Superluminal communication through entanglement

[swansont]

I think that it means that the derivations are not inherently quantum mechanical in nature.

[page russell]

I think I can add to the complexity of this discussion in two ways. first the experimental setup provided by bascule is unclear, and second by pointing out that delayed earasure is possible.

 

First off, the setup. It ought to look like this: A laser beam entangles pairs of photons using spontaneous parametric down conversion (SPDC - dont ask me for a more complicated discussion of that, however, it is important to note that what we mean by "entangled photons" is that the two photons have and will always have corresponding polarizations.) and then that laser beam is then split. Call the resulting beams S and P. The S beam is sent off towards a Young double slit setup, and the P beam is sent towards a simple detector which will register the polarization of the P beam (and thus we can infer the polarization of the S beam).

 

At this point, we have no way of determining any so called "which-way" information, and so running this experiment will produce an interference pattern as a result of the S beam traversing the double slit experiment. However, if we were to add quarter wave plates directly in front of the two slits, we could gain the possibility of infering which-way information. This is becuase the quarter wave plates change the polarization of the S beam, which results in a correlated change in the P beam's polarization. Thus we ought to be able to infer the which way information without disturbing the S beam in any way (it is important to note that tchanging the polarization has no effect on the interference pattern emerging or not, as will become doubly clear when I discuss delayed erasure in a moment).

 

Alas, the interference pattern is destroyed and replaced by the additive pattern. That is pecular enough, but at this point we still do not have the possibility of superluminal signaling as bascule has in mind, because the two measurement stations must still communicate results to each other in a classical way.

 

But consider the following; placing a polarizer in the path of the P beam will destroy the possibility of which-way information, and incredibly the interference pattern returns. This is the point at which bascule thinks we can communicate superluminally, and I agree with him. Now we have a scenario in which purposeful, systematic and deterministic action at one location will result in clear results in another, and the notion of locality seems to be violated (if you dont think so, ask and I will attempt to explain how and why bells theorem implies that either locality or determinism must be false, or at least irrelevant to quantum mechanics).

 

Thus what bascule has in mind is this: Alice at the P beam can place and remove the polarizer, corresponding to a 0 and 1 repectively, and Bob at the S beam can observe either an interference or an additive pattern, correlating to a 0 or 1, respectively.

 

Here is a nice graphic, which was obtained from the following site, which has a splendidly clear explanation of this setup: http://grad.physics.sunysb.edu/%7Eamarch/

 

Now, for those of you who dont yet beleive, consider delayed erasure. In this same experiement, the researchers went on to lengthen the P beam. This had the result that the S beam photons entered the double slit apparatus and did whatever they do there before the P beam photons either hit or did not hit a polarizer. The interference pattern still disappeared if there was a mere possibility of which-way information. Thus there appears to be some sort of backwards causality, which is only possible given superluminal speeds.

 

So consider this all a comlex smoke signal. Alice sits there and puts the polarizer over the p beam for some time and then removes it, and then puts it back, and so on. Bob sits tehre and observes alternating interference patterns and additive patterns correlating perfectly to the action of alice. What could we say about this other than superluminal signaling, assuming alice and bob are outside of each others light cone?

[swansont]

So consider this all a comlex smoke signal. Alice sits there and puts the polarizer over the p beam for some time and then removes it, and then puts it back, and so on. Bob sits tehre and observes alternating interference patterns and additive patterns correlating perfectly to the action of alice. What could we say about this other than superluminal signaling, assuming alice and bob are outside of each others light cone?

 

The detection of the presence or lack of an interference pattern takes several measurements — you have to move the detector, and make measurements. The graphs show ~25 data points, each of which will take some time to measure. I'm not sure to the extent that's a limiting factor here or not, but the removal of the polarizer is not instantly recognizable. That's not what the experiment was showing.

[page russell]

the removal of the polarizer is not instantly recognizable.

 

who said anything about instantaneous? yes, multiple measurements were required in the setup done by walburn et al, and most of those measurements required the detector to run for 400 seconds. But this isnt about instantaneous signals, but super luminal signals.

 

obviously this set up i have proposed would have no practical implications for any distance we are now able to traverse. you couldnt do this on the surface of the earth, let alone a table top. but the theoretical possibility of signaling from distances in such a way that the information must travel outside of the light cone still seems possible. who cares if it takes an hour of measurements to send a very simple message if that message comes from a source a billion light years away?

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Merged post follows:

Consecutive posts merged

let me put it this way: what the walburn et al experiment is doing in general is using entanglement to infer which-way information. It turns out, experimentally, that the availability of this information destroys the interference pattern. All I am suggesting is it ought to be possible to use that general mechanism to communicate faster than light. various set ups are irrelevant; the thought experiment still stands.

Edited March 10, 2009 by swansont
Consecutive posts merged. swansont: fix quote tag

[swansont]

who said anything about instantaneous? yes, multiple measurements were required in the setup done by walburn et al, and most of those measurements required the detector to run for 400 seconds. But this isnt about instantaneous signals, but super luminal signals.

 

But if the two detectors aren't more than 400 light-seconds apart, the communication is not superluminal.

 

 

 

let me put it this way: what the walburn et al experiment is doing in general is using entanglement to infer which-way information. It turns out, experimentally, that the availability of this information destroys the interference pattern. All I am suggesting is it ought to be possible to use that general mechanism to communicate faster than light. various set ups are irrelevant; the thought experiment still stands.

 

That's the problem with thought experiments, though — your thinking can be wrong. Until you actually do it, you can't be sure. The experimental setup wasn't trying to measure a signal delay from trying to convey information as you describe.

[lakmilis]

But if the two detectors aren't more than 400 light-seconds apart, the communication is not superluminal.

 

Ok.. so what if they were more than 400 light seconds apart? (1 AE for example)?

 

(sorry, I was reading the whole post but QM has long abandoned me , the little I knew. But was an interesting chat, then those few last posts really got me.

 

(Hope me asking that, isn't entirely absurd)..as in I see your point swansont, but could* they be more than 400 lightseconds away in this example (assuming measurements took 400 seconds) and if so, would it indeed be superluminal then?

 

OOps.. language mix-up.. I believe it is AU in English. (~480 light seconds).

[swansont]

I'm not sure exactly what would happen. But I recall an experiment that was set up to measure the time delay of entanglement, which confirms that the entangled particles collapse to their states essentially instantly (the distance/time is > 10,000 c)

http://blogs.scienceforums.net/swansont/archives/662

[lakmilis]

I'm not sure exactly what would happen. But I recall an experiment that was set up to measure the time delay of entanglement, which confirms that the entangled particles collapse to their states essentially instantly (the distance/time is > 10,000 c)

http://blogs.scienceforums.net/swansont/archives/662

 

cheers Swans (ps. ye , had a look at some of the cartoons of yours the other day, not as sharp as Gary Larsson generally but some did indeed make me crack up )

[Duda Jarek]

Hi,

Do You mean experiment like on:

http://www.newscientist.com/article/mg19125710.900-whats-done-is-done-or-is-it.html

If yes -the problem is that it will always give wavelike behavior.

If the photon will be measured on the longer path, this measurement meant that it has chosen this path - won't be seen on the shorter path.

 

There is needed some additional split of photon: look at 6th post in

http://www.scienceforums.net/forum/showthread.php?t=39787

[fferreres]

I was reading a book about the same experiments commented by page russell, and reached the same doubts (or ida) as him. He described it the same that had popped to my mind. The original poster had a similar idea, but was not very good at describing it (and I didn't see the need to have two pairs of entangled photons on different paths, when one would be enough). I see the question remains unanswered...

 

This is not about trying to detect the spin and leveraging the fact that the other photon will have a specific spin if measured along the same axis. I'd have to phone him for confirmation and be bound by speedligtht. This about working with a stream of entangled photons (A and B, or A and B streams) and measuring one stream (for example, the position). The idea is to use a measurement on the A stream to cause the B photons’ (part of the B stream) (the entangled peers) probability waves to collapse. I do not care what the measurement outcome is, and I do not care about any correlation between individual A/B entangled photons pairs (for which I'd have to use the phone or a fax to find out). I do care that the probability waves of photons of the B stream (say all the entangled peers going south) are collapsed. If I tried to work this out of a single pair of entangled photons, it would tell me nothing. As soons as the A stream starts to be measured (this means I start measuring the position of the A photon stream), the B stream running through two slits (light years away) suddenly stops forming the interference pattern, and starts forming additive patterns. After a number of photons, I should be able to detect the pattern.

 

What page rusell did is point to an experiment that implies that the interference pattern of the B stream does stop forming when a polarizer acts on the A photons, and that this happens at a speed faster than light.

 

What I was expecting to find when I googled for this topic, is why this reasoning is flawed, why is it a paradox, and what is causing the missunderstanding. But it seems to me that while probably flawed (because it's too obvious to not have been addressed earlier) nobody has been able to explain why/debunk. Some asked for the hard data. As rusell pointed out, the experiments used to make the reasoning have already been done. If for another reason in mind, the outcome of the experiment will not change. What’s the point in rerunning them?

 

If you can collapse the probability wave of a photon instantly (or other particle) through a distant entangled peer; and if collapsing the waves a photons part of a stream preventing an interference pattern to form (and the inverse when not collapsed), then you can apparently pass information faster than light, provided you have created the stream with sufficient time in advance (ie: to communicate anything at speeds faster than light to Alpha Centaury, I'd first need to be in between the sun and AC, and create entangled particles directly pointing at each star. After two years have passed I could send the first "instant message").

 

So if look at it, this is not so much about “faster than light communications”, but about "delayed messages". I send the photons unknowingly of what the message will be. So in the future, I'll be able to alter the stream (measuring/not measuring at different intervals, and having waves collapsed/not collapsed streams at the other end) to "modulate" my message. You can see this as the message traveling at the speed of light, (entangled photons separating from each other at the speed of light) but having the final message defined at a later stage (when they arrive Earth and I dcide to measure/not measure the stream for a few seconds). But clearly cannot send a message at a speed faster than light to anywhere I haven't prepared in advance.

 

Now, if measuring a stream of entangled photons at one end did nothing to alter the interference patterns forming at the other end (ie, the rusell reference and the robert green book and other books, etc are all wrong), then all this would be nonsense.

 

--

Now the reason why I thought this was possible is that the information is not traveling faster than light really. It's traveling through time. Just like delayed quantum choice, a decision in the future seems to affect something in the present (or past), which seems add odds with common sense. It’s as if "the reality” knew what that future choice would be (deterministic), and this “superluminar” example sits in the same theater.

 

If you think it through a different lens, you could say the photons travel at the speed of light, and that makes them timeless. It's the same now and 10 years from now for them. So from the photon perspective, both things occur instantly (being born, and being detected). For us, they may travel for a millisecond, a second, a year, etc. So it's no wonder that what for us "seems to be the future", for the particles its always the present. So the particle and the entangled peer always knew what we’ll do in the future…it’s the same exact moment for them.

 

When you use the entangled particles streams to modulate a signal (stream B collapsed/not collapsed as shown by interference pattern, when the A stream is measured/not measured) the same happens. When the particle (actually the entangled pair) is born, it already knows if it will fall in the "measured photons" bucket or the "not measured photons" bucket. Accordingly, the "spooky action at a distance" does not mean information traveling faster than light, but it means information traveling across time, which means that for us, a change here instantly produces a change there (non local), but what really happened is that the photons already knew our (we believe…future) choice. Particles such as photons can predict their future with 100% accuracy. And that means they know what we'll do in what we call the future.

 

So the faster than light information that rusell and the original poster described enable me to ask a friend in Alpha Centaury how much is 2 x 2, and have him answer my call instantly (or is it a few seconds or minutes?). But of course, I'd have needed to send entangled photons from a location half across Alpha Centaury and our Sun. And also agreed what the collapsed/not collapsed sequences would mean (putting something at half the distance would take >2 years and the entangled particles would take another 2 years to reach Alpha and Sol. And an agreement on what the sequences means would also take >4 years to reach Alpha Centaury.

 

Basically, you could see the superbowl live from any place on the galaxy provided you planned it with sufficient time in advance (1 year in advance, 1 light year distances, etc). The photons ability to “read their future” would allow the exact contents of the message to be determined in what it’s for us "the future".

[michel123456]

Here's a highly technical writeup on the delayed choice quantum eraser experiment:

 

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

 

Of course quantum erasure doesn't come into play in this, merely the delayed choice aspect. Hopefully one of the real physics people here can give you a layman's description, as I wouldn't want to try to explain it incorrectly (obviously I'm misunderstanding something here, or I wouldn't be asking this question)

 

 

 

They don't have to. Guess I need to bring out my diagram again:

 

 

Okay, here we have a device at B involving a laser (which is magically spitting out single photons at a fixed interval) whose beam is passing through a beam splitter (BS), both outputs of which are passing through down converters (DC) which generate entangled photons.

 

These entangled photons travel along until they arrive at destinations A and C. At A, we have "which path detector" (WPD). At C, we have optics which cause the two beams to cross back over each other.

 

Now, (assuming the WPD is off) we know from the traditional double slit experiment that the probability waves of the two paths will interfere. Thus if the little blue bar at C is a CCD hooked to a computer which registers where each photon hits and builds a composite image, the photons will eventually trace out a pattern that looks somewhat like this:

 

 

(note: This image assumes that the probability wave will collapse due to outside interference about 50% of the time)

 

Now, if we were to switch on the WPD, the interference pattern would be destroyed, and we'd get a picture more like this:

 

 

Is this correct? Am I misunderstanding something? Because if so, it would seem that we could use these two very different looking pictures to differentiate between two different types of signals. Would this not be an example of using delayed choice to send information?

 

Intuitively, this is not what you will see: the second image will not be different from the first one. I really don't understand why the image would change due to the turning on of the WPD. The WPD is a detector, not a cause. The collapsing of the wave is a detection, or did I understand nothing (again)?

[swansont]

Detection, with the WPD, can cause the wave function to collapse. The collapse of the wave function is not detection; it's the other way around. Detection causes the wave function to collapse.

[michel123456]

Detection, with the WPD, can cause the wave function to collapse. The collapse of the wave function is not detection; it's the other way around. Detection causes the wave function to collapse.

 

You say it, I read it a thousand times, I respect it, but still I cannot swallow it.

 

As I understand, detection let us know the particle's exact situation, something that we could not know before detection: this is my interpretation of the collapsing. When you transform that into "Detection causes the wave function to collapse" it is just like transforming the effect into cause, something I cannot swallow.

Edited December 3, 2009 by michel123456
particle not wave

[swansont]

You say it, I read it a thousand times, I respect it, but still I cannot swallow it.

 

As I understand, detection let us know the particle's exact situation, something that we could not know before detection: this is my interpretation of the collapsing. When you transform that into "Detection causes the wave function to collapse" it is just like transforming the effect into cause, something I cannot swallow.

 

You have a wavefunction that allows for more than one state (in this case, trajectories), but when you detect it, the particle must be in an eigenstate. We say the wavefunction has collapsed. The detection does this.

[michel123456]

I am a fugitive in a stolen car.

The Policemen know the kind of car (a corvette), its maximum speed, and its possibilities (no off-road). They don't know the direction (I am radiating) but they know the starting point (the prison I escaped). They also know the starting time, and they have a map. They can mathematically evaluate the probability of finding me at any point on the map, depending on time. They can considerate the fugitive as a probability wave.

They are blocking all roads at the right distance from the jail, and catch me. They don't know where they will find me, but in any case, i will be detected. At the time of the detection, the "probability wave" will collapse, in the sense that only at that moment, the police will know exactly where I am (the eigenstate)

Does that mean that I am a wave?

The term "collapsing" means that the mathematical evaluation of my position simply ends. I do not "collapse" (i just go to jail again....)

Edited December 4, 2009 by michel123456

[swansont]

Does that mean that I am a wave?

 

Can you interfere with yourself? If the answer is "no," then the analogy to QM doesn't work.

[michel123456]

O.K. good comment.

I suppose that even in QM you need more than one particle in order to interact.

So let's say that the entire prison escapes. 3 thousands convicts spreading into the nature. The Pauli's exclusion principle applies.

 

Anyway, an analogy is never but an analogy. i hope you understand my comment, as much I understand yours.

[ethana2]

I suppose that even in QM you need more than one particle in order to interact.

 

There's the problem. You suppose wrong. The fact that we've observed single particles interfering with THEMSELVES is the basis of quantum theory. You're not going to understand any of these comments until you realize that. It *doesn't make sense* but it's *proven fact anyways* because we've seen it. Einstein didn't like it either, and it's a screwy concept for everyone. Reality doesn't make sense. The fact that the two theories we've developed to explain it best are at odds with eachother is the biggest problem in modern physics. Things like photons are waves and particles at the *same exact time*. Generally speaking, the interaction between radiation (with or without mass) and spacetime is decidedly wavelike, and its interaction with matter is decidedly particulate. Forcing interaction with matter collapses the wavelike nature of radiation both directly and through entanglement.

 

 

I don't expect you to comprehend this, no one can yet-- but do you understand it?

[michel123456]

Woups a resuscitated thread. You made me read it again from the beginning. It makes me see I still agree with my own comments, what a pleasant surprise!

No I don't comprehend and I don't understand.

I strongly believe that some kind of logic must be maintained when observing phenomenas. When someone in his lab makes an experiment from which it appears that a single particle interacts with itself, to me logic says something is wrong. Maybe the problem is in the experiment, maybe in the measurement, maybe in the way we take conclusion, maybe in the Theory, there are many possibilities. But it may also be hidden in some basic statement, so basic that we cannot figure we made that statement at all. I think we should focus on this last point. Anyway, I am not prepared to say logic is wrong.

[swansont]

Logic from a flawed premise is not valid. Investigation that's rejected unless it confirms a preconceived notion is not science.

[michel123456]

Hm.

O.K. let's see it that way.

What is the "flawed premise" in this particular case? What is the preconceived notion that contradicts investigation?

Edited January 4, 2011 by michel123456

[Sisyphus]

You said that if we get a certain result, we must be wrong somehow. But science doesn't work that way. Reality trumps intuition.

[swansont]

Hm.

O.K. let's see it that way.

What is the "flawed premise" in this particular case? What is the preconceived notion that contradicts investigation?

 

That the answer has to make sense to you in order to be correct.

[michel123456]

That the answer has to make sense to you in order to be correct.

 

This is the best answer I ever dreamed of. You just made history.

Edited January 5, 2011 by michel123456

[steevey]

I've been through this before, and despite all the physics experts that seem to be around I never received a satisfactory answer. So someone who actually knows the math behind this and can help out someone venturing into quantum from a purely conceptual perspective, I'd really appreciate your help in pointing out what's wrong with my reasoning here because, as far as everything I've ever read from physics has told me, I must be wrong...

 

Okay, we have Einstein saying that you can't have anything causal occur at superluminal speeds... state changes within the universe propagate at c and that's all there is to it...

 

Yet we have ESR "spooky" action which allows certain things to happen non-locally, quantum entanglement in which the same events can occur "simultaneously" across distances which it would take light (in a vacuum) a considerable amount of time (interpret that as how you see fit) to traverse.

 

We have the delayed choice quantum eraser experiment demonstrating that collapsing the probability wave of an entangled particle simultaneously collapses the probability wave of the particle it's entangled with.

 

Okay, let me say that again, and if it's not correct, please let me know why, because that statement is the crux of this entire question:

 

Collapsing the probability wave of an entangled particle simultaneously collapses the probability wave of the particle it's entangled with.

 

If this is so, do we not then have superluminal causality through ESR spooky action? I'm aware that there is an occurance of spontaneous collapse of an otherwise uncollapsed probability wave, but as far as I'm aware it is not possible for the opposite to occur, namely a collapsed probability wave spontaneously reverting to uncollapsed.

 

So if this is the case, shouldn't statistical analysis of the behavior of an entangled particle on one "side" reveal whether or not the other "side" has chosen to collapse the probability wave or not? Can we not sample for awhile and discover that the probability wave was uncollapsed a whole bunch of times, leading us to the conclusion that the other "side" was sending the "uncollapsed" signal? Similarly, if we sample over a window in which we expect the probability wave to remain uncollapsed at least a few times and observe only the collpased probability wave, can we not infer that the other side is sending the "uncollapsed" signal? Sure, that inference may be lossy, but computer science has been dealing with sending signals reliably over lossy media for decades, and could they not work out reliable communication over such a medium?

 

I detailed my idea of how this could actually work based on the delayed choice quantum eraser experiment, and never received the debunking I had hoped for.

 

Please, I'm crazy, I'm saying that causation can occur at superluminal speeds! Debunk me!

 

 

Currently, quantum mechanics does not allow for faster-than-light communication, only vastly superior processing capabilities. This is a common misconception since entangled particles although can be separated by distance, are still the same particle when they are entangled and therefore cannot contain separate information on both ends.

 

Although I suppose if you could measure the wave function WITHOUT collapsing it, you might be able to mess with some properties and do some kind of quantum Morse code.

Edited January 17, 2011 by steevey