At New York Review of Books:
http://www.nybooks.com/articles/archives/2013/nov/07/physics-what-we-do-and-dont-know/
Since the 1980s a tremendous amount of mathematically sophisticated work has been devoted to the development of a quantum theory whose fundamental ingredients are not particles or fields but tiny strings, whose various modes of vibration we observe as the various kinds of elementary particle. One of these modes corresponds to the graviton, the quantum of the gravitational field. String theory if true would not invalidate field theories like the standard model or general relativity; they would just be demoted to “effective field theories,” approximations valid at the scales of distance and energy that we have been able to explore.
String theory is attractive because it incorporates gravitation, it contains no infinities, and its structure is tightly constrained by conditions of mathematical consistency, so apparently there is just one string theory. Unfortunately, although we do not yet know the exact underlying equations of string theory, there are reasons to believe that whatever these equations are, they have a vast number of solutions. I have been a fan of string theory, but it is disappointing that no one so far has succeeded in finding a solution that corresponds to the world we observe.
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Inflation is naturally chaotic. Bubbles form in the expanding universe, each developing into a big or small bang, perhaps each with different values for what we usually call the constants of nature. The inhabitants (if any) of one bubble cannot observe other bubbles, so to them their bubble appears as the whole universe. The whole assembly of all these universes has come to be called the “multiverse.”
These bubbles may realize all the different solutions of the equations of string theory. If this is true, then the hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our big bang is doomed, for their values would be an accident of the particular part of the multiverse in which we live. We would have to content ourselves with a crude anthropic explanation for some aspects of the universe we see: any beings like ourselves that are capable of studying the universe must be in a part of the universe in which the constants of nature allow the evolution of life and intelligence. Man may indeed be the measure of all things, though not quite in the sense intended by Protagoras.
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Such crude anthropic explanations are not what we have hoped for in physics, but they may have to content us. Physical science has historically progressed not only by finding precise explanations of natural phenomena, but also by discovering what sorts of things can be precisely explained. These may be fewer than we had thought.
Another productive day in the world of materialist cosmology.