How many extraterrestrial civilizations can exist nearby?

If aliens exist, why don't we "hear" them?

In the 12th episode of Cosmos, which aired on December 14, 1980, co-author and host Carl Sagan introduced viewers to the same equation of astronomer Frank Drake. Using it, he calculated the potential number of developed civilizations in the Milky Way that could connect with us using the extraterrestrial equivalent of our modern radio communication technology. Sagan's estimate ranged from "pathetic few" to millions. If civilizations do not always destroy themselves soon after the opening of radio astronomy, then the sky can literally buzz with messages from the stars, - said Sagan in his inimitable manner. Yet Sagan was pessimistic that civilizations were able to survive their own technological "Youth" - a transitional period when the development of a culture of, say, nuclear power, bioengineering or a myriad of other powerful possibilities could easily lead to self-destruction.

Life in the Universe

Sagan and other scientists have speculated that the appearance of life on planets should be cosmic inevitability, because according to geological data, it originated on Earth amazingly quickly: more than four billion years ago, almost immediately after our planet cooled down enough. And if, as in our world, life on other planets emerged quickly and evolved, becoming increasingly complex over time, perhaps intelligence and technology could also be ubiquitous throughout the universe.

In recent years, however, some skeptical astronomers have tried to give more empirical weight to such statements, using what's called Bayesian statistics. But even with such estimates, astronomers disagree on what they mean for life in other parts of the cosmos.

Drake's equation, introduced by an astronomer in 1961, calculates the number of civilizations in our galaxy that can transmit or receive interstellar messages using radio waves. It is based on the multiplication of a number of factors, each of which quantifies some aspects of our knowledge of the galaxy, planets, life and intelligence. These factors include: stars with exoplanets; number of habitable planets in the exoplanet system; the number of inhabited planets on which life is born and so on.

Maybe we'll never know if there's life beyond Earth.

Today we know that the worlds around stars are the norm, and Earth-like worlds are a common phenomenon in the universe. However, one of the biggest uncertainties in the whole chain of factors is the likelihood that life originating in other worlds is making a leap from chemistry to life. Ignoring this uncertainty can lead astronomers to make rather bold statements.

For example, astronomers at the University of Nottingham in England recently made headlines when they calculated that there should be at least 36 intelligent civilizations in our galaxy capable of communicating with us. However, it is difficult to answer questions about the probability of abiogenesis and the emergence of intelligent life, because scientists have only one information: life on Earth.

Another problem with assumptions based on what we see locally is the so-called selection bias. Imagine that you buy lottery tickets and win with the 100th attempt. In this case, it would be reasonable to designate the probability of winning the lottery as 1%. This incorrect conclusion is, of course, the bias of selection that arises if you only poll the winners and none of the losers (i.e. tens of millions of people who bought tickets but never won the lottery). When it comes to calculating the probability of abiogenesis, exactly the same thing happens, because scientists simply do not have information about all the worlds where life never appeared.

Probability of abiogenesis

If you apply Bayes' theoremo to calculate the probability that an event, such as abiogenesis, will occur, astronomers first come up with a probabilistic distribution of this event - a better guess if you like. For example, it can be assumed that abiogenesis is as likely between 100 and 200 million years after the formation of the Earth, as it is between 200 and 300 million years after that time or any other 100-million-year period of our planet's history. Such assumptions are called Bayesian ap priori,. Statisticians then collect data or evidence and combine previous and actual data to calculate the posterior probability.

The answer to the question of how widespread life in the galaxy remainsUnknown

In 2012, astronomers from the Princeton Institute for Advanced Studies were the first to be abiogenesis. According to their approach, life on a planet like Earth orbiting a star like the Sun does not occur until some minimum number of years (tmin) after the formation of this world. If life does not arise before some maximum time (tmax) then as its star ages (and eventually dies), the conditions on the planet become too hostile for abiogenesis.

However, it is not without its drawbacks. For example, some researchers question the assumption that intelligence originated at a certain time after abiogenesis. This a priori can be another example of the bias of selection, a concept influenced by the evolutionary path by which our own intellect originated.