Re: Faster than light signalling via an EPR type mechanism
- From: of_1001_nights@xxxxxxxxxxx
- Date: Mon, 30 Mar 2009 08:15:37 +0000 (UTC)
On Mar 28, 12:19 pm, John Hudson <johnhudson20032...@xxxxxxxx> wrote:
I believe I have a way of sending faster than light messages via
an EPR / Bell Inequality type mechanism, and would appreciate some
feedback (and hoping this is a suitable venue).
I am aware that faster than light signaling is generally not
considered possible by this process. The reason being that the
inequality is determined by comparing the spins of the correlated
pairs of photons at the two stations ... and this can only be done
at the end of the run at subluminal speeds.
However my proposal -- although it employs a typical EPR type set
up -- is not used to demonstrate the Bell Inequality, and does not
require post-test communication between the two stations. All the
information required can be obtained from the 'receiving' station
alone.
The set-up and procedure are described in more detail below, but
briefly it involves measuring the difference in the number of
'spin-up', and 'spin-down' photons passing through a nominated
'receiving' filter for a large number of counts, over a series of
runs, and then calculating the standard deviation for the series.
The test is carried out at different angles between the two filters
and the standard deviations compared for the various angles.
I am predicting / expecting that the standard deviation will increase
as the angle between the filters is increased.
This prediction is not correct (although you are in good company).
The statistics of measurements made solely at the 'receiver' filter
are completely independent of the orientation of the 'sender' filter
and of any correlations between the photon pairs. Any observable
dependence on the relative angle of the filters is only obtainable
from joint measurements.
This 'locality' or 'no-signalling' property of quantum mechanics is
well known. Basically, it is not difficult to show that if a joint
system such as the one you mention is desribed by some some
density operator rho, then, if one only deals with measurement
data from the first system, the statistics of the data are exactly
those arising from the reduced density operator
rho_1 := tr_2[rho],
obtained by tracing out the second system. This is the case
no matter what might be physically done to the second system
(eg, adjusting your "sender" filter). Note that the reduced
density operator is independent of any correlations.
The philospher Karl Popper suggested an experiment of a
similar sort, where correlated position and momentum states
(EPR states) would be generated, and a "sender" filter was
set to be a wide slit or a narrow slit. Popper assumed that if
the slit was narrow, there would be strong momentum dispersion
at the sender, and hence, due to correlations, strong momentum
dispersion at the receiver. Popper didn't know much quantum
theory of course, as this assumption is not consistent with it!
However, forms of the experiment were eventually carried out,
and it was shown he was incorrect (see, eg,
http://en.wikipedia.org/wiki/Popper's_experiment for refs).
.
- References:
- Faster than light signalling via an EPR type mechanism
- From: John Hudson
- Faster than light signalling via an EPR type mechanism
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