If light is a wave, what is waving?

[jtbell]

OT, but this is a good place to ask:

If one is interested in the metaphysical aspects of physics, or in any other philosophy of science, is there a good place to discuss it?

We do have a Philosophy forum here:

https://www.physicsforums.com/forumdisplay.php?f=112

 

[EskWIRED]

We do have a Philosophy forum here:

https://www.physicsforums.com/forumdisplay.php?f=112

Ack! I should have known!

Thanks.

 

[jtbell]

'Interpretation of quantum mechanics by the double solution theory - Louis de BROGLIE'
http://aflb.ensmp.fr/AFLB-classiques/aflb124p001.pdf

“any particle, even isolated, has to be imagined as in continuous “energetic contact” with a hidden medium”

The hidden medium of de Broglie wave mechanics is the aether. The “energetic contact” is the state of displacement of the aether.

I note that de Broglie himself did not use the term "aether" in that 41-year-old paper. De Broglie-Bohm theory is indeed a currently-researched topic under interpretations of quantum mechanics. We have at least one person who works in this area, and who posts regularly in our Quantum Physics forum. I don't think I've ever seen him refer to the dB-B "quantum potential" as an "aether" or a physical medium in the sense that people usually understand the word "aether".

 

[mikelepore]

I think the answer to the original question is:
It starts out with a charge being accelerated or having a quantum jump.
Then Maxwell's 4th equation says a changing electric field produces a magnetic field.
Then Maxwell's 3rd equation says a changing magnetic field produces an electric field.
Then Maxwell's 4th equation says a changing electric field produces a magnetic field.
Then Maxwell's 3rd equation says a changing magnetic field produces an electric field.
Repeat.
This pattern of a change generating another change is self-perpetuating.
Observers some distance away see sinusoidally changing electric and magnetic fields arriving at their location.
Is this correct? Please criticize. Thanks.

 

[harrylin]

This seems reasonable to me. If we say that "X is a wave" then that implies that associated with X is some A such that:
\frac{\partial^2 A}{\partial t^2}=k^2 \nabla^2 A

Then the question "what is waving" is asking "what is A", so it is a legitimate question which is not assuming anything beyond what is implied by the statement that X is a wave.

So for X=light you can answer the question either A=the electric field or A=the magnetic field, since both are associated with light and both obey the wave equation. That then reduces the problem in understanding to the static case that ZapperZ mentioned. If they are comfortable with electric and magnetic fields, then they should be comfortable with that answer. If not, then they need to get comfortable with static fields first.

Yes indeed. However an answer like that ignores that those two kinds of "fields" themselves were supposed to be, similarly as the wave description, properties of something and not things on their own - it complicates the original concept behind the equations by replacing the medium with a plurality of media or entities.
So, it's certainly very useful not to ask (or answer) the questions in isolation, as the three questions are related.

 

[harrylin]

[..] If one is interested in the metaphysical aspects of physics, or in any other philosophy of science, is there a good place to discuss it?

By "good", I mean like this place. I mean a forum with well-versed people willing to countenance simple questions and give relevant information. Not a place full of crackpots, but maybe a place where references to more information are routinely given.

We do have a Philosophy forum here:

https://www.physicsforums.com/forumdisplay.php?f=112

That's true. Regretfully it works like a black hole - hardly any physicist looks there. Not even those who make strong philosophical statements elsewhere (I tried it!). So, regretfully it's not a "good place". Perhaps it's too much hidden and far away? And I guess that such a question also needs to be discussed somewhere else.

 

[Dale]

those two kinds of "fields" themselves were supposed to be, similarly as the wave description, properties of something and not things on their own

Huh? Where did you get that from?

 

[harrylin]

Huh? Where did you get that from?

Where did I get what from? That "field" referred to some kind of state of a medium that exercises a force? As far as I know that concept originated with Maxwell and contemporaries.

ADDENDUM: That turns out to be incorrect (imprecise), and interestingly, already Maxwell spoke of "the electromagnetic field" (in singular).
That's in a paper* of 1864, and it sounds as if there he actually invented the term "field" for something that "has to do with the space in the neighbourhood of the electric or magnetic bodies [...] The electromagnetic field is that part of space which contains and surrounds bodies in electric or magnetic conditions". And he argued that this indicated the existence of a medium: "actions which go on in the surrounding medium as well as in the excited bodies".
*A Dynamical Theory of the Electromagnetic Field, 1864

Synonyms for "electromagnetic field" would thus be "electromagnetic area" or ""electromagnetic region".
So, thanks for asking!

 

[Dale]

Where did I get what from? That "field" referred to some kind of state of a medium that exercises a force? As far as I know that concept originated with Maxwell and contemporaries.

ADDENDUM: That turns out to be incorrect (imprecise), and interestingly, already Maxwell spoke of "the electromagnetic field" (in singular).
That's in a paper* of 1864, and it sounds as if there he actually invented the term "field" for something that "has to do with the space in the neighbourhood of the electric or magnetic bodies [...] The electromagnetic field is that part of space which contains and surrounds bodies in electric or magnetic conditions". And he argued that this indicated the existence of a medium: "actions which go on in the surrounding medium as well as in the excited bodies".
*A Dynamical Theory of the Electromagnetic Field, 1864

Synonyms for "electromagnetic field" would thus be "electromagnetic area" or ""electromagnetic region".
So, thanks for asking!

OK, so that is an historically correct "when did you stop beating your wife" question.

 

[PhilDSP]

It seems that the field concept originated with Faraday and Maxwell resisted making it a structural component of his theory in some ways. At least, if Maxwell could identify a non-field component as something else: force, polarization or positional displacement for example, he would prefer that to a field value. That makes sense to me as it seems Maxwell tried to make his theory as physical as possible. (His mentor William Thompson AKA Lord Kelvin didn't really fully approve of Maxwell dabbling with the metaphysics of fields)

 

[Drakkith]

I feel any answer other than "the electric and magnetic fields" is more philosophical than scientific currently.

 

[Studiot]

Talking of metaphysics the difficulty is this:

If we require a medium (field, aether, whatever) for the wave to travel in it implies that the medium was present before the wave started traveling and the wave spreads through the undisturbed medium at the wave velocity.

So if this medium is an electric or magnetic or electromagnetic field for a light source, how did the medium get there before the first emission of the light?

Alternatively we can suggest that the wave somehow takes its medium along with it or creates its own medium as it goes along.

 

[harrylin]

OK, so that is an historically correct "when did you stop beating your wife" question.

Yes indeed - I didn't mention or elaborate on that as I didn't want to step on someone's toes.

 

[harrylin]

[..] So if this medium is an electric or magnetic or electromagnetic field for a light source, how did the medium get there before the first emission of the light?

A similar "difficulty" would be raised by the question "if the light source is an atom that will emit the light, how did the atom get there before the first emission of the light?".

Alternatively we can suggest that the wave somehow takes its medium along with it or creates its own medium as it goes along.

That's a very different model - can you illustrate that with a water or sound wave? Or do you know an article describing that model?

 

[Studiot]

That's a very different model - can you illustrate that with a water or sound wave? Or do you know an article describing that model?

Well for an ordinary common or garden type Hawaiian surfing wave the water is there beforehand for the wave to propagate in(to). The wave can be seen to develop, travel and finally disappear.

On the other hand, when a tidal bore or surge occurs there is little or no water present beforehand. Certainly not enough to accommodate the oscillation amplitude. The bore front takes sufficient water along with it.

 

[PhilDSP]

I feel any answer other than "the electric and magnetic fields" is more philosophical than scientific currently.

Isn't it worth considering fields to be transformational devices that manipulate and transform other qualities such as force, energy, potentials, momentum, polarization potentials, etc., in a consistent and physically meaningful way? In a sense, fields collect what we know about how those other qualities are coupled together. In analogy to house construction, they provide the blueprint, but not the physical realization of the framing, plumbing, electrical system and so forth.

 

[Swetasuria]

First of all the field doesn't travel. It's just there, everywhere, including the vacuum. In reality the field is made up of quantized harmonic oscilators.

Fields can have directions, right? If they are "just there, everywhere", then how come fields are vector qantities?

 

[harrylin]

Fields can have directions, right? If they are "just there, everywhere", then how come fields are vector qantities?

Forces have directions, and as you can see in post #68, it seems that originally (and technically), fields were just the neighbourhoods where those forces were felt. However, later the term field strength (and sometimes simply "field") was used as a measure for the strength of forces that can be felt in the field - which includes directionality.

PS I'm amazed by this accurate intro in Wikipedia (although it is messed up by the there following "qualitative description"):
"In physics, an electric field is the region of space surrounding electrically charged particles and time-varying magnetic fields."
-http://en.wikipedia.org/wiki/Electric_field