general relativity - The Sun as a gravitational lens

Since the Sun is a gravitational lens with as focal length of 550 AU for visible light, with an immense amplification factor, shouldn't it light up objects hanging out there?

We should get solar sails up there to finally boost them somewhere interesting, opposite to the star that is emitting the light.

For neutrinos, I've heard that the focal length is 110 AU. Wouldn't a massive neutrino bombardement trigger interesting nuclear reactions (maybe that's not an astronomy question)?

Answer

There will not be any kind of "massive bombardment" and it won't "light up" things out there.

However, using the Sun as a gravitational lens for a radio telescope is a real possibility. There was a lecture at the SETI Institute on 11/25/2009 titled "Deep Space Flight and Communications: SETI, KLT and Astronautics in a 2009 book" by Claudio Maccone, Co-Vice Chair of the SETI Permanent Study Group, International Academy of Astronautics. Here is a link to the page where you can download a .zip file of the presentation and you can see the video of the talk here.

The problem is that you have to choose exactly the one target that you want to image and then launch a space craft with, say, a radio telescope out to between 550 and 1000 AU on exactly the other side of the Sun from that object. For example, you might use this to image the black hole at the center of our galaxy, or if you got a SETI signal from a star you might deploy a satellite to examine it closely.

The reason it won't light up objects is because the gravitational field of the sun does not act like a perfect lens. In particular, at the distance of 550 AU only radio waves that just miss the limb of the sun will focussed to that point. At a greater distance, say 700 AU, there will be a ring further away from the sun where all rays will be focussed. The width of the ring that is focussed depends on the size of the radio dish on the satellite. The big advantage for signal gathering capabilities is that it is like having many many dishes of that size forming a ring around the sun!

UPDATE: For any of you in the south San Francisco Bay Area, there is a special colloquim at SLAC tomorrow (Friday, Dec 2, 2011 at 11AM). See the announcement and details here.