SETI is an acronym for Search for Extra Terrestrial Intelligence. The possibility that intelligent life might exist elsewhere in the universe arises from the fact that the universe is made up of billions and billions of galaxies each containing billions of stars. Even if small fractions of this large number of galaxies have planets with conditions suitable for the appearance of life, there could be several million planets with intelligent life capable of communicating with us using radio signals which are the simplest media of communication. Also radio waves are capable of travelling across astronomical distances carrying substantial information.
However, most of the stars are so far away from us that even at the speed of light it will take several tens or hundreds or even thousands of years for a radio signal from a planet of a distant star to reach the Earth. Another problem is, because of the large distance even the strongest signals would have become extremely weak by the time they reach us. But astronomers are hopeful. They are using very large antenna to collect radio signals however weak from alien civilizations. Scientists at the National Aeronautics and Space Administration (NASA) who launched SETI in 1993 are sending out powerful bursts of coded messages to outer space in the hope of getting a response. But it is too early to say if their efforts will bear fruit.