# Double slit experiment

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

1. ### exchemistValued Senior Member

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Well actually the intent of this thread was to challenge the interpretation of, and to find out more on, the double slit experiment. Which we did quite successfully in the early part of the thread. You may have an agenda of unknown universal constants, but that did not appear in the OP.

But anyway, now that the junk has been edited out of the thread by rpenner, we can get back to some science, at least.

3. ### rpennerFully WiredRegistered Senior Member

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The conventional explanation is a predictive one in that for undisturbed monochromatic light (or mono-energetic electrons) (1) there is a characteristic length (called a wavelength): λ and (2) that half-integral path differences (when measured in wavelengths) interfere destructively while integral path differences interfere constructively. And (3) as light and electrons can be detected as particles, their arrival frequency is highest where constructive interference happens and smallest where destructive interference happens.

So since the position on the screen away from the center line, x, gives rise to a certain path difference ΔL, from two slits positioned distance d apart and distances D from the screen.

Geometry tells us $\Delta L = \sqrt{ D^2 + \left( x + \frac{1}{2} d \right)^2} - \sqrt{ D^2 + \left( x - \frac{1}{2} d \right)^2}$
which means that the dark fringes are located at $x = \pm \left( n + \frac{1}{2} \right) \lambda \frac{ \sqrt{ 4 D^2 + d^2 - \left( n + \frac{1}{2} \right)^2 \lambda^2 } }{2 \sqrt{ d^2 - \left( n + \frac{1}{2} \right)^2 \lambda^2 } }$ which (since typically $0 < \lambda << d << D$ is $x \approx \pm \left( n + \frac{1}{2} \right) \frac{ D \lambda}{d}$. Likewise the bright fringes are approximately at locations given by: $x \approx \pm n \frac{ D \lambda}{d}$ So, to some good precision, a regular pattern of fringes is expected from geometry and the assumption above.

That the spacing is so regular seems to me to be very strong support for (1) and (2). That it happens the rate of photons or electrons is as low as 1 in the experiment at a time seems like good support for (3).

Further, the interference hypothesis also explains the pattern of light from any number of slits, or other regular patterns like the pattern on a compact disk and iridescent beetle shells and feathers and can even explain how much diffraction one gets from a single slit, by modeling it as a continuum of slits placed next to each other. Thus (1) and (2) show light acts like a diffuse wave propagating through space, while (3) shows it still acts like a particle arriving at discrete and very localized places on the screen, one at a time. This shows our human notions of waves and particles don't apply and we need physics like quantum mechanics to explain similar phenomena. As it turns out, quantum phenomena has a certain characteristic scale so that most of the time we don't notice them.

5. ### Write4UValued Senior Member

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Yes, and of course it should be human science, not "alien"...

7. ### Write4UValued Senior Member

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I am not sure if this is related, but it seems to be a discovery with major implications.

https://www.sciencenews.org/article/whirlpools-might-have-stirred-baby-universes-soup

8. ### Xmo1Registered Senior Member

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Fringe is appropriate, but not to be discounted. For me, about 80 percent of what La Point said could be true. What he illustrated was persuasive. The source of power rather than magnetic could be mysterious to us. So as with similar - mix some humility. The fringe has time and again proved it has value, and sometimes even extreme value. To rpenner: I know your responsibility, but someone said to me, 'Who died and made you master of the universe?" - saying that you might take it down a notch. Vehement (fierce) objection is intellectual violence. Pet the cat as you nudge it off your thesis. Cats have feelings too.

9. ### exchemistValued Senior Member

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Well Fringe is where it now is, so we can all be happy.

I do stick to my guns, however, on my point about YouTube videos from unknown people. You can waste your entire life watching shit that way,if you're not careful.

10. ### Write4UValued Senior Member

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Keyword: "careful" sifting can also yield unusual and interesting tid-bits.

11. ### rpennerFully WiredRegistered Senior Member

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It's put in the fringe sub forum because staff discounts it. So discounting it because it is found in a curated fringe makes good sense as opposed to the wider fringe of non professionals expressing their opinions on topics where they would have no more chance of being wrong than average opinionated laymen.
So if you are acknowledging that about 20% can not possibly be true, isn't La Point a reality-challenged source with low credibility.
Yes, they laughed at Galileo, but they also laughed at Bozo the Clown. To show humility is to let the behavior of nature be your guide to how much you understand nature. See my post above.

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deleted: Off topic.

Last edited: Nov 27, 2016

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Brian Greene : What's Beyond The Double Slit Experiment ?

14. ### Xmo1Registered Senior Member

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I'm reminded of the box of pegs stood up, and having dropped a ball bearing from the top the probability is that it will arrive at a position under a bell curve. Maybe the bumps shown in the thumbnail above are not interference patterns caused by waves moving through slits, but that this is the texture of spacetime itself given that things have been moving through it since the beginning (sic). Anything small enough (quantum particles) moving through spacetime will be affected by its texture. There may be a compression factor. There was an article about Voyager detecting a magnetic bubble density near the edge of the solar system. https://www.nasa.gov/mission_pages/voyager/heliosphere-surprise.html - Could be a locally compressed version of the spacetime texture. But if you shoot electrons without the slit barrier the probability wave does not exist? Maybe that is because spacetime is not compressed enough to cause it, but the slit barrier acts to compress it. Does the probability wave begin on the far end of the slit barrier? The energy wave (the particle) is being compressed by the slits, and the particle representation is moving along it's fabric? This is difficult. BTW please forgive. I have not looked at the math, which may totally answer this. I just have a few minutes to visit the forum.

Last edited: Jan 6, 2017
15. ### karenmanskerHSIRIBanned

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Returning to the original OP inquiries and applying a relatively simple and logical philosophic (yet experimentally-testable) approach:

Diffraction (from 1, or 2, or multiple slits) involves simple particle (or wave motion of particle) interaction with edges of the slit(s). Interactions (impacts, glances, near-field, crystal/mass structure, etc.) result in interacting particles imparting energy (loss) to the slit mass - which energy is then manifest (released) as systematic distortions/interferences (with/to) the original particle's path or emanation of energy from the slit. IMO, Since we 'see' diffraction (interference) 'patterns' at the detector interface, such patterns are likely produced from shallow interaction crystallographic (atomic) structure of the slit material with moving particles (photons, etc.). Considering (again, IMO) this (these) simple interactive mechanismis, diffraction is not so mysterious a phenomenon.

Last edited: Jan 6, 2017
16. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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An even simpler approach is that it is just an intereference pattern from waves and not mysterious.
Not sure how you think particles interacting with the edges of a slit are going to give an intereference pattern. Your explanation seems to be mostly had waving.

17. ### karenmanskerHSIRIBanned

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Thanks Origin . . . Perhaps yes - just "ha(n)d" waving . . . . I have always been intrigued by "what" physically happens during diffraction processes. IMO, Energy absorbtion (interaction from, e.g., photo, electron, etc.) by slit (edge). Edge (outer-shell?) electrons of slit material are pumped to higher energy level by interaction(s). Re- emission of absorbed energy as secondary emissions (either in/out of phase with original particle motion) as slit-edge electrons return to stable energy levels. Not unlike well-demonstrated and understood X-ray diffraction mechanisms. Similar interactions (interaction/re-emission of energy) appears (to me) to be the dominant role in simple water wave interactions as well - energy from wave impacts/interactions are re-emitted as secondary wave-fronts that are either in- out- of phase with the primary wave - thus we 'observe' typical interference (and bending, etc.) of composite waveforms. To me it is a simple interaction of motion (energy) with non-motion (static mass) - water is an analogy to slit mechanics. H-m-m-m-m - maybe an interaction of inertia (energy) in motion v. inertia (mass) at rest?

18. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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If secondary emission from atoms of the slit material had anything to do with it, then energy of the secondary photons would be dependent on the composition of the slit. Not to mention that the secondary emission of photons would not give a pattern they would be emitted in a random pattern. Also how would that explain the interference pattern of electrons?

What is happening is that there is an interference pattern from the waves. Simple.

19. ### arfa branecall me arfValued Senior Member

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Waves and wave mechanics aren't simple, they're complex. Frequency dependent behaviour of all kinds of physical systems is much easier to analyse when you use complex, instead of real, dimensions.

Which is another way of saying waveforms in general have complex representations. You won't get around this with any diffraction experiments.

Well, that isn't exactly true, there is Bragg diffraction which treats the sample material as being like a rigid kind of diffraction grating.

20. ### karenmanskerHSIRIBanned

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Thanks arfa brane . . . . .Yes . . . .Bragg-like diffraction is what I was trying to suggest for shallow interactions with the slit-edge material . . . only a much shallower (i.e., weaker - say a few angstroms, or so) effect than more penetrating X-rays (Bragg). I guess I am visualizing something like 'electron microprobe' (EM) penetration depths - which also relies on the Bragg relationship (re: for 'wavelength dispersive' detection*) from electron penetration and secondary energetic emissions from the target substrate. [*Note: most EM analysis these days is via 'energy-dispersive' detection)]

Last edited: Jan 7, 2017
21. ### Xmo1Registered Senior Member

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That's why I was going to check into the optical side.

Last edited: Jan 8, 2017
22. ### Confused2Registered Senior Member

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The Double Slit Experiment has a history to it. It is also known as Young's Slits after Thomas Young (1773-1829) who gave his name to the experiment ( https://en.wikipedia.org/wiki/Thomas_Young_(scientist) ).

The feature that makes Double Slit Experiment so special is that you can work out the wavelength of light knowing no more than the distance between the slits, the distance to the display screen and the separation between the peaks. Before Young had his breakthrough nobody knew what the wavelength of light was (or if it was even waves) because they didn't know how to measure wavelength.

A consequence of the origin of the Double Slit Experiment is that a certain amount of reverse engineering is required. That is to say that if you can't tell the wavelength of the light knowing no more than the distance between the slits, the distance to the display screen and the separation between the peaks then the experiment can't be regarded as a true Double Slit Experiment.

A key component of a true Double Slit Experiment is that an aperture (or slit) that restricts a beam will cause it to spread out out - this is because restricting the aperture of a beam causes it to spread out - because it does - see for example http://labman.phys.utk.edu/phys222core/modules/m9/diffraction.htm .

23. ### Confused2Registered Senior Member

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Taking a random choice of True Double Slit Experiment suppliers ( http://www.teachspin.com/two-slit-interference--one-photon-at-a-time.html ) we have sample results like:
http://nebula.wsimg.com/0317f0d9b14...45B40D076900BE550&disposition=0&alloworigin=1
where the slit width is 0.09mm and the 'bright bit' has spread out to a total of about 5mm after one metre. Since the slits are about 1mm apart there is an overlap of around 4mm where light from both slits (two paths) reaches the same point - which is where the True Double Slit Effect lives.