Has FTL communication really never been tested in this way?

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P

PostReplies

http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]

I know most physicists reject the idea of FTL communication but I'm
surprised the experiment described in this article wasn't carried out
years ago or has it? Wouldn't the existence or absence of an
interference pattern created by entangled beams of photons be a way to
receive a signal instantaneously?
I'm not a physicist, just an interested lay person.
 
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U

Uncle Al

PostReplies wrote:
>
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.


Chirping a wave packet does not convey information superluminally.
Ditto diddling group vs. phase velocities. The
Einstein-Podolsky-Rosen paradox is *instantaneous* wavepacket collapse
into an observable across arbitrarily large distances and volumes
(throughout the entire universe if so configured). No information
exists until sender and recipient compare data - and that is limited
to lightspeed.

The universe is strictly causal. There is no superluminal conveyance
of information. Conceivably one could cleverly pull a Star Trek or
whatever and get from here to there in violation of Special
Relativity. One doubts there would remain opportunity to do it a
second time.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2
 
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D

Dirk Bruere at NeoPax

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>


Isn't this a type of Wheeler Paradox necessitating a Quantum Eraser type
measurement on the short time-of-flight beam?

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
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M

Marc Millis

In <7jhah3p1vo77562fircdttj8gvuq1lgsm3@4ax.com> PostReplies wrote:
>=20
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>=20
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>=20


Right now, experiments are being conducted at the Univ. of Washington in=20
Seattle, by Dr. John Cramer, to directly attack the causality issues of=20
quantum entanglement and "nonlocality" in communication. I suspect data=20
will be forthcoming later in 2008. Even then the results will be=20
difficult to comprehend.

The difficulty with this and even more-so with prior experiments is to=20
clearly define the connections between the parts of the system and=20
causal relations (cause precedes effect). In short, it is hard to=20
follow the chain of logic and facts in these experiments, and to be able=20
to distinguish facts from interpretations. Our notion of time and=20
causal connections is so innately ingrained in us, that it makes it very=20
hard to break from these notions and just to look at the physics in its=20
raw form, and then to try and decipher what is really going on. It is=20
confusing for physicists too, which is reflected in the ongoing=20
publications. Even the terminology varies from one approach to another (
e.g. the terminology used in quantum tunneling, electrical engineering,=20
retarded potentials... ).

Regarding FTL 'travel,' the situation is still theoretical, not=20
experimental. To get passed the light-speed limit of spacetime,=20
concepts of "warp drives" (Alcubierre) and "wormholes" (Visser) toy with=20
the idea of manipulating 'spacetime' itself to circumvent the limits. =20
In short, even these FTL concepts appear to evoke "closed time-like=20
curves" which means possible causality violations (Arrive back from your=20
trip before you departed to stop yourself from taking the trip...). =20
Given how little is known about spacetime itself and other unknowns=20
dubbed "Dark Matter" "Dark Energy" etc, it is safe to say we all have a=20
lot more learning to do.

In short, we are beginning to get smart enough to ask the right=20
questions on such deep unknowns, but it is likely to take a while before=20
we fully comprehend how nature works - if ever. It is fun to work on it=20
and to ponder the possibilities.

Regrettably, I do not have good books to recommend to you on this=20
subject that are deliberately written for the non-scientist.

Marc
 
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I

Ian Parker

On 17 Oct, 21:48, PostReplies <nom...@pleasepost.net> wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.


We can look at this from a number of viewpoints. Looking at this
mathematically I can write down Maxwell's equations, I can then
perform a Lorenz transformation and I will still have Maxwell's
equations. The Universe still looks the same. I may write
Schroedinger's equation as

H = E an equation which is invariant under Lorenz transformation.
Quite clearly from this FTL communication is impossible.

How then does this seeming paradox arise? Basically because if
something were a million LY distant it would

a) Take a million years to set the experiment up.
b) H = E we will have 2 eigenvalues of H which differ by 1/
(10^6*365.25*24*3600) Hz. Hence although we have (instantly) changed
state the uncertainty principle prevents us from seeing it.

This question of the UP to me at any rate resolves the paradox.

- Ian Parker
 
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S

scerir

"PostReplies":
> I know most physicists reject the idea of
> FTL communication but I'm surprised the experiment
> described in this article wasn't carried out
> years ago or has it?


Something similar many times, but not exactly
that one. I.e. see this one below
http://www.arxiv.org/abs/quant-ph/0106078

Next step would be (imo) to remove the coincidence
circuit and to have a "100% clean" source of
entangled pairs, i.e. something which emits
entangled pairs *only*, and with the *right* timing
:-)

(I do not know if prof. Cramer is trying something
like that, maybe he is interested in the 'bilking'
effect).

> Wouldn't the existence or absence of an
> interference pattern created by entangled
> beams of photons be a way to receive a signal
> instantaneously?


FTL signal? Uncontrollable FTL signal?
FTL causation? FTL influences?
It is a bit obscure (to me) what is allowed
by SR, and what is not.

Anyway standard QM seems to forbid that.
But there are (more or less) interesting papers
saying something different (are those theorems
circular?).

-J. B. Kennedy. On the empirical foundations
of the quantum no-signalling proofs.
Philosophy of Science 62, 543-560 (1995).

see also this one
http://www.arxiv.org/abs/quant-ph/9906036
 
K

Knud Soerensen

Maybe you should take a look at
www.cheniere.org/references/G-COM 3.pdf

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>
 
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D

Dirk Bruere at NeoPax

Knud Soerensen wrote:
> Maybe you should take a look at
> www.cheniere.org/references/G-COM 3.pdf
>


Never mind the PR BS, where's the circuit diagrams so we can test this
for ourselves? Or is this another inventor who wants $1billion up front
before he reveals the 'secrets'? Clearly a Nobel prize isn't a big
enough reward.

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
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D

Dirk Bruere at NeoPax

Knud Soerensen wrote:
> Maybe you should take a look at
> www.cheniere.org/references/G-COM 3.pdf
>


Never mind the PR BS, where's the circuit diagrams so we can test this
for ourselves? Or is this another inventor who wants $1billion up front
before he reveals the 'secrets'? Clearly a Nobel prize isn't a big
enough reward.

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
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D

Dirk Bruere at NeoPax

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>


BTW, isn't this expt similar to something Sarfatti suggested years ago?
Not saying he originated it but I seem to recall reading about him
mentioning it.

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
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P

PostReplies

On Sat, 20 Oct 2007 10:09:25 +0000 (UTC), "scerir" <scerir@libero.it>
wrote:
>
>Next step would be (imo) to remove the coincidence
>circuit and to have a "100% clean" source of
>entangled pairs, i.e. something which emits
>entangled pairs *only*, and with the *right* timing
>:-)


That's what Cramer is doing. Here's another page I found explaining
his experiment and a very interesting earlier experiment which was
encouraging:

http://www.paulfriedlander.com/text/timetravel/experiment.htm
 
D

Dirk Bruere at NeoPax

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>


BTW, isn't this expt similar to something Sarfatti suggested years ago?
Not saying he originated it but I seem to recall reading about him
mentioning it.

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
Last edited by a moderator:
P

PostReplies

On Sat, 20 Oct 2007 10:09:25 +0000 (UTC), "scerir" <scerir@libero.it>
wrote:
>
>Next step would be (imo) to remove the coincidence
>circuit and to have a "100% clean" source of
>entangled pairs, i.e. something which emits
>entangled pairs *only*, and with the *right* timing
>:-)


That's what Cramer is doing. Here's another page I found explaining
his experiment and a very interesting earlier experiment which was
encouraging:

http://www.paulfriedlander.com/text/timetravel/experiment.htm
 
S

scerir

"PostReplies":
> Here's another page I found explaining
> his experiment and a very interesting
> earlier experiment which was encouraging:
> http://www.paulfriedlander.com/text/timetravel/experiment.htm


There is an interesting quote,
about Dopfer experiment, from that link.

"The important conclusion is that,
while individual events just happen,
their physical interpretation
in terms of wave or particle
might depend on the future"
[A.Zeilinger]

Now it seems to me that the key word
is "interpretation". Meaning that
from a *single* spot, on the two-slit
screen, produced by one photon passing
through the interferometer, one cannot
realize if that *single* spot is part
of an inteferential pattern *or* part of
a smooth pattern. One can realize precidely
that only after he measures, in the future,
the position *or* the momentum of the other
photon entangled with the first one.

(The third possibility, that one doesn't measure,
in the future, the position or the momentum of
the entangled photon is interesting but it is not
relevant here).

But the problem I see (since long time) is this.
Imagine there is not a *single* spot (produced
by one photon) on the two-slit screen, but 1000
distinct spots (produced by 1000 distinct photons).
Can we still say that nobody can realize whether
these 1000 spots form an interferential pattern *or*
a smooth pattern, untill one measures,
in the future, the position *or* the momentum
of all the other distinct 1000 photons, each one
of them entangled with a photon producing the spot?

s.
 
G

Gerry Quinn

In article <7jhah3p1vo77562fircdttj8gvuq1lgsm3@4ax.com>,
nomail@pleasepost.net says...
>
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx [Broken]
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.


<QUOTE>
So what happens when the beams go their separate ways, and you conduct
a wave-vs.-particle measurement on one beam? When someone else checks
the other beam, the same measurement should yield the same result. In
fact, you could visualize using the wave-vs.-particle toggle as a means
for communicating information, sort of like Morse code.
</QUOTE>

It's an interesting experiment. The key is that he is using beams
rather than single entangled pairs of photons. With single entangled
pairs it clearly won't work, because the determination between wave-
like and particle-like properties depends on the statistics of multiple
measurements.

So, what precisely is the measurement protocol he is using to make wave
or particle like measurements on the particles constituting each beam?
Has anyone got a link, because the article doesn't say, and googling
failed to find anything? If we knew that, we could write down the sort
of statistics we would expect from a succesful 'bilking' experiment,
and perhaps that would considerably elucidate the situation (most
likely by proving that bilking is actually undetectable).

- Gerry Quinn
 
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S

scerir

"Gerry Quinn"
> So, what precisely is the measurement protocol he is using to make wave
> or particle like measurements on the particles constituting each beam?
> Has anyone got a link, because the article doesn't say, and googling
> failed to find anything? If we knew that, we could write down the sort
> of statistics we would expect from a succesful 'bilking' experiment,
> and perhaps that would considerably elucidate the situation (most
> likely by proving that bilking is actually undetectable).


Only this one
http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

As far as I remember (?) you need few photons (something
like 50 or less) to realize if it is a diffraction
pattern or an interferential one. But you also need
a very clean source of entangled photons imo.

There is an interesting paper here
http://www.arxiv.org/abs/quant-ph/0506027
about that 'bilking' or, to say it better,
once the future has unfolded, it cannot change
the past.

s.

"In the early 1950s, my mentor Fritz Bopp
pondered the question as to why the probabilities
in quantum mechanics appear as absolute squares
of complex amplitudes. This led to a series
of papers with titles such as "Dice Games Whose
Tokens Move Quantum Mechanically". In 1953, I had
the great chance to spend a year in Copenhagen.
One day Niels Bohr came to me saying: "I received
again a manuscript by Professor Bopp. I do not
understand why people occupy themselves with
questions which have been clarified for decades
while there are so many unsolved interesting new
problems around." My imprudent answer: "Maybe things
are not so clear", prompted a series of discussions."
- Rudolf Haag
 
G

Gerry Quinn

In article <XcHUi.153401$%k.295389@twister2.libero.it>,
scerir@libero.it says...
> "Gerry Quinn"
> > So, what precisely is the measurement protocol he is using to make wave
> > or particle like measurements on the particles constituting each beam?
> > Has anyone got a link, because the article doesn't say, and googling
> > failed to find anything? If we knew that, we could write down the sort
> > of statistics we would expect from a succesful 'bilking' experiment,
> > and perhaps that would considerably elucidate the situation (most
> > likely by proving that bilking is actually undetectable).

>
> Only this one
> http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf


Thanks!

> As far as I remember (?) you need few photons (something
> like 50 or less) to realize if it is a diffraction
> pattern or an interferential one. But you also need
> a very clean source of entangled photons imo.
>
> There is an interesting paper here
> http://www.arxiv.org/abs/quant-ph/0506027
> about that 'bilking' or, to say it better,
> once the future has unfolded, it cannot change
> the past.


Yes. I find the argument quite convincing. It does raise some
interesting questions with regard to quantum computation.

The authors propose that you cannot go back in a time machine and shoot
your father because, essentially, the observation of your own existence
demands that a chain of events, no matter how unlikely, led up to it.
If you try to do it, you will perhaps shoot someone you think is your
father but actually isn't, or your gun will misfire, or some such
possibility.

>From now on I will revert to the traditional experiment of shooting

your grandfather, as it somehow seems less brutal than simple
patricide. Anyway, I propose the Grandfather Computer (TM),
constructed using a grandfather, a time machine, pen and paper, and a
deadly weapon of some kind.

You travel back in time, locate your grandfather, and demand that, on
pain of death, he write down the answer to some difficult computation,
say the factors of a large number (incidentally, you don't need to tell
him the number). If you have so arranged matters that this is the only
remotely possible way he could survive, it seems like a way to leverage
the evolution of the wave function of the universe to carry out this
computation; in short, it is a form of quantum computer.

Probably the flaw in the this scheme is the difficulty of actually
removing all feasible alternative courses of events that will lead to
your grandfather's, and thus your, survival. In reality it will not be
possible to remove all low-probability events (weapon malfunction,
mistaken identity, a random police visit, etc.) that will still be much
more likely than his correctly guessing the answer to the computation.

I wonder if this argument can be extended to investigate the
possibility of quantum computation in general. While I know that
devices have been built that are technically working quantum computers,
there are none that can do general computations that are not feasible
by other means. For example, the system using caffeine molecules needs
a lot of molecules, and does not perform better in practice than a
classical molecular computer based on simple parallel computation.

My feeling is that quantum computation as usually advertised is also a
sort of 'bilking' attempt, albeit not exactly in the same sense as
Cramer's experiment. It might be that both forms of bilking are
impossible, and that the reasons are related. Ways in which you might
be born despite your grandfather not solving the computation are the
equivalent of errors in typical quantum computation schemes. Both can
be seen as resistance on the part of the wave function of the universe
to being squeezed into a very narrow range of selected outcomes after a
considerable amount of complex unitary evolution.

- Gerry Quinn
 
D

Dirk Bruere at NeoPax

Gerry Quinn wrote:
>
>> http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

>
> Thanks!
>
>> As far as I remember (?) you need few photons (something
>> like 50 or less) to realize if it is a diffraction
>> pattern or an interferential one. But you also need
>> a very clean source of entangled photons imo.
>>
>> There is an interesting paper here
>> http://www.arxiv.org/abs/quant-ph/0506027
>> about that 'bilking' or, to say it better,
>> once the future has unfolded, it cannot change
>> the past.

>
> Yes. I find the argument quite convincing. It does raise some
> interesting questions with regard to quantum computation.
>
> The authors propose that you cannot go back in a time machine and shoot
> your father because, essentially, the observation of your own existence
> demands that a chain of events, no matter how unlikely, led up to it.
> If you try to do it, you will perhaps shoot someone you think is your
> father but actually isn't, or your gun will misfire, or some such
> possibility.
>
>>From now on I will revert to the traditional experiment of shooting

> your grandfather, as it somehow seems less brutal than simple
> patricide. Anyway, I propose the Grandfather Computer (TM),
> constructed using a grandfather, a time machine, pen and paper, and a
> deadly weapon of some kind.
>
> You travel back in time, locate your grandfather, and demand that, on
> pain of death, he write down the answer to some difficult computation,
> say the factors of a large number (incidentally, you don't need to tell
> him the number). If you have so arranged matters that this is the only
> remotely possible way he could survive, it seems like a way to leverage
> the evolution of the wave function of the universe to carry out this
> computation; in short, it is a form of quantum computer.
>
> Probably the flaw in the this scheme is the difficulty of actually


One obvious flaw in the scheme is that it should be the grandmother:-)
Also, isn't the above quantum computer just a form of random number
generator plus quantum suicide?

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
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A

Andreas Most

scerir wrote:
> "PostReplies":
>> Here's another page I found explaining
>> his experiment and a very interesting
>> earlier experiment which was encouraging:
>> http://www.paulfriedlander.com/text/timetravel/experiment.htm

>
> There is an interesting quote,
> about Dopfer experiment, from that link.
>
> "The important conclusion is that,
> while individual events just happen,
> their physical interpretation
> in terms of wave or particle
> might depend on the future"
> [A.Zeilinger]
>
> Now it seems to me that the key word
> is "interpretation". Meaning that
> from a *single* spot, on the two-slit
> screen, produced by one photon passing
> through the interferometer, one cannot
> realize if that *single* spot is part
> of an inteferential pattern *or* part of
> a smooth pattern. One can realize precidely
> that only after he measures, in the future,
> the position *or* the momentum of the other
> photon entangled with the first one.
>
> (The third possibility, that one doesn't measure,
> in the future, the position or the momentum of
> the entangled photon is interesting but it is not
> relevant here).
>
> But the problem I see (since long time) is this.
> Imagine there is not a *single* spot (produced
> by one photon) on the two-slit screen, but 1000
> distinct spots (produced by 1000 distinct photons).
> Can we still say that nobody can realize whether
> these 1000 spots form an interferential pattern *or*
> a smooth pattern, untill one measures,
> in the future, the position *or* the momentum
> of all the other distinct 1000 photons, each one
> of them entangled with a photon producing the spot?


Yes, we can say this. Without any coincidence unit you will
not be able to see an interference pattern. Actually, by
involving the correlation with the entangled partner you choose
about 500 of the 1000 initial photons. The way you perform the
selection, either momentum or position measurement, determines
whether there is an interference pattern or not. What is actually
disturbing is that it does not depend on when you perform the
measurement on the entangled partner. But that is the way it is in
quantum mechanics.

I think Cramer is wrong in assuming he could send a signal backwards
in time because the interference pattern occurs when the correlation
between the two entangled photons has been established (and of course
roughly 500 events have been rejected) and not the moment the photons
hit the screen.

Andreas.
 
D

Dirk Bruere at NeoPax

Andreas Most wrote:

>>
>> But the problem I see (since long time) is this.
>> Imagine there is not a *single* spot (produced
>> by one photon) on the two-slit screen, but 1000
>> distinct spots (produced by 1000 distinct photons).
>> Can we still say that nobody can realize whether
>> these 1000 spots form an interferential pattern *or*
>> a smooth pattern, untill one measures,
>> in the future, the position *or* the momentum
>> of all the other distinct 1000 photons, each one
>> of them entangled with a photon producing the spot?

>
> Yes, we can say this. Without any coincidence unit you will
> not be able to see an interference pattern. Actually, by
> involving the correlation with the entangled partner you choose
> about 500 of the 1000 initial photons. The way you perform the
> selection, either momentum or position measurement, determines
> whether there is an interference pattern or not. What is actually
> disturbing is that it does not depend on when you perform the
> measurement on the entangled partner. But that is the way it is in
> quantum mechanics.
>
> I think Cramer is wrong in assuming he could send a signal backwards
> in time because the interference pattern occurs when the correlation
> between the two entangled photons has been established (and of course
> roughly 500 events have been rejected) and not the moment the photons
> hit the screen.


And at what point is that?
Precisely *when* does the interference pattern exist?

--
Dirk

http://www.transcendence.me.uk/ [Broken] - Transcendence UK
Remote Viewing classes in London
 
Last edited by a moderator:
A

Andreas Most

Dirk Bruere at NeoPax wrote:

> And at what point is that?
> Precisely *when* does the interference pattern exist?


The point is that you can only decide on whether you see
an interference pattern when you have established the correlation
of the measurements on the two entangled photons which means
after the measurements have been performed. Quantum mechanics
tells us something about the probabilities of the outcomes of a
measurement. It does not tell us anything about when an outcome
exists.
"Interference Pattern" is not a hermitian operator but you could
modify the experiment into a Bell-type experiment. From the
violation of Bell's inequalities it is then clear that the
interference pattern could not "have existed" before the second photon
is measured and the correlation between the entangled partners
is established.

Andreas.
 
P

PostReplies

On Fri, 2 Nov 2007 06:06:07 +0000 (UTC), Andreas Most
<Andreas.Most@nospam.de> wrote:

>Quantum mechanics
>tells us something about the probabilities of the outcomes of a
>measurement. It does not tell us anything about when an outcome
>exists.


I don't follow that. On the sender side of the communication channel
you have dual slits and a "which slit" detector that can be turned on
or off. On the receiver side of the channel (the other beam of
photons) you have dual slits but no which way detector. The sender
leaves his detector off. The receiver should be seeing an interference
pattern. The sender turns on his "which slit" detector. His beam and
the entangled receiver beam should no longer exhibit an interference
pattern until he turnes his detector back off. In other words, the
timing is based on the sender turning his detector on and off. That's
a simplification but I think is pretty much the crux of the experiment
in FTL communication. If that works (which would violate relativity so
likely won't) Cramer will then lengthen the sender side so the sender
side is lagging the receiver side in order to check for backward
causality--again likely won't work but I have no idea what the mode of
failure will be.
 
B

Ben Rudiak-Gould

PostReplies wrote:
> That's what Cramer is doing. Here's another page I found explaining
> his experiment and a very interesting earlier experiment which was
> encouraging:
>
> http://www.paulfriedlander.com/text/timetravel/experiment.htm


There is no nonlocal communication here. It's critical to understand that
the four graphs (around halfway down the page) do not represent images
recorded on a photographic plate or CCD. Rather, they represent hit rates on
a yes-or-no detector (think Geiger counter) as it's physically moved across
the detection field while the other detector is held fixed, and only the
cases where both detectors register a particle are counted. This makes a big
difference! You will get completely different results this way than with
photographic film. To see why, consider a simplified experiment in which
each detector can be moved to four different locations (D1 in locations
11,12,13,14 and D2 in locations 21,22,23,24). Suppose our light source is
such that all the light beams it generates pass through locations whose sum
is even -- for example, it will generate beam pairs going through 11 and 21,
but never through 11 and 22. The possible combinations are marked with "X"
below.

21 22 23 24
11 X X
12 X X
13 X X
14 X X

Now suppose D1 is held fixed (at any position) while D2 is moved, and
simultaneous clicks of D1 and D2 are recorded. Regardless of the fixed value
of D1, you will get a bright, dim, bright, dim pattern, which is our
simplified discrete version of an interference pattern. But if you consider
only the data from D2, without the coincidence counter, there will be no
interference pattern, just an equal distribution over all four locations.
Similarly, if you replace the detectors with photographic plates, there will
be no interference pattern on either plate.

Now in front of D1 insert a scrambling device that perturbs each incoming
photon so that, regardless of where it was originally headed, it's now
equally likely to go to any of the locations 11,12,13,14. Now, when you
again hold D1 fixed while varying D2 and counting coincidences, you will no
longer see an interference pattern. But the raw data from D2 has not changed
at all -- all that has changed is which raw detection events we subsequently
threw away at the coincidence counter.

This is what's going on in Dopfer's experiment (as both Dopfer and Zeilinger
realize).

-- Ben
 
B

Ben Rudiak-Gould

I've now read Cramer's three-page description of his experiment, found here:

http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

One of his coauthors is an undergraduate and the other claims to have only a
faint understanding of the work:

http://seattlepi.nwsource.com/local/292378_timeguy15.html

so I'm going to treat this report as Cramer's alone.

Um. Cramer apparently thinks that the coincidence counter in experiments
like Dopfer's is just an engineering detail, and that by throwing it away
you can get a superluminal communication device. Cramer's ideas have always
seemed a bit odd to me, but this is the first time I've seen what appears to
be clear evidence that he's incompetent as a theorist. I can't tell whether
he thinks quantum mechanics predicts superluminality in this experiment or
whether he thinks previous work fully consistent with quantum mechanics
somehow points the way to a result violating quantum mechanics -- but either
way his understanding of QM appears to match Herbert Dingle's understanding
of special relativity and Tom Van Flandern's understanding of classical
field theory. Seriously, what the heck?

The P-I article quotes Cramer saying that "there's no obvious explanation
why this won't work" and that "even if it doesn't work, we should be able to
learn something new about quantum mechanics by trying it." I can't imagine
what he expects to learn.

-- Ben
 
A

Andreas Most

PostReplies wrote:
> On Fri, 2 Nov 2007 06:06:07 +0000 (UTC), Andreas Most
> <Andreas.Most@nospam.de> wrote:
>
>> Quantum mechanics
>> tells us something about the probabilities of the outcomes of a
>> measurement. It does not tell us anything about when an outcome
>> exists.

>
> I don't follow that. On the sender side of the communication channel
> you have dual slits and a "which slit" detector that can be turned on
> or off. On the receiver side of the channel (the other beam of
> photons) you have dual slits but no which way detector. The sender
> leaves his detector off. The receiver should be seeing an interference
> pattern. The sender turns on his "which slit" detector. His beam and
> the entangled receiver beam should no longer exhibit an interference
> pattern until he turnes his detector back off.


No. Nothing you do on the sender side will change the outcome of the
measurement on the receiver side. See
http://en.wikipedia.org/wiki/No-communication_theorem

You need to know the correlation additionally (i.e. coincidence).
This piece of information can only be transmitted classically, i.e.
at best with speed of light.

The "spooky action at a distance" is about correlations of measurements
and not about single measurements. People tend to forget this sometimes.

> In other words, the
> timing is based on the sender turning his detector on and off. That's
> a simplification but I think is pretty much the crux of the experiment
> in FTL communication. If that works (which would violate relativity so
> likely won't) Cramer will then lengthen the sender side so the sender
> side is lagging the receiver side in order to check for backward
> causality--again likely won't work but I have no idea what the mode of
> failure will be.


Definitely, he will see no interference pattern whatever he does on the
sender side. Only if he correlates his actions on the sender side with
the measurements on the receiver side he is able to extract an
interference pattern.

Andreas.
 

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