From: *Brent Meeker* <meeke...@verizon.net>
On 4/21/2018 9:45 PM, smitra wrote:
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.
But he will find himself in one of only two states, correlated with
the two Alices. The other two of the four possibilities are verboten,
a non-local effect since they are zeroed even at spacelike interval.
Right. It is clear that you get it, Brent, but it is also clear that
Saibal and Bruno are far from getting it.
Let me try again, and reproduce the quantum calculation, as given by MWI
advocates such as Tipler and Lockwood. We start with the singlet state
(ignoring normalization factors):
|psi> = |+>|-> - |->|+>.
where the order of the kets refers to the two particles of the singlet
state. This now interacts with measuring devices, observers, and the
wider environment at both ends. (We assume for this example that the SG
magnets are aligned, so both Alice and Bob measure spin along the same
axis.) Let us summarize the effects at Alice's end by the ket |A>, and
similarly for Bob, |B>. The singlet interacts with these wider system:
|psi>|A>|B> = (|+>|-> - \->|+>)|A>|B>
= (|+>|->|A+> - |->|+>|A->)|B>,
where we have arbitrarily assumed that A measures here particle first,
and |A+> means Alice saw '+' and so on.This happens by entirely local
interactions at Alice's location. We now let Bob interact with the
system. This leads, again by purely local unitary evolution, to
= |+>|A+>|->|B-> - |->|A->||->|B->,
which means that when Alice saw '+', Bob saw '-' and vice versa, in
accordance with standard QM and conservation of angular momentum. As we
have both said, there is no component of the wave function in which
Alice and Bob both see the same result, they are always opposite. This
is clear from the above. There is no "action at a distance" is the sense
of some superluminal action that sees Bob's particle change after
Alice's measurement. Alice's measurement simply picks out a different
part of the wave-function for +' or '-' results. And Bob is left with a
modified wave function. Note that this effect is independent of whether
you assume a collapse model or a no-collapse model. In the collapse
model, after Alice's measurement the portion of the wave-function
corresponding to the result other than the one she got simply drops out.
In the many-worlds case, that piece of the wave function continues to
come along for the ride, but the net result is the same in both cases.
So it is simply not true that many-worlds changes the interpretation in
terms of non-locality. The change in the wave function on one
measurement is the same, and non-local, in both collapse and no-collapse
models, because it is a change in the wave function, not a physical
"action at a distance". The wave-function itself in these non-separable
cases is non-local.
I don't know if this will help Saibal and Bruno, but that is as clear as
I can make it. Maybe you, Brent, can explain it more clearly.
Bruce
--
You received this message because you are subscribed to the Google Groups
"Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
