How to Sort Signs of Artifical Life from the Real ThingJan. 30, 2003
by Seth Shostak - Senior Astronomer
Picture Jodie Foster, her eyes closed and a mildly bored look on her face. Shes wearing earphones and listening to the dull roar of the cosmos.
Now imagine Jodie 20 seconds later, when she hears something sounding like an unpleasant accident in the Boston Pops percussion section. Jodie knows shes scored big: the aliens are on the air.
Still, how can she be sure shes picked up intelligence, and not just the cosmic gurgle of a completely natural object? How can she know shes not merely harkening to the ticking beat of a pulsar, the whoosh of a quasar, or perhaps the lasing bray of a molecular gas cloud?
Because the signal sounds artificial, thats how.
But what does that mean? In a universe awash in electromagnetic radiation filled with light and radio from a slew of objects that either emit or transmit how do we recognize a signal thats produced by intelligence? How do we define artificial?
This problem is well known in the context of SETA, the Search for Extraterrestrial Artifacts. For example, any astronomer would be impressed by finding a stellar grouping in which a hundred stars were precisely arrayed on a 10 by 10 grid, and quickly conclude it was an alien construction. But not all artifacts are so unambiguous. For decades, the Viking Orbiter photo of the "Face on Mars" lathered up a lot of folk who felt that the appearance of this feature as seen by the Orbiters camera met the criteria of artificiality. Surprisingly, despite recent high-resolution photos of the "Face" that strongly challenge this conclusion, the debate continues. (Crop circles are a different matter: here the arguments fly not over whether or not theyre artificial, but who makes them.)
In the late 1940s, a Soviet engineer and science fiction writer, A.P. Kazantsev, suggested that the famous Tunguska meteor of 1908 (which leveled trees as far away as 25 miles, and roasted 200 reindeer) might have been an exploded, nuclear-powered spacecraft from another world making this the Siberian equivalent of a purported alien landing mishap in Roswell, New Mexico four decades later. (Extraterrestrial rocket jockeys apparently can manage interstellar travel. Its the landings that befuddle them.) But the failure to find any tell-tale indications of nuclear reactions at Tunguska, and a better understanding of how rocks from space can explode in our atmosphere, has quelled this idea.
Artificiality, some property that belies intelligence, is a crucial, and somewhat unresolved, concept in SETI. In the mid-1960s, a radio source found and cataloged by Caltech radio astronomers, CTA-102, piqued the interest of researchers in the U.S.S.R. This source apparently varied on time scales of 100 days and even less. Such relatively rapid change was, the Soviets averred, the result of deliberate signaling, and the famous physicist Nikolai Kardashev encouraged further study of CTA-102 as it might be a highly advanced society. As it turns out, CTA-102 is a quasar, not a beacon.
In 1967, British radio astronomers found some radio sources that emitted exquisitely regular pulses, and tentatively labeled them LGMs: Little Green Men. Their methodical signals certainly seemed to be mechanical, rather than natural. Once again however, the transmitters were found to be odd but not alien. The Cambridge astronomers had discovered pulsars.
Its worth asking what about a pulsars precise radio heartbeat would tell you that theyre not artificial. To begin with, the emissions occur over a wide bandwidth; a broadcast splatter rather similar to the static caused by lightning, and clearly a very inefficient way to transmit information. In addition, endless, regular pulses dont convey any information. Just as an interminable flute tone would not be music (except, perhaps, to Andy Warhol), so too is an unceasing clock tick devoid of any message. Within a few years of their discovery, pulsars were found all over the sky, a further indication that they were natural members of the galactic bestiary, rather than some societys attempt to get in touch. Finally, astrophysicists quickly confected a plausible theory that pulsars are collapsed stars. This has not only explained their radio (and optical) ticks, but also even predicted the fact that they slow down.
Is the ambiguity of the Cambridge discovery a thing of the past? Do contemporary SETI researchers have such a firm hold on what constitutes an artificial signal that theyll immediately recognize it when they find it? Or need we be concerned that, in the words of The Who, "we dont get fooled again"? Jodie Foster heard a rhythmic sound effect that was, to her, unambiguous. Soon thereafter, her SETI team discovered an old TV broadcast layered into the received signal. This firmly tagged it as not only extraterrestrial, but intended for us. No uncertainty there. But the scenario depicted in Contact could only play out in real life if aliens are very nearby.
A true SETI signal is likely to be quite different from its fictional counterparts. In the next installment of this two-part saga, well explore how todays SETI scientists hope to discern that most intangible cosmic commodity: artificiality.