Sunday, February 22, 2009

Bracewell probes: part one

Lately much speculation has trended away from the "classic" SETI paradigm and into the domain of hypothetical ET devices such as self-replicating spacecraft and automated communications platforms (an idea proposed by astronomer Ronald Bracewell in his book "The Galactic Club").

Both concepts come as invigorating alternatives to the original SETI paradigm championed by Frank Drake and Carl Sagan. On the other hand, they leave us faced with the unnerving prospect of a galaxy bereft of intelligent life. If deep-space is impregnated by the robotic emissaries of far-flung galactic intelligences intent on achieving long-range contact, we have yet to receive an irrefutable signal. No sign of the telltale prime numbers celebrated in Sagan's "Contact." No invitations to subscribe to the "Encyclopedia Galactica," however hard we might wish to mingle with our elders in the local stellar neighborhood.

Or so it might seem.

SETI pundits tend to assume that contact with an alien device would unfold basically along the lines as direct contact with an actual civilization. Even Bracewell, an adventurous thinker in many respects, assumed that his eponymous probes would be little more than advanced versions of our own far-flung exploratory spacecraft; he envisioned Earth-based radio astronomers striking up a rudimentary dialogue with a probe dispatched to monitor the potential emergence of intelligence in our sector of the galaxy. After learning of our technological capacity (specifically, our ability to communicate via radio transmissions), the probe would then alert its makers, who might then choose to communicate in "person."

If this scenario sounds cumbersome, that's because it's inherently limited by the speed of light. Any ET civilization capable of wafting its cybernetic spore into the galaxy might very well have achieved effective immortality, but prospects for our own world are less certain; we could self-destruct or succumb to environmental catastrophe many thousands of years before achieving a meaningful long-distance relationship. Knowing we're not alone may come with a certain existential comfort, but, by itself, would be of no specific practical value.

And although SETI advocates almost invariably expect that ET societies will want to communicate with us, we shouldn't dismiss the unflattering possibility that we're subject to some form of quarantine. Perhaps we're being watched by "intellects vast, cool and unsympathetic," if not as war-like as H.G. Wells' imperious Martians.

Does the apparent absence of Bracewell probes prove that we have yet to be visited? Hardly. Maybe we're just not looking hard enough. Maybe we need to think like an alien.

This piece originally appeared at aboutSETI.com.