Experiments Proving Electromagnetic Waves are Real

[mal4mac]

But those graphs are only abstract representations of the physical phenomena. In an electromagnetic wave the electric and magnetic fields are actually transverse to the direction of propagation of the wave; and in a sound wave in air, the oscillating motion of the molecules is actually parallel (longitudinal) to the direction of propagation of the wave.

But surely EM waves are less actual than sound waves? Sound waves are waves in physical matter (air molecules). EM waves are ... er... not.

 

[Mårten]

Being pedantic, amplitude is "the maximum displacement of a periodic wave" [Princeton wordnet], therefore you can't strictly define it at a "point", can you? Also the field concept is tricky. In sum might it be better to say, "what is oscillating are the *magnitudes* of the electric and magnetic *forces* at a given point"?

Ah, exactly how I look upon it. Therefore, that magnitude is like any other magnitude of some other property, e.g. pressure.

Don't be too hard on your teachers Mårten, this concept is often introduced using the wave on a skipping rope metaphor, as that's a more concrete example for people to understand than a regular oscillation of a magnitude at a point. (Gribbin uses it in his "kittens" book).

Hrm... I think that's just the kind of analogies one should avoid. Better to make an analogy with the sound waves then. And then, in a later stage draw the distinction between transverse and longitudinal waves. A distinction which, when it comes to light, I haven't understood yet, as this is the subject of this thread...

why would anybody compare a transverse EM wave to a longitudinal sound wave. why not to a transverse sound wave.

But the transverse sound wave makes physical extensions in the transverse direction, like waves on water - that's not the case for EM waves, is it?

 

[granpa]

But the transverse sound wave makes physical extensions in the transverse direction, like waves on water - that's not the case for EM waves, is it?

how much of an extension in the transverse direction do you think a transverse sound wave makes when moving through solid rock?

 

[jtbell]

No, what is oscillating are the amplitudes of the electric and magnetic fields

Being pedantic, amplitude is "the maximum displacement of a periodic wave"

Oops, you're right. I meant "magnitude" as in "magnitude of a vector." I've corrected my original post. Thanks for catching this.

(Of course, the directions of \vec E and \vec B also oscillate, or rather, flip-flop.)

 

[jtbell]

In an electromagnetic wave the electric and magnetic fields are actually transverse to the direction of propagation of the wave;

You mean transverse in the meaning - and nothing else - that the vectors of the E-field points 90 degrees in relation to the wave propagation?

Yes.

My problem here, is that for the sound wave, we have actual molecules oscillating back and forth parallell to the propagation of the wave. But for the EM wave we don't have anything that concrete. The E-field is also abstract (I thought at least). It doesn't really exist until you have a test charge which can be affected by it.

But surely EM waves are less actual than sound waves?

I think most physicists take the view that E- and B- fields are "real" in some sense, and exist independently of the real or imaginary test charges that we use for observing them. It's also possible to take the view that the fields are merely mathematical devices for simplifying the calculation of what is ultimately a complicated "action at a distance" of some kind. It's impossible to distinguish between these two interpretations experimentally, so the choice is a matter of personal preference and philosophy, and depends on exactly what one means by "real." People can and do debate such things endlessly. I get bored with it pretty quickly, myself.

If you have problems like this with the classical electromagnetic field, just wait until you get to quantum mechanics! In the Quantum Physics forum, people go at each other with hammer and tongs over questions of interpretation.

 

[granpa]

the question of what is actually oscillating is moot. it follows all the same laws. that is all that really matters.

 

[granpa]

the FIELD of a single photon extends 1/2 wavelength to the side. that's not the same as saying that the displacement is 1/2 wavelength.

 

[Mårten]

I think most physicists take the view that E- and B- fields are "real" in some sense, and exist independently of the real or imaginary test charges that we use for observing them. It's also possible to take the view that the fields are merely mathematical devices for simplifying the calculation of what is ultimately a complicated "action at a distance" of some kind. It's impossible to distinguish between these two interpretations experimentally, so the choice is a matter of personal preference and philosophy, and depends on exactly what one means by "real." People can and do debate such things endlessly. I get bored with it pretty quickly, myself.

Okey, if we on the border to philosophy, then I think I'm satisfied with my understanding so far. Maybe what disturbs me is how to interpret the word "transverse waves". Longitudinal and transverse should be reserved for positional displacement, as we talked about here. Since no displacement occurs for EM waves, that kind of waves ought not to be called that. They are more like continuous pulses of some sort. But I don't know the exact definition of the word transverse really.

If you have problems like this with the classical electromagnetic field, just wait until you get to quantum mechanics! In the Quantum Physics forum, people go at each other with hammer and tongs over questions of interpretation.

LOL! That day, that sorrow, as we say in Sweden...

 

[mal4mac]

The E vector, at the point, points upwards and downwards, rather than forwards and backwards. Think of that as "defining" the E as transverse. It's a wave because, if you plot the magnitude of E against time, you get a wave-like pattern.

Check the first section of:

http://en.wikipedia.org/wiki/Field_(physics )

To get some not-too-philosophical stuff on the reality of the field (quotes from Wheeler and Feynman!)

 

[granpa]

EM waves are considered to be transverse because they can be polarized.

your link is broken
http://en.wikipedia.org/wiki/Field_(physics)

 

[Mårten]

the FIELD of a single photon extends 1/2 wavelength to the side. that's not the same as saying that the displacement is 1/2 wavelength.

Hm... Need to think about that. I thought we were talking about field strength here at different points on the x-axis. Photons are too advanced for me to understand by now, I leave that for now, probably come back to that later...

Check the first section of:
http://en.wikipedia.org/wiki/Field_(physics))
To get some not-too-philosophical stuff on the reality of the field (quotes from Wheeler and Feynman!)

Interesting!
(It's the forum software which strips the closing parenthesis from the URL.)

 

[Phrak]

Mårten, I think the most concrete you might get with electromagnetic waves is to consider a very low frequency, so that you have a very slowly changing field--so slowly that the electric and magnetic fields are nearly fix.

It should be clear that if you put a compass needle in the field it will align itself transverse (or perpendicular) to the direction of propagation, and repeatedly reverse direction. A test charge will have upon it a force exerted in the other transverse direction.

I recall in my freshman year that there were a few who didn't take well to the idea of transverse waves, while the rest of us blissfully took them on faith. If it helps, they don't persist in other mathematical models.

ganpa,

the FIELD of a single photon extends 1/2 wavelength to the side.

That's not actually true, I am afraid, but your references to shear waves in solid media,were well taken.

 

[granpa]

ganpa,
That's not actually true, I am afraid, but your references to shear waves in solid media,were well taken.

well I may well be expressing it wrong but it is a fact that a photon can't get through a lattice of wires spread 1 wavelength apart.
http://en.wikipedia.org/wiki/Polarizer#Absorptive_polarizers

 

[granpa]

if you consider how electrons oscillating in an antenna produce a radio wave I would think that the EM wave is not actually produced directly at the antenna itself but rather it is produced in the space immediately surrounding it (by the expanding and collapsing electric and magnetic fields which are out of phase with the moving electrons). so I suppose that the reverse process of absorbing the EM wave doesn't actually require the wave to strike the antenna directly. I suppose its absorbed by the field that immediately surrounds the antenna that is produced by the oscilating electrons inside the antenna itself.

so perhaps the photon doesn't extend 1/2 wavelength to each side after all. (of course, being a wave, it will tend to spread out)