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Wave/Particle Duality in Light Quanta

  1. Aug 28, 2003 #1
    [SOLVED] Wave/Particle Duality in Light Quanta

    All right. It's a really simple concept to state, but a really difficult one to grasp. I'm curious about a specific experiment, if someone could explain it to me. The one slit/two slit experiment. With a constant stream of light and one razor-thin slit, we see an illuminated circle on a wall. With two slits, we see alternating bars of light and dark, the light ones decreasing in intensity as they extend toward the periphery. This is not what I am curious about. What I want to know is why, when a single photon is fired through the single slit (call it Slit One), it can land on a spot where, during the first experiment, there would be a black bar, (we can track it using a photographic plate), but if we open up Slit Two and fire the photon through Slit One the photon lands elsewhere.

    The information I have regarding the phenomenon is slightly outdated, so I was wondering if there was a new theory.
  2. jcsd
  3. Aug 28, 2003 #2


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    In the first case (one slit) ther is no challenge to the photon's particle nature, so it behaves like a particle and hits the target.

    In the second case (two slits), the photon is given a chance to interfere with itself. This is only possible to a wave, so its wave nature takes over and it produces the interference pattern.

    In the wave particle duality, the entity will exhibit whichever nature is appropriate to the experiment at hand.
  4. Aug 29, 2003 #3
    Huh. How exactly would a single photon interfere with itself, one, and two, how would that photon be able to discern between the two experiments? Or is that why you referred to it as an "entity?"
  5. Aug 29, 2003 #4


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    We don't really know.

    We don't really know.

    Well... there are theories, but we really don't know for sure. Most think that the photon exists as a probability waveform and so goes through both slits at the same time, and then "collapses" when it hits the barrier and is measured.
  6. Aug 29, 2003 #5

    Chi Meson

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    This does not actually describe what is seen . THe one slit diffraction/interference pattern is similar to the two slit in that there are alternating dark and light "fringes" of light on the target screen. The pattern depends on how wide the slit is.

    The two slit pattern itself depends on the separation between the slits.

    WHen light goes through two slits, we see a combination of the two patterns superimposed on each other. One pattern due to the slit width, and the other due to the slit separation.

    Could someone else verify if this is right? I'm positive that if you get a dark fringe in the single slit pattern, then that dark fringe will overlay (wipe out) any bright fringe due to the double slit pattern.
  7. Aug 30, 2003 #6


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    Greetings !

    One conceptual way to look at wave-particle duality and
    QM in general is to say that: When we're "not looking"
    nature will take all possible courses - use up all the
    possibilities that its fundumental laws dictate, however
    when we look at it - we will only see a single solution.

    The tricky part is that "cheating" doesn't work - you
    could observe the same event at different times and at different
    locations including cases when it would require the info
    to travel at faster than c and even instantly between your two
    different points in space-time and yet the solution into which the
    "unobserved" nature = all possibilities, collapses into,
    will still remain the same every time.

    One way to look at it is simply ignore the problem -
    say that it's not real and in fact nature does not
    follow every path. But then you must ignore the evidence
    AND come up with an alternative which no one has yet,
    apparently, succeeded to do.

    Another way is to simply accept the problem and keep on going.
    The problem with that approach is that we can not
    build all of our science using reasoning which our
    science rejects and we do not appear to be capable of, so far,
    finding the appropriate reasoning that we should use
    because QM violates the fundumental premises of
    any kind of reasoning we're capable of, so far.

    In short, it's QM - quantum mess.

    ( Nope, I'm not a philosopher, though I do have some
    related inherent tendancies... )

    Live long and prosper.
  8. Aug 30, 2003 #7
    You could say that all this is leading back to where the argument originally started , qunatum field theory states that electromagnetic fileds exist at every point in space. Surprisingly this fact is not considered when contemplating the two slit experiment.
  9. Aug 30, 2003 #8


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    This is incorrect. The field which exists at any point in space is a superpostion (sum) of all sources acting on that point. This is a vector sum, so that various contributions can total zero.
  10. Aug 30, 2003 #9


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    I tend to believe that the concept of a photon being a "particle" localized in space is a vast oversimplification. It is this flawed model which creates the problems. A better model would be to see a photon as being energy in the form of a wave packet spread across a region of space, when this energy encounters an atom, it causes electron transitions. The energy is only localized at the instant of enteraction. It has a spatial extent and is thus able to interfere with itself in the double slit experiment. Just where that bit of energy will be adsorbed by the screen is a matter of probability.

    Free yourself of the image of subatomic particles as small billiard balls, it just aint so. As long as you maintain that visual image QM simply does not and cannot work.
  11. Aug 30, 2003 #10
    You are discussing the famous Taylor's experiments. On of the great
    puzzles in quantum mechanics. The new theory that addresses this experiment from the point of the fractal structure of matter is Eugene Savov's theory of interaction (http://www.eugenesavov.com)
  12. Aug 30, 2003 #11
    It has already been said above that it is the picture of a particle that is at fault here. Observations should lead the model, and in this case, observations tell us that when we speak of a particle like a photon, we are not referring to a bb (i.e., some hard object with definite shape or even of zero size and definite trajectory). Hence the language leading to confusion: "the photon interferes with itself".
    Let's use the photon and double slit thought experiment. In quantum mechanics, you can still think in terms of definite point particle paths (as Richard Feynman did -see Feynman Lectures in Physics Vol 3 ch.1-), but with the following caveats: (1) you must draw all the possible paths the particle can take from the starting point to the photographic plate, and
    (2) to each path is associated a phase (this is the contribution of what is called the "wave nature of particles" in popular accounts).
    You can consider just two types of path for simplicity: one path for the particle to go through one opening, the second path for the particle to go through the other opening. When the paths meet at some point on the photographic plate, they will have a relative phase. If the relative phase is zero (i.e., they are in phase), then this point on the plate is a possible point where the photon will end up. If they are exactly out of phase with each other at some other point on the plate, the photon will never end up at that point.
    If you send a bunch of these photons one by one, they will appear most often at the points on the plate where the two paths were perfectly in phase, never appear at points on the plate where the paths were exactly out of phase, and will appear somewhat often at points where the two paths were "somewhat in phase" (between exactly out of phase and perfectly in phase).
    Those are the rules of quantum mechanics in terms of a point particle picture. If you ask *why* particles behave according to these rules, *that* we don't know. But if you accept these rules, we can explain and predict the behavior of more complicated systems in terms of them.
    Now, I mentioned that you can view quantum mechanics as 'point particles taking all paths possible', and this was a common view even by the developers of what is now quantum field theory. However, this is not the modern picture of what a particle is. Modern quantum field theory does not have point particles as the fundamental entities zipping around. Rather, something known as a "quantum field" is viewed as the more fundamental entity (don't attach any metaphysical meaning to 'entity' here). Particles are excitations ("quanta") of these quantum fields (and as a consequence are what we described as quantum particles above). When you calculate how e.g. an electron will interact with another electron electromagnetically, you start with two electrons (never mind that they are quanta of their own field, an "electron field")...then having a "photon field" present, you can compute how the electrons will will interact with each other via the photon field. If you want you can say that the electrons interact via fundamental units of the photon field: photons are exchanged in all possible ways and numbers between the electrons.
    The notion of a qauntum field, its quanta, and their interactions is subtle conceptually, and the best way to understand it, unfortunately, is to study it. But perhaps all that's important is the following: new rules were needed in the early 1900's for how things like electrons act fundamentally (and therefore how we view what a 'particle' is). Then we later found out that to describe interactions among these particles required newer rules that again modified our view of what particles were. The modern view is given in terms of quantum field theory.
    As a final note, these fields should not be confused with classical fields like the electromagnetic field. An electromagnetic field that exists in some region of space is a quantum state of a bunch of photons (I say quantum state to emphasize the fact that the collection is not a bunch of individual bbs...in fact the number of photons is not a definite thing for this macroscopic field).
  13. Aug 31, 2003 #12
    Regarding the Double-Slit experiment:

    What about the background? Has anyone surmised that there could be a connection with the atomic arrangement of the background that necessarily produces interference patterns?

  14. Aug 31, 2003 #13
    Answer me this... Physicists have no problem accepting the fact that the photoelectric effect can't be explained with a wave model of light. So why can't they accept that interference can't be explained (satisfactory) with a particle model of light ? And why this focus on the double slit ? Why isn't anybody trying to explain photoelectricity with a wave model ?
  15. Aug 31, 2003 #14


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    Er... we do?
    Because it shows both to be insufficient, thus giving rise to wave-particle duality?
  16. Aug 31, 2003 #15
    What would it take for a wave model to be sufficient ? .. Explanation for single photon hits resulting in interference patterns ? Anything else ?

    Exactly what is the problem with a wave setting off a single photocell in a detector ?
    Last edited: Aug 31, 2003
  17. Aug 31, 2003 #16
    Javier - Nice explanation. Thanks!
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