Why I am REALLY disappointed about string theory

mitchell porter

Tom, I am not an expert from the string community. I am a self-taught outsider and there are big gaps in my knowledge. Perhaps you can think of my level of knowledge as like a PhD student during the literature review stage of their thesis.

Marcus, despite how I phrased it, GUT phenomenology can't be restricted to field theory only. Field theory by itself is still a big source of ideas, but working within a class of string phenomenological models also offers guidance.

The standard way to go beyond the standard model is GUT, supersymmetry, string theory, and the world may in fact work exactly like that. My opinions lie in dynamic tension between this much-explored orthodox framework, and a few heterodox ideas (Alejandro Rivero on supersymmetry, Marni Sheppeard on M theory). A lot of the fuel for my own studies comes from the desire to be able to resolve that tension, and to actually know who's right.

tom.stoer

Perhaps we can find somebody else?

Haelfix

Tom, the answer to many of your questions is 'No one knows'. Perhaps it would be best if you focused on one question, formulated into a sufficiently precise statement (by doing a few minutes of self research to get the basics) and asked them one at a time.

For instance, how one gets particle generations out of string theory is somewhat involved, but often quite beautiful. There can be a very precise answers, but as usual depend on which type of vacua you live in. As an example, in heterotic string theory, the number of particle generations (minus the number of anti generations) is a topological invariant.

So suppose you are given the exact metric, and look for smooth continous changes of the geometry (induced by say quantum effects). The massless spectrum might change as the generations and anti generations pair up to become massive or vice versa, however the actual total number is always fixed, implying that the net number of generations in some corner of the configuration space is stable.

You might get a different mechanism elsewhere. But what is weird is that the *structure* and many if not most qualitative features of the physical world seems to fall out of the mathematics, even in unrealistic models. I mean why should a theory of strings, know anything about particle generations?

smoit

Just to add to the above, family replication is just one manifestation of a topological complexity which leaves its imprint in the particle physics spectrum. This is part of a very generic trend, namely the multiplicity of particles with similar or identical properties in a given string compactification. The other examples are: multiple geometric moduli, multiple axions, multiple non-Abelian hidden sectors, multiple U(1)s. This multiplicity looks rather mysterious within a strictly 4-dimensional theory but occurs generically in string theory. Particularly interesting and potentially testable is the idea of multiple species of string-theoretic axions, so-called "String Axiverse", where the QCD axion is just of of the many http://arxiv.org/abs/1004.3558, http://arxiv.org/abs/0905.4720.

atyy

Bosenova

tom.stoer

In-depth clarification of individual details was not my intention for this thread. I know some of these results, either from papers or from discussions here in this forum, sometimes with active string theorists. And if I want to understand more regarding one specific problem I will ask this question in an own thread.

My intention was different: I started this thread with a rather broad and generic criticism regarding the string theory research program as a whole. A reply should be equally broad and generic leaving out many interesting details, but it should address my generic criticism and of course summarize gain knowledge achieved during this discussion. Just as an example: it is not relevant that heterotic string theory has topological invariants which can be identified with fermion generations; that's interesting mathematically but irrelevant physically as long as we do not know "why heterotic string theory instead of something else" and as long as we have no good reason why the results should be three. But the find that "string theory" is not like QCD but more like "gauge theory", i.e. a framework instead of one individual theory, combined with the idea that string theory harmonizes different theories (like gauge theories, SUSY-GUTs and SUGRAs) into one single theory with different vacua is a very deep result and profound knowledge.

So my idea was to let one expert answer the generic questions I started with and to summarize the main achievements, both of the theory and of course of our common understanding developed together as part of this discusson. This seems to be fair.

If the answer to some question is "we don't know yet", then this answer should be part of the reply, why not?