Brief History of Physics

By a curious numerological coincidence, Stephen Hawking was born 300 years to the day after the death of Galileo Galilei, the man who did most to usher in the era of modern science. But for the purposes of this story, it is best to start with Sir Isaac Newton, who was the first truly mathematical physicist and thus a direct ancestor of Stephen Hawking. The first great achievement of theoretical physics was Newton's theory of mechanics (see 'Key Ideas' at the end of this book), which is encoded in three simple laws that are probably still remembered even by those who haven't studied physics since their school days:

(i) Every body continues in a state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.

(ii) Rate of change of momentum is proportional to the impressed force, and is in the direction in which this force acts.

(iii) To every action, there is always opposed an equal reaction.

These three laws of motion are general, applying just as accurately to the behaviour of balls on a billiard table as to the motion of the planets around the Sun. It was Newton's insight into the problem of planetary motion that it could be described by the same mathematical law as objects on Earth, such as apples falling from a tree. Newton realised that a body orbiting in a circle, like the Moon going around the Earth, is experiencing a force in the direction of the centre of motion (just as a weight tied to the end of a piece of string does when it is twirled around one's head). An apple feels a downward force towards the centre of the Earth. Based on this idea, Newton developed a theory of Universal Gravitation that could explain the motion of the planets discussed by Johannes Kepler more than a century earlier. This was the first proper example of apparently disparate phenomena being unified, i.e., incorporated in a single mathematical theory.

The idea of a universe governed by Newton's laws of motion was to dominate scientific thinking for more than two centuries. But wider than that, Newton's achievements suggested a perfectly predictable cosmos whose behaviour was as regular as clockwork. Once one knew the state of the Solar System at any time, one could predict its state at any time in the future with total confidence. Newton, a profoundly but unconventionally religious man, had unwittingly changed the role of God. Instead of intervening in the daily running of the world, He simply had to wind it up and let it go.

This view of a rigidly predictable universe was to hold sway until the end of the 19th century. But in the meantime, other branches of science came under the scrutiny of mathematical physicists inspired by Newton's example. Chief among these was the theory of electricity and magnetism. It was known that objects could be charged and that objects of opposite charge tend to attract each other, while particles of the same charge tend to repel. Coulomb's law of electrostatics, which accounted for these phenomena, was very similar to Newton's law of gravitation. Michael Faraday (through no fault of his own, Margaret Thatcher's favourite scientist) had done marvellous experimental work which showed that electricity and magnetism were related in some way. Moving charges generate magnetism, which in the early history of physics was thought to be a different kind of phenomenon altogether. James Clerk Maxwell was the first to elucidate the character of these interactions - now known as electromagnetic interactions - in a set of mathematical laws known as Maxwell's equations. These showed further that electricity and magnetism could fluctuate together in waves that travel at the speed of light. This led to the realisation that light was a form of electromagnetic wave, and that other forms of electromagnetic wave would be possible (such as radio waves).

So successful was this programme that physicists at the end of the 19th century were filled with confidence that soon all physical phenomena would surrender to a Newtonian treatment. This confidence was soon to be shattered.

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