Re: Delayed choice experiment and interstellar signalling

Williamknowsbest wrote:

Take a light source and a *** of material with two fine closely
spaced lines cut in it and a projection screen. These are the
elements of a double slit experiment.

The two slits will become sources of radiation that are in phase with
one another, and the radiation from both slits will interfere creating
an interference pattern on the screen.

http://en.wikipedia.org/wiki/Double_slit

Now, HOW you observe the experiment will determine what sort of
interference pattern occurs! This happens even if you decide how to
observe the experiment AFTER a photograph is taken of the interference
pattern.

Choose to observe a variable in the experiment after a photograph
showing the result of the experiment and you will always see the
photograph a certain way. Chose to observe another variable and the
photograph will always be seen a different way. Even after the
experiment is over.

This is called a delayed choice experiment

http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment

This apparatus can also be used to create a quantum eraser

http://en.wikipedia.org/wiki/Quantum_eraser_experiment

Now, my question is, can a variation of this process be used for
instantaneous interstellar signalling?

Imagine that two microwave sources send microwave signals through an
interference device and also send entangled microwave signals across
an interstellar distance after polarizing one of the sources.

A party recieving the signal can decide to measure the polarization or
not,changing the result of the observed interference pattern of the
entangled microwaves at the point they are interfered - even though
the later observation took place years or decades later. In some
sense, by choosing what parameter to measure, the receiving party can
change the result the sending party sees AT THE TIME THE MICROWAVES
WERE SENT. The sending party can reply to the receiving party by
placing another microwave signal in the beam and switching it on and
off. Thus instantaneous interstellar communications are achieved.

Also, if the receiving party sends a similar signal back to the
sending party, the sending party can send signals back in time to
itself. And by sending signals back to the receiving party, the
receiving party too, can send signals back to itself in time.


It's also a pretty compelling argument for the whole thing being misconceived.

I think it reflects a misunderstanding of what entanglement involves. QM predicts that *if* the two observers of a pair of entangled photons measure the polarisation at the same angle, they will get the same result. It also predicts what level of correlation will be found if they choose to measure the polarisation at different angles.

In no sense does it imply that measuring one of the entangled photons has any effect whatsoever on the other one.

It's undoubtedly a strange result, but it does not involve transmission of information. For one thing, if the measurements are separated by a spacelike interval, the order of the measurements is not well defined - you wouldn't be able to say whether the information was transmitted from measurement A to measurement B, or vice versa.

See http://en.wikipedia.org/wiki/EPR_paradox

and

http://en.wikipedia.org/wiki/Bell%27s_theorem

Sylvia.
.