
#1
Dec1112, 02:45 AM

P: 68

This thread is to move this discussion away from another thread in order to talk about it in more detail, so here is a brief recapitulation of how that went to make an opening for the discussion...
http://en.wikipedia.org/wiki/Photon According to this picture there are indeed only one electric and one magnetic field and what's perpendicular about them seems to be the plane of their oscillation. However, it doesn't seem to me energy of the photon defines the strength of those fields, but rather the other way around. That is the amplitude/wavelength of their oscillation is what defines energy of the photon, where the strength of the fields remains constant. Furthermore, how could there be a single magnetic field on its own, wouldn't that be a monopole? And also, how could there be an electric field in motion without creating yet another magnetic field directly around itself, which can't be the same one that is oscillating perpendicularly to it? 



#2
Dec1112, 03:50 AM

Sci Advisor
P: 2,470

At any rate, energy is related to the square of the amplitude, because energy density is given by (EČ+BČ)/2. [tex]\nabla \cdot E = 0[/tex] [tex]\nabla \times E = \frac{\partial B}{\partial t}[/tex] [tex]\nabla \cdot B = 0[/tex] [tex]\nabla \times B = \mu_0 \epsilon_0 \frac{\partial E}{\partial t}[/tex] These are given in electromagnetic wave article you've already linked. As well as derivation of following from the above. [tex]\nabla^2E = \frac{1}{c^2}\frac{\partial^2B}{\partial t^2}[/tex] Similar equation is derived for the B field. The solution is the electromagnetic wave given in the article. 



#3
Dec1112, 06:11 AM

Mentor
P: 16,477

http://en.wikipedia.org/wiki/Maxwell...speed_of_light Btw, none of your questions are actually about photons. They are about classical EM. Specifically, you want to know how you can have EM fields in the absence of a charge. Maxwells equations in vacuum are the answer to that question. 



#4
Dec1112, 07:27 AM

P: 977

Photon is electromagnetic field, right?
photon arises when one quantizes the electromagnetic field using creation and annihilation operator.All properties described to electromagnetic field can be ascribed to photon,if one quantizes it.you can see here for this quantization
http://en.wikipedia.org/wiki/Quantiz...magnetic_field 



#5
Dec1112, 09:41 AM

P: 68





#6
Dec1112, 09:51 AM

P: 68





#7
Dec1112, 10:49 AM

P: 68

In any case the question was about this sentence: "electromagnetic field is a physical field produced by moving electrically charged objects". So ok, I'll take it from here that photons are indeed electromagnetic fields, but the question is then how come electron is not electromagnetic fields, even more so since it fits that description better? 



#8
Dec1112, 01:21 PM

Sci Advisor
P: 2,470

In reality, electron does not simply jump from one state to another. An atom that's emitting EM radiation as electron transitions from one energy level to another exists in superposition of two energy levels. While pure states have zero dipole moment, the superposition of any two states has a dipole expectation which rotates. So you actually do have an oscillating charge in an atom while it emits radiation. Unfortunately, there aren't many good references on that. Pretty much the only thing I can suggest is taking the hydrogen atom solutions, composing a superposition state, say (1s+2p)/√2, and computing the dipole expectation. There is nothing really difficult about it, but it's about a page of notes. Everything in physics that is part of standard theory has overwhelming experimental support. If you think something doesn't, you are simply ignorant of that branch of physics. Feel free to ask, though. Keep in mind that a lot of things are confirmed indirectly. What's interesting is that charge can only change by a unit of 1e, which is why electron charge is also known as elementary charge. It's like spin. You can have a fraction, but you have to change it by a unit. Maybe you would understand EM propagation better via retarded potentials. You can think of EM wave as being caused by electric field of the distant charge that oscillates, and because "information" about position of the charge is delayed by speed of light, you get an oscillating EM field. This is really a carriage before horse explanation, because speed of light is handwaved into it, but again, if it makes it easier for you to understand, maybe it's worth for you to take a look. Retarded Potential. 



#9
Dec1112, 03:14 PM

Mentor
P: 16,477





#10
Dec1112, 04:18 PM

PF Gold
P: 11,036

Imagine for a moment that ships in the ocean could cause other ships to be pushed away by pushing all nearby water outwards or pulled in by pulling all nearby water towards themselves. Would you say the water itself has this property? No! Clearly it's just the ships themselves that do this. The water far away from any ships does nothing. How about a wave in this water, would it have this curious pushing or pulling property? Of course not! 



#11
Dec1112, 11:41 PM

P: 977

http://www.physicsforums.com/showthread.php?t=654329 



#12
Dec1212, 02:58 AM

P: 68

http://en.wikipedia.org/wiki/Electromagnetic_field : An electromagnetic field is a physical field produced by moving electrically charged objects. Does that mean electron, for example, is considered to be "electromagnetic field", or should they have better said: "electromagnetic radiation is a physical field emitted by moving electrically charged objects"? 



#13
Dec1212, 03:20 AM

Sci Advisor
P: 2,470

How can I explain to you that electron emitting a photon and photon being electronpositron pair are exactly the same thing, when I can't rely on your understanding of basic, nonrelativistic field theory? This is advanced stuff that's built on more advanced stuff that's built on several different branches of fundamental stuff. You can't just jump to the end and hope to understand any of it. 



#14
Dec1212, 03:46 AM

P: 68





#15
Dec1212, 04:03 AM

P: 68





#16
Dec1212, 04:23 AM

P: 68





#17
Dec1212, 04:28 AM

P: 68





#18
Dec1212, 04:36 AM

Sci Advisor
P: 2,470




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