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Right all along: Albert Einstein in 1921, the year he won the Nobel Prize in Physics

The great Scottish biologist D’Arcy Wentworth Thompson once said that Aristotle was the first person who “made [biology] a science, and won for it a place in Philosophy”. In a recent book on Aristotle’s biological work Armand Leroi even credits him with being the world’s first scientist, while disparaging Plato and modern physicists who come up with theories unjustified by experimental observation.

He has a point, yet quiet contemplation and thought experiments can play a huge role in physics, and Einstein was a master at it. Justifications can be sought later, and applications may appear later still. The modern GPS system is a great example. Facilitated by satellites orbiting the earth, it is essential their clocks be harmonised with those on earth, taking into account a 38-microsecond difference per day, otherwise you could soon find yourself standing in a muddy field rather than Piccadilly Circus. Why 38 microseconds? The clocks go seven microseconds slower because of the satellite’s speed — as predicted by Einstein’s Special Relativity — and 45 microseconds faster because of the weaker gravitational field — as predicted by his General Relativity. This raises the question of why there are two Relativity theories. But actually there is only one: the General subsumes the Special in the absence of gravity, but in order to explain both let us start at the beginning.

Aristotle formulated a theory, extended by the astronomer Ptolemy and much loved by the medieval Church, with the earth at the centre of the universe. Extraterrestrial bodies existed on various concentric spheres surrounding the earth, and the strange motion of the planets was explained using epicycles — each planet orbiting an invisible point that orbited the earth. Lovely stuff, but slight discrepancies with observation enforced added complications, and apart from divine dictate there was no obvious reason for this explanation.

During the Renaissance, attitudes started to change. Copernicus in Poland explained planetary motion as being centred on the sun, and the superbly detailed observations by Tycho Brahe in Denmark allowed his student Kepler to formulate a system in which the planets orbited the sun in ellipses. The sun lay at one focus of each ellipse and during the 17th century Kepler’s precise mathematical formulation finally found its natural basis in Newton’s law of gravitational attraction. As Pope wrote:

    Nature and Nature’s Laws lay hid in night:
    God said, “Let Newton be!” and all was light.

With Newton’s laws there was no longer a reason for everything to be centred on the sun. There was no point of absolute rest — the universe was not moored at the earth, the sun, or anywhere else. All motion was relative.

Fine — until electromagnetism caused a rethink in the 19th century. Electricity moving down a wire creates a magnetic field, and a moving magnetic field creates electricity. A synthesis of electric and magnetic fields emerged, mathematically formulated by James Clerk Maxwell. His equations involved electromagnetic waves moving at the speed of light, which had to be measured as the same by all observers no matter how fast they were travelling relative to one another.

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