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!
Bruce
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