Date: 2022-08-23 | Article ID: 30983 |

Quantum field theory (QFT) is the foundation for our current understanding of particle physics. One important thing to understand about QFT is how the phenomena we call *particles* arise naturally from a model that is defined entirely in terms of quantum fields, and a first step toward that understanding is to formulate the concept of *particle* using only observables that are available in QFT. This can be done by using observables that act like detectors. This article uses the relativistic free scalar model as an example.

One key message is that in relativistic QFT, perfectly reliable detectors cannot be perfectly localized in any finite region of space. This might not matter much in practical applications, which are never perfect anyway, but it is fundamental to a good understanding of relativistic QFT.

**2022-08-23**(added clarification about the context of the "position operator")**2022-08-21**(first version)

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