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To radiate or not to radiate

  1. Jan 14, 2010 #1
    An isolated charge, accelerated by a constant force, theoretically radiates (Larmor). Does the same charge, held at rest in a gravitational field, constantly radiate?
  2. jcsd
  3. Jan 14, 2010 #2


    Staff: Mentor

    How do you intend to detect the radiation?
  4. Jan 14, 2010 #3
    It does not radiate because the large distance behavior of the fields in the latter case do not correspond to what you would get from the former case by applying the equivalence principle.
  5. Jan 14, 2010 #4


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  6. Jan 14, 2010 #5
    If you refer to the radiation emitted (or not emitted) in the gravitational field, I envisioned placing the charge in a blackened container and monitoring to see if the container's temperature increases (up to a limit).
  7. Jan 14, 2010 #6


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    Great question! [strike]The equivalence principle DOES apply, and the charge does radiate.[/strike] Reference given by atyy.
    Caveat: This paper is new to me. I am going to check up further and make sure I understand it better; hence this answer should be understood simply as the answer of the reference, and not of me personally. I do not have the expertise to confirm it independently. But I'll check around.

    Cheers -- sylas

    Postscript. I am speaking above out of turn; and so I have struck out the sentence above for which I do not have sufficient confidence to give such a definite answer. Apologies, I shall focus for now on reading rather than giving answers.
    Last edited: Jan 14, 2010
  8. Jan 14, 2010 #7
    I think this paper is better:


    Put simply, if you're going to apply the equivalence principle, you have to do it correctly, and thus also look at the proper boundary conditions/initial conditions on the fields.
  9. Jan 14, 2010 #8
    And that paper can now be updated as the ancient problem of the electromagnetic self force which is responsible for the radiation of accelerated charges, has now been solved rigorously:

  10. Jan 14, 2010 #9


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    Yes, I like Parrott's paper, which is one of those I had in mind when I said the EP does not apply to charged particles (but you seem to have drawn a different conclusion?). In my understanding the Harpaz and Soker paper is just an amusing case where by accident the EP "applies" to a charged particle.
    Last edited: Jan 14, 2010
  11. Jan 14, 2010 #10


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    Thank you! I withdraw my previous answer, and shall look further.

    Cheers -- sylas
  12. Jan 14, 2010 #11


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    Some more references, in addition to Parrott's given above by Count Iblis.

    The qualification "uncharged" in the statement of the EP in http://arxiv.org/abs/0707.2748

    The EP does not apply to charged particles (unless it's a gravitational charge):
    http://relativity.livingreviews.org/Articles/lrr-2004-6/ [Broken]

    Also in Rindler's and J L Martin's GR texts (though Fredrik has now made me suspicious of Rindler ... :rolleyes:)
    Last edited by a moderator: May 4, 2017
  13. Jan 14, 2010 #12
    If the charge is held at rest, meaning mgh = constant, then where would the radiated energy come from? Wouldn't conservation of energy apply?
    Bob S
  14. Jan 14, 2010 #13
    Why the restriction 'isolated' charge?

    I think it does the same for all charges in any configuration subject to acceleration.

    And as all matter of the Universe is accelerated at all times ....
  15. Jan 14, 2010 #14
    the conservation of energy...
    the energy has to be of such a tiny value that I think is not measurable, and all instruments of measure also vary in the same way (the reference atom at lab) giving a null result.
  16. Jan 15, 2010 #15
    Those were questions I had in mind when I submitted the thread. The majority consensus seems to be that the EP doesn't apply to electric charge. I'll give some thought to an analogous situation that doesn't involve electric charge. Thanks to all for responding, and for the links.
  17. Jan 18, 2010 #16
    Is it just coincidence that the article http://www.maxwellsociety.net/Charge%20and%20the%20Equivalence%20Principle.html" [Broken] that you linked to in the other thread on the same subject, was authored by someone called G.R.Dixon?
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  18. Jan 18, 2010 #17


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    I've spent some time studying this, and although the mathematical treatment of charged particles moving in curved spacetime is heinously complex, and I haven't dug into it, I think I've gotten to the point where I understand the issue reasonably well at the conceptual level. Of course this stuff is very subtle, so most likely I'm making mistakes :-), but I felt confident enough to incorporate an explanation in my book, http://www.lightandmatter.com/html_books/genrel/ch01/ch01.html#Section1.5 [Broken] .

    Here's a slightly edited version of what I'm saying in the book:


    The equivalence principle is not a single, simple, mathematically well defined statement. As an example of an ambiguity that is still somewhat controversial, 90 years after Einstein first proposed the principle, consider the question of whether or not it applies to charged particles. Raymond Chiao http://arxiv.org/abs/quant-ph/0601193v7 proposes the following thought experiment. Let a neutral particle and a charged particle be set, side by side, in orbit around the earth. If the equivalence principle applies regardless of charge, then these two particles must go on orbiting amicably, side by side. But then we have a violation of conservation of energy, since the charged particle, which is accelerating, will radiate electromagnetic waves (with very low frequency and amplitude). It seems as though the particle's orbit must decay.

    The resolution of the paradox, as demonstrated by detailed calculations by Gron and Naess http://arxiv.org/abs/0806.0464 is interesting because it exemplifies the local nature of the equivalence principle. When a charged particle moves through a gravitational field, in general it is possible for the particle to experience a reaction from its own electromagnetic fields. This might seem impossible, since an observer in a frame momentarily at rest with respect to the particle sees the radiation fly off in all directions at the speed of light. But there are in fact several different mechanisms by which a charged particle can be reunited with its long-lost electromagnetic offspring. An example (not directly related to Chiao's scenario) is the following.

    Bring a laser very close to a black hole, but not so close that it has strayed inside the event horizon at rH. It turns out that at r=(3/2)RH, a ray of light can have a circular orbit around the black hole. Since this is greater than RH, we can, at least in theory, hold the laser stationary at this value of r using a powerful rocket engine. If we point the laser in the azimuthal direction, its own beam will come back and hit it.

    Since matter can experience a back-reaction from its own electromagnetic radiation, it becomes plausible how the paradox can be resolved. The equivalence principle holds locally, i.e., within a small patch of space and time. If Chiao's charged and neutral particle are released side by side, then they will obey the equivalence principle for at least a certain amount of time --- and "for at least a certain amount of time" is all we should expect, since the principle is local. But after a while, the charged particle will start to experience a back-reaction from its own radiated electromagnetic fields. Since Chiao's particles are orbiting the earth, and the earth is not a black hole, the mechanism clearly can't be as simple as the one described above, but Gron and Naess show that there are similar mechanisms that can apply here, e.g., scattering of light waves by the nonuniform gravitational field.


    The equivalence principle says that electromagnetic waves have gravitational mass as well as inertial mass, so it seems clear that the same must hold for static fields. In Chiao's paradox (p. 26), the orbiting charged particle has an electric field that extends out to infinity. When we measure the mass of a charged particle such as an electron, there is no way to separate the mass of this field from a more localized contribution. The electric field "falls" through the gravitational field, and the equivalence principle, which is local, cannot guarantee that all parts of the field rotate uniformly about the earth, even in distant parts of the universe. The electric field pattern becomes distorted, and this distortion causes a radiation reaction which back-reacts on the particle, causing its orbit to decay.


    So if the question is whether the equivalence principle applies to charged particles or not, I think the answer is not a yes/no answer. You have to keep in mind that the equivalence principle only applies locally, and this can be tricky to translate into actual experiments. A lot of thought experiments that might seem purely local are actually nonlocal, so they appear to violate the equivalence principle. If the e.p. were a mathematically well defined statement, then we'd be able to define "local" and "nonlocal" in a rigorous way, and probably everyone would have to agree on a yes/no answer. However, nobody has ever succeeded in stating the e.p. in a mathematically rigorous way.

    I like Chiao's thought experiment a lot better than the ones involving linear acceleration. I think it's conceptually simpler. For one thing, there's been some debate over the significance of horizons in the linear case. Boulware says that the radiation disappears behind the event horizon of the accelerated observer, where it can't be observed, but Parrott argues that this is wrong.
    Last edited by a moderator: May 4, 2017
  19. Jan 18, 2010 #18


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    Well, I don't know about using heat to measure it. I was thinking about measuring it with an antenna. My guess is that a charge accelerating past a stationary antenna will induce the same signal in the antenna as an antenna falling past a stationary charge.
  20. Jan 18, 2010 #19
    Very interesting comment. My guess is that a charge moving past a stationary antenna at constant velocity will induce the same signal in the antenna as an antenna falling past a stationary charge at constant velocity. Both are Faraday induction of an electric field.

    So how does the charge then radiate energy?

    My guess is that a charge accelerating past a stationary antenna will radiate the same signal in the antenna as an antenna accelerating past a stationary charge.

    Bob S
  21. Jan 18, 2010 #20
    I have not read the linked papers yet. (I'm in the middle of "A Rigorous Derivation of Electromagnetic Self-force". So far, so good.) But here is my question.

    If we are asking, "Does a charged particle radiate when it accelerates?" we have to specify: accelerate relative to what?

    If it has to be relative to an inertial frame, ... well, the only (locally) inertial frame we have in a (non-uniform) gravitational field is the freely falling one. So I would say that the equivalence principle has to hold because .. what else is there?

    This business about local vs non-local.. the equivalence principle is local, Maxwell's equations are local (curved spacetime or flat), so what is there that is nonlocal?
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