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A stationary charge "creates" a constant (but not uniform) electric field around it, and a moving charge "creates" a variable electric field around it. What "carries" the information about the existence of a stationary or a moving charge through space?

In particular, are photons necessarily involved in the process?

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    $\begingroup$ The electromagnetic field. Charges are the sources and the fields are the response. In turn charges respond to the fields... and that's where calculating things turns into hard work. You are probably looking for a less trivial response, though? $\endgroup$ – CuriousOne Feb 25 '16 at 23:41
  • $\begingroup$ @CuriousOne do you simply mean "a force from a distance"? $\endgroup$ – Sparkler Feb 26 '16 at 1:44
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    $\begingroup$ @CuriousOne Wikipedia: The concept of locality is that, for an action at one point to have an influence at another point, something in the space between the points, such as a field, must mediate the action. To exert an influence, something, such as a wave or particle, must travel through the space between the two points, to carry the influence. - what exactly is this "mediation" and what is the "something"? $\endgroup$ – Sparkler Feb 26 '16 at 2:45
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    $\begingroup$ @CuriousOne no underlying particles whatsoever? $\endgroup$ – Sparkler Feb 26 '16 at 3:22
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    $\begingroup$ @CuriousOne If no particles, where's the field energy coming from? $\endgroup$ – Sparkler Feb 26 '16 at 3:29
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Macroscopically we speak of electric and magnetic fields which follow the classical theory of Maxwell's equations.

A stationary charge "creates" a constant (but not uniform) electric field around it, and a moving charge "creates" a variable electric field around it. What "carries" the information about the existence of a stationary or a moving charge through space?

In the classical theory there is no necessity for "carrying" stationary electric and magnetic fields, the equations attribute fields to charges and magnetic moments which need no "carrying". The fields describe the behavior of charges and dipoles . For variable fields, classical theory's mathematics accepts that they propagate as electromagnetic waves in vacuum with velocity c, with no need of a medium. The theory fits the data perfectly.

In particular, are photons necessarily involved in the process?

Classical electromagnetism is an emergent theory from the underlying quantum electrodynamics.. There the photon is an elementary particle of zero mass and spin 1, traveling with velocity c and is the carrier of electromagnetic interactions, either in virtual form or real. An enormous number of photons build up the classcial electromagnetic wave, as demonstrated here.

So yes, photons are necessarily involved as the carrier of the EM interactions.

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  • $\begingroup$ Is that the reason that all objects emit in the IR? (Vibrational energy of the atoms creates alternating electromagnetic field) if yes, how is this consistent with non radiative relaxation (internal conversion, vibrational relaxation)? $\endgroup$ – Sparkler Feb 26 '16 at 13:06
  • $\begingroup$ @Sparkler Yes, black body radiation is photon emission from spill over fields in bulk matter. Internal conversions etc would belong to virtual photon exchanges. $\endgroup$ – anna v Feb 26 '16 at 17:41
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This question illustrates a crucial difference between fundamental fields and everyday ones.

Everyday fields are made by averaging over many particles. For example, consider a sound wave in air. It's nothing more than a ripple in the 'displacement field': it says that at one point, the air is stretched, while at some other point, it gets be squeezed.

But if you zoom all the way in, to the level of individual air molecules, you'll find that nothing is getting stretched or squished at all! In fact, if you look at a single air molecule at some moment, you won't have any idea whether it's in a sound wave or not. We only see sound waves when we average over the positions over many air molecules; the individual molecules make up the field.

Fundamental fields, like the electromagnetic field, are not the same way. Such fields are not made of anything; we do not construct them out of smaller pieces! In particular, the electromagnetic field is not "made of" photons like air is made of air molecules. This misconception suggests that if you got rid of all the photons in a region, the electromagnetic field would cease to exist there, which is totally false.

A better analogy is that photons are like sound waves. You can quiet a room, but the air will still be there; similarly, the electromagnetic field as an entity still exists when there are no photons.

So, to answer your question: the electromagnetic field carries electromagnetic waves. The waves are made of photons; they are not being carried by photons.

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  • $\begingroup$ So the electric and the magnetic field are made from nothing? Not to think about this does not mean that it is so. The question shows our not-knowledge about this phenomenon. $\endgroup$ – HolgerFiedler Feb 26 '16 at 12:02
  • $\begingroup$ Under standard physics. Since the OP asked about photons I assume that's what they wanted to know. $\endgroup$ – knzhou Feb 26 '16 at 16:55
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Currents and magnetic fields are what creates changes in electric fields. Without them, electric fields stay the same.

And it is electric fields that make magnetic fields change, without them, magnetic fields stay the same.

In particular here are the explicit equations for the electromagnetic field and $$\frac{\partial \vec E}{\partial t}=\frac{1}{\epsilon_0}\left(-\vec J+\frac{1}{\mu_0}\vec \nabla\times \vec B\right)$$ tells the electric part of the field how to change and $$\frac{\partial \vec B}{\partial t}=-\vec \nabla\times \vec E$$ tells the magnetic part of the field how to change.

So if you had a charge that had always been at rest at the origin the magnetic field might initially be $\vec 0$ and the electric field might initially be $\frac{q\hat r}{4\pi \epsilon_0|\vec r|^2}$ and then if you moved the charge it would have a current which would make the electric field change and the new electric field could make the magnetic field change. The new magnetic field could make the electric field change and so on, like a ripple effect that expands at the speed of light. Each field changing in time based on the spatial variation of the field values around it.

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  • $\begingroup$ are photons involved in the process? $\endgroup$ – Sparkler Feb 26 '16 at 1:42
  • $\begingroup$ @Timaeus, you are only citing 2 out of the 4 Maxwell equations. All 4 are needed to describe the EM field ! en.wikipedia.org/wiki/Maxwell%27s_equations $\endgroup$ – Anael Feb 26 '16 at 2:32
  • $\begingroup$ @Anael I cited the evolution equations, the constraint equations are impositions on the initial conditions, they don't tell you how things change in time. And the question wasn't about which kinds of initial conditions are allowed, it was about how things are created. The new fields are created out of the old fields according to the evolution equations. $\endgroup$ – Timaeus Feb 26 '16 at 2:35
  • $\begingroup$ @Timaeus, My bad, it wasn't clear on my first read! $\endgroup$ – Anael Feb 26 '16 at 2:44
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    $\begingroup$ @Anael so photons are always involved? in other words, is the photon the carrier of electric field? $\endgroup$ – Sparkler Feb 26 '16 at 2:48
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I would say photons as they are mediators of electromagnetic interaction. But I wouldn't say something is actually carrying information about the moving or stationary charge, it's just the field itself, as it is changing at the origin and is propogating at c. So the change of field at a certain point away from source is not immediate, there is a delay. And only after that time you find out that the charge is moving.

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  • $\begingroup$ are photons necessarily involved in the process? $\endgroup$ – Sparkler Feb 26 '16 at 1:43
  • $\begingroup$ If the answer is expressed in term of quantum field theory, then yes, photons are involved: virtual photons. Read Richard Feynman's "QED: The strange thory of light and matter"; videos of the original lectures are available online. $\endgroup$ – Peter Diehr Feb 26 '16 at 2:06

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