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Blog Entries: 27

If light is a wave, what is waving?

 Quote by 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".
But see, that is why I asked why the question of the electrostatic case wasn't asked! If one has no conceptual difficulties in accepting the presence of an electrostatic field in vacuum, with no medium, then why would one has issues accepting a "waving E field"? I'm trying to understand the "flow" of knowledge here of the person who asked the question, because I need to established what is known, and what is accepted by the person asking the question. Jumping in and simply providing an answer, to me, is usually not how I would normally proceed.

Zz.

 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!
I agree 110%

Which is why I was aghast when I discovered that the modern UK GCSE physics syllabus introduces force by using friction as the example force.

 Recognitions: Gold Member Thanks for the post ZapperZ! Very well done!

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 Quote by harrylin 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!
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.

 I'm wondering if what you really mean by "Nothing is waving" is that nothing physical is waving, or at least nothing that we commonly recognize as having physical substance. If you consider the equations for E and B fields, either static or dynamic, that certainly seems the case. But often electromagnetic potentials are placed in the equations along side E or B or even alone. Granted, physical substance is not normally associated with potentials either, even potential energy or gravitational potential. And yet they have a structural place that seems to generate physical effects. So there is a bit of a paradox there.
 What I'm about to say is really only based on something I've read in a vulgarization book so I have absolutely no idea how much of it is true or not (I have never studied Quantum Electrodynamics), but it suggests an answer to the question of "what" is being transferred in the field. I've read that, basically, there is a bunch of "virtual particles" that get created and destroyed almost momentarily along the whole path between one charged particle and the other charged particle. These "virtual particles" supposedly carry the interaction between the charged particles. Can someone who is more knowledgeable on the subject explain this more precisely, or tell me if it is absolute nonsense?
 Particles of any description are not required for wave theories. Particles are for corpusculer theories, and yes there are corpuscular theories as successful as wave theories that involve 'virtual particles'. However we are talking about wave theories here and this is the classical mechanics forum.

 Quote by ZapperZ Your question has nothing to do with "If light is a wave, what is waving?", does it? In fact, this is beginning to sound like a philosophy question, which doesn't belong here. Zz.
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?

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.

 Quote by timmyteapot for a number of years I have been questioning our understanding of EM waves and remain unconvinced that anyone truly understands the reality of the nature.
I share your dissatisfaction with physics. Too often, the answers are to questions of what and how. Too seldom, the answers encompass the question of "why". Too often the answer is "Nobody knows".

But the subject is fascinating nevertheless. And at this point in history, no fault can be laid on our inability to answer the "why' questions. They are hard questions.

But the inability to answer is frustrating to me. Too often, the answers are mere tautologies. For example, I wonder why one can decrease wire gauge when transmitting electricity at higher voltage. I wondered what this voltage thing was. When I found out that voltage is the ability of electricity to overcome resistance, I was disappointed. Higher voltages can carry current through wire with higher resistance because - voltage means the ability to overcome resistance. IOW, increasing the voltage allows the electicity to flow through smaller wires, and that is why increasing the voltage allows...

So much of physics, at the levels that I currently understand, has a tautology at its base.

I find that the more I ask "why', the less satisfied I am. Physics can explain a lot of "how it works", but very little of "Why is it like that".

I have heard that the philosophy of science is of little help in these matters. Some physicists have very disparaging things to say about the current state of the philosophy of science. But IMO, for my taste, I would love to know more "why" to supplement physics' answers to what and how.

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 Quote by EskWIRED 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:

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

 Quote by jtbell We do have a Philosophy forum here: http://www.physicsforums.com/forumdisplay.php?f=112
Ack! I should have known!

Thanks.

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 Quote by adt755 '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".

 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.

 Quote by DaleSpam 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.

 Quote by EskWIRED [..] 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.
 Quote by jtbell We do have a Philosophy forum here: http://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.

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 Quote by harrylin 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?

 Quote by DaleSpam 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".