Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is there mass if there is no gravity?

  1. Oct 23, 2014 #1
    Hi, I am not a trained scientist, but I have a deep interest in spacetime, gravity, quantum mechanics (including string theory) . Can one assume that if there is no mass (or matter), there is no gravity (or gravitational force)?
  2. jcsd
  3. Oct 23, 2014 #2
    Welcome to PF jines!
  4. Oct 23, 2014 #3


    User Avatar

    Staff: Mentor

    In general relativity, "gravitation" (curvature of spacetime) is produced by the stress-energy tensor.


    Mass contributes one form of energy, but there are other forms of energy, so it's possible to have gravitational effects without mass. For example, two antiparallel light beams (traveling in opposite directions) attract each other.
  5. Oct 23, 2014 #4
    Thank you, jtbell. For purposes of clarification, would these two anti-parallel light beams (i.e, photons which are massless) be traveling on a "flat space-time", and they attract each other (newtonian), or would these light beams be travelling on a curved spacetime, and this curvature is due to their presence (general relativity)?
  6. Oct 23, 2014 #5


    User Avatar
    Science Advisor
    Gold Member

    The latter.
  7. Oct 24, 2014 #6


    User Avatar
    Science Advisor

    The question you ask in your title, "Is there mass if there is no gravity", is the exact opposite of the question you ask in your post, "is the gravity if there is no mass".

    Certainly, we could still talk about "intertial mass" if we had no gravity. There is, as yet, no good reason why the inertial mass (the "m" in F= ma) is the same as gravitational mass (the "m" in F= GmM/r^2).
  8. Oct 25, 2014 #7
    In that case there actually is mass.
  9. Oct 25, 2014 #8


    User Avatar
    Science Advisor
    Gold Member

    Well, per SR, two oppositely directed beams of light have positive invariant mass, while two beams in the same direction have zero invariant mass. This is just suggestive for GR, where the SET is EFE source term, but in this case it corresponds with what is known directly from the EFE (oppositely directed beams attract, parallel beams to not attract each other). Note that a single light beam exerts attraction on test body near it, though the effect is too small to ever likely be detectable (though it is directly calculable).

    Anyway, I expressed no opinion on "what is mass?" because the while question of mass in GR is unresolved except for special cases. I just answered the question asked: can light produce curvature in GR (yes). Given that, since it is trivial its path is affected by curvature (light deflection), it follows that there should be (and are) circumstances where light both produces and responds to curvature.
  10. Oct 27, 2014 #9
    I read somewhere that in the quantum world, inertial mass and gravitational mass are not the same.
    If mass is equivalent to energy, then even a light beam (photon particles) would have some mass, right?
    I thought that mass is just condensed energy. The bigger the condensed energy (i.e. more massive) the greater the gravitational force or the spacetime curvature. The lesser the condensed energy (i.e. particles) the more negligible the gravity. Is this too simplistic?
    Relating it to string theory, does the frequency of vibration of each string (of energy) determine the type of particle and the energy density (i.e., mass) of a particle?
  11. Oct 28, 2014 #10


    User Avatar
    Science Advisor
    Gold Member

    I suggest you can start off reading this thread: https://www.physicsforums.com/threads/do-photons-have-mass.511175/ [Broken]
    Last edited by a moderator: May 7, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook