'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

Now imagine a second - whatever, human? - device, 
to measure a specific observable of the above 

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?)

'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.'



[It is too late here, I cannot write more, and I cannot
check the above :-)]

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