In ‘The Elegant Universe", Mr. Greene hints at the fact he is writing a book about the meaning of space and time, the twin brothers that define the physical universe. That’s actually a book very well worth writing, since space and time are entirely strange concepts in physics, radically different than anything else.
For one, the twins are usually constructed a priori and simply entered into equations without discussion. That’s troublesome enough. Then, to make things worse, we know there is something called equivalence of space and time, but if you go and look at the equations, there is a strange commutation factor in time that makes the whole equivalence look frightening.
Finally, the twins behave largely different with respect to human life: while we can move freely in space, we seem to be forced to move in time in one particular direction. And this direction of time is what moves the book. There are several, all unconvincing explanation of the directedness of time. The problem is that none of the laws of physics differentiates strongly between positive and negative time, so that there is really no physical reason why time should flow only in one direction.
After studying the problem for a while, Mr. Greene throws us into the depths of string theory (his favority topic), where he explains that certain properties of certain equations work only in circumstances in which time flows in one direction. In particular, the expansion of space and time at the beginning of the universe might have grossly violated the law of constant energy and thus created a universe that was initially much more ordered than it should have been.
Things move from ordered to unordered states, so that the laws of physics follow those of thermodynamics. In short, the quest of string theory is to come up with unusual properties of string equation that can be used to explain things we do not understand.
Now, when we talked about the beauty of physics, we used to mean that it was marvelous how something that was entirely theoretical would converge with reality if you just gave it a chance. In string theory, no such thing occurs (at least not when these extraordinary explanations are invoked). Instead, the reason something is a particular way seems to derive from a mathematical property of certain equations.
I leave it up to you to debate this. String theory may very well the unifying theory of the universe, but whoever comes up with an explanation that puts the mathematics behind reality and not in front of it, will have my vote.