The uncertain Universe - An introduction

5 The uncertain Universe

An introduction to the uncertain Universe

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Planck found that, in order to account for the observed pattern of emission from hot bodies, he had to assume that energy was transferred from the heated surface to the emitted radiation in a 'grainy' way. Corresponding to each particular colour of light there was a minimum amount of energy - a quantum of energy - that could be carried away from the surface by the light. The size of this quantum of energy depended on the colour of the light; an energy quantum of violet light was almost twice as energetic as an energy quantum of red light, and every other colour had its own charac-teristic quantum. Planck was able to write down a law that related the quantum of energy corresponding to any particular colour to the physical property (frequency) which determined that colour. In doing so he introduced a new fundamental constant of Nature - now called Planck's constant (h = 6.626 * 10^-34 1 joule seconds). The appearance of Planck's constant in a calculation can be taken as a clear indication that quantum physics is involved.
figure 1.29, the relative intensity of each colour in the light emitted by ideal emitters at different temperatures
^{Figure 1.29 Graphs showing the relative intensity of each colour in the light emitted by ideal emitters at different temperatures.}^{Click here for larger image (9.90kb)}
Planck's law was used with great success over the following quarter of a century, in a variety of contexts. Einstein used it in his 1905 paper explaining the photoelectric effect, and so did the Danish physicist Niels Bohr (1885-1962), in 1913, when he formulated a theory of the inner workings of the atom that achieved some remarkable successes in spite of a number of unsatisfactory features. It showed up again in 1924 in the doctoral thesis of Louis de Broglie (1892-1987), who suggested that entities which are normally thought of as particles, such as electrons, actually have a wave-like aspect to their behaviour. Einstein, Bohr and de Broglie all received Nobel Prizes in recognition of their work.

These early developments were strikingly out of step with conventional classical physics. They might even be described as revolutionary, but the real revolution was still to come.

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5.1 Quantum mechanics and chance