# The Double Slit Defraction Experiment

Discussion in 'Physics & Math' started by Erring Flatley, Apr 18, 2004.

1. ### Erring FlatleyErring FlatleyRegistered Senior Member

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The Double Slit Diffraction Experiment

Those of you who took physics in school remember the double slit diffraction experiment. It is an obvious infraction of the law of cause and effect for a single photon to pass through two slits at once. So there must be at least two photons passing through the two slits at once in order for the diffraction pattern to appear. We may ask, is the source of light really producing single photons or is it producing single packages of photons? Obviously it is the latter. This would indicate that light produced by a chaotic source such as the sun or incandescent bulb, is really semi-coherent. And of course it must be for us to see it. If it were truely noncoherent all the waves would cancel each other out statistically and we would not be able to see it at all. So the answer to the situation is that the source of light with its filters are producing packages of photons which then are broken down into smaller packages at the first slit by diffraction and then after some of the smaller packages pass through the second two slits they interfer on the photographic plate producing the diffraction pattern.

Last edited: May 27, 2004

3. ### RedroverRegistered Senior Member

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I don't find that really obvious. From what I remember, photons are produced when an electron loses a quantum of energy. The chance that a whole bunch of electrons releases their energy at exactly the same time is pretty small, so wouldn't light be a continual stream of photons instead of "packages"?

If the light was noncoherent, wouldn't the intensity of the light be an average of the peacks and the valleys of the wave, effectively giving us a light of constant instensity?

So if we set up a long enought series of slits, we could arrive at a point where the "package" of photons was split up so many times that they contain only one photon, right? So where does that photon go if we put it infront of the two slits?

5. ### James RJust this guy, you know?Staff Member

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Since the energy of the light emitted is measurable, and the energy of each photon is known, it is easy to determine if single photons or groups of photons are emitted from a source. The diffraction experiments done with single photons still produce interference patterns as predicted by quantum theory.

Also, the experiment can be (and has been) done with electrons or atoms instead of photons, and the results are the same.

7. ### PhysMachineMALLEUS SCIENTIARUMRegistered Senior Member

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The flaw in your arguement is that you automatically assume that the photons are particles that have to obey the same physical principles as marbles. You would be correct if this were a classical situation, but it's obviously not. The reason this diffraction pattern happens is that if you just look at the diffraction pattern, you can't know which hole the photon passed through, so it effectively passes through both and you get that diffraction pattern. If you try to observe which hole the photon passes through, you get the diffraction pattern of a particle coming through that one hole. This is one of the more peculiar things about quantum mechanics, is that if you attempt to measure one thing you can't have any knowledge about something else at the same time.

Why does it do this? Well, there's the whole wave equation thing, that the photon is in fact a wave that is incumbent upon the barrier, but really nobody knows all that much why matter-waves exist, or at least I've never heard an accepted reason for it.

8. ### Erring FlatleyErring FlatleyRegistered Senior Member

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Photons are produced ("relaeased their energy") all at once most of the time. A clearcut example is the laser, which operates on the principle. What is happening on the sun is much more like a laser than most realize.

In your example of repeated slits until only one photon from a package remains, The last remaining photon would go through one slit, the other slit, or hit the slit partition.

9. ### Erring FlatleyErring FlatleyRegistered Senior Member

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May I have a citation of an experiment from you, doing the experiment with a single photon emiter? Or, with a single electron emiter? Or, a description of an experiment you have done yourself showing the result? I know of no known such experiment. Many people assume that a bright source of light such as the sun, or a lamp, or a flame filtered with a carbon black filter produces single photons, but they do not. Such an apparatus produces single packages of photons. A package of photons consists of a single wave-particle of high amplitude. A filter or slit will only break the package down to smaller packages of lesser amplitude, not to single photons.

10. ### Erring FlatleyErring FlatleyRegistered Senior Member

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I assert that the photon is a wave-particle. A wave-particle is a three dimentional array of electromagnetic value. It is centered about a single point. That gives it a particle property. But it is distributed about the point like a wave. That gives it wave properties. Imagine an out of focus marble. It does not have a defined edge but fades in value the greater the radius from the central point.

You are making reference to Heisenberg's uncertainty principle. Heisenberg created the uncertainty principle to explain the double slit defraction experiment. Einstein objected to this and so do I. The uncertainty principle defies the law of cause and effect. One photon cannot pass through two holes at the same time. You said, "nobody knows...why...". Einstein knew uncertainty was wrong but the world sided with the uncertainty principle and said, "nobody knows...why...". (At this point I must ask, may I logically criticize publicly accepted theory? And may I have it not be considered an attack upon a person associated with the theory?) The reason why is that the light source is putting single packages of photons through the slits, not putting single photons through. The uncertainty principle is wrong. It does not explain the experiment.

11. ### James RJust this guy, you know?Staff Member

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38,035
The uncertainty principle is an inevitable consequence of wave mechanics. You cannot have photon "wave-particles" and not have the uncertainty principle. If you want to throw away the uncertainty principle, you will need to come up with an alternate explanation of quantum tunnelling and radioactive decay, while you're at it.

12. ### PhysMachineMALLEUS SCIENTIARUMRegistered Senior Member

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http://physicsweb.org/article/world/15/9/1

This is a link to an article discussing the first double slit experiment. Read all you want, and if it doesn't satisfy you feel free to research more yourself.

As for the uncertainty principle, it's deriveable a multitude of ways, from Fourier series analysis to the non-commutation of linear operators, but the results end up the same. If you don't like it, you'd have to completely reformulate quantum mechanics, which considering its success is a rather wasteful thing to do.

The fact is, nobody knows why the particle-wave duality exists, but saying the Einstein opposed it is not sufficient to sweep aside the theory. Experimental results have supported quantum mechanics time and again, and considering how odd it is, it's probably the most scrutinized theory ever. People have won Nobel prizes trying to find a point where the theory is wrong, and failed. Just because you have trouble conceptualizing the results in a classical mindset does not make the theory wrong. In fact, the theory does return classical results under classical circumstances, so it is also consistent with experience.

The theory matches experiment on its own level, and when taken to the proper limits duplicates everyday experience. That's a very good theory, and to brush it aside is not something to be done lightly.

I'm just curious as to your actual amount of background in quantum mechanics, as in books on the subject etc. Erring Flatley

13. ### OmnignostRegistered Senior Member

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I just have a little comment on your way of looking at light

You say that if a light source was truly non-coherent we wouldn't see it at all. I think you should think a little extra moment on this. Assume that we have a number of excited atoms of different kinds and energy releasing their photons in random order. This would give an oscillating electromagnetic field whith an amplitude varying like noise. It would, indeed be fainter than coherent light but not dramatically. It would simply be what we call white light.

14. ### Erring FlatleyErring FlatleyRegistered Senior Member

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Quantum mechanics is founded upon wave mechanics. Wave mechanics was introduced (Schrodinger, 1926) before the uncertainty principle. Wave and quantum mechanics will go on without the uncertainty principle. Quantum mechanics acquiesces to the uncertainty principle; quantum mechanics is not founded on the uncertainty principle. Quantum mechanics is successful in spite of the uncertainty principle, not because of it.

15. ### Erring FlatleyErring FlatleyRegistered Senior Member

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Let us assume that each atom in a source of light such as the sun is producing a steady output of light as a sign wave. As individual photon waves they would all be of the same amplitude, differing only in wavelength and equally energy. We may simulate this occurence on a computer. Let us have the computer generate a sine wave of random wavelength to represent one atom. Then let it generate another random sine wave and another, and another and have the computer add them all together. At first the result would be a wave which had as its Fourier transformation the sine waves put in. And it would have the appearance of white noise. But, as time passed the resulting wave would become flatter and flatter. The more sine waves added, the more they would average out and the flatter the sumation wave would become. Its totaled energy would approach zero and we would not be able to see it if it were a light wave. Another way to think of it: Imagine a computer program to randomly select whole numbers between -101 and +101, and average them. At first the average would jump greatly. But with time the average would approach zero. Only by adding a coherency factor would the first program produce a wave that deviates from zero and the second an average that deviates from zero. The fact of the matter is when light is produced in the sun, a phenominon like that in a laser is occuring between many atoms. The light from the sun can be thought of as a sum of the light from many, many lasers. And all these lasers tend to be activated in coherency together, and this prevents the photon waves from cancelling each other out. A laser might be compared to a "soloist", whereas the sun might be considered as a "choir" responding as a multitude of lasers together in a harmony.

16. ### RedroverRegistered Senior Member

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Now for the 64,000\$ questions: If we arrive at the point where we have one remaining photon hitting the slit, could we predict which slit the photon would go through, knowing the inicial characteristics (direction, energy, etc) of the photon?

17. ### Erring FlatleyErring FlatleyRegistered Senior Member

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There is equal probability that the photon would go through either of the double slits if the slits are identical. Though more likely it would hit the slit partition. If a person knew all the initial conditions (an imposibility for a mortal), the path of the photon could be predicted exactly.

18. ### 1100fBannedRegistered Senior Member

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The uncertainty principle is a direct result of the postulates of quantum mechanics. Quantum mechanics cannot go on without the uncertainty principle.

Furthermore, wave mechancs is just one representation of quantum mechanics.

19. ### OmnignostRegistered Senior Member

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Regarding coherency of thermal radiation. You are, of course, correct in that the average of a random function will aproach zero, eventually. It is just not relevant. Try taking the absolute value of your random function and you will see that the amplitude doesn't approach zero. This means that for each short time period there will be white noise and it will go on as long as the sun shines. The Fourier transfor may be a little devious when looking at these things. Try a wavelet transform. It is a nice way of getting a snapshot image of the frequency contents of an ongoing signal.

20. ### Erring FlatleyErring FlatleyRegistered Senior Member

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Heisenberg proposed the uncertainty principle in 1927, years before quantum mechanics formed. Schrodinger had proposed his wave equations in 1926. Quantum mechanics is based upon the wave equations and will not disappear for lack of an uncertainty principle.

21. ### Erring FlatleyErring FlatleyRegistered Senior Member

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When photons 180 degrees out of phase superimpose they cancel each other out. They do not take absolute value. If there was absolutely no coherency to sunlight, the photons would almost all cancel each other out and the sun would be almost invisible.

22. ### PhysMachineMALLEUS SCIENTIARUMRegistered Senior Member

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Focusing more on the uncertainty principle:

What you're missing is the uncertainty principle is a consequence of wave mechanics (or matrix mechanics, as you will), not an addendum to them. It's like arguing that you don't need to conservation of charge law to have Maxwell's equations, but you can derive it from them. It's a logical consequence of the laws, not an unrelated addition. In fact, here is a derivation of the uncertainty principle. It's not the most well-explained derivation (for that I recommend Shankar or Sakurai, but really any quantum mechanics book worth its salt will derive the uncertainty relations from first principles), but it shows that you can get it from the basic definition of an expectation value and a state vector. I have yet to see it derived using Fourier analysis (as Heisenberg derived it) but I do know there is a similar thing for classical waves, and if you dig you could find it yourself. You could probably find an English translation of Heisenberg's paper if you look enough.

Anyways, I diverge from the point. You're attempting to tear down a well-established (though admittedly difficult to understand) principle of physics because you have trouble accepting it, even though it flows logically from the part of the theory that you do accept.

23. ### Erring FlatleyErring FlatleyRegistered Senior Member

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I am not attempting to tear down the work of the last century. I am asserting that if the double slit defraction experiment were done with a light source that was designed specifically to produce single photons (or single electrons) that the defraction pattern would not form. (All published works use beams of light or electrons and then filter them to low levels.) There is no published experiment I could find that does this and for the sake of completeness it ought to be done. Doing the experiment with undeniably single photons would lay all questions to rest. I am forced to be a theoretician. I put the idea forward for an experimentallist. (And it would make a nice paper for anyone.)
It is true that it would force the modification of the uncertainty principle, but I am not willing to dump the law of cause and effect for the uncertainty principle.