Using a naked black hole as a mirror?

http://www.youtube.com/watch?v=ENd8Sz0AFOk The YouTube video is a good example how the gravity of this merging binary black holes bend light around themselves.

• @CuriousOne why not make it an answer (though a diagram would help!)? Commented Dec 24, 2015 at 9:53
• @RobJeffries: I can make it an answer, but there won't be a diagram... my graphical skills are really, really bad... Commented Dec 24, 2015 at 9:54
• @CuriousOne If you Google "black hole light trajectory", I think you will find a suitable illustration. Commented Dec 24, 2015 at 9:59
• In the added video you can clearly see whole stars are any of those ours?
– Muze
Commented Dec 24, 2015 at 9:59
• @RobJeffries: I added the lunar laser ranging to my answer to illustrate the signal attenuation problem for a passive mirror experiment. Commented Dec 24, 2015 at 10:01

No, you could not make this work. A black hole acts like somewhat like a lens. The effective focal length for beams that get bent by 180 degrees would be rather short, so the nearly parallel beam emitted by you standing far away would be scattered long before it made it back to you. In practice it would be practically impossible to make even a real mirror flat enough and large enough (because of diffraction) to avoid that problem in general.

To give you an idea what can be achieved with mirrors, take a look at the lunar laser ranging experiments: https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment. These are using an optimized type of mirror and a fairly large telescope with a laser beam to send light to the moon and measure the time it takes to return to us. The powerful laser beam has a roundtrip time of about 2.5 seconds, but only about one photon makes it back per second. The experiment is basically close to what is physically possible with a passive mirror.

One could do a little bit better than this with larger corner cube reflectors that have less diffraction, of course, but even that would only improve the distances by a trivial factor, nothing that even comes close to what you mean by "far in the past".

According to this black hole light trajectory maybe with new breakthroughs like the Event Horizon Telescope https://en.wikipedia.org/wiki/Event_Horizon_Telescope, but the telescope will need to be an optical telescope and may have to be much bigger than the Hubble and see much farther. No for now as well.

http://jila.colorado.edu/~ajsh/insidebh/schw.html Dr Andrew Hamilton, U.Colorado This picture illustrates how Earth would appear if the black whole was the same size of Earth, next to Earth and from Earth.

• I didn't know that one can ask a question and then accept his own answer given 10 minutes later, maybe I should do that too to gain some reputation [: Commented Mar 28, 2016 at 2:17