Photon is electromagnetic field, right?

[K^2]

You aren't just confused on a couple of definitions. You have no clue what you are talkinga bout at all. You need to read a textbook. That was probably the most helpful suggestion you got.

 

[fluidistic]

Barry, you seem to miss the point that K^2 tried so hard to show you.
There can be an electric field without any charge. An EM wave is just this.
For example the light coming from stars contain no charge at all, yet it is made of an electromagnetic field. All the light is like this.
You can take a region in space with an electric field and find no charge.

Ok, they are not synonyms. Field is a concept that describes geometrical distribution of charge, and charge just refers to the magnitude. For example, a field can have shape, described by gradients, while charge is a scalar number, a measure of the magnitude at some point in the field.

I'm not really understanding this.

The point is, when you ask whether there is electric field present at some point in space, it is just about the same as asking whether there is electric charge there, so in that sense they are synonyms, and you can see those two articles use the two terms interchangeably.

No, absolutely not as explained above.

 

[elfmotat]

Ok, they are not synonyms. Field is a concept that describes geometrical distribution of charge, and charge just refers to the magnitude. For example, a field can have shape, described by gradients, while charge is a scalar number, a measure of the magnitude at some point in the field.

The point is, when you ask whether there is electric field present at some point in space, it is just about the same as asking whether there is electric charge there, so in that sense they are synonyms, and you can see those two articles use the two terms interchangeably.

After reading through this and the thread you hijacked in the Relativity section, I must be the fifth or sixth person to tell you what I'm about to tell you:

There does not need to be any charges for there to be an electric field. Maxwell's Equations tell us that a changing magnetic field will also produce an electric field. Similarly, you don't need any currents for there to be a magnetic field; a changing electric field will produce a magnetic field. Essentially what happens in an EM wave is that a changing electric field produces a changing magnetic field which produces a changing electric field which... This allows the electric and magnetic fields to propagate without any charges or currents. I'll even go through how this is derived, step by step:

As you've been shown numerous times, Maxwell's Equations in a vacuum are:

$$\nabla \cdot E=0$$
$$\nabla \times E = -\frac{\partial B}{\partial t}$$
$$\nabla \cdot B=0$$
$$\nabla \times B = \frac{1}{c^2} \frac{\partial E}{\partial t}$$Now, using the following "curl of the curl" identity: $\nabla \times \nabla \times A=\nabla(\nabla\cdot A)-\nabla^2A$
$$\nabla \times \nabla \times E=\nabla(0)-\nabla^2E=-\nabla \times \frac{\partial B}{\partial t}=-\frac{\partial }{\partial t}[\nabla \times B]=-\frac{\partial }{\partial t}[\frac{1}{c^2} \frac{\partial E}{\partial t}]$$
Simplifying you get:
$$\nabla^2E=\frac{1}{c^2}\frac{\partial^2 E}{\partial t^2}$$
You can apply the same identity to the curl of the magnetic field to get the following:
$$\nabla^2B=\frac{1}{c^2}\frac{\partial^2 B}{\partial t^2}$$
The solution to both of these differential equations is a sinusoidal wave which moves at a velocity c.
You accuse K^2 of being condescending, yet he's absolutely correct. You have no idea how basic electrodynamics works. What you need to do is stop pretending you know what you're talking about and go pick up a textbook. You're arguing about things you don't understand.

 

[Barry_G]

The equations are satisfied. Therefore, the electromagnetic field of a beam of light simply adds on top of electromagnetic fields already present without any kind of distortion. In other words, photons are not affected by electric or magnetic fields.

Are we finally done with your "charged photon" nonsense?

That's ridiculous. There are no any photons, nor external fields in those equations except for Faraday's law. Gauss's law is about electric flux through closed surface, it's analogous to Coulomb's law. Gauss's law for magnetism describes geometry of magnetic field. Faraday's law of induction is about generating voltage by external magnetic field in closed circuit loops. Ampère's circuital law is about magnetic field generated around close circuit loop.

That's only because you have no idea what spin is. I strongly suggest reading an article on that. Pay attention to quantization axes and connection to angular momentum.

Pay attention to what I said and if you have any idea what spin is you might understand.

You aren't just confused on a couple of definitions. You have no clue what you are talkinga bout at all. You need to read a textbook. That was probably the most helpful suggestion you got.

You aren't just confused about actual application of Maxwell's equations, but you also have no clue what you are talking about at all. You need to wake up.

 

[elfmotat]

That's ridiculous. There are no any photons, nor external fields in those equations except for Faraday's law. Gauss's law is about electric flux through closed surface, it's analogous to Coulomb's law. Gauss's law for magnetism describes geometry of magnetic field. Faraday's law of induction is about generating voltage by external magnetic field in closed circuit loops. Ampère's circuital law is about magnetic field generated around close circuit loop.

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

You aren't just confused about actual application of Maxwell's equations, but you also have no clue what you are talking about at all. You need to wake up.

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.

 

[Dale]

Does electric field not imply electric charge?

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

All I am asking is if photon is electromagnetic field why can not be influenced by an external electric or magnetic field. What equation explains that?

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

 

[Dale]

Maxwell's equations do not explain how can photon have electric and magnetic field and yet we can not bend a beam of light by external electric or magnetic fields.

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

 

[Barry_G]

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

They do not describe photons (em waves).

http://en.wikipedia.org/wiki/Maxwel...ell_equations_as_a_theory_of_Electromagnetism : 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.

 

[Dale]

They do not describe photons (em waves).

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]

They do not describe photons (em waves).

http://en.wikipedia.org/wiki/Maxwel...ell_equations_as_a_theory_of_Electromagnetism : 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.

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.

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.

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.

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.

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]

As I said back in post #3 none of your questions are actually about photons. They are about classical EM waves.

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.

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.

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.

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.

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]

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.

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.

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.

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

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

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]

Maxwell equations are about electric currents in wires,

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!"

nothing to do with any EM waves,

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

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.

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

 

[elfmotat]

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?

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]

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.

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]

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?

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.

http://en.wikipedia.org/wiki/Maxwell's_equations

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

 

[sophiecentaur]

http://en.wikipedia.org/wiki/Maxwell's_equations

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

Ummm.
Do YOU understand?

 

[Barry_G]

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!"

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.

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

Not even electromagnetic wave equation describes any waves.

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

Nonsense.

 

[Nugatory]

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.

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]

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.

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

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.

Not even electromagnetic wave equation describes any waves.

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

Nonsense.

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.

 

[jtbell]

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?

The normal method is to use the "Report" button on a problematic post. In this case, however, you can consider it done.