The Reference System Loophole
Abstract
I argue that the Extended Wigner's Friend theorems have a `loophole' that has previously not been appreciated, coming from the fact that we typically need a reference system to give meaning to a measurement outcome. I show how this loophole can be used in the context of the Local Friendliness theorem to preserve the predictions of quantum mechanics without denying Local Friendliness, and I respond to some objections. I explain how the loophole arises naturally from consideration of relevant symmetry transformations, and then I appeal to the partial/complete observables formalism to argue that the same loophole will work for any type of Wigner's Friend experiment, even if it is performed with a variable that does not obviously look like it requires relativization to a reference system. Finally I compare the loophole to other possible responses to the Local Friendliness theorem, and I show that the same loophole will work for other Extended Wigner's Friend theorems too.
Related
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BibTeX
@misc{adlam2026reference,
title = {{The Reference System Loophole}},
author = {Emily Adlam},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.02283},
eprint = {2611.02283},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.02283},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Reference System Loophole
AU - Emily Adlam
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.02283
UR - https://arxiv.org/abs/2611.02283
AB - I argue that the Extended Wigner's Friend theorems have a `loophole' that has previously not been appreciated, coming from the fact that we typically need a reference system to give meaning to a measurement outcome. I show how this loophole can be used in the context of the Local Friendliness theorem to preserve the predictions of quantum mechanics without denying Local Friendliness, and I respond to some objections. I explain how the loophole arises naturally from consideration of relevant symmetry transformations, and then I appeal to the partial/complete observables formalism to argue that the same loophole will work for any type of Wigner's Friend experiment, even if it is performed with a variable that does not obviously look like it requires relativization to a reference system. Finally I compare the loophole to other possible responses to the Local Friendliness theorem, and I show that the same loophole will work for other Extended Wigner's Friend theorems too.
ER -