Godfrey: 'MWI + Projection postulates should reproduce regular Copenhagenian QM since MWI is basically QM - Projection Postulates!'
Imagine a superposition like this |'spin_z' +1> |'detector' +1> + |'spin_z' -1> |'detector' -1> It describes a superposition of spin up/down states, and the entagled (or relative) states of a detector. Now imagine a second - whatever, human? - device, to measure a specific observable of the above superposition. Let this observable be such that the ray generated by the above superposition state is an eigenspace of this observable, corresponding to a definite eigenvalue, the eigenvalue 'yes'. Since neither component of the above superposition state lies in the eigenspace of this observable, this observable fails to commute with the 'spin_z' observable, and fails to commute with the 'detector' observable. We can write (canonically) ... |'z-spin' +1> |'detector' +1> |yes> + |'z-spin' -1> |'detector' -1> |yes> In a MWI, a world should instantiate an eigenvalue for an observable if the superposition term associated with that world is an eigenstate of the observable corresponding to that eigenvalue. So, after the (second) measurement, what would an Everettista write? This one? |'z-spin' +1> |'detector' +1> |?> <=> world A |'z-spin' -1> |'detector' -1> |?> <=> world B (Since, in each world, the observable measured by the second - whatever, human? - device does not commute with the 'spin_z' observable, so it has no predeterminate value, that is to say that the outcome of the (second) measurement must occur by chance.) Or this one? |'z-spin' +1> |'detector' +1> |yes> <=> world A |'z-spin' -1> |'detector' -1> |yes> <=> world B (In this case the fact that the second device would later record the state |yes> seems to be fixed ... in advance of the measurement itself. And this is magic. White Rabbit? What else?) Godfrey: 'I believe that YD is incompatible with the whole formalism of QM which I don't quite think is simply reducible to Unitary Evolution plus Collapse, by the way.' Maybe. s. [It is too late here, I cannot write more, and I cannot check the above :-)]
