Where is ET? Fermi's Paradox Turns 65

Introduction
65 years ago, in 1950, while having lunch with colleagues Edward Teller and Herbert York, Nobel physicist Enrico Fermi suddenly blurted out, "Where is everybody?" His question is now known as Fermi's paradox.

Fermi's line of reasoning was the following: (a) Most likely there are numerous (maybe millions) of other technological civilizations in the Milky Way galaxy alone; (b) if a society is less advanced than us by even a few decades, they would not be technological, so any other technological civilization is, almost certainly, many thousands or millions of years more advanced; (d) within a million years or so (an eye-blink in cosmic time) after becoming technological, a society could have explored or even colonized most of the Milky Way; (e) so why don't we see evidence of the existence of even a single extraterrestrial civilization?

Clearly the question of whether other civilizations exist is one of the most important questions of modern science. And a discovery of such life, say by analysis of microwave data, would certainly rank as among the most significant and far-reaching of all scientific developments. For one thing, it would lend credence to the suggestion by some eminent scientists, such as Freeman Dyson, that the universe is primed for intelligent life.

But after 50 years of searching, the bottom line is that nothing has been found. If there are indeed numerous technological civilizations in the Milky Way, why have we not been able to detect any signals or other evidence of their existence? Why are they making it so hard for us to find them? In Fermi's parlance, "Where are they?"

Proposed solutions to Fermi's paradox

1. They are under strict orders not to disclose their existence. Rejoinder: This explanation falls prey to the inescapable fact that it just takes one small group in one extraterrestrial society to dissent and break the pact of silence. Given our experience with human society, it seems utterly impossible to think that a ban of this sort could be imposed, without a single exception over millions of years, on a vast extraterrestrial civilization dispersed over multiple stars and planets.
2. They exist, but are too far away. Rejoinder: Such arguments typically ignore the potential of rapidly advancing technology. For example, once a civilization is sufficiently advanced, it could send "von Neumann probes" to distant stars, which could scout out suitable planets, land, and then construct additional copies of themselves, using the latest software beamed from the home planet. Simulations of this scheme indicate that a single society could explore (via its probes) the entire Milky Way galaxy within at most a few million years, which is a tiny fraction of the galaxy's lifetime. Communication can similarly be greatly facilitated by futuristic, but entirely feasible, high-tech means.
3. They exist, but have lost interest in interstellar communication and/or exploration. Rejoinder: Given that Darwinian evolution, which is widely believed to be the mechanism guiding the development of biology everywhere in the universe, strongly favors organisms that explore and expand their dominion, it is hardly credible that each and every individual, in each and every distant civilization forever lacks interest in space exploration, or (as in item #1 above) that a galactic society is 100% effective, over many millions of years, in enforcing a ban against those who wish to communicate or explore.
4. They are calling, but we do not yet recognize the signal. Rejoinder: While most agree that the SETI project still has much searching to do, this explanation doesn't apply to signals that are sent with the express purpose of communicating to a newly technological society, in a form that this society could easily recognize. Indeed, the current SETI project program assumes that the remote civilization is making some effort to signal its existence using technology we can detect. And as with item #1, it is hard to see how a galactic society could forever enforce, without any exceptions, a global ban on such targeted communications.
5. Civilizations like us invariably self-destruct. Rejoinder: This contingency is already figured into the Drake equation in the L term (the average length of a civilization). In any event, from human experience we have survived at least 100 years of technological adolescence, and have not yet destroyed ourselves in a nuclear or biological apocalypse. Global warming presents a major challenge at the present time, and has recently been explicitly suggested as a negative solution to Fermi's paradox. But we now understand the situation fairly well and are rapidly developing affordable green technologies, leading some, including Al Gore, to change their minds and be cautiously optimistic. Additional, more exotic, technologies are in the works, and at least some of them may bear fruit. In any event, within a decade or two human civilization will spread to the Moon and to Mars, and then its long-term existence will be largely impervious to calamities on Earth.
6. Earth is a unique planet with characteristics fostering a long-lived biological regime leading to intelligent life. Rejoinder: The latest studies, in particular the detections of extrasolar planets, point in the opposite direction, namely that environments like ours appear to be quite common.
7. WE ARE ALONE, at least within our home in the Milky Way galaxy. Rejoinder: This hypothesis flies in the face of the "principle of mediocrity," namely the presumption, dominant since the time of Copernicus, that there is nothing special about Earth or human society. This may be a philosophically satisfying answer to some, but scientifically speaking it is rather disquieting.

One disquieting aspect of this line of thinking is that it then follows that either (a) we are first such technological society, since the great filter is behind us, or else (b) we are in deep trouble, since the great filter, possibly a great catastrophe, is still ahead of us. Along this line, Nick Bostrom, among others, hopes that the search for extraterrestrial life, either on Mars or on an extrasolar planet, comes up empty-handed, because if life were found, either ancient or present-day, this would reduce the number of possible candidates for the great filter being behind us, and it would increase the likelihood that the great filter still lies ahead of us.

Conclusion
With every new research finding of extrasolar planets in the habitable zone, or of potential life-friendly environments within the solar system, the mystery of Fermi's paradox deepens. Indeed, "Where is everybody?" has emerged as one of the most intriguing scientific questions of our time. There is no easy answer.

We will continue this discussion in a subsequent blog. Stay tuned!