How it works : Quantum Theory

QUANTUM THEORY Towards the end of the 19th century, scientists were well pleased with the neat, self-consistent picture of physics which they had built up. In particular, the theory of the nature and production of ELECTROMAGNETIC RADIATION was in a highly satisfactory and elegant state, James Clerk MAXWELL having recently shown that the ultimate source of radiation is an accelerated electric charge which emits waves travelling at the speed of light. But problems began to arise when scientists considered the radiation produced by a class of objects called black bodies, which have the property of totally absorbing all radiation they receive. Several experimenters had measured the radiation emitted from black bodies, and there were a number of attempts to fit a mathematical form to the way in which it was distributed among different wavelengths. None of these attempts was completely successful, and it was rather disturbing to many physicists that when Lord Rayleigh used rigorous classical physics in his efforts to find a solution, he failed as badly as the rest. The quantum In 1900, the young German physicist Max PLANCK provided the answer, but only after he had been forced to conclude that the classical approach was not valid in this case. He daringly proposed that energy is radiated not as a continuous flow, but in 'bundles' which he called quanta (singular, quantum). The energy carried by each quantum is proportional to the frequency of the radiation, so that Energy = Frequency x Constant The constant, written h, became known as Planck's constant. Planck assumed that his quanta of radiation spread out like waves on the surface of a pond after leaving their source, allowing the wavelike behaviour of radiation to he explained. The photoelectric effect Experiments soon confirmed the quantized nature of radiation, especially light. Hallwachs had noted that some metals lost negative charge, later found to be ELECTRONS, when exposed to ultra-violet light, which has a higher frequency (and therefore higher energy) than visible light. As the intensity of the light increased, more electrons left the metal, but it was observed that there existed a certain limiting 'threshold frequency' of the incident light, below which no electrons were emitted. These facts were very difficult to account for on the classical wave theory of light, but in 1905, EINSTEIN, in his first fundamental piece of work, was able to explain them in terms of the newly-developed quantum theory. He extended Planck's ideas by proposing that quanta of radiation (called photons in the case of light quanta) do not spread out like waves after leaving the source, but maintain their character as discrete bundles of energy. Thus, the high energy photons of ultra-violet light are able to eject electrons from the surface of a metal by bombardment. An increase in the light intensity simply means that a large number of photons hit the surface and consequently eject more electrons, but these photons must be sufficiently energetic. If the frequency of the light falls below the threshold frequency, the photons will not have enough energy to eject any electrons. The Bohr atom The new concept of quantized radiation paved a way to the interpretation of the dark lines observed in the spectrum of the Sun. These lines result from the presence of certain elements, particularly hydrogen and iron, in the Sun's atmosphere, and although they had been discovered and mapped more than a century before by Fraunhofer and others, there was no ready explanation of them. Niels BOHR, a young Danish physicist working at Cambridge, combined the new model of the atom proposed in 1911 by RUTHERFORD with the Planck-Einstein quantum theory, and in 1913 was able to account satisfactorily for the spectral lines of the hydrogen atom. Bohr's model retained the massive, positively charged nucleus and diffuse cloud of electrons of Rutherford's theory; its novelty lay in his non-classical assumptions. Firstly, Bohr proposed that the electrons travel in orbits around the NUCLEUS, but he suggested that only certain orbits were possible, each having a specific amount of energy associated with it. These are called energy states or energy levels. He secondly maintained that no energy is radiated or absorbed by an electron unless it 'jumps' between two of these energy levels. The energy of the radiation then emitted (or absorbed) in the form of a photon is equal to the energy difference between the two levels. In this way, Bohr avoided a problem of classical physics; that is, a normally orbiting electron should emit radiation by Maxwell's laws, lose energy and spiral into the nucleus. Bohr was then able to explain spectral lines as being due to radiation emitted or absorbed when an electron jumps between two energy levels. Far left: a pulse of laser light, photographed in a hundred thousand millionth of a second as it passes through a bottle of water from right to left. A photon of light can be thought of as similar to, but much smaller than, such a pulse. Above right: the Balmer series of spectrum lines produced by excited hydrogen. These are seen in the visible region of the spectrum: the primary lines, the Lyman series, are in the ultra-violet. When an electron jumps from the third orbital back to the second it emits red light; when it jumps from the fourth back to the second, the blue-green (higher energy) light is emitted, and so on. This series has a limit in the ultra-violet. This is a negative print. Bohr's theory was refined by Sommerfeld to allow for elliptical electron orbits, but even then it was difficult to apply it to atoms which were more complicated than hydrogen. But Bohr had paved the way for the far-reaching conceptual advances which were to put the quantum theory on a firm mathematical basis ten vears later. Quantum mechanics In 1924, Prince Louis deBroglie, working from the postulates of quantum theory, proposed that matter must also possess wavelike properties, and that Bohr's model of the atom could be more easily explained by considering the electron as a wave. The following year, Schrödinger, in Zürich, developed this concept into the mathematical form of wave mechanics in which he demonstrated that quantization of energy occurs naturally, and without recourse to artificial assumptions. Simultaneously Heisenberg, in Göttingen, introduced the technique of quantum mechanics, having assumed that energy is quantized, and showed that the wave nature of matter followed as a natural consequence. It was a triumph for the theory of wave-particle equivalence that these two scientists, working independently, and from two opposite approaches, were able to produce complementary solutions. Heisenherg's work led to the formulation of his 'uncertainty principle' in 1926, in which he demonstrated that the wave-particle nature of matter makes it impossible to measure simultaneously and absolutely accurately both the position and momentum of a body. This restriction on measurement is fundamental, and is of philosophical as well as scientific importance. The effects of the uncertainty principle are observable only on a submicroscopic scale, as in the case of an electron, for example, whose momentum and position can never be simultaneously known. A related phenomenon, the tunnel effect, allows electrons to penetrate barriers up to 100 atoms thick which, on classical theories, would be expected to stop them. This property has been exploited in making tunnel DIODES for ultra-fast switching purposes. Above: de Broglie's notion, based on the quantum theory, that matter particles ought to behave like waves was proved by reflecting an electron beam off a nickel crystal. The beam diffracted into zones because of its wave-like properties. Above: one result of the quantum theory is that a strong electric field should result in an increase in the number of energy levels between which electrons can jump. This is clearly shown here in the splitting of spectral lines, known as the Stark effect. The quantum mechanics of Schrödinger and Heisenberg was extended in the form of quantum electrodynamics by P A M Dirac, whose work in the late 1920s forms the basis of present day research. Among other successes, Dirac predicted the existence of a particle which had the same characteristics as the electron, but a positive charge. This particle, the positron, was discovered in 1932. Many physicists nowadays believe that the four fundamental forces of nature (see FIELD AND FORCES) owe their origin to quantum interactions. A force is thought to result when an 'exchange particle' is emitted by one subatomic particle and absorbed by another. In the ease of the electromagnetic force, virtual photons are exchanged, while pi mesons give rise to the strong nuclear force. Gravity is thought to originate from interactions involving gravitons, although these have not yet been detected. There is a growing body of experimental evidence to support the existence of the intermediate vector boson, the particle responsible for the weak nuclear force.

Reproduced from HOW IT WORKS p1895