What propagates in wave motion?

BINGO.

And no.

 

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So for a wave propagating with velocity \( v\) in a material medium, the intensity is just \( I = v\mathsf E\), where \(\mathsf E\) is the energy density.

Obviously, this equation says nothing about what a wave looks like. It says nothing about vertical displacement; it's just something very general, but otherwise precise, about a particular wave.

But since the amplitude will vary as the wave moves through a fixed area transverse to the wave's forward velocity, we have \( (dE/dt)_{ave} = IA = v\mathsf {E} A\).

 

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What are we talking about when we talk about energy or work?

We have four fundamental interactions in physics, so we have four fundamental forces. We can understand easily the concept of moving a material object a distance, not so easily the concept of something with no mass doing the same thing. That is, we can understand electron pressure, but photon pressure sounds a bit ridiculous.

But the photons "become" electron pressure (i.e. momentum), when material objects absorb photons; we seem to be stuck with the idea that somehow photons are material after all. They have "abstract" mass.

This mass has something to do with an oscillating electromagnetic field; but this is actually the photon itself.

 

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So the idea that EM radiation is a thing that propagates in an existing EM field is misleading because:

1) A broadcast antenna generates an independent EM field; if the electrons don't oscillate there is a EM field with zero magnetic potential, which propagates at c.
2) Individual photons can propagate in a field-free region of space, i.e. one which is free of sources of the field.

Given 1), all the electrons that get "pumped" into the antenna were somewhere else before that happens; the individual electrons all have electric fields, propagating at c since the electrons existed.
But we know that the charge field (or Coulomb potential) is attenuated near a source of positive charge. In general metal atoms in an antenna are neutral.

So an applied electric field at one end of this antenna will polarize the atoms, stretching out the electron orbitals. Doing this periodically makes the electrons emit photons.
Each photon is a small quantum of the EM field, in a packet. Likewise each electron is a quantum of a field. Electrons are created by, for instance, weak decay processes, so presumably the inverse holds and electrons can be annihilated by "inverse" weak decays.

The electromagnetic field oscillations are so unlike the oscillations in water waves that they should warn you about making comparisons. In fact, I have an old physics textbook (full of misconceptions according to James R), which does just that.

 

The right idea is, I think, that EM radiation as a field quantum, is a thing that is physically real (but only since we can measure momentum--as a dot, or a click so a definite signature of a small finite object), but which we have to theorize the structure of (between measurements). This might be what Feynman was really saying; as a form of radiant energy, nobody really knows what it's doing; we do know a lot more about light than when Feynman invented his diagrams.

The theories don't seem to be able to predict everything, every effect that we know about concerning the interaction of this radiation with matter is largely due to experiment and "discovery". It radiates, in some sense, through electronic circuits where the oscillations are large compared to the "underlying" field--an electric fluid or electron gas.
Electronic signals propagate at c through a copper wire; electrons aren't "free" particles in this gas, it's easier to consider them as also interacting with an underlying field as quasiparticles. This is also helpful when the signals propagate through semiconductors.

At the point you see that the flow of electrons (or the charges) has an equal and opposite flow of "holes" (with opposite charges), what is a particle gets slightly clearer: it's a quantized state in a matter-field. A thing which can carry "energy" from place to place, which is itself a form, or excitation, of a more fundamental field (interaction).

So "everything" that the four fundamental fields do when they interact, with themselves, or each other, is the entire family of particles. But we still don't know about most of the apparent energy or matter in the universe, what we do know only explains about 3%