A few posters are saying that there are many string theories. It would be more accurate to say that there's only one theory, but many models - if we define a theory by an equation of motion, and a model by a choice of initial conditions. For string theory, that means describing the space that the strings move through.
At the level of theory, string theory still doesn't have its fundamental universal equation. Probably that will come from the study of M-theory, the eleven-dimensional phase of the theory. Apart from M-theory, what we have are various fundamental equations for strings in ten dimensions, which correspond to different limits of M-theory. In the development of the subject, the ten-dimensional string equations were constructed first, and then the existence of M-theory was extrapolated from those.
To make predictions using string theory, you pick one of these fundamental starting points, and then you specify the geometric background that the strings will inhabit, and then you calculate their properties in that background. For example, you might look at "Type IIA string theory compactified on a T6/(Z2xZ2') orientifold". There is a very large number of possibilities.
So far, it hasn't even been proven that string theory can completely match experiment. This is because of the subject's mathematical difficulty. There are many models - many choices of the background space - which qualitatively resemble the real world, but it is technically very challenging to determine the exact parameters at which a particular geometric background will stabilize, and those parameters in turn determine observable properties like particle masses. It will be a big day for string theory when a class of models is found in which those geometric "moduli" are provably stable and in which the known particle masses come out right.
At a higher level, you might also hope that the fundamental equation of motion will actually determine the choice of background space, and not just how the strings behave on it. This is an even harder problem than moduli stabilization, though the two problems are connected; the moduli are the geometric parameters describing a space, and if they change enough, the space will become another space (e.g. it will change topology). So in both cases, to solve the problem, you need to understand the dynamics of the moduli. As things stand, the idea that string theory dynamically favors just one background seems like a naive hope from the early days of the theory. If we *are* living in a particular string model, some of its features may have been imposed by the anthropic principle, and the other features may just be "random". But we won't know for sure how to think about the theory at this level until we understand its dynamics much better than now.
So, when it comes to confronting string theory with experiment, there are two paths forward. First, you can hope for a spectacular qualitative indication that string theory is correct, such as the detection of supersymmetry or extra dimensions. Second, you can wait for the slow difficult advance of mathematical understanding to produce realistic models with stabilized moduli, or even a dynamical prediction of the background space. The LHC is guaranteed to show us something new - the cause of electroweak symmetry breaking, whether that's a Higgs boson or something else - but whether it will be spectacular enough to change the situation is unknown. If it doesn't, then testing string theory will depend on achieving those fundamental theoretical advances.
Slashdot Mirror
A few posters are saying that there are many string theories. It would be more accurate to say that there's only one theory, but many models - if we define a theory by an equation of motion, and a model by a choice of initial conditions. For string theory, that means describing the space that the strings move through.
At the level of theory, string theory still doesn't have its fundamental universal equation. Probably that will come from the study of M-theory, the eleven-dimensional phase of the theory. Apart from M-theory, what we have are various fundamental equations for strings in ten dimensions, which correspond to different limits of M-theory. In the development of the subject, the ten-dimensional string equations were constructed first, and then the existence of M-theory was extrapolated from those.
To make predictions using string theory, you pick one of these fundamental starting points, and then you specify the geometric background that the strings will inhabit, and then you calculate their properties in that background. For example, you might look at "Type IIA string theory compactified on a T6/(Z2xZ2') orientifold". There is a very large number of possibilities.
So far, it hasn't even been proven that string theory can completely match experiment. This is because of the subject's mathematical difficulty. There are many models - many choices of the background space - which qualitatively resemble the real world, but it is technically very challenging to determine the exact parameters at which a particular geometric background will stabilize, and those parameters in turn determine observable properties like particle masses. It will be a big day for string theory when a class of models is found in which those geometric "moduli" are provably stable and in which the known particle masses come out right.
At a higher level, you might also hope that the fundamental equation of motion will actually determine the choice of background space, and not just how the strings behave on it. This is an even harder problem than moduli stabilization, though the two problems are connected; the moduli are the geometric parameters describing a space, and if they change enough, the space will become another space (e.g. it will change topology). So in both cases, to solve the problem, you need to understand the dynamics of the moduli. As things stand, the idea that string theory dynamically favors just one background seems like a naive hope from the early days of the theory. If we *are* living in a particular string model, some of its features may have been imposed by the anthropic principle, and the other features may just be "random". But we won't know for sure how to think about the theory at this level until we understand its dynamics much better than now.
So, when it comes to confronting string theory with experiment, there are two paths forward. First, you can hope for a spectacular qualitative indication that string theory is correct, such as the detection of supersymmetry or extra dimensions. Second, you can wait for the slow difficult advance of mathematical understanding to produce realistic models with stabilized moduli, or even a dynamical prediction of the background space. The LHC is guaranteed to show us something new - the cause of electroweak symmetry breaking, whether that's a Higgs boson or something else - but whether it will be spectacular enough to change the situation is unknown. If it doesn't, then testing string theory will depend on achieving those fundamental theoretical advances.
http://singularity.posthuman.com/singularitarian/P tS/plan.html