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Laser photon

  1. Jun 18, 2014 #1
    Question I cannot seem to understand regarding the light shot through a slit. Changing when observed.
    How does anyone know what the light does when it's not observed... If they aren't observing it?
    "Oh the light passes through both when not observed but when we so observe it, it acts like a single photon." Well if we can't see it being in two places at one time, how did anyone know it was in two places at once? Unless someone observed it?
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  3. Jun 18, 2014 #2


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    We allow the light to illuminate a screen behind the barrier with the two slits. We observe the interference pattern that is produced by light waves passing through both slits.

    Next we reduce the intensity of the light until it is so weak that on average there is only one photon in flight at a time. Now of course we don't see an interference pattern; we just detect individual photons reaching the screen and we have no idea which slit they passed through on the way to the screen. But if we let this run for a long time, we discover that the pattern of photon arrivals gradually builds up the same interference pattern; we get many photon arrivals in the areas where the interference pattern would have bright and none where it would have been dark.

    Finally, we put a detector in one of the slits so that we know which slit each photon passed through. And (somewhat incredibly) the photons stop building up an interference pattern - instead we get a bunch of hits behind each slit, exactly what you'd expect if every photon just went through one slit or the other and hit the screen behind the slit.

    Google for "double slit quantum" and you'll find many more detailed explanations.
  4. Jun 18, 2014 #3
    Okay so by observed they mean, "a detector" on a slit?
    So the photon is sensing the slit somehow and than only choses to go through one, instead of both?

    Also, after you let the interference build up, if you went behind the slits would the interference disappear?

    When it boils down, is the big problem here, trying to figure out how the photon itself can detect something observing it?
    Or the fact that it can be in 2 places at once?
    Thanks for the response, googling the experiment as I respond. :)
  5. Jun 19, 2014 #4


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    They don't. The only property it has is the quantum state, which is simply a bookkeeping device to calculate probabilities if and when you were to observe it.

    To see this more clearly I will present QM a different way than is usually done.

    First I need to define a POVM. A POVM is a set of positive operators Ei ∑ Ei =1 from, for the purposes of QM, an assumed complex vector space.

    Then we need the foundational axiom:

    An observation/measurement with possible outcomes i = 1, 2, 3 ..... is described by a POVM Ei such that the probability of outcome i is determined by Ei, and only by Ei, in particular it does not depend on what POVM it is part of.

    We can now use this nifty theorem, Gleason's theorem, to prove a positive operator P exists such that the probability of outcome i is Trace (P Ei).

    You can find the proof here:

    P by definition is called the systems quantum state. It's simply a logical requirement of the fundamental principles of QM. Basically, like probabilities, its simply a bookkeeping device to allow us to predict the long term averages of the outcome of observations. It's not real like say an electric field is real. That is the only property that can be associated with a quantum system when not observed.

    I need to add this is purely from the formalism - interpretations have their own take.

    Last edited: Jun 19, 2014
  6. Jun 19, 2014 #5


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  7. Jun 19, 2014 #6


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    Yes, observed means you put a detector somewhere, in a state such that it can detect which slit the photon went through. In such a scenario, there will be no interference pattern (you don't have to look at the data afterwards, you can discard the data if you like, but so long as you set up a detector that CAN detect it, the photon will act like it's being detected).

    It's not so much that the photon will *know* it's being seen, or that it "senses" the slits. A photon is not a conscious entity. But the answer is that really, there's no such thing as passive observation. You detect something by interacting with it. It is this interaction that destroys the interference pattern. Quantum mechanics is weird, but it's not mystical. It's not like the photon can "know" it's being observed. The photon is interacting with the detector, so it loses its interference properties.

    We can never observe anything being "in two places at once". Fundamentally if we saw two simultaneous hits on a particle detector, we would assume that those are 2 particles, not 1. It's just that the photon behaves AS IF it went through both slits, we can't know for sure if it did or not.
  8. Jun 19, 2014 #7
    Why can't the photon go through one slit? And than act as it went through both?
    Also, is there a way someone could make a sensor so small that it wouldn't interfere with the photon going through the slit?
    Thanks for the answers guys, going to look at the links posted.
  9. Jun 19, 2014 #8


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    This makes no sense.

    We KNOW what the pattern looks like when photons go through only one slit. It's called the diffraction pattern! So if a photon that goes through one slit, can somehow act as if it when through both slits, it should also behave that way when we only have ONE slit. After all, that other slit is really not needed, is it?

    Did you even look at the link/reference you were given earlier?

  10. Jun 19, 2014 #9
    As photons pass through the double slit and are measured at the projector screen, the pattern that emerges on the screen is a function of the size and shape of both slits. So the photons are responding to both slits.
    Since they are not being measured at the slits, the fact that they are acting as if they did is all that matters. Once they appear at the screen, it is a purely philosophical point whether they had ever traversed either slit.

    It isn't the size of the measuring device that matters - only whether a measurement was made. The problem isn't with technology, it's an inherent part of the physics. Even if God made the measurement, He would not be able to do so without effecting the experiment. A measurement has been made as evidenced by the fact that the result is now known by God. A photon with a known position is inherently different than a photon with no known position.
  11. Jun 19, 2014 #10

    Zapper yes I looked. Like I said in my post. I replied before I looked at the links. I said that because we cannot understand why it can be in two places at once. So why is it so absurd to think it can go through one slit and look like it went through two?
  12. Jun 19, 2014 #11
    That paper that was published. I don't understand it, my math skills cannot follow that paper yet. Would observing it with your eyes be the same as, using a detector?
    My problem comes from, when the photons make a wave pattern, how can we know that a human wasn't sitting far away watching that happen?
    So the mere presence of eyes watching the experiment from 20 meters away effects the photon... How come when it acts like a wave, those photons some how can sense that there is nothing watching them?
    I know you guys kind of answered this already. But setting up sensors on the slits, or watching from a distance, still count as observing?
  13. Jun 19, 2014 #12


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    Why do you think its in two places at once?

    Why do you think it has the property of position, which is what being in a place means, when not observed?

    I carefully explained the only property it has when not observed is this thing called the state, which is simply, like probabilities, a theoretical device that allows us to calculate the expectation value of observations if one was to be done.

    Exactly what didn't you get about that?

    See the link I gave on the quantum analysis of the two slit experiment.

    What's going on is the photon leaves the emitter. Its in a certain state - it doesn't have the property of position - only this property of state. It hits the screen with the slits. That is an observation and if it doesn't get absorbed by the screen has the state of the sum of two Dirac delta functions after. That state changes with time to mathematically be similar to interfering waves - its not - its simply similar mathematically. It then is observed at the screen for its position and the state gives the probability of that position. It is not in two places at once, it does not go through both slits at the same time. Its none of those things - its simply obeying the laws of QM which doesn't ascribe any property to it until measured.

    Last edited: Jun 19, 2014
  14. Jun 19, 2014 #13


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    Precisely why would you think it would make any difference if a human being was present or not?

    Observations occur in an assumed common-sense classical world independent of human observers.

    I suspect you have been reading pop sci accounts.

    Forget them - they often are rubbish.

    You will get the real deal here.

    BTW don't feel bad if it isn't gelling. It takes a while for exactly what's happening to sink in so to speak. And if you need to 'unlearn' pop-sci stuff that doesn't help.

  15. Jun 19, 2014 #14
    You can't see a photon going through a slit 20m away, the only way you can see a photon is if it hits you in the eye, and then it isn't going through the slit.
  16. Jun 20, 2014 #15
    But the realness of quantum states is not fully decided on yet is it, it's still a part of interpretations. I mean local realism has been ruled out as per Bell, but non-local realism is still possible right?
  17. Jun 20, 2014 #16


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    What I am speaking of is the formalism, which every interpretation agrees on. In some interpretations its exactly as I describe, in others its very real, in still others a different tack is taken.

    To understand what's going on first understand the formalism.

  18. Jun 20, 2014 #17


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    Because of what I had argued! If It can go through ONE slit and make it look as if it went through two, then the SECOND SLIT isn't needed, and we will see the same pattern with just ONE slit as well! Furthermore, if it goes only through one slit, then it doesn't matter if I put a detector on one of the double slit or not, because I should still get the 2-slit interference pattern even if I detect the particle going through one of the slits. After all, according to you, even if goes through only one slit, it can still make it "look like it went through two" slits.

    This, we do not see, and thus, your conjecture has no experimental support.

    Being in "two places at once" is based on what we can interpret, because there are many experiments that showed the existence of such superposition.

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