Chicago. Dramatically and tragically, there followed nuclear bomb explosions in 1945. Mass had been transformed into enormous amounts of energy. We can now produce such energy in controlled fashion in nuclear reactors and we also know that nuclear reactions are the source of the heat and light of the sun.

The relation E = mc2 appears to be completely unrelated to the principles of special relativity. It seems quite mysterious how it could follow from arguments involving symmetries of space and time! In this chapter we try to follow the chain of logic step by step. Once the way has been pointed out, a rudimentary knowledge of mechanics is all that is required to go from the principles of relativity to mass-energy equivalence. The existence of nuclear energy is indeed a very convincing 'proof of the pudding'!

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