Do you hear anything? Me neither.

It's pretty deserted out here.

For the past 50 years, astronomers have been using radio dishes

to eavesdrop on the cosmos.

They are listening for signals from alien civilizations.

But so far, it's been pretty silent.

That begs the question: Are we alone in a cosmic desert?

In 1961, astronomer Frank Drake,

came up with a way to approach the question mathematically.

He created an equation that estimates how many alien cultures inhabit our galaxy.

The equation depends on a bunch of factors.

First, the number of new stars born each year,

times the fraction of stars with planets,

times the number of planets that could potentially support life

in each planetary system,

times the fraction of life-supporting planets where life actually takes hold,

times the percentage of planets with life forms that develop intelligence,

times the fraction of intelligent species that develop the ability

to broadcast signals across interstellar space,

times how long those civilizations are likely to survive,

or at least how long they'll broadcast their signals into space.

Multiply all those together and you get "N",

the number of detectable civilizations in the galaxy.

It's a long equation and astronomers only have estimates for some of the factors;

the ones we can observe from Earth,

like how many stars are born each year.

On the other hand, we don't know much about how life develops on other worlds.

Many experts think that alien microbes and slime molds may be common.

But little green men? Not so much.

Perhaps the biggest question is the value of "L" in the Drake equation.

How long will a given civilization last before dying out or self-destructing?

"L" could be hundreds of years

or it could be billions.

Humankind has only used technology; like radio broadcasts for a century or so

and in that short time, we've already had a few close shaves.

So the lifetime of a given civilization could be rather short.

If that's the case, we may be alone in the cosmic desert after all.

For Scientific American's Instant Egghead, I'm John Matson.