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EM Wave in Vacuum

  1. Sep 26, 2006 #1
    How does Electromagnetic waves travel in vacuum ?
  2. jcsd
  3. Sep 26, 2006 #2
    I don't think there is a fundamental reason for this. Your question can be replaced with "Why does a coulomb field expand through space?". "Why does a gravitational field travel through space". It simply does.
  4. Sep 26, 2006 #3
    You can consider an EM wave in terms of a disturbance in the electromagnetic field, but it isn't a 'real' thing, in the sense that it's the photon you observe and detect, not the field it represents.
  5. Sep 26, 2006 #4


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    You seem to be thinking that an EM wave is a wave in some "substance". It isn't. It is just a wave in the electromagnetic fields of charged particles that permeates everything- even vacuum.
  6. Sep 26, 2006 #5
    LOL,This is not an answer, your replay could be given any time we fail to explain a scientific phenomena

    Why do people die? They simply do.
    Why is the space black? It simply is.

    I'm not a physicist so please please excuse me for my ignorance, but is there an Electromagnetic field surrounding us that it's disturbed every time there is a source of EM waves, and even if there is, would it's effect be found in a vacuum space?
  7. Sep 26, 2006 #6
    What do you mean by "wave in the electromagnetic fields of charged particles" ?
  8. Sep 26, 2006 #7


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    There have been this type of question, and various incarnation of it on PF. You might want to spend some time trolling through the various forums. One such recent thread is this:


    You must also keep in mind that, at some point, the question you asked may not have a direct, clear answer IF you only want it in the simple, layman's terms. For example, we still have no exact explanation for the existence of "charge", "gravity", "spin", etc. We can invoke quantum electrodynamics to answer your question, but since you have posted this in the classical physics forum, and considering that your ability to comprehend the language of QED may be limited, using that to address your question would be meaningless to you.

    What I'm trying to say here is that at some point, unless you're willing to do you own background research, don't be surprised if things will simply be told to you without justification to answer your question.

  9. Sep 26, 2006 #8
    ZapperZ,first, could you plz recommend some books for me discussing this topic?
    The problem is that this question popped into my head during an Antenna lecture and when I asked the professor about it, he had no explanation, although he told me that he'll look it up I now doubt that he will find anything -since he's an EE and your saying that it needs QED for the explanation- but because I'm considering doing my master's in EM I would really like to understand why this happens.
  10. Sep 26, 2006 #9
    abdo375, you did not take my answer seriously. There are certain phenomena that are not currently explicable in terms of more fundamental concepts. Here is an example:

    Q: Why does proper time slow down for a moving person?
    A: Because the speed of light must be constant.

    Q: Why must the speed of light be constant?
    A: No one knows. That is a postulate.

    You will look at an antenna, conclude that the EM waves propagate because of the oscillations of the electrons, then you will learn about displacement current and then you will ask "but why does it propagate at all in the first place?" And you will re-derive my answer.
  11. Sep 26, 2006 #10


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    Why it happens has no bearing on your ability to do your Masters.

    I'm sure you are fully aware that you just simply don't pick up a physics text out of the blue and read it. Practically all areas of physics require a series of prerequisites to be able to understand them. You can't understand QED without first understanding QFT, and you can't do QFT if you haven't had a solid background in QM, etc... etc.

    EM "interaction" travels through vacuum without needing a medium. You can use the classical wave description, the QM photon description, or the more sophisticated QED description, to describe such interactions. Beyond that, it either has no answer yet, or still a highly-intense research-front topic. I highly doubt that this limitation would hinder your study of EM in any way. I mean, it is not as if there aren't more important things to care about that one can research on in that field of study.

  12. Sep 26, 2006 #11
    Thank you ZapperZ for your time and response.

    actionintegral, I'm very sorry that you got the idea that I didn't take your answer seriously but I find it very hard to accept somethings in this world as it is, without an explanation.
  13. Sep 26, 2006 #12
    No offense taken. Some people treat these boards as places to tell jokes but I try to learn as much as I can. I, too, try to understand the underlying "why" of things as much as possible. Differentiating between what is 'given' and what is 'derived' is usually the first challenge I address.
  14. Sep 26, 2006 #13


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    Indeed, that sort of attitude is what leads to further progress in our fundamental understanding of physics. Nevertheless, at the end of any chain of explanation, there is always something that is unexplained, at least at the present time. And if we someday do come up with an explanation for that "something," it will surely depend on something else that is unexplained.

    I cannot even conceive of a physical explanation that doesn't have yet another "why" question lurking beyond it!
    Last edited: Sep 26, 2006
  15. Sep 26, 2006 #14
    In light of the foregoing discussion, I think the original poster should re-phrase the original question.
  16. Sep 26, 2006 #15


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    actually, i would not call the constancy of c an axiom for which there is no idea why such exists (and we just notice it experimentally). it's because we can detect no intrinsic difference between different inertial frames of reference (two observers moving at constant velocities relative to each other both have equal claim to being "stationary") and that the laws of physics, namely Maxwell's equations, apply to both frames of reference equally. if two different observers, neither accelerated and moving relatively to each other, are examing the very same beam of light (an electromagnetic wave), for both observers, when they apply and solve Maxwell's equations for the propagation of the EM wave, they both get the same speed of c out of solving Maxwell's eqs.

    so we do have a good idea for why the speed of propagation of E&M is the same for all inertial observers that may or may not be moving relative to each other. it's because, we cannot tell the difference between a "moving" vacuum and "stationary" vacuum, that there is no difference between a moving and stationary vacuum and then there is not apparent reason for the observed speed of light to be different.
    Last edited: Sep 26, 2006
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