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Stupid Question of the Day:

What advantage does string field theory have over string theory? In all

the standard texts I own, there is very little reference made to string

field theory yet I know it exists.

Can anyone help me understand the motivation for it or point me to a good

primer?

Thanks

========================

Lubos Motl (moderator):

Let me start with an answer. I hope that people will post other answers,

too. String field theory is a tool to formulate string theory in a

language that is as similar to regular quantum field theory as possible.

(But it has infinitely many component fields.) Unlike the usual rules of

string theoretical CFT, string field theory allows one to compute

off-shell quantities.

On-shell quantities are the scattering amplitudes, off-shell quantities

are e.g. the Green's functions, to simplify it a bit. String field theory

is only simple and predictive for open strings; the closed string field

action must be corrected by new terms at every order in the Taylor

expansion in g. Because of the off-shell character of (open) string field

theory, string field theory is very good to study questions such as the

tachyon condensation - to check Sen's conjecture that the minimum of the

tachyon potential corresponds to a total destruction of the D-brane.

String field theory used to be believed to be useful for nonperturbative

treatment of string theory, but evidence supporting such a far-reaching

claim has been very limited so far.

String field theory has played virtually no role in the second

superstring revolution, but it was useful to study the tachyons, and I

think that a good review is the following:

http://arxiv.org/abs/hep-th/0311017

Cheers,

Lubos

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# String Field Theory?

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