Enlarge /. The moon can be seen behind an antenna at the location of the Nancay radio telescope on October 3, 2019 near Vierzon, central France.
Guillaume Souvant | Getty Images
When looking for extraterrestrial intelligence, there is really only one big question: where are they all? This question has been pursued by extraterrestrial hunters since Nobel Prize winner Enrico Fermi asked them some of their colleagues over lunch 70 years ago. There are billions of sun-like stars in our galaxy, and we now know that most of them are home to planets. But after decades of searching, astronomers have found none that seem to house life. This is the so-called Fermi paradox: Our galaxy appears to be full of alien civilizations, but we cannot find a single one.
Researchers working on the search for extraterrestrial intelligence (SETI) have proposed a number of solutions to the Fermi paradox over the years. But the most convincing answer is also the most obvious: perhaps intelligent life is far less common than we thought.
How rare? Many scientists have tried to answer this notoriously tricky question. Based on their conclusions, there are between zero and 100 million extraterrestrial civilizations in the Milky Way. This is not a particularly helpful range of estimates, so two physicists in the UK recently made another attempt and came to a remarkably specific conclusion. As described in a new article published this week in the Astrophysical Journal, the duo calculated that there should be at least 36 communicating extraterrestrial civilizations in our galaxy.
That's not much, of course, and it has some depressing effects. According to the paper, this would mean that we will likely have to spend hundreds of years looking for an alien civilization before we find one, and it also suggests that our closest neighbors could be up to 17,000 light-years away. "We have moved from a fairly bullish life in the universe to a more pessimistic one," said Christopher Conselice, astrophysicist at the University of Nottingham and one of the authors of the paper. "I think that's 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 spread of intelligent life in the galaxy date back to the beginning of modern SETI. In 1961, just a few months after completing the world's first radio search for ET, planetary astronomer Frank Drake called a small meeting of leading American scientists to discuss the future of SETI – or whether there should be a future at all. To organize the meeting, Drake made a list of questions that he thought were relevant to determine the likelihood that the search would be successful.
Scientists were able to answer some of these questions – such as determining the average star formation rate in the galaxy and the number of stars that house planets – before the first contact. Others – such as which part of the planet produces intelligent life and how long this life sends messages into space – could only be guessed. But Drake realized that if you multiply 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 safely fill in some of the gaps in the Drake equation, e.g. B. How many stars have planets (most of them) and how many stars form on average in the galaxy (a handful per year). And when a new generation of exoplanet telescopes like the James Webb Space Telescope go online, we also have a better idea of how many of these planets are in the habitable zone of their star. This means that liquid water could exist on these planet surfaces, which, to our knowledge, is a requirement 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 extraterrestrial civilizations in our galaxy is a statistical estimate and, like all statistical estimates, can vary widely depending on the assumptions used. In the Drake equation, about half of the unknowns are about alien civilizations. Since we know nothing about ET, astronomers have to make some guesses. And in their new work, Conselice and his colleague, engineer Tom Westby from the University of Nottingham, make two very big assumptions when revising the Drake equation.
First, the researchers looked at the only planet that we know is known to have created intelligent life – our own – and used it as a model for any other planet that could harbor alien intelligence. Approximately 4.5 billion years after Earth was formed, people appeared and began to spew radio waves into the cosmos. Conselice and Westby therefore assumed that this would also be the case on other earth-like planets. But they went further and assumed that all earth-like planets in the habitable zone of their star would inevitably produce intelligent life after about 5 billion years.
"Saying that all Earth-like planets will produce intelligent life is a big assumption and has some serious problems," said Seth Shostak, senior astronomer at the SETI Institute in California. “The habitable zone of our own solar system includes Mars and – depending on whom you ask – Venus. But they are not populated by intelligent beings, even though they sit around as long as the earth. "
One way statisticians learn about a large, unknown population is to take a small sample and extrapolate to the larger population. This is essentially what Conselice and Westby did in their newspaper. The problem is that they are extrapolated from a sample, which is a bit like trying to predict a national election by just asking yourself. Small sample sizes lead to a greater variance of the results, which is why the Drake equation reliably provides such different estimates of the prevalence of extraterrestrial intelligence. In fact, this was demonstrated by Conselice and Westby in their own work.
The researchers made two hypotheses – a strong and a weak one. In the strong hypothesis, the researchers assume that an earth-like planet must produce an intelligent species if it is between 4.5 and 5.5 billion years old. This is what happened on Earth, where people began to master technology after about 4.5 billion years. The weak hypothesis relaxes the time frame a little and assumes that an earth-like planet can bring life anytime after 5 billion years. Given the average age of the stars in the Milky Way galaxy of around 10 billion years, a larger pool of alien societies is emerging that could still exist today. (This assumes that extraterrestrial societies don't exist for 5 billion years – more on that in a moment.)
The strong hypothesis leads to an estimate of at least 36 alien civilizations in the galaxy, but with a very large error rate. The researchers calculated that the lower limit of the strong hypothesis could be between four and 211 alien civilizations in the Milky Way. With the weak hypothesis that the lowest number of possible alien societies is between 100 and 3,000, things are more hopeful.
That's a pretty big spread, but even the most optimistic lower limit of 3,000 societies is still pretty small given the size of the Milky Way. If most of the 250 billion stars in the galaxy are home to planets and some of these planets are habitable, you could still estimate that there are millions of civilizations out there. Why do both the strong and the weak hypothesis provide such small estimates? It all comes down to one important variable in the Drake equation: L or the lifespan of a broadcasting civilization.
N = L.
"The lifespan of an alien intelligence – how long it stays on the air – is at the heart of the dispute," says Shostak. "All other terms in the Drake equation tell you how many societies are created, but maybe they won't be in the air for very long, because once they invent the radio, they also invent the H-bomb and self-destruction."
In fact, Drake himself has reduced his namesake formula to "N = L", or the number of civilizations in the galaxy corresponds to the lifespan of these civilizations. (He even put that equivalence on his license plate.) So the longer you expect from an intelligent type, the more you expect. Humans have only had radio technology for around 100 years, and during this time we have also created existential threats such as nuclear war and climate change. How long will human civilization last? It depends on how we deal with the dangers we pose to ourselves.
In their work, Conselice and Westby are pessimistic for their second big assumption. In both their strong and weak hypotheses, they assume that all extraterrestrial civilizations will only transmit their existence to the galaxy as long as humans have had radio – about 100 years. This basically assumes we're on the verge of annihilation, but choosing a different estimated lifespan for extraterrestrial civilizations would also be completely arbitrary. Do intelligent civilizations typically last 500 or 10,000 years? The only way to know for sure is to find one.
"Humans have technology for a very, very short period of time in relation to the age of our galaxy," said Andrew Siemion, director of the Berkeley SETI Research Center, who was not involved in the research. This means that if – or when – we contact an extraterrestrial society, chances are that it is much older than our own. "It would tell us that it is possible for a technologically capable civilization to last," says Siemion. "It is very encouraging."
Siemion is the lead scientist on the Breakthrough Listen project, the largest SETI experiment ever conducted. Breakthrough Listen was $ 100 million funded by Russian-born billionaire Yuri Milner and has used some of the world's most powerful radio telescopes to search the cosmos for signs of life in recent years. If we ever find extraterrestrials, Siemion and his colleagues are the most likely to do it. While he recognizes that trying to estimate the prevalence of extraterrestrial life is an interesting thought experiment, there is no way to determine whether one estimate is better than another until we get in touch.
"No argument from the start should serve as a substitute for an experimental program," says Siemion. "The only way to ever answer that question is to look for extraterrestrial intelligence."
This story originally appeared on wired.com.