• Choose a category
• All
• Classic-Fiction

By Root 294 0

certain other particles of spin 3/2, 1, ½, and 0. In a sense, all these particles could then be regarded as different aspects of the same “superparticle,” thus unifying the matter particles with spin ½ and 3/2 with the force-carrying particles of spin 0, 1, and 2. The virtual particle/antiparticle pairs of spin ½ and 3/2 would have negative energy, and so would tend to cancel out the positive energy of the spin 2, 1, and 0 virtual pairs. This would cause many of the possible infinities to cancel out, but it was suspected that some infinities might still remain. However, the calculations required to find out whether or not there were any infinities left uncanceled were so long and difficult that no one was prepared to undertake them. Even with a computer it was reckoned it would take at least four years, and the chances were very high that one would make at least one mistake, probably more. So one would know one had the right answer only if someone else repeated the calculation and got the same answer, and that did not seem very likely!

Despite these problems, and the fact that the particles in the super-gravity theories did not seem to match the observed particles, most scientists believed that supergravity was probably the right answer to the problem of the unification of physics. It seemed the best way of unifying gravity with the other forces. However, in 1984 there was a remarkable change of opinion in favor of what are called string theories. In these theories the basic objects are not particles, which occupy a single point of space, but things that have a length but no other dimension, like an infinitely thin piece of string. These strings may have ends (the so-called open strings) or they may be joined up with themselves in closed loops (closed strings) (Fig. 11.1 and Fig. 11.2). A particle occupies one point of space at each instant of time. Thus its history can be represented by a line in space-time (the “world-line”). A string, on the other hand, occupies a line in space at each moment of time. So its history in space-time is a two-dimensional surface called the world-sheet. (Any point on such a world-sheet can be described by two numbers, one specifying the time and the other the position of the point on the string.) The world-sheet of an open string is a strip: its edges represent the paths through space-time of the ends of the string (Fig. 11.1). The world-sheet of a closed string is a cylinder or tube (Fig. 11.2): a slice through the tube is a circle, which represents the position of the string at one particular time.

Two pieces of string can join together to form a single string; in the case of open strings they simply join at the ends (Fig. 11.3), while in the case of closed strings it is like the two legs joining on a pair of trousers (Fig. 11.4). Similarly, a single piece of string can divide into two strings. In string theories, what were previously thought of as particles are now pictured as waves traveling down the string, like waves on a vibrating kite string. The emission or absorption of one particle by another corresponds to the dividing or joining together of strings. For example, the gravitational force of the sun on the earth was pictured in particle theories as being caused by the emission of a graviton by a particle in the sun and its absorption by a particle in the earth (Fig. 11.5). In string theory, this process corresponds to an H-shaped tube or pipe (Fig. 11.6) (string theory is rather like plumbing, in a way). The two vertical sides of the H correspond to the particles in the sun and the earth, and the horizontal crossbar corresponds to the graviton that travels between them.

String theory has a curious history. It was originally invented in the late 1960s in an attempt to find a theory to describe the strong force. The idea was that particles like the proton and the neutron could be regarded as waves on a string. The strong forces between the particles would correspond to pieces of string that went between other bits of string, as in a spider’s web. For this theory to give the observed