How Would Aliens Communicate? f_c

Suppose that extraterrestrial intelligences are rare but do exist. Could we expect them to communicate with us through radio signals? What fraction of civilizations are able — and motivated — to broadcast in a way that creatures like us can detect? In other words: what is the value of f_c?

SETI advocates tend to believe that it is large: that sooner or later, any civilization curious and inventive enough to become technological at all will discover that radio is an efficient way to communicate over astronomical distances, and will choose to do so.

Might there be a naive form of anthropocentrism at play here? Is it reasonable to expect that wildly different beings on another planet, even if they are far older, smarter, and more capable than us, will choose to build radio telescopes and send signals to the larger universe? Maybe we just don't appreciate the true diversity of biological evolution, or the uniqueness of humans' monkeylike curiosity. Or maybe radio is hopelessly primitive compared to something we have yet to discover.

Lifetimes, L

With f_i and f_c completely undetermined, we're left with the last term of the Drake equation: L, the average lifetime of communicating civilizations. Here also, optimists and pessimists are far apart.

The optimists claim that a stable, intelligent society could last for tens of millions of years, if not forever. This would certainly mitigate the effect of any bottleneck earlier in the Drake equation. In addition, a long-lived species might have time to spread to many stars, multiplying its presence. The pessimists point out that humans invented radio technology only a century ago, and that the human race has been on the verge of destroying itself as an advanced civilization (through nuclear war or ecological overshoot) for much of that time. The same technological power that enables interstellar communication also enables self-destruction.

But others have pointed out that the human animal (as opposed to human civilization) would be almost impossible to kill off completely at this point. People have become too widespread and too capable; a few pockets of individuals would find ways to survive almost any conceivable war or global catastrophe. Even a few survivors would be enough to fully repopulate the Earth, to numbers in the billions, in just a few thousand years. And a second technological civilization would arise more readily than our first one has done, because there would be a precedent. Maybe this will happen many times.

Which brings up a little-noticed point. The value of L properly does not refer to the lifetime of one radio-transmitting civilization, but instead to the sum of all those that ever appear on a planet once it develops its first.

The long-term future of humanity and Earth's biosphere is explored in Peter Ward's book Future Evolution (2001).

The Fermi Paradox

The pessimists' most telling argument in the half-century SETI debate stems not from theory or conjecture but from an actual observation: Earth has not already been overrun with aliens (contrary to some popular opinion). This is a more profound observation than it might seem.

A civilization lasting for tens of millions of years would have plenty of time to travel anywhere in the galaxy, even at the slow speeds foreseeable with our own kind of technology. The tendency to fill up all available territory seems to be a universal trait of living things. And yet Earth shows no sign in its long fossil record of ever having been colonized by an alien high technology, much less today. This is known as the Fermi paradox, after the nuclear physicist Enrico Fermi, who as early as 1950 asked (in a lunchroom discussion about aliens at, ironically, a nuclear weapons lab), "Where is everybody?"

(UFO believers might reply that we are being overrun right now. But scientists and other careful investigators who have examined the UFO movement's claims conclude almost universally that nothing is going on here but human misperception, tale-telling, and willful folly. More than 60 years after it was born, UFOlogy remains barren of a single tangible result despite thousands of loud claims — which suggests that you can sit out the next 60 years of it and not miss anything.)

Optimists have replied to the Fermi paradox in many ways. Maybe any culture that is civilized enough not to destroy itself turns away from imperialism, or maybe the imperial drive runs out of steam after settling just a few thousand planets. Maybe this always happens after cultural evolution replaces biological evolution as the dominant source of change and examples drawn from nature no longer apply.

Or maybe we live in an uninteresting area of the galaxy — the equivalent of a backwoods area in the United States, a country that has been "completely settled" since the frontier was officially closed in 1890, but where you can find plenty of places where no other person is in sight.

Or maybe aliens are thickly settled around us but obey, as in Star Trek, a prime directive "not to interfere" with living planets, which are kept off limits as nature preserves. This is the so-called zoo hypothesis. Or perhaps interstellar travel is even more expensive in effort and energy than we now imagine, and anyone capable of it has better things to do with the resources — such as investigating the universe by astronomy or radio.

A more sophisticated rejoinder to the Fermi paradox was published by William I. Newman and Carl Sagan in Icarus for September 1981. They analyzed how fast a spreading interstellar civilization would actually expand through the galaxy, based on mathematical models covering everything from the diffusion of molecules in a gas to the spread of animal species introduced into virgin territories on Earth. They found that how fast the galaxy fills up depends surprisingly little on the speed of interstellar travel; there are too many planets to be settled and populated along the way. "The expansion velocity of the colonization front is several orders of magnitude smaller than had been previously anticipated," they wrote; filling the galaxy might even take a time comparable to the age of the universe. To sum up, they quipped "Rome was not built in a day, although one can cross it on foot in a few hours."

But others have called this argument a stretch, because it assumes that population growth rates are never very high (no more than humanity's recent growth rate of about 2 percent per year). On the other hand, the assumption that exponential growth through the cosmos continues forever — at whatever rate — is challenged by Jacob D. Haqq-Misra and Seth D. Baum in their 2009 paper "The Sustainability Solution to the Fermi Paradox." Nowhere in the real world, they point out, does exponential growth ever continue indefinitely. A National Public Radio writer called this "my Sort-Of-Best-Unheralded-Scientific-Paper of 2009".

The SETI Institute's Seth Shostak writes, "I just checked the parking lot outside the Institute, and I see no large animals with long, prehensile noses. The conclusion a la Fermi is that elephants don’t exist on Earth, right? After all, any putative pachyderms have had plenty of time to get to my office, even if only a few of them are so inclined.

"To use the Fermi Paradox as a reason for the lack of a SETI signal is to make a very big extrapolation from a very local observation. Seems chancy to me."

Milan M. Cirkovic and Robert J. Bradbury propose a solution to the Fermi Paradox in light of post-biological evolution, arguing that advanced civilizations will indeed migrate, but to a galaxy's outermost regions.

But maybe all this is grasping at straws. In the end, the fact that aliens are not camped in your backyard right now may truly mean that we are alone in the entire Milky Way. Perhaps almost every galaxy is either completely barren or settled in every inch.

For more on this topic, see David Brin's influential 1983 essay "The Great Silence" (PDF format, 2.1 megabytes), Geoffrey Landis's analysis of partial, patchy galactic colonization based on percolation theory, and Stephen Webb's book If the Universe Is Teeming with Aliens. . . Where Is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life (Copernicus Books, 2002).

The Great Filter

And here is perhaps the most important point of all: the Fermi paradox turns the definition of "optimist' versus "pessimist" on its head when it comes to life in the universe.

If star-traveling intelligences are extremely rare or nonexistent, despite an abundance of planets where life could begin, there must be some kind of "Great Filter" that prevents the emergence of interstellar colonists. Is the Great Filter something in our past, or in our future? If we've already passed it — that is, if the filter is the origin of life, or the leap from prokaryotic to eukaryotic cells, or the leap from single-celled organisms to large multicellular animals, or from animal brains to technology-capable brains — then the great test is behind us. We have already passed it, and our way is open to spreading to the stars.

But if the Great Filter lies ahead of us — for instance, if technological civilizations arise often but always destroy themselves — then we are doomed. We will never get to the stars. Because (by definition) we are extremely unlikely to beat the odds that filtered out all who made it as far as we are now.

This reversal of optimism and pessimism is well presented by Nick Bostrum, director of the Future of Humanity Institute at Oxford, in Technology Review for May-June 2008.

Seen from this perspective, finding fossil evidence of life originating on Mars would be "the worst news ever printed," Bostrum writes. Finding a whole fossil creature on Mars would be even more terrible, while success of a SETI search would be the best news conceivable for our own future. Because if others have made it, so can we.

"Success Can't Be Predicted"

Where does all this leave us? Can we still believe that N = L, as Frank Drake long ago proposed? Probably not. What about N = 0? To many people that extreme is inherently unacceptable, but of course the universe isn't obliged to live up to our hopes and expectations. Maybe there is truth in the saying that nothing happens only once. Maybe alien civilizations are out there, and some are trying to announce themselves via radio transmissions. But their number could be very, very small.

In the preface to Is Anyone Out There? Frank Drake wrote that he wanted to "prepare thinking adults for the outcome of the present search activity — the imminent detection of signals from an extraterrestrial civilization. This discovery, which I fully expect to witness before the year 2000, will profoundly change the world." That was written in the heady days when NASA's long-aborted radio searches were about to get under way. In July 1996, at the fifth international bioastronomy conference in Capri, Italy, Drake confessed: "Maybe I was a little bit too optimistic. Success can't be predicted." Cocconi and Morrison already told him so in their 1959 Nature article: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."

Meanwhile, the Drake equation still stands as the best-known icon of one of the most forward-looking endeavors of the intelligent species here on Earth: the scientific search for coinhabitants of the dark emptiness of the cosmos, and for a wider, truer perspective on our place in space and time and on the meaning of our life. The "alien equation" has served this effort well by providing a rational basis for the search, by focusing our attention on the fundamental issues, and by defining a clearly visible goal.

We're a long way from that goal. The first term, R, has been known for decades; we now have good grip on the second, f_p; and third, n_e. That leaves us with one medium-size question mark and three big ones — and a lot of speculation.

Moreover, the equation is showing its age. It is looking a little frayed around the edges by not explicitly treating newer issues that we now consider important, such as the rates of planetary catastrophes or the effects of slow, one-way changes in the universe itself that could either boost or diminish the abundance of aliens in our present era (see for instance the paper about this by Milan M. Cirkovic).

But maybe the Drake equation isn't to be solved after all. Its real value may lie in those thought-provoking question marks. Uncertainty and curiosity will keep the search going for years and centuries to come. Maybe the real payoff for SETI will not be to yield a yes-or-no result, at least not in our lifetimes, but to help us discover more about ourselves.

Alan M. MacRobert is a senior editor of Sky & Telescope. Govert Schilling is an astronomy writer in Utrecht, The Netherlands.

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