
#1
Sep507, 05:38 AM

P: 2

Is it only when the acceleration is negative? If yes, when it is positive it absorbs a photon?




#2
Sep507, 06:09 AM

Sci Advisor
P: 4,491

Perhaps you should first understand why classical (not quantum) charge emits electromagnetic radiation.




#3
Sep507, 07:27 AM

Mentor
P: 28,795

This thread has been moved from QM subforum to the classical physics subforum. I agree that the OP needs to first understand this from the classical E&M perspective.
Zz. 



#4
Sep1007, 12:39 AM

Sci Advisor
HW Helper
P: 6,564

Why do accelerated charges emits a photon?
This is an important question and, remarkably, one that has no definitive answer as far as I can tell.
A charged particle that is accelerated by gravity does not emit an em wave/photon. The only other way to accelerate a charged particle is to apply a mechanical or electromagnetic force to it over some distance. And a mechanical force is nothing but an em force. Thus, the observational evidence amounts to: a charged particle does not emit an electromagnetic wave or photon except when it receives electromagnetic energy.So this leads to a reformulation of the question: does the charged particle emit a photon because it accelerates? Or does it emit a photon in response to receiving a photon, which incidentally causes it to accelerate? The question seems to be still unresolved: See this site, for example. AM 



#5
Sep1007, 02:02 AM

HW Helper
P: 1,273





#6
Sep1007, 05:48 PM

P: 1,583





#7
Sep1007, 07:06 PM

HW Helper
P: 1,273

Consider a man standing on a scale on earth. The scale reads, say, 170 pounds. Consider a man in completely empty space in a rocketship accelerating at 9.8m/s^2. The man is standing on a scale. The scale reads 170 pounds. Now, consider the exact same situation but now with a point charge stapled to the man's nose. If the principle of equivalence holds then... well, shouldn't the stationary point charge in a gravitational field be radiating? Regardless of whether or not it "should", it does not. 



#8
Sep1107, 11:40 AM

Sci Advisor
HW Helper
P: 6,564

A charged particle that is stationary in a gravitional field is locally equivalent to a charged particle accelerating in a gravityfree space. So it should radiate. This is a bit of a problem, because as olegranpappy points out, it is not observed. Here is a paper that explains why this occurs  it suggests that the radiation is a relative phenomenon that depends on the relative acceleration between the observer and the charge: 



#9
Sep1107, 05:33 PM

P: 1,583





#10
Sep1107, 05:40 PM

HW Helper
P: 1,273





#11
Sep1107, 06:17 PM

Sci Advisor
HW Helper
P: 6,564

AM 



#12
Sep1107, 10:14 PM

Sci Advisor
HW Helper
P: 6,564

AM 



#13
Sep1207, 02:02 PM

HW Helper
P: 1,273

Oh, I finally found that book I was talking about earlier. Here's some highlights:
Although the formula for the rate at which a small object loses energy via radiation is [tex] \frac{dW}{dt}=\frac{2e^2}{3c^3}a^2 [/tex] where a is acceleration. And this gives the right answer for total loss. It can not be the right expression for instantaneous rate of loss of energy. blah blah blah. For periodic or other types of motion we can use instead the expression [tex] \frac{dW}{dt}=\frac{2e^2}{3c^2}v\dot a [/tex] where v is velocity and [tex]\dot a[/tex] is "jerk". Unfortunately, further thought shows that this rewriting does not resolve the paradox... actually, it gets a bit complicated. A careful discussion was given by Rohrlich and Fulton in Ann. Phys. 9, 499 (1960). But the paradox was not truly resolved until arguments by D. Boulware came along (unpublished as of 1979, but given by Peierls in his book). 



#14
Sep1207, 05:52 PM

P: 46

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? 



#15
Sep1307, 08:40 AM

Sci Advisor
HW Helper
P: 6,564

AM 



#16
Sep1307, 06:55 PM

P: 46

So, it would take really high frequencies of light to accelerate a proton? And would the proton reemit this frequency of light as well?
A related question: If we were to accelerate a proton using a very weak magnetic field, would it emit radiation? 



#17
Sep1307, 07:27 PM

P: 2,050

If a charge is released to fall freely past the apparatus (and viceversa), is radiation then detected? If the apparatus is placed on (say) an accelerating train, will it continue not to detect radiation from coaccelerated charges? 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). 


Register to reply 
Related Discussions  
Compton scattering: electron absorbs then emits a photon?  Quantum Physics  13  
Raman Scattering  what emits the photon?  Advanced Physics Homework  8  
Accelerated charges radiate energy  Introductory Physics Homework  1  
Accelerated charges loose energy  Special & General Relativity  2  
why do accelerated charges emit e/m radiation?  General Physics  1 