On 12 Nov 2017, at 23:23, Bruce Kellett wrote:
On 13/11/2017 4:13 am, Bruno Marchal wrote:
On 12 Nov 2017, at 03:47, Bruce Kellett wrote:
On 12/11/2017 4:34 am, John Clark wrote:
On Fri, Nov 10, 2017 at 7:08 PM, Alan Grayson <agrayson2...@gmail.com
> wrote:
>> That's not the measurement problem, its determining
if how and why observation effects things.
> Not to split hairs, but why we get what we get in
quantum measurements, and how measurement outcomes come to be
what they are, are the same problem IMO.
The measurement problem is not the ability or inability to
predict exact outcomes, the measurement problem is
defining what is and what is not a
measurement and finding the minimum properties a
system must have to be an observer. Nondeterminism
is not a problem and there is no inconsistency at all regardless
of what turns out to be true ; if some effects have no
cause and true randomness exists then that's just the way things
are are and t here is no resulting paradox and no
question that needs answering.
The title of this thread is about the consistency of Quantum
Mechanics, but far more important than QM is the ability of ANY
theory to be compatible with experimental results, and one of
those experiments shows the violation of Bell's Inequality. And
that violation tells us that for ANY theory to be successful at
explaining how the world works AT LEAST one of the following
properties of that theory must be untrue:
1) Determinism
2) Locality
3) Realism
Is Many Worlds deterministic? Yes in the sense that it just
follows the wave function and that is deterministic, it's only
the collapse of the wave function that is nondeterministic and
that never happens in Manny Worlds.
Is Many Worlds Local? Some say yes but I would say no because
those other worlds are about as non-local as you can get, you
can't get there even with infinite time on your side. But even if
I'm wrong about locality Many Worlds would still be in the
running for a successful theory because it is certainly not
realistic.
I would agree with you that the many worlds account is non-local.
The problem that MW faces is that the separate worlds split off
when measurements are made at either end of the EPR experiment
must somehow be made to match up appropriately when the two
experimenters communicate. This requires coordination of separate
worlds, which, as you say, is about as non-local as you can get.
OK, but without action at a distance. If you take into account the
local propagation of the observers (treating them quantum
mechanically), and the same for their "future" counterparts. The
coordination is just kept locally by the observers. There is a
strong "local" first person sharable non locality, but yet no
physical action at a distance, nor problem with physical realism
(albeit multiversal).
Non-locality just means that there is a non-local influence -- what
happens to one member of an entangled pair influences the behaviour
of the other. No model is proposed for how this happens, because any
local causal model would have to be of the 'hidden variable' type,
and Bell has ruled out such local hidden variable accounts. The
'Quantum Mechanics is incomplete' route is ruled out. Maudlin
explores this in considerable detail in his book.
The problem becomes particularly apparent if you consider an EPR
experiment with time-like separation. Let Alice prepare an EPR
pair in her laboratory, then measure the spin of one of the pair
in some defined direction. She then takes the other member of the
EPR pair down the corridor to her partner, Bob, and gets him to
measure the spin projection in the same direction. If the two
particles are independent,
then both measurements give 50/50 chances for up/down.
OK. But they are not independent. After her measurement she is in a
class of worlds with some definite result for both particle, with
respect to the base up/down.
There is no 'class of worlds'. There are two worlds, one
corresponding to each of the possible results for Alice's measurement.
After Alice measures her particle, she splits into Alice_up and
Alice _down according to her result. Both copies then go to Bob's
laboratory, which by then has also split according to Alice's
result.
OK.
So Alice_up meets Bob, but when he measures his particle, he still
has 50/50 chances of either result.
I don't think so. Only if he got the time to do it before Alice
splits has not rich him.
Locality says unequivocally that what I say is correct: the particle
that Alice presents to Bob (in each world) is in exactly the same
spin state as when produced.
But he would propagate a possibly "violating Bell" result to a
different Alice, just by tyhe lienarity of the tensor products and
evolution.
I don't understand this comment. Alice and Bob have communicated,
she has told him her measurement angle and the result she observed.
All of this before he makes his measurement. But locality says that
the fact that he has this information cannot influence the
measurement he is about to make, so he still splits according to
'up' and 'down', and Alice becomes entangled with his result along
with him. Unfortunately, this also involves combinations of results
that violate angular momentum conservation.
Unfortunately, the only result that is consistent with spin
conservation is that if Alice got 'up', he must get 'down', and
vice verse (remember that the measurements are aligned by design).
Yes.
Since Alice_up can't meet a Bob_up, there must be a non-local
influence that determines Bob's result according to which Alice he
meets. This is not removed be assuming no collapse and many worlds.
Of course, with time-like separation, the results can be explained
by a local hidden variable, but no such explanation is available
for space-like separated measurements, and the same explanation
must be available for both cases.
But it is. Because the Alice and Bob moves locally, causally and
lives always in the partition dictated by the result of
ùeasurement, which propagate locally. In a pure space-like
separation, you cannot even defined the identity of the observers
with respect to their counterparts.
Bullshit. I have taken great care to design a scenario in which the
participants are always alongside each other so that they have no
doubt about the identity of their counterparts.
Since non-locality is still present for time-like separations, it
must be present in all cases. So many worlds do not eliminate non-
locality in Bell-pair measurements.
It does not eliminate the apparent non-locality, or Bell's results,
but it eliminate the "physical action at a distance".
What has 'physical action at a distance' got to do with it? Non-
locality involves instantaneous influence at a distance. Physical
action at this distance would be a local theory. Read Maudlin's book!
You are playing with word. "instantaneous influence at a distance" is
another terming for "physical action at a distance".
I read and quoted already Maudlin; he says exactly what we are saying
here: there is "non locality", but without any instantaneous action at
a distance once you eliminate the collapse of the wave.
The violation of Bell's inequality only show that each branch is non
local, and that there *would* be action at a distance, if they were
unique. But none are unique, and in the global picture, the appearance
of non-locality and indeterminacy is explained in term of only causal
interaction propagating at speed < c.
Bruno
Bruce
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