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How does a photon carry force?

  1. Apr 16, 2010 #1
    I've looked through about 40-50 of the threads dealing with photons and have not found one that describes how the photon transmits the force of the EM field.

    In Feynman's QED, he talks about how a nucleus keeps an electron in orbit by exchange of photons, but I don't see how a photon can provide a push, much less a pull.

    If I think of photon as a light quanta and consider light as a perturbation in the EM field, I'm stymied by the fact that the ripples in the EM field are transverse to the direction of propagation. A particle at (1,0,0) transmitting a light quanta to a particle at (0,0,0) creates an EM wave that oscillates in the y- and z-directions, so this does not seem availing to a force in the x-direction.
  2. jcsd
  3. Apr 16, 2010 #2


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    Hi FireBones! :smile:

    (leaving aside the question of whether photons do transmit EM force …)

    The ripples look longitudinal to me, just like the ripples when you throw a stone into water.

    And the energy flow of an EM field with (transverse) electric and magnetic components E and B is in the direction of the Poynting vector, E x B.
  4. Apr 16, 2010 #3
    Consider an electron some distance, much more than the size of an atom, from a proton. (Let's not worry for the moment about what happens at the atomic scale.)

    The reason the electron accelerates is because of interference between the case where a photon is exchanged, and the case where it isn't.

    To start, do you understand why the electron doesn't accelerate if the proton isn't there? Why it approximately obeys Newton's first law?
  5. Apr 16, 2010 #4
    I'm not concerned with energy flow here. I'm interested in force.

    Ripples caused by a stone thrown into water are transverse.
  6. Apr 16, 2010 #5
    Jeblack, I don't really know what to make of your question... I guess I don't know why an electron wouldn't obey Newton's first law...

    But I am interested in understanding how "interference between the case where a photon is exchanged and the case where it isn't" causes the acceleration.
  7. Apr 16, 2010 #6
    An electron pushes on all other electrons around it.

    If electron1 is pushing on electron2 and e1 accelerates in a perpendicular direction the pushing force of e1 on e2 will now have an additional component in this perpendicular direction.

    This component is what we measure as the force of the electromagnetic wave on another electron. The original electric force is not commonly included in equations, though it comes up every once in awhile.

    The act of applying a force in a way is like making a connection between you and the object. If you are pushing on a wall and you simultaneously start pushing up with your legs you will find that you are now pushing the wall both away from you and up. Not that it will go anywhere.

    For a complete understanding see http://web.mit.edu/smcs/8.02/
    Last edited by a moderator: Apr 25, 2017
  8. Apr 16, 2010 #7
    The photon provides kinetic energy, the direction does not matter. The reason why is because there is no physical state that exists where the electron crashes into the nucleus. This means that if an electron is given kinetic energy it will always be beneficial to keeping the electron in orbit.
  9. Apr 16, 2010 #8
    I don't see how this relates to my original question.

    Yes, if one electron moves perpendicular with respect to the displacement vector between them, the new force will have a component perpendicular to the old displacement vector...but what does that have to do with anything?

    My question is "how does a proton (at rest) exert force on an electron by means of a photon." How does a photon "carry force" in the x-direction if its EM perturbation is in the y/z plane?
    Last edited by a moderator: Apr 25, 2017
  10. Apr 16, 2010 #9
    The same way the force from an electron still maintains a force in the x-direction when the EM is in the y/z. In an EM wave there is always a force in every direction, because of the pure electric and magenetic forces.
  11. Apr 16, 2010 #10
    I'm not interested in "why doesn't the electron crash into the nucleus" questions...I'm interested in how the photon allows the proton to exert force on the electron. And I fail to see how force [a vector quantity] can be articulated upon an object purely by its receipt of energy. Clearly direction does matter since force is not a scalar.
  12. Apr 16, 2010 #11
    Nothing you have said has explained how a photon allows one charged particle to exert forces on another.
  13. Apr 16, 2010 #12
    Doesn't, "By adding energy and possibly causing emission" answer that question? It seems fundamental to the issue of emitters and absorbers, and I'm not clear where your confusion is coming from.
  14. Apr 16, 2010 #13
    How charged particles exert forces on each other I do not think we have an explanation for.

    What you are asking is how does the electric force exist.

    There may be a QM explanation for it, but its way beyond me if there is. From what I understand we only know the properties of the electric force.
  15. Apr 16, 2010 #14
    The energetic (non virtual) photon is the one you describe and it would transmit a transvers force. These are waves.

    The force you describe is longitudinal but the photons have no energy. They are virtual with infinite wavelength (or perhaps an undefined wavelength?).

    A virtual photon is not a free quantum like the light emitted by an atom. It has zero energy.
  16. Apr 16, 2010 #15
    Because simply adding energy does not explain the exertion of a force.

    Forget the proton/electron example for now. Just consider two electrons in free space. One electron [A] at (0,0,0), the other at (1,0,0).

    Now, how does a photon traveling from A to B pull B toward A? Simply saying that "B absorbs the photon, increasing its kinetic energy" doesn't give any indication of why B should be accelerated in the direction of A.

    energy is a scalar, so you cannot explain a force exerted by an object as purely due to the energy... To use a rather silly personification: an electron that gains energy might know it should be speeding up, but it won't know in what direction.

    Furthermore, if electron B happens to be moving with respect to electron A, this force might not be increasing its kinetic energy.

    The issue here is not ionization energy and atomic physics. I was just using Feynman's book as an example of someone talking about force exchange through photons. I'm talking about the very general question of "how does a photon exert force in a direction parallel to its transmission?"
  17. Apr 16, 2010 #16

    The only current answer I know of is that involving the exchange of virtual photons, which to me is the same as "We can make it work on paper, but this isn't real life." As for assigning an electron a direction... that seem to fly in the face of how an electron is defined in QM.
  18. Apr 16, 2010 #17
    That helps a little...at least it basically says "Anything you know about photon as a light quanta has nothing at all to do with how photon act as force carriers."

    But it also seems to me that you are saying we have no idea how or why photons exert force...but rather that they simply do.

    Do these virtual photons have momentum? It seems like they must since they are essentially transferring momentum from one particle to the other. Except it would also mean that, in some cases, the photons have momenta in the direction opposite to their motion

    Can anyone elaborate/confirm/correct this?

    it seems pretty strange for photons to be conceived of in these two completely different ways...
  19. Apr 16, 2010 #18
    I think we need a mentor or advisor on this one. Tiny Tim... Don't go tiptoeing through the tulips yet!
  20. Apr 16, 2010 #19
    The way photons exert forces on charges is by modifying the phase term in thei QM wave function. The presence of an electric field will shift the position and momentum of the electrons wave packet. One might say there's no force, just a readjustment to the dynamic variables of the electron. In doing this the photon is absorbed (including it's energy). This is how it works for virtual photons too except the QM phase change is pointed longitudinally and the energy gained doesn't come from the virtual quantum but from the radiation fields of the accelerating charges.

    In other words no energy is expended by pulling on something. It has to move as a result of your pulling on it.
  21. Apr 16, 2010 #20
    ... So this is just another aspect of Perturbation in a QFT?
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