Two major revolutions in physics took place at the beginning of twentieth century: the discoveries of quantum theory and general relativity. Both theories are extremely successful in their domains of applicability, and yet they are incompatible. Therefore, a deeper theory which would give quantum theory and general relativity as suitable approximations is needed. But attempts to obtain this deeper theory, called quantum gravity, which we hope would also unify all the fundamental interactions, have so far not been successful, despite the work of many brilliant physicists for more than seven decades.

While there are no conceptual problems in understanding general relativity, this is not true of quantum theory. The real difficulty in understanding and interpreting quantum theory may be the reason why we have not yet obtained the deeper theory. One of the first conceptual problems to arise during the creation of quantum theory was the wave/particle duality of light and matter. For example, when a photon strikes a photographic plate, it creates a localized spot as if it were a particle. Yet the same photon when it is constituent of a light wave has a wave aspect. All other particles, such as the electron, neutron and proton, exhibit this wave/particle duality as well.