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I'm a newbie here so have mercy.

I'm studying electromagnetic waves. This is the propagation of energy via the vibration of charged particles, as I understand it.

A charged particle could be like a proton, which has a positive charge. So I'm imagining a simplified "universe" that just exists along a two dimensional line, a long string of protons.

You can send energy from one end of this space to the other by vibrating this string of charged particles. Maybe this could be done by "lifting" one proton at one end of space, and that would effect the next proton along the string in a similar way, only less so, to create a wave.

You can move your end of space up and down as fast as you'd like. If you move it slowly, you create long waves (from crest to crest). If you move your proton at your end quickly, you produce short waves.

But no matter how quickly or slowly you lift your proton up and down, the wave you create (either longish or shortish) will always reach the other end of space at the same time. In other words, you can't control how fast your wave travels through space, you can only control the shape of the wave itself. The energy transmitted always travels the same speed (C, the speed of light).

Lift your proton up and down slowly, and you produce a radio wave. Lift it up and down really quickly, and you'll produce an x-ray wave. Lift it up and down somewhere in the middle, and you get a light wave.

Here's one problem I'm struggling with. If we can see light emitted from stars that are light years away from us, that must mean that every part of space along that line of sight is full of charged particles right?

If that's so, what are these charged particles? Is space full of particles in every direction, in every place? They aren't protons and electrons right, maybe more elementary charged particles like quarks?

So space isn't empty, it's absolutely jam packed with charged particles. The "fabric" of space time is "made of something," and that something must be charged either positive or negative.

Is this a correct line of thinking, or am I missing some fundamental idea here? Thanks for your help.

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2 Answers

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Nope, an electromagnetic wave is not a vibration of charged particles. It's a vibration of the electromagnetic field. So in order for an EM wave to propagate through space, you don't need any particles in the space; all you need is an electromagnetic field, and it certainly is the case that an EM field fills all of space. (That's the physical definition of a field, in fact: something that associates a value with every point in space) Vibration of a charged particle can start an electromagnetic wave, just like shaking the end of a rope can start an oscillation in the rope, but once it's started, the wave keeps propagating regardless of what happens to the particle.

The scenario you're describing, where vibrating protons transmit energy to one another, is really more like a sound wave.

One thing that bears mentioning: quantum field theory does predict that electromagnetic waves can cause vacuum polarization, which you can (roughly) think of as particle-antiparticle pairs popping in and out of existence throughout space as the wave passes through. But the wave is not carried by vibrations of these particles, so the existence vacuum polarization doesn't change what I said in my first paragraph.

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Ok great, that helps actually. So energy can be propagated through an EM field. You don't need charged particles to carry the energy. You just need the EM field, which space is certainly filled of (I'm assuming here, from your answer). I guess I'm struggling with the idea of a "field" then as being a mass-less thing. If energy is mass times velocity, you can't have energy without mass right? I know I'm lost here, but you're getting me along the right track I think... –  dvanaria Sep 20 '11 at 6:01
    
Well, you'll just have to get used to the idea that there are things in the universe which don't have mass ;-) Take a look at this question and perhaps this one for more about fields. Also, there are many different kinds of energy, only some of which are related to mass. So you certainly can have energy without mass. If you're still confused, feel free to bring this up in the chat room. I'd be happy to discuss it in detail when I have some time. –  David Z Sep 20 '11 at 6:12
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No!

You don't need charged particles for propagation of EM waves. The mechanism you are telling about vibrating charges is for physical media like glass or say water. Even in vacuum, EM waves can travel.

For how, you need to look for maxwell's theorem where he explains the propagation of EM waves via regenerating electric and magnetic fields.

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Ok thanks, I will look into Maxwell's theorem. I know that any charged particle emits an electric and magnetic field (which can be visualized as going in different directions, say electric field along the x-axis and the magnetic field along the y-axis). But still, doesn't that bring us back to the start? There has to be a charged particle there to emit those fields... –  dvanaria Sep 20 '11 at 5:52
    
no, actually changing electric field creates magnetic field and vice versa. In vacuum, you don't need anything to create them, they are mutually self sustaining. –  Vineet Menon Sep 20 '11 at 5:55
    
What generates the EM field though? Are you saying that energy can be transmitted in a vacuum in a "space" that is bordered by charged particles? The EM field must come from somewhere... –  dvanaria Sep 20 '11 at 6:03
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