[ Pobierz całość w formacie PDF ]
.This title is rather an exaggeration: the resultant theoriesare not all that grand, nor are they fully unified, as they do not include gravity.Nor are they really complete theories,because they contain a number of parameters whose values cannot be predicted from the theory but have to be chosento fit in with experiment.Nevertheless, they may be a step toward a complete, fully unified theory.The basic idea ofGUTs is as follows: as was mentioned above, the strong nuclear force gets weaker at high energies.On the other hand,the electromagnetic and weak forces, which are not asymptotically free, get stronger at high energies.At some very highenergy, called the grand unification energy, these three forces would all have the same strength and so could just bedifferent aspects of a single force.The GUTs also predict that at this energy the different spin-½ matter particles, likequarks and electrons, would also all be essentially the same, thus achieving another unification.The value of the grand unification energy is not very well known, but it would probably have to be at least a thousandmillion million GeV.The present generation of particle accelerators can collide particles at energies of about one hundredGeV, and machines are planned that would raise this to a few thousand GeV.But a machine that was powerful enough toaccelerate particles to the grand unification energy would have to be as big as the Solar System and would be unlikelyto be funded in the present economic climate.Thus it is impossible to test grand unified theories directly in the laboratory.However, just as in the case of the electromagnetic and weak unified theory, there are low-energy consequences of thetheory that can be tested.The most interesting of these is the prediction that protons, which make up much of the mass of ordinary matter, canspontaneously decay into lighter particles such as antielectrons.The reason this is possible is that at the grandunification energy there is no essential difference between a quark and an antielectron.The three quarks inside a protonnormally do not have enough energy to change into antielectrons, but very occasionally one of them may acquirefile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/d.html (6 of 8) [2/20/2001 3:14:54 AM] A Brief History of Time - Stephen Hawking.Chapter 5sufficient energy to make the transition because the uncertainty principle means that the energy of the quarks inside theproton cannot be fixed exactly.The proton would then decay.The probability of a quark gaining sufficient energy is solow that one is likely to have to wait at least a million million million million million years (1 followed by thirty zeros).This ismuch longer than the time since the big bang, which is a mere ten thousand million years or so (1 followed by ten zeros).Thus one might think that the possibility of spontaneous proton decay could not be tested experimentally.However, onecan increase one s chances of detecting a decay by observing a large amount of matter containing a very large numberof protons.(If, for example, one observed a number of protons equal to 1 followed by thirty-one zeros for a period of oneyear, one would expect, according to the simplest GUT, to observe more than one proton decay.)A number of such experiments have been carried out, but none have yielded definite evidence of proton or neutrondecay.One experiment used eight thousand tons of water and was performed in the Morton Salt Mine in Ohio (to avoidother events taking place, caused by cosmic rays, that might be confused with proton decay).Since no spontaneousproton decay had been observed during the experiment, one can calculate that the probable life of the proton must begreater than ten million million million million million years (1 with thirty-one zeros).This is longer than the lifetimepredicted by the simplest grand unified theory, but there are more elaborate theories in which the predicted lifetimes arelonger.Still more sensitive experiments involving even larger quantities of matter will be needed to test them.Even though it is very difficult to observe spontaneous proton decay, it may be that our very existence is a consequenceof the reverse process, the production of protons, or more simply, of quarks, from an initial situation in which there wereno more quarks than antiquarks, which is the most natural way to imagine the universe starting out.Matter on the earth ismade up mainly of protons and neutrons, which in turn are made up of quarks.There are no antiprotons or antineutrons,made up from antiquarks, except for a few that physicists produce in large particle accelerators.We have evidence fromcosmic rays that the same is true for all the matter in our galaxy: there are no antiprotons or antineutrons apart from asmall number that are produced as particle/ antiparticle pairs in high-energy collisions
[ Pobierz całość w formacie PDF ]