We hear about aliens all the time, whether in books or movies or tv shows, and this leads us to ponder our own existence. Are we alone, or is there other life out there? This is a question that has plagued mankind since begun exploring interstellar space, and started looking for life in our own Solar System. If it turns out that life is actually out there, it is natural to wonder if there is other intelligent life in the universe. Would there be other human civilizations, or is Earth the only planet in the entire universe that harbors intelligent life, and we really are all alone. And even if there are other human civilizations out there, how would we find them? Scientists now speculate that we might be able to detect other civilizations simply by listening for signals that they are sending into interstellar space. By this method, efforts to search for extraterrestrial intelligence can only succeed if advanced civilizations are broadcasting signals right now.
The astronomer Frank Drake wrote a simple equation that summarizes factors that would determine the number of civilizations we might be able to contact. This equation, known formally as the Drake equation, gives us a simple way to calculate the number of civilizations in our galaxy that are sending signals into outer space. The Drake equation relates the number of possibly habitable planets in our galaxy, fraction of habitable planets that actually have life, fraction of life-bearing planets on which a civilization capable of interstellar communication has arisen at some time, and the fraction of life-bearing planets that are communicating now. In short, the Drake equation tells us how many civilizations we could possibly contact right now. Because we do not actually know how many possibly habitable planets there are in our galaxy, we cannot accurately calculate the number of civilizations that are communicating right now. However, the Drake equation is still useful as it relates all of the factors necessary for us to determine the number of communicating civilizations, and we can therefore make a general estimate of the number of civilizations we could possibly contact today.