Talk:Double-slit experiment/Archive 8

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Feynman quote

Feynman has some good quotes about the double slit experiment in his Lectures on Physics based on his introductory physics class at Caltech. I think at least part of this quote, from the beginning of his double slit lecture, could be put in the article

We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality it contains the only mystery. We cannot make the mystery go away by "explaining" how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics.

— Richard Feynman, The Feynman Lectures on Physics, Vol. 3, 1965, p. 1-1

Maybe it would reduce the number of people telling us on the talk page that the experiment CAN'T work that way and that therefore we've got it all wrong. :) What do you think? --Chetvorno^TALK 02:54, 14 July 2013 (UTC)

Light "Absorbed at the Screen Behaves as Particles?"

Hi Jordgette -- looks like you have put your sentence back in but modified:

"However, even when there is an interference pattern, the light is always found to be absorbed at the screen as though it were composed of discrete particles or photons.[1][2] This result establishes the principle known as wave–particle duality."

What are you trying to say here? That photons or electrons, fired at the screen one at a time, will ... what? Single particle buildup into the interference pattern is covered in detail further down in the article with two great pictures, "Interference of Individual Particles". When you say "light is always found to be absorbed at the screen as through it were composed of discrete particles or photons", to what exact property of particles are you referring? That the particles or photons or electrons will hit the screen and have a measurable location x,y on the screen that is the point of impact? That the screen will show dots meaning something quantized hit the screen?

Also, you say "the light is always found . . ." Remember the experiment works with any particle small enough to have measurable wave characteristics. Not just photons.

Your version of the sentence, to me, sort of dilutes the whole point of the two-slit experiment. I would either delete it entirely, or replace it with:

"It is possible to do the two slit experiment by firing one particle at a time. Even in this case the interference pattern emerges (see below)."

BTW link [1] above seems to be faulty, missing title.

You've put a lot of work in on this article so I won't change it any more, but as a first time reader of the article that sentence in the intro gave me serious pause so I tried to contribute.

The elephant in the room now is the Ralf Menzel, Dirk Puhlmann, Axel Heuer, and Wolfgang P. Schleich experiment, footnote #24. I haven't been able to figure out what the physics community makes of this experiment. I've read so many treatments of the double slit experiment that claim that if you know which slit the particle goes through you lose interference, and that breaking this would break most current interpretations of QM.

Bluepost22 (talk) 20:47, 13 July 2013 (UTC)

It's a difficult concept to get across really well, so I'm sure it can be approved. If we fire enough particle-waves through a double slit, there will be an interference pattern of light and dark bands on the screen, but looking closely, those bands will be made of individual quantized dots. Each dot is attributed to the particle aspect of one particular particle-wave; however, the larger ensemble of these particles exhibits wave-like behavior with the interference pattern (as does any individual particle viewed in the context of a larger ensemble). This is a very interesting and counterintuitive result which very much captures the bizarre nature of quantum mechanics, which really starts with wave-particle duality, so I think it needs to be in the lede. The "one particle at a time" treatment is problematic because we don't really see the discrete particle aspect until the photons hit the screen. I made a couple of adjustments, and added a sentence about other particles (the lede is otherwise discussing the basic or light-based version of the experiment). Please take a look and let me know what you think. -Jordgette [talk] 23:43, 13 July 2013 (UTC)

I agree; it's a crucial point that must be in the introduction for nontechnical readers to understand the paradoxical nature of the experiment. The point is both matter and energy are always emitted and absorbed as individual particles, and are always found passing through the slits as particles. They don't "split in half" to pass through both slits. However, I prefer something like the version of the introduction before Bluepost22 tampered with it; I feel Jordgette's new sentence

"...the...pattern...is always found to consist of individual spots, as though light were composed of discrete particles or photons"

is too weak, and the term "spots" may be confusing to general readers. I'd suggest something like

"The light, electrons or other particles used in the experiment are always found to be absorbed at the screen at discrete points, as individual particles, not "waves". The interference pattern appears in the varying density of particles hits on the screen."

Something like this might be added

"In addition, experiments which look for the particles at the slits always find that each particle passes as a particle through one or the other slit, not as "waves" through both slits"

I think you're wrong, though, when you say its a difficult concept to get across. It's THE MOST difficult concept to get across. I think writing articles for general readers about QM, which are true to the science while explaining it coherently, is extremely difficult. And this is the most opaque concept in QM, which as Feynman said, contains its "central mystery". Kudos to you guys attempting it. --Chetvorno^TALK 01:28, 14 July 2013 (UTC)

Bluepost22 here -- I thought I could do a little wordsmithing (tampering?) on the intro and be on my merry way. Instead I've spent the better part of the last two days thinking about the import of the double slit experiment during the various epochs of its long life.

I think the double slit experiment should be presented as an experiment that exposes the wave nature of all light and matter. It's true for photons, electrons, and heavier particles on up to C60 as you say. The interference patterns will show up whether you are using a 5 kw continous laser or firing photons one at a time. When you say:

"However, the wave-like bright-and-dark pattern at the screen is always found to consist of individual spots, as though light were composed of discrete particles or photons"

I think you are distracting the reader. This experiment isn't about trying to show the particle nature of light -- that is the province of the photo-electric effect experiment and a host of other experiments that showed light and atoms were composed of smaller "particles". Equipment capable of detecting and creating a dot for each photon is quite specialized. A 1 watt blue green laser is going to be putting out about 10^18 photons per second. Yes we know the dots are there but that's not the point and the emphasis stands out as atypical compared to the now 20 other treatments of the double slit experiment I've read since I tried to do a driveby edit.

Here is an expanded version of the "Most beautiful Experiment" article: http://www.physics.rutgers.edu/~steves/501/links/double_slit_experiment.pdf

One could say that the importance of the double slit experiement in the last half century has been more as a platform for gedanken than for new experimental results. After all, electron diffraction was discovered in 1927, but electron interference in the double slit apparatus was only confirmed in 1961. It does seem that the currency of the experiment derives from the assertion that if you know which slit the electron goes through you destroy the interference pattern. Was Feynman saying that if this assertion is violated that QM is destroyed? (last para. pg 1-11 Vol III) Is that the current view among QM experts? If so what is the reaction to the 2012 result by Menzel et al that you cite?

Originally I was going to argue that putting the "which slit" conundrum in the intro would be too upsetting for the WP audience, and that you should build up to it chronologically in the article. Now I think that the "which slit" issue is central to the experiment's currency and should be mentioned in the intro as Chetvorno says. The paragraph from Brukner and Zeilenger seems opaque to me. I'd take Feynman or anything more blunt.

Suggest you go with a more chronological flow. You have two purely classical sections after you have waded into deep QM waters: "Point-by-point computer simulation with a massless wave packet in 2D", and "Classical wave-optics formulation". You do mention that Young overturned Newton's orthodox view of light as corpuscles -- I think it would be nice at that point to mention that Huygens anticipated this result much earlier but was apparently ignored. Bluepost22 (talk) 17:23, 15 July 2013 (UTC)

The experiment could be presented mostly as a wave experiment in the introduction, as you suggested, and the wave-particle conundrum left to lower down. But that is not its notability. If it was just an innocuous optics experiment about the nature of waves and interference, I don't think every student who has ever taken a general science course would have heard of it, as they have. It's the paradoxical QM aspects that make it notable. And the QM aspects unavoidably involve regarding the things going through the slits as particles sometimes. So I think those aspects, such as what happens when you try to determine "which slit", have got to be in the introduction in some form, although I don't know how to write it, and I absolutely agree with you that those aspects will be "upsetting" to general WP readers. It's an upsetting experiment. --Chetvorno^TALK 21:20, 15 July 2013 (UTC)

I pretty much agree with Bluepost22 about making the body of the article more chronological, or at least separating the classical and quantum versions of the experiment. Here's an idea of how it could be organized (actually don't read this, just read Feynman Lectures on Physics Vol.1, Ch.37 or Vol.3, Ch.1, that's where I got it):

• A section on the classical Young's experiment, which could also include the history up to the 20th century. It should include a simple version of the mathematics of wave interference (which is not really presented clearly in the current article) with diagrams, so readers will have a clear grounding in the "wave" part of the experiment.
• A section on "Quantum aspects of the experiment", which details the "particle" aspects. It should mention 20th century evidence that light is always emitted and absorbed as quanta, that matter particles have a wave nature and the experiment has been done with them; that when the photons and other particles are looked for at the slits they always seem to pass through one or the other slit as a particle, not through both as a "wave". Then alternate versions of the experiment; in an experiment which is able to distinguish "which slit", the interference pattern disappears and the particles act mathematically like particles; the intensity at any point on the screen is equal to the sum of the intensity when the left slit is closed and the intensity when the right slit is closed: $I=I_{1}+I_{2}\,$
• Then finally (following Feynman Vol. 1 p. 37-10) show how these two aspects reveal the fundamental law of how all wavefunctions behave. The wavefunction of the particle or photon is a complex number $\Psi \,$, if you add two wavefunctions the components can subtract as well as add, there will be interference. The probability of a particle striking the screen at a given point is equal to the square of the absolute value of the wavefunction $P=|\Psi |^{2}\,$. The probability is a positive real number between 0 and 1; if you add two probabilities the sum at any point can never be less than the two contributing probabilities. The fundamental law is:

• If a quantum event can occur in several alternative indistinguishable ways (like a particle reaching a point on the screen by passing through the right or the left slit) the probability of an event is given by the sum of the wavefunctions

$P=|\Psi _{1}+\Psi _{2}|^{2}\,$

The wavefunctions can subtract as well as add, there is interference, so the experiment shows wave behavior.

• If an experiment is capable of distinguishing which alternative is actually taken (like an experiment which can determine which slit the particle went through) the probability is given by the sum of the probabilities

$P=|\Psi _{1}|^{2}+|\Psi _{2}|^{2}=P_{1}+P_{2}\,$

There is no interference, the pattern on the screen is just equal to the sum of the patterns when each slit is closed $I=I_{1}+I_{2}\,$, so the experiment shows particle behavior.

• Then additional sections on the advanced stuff: delayed choice, quantum eraser, Englert-Greenberger duality relation, etc. --Chetvorno^TALK 00:09, 16 July 2013 (UTC)

I definitely agree we should cover the Young/classical two-slit experiment before getting into the quantum effects. After that, follow Feynman or any other reliable source that starts with classical. Dicklyon (talk) 01:38, 16 July 2013 (UTC)

Misleading picture removal

I vote that the first picture of the page consisting of the comparison between a single slit and a double slit diffraction be replaced by the pictures from the Fraunhofer diffraction page.
This is because at its current form the single slit picture, even when zoomed in, displays a single line, which just strengthens the already rampant belief that light through a single slit acts singularly as a particle by "piling up" at the screen.
We need a picture that clearly shows the shadow zones and light side-lobes in the single slit experiment to counter this. And showing the semi shadow zones inside each lobe of the double slit.
There is also a very nice photo in the hyper physics site at ending /hbase/phyopt/dslit.jpg which shows what the picture should resemble. 109.160.244.46 (talk) 00:54, 24 July 2013 (UTC)

Actually, the original photograph that I took for the article showed this; see for example this older version of the article:[1]. It was later edited and "cleaned up" by someone, but I always preferred the original, for the reasons you mention. -Jordgette [talk] 17:37, 24 July 2013 (UTC)

My feeling is this is kind of an unnecessary, irrelevant complication in an experiment that is already hard to understand. In the interest of presenting the experiment honestly I guess the original photo could be used, but I hope not much is said about the single slit interference, just a note in the caption that there is a secondary interference pattern due to the finite width of the slit. --Chetvorno^TALK 23:38, 24 July 2013 (UTC)

The point of the experiment is that interference appears when there is uncertainty about the path of the particle, and the interference pattern goes away in an experiment that gives "which path" information. The single slit pattern is due to the uncertainty of which part of the (finite width) slit the particle went through, which is present throughout the experiment. If the width of the slit were reduced to an infinitesimal line, or an experiment were done which detected which part of the slit the particle went through, that interference pattern would go away, too. But this is way too complicated explain at the top, where the picture is. --Chetvorno^TALK 23:38, 24 July 2013 (UTC)

Delete the unnecessary graphics?

This article is getting heavy with graphics, it takes a long time to load. This is mostly due to the animation of the Gaussian wave packet, which is a whonking 7.2 MB. It looks groovy, but it's also misleading, because it makes it look like a single particle can create an interference pattern. If it represents a single particle, in spite of the pretty multilobed interference pattern that propagates out of the slit it will still be absorbed at a single point on the screen. There are no sources, and no explanation of what it represents or how it applies to the article. I'd suggest deleting and putting a link to it in "External links". --Chetvorno^TALK 22:55, 19 September 2013 (UTC)

Support. There are already plenty of graphical representations in the article. -Jordgette [talk] 00:04, 22 September 2013 (UTC)

Adding a new chapter

I have added a chapter which solely shows the results and basic conclusions from the experiment. It is intended to show in a simple and graphical way the remarkable proterties of the photon-wave. This make it also more clear for those who are no familiar with the experiment and quantum mechanics. So please hold it together, don't add quantum theory or interpretation or explantions or details which are not relevant for the shown effect. That is explained in the other chapters.DParlevliet (talk) 20:44, 17 September 2013 (UTC)

I don't care for it.

1. The section appears to be original research, therefore even if this explanation were accurate, it is inappropriate for Wikipedia without reliable sourcing.

2. It's long and complex, with the variously numbered diagrams, 2 vs. 2a, etc. that the reader must follow. I don't know what a lay person could glean from the section that couldn't be gleaned from some of the other graphics already in the article.

3. It's dubious in diagram 2 to show a particle-photon striking the slit. Showing a wave instead there explains why diffraction occurs. If I were unfamiliar with wave-particle duality, I would be confused by diagram 2 as to why diffraction happens at all.

4. In diagram 3 it's unclear why the waves are "extinguishing" each other (particularly because the diagram uses a particle-path portrayal). Again, in other graphics in the article, the waves are actually shown interfering.

5. I don't know what's happening in diagram 3a. The photon is said to "turn off and arrive somewhere else." Of course that is not what actually happens; the photon is behaving like a wave. This would be very confusing to a lay reader, even if it's some kind of visualization device.

6. Later you say the wave is "travelling with the photon"? Again, confusing!

I recommend that the section be removed. Perhaps it can be reworked here on the talk page, but it's still original research. -Jordgette [talk] 00:59, 18 September 2013 (UTC)

1. How can there be original research with this experiment? I suppose all it shows is well known (otherwise it is wrong). It is just a way of showing the well down results of the experiment. More detailed explanations are found in all other chapters.

2. It is not written for lay persons. Numbers has a kind of setup, but if confusing can be changed.

3. I show a photon because it is there (if we would use slit detectors). The diffractions explanation is a part I want to add later, but first check if this part is right. I start with only a particle, no wave. The experiment then shows there is also a wave.

4. Some diagrams are repeated in this chapter to compare all on the same scale.

5. It is anyway always a particle, moving as/with a wave. You cannot remove the quantum because at that momnet it does not fit.

6. See 3. I don't start with a well known wave-particle, that is interpretation and theory afterwards. And yes,the strange properties of nature are confusing DParlevliet (talk) 06:58, 18 September 2013 (UTC)

The diagrammatic explanation is original research in the sense that it has never appeared in a secondary source (I am assuming). If no one has put these diagrams in this order with these descriptions, then it's an original creation by a Wikipedia editor and therefore original research, or, at best, synthesis. That doesn't make it wrong, it's just not in accordance with a pillar Wikipedia principle.

Aside from that important difficulty, yes the double-slit is confusing, but our job is to make it as clear as possible, including to lay readers. I personally believe the reader is better off by thinking about wave behavior to start off; that makes the easily visible diffraction and interference understandable. Then they learn that particle behavior also manifests, in different ways and under various circumstances. I don't like the suggestion that a particle moves "with" a wave. The entity is singular and is best described as both a particle and a wave at the slits; which behavior manifests to the observer depends on how they interact with the entity. A wavefront and its particle complement are both the same quantum; the classical assumption that waves and particles are ontologically different objects may be part of what makes this explanation problematic. Diagramming particles veering off and whatnot, to make a point, just muddies the water IMO. -Jordgette [talk] 22:50, 18 September 2013 (UTC)

Original research is not allowed because its conclusion is new, different then published. But in this chapter the diagrams, description en conclusion are all conformal with many years old common quantum knowledge. Only the explaining is different, and most probable not new. Every book about quantum uses these kind of thought experiments to show what experiments learn. If you describe the experiment and directly jump to quantum theory, you overlook the results of the experiment where this theory is build on. There should be a distinction between what proves directly from the experiment, and the interpretation. I think this is what the chapter adds to the article. But I will add a description of quantum interpreteation. Of course wave and particle are one, but their properties can be measured independently. Start with basic zero, building step by step, without anticipating on the final theory. DParlevliet (talk) 19:08, 19 September 2013 (UTC)

I am sorry, DParlevliet, but I don't think the added section should be in the article either. It is unsourced, it is unnecessarily complicated, it is confusing; it is not clear what conclusions, if any, are being demonstrated; and the combination of the particle (photon) and wave description in a single diagram is misleading. The description of the pattern when both slits are open is purely due to wave interference; introducing particle trajectories is false and is bound to confuse people. The basic lesson of the experiment is that particle and wave explanations are mutually exclusive and cannot both apply to the same experiment. It is very difficult to write clear explanations of this experiment; I wouldn't want to try it. It has to be explained in a certain way, with correct terminology to avoid misconceptions. We're better off sticking to the approaches in the textbooks. --Chetvorno^TALK 22:26, 19 September 2013 (UTC)

You may be confusing original research and synthesis; the latter is the case when added elements are sourced but conclusions are original. I'd also argue that diagram 2 is wrong -- a single slit doesn't result in "a wide single wave on the detector," as there is a diffraction pattern due to the nonzero slit width. This would not be explainable via diagram 2's particle representation. -Jordgette [talk] 22:50, 19 September 2013 (UTC)

I'm sorry but his English is so bad that I can't understand what the conclusions are.

"Conclusion: the waves have the same amplitude at a distance of the photon"

"Conclusion: the new wave directs the new photon, not the other way around."

"Conclusion: there is a wave before and after the photon, with the same amplitude."

I'd certainly call it WP:SYNTH but it's not a correct synthesis. For example, in 3a, he points out (correctly) that when the second slit is opened, the incidence of photons hitting the screen at some points, (B) decreases. He says that this is because "something" from the first slit changes the trajectory of the photon from the second slit heading for B, makes it go "elsewhere". But how can it? The only thing going through the slit is photons, and he has assumed that only one photon goes through the apparatus at a time. We know a photon cannot "split" and go through both slits. The photons are totally independent. If each photon goes through one slit, as he has tacitly assumed, how can opening a second slit possibly decrease the number of photons hitting any point? This shows the photon point of view cannot explain what happens when both slits are open. His explanation is completely wrong. The entire section commits this elementary mistake of combining wave and particle viewpoints. There are a few correct observations, but I think the section is not even close to good enough to keep. --Chetvorno^TALK 00:40, 20 September 2013 (UTC)

There is no "combine material from multiple sources" (which sources?) and it does not "reach or imply a conclusion not explicitly stated by any of the sources" because all conclusions are comformal with standard quantum theory. Confusion or bad Englisch are according Wiki rules no reason to delete, but to edit/improve. If descriptions are not clear, then explain. The normal physical particles and waves we know are mutually exclusive but the stange property of photons is that they are particle and wave together, at the same time. So they can be drawn an argued together (see Wave–particle_duality) Interference with a single extreme narrow slit (that is always supposed) is minor and not important for that conclusion. I don't say that the photon at point (B) decreases, but is detected at another position.DParlevliet (talk) 10:20, 20 September 2013 (UTC)

All particles have wave-particle duality, not just photons, but you can't combine particle trajectories and wave descriptions - that's the whole point of the dual-slit experiment. And all contributions to WP must be sourced (see WP:VERIFIABILITY). I'm sorry, your addition may make sense to you, but it's not making sense to others. --Chetvorno^TALK 16:33, 20 September 2013 (UTC)

I have showed the chapter to a friend and noticed that he also made a wrong conclusion, so I realised that the description can give a wrong impresssion. Therefore I rewrote it partly to be sure that no reader thinks that I seperate wave and particle. If the photon is a particle it follows an (average) track (with uncertany). This experiment shows that interference of the wave determines its track, so shows that both are combined. Which is logical if a photon is both particle and wave which you cannot seperate. Sourcing is needed when additional views are presented. I don't do that: it is just an educational way to show the standard quantum theory. DParlevliet (talk) 18:07, 20 September 2013 (UTC) The fact that the wave changes

No; the wave does not determine the "track" of the photon. If there is interference the wave must pass through both slits so there is no trajectory or "track". Photons are indivisible so they cannot pass through both slits to cause interference. Quantum particles like photons do not have a "track" because they can only be localized by interaction with other particles. In between interactions the only information about their position is the wavefunction. The wavefunction determines the probability of finding a photon at a particular point. You do not understand the subtleties of this experiment, or QM. And every contribution to WP must be sourced if another editor requests it, read WP:VERIFIABILITY. --Chetvorno^TALK 20:47, 20 September 2013 (UTC)

DParlevliet, maybe a better place for this contribution is on Wikibooks? There is no requirement for sources, and you could write an entire article on the experiment. --Chetvorno^TALK 22:34, 20 September 2013 (UTC)

The fact that you don't have information does not mean that is does not follow an (unknown) path. In (2a) the photon follows a path. We can measure that with detectors on its path and the time of detection is as predicted by its speed. Also without detector it follows this path because it goes through the slit and is detected. Now open the second slit: is the path gone now? Then where is the particle in between? It does arrive at the detector at the expected time (according its speed).DParlevliet (talk) 20:27, 21 September 2013 (UTC)

If it has a path, which slit does it go through when both slits are open? --Chetvorno^TALK 23:32, 21 September 2013 (UTC)

Deleted the section. --Chetvorno^TALK 00:23, 22 September 2013 (UTC)

Through one of the slits. It shows when you place detectors. Not through both slits because then the particle has to split itself, which is against its definition (and is never shown in experiments). Or the particle is dissolved in the wave but then is no particle anymore. But that is an older opinion.

And now your answer on my question: where is the particle?DParlevliet (talk) 11:49, 22 September 2013 (UTC)

If it only goes through one slit then how does it cause interference? You have completely misunderstood wave-particle duality. Quantum objects are both wave and particle - but not at the same time. This is the principle of complementarity. When both slits are open the photon is emitted from the source and absorbed at the screen as a particle, but travels through the slits as a wave. There is no "trajectory", no "track". The probability of the photon appearing at a point on the screen is determined only by the wavefunction, by the entire wavefunction, by the sum of every possible trajectory, as Feynman pointed out. For example, if you blocked off the left half of the interference pattern with a barrier, it would change the probabilities of finding the particle on the right half of the screen. In QM, whenever an event can happen in two or more indistinguishable ways (like the photon passing through either slit), there is always interference, the particle view fails, and only the wavefunction can explain the probabilities. --Chetvorno^TALK 16:00, 22 September 2013 (UTC)

If you place a detector at the slit to determine which slit the photon goes through, you have changed the experiment. Now there is no uncertainty as to path, now you know the particle's "trajectory", the "track", so the interference pattern disappears. There is no interference, so the photon acts like a particle, not a wave. In this experiment, when you open the second slit, the probability of finding the particle at any point on the screen increases, there are no points where it decreases, like in the first experiment. The probability of the particle hitting any point x on the screen is just equal to the sum of the probability with the left slit open and the probability with the right slit open: P₁(x) + P₂(x) = P(x). This is the property of particles. See? The photon can act as a particle, or as a wave, but not both at the same time. --Chetvorno^TALK 16:30, 22 September 2013 (UTC)

I have looked around and found out that the experiment follows the pilot wave model of Louis de Broglie and David Bohm. Remarkable is that the experiments leads to the conclusion that the wave has non-locality, what is also the most disturbing property of the Bohm model. So now I could use Bohm as reference... but that would not be right. It is not main stream and most probably the experiments are indeed new or anyway no published, so cannot be placed in Wikipedia. So Wikibooks is a nice place, where I could also go futher which I did not dare here.

And you still did not answer my question. Suppose (1), an electron, no slits: where is elecytron, its mass, its energy in the wave? DParlevliet (talk) 18:55, 22 September 2013 (UTC)

Yes, your description of the experiment is like the Bohm theory, which as you say is not an accepted part of QM. To answer your question: the mass, energy, momentum and position of the electron are given by the wavefunction, and because of the wave nature are subject to the uncertainty principle. Whether the electron has a well-defined energy, momentum, or position depends on the experiment. For example, when the electron hits the screen we know its y-position exactly - therefore its y-momentum (momentum parallel to the screen) is uncertain. --Chetvorno^TALK 01:00, 23 September 2013 (UTC)

Accepted enough to have its own article and description in duality.DParlevliet (talk) 10:56, 24 September 2013 (UTC)

Probability wave

I have stil one question I could not find the answer. The wave of the photon, is that the propability wave? For instance: right after a single atom in space at A emits one photon. How does the wave look like?

• Is it everywhere in the universe or radiating from A
• Is the amplitude everywhere the same or decreasing on distance from A
• Is it flat wave fronts running one direction or does it radiate spherical from A as centre DParlevliet (talk) 18:48, 24 September 2013 (UTC)

Questionable formulation

The text currently says: "Furthermore, versions of the experiment that include particle detectors at the slits find that each bit of light passes through one or the other slit (as classical particles would), but not through both (as waves would)." That statement goes beyond the evidence. If a "particle catcher" is placed immediately on the other side of each slit, a photon or an electron will "show up" in one or the other detection device. There is no question about that experimental result. Chetvorno's statement is correct: "When both slits are open, the electron wavefunction goes through both slits...." It follows that if there is a detector right up against the slits on the far side, both detectors will be "washed" by its own portion of the wavefunction. The probabilities associated with the wavefunction are not altered. If there happened to be a 90% probability that the electron would show up on the left detector, in 100 runs of the experiment an electron would show up in the right detector. Saying that the electron "went through" one slit or the other is a sort of retrograde thinking that goes back to our ordinary expectations. If we threw a baseball at the center bar between two windows and the baseball did not hit on center it would break either the left window or the right window because that is the way "particles" behave on the macro level. But we can't let that determine the way we make confident assertions about what happens at the micro level.P0M (talk) 18:01, 26 October 2013 (UTC)

Checking the citations to the above-quoted conclusion, the first one says: 5. Feynman, 1965, p. 1.7. However, there is no page 1.7. Is it a typo for p. 137???

The second footnote (6) leads to a reasonable place. It says, after a bit of wind-up, "This disposes of the crazy notion that a photon somehow splits into two pieces, and one piece goes through slit 2 while the other goes through slit 1." The author does speak of registering "which slit the photon went through," The author is making the same mistake I cautioned against above.P0M (talk) 19:13, 26 October 2013 (UTC)

The book cited in the third footnote (7) says, "A photon is registered only in one detector, not in both — hence it cannot split itself....we identify the attempt to determine which slit the photon passes through with the observation of its position coordinate q," This statement is precise and indicates that it is our thinking that places the photon in transition in a trajectory between laser and detection screen.

The fourth book cited (8) speaks of devices that let us "know which slit each quantum object passes through," again using the baseless logical step brought in from everyday experience. The author ought to have said "deduce" instead of "know."

Feynman makes it clear that we know the point from which a photon is emitted, the laser or whatever, and the time at which it is emitted (both to pretty tight specs), and we know the point where the photon shows up on a detection screen (at least to the precision of the size of a grain of silver in an emulsion or a CCD cell in an electronic detector. However, in the middle we know nothing unless we intercept the photon somehow. Doing so will change what we are trying to learn about. P0M (talk) 19:43, 26 October 2013 (UTC)

My statement may have been a little ambiguous. Here's what I meant. In the dual-slit experiment:

• When there is no detector to determine which slit the electron goes through, the electron wavefunction goes through both slits, and produces an interference pattern. Opening the second slit causes the number of electrons arriving at some areas of the screen to decrease; there is subtraction as well as addition. These are the attributes of a wave.
• When there is some detector able to determine which slit the electron goes through, there is no interference pattern. Only one of the detectors will be triggered for each electron that goes through, so each electron goes through one slit, not both. The total number of electrons N arriving at any area of the screen is simply equal to the number that arrived from slit 1, N₁ plus the number that arrive from slit 2, N₂: N = N₁ + N₂ This is the behavior of particles.

The general rule is: when the experiment can distinguish "which path", the object acts like a particle. When the experiment does not distinguish "which path", the object acts like a wave. Most of this is spelled out in Feynman. --Chetvorno^TALK 20:26, 26 October 2013 (UTC)

I disagree when you say: "... so each electron goes through one slit." I do so because there is no observation made on the electron while it is in flight. ADDED LATER: The crucial point is that you know which path an electron is on when, and only when, you detect it on that path. In a discussion in which he has been discussing flooding the volume of space immediately beyond a slit with light, Feynman (The character of Physical Law, p. 144) says: "If you have an apparatus which is capable of telling which hole the electron goes through (and you can have such an apparatus), then you can say that it either goes through one hole or the other. It does; it always is going through one hole or the other — when you look. But when you have no apparatus to determine through which hole the thing goes, then you cannot say that it either goes through one hole or the other." But you don't know which path an electron or a photon is on until you make a measurement, and if you delay making the measurement until the particle is almost to the detection screen you won't know which path the photon is on until it arrives at that point in space and time. You can argue that it must have been coming from where it appears to be coming from, but that conclusion is dogmatic in the sense that it depends only on our awareness of how macro objects like baseballs behave. (The pitcher reached out with his ungloved right hand and suddenly there was a baseball clutched in his stinging hand. "What in hell!" the catcher yelled, "I didn't throw that." "Right! It came from over there somewhere. Funny, I didn't see it until it hit my hand.")P0M (talk) 01:35, 27 October 2013 (UTC)

The Plotitsky book seems to have a good discussion of the approaches various people in the field have taken to this part of the puzzle. Unfortunately I can see part of it in Google Books, but not the part in the middle.

In the case of a photon, when both slits are open and unobstructed by detectors we argue that something (maybe "thing" is too strong here) goes through both slits. It makes no sense to insist that if a detector is placed opposite to one of the slits that somehow draws all of the "something" (the wavefunction or whatever you want to call it) down that one slit and that nothing goes down the other one. That reminds me of your question — "If the detector is on the far side of the slits, how does the electron "know" before it passes through whether the detector is there or not, and thus whether to split or not?" Does it go through the slits one at a time and then decided what slit to really go through? Is there something about a slit with no detector behind it that tells the photon that the other slit has a detector handy so the photon should back out and go the other way? Is there something about the slit with the detector behind it that tells the photon that the other slit doesn't have a detector behind it and so it should go ahead and "collapse" in this detector? If both slits had detectors behind them, would the photon go through one of them at random and "know" that there was a detector behind the other one, and then choose which one to really show up in? To me it seems like special pleading to withdraw the affirmation that seemed appropriate when both slits gave free access to the detection screen only when one of the slits is understood to have a detector behind it.

Let's make the experiment schematically easier to understand. Suppose we test a regular double-slit apparatus and discover that it behaves as we expect it to behave. Then we erect another wall, one that splits the left slit from the right slit all the way out to the detection screen. From the top side we would have something like a capital I with the laser being at the bottom and the detection screen being at the top. So if the left slit is open then a photon would move through the left slit and down to the left half of the detection screen. If the right slit were open then a photon would move through the right slit and down to the right half of the detection screen. If both slits are open we have two slits, each with a detection screen behind it. Interference cannot occur because nothing can go through the newly fabricated barrier. So are we to say that the laser emits a single photon, the photon gets close to the double slits, the photon then "decides" that it has to choose to go one way or the other way because there is a detector behind each of the slits? On what basis is it going to make that decision? It has yet to encounter either detection screen, and it has nothing in the equations that govern it to determine which slit to take. The equation just tells us what the probabilities are for the photon to manifest at various points on the detection screen. Do equations have a location in space and time? Or is there a quasi-physical something called a wave-function that can stay unitary or can split in a sort of retro-causal way? If I am not mistaken, all the wavefunction, all the equations tell us is that there are a series of values for the amplitude of a wave associated with the photon, and using those amplitudes we can calculate the probabilities that a photon will "show up" at the various points on the detection screen. Isn't that the way that Occam would pick?P0M (talk) 21:25, 26 October 2013 (UTC)

Raising the wall is equivalent to putting a detector at the slit; it distinguishes which path the particle takes.

That was my intention in setting up this particular thought experiment.P0M (talk) 00:18, 27 October 2013 (UTC)

You asked how the electron "knows". Consider the dual-slit experiment with a detector at one of the slits. Before it reaches the detector, the electron is represented by a wavefunction that is a superposition of two components or eigenstates; one of the electron passing through the lefthand slit, and one of the electron passing through the righthand slit. But - here's the important thing - the wavefunction is unobservable because any interaction with it collapses it. The only way you can observe a wavefunction is by the pattern of particles left behind after it collapses. The effect of the detector is to collapse the superposition (whether or not the electron ends up passing through that slit and triggering the detector). One of the two components randomly disappears - retroactively! - all the way back to the electron gun! - leaving only one eigenstate, representing the electron passing through one of the slits. I'm not sure about the mechanics of how the collapse progresses; I assume it propagates at the speed of light from the detector. The point is, from the standpoint of the outside world, it is as if there were never two possibilities, as if the electron always had a path through the one slit. In other words, the electron acts as if it was a particle. --Chetvorno^TALK 23:06, 26 October 2013 (UTC)

The question of how the electron "knows" was the one you asked Fartherred. So I guess he can now appropriate your answer, no?

How very convenient that a particle has, at creation if it happens to be a photon, exactly the superimposed wave functions that will be crucial to helping determine how it progresses through an experimental device! How can you support the idea that providence so richly provides for the future of particles? What would happen if, after the electron was torn out of its cathode or the photon was emitted by a falling electron, somebody jerked the double-slit apparatus out of the way? I suspect that a more correct way to formulate the idea you report (? Citation?) would be to say that any electron or other particle has a psi-function that could be mathematically analyzed to represent a large number (probably an infinity) of superpositions of various other psi-functions. We are only interested in the ones that happen to fit what our experimental apparatus throws in front of them.

I know that Wheeler et al. have entertained the idea of retrocausality, acting over millions of years if necessary, but I don't believe that is the only reasonable way to explain the things Wheeler was trying to understand. Certainly we can't take the idea of retrocausality on authority, and particularly not if it functions as a deus ex machina.

To be consistent, the idea of superpositions collapsing at the double-slit apparatus would have to apply equally to cases wherein there happened to be no detectors between the slits and the main detection screen, no? P0M (talk) 00:18, 27 October 2013 (UTC)

I like the new quotation you added because it says, "It seems that' light passes through one slit or the other ... if we ... detect which slit..."

One of the reasons I like the example of the experiment set up so that whatever goes through one slit is physically separated from whatever passes through the other slit is that there does not seem to be any "detection" until a photon shows up on the detection screen. There is no chance for interference to occur because the "waves" that spread out from the right slit can never be superimposed over those that spread out from the left slit.

We would like to know when the wavefunction collapses and determines which side a particular photon is going to show up at. What would be the result of dismantling the first x inches of divider wall on the end near the double slits? What would be the result of dismantling the last y inches of that wall on the part nearest the detection screen? Is there something special about this last-constructed wall in the lab such that it sets off collapse whereas the top, bottom, and original two sides of the room were contacted but would not set off collapse? I can't see it.

The only thing that seems to me to matter is whether there is a superposition of wavefunctions where the photon shows up. There is no superposition if the photon disappears as an electron is raised to a higher energy state in a fleck of dust so near to one or the other slit that there is no overlap of wavefunctions. There is no superposition if the wavefunctions from the left and right slits are separated by a barrier. Even though it is "divided," the wavefunction of the left and the wavefunction of the right are still the same wavefunction, which means that a single photon will show up somewhere along the original detection screen. It never happens that we get two photons of half the energy of the original photon. So we get a photon to show up either in the left half or the right half of the apparatus. We could argue that "the photon" went through the slit that belongs to that half of the apparatus. But that would be to ignore the fact that something else was going on.

Wheeler's cosmic version of the same experiment divides a photon around a gravitational lens. In an observatory on earth astronomers aim their telescopes at the same point in the sky and each telescope sees two stars, or actually they see two views of the same star. If they take light from the two stars and, using mirrors, direct the outputs of their telescopes so as to form an overlapping that yields one image of the one star, then they can detect interference phenomena. There is no "which way" information available for photons that are coming in to form that image. If, however, they direct light from the two telescopes to different pieces of photographic emulsion or to different CCD screens, then they get single photons coming in at different times to make up each image, and there is which path information and no interference phenomena.

So it seems that Wheeler says that even though the star is 8000 light years away from us, when an astronomer redirects light from the telescopes to make a composite image, then a command goes back 8000 years in the history of the universe and tells the appropriate photons to split themselves and go both way, and somewhat later in earth time the astronomer redirects telescope outputs, keeping the two outputs from intersecting on a single screen, and at that instant a command goes back 8000 years telling the apropriate photons not to split themselves but to only go one way or the other.

Two parts of the photon's original wavefunction have to be going through the double-slit apparatus sans detection tattle at the midpoint, so why not admit that two parts of the next photon's original wavefunction will also be going through the double-slit apparatus even if a tattle device is used. In the kind of device I described, the two components of the original wavefunction arrive at two parts of the original detection screen and then "collapse" occurs somewhere on the original detection screen, a single photon shows up, and that is the end of the matter. There is another possibility, and that is one in which the two components of a wavefunction go through two slits and immediately thereafter one of them encounters a detection device, the wave function (both parts included) collapses, and an electron is boosted to a higher orbital in the mid-stream detection device. Thereafter, the boosted electron resumes it equilibrium state and a new photon proceeds to the detection screen. I am not sure whether the article as currently written envisages some kind of detection device adjacent to a slit that somehow does not terminate the life of the original photon.

Maybe the idea was to use something like a horizontal polarizer in one slit and a vertical polarizer in the other slit. That way you could never get interference, and you could screen out everything with one polarity and look at what got through. You could know that if the wavefunction was aligned vertically it must have come through the slit with that kind of polarizer. You would have to have many runs of the experiment with a horizontal back-wall polarizer and then switch to using a vertical back-wall polarizer. But in that case, there would be no reason to believe that photons only went through the slit with a certain kind of polarizer. Even though you received a photon that was identified as associated with a horizontal polarization path, you would have no physical reason to believe that nothing went through on the vertical polarization path.

I should have stayed asleep. That's enough for now because I am too sleep to play thought experiment chess and ask what would happen if you diverted the path that went through a vertical polarizer off to the left somewhere and diverted the path that went through a horizontal polarizer off to the right somewhere. P0M (talk) 08:30, 27 October 2013 (UTC)

Unsupported statements

The article ascribes certain properties to particles and expectations of unspecified agents as to the properties of particles. I do not agree that particles have these properties and I do not share these expectations. There should be some reference to back up ascribing properties to photon and electron particles so that I can write a letter to the publisher and ask how they justify ascribing such properties to the particles. For instance, the text includes the statement: "The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen—a result that would not be expected if light consisted strictly of particles." I would expect that behavior of light consisting strictly of particles because that is the behavior observed. Further the article states: "...versions of the experiment that include particle detectors at the slits find that each bit of light passes through one or the other slit (as particles would), but not through both (as waves would)." Who writes that electrons or photons would pass through one slit or the other if they were particles? What experiment has shown that they do pass through one slit or the other? It seems to me that I read that measuring which slit an electron or photon passes through destroys the interference pattern and so such an experiment is irrelevant. Naturally a citation is needed to see just what the experiment actually demonstrated. - Fartherred (talk) 23:36, 24 October 2013 (UTC)

I added citations that support those statements. When you put detectors at the slits it is always found that the light is detected as particles (photons), passing through one or the other slit, not both. Yet when the detector is removed, the light forms an interference pattern, indicating it is passing through both slits. This paradoxical behavior is simply how physics works on a small scale. --Chetvorno^TALK 03:57, 25 October 2013 (UTC)

You claim that this is a paradox, but it seems in no way paradoxical to me. There are fundamentally different experiments one with the detectors at the slit locations that does not show an interference pattern, one with detectors at a screen behind the slits that does, and intermediate versions of these experiments. These experiments in no way demonstrate that an electron cannot go through both slits to form the interference pattern. The article is deficient in suggesting in the lead that particles do not go through both slits and only later in the article stating: "the electron had to be going through both slits at once[18]" and further relating what sources write about an electron or photon following a known path to the detector or not. This statement: "Furthermore, versions of the experiment that include particle detectors at the slits find that each bit of light passes through one or the other slit (as particles would), but not through both (as waves would)." should be removed from the lead section.- Fartherred (talk) 15:21, 25 October 2013 (UTC)

If the detector is on the far side of the slits, how does the electron "know" before it passes through whether the detector is there or not, and thus whether to split or not? --Chetvorno^TALK 18:42, 25 October 2013 (UTC)

@Chetvorno: See your own answer to this question in your reply to me in the next section. There you say that there is a superposition of states, that the electron hits the twin slits, and that the superposition "collapses" even before anything has gone through the double-slit apparatus.P0M (talk) 00:22, 27 October 2013 (UTC)

@Chetvorno: The Wheeler thought experiment makes your point very well. I mean the one that looks at streams of individual photons that originate in a distant star that encounters a gravitational beam splitter that results in our seeing two stars when there is really only one. Depending on how an observatory sets up its apparatus it can see photons that show up as though there was only one path available to them (no interference pattern) or photons that show up as though there were two paths available to them (interference being present). To say that the photon "knows whether to split or not" implies that something done in the observatory in our present affects what the photon "decides" to do hundreds or thousands or more years in our past. The other narrative we could impose on the experimental facts would be that some "thing" passed one way, and its counterpart "thing" passed the other way. That doesn't sound so bad with photons, which are a bit spooky to begin with, but if you do the same experiment with electrons and say that some "thing" goes one way and its counterpart "thing" goes the other way, then ordinary language and ordinary concepts balk at the problem of explaining the path of the mass of the electron. Can the mass divide and part pass one way and another part pass the other way? Sounds crazy. Can the mass stay with one "thing" and the other "thing" be massless and still interfere with its counterpart? Crazy idea also.

@Chetvorno: There does not seem to be any good way of making an "ordinary language" description of the flight of a photon. My solution is to say that neither photons nor electrons exist between the time they are emitted and the time they are detected, but I'll bet not many people will like that way of conceptualizing things.P0M (talk) 23:45, 25 October 2013 (UTC)

The only people I know of who claim that the electron splits in the double slit experiment are the many worlds superposition people and the objective collapse people. They claim that somehow the detection of the electron makes phantom electrons disappear. However, the existence of these phantom electrons is a matter of faith, because, according to theory no one can ever see the phantoms because they always disappear when you happen to see the one electron that turns real. There is no need for phantoms. All that is ever measured for electrons is their cross section for reaction for various reactions. Electrons at 420 MeV can interact with atomic nuclei revealing details around 2.95 * 10^-15 meters (according to the McGRAW-HILL ENCYCLOPEDIA OF science & Technology, 8th Edition, (c)1997, vol 12, p 212). Electrons in the double slit experiment react with the whole double slit apparatus by going through both slits. Any measurement of the electron affects the electron. Measuring that it interacts with one or the other detector at the slits causes some change in its position and its velocity so that it is no longer on the way to the position of a detector at a screen behind the slits. The effect of an interaction of the electron with the double slits without the detectors at the slits is to randomly alter the electron's velocity in a way that has certain calculable preferred values resulting in the diffraction pattern. There is no splitting. What is so hard about that?

In any case the article is not about the quality of arguments put forth by Chetvorno & Fartherred. There is a reliable source that claims the electron must go through both slits at ounce and to state that it goes though only one slit in the lead misleads the reader as to the state of reliable commentary. - Fartherred (talk) 00:13, 26 October 2013 (UTC)

The accepted way of describing this, used in the sources I gave, is that when there is more than one path the "particle" viewpoint fails; the light (or electron) must be regarded as a wave. This is the principle of complementarity; light (or electrons) are both a particle and a wave, but not at the same time When both slits are open the photon travels through the slits as a wave, the wavefunction, which determines the probability of finding the particle at a point on the screen. Is that what you were saying? --Chetvorno^TALK 00:51, 26 October 2013 (UTC)

@Chetvorno: It appears to me that the accepted way of describing this stuff is probably about as good as is possible using macro-scale language, ordinary English. But what we "regard" them as and what they "are" are not necessarily the same. Part of the time we "regard" them as waves, and part of the time we "regard" them as particles. There are two "viewpoints" or what I would call constructions. We impose them on the things we experience. They each fail part of the time. Nature does its own thing, and we try to characterize nature by imposing our constructions, our models, on nature. Surely the photons, electrons, et al. do not change depending on what construction we place on them. What happens is that by different kinds of experimental interactions (photon with double slits, photon with detection screen, electron with double slits, electron with detection screen) we induce them to reveal one or another aspect of whatever they really are. We never get the whole thing. Calling a photon a wavicle, for instance, doesn't really provide us with any clearer a picture or intuition of what is going on. All it does is remind us that our models are based on macro phenomena that don't really correspond well to quantum-level phenomena.

@Chetvorno: Even so, I think your way is the best way of putting the item on the consumer's menu. When the consumer eats the dish s/he will get a better idea of what the peculiarities of the description are trying to deal with.P0M (talk) 01:36, 26 October 2013 (UTC)

I believe PatrickOMoran's analogy based statement above is in support of keeping in the lead section the reference to particles going through only one or the other slit as an established fact even though it is disputed in the body of the text. Summarizing the article by stating only one of the views presented in the article is not the function of the lead section. A reliable source is referred to in the text that claims the electron, a particle, must go through both slits at ounce. So any weak soup of false implications that PatrickOMoran suggests should stay on the menu should rather be removed. - Fartherred (talk) 13:00, 26 October 2013 (UTC)

What Chetvorno accepts as a way of describing electrons is that an electron cannot be both a wave and an particle at the same time. I, Fartherred, do not accept this. Clearly electrons exhibit qualities in some experiments that are best represented by a wave model. In some experiments they exhibit qualities that are best represented by a particle model. Who says that a model cannot have both wave and particle characteristics, God? I certainly want to know what God is telling us. As I listen he seems to say by means of the properties of his creation that an electron is both a wave and a particle. I claim that it is reasonably likely that the boundaries of a single integral electron can be big enough to encompass both slits of double slit diffraction experiment, the electron passes through both slits at the same time and the material between the slits does not hinder it. A property of the dual slit experiment is that it is a measurement of the size of an electron. - Fartherred (talk) 08:40, 26 October 2013 (UTC)

@Fartherred: You said: "The only people I know of who claim that the electron splits in the double slit experiment are the many worlds superposition people and the objective collapse people." But a moment later you said, "Electrons in the double slit experiment react with the whole double slit apparatus by going through both slits." How am I supposed to understand this series of conflicting statements? P0M (talk) 01:48, 26 October 2013 (UTC)

Since PatrickOMoran does not specify what the contradiction is that bothers him, I will make my best guess. He seems to think that an electron is a tiny ball on the order of the dimensions of an atomic nucleus and that it has very well defined sudden boundaries and emits electric and magnetic fields. Further he believes that no part of the electron can pass through any portion of the experimental apparatus except the slits. With these assumptions an electron cannot be expected to pass through both slits of a dual slit experiment, but the electron seems to pass through both slits, so the assumptions of PatrickOMoran, whatever they are, seem to be wrong. - Fartherred (talk) 09:21, 26 October 2013 (UTC)

@Fartherred: I think you have some problems with fundamental ideas.

I have a problem with people who insist on interpreting experimental results as agreeing with their preconceived notions without regard to obvious contradictions. Anyone who prefers the majority view to experimental results should leave physics alone and concentrate on politics or religion. - Fartherred (talk) 09:42, 26 October 2013 (UTC)

@Fartherred: You said, "I would expect that behavior of light consisting strictly of particles because that is the behavior observed." This statement is dogmatic. What grounds, other than your own expectations, do you have for asserting that light consists strictly of particles? I don't know of anybody else who believes that idea, and I am more inclined to believe the physics community than to believe you.

PatrickOMoran whose arguments are nothing but dogma from beginning to end calls my statement dogmatic. Let it be well understood that it is PatrickOMoran that appeals to the community opinion to support his statement and that dogma is based on community opinion. Experiments constantly and continuously show that electrons are single entities, not to be destroyed nor created without about 1.03 MeV for an electron positron pair. So they are particles. This is the fundamental meaning of "quantum" in quantum mechanics. Yet the electron seems to go through two slits at once as demonstrated by countless experiments and reported by Brian Greene in The Elegant Universe, p. 110. The reason I expect that behavior [producing bright and dark bands on the screen] of particles is that it is experimentally demonstrated behavior. Let us clear up the definition of terms. Believing in community opinion because it is community opinion is dogmatism. Believing in experimental results is physics. - Fartherred (talk) 10:46, 26 October 2013 (UTC)

@Fartherred: You said, "Who writes that electrons or photons would pass through one slit or the other if they were particles?" Richard Feynman and every other physicist whose treatment of the double-slit experiment I have read. See The Character of Physical Law starting from page 127.

Brian Greene in The Elegant Universe, p. 110. reports that in some sense the electron had to be going through both slits at once. This is in perfect agreement with the results that I have read about dual slit diffraction. - Fartherred (talk) 11:06, 26 October 2013 (UTC)

@Fartherred: What you seem to want to do is to define "particle" as something other than what is generally meant by the word in English. Your "particle" would seem not to have a single location in space at any one time but to have two positions or as many positions as there are slits in a wall, perhaps.P0M (talk) 02:37, 26 October 2013 (UTC)

I do not define "particle". I use the definitions that already exist. Particle board is made up of particles that can be a few millimeters across. A carbon 12 nucleus is a particle that has a cross section for some reactions of about 2.7 * 10^-15 meters. Particles are single things and their size can vary. Now some people have decided to stick with a particular size of particle no matter what the reaction involved. Then they explain a reaction of elementary particles which according to their model do not touch by introducing the concept of tunneling. However, when particles react they are within the distance of their mutual cross sections for that reaction which is what is measured. Size for an elementary particle is not definable by a single simple number. Tunneling as a concept is an unneeded complication. For the reaction of an electron with the dual slit apparatus its cross section is large enough to include both slits since it is observed to go through both slits. Since electron orbits are known to overlap each other considerably in crystals, it is not surprising that the orbit of the electron which goes through both slits can overlap the material between the slits. I claim that a single electron going through a dual slit apparatus has a single location at all times and that it has a cross section large enough to overlap both slits. The two positions that you imagine are only in your own mind. - Fartherred (talk) 12:15, 26 October 2013 (UTC)

P0M, could you clarify to whom the above statements are addressed: Fartherred or me? Thanks. --Chetvorno^TALK 04:20, 26 October 2013 (UTC)

Anything indented one step beyond your indentation level is directed at your comment. Anything indented one step beyond Fartherred's level is directed at him. I guess that wasn't clear so I'll go back and @ them. P0M (talk) 05:17, 26 October 2013 (UTC)

Fartherred,

• When both slits are open, the electron wavefunction goes through both slits, and produces an interference pattern. Opening the second slit causes the number of electrons arriving at some areas of the screen to decrease; there is subtraction as well as addition. These are the attributes of a wave.
• When only one slit is open at a time, or you put a detector at the slits, there is no interference pattern. The total number of electrons N arriving at any area of the screen is simply equal to the number that arrived from slit 1, N₁ plus the number that arrive from slit 2, N₂: N = N₁ + N₂ This is the behavior of particles.

--Chetvorno^TALK 14:42, 26 October 2013 (UTC)

<-Unindent

I think the big problem here is how to keep discussion straight on when one is using the classical or "common English" understanding of "particle," and the quantum physics understanding of "particle." Maybe using "wavicle" on this discussion page for the QM characterizations would help.

Most articles start with the classical view and explain to readers why the classical "chunk of stuff" definition of "particle" leads to big problems, and then the discussion is carried forth to explain the QM understanding of what "things" like electrons and photons are.

It won't work for the average well-informed reader to begin by assuming the QM-relevant meanings of wave and particle. The average reader won't have any basis for statements that appear to say that a "chunk of stuff" simultaneously goes through two slits without slicing itself in half in the process.P0M (talk) 16:34, 26 October 2013 (UTC)

The terms "particle" and "wave" are universally used to describe the experiment, and encapsulate well the two incompatible aspects of quantum objects; I don't think "wavicle" will help. When they are used consistently, they explain the experiment as well as it can be "explained". The problem, as you pointed out, is that an enormous amount of blather is written about the experiment, and people who may not be familiar with the classical physics models of "particle" and "wave" misuse the terms. Such as Fartherred insisting that a "particle" can pass through both slits at once. --Chetvorno^TALK 19:50, 26 October 2013 (UTC)

I agree with you that the general reader will be confused by jumping straight into QM-relevant meanings of wave and particle. I believe you wrote a good deal of the current article, which does a decent job of explaining it. Many textbook explanations start off by describing a dual-slit experiment using water waves, and then one using bullets, to show how classical waves and particles behave. I think ideally the article should take that approach. However, as I'm sure you know, no explanation will really "explain" it. Quantum-scale objects just do not behave like macroscopic objects, and so cannot be "visualized" like them. Some people just cannot accept the paradoxical aspects, and will try to "explain them away" to get something more familiar, which inevitably involves making an error in the physics. --Chetvorno^TALK 19:50, 26 October 2013 (UTC)

One of the things that I really love about Einstein and Heisenberg is how they can write for the general reader and never make a careless formulation that can lead anybody the wrong way. Schrödinger, on the other hand, is frequently ironical if not snide and it is hard to tell what he is saying straight on and what he is mocking. It's fine for him to show off what a clever guy he is, but it is counterproductive if he is trying to convey what he thinks to other people.P0M (talk) 20:10, 26 October 2013 (UTC)

I have never read them, I didn't know that they wrote anything for general readers. Or I guess I knew Einstein did. I think it is very difficult to write articles about QM for general readers; the terms need to be used carefully to avoid misconceptions. And this article tackles the most difficult concept in QM. Kudos to you guys for attempting it. --Chetvorno^TALK 21:15, 26 October 2013 (UTC)

You could start with Neils Bohr's Atomic Physics and Human Knowledge. It was copyrighted in 1958.

I think one concept absent from the article which is causing misunderstanding is complementarity. The dual-slit experiment clearly demonstrates not only that quantum objects have both wave and particle aspects, but they don't have both at the same time. Many people on this page, including DParlevliet and Fartherred above, make the mistake of trying to talk about wavefunctions and particle trajectories together. When there is an interference pattern, there is no particle, particle trajectory, particle location, etc. except at the screen. In contrast, when there is a detector at one of the slits to determine which slit the object goes through, it only goes through one slit, not both, acting like a particle and not showing wave characteristics. It is very difficult for people to accept that merely the presence of a detector at the slit changes the entire behavior of the object, and people would like to combine the wave and particle views into something they can "visualize" better. --Chetvorno^TALK 00:23, 27 October 2013 (UTC)

Humans are the ones that can see things at the macro level only in terms of waves or of particles. To me it seems a kind of solipsism to claim that an electron is a wave or a particle depending on how some human looks at it. It seems to me that the mathematical work of Dirac that lets one equation be used in such a way that it produces either a particle or a wave solution is the better way to think about things. Electrons, protons, etc., have wave-like characteristics and they have particle-like characteristics and by making different choices in how we "interrogate" or measure them, we can gain knowledge of one or the other characteristic. It's hard to believe that a photon is a wave at the double-slit wall and is a particle at the detection screen. If we really believed that we would have to think about when it changes from one nature to the other.

Humans apply models to the objects of physics inquiry, and according to which model they use they will get a corresponding answer. The model is not the reality. There is no way to visualize an electron. Thomas Aquinas thought about things in a rather prescriptive way. He was interested in what kinds of things might be possible. He conceived of something analogous to two statements that are contradictory. Contradictory statements don't work because if you affirm that the swimming pool is full of water and you also affirm that the swimming pool is bone dry you have given with one hand and taken it away with the other. But Aquinas talked about a kind of contradiction that applies to characteristics of things. He called that kind of contradiction a "conflict of notes." For instance, he would not allow the idea of a pot that was full of boiling water and simultaneously full of ice. You can't have water that is at a rolling boil and simultaneously frozen rock solid. The wave-particle nature of light and other such quantum scale entities give us conflicts of notes, but we don't have the out that Aquinas set up. We just have to accept that quantum "particles" are only like waves and only like chunks of stuff, and that whatever they really are escapes our awareness, leaving only an impression of one aspect or the other.

Our awareness of light as a wave-like phenomenon complements our awareness of light as a chunk-like phenomenon. It's our awareness that is in need of some filling out. Nature has no such need.P0M (talk) 02:05, 27 October 2013 (UTC)

Some of the quantum erasure experiments work by blocking interference with crossed polarizers, but the effect can be "erased" by putting in more polarizers before the detection screen is reached. My guess is that you can only terminate a photon's potential to "show up" by actually getting it to light up a detection screen of some kind. I have yet to read the latest attempt with entanglement to try to get which-path information without disturbing interference. Maybe the safest policy is to report on what people would actually see in an experiment and not try to characterize what "really" happened.P0M (talk) 16:41, 27 October 2013 (UTC)

Better citation about detectors

For the conclusion "Furthermore, versions of the experiment that include particle detectors at the slits find that each photon of light passes through one slit (as would a classical particle), but not through both slits (as would a wave)" there are 5 citations. But I think none of them is (or refers to) a publication of a real experiment. Two waves only give an interference pattern if they are coherent. If the technical working of detector disturbs the coherence of the wave, then the explanation is just technical/physical. If one claims the principle that when the experiment distinguish "which path", there is no inference pattern, then it must show from the description of the detector that it only detects the photon and does not change the wave. DParlevliet (talk) 17:59, 24 December 2013 (UTC)

Maybe you are thinking that it is possible to detect a photon and then let it go on about its business. The way to learn where a photon is at x, y, z, t is to have it boost an electron to a higher orbital and then detect that change (e.g., by having the photon hit a CCD). But then the photon is gone. Getting a photon to go on from that point would involve some process wherein that electron fell back to its equilibrium state and emitted a new photon. It would not be coherent with the wave of the original photon.

What would an experiment to test the quoted statement look like? The conceptually easiest one would be to put a detector immediately on the other side of each of the double slits, reduce the light source output to one photon at a time, and then see whether the two detectors ever went off at the same time. Physically that would be a trifle difficult to achieve. Most experiments that do something like that have a way of diverging the two paths away from the double slits before something as bulky as a detector is positioned to terminate each beam. There are experiments of the quantum eraser type that set something like that up. They are more complicated, but, if my memory is correct, the one I am thinking of has symmetrical paths that each end up in one detector, and it would have been noticed by now if one photon ended up in two places. Anyway, I could highjack their lab apparatus, take a couple of things out, and run the simpler experiment. I would be famous or else get committed if I could get two photons out for each one put into such a simple apparatus.

I'm not aware of recent experiments with particle detectors at both slits. That kind of thing was being reported early in the history of quantum mechanics. I have some old books, but people like Heisenberg tended not to describe lab apparatus and details of experiments, so I'm not sure what I will find. On the other hand, if a single photon were discovered to be passing through two slits and activating two detectors, I think somebody would have thought to inform the world that the fundamentals of physics had suffered a grievous blow.P0M (talk) 08:38, 27 December 2013 (UTC)

"Everybody tells" is no reason for not needing a citation, also not early in the history. You also describe that the loss of interference pattern is caused by the technical behaviour of the detector, making it not useful for proving a fundamental of physics. Wikipedia is not a place for original reseach, but just has to show the reference or mention that this measurement actually has never been reported. DParlevliet (talk) 09:22, 27 December 2013 (UTC)

Can you suggest the design of an experiment that would successfully answer your own question?P0M (talk) 19:50, 27 December 2013 (UTC)

Perhaps, but Wikipedia is not the place to discuss that, because I have no reference. Regarding the often mentioned detector-before-each-slit experiment I don't know detectors which detects photons whithout disturbing the wave. And it seems that nobody knows a publication of this measurement actually done. If not, then it should be added that a publication is not (yet) known. I can propose something. DParlevliet (talk) 21:20, 27 December 2013 (UTC)

What can you propose?P0M (talk) 01:15, 28 December 2013 (UTC)

O.K. I've been going back through the discussion above, and I see where the problem is. Some people would deduce that if a baseball is thrown trying to hit the mid bar between two open windows and the baseball shows up in a basket behind the left window then it must have gone through the left window, and if it shows up in a basket behind the right window then it must have gone through the right window. That works fine for Newtonian physics and Newtonian physics works fine for baseballs. However, it is not a conclusion that can be supported by quantum mechanics. See http://www.science-bbs.com/161-physics/2b7111150a0a93d8.htm and search for "tennis ball." And in The Quantum Challenge, p. 16, Greenstein and Zajonc say that "quantum mechanics regards the very concept of a trajectory as deeply suspect."

It seems very natural to assume that if a photon shows up at the end of one path and not the end of a second path then it must have travelled down that one path. However, we run into situations that require a great deal of sophisticated reasoning to explain. One would be the results of Wheeler's cosmic double-slit experiment involving a distant star, a gravitational lens, and the altering of thousands of years of history depending on whether two telescopes project their images one over the other or keep them separated. We only get into trouble if we say that the arriving photons have trajectories providing that we keep the images from two telescopes from merging.

Rightly or wrongly, asserting that photons behave like baseballs in the double-slit experiment with added detector(s) is a conclusion based on an analogy.

I am going to make a simple change that I think will avoid the trouble. P0M (talk) 03:16, 28 December 2013 (UTC)

The sources provided are pretty clear. Where are your sources that contradict them? --Chetvorno^TALK 05:30, 28 December 2013 (UTC)

I'd like to be clear on what your objection to the experiment is. You don't believe that when single electrons pass through a dual-slit apparatus with detectors behind the slits, both detectors will register "hits", do you? Your objection is that a "hit" on one detector, say the righthand one, does not necessarily indicate that the electron has passed through the righthand slit; is that it? --Chetvorno^TALK 06:26, 28 December 2013 (UTC)

I have no objection to the experiment. I object to interpretations that go, by reason of expediency or carelessness, beyond what the experiments show. I think that you have not understood what I wrote.

The objective evidence we have is the diaphragm with two slits, the several arrangements of detection screens on the far side of the double slits, and the locations of "hits" seen under the several arrangements. We cannot argue from the way that classical objects work to the way quantum objects must work. I have already made reference to something by Martin Hogbin and something in Greenstein and Zajonc that make this point in different ways. Furthermore, if you read the sources in the footnotes to the passage we are arguing over you will see that Lederman and Hill support your position, but that other articles cited do not. (I haven't found my copy of Feynmann yet.)

The evidence shows that: (1) With one detection screen at a fair distance from the two slits an interference pattern will be seen. (2) With an additional detector nearby on the far side of the double slits equal numbers of photons will be detected on the near detector and the far detector. Moving the near detector to the other slit will not change the results. (3) With two detectors, one right behind each of the two slits, equal numbers of photons will show up in each detector. If we look for signs that a photon has split itself we find that we never get two hits when only one photon has been emitted. If we are using a laser that produces a certain frequency of photon, we never find pairs of photons whose frequencies sum up to the laser frequency. So the photon does not split itself.

We have no knowledge of the photon between its leaving the laser and its turning up on a detection screen.

We cannot deduce from the behavior of balls in classical physics what must be the behavior of photons or other particles in the quantum domain.

The third footnoted text, Miller-Kirsten, avoids saying anything about which slit a photon went through:

Every attempt to single out either of these aspects requires a modification of the experiment which rules out every possibility to observe the other aspect. This becomes particularly clear, if in a double-slit experiment the detectors which register outcoming photons are placed immediately behind the diaphragm with the two slits: A photon is registered only in one detector, not in both — hence it cannot split itself.

The fourth footnoted text, Plotnitsky, gives a rough characterization on the pages cited, but then on p. 82ff formulates things more carefully:

One could speak of a single photon as "passing through a slit" in the sense that the corresponding event could be registered by a "which-path" measuring device, but only in this sense.

I don't have time to copy out stuff from the other cited texts. I haven't re-read them yet. However, it should be clear that one of the most characteristic features of quantum mechanics is that things in that domain do not necessarily behave the way things do on the macro scale, so we cannot make implicit use of macro-scale behavior to say what must be happening on the quantum scale. Do the logic. What do you need to know to affirm, with logical consistency, that a particle that ended its existence in one detector went through the nearby slit associated with that detector but did not go through the other slit? P0M (talk) 08:24, 28 December 2013 (UTC)

There was only one more:

It seems that light passes through one slit or the other in the form of photons if we set up an experiment to detect which slit the photon passes, but passes through both slits in the form of a wave if we perform an interference experiment." Rae, Alastair

Note the "seems." That's the same change that I wanted to make in the article.P0M (talk) 17:49, 28 December 2013 (UTC)

So your point is that if two slits are open, it is not possible to say (or it is meaningless to say) which slit a photon "passes through", even if there are detectors behind the slits? By the way, as far as I can see, your second and third quote above supports the statement in the introduction, and your first quote does not contradict it. --Chetvorno^TALK 17:51, 28 December 2013 (UTC)

There is no evidence possible regarding which slit a photon "passes through." P0M (talk) 21:00, 28 December 2013 (UTC)

Why no response? I don't agree with your "by the way." Look at those quotations again. Only the guy writing for poets makes an unqualified statement about where the photon goes, and makes it seem that the photon was on the path and only on the path that terminates where it shows up. That kind of conclusion works in the macro world, but not in the micro world.P0M (talk) 04:26, 7 January 2014 (UTC)

Take a look at this:

Wheeler's Thought Experiment

Tell me what the grounds are for declaring that a photon has taken any one path. P0M (talk) 08:31, 7 January 2014 (UTC)

Sorry, I've been busy at work. I wanted to look up some sources on the issue first. --Chetvorno^TALK 20:30, 7 January 2014 (UTC)

No problem. I'm still trying to navigate between Copenhagen protocols and any possible way to say things to people beyond the math, so any clarification or neat ways of thinking about things would be helpful. I think Wheeler wanted to say that history was rerouted over a distance in space and time of x number of light years between the distant "doubled star" that telescopes on earth image and those telescopes.

Have you seen any discussions of experiments wherein the beams from a traditional double-slit experiment are diverged so that experimenters can check their intuitions about what ought to happen when two copies of the same wave-function fall upon separated detection screens? There should be a diffraction pattern on each of them, just as there would be a diffraction pattern of the basic apparatus with one or the other slit closed off. But that is my intuition talking, and I'd like to know what happens when you actually do the experiment. Thanks.P0M (talk) 21:59, 7 January 2014 (UTC)