In my Astronomy SE question What (actually) does “peculiar periodic spectral modulations” refer to in this preprint? I link to the October 2016 ArXiv preprint Discovery of peculiar periodic spectral modulations in a small fraction of solar type stars. The abstract reads:
A Fourier transform analysis of 2.5 million spectra in the Sloan Digital Sky Survey was carried out to detect periodic spectral modulations. Signals having the same period were found in only 234 stars overwhelmingly in the F2 to K1 spectral range. The signals cannot be caused by instrumental or data analysis effects because they are present in only a very small fraction of stars within a narrow spectral range and because signal to noise ratio considerations predict that the signal should mostly be detected in the brightest objects, while this is not the case. We consider several possibilities, such as rotational transitions in molecules, rapid pulsations, Fourier transform of spectral lines and signals generated by Extraterrestrial Intelligence (ETI). They cannot be generated by molecules or rapid pulsations. It is highly unlikely that they come from the Fourier transform of spectral lines because too many strong lines located at nearly periodic frequencies are needed. Finally we consider the possibility, predicted in a previous published paper, that the signals are caused by light pulses generated by Extraterrestrial Intelligence to makes us aware of their existence. We find that the detected signals have exactly the shape of an ETI signal predicted in the previous publication and are therefore in agreement with this hypothesis. The fact that they are only found in a very small fraction of stars within a narrow spectral range centered near the spectral type of the sun is also in agreement with the ETI hypothesis. However, at this stage, this hypothesis needs to be confirmed with further work. Although unlikely, there is also a possibility that the signals are due to highly peculiar chemical compositions in a small fraction of galactic halo stars.
The idea here would be that you could modulate sunlight somehow (their method is a little complicated) in time (imagine giant vibrating mirrors or optical shutters) and this modulation would cause a spectral feature in wavelength as well time. You imprint your simple information as a tiny wiggle in the spectrum of the sunlight somehow reaching another star. That means whenever they look with their telescope and spectrograph, they will see that the blackbody spectrum is wiggly. They don't need a time base or to look at the right moment, a conventional spectral survey of stars might now it up.
Phys.org's Existing laser technology could be fashioned into Earth's 'porch light' to attract alien astronomers links to the recent paper in the Astrophysical Journal Optical Detection of Lasers with Near-term Technology at Interstellar Distances but a quick search for terms like "interstellar", "SETI" and "laser" will turn up many papers.
Clark started looking into the possibility of a planetary beacon as part of a final project for 16.343 (Spacecraft, and Aircraft Sensors and Instrumentation), a course taught by Clark's advisor, Associate Professor Kerri Cahoy.
"I wanted to see if I could take the kinds of telescopes and lasers that we're building today, and make a detectable beacon out of them," Clark says.
He started with a simple conceptual design involving a large infrared laser and a telescope through which to further focus the laser's intensity. His aim was to produce an infrared signal that was at least 10 times greater than the sun's natural variation of infrared emissions. Such an intense signal, he reasoned, would be enough to stand out against the sun's own infrared signal, in any "cursory survey by an extraterrestrial intelligence."
He analyzed combinations of lasers and telescopes of various wattage and size, and found that a 2-megawatt laser, pointed through a 30-meter telescope, could produce a signal strong enough to be easily detectable by astronomers in Proxima Centauri b, a planet that orbits our closest star, 4 light-years away. Similarly, a 1-megawatt laser, directed through a 45-meter telescope, would generate a clear signal in any survey conducted by astronomers within the TRAPPIST-1 planetary system, about 40 light-years away. Either setup, he estimated, could produce a generally detectable signal from up to 20,000 light-years away.
Both scenarios would require laser and telescope technology that has either already been developed, or is within practical reach. For instance, Clark calculated that the required laser power of 1 to 2 megawatts is equivalent to that of the U.S. Air Force's Airborne Laser, a now-defunct megawatt laser that was meant to fly aboard a military jet for the purpose of shooting ballistic missiles out of the sky. He also found that while a 30-meter telescope considerably dwarfs any existing observatory on Earth today, there are plans to build such massive telescopes in the near future, including the 24-meter Giant Magellan Telescope and the 39-meter European Extremely Large Telescope, both of which are currently under construction in Chile.
Clark envisions that, like these massive observatories, a laser beacon should be built atop a mountain, to minimize the amount of atmosphere the laser would have to penetrate before beaming out into space.
Here the use of a laser is twofold
- A huge amount of power in a very small etendue or phase space. The expanded beam will diverge slowly.
- A huge amount of power in a very narrow spectral feature. Unlike sunlight, the laser will have a very narrow spread in wavelength, and so it could show up in a spectrograph as a little blip.