The Drake Equation

For several years all of my computers have been running the SETI@home project. This means that any spare CPU power is used to try and find aliens.

No, I’m not joking.

The program downloads data collected by a massive radio telescope in Puerto Rico, and analyses it to try and find a signal from the stars. Since the project began in 1999 there have been a few candidates, but no joy.

So where the hell are all the little green men, and why aren’t they talking?

One of the pioneers of the search for extra terrestrial intelligence (SETI) is Frank Drake. Among other things he is famous for creating what is now known as the Drake Equation. This equation predicts the number of our galactic neighbours capable of communicating with us at any given time.

The equation is stated on wikipedia as:

N = R x fp x ne x fl x fi x fc x L


N is the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time


R is the rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is 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
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of the above that are willing and able to communicate
L is the expected lifetime of such a civilization

If that all sounds like a bunch of gobble-de-gook then let me explain it using a different example.

N = R x fp x ne x fl x fi x fc x L


N is the number of good looking people who would actually go home with you on a saturday night


R is the number of towns in your area
fp is the fraction of those towns that have pubs
ne is average number of pubs that actually have customers
fl is the fraction of the above that are of your sexual preference.
fi is the fraction of the above that are good looking
fc is the fraction of the above that are drunk enough to actually go home with you
L is the expected time they would spend in the pub before being taken to emergency for alchohol poisoning

Put simply the Drake Equation gives us the likely number of our galactic neighbours we could “pick up”. As it turns out, that number is very high, or very low, depending on what values you plug in to the equation. This is where it gets tricky – those values vary greatly depending on who you ask.

Perhaps the most disputed parameter is L – the average lifetime of your average galactic civilisation. Apart from natural disasters and thermonuclear annihilation, other factors may influence this value. Some have argued that L should be limited to the amount of time a civilisation broadcasts or receives radio frequencies, after all, that is how we are looking for them with projects such as SETI@home. Our own civilisation has been listening to extra-solar radio waves since Grote Reber‘s work in 1938.

This makes things interesting, because every second that we are using radio waves increases the value of L. With this in mind I created a Drake Equation Clock of sorts – calculating L as the amount of time that has elapsed since 1938, and using very pessimistic values for the other parameters:

Current estimate of N: 0.00000005349419786340786
Years until N reaches one: 1282051213


var timerID = 0;
var strDivCurrentN = “Current estimate of N: “;
var strDivYearsToN1 = “Years until N reaches one: “;


function Start()
document.getElementById(“YearsToN1”).innerHTML = strDivYearsToN1 + Math.round(Estimate_NumYearsTillNOfOne());
timerID = setTimeout(‘UpdateEstimate()’, 1);

function Stop()
timerID = 0;

function UpdateEstimate()
document.getElementById(“CurrentN”).innerHTML = strDivCurrentN + NumberToStringNoExponent(Estimate_N());
timerID = setTimeout(‘UpdateEstimate()’, 1000);


As you can see, we have a very long way to go (unless you happen to be reading this in the year 1282049207 AD). In contrast, the more optimistic values Frank Drake suggests give us a number of more than ten thousand. This uncertainty does not devalue the equation’s worth, on the contrary – finding out the correct values of those parameters is more important than ever.

Ultimately the Drake equation is so simple, logical and obvious that it is hard to refute it. The only thing you can question are the numbers you plug into it – and people will be arguing about them until ET really gives us an interstellar booty call.


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