On 4/15/2022 2:36 PM, George Kahrimanis wrote:
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.
"In George's description" means George knows...what? Does he know the
setting of Alice's polarizer? Does he know that she got 1 or know that
she got 0, on only that she got a result? Some of these he can know
without being the forward light cone of Alice's measurement.
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.
That seems pretty much the same as Bruce's account. Whatever the
polarizer settings, there are four result pairings, while QM only
predicts two in case the polarizers are parallel.
Brent
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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