On 4/22/2018 3:42 PM, smitra wrote:
On 22-04-2018 07:46, Brent Meeker wrote:
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.
Brent
In the MWI it is just like drawing balls from a box containing a white
and a black ball. If the two balls are sent to a distant location to
Alice and Bob, and Alice performs her measurement she'll know what Bob
will find. Here too there are two possibilities for Alice and Bob, yet
two of the four = 2 times 2 possibilities are excluded. This is a
non-local effect, but an entirely trivial one that is the result of a
local common cause effect.
If it were that simple. But the balls aren't black and white. If that
were the case Bob would find his ball was either black or white no
matter what Alice found. The problem is that each measurement is in a
2-space, e.g. (x,y) So if Alice measures x +, then Bob get + - equally
for y; showing that it didn't have some common cause value of y. But if
Alice measures y+, then Bob gets y-. So what Alice chooses to measures
affects Bob's result, but it's not a common cause.
It is only in collapse interpretations where there is a serious
non-locality problem. In addition to the common cause effect due to
the creation of the entangled spins, a collapse of the wavefunction is
supposed to happen when Alice measures her spin. This kills one of the
two possibilities for Bob, even though he is space-like separated from
Alice. This non-local effect cannot be explained in terms of only
local interactions. In the MWI, there is no such effect, there is only
a universal wavefunction which also describes the states of Alice and
Bob and this evolves in time in a local way.
You then do end up with Alice and her local environment being
entangled with Bob's local environment, and such a state is obviously
non-local. But it's not a problem
Nobody said it was a problem. I said it was non-local.
to explain how this non-local situation came about from within the
theory where there exists only local interactions.
It's explained by Schoedinger equations evolution of the states where
decoherence causes some global states to have zero probability. Sure
the meausurement and decoherence interactions are local, but the wave
function isn't local and it is changed globally by the local interactions.
Brent
Such an explanation does not exist for collapse models.
Saibal
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