Search for Nanosecond Optical Pulses from Nearby Solar‐Type Stars

With "Earth 2000" technology we could generate a directed laser pulse that outshines the broadband visible light of the Sun by four orders of magnitude. This is a conservative lower bound for the tech- nical capability of a communicating civilization; optical interstellar communication is thus technically plausible. We have built a pair of systems to detect nanosecond pulsed optical signals from a tar- get list that includes some 13,000 Sun-like stars, and have made some 16,000 observations totaling nearly 2400 hours during five years of operation. A beamsplitter-fed pair of hybrid avalanche photode- tectors at the 1.5 m Wyeth Telescope at the Harvard/Smithsonian Oak Ridge Observatory (Agassiz Station) triggers on a coincident pulse pair, initiating measurement of pulse width and intensity at sub-nanosecond resolution. An identical system at the 0.9 m Cassegrain at Princeton's Fitz-Randolph Observatory performs synchronized observations with 0.1 µs event timing, permitting unambiguous iden tificationof evena solitary pulse. Amon g the 11,600 artifact-free observation s at Harvard, the distribution of 274 observed events shows no pattern of repetition, and is consistent with a model with uniform event rate, independent of target. With one possible exception (HIP 107395), no valid event has been seen simultaneously at the two observatories. We describe the search and candidate events, and set limits on the prevalence of civilizations transmitting intense optical pulses. Subject headings: extraterrestrial intelligence — astrobiology — instrumentation: detectors