Talk:Delayed-choice quantum eraser/Archive 2

This is an archive of past discussions about Delayed-choice quantum eraser. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archive 1

Archive 2

Archive 3

Archive 4

Archive 5

Wow! Great Job!

I added this article a few years ago and when I look at all the contributions people have made and all the improvements I am very pleased. I love the diagrams, which I was too lazy to do myself, and much improved text and lots of lively discussion on the talk page. Hooray! Thanks all for all the wonderful contributions. PS I DO think this can be used as the basis of a device that can transmit signals backwards in time. I do not believe that QM has any respect for the arrow of time nor that any impossible paradoxes would be created by this. Obviously that was my point when I added this articel a while back. I see lots of lively discussion, maybe some of the contributors are REAL physicists. If you are, back up your words with actions and DO THE EXPERIMENT. ;-) Rich.lewis (talk) 19:34, 13 March 2008 (UTC)

Glad you like the changes, I too was happy to see that a diagram was added. But as to your point about sending signals back in time and the need to "do the experiment", the delayed choice quantum eraser experiment has already been done, and it's been seen that, as noted in the article, the D0/D1 coincidence count and the D0/D2 coincidence count are out-of-phase (the part in the article about the peaks of one matching up with the troughs of the other) so that their sum gives a non-interference pattern, which means that the total pattern of signal photons does not show any interference, and an interference pattern can only be recovered after the idlers have been measured and a coincidence count is done (at one point a while ago I emailed one of the authors of the paper to make sure I was understanding this correctly, and he confirmed it). Did you follow that section of the article? Of course that doesn't mean some new "DCQE-esque" experiment might not be done which does allow sending signals back in time (although as noted in the previous section, Eberhard's theorem shows that if the current theory of QM is correct, this should be impossible), but I think it's pretty unambiguous that the results of this experiment show it wouldn't be useful for that purpose. Hypnosifl (talk) 20:25, 13 March 2008 (UTC)

Then what happens to QM (or certain flavors of it) if some experiment such as the Cramer experiment works?

"In any case, our minds are designed to assume this [arrow of time], and cannot possibly learn to live with any other point of view.” Wrong, wrong, wrong. There is no more reason for the arrow of time to be a universal invariant at all levels of nature than there is for the direction “down” to be. And the human brain is perfectly capable of learning to make predictions based on a more powerful model.

Werbos http://www.werbos.com/reality.htm is looking for a way to ground the QM discoveries in something more basic.

Some of the URLs this discussion has turned up have been fascinating. Thanks. P0M (talk) 01:14, 14 March 2008 (UTC)

I've archived the earliest parts of this discussion. Nothing has disappeared, just click on the link at the top of this page. (And if you need anything, just copy it back.) P0M (talk) 02:01, 14 March 2008 (UTC)

Associated questions

Have a look at: http://www.quantumphil.org/history.htm P0M (talk) 11:07, 15 March 2008 (UTC)

oh cool! I think wikipedia now needs an article on Multisimultaneity Rich.lewis (talk) 17:42, 25 March 2008 (UTC)

quote from http://www.quantumphil.org/history.htm "The final results of the experiments with moving measuring devices (see experiments) rule out the possibility to describe the quantum correlations by means of real clocks, in terms of "before" and "after"; nonlocal quantum phenomena cannot be described with the notions of space and time. This means that there is no time ordering behind nonlocal correlations, so the causal order cannot be reduced to the temporal one. " Rich.lewis (talk) 17:49, 25 March 2008 (UTC)

I had been assuming causality and time were both illusions of our perception. turns out causality may be real, but time itself does seem to be an illusion. Rich.lewis (talk) 17:50, 25 March 2008 (UTC)

One way to look at things has been suggested by people in ancient China who tried to think about the logical consequences that follow from "all is one." If that statement is true, what does that imply with regard to all of the statements we make about "this photon" at "that time," etc.? They came to the belief, and it is a belief that is shared by modern Western philosophers of science and/or phenomenology, and the people like the Vienna Circle, that there is "something out there," but the way we try to understand it is to reach out with our minds and impose our structures on that external reality. Newton did it one way, and was extraordinarily successful. But there were some problems and so another system of "convenient fictions" came to the fore. Whenever we make a better set of fictions we tend to get excited by them and imagine that the real thing is no different from the models that we make to understand that real thing.

Some things that we believe turn out to be false in very palpable and obvious ways. Walking out in the yard at night I reach down for another length of the black polypropylene cord that the guy I bought my house from has scattered all over the place. It slithers away from my hand and I realize that it is a baby black snake. Other things turn out to be false, but in much harder to discriminate ways. Time is one of those "things" that seems perfectly obvious and undeniable. It only turns out to have problems when we think very carefully about extreme cases. And when we do that thinking we do not really destroy our old ways of thinking about time as much as refine them in unexpected ways.

Evolution has tuned our abilities to perceive accurately, and technology has multiplied our natural abilities. We have very good abilities to penetrate camouflage, for instance. We have innate abilities to perceive patterns even when they are mixed in deceptive ways with other patterns. To help us survive we also have good abilities to perceive patterns in moving systems (like the predator sneaking across the grass and brush toward us). Essentially what we do (even if it is only in our gel-ware) is to create a single enduring pattern that maps a series and sequence of images (in the most general sense that might include sounds and smells as well as sights). Some of the ways our environments change us leave records of what has befallen us that may persist for even millions of years. A giant predatory marsupial may have broken a bone a million years ago, but when its bones are recovered a close examination will show traces of that accident. There do not seem to be any serious problems with that kind of idea of time. But it is most dependable when it is possible to keep a single physical entity that shows the sequence of events in its own structures. A dependable record could be anything from a spool of motion picture film to a layer of sandstone that was laid down over thousands of years and then turned to stone under heat and pressure.

The idea of time appears to start to break down when high speeds are measured between two systems and sequences in time are attempted across inertial systems.

The idea of cause is not as simple as it ordinarily seen to be. We tend to think of the cause of an accidental shooting as something like the hound dog that puts its foot down on the trigger, but that's really only one facet of a complicated event. It's impossible (at least in most cases) to find an isolated or discrete event.

The importance of Einstein's work was to show how times as measured by different people in different inertial systems would work out. As long as any process involved is governed by the speed of light, we are on grounds that people over the last century have gradually accustomed themselves to accept. The idea of entanglement was rather an affront to Einstein, but it turns out that what he thought was the death knell of quantum mechanics turns out to be a reality that people have to explore and then will have to learn to accept. P0M (talk) 08:03, 26 March 2008 (UTC)

Take a look at http://www.signandsight.com/features/614.html Zeilinger has a clear statement about how much of what we discuss are actually constructions (or "convenient fictions," or "fish traps and rabbit snares," etc.) P0M (talk) 13:00, 27 March 2008 (UTC)

Back to basics

The discussion so far has been very helpful to me in trying to see what the real issues are. I have started to go back and look for the fundamental articles to see whether secondary sources have been leaving anything crucial out.

While starting with the first article I dug out this afternoon I started to wonder whether there might not be a way to take advantage of the Young experimental apparatus. One of the advantages it has is that it provides such a simple way of producing interference fringes. So examination of a related thought experiment might help us see the experiments being done today in a clearer way.

The crucial element of many of the quantum eraser experiments is the use of entangled photons. The hope is to be able to use one of the entangled photons to see which path the other entangled photon has taken. So suppose that we make two Young apparatuses. In the one that is in the top of the following diagram, there is no attempt to interfere with the development of the expected interference pattern. In the bottom of the two, which one might imagine first being tested as an exact duplicate of the top one, there are a couple of ways of providing "which path" information. The first way is to simply plug one of the two slits. The other way is to insert a wall as diagrammed. Either way, if anything comes out of slit b it cannot interact with anything coming out of slit a -- either because nothing is coming out or because whatever is coming out is coming out in the other room.

I've only drawn light beams from the BBO to the two double-slit devices. I have also included an idea from Cramer -- to insert a long glass cable between the BBO and the double slit apparatus.

If this experiment works as the others appear to work, whether the entangled photon in the lower part of the total apparatus is permitted to interfere with itself or not will have a potent influence on the the behavior of the "twin" in the upper part of the apparatus. It appears that making the bottom photon unable to interfere with itself will make the photon up top to also be unable to interfere with itself. The results on the detector screens should be unambiguous. There either will be an interference pattern or there will be a diffraction pattern. Without even considering whether changes are made in the bottom path after the photon is observed in the upper path, it is an obvious paradox just to have the choice of "one path or two paths" in the bottom part of the apparatus determine whether the photon in the upper part of the apparatus looks like it had one path or two paths. It obviously has two paths, so how could it be influenced to behave as though it onmly had one path open?P0M (talk) 03:30, 19 March 2008 (UTC)

An experiment sending a pair of entangled electrons through two different double-slits is discussed on p. 290 of this article by Zeilinger (p. 3 of the PDF). He says you won't observe an interference pattern behind either slit, but then points out that if you measure one of the photons in a way that erases any possibility of using it to determine which slit the other went through, then subsets of the hits at the the other double-slit can show interference; he uses the Dopfer experiment as an example of this. However, he says on the next page (p. 291) that the interference will never be seen in the total pattern of photons at the other slit, an interference pattern can only be seen by doing coincidence-counting. Hypnosifl (talk) 04:08, 19 March 2008 (UTC)

Why only one? Why not measure all of them so that their "which path" information is destroyed? Just wondering...P0M (talk) 05:01, 19 March 2008 (UTC)

I didn't really mean that only one would be measured (that isn't suggested by Zeilinger), I should have said something like "if you measure any one of the photons in a way that erases any possibility of using it...", i.e. you'd be free to do it for all of them. Hypnosifl (talk) 06:12, 19 March 2008 (UTC)

So if you measure all of them so that which path information is destroyed, which is what the apparatus I've dreamed up is meant to do, then there should be a simple interference pattern in the other detector, no?

Yet according to what you report, you would only get an interference pattern if you did matching? Why should matching be necessary if everything is constant in the experiment except for the probabilistic arrivals of photons at fringe maxima? (It might be interesting to see whether a photon arriving in the third fringe to the left of center in one apparatus is matched by one appearing in the third fringe to the left of center in the other apparatus, but what could possibly distort the interference patterns, and how would matching photon arrivals with photon arrivals unscramble anything if that were the case?) I'll have to work through the article more carefully.

To put this another way, suppose that we started with two identical Young apparatuses and fed them with individual lasers. Unless the world is coming to an end or our lasers are really only LEDs, we will probably get two expected interference fringe patterns. Now link the photons being fed to each apparatus by contriving that they be entangled somehow. What happens to destroy one of the two interference patterns? Or are both interference patterns destroyed? Why would that happen? It's not because we now have "which path" information. It might be because the BBO produces photons of complementary polarization, even though I can't intuit any basis for that. Maybe the math would show it. But then we could put in polarizing elements to fix things as is done in some of the quantum eraser experiments.

It might cost me as much as a couple thousand dollars to buy the appropriate laser, BBO, and protective glasses, but it would be almost worth it to see this thing work. Then I could answer interesting questions like, "What happens if polarizing light traps are put on the outputs of the BBO that would otherwise head to the lower (or upper) set of double slits?"

In the experiment with the five detectors and multiple beam splitters, there is a clear reason why the d3 and d4 detectors will pick up complementary diffraction patterns. It's because of a vanilla flavored optical phenomenon, the change of phase caused by going through the glass side of a beam splitter and then being reflected out the same side. But what is going on in this experiment to produce a similar effect? I'm puzzled, probably because I haven't digested the article yet.P0M (talk) 19:27, 19 March 2008 (UTC)

I just had another look at the Zeilinger article. He says:

If the Heisenberg detector is placed in the focal plane of the lens, it projects the state of the second photon into a momentum eigenstate which cannot reveal any position information and hence no information about slit passage. Therefore, in coincidence with a registration of photon 1 in the focal plane, photon 2 exhibits an interference pattern. On the other hand, if the Heisenberg detector is placed in the imaging plane at 2 f, it can reveal the path the second photon takes through the slit assembly which therefore cannot show the interference pattern (Dopfer, 1998).

The above statement should be true for all cases, i.e., for all photons originating in the experimental apparatus. As far as I can make out so far, he is substantiating, and even going beyond, what I have been saying. Can you give me an exact quotation of what you are referring to on page 291 of the article? P0M (talk) 21:21, 19 March 2008 (UTC)

"So if you measure all of them so that which path information is destroyed, which is what the apparatus I've dreamed up is meant to do, then there should be a simple interference pattern in the other detector, no?"

I don't think so, no. After all, you could easily do something like that with the DCQE, removing the beam-splitters BSa and BSb so that most of the idlers would end up at the which-path-erasing detectors D1 and D2. But since the D0/D1 interference patter is out-of-phase with the D0/D2 interference pattern, so that the sum of the two interference patterns is a non-interference pattern, then that suggests that even if all the idlers end up at D1 or D2, the total pattern of signal photons at D0 won't show interference. I would imagine that in any experiment where you erase the which-path information of one set of photons (let's keep calling these the 'idlers', even if the setup is different than the DCQE) so that there's no way to tell which slit their entangled twins (call them the 'signal photons') went through, something similar will be true; there will be different possible locations you might measure the idlers, and if you look at the coincidence graph between idlers at one of these locations and the corresponding signal photons, you'll see interference, but the sum of all these interference patterns in the coincidence graphs will not show interference, so the total pattern of signal photons behind the double-slit will be a non-interference-pattern.

Just to be clear, are we just talking about what would be predicted by orthodox QM if we actually did the math to find out its theoretical predictions for any given setup? If we drop the assumption that orthodox QM is correct, then of course pretty much anything could happen in any experiment that hasn't been performed. But as long as we stick to the predictions of QM, it seems to me that Eberhard's theorem demands that the total pattern of signal photons behind the double-slit can't vary depending on how the idlers are measured (whether their which-path info is preserved or erased), since if it did then just by looking at this total pattern you could gain information FTL. Do you agree? If so, I think you must agree that orthodox QM can't predict an interference pattern behind the double-slit even if the entangled idler photons are all measured in a way that erases their which-path info. And yet, as I said above, the DCQE suggests an elegant way that orthodox QM can both avoid the possibility of FTL information transfer but also avoid violating the principle of complementarity which leads us to think we should see an interference pattern when the which-path info is erased; the resolution would just be that you'd see an interference pattern in the subset of signal photons that correspond to idlers that were measured at one particular location, but that when you sum up all the signal photons whose idlers were detected at a range of locations, the interference patterns for each location are out-of-phase in a way that allows their sum to show no interference.

"To put this another way, suppose that we started with two identical Young apparatuses and fed them with individual lasers. Unless the world is coming to an end or our lasers are really only LEDs, we will probably get two expected interference fringe patterns. Now link the photons being fed to each apparatus by contriving that they be entangled somehow. What happens to destroy one of the two interference patterns? Or are both interference patterns destroyed? Why would that happen?"

Yes, I think both interference patterns would be destroyed, simply because entangled particles have different behavior from non-entangled ones; have a look at the discussion threads Does a beam of entangled photons create interference fringes? and entanglement and which-path from physicsforums.com, where various knowledgeable posters say that entangled photons don't show interference fringes, and give plenty of links to papers by scientists to support these assertions.

"Can you give me an exact quotation of what you are referring to on page 291 of the article?"

Yes, I was referring to the paragraph where Zeilinger wrote:

We note that the distribution of photons behind the double slit without registration of the other photon is just an incoherent sum of probabilities having passed through either slit and, as shown in the experiment, no interference pattern arises if one does not look at the other photon.

Does this not mean that if what he calls "photon 1" is not detected at the focus of the Heisenberg lens then what he calls "photon 2" will not register in the "double slit detector" at a position consistent with interference?P0M (talk) 03:19, 20 March 2008 (UTC)

When you say photon 1 is not detected at the focus (and here Zeilinger seems to be talking about the setup of the Dopfer experiment, although he doesn't use that name), do you mean it's not detected there because 1) the detector D1 is placed at some other position than the focus (say, at distance 2f in the image plane rather than distance f in the focal plane), or do you mean 2) that the detector D1 is placed at the focus, but we look at the subset of photons that registered at D2 where there was no corresponding photon hit at D1? Either way, I feel pretty confident that orthodox QM would predict that the total pattern of photon hits at D2 (when we don't do any coincidence-counting) will never show interference, again because of Eberhard's theorem. Hypnosifl (talk) 03:43, 20 March 2008 (UTC)

I primarily had in mind what happens when "photon 1" is not detected at the focus of the Heisenberg lens because the Heisenberg detector is moved to its more remote position. (I guess it is theoretically possible that the detector could be at the focus of the Heisenberg lens but a photon still would not be detected for some reason, but I don't know how anybody could be sure that a photon had gotten through or around the detector somehow.) What he appears to me to be affirming is that if the Heisenberg detector is at the focus of the Heisenberg lens, then "photon 2" must be found at some position consistent with an interference pattern. "A double-slit interference pattern for photon 2 is registered conditioned on registration of photon 1 in the focal plane of the lens." (p. 290) The converse would be that the lack of a double-slit interference pattern for photon 2 is registered conditioned on "placing the detector for photon 1 into the imaging plane of the lens" (p. 290) -- or simply removing the detector and letting the photon show up wherever it may. It would still be possible to determine its path, so that would imply which-path information for photon 2 and no interference pattern.

There would be no need for coincidence counting if the Heisenberg detector were left at one extreme (at the focal plane) or the other (at the imaging plane) unless there are non-entangled photons entering the experimental apparatus somehow.

He summarizes the two extremes as follows:

It is sufficient to destroy the interference pattern, if the path information is accessible in principle from the experiment or even if it is dispersed in the environment and beyond any technical possibility to be recovered, but in principle still ‘‘out there.’’ The absence of any such information is the essential criterion for quantum interference to appear.

P0M (talk) 07:37, 20 March 2008 (UTC)

On the other hand, in the section you quoted he was clearly just talking about an interference pattern in the coincidence count, which is why he said "Therefore, in coincidence with a registration of photon 1 in the focal plane, photon 2 exhibits an interference pattern." Also note on p. 290 where he talks about the experiment of sending two entangled photons through opposite slits, and says:

"Will we now observe an interference pattern for particle 1 behind its double slit? The answer has again to be negative because by simply placing detectors in the beams b and b' of particle 2 we can determine which path particle 1 took."

Note that he isn't saying you actually have to place those detectors in the beams of particle 2, the mere fact that you could do such a thing is sufficient to explain why you never see an interference pattern for particle 1 behind its slit (again, if the pattern for particle 1 depended on what you actually did to particle 2, this would give you an easy way to make an FTL telephone, forbidden by Eberhard's theorem). Hypnosifl (talk) 02:48, 20 March 2008 (UTC)

Theory does not rule over the universe, and physics makes progress by discovering places where theory is disproven. Right now it would be helpful to see what are the questions of real import and what are the questions that relate to artifacts of experimental design.P0M (talk) 16:56, 21 March 2008 (UTC)

But you are saying, correctly, that what the experimenter does not do (that is, the cases when the experimenter does not place any detector in the beams b and b') absolutely determines the lack of an interference in the other part of the apparatus.

"One problem in all experimental situations thus far is due to technical insufficiencies, namely that only a small fraction of all pairs emitted by the source is registered." (p.293) I think the only function of the coincidence counter, in this experiment, is to strain out the noise. If it is true, as he says, that "by simply placing detectors in the beams b and b' of particle 2 we can determine which path particle 1 took," then in that case the experimenter has definitely constrained the interference pattern from manifesting in photon 1. It's unclear to me what function the coincidence counters play other than to eliminate random photon hits. If some photon is registered in one half of the apparatus and is not registered in the other half, then the experimenters rule those appearances out since they cannot be caused by one of the entangled photons. I think that the BBO does not manage to convert every photon that is pumped into it. And I recall that some of the experiments use a filter on the detectors to eliminate photons that are not of the expected frequency.

The Kim experiment has a need for the coincidence counter that could not be reduced by improved experimental design intended to eliminate "rogue" photons -- in that experiment, entangled photons in the signal path are randomly related to photons detected and d1, d2, d3, and d4. It doesn't much matter whether the photons are detected at d1 or d2 because either way there will be no interference pattern manifested in d0. But photons detected in d3 and d4 are related to complementary interference patterns that would merge into an evenly illuminated screen if they were all added together. The Dopfer experiment doesn't have this complication. It's either "interference" or "non-interference" P0M (talk) 08:25, 20 March 2008 (UTC)

I just had a look at p. 3. It seems plausible, at least if you believe that photons are particles that have position even when the operation needed to determine position would also change the event (making the event emitter to point where the position was detected rather than emitter to detector screen). If we leave out the alleged particle, then we have paths to deal with. If a photon is detected somewhere on the screen beyond the double slit, the we know that the photon took path a or path b (or both). If the other photon is not observed we have no help in understanding whether the first photon took one path or the other. If it is observed, then that means that we have put a couple of detectors along paths a' and b', and one of the detectors picked up a photon.

On the other hand, suppose we put up a mirror double slit on the a' b' side? Then we lose "which path" information, no? Then it would be consistent to get interference on both detector screens. I'm out of time, so I haven't read what the author thinks/has discovered about this alternative. More later.

It seems that creating two paths is necessary to get interference, and creating two paths and then isolating them prevents interference as surely as if there were only one path. Or, to put it another way, now instead of one single path we have two single paths.

However, turn things around and they appear not to be symmetrical. If we had two paths for one entangled photon we would not have interference unless the two paths were merged. If the entangled twin photon is in a situation that enforces there being two paths for it, then attempting to merge the first pair of paths does not result in interference. In some ways the following argument seems persuasive: If the a'b' paths are separated by a wall, it would amount to "magic" to be able to merge a' and b' despite the wall. If a' and b' were heading off in opposite directions, it would also be "magical" to get them together just by operating on the a and b path events.

No more time. Maybe tomorrow...P0M (talk) 04:58, 19 March 2008 (UTC)

True or false?

I know that all of this is discussed above, but it's simply waaay too long to read it all and it would only confuse me even more. If you came to a conclusion above, the answer to these questions should be no more than "yes" or "no". So please answer short and to the point, otherwise who knows how long this will become again.

Now the questions:

a) Replacing BSa and BSb with mirrors would give a pure "linear" (non-interference) pattern at D0?
Yes. a diffraction pattern as in the top image above

b) Removing BSa and BSb would give a pure interference pattern at D0?
No. This way also gives a diffraction pattern.

c) (Only if both answers were yes:) Even if the mirrors were placed/removed after the light hit D0?
A "yes" and a "no," so you don't need an answer to this one.P0M (talk) 03:27, 22 December 2008 (UTC)

Apography (talk) 16:30, 20 December 2008 (UTC)

I'll try to answer your question, with the caveat that I don't have access to the apparatus and thinking about what "ought" to be can be deviled by many unanticipated complications.

First, if we took out the BBO then we would just a beam of laser light coming into the double slit, and a detection screen placed directly in front of that apparatus would show what every properly set up Young double-split experiment would show. (I.e., it would show an interference pattern.) The complications come in when the BBO creates two diverging paths from slit a and slit b. So if we could eliminate the possibility of "path knowledge" coming down to the place where the Glen-Thompson prism is, then reason says that we ought to to get an interference pattern at D₀. And if we could absolutely determine which-path information, let's say by building a wall between the "a" path and the "b" path and putting a detector on each side of the wall, then because of the way the photons are entangled it must be impossible for interference to be observed in any of the detectors.

So in your case a one photon would either show up at D₄ or it would show up at D₃. "Which path" information would be unambiguous in that part of the entire apparatus, and if "which path" information is available then interference cannot occur. So we would get no interference pattern at D₀.

(My mistake. I should not have said "diffraction pattern" here last time. You would get a diffraction pattern, although most people regard it as just a single spot of light because diffraction produces only a slight flaring as shown in the top photo above.P0M (talk) 03:27, 22 December 2008 (UTC)

In your case b a photon would either show up at D₁ or at D₂. As before, same reasoning, and no interference pattern would be seen at D₀.

Either way, no interference pattern at D₀.P0M (talk) 03:27, 22 December 2008 (UTC)

Well, P0M, our interpretations of "short" are a bit different ;) but I don't mind. What you said made sense. However, what I used the wrong words to ask my question. I said "diffraction pattern" where I actually meant "non-interference pattern" (what is the right word for that anyway?). So I've removed your responses and now ask you the same with these now slightly modified questions.

The short answers are given above. I should just stop with a "yes" and a "no." But you went on to suggest some other concerns.

You were correct in calling the "smudged spot" a diffraction pattern. P0M (talk) 03:27, 22 December 2008 (UTC)

Also, in (c) I don't mean that you should constantly place and remove the mirrors every time a photon fires. I mean that you extend the distance between the BBO and the prism so long that first all photons hit D0, then you decide to place/don't place the mirrors, and then the photons arrive at the other detectors. Also make the cable between D0 to the counter so long that the coincidence counter still works. I don't care if all the technical details are correct, but reduce the question to "can a choice (by the researchers, not the photon) now influence the outcome of an event that happened in the past?"

That's the way it looks to human beings, and it bugs the hell out of most of them.P0M (talk) 03:27, 22 December 2008 (UTC)

Actually, it doesn't matter when you put the mirror in place of the beam splitter or when you remove it. What counts is whether something shows up at D₀ before it would have time to get to where there was originally a beam splitter. But if you believe that information might travel from the beam-splitter/mirror/gap location back to the double slit and influence the photon at the time that it comes through there, then you can eliminate that possibility by waiting until the photon is almost to that location and randomly put in a beam-splitter, a mirror, or a transparent medium. P0M (talk) 03:38, 22 December 2008 (UTC)

The experiment was set up so that the difference in distance between the BBO and D₀ could be made less than the distances between BBO and the other detectors. Your idea is to remove the beam splitters and replace them by mirrors, or just remove them. So you would have to make a somewhat greater difference in the locations of the parts in the two parts of the total experimental apparatus.

The way I conceptualize it is something completely different though. You can read my full article here, but I'll summarize my thoughts: the laws of physics are easy and intuitive, unless you experiment with extremely large (speed-of-light-ish) or small (photon-sized) scales, because then you get theories that are as logical as "2+2=5 for extremely large values of 2". However, there is no reason that why speed of light is not just infinite, and atoms could just as well be infinitely small.

Nobody knows why universal constants like c are as they are. People working on the "grand theory of everything" hope that they can close in on the issues. People like George Greenstein are fascinated by the way these universal constants have to have "come out" within very tiny limits to make the whole universe as we know it work so that planets, water, etc. are possible -- not to mention human life. For whatever reason, the speed of light is not infinite, and atoms have small but finite dimensions.

That made me think that those simple laws of physics are the only "real" ones and that this complex quantum/relativity weirdness are only (consequences of) limitations of our universe. One possibility is that our universe is calculated/simulated and that these limitations are imposed by the available computing power. Smaller atoms = more atoms = more calculations. Higher speed of light = more interactions = more calculations.

I don't think that will work. Do you know about the ultraviolet catastrophe? Classical physics yields some results that are beyond orders of magnitude wrong. P0M (talk) 03:27, 22 December 2008 (UTC)

The double-slit experiment fits in smoothly: instead of calculating every photon one by one (which would require a lot of calculations) the program just waits until it's necessary to calculate them, for example when something (photons or results) is observed. Then it instantly calculates the movement of all the photons as one wave. This is equal to a collapse of the waves of probability, and this both explains why photons can interact with themselves and how the choice of the photon's path through the splitters can influence the "earlier" forming of a pattern.

As a "myth" to explain how things work, this idea sounds pretty good to me. The flip side may be that some calculations are so complex that it is often easier to let nature do her thing. I'm thinking of the way soap bubbles were stretched over wire frames of various configurations as ways to solve equations that were too hairy to do the real way until we had big electronic computers. But why assume that some God needed soap bubbles to do some (even for computers) relatively simple computations?

This is a key point in my theory, and that's why I'm now researching if this "buffering" of photons is actually a possible explanation for the results of these experiments. If the answers to my (modified) questions were three times true, then my theory would hold. So what my real question actually is "is it possible that the movement of the photons is not calculated until observation?" That is all I am currently interested in, no time travel, no superluminal information travel, etc.

You might want to find the book by the neurophysiologist who suffered a stroke and lost all the functioning of the "logical" side of her mind. It's fascinating and is entirely in line with the idea that Lao Zi and Zhuang Zi taught (see the bottommost item at http://www.wfu.edu/~moran/zhexuejialu/philos_router.html to get a sample). The universe is a unity and we divide it up by active processes of mind. We make what modern philosophers of science call "models" by which to understand the universe. The lady doctor, who eventually had her stroke treated and then had to relearn language and her whole specialty, describes the part of the mind she lost temporarily as "creating the universe and then understanding it." She was not being solipcistic, just saying that "the universe" and any "thing" in it is something that our minds produce out of what is really out there. There are photons and they do their own thing. We reach out and either stuff them into the "skin" of a particle or the "skin" of a wave. Then we get pissed off because the two models are contradictory. But the photons are not at war with themselves. It is our models that refuse to harmonize seamlessly with each other.

So, yes, taking your statement above as a "model," what you are really reflecting is the fact that the photons do their own thing, which is totally out of our ken, and we catch up with things and try to put them into the context of our models. That is like trying to make a model of a cat out of tinker toys and then getting flummoxed because the cat does things better than our model can do and even does things that we can't get our model to do.

We would like photons to do their thing in a way that is consistent with our ideas of time and sequence, but where do we get our ideas of time from? Basically, we get our measurements of time first from biological processes of a fairly regular rate, then we notice that humans work at individual rates and we look to more regular processes, and recently we have gotten so refined that we count the number of times some atomic process takes place and call that a second. But all of these processes are going on through interactions that are ultimately governed by light speed-limited events. So light is doing its own thing to begin with and we take a part of what it can do (e.g., travel from one end of a meter stick to the other end a gazillion times) and make that into our refined idea of what time is. Then we discover that nasty old light has been doing some other things that it never told us about, and when "the same photon" is not traveling between two places but from one place to two or more places it behaves differently from the way we have grown used to it behaving in context we have become familiar with, and we get all flummoxed about that fact.

Maybe we could say that the action is the calculation, and that the calculation is the action, and that it isn't "over" until it is "over" -- whatever "over" means in this new world.

I have the feeling that light probably "travels" outside of time, and if that is true I think it has to travel outside of space-time. We ordinarily mean by "X exists" that we can go to some space-time coordinate, xyzt, and discover X sitting there. Do fairies exist? You think so? Fine. Tell me when and where I can observe one. You think Queen Elizabeth II does not exist? I have photographic evidence that shows her in a long string of appearances. She is an inter-subjective object. Now, how about that photon? We programmed the single-photon emitter to fire one at xyzt, and we expected to see a blip on a detection screen at x'y'z't'. Sure enough, the meter ticked over right on schedule. But, tell me, what happened between t and t'? In that time interval no observations could be made, so by the strict definition of existence given above, we cannot say that it existed.

Let's not make life complicated for ourselves by imagining some God who has an unseen motive for making light make that little trip. All we have done is create another entity that does not appear at any xyzt to explain something that happens when nobody can observe it.

Some time between 1130 and 1200 a famous Chinese philosopher had a discussion with one of his students who, basically, asked him why a certain individual was not a model citizen. The philosopher, Zhu Xi, said that the rather unworthy individual was endowed with very large diameter facial hair, a stout build, a tremendous appetite, a disregard for the feelings of anyone who got between him and his objectives... In short, the man was a swine. "He has the lifebreath of a pig, therefore he is a pig." And he would have given the same explanation for any other trait. Why was Newton such a great scientist? Well, he had the lifebreath of a genius. Why was Hitler such a destructive figure in mid 20th century life? Well, he had the lifebreath of a despot. Etc., etc. The trouble with is that all these expressions are tautological. Luigi is an xxx because he is the product of the causal factor that makes anybody become an xxx. It doesn't do us any good unless we have an analysis of the causal factor in terms of something more basic. In the case of the "swine," maybe it would be helpful to have a print-out of his genome. Or maybe we need an analysis of his childhood enculturation processes.

PS: No, I do not want to become "the one". The Matrix movie really killed this theory as now everybody thinks that a simulated universe is a "prison for the mind" out of which we can break free. I say nothing about being "plugged in" somewhere in a different universe. All I say is that we are calculated. Turing-complete. Apography (talk) 22:57, 20 December 2008 (UTC)

The one? I thought that was Obama. ;-) P0M (talk) 03:27, 22 December 2008 (UTC)

Thank you, P0M, that was a very enlightening response. It is however of no scientific value to this article and this page is already long enough, so I'll remove it. I hope you find it back in the discussion page's history (which apparently you did ;) Apography (talk) 16:33, 22 December 2008 (UTC)