In the search for extraterrestrial intelligence, theres really only one big question: Where is everybody? This question has haunted alien hunters ever since the Nobel-winning physicist Enrico Fermi posed it to some colleagues over lunch 70 years ago. There are billions of sun-like stars in our galaxy, and we now know that most of them host planets. But after decades of searching, astronomers havent found any that appear to host life. This is the so-called Fermi paradox: Our galaxy seems like it should be teeming with alien civilizations, but we cant find a single one.
Researchers working on the search for extraterrestrial intelligence, or SETI, have proposed a number of solutions to the Fermi paradox over the years. But the most persuasive answer is also the most obvious: Perhaps intelligent life is just far more rare than we thought.
How rare? Many scientists have attempted to answer this notoriously tricky question. Based on their conclusions, there are between zero and 100 million extraterrestrial civilizations in the Milky Way. That is not an especially helpful range of estimates, so a pair of physicists in the UK recently took another stab at it and arrived at a remarkably specific conclusion. As detailed in a new paper published this week in the Astrophysical Journal, the duo calculated there should be at least 36 communicating extraterrestrial civilizations in our galaxy.
That is … not a lot, obviously, and it has some depressing implications. According to the paper, this would mean well likely have to spend hundreds of years searching for an extraterrestrial civilization before we find one, and it also suggests that our closest neighbors may be up to 17,000 light-years away. “Weve gone from being quite bullish on there being life in the universe to being a bit more pessimistic as time goes on,” says Christopher Conselice, an astrophysicist at the University of Nottingham and one of the authors of the paper. “I think thats natural, but now we have the kind of information we need to make some real estimates based on reasonable assumptions about how life could form on other planets.”
Attempts to estimate the prevalence of intelligent life in the galaxy date back to the very beginning of modern SETI. In 1961, just a few months after wrapping up the worlds first radio search for ET, the planetary astronomer Frank Drake convened a small meeting of leading American scientists to discuss the future of SETI—or whether it should have any future at all. To organize the meeting, Drake made a list of questions that he deemed pertinent to determining the odds that the search would be successful.
Some of these questions—like figuring out the average rate of star formation in the galaxy and how many stars host planets—were possible for scientists to answer before first contact. Others—like what fraction of planets produce intelligent life and how long that life broadcasts messages into space—could only be guessed at. But Drake realized that if you multiplied the answers to these questions together, they could be used to get a rough estimate of the number of intelligent civilizations in the galaxy. This formula is known as the Drake equation.
Today, astronomers can confidently fill in some of the blanks in the Drake equation, like how many stars have planets (most of them) and the average rate of star formation in the galaxy (a handful per year). And as a new generation of exoplanet telescopes like the James Webb Space Telescope come online, well also have a better idea of how many of these planets are located in the habitable zone of their star. This means that liquid water could exist on those planetary surfaces, which as far as we know is a prerequisite for life—intelligent or otherwise.
A bit of a guessing game
But “as far as we know” is exactly the problem with the Drake equation. The number of communicating alien civilizations in our galaxy is a statistical estimate, and like all statistical estimates it can vary a lot depending on the assumptions that are used to make it. In the Drake equation, about half the unknowns are about extraterrestrial civilizations. Since we know nothing about ET, astronomers have to make some guesses. And in their new paper, Conselice and his colleague, University of Nottingham engineer Tom Westby, make two very big assumptions in their reworking of the Drake equation.
First, the researchers looked at the only planet that we know for a fact has produced intelligent life—our own—and used it as a model for every other planet that could host extraterrestrial intelligence. Humans cropped up and started spewing radio waves into the cosmos about 4.5 billion years after Earth was formed, so Conselice and Westby assumed that it would also be the case on other Earth-like planets. But they went even further and assumed that all Earth-like planets in the habitable zone of their star inevitably produce intelligent life after about 5 billion years.
“To say all the Earth-like planets will produce intelligent life is a huge assumption and has some serious problems,” says Seth Shostak, senior astronomer at the nonprofit SETI Institute in California. “The habitable zone of our own solar system includes Mars and—depending on who you ask—Venus. But theyre not populated by intelligent beings, even though theyve been sitting around just as long as the Earth has.”
One way statisticians learn about a large, unknown population is by taking a small sample and extrapolating to the larger population. This is, essentially, what Conselice and Westby did in their paper. The problem is they extrapolated from a sample of one, which is a bit like trying to predict a national election by surveying only yourself. Small sample sizes lead to greater variance of results, which is why the Drake equation reliably produces such wildly different estimates of the prevalence of extraterrestrial intelligence. In fact, this was demonstrated by Conselice and Westby in their own paper.
The researchers put forth two hypotheses—one strong, one weak. In the strong hypothesis, the researchers assume that an Earth-like planet must produce an intelligent species when it is between 4.5 billion and 5.5 billion years old. This is how it went on Earth, where humans started mastering technology after about 4.5 billion years. The weak hypothesis relaxes the time frame a bit and assumes that an Earth-like planet can produce life anytime after 5 billion years. Given that the average age of stars in the Milky Way is about 10 billion years old, this creates a bigger pool of extraterrestrial societies that could still exist today. (This assumes that extraterrestrial societies dont last for 5 billion years—more on that in a moment.)
The strong hypothesis results in an estimate of at least 36 extraterrestrial civilizations in the galaxy, but with a very large margin of error. The researchers calculate that the lower bound on the strong hypothesis could be between four and 211 extraterrestrial civilizations in the Milky Way. Things are more hopeful with the weak hypothesis, which estimates that the lowest number of possible extraterrestrial societies is somewhere between 100 and 3,000.
Thats a pretty big spread, but even the most optimistic lower bound of 3,000 societies is still pretty small considering the size of the Milky Way. If most of the galaxys 250 billion stars host planets, and some fraction of those planets are habitable, you still might estimate there to be millions of civilizations out there. So why do both the strong and the weak hypothesis both produce such small estimates? It all comes dowRead More – Source
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