Electromagnetic waves use spacetime as a medium?

Discussion in 'Physics' started by Werapon Pat, Feb 22, 2018.

  1. MrAl

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    Hi again,

    Now that i think about it, and more to the original point of the thread, then the real answer to if light uses spacetime as a medium depends on how we view spacetime. If light uses spacetime as a medium then we ALL use spacetime as a medium because we are all passing though spacetime, even as we read this passage.

    Since most of us are probably still compared to the speed of light, that means we are mostly traveling though time and not so much space, but we can change that by moving around, although to make a big difference we'd have to go pretty fast. However, because we exist we are traveling through spacetime.

    So if spacetime is a medium, just what kind of medium would it be. I dont think it is a medium like we usually think of one, unless we can find a spacetime quantum.

    Also, for light to use spacetime as a medium, i think the answer is 'no' for another reason. That is because light does not travel through time it only travels through space. That's why there is nothing that can go faster than the speed of light, because it's already "ignoring" all the possible time that it can ignore, there's nothing left to ignore in spacetime.
     
    Last edited: Mar 8, 2018
  2. Glenn Holland

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    The propagation of electromagnetic waves is a "self contained" process that does not require any medium. Energy is transferred back and forth between the electric and magnetic fields in accordance with Maxwell's law.
     
  3. nsaspook

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    In vacuum the electric and magnetic parts of the wave are in phase. They're two "aspects" of the same thing. It is correct to say that changing magnetic and electric fields generate each other. (simultaneously) There is no energy transfer between the two, only a relationship that both, together, simultaneously carry the energy.
    http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/emwv.html#c2
    [​IMG]
    https://en.wikipedia.org/wiki/Electromagnetic_four-potential
    https://en.wikipedia.org/wiki/Jefimenko's_equations#Discussion
     
    Last edited: Mar 8, 2018
  4. MrAl

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    Hello,

    I like to leave it as a sort of open question though because when we look at it electrically we find constants that relate to electrical constants that appear to be limiting the velocity of propagation. The constants appear in the notorious wave equation and are of course u0 and e0 (micro zero and epsilon zero):
    μ0 ε0

    The permeability acts in series with the velocity, the permittivity acts perpendicular. They may be theoretical but they act like electrical constants do just like in electrical circuits, and that limits behavior in electrical circuits as it looks almost like a medium of some sort. Without that we'd have no circuit.
     
  5. nsaspook

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    The vacuum of free space is the reference medium for c.
    0f03fbd13a7575de6344420c90dbd26a1c6f9784.png

    These constants are the properties of a transmission line (vacuum is devoid of matter but it still has electrical properties) for electrical energy that is resistive (a fictitious resistance, more properly a ratio between the E and H) but non-dissipative in nature. The energy does not transform into another type, instead it is dispersed in the the volume of space at a finite speed.
     
    Last edited: Mar 9, 2018
  6. MrAl

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    Hi,

    The way i like to look at it is that space is like a 3d transmission line. The 3d wave equation would use that c as the velocity. So if that could be accepted as a sort of medium then there we have it. However, that's just a theoretical idea i believe. Current theory would tell us that the two parts of the wave dont need any medium, but it is interesting how electrical circuits do (well, most anyway).
    What is interesting about free space is the relative permeability is 1, which means the magnetic part does nothing to change the environment except for it's field, just like an air core coil. So in that view there's nothing else needed.

    Of course that's not usually the end of all discussion, as happens so often in physics. We still might find more about this phenomenon. For now it does not seem to be relevant, but to understand more about space travel and the like we may need to know or at least take some guesses as to what else might be there.
     
  7. joeyd999

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    The problem is that such a "medium" would have to have (or be!) an absolute frame of reference. But it is clear that the speed of light is constant, regardless of your velocity or direction. This is pretty much absolute proof that a medium does not -- and cannot -- exist.
     
  8. MrAl

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    Hi,

    Well there are problems, but maybe you could elaborate on what you are saying there.
    Yes the speed of light is constant, through the same medium, but what limits the speed might be said to be something that is there.
    But elaborate if you can a little.
     
  9. joeyd999

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    That "something there" would have to follow each observer, independent of each other.

    Please refer to the Michelson-Morely experiment.
     
  10. nsaspook

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    Historically we sometimes look at the wave as having two parts because most of the early history of electricity taught them as separate entities in electrical systems. It's clear now that the magnetic field component is a frame of reference transformation result of relativity and charge flow from sources and currents, not a separate energy field from the electric component. The traveling EM energy that moves away doesn't disappear and reform from nothing at the E H null crossings as it propagates, the values are a result of the traveling EM wave origin from sources and currents. The instantaneous value (as momentum in photons) at that point in the waveform pattern of the wave is moving away from the source is a steady state if you were an observer traveling at c (any particle with rest-mass zero travels at this speed) with the wave.

    [​IMG]


    The waves are self propagating as fields in classic EM or moving as photons in QM. There is no need for medium to continually push-off from or travel through because the push-off point was the origin of the wave as a real physical entity in the universe.

    The speed limit is a limit on the transfer of information, things that don't transfer information (like the dielectric polarization current density in Maxwell's equations) can actually have FTL group speeds.
    http://casa.jlab.org/seminars/2004/slides/singleton_040629.pdf
    http://digitalrepository.unm.edu/cgi/viewcontent.cgi?article=1044&context=math_etds

    The generated EM waves travel at c here but is forever shaped by the speed of whatever generated it.
    https://blogs.scientificamerican.co...ight-electric-currents-could-explain-pulsars/
     
    Last edited: Mar 9, 2018
  11. MrAl

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    Hello,

    I am not saying that there "IS" a medium, i am saying that there is something that "ACTS" like a medium.
    For this view it doesnt matter if we are stuck in one frame of reference because that's the frame we choose to use at the time.
    In other words, it's like a lossless transmission line.
     
  12. nsaspook

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    An EM wave is its own medium (the electromagnetic field ) in space. It's the actuality that transfers real momentum. The EM wave exists in QED because matter somewhere has changed a energy state (EMF) causing a excitation of photon emission. If you think the electric wave E induces the magnetic wave B which creates the electric wave E in some fashion then the need for a transfer medium makes some sense ( but then you have a instantaneous causality action across spacetime problem) but that's not how it works. It's an electromagnetic wave, not E and B waves.
     
  13. MrAl

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