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Despite the impact of relativity, the greatest source of change in the scientific world-view in the twentieth century has undoubtedly been the development of quantum physics. This is the branch of physics that is mainly concerned with microscopic entities such as atoms and molecules, and their constituents.

by far the most quantitatively accurate part of science, routinely providing predictions that are correct to just a few parts in a million

It is by far the most quantitatively accurate part of science, routinely providing predictions that are correct to just a few parts in a million. Quantum physics is also of enormous technological importance since it provides the scientific underpinning for the modern electronics industry which brings us devices ranging from TV sets and transistor radios to CD players and computers.

So great has been its effect that it is now conventional to divide physics into two parts; quantum physics and classical physics, where, by classical physics, we mean anything that is not quantum physics. To be fair, it should be noted that some authors prefer to define classical physics as consisting of those subjects that were already well-defined by the year 1900, together with their direct developments in the twentieth century. In this way they include mechanics, thermodynamics and electromagnetism, but they exclude special and general relativity. Most physicists, however, would not hesitate to say that general relativity was a classical theory of gravity, and would regard relativity as the culmination of classical physics rather than a step beyond it. In any event, there can be no doubt that the development of quantum physics has demanded a fundamental change in outlook by physicists.

Quantum physics was born in 1900, but it took about twenty five years to reach maturity. During the first quarter of the twentieth century it had a rather rickety feel; there was not really any coherent theory of quantum physics, just assorted quantum ideas that were so successful in solving certain outstanding puzzles that it seemed there had to be something behind it all. The strongest characteristic of quantum physics during this early period was an emphasis on graininess or discreteness.

Indeed, the word quantum actually comes from the Latin for 'unit of quantity' or 'amount' and was introduced into physics by the German scientist Max Planck (1858-1947), in the course of his investigations into the emission of electromagnetic radiation from hot surfaces.
Crudely speaking, Planck was looking into why hot things glow. He knew that the light given off by a heated object is a mixture of all the possible colours of light and he wanted to predict the relative brightness with which each colour would be emitted from an object at a given temperature. It was changes in these relative brightnesses as temperature increased that explained why objects went from being red-hot at fairly low temperatures to white-hot or blue-hot at fairly high temperatures.

Figure 1.28 The changing colour of a heated body. The emitted light is a mixture of colours. As the temperature rises the relative brightness of each of the constituent colours changes. Click here for larger image (11.88kb)

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