How Searching for Extraterrestrial Messages in the Microwave Band Is Done

While the search for narrowband beacons from extraterrestrial civilisations is now more than six decades old (Project Ozma began in 1960) and ongoing, other approaches are now being explored. It is relatively easy and inexpensive to build a laser and telescope assembly that can send short (millisecond or less) monochromatic light pulses that outshine a star and can be detected at interstellar distances. Further, to explore a wide range of possible wavelengths simultaneously, all you need is a prism or diffraction grating and line detector with good time resolution. The SETI Institute’s Laser SETI project is developing a system to search for such pulses, with the ultimate goal of being able to monitor the entire sky visible from the night side of the Earth. Fourmilab contributed start-up funding to this project.


An interesting history. The missing part seems to be a hypothesis about why any technologically-advanced civilization would invest the effort to send out a signal. What would be in it for them? So far, the inhabitants of planet Earth have decided against making any significant effort to send out a signal.

There is the obvious disincentive to advertising one’s location, in that an advanced civilization which picks up the signal may decide to err on the side of safety and shoot first. That is a basic assumption explored in Cixin Liu’s “Three Body Problem” novel.

Even if we assume that other civilizations would be non-threatening, what would be the value to that other civilization of establishing a very low bandwidth communication pathway with a 2-way response time of decades (at best) or more likely millennia?

One hypothesis – what if a civilization could see the writing on the wall? What if they knew they were facing cosmological extinction from a nova or some such? They might want to memorialize their civilization by transmitting their history and achievements to the universe. However, the magnitude of that task would more likely call for sending out probes loaded with a civilization’s worth of data.

It would be advisable for those probes to draw attention to themselves by broadcasting some kind of signal, in the hope that a similarly advanced civilization would be able to capture a probe & analyze it. But if that receiving civilization has the space-faring ability to capture the probe, would sending out such probes be simple vanity on the part of the dying civilization?

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And yet the first four spacecraft launched onto trajectories that escape the solar system (Pioneer 10 and 11, Voyager 1 and 2, carried plaques that identify their senders and give directions to the solar system, and the Voyagers contain a record with a collection of “Homo sapiens’ greatest hits”.

The response time for two-way communication would be slow, but once contact had been made and the channel and modulation communicated, the bandwidth need not be low. Using optical or spread spectrum high frequency microwave, sufficient bandwidth to send all of Wikipedia in a matter of a few years should be achievable using an existing radio telescope as the transmitter and a similar instrument on the receiving end. The value of seeing what another entirely different civilisation which, statistically, would almost certainly be much older than our own, has leaned would be incalculable.

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Not to be argumentative – that would indeed be great for us, but what would be in that information transfer for the other party?

Putting on my wannabe SciFi author hat, it would be a smart move for a highly advanced expansionary society to pretend to be the new kid on the block, and encourage the other planet to share all its knowledge. And then use the information to exploit the unfortunate Earthlings. But I am sure someone has already used that plot line. :grinning: