Where are they, indeed?

kw: book reviews, nonfiction, radio astronomy, space aliens

Fifty years and counting. In April 1960 Frank Drake first listened to space using a radio telescope at Green Bank, WV, initiating Project Ozma, which has since morphed into SETI. For a few years around 2003 I ran the Seti@home "screen saver" application (actually, I soon turned off the screen saver part because it runs three times faster without it).

It has been estimated that the combined computer power of perhaps a million people running this application is about a petaflop (10¹⁵ calculations per second). That was several years ago, and it would not surprise me if the current figure is closer to an exaflop (10¹⁸/s). I do not run it at present because I don't run my computer when I'm not there. That may change now that I have a computer that is a bunch faster than the one I had in 2003, and a better Internet connection.

The fiftieth anniversary of Project Ozma is also the occasion for Paul Davies, who chairs the SETI Post-Detection Taskgroup, to issue his book The Eerie Silence: Renewing Our Search for Alien Intelligence. The book could be taken as a long riff on the question Enrico Fermi asked in about 1946: "Where is everybody?" It might seem that the prevalence of UFO reports beginning the following year provides an answer, but so far, these stories tell us lots about human nature, and nothing about space aliens. Dr. Davies takes this up as he closes his first chapter, "Is anybody out there?"

Perhaps you know of Dr. Drake because of the Drake Equation:

N_c = N* f_p N_e f_l f_i f_c L_c

These are, in order, the Number of (possibly) Communicating civilizations, a product of the Number of Stars formed yearly, the fraction thereof with planetary systems, the number of planets per system that are potentially habitable, the fraction thereof that develop life, the fraction of planets with life that develop intelligence (of a type and level that can build radio telescopes and powerful transmitters), and their average Lifetime as interstellar communicators.

Only the first and last component of N_c can be fairly estimated from data at hand. N* in our Galaxy is about seven per year at present, though it was a little higher a few billion years ago. L_c for Earth is less than 100 years for our accidental emissions of broadcasts such as old Amos 'n' Andy or I Love Lucy until the end of high-powered analog TV broadcasting just last year. But most of us have been getting our TV from cable for a decade or more already (ours is the last house on the block to use antenna reception for all the TV we watch). I use TV as the standard because VHF and UHF signals get out of the atmosphere much more readily than AM radio, and have quite a bit more power than FM radio, which is also VHF. Yet if we decide to become "radio loud" so as to attract attention, then L_c for Earth might be as long as we last without blowing ourselves up. Whether that is another generation or many millennia, who knows? But the Sun will remove civilization in about half a billion years, so that's our extreme limit.

As the author describes, the opinions about the other factors range the map, but are clustered about pessimistic and optimistic, with little to go on for a middle ground. By the Copernican principle—we're pretty average, not special in any way—we might conclude that life abounds in the cosmos, and most stars with habitable planets have smart "people" living there. Using some fairly optimistic numbers in the Drake Equation yields an estimate of about 20 million planets out there, all trying to communicate with their nearest neighbors. However, that puts the average spacing between communicating planets at about 50 light years.

Then there is the opposite view, the "rare earth" analysis, based on the notion that for life to form might be harder than we think, and for intelligent life (that can build big radio dishes) to evolve might be even harder. Using one-in-a-million for a couple of the parameters in the Equation indicates that the chance for even one Earth to arise (that would be us), is a one-in-a-billion or -trillion shot, so that we are a lucky fluke. Which is it? We won't know, and can't know until we either hear from someone or find other evidence that someone is out there.

Could we receive old TV shows from our planet if we put one of our radio telescopes on a planet around the star Sirius? Not yet, except perhaps with the Arecibo radio telescope, which is half a kilometer in diameter, but can't be steered (it is anchored to a bowl-shaped valley in Puerto Rico). How about with this array:

Maybe, maybe not. The Allen Radio Telescope Array at the Hat Creek Radio Observatory is online now. While each dish is relatively small, there are, so far, 42 of them. This array is sufficient to detect a signal of the type we could produce using Arecibo and a kilowatt transmitter, pointed right at a star of interest. If smart aliens are sending a pulsed signal, say with a peak power of a megawatt, 0.1 second on, 99.9 seconds off for an average power of a kilowatt, we'd hear it from as far away as a hundred light years or so.

What does it mean that, after fifty years of listening, we've heard nothing we can clearly say, "This is them!"? Firstly, we've only listened in the direction of about 0.01% of the stars within 100 light years. There are lots of stars out there! Secondly, we've only listened in a small part of the potential radio (and light) spectrum. There are lots more wavelengths out there!!

From the ground, the Allen array looks big, but this aerial photo (from Google Earth) shows that there is not much "capture area". The Allen dishes are the white blobs that are casting shadows. The larger things are part of the older radio observatory. Spreading them out over a large field makes it possible to focus sharply on stars of interest, but doesn't strengthen the signal. The whole array has less capture area than Arecibo, but has the virtue of being steerable.

Is there something besides radio signals that would indicate intelligent life? We are not yet capable of getting the spectrum of an exoplanet's atmosphere, so we can't even detect an oxygen/water atmosphere or photosynthesis, both of which would at least tell us "life happened". Could we detect human activity on Earth, even from a nearby place like Neptune? Dr. Davies has a chapter on such questions. In fact, he is very thorough in his analysis of all the factors that could tell us of life unlike our own. For example, we might someday find creatures on Earth (probably bacteria) with a different code for turning DNA into proteins.

I mean really different. There are at least seventeen variations on the "standard" code used by all Eukaryotes, found among Bacteria and Archaea. But they are all very similar and probably indicate branches off the "standard" tree, rather than separate life-initiation events. So we're talking a completely different code, and perhaps a basis in four rather than three DNA bases per codon, or a set of quite different amino acids, other than the twenty or so in use by all (so far) known life. Such a finding would help us say that life wasn't so hard to get started, because it did so more than once on Earth…or, alternatively, that it also started somewhere nearby such as Mars and came here on a meteorite (or spacecraft?) and flourished once it got here.

The author proposes widening the SETI search. He really tries to think outside the box. The way we are searching now is based on what we might do if we were trying to send a signal for others to receive. What if, instead, we look for signals that accidentally come our way, from one planet to another who are already in contact? Could we eavesdrop? We'd have to be close to a direct line between them, or the beam being used would miss us. The narrowest beams would be not radio signals, but IR or visible laser beams. It is a tossup whether light is better, though, because stars are bright in the visible and near-IR spectra, compared to radio.

Then there are artifacts. Will we one day find a black obelisk on the moon? How might it signal its presence, if it wants to be found, but is waiting for us to become spacefaring, à la the film 2001: A Space Odyssey? We've already blown that chance until at least 2030. But there are lots of possibilities and the book outlines everything I might have thought of, and more.

The last question for the last chapters is, should we ever receive a definite interstellar signal, at Allen array or elsewhere, then what? Do we crank up a transmitter and shout "Hello!"? Is that smart? How will it affect our culture and politics, and particularly, our religions? The various major religions of Earth have survived the Copernican revolution, and are currently working through the Darwinian revolution. What would an Alien revolution be like? Would it be the end of our civilization, or the trigger for a better one? Probably neither, at least not in the short term. People get over things. Since Giordano Bruno was burned at the stake for espousing that other planets had people, even religion has gotten over it, to some extent, and most religious leaders make sounds like they could accept aliens as just another kind of people who need faith.

What'll we do if the first interstellar communication we receive is evangelistic in nature? The author doesn't ask this, but it naturally follows from understanding that it is quite like one of our wealthier religious leaders to build a dish and buy a big transmitter and send an evangelistic message outward. How much would they really be like us?

Lots of questions simply have no answer yet. The book ends by saying that we simply don't know.