A. Neumaier
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Well, in the Heisenberg picture, your 3D N(x) clearly changes with time, hence should be written N(x,t), or in 4D notation, again N(x); the same holds for a(x). The only well-defined a(x) is the one Weinberg defines - with different notation - in (5.1.4), and it is covariant, as he states in (5.1.6).meopemuk said:I don't think so. It is known that the Newton-Wigner position operator does not transform covariantly under boosts. So, I wouldn't expect covariant transformations of the corresponding operators a(x) and a^*(x).
If you don't agree, please write down a fully precise definition of your version of a(x), so that we can discuss its properties.
But this doesn't mean that you can change the traditional terminology to suit your narrow focus. If you fill standard concepts with your private meaning you don't need to be surprised that misunderstandings result.meopemuk said:First, I must apologize for not being clear. I am interested only in relativistic quantum fields here.
Assuming it were so, why then does Weinberg talk about annihilation fields and creation fields? And why does wikipedia in the link given talk about ''composite fields, which are usually nonlocal, are used to model asymptotic bound states''? Both statements refer to relativistic quantum field theory!meopemuk said:Second, the covariance and the space-like (anti)commutativity are absolutely essential for the definition of relativistic quantum fields.
It is needed _only_ for those fields used (p.198 top) ''to construct a scalar interaction density that satisfies the'' [properties derived in earlier chapters]. But there are many other fields, with other uses. In particular, these other fields are used to construct the fields that satisfy your (1) and (2). Indeed, to achieve this, Weinberg proceeds ''to combine annihilation and creation fields in linear combinations:'' (5.1.31).meopemuk said:Only if these two conditions are satisfied, one can build interacting Hamiltonian density with properties (5.1.2) and (5.1.3) as products of fields (5.1.9). Note also the sentence at the bottom of page 198: "The point of view taken here is that Eq. (5.1.32) [the space-like (anti)commutativity] is needed for the Lorentz invariance of the S-matrix, without any ancillary assumptions about measurability or causality."
Since the field N(x) is not needed to construct the interaction density, it is not restricted by Weinberg's considerations on p.198.