Talk:Double-slit experiment/Archive 2

Interference

1. In the double-slit experiment, does it matter whether more than one light source is used?
2. Has there been any significant discussion regarding the possibility that photons fluctuate between positive and negative intensity?

--ηυωρ 08:14, 19 Nov 2004 (UTC)

What exactly is changed as light negotiates a single diffracting slit?

In the current article it is stated:

The waves interfering must be coherent, i.e., the light has the same frequency and is in the same phase. In Young's experiment, this was achieved by passing the light through the first slit, and thereby diffracting it, producing a coherent wave; this is more typically achieved now by using a laser, and removing the first slit.

Was the above procedure fully explained and justified by classical wave mechanics? According to classical wave mechanics, light consisting of a spectrum of colors is a superposition of electromagnetic waves with different wavelength. Does classical wave mechanics allow superposition of electromagnetic waves that closely match in wavelength, but are not in the same phase? Does classical wave mechanics predict interference patterns when the lightsource is a narrow band of the spectrum, but not perfectly monochromatic?

Quantum electrodynamics allows unlimited superposition, so according to quantum electrodynamics incoherent lightsources remain incoherent after the first (single) slit. What does classical mechanics allow, and should an article on double slit experiment present an interpretaton both in terms of classical wave mechanics and in terms of quantum electrodynamics? -Cleon Teunissen | Talk 09:44, 23 Mar 2005 (UTC)

In the 19th century, interferometry was used to measure the wavelength of spectral lines of elements. Sodium light is dominated by a very narrow band of the spectrum. Light from a Sodium lamp is incoherent, and it will form an interference pattern. The purpose of the first slit is to get a point source of light, that is the necessitiy. I don't think coherency matters at all. --Cleon Teunissen | Talk 10:28, 23 Mar 2005 (UTC)

coherency and pinholes

Cleon Teunissen | Talk 10:24, 7 Apr 2005 (UTC) In the section on the necessary conditions to produce an interference pattern I write that coherency of the light is not a requirement. I don't know how Newton's rings used to be explained in terms of classical wave dynamics.

In quantum electrodynamics Paul Dirac made the following observations about the quantum of action of the electromagnetic field, the photon "Each photon interferes only with itself. Interference between two different photons never occurs." (source: external link: Science week article on entanglement quoted from: Dirac, P. A. M. The Principles of Quantum Mechanics (Clarendon, Oxford, 1982))

I don't think that Thomas Young needed coherent light for his first slit experiment, and I don't think the light he used was coherent. However, it seems that according to classical wave mechanics inferference will occur only if the waves are coherent.

The first holograms by Dennis Gabor were obtained by using light from a mercury line that was cast on a screen with a pinhole in it. External link: history of producing holograms It seems to me that if the pinhole is small enough the light does not have to be coherent, pathlength will ensure that a hologram is produced.

Many lasers are designed in such a way that the light leaves the laser cavity through a pinhole. (Inside the cavity the light is "bouncing" in all directions.) Coming from a point source, the diverging laser light can be refracted to a parallel beam with the help of a lens. (Holography requires diverging light coming from a point source.)

It is not clear to me whether the concept of coherency has any meaning in Quantum Electodynamics (QED). Due to the way they are produced, the photons of laserlight are in a specific form of quantum entanglement, in a sense they are all "clones" of one starting photon. external link: Site with answers. See section: laser light is "in phase" light?

It seems to me that some interferometric experimental results just cannot be explained by classical wave mechanics. I think that only path length matters, as is illustrated by the sum-over histories approach to QED calculations developed by Feynman. I think coherency is an attempt to explain in terms of classical wave mechanics something that cannot be explained in classical wace mechanics. --Cleon Teunissen | Talk 10:24, 7 Apr 2005 (UTC)

possible gap here

Double slit free of time

Just throwing this out there, but what if interference pattern that occurs from the double slit experiment is independent of time? That is to say that while we perceive the electrons/photons/whatever to be shot one-at-a-time, the particles themselves do not due to their high-speed and therefore the Lorentz contraction of time. So while we see a "gun being fired", the particles behave as if they were in a wave of particles all being shot at once. So, it's not so much a probability-wave as a time-independent-wave (atemporal-wave). That would seem to coincide with general relativity and the double slit in time experiment mentioned below. Any answers? —Preceding unsigned comment added by 159.153.140.10 (talk) 00:42, 29 September 2007 (UTC)

Double slit in time

can anyone tell me if its true when you observe the double slit using detectors you get different results? to say one shows a wave function, and then when detectors are introduced you get non interference patterns.

If you have a source of electrons or photons and a detector screen with the double slits in between. then you have one event every time an electron is fired off and reaches the screen. Even when the emissions are made one at a time, an interference pattern still emerges. So from emitter to detector is one event. We can tell when the electron or particle is emitted, and we can tell when it hits the detector, and those two facts constitute an event. The only way to know whether something goes through one slit or the other is to put something in the slit that interacts with it. But that "thing in the middle" has to be a detector. So then we have one event from emitter to detector and another event from mid-way detector and the detector screen on the other side. The photon or electron that is detected in slit A or in slit B goes into A or B. It does not go into both A and B. And what comes out of A or B does not pass through two slits either. The implication seems to be that when A and B are not obstructed by anything that could detect an electron or photon, then the electron or photon behaves as though it had gone through both slits. We make that judgment because only something that goes through two slits could interfere with itself. But the way things work we can have knowledge only of emissions and detections. Actually, we infer the emission, and we can do so reliably, because we supply a short burst of energy to the emitter and at a calculable time interval later we detect something arriving at the detector screen.

The explanation is rather strange, but as I understand it the experimental results have been confirmed over and over again. In our everyday lives we get used to the idea that, if we have a fast enough movie camera, we can watch the baseball instant by instant from the pitcher's hand to its contact with the bat or the catcher's glove. But in quantum baseball you can see the windup, and you can see the arrival impact, but nothing can be seen in-between.P0M 05:41, 17 February 2007 (UTC)

While possibly a little early to include in the main text, the double slit in time experiment http://physicsweb.org/articles/news/9/3/1/1?rss=2.03Cbr20/E is surely experimental evidence that that matter/energy is not just quantised over spatial dimensions (as shown by the classical double slit experiement) but over the time dimension as well. ie: spacetime does have a quantum nature.

http://xenz.stumbleupon.com/

That's a really mind-expanding experiment. It is equally possible that either maximum trips the emission of a photon, so the event includes a 50-50 starting point and interference between the two quasi-starts at the detector. So with slits in a screen we don't know where it is, and with the other device we don't know when it is. In our everyday life we have to catch the 9 o'clock bus or the 10 o'clock bus. So we arrive at either 4 o'clock or at 5 o'clock. In the experiment reported above it seems that it is indeterminate when one will arrive, and that there is an interference pattern in time rather than in space resulting in a varying pattern of intensity at the detector. I hope I have visualized this experiment correctly.

To put this information into the article we would need to make some good diagrams. P0M 06:16, 17 February 2007 (UTC)

"Thought Experiment" review needed

Can someone who knows more than I do please review this passage:

"If either slit is covered, the individual photons hitting the screen, over time, create a pattern with a screen. But when the experiment is arranged in this way, the fringes disappear -- for reasons related to the collapse of the wavefunction."

Specifically, "a pattern with a screen" is meaningless to me. A fringe pattern? A single peak?

And "when the experiment is arranged in this way" - presumably meaning still a single electron, but it is one slit still covered? No mention is made of uncovering it.

What happens when both slits are uncovered and a single photon is used? That's what I came here to find out, and what this thought experiment appears to be addressing, but this explanation is not helpful.

Apparently there used to be more explanation, but it was removed, and I'm not sure if it was done because it was incorrect: http://en.wikipedia.org/w/index.php?title=Double-slit_experiment&diff=9310882&oldid=9119157

That extra explanation is completely correct and I'm uncertain why it was removed. --Laura Scudder | Talk 05:15, 9 August 2005 (UTC)

Looks like it has been put back in --ssam 22 Oct 2005

Why is this bit called a though experiment? I have performed the experiment with single photons. --ssam 22 Oct 2005

Beams of electrons or atoms?

The article says that the experiment can also be performed with beams of electrons or atoms; it would be nice if some more detail could be added for this (when were the experiments performed, what kinds of atoms were used, etc.) Cwitty 23:54, 4 November 2005 (UTC)

• Gerhard Paulus group

Revert

I reverted the recently added text, in part because it made incorrect statements, such as:

Since the photons are emitted one at a time, the photons cannot be interfering with each other -- so exactly what is the nature of the "interference"?

Most of the new text wasn't bad, but a few things like this spoil it.linas 15:18, 6 February 2006 (UTC)

Can this change be clarified: the photons cannot vs. the photons are not - i don't understand how these statements were substantially different. And: a few things like this... why not remove/edit/clarify those things which are incorrect? I added two book references from which the added text was drawn, and the added text appears to be accurate according to these references.

A single photon can and does interfere with itself. The word "not" needs to be striken. linas 04:42, 7 February 2006 (UTC)

My apologies; it seems that sentence was there all along. I must have gotten confused. I have removed that sentence in its entirety. linas 04:56, 7 February 2006 (UTC)

The reason i think the added text belongs in here is that:

• the text in it's original form does not illuminate the fact that it is a particular interpretation which treats the wavefunction collapse as a physical reality. It also refers to this as "modern quantum physics", when the Copenhagen interpretation is not strictly "modern" (i.e. there are more recent interpetations being debated alongside it) - so it makes more sense to just link directly to Copenhagen interpretation.

What part of the text implies that its a Copenhagen interpretation? linas 04:42, 7 February 2006 (UTC)

Again, never mind, it looks good now. linas 04:56, 7 February 2006 (UTC)

• the reference to QED adds depth to the description of the experiment which i think was lacking. It'd be nice if someone working in the field went through this section of the double-slit experiment article to include more information. I haven't found any other single source that collates everything about it.

Double-slit can be explained, computed, discussed, manipulated, etc. without requiring QED. Or QFT, for that matter. I think you are trying to say that "path integrals can provide insight." linas 04:42, 7 February 2006 (UTC)

I changed QED to path integral formulation which is what you want. linas 04:56, 7 February 2006 (UTC)

• The title of the section in dispute: The Thought Experiment - seems problematic: It implies that something described in this section is only a thought experiment - but it doesn't say what. One of the book references provided, "Q is for Quantum" by John Gribbin, explicitly states that the modified double-slit experiment with a detector placed at one or both of the slits is, in fact, a real experiment which has been successfully carried out; if this is the case, then naming this section "the thought experiment" is misleading.

Yes, that title does seem out of place. Fix it. Just please be very very careful with your edits. The articles on quantum tend to attract a lot of editors who have never actually studied QM, but think they can explain it anyway. A lot of subtle errors result, making more work for me and for others who try to tend to this article. If you are not absolutely, totally sure of what you are doing, don't do it. linas 04:42, 7 February 2006 (UTC)

Sorry, its possible I shot first, asked questions later. It looks good now, I think. linas 04:56, 7 February 2006 (UTC)

Thank-you for reconsidering and explaining all of that. I do promise to be careful with articles involving specialist knowledge; --Dissembly

--User:Dissembly 7th Feb 2006

at home

Say if we can do this experiment at home. (anon User:210.201.31.246 on 21 Feb 2006)

Yes you can. linas 00:12, 22 February 2006 (UTC)

very easily in fact in some countries people have thin textile before the windows, so you can look out the window, but people won't be able to look inside. Trough these fine textile look to a sunny object you will see an interference pattern

A basic question

I have a PhD in QM, but it's some years (20 plus) since I used it, so I'm rusty. Can I ask you guys for feedback on a basic question? One thing that always bugged me about most explanations of electron diffraction in the two slits experiment was that they were based on the single-particle approximation, with the electron being the particle and the slits being modelled as an external potential, so the only wavefunction being considered is that of the single electron. Now, in reality the slits are gaps in some kind of physical device made of, guess what, electrons and nuclei, so the electron that is passing through the slits is actually interacting, under the influence of electromagnetic forces, with all of the other electrons and nuclei that form the boundaries of the slits. That interaction will be mediated by photon exchange, in a way that is linked to the relative speed of the electron being diffracted, and it should be more properly modelled as a many-body wavefunction. Has anyone investigated whether the diffraction effects could reflect the wave-nature of the electromagnetic intereaction between the electron and the screen containing the slits? It seems to me that this could, in principle, allow for diffraction effects even if each electron definitely went through only one slit. (unsigned question from User:81.155.67.43 on 31 March 2006)

Interesting question. Several answers:

-- As long as the simplest theory of twoslit diffraction is sufficient to explain the outcome, there is no incentive to look for more complex explanations. Occam's razor.

-- You might find that the Mott problem to be the many-body treatment that you are looking for, although it is for cloud chambers and not the double-slit. Although it actually solves the opposite problem: it explains the abscence of a "diffraction effect", as it were. You might also find the Renninger experiment intriguing.

--There are quantum effects when the size of the particle being scattered is non-negligable with regard to the size of the slit. These have been experimentally measured and theoretically confirmed for the diffraction of large molecules through very very small slits.

But your suggestion founders: a many-body wavefunction is inherently quantum mechanical; and adds to the confusion of "wheres the electron" the additional confusion of "which electron are you talking about, they're all indistiguishable?" In a certain sense, many-body quantum mechanics aka QFT is even more mired in subtle confusions when it comes to the interpretation of quantum mechanics. linas 23:51, 31 March 2006 (UTC)

Does that mean 'No'? I would agree with your appeal to Occam's razor if the conventional explanation of the two-slit experiment was sufficient- but it is far from sufficient. The conventional explanation relies on the Copenhagen interpretation, and indeed is often cited as an illustration of the counter-intuitive implications of the Copenhagen interpretation. I'm glad that you spotted that a many-body wave-function is inherently quantum-mechanical- it would have worried me if you said it wasn't. To make myself clearer...I am unhappy with the conventional explanation of the two-slits experiment, in relation to electron diffraction, precisely because it depends upon the Copenhagen interpretation. It seems to me that there is another potential explanation of it, to do with the photon-based interaction between the electron and the screen containing the slits, that does not require the electron (or its wavefunction, more precisely) simultaneously to pass through both slits, which is the implication of the conventional explanation. I wondered if anyone had tried a calculation along these lines (my QFT/QED is far too poor for the job).

I see your question, and raise you one. What effect do the particles hiting the mask have on quantum state of the particles making up the edge of the slit? Is there any way to seperate the 'negative' effect of a hole in the mask material not gaining the charge of the particles from the 'positive' effect of the particles selected by the slits?Shalaid 03:39, 18 September 2007 (UTC)

Request for more description of Young's experiment

I would like to know how large/small the slits need to be. Actually there could be several articles referenced from here relating to the different variations of this experiment (and well yes, there are many references:-)

According to basic description the slits should be less than a wavelength. I think we were told in College that slits could be made (or were made) by cutting in covered glass.

But I have a recollection that the experiment at that time (1964) was meant to demonstrate that light after the slits would behave differently, as if one beam.

So you can see this is more a request for more explanation.

Thank you for this article, the many references and the interesting inspiring writing.

   Donald Axel

--d-axel 09:20, 12 May 2006 (UTC)

Why doesn't the article give Hugh Everett's Many Worlds resolution to the double-slit experiment? The article seems to favor the Copenhagen Interpretation point of view, which violates Wikipedia's rule that articles must be neutral and not have a point of view.

Michael D. Wolok 13:29, 15 May 2006 (UTC)

OK some other basic questions

Hopefully this isn't common knowledge.
1) How do the detectors that can be set up at either slit work?
2) What happens if you have 3 slits in a line? in a triangle shape? unevenly spaced? what if they are round pinholes rather than slits? I've never seen these variations mentioned. I assume that a different pattern appears on the screen for each arrangement of slits/holes.
3) Water waves sent through small gaps make a moving pattern on a screen as the waves move. Does the electron/photon pattern vary over time, or is it static? I don't remember from class, it's been a while!
4) If you gradually increase the separation of the slits, does there come a time where the photons/electrons no longer pass through both? Does each particle pass through both anyway? If not, then how, if you send them through 1 at a time, does a particle which passes through 1 slit only know the other slit is there? 5) If detectors on both slits detect 1 photon, I assume they slow it down but don't stop it, which makes it not coherant with say, another photon sent through the other slit at the same time? But the article says that the light doesn't have to be coherent for the diffraction pattern to show up.??

Thanks for any responses!

Double Slit experiment with only one detector?

What if you were to only place a detector in one of the slits instead of both? If a particle acts like a wave until it is detected, wouldn't the single detector always detect electrons(or whatever particle) since the wave always travels through it?

graphics

Please review wikimedia-diffraction for some nice graphics suitable for this article.

Computational Complexity

I'm curious what would be easier to model from beginning to end, a wave function or a 'particle'. It seems to me that this paradox is the result of a computational shortcut that only resolves the wave function upon observation. MarkchenNY 21:09, 20 December 2006 (UTC)

How do you define "to model"? Generally speaking, all scientific theories are typically described as models. Before the 20th century many people thought it possible to experience "the real thing," see the gears in the clockworks of reality. I. Kant started with the general ideas of Newtonian physics, explained perception in terms of a chain of presumed interactions between object, sense organ, brain, etc., and then asked where is the "thing in itself"? By the time he was finished he had pretty well deconstructed the foundations of his own philosophy. That result left practical scientists with some things to think about. So they had to think very carefully about what they actually experienced (typically, readings on meters, exposed spots on photographic film, etc.) and what their reasoning was that led them to picture reality in a certain way. In the case of the double-slit experiment, there are two models we can take from our experience of the macro world, maybe more. These models were used to talk about what light "really is" before this experiment emerged. One model says that light is composed of "corpuscles" (or what we would call particles today), and another model says that light is a wave phenomenon

If one starts with a naive model of the particle sort, the model makes bullets the analogs of photons. We ask what happens when a gun is trained at a wall and locked in place. Within limits of gun design and ammunition, a pattern emerges directly across from the gun. If we then build a wall to interrupt the bullet's flight nothing will get through unless we make a hole. If we made a bullet-sized hole not many bullets would get through because of the spread. If we made two larger holes side by side we would expect to get back the original pattern minus any bullets that might happen to strike the very center of the barrier. I could try this with my Crossman air pistol, and I'm pretty sure of the results I'd get. Everybody else has the same expectation as far as I know. But when photons or electrons are used in place of bullets (and the test apparatus is kept to scale of course) the model fails. So modeling a particle solution does not work.

If one starts with a naive model of the wave sort, the model makes waves propagated in some physical medium the analog of light waves. We need a physical vibrator in one end of the apparatus and some kind of detector apparatus at the other end. To match the experiment we will do with light as closely as possible, we need some way of creating a narrow beam that will show up as an analog of a dot of light on the detector screen. Then we ask what will happen if we build a wall midway. At this point some differences appear. A single hole in the wall will produce a blurring and an increase in diameter of the "spot" on the remote wall, due to diffraction. A double slit will produce a diffraction pattern. So modeling a wave solution does work.

Now what happens if we fix our agitator so that it makes only one movement? For instance, we might release one tethered ping-pong ball from under water so that it stops just as it displaces water on the surface. One wavefront heads off toward the two slits. If the wavefront is wider than the distance between outside edges of the slit, then two waves appear at the opposite side. But what happens if the beam is so narrow that we can send one wavefront through one slit? Then we will get the ordinary diffraction pattern. Is a wider beam or a narrower beam the correct analog for light at visible wavelengths? If it's a narrower beam, then modeling a wave solution does not work.

One of the difficulties of vetting these models, of course, is that while we could even observe the passage of bullets through our first model apparatus if we had the high-speed motion picture equipment, and we could do the same thing with water and other physical waves, we cannot take pictures of the progress of light through its apparatus any more than we could monitor the progress of a bullet by firing bullets at it.

How is this analysis so far? I am still rather confused by your question. P0M 23:39, 21 December 2006 (UTC)

Does not convey a clear meaning

One sentence in the article is too vague to be meaningful.

A very easy way to see the interference pattern is to look at light in the background through a slit made by common objects such as two pencils. The interference pattern of lighted bands and obscure bands is evident to anyone.

1. What is "the interference pattern" supposed to refer to? The aforesaid pattern produced by double slits? You don't need additional apparatus to see what is there, so the sentence most refer to some other interference pattern -- but this sentence leaves the reader to depend on total guesswork.

1. "Look at light in the background"??? Nobody can see light. One sees things of a certain magnitude by means of reflecting light off of them. So what is the author really trying to say? My guess is that s/he refers to looking at some lighted surface that stands at some distance from the observer.

1. Now we are instructed to look at this "light" through a single slit made by two pencils. Pencils have become animate, and operate in pairs. They go around carving slits in things? Probably not. Probably the author is trying to say that one should hold two pencils close together so that there is a very narrow gap between the two pencils.

1. If all this guesswork, which should not be necessary, is correct then the author is describing a single slit experiment, and asserting that "the interference pattern... is evident to anyone." According to normal English syntax, "the interference pattern" has to refer to an earlier mention interference pattern. (A little dog came walking over the hill. The dog was a Corgie.) But the earlier interference pattern was produced by interference between light transmitted through two slits.

The existence of a diffraction pattern produced when light is passed through a narrow gap is well known. But that phenomenon is different from the interference pattern produced by two slits illuminated by coherent light.P0M 07:42, 29 August 2006 (UTC)

Agreed, this article almost seems like it was made to confuse interested viewers, coherence is a task a 5 year old could pull off for goodness sake. Deepdreamer 21:21, 2 October 2006 (UTC)

Check Dates

The article currently reads: "...Although the double-slit experiment is now often referred to in the context of quantum mechanics, it was originally performed by the English scientist Thomas Young some time around 1805..." but the Thomas Young article says c. 1801, as does Tipler's "Physics" (as referenced in main article, page 1131). It's a minor point, but if anyone can verify it, it probably should be changed for accuracy's sake! Dom 15:41, 12 January 2007 (UTC)

Why not fix it then? P0M 07:06, 13 January 2007 (UTC)

ok 129.234.4.1 21:08, 13 January 2007 (UTC)

When was the first "single particle at a time" version of this experiment run? Does "Interference fringes with feeble light." by G. Taylor 1909 qualify as "single particle at a time" for photons? If so, was it the first? --70.130.45.233 05:19, 22 September 2007 (UTC)

How the trick is done

The double slit experiment uses single photons, quotes the dogma. A photon is a wave. How big is a wave? On our scale, it can be a tsunami or a wave where kids are paddling. A wave is divisible so the single photon/wave can becomes two photons/waves or ten photons/waves, etc and so go through all places at the same time. Kaneda.

Just Wondering

When the interference pattern occurs, is it because a single electron is interfering with itself as it splits and passes through the slits, or is it because the electron exists as a wave of potentials, and as the wave passes through the slits the two wave of potentials interfere with each other to make nodes and antinodes?

Thanks —The preceding unsigned comment was added by 128.189.235.186 (talk) 17:46, 26 February 2007 (UTC).

Technically, the electron can't split into anything smaller, so it's really all about probabilitiy waves. What you're really asking about gets down to wave-particle duality, and the answer is that the electron is both a particle and a wave, but not at the same time. So it's a probability wave when I'm not looking, and a particle when it hits the detector and I'm looking at it.

One way to think of it is to compare the electron to a person, who are usually indivisible when still alive. Before I go looking for him, a guy may have a 50-50 chance of being in either the living room or the dining room, but as soon as I turn on the lights and look for him, he's only in one or the other. But until I hit the lights I have no way of knowing, so his location is just a set of probabilities. That doesn't mean that until I hit the lights his left half is in the living room and the right in the dining room.

You might be interested in the Copenhagen interpretation and the many-worlds interpretation. — Laura Scudder ☎ 23:54, 26 February 2007 (UTC)

Notice that users Kaneda and users Scudder have opposite opinions about what they cannot see. That fact indicates the general truth that two reasonable people can infer different inter-phenomena from the same set of actual observations. We can get certain limited kinds of information about electrons and how they act under various circumstances, and we put different constructions on that information. To keep out of trouble we have to remember what the empirical information actually is, and what stories we weave around the information to make it easier for us to predict future instances.

That being said, the idea that a single electron is a something of small enough dimensions to get through one slit, but that it "splits and passes through the [two] slits" would seem to imply that somewhere between the cathode and the screen with the slits in it, the electron says, "Yikes, two slits up ahead! Gotta split!" Conceptualizing an electron as a smaller B-B gets us some useful explanations, but also gets us into trouble. Conceptualizing an electron as a wave is sometimes helpful, but also sometimes gets us into trouble. So we learn the lesson that our conceptualizations that are in terms of macro-world phenomena like bullets and ocean waves are not suited for describing phenomena on the atomic scale. P0M 01:01, 27 February 2007 (UTC)

sound

This doesn't work for sound waves does it? How do we calculate the fringes between constructive interference with a two source sound waves? Tourskin 06:20, 8 March 2007 (UTC)

I'm not sure what "this" refers to. Do you want to know whether sound waves have interference? That kind of interference definitly happens. It's even been put to commercial use recently in the noise cancelling headphones. They have a microphone that samples the sound that is going to hit your ears and they generate a sound that "mirrors" it so that when the outside sound is compressing the air the generated sound is rarefying the sound. It's like one little man was pushing on your eardrum with a stick and another little man was opposing the motion of the stick just enough that it can't move.

Piano tuners use sound interference to tune pianos. They hit two keys on the piano and count the "beats" (destructive interferences) per unit of time to judge the difference in frequency between the two notes.

Two sounds that are close in frequency can produce interference effects that are unpleasant to the human ear. Here is a sample:

http://www.wfu.edu/~moran/discord.html

In musicology the idea of harmony is very important. Sounds that go together harmoniously are ones that are related by a few simple ratios. Actually, there are different ways of coming up with more-or-less harmonious combinations. Some frequencies go together badly, and one bad result is the production of what is called a "wolf frequency," interference that produces very high intermittent "yelps" as though a wolf's high howl were coming in through the window somehow. P0M 22:14, 8 March 2007 (UTC)

I greatly appreciate your response. What I was wondering was, if we knew the wavelength of two identical sound sources, the distance to a "screen" and the first maxima, how could we accurately calculate teh next constructive fringes? Heres an image below of what I mean:

Tourskin 06:58, 9 March 2007 (UTC)

I'm getting ready for work right now so this will be quick. Your diagram gives you the answer. If you know the speed of sound you can calculate how long it takes each "puff" to reach the detector screen at different points. Then you would look for (solve your equations for) points on the wall where the time to wall of some "puff" was the same as the time to wall of some "puff" from the twin emitter, and points on the wall where a maxima from one speaker corresponded with a minima from the other speaker.

There is an animation in one of the footnotes where they show how it works with water waves. It's lots easier to think about something when you can easily visualize it. P0M 16:11, 9 March 2007 (UTC)

MOMMY!

Now I remember why I dropped out of quantum mechanics.

Until modern physicists can bridge the gap between human brain conceptualization and observed phenomena this stuff is going nowhere fast.

“Nobody understands quantum theory.” THANK YOU!

Can this experiment be reproduced with a piece of paper, a pin, and a light bulb? It would be interesting to see what would happen with 2 different light sources were separated by a solid object BEFORE HITTING THE SLITS. Can the light be of different wavelengths? White light? Don't throw any math at me, it's been 10 years since I was in school.

I've tried different ways of setting up the apparatus. The best way I've found is to use brads "↑" arranged this way |||↑↓↑||| and glued to plastic small plastic model railroad tracks -- with something at the two ends "|" to block light from those two ends. However, to the the experiment easily you would probably need to get two tiny lasers that could be positioned so that each one shoots through a single slit in a straight line. The brad heads give you automatic spacing. Be careful because the brads can reflect laser radiation right back into your eyes. You would not like the result if it burned holes into your retinas. (I'd love to find black brads.) P0M 15:52, 1 November 2007 (UTC)

I just tried to make another apparatus, and this time I'm having trouble. It's tricky to get the slit sizes and distances right and to get the laser aimed right, all while taking care not to look into the laser. More when I take another look at my original apparatus. Meanwhile, Sears, Optics, has a good set of photos of the patterns that are produced from one, two, three... slits. Being in a first rate physics department I guess he had some really nice apparatus to work with. The photos are beautiful. Even the shadow of an old-style double edged razor blade shows a beautiful diffraction pattern. P0M 03:48, 2 November 2007 (UTC)

Use the smallest diameter brads (1.25 inch x 18). Prepare a strip of printer paper 3/4 the length of the brad. Hang the strip from one brad like a bath towel draped over a clothes line. Position the other two above and below. Pinch the whole assemblage so they are arranged as compactly as possible. Put a drop of fast drying ("clamp for one minute") household glue (of the kind that contains acetone or some other volatile solvent)on each end. Let it get dry enough that things won't fall apart or get twisted awry when you let go. Remove the paper. My brads weren't totally straight so I had to use a mosquito hemostat to rotate them a little to even out the bulges. When it is thoroughly dry you can mount it in a frame of some kind. I used the last segment of plastic railroad track (tracks are separated about an inch) and the three brads just about filled the space between two of the railroad ties. Then I glued short strips of black plastic to block light on the two sides of the brads. I am going to try spray painting it black to reduce reflections from the nails that might otherwise hit my eyes. Plastic black electricians tape can be used to make a curtain over one or the other slit. My brads are not straight, so the slits are not perfect, but I get one kind of pattern with one slit open, and a different kind of pattern about four times the width with both slits open. P0M 18:04, 3 November 2007 (UTC)

As for two light bulbs, it won't work. LEDs are at least monochromatic, and a single LED won't work. Imagine that you could operate on an incandescent light bulb. You cut the filament in half and put in two new leads from the hot wire and the cold wire of the incoming AC. Have you changed anything? You would only change something if the two filament sections were producing coherent light. But incandescent lighting amounts to heating up lots of atoms or molecules and then having electrons getting kicked into higher orbit and falling to lower orbits in a very unregulated way. The molecules to the left, right, top, etc. don't have their radiation determined or affected by the one in the center -- or any other molecule for that matter. So the result is a pretty broad spectrum of radiation, i.e., all colors, all frequencies, all energies, however you want to look at it. Different frequencies of light go through an optical apparatus in different paths. That's why we get chromatic aberration. To get a clearly focused photographic image we have to take the "mush" out by designing multiple element lenses to compensate. So to the extent that any interference bands might be produced by light from an incandescent source the patterns would be mushed out by competing bands immediately adjacent to them. If you produce the apparatus I've described above, shine a flashlight or a LED through it and see what happens.

In the original experiment they used sunlight. The Sol is so distant from Earth that the wavefronts of light from that source are nearly perfectly straight. A pebble dropped into a pond makes a wave whose crest is obviously curved near that point, but the bigger it gets as it spreads the straighter any short segment of it looks. They directed sunlight through a pinhole, so the result was a "point" source of light that delivered flat wavefronts. There would still have been some slop in the resulting image due to chromatic aberration, but the experiment still worked well enough to understand what was going on.

If you wanted to do your experiment with two lasers, the best apparatus probably would put two tiny mirrors at a 45 degree angle to the screen with slits in it. It would probably be easier to get mirrors that close together than to get lasers that close -- unless you can get a hold of some of the newest lasers. I think those tiny lasers are mounted on surface-mounted chips. So you would have an apparatus made of parts around the size of the electronic components of digital watches. The laser light produced would not produce a very easily observed spot or bands on a screen. With mirrors you could get stronger beams of light. P0M 19:20, 3 November 2007 (UTC)

A long time ago I was on a tramp steamer in the Pacific with a newish transistor radio. Another passenger had a similar radio. We tried to tune to the same station and discovered that the instant we got the exact tuning both radios went dead. That was an inadvertent experiment in interference. (The radios apparently were both of the superheterodyne design. I forget the details, but that design re-radiates radio frequencies as part of the way they select and amplify the received signals.) The two radios were out of phase, so their external fields canceled each other. If, however, we detuned each radio slightly we could get the stations back -- because the signals were no longer sort of mirror images of each other. If we had tuned to two different stations playing tapes of the same program then the signals would have been out of agreement by some time lag. If we tuned to different stations we both had normal reception. I think that's the kind of thing you will find if you manage to use two independent sources of light. That's also the reason that putting something in one slit to detect the passage of a photon interrupts the interference phenomenon. It's a little like when two radio station engineers decide to play the same tape starting at 12:05 p.m. and they each watch their wrist watches and try to hit the buttons on the mark. They will most likely start the tapes at different times -- enough different that you would notice an "echo" if you had two radios receiving the two stations. P0M 16:23, 1 November 2007 (UTC)

Is this the direct result of the Heisenburg Uncertainty Principle? Close one slit, then you're measuring its position, and BLAM, only the momentum can change before hitting the optical plate. —Preceding unsigned comment added by 68.106.248.211 (talk) 02:07, 2 November 2007 (UTC)

I'm not sure what "this" you are talking about. I guess it's not the radios. The interference patterns do not need quantum theory to explain them. The unexpected result comes about when one tries to determine which slit a photon has gone through. Saying it that way implies that there is "a" discrete photon and that it has "a" linear path. That's one problem.

How does one measure the location of a photon? Does the unimpeded passage of a proton perturb any thing in any way? The ways I know about involve things like photographic plates, exposure meters in cameras, etc. They in turn involve the disappearance of the photon as it is absorbed in the electron cloud around some atom and boosts one of those electrons to a higher orbit. If there is a photon that is then emitted it must be because some electron has found a lower orbit and a photon has gone out from that transaction. So there has been a lapse of time, the photon is not the same photon, etc. So, to your point, the location x,y,z,t must be known to a very high degree. As far as the disappearance of the interference phenomenon goes, anything that indicates that a photon was seen in slit A seems certain to mean that it cannot have been at slit B. Since it is at one place or the other it cannot be both, and because it is not at both it cannot interfere with itself.

I think the problem is deeper than simple Heisenberg considerations, at least in the case of photons. But maybe one is making an assumption to say that an electron roving free between cathode and screen in a CRT has a single position at any one time or a single trajectory through which it reaches the screen. Experimenters have to interfere with whatever is going on to try to catch one, or catch sight of one, at any intermediate position, and their interference is not negligible.

The old wave way of modeling this phenomenon does not stand up very well to what we know about the delivery of energy by a photon. The old way says that something happens in the light emitter, and that starts the "aether" to vibrating. A spherical wavefront moves out from the emitter, contacts the screen, and then continues as some number of new spherical wavefronts that emergw from wherever the barrier wall is breached. If that were a true picture then one would expect that most of the energy of that wave would be absorbed by the barrier wall, and that the result of a single emission event would have one energy value if measured over the total surface of the barrier screen and a much smaller value (because most of the wave got left behind) at the detector screen. But we know that light travels as photons, that each photon has a certain quantum value of energy, that the frequency of the photon is related to the energy carried by the photon, etc. We also see that the light that hits the detector screen has not changed color, which implies that photon by photon energy either reaches the detector screen or it doesn't. Yet there is something that spreads out so that the "photon" can be "at" slit A and slit B, and yet the photon that we measure at the detector screen has not lost energy (changed color). And although the path was at least broad enough to encompass both slits, the photon that hits the detector screen does so at a single, narrowly limited, area. We see it as a scintillation at one point, a grain of silver affected in a photographic emulsion, etc.

The discussions I have seen about "detecting a photon in one or the other slit" never seem to be specific about how the experimenters have done this job. It would be illuminating to learn about that part of it.

P0M 03:48, 2 November 2007 (UTC)

Questions about the double slit experiment

Let's say that someone does the double slit experiment as follows:

The particle emitter being used is a photon gun. Each time a button is pressed, one and only one green photon is fired. The gun is aimed in such a way that the green photon goes through the middle of the left hand slit and strikes the detector screen at exactly the same spot every time. Since the gun is aimed through the middle of the left hand slit, what difference does it make if the right hand slit is open or closed? The width of the slit is much greater than the wavelength of the photon. All that happens is that you end up with many hits at a single point on the detector screen.

Please let me know what is wrong with this way of thinking about the double slit experiment.

By the way, what is the usual width of the slits in this experiment? Any slit visible to the naked eye would have to have a width many times the wavelength of any visible light photon. n5i124 Honeychurch 09:13, 30 August 2007 (UTC) ____________________________________________________________________ —Preceding unsigned comment added by Honeychurch (talk • contribs) 09:10, August 30, 2007 (UTC)

You have an interesting idea, but how are you going to implement it? Experiments deal with the real world, and in the real world the way you tell where your laser (our best "particle emitter" ) is aimed is to see where a photon lands when you fire the laser. If you can throttle the laser down to one photon at a time somehow, and close off one slit, you might think that you could successfully aim the the laser so that each photon would travel in a straight line directly through the open slit. But you can see the result of shooting photons through one slit in the top photo of the article. You get a diffraction pattern. That means that photons "spread out" when they go through a narrow slit. (Actually, they spread out when they go past any edge.) The fact that they and/(or their probability waves) spread out is what makes the inteference pattern possible when there are two slits open.

The width of the slit has to be wider than the light that is trying to get through it. If I remember correctly, the reason that you can see into your microwave oven but the microwaves cannot come out and cook you is that the holes in the metal screen covering the glass oven door are large enough to let visible frequencies of light through but not big enough to let microwaves (which, despite their name, are bigger than light waves) get through.

The original experiment was done with the equivalent of a double-blade razor blade stuck into the beam of sunlight coming through a pinhole. The widths of the "slits" were huge. It was the narrowness of the beam of light (coming through a pinhole) that limited the effective width of the two beams of light going beyond the thin partition.

Now here is the trick. (See the illustrations near the top of the article.) The narrower the slit (down to the width of the photon, anyway) the wider the spread beyond the slit. (You could work this out with a compass.) All that matters is that there be two beams (i.e., that you've split one beam) and that they start out from would would ideally be a single point (rather than a wide band). I.e., It's better to have just one photon getting through. Two photons going through side by side would make a close approximation to a single point of origin. A slit an inch wide would make a pretty close approximation to no slit at all (no spread that you could easily notice). Somewhere in the middle is the kind of situation you suggest, where you have a slit that might let several photons through "side by side."

The slits do not interact with the light that goes through the slits. All the slits do is lop off the sides of a wider beam. What is required is a narrow beam. With a narrow beam you get a diffraction pattern that is pretty clear. With a wide beam you get light that looks almost perfectly like light that had not gone through any slit at all. And that's all there is to it. It sounds weird. But check out the discussion given by de Broglie in The Revolution in Physics. If you can get the book, just look up "interference" in the index. Hu;ygens had a strange way of figuring out how beams of light would spread, how beams of light would act passing through lenses and other optical devices. His ideas were based on a vast oversimplification of what is going on, but he got such good predictions that they are still good today. (At least that was still the way they were teaching phyhsics when I was a physics major in the university.) It does not describe a "reality," but it is a model that is good enough for Nikon and Leica to make expensive optical equipment with. If you tqake his one basic idea and just draw a wave front, make little circles on lots of points on a wave front, and then connect the resulting little circles by drawing their tangents you will get an exact representation of where the wave front will be found later on. And you next do the same thing with the second wavefront. That's it. Simple, and in the time of Huygens there probably wasn't any real explanation for why it should work.l But if you look at what happens when a flat wavefront passes through a slit, and start drawing your little circles across that wavefront, then pretty soon you will have a curved wavefront, and each succeeding "generation" will more closely approximate to a circle.

It's really quite difficult to think about the behavior of light, or of photons to be more specific, We've never seen light. All we get through our senses are various effects. We light a candle and rods and cones in our retinas fire off as a result. We stick our fingers in the flame and pain sensors fire off. We compare light to water waves, but on that analogy the energy of a single wave passing through a slit should fan out and impact the entire target wall the way a water wave will fan out from a narrow opening in a sea wall and hit all along a distant shore. But it doesn't work that way. A single photon goes through the slit and a single photon hits the detector wall somewhere. A photon, if it is a particle, might be imagined to be spherical, but light is polarized and a harrow grate can pass or block light depending on its polarity.

So, point by point:

The particle emitter being used is a photon gun.

A laser is the closest thing I know of to a "photon gun." It's more like one of those multiple rocket firers or like a fighter that has multiple guns that all fire together when the trigger is operated. Real-world lasers operate more like shotguns than rifles.

Each time a button is pressed, one and only one green photon is fired. 

It's more like the guy who has a flock of hundreds of pigeons and he would like to get them to fly off one by one. Probably the best he can do is to put up lots of hets that trap most of the pigeons and then hope he can calibrate his nets well enough so that he can figure that if 24 pigeons make it out in a day then most of the time they have made it out individually.

The gun is aimed in such a way that the green photon goes through the middle of the left hand slit and strikes the detector screen at exactly the same spot every time. Bullets go straight. If you try to shoot bullets near the edge of a building or through narrow slits in a brick wall, then they either to past the edge of the wall, through one of the narrow slits, or hit the wall. But light "fans out" whenever it goes through a slit or goes by an edge.

Since the gun is aimed through the middle of the left hand slit, what difference does it make if the right hand slit is open or closed? 

Photons are not like bullets because bullets do not have a "wave nature." The wave, maybe I should call it the probability distribution "wave", is much broader than the single slit or even the double slits. Where the "particle" will be when the slit wall is reached is a matter of probability. If the laser happens to be aimed at the center point between the slits, then the probability that it will ge through the left slit is equal to the probability that it will go through the right slit. If it goes through, then it goes through. But you can't tell which slit it went through because the probabilities are equal. (Yeah, I know that doesn't make sense in the macro world, but it is shown true by experiments when time rather than space is involved as the "separater.") And it makes a difference when the right slit is open because then the wave goes through the right slit "as much as" it goes through the left slit. And that makes a big difference because then you actually get interference.

The width of the slit is much greater than the wavelength of the photon. All that happens is that you end up with many hits at a single point on the detector screen. All that happens is that you don't get that kind of result at all

If we were dealing with bullets being fired through a single lit several times the diameter of a bullet, you would get a band of bullet holes on the target wall because guns all have their "spreads." You pay big bucks more for a gun that shoots a one inch pattern instead of a five inch pattern. Robin Hood could shoot an arrow into a target and then shoot another arrow that would split the first arrow right down the middle. But even he couldn't do it consistently. And his skill was legendary. But photons are not bullets and if two slits are open the "bullet" (or something about the bullet that determines something about where it hits) goes through both holes. The bullet analogy breaks down. Remember that we start with the real world where interference occurs (see the new photos in the article. I made those photos and even though common sense says it should not happen, that is the same kind of results that people ahve been getting since 1801. So we start with that reality and then try to make models to explain it, talking about wavy bullets, wavicles, or waves that behave more like sawed off solitons. —Preceding unsigned comment added by Patrick0Moran (talk • contribs) 08:24, 12 November 2007 (UTC)

Single slit diffraction

I have a problem with this statement in the article "When only one slit is open, the pattern on the plate is simply a dark line where all the photons accumulated off to one side of the center, as if bullets had been fired at the plate. This suggests that light is made up of photons and that photons are particles. " This is not true because a single slit produces a diffraction pattern having a central peak with a series of smaller peaks on each side.Harold14370 12:09, 25 October 2007 (UTC)

I think you are correct in this criticism. If one believes that light is "corpuscular" (i.e., comes in discrete particles), then one would predict that light shining through one slit would end up in a single narrow band on the target screen. Since that is not what happens, since a diffraction pattern is created, some other explanation has to be given, and modeling light as a wave phenomenon gives satisfactory results.

So why is the double slit experiment important? If I remember correctly, the interference effect was first noticed when someone put a thin vertical strip down the center of a beam of light coming from a very remote source of light. (The remoteness of the light source ensured that the wave fronts were almost perfectly parallel to the target screen.) An interference pattern was noticed. Blocking half the incoming beam with a wide verticle strip would have produced a diffraction pattern, but it would not have produced an interference pattern. So the narrow vertical strip could be expected to produce one diffraction due to its left edge and another diffraction pattern due to its right edge.

If one accepts the idea that light is a wave phenomenon, i.e., if one operates on a wave model of light, then the interference produced when one wave coming from the light emitter is split into two waves when part of the original beam passes through the left slit and part of it passes through the right slit is predictable and certainly not remarkable.

Interest in the double slit experiment picks up when it is observed that anything done to determine whether light has passed through one slit and/or the other will destroy the interference pattern. Even more interest is garnered when it is discovered that a single "particle" of light, i.e., a single photon, is permitted to be emitted at any given time, and yet the same interference pattern is eventually observed as more and more single emissions occur.

Our macro-world thinking makes this result seem particularly spooky since we are accustomed to being able to use measuring devices on macro objects that do not disturb their behavior. For instance, if one were to construct a double-slit apparatus to produce interference between waves of water it would not affect the experiment to train a motion picture camera on the water waves as they pass through the two slits.

Making something happen in a way that leaves a trace demands the transfer of energy. The energy transferred to the water waves, is reflected, and then produces a physical result in the camera is small enough to be negligible in any case, and is affecting the water waves whether there is a camera in operation or not. But the energy used to activate a tell-tale of some kind in one or both slits in the light experiment comes from the incoming photon itself. That seems to be the crux of the matter.

The article probably needs to be modified to speak of the expectations of those who think of light as a particle phenomenon. It might be worthwhile to prepare a photograph of what actually is observed when a laser is shown through a single slit.

I don't have time to look now. I will try for a photo soon.

P0M 09:37, 27 October 2007 (UTC)

The Diffraction article has a good photo. It is, however, much clearer than the kind of results I can get with a couple of razor blades for a gate and a laser level beam for the light, which makes me think that people observing single-slit transmission could account for the visible results in a couple of ways that would be consistent with crude experimental results. I'll try to get a photo posted. I'm not arguing that the article as written is currently accurate. I'm just saying that if one were to shine a dimmer light than what I have, one produced by sunlight coming through a pinhole, what one would see after the light had passed through a narrow vertical gap would be a band that is wider than the gap through which the light shines. The dimmer side bands might well not be visible, and if they were visible one might imagine that some of the light particles had struck the narrow surface in the general direction of the particles' flight, and so had bounced through the gap rather than going straight through without touching. If one were shooting machine gun bullets at a wall and some went through a narrow slit in the wall you would also get a band of bullet holes wider than the slit on whatever was standing behind that slit. The occasional glancing transit might even produce some double bounces somehow. But the article should probably just reflect the best experimental results, not the possible misunderstandings.

I am now wondering whether it is interaction with the physical apparatus at the gap that produces the effect or whether the effect is rather a function of the limited range of paths possible for the photons that make it through the screen. The latter, I think, but I can't remember having read any direct reference to this question. In predicting the path that light waves will take, one calculates the expansion of a wave front by imagining that each point on the wave front at any one time is the source of emission of a spherical wave front. That idea was presented as a mathematical fiction that got good results, but it must reflect a deeper, a quantum physics, theoretical consequence. Photons leave a laser and have a high probability of striking a remote screen in a spot that is about the size of the exit port of the laser and in a straight line out from the long axis of the laser. If, somewhere in the middle, there is a screen with a slit in it that is narrower than the wave, the photon will not go through. (For instance, visible light gets in and out of a microwave through the small holes in the metal screen covering the glass door, but the microwaves do not escape.) If the slit is wider than the wave, the photons will get through if they do not intersect the edges of the slit. But what does that really mean? The "position" of a photon is a matter of probability. On the way out of the laser the photon might have "materialized" at an appreciable distance away from the straight line path that we assume it should take. But if it gets through the slit then it had to have surrendered its potentiality to impact the screen, no? So, for a very brief time, it has a very narrow range of possible locations. From that time on, however, the probabilities of future locations are to be computed from that starting spot. And if it eventually hits the target wall, we know only that it had to have had a position at some time in the "canyon" of one or the other slit. But either of the two positions is equally valid. So we can calculate the probabilities of its striking the wall at various points when going through slit A, and we can calculate the probabilities of its striking the wall at various points when going through slit B. And, mirabile dictu, it turns out we can calculate the combinations of those probabilities, and those calculations are the ones that accurately predict the striking points of a rain of photons.

I guess I shouldn't be thinking out loud here. If anybody objects I will copy it off to somewhere appropriate, but I don't want to lose it just yet. If anybody sees errors in what I have written, please enlighten me. We start out playing with model airplanes and forget that they are not real, and end up playing with models of physical reality and believing that they are real. P0M 07:03, 29 October 2007 (UTC)

Out of time for the night, but I just found what I was thinking about yesterday, a reference to "Huygens' principle": "Every point of a wave front may be considered as the source of small 'secondary' wavelets, which spread out in all directions from their centers with a velocity equal to the velocity of propagation of the wave. The new wave front is then found by constructing a surface tangent to the secondary wavelets, or, as it is called, the envelope of the wavelets." (Francis Weston Sears, Optics, p. 5) Note that this discussion follows a traditional wave model of light. A light source acts in a way analogous to a rising and falling bobber in a pond, sending out ripple after ripple as long as it keeps moving up and down. A "wave of light" is analogous to a wave spreading out from a pebble dropped in a body of water. That wave theory has nothing to say about photons or quantum effects, but since it very accurately predicts what light will do as a beam is shined through a lens (for instance) any superior theory is going to produce results that agree with the experimental results that the more primitive theory has already gotten right.

What I am thinking of is that a "wave front" would move out from the laser toward the wall with one or more slits in it. Everything being equal in the propagation department, the wave front would be spherical until it reaches the wall. At places where a slit is found, the wave front, what is left of it, would move through the wall and on toward the more distant detector wall. But it would from that point on behave as though there were two lasers, one in each slit, that would then each emit a spherical wave front, but the two spherical wave fronts would have different centers. There would be a difference, however, because of the difficulty of getting two separate lasers to flex their muscles in unison (so to speak). If parts of two wave fronts reach the same point of the target wall they can interfere only if they get there at exactly the same time. If we are dealing with the same wavefront that gets divided at the barrier wall, the two remainders of the original wavefront will still be moving together. So at some points on the target wall two maxima may arrive at the same place and time, and at other points on the target wall a maximum and a minimum may arrive at the same place and time, and the overall result will be an interference pattern.

The original observation of the double slit experiment was that there is such an interference pattern, and the conclusion was that light must have something about it that is described, however crudely, by Huygens' principle, i.e., in some sense light is a "wave" phenomenon. Imagining that light is corpuscular (or of a particle nature) does not permit the explanation of the interference pattern. It also does not explain the appearance of a single slit's diffraction pattern. Later on, when it became possible to get experimental equipment that could send out single photons or single electrons, people were surprised to discover that a single one of either kins would interfere with itself, and that running the experiment with a large number of single emissions would produce a clearly visible interference pattern. It seems counter-intuitive to people that "observing" the passage of the electron or photon through one or the other slit would destroy the interference pattern. But anything that anybody has done to detect passage has involved causing the photon to be absorbed, kick an electron into higher orbit, and then the electron may fall to a lower orbit causing the emission of a new photon, one that goes on to hit the detector wall. But that photon (or its "wave front") does not itself pass through two slits, so there is no interference. If that much is correct then this experiment is another one that should cause us to doubt the suitability of our macro ideas of simple location to quantum realm interactions. The impact event is "particulate" but the passage process is of the nature of an expanding spherical (and continuous) "wave front." P0M 08:04, 1 November 2007 (UTC)