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Mass of light

  1. May 24, 2015 #1
    We know that light get bent near stars (because of its gravitational force), and in fact it cannot escape the gravitational forces of black holes.

    We also know from Newton's laws of gravity that the gravitational attraction between any two objects is ZERO if any mass is ZERO.

    We also know that photons (light) have ZERO mass.
    So, if light has no mass, then why is it affected by any gravitational force ?????
    Am I missing something ?
  2. jcsd
  3. May 24, 2015 #2
    Only Einstein's theory of general relativity :wink:
  4. May 24, 2015 #3


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    Even in Newtonian mechanics, light was expected to be affected by gravitational fields. While there is no force, no force is necessary to accelerate an object with no mass. The idea in Newtonian mechanics would be to take ##mg = ma## and taking the limit ##m\to 0## would still give ##a = g##.

    Now, Newtonian mechanics does not describe the bending of light appropriately and quantitatively, you need general relativity for that. In GR, mass is not the source of gravity and gravity is not even a force, so you should not have any expectation of gravity based on masses.
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