An Introduction to String Theory

Slide 19 of 37
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Even without the difficulty of quantum gravity, particle physicists have believed for many years that the Standard Model is probably not fundamental. The concern about its many arbitrary parameters is largely an esthetic one. For that reason, it may carry less true scientific weight, but it is still worth thinking about (especially as the values measured for some of them are very odd).

But it is the unexplained patterns in the model that really seem to point to some deeper theory. For example, the fundamental quarks and leptons of the Standard Model are arranged neatly into three nearly identical "generations", which differ mainly in their increasing mass. Even the fact that the quarks and leptons match up into these generations is an accident. But if it weren't for the details of that accident, it turns out that the Standard Model QFT would be subtly inconsistent. To many theorists, that is too much of a coincidence to believe. And that is just one of many deep patterns in nature that the Standard Model leaves entirely unexplained.