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Opinions on whether light can be mass

  1. Nov 8, 2015 #1
    While researching possible culprits of dark matter, i had a curious thought. What if the emmited light from stars and galaxies effected the curvature of space without measurable mass. One google later i discover that this was asked by several other researchers on it but i couldn't find much or anything conclusive. I guess my question is two things: what are your ideas on the subject, and what, if any, information exists about experiments in this direction?
  2. jcsd
  3. Nov 8, 2015 #2

    Simon Bridge

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    Then you've probably seen this:

    When we look for the "missing mass" we are using the word "mass" as a synonym for the word "energy".
    To curve space-time you need energy - mass is a very dense concentration of energy so it has a big effect.
    What we can detect of dark matter seems to concentrate in much the way that matter does ... which would be very funny behaviour for light.
    I would be surprised if anyone has done any experiments for this since afaik there are no models for how light could be made to behave like this to test.
    Oh but see:

    You'll notice in that last article a calculation was done to see if the "heavy photons" would be heavy enough to account for dark matter effects observed... the answer was "no", so no actual experiment was done to test the idea. In general, before looking for something, it is a good idea to check that there is some reason to think it may exist in the first place.
  4. Nov 8, 2015 #3


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    All of the photons (and neutrinos) in the universe contribute to the cosmological evolution, see, for example, http://hyperphysics.phy-astr.gsu.edu/hbase/astro/denpar.html. Only a fraction of these photons are those emitted from stars. Furthermore, the current photon density is too small to have a significant effect on the current evolution. However, in the distant past, when the universe was much smaller, the photon density was large enough to dominate the expansion, see https://en.wikipedia.org/wiki/Radiation-dominated_era. However, that was so long ago that stars had not yet formed, so all of these photons should really be thought of as relics of the big bang and other high energy processes occurring in the early universe.

    In the analysis of the standard cosmology being referred to above, the photons are assumed to have zero mass. It is their energy that couples them to gravity.
  5. Nov 8, 2015 #4
    Now this explanation i like! Thank you, this answersy question completely!
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