General relativity considers gravity as the curvature of space-time instead of a force. Therefore, what is bent around an astronomical object is the space-time itself. The light just follows the path as regular.

In classical physics however, I think we would interpret gravitational lensing as the light's being pulled by a massive object (correct me if I am wrong). This implies some centripetal acceleration on light.

The question is does the frequency of light have any effect on the amount of bending in gravitational lensing? Is it possible to separate the light into its components by gravity, like a glass prism would do?


2 Answers 2

  1. Resorting to classical physics would result in no bending of light as light as no mass.
  2. But as you pointed out distorted spacetime curvature results in light travelling a bent path which is gravitational lensing.
  3. Splitting light by gravity is called Rainbow gravity theory, which is highly controversial..

I think I can't say for sure whether Rainbow gravity theory is right. And hence cannot answer your question.

  • $\begingroup$ Thank you. Regarding the top sentence in your answer, wouldn't some classical physicist, who observed gravitational lensing, interpret the phenomena as light being pulled by gravity? $\endgroup$
    – Xfce4
    Sep 18, 2021 at 21:01
  • 1
    $\begingroup$ They might. But they would also assume that different wavelengths or energies should be effected by the same amount. For example, a particle of mass 1kg travelling at 10m/s and 20 m/s will have different kinetic energy but they would both be falling with an acceleration of g. $\endgroup$
    – bitsabhi
    Sep 18, 2021 at 21:21
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    $\begingroup$ All the observational evidence suggests that "Rainbow gravity theory" is wrong. We can observe a huge range of photon wavelength from gravitationally lensed quasars: radio, visible light, and x-rays, but those signals all get deflected by exactly the same amount. $\endgroup$
    – PM 2Ring
    Sep 18, 2021 at 23:38
  • $\begingroup$ Let us take a closer look at the reality. I just searched Google to get some gravitational lensing images. While I can not be as certain as you to reject this idea, it seems like a clear separation of blue and red lights. cdn.mos.cms.futurecdn.net/EWYzfDWcBsbuj9czkBjXva.jpg newscenter.lbl.gov/wp-content/uploads/2021/02/… th-thumbnailer.cdn-si-edu.com/E96-GEa7IcS6dSM30JDE3shDtP0=/… $\endgroup$
    – imz22
    Sep 23, 2023 at 6:34

Do you know that any mass with the the velocity of earth would take the same orbit?

so since "mass" for light in the more classical approach ist E/c^2 different light has different mass but since all have speed c the same path.


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