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A. Neumaier

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Steven Weinberg wrote: ''In its mature form, the idea of quantum field theory is that quantum fields are the basic ingredients of the universe, and particles are just bundles of energy and momentum of the fields.'' (see p.2 of his essay, ''What is Quantum Field Theory, and What Did We Think It Is?'' http://arxiv.org/pdf/hep-th/9702027v1)

This thread is intended to discuss questions related to the above quote. Such questions came up in other threads ( https://www.physicsforums.com/showthread.php?t=471125 and https://www.physicsforums.com/showthread.php?t=474537 ), but to focus the discussion, I respond to them here.

More precisely, the question to be discussed is; Which fields (or other observables) represent the operational content of relativistic quantum field theory?

It will take some time till I have prepared the proper background for a full discussion.

For the moment, I just begin by commenting on the following, which assumes the QED electron field Psi(x,t):

This thread is intended to discuss questions related to the above quote. Such questions came up in other threads ( https://www.physicsforums.com/showthread.php?t=471125 and https://www.physicsforums.com/showthread.php?t=474537 ), but to focus the discussion, I respond to them here.

More precisely, the question to be discussed is; Which fields (or other observables) represent the operational content of relativistic quantum field theory?

It will take some time till I have prepared the proper background for a full discussion.

For the moment, I just begin by commenting on the following, which assumes the QED electron field Psi(x,t):

This is not an ''abstract object'' but a measurable observable, which persists in the macroscopic (classical) limit and is there identical with the current figuring in the Maxwell equations, describing the current density at time t and position x in ordinary space-time.we use this operator to construct another abstract object called "current density 4-vector"

[tex] J^{\mu} (x,t)= \overline{\Psi} (x,t) \gamma^{\mu} \Psi(x,t) [/tex]

I don't know what you are talking about. The parameter x in the standard Maxwell equations in flat space-time has the standard meaning of the physical position. Quantization doesn't change this.Moreover, parameters x are just integration variables. There is absolutely no reason to identify them with physical positions. Parameter t is set to 0, so it has not direct relevance to measured time.

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