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Other ways to create an interference pattern?

  1. May 9, 2013 #1
    Another newbie question if I may....

    I have previously questioned some of the general assumptions made about the double slit experiment. Particularly the idea that particles of matter cannot create an 'interference pattern'. I always read that if you shot matter at a double slit that you would get just 2 bands on the back wall, which I dont believe is always correct as I think there is a misinterpretation of the scale of the experiment here. We as humans look at the 2 tiny slits, and do not consider that the inside of the slits are at all important to the experiment. We seem to only consider the slits in relation to what happens to the waves when they leave the slits. However to an electron, something which would be at least millions of times smaller that the length of the slit....perhaps it isnt so irrelevant. An electron would have to perceive each slit as a long cavernous tunnel, rather than a short slit....and as it is travelling at a angle to the slit, as it was fired from a gun in a central location, a number of the electrons would make their first contact not with the back wall but with the inside of the slits themselves. Now *if* electrons were made of matter.....if the matter was of a sticky nature - ie when it hits something it goes no further....I could see it creating the 2 band pattern most commonly attributed to all matter. However if the matter had any ability to bounce, it would create an 'interference pattern'. I did run that experiment in my backyard and did easily create that pattern ().

    So since then, I have been looking for alternate ways to create an 'interference pattern' with matter. I have noticed that iron filings line up around a magnet...but they dont seem to only just align themselves with the field - they group together a little and separate from other groups creating little bands and I am trying to understand why. Perhaps it is just that the opposite poles of nearby iron filings attract each other, and the similar poles repel. It strikes me though that if you take a cross section through this pattern.....it could look like an 'interference pattern'. From what I understand a moving electron can produce a magnetic field.....so perhaps they are just attracting and repelling in the same way? I believe there has been some experiments where there is supposed to be only one electron travelling in the experiment at any one time....but perhaps a magnetic field could linger to affect later electrons? Some of the double slit experiments I have seen on Youtube (the only ones I have access to).....can have up to 5 electrons hitting the back wall for every frame of the movie....and yet the 'interference pattern' remains...even though their waves would be interfering which each other in such a convoluted way that no neat 'interference pattern' should result.

    Once again I am fully expecting to be thoroughly flamed for my wild ill-informed ideas. :-)
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. May 9, 2013 #2


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    But this is incorrect. People have already shown interference in a double slit experiment with buckyballs and slightly larger stuff. I think the current record-holding experiment used molecules consisting of around 114 atoms. The problem with matter interference is that you need to avoid decoherence. As soon as the molecule interacts with something along the way, that will leave some mark and position information and will therefore destroy any superposition state and the interference pattern. A photon is a single weakly interacting particle. Getting interference with that is easy. A single atom interacts rather strongly with its surroundings, but there is still a good chance that ait does not interact while one sends it through the double slit. For large lumps of matter none of the constituents may interact along the way. The chance of that happening goes down more or less exponentially with the number of constituents. This is what makes interference of large chunks of matter unrealistic.

    The rest of your speculation seems way off. It might seem tempting to speculate, but the only sensible way lies in learning about how interference works, first and think of how to do something new afterwards. There are lots of 'could look likes' in physics, even for professional physicists in everyday work. It is very important to resist walking into that trap and see only what is really there, without biasing towards what one wants to see.
  4. May 9, 2013 #3
    As noted already this is simply incorrect....

    You can verify that here:

    so I did not read the rest of your post.....but I noticed your youtube 'experiment'....and watched
    BRAVO!!!!!!!...that was great.....trying to match up observations with theory is big stuff.....
    good for you....

    You might find the subjects 'de Broglie wavelength' and delayed choice quantum erasure'
    very interesting if you have not already read about them....both are linked to in the above Wikipedia article...the implications are mind boggling as you will discover if you learn more and more....
  5. May 10, 2013 #4
    Thankyou both for your reply. I agree with you that the initial statement was incorrect. That was the point I was trying (albeit very badly) to make. Every introduction to quantum physics seems to start the same way....with the double slit experiment and then statements like matter produces just two bands, and waves produce an interference pattern. They are the statements that I am questioning as the experiments (or simulations) that you invariably then see are not doing an apples for apples comparison in regards to the scale of the experiment.

    Even the examples shown of waves in tanks do not have the experiment at the correct scale. They generally show images of waves interfering in a large tank with no back wall to inhibit the waves interference. You are left to infer what would naturally happen if a back wall was in its proper place. However a back wall in that location would then cause water waves at least to flow back towards the double slit wall interfering with any other waves coming through (assuming enough time hasn’t passed). Again I know there have been experiments where there is supposed to be only one electron in the experiment at a time, but it is the experiments that produce the same result when there are more than one electron in the experiment at a time that makes me wonder how the waves from each electron are not interfering in a messy way with waves from other electrons moving through the experiment at the same time ie the example I mentioned above with 5 electrons hitting the back wall every 1/25th of a second.

    …and thanks Naty1, I will read up on the de Broglie wavelength.
  6. May 11, 2013 #5
    Son... I was unsure if you were familiar with de Broglie wavelength...if not, when you do any reading also be sure to check 'wave particle duality'...in essence, particles behave not only as infinitesimal points but also as waves...spread out waves....

    What do you refer to here??
    Electrons as charged particles interact mostly via the electromagnetic field...one can ignore gravity since it is billions of times weaker.....but gravity becomes important at very short distances...like electron degeneracy....and neutron stars. When electrons are in close proximity, interactions are measurable/observable....across the galaxy, not so much.

    You might find this discussion of interest:

    A Night with the Stars [Brian Cox on Telly]

    Cox is the British Physics guy..teach at Cambridge..and he posted in the above discussion..

    He claims every particle of a like kind [say all electrons] in the universe are instantaneously linked....that is, when one changes energy level they all do....
  7. May 11, 2013 #6


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    And you believed him? :smile: Seriously, Naty1, common sense must come to bear at some point. I hope that Cox did not really say this. I look out my window and do not see the energy levels of all 1023 atoms changing in synchrony with each other.

    The correct statement is that all electrons in the universe are excitations of one and the same electron field. And that if you choose to describe them by a single multiparticle wavefunction, that wavefunction must be totally antisymmetric. Meaning in particular that no two electrons can occupy the same state. But electrons on Earth and electrons on Mars are not in the same state, and so do not affect one another. Doing an Earth experiment, one does not need to worry about Mars electrons changing the result!
  8. May 11, 2013 #7


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    Any particle (bucky ball, electron, photon...) that undergoes continuous periodic change can be made to cause an interference pattern. Consider a group of particles that expand and contract at a specific frequency in phase with each other (coherent) bouncing back and forth in a small cavity. Watch the interference pattern on the edges of this cavity as 40 photons bounce back and forth. The "phase" of the particle is represented as its "size" at any one time.

    Single particle: http://youtu.be/yPC5lxCXOkM

    Burst of particles: http://youtu.be/Fg3ifqBNDB8
  9. May 11, 2013 #8

    See Cox's post beginning with #14 in the above link....He IS serious, I may not be....
    I believe he says that is what they teach at Cambridge.....
    let me double check....

    yes..that is exactly what he says:

    Fredrik comments just before that post :
    As I recall, somewhere it is mentioned such energy differences are so small as to not be observable....so I did not worry too much about it....But I will note
    is not applicable to quantum mechanics.

    edit: "Meaning in particular that no two electrons can occupy the same state..." I think that is what Cox says...but I'd be interested in what you think about his text excerpt.
    Last edited by a moderator: May 6, 2017
  10. May 11, 2013 #9
    I just skimmed that thread again:

    In Post # 24 of that thread Fredrik explains Cox's perspective:

  11. May 11, 2013 #10


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    Ok, my disagreement is with Cox then. The irony is that his web page argues that "distinguishable particles may be treated as distinguishable" because the distance between them makes the overlap between their wavefunctions negligible [quite true] but his PF comment says exactly the opposite: The essential point is that two widely separated hydrogen atoms should not be treated as isolated systems. [quite false!]

    Yes, even in quantum mechanics, and I think this is what his Manchester students may be missing. Cox gives an example where the overlap is in the 50th decimal place. Common sense says this is negligible!

    But to drive home the point, he should next ask his students: what's the overlap in a real world situation? How much do the electron wavefunctions overlap in a pair of atoms that are, say, 1 cm apart. The ground state wavefunction in a hydrogen atom falls off exponentially, exp(-r/a) where a is the Bohr radius. For r = 1 cm and a = 10-8 cm, that's exp(-108). That's not just the 50th decimal place, that's the 100000000th!

    An important part of physics is knowing how to make estimates and how and when to make approximations. Telling students that "two widely separated hydrogen atoms should not be treated as isolated systems" is simply wrong advice, and IMO detracts from their education.
  12. May 11, 2013 #11
    I was referring to at least one example I had seen on YouTube (which again is all I currently have access to).

    Start at 2:56. I do not know if this is a real experiment or a simulation for the video, but if you move through this video one frame at a time (which you can do with the left and right arrow keys with some browsers ie Firefox), you can see multiple dots arriving at the back wall in almost every frame of the video. There are up to 5 electrons at some times, of almost equal brightness, implying I believe that they arrived soon after one another. I am trying to understand how their waves would not be interfering with each other in a messy way, and destroying the neat interference pattern we are seeing. Perhaps they just don't behave like normal waves at all?

    …and thanks again naty1 for your reading ideas. I have investigated wave particle duality before, and do understand it is the established view. I was however still interested in looking for other possible ways to make these patterns given some of the assumptions that I felt were incorrect in every introduction to quantum physics that I have come across…even in University lectures I have found online.
    Last edited by a moderator: Sep 25, 2014
  13. May 12, 2013 #12

    One confusing thing in that youtube video you have, is that sometimes the slits are horizontal and sometimes vertical...which means the patterns also shift by 90 degrees!!

    and when the tech sketches the results on the chalkboard I think his pattern does not match his slit orientation as usually depicted....I only watched once......it's not wrong, but that is not the usual pattern of major interest.

    for the usual pattern, slits are vertical and side by side...

    and that pattern is the same whether photons are fired one at a time or many simultaneously...

    Have you seen the three illustrations of the double slit experiment in Wikipedia:
    It's pretty good.

    The third is an animation that shows wave interference

    Note that in the double slit experiment, the detection pattern is the same whether individual photons are fired one at a time or many are fired simultaneously....

    well, they do in a sense...You can't predict where the next photon [wave packet] will 'blip a display on the screen'...no pattern is visible until a large enough sample of photons passes the detectors..it's a statistical distribution pattern...not unlike rolling dice where even with an equal probability of heads or tails, you can sometimes roll 'three in a row'...
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