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What doesn't have gravitational mass?

  1. Jul 15, 2008 #1
    Is there anything in the universe that doesn't have gravitational mass? My understanding is that both rest mass and "relativistic mass" are gravitational masses, i.e. both are subject to the force of gravity. Further everything must have at least one or the other (if not both). Anything that doesn't have rest mass must move at the speed of light in order to exist in our universe. This in turn gives it a relativistic mass.

    Is my reasoning correct? (be gentle:eek:))
  2. jcsd
  3. Jul 16, 2008 #2


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    Science Advisor
    Homework Helper

    Hi Usaf Moji! :smile:
    Yes … energy has mass (or energy is mass), so anything with energy of any sort has gravitational mass.

    (Assuming, of course, that gravitational mass and inertial mass are the same. :rolleyes:)

    And it's difficult to imagine anything without energy. :smile:
  4. Jul 16, 2008 #3


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    Staff: Mentor

    In general relativity, the concept of "gravitational mass" is not fundamental. The effects of gravity are described using the curvature of spacetime.

    The curvature of spacetime is determined by the stress-energy tensor, whose components at each spacetime point are derived from the energy and momentum density at that point.

    I'm not an expert in GR by any means, but I understand from reading various threads here that one can define various kinds of "gravitational mass" for an extended object, depending on which gravitational effects you're interested in. In some cases these do reduce to what we call "relativistic mass" in special relativity, but it is dangerous to use "relativistic mass" blindly as "gravitational mass."
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