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Vacuum impedance

  1. Jul 21, 2003 #1
    What is Vacuum Impedance?

    In electronics, I would assume that it means some ohmic resistence to alternating EM signals. In vacuum, I understand it is necessary property, elasticity, for vacuum to be able to transfer energy waves. Is it some universal constant?

    In wires, conductor needs to be ~3 times lower resistence than insulator to become main carrier of electric current. What puzzles me and I can't find, what would be implications of vacuum impedance change in some specific direction, say tube shaped, would this create prefered path for photon EM waves, creating sort of virtual conductor tunnel into sea of insulation?

    Is there any reason to suppose impedance changes can occur? And if yes, then does some matter in the path of photon have any influence on vacuum impedance on that path? Basically, can matter in the path between source and target affect imedance in such a way as to result preference of alternate path between source and target where matter is no present?
  2. jcsd
  3. Jul 21, 2003 #2
    Here's how to understand vacuum impedance.

    In an electrical circuit impedance or resistance has the dimensions of velocity. Now it's very odd to think of a 10KΩ resistor as in some way having a velocity associated with it, but when you consider the impedance of the vacuum that view makes much more sense because the velocity in question is the velocity of light. The vacuum impedance is 4pi times the velocity of light in rationalized units, and therefore 377 ohms.
  4. Jul 21, 2003 #3
    Thanks. I seek to understand what it means for space to have impedance. Is frequency of photons basically vibrations of reflexible spacetime structure?
    Can we say that in any medium impedance is lower than in vacuum?

    In electrical circuit, impedance is also subject to influence by resonances of its elements, like LC circuit. Resonances can both increase and reduce impedance. Reflections from borders between different impedance can cause points with varying effective impedance. Are similar effects valid for space impedance?

    Does having 377 ohms impedance mean vacuum is also subject to thermal johnson noise?
  5. Jul 25, 2003 #4
    The factor of 3 times lower will be tough. Index of refraction is related to wave impedance. As you go from air (enough gas to keep us alive) to the vacuum of space index of refraction changes from 1.000000 something to 1.000000....
  6. Jul 25, 2003 #5
    There are two kinds of impedance. One is characteristic impedance, the ratio of Electric to Magnetic field for a single traveling wave. It is a constant of the medium (377 ohms for free space). It dissipates no power and does not contribute johnson noise.

    The other is total impedance which is the impedance talked of in a circuit. If there are two waves of the same frequency traveling in opposite directions then total impedance is (Ei + Er)/(Ii - Ir). This way of looking at a circuit is cumbersome so only used when necessary.
  7. Jul 28, 2003 #6
    Yes, vacuum impedence is given by a relation between the two (electric and magnetic) 'constants' of free space; namely, e, the permitivity and u, the permeability of free space:
    I = [squ](u/e) where u is mu; e is epsilon

    Just like Maxwell's crowning achievement which showed that these two 'contants' define the value of the speed of light, even so they define the value of Vacuum impedence as 377 ohms (as mmwave said).

    Excellent question again; and the answer is yes. Any relative change between the permeability and the permittivity will not only change the impedence but also the speed of light since
    c = 1/[squ](ue).

    This is exactly what happens in a material medium, both impedence and the speed of light change.
    However, I suspect you are looking for a change in the vacuum values of these 'constants'.

    I will answer that later if you are still interested and if I can find time to be thorough.

    Again, yes; there is a specific instance where this appears to happen.
    If I have time later, I'll try to get back to it.

    Last edited: Jul 28, 2003
  8. Jul 29, 2003 #7
    Re: Re: Vacuum impedance

    Yes. If fields can influence vacuum, I wonder if localised changes are possible. Why I ask is because I've got some weird thoughts in regards to this:

    I thought what if few first photons that go through as particles, interact with splitters or space and cause impedance changes on alternate paths, causing next photons take alternate path that again interact with splitters, causing impedance changes in other path again. This would be wavelike minute resonant oscillation of 'circuit' impedance when path is 'undetermined', being outofphase with photons, and perhaps interacting with them so that at detectors interference occurs, either from cancellation or from photon trajectory fluctuations.

    When one of paths is obstructed, resonant oscillation is avoided and does not anymore interact with photons, but instead preferred channel due to impedance changes by obstructions is formed.

    Thoughts are of course inspired by a need to find intuitive explanation to photons 'knowing' that information about their path is potentially exposed. So I wondered if photons could leave traces in space as impedance changes that could resolve the paths without mysteries.

    This obviously sounds fishy, but I'd like to know if there could be anything along these lines possible.

    ps. Can gravity be expressed as changes in permitivity and permeability of free space?
  9. Jul 30, 2003 #8
    Re: Re: Re: Vacuum impedance

    Hmmm, Well gravity can bend light and a change in permitivity or permeability can bend light so this is interesting. But when the light enters normal to the boundary it does not bend. Does this fit with gravity bending light? I suspect not. Also, I can't see how it would explain gravity's effect on matter.
  10. Jul 31, 2003 #9
    Re: Re: Re: Re: Vacuum impedance

    How does gravity bend light that goes straight towards center of mass, ie normal to field lines? What is matter? What would be effect on it if it spreads over several field lines?
  11. Jul 31, 2003 #10


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    Staff Emeritus
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    I believe you mean 'equipotentials,' not field lines.

    Gravity will obviously not bend light which is moving directly toward the center-of-mass, i.e. normal to the equipotentials.

    However, gravity WILL redshift or blueshift the light, changing its frequency (and energy).

    - Warren
  12. Jul 31, 2003 #11
    Space impedance change would cause c change. Can that account for redshift/blueshift?
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