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Schrodinger's Twin Slit Experiment

Making quantum mechanics fess up.
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This isn't really an invention - it's a proposal for an experiment. Skip to the sixth paragraph if you already know about the two-slit experiment. The classic two- slit experiment is a manifestation of the weirdness of quantum mechanics. You start with a light source shining on a detector (for instance, a piece of photographic film or a sensitive camera). Between the light and the detector, you place a barrier which has two thin, vertical slits in it to let light through.

If light behaved like a particle, you'd just see two bright strips on the detector, as the light shone through the two slits. However, because photons have wavelike properties, the light coming through the left slit interferes with that coming through the right slit. It's a bit like two sets of ripples on a pond interfering with eachother: in some places, the light waves coming through the two slits are 'in phase' and add up; in other places, they are 'out of phase' and cancel. The result is that, on the detector, you get a series of many light and dark stripes where the light adds or cancels.

So far so good. Now you turn down the intensity of the light, right down, to the point where it's only giving out one photon every second. Now, of course, each photon has to go through either the left or the right slit, and there is no other photon for it to interfere with - they're going through alone - so there can't be an interference pattern. But the spooky thing is that there is *still* an interference pattern. It's as if each photon, going through alone, splits into two and goes through both slits, interfering with itself.

OK. So now you add something. It's possible to build a detector which can 'see' a photon going past it. So, you put one of these detectors next to each slit, to see which slit each photon goes through. When you run the experiment (still with only one photon at a time), you find that each photon goes through *either* the left slit *or* the right slit - never through both. And now, spookily, there's no interference pattern.

In other words, when you *don't* follow each photon, it splits in two and goes through both slits at once and interferes with itself. When you *do* follow each photon, you see it going through one slit or the other, and there's no interference. This experiment has been done, and all of this really happens.

OK, so here's the experiment I want to do. We build our two "photon watchers" to detect the passing photons, but we make them differently. Each detector records, in itself, when a photon goes past, and it stores this information internally. At the end of the experiment, we can either download this information and get a record of which way each photon went, *or* we can press a button which erases the information, so we don't know which way each photon went.

If we download the information, then we have 'watched' each photon to see where it went, and we should therefore abolish the interference pattern. But if we press the 'erase' button, we haven't 'watched' each photon, and therefore we should get an interference pattern. But, we don't decide whether to download or erase until *after* we've looked to see whether there is, in fact, an interference pattern.

This system should, therefore, be able to predict whether we're going to download or erase the data. If we see an interference pattern, we know that we are going to press the 'erase' button. If we see no interference pattern, we know that we are going to download the data about which way the photons went (which, of course, means that we've observed them and therefore abolished the interference pattern).

But then, suppose we automated the system so that, if we get an interference pattern, we automatically download the data; and if we see no interference pattern, we erase the data.

MaxwellBuchanan, Nov 28 2007

(?) Bacon http://www.constitu...r/baconsemiosis.htm
[pertinax, Nov 28 2007]

Can't resist... must...self promote... Schr_f6dinger_27s_20Dyson_20Shell
For [Maxwell], not for me, honest. [theleopard, Nov 29 2007]

A REAL causality experiment being done http://cosmiclog.ms...7/07/17/274531.aspx
QM is spookier than most folks imagine, heh. [Vernon, Nov 30 2007]

Lotsa stuff about virtual particles http://www.nemitz.net/vernon/GHOSTLY.pdf
Much more detail about something mentioned in an annotation here. [Vernon, Nov 30 2007]

Another experiment, already done http://www.analogsf.../0412/altview.shtml
This one boggles, too. [Vernon, Nov 30 2007]

The Paper describing the preceding experiment http://www.irims.or...mentarity%20All.PDF
Includes diagrams toward the end of the document. [Vernon, Nov 30 2007]

Wikipedia describing the same experiment http://en.wikipedia...i/Afshar_experiment
The explanation is less technical and the diagrams are in color. [Vernon, Nov 30 2007]

What I was thinking of... http://arxiv.org/PS.../0610/0610241v1.pdf
commonly referred to as a delayed choice quantum eraser, first proposed by Wheeler. [4whom, Dec 01 2007]

the wiki on delayed choice quantum erasers http://en.wikipedia...ice_quantum_eraser.
includes some errors [4whom, Dec 01 2007]

Airy Optical Beams http://www.physorg.com/news115556629.html
Photon, meet hole (CCD version of Whaka-mole) [reensure, Dec 03 2007]

Many-worlds interpretation http://en.wikipedia.org/wiki/Many_worlds
This cab driver believes the "wavefunction collapse" is a bit irrelevant. [ed, Dec 03 2007]

Soliton waves http://en.wikipedia.org/wiki/Soliton
Persitant propagating wavefronts. [8th of 7, Dec 03 2007]

Ten Commandments http://en.wikipedia...ki/Ten_Commandments
No other gods/do not worship idols. [8th of 7, Dec 04 2007]

Schrodinger's Comic http://xkcd.com/45/
The observer decides the outcome. [Vernon, Dec 06 2007]

[link]






       The mere capability of recording data will erase the superposition. Sorry. If you don't believe me, read "Entanglement" by Amir Aczel for a good source on this peculiar facet of quantum mechanics.   

       And, why is it Schrodinger's? Young was the inventor of the experiment. At least attribute the new experiment to the founder of the download: Al Gore, of course.
daseva, Nov 28 2007
  

       Thank you, [MB], for providing my daily fix of awe and wonder.   

       It's interesting (to me) that your subtitle echoes Francis Bacon's metaphor of experimentation as a form of torture, with the aim of extracting confessions from nature.   

       [daseva], if I've understood you correctly, the interference pattern is prevented at the point when one or other of the built 'photon watchers' spots a photon, whether or not the information is forwarded to a human observer; so, you are predicting that this experiment will never yield an interference pattern. Is that right?   

       <somewhat out of my depth>
[MB], if that is right, does it mean that you were supposing that the role of 'observation' in quantum mechanics necessarily implied human (or other 'conscious' or 'intelligent') observation, whereas in fact (IIRC), 'to be observed' in quantum terms is simply 'to have a measurable effect on something' (which itself might or might not subsequently be observed by a human)?
</soomd>
  

       <slightly trollish whimsy>
If all of the above is correct, we may return to the 'confession' theme by affirming that the extraordinary rendition of the photon into the non-human jurisdiction of the photon-watching devices (while we look the other way) yields no actionable intelligence beyond what the photon would confess under our immediate oversight. This proves nothing, but struck me as darkly humorous.
</stw>
pertinax, Nov 28 2007
  

       I'm a little curious how you detect the passage of the photon without affecting it's passage.   

       I'm sure Heisenburg would have had something to say about that. - So how is it done?
Custardguts, Nov 28 2007
  

       //The mere capability of recording data will erase the superposition.// I'm not sure this is the case - at least it's something that's heavily disputed. The argument is the same as for Schrodinger's cat (hence the title). In that case, the quantum event is the decay of the atom, and it determines the life or death of the cat; but the cat is meant to be in a superposition of states until it is observed by opening the box. More generally, there is the question of exactly what is needed to collapse a quantum state. Clearly, it is not "any" type if interaction; yet observation by a human is sufficient to collapse it. There is no reason to assume that a detector cannot be built which becomes entangled with the passing photons but then erases that information without any external manifestation. Such a detector would not collapse the state; the basis of quantum computing is that quantum- scale interactions don't collapse things until they are coupled to the macro world.   

       //And, why is it Schrodinger's// Because the philosophy is the same as his cat. You could, for example, use a series of cat-boxes to record the passage of each photon, and then decide whether to open the cat boxes and look inside.
MaxwellBuchanan, Nov 28 2007
  

       //I'm sure Heisenburg would have had something to say about that//
But how can you be sure?
AbsintheWithoutLeave, Nov 28 2007
  

       //does it mean that you were supposing that the role of 'observation' in quantum mechanics necessarily implied human (or other 'conscious' or 'intelligent') observation,// That is one of the fundamental paradoxes in QM - observation collapses the state, at least from the observer's perspective. The 'cat' experiment is designed to highlight this paradox - can the cat not collapse the state itself? What happens if the whole experiment is done in a closed room - does the state of the cat remain ambiguous until the experimenter leaves the room and reports the results to someone else?   

       The experiment I designed is intended to force this paradox to show itself - clearly there has to be a contradiction at some point, but it's not clear where the paradox lies, in the path from quantum event to human observation of a diffraction pattern.
MaxwellBuchanan, Nov 28 2007
  

       //I'm a little curious how you detect the passage of the photon without affecting it's passage// Me too, but I'm assured it is possible, and has been done. Moreover, if your detectors are not very efficient, then you see a weak diffraction pattern - midway between the 'observed' and 'unobserved' states.
MaxwellBuchanan, Nov 28 2007
  

       Very clever, [MB] - bun for sheer creativity. I'm sure [daseva]'s first comment is right, but I'd like to try this just to make sure. After all, that's the point of experiment.
wagster, Nov 28 2007
  

       It's not self-evident that recording data should collapse the state. In many situations, you have a series of quantum events interacting, and it's only when (and if) you read out the final outcome that the intermediate states collapse. Likewise, it should be possible to have a photon-passage- detector which puts an internal device into a state which can either be read out (collapsing everything), or annhialated. For instance, you could encode the passage of a photon in the spin-state of two nuclei - up/down or down/up; then you either read the spin states and extrac the data, or let the two nuclei interact in such a way that you can no longer tell which is which, in which case the data has been lost with no possibility of recovery.
MaxwellBuchanan, Nov 28 2007
  

       That's fascinating. I'm sceptical, but can't deny your logic. Bun, for making me think (not that that's a hard thing to do, but I enjoy the opportunity!).
david_scothern, Nov 28 2007
  

       If I believe you can prove that you have not looked at your idea again [MB], I will give you this croissant, so have I given it or not?
xenzag, Nov 28 2007
  

       As you'll see at the top of the page, [xenzag], I have been given both a bun and a non-bun. It would be foolish to suppose that these result from two different bakers - the only sane interpretation is that they arise from a single baker in a superposition of states.
MaxwellBuchanan, Nov 28 2007
  

       [Pertinax] - sorry, I missed your annotation. In regard to // <somewhat out of my depth>...// the whole question of what constitutes "an observer" capable of collapsing the states is a very spooky one. Certainly, with no "observer", there really is a superposition of states. At the other extreme, with a human "observer" the states collapse. Schrodinger's cat is an oddity - it surely should qualify as an 'observer' of its own life, but from the perspective of someone outside the box, it doesn't. The idea of this experiment was really to force quantum mechanics into a corner; clearly, QM should not be able to "predict" whether we are going to download or erase the data, but pinpointing the flaw in the argument would be informative.   

       I suspect (in a very fuzzy way) that the answer to all this lies in a sort of "quantum relativity", whereby a quantum state can collapse from the perspective of one observer, yet remain uncollapsed from the perspective of another. But this is almost certainly bollocks.
MaxwellBuchanan, Nov 28 2007
  

       I think this was previously alluded to in a dyson shell implementation, I shall leave the author to link to it.
4whom, Nov 29 2007
  

       More people failing to be unconvinced by David Hume *yawn*...
Ned_Ludd, Nov 29 2007
  

       //alluded to in a dyson shell implementation// Can you expand a little, [4whom]?
MaxwellBuchanan, Nov 29 2007
  

       //We build our two "photon watchers" to detect the passing photons// - as [CustardGuts] says, this is why your experiment is doomed to fail. You can't detect the presence or absence of a photon without affecting the photon itself. It will *have* to give up energy to your detector.
hippo, Nov 29 2007
  

       This idea contains exctly the right amount of science. Enough to make me feel clever but not enough to make me realise I'm not.
wagster, Nov 29 2007
  

       //You can't detect the presence or absence of a photon without affecting the photon itself.// Yes, I too am unconvinced by this point. And by inferrence, the whole "whoooo - wave/particle duality" mysticism that appears to surround it. It could easily be that I'm missing something, but if so, I'd love to know what it is.
zen_tom, Nov 29 2007
  

       ([zen] - what? - you think you *can* detect the photon without affecting it, or you agree you can't?)
hippo, Nov 29 2007
  

       The author has linked, MB.
4whom, Nov 29 2007
  

       //this is why your experiment is doomed to fail.// My understanding is that that's not the case, counterintuitve though it may seem. For one thing, it is possible to perform "interaction free measurement" - people have produced images despite the relevant photons not having interacted at all with the imaged object - but having had the potential to interact. All very spooky. The detection of the passing photon is a sort of inverse of this.   

       In any event, suppose that the photon *does* have to give up some energy - it's still not a problem. Imagine we have material which absorbs a photon at wavelength A, and emits a photon at a longer wavelength, B. In doing so, an atom of the material is kicked into an excited state. This is just fluorescence.   

       Now imagine that each of our slits has a window of this fluorescent material. You'll still get interference: it'll be interference between the longer- wavelength emitted photons rather than the original incoming ones. So now, each "window" has absorbed a tiny amount of energy from the passing photon. We can then choose whether or not to detect this absorbed energy and, by inference, the passage of the photon.
MaxwellBuchanan, Nov 29 2007
  

       //we have material which absorbs a photon at wavelength A, and emits a photon at a longer wavelength// - but how do you know it's emitted a photon? The answer is, you don't - that's the point.
hippo, Nov 29 2007
  

       [hippo] //eh - what? etc // I agree that it can't. However, I do remember specifically the bit of the experiment where someone starts 'measuring' which slit the individual photons go through, and the subsequent collapse into particulate behaviour (over time). And have always wondered about this.   

       What I've never understood is how this measurement gets to take place - perhaps it's by means of this fluorescence thing [MB] is talking about - but that doesn't quite add up to the same thing - not sure exactly why - it just doesn't feel as though it does. Maybe Heisenberg and all that.   

       It's not that 'observation' is some magical act that collapses wave functions, or kills/unkills cats, it's that at such tiny scales, the only way of observing what's going on is akin to a blind man observing where other road users are by colliding with them at full speed, in a tank. The observation (read fatal traffic accident) will tend to significantly effects the results. It's not mysterious, it's just that small delicate things get so small and delicate that we just can't tell they are there without mashing them all up.   

       So fluorescence as a test, might be a bit like having a bus so packed full of people that, when Godzilla throws a hapless human in at one end, another pops out the other side, screaming and running directly for shelter, in a particulate manner.   

       //We can then choose whether or not to detect this absorbed energy and, by inference, the passage of the photon.// I suppose my question here is - is it the windows that causes the change (I imagine it will be) or is it the act of looking at the windows? If two scientists at opposite ends of the room look at two different parts of the experiment; Scientist A is looking at the projected interference patterns/line pairs, and Scientist B is looking at the fluorescent windows. Scientist B is distracted for a moment by an abandoned sandwich - does this mean that the twin lines on the wall suddenly revert to interference patterns while Scientist B is temporarily indisposed? I think not.
zen_tom, Nov 29 2007
  

       //but how do you know it's emitted a photon// because the emitted wavelength is longer than the absorbed wavelength, which means that the material has absorbed some energy, which must in theory be detectable.   

       Failing that, there are "wavelength doublers" which absorb one photon and emit two of longer wavelength. There's nothing to stop you detecting one of these two photons whilst its twin goes on to diffract (or not).   

       It's not true to say that *any* interaction collapses the waveform. That's why quantum computing works - you can couple quantum events without collapsing them. The question is this: in between quantum-scale interactions which don't collapse the waveform, and macro-scale interactions which do, at what point does collapse happen?
MaxwellBuchanan, Nov 29 2007
  

       I will try and find the link to a paper that tries to resolve this. It is quite an elegant "solution". If I have not done so by tommorrow 19h00 GMT, give me a reminder, I will have more time on the weekend.
4whom, Nov 29 2007
  

       Thanks, [4whom]. [zen tom] can you remind me to remind [4whom]?
MaxwellBuchanan, Nov 29 2007
  

       MB, Tom as a calendar? Try Tim. He is less difficult to spot.
4whom, Nov 29 2007
  

       Good point. Can we get Tim to remind Zen Tom to remind me to remind you?
MaxwellBuchanan, Nov 29 2007
  

       // treat light as an EM field which can be absorbed in quanta// Yes, that's precisely the point (or a point, anyway). Light has both field properties and quantized (i.e., particulate) properties, as indeed do electrons and elk.   

       However, that's not the real spookiness. The real spookiness comes back to this question of what consititutes "observation" sufficient to collapse the behaviour of the system. Simple quantum interactions *don't* (hence the possibility of quantum computing); measurement and human observation *does* (hence the two-slit result). But any observer and any measuring device is just a whole pile of quantum interactions anyway. So where is the threshold between a non-collapsing quantum interaction and a collapsing one? (I am using the verb 'collapse' transitively here.)
MaxwellBuchanan, Nov 29 2007
  

       "// treat light as an EM field which can be absorbed in quanta//" This is the exact start of the precipitation of quantum theory, a description proposed by Einstein to explain the photo-electric effect. It is damn near verbatim. If it was intentional, I appologise, but me thinks not.
4whom, Nov 29 2007
  

       // other than the probability of extracting a quantum from the field// Exactly. But the bottom line is that the system behaves differently - in a tangible physical sense - when it's being watched.
MaxwellBuchanan, Nov 29 2007
  

       I hate coming to someone's defense but I will say this:   

       Is it impossible for a broken glass, that has fallen from a table, to reconstitute itself? Nay! And this is correct. But only because the *probabilty* that it will not, completely outweighs the *probability* that it would.
We are not afforded this luxury in the quantum world. All probabilities exist equally (in the quantum environs). It is a consequence of the speed of light and its time dilation effects.
The critique of "What constitutes observation?" remains valid (IMHO). Unfortunately, previously covered in theleopard's previous iteration.
4whom, Nov 29 2007
  

       //Rubbish. You're not psychotic at the moment are you ? // Not as far as I know.   

       Why do you ask?
MaxwellBuchanan, Nov 29 2007
  

       bigsleep, far be it for me to correct your view, and/or misinterpret another's post. But having said that, I think you underestimate our spotted friend.   

       The question of observation is an open one. I think theleopard had a good shot at it, (at least the best I have seen that exists outside the realms of possibilty). MB has taken another good crack at it (within the realms of possibilty).
  

       I challenge you to read *and* understand theleopard's last anno on his idea.
Post Script: Hey, at least I remind you of someone!
4whom, Nov 29 2007
  

       Fresh observational eyes are needed . Temporarily forget particles , temporarily forget waves , think total anew . A new 'wrong' approach might just uncover the insight needed .
wjt, Nov 29 2007
  

       Knowing how arm moves through its degrees of motion (behaviour) and the neuron muscle firing chemistry (structure action) are two different things .
wjt, Nov 30 2007
  

       <figuratively a taxi driver>   

       [bigsleep] (or another well-informed person), would you be willing to offer an answer to the question in my first anno which I originally addressed to [daseva], but which [daseva] decided (perhaps rightly) was beneath him?   

       'Ere, I 'ad that Professor Schroedinger in the back of the cab once... or did I?   

       </fatd>
pertinax, Nov 30 2007
  

       No, sorry. You are correct. I stand that there will exist no interference pattern. The only way to have the interference is not to watch it. You can't watch it, and then decide not to. You have watched it and you know which slot it went through. Erasing the data before checking it out is not going to change the fact that information was collected. You gotta burn that data, spread the entropy, and now you data is an element of reality wether it wants to be or not.   

       Also, this idea is somewhat magical in the fact that if led to further conclusions, one might be able to just backup the data before erasing it. Now, you see an interference pattern, but you're damn sure they were single particles going through one or the other hole like bullets: you recorded it!   

       So, you're going to force measured particles to behave like waves. Now, that would be cool. This is essentially forcing real particles to behave as if they were superimposed waves. Get in line, little particles! you are all going to pretend to interfere and not go in a probabilistic straight line but move over here and there in a cool pattern!!!   

       I doubt their going to listen ;)
daseva, Nov 30 2007
  

       [MaxwellBuchanan], are you familiar with the "virtual particles" that fill the vacuum between ordinary particles? Ordinary and virtual particles are always interacting with each other, and much of QM can be explained as a consequence of that. For example, an ordinary particle might temporarily transfer some of its momentum to a virtual particle, and get it back less than a nanosecond later. This can explain the two-slit experiment completely. The ordinary particle (ANY type) goes through one slit, and the virtual particle goes through the other slit. The interference pattern is the natural result of the momentum getting back into the ordinary particle afterward, since the MOMENTUM went through the two slits simultaneously, and not either particle.   

       If you perform some interaction with the ordinary particle, to detect which slit it went through, you ALWAYS change its total momentum, and that's why the interference pattern disappears.   

       Now I do understand that you would like to do some kind of interaction that does not mess up the momentum of the particle in question. This could actually be done, if you could control virtual particles. The trouble is, you are not allowed to DETECT virtual particles, so even if you did use them to detect the which slit the ordinary particle went through, you couldn't get the information/result from the virtual particles that did the interacting!   

       I've added some links. There is an experiment in which the photon's path is determined AFTER the interference pattern has been created.
Vernon, Nov 30 2007
  

       Does this count as interaction-free observation? Start with the two slit experiment. There will be places in the interference pattern that get no photons due to the interference. Let's call one such place Fred. Now cover one slit with a photon detector. Fire a photon. Perhaps it hits the detector. Obviously in this case there is interaction. On the other hand, it may not hit the detector. In this case it may go through the other slit and hit Fred. If so, there was no interaction with the detector and yet the observation (that the photon didn't hit the detector) destroyed the interference pattern so the photon could land on Fred, which would otherwise be impossible. Just the potential for interaction changed something, even when the potential interaction turned out not to happen.   

       [edit] I'll note that you could just as easily block the slit without using a detector, and it would still let photons hit Fred
caspian, Nov 30 2007
  

       Oh my. I just had a memory of the last scene of back to the future and I know exactly what's going to happen if you try this. It will look like an interference pattern, and then you'll download the data and it will start to change into a normal pattern and then Marty will start playing real bad on the guitar and someone will get punched in the face and falls on the 'erase' button! And the interference comes back and Marty freaks out on a killer solo but nobody really gets into it.
daseva, Nov 30 2007
  

       If I may throw my two small monetary units into the mishmash of discussion...   

       I deny that it is possible to make a device which can detect the passage of a photon without affecting it. The double-slit experiment is itself proof of this--if you put a detector at the slits themselves to watch the passing photons, interference no longer works, therefore the photons are being affected by the detectors. it doesn't matter if we actually see the output of those detectors, their presence is enough to throw things off. It's not a question of observation, but an interaction which forces the presence (or non-presence)of a single photon--there's nothing magical about the fact that we actually see what happened.   

       [MaxwellBuchanan]. You assert that interaction-free observation is possible, but don't provide any evidence beyond your own assertion and the claim that someone told you so. (links?) Until you do, I'm going to assume that what I know is correct. As for [Vernon] and the Afshar experiment, it's very intriguing but still being very hotly debated as to whether Afshar's interpretation is true or not.   

       Anyway, The common misconception with the two-slit experiment is that this distinction of interference/non interferences is made at the point where the photons are collected. This is false. The distinction is made at the slits--whether you have one or two slits open to (unimpeded) progress of the photons.   

       If one slit is open, you get no interference pattern. If two slits are open, you get an interference pattern. The weirdness is that if you fire photons at the two slit version one at a time, you still get an interference pattern. The photons arrive at the far side one at a time, in individually distinct events, but they arrive in locations that correspond to an interference pattern. If you fire one photon at a time several hundred times, you'll see the pattern emerging. Somehow, the photon is producing an interference pattern by itself, which most people rationalize by saying it's acting like a wave. But the fact of each arriving one a time belies this--something else (quantum mechanics!) is going on.   

       The way this applies to this experiment: If the detectors were physically possible (doubtful) you'd see an interference pattern every time, because both slits are open. End of story. The business with the detectors is irrelevant, because the relevant portion of the experiment is the slits, not the detectors. What I would like to know is what the detectors would show.   

       Anyway, everyone should just read Feynman's Q.E.D. (Quantum electrodynamics) It's actually pretty easy to understand, and explains the entirety of light without actually worrying about the wave or particle nature of light at all. The whole argument is just a case of people using two equally bad metaphors for the way light (and other quantum objects) works.   

       [edit: I've changed my mind. No interference pattern will appear. scroll down for explanation.]
5th Earth, Nov 30 2007
  

       Wasn't I supposed to remind someone of something?
"Facking par'icol fysiks! Dannah they're fackin' born. Iv ahrv said it once ahrv said i' a faaasand times, facking virtuul par'icols innit. Ferfacksake. Do you want to go by London Bridge, or Tower? Ken Livingstone? Don't facking tawk to me abaht Ken Fackin' Livingstone!"
zen_tom, Nov 30 2007
  

       [Bigsleep] I'm not really following your argument here, but never mind. You seem to think I'm winding you up. All I'm doing is presenting a well-known phenomenon and question in QM, and suggesting an experiment which would lay the question bare. Sorry if that causes you a problem.   

       [5th earth] Re Interaction Free Measurement - just Wikipedia for this term, and you'll find plenty of references therefore. We also had a seminar here (at the cab-driving laboratory) a while ago where the guy showed images obtained in this counterintuitive way. I was taking this as a given rather than as a contentious point.   

       [others] I'll catch up with later. Got a fare, have to dash, you know how it is.
MaxwellBuchanan, Nov 30 2007
  

       MB - I'm head-hunting you as a driver for my own Taxipedia Cab company.
xenzag, Nov 30 2007
  

       [zen_tom], sorry, I was supposed to remind you.
theleopard, Nov 30 2007
  

       OK, let me know when you do.   

       All this talk reminds me of a book I recently read on holiday - most reccomended; Will Self's "The Book of Dave" - set in a post-global warming future where sea levels have risen over most of the UK, and a whole culture and religion has formed based on the discovered writings of a London Cab driver called Dave.
zen_tom, Nov 30 2007
  

       //I challenge you to read *and* understand theleopard's last anno on his idea.//   

       Indeed, bit of a conversation stopper that one.
theleopard, Nov 30 2007
  

       The one about curiosity killing the universe? Seems reasonable to me.
zen_tom, Nov 30 2007
  

       //If the detectors were physically possible (doubtful) you'd see an interference pattern every time, because both slits are open. End of story.// Yes, but that's not the case. The detectors are physically possible, and you don't see interference. This is pointed out in Feynman's QED, and also in at least one recorded lecture by him (and others). He also explains how, if the detectors are "inefficient", you get an intermediate case - a weak diffraction pattern.   

       Incidentally, my understanding is that the same experiment can be done with electrons, which also diffract, and for which the experimental setup is a bit simpler.   

       I thought we were taking all this as the starting point?   

       The key question here, to cut to the chase, is this. As long as everything is kept 'quantum', the system exists in a superposition of states. Interaction with the 'macro' world is supposed to collapse it. So, at what point does the transition occur from 'quantum' interaction to 'macro' interaction? Is this point the same from all perspectives? This is the nub of why there has been no satisfactory interpretation of QM, and why there may never be one. I'm just trying to devise an experiment that makes this weirdness even more explicit.
MaxwellBuchanan, Nov 30 2007
  

       I'm still wondering why all these photons are all collapsing into a state. Neurotic photons? I suppose that explains why they don't know whether to wave or not.
Ling, Nov 30 2007
  

       Perhaps light rather 'prefers' to be quantized but can be coaxed to separate under the right circumstances...
RayfordSteele, Nov 30 2007
  

       Well my theory, seeing as we seem to be doing theories for the moment, is that photons actually move in the opposite direction, temporally speaking, to us and therefore they know what you are going to do before you do it. Hence their ability to befuddle the experiment. Alarmingly, the corollary to this is that photons do not originate from a light source but from the observer and that they accumulate around what appears to us to be the lightsource afterwards, hence the reason that observation has an effect upon outcome. After that my rather dodgy theory becomes a bit sketchy as it's too complicated for my poor brain to work out the details.
DrBob, Nov 30 2007
  

       Your appellation of the term "dodgy" to this pot of crack is a grave insult to Dodge.
globaltourniquet, Dec 01 2007
  

       I've reviewed the Wikpedia articles I think we're just quibbling over definitions of "non-interaction". I've got a non-standard one, so I'll admit I'm sort of wrong. (well, more than sort of--I've reversed my opinion, as I will explain in excruciating detail)   

       All the experiments basically rely on setting up a system where one of two events can occur, and then observing only one of them. If it doesn't occur then the other must have happened. Oddly enough, it's this exact principle which, I think, shows this experiment won't work.   

       I may be wrong. Working this out was tricky.   

       With your hypothetical detectors, you are assuming, at the detectors, that the photon is acting as a particle. On the other hand, at the film at the back with an interference pattern, you are assuming it's a wave. So you have a two-outcome scenario: 1, the photon acts as a particle and is detected at one or the other of the slits, or 2, it is NOT detected, acts like a wave, and makes an interference pattern (IP).   

       If you observe an IP, then detection at the slits did not occur--you yourself state this in the description of the experiment. Conveniently, you don't need to actually look at the results of the slit detectors to know this--the presence of the IP proves they didn't work, they recorded nothing.   

       Conversely, if you do not see an IP, you know that the slit detectors worked, and they recorded something. Conveniently, you don't need to actually look at the numerical results (which slit) to know that they worked--the absence of an IP proves it.   

       So, (if I've worked this out right) the knowledge of which way the photon actually went is irrelevant to the presence or lack of an IP. If the slit detectors work, no IP will appear, regardless of whether or not we actually look at the data they record.   

       You'll note I've changed my opinion--you'll get no IP, no matter what. I think. Go find a quantum physicist, or get a government grant and actually do the thing.
5th Earth, Dec 01 2007
  

       "Each detector records, in itself, when a photon goes past"
...and therein lies the fundamental problem of quantum mechanics. You can't think of it the same way as you thing of things in a classical scale. The reason why the observer perturbs a quantum mechanical experiment is that the observer *must* participate in order to observe. You *cannot* detect a photon going past you. You detect a photon by stopping it dead and looking at where it hit.
  

       Think of it like this:
Classical mechanics: I open my front door and look down the street. I see a red car. It's red. I can blink and it's still there. It's still red. I shut my eyes, turn around and it's still red. I ask hippo over to look at the car. He agrees that it's red.
Quantum mechanics: I blindfold myself and walk out of my front door until I trip over. I assume what I have tripped over is a car. I chip a bit of paint off the corner at which point the car spontaneously combusts and crumbles to a pile of ash. I return to my house and conclude that there probably had been a red car outside.
  

       The more you try to apply classical rules to the quantum world the deeper and more confusing these paradoxes will be. You won't crack them. You won't solve them. You'll just highlight and perpetuate the paradox that exists when you approach quantum mechanics with the laws of classical mechanics.   

       If you realise that the two instructions: "Tell me the colour of the car outside my house" and "drive it down the shops and get a pint of milk" are not both possible in the quantum world your head will be less likely to explode.
st3f, Dec 01 2007
  

       Thank for the heads up, the brothers Kimm.   

       "So, at what point does the transition occur from 'quantum' interaction to 'macro' interaction?" ~ MaxwellBuchanan,
Well why don't you take a look-see and find out. :-)
  

       My favourite quote, in relation to this: "Delayed-choice experiments of this type have actually been performed, and the interference pattern detected when D (the detector) is up is not affected by the delayed choice. Somehow the photon can retroactively arrange to go through one slit or two depending on which measurement is ultimately made."   

       You want the truth? You can't handle the truth! ~ Col. Jessop.
4whom, Dec 01 2007
  

       To make it a bit easier to understand why we don't/can't understand these phenomena, I propose the following story (originally to have been posted under the Herge's Twins Paradox (you know.... for massless twins)):   

       It starts with a paradox proposed by Einstein. The Twins paradox. Those unfamiliar with it can wiki it, if needs be. It deals with the age difference of twins, one travelling at rest, with respect to a certain frame of reference, and the other travelling at relativistic speeds, with respect to the same frame of reference. When they eventually meet again the "rest" twin is older than the other.   

       Now consider twin photons. One is fired distance d to a detector and the other (simultaneously (or not, as it turns out)) distance x.d to the same detector. The first casualty of this experiment is the frame of reference (which previously resided with the "rest" twin). So let us just sweep that under the carpet. Shirley, each photon will have its own frame of reference and we can compare those. Photon twin A arrives at the detector at time a and photon twin B arrives at the detector at time b=x.a (for us). We perceive no age difference, but what if we asked the twins their ages? Presumably twin photon B is slightly (or infinitely) older the photon twin A. However, at the speed of light , time has dilated to the extent that it becomes a meaningless entity mathematically, so a calculation as to their ages relative to one another becomes just as meaningless. To put it bluntly, although it may seem to you that your photons arrived one after the other, for them it was a peculiar coincidence that they both arrived at the same time.
4whom, Dec 01 2007
  

       //Somehow the photon can retroactively arrange to go through one slit or two depending on which measurement is ultimately made.// Where was that quote from, [4whom]? What was the context?
MaxwellBuchanan, Dec 01 2007
  

       If memory serves, it was by Cramer, can't remember the specific article. It stems from this earlier gem from John Wheeler:
"We have a strange inversion of the normal order of time. We, now, by moving the mirror in or out have an unavoidable effect on what we have a right to say about the already past history of that photon."
Wheeler made the statement without his experiment being ratified. Cramer made his statement post "ratification", as several experiments conducted showed this phenomenon of photons choosing their paths/natures post facto.
  

       There is an even bigger bug-bear to consider. In recent experiments (circa 2005-2006) certain arrangements have been detecting photons *before* they have been emmitted!
4whom, Dec 01 2007
  

       //certain arrangements have been detecting photons *before* they have been emmitted!// That sounds very much akin to the "interaction free measurement", in which a real image can be obtained as a result of the _possibility_ of photons interacting with an object, despite the fact that they don't actually do so.   

       It's all enough to make a poor psychotic trolling cab driver's head spin.
MaxwellBuchanan, Dec 01 2007
  

       We are deeply dissapointed that this idea, however fascinating, contains no reference to forcing cats through narrow slits, nor placing them in boxes with vials of poison and radioactive sources. Therefore, though clever, it is of little practical benefit.
8th of 7, Dec 02 2007
  

       [1.142857..] I have to disagree. If I may quote an esteemed cab driver's annotation, from the foregoing:   

       //You could, for example, use a series of cat-boxes to record the passage of each photon, and then decide whether to open the cat boxes and look inside.//
MaxwellBuchanan, Dec 02 2007
  

       The cat sat on the matter.
xenzag, Dec 02 2007
  

       By pure coincidence (or is it?), cats have vertical slits in their eyes...
Ling, Dec 03 2007
  

       // cats have vertical slits in their eyes..   

       "Igor, bring me the laser .... no, stupid, not the little hundred-watt one, the good one ...."
8th of 7, Dec 03 2007
  

       Ooh, hello stranger!
DrBob, Dec 03 2007
  

       <points at 8th of 7>
I know him!
Quickly, check Schrödinger's cat. There's a far greater than 50/50 chance that it's dead... or blasted into space on the back of a rocket...
<stops pointing>
  

       Er... Hi!
Jinbish, Dec 03 2007
  

       large cats have round pupils, Ling.   

       hi 8th!
po, Dec 03 2007
  

       I find it interesting that quantized third dimensional forces (read that "normal physical properties of the macro physical environment that can't be measured in subatomic milieu, therefore don't exist") defy probability and continue to manifest in the behavior of photons (link to "airy optical beams").   

       The authors depict special photon behavior, under conditions that simulate classic 'falling frame of reference' observations but rather than moving a reference point in tandem to a photon, the photon travels unguided in an arc and is registered across points in a CCD field. How is this possible? Does the photon's 'spin' travel at a slower speed than its propagation wave? Does the less-than-ideal milieu impose on each undispersed photon a horizon of sorts, creating spin (third dimensional vertex) from an otherwise pristine 2D wavefront? Is this all consistent with Heisenberg, in that CCD fields are sufficiently weak to allow quantum level flux at a detectable distance but not so weak as to cause no interference if detection happens?   

       Must have the seer read some cat entrails.
reensure, Dec 03 2007
  

       Sounds like a Soliton (Saltire) wave propagation effect to us .... <link>
8th of 7, Dec 03 2007
  

       dog chasing tail . An experiment of water by manipulating water with water in a box of water and trying isolate what the water is made of . outside leak in .
wjt, Dec 03 2007
  

       Yes.
MaxwellBuchanan, Dec 03 2007
  

       The Tao of Physics: An Exploration of the Parallels Between Modern Physics and Eastern Mysticism (1975) by Fritjof Capra. Still a thoughtful gem.
xenzag, Dec 03 2007
  

       The problem is, when something is sufficiently vague, it has good analogies with everything. Tao schmao.
MaxwellBuchanan, Dec 03 2007
  

       //large cats have round pupils, Ling.//   

       But smaller cats are easier to get into boxes.
Ling, Dec 04 2007
  

       // But smaller cats are easier to get into boxes. //   

       Yea verily, for it is easier for a Camel to enter the Kingdom of heaven than it is to cram a large cat into a small box .....
8th of 7, Dec 04 2007
  

       One is always south of the river. It is only a question of which river.
MaxwellBuchanan, Dec 04 2007
  

       [Bigsleep], I really don't follow this. If it's a running joke, I don't get it.
MaxwellBuchanan, Dec 04 2007
  

       Well, fair enough, and if everyone else agrees I'll delete this. The idea was for a photon detector which could store information until it was either downloaded or deleted, as a means of understanding what's happening in the twin-slit experiment. It's certainly halfbaked, and I'm not professionally qualified in quantum physics. I don't see the philosophy side myself, but there you go.   

       I still don't get the spoon reference - perhaps you could explain?
MaxwellBuchanan, Dec 04 2007
  

       I have a theory and you may laugh - anything that reduces that little twiddly thing that you use to scroll down an idea - see to your right; If it's really weeny and you have to squint to see it at the top/bottom then its an idea is worth keeping!
po, Dec 04 2007
  

       Well, thanks [po]. However, I suspect [bigsleep] is a physicist, and I've obviously pressed a wrong button without realizing it, so no worries. It can go.
MaxwellBuchanan, Dec 04 2007
  

       // I've obviously pressed a wrong button without realizing it //   

       You are George W. Bush and we claim our five dollars ...
8th of 7, Dec 04 2007
  

       Ah well, there you go. Bless you for pointing out my mistake, [Bigs].   

       And //Its the whole Community/ Halfbakery dilemma.// - marked for tagline, shirley?   

       Still not following the spoon business, though.
MaxwellBuchanan, Dec 04 2007
  

       // Still not following the spoon //   

       Good, because follwing a spoon would be Idolatry, which is a cotravention of the first or second commandment, depending on which version you go for (link).
8th of 7, Dec 04 2007
  

       [po], the slider thingy in the scrollbar is called a "thumb". I don't usually use it, myself. When scrolling a long page, I usually use the Page Down key on the keyboard. Sometimes it doesn't work, and then I position the mouse cursor near the bottom of the scrollbar, not on the button at the very bottom, and left-click that spot. It's an alternate way to get a page-down effect, one screen-full at a time. Either way, I don't have to worry about how teensy the "thumb" is.
Vernon, Dec 04 2007
  

       [8th], I'm not sure, but I presume [Bigs] was using the spoon in a more Gellerian sense.
MaxwellBuchanan, Dec 04 2007
  

       // I presume [Bigs] was using the spoon in a more Gellerian sense //   

       Possibly, but we hold to the view that what a person does with their own spoon in the privacy of their own home is their business, and we shoud not interfere or criticise. Do we, after all, hold the moral high ground where spoons are concerned ?   

       Can we have our five dollars now ?
8th of 7, Dec 04 2007
  

       //Can I have my five dollars now ?// I strongly refutiate any insemination that I am or ever have been George Bush. To suggestify such a preposition is beneath content.
MaxwellBuchanan, Dec 04 2007
  

       I didn't remember the bent-spoon-in-the- ass reference from the Matrix, but it was an OK film. The whole "reality is just a computer simulation" thing is a bit hackneyed, but each to their own. So, what exactly was the significance of the spoon in the film?
MaxwellBuchanan, Dec 04 2007
  

       Walk into any intro to philosophy class and you'll be suprised to find a topic that some blindly led, beer fed college soon-to-be-dropout won't respond to with: "Oh, like in the Matrix!"   

       That's why I hate the movie.
daseva, Dec 04 2007
  

       // Scriptwriters needed a device to reinforce Neo's disassociation from "reality" before introducing "the vase" paradox by the oracle.// They don't write 'em like that any more.
MaxwellBuchanan, Dec 05 2007
  

       //if everyone else agrees I'll delete this. //   

       No, don't delete it. I'd like some more time to reconcile the actual experiment in [4whom]'s link with the stuff in the annotations.   

       By the way, whose spoon is this?
pertinax, Dec 05 2007
  

       What frustrates me as a non physicist is the lack of good solid common public metaphors that are non mathematical for atomic objects . What is an electron - A negatively charged particle that whizzes around the nucleus of an atom . What is a negative charge ? - A charge that has more electrons than protons . See my point .
wjt, Dec 05 2007
  

       //What frustrates me as a non physicist is the lack of good solid common public metaphors that are non mathematical for atomic objects .//   

       Physicists have the same problem. But the basic problem is, quite possibly, that there *is* no metaphor for the behaviour of objects on that scale. To paraphrase Richard Feynman, they just don't behave like anything you're familiar with. A photon really isn't like a little bullet, and it really isn't like a ripple on a pond - it's a photon, and it behaves like a photon.   

       Quantum mechanics gives a set of mathematical rules which describe, with greater accuracy than any other theory, how things actually behave at that scale. It really works, and you can do wonderful things with it. But translating that into analogies or metaphors that we can "understand" is, so far, impossible. Most physicists have given up trying to find a "classical" interpretation that fits with quantum mechanics, and just accept that the equations work even though they don't fit with "common sense". Other physicists (quite a few) argue that there are underlying mechanisms which, if we could find them, *would* make "sense", and would explain quantum behaviour. For example, the "parallel worlds" school of thought says that the photon always goes one way or the other, but makes different choices in different worlds - we see the combined influence of all these parallel worlds until we make a measurement that "catches" one world or another - collapsing things into a classical outcome.   

       The idea of all these quantum experiments (including, for instance, Wheeler's Delayed Choice experiment - which you should google - it's fun) is to try to lay bare the paradoxical behaviour of things at the quantum level, in an attempt to make the paradox understandable. But we may well be faced with the fact that quantum behaviour is not analogous to macro- scale behaviour, any more than imaginary numbers can be represented as a number of beans.
MaxwellBuchanan, Dec 05 2007
  

       //Wheeler's Delayed Choice experiment - which you should google //
Later, maybe.
AbsintheWithoutLeave, Dec 05 2007
  

       //charm the water from the taps//   

       I really like that.   

       Maybe we could halfbake that idea and rebuff claims of [mf*]:magic with a "Quantum Mechanics Defence".
Jinbish, Dec 05 2007
  

       [Jinbish], "Any sufficiently advanced technology is indistinguishable from magic. " (Arthur C. Clarke, "Profiles of The Future", 1961 {Clarke's third law}), English physicist & science fiction author (1917 - )
8th of 7, Dec 06 2007
  

       "Any sufficiently advanced technology is impossible to repair." (Buchanan's Second Law; Maxwell Buchanan, freelance consultant topologist, 1962-).
MaxwellBuchanan, Dec 06 2007
  

       "If it ain't broke, it don't have enough features yet." (Scott Adams)
8th of 7, Dec 06 2007
  

       // it needs more monkeys. //   

       An infinite number thereof ... but who will provide the typewriters ?
8th of 7, Dec 06 2007
  

       PLease, don't badger us ...
8th of 7, Dec 06 2007
  

       [Bigs] I've finished with your spoon if you'd like it back.
MaxwellBuchanan, Dec 06 2007
  

       I hope there are no cases where the formula works but points the understanding in the wrong direction .   

       Things can be done without understanding . You don't really need to know about magnetism to generate electricity in a wire just a magnet and a wire . I suppose theorising allows leaping of all that time consuming trial and error experimental reality stuff .
wjt, Dec 08 2007
  

       // I suppose theorising allows leaping of all that time consuming trial and error experimental reality stuff .// Also, people need to understand how things are.
MaxwellBuchanan, Dec 08 2007
  

       //people need to understand how things are//   

       <saddles up weary old hobby horse>
Actually, that's not universally true (though it's probably true of most half-bakers). People who are good at 'living in the moment' don't really have the same felt need for this.
</suwohh>
pertinax, Dec 09 2007
  

       I think a hand fondue is a gross idea. I certainly wouldn't want a second one.
MaxwellBuchanan, Dec 09 2007
  

       // a hand fondue is a gross idea //   

       Maybe not gross, but a pocket fondue would be much more practical.
8th of 7, Dec 09 2007
  

       I think a fondued pocket would taste even worse than a fondued hand, frankly.
MaxwellBuchanan, Dec 09 2007
  

       That's a kind offer, [custard], but one which I shall politely decline.
MaxwellBuchanan, Dec 09 2007
  

       Long ago, [hippo] said //You can't detect the presence or absence of a photon without affecting the photon itself.//   

       Can't detect the presence of a photon without affecting the photon itself? Fine. Can't detect the absence of a photon without affecting the (nonexistent) photon itself? That doesn't make any sense to me. Mind you, quantum mechanics is weird enough that it could still be true. Something to do with virtual photons maybe. And the twin slit experiment might suggest it's true.
caspian, Dec 28 2007
  

       // That doesn't make any sense to me //   

       It makse sense to everyone else. When you "observe" the photon, its wave function collapses; energy is transferred, causality is conserved.   

       When there is no photon there is no signal. This is a presence/absence system, not a logical system where zero is an included value.   

       The system is still determinate, even though one of its states id defined in terms of the absence of the other state. It is still possible to disambiguate the state of the system at any given Planck interval.
8th of 7, Dec 28 2007
  

       Two things I'm not convinced about here: (not that I know much about QED):   

       1. (Already mentioned) - can you really measure a 'passing photon' without interfering with it?   

       2. Won't the 'erasure' operation also interact with the data?   

       What's really the difference between downloading the information and never looking at it and erasing it before downloading it ? (I've not quite followed your logic, but I assume you had to put in the 'erase' device to make this work).   

       Another way of putting it: isn't your 'eraser' an idealized device:in practical terms would it be possible to build such a machine ? Would it not have to interact with the stored information ?
monojohnny, Sep 17 2008
  

       Two things: 1[ there’s got to be a solution to quickly catching up on a huge thread on the inteqwberebtwittefrororumnet thing without you know having to actually read much of it, but yet, kind of, actually getting the gist of all of it without missing out vital minor details that some arsehole will catch you out on later as if you were supposed to sit here and read it all first.   

       B):- the more , hang on, let me start with a capitalisation. The more I think about it these days, the more I’m convinced that observation is not just a large part of things like this, but it really is everything. At first I thought it was an uninteresting incidental side effect not even worth taking into consideration — of course you’d observe it, how would you observe it otherwise, it goes without saying.   

       Take the subject of phenomenon vs noumenon. A phenomenon is that which can be apprehended by the senses, whereas noumenon is knowledge that is known independent of the senses. Well, I think it’s fair that I should point out to the populace that there is no thing that is known independent of the senses, or if there is, it is isn’t. The only thing that is, is that which isn’t. Nothing can be known without perception, and that is fitting. The outside of the set of things that may be known is the stuff that may not be known and the way they may not be known is by not knowing about them.   

       I think we’re all in agreement here that we can draw a line and leave noumenon over there on the fictional perceptual error side of things, with all the other nonsense. Which leaves phenomenon. That which we can only know through sensory information. Well, that’s everything. This goes further. Everything doesn’t exist until we sensory it. Then it is. That is why I elevate observation up to the highest incidence of reality. Nothing is until it is observed. In fact, there is no “is-ness” in anything out there. All there is is the effect it may have upon our sensory perception, there doesn’t need to be the thing itself.
Ian Tindale, May 06 2016
  

       But see Kant, FWIW.
pertinax, May 07 2016
  

       //observation is not just a large part of things like this, but it really is everything.//   

       //Everything doesn’t exist until we sensory it.//   

       What you are doing there, [Ian] old bean, is making a fundamental error. It is the same error that continues to be made by physicists and philosophers.   

       People tend to assume that quantum states "collapse" at some point - either through "observation" or interaction with a larger system (which is the same thing). This is silly and very Newtonian.   

       What actually collapses is the _combination_ of the "observed" event and the _observer_ collapse. Nobody seems to appreciate that the observer gets collapsed as much as the observee does.   

       Moreover, this dovetails nicely into the other little-appreciated truth, namely that quantum collapse is relative. The state of the cat and the state of the observer both collapse when the box is opened, but only relative to one another. Relative to someone outside the room, both are still uncollapsed.   

       This also (if you follow it through sensibly with a glass of something agreeable) explains all the tombollockry of "spooky action at a distance", which of course isn't.
MaxwellBuchanan, May 07 2016
  

       // Nobody seems to appreciate that the observer gets collapsed as much as the observee does. //   

       Werner Heisenberg did...
8th of 7, May 07 2016
  

       And how is he doing?
MaxwellBuchanan, May 07 2016
  

       No-one's sure ...
8th of 7, May 07 2016
  

       That's odd: even though I can't pinpoint [8th]'s location precisely, nor tell his exact mass, I knew for a fact that he would say that.
MaxwellBuchanan, May 07 2016
  

       I just thought of a way to really complicate things... It's closely related to this, so I don't think it merits a new post:   

       Instead of detecting which slit the photon passes by measuring at the slit, add a pocket on the receiving plate. The pocket is designed so a photon which reaches the bottom can only originate from one slit. The rest of the plate is the same as before.
Ling, May 07 2016
  

       Oooh, but I did just think of a variant.... Probably easier using particles (electrons probably) than photons.   

       Instead of trying to detect the photon/particle without affecting it as it passes through the slits - do something different at the target. Put the target plane at a non- orthogonal angle to the slits and use particle detectors on the target to precisely measure the time of the impact.   

       By knowing when the particle was emitted, and knowing when the particle strikes the target, and by having the target plane at an angle (different distance to any point on the target from each slit) - you can infer which slit the particle went through.
Custardguts, May 08 2016
  

       Been done [Custard]. Particle-wave duality has been observed in molecules containing over 100 atoms.
LimpNotes, May 08 2016
  

       What's required is the optical equivalent of a flip flop. If one hits before another, it can inhibit the other first. Or code it in such a way as to ascertain sequence. Then the diffraction grating pattern on the collecting plate that isn't collapsed at all, it's all still standing up perfectly straight but there it is on the holographic film, can have some interpreted meaning.
Ian Tindale, May 08 2016
  

       [Limpnotes] - I don't think that has anything to do with what I suggested.   

       I'm perfectly happy with the notion that the waveform collapses when observed. My contention is that it is the observation that causes it - bouncing a photon off, or measuring infinitesimally changes to a magnetic field from, etc the wave is probably what does it. I contend that if you do the observing after the fact - by geometry make it so you can tell which slit the particle went through - you haven't interacted with the particle until after the experiment is over.   

       A very simple way to do it is have the "target" as a W- shape - angled so that a particle coming from slot A hits a different part of the target than a particle from slot B, etc.   

       Anyhow, it's been said that Quantum stuff is not meant to be understood anyway, and anyone that pretends like they do is either crazy or lying. Certainly our brains are wired to instinctively think in terms of causality and Newtonian physics, neither of which are of any concern to quantum stuff.
Custardguts, May 08 2016
  

       Thanks for clarifying [Custard]. I'd be curious to see how that would work out. If the target was close enough so that the middle point on the W effectively prevented interference between the two compartments (the V's of the W), I think it would detect which slit the particle came through. But there would be no interference. As you moved the target further away, and interference occurred, would the interference pattern be equally distributed... even if the particles were shot through one at a time? I'd guess that, yes, if the source was not favoring one slit over another, the interference pattern would be equally distributed, and so, would prevent knowing for certain, which slit it had traveled through. The premise being of course, that it travels through both simultaneously.

The target would have two places where it could hit within each compartment for a given time of travel... for a total of four locations. That it hit the right wall of the right compartment would mean nothing because it could have hit the left wall of the right compartment, or either wall of the left compartment, with equal probability.
LimpNotes, May 09 2016
  

       // anyone that pretends like they do is either crazy or lying //   

       Why don't you try arguing that one with Stephen Hawking ? Let us know how that works out for you ...
8th of 7, May 09 2016
  

       To quote Jeremy Hardy, you shouldn't trust Stephen Hawking too much as he's subject to interference from minicab radios.
MaxwellBuchanan, May 09 2016
  

       // Why don't you try arguing that one with Stephen Hawking //   

       I'm relatively familiar with some of his more popular works. You find me a passage where he says he, or anyone else for that matter fully understands quantum physics well - and I'll eat my hat. I'll eat your hat. The point is we have some equations that seem to to work most of the time, and the beginnings of some technologies that utilise some aspects, but the research and experimentation is hardly over, is it?
Custardguts, May 09 2016
  

       //equations that seem to to work most of the time, and the beginnings of some technologies that utilise some aspects//   

       That's a huge understatement. We have equations that, in all but a very few peculiar situations, work to a higher degree of precision than any other equations used to describe the physical world - the only limit on their precision is our ability to measure the physical world for comparison. They have been tested to far greater precision than relativity; and Newtonian mechanics is an order of magnitudes of orders of magnitudes less true.   

       And as for "the beginnings of some technologies that utilise some aspects" - pretty much every electronic device made in the last 50 years not only depends on quantum mechanics but includes materials and structures designed on the basis of quantum mechanics. Fission reactors are designed from quantum mechanics upward. Many chemical processes were designed on the basis of our understanding of quantum mechanics. Every modern microscope and telescope is designed with an element of quantum mechanicry in mind.   

       Biology is the only major field that has been slow to go quantum. But it turns out that photosynthesis, respiration, magnetoception and probably most other enzyme reactions work by specifically exploiting quantum mechanical tricks.
MaxwellBuchanan, May 09 2016
  

       Most interesting to me is the generation of observable Bose-Einstein condensates. Super-cooling of certain atoms to near absolute zero where the individual atoms lose their identity and the whole lot coalesces into a quantum goo not unlike a quantum wave.
LimpNotes, May 09 2016
  
      
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