It may seem fantastic to assume that advanced alien civilizations are beaming signals across the cosmos, but the possibility has intrigued respected astronomers enough that several have taken up the task of finding those messages. While SETI opponents say that technologically advanced life is extremely rare in the universe, there is also a popular belief that there are simply too many stars and planets for life to be limited to one planet, Earth. A search, therefore, would be justified. Optical SETI is a particular branch of the search that involves setting up ground-based telescopes to look for laser signals hitting our planet. The telescopes are set up to look for a continuous, powerful beam or a laser that pulses every billionth of a second.
Lasers have the advantage that they can send a lot of information.
You could send your whole Library of Congress, all your music, poetry, literature, science, medicine. The search for lasers is only one method in a broader SETI search. Early searches, dating back 40 years, involved scanning the sky for radio signals. Those kind of searches continue to this day, involving some of the world's most powerful radio telescopes. Radio signals carry less information than laser signals, but searching for them takes advantage of the possibility that even if there has been no attempt by aliens to communicate with us, advanced civilizations might inadvertently broadcast radio signals during the course of their own intraplanetary communication.