"But!" He straightened up. "We will now tackle the equation itself. Here it is." He tapped the blackboard once more.
N = R* x fp x ne x fl x fi x fc x L
"First of all, the obvious.
N is the end result, the number of alien civilisations with which we might be able to communicate." He tapped at the letter with a finger. "The other letters are more or less those factors we already discussed earlier." His hand slid lower to indicate the list the students and he had put together. "
R* is the rate of star formation. And then, one after the other, stars with planets, planets that can support life, planets that actually
have life, planets with intelligent life, that's
fi,
fc is the intelligent life that sends out signals into space, and finally,
L is the length of time an alien civilisation actually sending signals."
He paused. This sounded very confusing, but, with the equation on the blackboard, hopefully not impossible to understand. "There is one more subtlety here that I need to talk about. These values are all built upon each other. We sort of neglected this in
our discussion, but the original Drake equation did not, which complicates things. See the little
f signs? This means we are talking about
fractions. For example, if you take the first fractioned value,
fp, then this is the fraction of
all stars that have planets, not the number of stars with planets itself."
He paused again. "And so, further on,
fl isn't just the number of planets with life, it's the fraction of
all planets, and
fi is not the sum of all intelligent life in the galaxy but the
fraction of planets with any kind of life in them. This makes figuring out the values even more difficult since, as I said, they all rely on each other."