In a recent paper outlining a novel strategy for SETI, Michael Gillon (Université de Liège) makes a statement that summarizes what Robert Forward began saying back in the 1970s and even earlier. Interstellar flight is extraordinarily difficult, but not beyond the laws of physics:
Our technology is certainly not yet mature enough to build a probe able to reach one of the nearest stars in a decent time (i.e. within a few decades), but nothing in our physical theories precludes such a project. On the contrary, the constant progress in the ﬁelds of space exploration, nanotechnology, robotics and electronics, combined with the development of new possible energy sources like fusion reactors or solar sails, indicate that interstellar exploration could become a technological possibility in the future, provided that our civilization persists long enough.
That last issue about the survival of our society is the L variable in the Drake equation, referring to the lifespan of any technological civilization. We don’t, alas, have any idea what its value is, which means we have no assurance going forward that any civilization can expect to have a lifetime long enough to explore the stars. Finding another functioning technological society would give us hope that such entities don’t necessarily destroy themselves.
But Michael Gillon is after other game in this new paper, titled “A novel SETI strategy targeting the solar focal regions of the most nearby stars.” After running through the Fermi question and noting that self-replicating interstellar probes of the kind posited by John von Neumann could fill the galaxy within, at most, hundreds of millions of years, Gillon asks whether any such probe in our own Solar System would be detectable. He’s interested in the question of communications and invokes the Sun’s gravitational lens, first discussed by Von Eshleman in terms of astronomy and richly examined by Claudio Maccone, as the key to how any interstellar probes would communicate.
Just how big a difference the use of a gravitational lens could make is outlined in Maccone’s Deep Space Flight and Communications (Springer, 2009) and an earlier paper (see The Gravitational Lens and Communications). Bit error rate, a measure of the quality of a radio signal, becomes problematic (to say the least) if we try to communicate with a probe at Alpha Centauri with existing technologies like the Deep Space Network. But a communications relay at the Sun’s gravitational focus — 550 AU and beyond, depending on the wavelength we want to work at and the need to avoid the Sun’s coronal effects — would radically improve the situation.
In fact, Maccone has shown that at 32 GHz, the combined transmission gain brought by using this kind of link between the Sun and Alpha Centauri is 1016, making it possible to communicate with such a probe using only low power transmitters. A mere forty watts of transmitting power produces an all but flawless bit error rate, and the situation improves even more radically if we assume a relay at Alpha Centauri’s own gravitational lensing distance. Flawless communications then become possible at a power of less than 10-4 watts using the two 12-meter spacecraft antennae Maccone plugs into his assumptions.
Image: Pulled from the cover of Claudio Maccone’s book, this image shows his proposed FOCAL probe with antennae deployed, ready to take advantage of the Sun’s gravitational lensing.
But assuming a communications relay somewhere between 550 AU and 1000 AU in our own Solar System, the work of a self-reproducing interstellar probe, how could we go about finding it? Gillon looks at traditional techniques of optical imaging and stellar occultation but finds that they would probably not be able to turn up so small an object — he assumes an antenna with a diameter of no more than one or two dozen meters — so he suggests looking for ‘leakages’ in any traffic between the two systems (he uses Alpha Centauri purely for illustrative purposes):
Attempts to detect the hypothesized ICD [interstellar communications devices] can still be performed now, basing on the very purpose of the device: not only to receive messages from Alpha Cen, but also to send messages to Alpha Cen and to one or several probes orbiting the Sun. An intense multi-spectral monitoring of the focal region of Alpha Cen with, e.g., the Allen Telescope Array, could in principle detect some leakages in these communications, depending on the used technology, communication frequency, and emission power.
Underlying the search is the hypothesis that self-reproducing probes would be unlikely to communicate with their original stellar system, wherever it happened to be. More likely is that communications would be networked among nearby stars. Gillon goes on to say:
A communication strategy based on direct connexions between neighboring systems would be a much better solution, with the extra-beneﬁt that the information gathered by probes would be spread among their whole network, without any loss even in case of collapse or migration of the original civilization. The ﬁrst part of our hypothesis is thus that the envisioned probes would use this direct communication strategy.
A galaxy fully colonized by self-reproducing probes should, in Gillon’s view, produce an interstellar relay in the focal region of at least one nearby star, leading to a series of SETI searches looking for incidental radiation from these devices. It’s an interesting notion (and I am much in favor of fully exploring the gravitational lens and its implications), but it’s hard to see how a civilization able to build interstellar probes and the communications tools to support them would be unable to shield its technology from detection if it chose to do so.
The paper is Gillon, “A novel SETI strategy targeting the solar focal regions of the most nearby stars,” accepted for publication in Acta Astronautica (preprint).