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Questions about the Double Slit Experiment

  1. Nov 18, 2013 #1
    The double slit experiment shows that light is a wave, unless observed at the point where photons enter the slits. If observed there, the result of the experiment shows light as particles. What I want to know is:

    1.) What instruments are used to observe the photons as it passes through the slits?

    2.) Is it possible that the instruments themselves cause the interference, instead of just the fact that they are being observed?

    3.) Why doesn't simply observing with the naked eye that light is going into the slits suffice to "change the physical state" of the light from waves to particles?
  2. jcsd
  3. Nov 18, 2013 #2


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    Good question.
    What happens when the screen is our retina?
  4. Nov 18, 2013 #3


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    The rule is: there is NO interference pattern if it is possible, in principle, to know which slit the photon takes.

    Imagine that the Left (L) slit has a polarizer oriented at 0 degrees in front of it, and the Right (R) slit does as well. A suitable source (say oriented at 45 degrees) will generate an interference pattern. Any photon arriving at the screen could have gone through either/both slits (although half of the light will not pass the polarizers).

    BUT... orient the R polarizer to 90 degrees (keeping the L polarizer at 0 degrees) and the interference pattern will disappear completely. The reason is that it is now possible to figure out whether a photon hitting the screen when L or R by checking its polarization. Note that it matters not whether you actually take this last step. (Half of the light still will not pass the polarizers.)

    So it is clearly not the polarizers themselves that individually affect the outcome, it is the *relationship* of the polarizers that controls.
  5. Nov 18, 2013 #4
    I have no background in physics whatsoever, so forgive me for such basic questions, but how does a polarizer detect which slit a single photon goes through? If the photon goes straight from source to destination without hitting the polarizer, how can it be detected?
  6. Nov 18, 2013 #5


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    You misunderstand. The half of the light that is blocked by the polarizers doesn't make it to the screen.
  7. Nov 18, 2013 #6
    I'm currently trying to figure out exactly what "polarized light" means. Before today, I've never heard of a polarizer. (and also what it means to orient one)
  8. Nov 18, 2013 #7


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    Have you heard about "polarized sunglasses"? It is the same thing. Light has a property called polarization.

    And while you may not understand it, the point is that experiments have already been done which support current theory and refute the idea that the measurement device itself creates or destroys interference. In fact, it is the entire setup which must be considered.

    I would recommend that you spend some time learning about this fascinating subject. I assure you it will not be wasted! :smile:
  9. Nov 18, 2013 #8
    That's why I'm here! :-D

    I have no doubts that my questions will have absolutely no impact on this field, i assure you ;)

    I spent some time researching this and also found out about the Quantum Eraser test, and the Delayed Choice Quantum Eraser Test, and my questions are now answered.

    I'm just a computer scientist but this quantum mechanics stuff in particular has been fascinating me for a long time. I'd like to go so far as to actually perform the test myself just for the sheer thrill of actually seeing this work if it wouldn't be terribly expensive. To flip a switch and see the light inside a box go from two bars to several simply at the flip of a seemingly unrelated button.... that would be amazing.

    I guess my next big question is how to quantum entangle particles - which I've heard can be done to light simply by shining it through some sort of crystal... I'm probably butchering the explanation all to hell, so feel free to correct me :)
  10. Nov 18, 2013 #9
    The double slit experiment can be created with things that you almost certainly already own, but it might get a bit fiddly.

    A quantum erasor setup would be pretty cheap. You might even be able to perform this with those semi-disposable 3d glasses that are given out at cinemas.

    Although creating entangled photons is going to be cheap, actually measuring the entanglement isn't going to be something you'd want to spend money on just to prove it to yourself, once.
    Last edited: Nov 18, 2013
  11. Nov 19, 2013 #10


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    Is our retina screen a which path detector?
  12. Nov 19, 2013 #11


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    No. Like a piece of photographic film or a collection of photodetectors, it will register whether or not it's being hit by light no matter what path the light took to get there.
  13. Nov 19, 2013 #12
    It depends where you put it. If it's actually at one of the slits then yes.

    If it's at a distance of the projection of interference pattern then no.

    Undoubtedly, you've already seen notable interference patterns detected by your own retina. Have you ever noticed the rainbow pattern when you look at a layer of oil on of a puddle?

    Interference patterns are even generated inside the eye. I remember as a very small child noticing in certain lighting conditions, little circular fringes drifting across my vision. You've possibly noticed them too. It was many years before I could explain what was actually happening.

    Obviously, be careful with bright light sources when using the eye as a detector.
  14. Nov 19, 2013 #13


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    Haroche gives the general answer to the coherence reapparance after a which path meaurement. read p 77 of "Exploring the Quantum: Atoms, Cavities, and Photons".
    The information about the path is stored in a huge number of atoms, photons in the environment.
    You would have to erase all the subsystems of the environment where the information is imprinted.
  15. Nov 19, 2013 #14
    Hm, I'm actually in the market for a good elementary book on QM. The Foreword (as I can read on Amazon) says the book is just about right for that purpose. Are there others you might recommend keeping in mind that I'm not terribly mathematically inclined? (I can bang out some trig but when it comes to more complicated things like calculus, I'd have to really hit the books...)
  16. Nov 19, 2013 #15
    The Quantum Universe ( https://www.amazon.com/Quantum-Universe-Anything-That-Happen/dp/0306821443 )
    Sneaking a Look at God's Cards ( https://www.amazon.com/Sneaking-Look-Cards-Revised-Edition/dp/069113037X )
    Quantum Reality: Theory and Philosophy ( https://www.amazon.com/Quantum-Reality-Philosophy-Jonathan-Allday/dp/1584887036 )
    The Quantum Challenge ( https://www.amazon.com/The-Quantum-Challenge-Foundations-Mechanics/dp/076372470X )
    Last edited by a moderator: May 6, 2017
  17. Nov 19, 2013 #16
    Is that the same for a rainbow too?
  18. Nov 19, 2013 #17

    Rainbows are caused by the dispersion of light when it undergoes refraction. We don't need multiple paths to explain this phenomenon.
    Last edited: Nov 19, 2013
  19. Nov 20, 2013 #18


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    Yes - but its worth it. You really need the math to understand it properly.

    Start with Lenny Susskinds book on Classical Mechanics:

    Then his new book on QM once its released:

    The video lectures associated with the books can be found here:

    Please do make the effort to come to grips with calculus - its will require a bit of effort, but well worth it.

    Check out the following as a start - its actually good enough to get you started:

    If you want to go a bit deeper, here is a nice free resource for learning it:

    Last edited by a moderator: May 6, 2017
  20. Nov 20, 2013 #19


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    Cool - then you and I have the same background and another shared interest :smile:.

    The basic double-slit experiment (with photons/light) is very easy to do; in fact I wrote a blog post on how to do it at home, since I noticed there are people interested in trying it. In my post there are also other links and instructional videos. (I have actually done new DSE experiments at home recently with a better laser module, but I haven't updated my blog yet, I'm going to try to get some really good pictures of the interference patterns if I can.) Anyway, I sincerely recommend doing the DSE at home - it's a real kick to see it in action.

    Quantum eraser: Scientific American had an article about a DIY experiment: http://www.scientificamerican.com/slideshow.cfm?id=a-do-it-yourself-quantum-eraser. I haven't tried that myself (yet :smile:).

    Entanglement experiments are a bit harder; you'll need a laser source, a special crystal, polarizers and photodetectors connected to equipment which counts detections (coincidence counting). You could check out this forum thread (Detecting entangled photons) for more info and a couple of links.

    Anyway, please note and always remember that lasers, and thus laser experiments, should be handled with care: Laser safety.

    This video should also be interesting; The Original Double Slit Experiment:
  21. Nov 20, 2013 #20
    Hi guys,
    I am just wondering: what is the minimal speed of let say an electron which is sufficient for the observation of the double slit experiment effect? Are there actually any experiments about this?

    Have spent some time googling it, but nothing obvious popped out.
  22. Nov 20, 2013 #21
    Historically, this was studied by De Broglie. He developed equations that will answer your question.

    You'll need to use them in conjunction with a little trigonometry to work out the scale of the experimental set up.
    Last edited: Nov 20, 2013
  23. Nov 21, 2013 #22


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    It isn't so much that "one can know" which slit, but that the particle going through a slit becomes different than a particle through the other slit, so they are no longer coherent. The loss of coherence means no interference pattern. When you learn the math you will see easily that if the paths are not identical, there will be no interference.

    I think the best introduction is actually Susskind's entanglement lectures. It's a fascinating self contained look at QM with a high level overview of the math.


    And there are lecture notes here.
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