Finora il valore della velocit� della luce nel vuoto non � stata superato da
alcuna altra *front* velocity. Da sempre la *group* velocity pu� superare
tale valore ma si ritiene che il *peak* non possa mai superare il *front*,
salvando la causalit� (light cone) e la relativit� (C indipendente dalla
velocit� della sorgente). Ci sono per� velocit� *quantistiche*, non
relativistiche, che superano C (fino a 10^7 C !). Come qui di seguito �
detto.
The media have not reported 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 showed 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 the 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 (Bellevue) and the
dynamics of the other entangled photon into the interferometer (Bernex) are
*identical*. Actually entangled photons behave *coherently* at very long
distances. This is 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 recorded 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 I�m
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 it is irrelevant; the
relativistic causality it 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 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 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 ?
swiss experiment
http://xxx.lanl.gov/abs/quant-ph/0002031
http://xxx.lanl.gov/abs/quant-ph/9704038
particle's infinite (or superluminal) tails
http://xxx.lanl.gov/abs/quant-ph/9809030
http://xxx.lanl.gov/abs/quant-ph/0003075
is quantum mechanics nothing but an information theory?
http://xxx.lanl.gov/abs/quant-ph/9806047
Received on Mon Jun 19 2000 - 00:00:00 CEST