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.
Why do four scientists reach four different answers about alien life? Show answer
The data are thin, so each answer depends on assumptions about unmeasured things, such as how easily life starts and how long civilizations last.
Which parts of the alien life question does Adam Frank say are now settled? Show answer
That essentially every star has planets, and that about one in five has a planet in the habitable zone.
What is Frank's pessimism line? Show answer
With about 10 billion trillion habitable-zone planets in cosmic history, we are the only civilization ever only if the odds per planet are extraordinarily small.
What does Frank's argument not claim? Show answer
It does not say any civilization is nearby or exists now. It concerns whether one has ever arisen anywhere.
Why does Frank say the silence proves little? Show answer
Searches have covered a tiny fraction of the possibilities, and civilizations with finite lifetimes would leave long-lasting empty regions.
What is Brian Keating's Mars argument? Show answer
Mars had liquid water and received material from a living Earth, yet shows no life so far, suggesting an impediment. He admits Mars is not fully searched.
How does Keating answer the argument from the huge number of planets? Show answer
Vast room shows that life is possible, not that it is probable. He sees it as filling a gap in knowledge with an assumption.
What is Sean Carroll's reading of the silence? Show answer
If civilizations were easy to create, very noticeable ones should exist. The simplest explanation is that they are not there.
What is Carroll's time argument? Show answer
Even if life always arises eventually, it might typically take far longer than the age of the universe, leaving us alone for now.
What search strategy does Carroll favor? Show answer
Looking for a quiet probe parked in our solar system, since a radio signal must arrive during the brief time we are listening.
What is Sara Walker's great perceptual filter? Show answer
A speculative idea that highly advanced life becomes so abstract that it cannot perceive other advanced life. She says she is not sure she agrees.
What do Frank and Walker both identify as the key unknown? Show answer
The origin of life. Walker thinks first contact with alien life may come in an origin-of-life experiment.