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I Question about double slit experiment results

  1. Jul 21, 2016 #1
    Is one particle detected on the detection screen for each particle shot in a double slit experiment? Or do some particles fail to be detected because they hit the barrier in between the two slits? In other words, is there an exact one to one correspondence of particles shot to particles detected? I was corrected by someone when I maintained that, in a double slit experiment, one never finds a particle in both places, or neither places, when you look. I think he is right, it could fail to be detected, but it hasn't been clear to me when reading the actual experiments.
     
  2. jcsd
  3. Jul 21, 2016 #2
    Only particles fired towards the slits and then onto the screen are detected. The others, as you say, may not be detected because they hit the barrier.
     
  4. Jul 21, 2016 #3
    Its an interesting question. I wonder if the interference pattern can also be deduced from what is missing rather than what made it though?
     
  5. Jul 21, 2016 #4

    bhobba

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    Most particles hit the screen.

    Here is a much better analysis of the double slit than found in beginner books or popularisations:
    https://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Thanks
    Bill
     
  6. Jul 22, 2016 #5
    I guess it is quite hard to express what is happening in classical words so not to give impressions, which aren't the case. Saying the particle goes from point A to point B is misleading because the quantum system is neither a classical particle (but nor is it a classical wave).
     
  7. Jul 22, 2016 #6
    The smaller the slits, the fewer particles will make it through.
    If you use a laser to illuminate the slits, you will see that some of the laser light hits the slit plate, and some of the light goes through and hits the screen behind it.
     
  8. Jul 23, 2016 #7
    Thanks for the reference. I wonder if I am misinterpreting the point of this paper; I am barely literate in math and physics, but I have read popular treatments of quantum theory for interested laypersons by Brian Greene and Ghirardi among others, and my impression was that an interference pattern is considered by physicists as a "smoking gun" for the presence of waves. Is this paper saying that there is no need to invoke a wave explanation in order to explain an interference pattern? So the double slit experiment proves nothing about wave/particle duality?
     
  9. Jul 23, 2016 #8

    Nugatory

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    "Wave/particle duality" is not a precisely defined concept so it's hard to "prove" anything about it. The double slit experiment does prove that waves are an essential part of the physics of quantum objects. It's up to you whether you want to attach the words "wave/particle duality" to this fact.... But before you do, you might want to reflect on any assumptions you're making about what a particle is.

    The history is important here. At the start of the twentieth century, physicists were starting with classical assumptions about the behavior of very small things - a grain of sand is a lot smaller than a cannonball or a planet but it obeys the same physical laws and has a classical position, velocity, mass, and momentum just like the cannonball. It seemed natural to extend the same thinking to objects even smaller than a grain of sand, so that's how we thought about the subatomic phenomena that were being discovered at the time - you'll see evidence of this mindset in the Physics Forum logo which shows electrons as little balls orbiting a slightly bigger ball. So when these things that were assumed to be classical particles like grains of sand except even smaller were found to be displaying wave-like behavior, we had wave/particle duality.

    During the following decades the mathematical basis of QM was discovered and fleshed out, and it became increasingly clear that these "particles" actually did not behave much like very small but still classical objects, and the concept of wave/particle duality became less useful.
     
  10. Jul 23, 2016 #9
    That's helpful. But that's also my question. Is this paper claiming that this isn't true: "The double slit experiment does prove that waves are an essential part of the physics of quantum objects." That an interference pattern can be explained completely by the behavior of particles?
     
  11. Jul 23, 2016 #10

    Nugatory

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    One of the unfortunate consequences of the history is that when a modern physicist uses the word "particle", it means something quite different than the plain English meaning of the word, which is pretty much equivalent to the classical physics meaning. Physicists started with the (then reasonable) belief that the new things they were discovering early last century were classical particles, so that's the word they used. By the time they discovered that they were really something quite different, it was too late - once a a word gets attached to something it sticks, even if it's not very sensible.

    So as I said above, you have to nail down your understanding of the word "particle" first. The interference pattern cannot be explained by any model in which a "particle" is a little tiny ball flying through space, but it is explained by a proper quantum mechanical treatment of the things that physicists call "particles" for historical reasons.

    BTW: Ghirardi is a much better starting point for understanding than Brian Greene.
     
  12. Jul 23, 2016 #11
    Thanks for your patience. I take it that I have misunderstood the paper, using the language and intuition of the classical world and applying it to the quantum world. I thought he was saying that interference can be explained by a 'model in which a "particle" is a little tiny ball flying through space'.
    The author is demonstratinging, then, that the interference pattern can be explained completely by quantum mechanics, (inherent in which is a "probability function") without any reference to classical wave optics. But
    isn't the probability function (what Brian Greene calls "probability wave") the same thing as the wave function? And a wave in the quantum sense, whatever that means, is still necessary for an interference pattern?
     
  13. Jul 23, 2016 #12

    bhobba

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    All it is, is in the usual treatment of QM you need to build up to it in stages.

    Along the way you learn and unlearn things.

    Once you learn the full machinery of QM then the wave particle duality idea you learned earlier is outmoded. That paper corrects an omission in most texts - going back and explaining the double slit without reference to the earlier idea.

    IMHO it would be best to start QM with a better basis from the start eg:
    http://www.scottaaronson.com/democritus/lec9.html

    But like all things there are pros and cons - there is no easy road to understanding QM.

    Thanks
    Bill
     
    Last edited: Jul 23, 2016
  14. Jul 24, 2016 #13
  15. Jul 24, 2016 #14

    bhobba

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    It illustrates the point again that as you become more advanced you need to learn and unlearn things.

    Thanks
    Bill
     
  16. Jul 25, 2016 #15
    Surely all 'particles' always have a degree of uncertainty and are always probabilistic waves. The 'particle' is a terrific mental/mathematical tool but it's all waves (fields).
     
  17. Jul 25, 2016 #16

    bhobba

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    Quantum objects are not probabilistic waves. QM is a theory about observations. Whats going on when not observed is anyone's guess. Probability waves simply describe the probabilities of observations.

    Thanks
    Bill
     
  18. Jul 25, 2016 #17
    Cheers Bill, I've always looked at it back to front, the object 'owned' the probability.
     
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