# Slow light and gravitational lensing

It has been proposed that the Sun's gravitational lens be used to observe distant objects, but according to relativity a spacecraft would need to be 550 AU away in order to take advantage of the effect. Would it be possible to instead slow down the light while it is being deflected using, for example, a Bose-Einstein condensate such as a large quantity of very cold sodium placed into orbit around an asteroid? An answer to this question mentioned that it is useless to do the same thing using an ordinary material with a high refractive index because of dispersion, but as I understand it the mechanism for producing slow light changes the group velocity, which is the speed at which energy is carried, instead of the phase velocity.

• @Tom: the point is that without the Sun the asteroid sized lens can capture some amount of light, call this $L_0$, and with the Sun present the asteroid can capture a bit more light, $L_1$, because of the Sun's focussing effect. However when you're near to the Sun its focussing effect is tiny, and $L_0 \approx L_1$ - i.e. the presence of the Sun makes no significant difference. If you go out to 550 AU the Sun will have strongly focussed the light and your asteroid sized lens will capture orders of magnitude more light i.e. $L_1 \gg L_0$. – John Rennie Nov 18 '13 at 16:11