On the fundamental level we have now identified four forces: the Standard Model describes three of them and the fourth – gravity – is described by General Relativity. The Standard Model is a quantum theory, while General Relativity is not. It is not surprising that with these successful steps towards unification in the description of particles and forces, the hope and expectation are that we are heading towards an all-encompassing theory of nature, including a quantum theory of gravity. This is considered the Holy Grail of fundamental physics.

At the moment the most ambitious attempt to formulate such a theory is string theory, in which the fundamental constituents of matter and all forces between them are manifestations of an underlying dynamics of strings. These strings are so tiny that there is little hope they will ever be observed directly. Credibility of the theory has to be achieved by indirect means. Predictions (or outcomes) of the theory are, that all elementary particles have supersymmetric partners, and that the world on a fundamental level would have ten, possibly eleven dimensions. The scale at which these features manifest themselves is still open to debate.

The quest for a unified field theory dates back to Einstein, who spent the later years of his life on this problem. Over the years many attempts have been made, with a varying degree of success. Examples are the highly symmetric Grand Unified Theories, which try to unify strong, weak and electromagnetic forces in a single model. These theories explain for example the simple experimental fact – so far never accounted for by theory – that the electron and proton have exactly the same but opposite electric charge. Another remarkable generic prediction is that the proton will decay (though extremely rarely) into lighter particles, implying that ultimately all matter would be unstable. Fortunately, the lifetime of the proton is more than 1032 years. Nevertheless, these theories would no doubt gain great respectability once such a decay would be observed.

Some people have started from the gravity side. It turned out that the methods of quantum field theory, which yielded a successful quantum description for the other interactions, failed miserably when applied to general relativity.