Re: Why does EPR need two observables?

Ilja Schmelzer wrote:
In the form used by Bell the "reality assumption" is very general, very
weak, does not even refer to spacetime. It may be argued that giving
it up is equivalent to give up parts of the scientific method (the search
for realistic explanations of observable effects.)

Einstein causality (i) is, instead, a quite special assumption about
spacetime. If we give up realism, the realistic version of Einstein
causality becomes meaningless anyway. The weak (probabilitistic)
version survives anyway. Thus, if we compare the two choices,
we only loose if we give up realism.

I don't know that one can be said to be definitively weaker than the
other - it possibly comes down to personal preferences. I find it hard
to give up either (i) or (ii) !

However, arguments for not giving up the locality assumption (i)
include
(a) "If we don't see it, why postulate it?": Locality is strictly
predicted/observed by quantum mechanics (operators with
spacelike-separated support commute). Hence, non-locality can only
consistently be brought in at some "deeper" level providing that this
level remains wholly unobservable for all known quantum phenomena.
This smacks of (unnecessary) conspiracy to me! Of course, it would be
a different matter if the deeper level did give rise to physical
predictions of verifiable nonlocality (while avoiding the shooting of
one's own grandparents).
(b) "Giving up locality is equivalent to giving up parts of the
scientific method": The main reason that physicists get away with
explaining so many things about the world around us is because we can
ignore so much of the world around us (we are also very good at picking
things that we can explain!). In particular, most interactions between
systems (and indeed most systems) can be ignored in modelling a
particular phenomenon - particularly if they are a long long way away
(eg, in the lab next door). If we suddenly say that there are in fact
nonlocal interactions going on between everything, we put this basic
principle at risk - how can I ever hope to understand the energy levels
of a single hydrogen atom if the electron is interacting with my Aunt
Jemima's hairdryer? Where is the noise?


Some people also note
that the root of the trouble might be the implicit use of a third
"non-conspiracy" assumption, that experimenters have free choice in
what they are going to measure.

If we follow this line, Einstein causality becomes meaningless.
Even a working FTL phone would no longer falsify Einstein
causality, because it could be explained with conspiracy as well.

I agree that conspiracy is rather too strong to please anyone. It is
more of an unpalatable logical possibility (bit like religion
explanations really!). There was a very amusing paper by Shimony and
others in Dialectica a long while ago, with this scenario: Alain and
Philipppe order their "Bell experiment" apparatus and set it up. To
randomise the measurement settings for each correlated pair of spins,
they get their secretaries to say "heads" or "tails", and set the
detectors appropriately. They verify the Bell inequalities are
violated, and publish in the latest fashionable journal. However,
unknown to them, the manufacturer had provided two sheets, each with a
sequence of "heads" and "tails" written on them, and the secretaries
had merely read out what was on the sheets in order. It was all
predetermined!

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