7.07.2020

On Earth, civilizations have limited lifetimes.

Roman civilization, for instance, lasted less than a thousand years from the founding of its republic to the fall of its empire (after a long decline). In the New World, Maya civilization spanned roughly two millennia (maybe a little longer depending on when you date its beginning). In the late Bronze Age, the Greek Mycenaean civilization lasted a mere five centuries or so. As for American civilization (as in the United States of), at the rate things are going it won’t last even that long.

For some reason, civilization is not a self-perpetuating state of affairs on this planet. And perhaps not on other planets, either. In fact, limits to civilization lifetimes may explain why extraterrestrial aliens have not yet communicated with Earthlings. A new analysis suggests that the entire Milky Way galaxy currently houses only a few dozen worlds equipped with sufficiently sophisticated technology to send us a message. They are probably scattered at such great distances that any signals sent our way haven’t had time to get here. And by the time a signal arrives, there may be nobody here around to hear it.

“We may imagine a galaxy in which intelligent life is widespread, but communication unlikely,” write Tom Westby and Christopher Conselice in the June 10 Astrophysical Journal.

Westby and Conselice, of the University of Nottingham in England, base their analysis on a modified version of the Drake equation, proposed nearly 60 years ago by the astronomer Frank Drake. At a time when most scientists didn’t take communicating with E.T. seriously, Drake identified the factors that would, in principle, permit an estimate of how many communicating civilizations might exist in the galaxy. His equation provided the framework for all subsequent scientific assessment of the prospects for extraterrestrial intelligence.

Westby and Conselice accept the Drake equation as “a tool for estimating the number of planets in our galaxy that host intelligent life with the capability of releasing signals which could be detectable from Earth.” (Such Communicating Extra-Terrestrial Intelligent civilizations are sometimes referred to by the acronym CETI.) But since some of its terms are impossible to measure today (such as how many stars have planets, and how many planets are capable of hosting life), Westby and Conselice adopt a novel approach by making assumptions that can circumvent the lack of data needed to fill in the Drake equation’s blanks.

Westby and Conselice begin by assuming it takes 5 billion years for intelligent, technologically advanced life to evolve — because that’s (approximately) how long it took on Earth. In some scenarios they assume that any habitable planet that lasts that long will, in fact, evolve such life. Given those data points, the task of counting galactic civilizations then involves figuring out how many stars are old enough and how many planets orbit those stars at a distance providing Goldilocks temperatures plus water and other raw materials needed to create and sustain biological beings.

For one thing, that means the stellar system must possess sufficient quantities of metals — in astronomers’ argot, elements heavier than hydrogen or helium. Carbon, oxygen, nitrogen and other more complex substances must be available for life to both evolve and build radio transmitters or lasers to send signals through space.

So in their new CETI equation, Westby and Conselice show how the number of intelligent, communicating civilizations in the galaxy today depends on how many stars the galaxy contains, how many of them are more than 5 billion years old, with how many habitable planets, and the average lifetime of an advanced civilization. Crunching all sorts of numbers about star formation rates and ages, results of planet searches and other astronomical studies yields estimates for each term in the CETI equation. It turns out that some of those factors don’t limit alien life’s prospects very much. Almost all the stars in the galaxy are older than 5 billion years, for instance (and their average age is almost 10 billion years).

Some of those stars would be ruled out as E.T. habitats because of a lack of raw materials. Assuming the most pessimistic scenario — that life requires stars to have at least as much metal as the sun — eliminates about two-thirds of the galaxy’s stars. Of those remaining, the fraction with planets in an orbit conducive to habitability is probably about 20 percent.

Since the galaxy is home to more than 200 billion stars, age, metal content and habitability limits still leave billions of possible CETI abodes. But that’s before factoring in civilization lifetime. It’s safe to say that a communicating civilization can last 100 years, since Earth’s technology has been emitting radio waves for that long. But if no high-tech society survives for more than a century, very few will be around at this particular time to communicate with us. With the strictest set of assumptions, assuming 100 years as the average CETI life span computes to only 36 communicating civilizations in the galaxy today. If so, far more movies have been made on Earth about alien civilizations than there actually are alien civilizations.

Among those 36, the closest neighbor would probably be about 17,000 light-years away, “making communication or even detection of these systems nearly impossible with present technology,” Westby and Conselice write. For an ambitious civilization lifetime of 2,000 years, the nearest CETI neighbor could still be thousands of light-years away. In a wildly optimistic case, with an average high-tech lifetime of a million years, the closest civilization should be within 300 light-years and maybe as close as 20.

“The lifetime of civilizations in our galaxy is a big unknown … and is by far the most important factor in the CETI equation,” Westby and Conselice note. “It is clear that … very long lifetimes are needed for … the galaxy to contain even a few possible active contemporary civilizations.”

If you’re wondering how different assumptions can affect the prospects for getting alien e-mail, you can check out a tool at the Alien Civilization Calculator websitecreated by physicists Steve Wooding and Dominik Czernia. Their tool permits you to plug in values to either the new CETI equation or the original Drake equation to see how different assumptions affect the galaxy’s population of alien civilizations.  

All such calculations are pretty imprecise. The uncertainty range for Westby and Conselice’s estimate of 36 civilizations, for instance, is four to 211. But the lack of precision is not as meaningful as the underlying message — the importance of civilization lifetime for the odds of receiving a message. And that message implies, as Westby and Conselice emphasize, that no news from E.T. is a bad sign for the lifetime of civilization on Earth.

Since most stars in the galaxy are much older than the sun, the absence of signals so far suggests that most communicating civilizations have already come and gone, like the Maya and Myceneans.  If that’s the case, an ability to communicate may signify an ability to self-annihilate.

“Perhaps the key aspect of intelligent life, at least as we know it, is the ability to self-destroy,” Westby and Conselice comment. “As far as we can tell, when a civilization develops the technology to communicate over large distances it also has the technology to destroy itself and this is unfortunately likely universal.”

In other words, Earth’s entire civilization will go the way of the Roman Empire sooner rather than later. There are plenty of likely roads to ruin. Nuclear holocaust is always a possibility, although nowadays it seems more likely that a viral pandemic will reboot the planet’s biosphere. Or climate change might do the job. If all else fails, there’s always social media.

Yet there is always hope that high-tech societies can survive longer. Maybe long-lived alien civilizations are not so far away after all, but simply have chosen not to communicate with use because we don’t seem to be sufficiently civilized.

Quelle: ScienceNews