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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


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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


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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


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    Feynman spoke about it that way.


    "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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