# Double slit experiment

Discussion in 'Physics & Math' started by Xmo1, Nov 15, 2016.

1. ### Xmo1Registered Senior Member

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I think the interpretation of the double slit experiment is wrong. If you aim your photon or electron at the center between the two slits it will go someplace (roll toward a slit lets say). In doing so it will leave part of itself (maybe just a charge) as a trail, and when it goes over the edge it will again leave part of itself (a bump) glued to the edge. The next time it squiggles along the same trail, it will hit that bump and bounce, but still fall through a slit without touching the inner edges of the slit. After doing that a few times it will hit the inner and opposite wall of the slit, and again bounce maybe toward some outer edge of the receptor.

It hits the wall (the receptor), not as a wave, but still as a particle. That is, the individual photons or electrons are hitting the wall. Otherwise, if a wave were hitting the receptor - the whole wave structure would be hitting the wall at nearly the same time. You would detect the whole wave hitting at once rather than the particle. Your slits would need to be thinner (front to back) than the particle to minimize the bouncing effect. I'll bet if you weighed the particle at start, and then at the wall you would find that it was lighter at the wall receptor due to leaving the trail and bump on the slit material.

3. ### Confused2Registered Senior Member

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Here is some information about the double slit experiment:-
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html
Notice particularly that the peaks (brightest bits) are found (approximately) where the wavelength * (Distance to slits)/(distance between slits) is a whole number.
Note that the material and the thickness of the material the slits are cut from has no effect - it can reasonably be assumed that any photons that don't actually strike the slits are either reflected or absorbed and play no part in the result.
Photons can't lose mass because they don't have any mass to lose. What they could do is lose energy which would cause them to change frequency. It is observed that the photons (in this experiment) don't change frequency therefore they have not lost any energy. You can probably imagine that as a wave spreads out it loses energy - this doesn't happen with light (photons) - the energy of each photon stays the same but there are less of them as you get further from the source.

Last edited: Nov 15, 2016

5. ### Xmo1Registered Senior Member

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If you are firing a stream of anything the stream is going to act as a fluid, and in accordance with conservation laws (axioms of fluid dynamics). To be accurate the particles must be fired singularly. If then you get an interference pattern, then my statement is that the interference is generated by the interaction between the particle and the slit screen. Another respondent to my inquiry said that 'the laser is fired at both slits.' If so, then my statement is that the experiment is flawed. If you pay attention to the slit screen and the slits, and do the experiment properly you won't get an interference pattern. Hence, no (probability) wave. I have no evidence to support my statements.

I have a problem when the physics community makes irrational 'discoveries.' I'm skeptical and ignorant, so I encourage people to educate me. 25 dimensions is about 20 too many for me, and multiverse is poof-candy.

Last edited: Nov 16, 2016

7. ### originTrump is the best argument against a democracy.Valued Senior Member

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That is a big issue. There is a massive amount of evidence that DOES support the interpretation that you dismiss!
To change the mind of scientists you need to have compelling evidence that your concept BETTER fits observations than the current interpretation. Since you have admitted you do not have the evidence your idea is just and idea that cannot go anywhere.
Another issue is that you must make up new physics to support your idea - photon 'trails'. There is no evidence of these trails and I see no way they could exist with out violating the conservation of energy.

Last edited: Nov 16, 2016
8. ### exchemistValued Senior Member

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The photons are sent individually in this experiment - that's the whole point. They are sent one by one, but an interference pattern nevertheless appears, over time, as the discrete points of arrival of each photon build up into a pattern. And that pattern is what you would get if waves were diffracting through the slits. So the "probability wave" of a single photon appears to be passing through both slits and interfering with itself, even though the photon can then only be detected at one precise spot.

You are right that the interference is created by interaction between the slits and the photon. Of course it is: no slits, no pattern. But the pattern observed is consistent with that interaction being wavelike diffraction and interference. If you think the experiment must be flawed, all we can tell you is that it has been repeated many times and it gives consistent results, results which are exactly in accordance with the predictions of quantum mechanics, counterintuitive though they certainly are. So you have to understand that, your personal scepticism notwithstanding, people who know what they are talking about are pretty convinced by it.

As a chemist, I am fairly used to thinking of electrons as wavelike, because that is how we model chemical bonding. The way I picture the dual slit experiment is that the photons are waves but, being quantum entities they can only interact - e.g. with a detector - in whole units, i.e. in quanta. I don't pretend this is remotely rigorous, but it helps me to visualise it. At the atomic scale, there seems little doubt that matter behaves in ways that are remote from everyday human experience. As a scientist one has to accept what the observations are telling you, once you have had them corroborated reproducibly. And if that does not feel "normal", tough, it seems to be the way the universe works, so get used to it. In fact this is what makes physical science interesting to many of us.

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9. ### Write4UValued Senior Member

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I understand that photons propagate as a *probability wave function*.

Question: If this is correct, why should we expect the photons to strike the exact same spot each time?
Is that theoretically even possible?

IMO, while each photon behaves as a probabilistic wave, I can visualize the photons striking the plate (behind the slits) at different probabilistic targets, but as they also behave as a wave function, the result will still show a wavelike interference pattern (the striped lines).

10. ### Xmo1Registered Senior Member

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386
I'm not trying to change the mind of anyone. I'm putting forth an idea of my own. If you are shooting center mass between the two slits: The element you are shooting, will behave to spread out, to bounce, or to roll. There will be no 'magical' behavior that splits the element so that it can behave as a wave. You say that you are shooting a single quantized particle. If not you might just as well be shooting a stream of water, rather than a 'bullet.' The pressure going through the slits will naturally produce interference patterns.

Basically, I'm saying that the experiment should exclude the possibility that a stream is being shot at the screen, and also that the element itself is not wide enough to cover both slits. If that single element (a photon or electron or buckyball or whatever) then produces a wave pattern on the receiving screen all at once and around the same time then I would say yes there is evidence of the particle acting as a wave. So far, I haven't seen an experiment that uses these constraints. Rather, they use lasers and other devices that produce a stream. So yes, as I said a stream of any type acts according to the laws of conservation, which produce interference patterns.

I might be seeing a dispersion of energy - like the photon going through a coke bottle. That is just saying that the particle is energetic, not that the particle itself is energy. It is the coke bottle that is interacting with the energy. If it were not there - there would be no reflections. Maybe the energy itself has a wave function that produces the probability of position of the particle. That's like saying that somewhere on the spider web there may be a spider. It doesn't mean that the spider and the web are the same thing.

There is an explanation here http://www.physlink.com/education/askexperts/ae444.cfm that explains that a single photon (itself) has a (big ? mark here for me) wave function that produces a diffraction pattern, but it is invisible until you shoot more photons (information is cumulative?). So that's about where I'm at. I'm still skeptical and ignorant.

Last edited: Nov 17, 2016
11. ### Xmo1Registered Senior Member

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What came first - the experiment or quantum mechanics? I'm far from understanding physics, but I do understand some of it. I've made a response (to origin) below (or above - don't know where its going to land) that continues my thought. BTW, awhile ago this experiment was said to indicate that light acted as both a particle and a wave. Now probability has entered the explanation. Seems the explanation as to what the experiment reveals has evolved.

Last edited: Nov 17, 2016
12. ### exchemistValued Senior Member

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QM came first all right, by about 70 years. It is only very recently, with the development of very sensitive detectors, that it has become possible to detect the arrival of individual photons. The experiment was initially just a "thought experiment", intended to illustrate how counterintuitive some predictions of QM would be. It was quite a sensation when it was finally done for real - and agreed exactly with prediction!

Actually, on the probability issue, I think you touch on quite a subtle point. As far as matter "waves" * are concerned, for example the wavelike properties of an electron, the "wave" is a wave of a sort of square root of a probability density. (Mathematically, the probability of finding the electron in a given volume is given by multiplying the wavefunction by its complex conjugate - the equivalent of squaring it - and then integrating over the volume of space of interest.) With light however, the wave is a true wave in the electromagnetic field. Nevertheless, the behaviour of this wave too, in the case of individual photons, corresponds to a probability of detecting the photon at point in space. It appears to me that this probability property that both these type of wave represent, in spite of their different nature, is something that is often rather glossed over. So thanks for highlighting it.

Probability is in fact fundamental to the quantum mechanical model of light and matter. When one learns quantum theory, the concept of "probability density" , as represented by the square modulus of a wave function, is one of the first ideas one encounters. I suspect that if probability was not mentioned in earlier explanations of the double slit experiment that you have across, it would be because it was assumed that the reader would already be aware of this.

* I put waves in quotation marks because the general form of Schroedinger's equation is strictly speaking a diffusion equation rather than a true wave equation, as it has (if I recall this correctly) only a 1st derivative of time rather than a 2nd derivative. However for stationary states, such as the orbitals of an electron in an atom, there is no time dependence, so the equation becomes that of a standing wave.

Last edited: Nov 17, 2016
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13. ### originTrump is the best argument against a democracy.Valued Senior Member

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No, it won't.
If you feel better making up an explanation because you do not understand the scientific explanation that is fine, it doesn't really matter one way or another.

14. ### rpennerFully WiredRegistered Senior Member

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Fact: Photons carry momentum and energy. (Einstein's 1905 description of the photoelectric effect pretty much defines what a photon is.)
Fact: Photons can be localized like particles. That's why low-light images are grainy. The earliest single photon detectors were crystals of photoemulusions, as the process of "developing" a negative turns each crystal activated by light into a dark blob of silver — the bigger the crystal, the bigger the target for each photon, the "faster" the film is to capture the scene given a certain light level, and the grainier the resulting image. https://en.wikipedia.org/wiki/Film_speed
Fact: Photons have a characteristic wavelength like waves. One of the ways we can manipulate light is to use interference, filtering or dispersion to treat different wavelengths of light differently. Monochromatic light is characterized not just by not being subject to any further fractionation by color but by having a single characteristic wavelength.
Fact: Photons interfere to produce dark places as well as light ones, like waves. This is the phenomenon behind iridescence, the colors of a soap film and reflections off a CD, holograms and both single-slit and double-slit interference patterns.
Fact: The momentum each photon carries is proportional to its momentum which is proportional to its frequency which is inversely proportional to its wavelength.
Fact: Except for the simple proportionality, all these facts apply to electrons, protons, etc.

Momentum (p) and Energy (E) and mass (m) and velocity (v) are related for a free particle (in special relativity) by:
$E^2 = \left( mc^2 \right) ^2 + \left( c \vec{p} \right)^2 \\ E \vec{v} = c^2 \vec{p}$
Wavelength (λ), angular wavenumber (k), frequency (f), angular frequency (ω), phase velocity (V) and group velocity (v) for any plane wave are related by:
$\lambda = 2 \pi \left| \vec{k} \right| ^{-1} \\ \omega = 2 \pi f \\ V = \omega \left| \vec{k} \right| ^{-1} = \lambda f \\ v = \partial \omega / \partial \left| \vec{k} \right| = \partial f / \partial (\lambda ^{-1})$
Relating the two is Planck's constant (h):
$\hbar = \frac{h}{2 \pi} \\ E = h f = \hbar \omega \\ \vec{p} = \hbar \vec{k} \\ \left| \vec{p} \right| = h / \lambda = \hbar \left| \vec{k} \right|$
Which means mass controls how a quantum particle's wavelength and frequency are related:
$(h f)^2 = \left( mc^2 \right) ^2 + \left( c h / \lambda \right)^2 \Rightarrow f = \frac{c}{h \lambda} \sqrt{ h^2 + m^2 c^2 \lambda^2 }$
Thus the phase velocity is:
$V = f \lambda = c \sqrt{1 + \left( \frac{ m c \lambda}{h} \right)^2 } = c \sqrt{1 + \frac{ (m c)^2 }{ \vec{p}^2} } \geq c$
while the group velocity is:
$v = \frac{\partial f }{ \partial (\lambda ^{-1}) } = \frac{ c }{ \sqrt { 1 + \frac{m^2 c^2 }{ h^2 \lambda ^{-2} } } } = \frac{ c }{ \sqrt{1 + \frac{ (m c)^2 }{ \vec{p}^2} } } = c^2/V \leq c$
And this group velocity is the rate at which signals and energy propagate, which means it is the same as the particle velocity (which is why I used the same symbol.

So for a massless particle, like a photon, we get: $m = 0 \quad \Rightarrow \quad V=v = c, \quad E = h f = h c / \lambda = c \left| \vec{p} \right|$.

Have you ever done this experiment? The separation between the slits is tiny so one may aim the source at the slits, but since the beam is of finite measure, it hits both slits.

Nothing works like that. The trail a snail leaves behind is made up of substances. But photons and electrons are the smallest parts of their respective phenomena.

Nothing works like that. Quantum particles don't have ambitions, goals, or make multiple attempts to reach a goal. The total intensity of light reaching the screen indicates the light not going through the slits on the first encounter is just plain blocked.

As illustrated above, quantum particles don't flip back and forth between two different behaviors. They have a single behavior which means they travel like waves and interact like point particles as classically imagined.

No. We detect particles because quantum particles always interact like point particles and the intensity is low enough to pay attention to individual events. When the intensity is high, we can't distinguish the individual events and so we only perceive the pattern of dark and light bands as in holograms.

We can do the experiment with a photon counter so we record individual photon strikes as we move the detector across the screen and get different patterns for the one-slit and two-slit setups. http://www.animations.physics.unsw.edu.au/jw/light/youngs-experiment-single-photons.html (Flash video)

Here they observe individual electrons:

Have you ever done this experiment? The thickness of the object with the slits is immaterial to the results.

All photons have the same mass, zero.

Particle physics, which is more fundamental than fluid dynamics, says you are making unjustified claims.

You get the identical pattern at all intensities, which renders both your fluid dynamics claims and "bouncing" ideas as debunked.

To support your statements with evidence means you care about reality and science. A scientific person who cares about truth and humanity would not pooh-pooh the work of others as "irrational" without demonstrating superior knowledge about the behavior of reality. A skeptical person would first be skeptical about their own conjectures. A worthy person desiring education would not form and express baseless opinions on the subject of their ignorance.

Teaching you would be a service your government already spent funds on, so you should expect to have to pay out of pocket from now on.

You lack a basis to take a position on either topic.

Last edited: Nov 17, 2016
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15. ### FarsightValued Senior Member

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• Parts of this are miseducation. Farsight is forbidden to post in the main science forums prior to January 15, 2017.
There's more than one I'm afraid. Some people will claim there's some many-worlds multiverse involved, which I think is pseudoscience nonsense. It is not scientific, it is not falsifiable, I reject it. Others will claim that the electron is some point-particle, and some probability field is at work that surpasseth all human understanding. I reject that too. Because it's quantum field theory, not quantum point-particle theory. The photon has an E=hf wave nature. We make electrons and positrons out of photons in pair production. The de Broglie hypothesis concerns the wave nature of matter, not the point-particle nature of matter. We can diffract electrons. We can even refract them, as per Ehrenberg and Siday’s 1949 paper The Refractive Index in Electron Optics and the Principles of Dynamics. And note that after noticing a point made by Hermann Weyl Erwin Schrödinger gave us the time-independent Schrödinger equation which “predicts that wave functions can form standing waves”.

No. It's a wave. A standing wave thing. Standing wave, standing field. The electron's field is what it is. Only when it isn't quite standing, the electron is moving. Through both slits.

It hits it as a wave, because it is a wave. The electron is not some point-particle that has a field, it is field. But the receptor/detector involves an interaction, and this interaction leaves a dot on the screen, so you might think the electron was a dot. Don't. Instead take a tip from the optical Fourier transform. See Steven Lehar's web page:

A convex lens performs an optical Fourier transform in real time. But you don't think the incident light was some point particle. You know that the light-lens interaction localized the light into something pointlike. Apply this logic to the double slit experiment. When you detect the electron at the screen, you convert it into something pointlike, and you see the interference pattern because the electron went through both slits and interfered with itself. When you detect the electron at one of the slits, you convert it into something pointlike, so it goes through one slit only. It then spreads out like the wave that it is. Then when you detect the electron at the screen, you convert it into something pointlike again. But this time you don't see the interference pattern because the electron went through one slit only. No many-worlds multiverse is required, and no magic either.

16. ### exchemistValued Senior Member

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Oh no.

That's the end of this thread, just when it was getting interesting.

17. ### Xmo1Registered Senior Member

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You have shown me, at least, that I must understand the thing first before I can disagree with it. I am going to understand this experiment, which for me includes putting the narrative explanations and the math together until it jell's. Thank you rpenner.

18. ### exchemistValued Senior Member

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Good plan.

(But do not take any notice of Farsight. He has a tendency to make his own physics up.)

19. ### Xmo1Registered Senior Member

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I'm matching up the math with the narrative a bit at a time, so many thanks for the descriptions and links. I wonder how long its going to take for my little bowl of gray matter to integrate the information into something meaningful. Honestly, I must use my calendar to study this. Using the optics and Fourier transforms stuck a nerve. So thanks.

20. ### Xmo1Registered Senior Member

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Looks like I was going down the same path. I found Farsight's information useful when pointing to the field of optics for further study. I'm going to look there. Also thanks origin, exchemist, and others for your lessons that put me on a better track to understanding these points of physics. Interesting to note how 'excited' people get when talking about it.

Last edited: Nov 17, 2016
21. ### rpennerFully WiredRegistered Senior Member

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Farsight is now banned from posting in the main science forums, including in reply to this thread, for repeated miseducation.
He has denied, without support, that the observed behavior of photons and electrons is, within experimental limits, like the point particles of quantum field theory. Yet he ignores that, in this thread, phenomena related to the particle-like nature of light are discussed. This makes his post resemble a cut-and-paste post collage of his earlier posts for which he was last warned about in July.

Because he has long advocated the baseless idea that it makes sense to assume the electron is composed of light (despite the mismatch of electric charge, weak interaction charge, angular momentum, and phenomenology and complete failure to generalize to other related phenomena), all ambiguous language in mentions of pair production is assumed to be attempting to again promote this worthless abandonment of actually useful electromagnetic physical theories.

The optical Fourier transform is a theorem of the operation of idealized thin lenses which convert between idealized plane waves and spherical wave fronts on the image plane. Thus, it's not a model for wave-particle duality as it does not localize in the indeterminate manner of photons hitting a screen. The math is straightforward to those with enough calculus to do Fourier transforms. http://web.mit.edu/2.710/Fall06/2.710-wk10-a-sl.pdf

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22. ### exchemistValued Senior Member

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Thank you rpenner, this will certainly help the serious science parts of the forum to become more credible to all readers.