**5 The uncertain Universe**5.2 Quantum fields and unification

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From its inception, quantum physics was concerned not just with particles such as electrons, but also with light and other forms of electromagnetic radiation. In 1900 Planck discovered the quantum in the transfer of energy from matter to radiation, and in 1905, Einstein's explanation of the photoelectric effect assumed that the transfer of energy from radiation to matter occurred in a similarly quantized fashion. It is therefore hardly surprising that the development of quantum mechanics was soon followed by an attempt to formulate a quantum theory of electromagnetic radiation. That meant, of course, combining quantum ideas such as Planck's constant and intrinsic probabilities with the field theory of electromagnetism. The result would be a **quantum field theory**.

The quantum field theory of electromagnetism is called **quantum electrodynamics**, or QED for short. Its formulation proved to be very difficult. The first steps were taken by the British physicist Paul Dirac in 1927, but the theory was not really sorted out until the late 1940s.

| ^{Figure 1.31sPaul Dirac (1902-194)Click here for larger image(13.62kb)Dirac, recognized as the 'discoverer' of antimatterClick here to learn more about Dirac} |

During the lengthy development of QED the following important features of quantum field theory became apparent.**Quantum field theory provides the natural way of combining special relativity and quantum physics.** Quantum mechanics, as originally formulated by Heisenberg and Schrödinger was inconsistent with the principle of relativity. Attempts were made to rectify this problem and significant progress was made by Dirac with his relativistic electron equation. However, despite many successes it became increasingly clear that relativistic quantum mechanics was ultimately self-contradictory and that quantum field theory provided the natural way of producing a relativistic quantum physics.

**Quantum fields may be regarded as collections of particles.**In the case of the quantized electromagnetic field these particles are called **photons**. Each photon of a particular colour carries a characteristic amount of energy: the quantum of energy used by Planck and Einstein. Emission and absorption of radiation corresponds to the creation and destruction of photons and therefore inevitably involves the transfer of complete quanta of energy. (Interestingly, Einstein realized as early as 1905 that the quantized transfer of energy would be explained if radiation actually consisted of particles, but the idea was not well received and he did not press it. Photons only became an accepted part of physics in the 1920s.)

**Quantum field theory can be used to describe all fundamental particles.** Electrons and positrons are normally regarded as examples of fundamental particles of matter. In quantum field theory all such particles are associated with quantum fields in much the same way that photons are associated with the electromagnetic field. The number of particles of a given type reflects the state of excitation of the field, and the particles are said to be 'quanta of excitation' of the field. Thus, although quantum field theory describes particles and the forces between them, it does so entirely in terms of quantum fields.

**Quantum field theory describes processes in which particles are created or destroyed.** When a quantum field becomes more excited, the number of quanta of excitation increases. This occurs because new particle-antiparticle pairs are created from radiation. When a quantum field becomes less excited, the number of quanta of excitation decreases. This is achieved by processes in which particles and antiparticles collide and annihilate one another to produce radiation. (Both of these processes are permitted by Einstein'sE=mc^{2}and were explicitly predicted by Dirac.)

These were all very attractive features of quantum field theory and raised the hope that it might be a truly fundamental theory capable of providing a new world-view. However there were serious problems within quantum electrodynamics that had to be overcome before such hopes stood any chance of being realized. Using QED to predict the value of a physically measurable quantity involved, in practice, working out the contribution made by several different sub-processes. Making sure that these sub-processes were fully identified was a problem in itself, working out their individual contributions was even worse. Even in the simplest cases determining the contributions was difficult, and in the more complicated cases the result was usually a meaningless infinity showing that something was wrong.

Continue on to Quantum fields and unification, part 2 of 3