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

Why are Photons affected by gravity?

  1. Sep 21, 2008 #1
    Hi there,

    I was wondering if anyone could tell me why a photon is affected by gravity? From my current understanding of gravity, only objects with mass are able interact with the warping of space time. Why then, would a photon or any other massless particles have there vectors be deviated by other objects around them?

    Thanks for your time in feeding my hungry mind.
  2. jcsd
  3. Sep 21, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think your current understanding of gravity needs to be revised. As lacking as my knowledge of general relativity is, I can nonetheless tell you that what your statement should have said was:

    "...only objects with mass are able to *cause* the warping of spacetime"

    The warping of spacetime, however, affects anything that travels through it, including photons. Photons in the vicinity of some object that *does* have mass and *is* warping spacetime will travel on curved paths simply by virtue of the fact that the region of spacetime in which they are travelling is curved.
  4. Sep 21, 2008 #3


    User Avatar
    Science Advisor

    Because photons only have zero rest mass. But since they are never at rest, they do have mass.

    Very strictly speaking, "photon" is a term from quantum theory, while the "gravitational mass of an electromagnetic field" is an idea from General relativity, and there is as yet no framework which incorporates both theories properly. So perhaps I should say that the electromagnetic field has a non-zero stress-energy tensor which enters into the Einstein field equation of General Relativity. However, lax language about the mass of a photon is traditional in physics:

    Apparent Weight of Photons
    R. V. Pound and G. A. Rebka
    Phys. Rev. Lett. 4, 337 (1960)
    http://prl.aps.org/50years/milestones [Broken]

    See also Count Iblis's post #48 at:
    Last edited by a moderator: May 3, 2017
  5. Sep 22, 2008 #4
    Thanks for the clarification on gravitational interaction of particles. Just one more question though, do Photons having mass or being massless give off gravity themselves?
  6. Sep 22, 2008 #5


    User Avatar
    Science Advisor

    Yes, a photon is affected by the curvature of space, and also causes space to be curved.
  7. Sep 22, 2008 #6
    time passes more slowly in a gravitational field. a light wave passing through will slow down. the light refracts exactly as it does when passing through any material, like glass, which causes light to slow down.

    thats my understanding of it anyway.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook