# Green, Michael Boris

### forces theory nuclear force

(1946– ) British theoretical physicist.

Educated in Cambridge and afterwards working in Princeton, Cambridge, Oxford and London, Green became professor of theoretical physics in Cambridge in 1993. He is best known for his work on the superstring theory, a novel approach to the nature of nuclear particles and the forces of nature.

As background to Green’s work, it should be noted that interactions within and between atoms depend upon four field forces. The electromagnetic force can be attractive or repulsive. The strong nuclear force is even stronger at very short distances (10 –15 m) within an atomic nucleus. The weak nuclear force, only 10 –4 as powerful, is also very important. Gravitational force is very much weaker (10 –40 ) and is only attractive: it is of course of great importance on the cosmic scale, as between planets or stars where, although distances are large, the bodies are uncharged and the masses are large.

From the 1970s grand unified theories (‘GUTs’) attempted to embrace the first three forces, with partial success: work by and has linked electromagnetic and weak nuclear forces in one theory. The strong and weak nuclear forces have resisted unification.

The simplest and most elegant of the GUTs requires the proton to decay with a lifetime of 10 30 years, and experiments to detect this have failed to do so. Gravity, also, which is not quantized (unlike the others) has resisted incorporation, so a ‘theory of everything’ (‘TOE’) has proved elusive.

The nearest to success has been superstring theory, devised in the 1980s with Green as the main proponent. This treats the interaction of subnuclear particles not in terms of points, but of one-dimensional curves (superstrings) having mass, and a length only 10 –35 m (ie 10 –20 the proton diameter), in 10-dimensional space-time (nine in space, plus time). The theory deals with all four field forces (ie it includes gravity), involves sophisticated mathematics (including symmetry properties) and has implications for cosmology as well as particle physics. It is consistent with special relativity and quantum theory. Despite its mathematical attractions, wider acceptance by physicists must depend on experimentally testable predictions evolving from it.

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