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Speed of light in a dark matter medium

  1. Apr 20, 2013 #1


    Staff: Mentor

    The speed of light in a vacuum is the universal constant c. The speed of light in a medium is something less than c; let's call it s.

    Has the positive difference (c-s) been observed as light travels through ordinary matter in space such as gaseous nebula or plasma nebula? If no, is it because the mean inter-particle distance is large compared to the wavelength of the light?

    Has the positive difference (c-s) been observed as light travels through clouds of dark matter? If no is it because dark matter does not interact with light regardless of density?
  2. jcsd
  3. Apr 20, 2013 #2


    Staff: Mentor

    To influence the speed of light, matter has to have some "weak"* interaction with the medium. Dark matter does not have this in a significant way.
    For the density of molecular nebula, I found 10^5 particles/cm^3 as estimate. This has to be compared with ~10^19 particles/cm^3 in our atmosphere, altering the speed of visible light by just 0,03%. Using the upper value of 600ly diameter in the Wikipedia article and assuming everything is linear, this corresponds to a time delay of roughly 50 nanoseconds. There is no way to see this, even if my estimate is wrong by several orders of magnitude.

    *incoherent scattering on free charges like in a plasma won't help, as it changes the direction of the light
  4. Apr 20, 2013 #3


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    The speed of light is altered by the refractive index of the medium through which it travels. The refractive index of a medium is dependent on the electric charge to mass ratio of its constituent particles. The charge to mass ratio of dark matter has been determined to be extremely small, so its refractive index is nearly [but not quite] zero. For discussion see 'Dark Matter Constraints from a Cosmic Index of Refraction', http://arxiv.org/abs/0904.1612.
  5. Apr 21, 2013 #4


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    This article “How Gravitational Lensing Shows Us Dark Matter” makes it clear that light does indeed interact with dark matter. The Bullet Cluster is the most famous example of this.


    Dark matter alters the null geodesic (curves the spacetime) owing to its mass, therefore the light from a distant source is “lensed” by its presence. Is it correct to call this a “change in the velocity of light”? This would imply that light climbing out of an overdensity of dark matter is red-shifted. Conversely, light would be blue-shifted falling into the gravitational potential well of an overdensity of dark matter.
    Last edited: Apr 21, 2013
  6. Apr 21, 2013 #5
    No its not a change of its speed.
    Lensing is a refraction effect or changes in direction.
    You are correct on the
    redshift aspects however redshift is a lengthening of a signals wavelength not the signals rate of travel. A 1 Ghrz signal travels the same rate of velocity as a 60 hertz signal.

    Mfb above gave some figures in regards to light slowing down due to a medium. From his figures its easy to see that the medium needs a substantally dense/cold medium before any measurable effect.
    Dark matter is not dense enough to slow down light measurably. However it does cause lensing and redshift. As the article Chronos posted mentions that refraction index is low enough not to significantly alter the speed of light
    Last edited: Apr 21, 2013
  7. Apr 21, 2013 #6


    Staff: Mentor

    Well, the gravitational influence of matter leads to the Shapiro delay. This has nothing to do with the electromagnetic interaction, however - it is just a result of the mass.
  8. Apr 24, 2013 #7
    What if dark matter has no mass at all because it is passing through the fabric of space it self from another universe and that is why we have no way to catch a particle yet?
  9. Apr 24, 2013 #8


    Staff: Mentor

    Dark matter is defined to be invisible stuff with mass.

    Do you have some theory in mind, which could be described as "[dark matter] passing through the fabric of space it self from another universe"?
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