http://www-conf.slac.stanford.edu/ssi/2005/lec_notes/Lykken1/default.htm(adsbygoogle = window.adsbygoogle || []).push({});

Every year Stanford SLAC has a summer institute. This summer Fermilab Joe Lykken is giving a series of talks called "String Theory for physicists". His first 7 slides summarize what he thinks are the stringy goods and bads.

In case anyone might want to discuss, question them, clarify, I will transcribe the list. The slides are handwritten--so in case the list is interesting to anyone here at PF, it may be useful to have it copied out. The rest of this post is a transcription of Lykken's list: "good/bad news about string theory"

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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).

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Good: String theory is unique. i.e. there is only one distinct consistent theory of "fundamental" strings

Bad: It has an infinite number of continuously 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.

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Good: String theory gives you chiral gauge theories, with big gauge groups, for free + 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 (lots of extra exotic matter extra U(1) gauge groups etc)

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Good: String theory predicts supersymmetry and extra dimensions of space

Bad: It's happy to hide them both up at the Planck scale

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Good: No length or energy scales are put in by hand; all scales should be determined dynamically

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)

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Good: String theory gives a microphysical description of (at least some) black holes, resolves their singularities

Bad: Doesn't seem to resolve the singularity of the Big Bang

(good for inflation, though)

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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

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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

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Good: AdS/CFT duality shows that 10 dim. String theory in a certain background is equivalent to a 4 dim. gauge theory!!

Use this e.g. to show that RHIC QCD physics maps onto quantum gravity/black holes.

Bad: Add 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 strings, etc.

Bad: The QCD community was already doing fine, thank you.

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# Joe Lykken's List of Stringy Goods and Bads

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