Let's resume the discussion of string theory, which I introduced in my August 15, 2006 posting. String theory is considered very controversial. To understand why, it will be useful to review the arguments in its favor and in opposition. Today's posting will cover the former, with the latter to come later. Much of my discussion of string theory will come from the book The Fabric of the Cosmos by Brian Greene, a well-known proponent of strings, but someone who seems open to the possibility that the whole idea could be wrong.
To many, the most compelling argument on behalf of string theory is its potential to unify two heretofore incompatible major ideas in physics -- general relativity and quantum mechanics -- in the spirit of Einstein's quest for a "theory of everything." (The original question motivating the development of string theory did not involve such unification, however.)
A few excerpts from Greene summarize the current state of affairs.
Years of astronomical observations have shown that general relativity describes the macro world of stars, galaxies, and even the entire expanse of the cosmos with impressive accuracy; decades of experiments have confirmed that quantum mechanics does the same for the micro world of molecules, atoms, and subatomic particles (p. 336).
...the incompatibility between general relativity and quantum mechanics rears its head in a very specific way. If you use the combined equations of general relativity and quantum mechanics, they almost always yield one answer: infinity. And that's a problem. It's nonsense. Experimenters never measure an infinite amount of anything... (p. 335).
Since [general relativity and quantum mechanics] works wonders in its own domain, why worry about combining them?... For physicists, though, the existence of a realm in which the known laws of physics break down -- no matter how esoteric the realm might seem -- throws up red flags. If the known laws of physics break down under any circumstances, it is a clear signal that we have not reached the deepest possible understanding... (pp. 336-337).
Enter string theory, which at the conceptual stage, makes the two theories compatible. Greene provides a detailed explanation of why this is so, on pp. 348-350, having to do with strings' putting a limit on how small entities can get, thus "avoid[ing] the catastrophic clash between quantum mechanics and general relativity."
Also, because general relativity is a theory of gravity, string theory integrates gravity with the other forces (electromagnetic, strong, and weak), which are all quantum theories within the Standard Model of particle physics.
String theory appears to look good, thus far. Its difficulties will be discussed in future postings.
[For additional background material, I wrote about general relativity on June 4, 2005, and about quantum mechanics in a series of entries from July 27-September 22, 2005. See the archives in the right-hand column of this page.]