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Photons as waves

  1. Aug 19, 2008 #1
    I have a question regarding the behavior of photons, or anything for that matter, when treated as a wave.
    When I think of waves, I think of sound, sea, earthquakes, etc... and a common theme amongst these is a medium through which they propagate (air, water and rock). In this way I have no problem understanding (destructive) interference, two adjacent waves both trying to move the same element of the medium in different directions, therefor canceling each other out.
    I have some problems applying these same principles to light. Would a light wave not require a medium to propagate in?
    If (as I suspect), there is no medium, how does a light wave exist? Is it simply that a photon vibrates? and as such behaves like a wave? In this situation how would interference occur?

    Apologies if this is obvious, but it's always confused me!
  2. jcsd
  3. Aug 19, 2008 #2
    I am no PhD but I will take a shot at answering this for you-

    I believe you are referring to wave-particle duality that photons appear to exhibit. The wave-like nature of light can be seen in the double-slit experiment where light intensities are varied due to interference.

    Photons behave in the particle sense as they can transfer energy upon interactions with other forms of matter. (E_photon = hv)

    Photons have no "rest mass" which is used in relativistic calculations, thus allowing the photon to travel in a vacuum.

    *if I am wrong please feel free to correct me and punish me accordingly :)
  4. Aug 19, 2008 #3
    There is no medium or vibration in space. The "wave" is an oscillation of the electromagnetic field over the course of the journey of the photon. Imagine a tiny little dot of light - a point - moving through space, oscillating back and forth between red and blue as it moves. If you plotted the EM field over x or t, you'd see a wave, but the photon is not vibrating in either space or time.

    As for other quantum particles, the "wave" has to do with the probability of finding the particle. The particle itself is not oscillating back and forth as it moves. The waves are purely mathematical constructs with no physical meaning except in one particular interpretation (Bohmian Mechanics).
  5. Aug 19, 2008 #4
    take an example: - Radio transmitter antenna. Electrons slosh backward and forward in the aerial, driven by the transmitter power.
    These electrons produce a sinusoidally varying electric field. propagated at a speed determined by the permittivity constant of the medium, usually air maybe vacuum.
    The electric field produces a complimentary magnetic field which propagates at the speed determined by the permeability constant.
    The magnetic field produces a complimentary electric field, etc, etc.
    So these fields ratchet themselves through space at a combined speed of 1/sqrt(Permittivity x permeability) this is the speed of electromagnetic propagation (speed of light).
    Photons are a confusion introduced by the way physics is taught. All we really know is that energy can only be extracted from an EM field in chunks equal to hf.
    Why this is so is a quantum mystery.
    Do not think of photons as little particles, but as wavepackets where the packet energy E = hf.
  6. Aug 20, 2008 #5
    They also have physical meaning in Cramer's Transactional Interpretation of Quantum Mechanics:
  7. Aug 20, 2008 #6
    In clasical physics, light is an electric "field" perpindicular to a magnetic "field". It isn't really known what a "field" is exactly other than something with energy that exists in spacetime that has the propensity to exert force. Light is a wave because the magnitude of the electric and magnetic fields vary in space, and in time, in a periodic fashion.

    In quantum mechanics, a photon is this same electromagnetic wave. It is just the smallest possible unit of an EM wave.
    Last edited: Aug 20, 2008
  8. Aug 20, 2008 #7
    peter0302 has an incorrect view of waves. You can show the waves on an oscilloscope, you can generate a standing wave, particularly in the antenna example I gave. You can demonstrate the waves physical existance, electrical and magnetic.
    What you can't do is show a photon.
  9. Aug 20, 2008 #8
    I think it is you who has an incorrect (or at least, hopelessly classical) view of quantum waves. Photons and electrons are particles. There is no wave moving through space like sound or water. The wave only appears when experimental data is plotted or probability densities are being calculated.
  10. Aug 21, 2008 #9
    I think the classical mathematical wave model is unusable not only for describe photons and other quantum objects, but for usual acoustic waves. If you remember, acoustic waves consist of quantum particles – phonons. Phonons obey the Plank’s statistical and have many photons’ properties. Thus, wave is rough classical approximation.
  11. Aug 22, 2008 #10
    in a system of springs and masses, wave energy passes back and forth between the springs and the moving masses. if the masses where replaced with completely massless charges then it would still work because it takes energy to move a charged particle. the energy going into the magnetic field. not just moving particles but any changing electric field produces a magnetic field. likewise a changing magnetic field produces an electric field. in a light wave energy passes back and forth between the electric and magnetic fields in a way that is similar to a simple system of masses and springs.

    the medium is simply called 'space'.
  12. Aug 22, 2008 #11


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    And a particle is a very very small ball? ;-)

    you are right that there are no such thing as an electron jiggling up and down while it is moving, but quantum mechanics also raises the question *what is a particle?*
  13. Aug 22, 2008 #12
    Electromagnetic waves travel WITHIN space-time. Space-time is your media.

    The interferometer was used to disprove the ether. It will be used again to show propagations of space-time in detecting gravity waves. (Google LIGO and LISA projects).

    If needing a media bugs you, consider the media space-time.
  14. Aug 22, 2008 #13
    In the double slit experiment, how do you explain your "billiard ball" electron acting differently when both slits are open, than when one slit is open?
  15. Aug 22, 2008 #14


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    An electron is an electron. The electron has billiard ball (classical particle) properties, and wave properties.

    BUT, the word 'particle' in context of QM often refers to this, that it has two properties. So one can say that 'an electron is a particle' if one by 'particle' mean: 'an entity that has has billiard ball (classical particle) properties, and wave properties'.
  16. Aug 22, 2008 #15
    1) I didn't say they are billiard balls.

    2) I cannot explain it. But we know, with a great deal of certainty, that electrons have virtually no, if not absolutely no volume in space, i.e., they're point particles in one dimension. We also know that they are always detected at one and only one point regardless of whether both slits are open. The only thing we don't know is *why* the probability of detecting an electron changes when one slit is closed versus both open. But the mere fact that we don't know *why* doesn't mean our other understandings are wrong, nor does it mean that a wave-only understanding is right simply because it's more heuristically appealing.

    But if you're going to advocate a wave-only view, I think the burden is on you to demonstrate how such a view allows you to make all the predictions (and ideally more) that orthodox QM does. Since physics was at a standstill until quantization was discovered (whereas people had been working with waves for a long time) I think that's pretty good evidence that we were on the wrong track before.
  17. Aug 22, 2008 #16
    I guess. But what is an electron but what we observe? And we know that -
    1) It has no internal structure that we've yet found, and
    2) It never appears in more than one place at a time

    If you want to impart any more meaning to the electron's behavior, then you have to deduce what/why it did while you weren't looking at it. But how can we ever know what something is doing when we aren't looking at it? Could it have morphed into Barney the Dinosaur before it hit the detector as long as nobody saw it? Maybe, but who cares.
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