Bending of light in a uniform gravitational field

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Homework Help Overview

The problem involves the bending of light in a uniform gravitational field, specifically examining the displacement of a light pulse emitted in the positive x direction while the gravitational field acts in the negative y direction. The context is set within an introductory cosmology class.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants express uncertainty about the equations needed to relate light's behavior in a gravitational field. Some suggest using concepts from General Relativity, while others question the applicability of Newtonian physics. There is discussion about the potential use of the equivalence principle and spacetime metrics.

Discussion Status

The discussion is ongoing, with participants exploring various interpretations of the problem. Some guidance has been offered regarding the need for General Relativity, but there is no consensus on the approach to take or the specific equations to use.

Contextual Notes

Participants note that the class has not covered the mathematics of General Relativity, which raises questions about the assumptions they can make regarding metrics and the applicability of Newtonian methods.

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Homework Statement


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 don't even know the equations relating the two. any help would be greatly appriciated.

i don't even know how to relate the two. we were given only one equation.


Homework Equations



uy = c (1 – v2/c2)1/2


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.
 
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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
 
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.
 
But how would the force help me... I am thinking that this can be treated like a free throw problem... but I am not sure...thnk u
 
Cragar is right. Using Newton would understate the effect by about 2:1.

This requires the General Theory of Relativity.
 
Last edited:
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?
 
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 shouldn't use Newtons method...
 

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