New Study Suggests Our Galaxy Is Either Crowded or Empty — and It's Not Clear Which Is Worse
Is there intelligent life in the universe? And if so, how common is it? Or perhaps the question should be: what are the odds that those searching for extraterrestrial intelligence (SETI) will ever encounter it? This is a topic that has been hotly debated by scientists for decades, and much ink has been spilled on it. Of the many papers and studies written on the subject, two main camps have emerged: those who believe that life is common in our galaxy (SETI optimists), and those who argue that extraterrestrial intelligence is either rare or nonexistent (SETI pessimists).
In his recent work David Kipping (a professor at the Cool Worlds Lab) and Geraint Lewis have looked more closely at this debate and have proposed a new perspective based on a form of probability analysis known as the Jane experiment. Applying this method to astrobiology and the Drake equation, they conclude that the existence of intelligent life in our galaxy is an “all or nothing” proposition. To quote the late and great scientist and science fiction author Arthur C. Clarke: “There are two possibilities: either we are alone in the universe, or we are not. Both possibilities are equally frightening.”
David Kipping is an associate professor of astronomy at Columbia University and a Carl Sagan Fellow at the Harvard College Observatory. He is also the principal investigator of the Cool Worlds Lab at Columbia University, which studies and characterizes exoplanet systems. Geraint Lewis is a professor of astrophysics at the Sydney Institute for Astronomy, part of the University of Sydney’s School of Physics. Their paper,Do SETI optimists have a fine-tuning problem?” has recently appeared online and is currently under consideration for publication in the International Journal of Astrobiology.
Drake equation
In 1961, famed astronomer Frank Drake held the first ever SETI meeting at the Greenbank Observatory in West Virginia. In preparation for the event, he created an equation that summarized the problems SETI researchers faced. It became known as the Drake equation and is mathematically expressed as follows:
N = R* × fp × ne × fl × fi × fc × L
Where:
N is the number of active, communicating civilizations in our galaxy.
R* is the rate of star formation in our galaxy.
fp — the proportion of stars with planets.
ne is the number of planets that could potentially support life per star with planets.
fl is the proportion of the above-mentioned planets on which life actually develops.
fi is the proportion of the above-mentioned planets on which intelligent life develops.
fc – the proportion of the above that develop the ability to communicate interstellarly.
L is the duration of the existence of such communicative civilizations.
The Drake equation was not intended to estimate the number of extraterrestrial civilizations (ETCs) in our galaxy, but to stimulate dialogue about SETI. Since Drake first formulated it, the equation has been criticized, expanded upon, and revised, often distorted in the process. As Professor Kipping explained to Universe Today via email, part of the problem is that the parameters are often assigned arbitrary values:
“Since we don’t know most of the parameters, this is pure speculation and needs to be clearly labeled as such. Another point that is often overlooked is that it represents an average number of civilizations, and is thus the expected value of some underlying distribution. Criticism of the Drake equation has become something of a sport these days. Of course, anyone who uses it as a calculator should be subject to fair criticism, but the basic idea is not wrong. There must be some number of civilizations, and we can, in principle, collect the appropriate parameters to calculate it. The problems arise in the precise formulation, which parameters to include, what they actually mean, and how to deal with nuances such as time variability.”
The Jaynes Experiment
Edwin Jaynes (1922-1998) was the Wayman Crow Professor Emeritus of Physics at Washington University in St. Louis. In 1968, he devised an experiment in which a person in a lab is given a jar containing an unknown and unlabeled compound (chemical X). Along the lab bench are a number of beakers filled with water, and the experiment is to test how often chemical X will dissolve in them. Jaynes argued that one should expect the compound to dissolve either almost always or almost never.
When plotted, the probability distribution will be shaped like a bowl, with values 0 and 1. As Kipping explains in more detail:
“Jaynes imagined a series of experiments that we call Bernoulli experiments, that is, experiments that give yes/no answers. These could be anything, but as an example, he imagined dissolving an unknown chemical in a series of beakers of water and then asking, what fraction of them will dissolve? Another scientist, the legendary John Haldane, had already suggested that an answer of ~50% was unlikely a priori. You would expect either almost all or almost none to dissolve.
“Jaynes proved this rigorously and pioneered many of the tools of objective Bayesian inference. We can equally replace the Bernoulli experiment in question with other questions, such as what fraction of stars will turn into black holes? Before any observations, an answer of ~50% would be surprising, implying that the distribution of stellar masses is finely balanced, with half above the critical mass threshold and half below. In fact, the answer is one in a thousand, consistent with Jaynes's position.”
Because of his enormous contributions to statistics, Jaynes is considered one of the founders of “objective Bayesianism.” Although his experiment was not intended as such, Kipping and Lewis saw potential applications in it for astrobiology.
All or nothing?
In his seminal 1983 paper, “The Great Silence: The Debate on Extraterrestrial Intelligent Life,” David Brin examined the ongoing debate over the existence of extraterrestrial life. From this, he identified two camps: “optimists” and “pessimists,” or, as Kipping and Lewis call them in their paper, SETI “optimists” and “pessimists” — those who believe that there are civilizations in our galaxy with which humanity can make contact, and those who believe that doing so is futile because humanity is alone in the universe.
If we apply Jaynes's experiment to the question of intelligent life in our galaxy, we would expect it to be either very common or very rare. In the middle, where the probability distribution is weakest (i.e., extraterrestrial life is semi-rare), is where the “fine-tuning problem” arises. In the context of cosmology and astrobiology, fine-tuning refers to the idea that the conditions for life can only arise when certain universal constants are within a very narrow range of values.
If even one of these fundamental constants were slightly different, the universe would not support the development of matter, large-scale structure, or life as we know it. As Kipping explained, this poses a problem for SETI optimists:
“Unlike the black hole example I gave earlier, there are no lower bounds for this problem. In the case of black holes, we know the smallest and largest allowable mass of a star from astrophysics, and these are only a few orders of magnitude. The threshold for a black hole must be somewhere in this fairly narrow range. In the case of aliens, the probability of their intelligence might be 1% or 0.000….00001% (add as many zeros as you like).
“With such a wide range of possibilities, SETI optimists are forced to believe the rather far-fetched notion that the percentage is not so high that we won't see anyone, but certainly much higher than the deep chasm of low probabilities that are plausible. So they essentially have a fine-tuning problem: they want the percentage to live in a fairly narrow corridor.”
If our galaxy were filled with extraterrestrial civilizations, there would certainly be undeniable signs that we would notice – radio signals, megastructures, Clarke belts [искусственные спутники у экзопланет / прим. перев.] and other “technosignatures.” If this sounds familiar, it's because this argument is at the heart of the Fermi Paradox. So Kipping and Lewis's argument can be seen as an example of SETI pessimism. Fortunately, the story doesn't end there.
New formalism
Faced with this result, Kipping and Lewis tried to develop a new formalism for the Drake equation that takes into account only two processes: the birth and death rates of civilizations. In this case, all the parameters of the equation (except L, the lifespan of civilizations) are reduced to one parameter: the rate of birth and death of civilizations (rc). Or, as it looks mathematically: NC = rc x LC. Kipping said:
“In the standard Drake equation, we often argue about which parameters to include (for example, whether to include a fraction for the probability of life evolving into multicellularity). But it is absolutely undeniable that every civilization must have a beginning and an end, and we can even set the mortality rate to zero, which corresponds to an infinite lifespan, if we want within that framework. In an ecological system, such as a petri dish, there is a well-defined maximum possible population size, which we call the carrying capacity. So we updated the birth-death version of the Drake equation to take this nuance into account.”
In this case, the probability distribution has become S-shaped (see the figure above), but the end result is the same: either the galaxy is crowded or empty. One solution to this problem is the idea that humanity may be living in a period when the EDCs were just emerging and spreading through the galaxy, and thus not yet detected by our instruments. However, as Kipping and Lewis showed, this option also suffers from the fine-tuning problem, since biology indicates that population growth is an accelerating phenomenon.
“You see, the phases of galactic expansion must happen relatively quickly on a cosmic time scale; in fact, it’s like the blink of an eye,” says Kipping. “So it’s unlikely that you’ll be alive during such a phase; you’re more likely to be alive when the galaxy is empty before it happens, or after it happens (which is basically impossible, since your planet is colonized).” And so the Fermi Paradox comes into play again, in which the most likely outcome is that humanity is either alone, early to the party, or one of the few civilizations currently existing in the Milky Way.
Hope for SETI?
But before you think this is all bad news, Kipping and Lewis emphasize that SETI is an important and vital experiment that deserves dedicated resources. “Although the odds of success seem small, such a success would likely be the most significant scientific discovery in human history,” they conclude. They also offer several reasons to remain hopeful, including Hanson’s hypothesis that “greedy aliens“, which states that humanity is at the midpoint of the S-curve and that we will meet the ITI in a few hundred million years.
Meanwhile, Kipping also thinks SETI could benefit from casting a wider net. If, as their research suggests, advanced civilizations are rare (or nonexistent) in our galaxy, then we should look to extragalactic sources. “I think the one I like best is that our galaxy is just unusually quiet, most others are busy and crowded, but in the Milky Way, we’re the first,” he added. “It seems unlikely, but maybe being born into a busy galaxy is impossible because the habitable real estate has already been gobbled up. That suggests we should be focusing more on extragalactic SETI as our best chance.”
