String theory explained
The first great achievement of string theory, according to the quantum gravity website which I check every day for news, was to give a consistent theory of quantum gravity which resembles GR at macroscopic distances. It doesn’t say what the second great achievement was. Neither does it help to try and get a mental picture of something which might resemble GR. Nothing springs to mind.
Let us examine this issue more closely. First, quantum gravity. This, we are told, is “a field of theoretical physics which tries to unify quantum mechanics with general relativity in a self-consistent manner or, more precisely, to formulate a self-consistent theory which reduces to ordinary quantum mechanics in the limit of weak gravity (potentials much less than c2) and which reduces to general relativity in the limit of large actions”.
It is difficult not to admire the “more precisely” in that description, as if what follows it is more precise than that which went before it. It isn’t. In fact, if we remove the meaningless words from that definition, we are left with “a field”, which is self-explanatory. The key word in the definition is ‘theoretical’ which, according to the dictionary, means “concerned with knowledge but not with its practical application” or, more precisely, nonsense.
Apparently, “a great hope was created that string theory would be able to unify all the known forces and particles together into a single theory of everything”. This didn’t materialise, however. What rotten luck.
We are told that the average size of a string should be somewhere near Planck length, which is about a millionth of a billionth of a billionth of a billionth of a centimetre. Hardly worth worrying about really.