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Q: Quantum Entanglement ( No Answer,   10 Comments )
Subject: Quantum Entanglement
Category: Science > Physics
Asked by: mongolia-ga
List Price: $15.00
Posted: 04 Sep 2006 09:28 PDT
Expires: 04 Oct 2006 09:28 PDT
Question ID: 762100
I have been reading some stuff lately on Quantum entanglment. It would
appear that the concept of Quantum Entanglement has been fully
verified by numerous experiments in the last few years. One thing
though which I cannot seem to get a straight answer on is as follows:

Using the concept of Quantum Entanglement is it possible to send
information faster than the speed of light?


There is no answer at this time.

Subject: Re: Quantum Entanglement
From: qed100-ga on 05 Sep 2006 09:46 PDT

   This is a very interesting question. If by "information faster than
light" you mean, in other words, "conduct a meaningful conversation
about the weather with someone on Proxima Centauri, without a time lag
of several years", then the answer is no.

   Entanglement is the circumstance that at least two quantal objects,
particles, are both features of a single wave function. Thus, what
happens to one also happens to the other, regardless of their
remoteness. This isn't, however, the same as conducting causality
across spatial distances at greater than c. When what happens to one
also happens to the other, it's because the wave function is an object
which cannot be reduced to its parts without destroying the identity
of the wave itself. By making an unambiguous measurement of an
entangled particle here, the wave function collapses, which has
implications regarding the more distant entangled partner.

   In other words, the distant particle must also select a well
defined state, since its wave function has collapsed. Nevertheless, we
here on Earth aren't there to physically measure the distant particle,
and vis versa for anyone at Proxima Centauri.
Subject: Re: Quantum Entanglement
From: willh-ga on 05 Sep 2006 13:58 PDT
Why do I think the same person both asked and answered this question?
Subject: Re: Quantum Entanglement
From: qed100-ga on 05 Sep 2006 14:19 PDT
How do you figure that?
Subject: Re: Quantum Entanglement
From: hillcd-ga on 06 Sep 2006 04:56 PDT
I am not being paid for this, but I would like to answer anyway.

The answer is: no.

Entanglement is only a statement about the correlations between two
particles. You cannot transmit any information via this entanglement.
However, if you make measurement of the two particles and look at the
correlations between the measurement records, then the correlations
are different to what you expect classically. In particular, for some
entangled states, they can violate a Bell inequality.

The most common example is that of
|psi> = 1/sqrt(2) |00>+|11>
which is a maximally entangled state. Imagine that Alice has the first
particle and Bob has the second. Alice's measurements are completely
random. 50% of the time she will measure a "0", 50% of the time she
will measure a "1". There is nothing that Bob can do to affect this.
No measurement (or rotation of his system) that Bob makes will affect
what Eve measures. Since Bob can't do anything to affect what Alice
measures, he can't pass her any information. It is only if Alice and
Bob communicate (which is limited by the speed of light) that they can
make use of the correlations provided by entanglement. So the answer
is no.

The best explanation I've seen of this is in Michael Nielsen and Ike
Chuang's book, "Quantum Computation and Quantum Information".

Of course, entanglement is extremely useful for communications. If you
and I share that state, |psi> we can communicate faster than if we
don't. In fact, we can communicate double as fast. That's known as
"dense coding". For every one bit of information that you send me, I
can find out two bits of information. But it certainly doesn't let me
communicate faster than the speed of light: We still need that
classical communication to send the first bit, and that is limited by
the speed of light.

Entanglement is also useful for teleportation. In teleportation, Alice
and Bob share an entangled pair. Alice then makes a measurement on her
system. Then she _sends_ the result of this measurement to Bob. That
classical information she sends is limited by the speed of light. Bob
then has to perform different actions on his half of the entangled
pair, based on the information that Alice has sent him. As you see, it
is the correlations that we are making use of. Once again, it is only
when Alice and Bob communicate (which is limited by the speed of
light) that they get any advantage.

The short answer is: no. Entanglement does not allow communication
faster than the speed of light.
Subject: Re: Quantum Entanglement
From: mongolia-ga on 06 Sep 2006 06:33 PDT
Dear willh-ga

It looks as though  as a result of my Question , I have almost become
"entangled" with qed100-ga :-).

I have to disappoint you however to let you know we are two different
entities.  Sometimes I do envy qed100-ga's in depth knowledge of
Physics and other things. I very much admire his (or maybe her!)
contributions to this forum.

As our Wave Functions have separate origins there is no danger of us
ever been found in the same Quantum State.


Subject: Re: Quantum Entanglement
From: qed100-ga on 06 Sep 2006 07:08 PDT
Hi Mongolia,

   In my case I'm a "he". It is hard to figure out sometimes what the
posters' genders are. I went for months thinking Pink was a guy. :)
Subject: Re: Quantum Entanglement
From: pafalafa-ga on 06 Sep 2006 08:01 PDT
That's why I chose the unambiguous name avoid any gender confusion!
Subject: Re: Quantum Entanglement
From: qed100-ga on 06 Sep 2006 08:15 PDT
pafalafa, did Saturday Night Live used to have a regular character
based upon you? Someone named "Pat"? :)
Subject: Re: Quantum Entanglement
From: scubajim-ga on 06 Sep 2006 12:51 PDT
I'm glad the poster allowed for the possibility that the answer was
from a she and didn't assume the answer was given by a man.  There are
also many intelligent women out there. (I am a he.)
Subject: Re: Quantum Entanglement
From: purpleprogrammer-ga on 20 Sep 2006 20:50 PDT
There is evidence that communication /can/ occur at a rate greater
than the speed of light.

Scientifically, this has only been so far as, maybe, the planck length
(a very, very small amount of spacetime)...  Due to uncertainty, some
data can arrive faster than light, even though the average speed is no
greater than the speed of light.  Apparently this has been verified in
experiment, but it really only gives you a faster transmission by a
totally useless amount.

Also, some photons are polarized in the time direction; they "wobble"
forward and backward in time, rather than horizontally or
vertically... so they toggle between going a little faster than light,
then a little slower than light.  It all averages out (like so many
other things in quantum science) to being *at* the speed of light, but
on a very small scale, it might be just a little ahead, or a little

So, to recap, it's only possible +/- the heisenberg uncertainty principle.

As a friend once put it, Quantum Science states this quite clearly:

Blah blah faster blah than blah blah light: False.

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