My current understanding of gravitational lensing follows
When a star or other massive body passes between us and another star, the phenomenon generally labeled 'gravitational lensing' occurs. The term 'gravitational lensing' refers to the bending of light rays due to the gravitational influence of the massive body. Since the bends concentrate light for the observer, they act like a magnifying lens.
OTOH
But since I was young I have pondered other explanations for such observations. One thought about what (else) might cause the observed effect is a cloud of gas around the massive body itself. The assumption being that any change in density between the thin gas of interstellar space and the thicker gas surrounding a massive object should form a refractive boundary. In the case of an active star, that gas & might be the heliosphere of the star, and for any massive object that is not creating an heliosphere, it could be due to clouds of gas that the massive object is sucking in.
I raised this idea with a physics teacher in high school & he assured me that any possible refraction from clouds of gas had been ruled out in observations of gravitational lensing. I never got to see papers on the subject, so I cannot confirm that (and am not convinced).
My questions are:
- Does anybody know of papers on gravitational lensing that discuss the refraction from clouds of gas around the lensing body?
- How could an observer/experimenter go about quantifying the likely refractive index of such a gas cloud.
Answer
One point in addition to the other answers. Refraction is almost always chromatic (i.e., different wavelengths refract differently). Gravitational lensing is achromatic. Some studies of gravitational lensing, particularly the MACHO-like microlensing studies, look specifically for achromaticity to test that what they're seeing really is gravitational.
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