Slide 22 of 24
The only currently viable theory that unifies Einstein's theory of gravity with the theory of the microscopic world (quantum mechanics) is string theory. In this theory there is no microscopic matter as we know it, nor is there any gravity. Instead, the Universe and all its diversity stems from a single, new fundamental object: a string. Strings are tiny, circular, one dimensional shapes, exactly like elastic bands, but with no width. All strings are identical. So why do we see so many different particles at the microscopic level, such as electrons and protons and neutrons, instead of identical strings? According to string theory, the different particles are simply the outward manifestations of the many different ways the strings can vibrate. We know that guitar strings have many vibrational modes that produce different sounds.
Well, these strings also have different vibrational modes, and each mode looks to us like one of the observed elementary particles. In this model gravity too is accounted for by one of the infinite vibrational modes of the string.
This theory does resolve in principle the problem connected with black hole evaporation: it tells us that once the black hole evaporates down to the size of one of these strings, the laws of gravity and quantum mechanics as we know them are no longer going to apply. In fact, even our notions of space and time will need to be revised. Once this happens, there are no rules to tell us what can or cannot come out of the event horizon, since the concept of an event horizon belongs to ordinary gravity theory, which no long applies. Although the details of what happens are far from worked out, string theory does seem to solve the black hole evaporation problem. The discord has been replaced by Harmony, this time in a very literal sense, by the Harmony of the Strings.