Computing Lagrangian for Light Propagation in Spacetime

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The discussion focuses on the Lagrangian for light propagation in curved spacetime, emphasizing the need to compute the action for null curves. It highlights that the approach is similar to that for massive objects, but requires using an affine parameter instead of proper time for parameterization. Participants express frustration over the complexities involved in this computation. The conversation underscores the importance of understanding the differences in parameterization when dealing with light versus massive objects. Overall, the thread seeks clarity on the mathematical formulation necessary for accurately describing light's behavior in curved spacetime.
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Does anyone know the Lagrangian for the propagation of light in curved spacetime? I'm disappointed to discover that I don't actually know how to compute the action for a given null curve.
 
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It's the same as for a massive object except instead of the proper time being your parameterization, you have to use just some random affine parameter.
 

Thread 'EPR revisited'

In an inertial frame of reference (IFR), there are two fixed points, A and B, which share an entangled state $$ \frac{1}{\sqrt{2}}(|0>_A|1>_B+|1>_A|0>_B) $$ At point A, a measurement is made. The state then collapses to $$ |a>_A|b>_B, \{a,b\}=\{0,1\} $$ We assume that A has the state ##|a>_A## and B has ##|b>_B## simultaneously, i.e., when their synchronized clocks both read time T However, in other inertial frames, due to the relativity of simultaneity, the moment when B has ##|b>_B##...
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