Insight | Are we alone? Four scientists split on how common alien life is
Adam Frank argues there is no Fermi paradox and that we are probably not the first civilization, while Brian Keating and Sean Carroll suspect the silence means life, or at least civilizations, is rare, and Sara Walker wonders whether advanced life could even perceive other advanced life. Seeing the same evidence read four ways shows how much of the alien question still rests on assumptions rather than data.
Nobody knows whether we are alone. Four scientists look at the same planet counts and the same radio silence and reach four different answers, because the data are thin and each answer still rests on assumptions about things no one has measured.
Astrophysicist Adam Frank is the optimist. He says the planet question is now settled: essentially every star has planets, and about one in five has one in the habitable zone. With roughly 10 billion trillion such planets in cosmic history, humanity is the only technological civilization ever only if the odds per planet are astonishingly low. That does not mean anyone is nearby or still alive. He also argues that the silence proves little. Radio searches have covered the equivalent of a hot tub in an ocean, and if civilizations have finite lifetimes, the galaxy would contain large empty regions for millions of years. His gut feeling is that simple life is common, while intelligent life may be sparse. He calls much of this armchair reasoning until we go and look.
Cosmologist Brian Keating thinks life is probably very rare. There is no evidence of it anywhere else, he says. Mars had flowing water and has received rocks from a living Earth, yet shows no life so far, which he reads as a sign of some impediment, while conceding Mars has not been fully searched. A vast number of planets shows room, not likelihood: "possibility is not probability."
Physicist Sean Carroll also leans toward absence. If civilizations were easy to make, he would expect very noticeable ones, and the simplest explanation for why we have not seen any "is that they're not there." Life might simply take far longer to arise than the universe has existed. He doubts that every civilization destroys itself. He would look for a parked probe before a radio signal, and he thinks communication would be solvable through shared math and physics.
Astrobiologist Sara Walker asks whether advanced life could even perceive other advanced life. She raises the idea of a great perceptual filter and says she is not sure she agrees with it. Her reminder is that humans missed microbes until the microscope. She believes solving the origin of life and finding alien life are the same problem.
They agree that there is no evidence yet, that planets are abundant, and that simple life and civilizations are separate questions. Frank and Walker both call the origin of life the key unknown. The core split is what the silence means. None of the four treats UFO reports as evidence.
The article's caution is that every position rests on an unmeasured quantity: how easily life begins, how long civilizations last, whether they expand, or whether we could recognize them.
What to do with it: separate the questions before arguing. When someone gives a confident number of alien civilizations, ask which unknown they filled in. Ask how much has actually been searched before reading anything into silence. Watch origin-of-life research and studies of exoplanet atmospheres, where Frank expects hard data within 10 to 30 years. Hold new claims to a high standard of evidence, and notice your own leanings.
