Last Monday Stephen Hawking gave a lecture at the George Washington University for the 50th anniversary of NASA. There he discussed the chance of a contact between our civilization and an extraterrestrial one. And he warned about the risks we may be facing.

Hawking (on the right during a gravity-free flight) listed the dangers: viral agents our immune system is unable to recognize; the superior weaponry of space travelers; or their nature of predators. The scenarios he envisioned are not too different from those of recent blockbuster movies, from "War of the Worlds" to "Independence Day". In the first case, the viruses actually end up helping us; in the second a representation is attempted of the kind of civilization we might be facing if we were found by aliens serially colonizing worlds to exploit them and leave.

The question is after all not a stupid one. Should we be more cautious with our own broadcasting of signals into space ? So far we have not considered aliens a risk, and we have actually taken a different stand: we sent probes around, signals with our radio-telescopes (famous the one sent by Arecibo to M13, see right). Fortunately, one might say, advertising in open space is not easy to perform -for the lack of a visible target.

Now, despite Hawking's arguments are hard to counter, I do not share his fears. I think that the chances of a physical encounter are exceedingly small. Because of the distances involved!

I have always thought that extraterrestrial life is common in the universe. Of course, the temptation of thinking at the matter in other than purely rational terms is high: a small part of the brain of each of us keeps screaming anthropocentrically about ourselves being the necessary product of the universe itself, regardless of our religious faith or absence of same. If we listen to that inner voice, we may reach opposite conclusions depending on whether we consider humanity special or typical.

Drake's Equation

Instead, I have always found convincing by itself the vastness of the universe and the sheer number of stars it contains. This has been the subject of considerable scientific debate ever since we realized how big is the cosmos surrounding us. In particular, things have become more quantitative after Frank Drake, in 1961, put together a simple equation to compute -or should I say guesstimate- the number of civilizations presently existing in our galaxy, ones with which we have a chance of coming in contact. The idea is that interstellar space travel is possible, while inter-galaxy travel is honestly too hard to envision; and radio broadcasting from other galaxies are similarly out of the question for the time being.

While Drake's equation is a good basis for systematic investigations of signals from extraterrestrial intelligences, I care little about the admittedly scarce possibility that we ever receive positive news from our SETI searches. I care more about the fact that, if we consider the whole universe instead than restricting to our small galaxy, and if we omit to require that other civilizations exist at present (whatever this means over billion-light-year distance scales), the probability becomes a certainty.

Let us first have a look at the Drake equation: it states that the number of civilizations in our galaxy which could in principle communicate with us, call it N, can be computed by multiplying several factors:

- R = the average rate of star formation per year in our galaxy;
- Fp = the fraction of stars having a planetary system (never mind peanut-shaped ones -sorry Pluto);
- Ne = the number of life-ready planets per planetary system;
- Fl = the fraction of planets that actually develop life at some point;
- Fi = the fraction of life evolution bringing in intelligence
- Fc = the fraction of civilizations that develop a technology releasing detectable signs of their existence;
- L = the length of time that such civilizations broadcast detectable signals into space.

According to wikipedia, the most recent estimates of those numbers are the following:
- R = 7/year
- Fp = 0.4
- Ne = 2
- Fl = 0.33
- Fi = 0.01
- Fc = 0.01
- L = 10000 years

With the above numbers, N becomes 2.1. Besides us, it is possible that there are two civilizations currently broadcasting a signal in the galaxy. A not too encouraging thought for SETI, or maybe a scary one for Hawking; in any case, admittedly a small number, subject to such wild uncertainties (from the unknown error in the parameters estimated above) that we cannot really draw any conclusion. It can be zero, or easily several hundreds.

So are we alone in the universe ? NO!

Despite the uncertainties, some conclusions can still be drawn. For let us take a step back: we might content ourselves with the notion of the existence of intelligent life anywhere in the universe. In that case we would be able to get rid of the Fc factor, and we would get the small bonus of summing over all the galaxies in the known universe: at least 125 billions, according to recent estimates enabled by the Hubble surveys.

The net result is that the meager N=2.1 becomes over 20 trillions! This means that there are presently 20 trillion civilizations around. 20 trillions. Okay, we might have dropped or added one factor of a hundred too many here or there, but the number is still enormous, no escape!

Is that not a sobering thought ? To me, that is both awesome and saddening. As far as awe is concerned, of course there is no need to explain it. But there is sadness too: for imagine the incredible, unfathomable number of things that we will never be able to know, constrained in our tiny planet, during our insignificant lives. Masterpieces, inventions, acts of bravery, adventures. But also wars, atrocities, catastrophes. The history of the universe will never be written - but it would be quite a read, I am sure.