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Energy density and gravity

  1. Apr 5, 2010 #1
    This might sound like a dumb question but I wanted to know if energy density has an effect on the gravitational field, for instance say there are two masses of equal density, one say is a low temperature neutron star and another is a white dwarf (please exclude for now any impossibilities inherent in the situation for the purpose of answering the subject in question). would the white dwarf have a greater or lesser gravitational field by virtue of its higher energy density? Or is energy density (in terms of thermal or electromagnetic energy) have absolutely no effect on gravitational fields beyond the extent to which they imply the transformation from mass into energy. (therefore a reduction in the strength of the gravitational field as a function of time)
  2. jcsd
  3. Apr 5, 2010 #2


    Staff: Mentor

    Yes, a hot object will have slightly more gravity than an otherwise identical cold object.
  4. Apr 5, 2010 #3


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    DaleSpam, I think you missed the part where the OP said, "beyond the extent to which they imply the transformation from mass into energy." Since the question contains this qualifying phrase, I think the correct answer is no rather than yes.

    Birkhoff's theorem says that if you expand or contract a spherically symmetric distribution of mass-energy, it has absolutely no effect on the exterior field, which is still a Schwarzschild field. That is, the density doesn't matter, only the total amount.
  5. Apr 6, 2010 #4


    Staff: Mentor

    I did miss that qualifying phrase. Thanks for pointing it out.
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