On 03-06-2017 05:42, Bruce Kellett wrote:
On 3/06/2017 11:38 am, smitra wrote:
On 03-06-2017 02:10, Bruce Kellett wrote:
On 3/06/2017 9:16 am, smitra wrote:
In a single universe theory, this implies non-locality, because of
the absence of local hidden variables. If local hidden variable
were to exist then you could say that Alice and Bob where to find
whatever they found anyway only due to their local interactions with
the spins and polarizers. But with that ruled out, whatever Alice
will find is information that just popped into existence when Bob
made his measurement.
What is ruled out by Bell's theorem is that local hidden variables
can
account for all possible correlations between the observations,
Bell's
theorem does not rule out the possibility of a local hidden variable
explanation in special cases, like that of polarizers set at the same
angle.
Either there exists a local hidden variable theory from which QM can
be derived or such a theory doesn't exist. If we assume that a local
hidden variable theory underlies QM, then we find that regardless of
the details, it cannot reproduce QM in certain cases. Bell
inequalities can be derived for such theories that QM violates. Then,
with QM confirmed and in particular the violations of the Bell
inequalities conformed, we can then discard any local hidden variables
theory.
This means that even in cases that do not involve violations of Bell
inequalities, we can still say that there are no local hidden
variables that can explain the results in those experiments, because
we've verified that there is no hidden variable theory that can
reproduce QM.
Reproduce all QM results. It does not rule out cases where local
hidden variable, or common cause, explanations are possible.
Then it suffices to consider a simple case of entanglement between two
spins where in a single universe interpretation there is non-local
behavior, e.g. Alice and Bob measuring the z-components of a system of
two spin 1/2 particles in the singlet state. You may consider more
complex cases where Alice randomly chooses another direction, but I
remember from a previous discussion that this led to a disagreement
about how to treat the source of this randomness.
Not that I remember. There might have been disagreement about how the
orientation of one polarizer became known at the other polarizer, but
randmoness was never an issue.
Ultimately all randomness has a quantum mechanical origin as pointed
out in this article:
https://arxiv.org/abs/1212.0953
There simply exists no known way to get to purely classical
randomness.
In the case where both polarizers have the same setting, the fact that
Bob knows what Alice will find poses a problem for locality because
local hidden variables have been ruled out.
No it doesn't. There is a simple common cause explanation available
for this case.
Which doesn't eliminate non-local effects, see EPR.
So, what Alice will find is random, new information will appear at
her place after she measures the spin that didn't previously exist
locally. But the fact that Bob could predict her result means that
this information did exist at Bob's place. This demonstrates the
non-locality aspect of single universe theories.
No, it simply demonstrates that when there is a common cause
explanation, there is no problem. Remember Bertlmann's socks.
No, see EPR argument.
Then in the MWI, Alice is identical in the two branches, so her
measurement result is not predetermined as she is not yet located in
either of Bob's branches. Her measurement result will do that.
Not so. When the polarizers are aligned, Bob's measurement has a
common cause with Alice's, so the agreement between results is
determined from the start.
Agreement via common cause does not eliminate non-locality problem in
single universe theories, as EPR have shown.
If we change the set-up by letting Alice choose different setting of
her polarizer to bring in the additional baggage of having to rule out
hidden variables within the same experiment so that non-locality
arguments have to be re-argued based on that, then that's not going to
add anything but confusion.
It might confuse you, but until you can do this you have not achieved
a local explanation of the general case in which the polarizers are at
arbitrary angles to each other.
It seems that you are making valiant attempts to avoid the case where
the shit hits the fan. Good luck with that, but I am not fooled.
Bruce
It's totally irrelevant, but I'll write out the GHZ state case with 3
entangled spins and 3 observers that measure the x and y components of
the spins to demonstrate step by step that there is no problem in the
MWI, while in the CI there is a non-locality problem later when I have
time.
But in general, the EPR thought experiment proves you wrong, the common
cause there does not save you from non-local effects in single universe
collapse theories. Then Bell proved EPR wrong in the sense that, there
is no local hidden variable way out of non-locality without violating
QM.
Then, if we assume that QM is correct then there is not even the need
to actually go about verifying the violation of Bell's inequalities. All
that this verification does is that it rules out local hidden variables
theories, regardless of whether or not QM is actually true or not
(suppose it get's falsified, then you would still have falsified all
local hidden variables theories).
Saibal
--
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.
