The electromagnetic interaction provides light and cohesion

The electromagnetic interaction provides light and cohesion

The electromagnetic interaction is responsible for a number of common phenomena in the world that surrounds us, such as friction, magnetism and the fact that neither we nor objects we lay aside fall through the floor.

The electromagnetic interaction that binds an electron and a proton in a hydrogen atom is the inconceivably large number of 10⁴¹ times stronger than gravity. Yet, in spite of the very large difference in strength between the two interactions there are several similarities. The interaction strength decreases with the square of the distance and has a long range. Both the electromagnetic interaction and the gravitational interaction are mediated by force carriers, the graviton and the photon (the light particle). In contrast to the photon, the graviton still hasn’t been found. Their long range can be shown to be due to the fact that they have no rest mass. The photons from the sun are necessary for life on earth. However, when the energy is produced from fusion at the centre of the sun the other two interactions in the Standard Model also play important roles. The photon has an important property; it is electrically neutral but couples with electrical charges. That is why photons do not interact with each other.

The electromagnetic interaction is described by the theory of quantum electrodynamics (QED), one of the most successful theories of physics. It agrees with the results of experiments with a precision that approaches one part in ten million. Sinitiro Tomonaga, Julian Schwinger and Richard Feynman were awarded the Nobel Prize for this in 1965. One of the reasons why it is so successful is that the equation contains a small constant, the so-called fine structure constant or coupling constant, a_em, with the value of 1/137, which is considerably smaller than 1. This makes it possible to calculate electromagnetic effects as a series expansion in the small constant, an elegant mathematical method called perturbation calculation that was much developed by Feynman.

One important property of quantum mechanics in the QED theory is that the fine structure constant could be shown to vary with energy; it increases with increasing energy. At today’s accelerators, for example the CERN LEP accelerator, the value has been measured as 1/128 rather than 1/137 at energies corresponding to approximately 100 billion electronvolts. If the energy dependence for the fine structure constant is depicted in relation to the energy, the curve slopes slightly upwards. Theoretical physicists say that the derivate, or the beta function, is positive.

The weak interaction — radioactive decay

The weak interaction is carried by the bosons, W^± and Z⁰, particles that, unlike the photon and the graviton, have very large masses (approximately 100 proton masses! ). That is why the interaction has a short range. It acts on both quarks and leptons and is responsible for some radioactive decays. It is closely related to the electromagnetic interaction and the two interactions are said to be united in the electroweak interaction, which was elucidated in the 1970s. Gerardus’t Hooft and Martinus Veltman received the 1999 Nobel Prize for the final formulation of this theory.

The strong interaction — charge and colour

It had been known since the 1960s that the proton (and the neutron) are composite and built up of quarks. However, strangely enough, it was not possible to produce free quarks. They are confined, a fundamental property of these building blocks. Only aggregates of quarks, two or three, can exist freely as, for example, the proton. Quarks have electric charges which are a fraction of the proton’s, –1/3 or +2/3, a strange feature which has not yet been explained. Each quark, in addition to an electric charge, also has a special property which, like its electric charge, is quantised, that is, it can only take on certain values. This property is called colour charge, owing to its similarity to the concept of colour.

Quarks can carry the colour charges red, blue or green. For every quark there is an antiquark in the same way as the electron has an antiparticle, the positron. Antiquarks have the colour charges antired, antiblue or antigreen. Aggregates of quarks, which can exist freely, are colour neutral. The three quarks in the proton (u, u and d) have different colour charges so that the total colour charge is white (or neutral). In the same way as electrically neutral molecules can form bonds (through the attraction between their positive and negative parts) the exchange of force between protons and neutrons in the nucleus occurs through the colour forces that leak out from their quarks and force-carrying particles.

The force between quarks is carried by gluons (from the word ‘glue’), which, like photons, lack mass. Gluons, however, in contrast to photons, also have the property of colour charge, consisting of a colour and an anticolour. This property is what makes the colour force so complex and different from the electromagnetic force.