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Definition Of A Photon?

  1. Sep 6, 2007 #1
    I have been told many different stories of what light is could someone clarify exactly what it is? At the moment i am being told it travels by displacing matter but doesn't this mean that it would lose energy and eventually die? I thought it was supposed to be stable. Also how does it travel through a vacuum where there is very little matter at all? I guess what i have been told isn't correct?
  2. jcsd
  3. Sep 6, 2007 #2
    As a rule: physics can't ever say what something *is*. It can only say what something *does*. However, we all like to have consistent mental models which does what the maths say, and a photon is one such model. A photon is just a particle, which is massless, travels at the speed of light, has spin 1, and interacts with electrons and other charged particles, as well as with itself but only weakly.
  4. Sep 6, 2007 #3
    I like that explanation better than all those people who complain about people using *why* in their questions. (except maybe for the 'can't' and 'only'--throw in a 'usually'?)
  5. Sep 6, 2007 #4
    (Sorry rewebster and genneth, but I'm going to use "is" in this, because the easiest way to describe the photon model of light is to say "a photon is").

    The photon is a single particle in the particle model for light.
    Before quantum theory, and the concept of wave-particle duality, the nature of light was disputed. Some argued in favor of the particle model, others argued in favor of the light model.
    Young's double slit experiment seemed to verify that light was a wave, until it was discovered that the same effects could be seen if light was let out very weakly (one photon at a time). The double slit experiment showed that light behaves occasionally as a particle (photon), and occasionally as a wave.
    An electromagnetic wave consists of an electric field and a magnetic field perpendicular to one another. Moving electric charges create magnetism, while moving magnetic fields induce electric charges. They can travel across vacuum because it is self-propagating in this way (each field produces the other). It doesn't require a medium, because it is its own medium (hope that's accurate).
    I don't know what your source means by "displacing matter." Photons have no rest mass (they do have mass proportional to their wave energy while moving).
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