From: *Bruno Marchal* <[email protected] <mailto:[email protected]>>
On 16 Jul 2018, at 03:57, Bruce Kellett <[email protected]
<mailto:[email protected]>> wrote:
Other directions are irrelevant to the measurement. The state is not
in a superposition of eigenvectors of every possible orientation.
After the measurement is done, and this makes sense only with
orthogonal measurement from Alice and Bob parts, if not, as I say
above, the notion of belonging to the same world makes no sense. You
tall like if a measurement determine which world they are both in,
which is true only for particular case, when they made the measurement
of the spin or polarisation in correspond direction.
This is simply wrong. The state is a superposition of up and down for
both observers, regardless of whether their measurements are along the
same axis or not. The measurement does determine the world in which they
will find themselves when they meet. Just think through the significance
of the facts that they record their measurements in their lab books as
they go along; they are in the same world when they meet; and they can't
jump between worlds at any point. The logical conclusion of this is that
the correlated Alice/Bob pair are always in the same world -- they can't
be in any other world because world-hopping is not allowed. When you can
get this point, all the mystery of EPR correlations vanishes -- they are
still non-local, but that is just a consequence of the non-separability
of the singlet state.
All your worries about FTL are irrelevant -- as I have pointed out many
times. It is all a lot simpler than you seem to want to make it.
..........
And then each tracks along a particular branching tree as recorded by
the sequence of up/down results recorded in their lab books. There is
absolutely no ambiguity here because neither Alice nor Bob can switch
between branches -- they must always be in the same branch.
But when non orthogonal measurement are done, this makes no sense.
It is the only thing that does make any sense. If you can't track back
through the history of your sequences of branches -- by referring to
your lab book if necessary -- then Everettian branching worlds make no
sense. And it does not matter whether the measurements are orthogonal or
not -- they always end up with Alice and Bob meeting in one world with
sequences of measurements made in that world. Once you can grasp this,
Bruno, you might gain some insight into both Everett and EPR.
I did. I referred also to Pirce FAQ for a good approximation.
The Price account assumes non-locality -- as I have pointed out on many
occasions.
You are the one invoking the FTL, so I think you are the one who
should explain where that comes from, and how to test it
experimentally. Aspect experience test non-locality or inseparability,
not FTL.
You keep accusing me of invoking FTL. I have never done any such thing.
All I have talked about is the non-separability of the state and the
fact that the spin measurements are made non-locally. If you invoke FTL
then you are invoking a non-local hidden variable. I see no need to do
this, and never have done. Stop reading things into my arguments that
are not there.
There are not an infinity of worlds, there are only 2^N of them. Of
course the correlations come out right for every Alice/Bob pair when
they meet. But you have not explained this locally.
That is exactly what the wave explains, when you dismiss all collapse.
The wave evolves purely locally in the phase space, which is the real
“mutiversal” reality (up to some gauge nuances).
The wave function, as Maudlin explains, is itself non-local. So you have
not magically restored locality by invoking the wave function. The wave
function for this state is non-separable = non-local.
In fact, the multi-branching tree forms a giant superposition, and
we have just singled out one component of this superposition. There
is nothing at all mysterious in this -- it is what physicists do
all the time when they perform calculations in momentum space -- on
just one component of the superposition that makes up a wave packet.
That makes sense.
That is what I have been saying all along, and this is what removes
your worry about 'collapse models' -- they are just a branch from the
many-worlds superposition.
I don’t see how you would do that when Alice and Bob makes non
orthogonal measurements.
You don't understand the underlying quantum mechanics in that case.
The "giant superposition", in so far as it exists, has been spirited
away by just looking at a single branch. There is nothing the "giant
superposition" can add to the conclusions obtained from just one branch.
? It changes the result of further possible measurement, notably
involving Alice and Bob possible amnesia, in theory. Collapse has to
be non local to make sense, as Eistein made already clear in 1927. It
is just worst with the singlet state, but here too, avoiding a
collapse avoid the need of FTL.
Avoiding the collapse does not change one thing in relation to the
understanding of EPR correlations, as my analysis of the superposition
of the 2^N copies of Bob and Alice in terms of a single branch within
this superposition shows. Collapse models do nothing more than pick out
one branch in the Everettian tree. Nothing magical about it. We do this
all the time when we disregard the other worlds in everyday life.
I don’t see at all why and how any FTL would be needed once we agree
that both the evolution and the tensor product are linear.
Stop worrying about FTL. There is no physical FTL mechanism
Then; like last time, se agree on everything!
I would like to think that this were the case, but you keep coming up
with irrelevancies that contradict the straightforward account of these
phenomena. If you forget about the metaphysics and just concentrate on
Alice and Bob making real measurements and recording them in their lab
books, then all these superfluities vanish. There are no
counterfactuals, no worries with other unobserved worlds, and Bell's
theorem goes through exactly as he intended. Many-worlds does not
invalidate Bell's argument. In fact, deflecting Bell's theorem would do
no more than allow for the possibility of a local hidden variable
account. That alone does not prove that many-worlds is local -- that
would still have to be established by developing such a local hidden
variable theory. No one has to date developed such a theory. But since
Bell's theorem has not been deflected, we do not have to worry about
such contingencies.
Bruce
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