Recent content by Javier

OK. The confusion here is in the two ways I was using "type of string". In your first sentence above, replace "type of string" by "choice of string model" (there are 5 in the case of superstrings). Once a *model* is chosen, there is then one type of string. Then each string in space looks...

"BPS saturated" is indeed when the equality holds. "BPS states" refer to (quantum) states that have such a mass. The concept originated in quantum field theory and was later extended to supersymmetric and string theories.

You're right when you say "suspect to say the least". I wouldn't bother trying to use that site to understand any facts that may actually be contained there.

The idea is roughly right: a single type of string can look like particles with different quantum numbers like spin, charge, etc. (However, the proton is a composite particle while a single string should correspond to an elementary particle.) But there are a great number of technical issues to...

The particle exchange picture is, in a precise sense, an approximation to what is happening. (1) Loosely stated, this approximation applies best when the dimensionless coupling strengths associated with any given interaction are less than 1 (so that perturbation theory is applicable). (2)...

The counting goes according to the number of independent ``coupling strengths" of the interactions. There is one coupling parameter for each ``simple or abelian" group factor in the full symmetry group representing the interactions. The symmetry group describing the four interactions, with...

The field theory of linearized gravity is a truncation of the full string description. If you expand the string Lagrangian out in terms of graviton fields, you get linearized gravity Lagrangian (which you can write in terms of the Ricci scalar...this is the Einstein-Hilbert Lagrangian)+ higher...

The determination of area in quantum geometry is the same spirit as for classical gr. In classical GR, there is a bare 4-dimensional manifold with some topology. You solve Eintein's field equations to determine the metric on this bare manifold. The metric gives you notions of geometry...

Upon quantization of the string, the characteristic string length L is related to the string "tension" T (the characteristic energy scale) by L=1/(sqrt{pi*T}).

You can construct systems of n photons in which the total spin angular momentum is any integer between -n and +n. This does not mean that single particle states with helicity n and -n are formed. A single particle state is defined as an irreducible representation of the little group of the...

First, let's clear up terminology. p-branes (where p=0,...,10) and "fundamental strings" are distinct objects in what we all "String theory". Fundamental strings are the ones appearing in the original formulation of string theory, but since the mid 90's, it was realized that there are also...

Superstringtheory.com has a good description of the things you would need for a fair to full blown study of the theory. I would be unsatisfied with Brian Greene's account as well, but that's because to really understand *why* the theory is studied and why it says the things it says, you...

I agree not to make this into a debate...I don't want to stray too far from the original question, but since there were disagreeing responses, I'll just state what the standard understanding is for the record, and leave it for others to decide (or ignore) for themselves. First, I'll get to the...

Agreed. If you don't study what a theory says (like GR) in terms of observational predictions and work out for yourself what other theories say in comparison, opinions about the validity of theories like general relativity are empty. If you're going to have an alternative theory replace an...

The situation should be treated with quantum mechanical care. The "backwards in time" business is not a modern physical interpretation, but a formal identification one can make for computational purposes. Set up the situation by having two charges particles at a large enough distance so...