Allan Wechsler <acwacw@gmail.com> wrote:

As Warren points out, we want the gamma-ray source to be as close to the disaster site as possible, and if it were *behind* the site, we could get it to within 0.2 light-seconds.

Nitpick: 0.18 light-seconds if you account for the cosmological red shift.

That increases the excitation by a factor of four (is that right?),

Half the distance is four times the power but only twice the strain.

which isn't enough to overcome Warren's objections.

I agree. Perhaps the event destabilized an orbit very close to the event, and the gammas were when that orbit decayed a few hundred milliseconds later. Perhaps a neutron star crashed into the new economy-size black hole, or was sent crashing into another neutron star or into a regular star. Or maybe it was a coincidence, and the two events weren't even in the same part of the sky. Or maybe the events were really simultaneous, but LIGO's clocks and the gamma ray observatory's clocks aren't synchronized. Perhaps one is on UTC and the other on UT0 or TAI. The most intriguing possibility is that gravitons have a small imaginary rest mass, hence always travel faster than c. If so, they could in principle be used to send messages back through time.