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Nature of the vacuum of space

  1. Oct 9, 2012 #1
    A few questions for anyone kind enough to reply and help me understanding this matter:

    1. Since electromagnetic waves in vacuum have a maximum speed is this speed limited by the permittivity and permeability of the vacuum?
    2. Or is it related to the density of the vacuum as it is for waves traveling through water and air?
    3. If so, does permittivity and permeability creates this apparent density?
    3. If so, is the vacuum of space even? Throughout the universe? or does it fluctuate for any reasons?
    4. And if this density fluctuates, is it possible that light in vacuum could go faster or slower than its presently recorded speed? in some part of the universe?

    Thanks you
  2. jcsd
  3. Oct 10, 2012 #2


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    I don't think this (permittivity and permeability of the vacuum) is a reasonable way of thinking about it.

    You may study different waves propagating on vacuum, i.e. in Maxwell's theory, classical linearized QCD and linearized gravity. In all cases you will find a set of hyperbolic partial differential equations defining characteristics, i.e. a light-cone structure. This is a unique property of the geometry of space-time itself, and it would not be appropriate to explain the same constant c defining the wave propagation in different theories using different constants; if you start with to explain c using permittivity and permeability of the vacuum in electrodynamics, how do you explain the same constant c for gravitational waves?
    Last edited: Oct 10, 2012
  4. Oct 10, 2012 #3
    As earlier explained, if we use the model of waves, waves (of whatever nature) are not "limited" by anything (you claimed to understand that in one of your last posts); instead their propagation speed is a property of what they are in. Permittivity and permeability are just characteristics for electromagnetism with which one models light propagation.
    And you already know that, as measured from earth (this is called a "non-local measurement"), light propagates slower near the sun, and faster far away from earth and other bodies; didn't you understand the animation? Here it is once more:

    However, nobody knows what "really" goes on in vacuum, on a sub-microscopic scale. Simple "pressure" models don't work. Speculation about such a model in Physicsforums is only permitted if it relates to an ongoing discussion about one or more models in any of the journals listed in the forum rules.
    - https://www.physicsforums.com/showthread.php?t=414380

    Relativity theory provides the equations to calculate ("predict") phenomena, and that gives only very partial insight in the "nature" of space. Perhaps you know the joke about describing an elephant, based on very different ways of observing; relativity is like understanding and predicting how fast an elephant can walk under certain conditions. That tells you a lot but is still very incomplete; it can't tell you what an elephant is made of, and what the nature is of the soil.

    Your question even touches on what cannot be measured at all ("metaphysics"), but perhaps some kind of string theory is in discussion (and maybe something else in a journal that I don't read).
    - http://en.wikipedia.org/wiki/String_theory
    You could ask about such theories on the general physics forum.
    Last edited: Oct 10, 2012
  5. Oct 10, 2012 #4


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    Permittivity and permeability are artifacts of the SI system of units, not attributes of nature. Maxwell's Equations can be written without them, the only natural constant in MLT units being c itself. In fact the so-called "permeability of the vacuum" has a defined value of 4π x 10-7.
  6. Oct 10, 2012 #5
    Actually that is a debated issue to which I alluded in an earlier answer to Serge; as a matter of fact, also c can be made to "disappear" from for example the Lorentz transformations (as Poincare first did) by a convenient choice of units. Isn't there yet a thread on the generic issue of the possibility to give any proportionality constant the value 1? If not, I will comment on that topic if you start it.
  7. Oct 10, 2012 #6
    And i did understand that for a wave to propagate a medium is required. And i understand too that the speed of a wave is irrelevant to its frequency but it is relevant to the density of the medium that propagates it. To me it sounds like the medium limits its speed. That is why i am asking if it is the same for an electromagnetic wave. But i believe that part of your answer suggest it is not ( pressure models are different)

    As far as light slowing down close to a large body, i understand the principle behind this delay and the apparent conclusion that it has slowed down. But is it related to my questions?
  8. Oct 10, 2012 #7
    Perhaps you forgot, here's your own answer:
    "I've got it. :-) No air, no water = no waves. No fabric of space = no wave. "
    The point which I thought you "got" is that a property is not a limitation but a feature. But in order to understand wave models, one should understand how mechanical waves work.
    - http://en.wikipedia.org/wiki/Mechanical_wave
    [edit: I now notice that that article is just a stub, and inaccurate; I don't know a good online article. A mechanical wave is a local oscillation of material that propagates. ]

    Note also that nobody knows what "fabric of space" is and many people hold that it is just nothingness; however there are vague notions corresponding to different models and/or fashion of words such as space, field, vacuum, spacetime, virtual photons (technically not all exactly the same meaning though).
    Certainly, in the way of the elephant illustration*. If you want to know an elephant by means of a few scarce observed properties, then you should know those observations and be able to interpret them. And different theories predict/describe different kinds of properties. For a general discussion, the General Physics forum would be best.

    Last edited: Oct 10, 2012
  9. Mar 27, 2013 #8


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    I ran across this article in a general interest forum. It may be interest to our members.


    Two forthcoming research papers question whether or not the nature of a vacuum remains static.

    In one paper, Marcel Urban from the University of Paris-Sud, located in Orsay, France identified a quantum level mechanism for interpreting vacuum as being filled with pairs of virtual particles with fluctuating energy values. As a result, the inherent characteristics of vacuum, like the speed of light, may not be a constant after all, but fluctuate.

    Meanwhile, in another study, Gerd Leuchs and Luis L. Sánchez-Soto, from the Max Planck Institute in Germany, hypothesize that physical constants, such as the speed of light and the so-called impedance of free space, are indications of the total number of elementary particles in nature.

    Vacuum is one of the most intriguing concepts in physics. When observed at the quantum level, vacuum is not empty, but rather, filled with continuously appearing and disappearing particle pairs such as electron-positron or quark-antiquark pairs.

    These ephemeral particles are real particles, but their lifetimes are extremely short. In their study, Urban and colleagues established, for the first time, a detailed quantum mechanism that would explain the magnetisation and polarisation of the vacuum, referred to as vacuum permeability and permittivity, and the finite speed of light. This finding is relevant because it suggests the existence of a limited number of ephemeral particles per unit volume in a vacuum.

    As a result, there is a theoretical possibility that the speed of light is not fixed, as conventional physics has assumed. But it could fluctuate at a level independent of the energy of each light quantum, or photon, and greater than fluctuations induced by quantum level gravity. The speed of light would be dependent on variations in the vacuum properties of space or time.

    The fluctuations of the photon propagation time are estimated to be on the order of 50 attoseconds per square meter of crossed vacuum, which might be testable with the help of new ultra-fast lasers. Leuchs and Sanchez-Soto, on the other hand, modelled virtual charged particle pairs as electric dipoles responsible for the polarization of the vacuum.

    They found that a specific property of vacuum called the impedance, which is crucial to determining the speed of light, depends only on the sum of the square of the electric charges of particles but not on their masses.

    If their idea is correct, the value of the speed of light combined with the value of vacuum impedance gives an indication of the total number of charged elementary particles existing in nature. Interestingly, experimental results support this hypothesis.

    Respectfully submitted,
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