Running the Drake Equation: Lex Fridman's Estimate of Alien Civilizations, From 0.7 in the Milky Way to Trillions in the Universe

Lex Fridman centers this solo video on the Drake equation, which multiplies several parameters to estimate how many communicating alien civilizations exist in the Milky Way and in the observable universe. He says the video is less about the numbers themselves than about the mysteries behind the question. He gives three estimates: an optimistic one, a pessimistic one and his own as he feels today.

The parameters are the number of new stars per year, the share with planets, habitable planets per star, the chance life develops, the chance intelligence develops, and the chance a civilization becomes technologically able to communicate. Then comes the lifetime of the communicating civilization. He adds an eighth, not in the original equation: how many times a civilization is born on a planet, goes extinct and is reborn.

He stresses that the optimistic and pessimistic values are not best and worst cases, just reasonable high and low values from recent publications he knows of. His own estimate can swing from day to day, or hour to hour, with his optimism.

For new stars per year, Fridman gives 1.5 as the pessimistic value and three as the optimistic one, and he sides with three. The figure comes from a NASA estimate about five years before the video, with a relevant 2015 paper. He notes that this parameter varies little compared with those later in the chain.

On the share of stars with planets, he says that since about 2012 almost everyone seems to believe that nearly all star systems have them. Someone might argue for a pessimistic value of 90%, or even 30 or 20%, but he thinks it barely affects the result. He says he likes big rocks and gravity.

The real debate begins with habitable planets per star. The optimistic view is that being in a star's habitable zone is simply a matter of distance, and Fridman adds that habitable need not mean Earth-like, since there may be kinds of planets we do not know support life. The rare earth hypothesis instead stacks constraints such as low radiation, high star metallicity and low density to avoid asteroid bombardment. He jokes that to an astronomer, carbon counts as a metal.

Fridman finds the probability of life developing especially exciting because it is one of the biggest open questions within reach of science. He points to possible evidence on Mars, Europa and Venus, where phosphine had recently been reported in the atmosphere. His estimate depends on how optimistic he feels about such discoveries: papers give 13% as the optimistic value and 0.1% as the pessimistic one, and today he says 1%.

The case for a high value is that life on Earth started quickly once conditions were right. The case for a low value is that there is no evidence of life originating more than once on Earth, meaning no very different kinds of ancestor organisms. This parameter stays conditioned on a planet already being habitable.

On intelligence, he sees it as close to inevitable once life starts, given enough time. The optimistic published estimate he saw was 1% and the pessimistic one 0.1%. He did not understand the 1% optimistic figure, so he doubled it to 2%. The open question, he says, is how long stable conditions must persist, and the counterargument is that humans are the only intelligent species among many that have lived on Earth.

Fridman treats communication as the share of civilizations that become technologically advanced, and he sees it as broader than electromagnetic signals. His optimistic reasoning is that complexity keeps growing, so technological advancement seems inevitable. On the pessimistic side, civilizations might isolate themselves or lose interest in exploration, or might communicate in ways we cannot detect. He settles on 20%.

The lifetime of a civilization in its communicating stage is, in his words, one of the most open parameters, and the one that changes the final estimate most. Optimists imagine one or more great filters that, once passed, make a civilization effectively immortal as it spreads and diversifies. The pessimistic case notes that humanity's advanced-technology stage has lasted only about 100 years.

His own estimate is 10,000 years. He is only in the first 100 to 150 years of advanced technology, with innovation accelerating, and he is optimistic because he believes the forces of good will out-innovate the forces of evil. He is uncomfortable with a figure like a billion years because it is impossible to imagine, and jokes that there are 9,900 years left to prove him wrong.

For the extra parameter, how often a civilization is reborn on a planet, Fridman puts the value at three, reasoning that a planet can be billions of years old. The original Drake equation implicitly used one, which serves as the pessimistic value. He then multiplies everything together, treating percentages as fractions.

For the Milky Way the result is 468,000 civilizations for the optimistic estimate and very close to zero for the pessimistic one. His own estimate comes to 0.7. Using the current figure of two trillion galaxies in the observable universe, the optimistic total is about 940 quadrillion, the pessimistic is 300,000, and his is 1.4 trillion.

He finds it entertaining that his estimate lands so close to one. If Earth and the Milky Way are fairly average, humanity may be the only communicating civilization in our galaxy while every galaxy has one.

Fridman's view is that we are not alone, but that we lack the tools and science to understand what it means to communicate with alien intelligence. He thinks aliens are very unlikely to be humanoid and that the variety of life is greater than we can imagine. That variety could invalidate the structure of the Drake equation itself, which rests on many cosmological assumptions.

He cites Carl Sagan's idea that life could exist at very different time and spatial scales, making its signals look like noise to us. Intelligence might operate in other conceptual spaces or layers of abstraction. For humans, communication lives in a space of ideas, but it could instead live in a space of experiences, or in whatever lies behind consciousness, and he muses that consciousness itself might be aliens communicating with us.

He grants that this sounds crazy scientifically, but says we understand little about the basic nature of the human mind. Understanding it could unlock new ways of communicating and a new idea of what an intelligent civilization is. For that reason he sees AI, and perhaps artificial consciousness, as a key to both understanding the mind and building artificial general intelligence.

Most Drake parameters, he says, can be illuminated by science and engineering. Life on Mars would show life is doable elsewhere, and AGI would show intelligence is doable beyond the human brain. Avoiding existential risk would show that survival is doable elsewhere. In a sense, understanding ourselves is one way to search for intelligent civilizations.

Fridman finds the idea of other civilizations inspiring and hopes governments will treat the search as something that unites rather than as a threat or a secret. He says we are humans first and Americans second, curious descendants of apes. Hope alone is not enough, he adds, and we need practical ideas for systems that move beyond the current rivalry between superpowers.

He thinks moving past that rivalry is essential for the survival of the human species, which is the point about extending civilization's lifetime. His thinking goes back and forth depending on how much evil he sees in the world, but for now he is optimistic about the fundamental good in human nature. He closes by saying science is the best tool we have while keeping an open mind to the mystery of the universe and of the human mind.