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Particle or wave question

  1. Nov 30, 2012 #1
    Hi to all
    This may be a stupid question.

    1.How much evidence is there proving light to be particles rather than waves? Like the double slit experiment, using a single photon. Are there other experiments with a similar outcome?
    And 2. How sure are we that we are actually firing a single photon in the said experiment? (it could, for example, be just the smallest amount of light that could still be considered light.. )
  2. jcsd
  3. Nov 30, 2012 #2

    Simon Bridge

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    Welcome to PF;
    Those are actually decent questions.

    1. Usually the photoelectric effect is cited as pretty definitive for this - a telling trait is that sometimes you get an ejected electron the second you turn the instrument on (i.e. the light wave has not had time to deliver enough energy.)

    What distinguishes the (classical) wave from the particle model is that the particles deliver their energy in one sharp hit.
    QM accounts for things like interference patterns by having the wave-behavior in the statistics rather than the actual object.

    2. You don't need to fire a single photon ... just like you don't need to isolate a single electron to prove that charge is quantized.
    http://www.cnrs.fr/Cnrspresse/en25a4.html [Broken]

    "The smallest amount of light that can be considered light" is the definition of "photon".

    But I think you are imagining switching a light sourch on and off fast enough that only one photon gets fired out? In fact, the light source is so dim that only individual photons traverse the apparatus in one time ... in a typical experiment, the average distance between successive photons is over a kilometer (i.e. the probability that two or more do so can be made vanishingly small). The experiment is repeated many time to collect statistics (in case two or three did make it through sometimes) and the photon detectors always register light energy in randomly spaced lumps rather than as a smooth delivery ... these days we can use detectors good enough to register a single photon hit.
    http://qubit.nist.gov/qiset-PDF/Nam.QISET2004.pdf [Broken]

    There is a semi-classical formulation that tries to account for this.

    I understand there is a purely wave theory as well ... also not classical waves.
    However, I think it is most useful when you are learning about particle physics to start thinking of everything in terms of fundamental particles: just to make the break with classical waves and particles and any lingering ideas about "duality".

    The following lecture series often helps people understand the concepts:
    ... Richard Feynman on QED and wave-particle duality.
    Last edited by a moderator: May 6, 2017
  4. Nov 30, 2012 #3


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    There are well established methods for doing this that are based on measuring correlation functions.

    It is imporant to understand that single-photonics is "everyday science" nowadays, and is nothing special as long as you are talking about photons with large energies (things get a bit trickier in the infra-red and microwave regimes because of technical problems).

    You can get single photon sources that can generate photons "on demand" ("machine guns" ) and detectors that can register single events (without much need for statistics). It is true that neither the sources not the detectors are perfect (problems with dark counts etc), but that is true of all experimental equipment and it is the job of the experimentalist to understand and work around those problems.

    Also, note that single photon sources/detectors have been used in commercial quantum cryptography equipment for a number of years.

    My main point here is that this is extremely well-established and well-understood science which has already resulted in relativly mature commercial technology.
  5. Nov 30, 2012 #4
    Thanks for the answers. The reason for my question, in essence, is; I find the wave particle duality, perhaps in ignorance, as sloppy results. As if Occam's razor alarms should sound.

    I can visualize the wave, moving, as with light, as radiation at a certain frequency.

    Traveling at the speed of light is easy to visualize for a wave but hard for a particle.

    I struggle to see how energy can be converted to a particle (and back) at will. For example, if stars were emitting particles, shouldn't the mass of the universe be affected at the very least?

    Most of all, I am baffled by the "single photon" going through 2 slits, except if observed or rather detected.

    Possible explanations I can think of:
    1.That it was never one photon particle that was fired, rather the smallest amount of radiated light wave that still has the original properties.
    2.That our understanding of the fabric of space is too limiting and that the propagation of light through the medium of space is hiding the answer *
    3.That the method of observing/detecting the light at the slit affects the experiment

    *Imagine light warped space in a similar way matter does, causing a tiny gravity-well in the space around it or even just a mere disturbance in the fabric of space; how would it affect the experiment when one photon is fired through through the two slits. The way I have it, we don't know much about the propagation of light, in the way we understand that sound is pressure waves in atmosphere.
  6. Nov 30, 2012 #5


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    As for evidence, we have plenty. Practically all of the which-way experiments (see http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf and B.J. Pearson and D.P. Jackson, Am. J. Phys. v.78, p.471 (2010).) and photon antibunching experiments are evidence for QM's photon picture of light. And note that this picture can also arrive at the so-called wave description of light without having to switch gears.

  7. Nov 30, 2012 #6
    Thanks, I shall look up the given. Good stuff,
  8. Nov 30, 2012 #7


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    You've missunderstood what the "duality" is all about. No one belives that light is "converted" from one form to the other, the duality referes to the fact that light has wave-like and particle-like features, and which one "dominates" depends on the experiment you are doing. But light is neither a "particle" (in the newtonian sense) nor a wave (in the "water" sense), it is something else we don't have a good intuition for.
    Moreover, this is only an issue when you try to interpret youre results, the mathematical description of light is consistent and never requires you "choose" between waves and particles if you use the full formalism.
  9. Nov 30, 2012 #8
    What I meant was: use energy to make a wire glow with heat (light bulb). The heat cause photon particles to appear, travel at just under 300 000km/s, hit an object causing heat on object. Converting energy into particle and back.

    Your explanation did shed more light on the subject though.
  10. Nov 30, 2012 #9


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    E = mc2. Happens all the time. :smile:

    Total mass is not conserved. Total energy (which includes mass) is conserved.
  11. Nov 30, 2012 #10

    Simon Bridge

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    Feynman addresses wave-particle duality in an accessible way.
    I think the main conceptual challenge concerns the habit of thinking in terms of classical particles and waves.

    Energy can get changed from one form to another ... the particles are another form of energy. For light ##E=hf##, and for massive particles ##E=\gamma mc^2##.
  12. Nov 30, 2012 #11
    In a classical context they are waves since we describe them with maxwells equations.
    The "particle" side only becomes useful in QFT, even in QM we never speak of photons in a rigorous way.

    I've always wondered how the term "particle wave duality" came to being, the schrodinger equation has nothing to do with waves.
  13. Nov 30, 2012 #12

    Simon Bridge

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    There are endless histories of the term online :)
    I think it came about with that matter-wave chap where is it, oh yes:

    de Broglie L. (1924) Recherches sur la théorie des quanta [PhD Thesis! Sorbonne]

    But the idea goes back to Newton's time.
  14. Nov 30, 2012 #13


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    If you are going to study quantum mechanics, you HAVE to get over the idea that concepts such as "I can visualise ... ", "It seems intuitively ... ", and the like are in any way helpful. They will just lead you astray.

    Our human experience is based on scales that do not include cosmological scales or quantum scales, so our "common sense" / "intuition" etc are not designed to be helpful at those scales.
  15. Nov 30, 2012 #14
    We really should stop using the term wave, QM has nothing to do with waves
  16. Dec 1, 2012 #15
    So in essence it is more of an abstract description of it's behavior, rather than what it actually is.. This I can visualize again, to some limited extend. It's hard to try to not visualize. And I suspect that the terminology wave particle duality must originate from an attempt to visualize the damn concept.
  17. Dec 1, 2012 #16
    Well particles are represented by states, if you project the state in a position basis you get this wave looking thing for some hamiltonians
  18. Dec 2, 2012 #17
    Is the propagation mechanics of light moving through space known? How it is "conducted" and how does it affect space. It is mass/energy after all = gravity or bent space? And the speed of light, perhaps a max amount of change possible in relation to mass and energy within certain constraints to conserve energy in the universe. (Almost e=mc2) This is probably not QM but certainly super relevant to QM. Forgive my possible blundering, I am untrained in this discipline.
  19. Dec 2, 2012 #18

    Simon Bridge

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    You mean "does light have gravity?" or, "do we know how a photon got from A (source) to B (detector)?"

    I think the speed that light travels at is understood as a property of space.

    ... light has gravity.

    (Off the Feynman lectures prev.)
    ... we consider the exact path of a particular photon to be uncertain. The classical trajectories are what you get on average.

    There is a relativistic formulation of quantum mechanics - usually covered at post-grad level.

    Does that help?
  20. Dec 3, 2012 #19
    Both. How does it move from A to B; There must be a form of conductance to carry the information.
    If light (a photon (I struggle to say a photon because in my mind it is counter intuitive)) is/has energy and a mass, (to my understanding e and m is almost interchangeable but not quite) it must affect space and have a gravity of sorts.

    Now I wonder how this gravity or affected space around a photon could affect what we would observe at the two slits. For example what if the two slits were further apart?

    I suspect (probably in ignorance) that the way light travels and gravity could tell us a lot about the structure of space.
  21. Dec 3, 2012 #20

    Simon Bridge

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    Well then - both questions have already been answered for you :)
    I have provided a "tldr" version in my previous post with a link for a more detailed version. You should read/view those links in order to better frame further questions.

    In the EM-wave model for light, the electric and magnetic fields sustain each other through space with no need for a conductor or a medium.
    In the standard (particle) model, photons get from A to B the same way as any particle. Do you have any trouble with the idea that an electron can travel through a vacuum without having a conductor there? The details of how light, or any particle, gets from A to B, is covered in the Feynman lectures linked to earlier.
    Light does not have mass.
    Mass and energy are interchageable through the mass-energy relation.
    We know how the 2-slit experiment is affected - you can look and see!
    You know that - just move the slits further apart and see.
    The way light travels does tell us a lot about the structure of space ;)

    You need to check out those links before you reply again.
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