Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Self-replicating fields in EM radiation?

  1. Jan 26, 2011 #1


    User Avatar
    Gold Member

    I see everywhere the explanation that electromagnetic radiation self-propagates this way: a changing magnetic field induces a changing electric field, which in turn induces a changing magnetic field and so on and so on.

    I do not understand that. I do not have a problem with saying (it is a legitimate conceptual model, a way of expressing what happens) that wherever an electromagnetic wave is present, there is an electric field and a magnetic field. However, I do not see how that can “explain” why the radiation propagates through vacuum.

    After reading this month about the subject, I come to the conclusion that all the chapters about the subject can be summarised, simplifying a little, as follows:

    a) There is a basic principle: a charge attracts/repels (depending on sign) another charge.
    b) Hence a set of moving charges, i.e. a current attracts/repels (depending on the sense of motion) another current (including in the concept of “current” both motion of free charges inside a conductor and aligned motion of bound charges inside a magnet).
    c) An oscillation of charges, i.e. an alternate current induces (by way of attraction/repulsion) another alternate current (including in this concept also a moving magnet).

    In a) and b) the phenomenon happens at a distance and I have not read that authors resort to self-replicating fields to account for that fact. Why do they do in the case of c)?

    After all, the three things are manifestations of the same phenomenon and one of the findings of SR is to make that clear, isn’t it?

    It seems simpler and more honest to accept that in the three cases, a), b) and c), the “influence” (attraction or repulsion or an alternative series of both things) just propagates, we do not know how. Would you agree to that?
  2. jcsd
  3. Jan 26, 2011 #2
    It doesn't make sense that the EM field was present (although unified into electroweak) during the initial expansion of the universe, and has continued to do so?

    Your question boils down to: where do fields come from, which boils down to: where did the universe come from.

    Physics doesn't go there really. Fields are present and quantized at each point in space... always have been, and expanded apace. What's the problem?
  4. Jan 26, 2011 #3
    This isn't the modern view of explaining propagation of em radiation, even though engineering books, and even some physics books are rife with this idea which causes confusion among intelligent people like you that even bother to think about this problem.

    It is moving charge that creates E and B at the source which propagates away from it at the speed of c. Neither E nor B cause one another. Classical Electromagnetic Radiation by Heald and Marion discusses this, giving the reference:

    Soodak and Tiersten, Am. J. Phys. 62, 907 (1994), and Heras, Am. J. Phys. 62, 949 (1994).
  5. Jan 27, 2011 #4


    User Avatar
    Gold Member

    Thanks a lot. I will try to find those references for detail, but what you say is already enlightening. I think that in this matter, like it often happens, there is a certain intellectual "inertia" in maintaining old mental habits... of the the time when it was thought that electricity and magnetism were different phenomena, with different origins, so one phenomenon could generate or induce another.

    It seems that modern physics, following SR, akcnowledges that there is a single phenomenon, called the electromagnetic field tensor, having E and B as components. Components in turn are a trick whereby a single thing whose magnitude is not directly measurable from a certain perspective is decomposed in parts that can be actually reached and measured from that perspective. But it would be absurd to say that the components "create" the observed thing... How do you see this analogy? Maybe someone mastering the concept of the electromagnetic field tensor could elaborate on the subject...
  6. Jan 27, 2011 #5
    Jefimenko also has done extensive analysis that shows no causal relation between E and B but rather in simple terms what you describe. He has several books available and an alternative derivation of his findings appears in a sub-chapter in Jackson's ubiquitous EM text book. One interesting point is that he has re-derived the definition of the vector potential in terms of retarded solutions to the Maxwell equations in such a way to give it definite observable meaning. (And thereby it becomes a "component" that generates a portion of the E field)
    Last edited: Jan 27, 2011
  7. Jan 27, 2011 #6


    User Avatar
    Science Advisor

    I am not a Physics student but I have wondered about the same thing. I may have this all wrong but here's the impression that I get.

    Maxwell's equations describe light as two fields that exist locally. They do not arise from a source. The local fluctuation of the electric and magnetic components at each point of space is what light is.

    One does not need moving charges to have light. Shine a flashlight into space then immediately shut it off. The light does not go away but continues on its own without a source.

    So it exists independently and without going into the philosophy of what it means for anything to exist it would seem that Maxwell's theory suggests that the fluctuating local fields generate each other. There is no need for stationary charges or currents.

    To me this is profound because it really seems to say that the fields are real and are not just mathematical devices for describing action at a distance as in Newton's theory of gravitation or in Coulomb's Law for the electric field around a charge.
    Last edited: Jan 27, 2011
  8. Jan 27, 2011 #7
    It should probably kept in mind that electric and magnetic fields are 2 observable quantities that together give a pretty useful description of light. But they are certainly not the only observable aspects of light and more show the behavior of light than what it consists of. It's probably more consistent with Maxwell's theory to say that light is a displacement current.
  9. Jan 27, 2011 #8


    User Avatar
    Gold Member

    Thanks... The wiki article on “[URL [Broken] equations[/URL] says this:

    Hmm… If I reached the same conclusion, having little mathematics, through another route, based on an understanding on the physics behind plus deriving logical consequences from it, that may be a proof we are all on the right track…

    Well, it is clear that the radiation “exists” after the source has disappeared. We see the light of stars that exploded time ago. But that does not mean that it does not have a source. It originates because of the oscillation of charges, although it keeps travelling after the charges stop oscillating.

    A different thing is “what” exists. You may think it is a perturbation or wave in something that was there beforehand (is that your idea of the “field” as something “real”, akin to a medium?) or rather a particle that had never been there or rather, as it seems to be the prevailing opinion, another concept that combines both ideas.

    In any case, leaving that issue aside, which is quite thorny, what the OP wanted to highlight is that “what exists” is a succession of double orders “I am positive (so please be repelled if you are also positive or be attracted if you are negative) – I am negative (…)” or “I am negative (…) – I am positive (…)”, i.e. an oscillation of electrostatic orders, which are not different from those emitted by a single static charge. And if, in order to account for the propagation of the latter orders you do not need pairs of causally connected fields, neither do you need such explanation to account for the propagation of the former orders.

    I still do not understand those concepts, I will have to study them, but maybe you can detect if they have a connection with my point, as I developed it above.
    Last edited by a moderator: May 5, 2017
  10. Jan 27, 2011 #9


    User Avatar
    Science Advisor

    I use real in the sense that it is not a mathematical device that attempts to replace the idea of action at a distance.

    Semantics is not my strong point but to me something that just exists through its own intrinsic dynamic has no source even if there is no way to originally create it without something else.
  11. Jan 28, 2011 #10


    User Avatar
    Gold Member

    No problem with that.

    Well, yes, I guess it would not be precise to say that in such case there is no source. A bullet shot from a gun would keep its state of motion by inertia for ever, in the absence of friction, and we would not say that it was not shot from the gun. The same happens with mechanic waves: they would continue expanding endlessly if they were not attenuated by friction in the medium (i.e. random motion introducing disorder in the the order represented by the direction of propagation). As to EM radiation, I think it only attenuates due to interaction with matter (through absorption or scattering), but not due to anything intrinsic to its way of propagation. But that only means that it finds no hindrance. However, the continuation of motion in the absence of hindrance, in itself, is not anything uncommon but what one would naturally expect to find. Rather the idea of a need for a continuous self-acceleration (one field creating the other) is what strikes the mind as abnormal. It reminds me of the old aristotelian concept of the necessity of a constant "mover" moving the arrow.

    But forget now for a moment about EM radiation. Think only about the attraction between like or opposite charges. How do you explain that the attraction / repulsion orders propagate across space? That takes time of course, it does not happen instantaneously. But how do the orders travel along space and time? Would you say that an E field (which is the only thing you have here) creates another E field, which creates another and so on?
  12. Jan 28, 2011 #11
    You're basically describing "polarization" which Maxwell used as the basis for his theory. He considered the EM medium to be polarizable where positive and negative charges (or current elements in a displacement current) are displaced from each other by an external force creating a tension because work must be done to separate them. That increases potential energy between the polarized elements. Tension in the medium is released when the polarized elements are allowed to approach each other again and in doing so the potential energy decreases.
    Last edited: Jan 28, 2011
  13. Jan 28, 2011 #12


    User Avatar
    Gold Member

    I have seen the term "polarization" referring, rather, to the influence exerted by a pure E field: the dipoles turn round so that that, say, their, + faces the - face of the E field, thus weakening it. What you describe (and I was also describing)..., shouldn't we call it "oscillation or changes in polarization"?

    In any case, it is clear that this "polarization" (meaning that all + o - particles of atoms or molecules of a piece of material get aligned in a certain direction) and "oscillation in polarization" take place at the level of matter. But the big question is how the "influence" that triggers polarization or changes thereof propagates across the space lying between matter pieces. I think there is no answer for that, all we know is that it just propagates, don't you agree?
  14. Jan 28, 2011 #13


    User Avatar
    Science Advisor

    In a) we describe the attraction by means of an electric field that each both charges generate. So there is are two equations;
    1) Electric charge generates electric field
    5) Electric field causes movement of charge

    Another of Maxwell's equations says
    3) Current and changing electric field generate magnetic field

    Because of 3) and its similarity to 1) we say that magnetic fields are generated by changing electric fields. If you reject this interpretation of 3), then you must also reject the interpretation of 1) that electric charge generates electric fields.
  15. Jan 28, 2011 #14


    User Avatar
    Gold Member

    Atty, I do not follow you. In fact, we are a level beyond the areas you mention. You have mentioned attraction/repulsion between static charges = pure electric fields (which is my level a) and also attraction between currents strictu sensu and currents in magnets = electric fields accompanied by magnetic fields (my level b). But we were talking about level c), oscillation of charges that induces a distant oscillation, after a trip time = electromagnetic radiation.

    It is for the latter that there is a widespread interpretation that "explains" the propagation of the EM radiation as one field causing another. The question is precisely: if you like that explanation for c), how do you explain the propagation of the "influence" in a) and b).

    Is your answer by chance that the charge generates an electric field or that the current (changing electric field) generates a magnetic field? Well, that would be a pure description of what happens, that would amount to saying the "influence" propagates across space and time following certain rules. But that is not an explanation.

    If you are contented with that, say for c) that the oscillating charge generates an electromagnetic field that propagates and thus save time. But not that the radiation propagates due to a mutual and recursive influence between the fields, because that is misleading: it pretends to be an explanation without deserving that title. Explanations (answer to why causes) are often unnecessary in science, but then better acknowledge the lack thereof instead of inventing pseudo explanations.
  16. Jan 28, 2011 #15


    User Avatar
    Gold Member

    Is there not a problem with (1) E 'causes' B or (2) B 'causes' E, because the E and B fields are exactly in phase, so it would not be possible to choose between (1) and (2) ?

    A simpler explanation is that something else causes both.

    My own view is that our limited understanding of the nature of EM makes this kind of discussion very speculative.
  17. Jan 29, 2011 #16


    User Avatar
    Gold Member

    Any theory about how EM radiation (or to go to the root of the issue, "electrostatic influence") propagates through vacuum, yes, that is far from solved, it is speculation. But arguing that the theory of causally connected fields is wrong does not seem to be so speculative: according to info given by some posters above, there are very authoritative peer-reviewed papers in that line. What may be fun is listing arguments to support it. Yours in fact looks quite reasonable to me:

  18. Mar 30, 2011 #17


    User Avatar
    Gold Member

    Above in this thread I argued that one of the reasons why the explanation criticized here (EM waves propagate because a changing E field creates a changing B field and so on) is odd would be the fact that, according to SR, the explanation of a certain event as caused by an electric or a magnetic field is a frame-dependent issue; thus it may happen that, given a certain event, an observer explains it as caused by an Electric field and force and another as generated by a magnetic field and force; hence... I am not sure now what should be the consequence with regard to the subject matter. For the idea to work fine, I suppose one should be able to say the following: you say that here there is a couple of mutually generating fields, but in my frame there is only one... But can that happen? Now I tend to think it can't, that whenever there is an EM wave (or a photon), there are both things, E and B field... even if different observers label them differently?
  19. Mar 30, 2011 #18


    Staff: Mentor

    You are correct, this cannot happen. Suppose in some frame you have E and B fields which satisfy Maxwell's equations and form a plane wave in vacuum (no currents or charges). If you boost those fields by the Lorentz transform you will get different fields, but those fields will still be a valid plane-wave solution to Maxwell's equations in vacuum.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Self-replicating fields in EM radiation?