Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Single electron ...two slits?

  1. Jan 7, 2016 #1

    I am a newby so please excuse my ignorance.

    With reference to the single electron double slit experiment, one thing has been bugging me. The videos that illustrate the particle or wave behaviour do not tell us how the single fired electrons are directed to either the left or right slit. If the electron gun is directed to the right slit and doesn't move surely the electrons will always go through the right slit and the same would apply if the gun was pointed at the left slit, they would always travel through the left slit. I imagine the path of the electron to be straight and fixed. i.e. (while observed) each repetitive firing would result in a single bar on the backplate behind either the left or right (wherever the gun was pointed). This would be the result if you set up a rifle to fire bullets through only one slit each time.
    Is the electron gun oscillating between the two after each single fire? the marble analogy shows us that the marbles are coming out randomly across the path of both slits, so therefore some hit the plate, some travel through the right and some through the left slits, hence the double bar pattern.

    What give the electron paths a similar random direct path from gun to plate/right/left slit...surely the gun must be moving in some way across the arc of the two slits.

    Also, has there ever been an experiment to fire two single electrons simultaneously from two separate guns, one to the left slit and one to right slit. If so what were the unobserved / observed results from that?

    Many thanks

  2. jcsd
  3. Jan 7, 2016 #2


    User Avatar
    Gold Member

    Beam from electron gun is wider than double slit. So it will be that some electrons hit the material of double slit while some electrons will go through one or other slit.
    I don't know about interference experiment using two electron guns but there can be interference between beams of photons from two lasers if they are mutually coherent. For example http://dx.doi.org/10.1103/PhysRev.159.1084 (paper is behind paywall but you can google with the title of the paper and you will find discussions that cite this experiment)
  4. Jan 7, 2016 #3


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Hello Tom, :welcome:

    You are missing the crux of the situation in this double-slit experiment: the Broglie wavelength of the electron and the separation between the slits have to be of the same order of magnitude to observe an interference pattern. So narrow and very close slits ! That means that the electron gun isn't aimed at one slit or the other, but at both. Just like with the analogous light experiment. The wave-character of the electrons makes that you can't decide which slit a single electron goes through.

    Here is a description that starts off with bullets and then treats the electron double slit experiment.

  5. Jan 7, 2016 #4
    Hi BvU

    Thanks for your reply, in the document you sent it mentions the a machine gun is spraying the area with bullets, i was kind of imagining a rifle fixed to a single point, so all bulleta fired always follow the same path.
    Is my understanding of how the single electron travels from the gun flawed, i am looking at it as (when observed) travelling like a particle from point to another inba straight line... Like a bullet
  6. Jan 7, 2016 #5


    User Avatar

    Staff: Mentor

    The electron does not have a precise bullet-like path through space. If you want a mental image to replace the bullet, you could imagine a very small and fast-moving cloud; we cannot specify its path at a scale smaller than the approximate size of the cloud - and the "size" of a cloud is an inherently fuzzy concept because a cloud has no hard and fast edges. This picture isn't quite right either ("quite right" would mean a fair amount of math) but it's a lot better than the "little bullet" picture.

    The word "particle" as used in quantum mechanics means something different from what the word means in ordinary English usage, and this leads to endless confusion; it's almost impossible to hear the word "particle" and not be thinking of small solid objects like a bullet or a grain of sand. The confusing wording is an consequence of the history by which we arrived at quantum mechanics; the word was being widely used to describe electrons and photons long before their essentially quantum nature was understood.
  7. Jan 7, 2016 #6
    Hi Nugatory,

    Thanks for this, understood, So essentially the size of the fuzz (or cloud) of the electron is covering both slits. when unobserved the resulting pattern made after repetitive firing results in a wave like form on the back plate but when the slits are observed to see which path the electron 'chooses' the resulting pattern is that of a shower of particles.

    I am no maths or physics expert but just the observation of what's happening here has me fascinated, the philosophical implications are what interest me most as do the variations of this simple experiment. We only ever hear of the classic 'gun / slits and backplate pattern'

    I have a few questions I am intriqueed to know about..

    1. I assume this experiment in done in a vacuum
    2. Does distance from gun to split to backplate affect the outcome
    3. Does the time between firing (seconds to weeks) affect the outcome
    4. what about 2 (or more) guns firing simultaneously or alternatively, what results then?
    5. My understanding of 'observing' the slits is any by means (humans senses, lab apparatus, sensors, knock on affect via some side of the slit etc...), is this correct?

    Again, please excuse my scientific ignorance, I would love to understand more and find out about other experiments that prove the same oddities

    One thing that really interests me from a philosophical point of view is how we perceive the world around us, when we think of nothing (blank our minds) our interaction and the potential of the world around us is in one state (wavelike) or full of potential.
    When we focus on an object or topic it becomes real in that moment and we kind of solidify it (particle like). As I write this this I am focused on the screen and the words I am writing, I've forgotten about everything else in the world around me momentarily, although I know it's all potentially still there

    Is the fundamental nature of mind and the universe that surround us essentially the same?

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook