Talk:Double-slit experiment/Archive 03

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Archive 3

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[edit] Special Request

This is a request for YOU: Please read #29 "Questions about the double slit experiement", and, if you are able to do so, provide a commentary. God bless you. Honeychurch 09:38, 7 November 2007 (UTC)

See above. Aquinas said abut God that it was impossible to use finite human cogitation to form accurate ideas of the infinite. He was aware of human limitations (and he was one of the least limited of humans). It turns out that finite human conceptual processes are not even very adequate for dealing with the very small (but still of some finite size) and the very large (but still in this finite Universe). So one works with models, or what somebody called "useful fictions" much as Aquinas said we get at some idea of God by denying of him all human imperfections -- which eliminates an awful lot. P0M 08:30, 12 November 2007 (UTC)

[edit] What does this mean

"However these experiments performed with detectors are dubious because we do not find, know design and working of these detectors." What does the italicized portion mean?

"There is no reliable source to verify that these detectors do not mess up path and properties of electrons and photons involved in experiments." Is this encyclopedic language? 212.179.210.204 04:12, 18 October 2007 (UTC)

I agree--these lines are vague and not encyclopedic-sounding. What the hell does it mean to say that we don't know the design and working of the detectors? Obviously SOMEONE does, because someone built them. I'm assuming they weren't gifts given to us by little green men. --Lode Runner 23:36, 23 October 2007 (UTC)

[edit] Doubting Rothman

The article cites as fact Rothman's doubt that Young actually carried out his experiment, spending a considerable amount of time on that issue. It would be helpful to know who else doubts it besides those who accept Rothman on blind faith. Repeating isolated theories as fact runs the risk of filling Wikipedia up with half-truths shouted at full volume. One could then sympathize with the man who wanted to charge $10 per printed Wikipedia article, who when confronted with the statement in Wikipedia that it is free replied, "You believe everything you read in Wikipedia?"

If Rothman is right about the experiment not having been performed, we would have the interesting situation of someone who, in a culture totally wedded to the corpuscular theory of light, guesses its wavelength and then describes a gedankenexperiment that if carried out would demonstrate quantitatively the wave nature of light. To accomplish this without actually doing the slit experiment, he or an accomplice would either have had to carry out some other experiment yielding that information involving for example Newton's rings (in which case why would he not report that experiment instead?), or be visited by a time traveler from the future. --Vaughan Pratt 23:48, 22 July 2007 (UTC)

Yeah, I'm not really sure why it's such a big deal for a scientist to reuse explanatory diagrams. That seems to be the foundation of the entire suspicion. — Laura Scudder ☎ 14:04, 23 July 2007 (UTC)
It seems that this problem was disposed of long ago. P0M (talk) 07:29, 23 November 2007 (UTC)

[edit] Improper use of copenhagen interpretation?

I'm surprised that no one has mentioned this earlier, but the following statement in the article I would claim is not true:

The Copenhagen interpretation posits the existence of probability waves which describe the likelihood of finding the particle at a given location. Until the particle is detected at any location along this probability wave, it effectively exists at every point.

Given that the Copenhagen interpretation is the primary explanation given in most physics courses, I have never encountered any explanation regarding the existence of a particle at every point due to the wave-function changing in time. Based on the copenhagen interpretation, we could claim that the probability distribution changes in time giving us the knowledge of where the particle is most likely at, but I do not think we can claim that the particle exists at all points at once... this would contradict the idea of the probability distribution. Any thoughts on this? —The preceding unsigned comment was added by 71.117.4.108 (talk) 22:50, 16 April 2007 (UTC).

I think I've fixed this problem. Have a look. P0M (talk) 07:27, 23 November 2007 (UTC)


[edit] fractal spacetime

What is the scientific consensus on El Naschie's theory of fractal spacetime? El Naschie says the theory explains the results of the double-slit experiment. --70.130.45.233 09:38, 21 September 2007 (UTC)

[edit] Electron Motion sensors to detect 'which-way' info.

Single electron double slit experiment produce interference pattern even though radio waves are present in room. But it is said that it is not possible to detect through which slit electron entered. Why don't they fix simple radiowave motion detector in one slit? The electron will break radiowave signal triggering alarm and we will know through which slit electron entered.

We can fix separate motion sensors in both slits and we can arrange second alarm with different sound. —Preceding unsigned comment added by 6E656F (talk • contribs) 10:12, 6 October 2007 (UTC)

I don't think there is any question that electrons could be detected. Whether there is a detector at one slit or at both slits does not really matter. It's a spooky way of talking about things, perhaps, but if something exists that leaves a sign of a particle having passed one way or the other, then interference will not occur. It's not that physicists have contrived some odd way that they think the universe ought to work, but that the universe works in certain ways and those workings drive the formulations that sound so improbable that people try to think of ways around them. If people could detect the presence of an electron in such a way as to indicate that it had come out of one slit or the other and interference still occurred, then that result would be big news. Your idea sounds like it would be easy to put into practice in the real world. Probably people have already done it. The experiments are probably on record somewhere, but the news value would be something like, "Female collie gives birth to five pups." It would be a small town's second newspaper that would bother to publish such news. P0M (talk) 07:20, 23 November 2007 (UTC)


[edit] A rumored variation of this experiment

The last chapter of the Scott Adams book The Dilbert Future mentions (page 231) a variation of the double slit experiment that goes something like this:

Detectors are put in place, with the familiar result. Then, the detectors are programmed to automatically delete the recorded data the moment after it's recorded, so that no one gets a chance to see it--the post-event act of data deletion supposedly altered the pattern the photons left. The self-purging detectors caused the photons to leave the same pattern as if there were no detectors at all.

I remain sceptical, because Adams freely admitted that he was explaining it all from memory and had probably gotten some things wrong, but if anyone can confirm that this variation of the experiment has actually been performed I think it definitely merits mention. If the experiment took place like Adams described, I think it would without a doubt support the (currently pretty shaky and pseudoscientific) consciousness causes collapse concept. If, on the other hand, Adams was mistaken and such an experiment never took place, then we can mention that in the book's article. --Lode Runner 09:47, 23 October 2007 (UTC)


Update: The video here http://www.youtube.com/watch?v=_OWQildwjKQ describes the experiment as Adams described it. I do not know whom the speaker is. --Lode Runner 10:43, 23 October 2007 (UTC)

creepy —Preceding unsigned comment added by 68.106.248.211 (talk) 10:31, 1 November 2007 (UTC)
This is "quantum eraser" stuff. It is very interesting. It reminds me of the experiments that show that when a time frame in chosen in which there are two maxima and one minima, and an interference effect can occur wheneven the maxima is there -- but you can't tell whether it is the earlier or the later maximum that is involved in the real-world phenomenon under observation. It's hard for me to digest this stuff because I almost have to write out a script with notes to myself about what we measure and what we know in all aspects of the experiment and at all times in one run of the experiment. It is very easy for me to let in some assumption from the macro world. I'm looking for clear accounts, accounts that will help me avoid stupid mistakes. Of course if anyone wants to write such an clear account and supply citations then I won't need to do the work, and there is less chance that I will make a mistake. I'm starting to wish for half-silvered mirrors, etc. P0M (talk) 07:44, 23 November 2007 (UTC)

[edit] A note on the templates

Just a quick note as to why I've tagged this article for cleanup and reference improvement. Large sections of this article are written less like an encyclopedic entry and more like a textbook--inline questions, enthusiastic descriptions of the experiments, and so on. Tidying up these sections should take care of the style issues.

The references tag should be pretty self-explanatory; six references in an article (with three whole sections having no references at all) of this importance is simply not enough. Given the relatively wide appeal of this experiment, the number of times it has been repeated, and the speculation it invites, I would imagine that a veritable cornucopia of references could be found.

I'll do my part once I get a shot, but I just wanted to put down the specific issues I saw to help ensure these improvements get made. --jonny-mt(t)(c) 09:21, 31 October 2007 (UTC)

I've asked this editor to give indications of what things still seem to require citations. So far nothing has been done. Is there still any reason to post a wolf ticket at the top of the article? P0M (talk) 01:31, 21 November 2007 (UTC)


[edit] Beginning to respond to the templates and critique

I have just wiped out the nonsense about "a single band" being produced by one slit. Strangely, even Sears made the same mistake in his Optics although his own photograph show otherwise. Anyway, wrong is wrong and lots of places have the right information. I will tuck in some of the references I have found when I get a little more time. I made my own photos of both the single and double slit results from the same basic apparatus, which gives the reader what one really will see when the experiment is done. The photos in Sears are better, but they are copyrighted. My equipment cost a couple dollars. I suspect that his equipment cost a bit more. P0M 04:15, 6 November 2007 (UTC)

I couldn't stop myself. I found some very useful diagrams on the Commons and was able to write out the basics of the experiment in a way that I hope will be correct but also clear to the average well-informed reader.

The rest of the article needs some editing both with regard to fact (one photon experiments were done very early, with the math to prove that the photons had to be arriving one at a time at least most of the time) and with regard to writing suitable for the non-expert. Frankly, there are things that ought not to be incomprehensible that I could not understand on a rapid reading. Since Sears and Heisenberg don't provoke these confusions and uncertainties in my mind I suspect that it is the writing that is at fault. (Sears is a great textbook writer, by the way.) P0M 05:35, 6 November 2007 (UTC)

[edit] problematical content

The paragraph that starts

A remarkable result follows from a variation of the double-slit experiment

Are there any citations to back this idea up? If one photon can go through two slits at the same time, what happens if two photons arrive at those two slits at the same time? At the same time? How does anyone know which instances of transit through the two slits occur at exactly the same time? And if one photon goes through two slits anyway, isn't it going to interact with itself regardless of what any other photon does? What would be predicted if two photons were entangled and one or both of them went through a detector that absorbed and re-emitted it? How else would one get two photons known to be temporally matched? P0M 06:35, 6 November 2007 (UTC)

If nobody can provide citations I will cut this part out. P0M (talk) 00:47, 21 November 2007 (UTC)


[edit] A sentence that does not make sense.

The article says:

In case two pinholes are used instead of slits, as in the original Young's experiment, hyperbolic fringes are observed. This is because the difference in paths travelled by the light from the two sources is a constant for a fringe which is the property of a hyperbola.

Does a hyperpobola have a property? Maybe, although I do not believe that mathematicians ordinarily speak of a definition that corresponds to the word "hyperbole" and maybe the lines that can be drawn on the basis of such a definition. And is the property supposed to be a fringe or a constant? The syntax is totally bonkers. Can somebody rewrite it so that it is a statement that is true or false? That would be a start. P0M 06:16, 12 November 2007 (UTC)

I will delete the sentence if nobody can fix it. -- P0M (talk) 21:41, 16 November 2007 (UTC)

I have deleted the sentence. P0M (talk) 00:46, 21 November 2007 (UTC)

[edit] present state of the article

Is anybody else monitoring this article? The request for more citations, etc., was very vague. Perhaps someone would like citations regarding specific issues. Please provide such notices.

Right now the article probably is suffering from its own lack of coherence. It is difficult to change things one by one without creating problems of repetition or even inconsistency. P0M (talk) 06:25, 23 November 2007 (UTC)

[edit] The observer role

The article states: "The remarkable consequence discovered by this experiment is that anything that one does to try to locate a photon between the emitter and the detection screen will change the results of the experiment in a way that everyday experience would not lead one to expect. If, for instance, any device is used in any that can determine whether a particle has passed through one slit or the other, the interference pattern formerly produced will then disappear."

I understand that this video: http://www.youtube.com/watch?v=DfPeprQ7oGc ... illustrates this phenomenon.

... but the article doesn't explain why that could be and what it means. (Whereas the full length version of the above video draws a meta-physical conclusion on the subject).

It's a little complicated, actually. What you are asking is sort of like asking to prove a negative. If the article has any defect as it is currently written, it is that is makes it sound as if you can absolutely prove a positive general proposition (e.g., "All swans are white."). What people know so far is that lots of things were done to try to detect whether a photon passed by way of one slit or the other, and that when those methods were tried the interference pattern disappeared. So let's say that 100 ways of detecting the passage of a photon have been tried and I say, "I guess that shows you!" Then somebody comes along with formula 101 and detects the photons somehow without ruining the interference pattern. Bingo.
One of the "metaphysical" questions is whether it is "human observation," or just doing something that will leave a mark that somebody a million years from now (or at any point in time) could choose to look at. Heisenberg thought that the essential thing was that when the dog went through the dog door the dog door slapped the dog's tail. Whether anybody heard or recorded the sound of the slap was irrelevant. Even a deaf dog would still be impelled to move his tail forward a little faster. (Sorry for the lame analogy.)
There are several attempts in motion to detect the slit that different photons pass through and then "erase" that knowledge so that the interference pattern will still appear. One such experiment is demonstrated by the following diagram:
Instead of dividing a stream of photons into two groups by putting a double slit in the middle of things and letting half the photons go through each slit, this experiment uses a mirror the is designed to reflect half of the incident and allow half to pass through. Then the two beams of light are led off in different directions, made to turn corners by fully reflective first surface mirrors, and finally their beams cross (which doesn't do anything to either) and the beams each end up throwing a single spot on the approriate detection screen. So from the time the light hits the first beam splitter (half reflective mirror), two groups of photons are going through entirely different paths. And when they get to the end of the free flight trajectories and hit a wall, they do just what anybody would expect them to do. They form a spot of light on the wall just as the original light would have formed a spot on the wall if there hadn't been any mirrors in its way. However, if a second beam splitter is put at the other end where the two beams split, the two beams get mixed and it is no longer possible to tell which beam a given photon originally belonged to. At this point it becomes possible to get an interference pattern.
To me what this experimental result shows it that to get an interference pattern you have to give photons two (or more) paths to follow and then combine those paths in a way that will let them interfere. But once you let them combine you inevitably lose track of which photon is which. Feynman argues that it is wrong to talk about photon one going by the left path and photon two going by the right path; instead, each photon goes by both paths -- and if you don't grok that it's your problem. He's not sympathetic because he doesn't understand it either.
There is another experiment that is probably lots harder to set up, but it is much more threatening to our everyday ideas. It even seems to upset our ideas of time flow and causality. If the double-slit experiment is done with electrons then a light shining on the other side of the slit will bounce off individual electrons and in so doing will be diffused. Because the light interacts with the electrons it changes them individually enough that they do not interfere with themselves and the expected interference pattern does not emerge. However, if a lens is used to collect the bounced light and direct it into a camera, the interference pattern returns. The most disturbing part of the experiment is that the optical path to the camera can be extended far enough to make the electrons reach the detection screen before the light reaches the camera. Why should this experiment work? Some people think that something goes backwards in time "after" the light reaches the camera. The camera appears to function as some kind of terminal point from which a time-inverse reflection propagages. I'm just going on memory, but you can look it up. (It's somewhere in the links to this article.)
The salient thing seems more and more to be that if one could have knowledge of something then it will affect the experiment, and if possibility of gaining knowledge of that thing is destroyed then the interference patterns come back. It sounds like some variant of solipsism, but there are ideas of multiple universes that don't make me any happier. (Conservation of matter and energy, well yes, but another entire universe appears every time a geiger counter clicks.)

If possible, I'd like someone qualified to complete this article and maybe give us plebeians further information on the generally accepted meaning of these results. Thanks! :D

The vanilla cases are pretty clear. To get interference patterns you have to put a divider of some kind in front of an emitter of light such as a laser and then you have to take whatever it is you have divided (and it is very much in question or even unknowable what you have actually divided) and let it come back together again in such a way that the two streams are not in phase. Then you will see an interference pattern. The only way you can have an interference pattern is is you have some kind of "peaks and troughs" deal so that the peaks can sometimes fill in the troughs, and sometimes peaks can stand on top of other peaks.
At least some of the ways to mess up the interference phenomenon are known. Anything that you put in one path or the other that interacts physically with what is passing by that point will let you say that "a photon was detected here" will destroy the interference pattern. In this case it is pretty clear what has happened because the vanilla case of a photon betraying its presence is that it is absorbed by an atom. It then is no more, but an electron in that atom rises to a higher level. Day-glo paint would be an example. Day-glo works because infra-red light is absorbed, and energy of a higher frequency is subsequently emitted. I am not sure, but it looks like what happens is that more than one infra-red photons are absorbed, boosting an electron up some for each infrared photon that is absorbed and then the electron falls down near where it started, but in one jump. Anyway, you could perhaps put a piece of doped glass in one or both slits and then observe the emission of the visible frequency light. But the photon that leaves that point was not the photon that arrived there. So it can't interfere with "its other half" because even if its other half went through a similar change it wasn't the same change.
As I understand it, the next attempt to mark photons somehow is to do something that will identify them without changing significant features of their identity that would prevent them from interfering with themselves. One idea is just to rotate photons with a polarizer, so you know if the photon went through slit A it was rotated and if it went through slit B it was not rotated. Then later on you could put in another polarizer and re-rotate the ones that were earlier changed. To me this seems rather lame because the next thing you are going to have to do is to merge the restored paths, and at that point there will be no "knowledge" of which one is which.
It is easy to reduce this picture to very simple terms, which makes it seem trivial.
If you make a wall that extends the area between the two slits so that the two resulting beams can never get together, then you can associate a photon detected on the screen with the slit on that side of the wall. You will know (or know with very high probabilistic certainty) that it didn't go through the other slit and then tunnel through the median wall. But you will not get interference. Tear down the wall and you will get interference, but you will be dealing with a mixed path situation again.P0M (talk) 05:01, 9 December 2007 (UTC)

EDIT: in particular the article does not mention if the measuring equipment can actually alter the movement of the photon, which would explain why the experiment results are different.

If you "measure" something you have to interact with it, and if you interact with it you change it. It's not clear that you alter the "photon's" "movement" because between the time you flick the switch on the laser and the time you see a flicker of light on the detection screen you have only inference and guesswork about what is going on. You know there is a time factor involved, you know there is a frequency factor involved, and you know that what happens depends on the widths of the slits and on their separation. But those are the only things you can actually observe. The fact is that lots of things that can be done to measure photons actually end up destroying photons. It may be that some things can be done that change part of the psi function of a photon. Maybe an example of that would be polarizing the photon. The hope of experimenters appears to be that they can identify, somehow, which slit a photon has gone through and do so without destroying its ability to interfere "with itself."
I think I agree with Feynman that it is presumptuous to say that there has to be "a particle" that either "goes through the left slit" or "goes through the right slit." That's the way baseballs and bullets work in the macro world, and we are more comfortable with thinking in such familiar terms. On the other hand I would be uncomfortable about thinking about a baseball that went through one window in my living room and that had to be accompanied by a "ghost" baseball that went through the other window. But at this point all I have is a point of view. P0M (talk) 05:01, 9 December 2007 (UTC)

xxxx

"Everything propogates like a wave, and exhanges energy like a particle." Tipler, ISBN: 0-87901-432-6, but pretty much anywhere you look. So when you obseve the position of the electron, you collapse the wavefunction describing the probability of it's position; i.e., because you have measured it to be in one physical location, the probability of it being anywhere else becomes zero.

Personally I agree with this point of view. But encyclopedia articles are not supposed to take sides. P0M (talk) 05:01, 9 December 2007 (UTC)

To detect something you must interact with it, and to interact you must exchange energy; therefore you collapse the wavefunction, the position of the electron is determined, and the interference pattern is removed.

Again, I agree, but it appears that physicists are trying to find ways of "marking" particles without changing irrevocably the aspects of them that would permit them to interfere. P0M (talk) 05:01, 9 December 2007 (UTC)

Now, the above is a little controversial. Mainly because I've described the collapse as a real change in the system. The majority would agree, but not all, and there is a discussion around 'real'. Secondly, some believe that it is observation by conscious beings that is important rather than interaction. This is a view that is in NS this month, as is also in the article in this commment:

"NS"? P0M (talk) 05:01, 9 December 2007 (UTC)

"Even less in line with the expectations of human scale interactions with nature, if the information about which slit a given particle came through is "erased" before a photon has time to interact with the detector screen interference will be restored."

The change to include this is recent, as far as I can tell. If this is true, then at the very least it needs references. This is what has prompted me to make this, my first contribution. Apologies for any Wikipetiquette broken. Davini994 21:12, 30 November 2007 (UTC)

No problem with etiquette at all. Good comments. I've been wondering how to work in the newer stuff about erasure. If you will take a look at the Wheeler's delayed choice experiment you will see an article that was apparently conceived and then abandoned by someone. I've been trying to get it straightened out, but I haven't had time to get Wheeler's book yet. What I found worthy of note about that article was how the mere assertion that somehow looking at two slits of the double-slit apparatus with "telescopes" directed at the slits could prevent the formation of an interference pattern could apparently be totally convincing to some people -- all without a shred of experimental evidence and obviously without carefully following through the individual steps of the thought experiment. Wheeler was an eminent physicist. I doubt that he could be a hazy thinker, but some of the assertions originally in the article were without any basis I could find. What he did say, from what I can make out from secondary sources, was quite deep and has to do with issues of the fundamental structure in space and time of the universe. Anyway, my point is that when writing an article on a question of fundamental importance like this one we need to be sure to hold ourselves to what the fathers of the field said and trace out the development of these ideas. Everything, ideally, should have a good clear citation to back it up. P0M (talk) 05:01, 9 December 2007 (UTC)

People, this article is on edge of dumbest false speculations when it comes to "observer role" or "quantum erasing". Why not ask Markus Arndt or Anton Zeilinger from Univercity of Vienna? They both are on the bleeding edge of quantum wave experiments, and their home pages and emails are well known by Google.

Please sign your postings.
As long as the article does not go over any "edges" but instead explores those edges, it will probably be o.k. I personally agree that Heisenberg was right in cautioning about any "metaphysical observer with sentient intelligence, blah, blah, blah." I don't think, however, that the article does anything more than report on how various people who are important in the field have expressed themselves.
I am in doubt as to what "quantum erasing" would actually prove, anyway. The salient feature of these experiments, which everyone seems to risk overlooking by worrying about "which slit the photon really went through," is that something is important about the second unobstructed slit. I personally am deeply agnostic about whether a "particle" goes through either slit. The operational definition of a particle has to do with sticking some kind of detector up and seeing whether it takes a hit or not. Absent that, there is no particle. But people with any sense will prefer to know what Feynman, Heisenberg, et al., have to say about it.
That being said, I still have to do a lot of study to get up to speed on the "erasures" business. Some of them involve entangled photons, measuring one entangled photon and drawing conclusions on that basis about the other one, etc. It will be difficult to think carefully through all of these things, and I think the reader will be better served if we put the history of conclusions about the double-slit experiment up in this article and let links to other articles lead readers to the newer and perhaps more unsettled stuff. P0M (talk) 05:01, 9 December 2007 (UTC)
I just had a look at the article on quantum erasure, which is pretty poor. I also had a look at the external link, which was to a school project somewhere. That site had a quite clear description of the physical set-up. If you don't start out by believing in the power of the human will to determine the universe, the importance of having a sentient observer to make quantum probabilities come to rest in the real world (which conscious thought creates I guess), then it does not look so wonderful at all. It's rather like a magic trick.
By using entangled photons, it is possible to change the polarization of one stream of photons by polarizing the other stream. That's disturbing because of non-locality. But the rest of the setup in almost transparent. The second stream of photons is divided and sent through two polarizing crystals. After that operation is performed, the two streams will not interfere because they are crosswise to each other. But now a second polarizing crystal is put into the apparatus. It is placed so as to make half of the incoming photons on the other path vibrate in the x direction ("horizontally") and half vibrate in the y direction ("vertically"). But doing that has the same effect on the entangled photons in the first path, so now rather than going either vertically or horizontally into the polarizers that turn vertical vibrations into one kind of circular polarization and horizontal vibrations into the other kind of circular polarization, the second stream goes in being half vertical and half horizontal, so the polarizing crystals on that path now each produce an equal number of circular "S" mode and circular "Z" mode polarized photons, so there are enough circular "S" mode coming through slit A to meet their "other halves" coming as circular "S" mode through slit B, and they interfere nicely. And there are enough circular "Z" mode coming through slit A to meet the appropriate circular "Z" mode ones coming through slit B, and they interfere nicely too.
The essential question in regard to interference and non-interference is whether photons (or whatever we really should call whatever "ghostly" thing it is that goes between both slits or through all possible paths (Feynman) are afterwards permitted to coincide where they can betray the fact that they are out of phase or whether they are not permitted to get together with themselves and are fenced apart one way or another.
It's probably actually more interesting that the "segments" of the original disturbance that is propagated through the apparatus seem to stick together. One of the major early figures in the study of this problem said that photons only interfere with themselves. (You get big problems, contradictions, if you assume they can interfere with others.) So the two "segments" found after going through two slits and encountering polarizers will coordinate their polarizations. That has to happen, else there could not be a good balance at the end of the erasure experiment when interference is reestablished. In the Wheeler experiment (the single and dual half-wave devices), if it were not this way then the two segments of one original disturbance propagating through the apparatus could end up each on a different one of the two detection screens at the end of the apparatus. P0M (talk) 08:17, 9 December 2007 (UTC)