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Question:
If gravity affects the path of light, does it affect the speed as well, meaning the speed of light relative, or is the change in the path the only transfer involved? Also, since gravity does bend light, is not the light that we assign to the physical location of a star flawed?



Replies:
The key axiom of relativity is that the velocity of light will be the same for all observers. This means that near a large mass both time and space must alter so the velocity of light is the same everywhere. The amount that light is bent near a "normal " sized star, like our sun is so small that we must use an eclipse to measure the deflection by getting light that passes through the strong gravity near the surface. The effect on star location should be small unless the light goes past a very massive object like a black hole. These effects are thought to be visible. In general, this location correction is small because most of space is empty, or not very dense.

samuel p bowen


Actually there is a situation where the distortion due to gravity of light is quite important. This is called gravitational lensing. When a massive object lies exactly between us and a distant galaxy, the light is bent around the object and refocused in front of it. Since we do not sit at the focal point but rather past it, we see a multiple image of the galaxy. You can tell it is a multiple image instead of several galaxies right next to one another because the character of the light (the spectra) are exactly the same. I believe looking for little lensing events as dark, massive objects pass in front of distant galaxies is one way in which people are searching for the MACHO candidates for the famous dark matter.

christopher grayce



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