Nearly 50 years ago,

a young researcher called Frank Drake led

a discussion at a meeting at

the Green Bank Radio Observatory.

It was an early discussion group

on the possibilities of life in the universe.

He went to the board and sought in real time

some way of organizing this difficult subject,

which involves many disciplines.

The equation he wrote on the board has

become a basis for astrobiology,

an organizing principle that most people recognize.

It's the way of calculating N,

the number of intelligent communicable civilizations

in our galaxy at any time.

Notice those modifiers.

We're looking for intelligence,

we're looking for technology,

and the ability to communicate,

and we're looking for some contemporaneous existence,

where they exist with us such that we can communicate.

Notice also, we're only talking about the Milky Way.

Logically, whatever the number N is,

we must multiply it by 100 billion for the number of

intelligent communicable civilizations in the universe.

But we assume that those are mostly at

distances far too far to even imagine communication.

What are the factors of the Drake equation?

The first factor is the rate at which

new stellar systems that could host planets

are being formed in the Milky Way,

the star formation rate of Sun-like stars.

This number is a few,

three to five, and it's well-determined by astrophysics.

The next number is the fraction of

those stars that have planets of any kind.

Exoplanet searches are pinning this number down.

This fraction is going to be close to one.

Already it's known to be

more than about 20 or 30 percent.

The third number is the number of

earth-like systems within each exoplanet system.

This is the subject of intense

investigation in astrobiology.

We don't know the answer,

but it's looking like N is one or two.

There are typically a couple of

earth-like planets in each solar system out there.

These numbers are all either determined or

about to be determined by astronomical observation.

The next factors we'll recognize as

completely indeterminate or uncertain.

The next one is the fraction of

those earth-like planets that have life.

We don't know what that fraction is.

We might imagine if a planet is habitable,

it's almost inevitable that it has life,

and so that fraction is close to one.

But some biologists argue that that's not the case.

That life's formation on the earth involved

some unlikely steps and so that fraction is small,

maybe only one percent or less.

We don't have the data to decide until we find life

elsewhere and get some statistical basis.

The next fraction is the fraction

of those planets where life form,

where an intelligent species eventually develops.

Again, we have no idea of that fraction.

Some argue that it's inevitable because it happened here,

others argue that it took a long time

or it involves a lot of contingency in evolution,

and so it's a low number.

There's no way to decide.

The next to last fraction is the fraction of

those intelligent species that develop

the ability to communicate or travel in space.

On the earth, we have a handful

at least of intelligent species,

only one of which, us have the ability to do that.

But again, we have no idea what

that fraction is in the general case.

The final number is critical because it involves time.

It's the lifetime of

that hypothetical civilization

in their communicative state.

Since N is proportional to L,

the entire product is driven

by L. If L is a small number,

then N may be a small number.

If N is large,

N may be large, what is L?

For us, it's hard to tell.

We've only had the communication ability in

space or to travel for 50 years,

say, 100 as a round number,

but people looking at human history

will realize we nearly extinguished ourselves

in that time when there were 40,000 nuclear weapons

in the world and the Cold War was in its darkest days.

So if human civilization with

intelligence and technology is unstable,

we don't know what the cosmic average of this number is.

Perhaps, civilizations always are unstable,

or perhaps where there are exceptions,

the immature ones who almost self-destructed.

When you get through your teething adolescence,

you live forever or for millions of years.

You can see the huge uncertainty in the Drake equation.

It's a simple trope of mathematics,

that the product of a series of numbers is as

uncertain as the largest uncertainty.

So the fact that we know

the first three factors through astronomy increasingly

well doesn't mean that the uncertainty

goes away for the factors where we know nothing.

Even Frank Drake recognized

the limitations of this equation,

and he called it a container for ignorance.

It is also worth pointing out logically that it's

thick number of intelligent communicable

civilizations in the Milky Way.

If some civilizations live a very long time,

they could have the possibility

to travel between galaxies.

At one tenth light speed,

the trip to Andromeda is perhaps 20 million years.

For newly eternal civilization,

this is not a long time,

and it's a small fraction of the age of any galaxy.

We're heading into extremes of speculation

here talking about the longevity of civilizations.

But it's also important to point out that

the Drake Equation involves an average,

and that the possibilities of

intergalactic civilization and communication may

depend on the longest lived members of

that club rather than a short-lived members.

You can see that the Drake equation

conceptually is a series of subsets.

There's the subset of

all the stars that have earth-like planets,

the subset of those that host life as

opposed to the ones that are sterile or stillborn,

and then the subset of those

where life develops sentience,

and then intelligence, and eventually technology.

As we winnow down

the very large number of sites for life,

the initial real estate

from the habitable planets searches,

we could end up with a very small number.

Logically, it's possible that N is one,

where it we're unique in the galaxy

as an intelligent technological civilization.

Also logically, and some have argued like Carl Sagan,

that N is more likely to be 100,000 or a million,

in which case, there's a large set of

potential Pen Pals out there.

Sociologists, and psychologists,

and evolutionary biologists have

also weighed in on this subject even though it's

mostly governed by an astronomy arguments.

Because when we're talking about

the biological and cultural aspects of the equation,

we have to do that kind of analysis of our situation.

But the uncertainty of the Drake

equation doesn't go away.

In the end, we simply have to do the experiment and look.

It's striking and

probably a coincidence that Thomas Wright,

a physicist living in England in the middle of

the 18th century speculated about

the size of the Milky Way Galaxy during

a time when Herschel was

mapping it out with his telescopes.

He came up with an estimate of

170 million habitable worlds in the universe,

which at that time was the galaxy.

This is actually close within a factor of two,

and certainly an order of magnitude of

the true modern astronomical estimate.

So people have been thinking in

these terms for centuries.

There are more sophisticated ways of

thinking about the Drake equation,

and there's a theoretical construct which stops

using the binary on off life exists or it doesn't,

it's intelligent or it's not,

and takes into account the contingencies of evolution and

the fact that all of these attributes

will exist on a spectrum,

and that some of the relationships

between the factors are not independent.

They are coupled. This is called a

Bayesian framework for estimation.

It's important that we use

as few prior assumptions as

possible in studying the Drake equation.

Either way, there's an enormous possibility for

intelligent civilizations in the universe.

The Drake equation seeks to organize our knowledge

and our ignorance of astrobiology in one formalism,

a simple equation to calculate N,

the number of intelligent communicable civilizations

at any time in one galaxy, ours.

The first few factors of

the equation are astronomical and they're

being determined by current and future observations.

The latter factors are completely

uncertain because until we find life elsewhere,

we won't know how likely it is for life to develop on

a habitable planet or subsequently

evolve to intelligence and technology.

Finally, the Drake equation is proportional to

the longevity of a civilization

in the communicable state,

and that number is also completely uncertain.

If N is a large number,

and there are many durable civilizations out there,

then we will have many potential Pen Pals.