The laws of the world of fundamental particles

The relevance of Einstein's theory of relativity is very obvious in the world of fundamental particles. The relation E = mc2 governs the creation processes of all particles, the new as well as the old. When these new particles subsequently decay into other particles the energy released is governed by the same equation. The relationship is now so well established that units of mass and units of energy are interchangeable. Things move fast in this world; speeds close to the speed of light are the norm. Time dilation is a significant factor for nuclear and sub-nuclear particles in motion.

Einstein was not the only one who could look with satisfaction at phenomena in the sub-nuclear world. Max Planck's 'act of despair' and 'revolutionary hypothesis' was that light energy comes in quantised units. Niels Bohr extended the concept to quantised angular momentum of electron orbits. We now find that quantisation also is the norm rather than the exception in the particle world. Particles themselves have intrinsic angular momentum, which comes in quantised units. Its value is independent of where they are or what they are doing. Finally, the philosophy of quantum mechanics, developed at Bohr's Institute at Copenhagen, rules throughout. Processes are based on probability and not determinism.

The Heisenberg uncertainty principle comes to life in a very real way. When written in the form DED t = h/2p it can be interpreted as saying that Nature allows energy conservation to be violated by an amount AE provided that it takes place over a short enough time interval, At. The principle takes on very real significance in the case of particles which decay via the strong interaction and have very short lifetimes. Such particles, also known as resonances, have a lifespan of the order of 10-22 s, which means that they have an intrinsic uncertainty AE in mass energy. This uncertainty, called the resonance width, can be as much as 20%.

Dirac's prediction of the positron as the antiparticle to the electron extends to all members of the particle zoo. There is a rule which applies very strictly to some classes of particles, in that they cannot be created singly; they must be accompanied by an antiparticle. We cannot add an extra proton to the universe without balancing it with an antiproton, or make a neutron without an anti-neutron. This rule does not apply to mesons, any number of which can be created (provided that the usual conditions apply).


The law of conservation of momentum is strictly observed in the sub-nuclear world. It had been invoked as far back as 1931 by

Wolfgang Pauli (1900-1958), when he was developing his theory of radioactive beta decay. He proposed a 'desperate remedy' to explain the experimental results of apparently missing momentum by postulating that it was carried away by a mysterious particle. He called it the 'little neutral one', or neutrino. The mysterious particle seemed to have neither mass nor charge and did not seem to interact at all. In fact it appeared to have no properties other than the ability to carry energy and momentum. Like the emperor's new clothes, the existence of the neutrino had to be taken on trust. Thirty years later interactions of neutrinos were seen at Los Alamos and subsequently at Brookhaven and CERN. In fact there was evidence for not just one, but two kinds of neutrino, which were labelled ve and v^. Pauli was vindicated.

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