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Homework Help: Bending of light in a uniform gravitational field

  1. Oct 1, 2011 #1
    1. The problem statement, all variables and given/known data
    This is supposed to be a simple problem that shows that light bends or gets pulled in a gravitational field.

    A light pulse emitted in the positive x direction travels 10 s in a uniform gravitational field of 100 m/s^2. the gravitational field points in the negative y direction. find the displacement of the light pulse in the y direction.

    help, i dont even know the equations relating the two. any help would be greatly appriciated.

    i dont even know how to relate the two. we were given only one equation.

    2. Relevant equations

    uy = c (1 – v2/c2)1/2

    3. The attempt at a solution

    dont even know where to start. i understand that light will be curved down. it travels at the speed c for 10 seconds so that gives me a distance.... but that as far as i can get

    this is an intro class to cosmology

    thank you for any help.
  2. jcsd
  3. Oct 1, 2011 #2
    Btw can anyone suggest some good intro books to comsology that has lots of examples. the one we are using is by ryder. there are no examples
  4. Oct 2, 2011 #3
    you can use E=mc^2 hf=mc^2 , to get an effective mass for the photon and then use newtons law for gravitation to get an estimate of the force between them. But i think its off by a factor of 2 from GR.
  5. Oct 2, 2011 #4
    But how would the force help me.... im thinking that this can be treated like a free throw problem... but im not sure....thnk u
  6. Oct 2, 2011 #5

    rude man

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    Cragar is right. Using Newton would understate the effect by about 2:1.

    This requires the General Theory of Relativity.
    Last edited: Oct 2, 2011
  7. Oct 2, 2011 #6


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    You could resort to using an argument based on the equivalence principle, but I think you'd be begging the question. You're probably expected to do the calculation using GR.

    Do you have the spacetime metric that corresponds to a uniform gravitational field?
  8. Oct 2, 2011 #7
    We didnt talk about metrics bc the class is supposed to not cover and mathematics of GR. Should i assume a metric.
    How would i go about solving it then if i shouldnt use newtons method...
  9. Oct 2, 2011 #8

    rude man

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    Last edited by a moderator: Apr 26, 2017
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