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Natural Science Forum / Physics / Research / November 2007Has FTL communication really never been tested in this way?
http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx 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. > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > 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.  SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > 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? SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > receive a signal instantaneously? > I'm not a physicist, just an interested lay person. Right now, experiments are being conducted at the Univ. of Washington in Seattle, by Dr. John Cramer, to directly attack the causality issues of quantum entanglement and "nonlocality" in communication. I suspect data will be forthcoming later in 2008. Even then the results will be difficult to comprehend. The difficulty with this and even more-so with prior experiments is to clearly define the connections between the parts of the system and causal relations (cause precedes effect). In short, it is hard to follow the chain of logic and facts in these experiments, and to be able to distinguish facts from interpretations. Our notion of time and causal connections is so innately ingrained in us, that it makes it very hard to break from these notions and just to look at the physics in its raw form, and then to try and decipher what is really going on. It is confusing for physicists too, which is reflected in the ongoing publications. Even the terminology varies from one approach to another ( e.g. the terminology used in quantum tunneling, electrical engineering, retarded potentials... ). Regarding FTL 'travel,' the situation is still theoretical, not experimental. To get passed the light-speed limit of spacetime, concepts of "warp drives" (Alcubierre) and "wormholes" (Visser) toy with the idea of manipulating 'spacetime' itself to circumvent the limits. In short, even these FTL concepts appear to evoke "closed time-like curves" which means possible causality violations (Arrive back from your trip before you departed to stop yourself from taking the trip...). Given how little is known about spacetime itself and other unknowns dubbed "Dark Matter" "Dark Energy" etc, it is safe to say we all have a lot more learning to do. In short, we are beginning to get smart enough to ask the right questions on such deep unknowns, but it is likely to take a while before we fully comprehend how nature works - if ever. It is fun to work on it and to ponder the possibilities. Regrettably, I do not have good books to recommend to you on this subject that are deliberately written for the non-scientist. Marc "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 >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 "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. > "PostReplies": >> Here's another page I found explaining [quoted text clipped - 39 lines] > 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. >> But the problem I see (since long time) is this. >> Imagine there is not a *single* spot (produced [quoted text clipped - 22 lines] > 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? SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > 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. >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. >> Quantum mechanics >> tells us something about the probabilities of the outcomes of a [quoted text clipped - 9 lines] > 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 [quoted text clipped - 4 lines] > 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. Andreas Most: > The "spooky action at a distance" is about correlations > of measurements and not about single measurements. > People tend to forget this sometimes. There is a general agreement that it is incorrect to say that the experimenter=92s arbitrary choice of measuring position or momentum of the idler photon on the right side of the apparatus somehow CAUSED a specific collapse of the signal photon wavepacket on the left side. Only nonlocal, instantaneous, UNCAUSED correlations-at-a-distance are predicted by quantum theory. Clearly, the collapse phenomenon is nonlocal and NONCAUSAL in nature. However whether or not fringes, in coincidence detection, show up on the left side of the apparatus, DEPENDS ON THE ARBITRARY CHOICE by the experimenter of measuring - even in the future (delayed choice) - on the right side of the apparatus, position or momentum of the idler photon. Now, what (imo) Cramer is trying to do is to *study* that apparent contradiction between 'UNCAUSED/NONCAUSAL' and 'DEPENDS ON THE ARBITRARY CHOICE ...'. For this purpose he needs a clean source of entangled photons, to remove the coincidence detection unit, to perform the *same* measurement on (say) 1000 idler photons, and see what happens on the other side. Is that correct? s. > Andreas Most: >> The "spooky action at a distance" is about correlations [quoted text clipped - 10 lines] > are predicted by quantum theory. Clearly, the collapse phenomenon > is nonlocal and NONCAUSAL in nature. "Collapse" is actually nothing physical. It is only a notion we have introduced to explain when we know "what" about the physical state. E.g., you cannot assign a time stamp to the collapse. By choosing an appropriate reference frame one could come to the conclusion that the collapse took place right after the creation of the entangled pair. > However whether or not fringes, in coincidence detection, show up > on the left side of the apparatus, DEPENDS ON THE ARBITRARY [quoted text clipped - 12 lines] > > Is that correct? You cannot remove the coincidence unit. This has nothing to do about whether you have a "clean source" of entangled photons. The measurement on the idler so-to-say selects the valid pairs. There is no way to settle this in advance. (Someone once came up, that it is a type of book keeping, that the idler photon determines which photons on the screen are taken into account. I don't like this type of picture, because it silently implies that the outcome of a measurement is fixed since the creation of the pair, which is disproved by the violation of Bell's inequality. But it is maybe not so bad as a guideline...) The only way Cramer could be right would be quantum mechanics being wrong in this respect. From my point of view, however, this would violate observations in quantum statistics that are well described by quantum mechanics. Andreas. Andreas Most: > You cannot remove the coincidence unit. > This has nothing to do about whether you have > a "clean source" of entangled photons. > The measurement on the idler so-to-say > selects the valid pairs. There is no way > to settle this in advance. If you want to measure the two-photon interference, you have to observe, and register, the behaviour of both entangled photons, the idler photon at its wing, the signal photon at its wing. This has much to do with Wheeler's (and Bohr's) saying: 'No elementary phenomenon is a phenomenon until it is a registered phenomenon.' Of course, because we are performing a two-photon interference experiment, we must use an appropriate source of entangled photons (this source must have a 'large' size). Now we can also ask something like this. Imagine we want to observe what happens at the signal photon wing only. That is to say: we want to see if there is some visible interference pattern, made by signal photons, on the two-slit interferometer screen, without looking at the other side (where somebody else is measuring the idler photons). (We do not use the coincidence detection unit here, because we are interested in whether there is some visible interference pattern, made by signal photons, on the two-slit interferometer screen, without looking at the other side, where idler photons are measured). And imagine we cannot see any interference pattern on that screen. (Apparently this is very strange because we have a two-slit interferometer and a beam of photons, and we do not see any interference pattern). What is the reason? The possible reason seems (to me) this one. Signal photons cannot cause their interference pattern on the screen because their momentum uncertainty is large. And their momentum uncertainty is large because the source of entangled photons we (must) use to perform a two-photon interference experiments has a 'large' size (divergence of the beam). > And imagine we cannot see any interference pattern > on that screen. (Apparently this is very strange [quoted text clipped - 11 lines] > a two-photon interference experiments has a 'large' > size (divergence of the beam). This has nothing to do with our poor set up of the experiment. Imagine a quantum eraser experiment with two slits where the left slit is covered with a horizontal polarization filter and the right slit is covered with a vertical one. If you shoot linearly polarized photons (at an angle of 45°) through it there will be no interference pattern on the screen because left and right linearly polarized electromagnetic waves do not interfere. Now, If you chose to measure circular polarization on the idler photon and select those events where the idler photon has e.g. right circular polarization then you would see the interference pattern for the selected signaling photons on the screen. (By choosing the left circularly polarized idler photons you would see a shifted interference pattern) You could argue now that it is possible to use circularly polarized photons from the very beginning in which case you would see an interference pattern on the screen without the need of any coincidence unit. The question is then, what is the use of the idler photon if not to decide on the type of measurement. And, does the type of measurement actually change the interference pattern? The answer by QM and experiment is definitely: No. Andreas. Andreas wote: > > Signal photons cannot cause their interference pattern > > on the screen because their momentum uncertainty is large. > > And their momentum uncertainty is large because the > > source of entangled photons we (must) use to perform > > a two-photon interference experiments has a 'large' > > size (divergence of the beam). > This has nothing to do with our poor set up of the > experiment. Since we were talking about the possibility of 'signaling', at a distance, using a two-photon interference set-up, having removed the coincidence detection unit, I was only pointing out that the usual set-up, i.e. the usual SPDC source itself, might not allow any single-photon interference pattern, at the signal wing, for essential reasons (divergence of the beam). More below. > Imagine a quantum eraser experiment with two slits where the > left slit is covered with a horizontal polarization filter and [quoted text clipped - 8 lines] > (By choosing the left circularly polarized idler photons you would > see a shifted interference pattern). Yes, I know these interesting experiments. http://www.arxiv.org/abs/quant-ph/0106078 http://icpr.snu.ac.kr/resource/wop.pdf/J01/1998/033/R04/J011998033R040383.pd f > You could argue now that it is possible to use circularly polarized > photons from the very beginning in which case you would see an [quoted text clipped - 3 lines] > of measurement actually change the interference pattern? > The answer by QM and experiment is definitely: No. Again, an important distinction is to be made. Two-photon interference and one-photon interference are obviously different phenomena. In the first case you need a coincidence detection unit of some sort (two clocks at least). In the second you do not need any coincidence device. It seems to me (I may be wrong of course) that these position/momentum correlated photons 'signaling' machines are based on a sort of ... fusion :-) of the one-photon and the two-photon interference phenomena (you perform a specific measurement on the idler photons and, at a distance, without checking the coincidences, an interference pattern would appear, or disappear, at the signal wing). Now it is known, since long time, there is a weird 'complementarity' principle between the one-photon and the two-photon interference. In the sense that the more you can see the first interference, the less you can see the second interference, and viceversa. See, i.e., these papers: M.A.Horne, A.Shimony, A.Zeilinger, 'Two-Particle Interferometry', Phys.Rev.Lett. 62, 2209 (1989). M.A.Horne, A.Shimony, A.Zeilinger, 'Two-Particle Interferometry', Nature, 347, 429 (1990). D.M. Greenberger, M.A. Horne and A. Zeilinger, 'Multiparticle Interferometry and the Superposition Principle', Physics Today 46 8, (1993). and these specific experiments ... http://www.arxiv.org/abs/quant-ph/0112065 http://josab.osa.org/abstract.cfm?id=35389 Since the 'complementarity' principles, in general, presuppose a 'smooth' transition from the visibility of a phenomenon to the visibility of the other, here we can also expect (imo) a smooth transition from the visibility of a single-photon interference to the visibility of a two-photon interference, and viceversa. If there is an intermediate situation in which both interferences are (badly) visible, and if - in this intermediate situation - it is possible to perform 'signaling' (in principle) experiments, I cannot say. Summing up. I think WE CAN AGREE that in the two-photon interference we need a coincidence detection unit, and in the usual single-photon interference we do not need such a device. I am pointing out that it is not just about the use of the coincidence unit, or the use of specific detectors. There is much more physics beyond, there are many essential principles involved here. Regards, s. >The possible reason seems (to me) this one. Signal >photons cannot cause their interference pattern on [quoted text clipped - 3 lines] >a two-photon interference experiments has a 'large' >size (divergence of the beam). Reading all the posts I still don't see why there wouldn't be an interference pattern. What causes the pattern is each photon interferring with itself. Since each photon that registers on the back of the aparatus had to go through a slit in order to even register, and since we don't know which slit, then according to QM it seemingly goes through both slits and interferes with itself. Isn't that the crux of the dual slit experiment? That the source of the photons just so happened to generate another entangled beam of photons I don't see how that affects the dual slit experiment. As far as I can see, polarization doesn't matter in Cramer's experiment since he's only using the particle vs wave complimentariness in the experiment--will a particle-like pattern show up or an interference patttern. Also, I don't think his experiment has anything to do with quantum erasers. If you make a which-slit detection and then send the photon through another 'which path did it take?' choice then you lose the intereference pattern--not too surprising given what is known about how QM works. But I don't see what that has to do with Cramer's experiment. Neither do I see what any kind of delayed choice experiment has to do with it since the experiment can be performed without that. I expect that the sender can at will turn on and off the interference pattern on his side in the same way as the classical dual slit experiment. Apparently, that will have no effect on the receiver side since that would violate Relativity. Perhaps once the results of the exxperiment are published it will be more understandable to me about why it failed. > Reading all the posts I still don't see why there wouldn't be an > interference pattern. What causes the pattern is each photon > interferring with itself. Since each photon that registers on the back > of the aparatus had to go through a slit in order to even register, > and since we don't know which slit, then according to QM it seemingly > goes through both slits and interferes with itself. That is not quite true. It doesn't matter whether we know or we do not know which slit the photon has taken. If we could possibly know which path the photon has taken there will be no interference pattern. That is a subtle but important difference and it is actually the reason why the coincidence unit in the quantum eraser experiment is mandatory. > Isn't that the > crux of the dual slit experiment? That the source of the photons just [quoted text clipped - 5 lines] > complimentariness in the experiment--will a particle-like pattern show > up or an interference patttern. "Polarization" is here a tool to tag the photons path. There are other ways of doing that but polarization is probably the easiest to accomplish. > Also, I don't think his experiment has anything to do with quantum > erasers. If you make a which-slit detection and then send the photon [quoted text clipped - 4 lines] > experiment has to do with it since the experiment can be performed > without that. IMHO the quantum eraser experiment exhibits all the features to understand what is going on in a delayed choice experiment and why Cramers experiment will fail. Actually, what Cramer sets up is a quantum eraser without a coincidence unit. Since the outcome is already known it is clear that Cramer will fail. > I expect that the sender can at will turn on and off the interference > pattern on his side in the same way as the classical dual slit > experiment. Apparently, that will have no effect on the receiver side > since that would violate Relativity. Perhaps once the results of the > exxperiment are published it will be more understandable to me about > why it failed. Andreas. >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. I thought I understood the classical two slit experiment but I must not (I'm not a physicist). In the classical experiment if you rig the experiment such that only photons that went through the slits can possibly hit the back wall of the aparatus you should see a definite interference pattern on the back wall. If you then put a detector on the slits to record their location information the pattern should disappear. The detector is causing a collapse of the wave function and therefore each photon no longer has wave characteristics so cannot interfere with itself. You can even slow the experiment down to firing one photon a second if you want to convince yourself that different photons aren't interferring with each other causing the pattern. I thought that the EPR paradox was a thought experiment in which if you take that experiment but add a second beam of entangled photons then when you collapse the wave function for a photon in beam A it would in theory collapse the wave function for the entangled photon in beam B. I thought according to QM that happens instantaneously no matter how far apart the two photons are. That is "spooky action at a distance" so Einstein considered it a paradox. I know that in theory FTL communication cannot happen. The "why" part confuses me. If the photons in the entangled beam actually do take on particle-like properties then wouldn't that beam no longer be able to produce an interference pattern? >> 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 [quoted text clipped - 20 lines] > matter how far apart the two photons are. That is "spooky action at a > distance" so Einstein considered it a paradox. The old Kopenhagen interpretation considered the so called collapse of the wave function as a physical process. This implies a lot of paradoxes not to mention that nobody has ever been able to prove that there is actually a collapse happening. Nowadays most physicists avoid the term collapse. Quantum mechanics does not actually describe a collapse but it describes the probabilities of possible measurement outcomes. This is called the minimal interpretation. An entangled pair cannot be considered as two seperate objects like you would do in classical physics. It is a superposition of states or loosely spoken a superposition of possible measurement results. It is actually worth noting about what you said about EPR, that the term instantaneously immediately raises the question: In which reference frame? In a certain frame the measurement on particle B might have been the first. So, which measurement actually collapses the wavefunction of the other particle? Going a bit beyond the minimal interpretation you could consider the wave function being the information content about a quantum system. This goes back to Heisenberg, I think already in the 1930s. > I know that in theory FTL communication cannot happen. The "why" part > confuses me. If the photons in the entangled beam actually do take on > particle-like properties then wouldn't that beam no longer be able to > produce an interference pattern? In an quantum eraser experiment the signaling photons do not make up an interference pattern. Only if you apply a certain measurement on the idler photon and set up the appropriate coincidence between the two photons (thereby selecting a subset) you will be able to see an interference pattern. The point is that you cannot control the measurement outcome of your idler photon. That is, you need to know the result in order to select or deselect the corresponding signaling photon for the interference pattern. By doing so you need to communicate classical information about the measurement on the idler which is not FTL. By no means Cramer will be able to circumvent the necessity of a coincidence unit. Andreas. > 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 > > That's what Cramer is doing. Here's another page I found explaining > > his experiment and a very interesting earlier experiment which was [quoted text clipped - 31 lines] > Similarly, if you replace the detectors with photographic plates, there will > be no interference pattern on either plate. I'm not convinced your 'simplified' version of the experiment is actually the same experiment at all! The primary function of the coincidence counter, as described in the linked URL, is to separate valid pairs of entangled photons. In principle, surely, a system could generate a stream of entangled photon-pairs at predictable intervals. In that case, the primary function of the coincidence counter would not apply. However, the coincidence detector also has an another important function, which is closer to what you are saying, but not quite the same. The two-slit portion of the apparatus only passes a small percentage of the photons that hit it. The coincidence counter allows only photons that passed the slits, and their entangled partners, to be considered. Given that most photons fail to pass the slits, this particular apparatus won't do anything very amazing without a coincidence detector. The question becomes: could we increase the percentage of photons passing the two slits (or a different obstacle), so as to achieve statistically interesting results without deploying a coincidence detector? The important point here is whether the coincidence detector is actually the key to generating the alternative patterns, or whether it could in principle be dispensed with. While you may well be right that the former is correct, I don't think your 'simplified version' does anything to demonstrate it. - Gerry Quinn > I'm not convinced your 'simplified' version of the experiment is > actually the same experiment at all! It's not intended to be the same, just to illustrate the importance of the coincidence counter. It's pretty similar though, aside from being discrete and classical and omitting the slits. > The primary function of the coincidence counter, as described in the > linked URL, is to separate valid pairs of entangled photons. That's wrong; it's a misunderstanding by the guy who wrote that page, and it's presumably the cause of all his other misunderstandings. The two detectors have very narrow detection cross sections, and deliberately miss most of the photons that pass them by. If the coincidence counting had the purpose you suggest, it would make sense to replace the fixed detector by one with a much wider cross section, since this would give you a much larger data set. In reality this would destroy the signal: the better the detector at D2, the less difference there will be between the two graphs labeled "Measurement at D1". These graphs do not show measurements at D1. They show a slice through the parameter space of the nonseparable function f(x1,x2) that relates detector position to coincidence count. With a wider detector at D2 you'd instead get integral f(x1,x2) dx2, which would look completely different. -- Ben 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 > 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. Well, on the face of it, you might! The question is whether it *can* be thrown away. Its role is important, but not quite so central as you make out in your other post. > 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 A middle ground is to expect that quantum mechanics doesn't predict superluminal communication, but that the way in which superluminal communication fails is due to just a little bit too much error- correction being needed in any scheme that might seem like it could generate superluminal communication. That's what I believe; what Cramer believes I obviously can't say. He hasn't stated that it definitely will create superluminal communication, though. He has observed that according to a certain argument - no claim is made that every possible ramification of quantum theory is fully taken into account - it appears as if it might. That is nothing out of the ordinary in physics, in my opinion. - Gerry Quinn Gerry Quinn: > That's what I believe; what Cramer believes > I obviously can't say. He hasn't stated that [quoted text clipped - 5 lines] > as if it might. That is nothing out of > the ordinary in physics, in my opinion. Since QM does not "explain" those correlations, that is to say it does not explain them in terms of space (see nonseparability) and in terms of time (see noncausality, see delayed choices, etc.) Cramer is trying to fix an ontology (in terms of advanced and retarded actions, via the source). Of course his ontology is not new: Huw Price, Costa de Beauregard, Klyshko, and many more, wrote papers about it. See his transactional interpretation of Dopfer's experiment on the slides here http://www.kathryncramer.com/kathryn_cramer/2006/09/retrocausality.html and also http://www.analogsf.com/0612/altview.shtml > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > 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 Maybe you should take a look at www.cheniere.org/references/G-COM%203.pdf > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > receive a signal instantaneously? > I'm not a physicist, just an interested lay person. > Maybe you should take a look at > www.cheniere.org/references/G-COM%203.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. SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > 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. SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx > [quoted text clipped - 4 lines] > 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 "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? [quoted text clipped - 3 lines] > 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 > "Gerry Quinn" > > So, what precisely is the measurement protocol he is using to make wave [quoted text clipped - 7 lines] > 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 [quoted text clipped - 6 lines] > 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 >> http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf > [quoted text clipped - 36 lines] > > 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? SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > > [Scerir wrote] > >> There is an interesting paper here > >> http://www.arxiv.org/abs/quant-ph/0506027 [quoted text clipped - 31 lines] > Also, isn't the above quantum computer just a form of random number > generator plus quantum suicide? No, it might look similar in some respects but it's actually quite different. The big difference is that there is no need to ignore all outcomes that do not involve the survival of a particular observer. You can see this by looking at it from the point of view of a third observer: In the quantum suicide case, he sees (almost always) just a pointless death. In the time-machine version, he always sees exactly what the machine operator sees; a bilking attempt that fails in some physically consistent manner. [Conceivably some outcomes would involve the death of the operator, but these not be numerous and there is no reason for them to be important.] So in the latter, there is a always a physical system equivalent to a quantum computer that can be examined to see how it succeeded or failed. There is no need to hypothesise the existence of other universes with a different outcome, or to assert that any hypothetical alternative universe has a status equivalent to the observed universe. - Gerry Quinn |
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