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I Clarification "Force Carrier" and "Radiation" Gravity and EM

  1. Sep 23, 2016 #1
    I need clarification on the difference between a “force carrier” and “radiation.”

    Imagine two electric charges separated by a distance “d” but not moving. They exchange “force carrier” photons which tell the electric charges to attract/repel. But you can’t see these “force carrier” photons unless the electric charges accelerate, like in an antenna. However, if the “force carrier” photons of attraction/repulsion are really present between the electric charges, it seems like they should be detectable. Are the photons of “force carrier” the same as the “photons” of radiation? If not, why are photons of force detection not able to be detected but the radiated photons emitted from acceleration are? It seems that photons are photons.

    This now applies to gravity waves. Similarly, I know that masses exchange gravity waves from which they know to attract. If such gravity waves really are present between the masses, shouldn’t I be able to detect them? Yet I know that accelerated masses are required for the detection of gravity waves. This seems like a contradiction. Aren’t the gravity waves always there, always moving between the masses, whether the masses accelerate or not? Or like, with electromagnetism, is there some fundamental difference I am missing between the force carries of gravity and the gravitational radiation we observe from binary black holes?

    It seems like the same "apparent contradiction" would hold up if I thought of the "force carriers" as a "fields" since I know that there are electromagnetic (or gravitational) fields causing attraction/repulsion and that radiated electromagnetic (or gravitational) waves propagate as fields.
  2. jcsd
  3. Sep 23, 2016 #2


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    photons only exist IF the charged particles are accelerating, is they are stationary as you said, then there are no photons emitted

    have a read of this ......


  4. Sep 23, 2016 #3


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    The "force carrier" photons are virtual photons, not real ones. And - they won't be detected by apparatus that is designed to detect real photons. If you need a better answer than this, you probably need the QM forum. "Force carrier particles" simply aren't a classical concept, they're a quantum mechanical (QM) concept.

    The whole concept of how a force carrier carries force in QM is a lot trickier than it looks. For instance, how do you explain the attraction between unlike charges with "force-carreirers"? And the direction of the "force" is not delayed, people often draw the incorrect conclusion about that from the oversimplified populraization.

    Not in classical GR. "photons" , "gravitions", and "force carrier" particles all belong to the realm of quauntum mechanics, not to the realm of classical mechanics. And general relativity (GR) is a classical theory.

    Since we don't have a complete picture of quantum gravity, we don't have a complete picture where gravity is caused by "carrier particles" might be. I'm not sure to what extent we have partial theories - another forum might be the best place to ask such questions.
  5. Sep 23, 2016 #4


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    I think that you are asking about the distinction between virtual particles and real particles. That may be better to ask in the quantum mechanics forum.

    I don't think that there is a well accepted theory describing gravitons. Probably all we can answer here will be related to EM or the nuclear forces.
  6. Sep 24, 2016 #5
    I'll go ahead and repost this in the QM section. I did not know the difference between real photons and virtual photons. I posted it here because it was really the gravity waves that I was interested in. Upon thinking about the gravity waves, I realized I did not understand the difference between detectable gravity waves through gravitational radiation and gravity waves as transmitting the force of gravity between masses. I tried to draw an analogy of the electromagnetic photon and realized I didn't understand between radiated photons and virtual photons.
  7. Sep 24, 2016 #6
    Does the logic above mean that we have discovered gravity waves in LIGO as from gravitational radiation but not the actual transmitter of gravitational force between masses?
  8. Sep 24, 2016 #7


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    That is correct. What we have detected is classical gravitational waves, not the hypothetical gravitational force carrier (called a graviton).
  9. Sep 24, 2016 #8


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    There is another confusing point and that is that there is no reason to think the 'virtual photons' would be at optical frequencies. There is a temptation to be looking for some sort of sparkling between charges as you move them relative to each other. Photons can be associated with waves of any frequency and I would imagine that the energy of virtual photons would relate to the Energy change involved with an infinitesimal change of Electrical Potential Energy. That could be very small and the photon frequency involved could be sub Hz.
    I was interested to learn that the frequencies of the gravitational waves that have been detected are in the middle of our audio range. Unbelievable when you think that 'classically' it could involve two massive objects orbiting each other on the way to merging, at hundreds of revs per second. Not an intuitive picture!
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