The Drake Equation, also known as the Green Bank Equation after the 1961 conference where it was first presented, is a formula created by American physicist Frank Drake to estimate the number of intelligent alien civilizations which exist in the Milky Way and which, given sufficient technology and time, humanity might one day be able to communicate with. Although it takes the form of a mathematical equation, it is important to note that Drake never intended it to be solved precisely. Instead, he offered it as a sort of thought-experiment, taking into account all of the factors which would go into the likelihood of intelligent civilizations existing elsewhere in the universe:
N = R* x f(P) x n(E) x f(L) x f(I) x f(C) x L
In his work, Drake assumed that all life would emerge independently, and naturally, through evolution. As a result, one can imagine the universe as a collection of billions of sets of dice, all of them being rolled over and over again. Everywhere the right set of numbers turns up, intelligent life will emerge. The Drake Equation is an attempt to think about those numbers.
– Physical Variables for Life –
The first set of three functions in the Drake Equation relate to the fundamental physical conditions necessary for life to develop. R* is the number of stars which form in the galaxy per year (some variants of the equation say the total number of stars in the galaxy). In a young galaxy, new star births can number in the thousands per year. In a middle-aged galaxy like our own, however, the Max Planck Institute for Astronomy says that only about one Sun-like star (not too large or small to support life) is born per year. The Milky Way Galaxy currently holds about 100 billion stars. This is the only function in the Drake Equation that can be known with any degree of mathematical certainty.
The second function, f(P), refers to the proportion of stars that have planets around them. Drake reasoned that life could not evolve without a planet for it to be based on, orbiting a star capable of supporting life with light and heat. How many stars have planets is unknown, mostly because we are only now developing the technology to find Earth-sized planets around other stars. According to the Scientific American, recent estimates are that 7-30% of stars have planets large enough for us to detect them (roughly Saturn-sized or larger). However, Rich Townsend at the University of Wisconsin-Madison says that if we factor in smaller planets, virtually every star is likely to have some – if we only knew how to look for them.
The third function, n(E), refers to the proportion of planets that are actually capable of supporting life. If we did not know the number of total planets, we certainly don’t know the number of life-sustaining planets. It’s tempting to set this number at 10% – after all, one in eight of the planets in our solar system (Earth) is known to have life on it. On the other hand, so far as we can tell, Mars, Jupiter’s moon Europa, and Saturn’s moon Titan all have the capability to support microbiotic life, either in the distant past (Mars), the present (Europa), or the distant future (Titan). Townsend says we should figure that one-quarter of planets supports life.
So far, with three variables accounted for, the Drake Equation suggests that one potentially life-supporting star comes into existence every four years, and that there are 25 billion candidate stars in the total history of the Milky Way Galaxy, which might plausibly have life on a nearby planet.
– Biological Variables –
The final four variables of the Drake Equation assume that life is actually capable of first coming into existence from non-living organic compounds, and then of evolving into more advanced forms. How likely any of these things are to happen is truly unknown, since all we have to work with are the bits and pieces of the Earth fossil record which paleontologists and evolutionary biologists have been able to piece together.
The first of these, f(L), refers to the likelihood that life will develop from non-living matter, a process called abiogenesis. There is no scientific consensus on the exact process by which this happened in the first place, so there really is no way of knowing how likely it is to occur somewhere else. However, this is another variable which Rich Townsend says is effectively 100%. After all, he reasons, life has billions of years to work with during the lifetime of a Sun-sized star. Sooner or later, if it’s possible for life to originate at all, it will do so.
The second variable, f(I), is far more problematic. This variable refers to the likelihood that intelligence will evolve. Paleoarchaeologists do have some idea how this might have happened among apes and humans, in terms of gradual increases in mental and social complexity. (So far they have less to offer in the instance of other somewhat intelligent species, like dolphins, Humboldt squid, and social insects.) Townsend, ever the optimist, says that this number is 100% as well.
But that may not be the case: the Earth has benefited from several unusual conditions, like our large Moon, our active geology, the lack of unstable stars nearby, and the helpful gravitational influence, which pulls most potentially dangerous comets and asteroids out of danger. All of these factors go into making the climate and the surface of the Earth relatively stable. If that weren’t the case, life simply wouldn’t have time to evolve far before one or another stellar or natural disaster simply wiped it all out. So, rather than taking Townsend’s figure, it seems appropriate to assume that only 10% of life-supporting planets will actually develop intelligent life. Even this figure may be a high one.
The third life variable, f(C), estimates what proportion of otherwise intelligent life will eventually go on to develop advanced technology. The best measure of this is the ability to communicate by radio waves. Here, again, there is much speculation. What is the likelihood that such technologies will be invented? What percentage of intelligent civilizations will never invent them, or, having invented them (at least theoretically), decide never to use any with sufficient power to broadcast radio waves out into space. Our current Search for Extra-Terrestrial Intelligence (SETI) involves listening for alien radio signals. If they aren’t sending any, though, we will never find them. Once again, optimists like Townsend say that 100% of intelligent alien species will eventually invent and use radios. This is probably optimistic, but let us accept it for the moment.
The final variable in the equation, L, is the most speculative of all. Advanced, radio-using civilizations probably have limited lifespans, predicted Drake: at least a large percentage of them will eventually succumb to climate change and go extinct, die out in a vast pandemic of contagious disease, or blow themselves up with nuclear weapons. (Drake was writing at the height of the Cold War, which explains his pessimism in this respect.) Townsend, unusually pessimistic himself, says that the average advanced civilization probably only survives for 200 years. That seems very pessimistic: after all, a civilization which survives its equivalent of events like the Cold War could plausibly survive for many thousands or even millions of years.
There is another factor to consider, however. Even if a civilization survives for many thousands of years, there is no guarantee we would be able to hear their radio signals for that entire time. The strongest signals being sent on the Earth are television carrier waves; these would also reach the farthest out into space, where aliens might hear them. But very shortly we will no longer be using television carrier wave signals: they’re just not necessary in the new age of digital and satellite communications. Civilizations much more advanced than ours might communicate in ways we haven’t even imagined yet.
– Where Are They? –
Because these figures are so speculative, it’s hard to solve the Drake Equation with any certainty – which is just as he intended it to be. Using the numbers here, though, we can say that some sort of intelligent civilization could be born in the Milky Way Galaxy as often as once every forty years. If each one survives for about 200 years, there are currently five civilizations in the galaxy with intelligence equal to or greater than our own. Optimists say there are actually far more: Townsend, for example, figures there are 25. The pessimists say there are less – in fact, we may be the only ones.
Accepting Townsend’s figure, though leads to another thought experiment problem, called the Fermi Paradox. If there are intelligent civilizations elsewhere in the galaxy, the odds are that at least some of them are older than ours. Given the rate at which our technology is progressing, even a century or two’s difference in ages could put them far ahead of us in terms of their technology. And if that’s true, we should be able to detect them. So where are they?
There are several possible solutions to the Fermi Paradox. Only one of them is an optimistic one: all of the advanced civilizations are so advanced that they’ve progressed beyond our ability to see what they’re doing. The other is pessimistic: civilizations tend to destroy themselves very quickly. Pessimists say that the reason we can’t see any alien civilizations is because, even if they did once exist, they’re already dead.
