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The double slit experiment

  1. Jan 13, 2008 #1
    While researching the double slit experiment and finding that even when only one photon is fired at a time and interference still occurs (so the photon must be traveling through both slits at once) i came up with a possible problem with the theory and wondered if anyone could help me see where i'm going wrong.

    Particle emitter at point A fires a photon over 299792458 meters which should take exactly one second if it remains at a constant speed (light speed).

    A timer is started exactly as the photon is emitted from the emitter, which is then stopped as soon as the particle is registered on the receiver at point B.

    If the particle has taken a perfectly straight path then the time on the clock should be exactly one second. However if the particle has taken all possible routes and thus caused interference then the time should register a slightly different time. Either showing it has taken a longer path than a straight line so taking longer or somehow arriving sooner and breaking the light speed barrier.

    All help greatly appreciated

  2. jcsd
  3. Jan 13, 2008 #2


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    Er.. it's not as if one photon goes through one and then zig-zag back to take the other one. It is the SAME photon going through BOTH slits simultaneously. There's no time difference.

  4. Jan 13, 2008 #3
    Yes but a perfectly straight line would restrict the photon to only traveling through one slit. to go through the other slit would require taking a longer path than the 1st slit.
  5. Jan 13, 2008 #4
    I researched the double slit experiment as well and i must tell you that what i ended up with is that the photon or the electron interferes with itself, no matter which slit it goes through or which path it takes, let that be straight line or not...it still interferes with itself and you get the interference pattern.
  6. Jan 13, 2008 #5
    Say the emitter pointed straight at the slit on the left then a straight line between point A and B would be directly through that slit. to interfere it would have to travel through the other slit as well, a longer distance. so being longer it would mean the photon had traveled slower than light speed. and since in a vacuum light speed is constant that is what i don't understand.
  7. Jan 13, 2008 #6
    " You have to stop thinking that you know what a particle is" our lecturer always tells us this.

    He means that the photon does not phyically go into both slits at once but it rather acts like it does, it interfers with itself by going through a single slit....but it does not go through both slits physically.

    It would take the path you suggested but it would still interfere with itself.
  8. Jan 13, 2008 #7
    so why does the interference disappear when only one slit is used?
  9. Jan 13, 2008 #8


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    It takes the same time for light to go through one of the slit as it does for the other slit. So why would there be a difference in time? Both slits are already very close to each other as it is already.

    Furthermore, we know what would happen if there is a "path-length" difference between the two path. This is the same principle in practice with the superconducting quantum interference devices (SQUIDs), where the flux through the closed loop will cause a phase shift in the current. No such thing is going on here.

  10. Jan 13, 2008 #9
    I think this is the particle-wave duality, i'm not on this but with single slits you don't have interference with waves and with particles but with double slits you get interference with waves but not with particles...however photons produce a wave interference pattern. Thus they are said to behave like waves and like particles.

    Trust me the photon only interferes with itself without physically going through both slits...

    EDIT : This is a reply to paul's post...
  11. Jan 13, 2008 #10
    So what if the emitter was positioned so that a parallel line between the emitter and the receiver put the path of the photon through the left slit. making the path through the right slit fractionally longer?
  12. Jan 13, 2008 #11


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    You need to keep in mind that the double-slit interference is due to the superposition of paths that a photon can take between the source to the screen. If you set it up in such a way that you can actually favor one slit over the other, than this might no longer be a superposition of paths since you could have removed the other path from contention. You can easily do this by increasing the separation between the slit, make them further apart. Then you'll see that if you place the source directly behind one of the slit, you'll see no more interference pattern, because the possibility of light going through the other is gone.

    Now, if you simply shift it slightly, you will still get an interference pattern, but it will not look identical to the one previously. One side of the interference pattern may be more intense than the other. I still do not think you can detect the "time delay", because you forgot that the interference pattern being produced has a "wide spread" laterally, so the photon is taking all kinds of paths after it passes through the slit, some time making a large angle away from normal. So your time "delay" can't distinguish between a photon going through one slit versus it making a longer path after passing through the slit.

  13. Jan 13, 2008 #12
    ok thanks think thats cleared it all up
  14. Jan 13, 2008 #13

    So why is there no interference when only one slit is used if they are not going through both slits simultaneously?
  15. Jan 13, 2008 #14
    Its because of what Zz said, in the double slit experiment there's a superposition of paths, but in the single slit experiment, the path is almost determined.
  16. Jan 16, 2008 #15
    read 'Schrodinger's Kittens' by ( i forget ) which offers understanding in layman's terms.
    It's a cheap paperback and I promise you it's a very interesting read.
    You will learn that as yet, there is no definitive understanding of the 2-slit and ALL theories so far are open to interpretation.
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