One doubt about string theory.

arroy_0205

I read somewhere that one power of string theory is that there is no adjustable dimensionless parameter in the theory. Why is this considered a strong point in favour of string theory?

Second, how does the theory help in solving cosmological constant problem? Again there is no adjustable dimensionless parameter which can be put equal to zero. (The article was written long ago, so the author thinks about putting CC=0 but now we know it has a very small positive value.)

tom.stoer

String theory should provide a way to "derive" the standard model of elementary particle physics with its different particle species and approx. 20 free parameters as a special solution of string theory where the particles and parameters can be "explained" somehow. It's like looking at quantum mechanics in solid state physics: it can explain in principle a huge number of solid states and their parameters like heat capacity, susceptibility, thermal and electrical conductivity etc.

Very interesting question. I always read that string theory has a problem with the sign of the cosmological constant, i.e. that AdS can be understood quite well, but that dS is problematic; I never fully understood why this is the case and why string theory is able to deal with AdS/CFT but fails to explain something like dS/xyz.

suprised

There are plenty of string models that have a positive CC, for example in the "KKLT scenario". And that dS is more problematic than AdS, is not a property of just strings but, I guess, for any theory of quantum gravity.

tom.stoer

OK, thanks for the hint. KKLT is of course well-known.

hm, why? it seems that in LQG and CDT they are rather close to dS; I remember a paper by Witten explaining why dS could be problematic, but it was not very convincing

suprised

This is what I had in mind, Tom.

At any rate, string theory is as consistent a theory of QG as it can get, at least in the domain where it can be properly defined.

arroy_0205

Thanks for the responses. However I still do not understand why it should matter whether the parameter we are talking about is dimensionless or dimensionful.