SETI signalling strategy

This article suggests a signalling strategy for interstellar communication
that, in its most basic form, would be feasible for even an
emergent technological civilization such as our own.

It implies that interstellar signals from other civilizations, if they exist,
would be received in a form similar to that of a slow pulsar.

Ian Ridpath

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SEARCHES FOR INTERSTELLAR RADIO SIGNALS have been underway since Project Ozma in 1960. These searches, which now go under the acronym SETI (the search for extraterrestrial intelligence), assume that there are advanced civilizations elsewhere in the Galaxy deliberately attempting to attract attention. But if a civilization wanted to send a signal to others, how would they go about it? Flashing a message at a star for a few minutes in the hope that someone will be listening before moving on to the next holds out no reasonable hope for success. An alternative is to have separate transmitting dishes pointing full-time at each of the 1,000 or so most likely target stars, as in the so-called Project Cyclops array suggested in the 1970s. But this seems inefficient. A third possibility is that some super-civilization will be transmitting with an omnidirectional antenna of enormous power. But I doubt that any civilization would want to squander such power, even if an antenna physically capable of handling the immense voltages involved could be constructed.

Here I suggest a more efficient strategy than either the fixed-dish or omnidirectional systems. It takes advantage of the fact that most target stars are arranged in the band of the galactic plane, about 20 degrees wide, which we see as the Milky Way. Therefore we could transmit to most of the stars in the sky by regularly sweeping the plane of the Galaxy like a lighthouse beam.

Starting now

A limited form of such a system could begin operation on Earth with current technology. A row of dishes would sweep along the galactic plane from one horizon to the other and back every few minutes, each dish covering a strip of sky a fraction of a degree wide. Such a system would look like an aperture-synthesis array except that, instead of being arranged east–west, the line of dishes would be ranged north–south, forming what is effectively a partially steerable transit radio telescope. Such a transit instrument would have powerful astronomical uses. It could also be used to scan for incoming SETI transmissions. If we wished to encourage international cooperation, any number of countries could fix up their own dishes to sweep an agreed band of sky.

Anyone within the beam who is patient enough to listen to us for five or ten minutes at a time, typically the length of time used by signal-searchers on Earth, would therefore know that we are here. Once they had detected the first flash of our radio beam, they would be able to discover our strategy by listening for further flashes. Eventually, they could extract as much information as if the beam had been continuous. Two lower-powered, omnidirectional antennae in space, each shielded from Earth, could cover nearby stars out of the galactic plane.

By trading off a small amount of time, and not being greedy in the amount of sky that we attempt to cover, we can therefore signal to the stars with almost as much chance of being detected as with continuous all-sky coverage, but much more cheaply and easily. The efficiency and practicality of such a lighthouse-beam system, combined with its easy alternative use for conventional astronomy, suggests that it should be the system of choice for interstellar communication by our own civilization and any like it.

Future expansion

The main drawback of such a ground-based system is the mechanical problem of continually driving dishes back and forth along the galactic plane. A more efficient alternative would be to site the antennae in space. Here, our beacon would consist of a row of antennae studded along the outside of a spinning cylinder. In the low-gravity environment of space, the antennae can be lighter and simpler than on Earth. The cylinder’s axis will be aligned on the galactic poles, so that the beam from the transmitters sweeps the galactic plane on each rotation. And plentiful sunlight is available to power the transmitters.

A fully rotating 360-degree lighthouse beam in space would produce flashes like a slow pulsar. In the decades to come, we could be operating such a beacon, and it does not seem implausible to imagine that civilizations only slightly more advanced than ourselves are already doing so.

What if we just listen?

Even if we choose for the moment to continue listening rather than begin sending, an understanding of the likely transmission strategy of another civilization gives us a better idea of what to listen out for. Those analysing the signals received by SETI projects should be on the lookout for spikes at intervals of a few minutes as the lighthouse beam of the transmitting civilization illuminates us for a second or more. Their outputs would be at lower power than normally anticipated for interstellar transmissions. There may be many weak radio pulsars with periods of a few minutes that are actually lighthouse beacons from other civilizations, but which have so far gone unnoticed. After rechecking on each star for six months to a year, to take account of possible eclipse of the transmitting beacon by its parent star, we would know whether there were any signals to be picked up from that target.

SETI searchers should be encouraged in their task by the knowledge that civilizations like our own could be using such a system at this moment, which vastly increases the potential number of signals available for us to discover.

Should the current SETI listening projects draw a blank, I believe we should consider setting up a transmission programme ourselves, on the assumption that someone is waiting for a lead from us. Interstellar communication can never succeed if everyone listens and no one transmits. Our first message to the stars will announce our emergence as a civilization of galactic significance.

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This article is based on a paper published in the Journal of the British Interplanetary Society, vol. 31, pp. 108–9, 1978. The ideas contained herein were originally presented at the Second BIS Conference on Interstellar Travel and Communication, London, 1977 April 4–5, and were developed further in my book Messages from the Stars, 1978.



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