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.