If light is a wave, what is waving?

PhilDSP

What is waving? In one simple word: energy.

EM fields might be considered conceptual crutches that allow us to determine the relationships between energy and force. (Maxwell regarded what we now call the E field as a force). More recently, the field idea replaced Maxwell's force in a consistent manner. EM fields are not directly observable and depend on the concept of having an artificial "test charge" to determine their values by observing the motion of a charge or set of charges that can be compared with that "test charge" by determining the force needed to move the mass associated with the charged particle(s) in such a manner.

If we assume that Poynting theory is correct then all fluctuations of EM fields can be reduced (or translated) to fluctuations of energy.

harrylin

Traditionally in classical physics, light was conceived as either particles in empty space or waves in a medium which got the name "ether". All considered particle models were disproved so that the wave model of light remained, but the medium looked less and less like "ether". In particular, the concept of a kind of thin medium that is pushed away by moving bodies was disproved, leaving as only model that of a "stationary" ether that includes particles as a kind of waves.
However, next the wave description was kept but the medium was considered superfluous by most - even if it doesn't make conceptual sense to have a vibration of nothing. Some people replaced it with the concept of Spacetime (also coined a 4D ether). In recent times medium concepts also came from quantum mechanics but that is out of the scope of this classical forum.
So, if you fancy it then you can use it to satisfy your personal logic, but be prepared to face the fact that a great number of people do not fancy it at all and that it is even dogmatically rejected in influential circles.

harrylin

Then it may please you to hear that at a certain time even Einstein admitted to a kind of ether of which the "fundamental" difference with the "ether of Lorentz" was that in the new model the state of the ether is affected by matter. However he later promoted the 4D Spacetime concept as hidden reality, which is quite a different philosophy. There are threads on those different interpretations, you may search for them and choose the one that according to your logic is more likely to be close to "hidden reality". So far neither model has been disproved beyond doubt, and it's not sure that this will ever be possible.

PhilDSP

Dirac also pushed very heavily for a reconsideration of the usefulness of an aether. And he is known to have, more or less, single-handedly defined the modern structural basis of QM. But again, what the "aether" represents is our lack of full recognition of what the rules are for how fluctuations of energy are related to fluctuations of force and what could sponsor those rules.

nikolafmf

Electromagnetic field has some energy, as has sound. Energy of sound comes from vibrations of molecules of air, say. Where does energy of EM field comes from? One would may be say from the oscillation of the field. But this is conceptually challenging for many people. We can see the molecules under the microscope and know how they look. But we can't see the EM field. So, people keep asking what is that field...?

ZapperZ

We get question like this rather often, and it is an interesting puzzle for me, as a physicist, on not the question, but why it is asked. Let me explain.

Let's say that you have a charge sitting somewhere, at a fixed location. Now, at a distance r away, you have another charge. This second charge feels a force due to the electrostatic field from the first charge. So far so good?

Now, no one seems to want to ask what MEDIUM that electrostatic field traveled in. Presumably, everyone seems OK with this lack of medium, for some reason, in the case where everything is static. After all, this was never asked in this thread. So I will assume that no one has any conceptual problem with this situation.

Just when things look fine and dandy, I decided to take that first charge, and then wiggle it up and down (or, if you prefer, sideways). What did I just do? I've created an electromagnetic radiation, i.e. "light"! If I look at the second charge, it will detect this oscillating field.

So my question is, if no one seems to find any issue with the electrostatic case, why is the oscillating case any different? Why is there an insistence on a "medium" to satisfy your conceptual understanding when you didn't find it conceptually difficult with the electrostatic situation? It would be malicious of Mother Nature if she doesn't require any medium to transmit electrostatic field, but then suddenly changed the rules of nature and now introduced a medium just because I decided to wiggle that charge! I find THAT to be even more conceptually troubling!

And BTW, if you think I've just made up this example out of thin air, think again. If you visit a synchrotron light source facility, you'll find electron bunches going through insertion devices called "wigglers" or "undulator" that essentially are making these electron bunches wiggle up and down, to generate electromagnetic radiation. In fact, free-electron lasers use this very concept to generate such radiation.

Zz.

nikolafmf

Not necessarily. On could ask, how is that that first charge tells the second it is present here. In other words, how is that force transmitted to the second charge? In macroscopic objects, I can push against the wall with my hands, of tight somebody with a rope and pull her to me in that way. There is always direct contact. But how can the second charge feel the force of the first without a direct contact? I have not studied quantum electrodynamics yet, so I want to know if that theory may be gives some answers?

ZapperZ

But you didn't ask that. You directly asked about "oscillating" wave. And my point is, why did you ask that and didn't ask why there was a force between them under STATIC condition in the first place? Was that situation understood intuitively already?

BTW, when you push against something, what you say is a "direct contact" is really the electromagnetic interactions of the atoms/molecules of your hand against that of what you're pushing against. That is your "direct contact", which is the same as what I've described earlier in the electrostatic case.

So now what? Do we need to take several steps back and now figure out what exactly gets transmitted between remote objects that do not require a medium? After all, one can make the same argument with gravity as well.

Zz.

jtbell

Ah, but when you look really closely at the interface between your hand and the wall, at the atomic level, the notion of "direct contact" becomes fuzzy, because you're dealing with "probability clouds" of electrons which are repelling each other via the electrical force.

[Zz beat me to it by a nose.]

nikolafmf

You are right.

nikolafmf

Yeah, that is a good question which could solve many dilemmas. What exact gets transmitted...? And if it is photons, how they make charges to feel force?

harrylin

Surely many people do! It's fascinating to see how the electrostatic force of a plastic pen can deflect "at a distance" the water flow from a tap and as a kid I wondered what "is there" that transmits magnetic force, as we can feel how repulsive forces interact at a distance.

This was also asked by the interviewer in the linked video:
http://www.youtube.com/watch?v=wMFPe-DwULM
I find Feynman's reply disappointing. In the middle he implied that he didn't know the answer which is OK, but mostly he was just looking for excuses - he spent much time explaining how difficult it is to answer a "why" question and said that he could not explain it in terms familiar to the interviewer.

ZapperZ

Oh, I'm fascinated by it. I'm saying that, especially in THIS thread, this issue NEVER came up. And I've seen many other threads where people dived in right into the more complex situation without bothering to step back and see if they've understood the simpler case.

This "incident" is well-documented, even by the interviewer. Still, at some point, I can understand Feynman's frustration, keeping in mind that this was the SAME guy that captivated the media and the public with his very simple demonstration during the Challenger disaster hearing.

As Einstein was known to say, explain it in simple terms, but not any simpler. One runs the risk to making inaccurate analogies when one tries to do many of these things at the pedestrian level.

This gives me the opportunity to kill 2 birds with one stone.

1. It is imperative that, in learning, one starts with the simpler, more basic understanding. This is why I puzzled at the question, because it seems to start with the more complicated time-varying problem of something oscillating. When we teach physics to students, we try to start with something simpler, just so they get an idea and a feel for the physics, before proceeding to more complex physics and situations. That's how one HAS to learn. Really, the cliche that one has to learn how to walk FIRST before attempting to run truly applies here!

2. Many people, especially crackpots, have accused physicists of wanting to stick "within the box", and that we are unwilling to work "outside the box". This is, obviously, utterly false. What happens usually is that we often discover that what we consider to be a "rule" turns out to be only true for a limiting case. For example, we thought we have a conservation rule for energy, and a separate conservation rule for mass. Of course, we KNOW know that the more UNIVERSAL rule is the conservation of mass(energy content)+energy. So we change our concept when that has been shown convincingly to be valid.

Now that last argument applies here with respect to the CLASSICAL FIELD (since this is posted in the Classical Physics forum). We originally thought that all waves must have a medium to propagate in. When Special Relativity came out, and upon further development and experimentation throughout the previous century, we realize that the concept of a medium for EM radiation is superfluous. It is not needed, and more importantly, it is not detected! So we drop the "universal" requirement that all waves must have a medium to propagate. Thus, Maxwell equations, the equations that describe classical EM interactions, no longer require one!

So now, instead of physicists being stuck to thinking only within the box, it now appears that the general public/laymen are the ones who can't go beyond this box. Why? Because it doesn't make sense, or conceptually difficult to accept? That isn't a very strong argument (it isn't a very weak argument either), because having something to "make sense" or conceptually acceptable requires that one is FAMILIAR and understand that something. There are many things that don't make sense but are true, simply because we did not understand it in the beginning.

So the question on "what is waving" cannot be answered other than saying "nothing". This is because asking "what is waving" assume that there's something out there that is waving, and you want to know what it is (other than, presumably, the electric and magnetic fields). One can't answer that question anymore than one can answer "So, when did you stop beating your wife?" Both made an a priori assumption that hasn't been verified.

If you ask "Does EM require a medium to propagate?", then the answer is NO. Once that is established, then the question on what is "waving" doesn't come up anymore, because it becomes moot!

Note that we haven't dealt with quantum field theory, or in particular, quantum electrodynamics. I am extremely hesitant to want to start delving into that, because I can see myself having to take steps backwards at every step along the way to explain the explanation.

I don't know if my response here is sufficient to satisfy the conceptual understanding of this. It may not be satisfying to hear that "nothing" is the answer. However, Mother Nature has no obligation to cater to our feelings or our needs. She does what she does.

Zz.

harrylin

Following your comparison, your answer on the second question is then that the wife was not beaten because no one knows if she was beaten - because for you answering in the negative is equivalent to not answering?!

ZapperZ

Er.... no. If you read further, I gave an example in which, apply to this, means that one must FIRST ask "Did you ever beat your wife?". If the answer is "No", then asking "When did you stop beating your wife" is moot and has no answer.

My first answer of "Nothing" to the original question was a demonstration on why many find that answer difficult to understand or accept.

Zz.

nasimrahaman

In short, the electric and magnetic fields are the ones waving. Their 'waving' is kinda coupled; i.e. a waving electric field induces a waving magnetic field (described by Ampere's corrected circuital law) and a waving magnetic field induces a waving electric field (see Faraday's law of induction).

Think about the pond again. The waves in a pond are very different from electromagnetic waves, but what makes the water 'wave'? You could almost think of it as the periodic vertical translation of the water molecules at every relevant point in space (or just the up and down movement of water molecules on the surface) which makes the wave a wave. ( http://upload.wikimedia.org/wikipedi..._animation.gif )
With light, a vertically waving electric field induces a horizontally waving magnetic field, which in turn induces a vertically waving electric field, the waves propagating in a direction perpendicular to both electric and magnetic field oscillations.

harrylin

OK, thanks for the clarification!

Still, the OP based his question on an already received answer (or assumption) "[yes], light is a wave". Based on that first answer, his logical next question was: "[well then,] What is waving?".
And you answered: "Nothing is waving" - which means in full, if you were answering the question: "Light is a wave of nothing waving". As I mentioned, I find it logical if many find that a difficult answer to accept!

But it is strange that we rarely hear the question "If a magnet has a magnetic field, what is it made of".