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Does the force of a single photon act in 1 dimension?

  1. Feb 5, 2013 #1
    If it does, is there a way to calculate what the force of a single photon would be if it were acting in 3 dimensions?
  2. jcsd
  3. Feb 5, 2013 #2


    Staff: Mentor

  4. Feb 5, 2013 #3
    I'm pretty poor with using the correct terminology. Maybe I can rephrase and the question will make more sense.

    The fundamental interactions, in this case the electromagnetic interaction, have a strength relative to each other.

    So maybe the word is 'strength' instead of 'force' (the photon is sometimes called the force-carrier of the electromagnetic interaction, which is why I got confused).
  5. Feb 5, 2013 #4
    Maybe this is better--dropping the reference to single photons:

    Gravitation occurs in 3 spacial dimensions. Does electromagnetism occur in only 1 spacial dimension?

    If I'm framing this question correctly and if electromagnetism does occur in 1 spacial dimension, would there be a way to calculate its strength if it were to occur in 3?
    Last edited: Feb 5, 2013
  6. Feb 6, 2013 #5


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    The formulation of a QFT depends on the spatial dimension D. For the el.-mag. interaction and the gravitational interaction you have a 'potential'
    D=1: linear
    D=2: logarithmic
    D=3,4,...: ~ r-(D-2)

    You cannot say anything referring the relative strength.
  7. Feb 6, 2013 #6
    I've seen in several places that gravitation as a fundamental force is weak compared to electromagnetism. If gravity=1 then EM=10^36.

    What I'm asking is: if EM is linear, could there be a calculation to find what it's strength...or potential would be if it were like gravity and acting in 3 spacial dimensions at once?
  8. Feb 6, 2013 #7


    Staff: Mentor

    I still dont understand why you keep going back to the 1 dimensional case. We live in a 3D world where gravity and EM emanate in all directions from a source.

    Are you thinking about a StarTrek tractor beam or a laser? These would be directed along a line.
  9. Feb 6, 2013 #8
    That's why I initially referred to a single photon, which goes in a single direction. Or, is absorbed at a single point. I know that it acts like a wave before that.
    Last edited: Feb 6, 2013
  10. Feb 6, 2013 #9
    Let's try it as a thought experiment (please forgive me if I use the wrong terms, I'll do my best to clarify if needed).

    If we have some massive object like a planet, a rock or something that is perfectly spherical and then measure out some radius from the center there would be an imaginary sphere at that distance around the object.

    The gravity of the massive object would be 'felt' equally by an object located at any point located on that spherical plane.

    This central massive object is also radiating light and if a single photon could and were to interact with the imaginary sphere this interaction would take place at a single point.

    I'm asking if that single photon radiated outward from the massive object in the way that gravity does, in all three spacial dimensions at once, what would be the difference in force, strength, or whatever term works here compared to the force or strength of the point-like photon?
  11. Feb 6, 2013 #10
    how do you know the photon is NOT propagating outwards in 3-D? Can you detect that photon while its in flight? no. can you detect that photon without destroying it? no. so how do you know that photon is not everywhere in 3-D all at once, and that once you detect it, its gone?
  12. Feb 6, 2013 #11
    I know that it is considered to be everywhere at once until it is detected. But when it is detected it is at a single point.

    The question is if it could be detected at all points on the imaginary spherical plane at once (which I know it can't) what would the difference in strength be as compared to the single point which we detect in reality?
  13. Feb 6, 2013 #12


    Staff: Mentor

    I think the best you're going to get is the nasa article I posted earlier about solar pressure where you could try to determine the effect of one photon on an object.
  14. Feb 6, 2013 #13
    So Wikipedia may be a hated thing to reference by some or all, but when I talk about strength there's a good chart here that shows what I'm talking about. The 'relative strength' mentioned in this chart is the same as what I've read in several other places about the befuddling weakness of gravity.

  15. Feb 6, 2013 #14


    Staff: Mentor

    Okay, I think the chart is referring to the force felt from a charged source or the magnetic field felt from a magnetic source not the force felt from a single photon.

    So for example, two electrons would repel each other 10^38 times more than they would attract.
  16. Feb 6, 2013 #15
    More than they would attract through gravitation?
  17. Feb 6, 2013 #16


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    yes electric field is 10^38 times stronger than gravitation
  18. Feb 6, 2013 #17
  19. Feb 6, 2013 #18


    Staff: Mentor

    JC I think this thread has run its course.

    We just don't know the answer to what you're asking. In the article you referenced yes photons are mediators of EM field but given that its QM you cant start to think in a Classical Mechanical way about a single photon pushing the other particle.

    Do you see what I mean? I can't explain it further I'm not a PhD level physicist.

    I do hope you got some insight out of the thread that can help answer your question in the future but I think we've run out of stuff to say.
  20. Feb 7, 2013 #19
    Well, thanks for trying.

    My disconnect comes when I read about the quanta of the electromagnetic field (photons) carrying the force between particles. That through the exchange of photons the energy contained in the electromagnetic field is moved from one place to another. I know that it isn't thought to bounce off of the other particle--that a photon is emitted from one particle then absorbed by another particle which then becomes more energetic or emits a photon of its own and so on. It is the means for exchanging energy between particles that are separated. It must do something.
  21. Feb 7, 2013 #20


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    The problem here is that you are asking about quantum field theory, but trying to learn it in a handwaving fashion. So inevitably, many of us try to answer it via analogies. While doing it this way might have some benefits, it really isn't a very accurate picture of the physics. So at some point, if you want to dig in deeper, you need to decide if you really need to learn the actual physics. There are limits to what can be done superficially here.

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