# To construct an action from a given two-point function

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This is really a basic question whose answer I guess may have to do with the way we construct Feynman rules and diagrams. The question is: Suppose I have been given a two-point function (found in some other ways, say for example some gauge/gravity duality or some symmetry in the theory). How can we construct the Lagrangian of that theory from there?

Is there a general rule for that? Can you give me a reference?

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user user1349
@user1349: Do you think you found the right starting point for constructing physical description?

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user Vladimir Kalitvianski
Of what theory? Lagrangian encodes properties and interactions of all particles. Two-point function gives you just one propagator of just one type of particle. How on Earth do you propose to recover the full Lagrangian from that?

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user Marek
Should I just vote up the given answers to my questions?

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user user1349
@user1349: no, you need to mark some answers as <accepted>. There's an OK checkmark to the left of each answer that you can click. You are supposed to do this whenever there is an answer that you consider to be THE right answer from your point of view.

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user Marek
@Marek: Thanks a lot again on your help on acceptance.

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user user1349
For the record, apparently the word from on high now is that we're not supposed to be pushing people to improve their accept rate. But I think friendly reminders are fine.

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user David Z
Also, this is an interesting question, though unfortunately I don't know quite enough to answer it. I hope somebody else does.

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user David Z

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I think the answer is that such a construction is in general impossible, for two reasons:

(1) The two-point function (or functions, if the field multiplet is not a singlet) says little by itself about the higher-order correlation functions of the theory. It does fully encode the theory if the latter is free (see (2) below).

(2) A two point function need not come from a quantum field theory given by a Lagrangian on the same space-time the fields live on. For instance, a conformally covariant scalar two-point function in Minkowski space-time with non-canonical scaling degree yields a well-defined, free field theory if we set the higher-order truncated correlation functions to zero. This quantum field theory has a dynamics which cannot be given by any Lagrangian in Minkowski space-time.

This post imported from StackExchange Physics at 2014-04-25 01:58 (UCT), posted by SE-user Pedro Lauridsen Ribeiro
answered Dec 11, 2012 by (580 points)

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