I had an idea, in which humanity sends signals out (via Laser Communication), in a search for extraterrestrial life. As a result of gravitational lensing, the laser signals are eventually turned back and sent to earth, which leads them to think they are communicating with aliens.

Is it possible for a series of conveniently located black holes to warp space-time enough for the lasers to point back at the point of origin?

  • $\begingroup$ why black holes? any massive object will do albeit to certain degree. $\endgroup$
    – user6760
    Feb 24 at 4:45
  • $\begingroup$ @user6760 Only a black hole can make a 180 degree direction shift. Maybe also better neutron stars. $\endgroup$
    – Gray Sheep
    Feb 24 at 12:39
  • 1
    $\begingroup$ Unrelated but won't humanity realize it's identical to a signal sent x years ago? $\endgroup$
    – Seggan
    Feb 24 at 15:10
  • 1
    $\begingroup$ @Seggan, no, some mumble, mumble, signal decay, mumble will occur $\endgroup$ Feb 24 at 21:29

1 Answer 1



Light rays that pass close to the black hole get caught and cannot escape. Therefore, the region around the black hole is a dark disk. Light rays that pass a little further away don't get caught but do get bent by the black hole's gravity. This makes the starfield appear distorted, as in a funhouse mirror. It also produces multiple images. You would see two duplicate images of the same star on opposite sides of the black hole, because light rays passing the black hole on either side get bent toward you. In fact, there are infinitely many images of each star, corresponding to light rays that circle the black hole several times before coming toward you. (Source, emphasis mine)

If they can go around multiple times, they can go around less than once. I suspect, however, that the "angle of incidence" (for lack of a better term) of your communication beam would have to be amazingly precise. But I could live a happy life not worrying about that.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .