When people first began thinking about quarks, a persistent question was why isolated quarks had not been observed. One idea was that quarks might be the ends of strings. If a particle was a string and the quarks were just the ends of the string, then it could be understood why one never found one quark without another. This idea did not gain much acceptance, but strings soon took on a life of their own.
A string is essentially a one-dimensional object in a space of four dimensions (counting time as a dimension). Physicists in 1970 turned to topology, the mathematics of knots and surfaces, to find what the implications might be of using strings instead of particles in their calculations. Surprisingly, strings simplified calculations. But some string theories had far too many dimensions — 26 in one version.
At about the same time, physicists working with other mathematical theories of particles and forces rediscovered an idea from the 1920s. In 1919 Theodor Kaluza had found a way to derive electromagnetism from Einstein’s theory of general relativity, but Kaluza’s derivation requires five dimensions and we only observe four. In 1926 Oskar Klein developed an explanation of why the fifth dimension is invisible — it curls up into a tiny circle — but very few people were impressed.
In the late 1970s, however, string theorists discovered that their theories became very simple if the universe has eleven dimensions with seven of them curled up in the way Klein suggested. Now physicists tried putting strings into spaces with more dimensions. The complicated mathematics of strings showed that they work best in Kaluza-Klein spaces of nine or ten dimensions, not eleven. Happily several physicists showed that spaces of ten dimensions could be transformed into spaces of eleven dimensions.
In 1974 the discovery that string theory could explain gravity in a way that reconciles with quantum theory caused many physicists to begin to study the new theory. The key realization here was that a string with a spin of 2 has a mass of 0, which is the same combination as that predicted for the graviton. Two years later gravity was reworked in terms of supersymmetry, a general type of theory that predicts that every fermion has a bosonic “superpartner,” and vice versa. The combination of supersymmetry with gravity became known as supergravity. By 1980 physicists also incorporated supersymmetry into string theory, resulting in superstring theory.
But the whole edifice was in trouble. For one thing, none of the effects predicted by superstring theory could be tested at energy levels accessible in even the biggest accelerators. Furthermore, all superstring predictions of effects in the early universe could also be explained by simpler theories. Worse, various mathematical results were clearly impossible. But in 1984, just as in the much older theory of quantum electrodynamics, it was shown that the mathematical anomalies cancelled each other out if handled properly.
By then, five different string theories existed and, while consistent within themselves, they were inconsistent with each other. At this point M theory or brane theory came to the rescue by picturing strings as the intersection of manifolds (M) in elevendimension space (which is an outgrowth of ten-dimension space). These manifolds correspond to membranes (M-branes) in fourspace.
Now all five string theories could be seen as different aspects of the same theory. Furthermore, brane theory showed a striking success in explaining aspects of black holes. So far, however, brane theory cannot be tested experimentally. Also, theoreticians have begun to devise new versions, such as D-brane theory.