Bruce wrote
> [...] Since I have not been able to formulate an argument that has
convinced Saibal, there seems little point in continuing the discussion.
Not yet, because I just got an idea of what went wrong in the
communication. Part of it was the understanding (maybe it was just me!) of
"local splits" as propagating on light cones, which now makes me laugh.
> The argument that MWI is strictly local is just a mistake, and not an
argument against MWI itself.
Let me try to back this claim. In the example with Alice and Bob, the
splits arise in the description ("wavefunction") of a superobserver,
George, who only knows that, first Alice then Bob, measure entangled spins
on two pre-set axes, without him knowing the outcomes. Instead of the
superobserver, you may think of an impersonal quantum description of the
whole system, but I wonder what does it mean, to assume a quantum
description without a subject, so I will keep speaking of a superobserver.
When Alice becomes entangled with the spin, and the record becomes
practically permanent (by decoherence in her head), then in George's
description the whole system splits. Not separately Alice or Bob; the whole
system. We should not be confused by the fact that at first the two Bobs
are exactly identical, until Bob eventually learns Alice's record -- but
the split had occured earlier, in George's description (of the whole
system).
When, in turn, Bob (that is, both Bobs) measures the spin with his
equipment, George's description splits again; so George counts four Bobs
now. (If the two axes are parallel, then one of the two components of the
second split has measure zero.) This split also affects each Alice, who
becomes two IDENTICAL copies (total: four!), until she (that is: each
Alice) learns Bob's record, and then there are no more identical Alices --
but the split had ocurred earlier.
So, "local split" makes no sense: each split arises in the description of a
superobserver. Without a superobserver, it is unclear whether Alice will
regard herself as split by her measurement. Bob will tell her "Alice, now
you are split", but so what? Alice (both of them) may reply "I know that I
am counted as double by anyone who knows of my measurement without knowing
the record, but I assure you that I am only one of the two Alices you have
in mind". (No wonder that many people hate MWI!) And she may add "If I am
split then you are too!".
I could say more about needing a superobserver -- maybe in another posting.
George K.
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