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B Why electromagnetic waves do not stand still?

  1. Aug 8, 2016 #1
    I know that the electromagnetic waves are alternating electric and magnetic fields. But I don't get why the fields can't stand still and alternate at the same point. Why they always move ?? I would prefer a logical and verbal answer rather than a mathematical answer. But if mathematics helps to explain the logical answer then it's fine.
     
    Last edited: Aug 8, 2016
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  3. Aug 8, 2016 #2

    BvU

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    Waves arise from disturbances that propagate, so they move. They can appear to be 'standing' if there are two waves moving in opposite directions. That can happen to electromagnetic waves as well as to water or sound waves.
     
  4. Aug 8, 2016 #3

    jtbell

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    They can. You can have standing EM waves inside a cavity with reflecting walls, just like you can have standing sound waves inside a room.
     
  5. Aug 8, 2016 #4

    anorlunda

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    That's true, but I think the OP was asking about photons.

    EM waves, in the form of photons, always move at the speed of light. At the beginners level, you can't ask why; it just is. At more advanced levels, you can study wave equations and Maxwells Equations.

    Similarly, you can't ask why gravity makes us fall down rather than up ; it just does.
     
  6. Aug 8, 2016 #5
    What in the OP makes you think that he asked about photons?
     
  7. Aug 8, 2016 #6
    Not always: http://extremelight.eps.hw.ac.uk/publications/Science-Express-slow-photons-Giovannini(2015).pdf [Broken]
     
    Last edited by a moderator: May 8, 2017
  8. Aug 8, 2016 #7

    anorlunda

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    Yeah, I seen similar claims. Some even claim to stop a photon completely. Frankly, I don't understand them. They seem to violate QM and Maxwells Equations. The paper cited says that it applies only to transversely structured photons, whatever that means. Other papers say that they only modified phase velocity, but this paper says both phase and group velocities. I would be grateful if someone can explain how they do this without violating the laws of physics.
     
    Last edited by a moderator: May 8, 2017
  9. Aug 8, 2016 #8

    sophiecentaur

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    Photons don't provide the answer that the OP needs, I think. He seems to want a 'wave' explanation and that is in no way inferior to an explanation involving photons. Also, the same basic description applies to any kind of wave - even a Mexican Wave.
    The 'movement' of a wave through space is because there is a delay in the value of the field at a point on the path of the wave, The current in an antenna changes and so does the local field but, the field thirty metres away will take 10-7 seconds to change - and so on, with increasing distance. That produces a variation in field with distance and that changes in time (along with the variations of the current in the antenna).
     
  10. Aug 8, 2016 #9

    Merlin3189

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    I'm not an expert like your earlier respondants, so don't rely on my ideas, but I don't understand why you say this. At least in the context of radio waves, which is what springs to my mind when someone says "electromagnetic waves", you can.
    If I stand still near an oscillating tuned circuit, then I can "see" (or detect, measure, observe on an oscilloscope, etc) an alternating electric field and an alternating magnetic field. The fields oscillate at a fixed point - indeed, at any fixed point, though their strength will be lower, the further away from the source I am.

    If by some magic power I were able to travel along with the radiating radio wave at the speed of light, then I think I would not observe any oscillation in the fields at all! I would see constant phase, though monotonically diminishing amplitude..


    If there is a constant current flowing in a wire near me, I expect to observe a constant magnetic field at my position. I also expect everyone else to observe the same constant magnetic field at their position - weaker if they are further away, stronger if they are nearer. Similarly if there is an electrically charged object.

    If that current now varies periodically, as I first said, I expect to observe a varying field. And I expect everyone else to now observe a varying field.
    So is your thought that they should not observe this varying field? That only I at some special position can observe it, but nowhere else can it be observed?

    Or is your question more subtle, about why they can't observe the variations at the same time as I do,? Is the problem not that they see the same field as I do, but that they apparently see it later than I depending on how much further away they are?

    So could you clarify a bit, what you mean by a field "standing still"? And perhaps whether you consider non-oscillating fields to be moving or standing still.


    Although I can't really imagine the situation you speak of, if we did have an oscillating magnetic field in some fixed region and nowhere else, would that not give rise to an electric field, just as a changing magnetic field causes an emf in a wire? And since a wire loop could be anywhere where the magnetic field passes through the closed surface defined by the wire, would not that electric field have to exist, if not everywhere, at least in places going away to an indefinite distance? So even if the oscillating field could be trapped, the electric field would inevitably escape.
    And I think maybe there is a similar argument for the electric field letting a magnetic field escape (though I don't understand that one, even in my naive way!).
     
  11. Aug 8, 2016 #10

    robphy

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    Adding to responses by @BvU and @jtbell ...

    (with chocolate)

    (with marshmallows)

    (with cheese)


    Neat Microwave demonstrations... (safety first!)...


     
  12. Aug 9, 2016 #11
    I think your claim is wrong. That's precisely the kind of question Newton is assumed to ask himself. And this question led him to formulate the Gravitation concept and theory. Asking question about things looking "obvious" is the root for real breakthroughs.
     
  13. Aug 9, 2016 #12
    Thanks a lot to ALL for their replies. I find now just one reply by 'sophiecentaur' that is somewhat on the same track of my question's intention. But I need to discuss more with 'sophiecentaur' about his reply.
     
  14. Aug 9, 2016 #13
    Propagating waves propagate... by definition.

    Standing waves stand....by definition.
     
  15. Aug 9, 2016 #14
    Thanks for your reply.

    Please let's leave antennas aside. Let's take just 1 atom in which an electron suddenly jumps to a lower orbiting level and so emits a photon (1 packet [quanta] of alternating electric and magnetic fields). There are no other currents or fields near this atom. So this emitted packet of alternating electric and magnetic (electromagnetic) fields is an independent packet of energy. As we already know, the electromagnetic waves (like the light) do not need any existence of any external electric or magnetic fields for moving in the Speed of Light. These waves move in empty space only by the virtue of their own alternating electromagnetic fields. So I am asking, why in the first place these alternating electromagnetic fields don't continue alternating at the same point ?? What makes these alternating electric and magnetic fields to displace their location with each cycle of their alternation ??
     
  16. Aug 9, 2016 #15

    sophiecentaur

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    The problem with jumping right into the quantum physics of a single atom transition is where you say "suddenly jumps". What do you actually mean by that? How long does the transition take and what is actually involved? You want an explanation about 'waves' so the explanation really needs to involve waves, surely.
    I was using the antenna description because it describes a wave phenomenon in terms of waves. You are wanting a wave / field phenomenon described in terms of particles. That is loaded with problems. How would you actually describe the photon in terms of waves (essential if you want it to work your way)? How big is it? Where is it? We know you can't treat a photon like a little bullet - or give it an 'extent', that's meaningful.
    This is a very fuzzy area in many people's minds and it will remain fuzzy unless people accept the mutually exclusive nature of the two models. It's QM and that means it will be hard / impossible to appreciate fully.
    At least, consider the wave answer (based, in my case, on RF frequencies and an antenna) and see where things fit together there. I have looked all over the place for a source that links the two models together in a satisfactory way and it isn't surprising that I can't.
    You ask why the fields don't keep continue alternating after the wave has 'left'. There is no energy left to sustain that oscillation as it has radiated away, being dissipated in the Radiation Resistance.
     
  17. Aug 9, 2016 #16
    Possibly beyond the scope of the OP question but a second quantisation approach resolves the problem ie a field mode excitation.
     
  18. Aug 9, 2016 #17

    sophiecentaur

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    I hoped I was answering the question at the appropriate level. The OP was asking for an answer that could be explained in terms of waves and was trying to reconcile things that are not reconcilable without digging pretty deep - if at all. :smile:
     
  19. Aug 9, 2016 #18
    In the wave, a changing electric field causes a changing magnetic field. That changing magnetic field then causes a changing electric field. Etc. Etc. So the wave propagates.
     
  20. Aug 9, 2016 #19

    sophiecentaur

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    But in a wave in free space, the two fields are in phase so where does that causal chain apply?
     
  21. Aug 9, 2016 #20
    What ia an "energy medium"?
    Are you claiming that we don't know what a wave is and there is a need for a different definition?
     
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