I'm trying to understand something regarding Noether's theorem - and with the given situation, my question isn't that much of a question, I'm rather just seeking confirmation whether I'm thinking right or not.
The situation:
Let L be a Lagrangian density, depending on some field ϕ, and its first derivative. Noether's theorem (naively) says that if ϕ(x)↦ϕ(x)+ϵδϕ(x) is a specific infinitesimal deformation of the field (a more precise thing would be to say that this is a smooth 1-parameter family of finite deformations - and we're interested in behaviours under d/dϵ|ϵ=0), such that L changes by a divergence (δL=∂μKμ), then the current jμ=∂L∂(∂μϕ)δϕ−Kμis conserved on-shell.
It is known that this can be recast in a different form, by making ϵ be a function instead of a parameter. Then the action won't be invariant in general, but the deformation of the action gives the same current jμ, and its conservation can be shown.
The problem is that this doesn't make sense, imo. To show this, consider the following, let ϵ(x) be the "infinitesimal" functional parameter, the variation is ϕ(x)↦ϕ(x)+ϵ(x)δϕ(x). Let us define ϵ′(x) and ϵ as ϵ(x)=ϵϵ′(x), where here only ϵ is "infinitesimal". Now the variation has the form ϕ(x)↦ϕ(x)+ϵϵ′(x)δϕ(x). Now we redefine δϕ(x) to δϕ′(x)=ϵ(x)δϕ(x), then the variation has the form ϕ(x)↦ϕ(x)+ϵδϕ′(x).
This is literally the same form we had before we assumed ϵ is a function.
So this begs the question - what do we mean on an "infinite-parameter" variation? The problem is clearly caused by the fact, that if δϕ(x) is specific, but reasonably arbitrary, then this still contains as many "free parameters" as the different possible values for x. Essentially, δϕ(x) already contains infinite parameters.
The resolution:
Looking at specific examples, such as an U(1) transformation of the free, massive, complex Klein-Gordon field, the finite transformation is ϕ(x)↦eiϵϕ(x). Infinitesimally, this is ϕ(x)↦ϕ(x)+iϵϕ(x), so δϕ(x)=iϕ(x).
Here we see, that δϕ(x) depends on x only through the unperturbed field ϕ(x) itself, so here the variation is truly 1-parameter.
If we do this for another archetypical example - spacetime translations - we get the same results.
The question:
Am I right in saying that the usual form of the Noether's theorem should be stated that we consider variations of the form ϕ(x)↦ϕ(x)+ϵδϕ[ϕ(x),∂ϕ(x)], where δϕ is a specific function of the field ϕ, and possibly, its derivatives, but not the coordinates x?
Because only then does it makes any sense to me to discuss whether the variation has finite or infinite amount of parameters.
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