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Light waves? Particles? Why?

  1. May 27, 2010 #1
    I understand that there are multiple tests proving that electromagnetic waves carry energy through photons, but what causes the electrons to emit both waves and particles? I also don't understand how light is self propelling if anyone could explain that. I know it is related to the creation of magnetic and electrical fields but don't both of those fields require electrons to be spinning? Ontop of that, isn't the energy of light just kinetic energy, and wouldn't that mean that anything with momentum must have mass, implying that photons do intact have mass? Help!! (keep in mind I'm only a highschool student)
  2. jcsd
  3. May 27, 2010 #2

    Char. Limit

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    I can help you out with a couple of issues, I think. I'm also a high school student.

    First, the energy of light, as far as I can remember, isn't kinetic energy. It's electromagnetic energy. And while photons do have momentum, the momentum is not described by p=mv, but by Einstein's Equivalence Equation (it's called something like that), which is...

    [tex]E^2=p^2 c^2 + m^2 c^4[/tex]

    Now, as you can see, if an object is at rest and has mass, the equation reduces to E=mc^2. If an object is massless, like the photon, then the equation reduces to E=pc, and a photon's momentum is defined by p=E/c.

    Hopefully someone more qualified than I (easy to do, I assure you) can resolve the other issues.
  4. May 27, 2010 #3
    An Electron which has been previously excited into in higher energy state, will jump back down into a lower state, emitting a photon.
    The is a quantum phenomena, because the possible energy states are discrete.
    This is one way of understanding the packet-like or "particle" property of emitted photons.
    These photons, however, can self-interact, as in the double-slit experiment.
    In quantum mechanics, all tiny particles can be seen to have this property.
    It's just how nature works that photons have dual properties of both particles and waves.
    It seems to me a mistake to say they are at once "both waves and particles", since they are neither!
    They're just photons!

    Light does not need propulsion.
    Light moves at the speed c as a consequence of Maxwell's Equations.
    Alternately, I could respond that it is a wave in the electric and magnetic fields.
    An electric field produces a magnetic field, and if these happen perpendicular to each other, the process is self-repeating.
    See: http://hyperphysics.phy-astr.gsu.edu/hbase/waves/emwavecon.html

  5. May 27, 2010 #4
    but since E = T + m

    and E^2 = p^2 + m^2

    then E = T = p

    for a photon..
  6. May 28, 2010 #5
    I thought it was E^2=(pc)^2+m^2c^4 , but maybe your talking about something different , and if a photon had mass then that means that we could slow it down and we cannot slow photons down they always travel at c .
  7. May 28, 2010 #6


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    Units where c=1.
  8. May 28, 2010 #7
    no I work in the units where c = 1 ... a common trick in physics to just work in those units where your expressions becomes simple... there are different units than SI you know ;)

    and the answer is still that the photon energy is indeed kinetic
  9. May 28, 2010 #8
    i see
  10. May 30, 2010 #9
    okay i understand a bit better now. This is a little bit off topic but for spacetime, all of the models that I see or read about have a 2-dimensional plane being bent by a 3 dimensional object. How can this happen when we live in a 3 (as far as we know it) spatial dimensional world with 3 dimensional objects in it?
  11. May 31, 2010 #10
    Ax3111, I think the pictures you are referring to are supposed to be graphical analogies. If you viewed them from the perspective above the drawing, and placed another body in those drawings, then the motion of those bodies as viewed from above will approximate reality and the equations.

    Just in case I am not being clear enough, if you placed a bowling ball on a trampoline, it'll prob roll to the middle and we now have a massive object. If we could view the trampoline from above, and we then placed a smaller bowling on the trampoline, assuming the trampoline was carefully crafted (such that its fabric modeled the "fabric of the universe"), the motion of that bowling ball around that other massive object, might be analogous to, say, the moon and the Earth.

    Does this make sense?
  12. Jun 1, 2010 #11
    I understand they're supposed to be analogies, but again you're taking a seemingly 2 dimensional plane (the trampoline) and putting 3 dimensional objects on it to make the plane bend into a 3rd dimension. But in the seemingly 3 dimensional plane that we live in and observe, how does a 3 dimensional object bend this plane to create gravity? Would it bend into an unobservable 4th dimension?
  13. Jun 1, 2010 #12
    The real situation is that 3D objects warp 3D Space, analogous to how a bowling ball warps a trampoline. How does it do this? Good question, maybe. It just does, as far as I know, i.e. that's how mass interacts with space-time. Mathematically, I think it is the case that the field's strength varies. I don't know if there is actually any "folding" taking place.

    I am not an authoritative voice on the matter (obviously) but if it helps any, that is how I have always perceived it. General Relativity is next quarter for me :P
  14. Jun 1, 2010 #13
    For more very similar discussions just search photons or wave particle duality here...or at Wikipedia.... ...
  15. Jun 1, 2010 #14
    I have issues with the "bowling ball on trampoline" analogies as well.

    I wonder if people have mixed up cause and effect. Einstein's equations describe the effect of gravity mathematically - as far as I'm aware they don't describe the cause.

    So the idea that objects themselves 'warp' time is..premature, perhaps?
  16. Jun 1, 2010 #15


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    Einstein's Equations describe how stress/energy gravitates -- they determine how spacetime responds to a source of stress/energy. The geodesic equation (which follows from Einstein's Principle of Equivalence) describes how matter moves as a result of spacetime curvature. So together, these equations come full circle -- matter tells space how to curve, space tells matter how to move. Nothing premature here.
  17. Jun 1, 2010 #16
    It doesn't emit "both particles and waves". Quantum objects have rules of motion that are similar to what you know about waves.

    I suggest you read QED: The Strange Theory of Light and Matter by Richard P. Feynman. I see it on Amazon for $2. You can also find the audio, as it was originally presented as 4 lectures.

    How is anything in motion? Photons move like any other particles.

    You mean the non-quantum model, a solution of Maxwell's Equations? A changing electric field creates a magnetic field; a changing magnetic field (including one that's changing because it's being brought up to strength or fading away again) creates an electric field.

    Looking at it as classical fields and looking at it as a collection of photons are different models. Don't try and mix them up. The classical view is just fine for applied radio engineering, because the photons are very weak and the waves very large, so it behaves like a continuous field just fine. This has been discussed in other threads recently. (you might look at threads I've posted on to get a small list)

    A created electric field (from a magnetic field) doesn't involve the presence of charged objects. That was a breakthrough, in realizing that the "field" can be understood separate from a physical object.

    In permanent magnets, the magnetic field is due to spin. That's another story.

    Momentum = M × V, but you are assuming that this is the only way to have momentum. For photons and other massless objects, momentum is h/λ. Both are special cases of the full relativistic formulation, which is discussed elsewhere on this thread. M×V is only an approximation for V being very slow relative to the speed of light.

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