From: *smitra* <smi...@zonnet.nl <mailto:smi...@zonnet.nl>>
On 22-04-2018 06:08, Brent Meeker wrote:
On 4/21/2018 8:39 PM, smitra wrote:
On 22-04-2018 02:05, Brent Meeker wrote:
On 4/21/2018 4:45 PM, smitra wrote:
Yes, collapse does imply non-locality, but note that
in the MWI there is no collapse. There is no real
"splitting of Worlds" in the MWI either, it's only an
effective splitting that can be interpreted as an
effective collapse as observed in the various
effective worlds.
And that observation is predicted by events spacelike
separated from it.
Brent
And that ability for Alice to predict what Bob will find,
poses a problem for single world collapse theories. Only there
does new information appear after a measurement and that then
happens in a non-local way when making certain measurements on
entangled pairs of particles.
There are only four cases without collapse and in every case Alice can
predict Bob's result. The very fact, which you have brought up, that
any hidden variable theory that explains the results must be non-local
(like Bohmian QM) shows that effect is non-local.
Brent
In case of a collapse theory, the non-local effect is far more
problematic. Alice then finds a result at her place and because there
is no other copy of her who found the other result, new information
has appeared. And that means that Bob's result is now also well
defined but the information about his measurement exists at a
space-like separation. In the MWI Bob may know that Alice has already
made her measurement, but he would also know that Alice exists as a
superposition of two copies who will have found two different results,
so there exists no information about what he is about to find later
when he will measure his spin at the distant location where Alice is
as that entire place is in a superposition.
Saibal
That is where the wave-function comes in: the wave function acts
non-locally to ensure that when Bob does make his measurement, he will
only obtain results that agree with angular momentum conservation -- his
results cannot be arbitrary because that would violate the basic
conservation law enshrined in the singlet wave-function. Bob's
measurement is independent of Alice, but the state that he is measuring
is necessarily correlated with Alice's -- changed by Alice's
measurement. Many worlds does not alter this basic fact.
Bruce
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