  The most observable leakage radiation from an advanced civilization may well
be from the use of power beaming to transfer energy and accelerate spacecraft.
Applications suggested for power beaming involve launching spacecraft to orbit,
raising satellites to a higher orbit, and interplanetary concepts involving
space-to-space transfers of cargo or passengers. We also quantify beam-driven
launch to the outer solar system, interstellar precursors and ultimately
starships. We estimate the principal observable parameters of power beaming
leakage. Extraterrestrial civilizations would know their power beams could be
observed, and so could put a message on the power beam and broadcast it for our
receipt at little additional energy or cost. By observing leakage from power
beams we may find a message embedded on the beam. Recent observations of the
anomalous star KIC8462852 by the Allen Telescope Array set some limits on
extraterrestrial power beaming in that system. We show that most power beaming
applications commensurate with those suggested for our solar system would be
detectable if using the frequency range monitored by the ATA, and so the lack
of detection is a meaningful, if modest, constraint on extraterrestrial power
beaming in that system. Until more extensive observations are made, the limited
observation time and frequency coverage are not sufficiently broad in frequency
and duration to produce firm conclusions. Such beams would be visible over
large interstellar distances. This implies a new approach to the SETI search:
Instead of focusing on narrowband beacon transmissions generated by another
civilization, look for more powerful beams with much wider bandwidth. This
requires a new approach for their discovery by telescopes on Earth. Further
studies of power beaming applications should be done, which could broaden the
parameter space of observable features we have discussed here.
