Where are all the extraterrestrials? Struggling and jostling, as we

If the galaxy is billions of years old and it took a few decades for humans to visit the moon and launch space stations, why did a single alien ship land on the White House lawn?

We must be the only beginner thieves in the cosmic neighborhood, as various astrophysicists have suggested for decades. But a new analysis of a team led by astrophysicist Adam Frank of the University of Rochester suggests a more nuanced resolution of the old paradox. Sweeping the Milky Way and establishing a unified galactic empire might be inevitable for a monolithic super-civilization, but most cultures are neither monolithic nor superb – at least if our experience helps us. Space ships fail. Hyperactive stars roast otherwise desirable targets. Economic crises and government closures kill the nascent spaceships before they take off.

Using a blend of theory and simulation published last week in the peer-reviewed PeXiv paper database, Frank and his colleagues explore the common ground between a sterile and overflowing galaxy – a civilization where some civilizations can succeed in becoming multistellar without establishing a spatial and temporal hold on the entire Milky Way.

Astronomers are just beginning to look for signs of technology in the stars (few people feel discouraged by the silence to this day) of seawater). What makes optimists stay awake at night, however, is the argument developed for the first time in 1975.

Even at the snail's pace, the galaxy is so ancient that any technological species should have enough time to radiate to all the stars of the Milky Way. Yet we see no evidence of past or present foreign cities on Earth – an observation dubbed "Fact A". Academics have flipped ink bottles to try to solve this puzzle, known as the Fermi Paradox, with explanations ranging from humanity in a reserve to all those who are waiting in hibernation for a universe which will cool their computers better.

Many of these solutions are based on assumptions about extraterrestrial behavior, which Frank never liked: "One of the things that has always bothered us when we talk about the Fermi paradox is that there was always so much imaginary exo-sociology.

Instead, Frank and his collaborators have built the most comprehensive model ever conceived to try to subtract fiction from science and answer a question: of all the possible galaxies in which we could live, what types are compatible with the made in?

A number of features distinguish this analysis. On the one hand, the researchers included in their simulations the fact that the stars really move. "Even if you do not have a ship, you're traveling free to the galaxy," says Jason Wright, co-author and astronomer at Penn State University. "Every 100,000 years or so, a new star is the closest, so jump to that one."

The team calculated that for a civilization using ships several tens to thousands of times faster than our old Voyager probes, the drift of stars shortened the time it would take to spread in the galaxy to a few hundred million dollars. years. In other words, if you reduce the galaxy's existence in a calendar year, anyone launching a ship by Jan. 7 could have reached each star system by Jan. 14. Now, on December 31, they are definitely late.

This fast passage time deepened the mystery of fact A until researchers add two more verifications of reality. First, as Frank says, it can be difficult to find good planets. Some stars do not have planets. Many planets may not meet your needs. Or, your dream planet can already be occupied.

Second, no habitat can last forever, as humanity quickly achieves. Spreading to other stars can extend the life of a civilization, but from the moment a new colony is born, its days are counted. It can last for hundreds of thousands, even millions of years, but at some point a disaster will almost certainly.

By taking into account a range of settlement lifetimes, appropriate planetary fractions, recharge times between launches and other factors, the team calculated the galaxy's share of billions of galactic stories. potential and found three broad categories. The first two represent common-sense solutions to the Fermi paradox: if there are many good planets and survival is easy, the galaxy should be brimming with life. If the opposite is true, no one goes very far.

However, many of their simulations fell into a third category – a Milky Way that remains partially resolved indefinitely. Fast speed in the galaxy by fast waves can be easy, but administering this territory eventually is almost impossible. Settlements extinguish, separate, and vast expanses of space become uninhabited again. Some are relocated, others not.

"You can end up with this loose network of settlements," says Wright, "where the whole galaxy is installed but where a given star at one point might not be."

We may be in one of the areas that has not been affected for at least millions of years (the authors point out that if the Earth were hosting a colony deeply rooted in its geological past, there would be no sign of today). If this is the case, fact A does not require any explanation.

Some still prefer other ways to solve the Fermi paradox, such as Anders Sandberg, a researcher at the Institute of the Future of Humanity in Oxford, England, who suggests that life could be extremely rare. He would like Frank's simulations to be expanded to cover a wider range of possibilities. "It's a lovely model," Sandberg wrote in an e-mail, "but the authors limit themselves to a rather limited corner of possibilities."

Astronomer Jill Tarter, whose work inspired Carl Sagan's novel Contact, praised the team's abilities but wondered how much theoretical analysis can go. "These are smart people who probably have understood the calculations," she wrote in an email, "no matter what it means in the absence of data."

The authors agree that no theory can replace real research, but this analysis reinforces their optimism about the possibility that technological life exists in our galaxy, as well as their belief that the astronomical community must go get her. Frank highlights the explosion of known planetary systems, with nearly 4,000 exoplanets discovered since 1992, and looks forward to researchers developing the ability to study these worlds in detail.

"We're going to look at these atmospheres and see if they have oxygen and methane," Frank explains, "and we risk tripping over a technosignature. For the first time in millennia, we discuss life on other planets. We live in a time when we will actually have data. "