Field Renormalization vs. Interaction Picture

In summary: But this means that perturbation theory doesn't give us any theoretical insights into the fundamental structure of the theory, since all the terms involved are just lumped together arbitrarily.
  • #36
A. Neumaier said:
In particular, there must be a factor before the derivative term.
Well, I think I just provided a derivation where this is not necessary...
A. Neumaier said:
The restriction 0≤Z≤10≤Z≤10\le Z\le 1 plays no role in the renormalization procedure and is never checked since ZZZ is a bare quantity without physical meaning.
This is indeed an issue that also bothers me quite a bit. The ##Z## factors are defined in terms of a real inner product between normalized states, so ##0\le Z\le 1## is a physical restriction, but in fact ##(1-Z)## is found to diverge to each order in perturbation theory. I have been told that this is the same issue as the existence of the Landau pole, that it indeed shows that theories such as QED and ##\phi^4## are incomplete at high energies, and that for asymptotically free theories such as QCD there is a geometric series over the orders in perturbation theory that shows that the factor goes to zero. I don't know whether any of this is correct.

You seem to be saying that the values of the counterterms in fact have nothing to do with the inner-product-based definition of ##Z##. But than leaves the question: why do we require that the vertices get a counterterm with the same value (to the appropriate power) as the propagator kinetic counterterm?
A. Neumaier said:
ppp is not a variable in the Lagrangian, so I don't understand your proposal about adding an interaction term proportional to p2+m2p2+m2p^2+m^2.
Of course I mean ##\partial^\mu \phi \partial_\mu \phi +m^2##. I wrote ##p^2+m^2##, being the corresponding vertex contribution.
A. Neumaier said:
This holds for a classical theory but not for a quantum theory when, as usual, ℏℏ\hbar is fixed at 1, since scaling the Lagrangian changes the expression in the functional integral.
Good point, my mistake. The CCR have ##[\phi(x),\pi(y)]=i\delta(x-y)##, where ##\pi(x)=\frac{\delta L}{\delta(\partial_t \phi)}##. So changing the scale of the Lagrangian changes the CCR, just as rescaling the fields would.

But this also settles the issue: The terms in the Lagrangian that include time derivatives of the field cannot be arbitrarily rescaled, because their scale encodes the commutation structure of the field operators. So the only way to get a ##p^2 +m^2## counterterm is to use the the ##Z## factors from the LSZ formula, either the way I described or with some other derivation involving a "renormalized field" (which I still don't understand).
 
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  • #37
maline said:
Of course I mean ##\partial^\mu \phi \partial_\mu \phi +m^2##. I wrote ##p^2+m^2##, being the corresponding vertex contribution.
This has nothing to do with ##p^2+m^2##, and doesn't even have meaningful units. I think you meant
##\partial^\mu \phi \partial_\mu \phi +m^2\phi ^2##
but this is just a special case of a weighted sum of ##\partial^\mu \phi \partial_\mu \phi## and ##\phi ^2##,
which are already present in the Lagrangian and provide two of the counterterms. Nothing new if you add your combination to the Lagrangian.
maline said:
But this also settles the issue: The terms in the Lagrangian that include time derivatives of the field cannot be arbitrarily rescaled, because their scale encodes the commutation structure of the field operators. So the only way to get a ##p^2 +m^2## counterterm is to use the the ##Z## factors from the LSZ formula, either the way I described or with some other derivation involving a "renormalized field" (which I still don't understand).
There are no renormalized fields on the level of perturbation theory - all this necessarily happens in a Fock space with the standard free meaning of the operators. The commutation can be chosen completely independent of the Lagrangian and still make mathematical sense. The bare coefficients have no meaning in the relativistic setting; the constraint on ##Z## is a remnant of the nonrelativistic theory where all renormalizations are finite and the interacting fields are series in products of the bare fields.

The textbook derivations are largely historical baggage motivated by the nonrelativistic theory (which breaks down in the relativistic case due to UV divergences). If you want to have a logically sound setting for the renormalization stuff you need to make a fresh start and work with [URL='https://www.physicsforums.com/insights/causal-perturbation-theory/']causal perturbation theory[/URL] as in the books by Scharf; I recommend the second edition of his QED book. Then everything is (perturbatively) well-defined and well-motivated in modern, logically sound terms.
 
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  • #38
A. Neumaier said:
I think you meant
##\partial^\mu \phi \partial_\mu \phi +m^2\phi ^2##
Yes, of course. That was a simple mis-type.
A. Neumaier said:
Nothing new if you add your combination to the Lagrangian.
I am not trying to add anything new. I'm sorry if my notation made this unclear. I am just asking whether we can get a counterterm that depends on ##p^2## by simply multiplying the ##\partial^\mu \phi \partial_\mu \phi## term in the Lagrangian by an arbitrary constant, as you seem to be suggesting. It seems to me that we cannot do this, because in the canonical formalism, it would imply a change in the CCR of the fields. Instead, we need a specific justification for where the ##(1-Z)(p^2+m^2)## counterterm "comes from".
A. Neumaier said:
The commutation can be chosen completely independent of the Lagrangian and still make mathematical sense
Sure, it can make sense, but it doesn't follow the canonical formalism. And that formalism is the only thing that given the Lagrangian a meaning.

By the way, I need to retract a wrong argument that I made above:
maline said:
But than leaves the question: why do we require that the vertices get a counterterm with the same value (to the appropriate power) as the propagator kinetic counterterm?
In gauge theory, this has a straightforward answer: the relationship between the fermion propagator and the interaction vertex is fixed by gauge invariance, specifically the Ward identity. For non-gauge vertices, including the ##Z## factors in the interaction terms has no real effect, because we will anyway include another counterterm proportional to each such interaction.
 
  • #39
maline said:
Well, I think I just provided a derivation where this is not necessary...

This is indeed an issue that also bothers me quite a bit. The ##Z## factors are defined in terms of a real inner product between normalized states, so ##0\le Z\le 1## is a physical restriction, but in fact ##(1-Z)## is found to diverge to each order in perturbation theory. I have been told that this is the same issue as the existence of the Landau pole, that it indeed shows that theories such as QED and ##\phi^4## are incomplete at high energies, and that for asymptotically free theories such as QCD there is a geometric series over the orders in perturbation theory that shows that the factor goes to zero. I don't know whether any of this is correct.
Isn't there a theorem (or is it rather a conjection?) that if ##Z=1##, the theory is non-interacting? Also in perturbation theory the formal ##Z## factors or the counterterms ##\delta Z## are infinite, but that's due to the improper handling of the distributions and products among them. The important thing is that (in some cases properly resummed) perturbative S-matrix elements don't violate unitarity constraints in the realm, where perturbation theory is applicable (i.e., for small renormalized couplings). For gauge theories, indeed the Ward identities are the key issue. If they are not fulfilled you annot guarantee unitarity, and that's why then the theory becomes inconsistent, because it means that gauge symmetry has been broken.
 
  • #40
maline said:
Sure, it can make sense, but it doesn't follow the canonical formalism. And that formalism is the only thing that given the Lagrangian a meaning.
In the canonical formalism you can specify an arbitrary Lagrangian with arbitrary parameters and the standard commutation rule.

If you want more than I explained then you simply try to put more meaning into an ill-defined recipe than is sensible.
 

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