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B Basic question about double slit interferance

  1. Feb 24, 2017 #1
    I am new here so I will start with a warming up question :)

    My question is very trivial here we go:

    In double slit experiment when shooting one photon at a time, we get interference pattern. Then when we put a sensor at one of the slits the interference pattern disappears because we now know which path the photon took.

    Now with same setup, if we switched from shooting single photons, to full source of light. In this case the sensor will register a swarm of photons passing by. So in this case the measurement will become meaningless and the which path information will become obscure because we cannot tell which photon took which path, consequently the interference pattern shall come back again.
  2. jcsd
  3. Feb 24, 2017 #2
    i can't see the question, maybe you could elaborate more on your problem with your statement?

    i think it helps if you think of the "swarm" as an electromagnetic wave that has an interference pattern when the wave fronts pass through the two slits. In the single particle problem, photons behave like quantum mechanical particles.
  4. Feb 24, 2017 #3
    My question: After switching from single photons source to full light source, while keeping measuring one of the slits, will the interference pattern come back again?
  5. Feb 24, 2017 #4


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    No. The multiple photons coming through does not obscure the effect, even though we might not be able to determine the "which-slit" information for every individual photon.
  6. Feb 24, 2017 #5


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    For a good but layman-friendly introduction to what's going on here, you might want to try Feynman's book "QED: The strange theory of light and matter". It shows how we can calculate, for any given photon, the probability of that photon landing at a given point on the screen when there are contributions to the probability amplitude from both slits (no detector) or only one slit (detector present so only one slit can contribute). This is calculated on a per-photon basis, so it doesn't matter whether they arrive all at once or one at a time.
  7. Feb 24, 2017 #6

    m k

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    Passing by can't be registered.
  8. Feb 24, 2017 #7


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    There's not an easy way to measure photons without destroying them. But since this example is didactic, let's just replace it with an electron double slit experiment.
    If you measure which way the electron went, then the interference pattern will not be present. Now, if it's possible to overload the sensor with too many electrons to measure, then it depends on how that sensor works. If the sensor still interacts with each electron in a way, but we can't keep up with the data stream, then the interference pattern will still not be present. It doesn't matter if a conscious observer can measure which way, but rather if interactions have taken place which can determine which way. If the sensor only catches a few particles, then the interference pattern should still be there, but perhaps with lower contrast.
  9. Feb 24, 2017 #8
    Good answer!

    This makes sense, if the sensor is overloaded then the result at the screen will be mixed, for example lets say the sensor measures only 50% of passing electrons, then 50% of the electrons will show interference while 50% will not.
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