# Gravitational lensing and red shift

jollett
Hi I'm new to the forums. I have a question I hope you guys can help me understand. When gravitational lensing produces multiple images of the same object, do they undergo any red/blue shift? And if so, is the frequency shift the same for each image?

meopemuk
Hi I'm new to the forums. I have a question I hope you guys can help me understand. When gravitational lensing produces multiple images of the same object, do they undergo any red/blue shift? And if so, is the frequency shift the same for each image?

No, there is no red/blue shift of light deflected by a massive body. At least, nothing of that sort has been observed. In principle, one can expect that photons passing near a massive body should lose a part of their energy (and frequency) due to the tidal effect. However, this effect is very small.

Eugene.

Lost in Space
No, there is no red/blue shift of light deflected by a massive body. At least, nothing of that sort has been observed. In principle, one can expect that photons passing near a massive body should lose a part of their energy (and frequency) due to the tidal effect. However, this effect is very small.

Eugene.

Are you saying that gravitational fields don't shift light? Do you mean that there is no difference between the redshift of two equidistant galaxies, one of them being lensed by a closer gravitational object? Is this always true or is this because the shift is so small it's undetectable, and if redshift is subject to the curvature of space, could there be a noticeble change in redshift if the lensing object is massive enough or the distance of the lensed object is even further away?

Don't gravitiational fields affect time? Surely an intervening gravitational lensing body would have some kind of time effect on the light being lensed around it as well as the distance the light has to travel being increased due to the curvature of space around the lensing object?

Gold Member
Usually, for gravitational lensing, the blueshift incurred while the light is "dropping into" the gravitational potential matches the redshift incurred while the light is escaping from the gravitational potential, so that the net effect is 0. This is not true if the gravitational potential changes during the time light is traversing the object. I think this happens when light is lensed by a very large object (e.g. a galaxy) rather than like a star, but I'm not sure. I do know that the anisotropies in the CMB are due to this effect (there's a paper on it, but I forget by who).

PatrickPowers
Usually, for gravitational lensing, the blueshift incurred while the light is "dropping into" the gravitational potential matches the redshift incurred while the light is escaping from the gravitational potential, so that the net effect is 0. This is not true if the gravitational potential changes during the time light is traversing the object. I think this happens when light is lensed by a very large object (e.g. a galaxy) rather than like a star, but I'm not sure. I do know that the anisotropies in the CMB are due to this effect (there's a paper on it, but I forget by who).

Right. If you were close to a black hole then light from distant stars would be blue shifted. If the light source is closer to the black hole than you are then the light you see will be red shifted.