The Drake Equation is a large and fairly vague mathematical system for working out how many other extra-terrestrial life forms there are in the universe that have reached a recognised level of civilization. That is not only other races but ones that meet specific criteria regarding their technology and scientific development. Algae at the bottom of a distance ocean on the other side of the universe my belief but what the equation is interested in is cultures that we would recognise from a distance or better still could come and visit us.

The equation is named after frank Drake and when written out looks like this:

N = N* fp ne fl fi fc fL

The equation can really be looked at as a number of questions:

N* represents the number of stars in the Milky Way Galaxy

Question: How many stars are in the Milky Way Galaxy?

Answer: Current estimates are 100 billion.

fp is the fraction of stars that have planets around them

Question: What percentage of stars have planetary systems?

Answer: Current estimates range from 20% to 50%.

ne is the number of planets per star that are capable of sustaining life

Question: For each star that does have a planetary system, how many planets are capable of sustaining life?

Answer: Current estimates range from 1 to 5.

fl is the fraction of planets in ne where life evolves

Question: On what percentage of the planets that are capable of sustaining life does life actually evolve?

Answer: Current estimates range from 100% (where life can evolve it will) down to close to 0%.

fi is the fraction of fl where intelligent life evolves

Question: On the planets where life does evolve, what percentage evolves intelligent life?

Answer: Estimates range from 100% (intelligence is such a survival advantage that it will certainly evolve) down to near 0%.

fc is the fraction of fi that communicate

Question: What percentage of intelligent races have the means and the desire to communicate?

Answer: 10% to 20%

fL is fraction of the planet's life during which the communicating civilizations live

Question: For each civilization that does communicate, for what fraction of the planet's life does the civilization survive?

Answer: This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th.

When all of these variables are multiplied together when come up with:

N, the number of communicating civilizations in the galaxy.

The real value of the Drake Equation is not in the answer itself, but the questions that are prompted when attempting to come up with an answer. Obviously there is a tremendous amount of guess work involved when filling in the variables. As we learn more from astronomy, biology, and other sciences, we'll be able to better estimate the answers to the above questions.