In November 1991, the National Radio Astronomy Observatory, located near Green Bank, West Virginia, USA, held an academic symposium on intelligent life outside the land. The American astrophysicist Drake proposed a famous equation, which was later called the "Green Bank Formula". This was the first attempt to make a quantitative analysis of the exploration of intelligent life outside the earth. Drake proposed the "Green Bank Formula". The formula" is like this: N=Rnefpnflfi fe L formula, N represents the number of detectable technological civilization stars in the Milky Way, which depends on the product of the 7 numbers on the right side of the equation; R represents the star formation of the sun-like stars in the Milky Way Rate (that is, the average number of stars born each year), it is generally believed that only stars like the sun can breed intelligent life; ne is the star that may carry (life) planets, and its ecological environment is suitable for life. Average number of planets in
fp represents the number of stars that may have biological existence (some call it "good sun"). In other words, "good sun" generally refers to those with constant luminosity that can shine for a long time to meet the needs of the formation of intelligent life evolution. The star; fl is the share of planets that have already had life in the planets that may have life; fi is the number of planets that have already had intelligent life, because the probability of low-level life evolving to intelligent life is after all very small; fe It is the share of planets with advanced intelligent life (such as interstellar electromagnetic wave contact) among these planets that have intelligent life; L represents the average lifespan (or continuation time) of a civilized world with advanced technology , Because only civilized planets that have been developing for a long time can do interstellar visits
The Green Bank formula is expressed in the form of a product. The exact size of these factors is currently unknown. The importance of each factor in the formula is the same. Some factors can take approximate values (such as R), and some factors are purely subjective (such as L). Some scholars believe that in addition to L, the product of the other factors gives the yield of detectable civilizations in the galaxy. Using the rough estimate of the lowest value for human calculation, N=40 can be obtained; using the maximum possible value of each item, Then N=50 million, that is to say, the number of advanced technological civilization planets in the galaxy is 400,000 to 50 million.
According to his own opinion, the famous American science writer Asi Inf put forward a formula similar to the Green Bank formula and estimated that there are approximately 530,000 civilized planets in the Milky Way, that is, every 1 million stars in the Milky Way. On average, there may be 18 high-tech civilized worlds.