4 ms·

You don't want the light reflected by 180 degrees - it needs to be reflected where Earth is going to be; a much harder problem. For the easy case quoted in the

by ISL 14y ago

You don't want the light reflected by 180 degrees - it needs to be reflected where Earth is going to be; a much harder problem. For the easy case quoted in the article of 6 light years, any collimated light would need to be properly directed at the sub-nanoradian (milliarcsecond) level (which is possible -- see sig and [0]). Interesting to think about, even if it is an incredible observational challenge. Fun application for the Arecibo radar, if it's still running. Locking into a modulated signal might yield the necessary SNR? [0] http://en.wikipedia.org/wiki/File:Diffraction_limit_diameter_vs_angular_resolution.svg http://en.wikipedia.org/wiki/File:Diffraction_limit_diameter...

andrewflnr 14y ago
I don't know if it's harder to get the light to where earth is going to be. There's still exactly one angle that will work, and we would have to trust to luck either way to send us the right photons. It's not like anyone is aiming.
guelo 14y ago
Actually it makes the problem easier because the angle is less than 180 degrees which means the lensing happens further away from the blackhole with less noise. By my quick googling and back o' the napkin calculations using MACHO-96-BLG-5, we, the Sun and the Milky Way, have traveled about 24 light years in 6,000 years giving an angle of 179.5 degrees! OK, that didn't change much.
andyjohnson0 14y ago
"it needs to be reflected where Earth is going to be; a much harder problem" The relative velocity of the ancient Earth, black hole, and present earth would also need to coincide in such a way that you'd get a continuous (or, less desirably, periodic) stream of photons making the complete round-trip. Such a coincidence seems pretty unlikely. Its a nice idea though.
- JoeAltmaier 14y ago
  Actually it seems pretty inevitable. Light is sprayed across the black hole from Earth; some of it ends up in every possible trajectory; some ends up in the critical 'orbit window' to be reflected back to where earth is going to be.

by ISL 14y ago

andrewflnr 14y ago

I don't know if it's harder to get the light to where earth is going to be. There's still exactly one angle that will work, and we would have to trust to luck either way to send us the right photons. It's not like anyone is aiming.

guelo 14y ago

Actually it makes the problem easier because the angle is less than 180 degrees which means the lensing happens further away from the blackhole with less noise. By my quick googling and back o' the napkin calculations using MACHO-96-BLG-5, we, the Sun and the Milky Way, have traveled about 24 light years in 6,000 years giving an angle of 179.5 degrees! OK, that didn't change much.

andyjohnson0 14y ago

"it needs to be reflected where Earth is going to be; a much harder problem" The relative velocity of the ancient Earth, black hole, and present earth would also need to coincide in such a way that you'd get a continuous (or, less desirably, periodic) stream of photons making the complete round-trip. Such a coincidence seems pretty unlikely. Its a nice idea though.

JoeAltmaier 14y ago
Actually it seems pretty inevitable. Light is sprayed across the black hole from Earth; some of it ends up in every possible trajectory; some ends up in the critical 'orbit window' to be reflected back to where earth is going to be.