0:01

Nearly 50 years ago, a young researcher called Frank Drake

Â led a discussion at a meeting at the Green Bank Radio observatory.

Â It was an early discussion group on the possibilities of life in the universe.

Â He went to the board and sought in real time some way

Â of organizing this difficult subject, which involves many disciplines.

Â The equation he wrote on the board has become a basis for

Â astrobiology, an organizing principle that most people recognize.

Â It's the way of calculating N, the number of intelligent,

Â communicable civilizations in our galaxy at any time.

Â Notice those modifiers.

Â We're looking for intelligence.

Â We're looking for technology and the ability to communicate.

Â And we're looking for some contemporaneous existence where they exist with us such

Â that we can communicate.

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

Â Logically, whatever the number N is, we must multiply it by 100 billion for

Â the number of intelligent, communicable civilizations in the universe.

Â But we assume that those are mostly at distances far too far

Â to even imagine communication.

Â What are the factors of the Drake equation?

Â The first factor is the rate at which new stellar systems that could host planets

Â are being formed in the Milky Way.

Â The star formation rate of sun-like stars.

Â This number is a few, three to five, and it's well-determined by astrophysics.

Â The next number is the fraction of those stars that have planets of any kind.

Â Exoplanet searches are pinning this number down.

Â This fraction is going to be close to one.

Â Already, it's known to be more than about 20 or 30%.

Â The third number is the number of Earth-like systems within each exoplanet

Â system.

Â This is a subject of intense investigation in astrobiology.

Â We don't know the answer, but it's looking like N is one or two.

Â There are typically a couple of Earth-like planets in each solar system out there.

Â These numbers are all either determined or

Â about to be determined by astronomical observation.

Â The next factors we'll recognize as completely indeterminant or uncertain.

Â The next one is the fraction of those Earth-like planets that have life.

Â We don't know what that fraction is.

Â We might imagine if a planet is habitable, it's almost inevitable that it has life,

Â and so that fraction is close to one.

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

Â That life's formation on the Earth involves some unlikely steps, and

Â so that fraction is small, maybe only 1% or less.

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

Â get some statistical basis.

Â The next fraction is the fraction of those planets where life formed

Â where an intelligent species eventually develops.

Â Again, we have no idea of that fraction.

Â Some argue that it's inevitable because it happened here.

Â Others argue that it took a long time.

Â Or it involves a lot of contingency and evolution, and so it's a low number.

Â There's no way to decide.

Â The next to last fraction is the fraction of those intelligent

Â species that develop the ability to communicate or travel in space.

Â On the Earth we have a handful at least of intelligent species,

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

Â But again, we have no idea what that fraction is in the general case.

Â The final number is critical, because it involves time.

Â It's the lifetime of that hypothetical civilization,

Â in their communicative state.

Â Since N is proportional to L, the entire product is driven by L.

Â If L is a small number, then N may be a small number.

Â If N is large, N may be large.

Â What is L?

Â For us it's hard to tell.

Â We've only had the communication ability in space or to travel for 50 years,

Â say a 100 as a round number.

Â But people looking at human history will realize we nearly extinguished ourselves

Â in that time when there were 40,000 nuclear weapons in the world.

Â And the Cold War was in its darkest days.

Â So if human civilization, with intelligence and technology,

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

Â Perhaps civilizations always are unstable.

Â Or perhaps we're the exceptions, the immature ones who almost self-destructed.

Â And when you get through your teething adolescence, you live forever or for

Â millions of years.

Â You can see the huge uncertainty in the Drake Equation, and

Â it's a simple trope of mathematics that the product of a series of numbers

Â is as uncertain as the largest uncertainty.

Â So the fact that we know the first three factors through astronomy

Â increasingly well doesn't mean that the uncertainty goes away for

Â the factors where we know nothing.

Â Even Frank Drake recognized the limitations of his equation, and

Â he called it a container for ignorance.

Â It is also worth pointing out logically that it's the number of intelligent,

Â communicable civilizations in the Milky Way.

Â If some civilizations live a very long time,

Â they could have the possibility to travel between galaxies.

Â At one tenth light speed, the trip to Adromeda is perhaps 20 million years.

Â For nearly eternal civilization, this is not a long time.

Â It's a small fraction of the age of any galaxy.

Â We're heading into extremes of speculation here,

Â talking about the longevity of civilizations.

Â But it's also important to point out that the Drake equation involves an average.

Â And that the possibilities of intergalactic civilization and

Â communication may depend on the longest lived members of that club,

Â rather than the short-lived members.

Â You can see that the Drake equation conceptually is a series of subsets.

Â There's the subset of all the stars that have Earth-like planets, the subset

Â of those that host life, as opposed to the ones that are sterile or still-born.

Â And then the subset of those where life develops sentience and

Â then intelligence and eventually technology.

Â As we winnow down the very large number of sites for life, the initial real

Â estate from the habitable planet searches, we could end up with a very small number.

Â Logically it's possible that N is one.

Â We're it.

Â We're unique in the galaxy as an intelligent, technological civilization.

Â Also logically, and some have argued, like Carl Sagan.

Â That N is more likely to be 100,000 or

Â a million, in which case there's a large set of potential pen pals out there.

Â Sociologists and psychologists and evolutionary biologists have also weighed

Â in on this subject, even though it's mostly governed by astronomy arguments.

Â Because when we're talking about the biological and cultural aspects of

Â the equation, we have to do that kind of analysis of our situation.

Â But the uncertainty of the Drake Equation doesn't go away.

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

Â It's striking and probably a coincidence that Thomas Wright, a physicist living in

Â England in the middle of the 18th century, speculated about the size of the Milky Way

Â galaxy during a time when Herschel was mapping it out with his telescopes.

Â And he came up with an estimate of 170 million habitable worlds in the universe,

Â which at that time was the galaxy.

Â This is actually close, within a factor of two,

Â and certainly an order of magnitude, of the true modern astronomical estimate.

Â So people have been thinking in these terms for centuries.

Â There are more sophisticated ways of thinking about the Drake Equation, and

Â there's a theoretical construct which stops using the binary on off,

Â life exists or it doesn't.

Â It's intelligent or it's not, and

Â takes into account the contingencies of evolution.

Â And the fact that all of these attributes will exist on a spectrum, and that some of

Â the relationships between the factors are not independent, they're coupled.

Â This is called a Bayesian framework for estimation.

Â It's important that we use as few prior assumptions as possible

Â in studying the Drake equation.

Â Either way, there's an enormous possibility for

Â intelligent civilizations in the universe.

Â The Drake equation seeks to organize our knowledge and

Â our ignorance of astrobiology in one formalism.

Â A simple equation to calculate N, the number of intelligent,

Â communicable civilizations at any time in one galaxy, ours.

Â The first few factors of the equation are astronomical,

Â and they're being determined by current and future observations.

Â The latter factors are completely uncertain because until we find life

Â elsewhere, we won't know how likely it is for life to develop on habitable planet or

Â subsequently evolve to intelligence and technology.

Â Finally, the Drake Equation is proportional to the longevity of

Â a civilization in a communicable state, and

Â that number is also completely uncertain.

Â If N is a large number and there are many durable civilizations out there,

Â then we will have many potential pen pals.

Â