1. Space is not empty they say, is it a new idea or has it always been like that?

  2. Can you explain what is the (current) real meaning of vacuum, which is just the Latin for empty

  3. Can you also explain why space is not (or can not be) considered a medium in which light propagates, at a speed that depends only on the properties of the medium?

  4. Could you also describe what would happen if space could be or would be found out to be a medium, whould the question of invariance of light and the equivalence principle be safe? What problems concerning light would remain unsolved? would space-time still be necessary to explain some issues?

  5. Probably it is necessary to specify that I am not suggesting an additional medium (a new form of ether ), I am asking why space itself (the vacuum and not space-time) cannot be considered the medium (made up by virtual particle, quanta or other...) that has the necessary properties that can propagate EMR. If it does not qualify, what properties are missing?


  1. comments seem to suggest that space can and is considered a medium, but that it cannot be considered the medium in which light might propagate. Can someone explain why that it is so?

closed as too broad by Qmechanic Feb 21 '17 at 12:35

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    $\begingroup$ What does it mean to "consider space a medium"? Some might say it is the medium for electromagnetic and gravitational waves, while others would disagree because of a lack of a frame associated with the medium. It depends on what you mean by "medium", it's word games, not physics. $\endgroup$ – ACuriousMind Apr 8 '16 at 13:40
  • $\begingroup$ @ACuriousMind, can you give me some links of physicists who think it is the medium for EM waves. ( air is the medium (a physics term?) for propagation of sound, ostensive definitions are best,.Please suggest a more suited physics term, if you can, or edit my question, I am sure you got the gist of it) $\endgroup$ – user104372 Apr 8 '16 at 13:48
  • $\begingroup$ @user11374: I would say pretty much everyone in the mainstream looks at it that way. We even have a field called "high energy physics", which does nothing else than to analyze the properties of this "medium". Having said that, nobody in the mainstream believes that the physical vacuum is the same thing as the aether. They are two independent and completely different concepts. $\endgroup$ – CuriousOne Apr 8 '16 at 19:00
  • $\begingroup$ @ACuriousMind, so you are saying that space is a medium alright, but not the medium in which light can propagate, is that so? Could you expand your idea in an answer? $\endgroup$ – user104372 Apr 9 '16 at 4:42

The premise "vacuum is not empty" could explain the Hubble redshift, in a STATIC universe:

http://www.eleceng.adelaide.edu.au/thz/documents/davies_2001_cha.pdf "As pointed out by DeWitt, the quantum vacuum is in some respects reminiscent of the aether, and in what follows it may be helpful to think of space-time as filled with a type of invisible fluid medium, representing a seething background of vacuum fluctuations. Although the mechanical properties of this medium can be strange, and the image should not be pushed too far, it is sometimes helpful to envisage this "quantum aether" as possessing a type of viscosity."

http://www.nature.com/news/superfluid-spacetime-points-to-unification-of-physics-1.15437 "As waves travel through a medium, they lose energy over time. This dampening effect would also happen to photons traveling through spacetime, the researchers found."

  • $\begingroup$ space being the medium would entail light-speed invariance? $\endgroup$ – user104372 Apr 8 '16 at 14:44
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    $\begingroup$ And that would, as @user11374 correctly points out, breaks Lorentz invariance, which has not been observed. $\endgroup$ – CuriousOne Apr 8 '16 at 18:54
  • $\begingroup$ @user11374: It's half correct, half wrong. As soon as the "invisible fluid" enters the picture, Lorentz invariance leaves. If Lorentz invariance stays, it's not a fluid... $\endgroup$ – CuriousOne Apr 9 '16 at 4:59
  • $\begingroup$ @user11374: Nothing is needed to keep invariance, but as soon as we throw "something" in there, invariance goes. Not even superfluids work, since they have eddies that allow us to select a preferred coordinate system. $\endgroup$ – CuriousOne Apr 9 '16 at 5:21
  • $\begingroup$ @user11374: spacetime is merely a parametrization used by physicists. The essential property the physical vacuum needs to have to propagate light is Lorentz invariance, the rest follows more or less (up to representations) from that symmetry. $\endgroup$ – CuriousOne Apr 9 '16 at 10:35