Cosmic
Communications

It's almost a given that remote civilizations will try to contact us using radio waves. The search for extraterrestrial intelligence was first mooted by Giuseppe Cocconi and Philip Morrison 45 years ago, and since then two million years of CPU time has been given to SETI@home in the spirit of that paper: the probability of success is difficult to estimate, but if we never search, the chance of success is zero. Christopher Rose and Gregory Wright propose a new take. It seems the "Sounds of Earth" gold disks aboard Voyagers 1 and 2 were on the right track. The message-in-a-bottle idea of sending physical objects across space is highly energy efficient, and we should search for artefacts in the Solar System now.

If that blurb made little sense, some very good science writers put it better — see the press coverage. A few even managed to drag the right words out of me during radio interviews, at least where they could contain my digressions and technospeak.

This work was an accidental outgrowth of research on terrestrial wireless. It is careful about the energetics, and it deliberately stacks the deck in favour of radiation — assuming things like infinite bandwidth — before deriving outer bounds on the ratio of energy needed to deliver a message by inscribed matter versus by radiation.

We worry about directed radiation, aperture sizes, and the fact that there are many targets to reach. Matter transmission still wins.

Later versions added the delivery problem (how do you slow the particle down at the destination?), the construction problem (how much energy to build the particle in the first place?), and a version of the civilization communication problem — civilizations are almost certainly asynchronous, whether through celestial calamity like meteor strikes or through competition and stupidity. We had wanted to hold these back, but various referees felt they were necessary to convince readers that evidence of ET might turn up in grains of sand, or on comets and meteors.

We also considered the ballistic aiming problem, since gravitational perturbations might knock a package off course. The effect seems small.

Of course, if energy is no object — consider Freeman Dyson's sun-herding, or building what amounts to a giant blinker around a star — our energy comparisons are irrelevant. But since such feats are not presently on our technical horizon, we are biased toward worrying about energy.

We ran a cartoon competition. Caroline Angelo took first place with a drawing that put together every element of a message hurled from very far away; Zhen Li and Xiangpeng Jing took second and third with animated GIFs. There are honourable mentions, an anonymous entry that was artistically my favourite but got the distances wrong, and a cut-to-the-chase effort from Dick Frenkiel — a conceptual genius, and about as good an artist as I am.

If civilizations at any given stellar location are short-lived — through celestial or geological calamity, or self-inflicted — there could be a survival benefit to seeding the cosmos with compelling records of a civilization, in the hope of eventual adoption by fledgling civilizations or absorption by older ones.

But under the assumption of impermanence, literal biological seeding of likely habitats — Crick's panspermia — seems much more likely, because it confers a selective advantage of sorts. Civilizations that seed may survive in some form; those that don't may perish completely.

I have a soft spot for comets as delivery vehicles. They release material during transit through the inner solar system and they are highly visible. The annoying fact is that their orbits are stable only for tens of thousands of years rather than hundreds of millions, which doesn't leave local denizens much time to acquire the technology to reach out and grab one. For biological seeding, though, comets look decent. More on this.

Michael Crichton made exactly this point — that life might be the message, and explicitly that it is more efficient to send a package than to radiate — in his 1969 novel The Andromeda Strain, pp. 222–225. Courtesy of Rich Howard.

One admittedly crazy image I cannot shake: arrays of computer chips using MEMS catapults to move information instead of optical or electromagnetic bus lines. Eventually I nagged myself into doing the calculation. Wafer-to-wafer communication over 10 cm at about 0.3 m/s, against infrared laser communication with 5 µm apertures.

The matter efficiency comes out at Ω = 5.2 × 10⁻¹⁴ρ. So tossing little magnetic chits back and forth would be roughly 5000 times more energy-efficient than laser communication — and if an STM tip inscribed the surface of an SiO₂ chit, more than a thousand times better still. Smart dust tossing inscribed dust may not be so crazy after all.

New Scientist once profiled a now-defunct service — racket — whereby you called a 900 number to leave voice messages for ET, to be beamed into space with a satellite dish. They contacted me, and since silence can imply complicity, I did the calculation. I suspect customers had little hope of getting their messages across. But people bought pet rocks in the '70s, so perhaps there were plenty of takers.

It's always nice when your work gets called seminal, whether it actually is or not. In truth, without quantifying the reception problem and figuring out where to look without spending billions, a search for inscribed matter will be a tough sell both inside and outside the SETI community — even if we never hear distant intelligent radio signals, at least SETI telescopes do useful astronomy as a sideline. My guess is that inscribed matter, if any is ever found, will turn up en passant during solar system exploration rather than through a dedicated search.

A special note of thanks to Leslie Sage and the editorial staff of Nature, who saw something of interest in an overlong and overmathy draft and shepherded a newbie through the process from presubmission enquiry to final paper. I would have thanked Leslie formally in the acknowledgements — he made substantive contributions; the Voyager comparison was his idea — but it seemed to go against Nature's policy. This was the most enjoyable editorial experience I have ever had, and it would have been even if the paper hadn't made the cut. Bravo Nature, and a special bravo Leslie.

Without countless invaluable conversations with G.J. Foschini, or N. Beaulieu's eye for potential and strong encouragement, I would never have approached Nature with this work. Thank you both.

Two misprints in the published acknowledgements to correct: special thanks are due to Paul Henry for being an invaluable sounding board early on, and to Stavros Toumpis for careful reading of an early manuscript. Their names were mangled in the paper, so they get capitals here.

A common question: how did an independent consultant-astronomer and a professor of wireless communications theory find each other? We're both part of the post-divestiture diaspora from the old Crawford Hill lab at Bell Labs Research — of Telstar and big-bang background radiation fame.

Tripped over a broken link? Please email me. No guarantees, but I'll try to get to it.