To Open The Sky

The grand questions of whether worlds like Earth exist elsewhere in the Cosmos, and if so, whether they support life and intelligence, have occupied us from the days of Ancient Greece. The answers we have on record — all speculations, of course — go both ways. But many thinkers have answered all three questions in the affirmative. In the nineteenth century, few people had trouble believing that planet-girdling canals had been observed on Mars, or that bat-men and other creatures had been seen on the moon. (The latter story was a hoax; the former, a mistake.) In the twentieth, the Mercury Theater put on a radio play that mimicked news broadcasts about an invasion from Mars, and panicked thousands of people. Indeed, only in recent decades has anyone argued on a scientific basis that humanity is alone in the universe.

Two schools of thought exist. One, to put it briefly and a bit floridly, holds that space is filled with life-bearing worlds, and upon them dwell a cornucopia of intelligent creatures weirder than we can imagine. I'll call people who hold this view "Teeming Worlders". To the other school, the fact that we have no valid evidence of any kind for the presence of extraterrestrial life proves that such life cannot exist, anywhere. I'll call them the "Great Silencers".

The dispute is mostly, at present, a game of numbers.

Teeming Worlders point to the fact that our galaxy has over 400 billion stars, and is but one of billions of galaxies. Such abundance, they say, makes it likely that millions of extraterrestrial civilizations exist, even if the laws of nature dictate that life arises only in a tiny fraction of star systems. It was Frank Drake who first put this on a somewhat scientific basis by creating what we now call the Drake Equation.

Great Silencers refute this with their own set of numbers. It is based on the observed fact that life forms tend to expand their territory when they can. They argue that if millions of intelligent civilizations, or even hundreds, exist, at least some of them are near enough, capable enough, motivated enough, and old enough to have reached us. Thus, the fact that none has proves that no such civilization exists. To quantify this argument, imagine that just one civilization, located far across the galaxy, has developed star travel and starts to expand, leapfrogging from one habitable world to another. Even with ships no better than we can build today, such star-farers could have occupied all the galaxy in a time far shorter than the span of the galaxy's existence. (Depending on the exact assumptions made, estimates of the time required vary from 5 to 500 million years — 0.05 to 5% of galaxy lifetime.)

There's some logic in both arguments, and what we've learned through science supports both to some degree. We can hope that continued scientific investigation will ultimately settle the question. But that resolution may be a long time arriving. In the meantime, speculation is fair game. Here I'll try to tip the scales toward the side I prefer: the Teeming Worlders.

Scientific support for the Teeming Worlders comes first from astronomy and physics: Telescopic observation that the lights in the sky are physical, imperfect worlds like our own, or distant stars like our Sun; that they obey the same physical laws; that they exist in vast numbers; and that we can now detect planets orbiting other suns than ours.

Biology also supports the Teeming Worlders. Radio astronomy techniques show that organic compounds float in clouds between the stars. Some comets and meteors also contain organics as well as water, and strong evidence exists for the presence of water on Jupiter's moon Europa, underground on Mars, and in sheltered areas at the poles of our moon. We've learned that simple organic chemicals like amino acids can form under the conditions of primitive Earth, and have seen primitive life surviving in very hostile conditions.

All of these discoveries suggest that life, at least in primitive forms, is plentiful:

• Suitable sites — planets — appear to be plentiful.
• Building materials — water, carbon, trace elements, organic compounds — are abundant.
• Life forms can thrive under a wider range of conditions than we suspected.

This encouraging chain of reasoning breaks because we don't yet understand how life begins. If there's a majority opinion among scientists today, it is that life will arise wherever suitable conditions persist for long enough. Of course, opinion doesn't make valid science. How life arises from non-life is one of the mysteries astrobiology seeks to unravel.

Another conundrum is what drives life forms to rise to our level. It's easy to see how raw intelligence helps a species survive in a state of nature — "Nature red in tooth and claw" — against creatures that are larger and swifter and stronger; creatures with fearsome features like, well, big teeth and long claws. A certain ability to cooperate in hunting, a knack for fashioning basic weapons, can compensate for the lack of innate physical superiority. But from where comes the evolutionary pressure to go beyond that level of intelligence — to develop advanced technology and culture such as we possess? Once present, they have obvious advantages. The puzzle is that those are intraspecies advantages: while they give one individual a leg up over others, they don't seem to help much with interspecies competition. I can't see what pushed primitive hominids across the gap from that raw intelligence level to our level. It's tempting to conjecture that once they reach a certain complexity, neural systems are self-organizing, and just naturally increase in capability. That's probably too self-congratulatory as well as too mechanistic.

Finally, there are the terms of the Drake Equation. Each rests on a number of assumptions. Some can be verified; others cannot. The critical one is the expected lifetime of an intelligent, technical civilization. The only one we know of has existed for a mere 6,000 years — and that's being generous. Clearly, if acquiring technology typically leads a species to self-destruct in such a short time, the likelihood of two existing near each other at the same time plummets. Viewed over the span of its existence, our galaxy may resemble a bed of hot coals upon which random snowflakes fall one by one, each flaring and vanishing in a relative instant, never knowing that other snowflakes exist.

Personally, I find that state of affairs too gloomy and forlorn to accept. I'm entitled to my opinion, of course, absent any hard evidence to debunk it. But can I defend my conclusion? I'll give it the old college try.

The Great Silencers argue that, if extraterrestrial civilizations can exist at all, the probability of at least one becoming galaxy-girdling star-farers approaches unity. This may be true; we certainly have the drive to expand, and very soon we will have the means to expand into our solar system. However, no one knows what changes human nature will undergo as we undergo that expansion. It may well be that we will attain a comfortable, fulfilling life for every human within those confines and, with greater maturity, realize that the greater struggle to reach the stars offers too little in the way of concrete advantages. It may be that the same can be said for any long-lived civilization. But on this point, I have to concede that the Great Silencers are probably right: Even if most ETIs nestle in their home systems, the likelihood that all of them do is miniscule, and one exception is all it takes.

It's incumbent on me, then, to resolve the Fermi Paradox another way. That's a difficult assignment, one that has confounded better minds than mine. See, for example, a series of three articles in the July 2000 issue of Scientific American. Nevertheless, like Mr. Briggs, I choose to accept it.

To accomplish this mission, I need to explain both the lack of physical contact and the absence of signals. I'll tackle physical contact first, since that is what inspired Fermi to propound his Paradox.

Galactic Travel

The solutions I can see fall into four classes:

The galaxy as minefield

We know of no physical law precluding interstellar travel. However, there may be more practical difficulties than we understand. In the long journeys from origin to destination, starships may run afoul of nearby supernovae or other natural hazards.

Economic disincentives

Interstellar travel is enormously expensive. As our domain expands into the solar system, which it will certainly do, our economy will grow along with it and the cost of star voyages will become less prohibitive. But it may be that the business plan never closes. With increasing ability to make use of diffuse local resources, through nanotechnology or other methods, and with control of population, it may be that humanity — or the typical civilization — simply decides star travel is profitless.

Cultural disincentives

This applies to both potential contactees and contactors. For the contactees, it may be that they choose to remain unnoticed. Perhaps they anticipate hostility, or simply prefer privacy. For any star-faring race, the same reluctance might obtain. They might have the equivalent of the Star Trek Prime Directive forbidding them to interfere with less advanced cultures. Or perhaps they're on urgent business and simply don't have time to stop and look around. The effect of artificially extended lifespan on cultural character should be considered too. Greater individual longevity might confer a contemplative, reclusive mind-set, or greater aversion to risk.

Competition

This could mean warfare, economic competition, or even genocide. An older civilization may, for good reason, fear being overrun by younger, more vigorous races and seek to nip the problem in the bud by exterminating those it encounters. Military and economic competition, as we understand them, call for faster-than-light travel (which is a violation of the physical laws we know) — or for fantastic dedication. Longevity might be a factor here as well. (But I would expect it to make long trips across space more tolerable, not less.)

Walt Kelly's marvelous cartoon character Pogo once opined, "I sometimes think the surest sign that intelligent extraterrestrial life exists is that none of it has tried to contact us."

Pogo's comment points up one more possible reason ET hasn't showed up on our doorstep. It could be that we have nothing they want. Resources? They'd have those at home, or from uninhabited worlds. Medicines, gadgets? They could manufacture anything they wanted. Art objects, exotic scenery, quaint local customs? These could be a draw — but would they be sufficient? Scientific study? Probably they could conduct as much study as they wished, or as much as their ethics permitted, using unobtrusive remotely controlled probes. As for the intangibles like legal principles, religions, philosophies, languages, even company or conversation, how surprising would it be to find they regard all those as repulsively primitive, much as we might regard having to share a hut with a New Guinea headhunter and his family. It's pure speculation — it cannot be anything solider — but perhaps there is a qualitative change we as a civilization will undergo that will remove that onus.

Interstellar Communication

Sending radio signals across interstellar distances is faster than sending ships. It also appears less expensive — but is it? The apparatus to achieve reliable two-way communication will include large antennas, to keep energy requirements down. They will probably be space-based. Thus the setup must include the antennas and support structures (probably steerable), the radio transmitters and related electronics, the power generators, plus living quarters with regular supply and transport runs as well as emergency services for the paid crew. If receiving and transmitting frequency are the same, the receiver installation will likely have to be physically isolated from the transmitters, effectively doubling the cost burden. And this burden must be borne for decades to millennia. Merely putting such large facilities in place requires a lot of capability in space transport and construction. That same infrastructure might be turned to building starships. Therefore, IMO, it's far from clear whether ships are more expensive at the system level.

Being mobile and locally controlled, ships are more flexible. Being large lumps of solid matter, they take far more energy to disrupt. And, for first-contact use, they are far more noticeable. This makes them a more robust method than radio, which is subject to all sorts of natural interference. This could be anything from attenuation by dust or dark matter to bursts of EMP from random supernovae. (I believe a supernova occurs once every century in our galaxy, on average.) Laser beams are not susceptible to EMP disruption; but cosmic dust attenuates them severely. Of course, if travel is impeded by fear of hostile races, or if older races turn inward and lose interest, there won't be much signalling either.

Now I think I've covered the possible de-motivators. But let's say everything is as easy as it seems. There are no mysterious hazards blocking interstellar travel or communication; ET civilizations are typically curious and expansionist, but not warlike; and they would find everything we do fascinating. As for signalling, a fair percentage of races are altruistic enough to announce their presence, and each of those picks the Water Hole frequency band. Even with such optimistic assumptions, there is still no reason to accept the Great Silencers' view.

Foremost among them is the simple fact that we haven't been looking very long. We've had radio for barely over a hundred years, radio astronomy for half that. Our SETI monitoring has been intermittent, on limited portions of one band of frequencies, and mostly targeted at selected individual nearby stars.

Next is the concatenation of improbabilities that ET would use omni-directional beacons, that they would be of galaxy-spanning power (or conversely of more modest power, but quite nearby), and were kept going for the millennia required until another civilization advanced enough to hear them.

There are other improbabilities. How likely is it that anyone, ever, would set up an omni beacon? If we can identify likely stars, then so can ET. Tight beams directed at those stars would save power and be almost as effective. How likely is it that ET would waste any power on beacons, when communications with daughter colonies and far-flung ships might use the entire budget allocation? If they only use radio to signal among themselves, then no matter how widespread they are, we would be incredibly lucky to catch the merest flicker of one of their signals. How likely is it really that the so-called water hole would be ET's natural choice? Even our scientists have proposed other frequencies. If lasers are more effective than radio, as we are just finding, why not use those? Finally, what if there are signalling techniques we haven't even dreamed of yet?

And, as I implied, the greatest imponderables of all are timing and spacing. ET civilizations may well be common in space, but still not exist simultaneously. They may arise frequently, but in widely separated locations. They may be rare in both space and time. These conditions all depend on the Drake Equation, and especially on its factor for the lifetime of a typical civilization.

To wrap this up, I can't say I've succeeded in debunking the arguments of the Great Silencers. Yet I'm sure of three things:

• There's still scope for hope — for believing ET is out there somewhere.
• No single answer can explain ET's apparent absence; but multiple answers may do so.
• Many of these answers, when we find them, will turn out to be stranger than we could have imagined.

There's a new book called Where is Everybody? which examines many of the most plausible solutions to the Fermi Paradox in detail. I recommend it, despite its conclusion. (There's an entry for it in my Reviews list.) Here's a link to the citation on my page of SETI references: