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EM wave medium?

  1. Jan 18, 2012 #1
    So, I have been doing a lot of reading. I have an 8 year old girl that has renewed my interest in physics, chemistry, biology, etc.

    I find it interesting that we say that Electromagnetic Waves travel through a vacum. We know that mechanical waves like those propagated in liquids and gas travel through a medium. It seems that photons can be created from a vacum if you add enough enegry (http://www.sciencedaily.com/releases/2011/11/111118133050.htm) so... where do they come from... there has to be some medium that they are traveling through doesn't there. We can say that electric and magnetic waves feed off of each other but... I'm not sure how that happens. Are there quantum particles that we simply cannot measure that the EM waves travel through? When we talk about Dark Matter... it's just "stuff" that we cannot bounce something off of, something that doesn't emit anything, and something that we cannot measure interacting with anything else right? So, if the Universe is filled with "Dark Matter" and if it would make sense if EM waves propagated through something... would "Dark Matter" be the medium for EM Waves. And is Dark Matter just a quantum soup? My understanding is that Photons travel through glass by being absorbed and "split out" by many layers of atoms as they travel through the glass. If there was no way for us to measure or "see" glass - and if we only knew about a smalled specturm of the EM waves that are not directly impacted by glass... I guess it would be called "Dark Matter" as well? Simplified but, just trying to get the thought across.
     
  2. jcsd
  3. Jan 18, 2012 #2
    When people first understood EM to propagate as waves, they worked on finding the physical medium and called it the Luminiferous Aether.

    A few key experiments about 100 years ago showed that there is no "Aether wind" as the earth spins and moves around the sun. This led people to question whether there could be a medium at all. That thinking eventually led to relativity and our modern understanding that space and time are connected; and it is space-time itself, not a material medium like water or glass that EM waves move through.
     
  4. Jan 18, 2012 #3
    I found this info interesting... http://open-site.org/Science/Physics/Modern/Luminiferous_Aether

    Maybe there is no medium *required* in our mathematical equations but if dark matter is so prevalent ... what prevents the idea that EM waves might propagate through dark matter?
     
  5. Jan 18, 2012 #4
    Nothing says that they can't, but nothing says they should. And we have incredibly successful physical theories that require nothing of the sort.
     
  6. Jan 18, 2012 #5
    Agreed :-) Thank you both for the responses.
     
  7. Jan 19, 2012 #6
    It depends on what you mean with "require nothing of the sort". Maybe not dark matter, but something nevertheless. For example, Einstein clarified that GR (and with hindsight, even SR) does require something like a medium ("unthinkable" without it), simply because it is a field theory according to which "empty" space has local properties.
     
  8. Jan 19, 2012 #7

    Dale

    Staff: Mentor

    Why does there have to be? Mechanical waves happen to have a medium, but why would that imply that all waves must have a medium?

    Cars have tires, and so do trucks, motorcycles, and bicycles, so obviously all vehicles must have tires. A boat is a vehicle so boats must have tires.
     
  9. Jan 19, 2012 #8
    That misses the point of what technically a wave is: waves do not just "have" a medium, they are medium oscillations. A more appropriate comparison would be that a boat is something that floats on water, so that logically water has to exist if boats exist.
     
  10. Jan 19, 2012 #9

    Dale

    Staff: Mentor

    No, waves are things which (at least approximately) follow the wave equation. Some waves are medium oscillations, others are not.
     
  11. Jan 19, 2012 #10
    To elaborate: similarly, one could say that boats are things that follow from shipyards and that some boats float on water, others not. Such a change of definition is possible of course, but it obscures meaning. I confess, I myself am guilty of the same when speaking of a plastic drinking glass. :tongue2:
     
  12. Jan 19, 2012 #11

    Dale

    Staff: Mentor

    No, it doesn't obscure meaning, it captures essential features of waves that your definition misses. It is a complete and general definition, your definition is neither complete (it misses essential features of waves) nor general (it doesn't apply to all waves).

    Specifically, if a part of a medium oscillates but that oscillation does not propagate in space to other parts of the medium then according to your definition it would still be a wave. You could then have waves that do not propagate, nor diffract, nor refract, nor transfer energy, nor any of the other things that we normally think of when we think of waves.

    The definition I provided ensures that a wave behaves as a wave is expected to behave (complete) and that things which behave as waves are not misclassified as non-waves (general) because of tangential concerns about the existence of a medium.
     
    Last edited: Jan 19, 2012
  13. Jan 19, 2012 #12

    sophiecentaur

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    Waves that involve a medium are waves in a medium which can oscillate. That does not in any way exclude waves that do not involve a medium. Yours is a 'proof by faith' which is not a proof and it's based on an unfounded assumption about the nature of a wave.

    A mathematical wave can be totally abstract - not even drawn or written down. It is still a wave.

    @mikewday : you didn't know what you're letting yourself in for did you?
     
  14. Jan 19, 2012 #13
    I think it's awesome.
    So, in my mind I think about how we can represent things with mathmatical equations but - does that equation equal a "thing" or just a representation of how that thing acts? You can have an equation for a wave but would a wave "as a thing" exist as something to be represented mathmatically without a medium to travel trough?
    When I look at images and pictures that represent EM waves, they always seem to look like one dimentional wavy lines but, that is just a cross section of the wave right? It's really spreading out equally in a plane of some sort until it interacts with something right? Like a pebble dropped in water? Except the energy might be from two hydrogen nucleus being pushed together to the point that the strong force takes over instead of a pebble...
     
  15. Jan 19, 2012 #14

    Dale

    Staff: Mentor

    It doesn't matter if a definition is in the form of english words or mathematical symbols, a definition is just what we mean when we use the defined term. The defined term is not equal to the "thing".

    When we say the word "wave" we mean something which follows the wave equation. We are not saying that the word "wave" nor that the wave equation is the "thing".
     
  16. Jan 20, 2012 #15
    That's wrong as the meaning of a word cannot be a "proof" - nor did I intend such a thing, and neither did I pretend a precise definition as Dalespam thought. As you indicated yourself, a mathematical wave is not the same as a physical wave.
     
    Last edited: Jan 20, 2012
  17. Jan 20, 2012 #16
    You hit the nail on its head: A mathematical wave - a wave equation - is merely a mathematical description of the propagation of something physical (waves/particles/something else?). As they say, the map is not the territory. :smile:

    The propagation of light is independent of the source; instead, its velocity is determined by the space location through which it propagates. We may infer that something has to be at that location, as nothingness can't do anything. Therefore Einstein claimed that 'space without ether is unthinkable; [..] in such space there [..] would be no propagation of light'. In that context, you may also be interested in "gravitational lensing" (just search for it).
     
    Last edited: Jan 20, 2012
  18. Jan 21, 2012 #17

    Dale

    Staff: Mentor

    Just to be clear, I never said that it was. I said "waves are things which follow the wave equation." I thought that I was clearly distinguishing between the thing and the equation.

    In general, I don't know why it is that when someone quotes a definition in English words people understand that we are not confusing the words of the definition with the thing being defined, but when someone quotes a definition in math people suddenly assume that we are confusing the math of the definition with the thing being defined. It seems to be common, but I don't understand it. Any insights? Any way I can phrase mathematical definitions so that this type of objection can be avoided?
     
  19. Jan 21, 2012 #18

    sophiecentaur

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    This is just human nature, I'm afraid. Our brains are constantly trying to make connections and Maths is so very often a very good model with which to describe real life situations. It isn't surprising that we want it to be more than that.
    It's strange when you think that a huge proportion of the population have been taught about quadratic equations and how many quadratic solutions only involve one root that can apply to the real life situations they can be used to solve. Yet they still want the results of wave analysis, Fourier analysis and other, harder Maths to apply absolutely to Science.
     
  20. Jan 21, 2012 #19
    Thanks for the emphasis; note that there I did not refer to what you said. Anyway, a discussion about words is quite useless. Let's stick to the topic which is evidently about the physical understanding of EM waves, or, let's say, our physical understanding of the propagation of radiation. :tongue2
     
    Last edited: Jan 21, 2012
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