What is the simplest way to describe the difference between these two concepts, that often go by the same name?
1 Answer
The Wilsonian effective action is an action with a given scale, where all short wavelength fluctuations (up to the scale) are integrated out. Thus the theory describes the effective dynamics of the long wavelength physics, but it is still a quantum theory and you still have an path integral to perform. So separating the fields into long and short wavelength parts $\phi = \phi_L + \phi_S$, the partition function will take the form (N.B. I'm using Euclidean path integral)
$$ Z = \int\mathcal D\phi e^{-S[\phi]} =\int\mathcal D\phi_{L}\left(\int D\phi_{S}e^{-S[\phi_L+\phi_S]}\right)=\int\mathcal D\phi_{L}e^{-S_{eff}[\phi_L]}$$ where $S_{eff}[\phi_L]$ is the Wilsonian effective action.
The 1PI effective action doesn't have a length scale cut-off, and is effectively looking like a classical action (but all quantum contribution are taken into account). Putting in a current term $J\cdot \phi$ we can define $Z[J] = e^{-W[J]}$ where $W[J]$ is the generating functional for connected correlation functions (analogous to the free energy in statistical physics). Define the "classical" field as $$\Phi[J] = \langle 0|\hat{\phi}|0\rangle_J/\langle 0| 0 \rangle_J = \frac 1{Z[J]}\frac{\delta}{\delta J}Z[J] = \frac{\delta}{\delta J}\left(-W[J]\right).$$
The 1PI effective action is given by a Legendre transformation $\Gamma[\Phi] = W[J] + J\cdot\Phi$ and thus the partition function takes the form
$$Z=\int\mathcal D\phi e^{-S[\phi] + J\cdot \phi} = e^{-\Gamma[\Phi] + J\cdot \Phi}.$$ As you can see, there is no path integral left to do.
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1$\begingroup$ Thank you for elegant and beautiful answer. As I understood, Wilsonian eff action has less information in it than full theory, but 1PI has even more information than needed. $\endgroup$Newman– Newman2011-12-01 17:24:04 +00:00Commented Dec 1, 2011 at 17:24
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8$\begingroup$ You need to be a little careful about saying that Wilsonian eff. theory contain less information than the full theory, since performing the path integral over $\phi_L$ you get the full result. The high energy (low-distance) fluctuations are integrated out, but the information about them is still there, but hidden in $S_{eff}$. Wilsonian eff action is describing how the theory effectively behaves at low energy (long-distances), but you can't just "remove" short-distance physics. You need to integrate them about since for interacting theories they couple and contribute to the low-energy physics. $\endgroup$Heidar– Heidar2011-12-02 13:31:15 +00:00Commented Dec 2, 2011 at 13:31
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3$\begingroup$ I just found these note (arxiv.org/abs/hep-th/0701053v2) which I think contain more detailed discussion about differences and connections between 1PI and Wilsonian effective actions. $\endgroup$Heidar– Heidar2011-12-02 13:43:49 +00:00Commented Dec 2, 2011 at 13:43
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2$\begingroup$ In fact, Wilsonian effective action is nonlocal. If you integrate out high ebergy degree of freedom, you just replace the short distance physics with a propagator. $\endgroup$thone– thone2012-06-23 12:23:23 +00:00Commented Jun 23, 2012 at 12:23
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2$\begingroup$ Could we say that the 1PI action is obtained when one integrates over all energies/momenta? Which is the same as saying there is no cut-off. $\endgroup$Siraj R Khan– Siraj R Khan2015-08-09 14:13:37 +00:00Commented Aug 9, 2015 at 14:13