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Question about the double slit experiment

  1. Sep 28, 2013 #1
    So I'm curious. When in one instance where light behaves as a wave and you get an interference pattern, and in another instance where light behaves as a particle and you don't, does the intensity of the inner most bands of light change at all? When we place a detector beside one of the slits and measure the particle, does that do anything to change the intensity of the two inner most bands of light?

    Lets say we have 2000 photons and fire 1000 photons each in two experiments, one with a detector and one without. You would think that when light is more scattered in the interference pattern there would be less intensity across the two inner most bands, as we have a limit of 1000 photons and maybe only 400 photons landed in either of those two inner bands. However when we place the detector next to the slit and light behaves as a particle, the intensity of light should be greater because there is no interference and thus all 1000 particles landed in those two inner bands.

    Is this the case? Can anyone shed any light (no pun intended!) on these types of observations?

    Thanks in advance!
  2. jcsd
  3. Sep 28, 2013 #2


    Staff: Mentor

    There is your problem from the start - it's neither wave or particle:

    What happens in the double slit experiment is probably best explained by Feynmans approach. Particles take all paths. With both holes open they can take both paths so you get interference. Close one hole and since it can only go via one path you don't.

    Now put a detector in one path and its like closing one hole - if it goes through that path it's detected so cant interfere - if it doesn't its not detected - but either way we know by which path it went so cant go via both paths.

    It's subtle, and its best to have it explained in detail from the start. I suggest having a look at Lenny Susskinds lectures on it:

  4. Sep 29, 2013 #3
    Why is putting a detector there like closing a hole? The hole is still open so if particles take all paths then a detector shouldn't make a difference.

    To address my original question, how does the distribution of light intensity on the back plate change with variations in this experiment?
  5. Sep 29, 2013 #4


    Staff: Mentor

    If you put a detector it cant go through both holes simultaneously - it goes through one or the other. Either its detected at the detector or not. If it is then it went through that hole - if it wasn't it went through the other hole.

    To understand Feynman's view and why you get dark and light bands check out his lectures:

    But just as an overview each path has a little arrow attached to it that twirls around. They sum at the screen - if they are opposite they cancel and you never get a particle there - if they are the same then they reinforce and you are more likely to get a particle there.

    I also STRONGLY suggest you view the Susskind lectures I linked to.

    Last edited by a moderator: Sep 25, 2014
  6. Sep 29, 2013 #5


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  7. Sep 29, 2013 #6
    Note that removing some of the electron won't necessarily make a band dimmer. It might make it narrower instead
  8. Sep 29, 2013 #7


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    The answer is yes, the intensity changes as you describe. :)
  9. Oct 1, 2013 #8
    That just blows my mind even more! So wouldn't it be fair to say that a force of nature is at play here? Would that force of nature simply be electromagnetism? The detector alone is capable of changing the physical properties and distribution of the light even though it isn't providing a medium for the light to become distorted, such as water or a diffraction grating, or interacting with it in any classical way. Does the wave/particle know it's being measured? Does it lose or exchange any particles in the process? Why is measurement so important to the behaviour of light and how can some non physical interaction change the behaviour and pattern of something without another force being behind said interaction?
  10. Oct 1, 2013 #9


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    Great questions. The answer is that the detector is not responsible for creating the interference pattern. This is a function of whether or not it is possible to know that the particle went through a specific slit.

    In one version of the experiment, polarizers are placed in front of each slit. This blocks half the light. When the polarizers are aligned parallel, there is an interference pattern. But none when aligned perpendicular. That is because it is possible - even if you don't attempt to find out - which slit the particle went through when the polarizers are perpendicular. The only variable here is the orientation of the polarizers!
  11. Oct 1, 2013 #10
    the interaction is physical.

    the detector, I think, interacts physically with the photon.

    Maybe someone can clarify this.

    good point.

    To add: The perpendicular polarizers create a difference in the direction of the oscillation of the two beams. The beams are now oscillating perpendicular/orthogonal to each other. Because of this the beams can no longer interfere.

    Before encountering the perpendicular polarizers, the two beams were oscillating in the same directions (axes).
    Last edited: Oct 1, 2013
  12. Oct 1, 2013 #11

    So is it possible for the wave/particle to change it's properties and distort the distribution of light without a force being applied to it? What does the conservation of energy have to say about this experiment? Is it relevant? For whatever reason, the act of measuring or knowing which slit the particle went through is enough to change the distribution of light, regardless of whether it interacts with it or not (from a classical view point). For instance if you tried to throw a hand full of balls up in the air and a gust of wind blew them to the side, they wouldn't land in the predictable fashion because the gust of wind had interfered with the experiment. However, placing a detector next to one slit doesn't seem to exert a force on the particle in this sense, but the distribution of light is still changed purely by knowing which slit the particle went through.

    So what does it mean to know? And why does knowing actually change the physical properties of light? Can that happen without a force? Or does that mean a material detector has more potential than just being able to detect things, the potential to actually change things? As in being a force carrier that causes the wave to collapse into a particle?
  13. Oct 1, 2013 #12


    Staff: Mentor

    It does - via the process of decoherence:

    Technically its changed from a superposition of both paths (loosely speaking) to what's called a mixed state that can be interpreted as having a probability of going via one path or the other - but not both.

    I wont hide from you looking at it this way is controversial and discussions on it can get a bit heated. I wont be drawn into that but simply give a link with the detail so you can make up your own mind - if you want to pursue it that is:

    BTW its not decoherence that's controversial - its implied by the formalism of QM and no one doubts it - its the interpretation that's at issue.

    Last edited: Oct 1, 2013
  14. Oct 1, 2013 #13
    I would advise you to read first four chapters of Feynman Lectures vol III. If you don't have one then buy one. It is really worth reading.
    You will find answers to all the questions you have asked.You don't need to be familiar with anything but high-school mathematics to understand those chapters.
  15. Oct 1, 2013 #14
    I will look into Feynman's work!

    Does force and energy have any significance in these arguments? Or is that kind of logic left behind in classical physics? Where does the particle get the energy to do these things and how is that energy changed once the wave function collapses and the particle takes one specific path? Are these kinds of questions relevant here?
  16. Oct 1, 2013 #15
    Yes they have significance but probably not in the way you are thinking. In Quantum Mechanics you do physics which is very much different from Newtonian Mechanics.
    If you take postulates of Quantum Mechanics (translated into a simple english by Feynman) for granted then you will find that the interference (so called wave nature) appears in absence of detector and it disappears in presence of the detector. You can actually see the interference disappearing through the equations, without considering any "extra forces" but the rules stated by Feynman.

    While I am not very much comfortable with collapse and how/why it happens, I can tell you that if you apply Feynman rules you can get the result without having to answer these questions.

    Of course it doesn't mean that Quantum Mechanics can't answer these questions. But then you will have to consider stuff like "de-coherence" which is not something you want to learn in the beginning.
    Last edited: Oct 1, 2013
  17. Oct 1, 2013 #16
    Dear m_robertson,

    Keep in mind that the detector and polariser are interacting in ery different ways with the photon.

    The detector is collapsing the wavefunction (decoherence),
    the polarizer is not, the polarizer is simpling changing the axis of oscillations.
  18. Oct 1, 2013 #17
    Thanks. I just think of some of the basic principles of physics, such as, that energy can not be created nor destroyed, and that for every action there is an equal and opposite reaction. To think of a particle taking multiple paths, while easy to picture is a very perplexing thought to understand. Where does the energy come from to collapse the wave function? How is that energy applied and in what form does it take? Are these questions that can even be answered right now?

    The way I imagine it in my own (uneducated) mind is as if the photon carries with it a particular particle, and if the photon gets measured by an instrument, it loses that particular particle which allows it to escape and prevents it from interfering with its sibling. The reason for that is because I don't understand how a particle can separate into multiple potentials and not interfere with its self on the other side, without some sort of energy being gained or lost. If the answer is that no energy is lost because energy remains the same no matter how many potentials exist, then that sounds like something which isn't even real (real in a physical sense) as it would imply a loss of things such as momentum and kinetic energy as the particle separates into multiple potentials, however we know it's real because we can see the effects in the changes of the way that light is distributed. It's very confusing and hard to know what logic to use.

    I'm particularly interested to learn how energy is distributed in this experiment.
    Last edited: Oct 2, 2013
  19. Oct 2, 2013 #18

    One thing is that:

    Particle interactions are assumed (and it might actually be that way) to be frictionless.

    For example: There is not energy loss during, for the photon during, say, decoherence or when the direction of polarization of a photon is changed etc.

    For QM - The brain has to be trained to not think classically and perhaphs think beyond time-space.
    Last edited: Oct 2, 2013
  20. Oct 2, 2013 #19


    Staff: Mentor

    This sort of stuff is very much tied up with interpretation.

    If collapse is even a physical process is far from certain.

    In some interpretations, such as the ensemble interpretation I hold to, and even Copenhagen, its like probability theory. Before you throw a dice there is a probability of 1/6 that any face can come up. This is represented by a vector with 6 entries and in a sense is the state of the dice. Now throw it and a face comes up - it is now a dead cert that face is up. The state changed from a vector with 1/6th in each entry to one with a 1 in one entry. The state has collapsed. But nothing in a physical sense changed - simply our knowledge about the dice. Some interpretations of QM view state in a similar way.

    If so questions like where does it get the energy to collapse are pretty meaningless.

    One view of QM is its simply an example of a generalized probability theory:
    http://www.mathematik.uni-wuerzburg.de/sommerschule2011/download/ln-janotta.pdf [Broken]

    Last edited by a moderator: May 6, 2017
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