Piling on Frauchiger&Renner is a sport that I've never felt much inclination for, but a paper in SHPMP, "The Frauchiger-Renner argument: A new no-go result?", https://doi.org/10.1016/j.shpsb.2019.12.002, Fortin&Lombardi, states F&R's premises neatly enough that I feel like messing with the field:
(Q) Compliance with quantum theory: Quantum mechanics is universally valid, that is, it applies to systems of any complexity, including observers. Moreover, an agent knows or is certain that a given proposition is true whenever the Born rule assigns probability 1 to it.
(C) Self-consistency: Different agents' predictions are not contradictory.
(S) Single-world: From the viewpoint of an agent who carries out a particular measurement, this measurement has one single outcome.
If we take an algebraic approach, we have, instead,
(Q) There is a collection of measurements (modeled by a collection of operators) and recorded measurement results for each of those measurements (modeled by real non-negative normalized linear forms over the algebra generated by the collection of operators) [there are no "systems" and no "observers", only measurements, models of the measurements, and results and models of the results].
(C) If recorded measurement results match the modeled measurement results, we're good, otherwise change the recorded measurement results [just kidding! It's a lot safer to change either our model of the measurements or our model of the measurement results.]
(S) Journal editors are suspicious of recorded measurement results that change after they have accepted an article [it is well-known that journal editors are the closest anyone can come to being God, but there are also plenty of physicists ready to pillory a cheat.]
F&R's rules don't seem an interesting game. The above may seem too operational, but we can optionally introduce a very beyond-the-operational in-between rule:
(B) We can introduce a continuum of models of measurements and of their results that are, in various ways, in-between the measurements we actually performed or intend to perform, and which we might perform, perhaps even unexpectedly [that is, metaphysically, we can imagine a continuum of measurements and their results.]
(Q) Compliance with quantum theory: Quantum mechanics is universally valid, that is, it applies to systems of any complexity, including observers. Moreover, an agent knows or is certain that a given proposition is true whenever the Born rule assigns probability 1 to it.
(C) Self-consistency: Different agents' predictions are not contradictory.
(S) Single-world: From the viewpoint of an agent who carries out a particular measurement, this measurement has one single outcome.
If we take an algebraic approach, we have, instead,
(Q) There is a collection of measurements (modeled by a collection of operators) and recorded measurement results for each of those measurements (modeled by real non-negative normalized linear forms over the algebra generated by the collection of operators) [there are no "systems" and no "observers", only measurements, models of the measurements, and results and models of the results].
(C) If recorded measurement results match the modeled measurement results, we're good, otherwise change the recorded measurement results [just kidding! It's a lot safer to change either our model of the measurements or our model of the measurement results.]
(S) Journal editors are suspicious of recorded measurement results that change after they have accepted an article [it is well-known that journal editors are the closest anyone can come to being God, but there are also plenty of physicists ready to pillory a cheat.]
F&R's rules don't seem an interesting game. The above may seem too operational, but we can optionally introduce a very beyond-the-operational in-between rule:
(B) We can introduce a continuum of models of measurements and of their results that are, in various ways, in-between the measurements we actually performed or intend to perform, and which we might perform, perhaps even unexpectedly [that is, metaphysically, we can imagine a continuum of measurements and their results.]
