Now that we’ve had a nice cross section of where we might expect to find life within and without our solar system, I’m bringing this alien life series to a close with the consideration of a very pressing question: if life may be so common in the cosmos, why haven’t we heard from anyone else?
This question is a good one. If the potential to give rise to intelligent life is common in the cosmos, we should be awash in artificial radio waves from the stars–shouldn’t we? Let’s take a closer look at what would be required for this to be the case.
You may have heard of something called the Drake equation. This equation, developed by Astronomy and Astrophysics professor Frank Drake, purports to offer a way to calculate the number of extraterrestrial civilizations in the Milky Way galaxy. It looks like this:
N = R* x fp x ne x fl x fi x fc x L.
Like any equation purporting to do such a thing (and most of the theories we’ve discussed so far), it’s rife with unknown variables. Each of the terms that are multiplied to produce N represent a variable related to the formation of stars and planets, the probability of planets supporting life, and the probability of this life becoming a technological civilization that’s detectable from space. Needless to say, almost all of these terms are unknown variables.
Each year we get closer to filling in a few. Astronomers now feel they may be able to make educated guesses about the rate of star formation (R*), for example, and maybe even the fraction of stars that support planets (fp). But the rest are total black boxes. “ne” is the number of potentially habitable planets per star; fl is the percentage of potentially habitable planets where life actually does develop. They only get more esoteric and unknowable (by present technology) from there.
The variable of the most interest to astronomers who are also philosophers is “L.” “L” is the length of time your average civilization releases detectable signals into space. If “L” is small, then the number of detectable civilizations in the Milky Way at any given time is small. If “L” is large and civilizations broadcast signals for millions of years on average, the number of detectable civilizations could be quite large (all other variables allowing).
For Carl Sagan, “L” was worrisome. The radio silence of the cosmos was worrisome. In his eyes, the complete lack of communication from extraterrestrials meant that there weren’t any intelligent civilizations out there who had been broadcasting signals for lengths of time equivalent to their distance from us in light years. To him this meant that technological civilizations may be, by nature, short-lived and self-destructive; each got to broadcast for only a few decades or centuries before destroying itself utterly.
Sagan’s fear was especially poignant during the 1980s, when the Earth stood on the brink of nuclear war. In those days, it was quite easy to imagine that Earth’s technological civilizations may soon render the planet unfit for civilization for millenia to come. So if the radio silence from the stars indicated that “L” may indeed be small–astronomers like Sagan felt that we should listen to the stars’ warning.
Today, the prospect of complete man-made annihilation is far more distant to us than it was to our parents a few decades ago. And, perhaps fittingly, some new possible reasons for the interstellar silence has been proposed.
Some very sensible mathematicians point out the problem of distance. One major obstacle to Earth-based interstellar missions would be the communication time lapse; actual travel time aside, it would take over four years to send a lightspeed signal one way from our nearest stellar neighbor, Proxima Centauri. It would take more than another four years to receive a response from Earth.
Expanding this principle more broadly, most stars in the Milky Way are dozens or hundreds of light years from Earth. And let’s remember that humans have been broadcasting radio signals at a level detectable from space for less than 80 years; any civilizations more than 80 light years from us would have no idea that a technological civilization exists on Earth.
A civilization 60 light years away may have picked up our radio signals 20 years ago and started answering them, but their response would not reach us for another 20 years. Likewise, for us to hear them, any given civilization would have to have broadcast our way in the astronomically extremely narrow temporal window of the last few decades. Not having heard from aliens since the 1970s does not mean that aliens do not exist.
Stephen Hawking recently pointed out another, perhaps even more compelling argument for why radio silence does not mean that the skies are empty.
Think about the history of Earth. When technologically advanced civilizations come into contact with more technologically primitive societies, the technologically primitive societies are virtually always the losers. Sometimes they just have their resources exploited to the point of sinking into permanent poverty and social chaos; other times they are actually wiped out altogether, by accident or design.
This doesn’t give us a lot of incentive to broadcast our presence to the cosmos. In fact, if patterns are similar on other planets (and from a perspective of evolutionary biology, they probably are), no civilization with a high enough IQ to build radio telescopes should want to let every potential advanced intelligence know exactly where they are.
“Oh look, Great Alien Leader, these radio signals lead back to a planet inhabited by a young and defenseless civilization that has not trashed their planet to nearly the extent that we as an older and more industrially developed society have trashed ours. What a marvelous opportunity for the expansion of our species.” (Photo of everyone’s favorite hostile alien race at left by Aaron Thomas.)
Would we put this kind of thinking past a human? Then why should we put it past anyone else?
Stephen Hawking has made headlines more than once by suggesting, in all seriousness, that we take care to avoid broadcasting our presence to space. And given the seemingly head-banging obviousness of this line of thought, what makes us think other civilizations–anybody with a good head on their shoulders, really–hasn’t also thought of it?
So it could be that if there are extraterrestrial civilizations, we won’t find out about them by hearing the noise they broadcast into space. It’s very possible that “L” lasts just a few decades–not because technological societies destroy themselves, but because they take a look at their own histories and wise up, deciding not to broadcast their presence as potential targets for exploitation.
If that’s the case, there might be alien spaceships swinging by Earth every day and hearing our radio clamour, thinking “Poor, silly humans. They must be new at this.”