Scientists have claimed that if we ever do find a signal from an advanced alien civilization, those aliens are likely to be dead by the time we hear it.
Published on arXiv, the team examined how long civilizations might stick around, and also how far away from us they are likely to be.
It’s thought there could 20 to 40 billion Earth-sized worlds in the Milky Way with habitable conditions. However, we can’t yet estimate how many of these could have spawned life so far, nor if they have been around long enough to “retain liquid water during Darwinian time scales,” the researchers write.
The key is the lifetime of a detectable technological civilization. In the famous Drake Equation, devised by SETI astronomer Frank Drake in 1961, this is the value L. Estimates for this lifetime range from 106 to 12,100 years or more.
The authors, which include Drake himself, note that humanity has only covered 0.001 percent of the Milky Way with signals in 80 years. Even at the upper limit of the lifetime for civilization, we will not have covered the entire galaxy.
“According to this estimate, SETI should detect on more than 120 different signals,” the researchers write, noting this is “a bounty compared to the present zero.”
But if any civilization has sent a signal, and has lasted less than 100,000 years – the width of the galaxy in light-years – then the chances of finding a civilization from a living galaxy are incredibly small.
“If the civilization emitted from the other side of the galaxy, when the signal arrives here, the civilization will already be gone,” Claudio Grimaldi from the Federal Polytechnical School of Lausanne in Switzerland, lead author on the study, told Science News.
They note that when a civilization stops emitting, their signals will continue traveling out in a “shell wall”, a sort of echo from the dead civilization. And it might be possible to spot these, but the odds are not necessarily in our favor of finding someone alive.
“The transmissions arriving at Earth may come from distant civilizations long extinct, while civilizations still alive are sending signals yet to arrive,” they write.
Still, the big factor here is we just don’t know how likely life is to arrive on a planet. There are plenty of potentially habitable world within 80 light-years of Earth, close enough to have received a signal from Earth.
Was intelligent life on our planet a fluke, though, or is it widespread? Until we answer that question, it’s difficult to jump to any conclusions just yet.