In the apartments of Pope Julius II in the Vatican, Raphael's fresco * The School of Athens * is a brilliant con-vocation of the sages and philosophers who were the founders of rational inquiry into the nature of the physical world. The throng includes Plato and his pupil Aristotle, Pythagoras, Euclid and Ptolemy of Alexandria. In this decoration for the philosophy section of Julius II's library, Raphael pictures truth acquired through reason. In doing so he assembles the great minds who began the quest to discover the fundamental laws governing the movements of heavenly bodies. The history of physics begins thus.

Systematic astrology, supported by careful observation, began in the 3rd millennium BC. During the Greek miracle, the savants dismissed the magical and mythical universes of Egypt and Mesopotamia, replacing them with geometrical models that explained the mechanics of the heavens. The heliocentric universe of Copernicus marks the beginning of modern physics because he believed it to be a true representation of the cosmos rather than a model. Galileo, Kepler and, finally, Isaac Newton refined the laws of motion to the point where orbits in the solar system could be defined with precision. The step from Copernicus to Isaac Newton marks the first great revolution in the history of physics.

In the two centuries after Isaac Newton, Roger Newton, the author of * A History of Physics * , identifies two dominant characteristics. Mathematicians developed the calculus into a powerful tool for investigating mechanics, and they developed a taste for expressing physical theory in rigorous mathematical language. Simultaneously, practitioners emphasised careful observation. A new view of the structure and mechanics of the universe was in place by the early 19th century. Some physicists then began to look inwards and probe the properties of matter. John Dalton provided the evidence that the variety of the material world arises from different arrangements of fundamental building blocks, atoms. Michael Faraday described electromagnetic phenomena in terms of fields without using a single equation - he left that to James Clerk Maxwell, whose theory of the electromagnetic field included light as one of its manifestations.

At the dawn of the 20th century, classical physics had reached the state in which it appeared to give a complete description of matter and of the universe. Albert Michelson and Lord Kelvin announced the death of discovery in physics with precise measurement of the known phenomena, the only task remaining. However, Maxwell's equations appeared to require a universe filled with ether, a concept then as mysterious as dark energy appears to us today.

Einstein's theory of relativity removed the ether puzzle. In the general theory of relativity, he showed how the laws of physics must operate in an accelerating frame of reference. In his application of this theory to the cosmos, he assumed a static closed universe, in accordance with the prevailing view. To bring his theory in line with presumed fact he introduced a fudge factor, the cosmological constant, which is now seen as central to modern consensus cosmology.

Newton tells the story of the quantum revolution with remarkable clarity.

Quantum theory destroyed the notion of determinism, a central plank of classical physics. It also provided the tools to understand the inner world of the atomic nucleus and to discover the link between particle physics and cosmology.

Newton's account is superb. He is magnificent at explaining the profound influence of mathematics on the development of physics. The historical relationships between subdisciplines, such as thermodynamics and statistical physics, are illuminated. Numerous biographical sketches add a lively dynamic to an enjoyable book.

Simon Mitton is a fellow of St Edmund's College, Cambridge University.

## A History of Physics

Author - Roger G. Newton

Publisher - Harvard University Press

Pages - 352

Price - £19.90

ISBN - 0 674 02337 4

## Have your say