You wouldn’t be human if you hadn’t looked up into the night sky and asked, do aliens exist? Are we alone in the Universe? Scientists have asked these same questions for centuries and have diligently worked to answer that question. Carl Sagan, who passed away in 1996, made it his life’s work to show us the way forward. In honor of his birthday today, we wanted to revisit one the topic of one of Sagan’s most widely known episodes of his iconic television program, Cosmos: the Drake Equation.
What is the Drake Equation?
In 1961, Dr Frank Drake was working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia. As a pioneer in the Search for Extraterrestrial Intelligence (SETI), Drake would listen to the sky for possible radio communications from an intelligence other than our own. As part of a systematic survey of the sky known as Project Ozma, Drake generated interest among other like-minded scientists in the SETI field and a conference was held at the Green bank observatory in 1961.
According to Drake, the equation was somewhat of a fluke, "As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy." This equation has proved to be the launching point for countless debates and academic papers in the decades since Drake first presented it, largely because of its straight-forward simplicity:
N = R*•ƒ(p)•Ne•ƒ(l)•ƒ(i)•ƒ(c)•L
What Does the Drake Equation Mean?
The equation is simply taking several known and estimated factors and applying different probabilities to arrive at the answer N, the number of estimated alien civilizations capable of radio communication with Earth. Several of the factors are known or can be reasonably estimated:
- R*: the rate of star formation in our galaxy
- ƒ(p): the fraction of stellar bodies that have planets around them
- Ne: the average number of habitable planets or moons in any given solar system
Other factors, however, are much more difficult to estimate
- ƒ(l): the fraction of habitable planets that produce life
- ƒ(i): the fraction of planets with life that are home to intelligent life-forms
- ƒ(c): the fraction of intelligent life form that develops radio communications
- L: the length of time such civilizations survive and continue to attempt communication
Response to the Drake Equation
Since it was purposed, the Drake Equation has fueled both excitement and controversy. As far as life goes, we have only ever found one planet that has developed even the simplest form of life. Homo Sapiens evolving to the point of interstellar radio communication could be the most mundane event in the history of intergalactic civilization—so much so that we may have gone completely unnoticed by tens of thousands of far more advanced civilizations—or it may be the only time it has ever happened, the end result of a long string of incredibly improbable events. In the end, we wouldn’t know the difference.
The last four factors of the Drake Equation reveal much more about the scientist than the science itself. The assumptions one makes about habitability, the speculation about the technological advancement of an alien civilization, and even the likelihood of a civilization’s survival upon reaching its nuclear age will yield vastly differing answers. Some have even argued that the odds of ours being the only civilization in the observable universe is as high as 39% while others propose that the odds of human beings being the only intelligent lifeform in the universe at abour 1 to 10 billion trillions.
Carl Sagan and the Drake Equation
For nearly two decades thereafter, scientists have offered their own estimates and argued with the opposing side as either too optimistic or too pessimistic about their assumptions, but this was largely confined to the academy and scientific journals that most people couldn’t even access, much less understand if they could.
This disconnect between the average scientist and the average man or woman on the street was uniquely filled by Carl Sagan. Sagan had a common touch and an unmistakable voice. He had all the brilliance of a leading scientist in his field and all the exuberance of a child looking through a telescope for the first time—and his excitement was infectious.
It isn’t surprising then that a 13-part miniseries on Public Television would garner such legendary acclaim with Carl Sagan at the helm. When PBS broadcast Carl Sagan’s Cosmos into living rooms across the country in the fall of 1980, American audiences were enthralled by Sagan’s dissection of the Universe into digestible pieces. The average American was introduced to concepts and ideas that had long been known and discussed by the scientific community but would never have reached their ears had it not been for Sagan’s work.
From galaxy clusters to the origins of life on Earth, Sagan wove a tapestry of popular science that blazed a trail for figures such as Bill Nye and Neal DeGrasse Tyson and gave Americans more science education than they’d had in years and, for many, their whole lives. So, when he introduced the Drake Equation in the second to last episode of Cosmos, Sagan—a SETI optimist—struck a decisive blow in the American consciousness for continuing the hunt for intelligent life, leaving an unshakable feeling in the culture that the long-awaited call from the stars is just around the corner.
Carl Sagan’s Legacy Of Optimism
More than anyone, Carl Sagan gave Americans reason to hope at the height of the Cold War, and his legacy has spread around the world since Cosmos first aired. He reduced the conflicts that threatened our very existence to their appropriate scale when viewed from the perspective of the universe. The “pale, blue dot” he popularized in Cosmos offered desperately needed perspective for humanity, reminding us that from all the way out there, our wars and our conquests were insignificant against the great ocean of nebulae, distant stars, and far off galaxies. And in his discussion of the Drake Equation, he showed us that it was that much more urgent that we lay down our guns, preserve our planet, and work together to build a civilization worthy of the call of the cosmos when it came. He was certain that it was coming and, even now, he makes us believe it too.