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B Waves in a medium versus waves in a vacuum (ie no medium)

  1. Nov 6, 2018 #1
    Famously em waves are an example of the latter case.

    The two cases (waves in a medium vs waves propagating without a medium) seem at first (to me) to be extremely different and perhaps only connected by their mathematical descriptions.

    I can think of two cases of waves without a medium (em and gravitational). Are there others?

    Is there a closer connection between the two kinds of waves than I imagine?
     
  2. jcsd
  3. Nov 6, 2018 #2

    sophiecentaur

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    The word "only" suggests that the Maths is just an incidental. All waves behave in very similar ways; the physical quantities may differ but time and distance are always there. I would say the Mathematical connection is highly relevant and says a lot about how the Universe actually works - it's always about the delay as energy propagates though space.
    I think you are right about EM and Gravity being the only waves discovered that require no medium (at least so far). Sub-space communications haven't been developed yet. :wink:
     
  4. Nov 6, 2018 #3
    I have heard quite frequently that energy is not a (physical) "thing" , more of an accounting process. In the same way as fields ... but I have been contradicted before when I mentioned it .

    Perhaps it is more of a point of interpretation than anything (some people like to think of it as something that could be "packaged" and so take it too far)
     
    Last edited: Nov 6, 2018
  5. Nov 6, 2018 #4

    sophiecentaur

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    That's a good description. It's always 'conserved' as long as you take account of all the inputs and outputs. But 'what it is' is not defined - except in the sort of terms that we teach kids. I always told 'em "Energy is necessary for anything to happen". That was followed by a chorus of suggestions about when that definition of mine was violated. I had to field stuff like "What about when someone dies, sir????"
    But all the things we take for granted - like mass and distance - are just as slippery.
     
  6. Nov 6, 2018 #5

    jbriggs444

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    Feynman spoke about it that way.

    http://www.feynmanlectures.caltech.edu/I_04.html

    "There are no blocks"

    "It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives “28" -- always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas."
     
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