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Is it only when the acceleration is negative? If yes, when it is positive it absorbs a photon?
Andrew Mason said:This is an important question and, remarkably, one that has no definitive answer as far as I can tell.
What is your basis for that assertion?Andrew Mason said:A charged particle that is accelerated by gravity does not emit an em wave/photon.
lugita15 said:What is your basis for that assertion?
Further to what olegranpappy has said, according to the principle of equivalence, a charged particle accelerating in freefall in a gravitational field is locally equivalent to a charged particle at rest in an inertial frame of reference. Since a charged particle at rest in an inertial frame does not radiate, the same must hold true for the charged particle in freefall. This is consistent with all observation.lugita15 said:What is your basis for that assertion?
From "Radiation from a Charge in a Gravitational Field" said:It is found that the “naive” conclusion from the principle of equivalence - that a freely falling charge does not radiate, and a charge supported at rest in a gravitational field does radiate - is a correct conclusion, and one should look for rdiation whenever a relative acceleration exists between an electric charge and its electric field. The electric field which falls freely in the gravitational field is accelerated relative to the static charge. The field is curved, and the work done in overcoming the stress force created in the curved field, is the source of the energy carried by the radiation. This work is done by the gravitational field on the electric field, and the energy carried by the radiation is created in the expence of the gravitational energy of the system.
But I thought that principle of equivalence contradicts special relativity, and by consequence, classical electromagnetic theory as well.Andrew Mason said:Further to what olegranpappy has said, according to the principle of equivalence, a charged particle accelerating in freefall in a gravitational field is locally equivalent to a charged particle at rest in an inertial frame of reference. Since a charged particle at rest in an inertial frame does not radiate, the same must hold true for the charged particle in freefall. This is consistent with all observation.
AM
lugita15 said:But I thought that principle of equivalence contradicts special relativity, and by consequence, classical electromagnetic theory as well.
The General Theory and the Special Theory of Relativity are perfectly consistent. The principle of equivalence is the cornerstone of GR. Maxwell's equations are consistent with both - certainly with SR. In fact, the belief that Maxwell's equations are valid in all inertial frames of reference was one of the things that drove Einstein to develop SR.lugita15 said:But I thought that principle of equivalence contradicts special relativity, and by consequence, classical electromagnetic theory as well.
I should have added after: "So it should radiate"... "IF accelerating charges radiate because they are accelerating.Andrew Mason said:A charged particle that is stationary in a gravitational field is locally equivalent to a charged particle accelerating in a gravity-free space. So it should radiate.
The force on, or rate of momentum change of, the proton would be 1800 times greater than that for the electron. This means that the electromagnetic interaction needed to cause this (ie. the momentum of the photon) would have to be 1800 times greater.idea2000 said:Hi,
I was reading this thread and I have an additional question to ask. If an electron and a proton were accelerating at the same rate, would they emit the same photon?
Andrew Mason said:This is a bit of a problem, because as olegranpappy points out, it is not observed.
idea2000 said:Hi,
I was reading this thread and I have an additional question to ask. If an electron and a proton were accelerating at the same rate, would they emit the same photon?
There is a difference between a charge in free fall in a gravitational field and a charge accelerating due to a non-gravitational force (ie. an electromagnetic force). In the first case it does not radiate. In the second, it does.cesiumfrog said:If a charge is released to fall freely past the apparatus (and vice-versa), is radiation then detected? If the apparatus is placed on (say) an accelerating train, will it continue not to detect radiation from co-accelerated charges?
I am not sure I understand what you mean here.Actually, my understanding was that the field lines of a constantly accelerated charge merely "droop". This isn't radiation if you can stay stationary with respect to charge and the direction of acceleration. I think the equivalence principle here is unscathed (until you add boundary conditions, but that feels like cheating).
Do you have evidence of that? How do you define "radiate"?Andrew Mason said:There is a difference between a charge in free fall in a gravitational field and a charge accelerating due to a non-gravitational force (ie. an electromagnetic force). In the first case it does not radiate. In the second, it does.
I say that to a classical detector-apparatus, "radiation" means any time-varying field at the apparatus.Andrew Mason said:I am not sure I understand what you mean here.
Sure. An electron whipping around a curved path radiates (synchrotron radiation). http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V3S-4N68NMP-7&_user=10&_coverDate=12%2F31%2F2007&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ea92b6de2f38ef51359814f6390924f5have shown that a falling electron does not radiate: .cesiumfrog said:Do you have evidence of that?
Emit a photon.How do you define "radiate"?
I never said that there was a problem with the principle of equivalence. There is no problem. The problem may be with the theory that a charge radiates because it accelerates. Outside a gravitational field, a charge cannot accelerate unless it interacts with a an em field ie. a photon. Perhaps a charge radiates because it interacts with a photon which, because it has momentum, causes the charge (which has mass) to accelerate.I say that to a classical detector-apparatus, "radiation" means any time-varying field at the apparatus.
Now, if an apparatus inside an elevator is stationary with respect to some charge, regardless of whether the elevator is "standing in a planet's gravitational field" or "accelerating constantly in space" the apparatus will detect no radiation (because in both cases the field at the apparatus is time-independent). Hence I disagree with your statement that there is some problem with the equivalence principle.
I presume you mean 'supported in a grav. field' as inThe measuring instrument is stationary wrt gravitational field
charges only radiate if they are driven by an electromagnetic force (not allowed to fall)?
Yes. If the moon had net charge it would not radiate merely because it was orbiting the earth. It would be a charge in free-fall in a gravitational field and according to the princple of equivalence, it is locally indistinguishable from a charge in an inertial frame of reference.cesiumfrog said:So you're saying you think charges only radiate if they are driven by an electromagnetic force (not allowed to fall)? So then, if the moon was given an electric charge, you think it also would not radiate (presuming one could detect light-month wavelengths)?
The principle of equivalence requires that an electrically charged moon not radiate.cesiumfrog said:OK, so you are insisting the electrically charged moon would not radiate.
Since the sun and the Earth and moon are moving relative to a distant star, the field of a charged moon measured from a distant star would be constantly changing.Nonetheless, do you agree that there would be a periodic variation in the electric field measured from earth? And from any distant star?
Andrew Mason said:The principle of equivalence requires that an electrically charged moon not radiate. [But] the field of a charged moon measured from a distant star would be constantly changing.
Andrew Mason said:An electron whipping around a curved path radiates (synchrotron radiation). http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V3S-4N68NMP-7&_user=10&_coverDate=12%2F31%2F2007&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ea92b6de2f38ef51359814f6390924f5have shown that a falling electron does not [..] Emit a photon.
Accelerated charges emit a photon because of their interaction with the electromagnetic field. As the charge accelerates, it creates a disturbance in the field which propagates outward as a photon.
An accelerated charge emits a photon by releasing energy in the form of electromagnetic radiation. This energy is carried away by the photon, which is a quantum of light.
Yes, there is a specific speed at which an accelerated charge emits a photon. According to classical electromagnetism, the acceleration must be non-uniform and changing in order for a photon to be emitted. This means that the charge must be constantly changing its speed and direction.
Yes, an accelerated charge can emit multiple photons. The number of photons emitted is directly proportional to the acceleration of the charge. This means that a higher acceleration will result in the emission of more photons.
The emission of a photon by an accelerated charge is significant because it is the fundamental mechanism behind the production of light and other forms of electromagnetic radiation. It also plays a crucial role in many important phenomena, such as electricity, magnetism, and the behavior of atoms and molecules.