The media have not reported (as far as I know) a beautiful swiss experiment
about the superluminal quantum *influence*. This experiment has tested the
s.c. *conspiracy* or *peaceful coexistence* between quantum mechanics and
special relativity and has shown that such an *influence* (spooky action -
or *passion*, or *fashion* - at a distance) acts with a velocity which is
greater than 10^6 C (one million times C).
Whether or not such an *influence* is actually carrying some kind of
information, or *is* itself the information (is quantum mechanics an
information theory?), or is nothing but a distant quantum state preparation,
or is supported by possible *preformed and non local* structures of quantum
objects, are still open questions.
The experiment. Two entangled photons are emitted (in Geneve), one directed
to a Franson interferometer (in the village of Bellevue), one directed to a
Franson interferometer (in the village of Bernex). The geometry (Geneve,
Bellevue, Bernex) is triangular and each side is about 10 kilometers long.
When both photons reach these interferometers a (second order) interference
pattern appears, violating Bell's inequalities and thus showing that *local*
hidden variables do not exist. This interference pattern means that the
dynamics of an entangled photon into the interferometer in Bellevue and the
dynamics of the other entangled photon into the interferometer in Bernex are
*identical*. Actually entangled photons behave *coherently* at very long
distances. This is called the nonlocality (or nonseparability) problem.
Nonlocality comes directly from: the superposition principle (entangled
photons states are superposed in a singlet state); the s.c. *collapse* (when
some *selective* or recordable measurement is performed) of the wavefunction
describing the entangled photons superposition; the indistinguishability of
the entangled particles; the wave-like nature of particles.
Now if we move, very fast, one interferometer (Bellevue) and-or the other
(Bernex) we must take into account the special relativity. These relative
movements can indeed lead to a tension between quantum mechanics and special
relativity, so that the interference pattern could breakdown. That is
because, due to the (relative) movements of interferometers, *each* of the
two natural reference frames (Bellevue interferometer, Bernex
interferometer) can realize that the local measurement takes place *before*
the distant one and can also think that the state of the distant photon is
knowable performing some local measurement (on the local photon). Of course
if we perform measurements in regions which are space-like separated (no
causal correlation) we lose even the lorentzian invariant way of determining
which measurement was carried out first, but the experiment Iım speaking
about does not require this assumption. (Whether or not operators localized
in space-like separate regions commute is an interesting question!).
In this context the term measurement means *collapse* (or sudden
decoherence) of the wavefunction describing both entangled photons
(singlet), hence a unique irreversible nonlocal process. This process is not
dependent on any time ordering of measurements, because it is based on the
superposition principle and on the indistinguishability of the entangled
particles. We can also say that while this process is not dependent on any
time ordering of measurements it could be linked to an absolute universal
order of succession: the Bohmian *quantum aether* or some other preferred
reference frame. So this quantum nonlocality could take us to a quantum
causality principle, based on that preferred universal (cosmic) reference
frame (and on the absolute time ordering of measurement).
Thus from quantum mechanics we get the following points: the measurements
can create events which are simultaneously correlated (even in space-like
separate regions); the time ordering of measurements is irrelevant; the
relativistic causality is not necessarily a rule; the superposition seems to
be the main principle and is based on the indistinguishability of entangled
particles; the (second order) interference pattern is predicted.
But from special relativity we get: the postulate which says that the laws
of physics have the same form in all inertial frames; the prediction which
says that in space-like separate regions causality does not exist (no
possible causal correlations); the postulate which says that the velocity of
light *in vacuo* is independent of the velocity of the source; the necessity
of some time ordering of measurements; the essentiality of the relativistic
causality; the interference pattern breakdown prediction.
Experimental results show no evidence for a breakdown of the interference
pattern. Is this a sharp violation of the relativity postulate which says
that laws have the same form in all inertial frames ? The answer is this:
actually the Hardy-Percival theorem has already shown that a quantum
*nonlocal* process needs a preferred lorentzian reference frame.
Experiments also show that the quantum *influence* velocity exceeds 10^6 C.
Is this almost infinite speed in conflict with relativity which says that
the front velocity of a signal cannot be greater than C, and the group
velocity of a signal can be greater than C (but the peak cannot overtake the
front) ? Is this almost infinite velocity in conflict with the relativistic
causality which says that the source cannot cause effects outside its
forward light cone ? Answers to these questions depend on the meaning of the
terms *influence* and *collapse* and also on the possible *preformed and non
local* structures of those entangled particles.
Anyway it is largely accepted that if a *perfect quantum cloning machine*
could create *identical* particles this machine could easily violate
Heisenberg's (uncertainty) principle. The perfect knowledge of all
observables (measuring many particles which are identical) could then allow
an effective superluminal quantum signaling. But that machine unfortunately
can not exist (there is the *no quantum cloning* theorem). So the
*conspiracy* is back again ! A new old fashioned principle could be on the
way (the quantum mechanics and relativity complementarity).
Since things are, at present time, so subtle, what about the very beginning
of the story when - as Boltzmann once said, commenting on the entropy law -
the universe was in a very unlikely condition ?
particle's infinite (or superluminal) tails
is quantum mechanics nothing but an information theory?
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