Joe Lykken on “Some good/bad news about string theory”

Joe Lykken, “String Theory for Physicists,” XXXIII SLAC Summer Institute, 2005, Lecture 1 [PDF]; Lecture 2 [PDF]; and Lecture 3 [PDF].

“Some good/bad news about string theory:

Good: String theory is a consistent theory of quantum gravity. Bad: It’s really a generator of an infinite number of mostly disconnected theories of quantum gravity, each around a different ground state. No background independent truely off-shell formulation of string theory is known (yet).

Good: String theory is unique, i.e. there is only one distinct consistent theory of “fundamental” strings. Bad: It has an infinite number of continously connected ground states plus a google of discrete ones. There appears to be no vacuum selection principle, other than the stability of supersymmetric vacua, which gives the wrong answer.

Good: String theory gives you chiral gauge theories, with big gauge groups, for free and complicated flavor structure at low energies is mapped into the geometry of extra dimensions. Bad: Doesn’t like to give the standard model as the low energy theory. A “typical” string compactification is either much simpler (with more SUSY and bigger gauge groups) or much more complicated (lot’s of extra exotic matter, extra U(1) gauge groups, etc.).

Good: String theory predicts supersymmetry and extra dimensions of space. Bad: It’s happy to hide the both up at the Planck scale.

Good: No length or energy scales are put in by hand; all scales should be determined diynamically. Bad: Appears to be too many (hundreds!) scalar fields (moduli) with too much SUSY to get determined dynamically; may be forced to appeal to cosmic initial conditions (the Landscape).

Good: String theory gives a microphysical description of (at least some) black holes, resolves their singularities. Bad: Doesn’t seen to resolve the singularity of the Big Bang (good for inflation, though).

Good: Lots of powerful dualities including weak ↔ strong coupling dualities and short ↔ long distance dualities. Bad: Can’t tell what are the “fundamental” degrees of freedom. String theory not necessarily a theory of strings.

Good: Unification of all the forces is almost for free, may need an (interesting) extra dimensional assist. Bad: In our most realistic string constructions so far, SU(3)C, SU(2)W, and U(1)Y have essentially nothing to do with each other: related to different features of complicated D-brane setups.

Good: AdS/CFT duality shows that 10-dimensional string theory in a certain background is equivalent to a 4-dimensional gauge theory!! Use this e.g. to show that RHIC QCD physics maps onto quantum gravity/black holes. Bad: Adds more confusion: can’t tell an extra dimension apart from technicolor.

Good: We are starting to use string theory to learn tricks for perturbative QCD, understanding the QCD string, etc. Bad: The QCD community was already doing fine, thank you.

That’s all folks!”

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