In 2006, Columbia University professor David Helfand gave a talk on the modern interpretation of Drake's equation. This equation is theorized to represent the number of intelligent societies in our galaxy. At the time of its creation, the equation had many unknown terms. Since then, many of these have been scientifically and statistically found to great accuracy. Helfand's talk on the Drake equation is informative and easy to follow. If you've got a half-hour to spare, I would definitely recommend it. Video embedded below (some feed subscribers may need to visit the website).