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Why is light affected by gravity?

  1. Jul 17, 2010 #1
    I've learned that a photon has no mass, and it carries the Electromagnetic force. So then how can gravity affect light? Gravity is the attractive force of mass, so if a photon has no mass, why does it work?
  2. jcsd
  3. Jul 17, 2010 #2
    gravity affects energy, not mass
  4. Jul 17, 2010 #3


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    To be more accurate, gravity interacts with the stress-energy tensor.
  5. Jul 17, 2010 #4


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    No. In general relativity, gravity is a manifestation of spacetime curvature. Objects moving only under the influence of gravity follow geodesics in spacetime, which are generally curves in ordinary space. (In flat spacetime without gravity, geodesics are simply straight lines.) This applies to all objects, whether they have mass or not.

    Going further, spacetime curvature is produced by energy and momentum, via the the stress-energy tensor. Mass is only one form of energy.
  6. Jul 17, 2010 #5
    I assumed, and correct me if I am wrong, that gravity distorts spacetime creating the curvature mentioned above.
    My logic, though probably flawed, then leads to the idea that since light enters this distorted spacetime it get pulled towards the object with mass because of the newly formed shape that spacetime is molded into by that massive object.

    It's as if the spacial coordinates in which the photons exist forms a pothole and the photons have no choice but to glide on in.

    So in that case a photon would not require mass but only to exist in the space that is being pulled.

    I hope that makes sense, but more importantly I hope I am understanding this correctly.
    These are raw assumptions on my part based on ZERO scientific expertise.
    Last edited: Jul 17, 2010
  7. Jul 17, 2010 #6
    Photons have energy and momentum, which also contribute to the Stress-Energy tensor; they are not merely passive. Gravity is the effect we attribute to geometry of spacetime, created by the interaction of spacetime and the elements of the SET (mass, energy, momentum...), not a force which changes spacetime.
  8. Jul 17, 2010 #7


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    Please read the FAQ thread in the General Physics forum.

  9. Jul 18, 2010 #8
    Gravity doesn't affect light in the sense that you're thinking. The photon travels in a straight line regardless of whether or not there is a massive object near by. The difference in the case with the massive object is that it changes the shape of the space containing the photon.

    It is easier to think of it in two dimensions. Think of a Mercator map of the Earth and draw a straight line on it. Now think of another map projection like the Mollweide projection. That line from earlier is now bent on this new projection. The gravity of a massive object does something similar but in more dimensions.
  10. Jul 19, 2010 #9
    Simplify, Simplify.

    All this talk about warped space and such has to do with the fact that gravity is different in all points of space.

    The question is just how gravity effects light and has nothing to do with the curvature of space due to round objects or even the stretching of space due to flat objects that extend to infinity. That is the subject of General Relativity.

    Lets just look at one small area of space where gravity is just the same as constant acceleration. Now it is easy to see how gravity effects light.

    If you shine a beam of light across an elevator that is accelerating the light will follow a straight path, which will cause it to curve in your elevator.

    To answer your questions about mass. Photons have mass, they have something called zero rest mass, which means all of their mass is pure kinetic energy. A normal massive object such as a baseball's mass is it's Kinetic Energy + Mass Energy. It is widely believed that Mass Energy in massive objects is just a hidden intrinsic Kinetic Energy of the system.
  11. Jul 19, 2010 #10


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    Widely believed by who?
  12. Jul 19, 2010 #11
    String theory basically states that the mass of an electron is due to specific vibrations which is Kinetic Energy.

    QFT is the study of everything being made up of perturbations in fields
  13. Jul 19, 2010 #12
    We were on the mark until post #7; then the posts go waaaaaay off base.
  14. Jul 20, 2010 #13
    If i take an electron and positron and collide them i will get light ,
    So my question for you is , where did the Gravitational field of the electron and positron go ?
  15. Oct 3, 2012 #14
    Gravity doesn't affect light in anyway other than changing the frequency of (the speed) the light is traveling (based on the observation point). The appearence of light being altered in linear direction is an illusion of the increased space-density generated by the increase of a gravitational field/body.
    As planets (and black holes for an extreme) have an increased concentration of mass, and as a result increase of gravity, the photonic path will be affected as the photon follows the straight path which is being sqeezed as the space (or spacetime if you like) itself is compressed by the increased gravity. If you were riding alongside of the photo, you would only see a straight line.
    Those watching from outside would see you curve around the increased gravitational field.

    I have no idea what I'm talking about, I stayed at a Holiday Inn Express last night.
  16. Oct 3, 2012 #15
    Hi Switchblade, welcome to physicsforums. :smile:
    [edit: Aargh, Switchblade never came back!]

    The answer is provided by GR, although different people formulate it differently. The speed of light going through space as determined from a single reference frame such as on earth, is a function of the presence of big masses. Gravity is not really modelled as attracting light, but as locally slowing down (or retarding) light.
    I found a nice summary with animations here:
    The animation shows the Huyghens construction, which is how Einstein calculated bending near the sun.

    ADDENDUM: Only half of the total bending is ascribed to time dilation, the other half is ascribed to anisotropy ("spatial curvature"). Both were accounted for with the Huyghens construction. For details, see: http://mathpages.com/rr/s8-09/8-09.htm
    Last edited: Oct 3, 2012
  17. Oct 3, 2012 #16


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    Necropost alert! :wink:
  18. Oct 3, 2012 #17
    Ah now I see it! :tongue2:
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