“Either we are alone, or we are not; either way is mind-boggling” –Lee DuBridge (Astrophysicist and Science Advisor to Pres. Eisenhower).
Is there life elsewhere in the universe? In our galaxy? In 1960 Dr. Frank Drake (Prof. at U. of Calf. Santa Cruz) developed the equation now known as the Drake Equation. This equation attempts to quantify the number of sentient civilizations in our galaxy with whom we might come in contact. The Drake Equation is not supposed to be an actual scientific prediction of alien life, but rather a mechanism to raise discussion around how much intelligent alien life there might be. Here is the equation:
where:
N is the number of civilizations in our galaxy with which we might hope to be able to communicate;
and
R* is the number of stars in our galaxy (actually, the true formula is based on the rate of star formation)
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fl is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
Most plausible inputs to the equation generate numbers greater than one (meaning that there should be at least one other civilization out there with intelligence and ability to transmit communications).
Okay, let’s run this baby. Assume that the Milky Way has 200 billion stars, that 30% of those stars have planets and that there is about one planet per star (that has planets) that lie in the habitable zone and are rocky – meaning that they can potentially support life. Further, assume that a mere 1% of those planets go on to develop life, and then 10% of those planets with any sort of life develop intelligent life. Then 10% of those planets with intelligent life develop a technology that releases detectable signs of life into space and they do so for 100,000 years. The Drake Equation says that with those assumptions there should be about 60 communicating civilizations in our galaxy.
All this raises the question – where is everybody? Carl Sagan proposed that the real problem with finding any other intelligent life out there is the last factor – the length of time that such civilizations release detectable signals into space. Consider that we have only sent signals into space with the first radio broadcast in 1906 – so we have only been broadcasting 101 years. So, our transmitting of signals have only reached a mere 101 light-years away. Our galaxy is 100,000 light-years in diameter and we are in one of the outer arms. The radio signals from another civilization broadcasting for thousands or tens of thousands of years from the other side of the galaxy would not have reached us yet unless they started broadcasting many thousands of years before.
Here is a link to a site that will allow you to plug in your own numbers into the Drake Equation: http://www.activemind.com/Mysterious/Topics/SETI/drake_equation.html
A compelling argument for the existence of life, but an equally compelling argument that communication between us and other civilizations based on our knowledge of signal propagation is pretty remote. Also, consider if we detect a communication form another civilization that was sent several 1000 years ago, how relevant is that communication.