TechBook

Link: http://news.bbc.co.uk/2/hi/science/nature/7870562.stm

Duncan Forgan, a researcher at the University of Edinburgh has calculated that there are somewhere between 361 and 37,964 intelligent civilizations in our galaxy. But how does a scientist come up with numbers like that and get away with it? On the face of it, those just look like arbitrary numbers. Even worse - they're not even very precise, we're talking 2 orders of magnitude here. Well I'm here to tell you what our friendly BBC article doesn't.

This is the infamous Drake Equation. Looks fancy doesn't it? OK, so here's whats actually going on:

• is the rate of suitable star formation in our galaxy
• is the fraction of those stars that have planets
• is the average number of planets that can potentially support life per star that has planets
• is the fraction of the above that actually go on to develop life at some point
• is the fraction of the above that actually go on to develop intelligent life
• 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

So you take each of these numbers, multiply them all together, and you get the number of civilizations in our galaxy that we might be able to communicate with. Wonderful! But that doesn't explain why Dr. Forgan gave such a wide range of values, after all this equation is a function and only gives one value per set of inputs. Well the problem is that we don't know all those inputs. In fact, in order to know all of the above, not only would we have to have some contact with other civilizations, we'd have to have contact with more civilizations than the equation predicts!

So why on Earth make this equation in the first place? Because we're curious, and the scientists who came up with it wanted to all get together and have a party, but wanted someone else to pay for all the drinks. In any case, it lets us stick values into it and see what would happen based on various guesses that we make. Dr. Forgan simply put several guesses into the magic equation and out popped those numbers. These guesses assumed different things, for example in one scenario he assumed that it was easy for life to form, but hard for it to evolve. In another, he assumed that it was hard for life to form, and easy for it to evolve.

What makes this headline news? Well its got new estimates for the variables, but other than that, nothing really. Its just an interesting tidbit. If you want to try it out with your own values, there is a calculator here. Dr. Randall Munroe has proposed adding another variable, , which you can read all about here.