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Gravitational Attraction, Electromagnetic Radiation and Dark Matter

  1. Dec 29, 2013 #1
    Since we can observe gravitational lensing and conclude that mass can affect the path of EM radiation it seems logical to me to assume that EM radiation will exert a slight gravitational attraction of it's own on a mass,- although I do not recall ever reading about this.

    Presumably the gravitational attraction that a single photon of EM radiation will exert is proportional to it's mass where m=E/c(squared) and the distance between the photon and the mass. Clearly we are considering an incredibly weak force if this is to be calculated.

    However, if the total gravitational attraction due to the total EM radiation within a galaxy is calculated this would now become significant. Could this explain the observed motion of the outermost objects within galaxies and therefore be a candidate for dark matter?

    Many thanks for reading.
  2. jcsd
  3. Dec 29, 2013 #2

    Jonathan Scott

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    Gold Member

    It is true that in theory the energy of EM radiation should act as a gravitational source, but the effect is incredibly tiny.

    Given that the energy of the EM radiation emitted by the material in a galaxy comes from its rest mass, I'd guess that the fraction of the total energy in the form of EM radiation would be extremely small.

    Why not do a calculation to estimate the ratio of EM radiation energy to rest energy, at least very roughly, by using the sun as an example? You can calculate the amount of energy the sun radiates away (look up "solar luminosity") on a time scale corresponding to the radius of a galaxy at light speed, then compare the mass of that energy to the mass of the sun.
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