Photon is electromagnetic field, right?

elfmotat

Your responses are quite painful to read. The "photon" in question consists of an electric and a magnetic field, which he denoted as [itex]E_{\gamma}[/itex] and [itex]B_{\gamma}[/itex]. Maxwell's Equations aren't just for the narrow range of applications you list above. They completely describe how electric and magnetic fields behave!


I really don't see how you could possibly be under the impression that you're the one here with the most understanding of E&M. You understand practically nothing. In fact, I'm starting to suspect this entire thread is just a giant troll. I don't see how you could realistically be this delusional.

DaleSpam

No, electric field does not imply electric charge. See Maxwell's vacuum equations.

Maxwell's equations. The key property of Maxwell's equations that lead to this is the linearity. The linearity of Maxwell's equations shows that EM follows the principle of superposition which in turn implies that an EM field won't be altered by passing through an external static electric or magnetic field.

http://en.wikipedia.org/wiki/Superposition_principle

DaleSpam

Yes, they do. Specifically, the linearity of Maxwell's equations shows this, as mentioned above.

Barry_G

They do not describe photons (em waves).

http://en.wikipedia.org/wiki/Maxwell...ectromagnetism : any phenomenon involving individual photons, such as... would be difficult or impossible to explain if Maxwell's equations were exactly true, as Maxwell's equations do not involve photons.


Electromagnetic wave equation is only DERIVED from them, and it is not to describe any properties of photons, but only to get to the speed of light. It also has nothing to do whether photons are neutrally charger or not, it is about working out that equation in a setup of vacuum and charge-free SPACE.

DaleSpam

As I said back in post #3 none of your questions are actually about photons. They are about classical EM waves. Specifically, you want to know how you can have EM fields in the absence of a charge. Maxwells equations in vacuum (and their associated wave solutions) are the answer to that question.

elfmotat

That article is talking about the failure of classical electrodynamics and why quantum electrodynamics is needed. Maxwell's Equations completely describe EM waves in the classical sense. Of course they don't involve photons as such, because a photon is not a classical concept. Notice how I put quotes around the word "photons" because what we were talking about wasn't photons exactly, it was EM waves. You're the one who keeps using the two interchangeably.

You're mixing classical and quantum ideas. Classical and quantum electrodynamics are two completely different frameworks. For the majority of this thread we've been discussing light in terms of classical EM waves. Now you're claiming I'm wrong because you suddenly decided to switch over to quantum. I'm not wrong, you're just inconsistent.

You just said that Maxwell's equations don't describe EM waves, and now you're saying EM waves are derived from Maxwell's equations. Obviously if EM waves are derived from Maxwell's equations then Maxwell's equations describe EM waves.

You derive the equations for light in charge-free space, so obviously light is charge-free. Is this really so hard to understand?

Barry_G

You mean "photon" is concept that belongs to QM and has nothing to do with EM fields? Ok, yes, so I shall call it EM waves instead of photons.


Maxwell equations are about electric currents in wires, nothing to do with any EM waves, only electromagnetic wave equation has anything to do with light, and it is not about having EM fields in the absence of any charge, it's simply about EM fields propagating thorough empty space, but it says nothing about how would those EM field be influenced or not if there were any external fields in that space they propagate through.


There is no any superposition if you have a single wave or a single electric/magnetic field. For superposition to neutralize that field you would need another field of opposite sign.

Barry_G

Maxwell's Equations do not describe any waves. Not even electromagnetic wave equation describes any waves, it's just gets you the speed of light.


Maxwell equations are about electric currents and wires, nothing to do with any waves.


No, it means SPACE is charge free. It means there are no any external fields that could influence those fields making up em wave. Is that so hard to understand?

Dead Boss

It seems quite obvious now that Barry does not want to learn anything and is trying to push his own personal theories. Is there a way to request thread lock?

elfmotat

NO NO NO NO NO!!! Maxwell's Equations tell us how electric and magnetic fields behave in any situation, with any charge and current distribution! They're not just about "electric currents in wires!"


NO!!! As you've been told and shown numerous times already, EM waves are derived from and described by Maxwell's Equations!

K^2 already showed you with a detailed post what happens in the presence of external fields!

elfmotat

Agreed, he doesn't want to learn at all. He seems to enjoy arguing about things he doesn't understand just for the sake of it.

sophiecentaur

I have just been wading through this thread and it strikes me that you are determined to approach the understanding of this topic entirely on your own ideosyncratic terms. You keep wanting to bend what you are told to fit your particular model. Of course you are free to believe anything you want to but, as this thread has demonstrated, you just won't get anywhere near the accepted understanding of EM waves if you don't follow the established approach.
You need to ask yourself whether you really believe you are right and that all the replies you've been given are flawed. Could you not consider starting at the very beginning and work towards some real sense instead of jumping in half way through, getting many things the wrong way round and then demanding to be given answers that make sense to you. This is a difficult topic and needs some Rigour if you want an understanding of it. I really don't think that you can accept (or even recognise) correct answers when you see them.
You need to learn the basic terms and definitions in full and not use your own interpretation of things.

(The title of the thread is slightly bonkers, by the way)

Barry_G

http://en.wikipedia.org/wiki/Maxwell%27s_equations

It's not about light or EM waves. It is about geometry of fields, about currents and wires. Do you understand?

sophiecentaur

Ummm.
Do YOU understand?

Barry_G

Educate yourself. If you want to work out electric and magnetic fields for any distribution you need to use equations for point charges. That is Coulomb's law, Biot-Savart law and Lorentz force equations, for point charges, and then you integrate.

Not even electromagnetic wave equation describes any waves.


Nonsense.

Nugatory

Ahhh.... Electromagnetic waves propagating through empty and charge-free space are described by Maxwell's equations.

In fact, electromagnetic waves were predicted by Maxwell's equation before they were ever discovered. The discovery that light was a specific example of the electromagnetic waves predicted by Maxwell's equations came later.

elfmotat

Coulomb's Law is derived from Maxwell's Equations (Gauss' Law for Electricity) under the assumption of static conditions (i.e. when [itex]\partial E / \partial t =0[/itex] and [itex]\partial B / \partial t =0[/itex]).

The Biot-Savart Law is derived from Maxwell's Equations (Ampere's Law and Gauss' Law for Magnetism) also under the assumption of static conditions.

Coulomb's Law and the Biot-Savart Law are only valid when the electric and magnetic fields are not changing in time. Maxwell's Equations are more fundamental, and will describe the electric and magnetic fields under any conditions. The Lorentz Force Law will then tell you how a test particle placed in these fields will behave.

I'm not really sure what you mean by that. It's a wave equation, so obviously it describes a wave.

Just because you say something is nonsense doesn't make it so. He showed you, step by step, how the linearity of Maxwell's Equations prove that external fields superimpose on the EM wave so that the wave itself is unaffected by the external fields.