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If I understand it correctly, in Quantum Field Theory, a vacuum state is a state with zero particles in each mode. However, if a photon is "created" with a specific momentum, it will be spread throughout all of space (i.e. photons are non-local). Since the total number of photons in the universe is enormous, does this mean that a vacuum state can never exist, and that the interstellar vacuum has nothing to do with the vacuum state of QFT? Or can the QTF vacuum state somehow be localized to a bounded region of space and correspond to what is normally thought of as "vacuum"? Sorry if the question is not clear, as I'm only learning QFT...

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As you are just learning QFT please keep in mind that QFT is a very successful mathematical tool that can describe data from many particle interactions, but it has also been used to model data from different areas of physics:

In physics, quantization (quantisation) is the process of transition from a classical understanding of physical phenomena to a newer understanding known as quantum mechanics. (It is a procedure for constructing a quantum field theory starting from a classical field theory.) This is a generalization of the procedure for building quantum mechanics from classical mechanics. Also related is field quantization, as in the "quantization of the electromagnetic field", referring to photons as field "quanta" (for instance as light quanta). This procedure is basic to theories of particle physics, nuclear physics, condensed matter physics, and quantum optics

You ask:

However, if a photon is "created" with a specific momentum, it will be spread throughout all of space (i.e. photons are non-local).

As real photons are localized , have a look at this single photon at a time experiment, the concept of wave packets is used to model free particles, including photons. As the basic fields of particle physics are plane wave solutions of the corresponding equations for the particles, creation and annihilation operators will be creating the un-physical (spread by probability over all space) particles; the wave packet is needed for free particles. The Feynman diagram visualization of the integrals in interactions makes this unnecessary for usual calculations.

you ask

and that the interstellar vacuum has nothing to do with the vacuum state of QFT?

QFT is a mathematical tool to fit observations . Observations depend on measurements, which have a specific accuracy. Modelling experimental results with a QFT depends on the level of accuracy of the measurements, and that accuracy would define your "bounded region of space" . As for interstellar space, I have not heard of a model trying to fit it with a QFT, except in cosmological models of the early universe, and there the energy density is such that the concept of "vacuum" is truly a mathematical hypothesis.

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