Measurement in QFT: Mapping Fields to Theory's Math Formalism

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Discussion Overview

The discussion revolves around the mapping of experimental measurements of quantum fields in quantum field theory (QFT) to the mathematical formalism of the theory. Participants explore the implications of measurements made in particle accelerators, the nature of eigenstates in QFT, and the relationship between position and momentum measurements, particularly in the context of the uncertainty principle.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions how to interpret measurements of particle tracks in accelerators, noting that position is not an operator in QFT but a parameter for field operators.
  • Another participant emphasizes that the states before and after scattering are approximately free states, suggesting they are eigenstates of the momentum operator.
  • Concerns are raised about the implications of precise position measurements on the uncertainty principle, questioning whether such measurements could violate it.
  • Some participants argue that the uncertainty principle is a statistical law and that the measurements in question may not be precise enough to violate it.
  • There is a discussion about how to conceptualize the collapse of fields during measurements and whether these collapses pertain to position or momentum.
  • A later reply suggests that the tracks observed in experiments do not provide sufficiently precise measurements to disrupt classical trajectories, indicating that the uncertainty principle may not be significant in those contexts.

Areas of Agreement / Disagreement

Participants express differing views on the nature of measurements in QFT, particularly regarding the relationship between position and momentum, the interpretation of eigenstates, and the implications of the uncertainty principle. There is no consensus on these issues, and the discussion remains unresolved.

Contextual Notes

Participants highlight limitations in understanding how measurements in QFT relate to classical trajectories and the implications of the uncertainty principle, indicating that the discussion involves complex and nuanced interpretations that are not fully settled.

  • #31
vanhees71 said:
But in your reference I don't see a concrete description of a TPC with the POVM formalism.
This would be a separate paper on its own. I just describe how track measurements actually performed with a TCP (no matter how it is built) can be interpreted as a POVM measurement of position and momentum.
vanhees71 said:
I doubt that my experimental colleagues actually building such instruments use the POVM formalism at all ;-)).
Neither did the spectroscopists before 1925 use the modern quantum formalism.

None of your statements mean that one cannot get correct descriptions in terms of POVMs respective energy levels.
 
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  • #32
I don't doubt the POVM formalism in any way. Of course spectroscopists before 1925 didn't use modern quantum formalism. To the contrary before at least 1913 with Bohr's "old quantum mechanics" it was an enigma how to explain the discrete spectra observed by Fraunhofer as well as Kirchhoff and Bunsen in the 19th century. It's only with modern QM that a complete understanding, including intensities, from a generally valid theory has been achieved.
 

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