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Propagation of light in a vaccuum?

  1. Jan 14, 2010 #1
    We have consistently been told that light propagates through a vaccuum. My question is twofold:

    What is this vaccuum vacant of? (Unless we could reach temperatures below -273C, I cannot imagine that there is somewhere that electromagnetic waves are NOT present)

    Where and when was this bold statement of propagation proven?
     
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  3. Jan 14, 2010 #2

    DaveC426913

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    Vacant of?

    The vacuum is vacant of matter; It is lousy with energy (EM radiation).


    The moment we concluded that the gulf between Earth and Sun was a vacuum. Presto. Light travels through a vacuum.
     
  4. Jan 14, 2010 #3
    Hi Dave, Is that the best you can do? You seem to be saying: Believe or the you will be put to death by the physics community.
     
  5. Jan 14, 2010 #4

    DaveC426913

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    Best I can do?? I answered your questions pretty conclusively.

    The only thing you have to "believe" is that, above the atmosphere of the Earth, there is vacuum. Do you have reason to think there is something else?
     
  6. Jan 14, 2010 #5
    You said it. It is seething with energy. Why would energy be distinguished from "matter" and be ignored as a ligitimate medium or fabric?
     
  7. Jan 14, 2010 #6

    DaveC426913

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    It is the energy that is doing the propogating - in the form of EM radiation (or photons), and it is doing so at the speed of light. Energy does not simply sit around in the vacuum.
     
  8. Jan 14, 2010 #7
    I disagree. The light or EM we are measuring is the energy we are trying to find out what it is propagating through. I'm talking about the background through which this specific EM wave is being propagated.
     
  9. Jan 14, 2010 #8

    DaveC426913

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    Do you feel your knowledge level of the subject is sufficent to refute established science?

    It is one thing to ask why your logic doesn't makes sense. It is another to think your logic makes sense when you haven't learned the field of study.

    Yes you are. And you have no basis for doing so because there is no reason to hypothesize that the universe works this way.
     
  10. Jan 14, 2010 #9

    Born2bwire

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    The reasons for the failure of the aether theories would be something that the OP would find instructive in familiarizing themself with.
     
  11. Jan 14, 2010 #10
    I am certainly not tryng to hypothesise a "new" theory, but merely trying to understand why the EM background was ignored as a propagation medium. Is this not the right forum for such questions... if so I'll move on.
     
  12. Jan 15, 2010 #11

    DaveC426913

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    It is a fine forum to discuss.

    But your question is leading; it is not well-formed. It contains some assumptions, for example about "why something was ignored". What I'm trying to do is root out such preconceptions and get at the real question you have.

    This EM background is not simply a field against which other things move. It is properly EM radiation. It radiates from hot (matter) objects and is abosrbed by other (matter) objects. At the speed of light.


    On the other hand, in a sense, your question can be easily answered: the medium by which EM radiation propogates is itself. Photons are their own medium. As they move away from a radiation source at c, they do so while voibrating at their characteristic frequency.



    I have a question: why do you think that EM radiation must have a medium through which it travels?
     
    Last edited: Jan 15, 2010
  13. Jan 15, 2010 #12

    Matterwave

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    You're asking why the EM background is not a propagation medium for EM waves?
     
  14. Jan 15, 2010 #13
    Hi there Born2wire. Excuse my ignorance, but who or what is the OP you are referring to. Do you also think that this forum is inappropriate for such questions? Please advise me.
     
  15. Jan 15, 2010 #14
    Hi Matterwave. Yes that seems to be what I am asking. Is water not the medium through which energy or wave or disturbance moves through in a wave form. Why is the energy disturbance propagating through a EM background any different?
     
  16. Jan 15, 2010 #15

    DaveC426913

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    OP = original poster = you

    This forum is perfectly appropriate. But you must be careful of making claims that you cannot back up. Ask questions instead of making claims.

    born2wire's advice is good; read up on aether theories. They have ebbed and flowed over more than a century, but ultimately failed to be reconciled with observation.

    The big failure with aether theories is this:
    If tghere were a medium through which EM radiation propogated, then we could determine Earth's velocity with respect to this medium - like tghe wind on a moving car. We should be able to look in the "forward" direction and see lgiht coming at us faster or more blue-shifted than it does if we look "rearward".

    We do not see this bias. No matter which direction we look, we see EMR coming at us the same. This means one of two things:
    1] Earth is dead-stopped wrt this aether. i.e. Earth is unique in the universe, being stopped while all other bodies (including the sun!) are not privileged
    2] the aether theory is false
     
    Last edited: Jan 15, 2010
  17. Jan 15, 2010 #16

    Matterwave

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    The problem with this analogy is that you're talking about waves moving through a medium rather than waves moving through itself.

    E.g. Would you say that water travels through the medium of water? The point is, the water is the water, of course when water travels there's water present. It's basically a tautology. What you're saying is effectively something along the lines of "water propagates through the medium of water". I mean...ostensibly, this may be correct. EM waves do travel as propagating EM fields (the E-field induces a B-field which in turn induces a E-field, etc), but it's not the same as saying EM fields travel within a medium.
     
  18. Jan 15, 2010 #17

    DaveC426913

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    This question is poorly-formed.

    "Energy disturbance" is not a real term.
    "EM background" is not a real term (in this context).

    A question that contains fictional terms cannot be sensibly answered.

    I'm not trying to be all shirty about it here (I realize that it may be coming across that way). I'm trying to correct some misconceptions you have. One of the ways to do that is disqualify the ambiguous terms you're using. It is these terms, and the false concepts they represent, that is confusing you.

    Simply put: water moving across a pond is not a good analogy for photons moving through vacuum.
     
    Last edited: Jan 15, 2010
  19. Jan 15, 2010 #18
    Thanks for the advice. Yes Dave, My common sense does shout out that a wave needs something to propagate through. My understanding of the current theory is that it involves "virtual" photons and the original question was based on the dilemma that I have that it is necessary to theorise anything virtual when gillions of photons are already present to do the job (by the way, that IS a question, but I understand if you say it could be leading ... but I don't know where else to involk such dialectic. Perhapa all questions are leading in a way)
     
  20. Jan 15, 2010 #19

    Matterwave

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    Virtual photons don't have anything to do with photon propagation, AFAIK. Virtual photons are the force mediator for the electro-magnetic force.
     
  21. Jan 15, 2010 #20

    DaveC426913

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    Why does a wave need something to propogate through?

    Why does your common sense come into play?

    Common sense is the application of knowledge we already have to a new situation. If we are to ever learn even the slightest new fact about the universe (rather than just rehash stuff we already know), it will have to run counter to our common sense. Right?

    A scientist gives up his common sense as the burden it is. A scientist merely observes what is, and modifies his understanding accordingly.


    No. Not sure where virtual photons came from in this topic.
     
  22. Jan 15, 2010 #21
    Hi Dave, I am a question behind you now. Perhaps it is wise to correct my terminology because it helps the discussion. What I meant by energy disturbance was the oscillation of current (or whatever) that instigated the disturbance. And by EM background I really meant the gazillions of EM waves propagating in all directions that are NOT the particular beam or waveset that we are evaluating.
    From my side, I must say that I think it is inappropriate to use "intimidating" and "paternalistic" comments to enlighten people. Life has cut me down to size already and knowlege is certainly not as absolute as you may think. Let's keep to the question at hand ... it is called dialectic.
     
  23. Jan 15, 2010 #22

    DaveC426913

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    I'm sorry if you see this as paternalistic or intimidating but, as you say, if this is a dialectic then it behooves me to point out logical fallacies in your arguments.


    Your questions are of this form:

    Have you stopped beating your wife yet? Please answer with a yes or no.

    (Yes: you were beating your wife but have now stopped)
    (No: you are still beating your wife)

    (Assuming you have not actually been beating your wife...) there is no way of answering this question truthfully without granting the accusation that is embedded in it.

    To sensically answer this question you must stop the questioner and tell him his question is poorly formed.



    You have been asking questions that contain assumptions that we cannot grant.
     
    Last edited: Jan 15, 2010
  24. Jan 15, 2010 #23

    DaveC426913

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    It is 2AM. I'm afraid I must away to bed.

    Perhaps the morning will see some resloution in the form of other users with more satisfactory responses.
     
  25. Jan 15, 2010 #24

    Born2bwire

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    Virtual photons is a different set of ideas. First, it is encompassed within relativistic quantum electrodynamic theory, like quantum field theory, and is not used in classical electrodynamics or even non-relativistic quantum mechanics. The latter here would be able to predict the background energy and black body radiators. Virtual photons arise only when we allow relativity. One way of thinking about it is that in relativity, we can go between matter and energy due to the equivalence principle (but this is not to say that they are both at the same time, I would not equate matter as being energy and vice-versa as you seem to be promulgating by stating that the background medium is the background radiation). This allows fields of high enough energy to create particles, like photons or electrons. An additional effect from quantum theory is that the energy of a field can vary due to the Heisenberg Uncertainty Principle with respect to a given length of time. Thus, over short time periods, a field can have a high enough energy to create a particle that is short lived. This particle is created and annihilated back to energy over very short time spans with accordance to the Heisenberg Uncertainty Principle. These are virtual particles. They do not persist in their existence long enough (or to any degree of certainty let's say) to be considered to be real. However, they do interact with a system and leave their mark in such a way.

    So a static field is non-radiating. In a way, we can thus simulate it as interacting via virtual photons. These virtual photons will propagate an electric/magnetic field, but since they are virtual in a sense they will not create a radiating field.

    The background radiation however is not made up of virtual photons. Virtual photons are not radiating waves nor are they meant to be taken as true physical phenomenon. Most of the time when we talk about virtual particles in quantum field theory, the processes that we apply them to are not meant to be taken as actual physical processes. Instead they are just a means of describing the problem mathematically.

    Probably the biggest thing to note though is that photons are indistinguishable boson particles. They do not interact with each other like a fermion (ie: electron) would. If I have a bottle with a photon in the energy eigenstate E_1, I can place another photon in the bottle with energy eigenstate E_1 and they both will be happy. This cannot happen with fermions like electrons because of the Pauli Exclusion Principle. So classically, electromagnetic waves follow linear superposition. If there is a background radiation field, this field does not interact in any way with other radiation outside of just being an additive field (though it will affect any sources but we are strictly speaking of a source free region here). If the electromagnetic waves required this background radiation to propagate, it would stand to reason that the behavior of the background radiation should impact the propagation. But I have yet to hear of differences in the propagation of electromagnetic waves as a function of background radiation. In addition, there could be other effects similar to what would arise with the aether of yesteryear, like the what Dave has mentioned previously, which have not been observed.
     
  26. Jan 15, 2010 #25
    Hi Born2wire,
    Thanks for your explanation. I appreciate the trouble you took to eliminate virtual photons from my confusion.

    So classically, electromagnetic waves follow linear superposition. If there is a background radiation field, this field does not interact in any way with other radiation outside of just being an additive field (though it will affect any sources but we are strictly speaking of a source free region here).

    What about standing waves? I also don't understand how superposition eliminates propagation. Does it not provide the mechanism of energy transfer in the form of the wave?

    If the electromagnetic waves required this background radiation to propagate, it would stand to reason that the behavior of the background radiation should impact the propagation. But I have yet to hear of differences in the propagation of electromagnetic waves as a function of background radiation. In addition, there could be other effects similar to what would arise with the aether of yesteryear, like the what Dave has mentioned previously, which have not been observed.[/QUOTE]
    What about the differences in the behaviour of the propagating wave being modified by the General Relativity changes of the "background radiation"
     
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