A forbidding prospect

The essential uncertainty of quantum behaviour might create the uneasy suspicion that anything goes in this world. But since the Universe has (on all observed scales of length, time, mass and so forth) a very definite structure, it seems extremely unlikely that everything is allowed. There must be at least some things that are more allowed than others.

As it happens, there are many things that are forbidden (in the sense that they are unlikely in the extreme), and it is the forbidding rules that give structure to the world. We will discuss quite a few of these rules, and it is important to bear in mind that the origin of these is essentially not understood. It is not known why the rules must be as they are; some day we may attain a deeper level of understanding, which will provide insight into the rules that govern the rules, as it were. But we are not at that stage yet, and for the moment all rules that have been discovered seem arbitrary and highly non-obvious. If you were to design your own universe, it might not occur to you to do it the way our world is, and we know of no reason why you should impose all the experimentally observed rules in order to obtain a working universe.