Extended Bodies in QFT

Stingray

Is string theory the (more-or-less) only way of dealing with 'extended bodies' in a theory like QFT? Or does it only deal with very specific actions, or ones representing 'particles' with special dimensionalities (e.g 1+1 only)? If it is general, is all the commonly-stated phenomenology (11 dimensions etc.) always necessary?

selfAdjoint

From your post, you seem to mean by Extended Bodie" extended elementary bodies. Regarding extended bodies made of elementary ones, such as protons, atoms, molecules, people, suns, etc., present theory contemplates them and string theory does no better (in fact, worse).

So extended elementary bodies. Strings are linear, but branes are of all dimensions up to the 10 or 11. There are even 0-dimensional branes in the theory. So in this sense stringy physics does extended bodies of general dimensionality.

Traditional physical theories of extended elementary particles didn't work. A number of physicists tried theories with little balls - I remember in particular Rudolph Peierls, who was important in the work leading up to the atom bomb, had one. The field theories always failed to close or predicted things that weren't so.

Stingray

Thanks for the answer. I did mean elementary bodies, by the way.

Are these objects restricted to having a particular type of action? I guess I'm asking more whether string theory provides a general framework for multidimensional extended (elementary) bodies, or whether it just studies some specific models. If the latter case is true, then is there any good argument (other than simplicity) why the chosen Lagrangian(s) should be correct?

Thanks. A quick search doesn't come up with much, though. Peierls has done too many things!

Anyway, these questions were motivated by looking at some of Dirac's old papers (mainly from the 60's). I don't know enough to say whether string theory solves the problems that he was stressing, or whether his ideas have since been considered misguided.

selfAdjoint

Good question, Stingray.

I don't know about branes, but strings are based on the Nambu-Goto action. This states that the action on a string worldsheet (two dimensional surface analog of particle world line) is proportional to the element of area of the worldsheet. This is analogous to the action for a relativistic particle, and Nambu-Goto is Lorentz invariant too. It also turns out to be Conformally invariant, and to possess Weyl symmetry as well. This free symmetry is part of the elegance string physicists perceive in their theory.

String physicists have never tried to do all possible theories, they are motivated to find the one correct theory. Their problem is that all possible theories, or a heck of a lot of them anyway, have fallen out of their models, and they can't tell which one is correct.