Misconceptions of Science: Issues With the Drake Equation

FROM THE LECTURE SERIES: Understanding the Misconceptions of Science

By Don Lincoln, Ph.D., University of Notre Dame

The Drake equation is believed to be a good estimator of the number of civilizations in our galaxy. How accurate is it? Does it take into consideration all possible parameters? Let’s find out.

Illustration of an alien city, with its skyline in the foreground and its moon visible in the background.
The Drake equation is supposed to provide an estimate of the number of civilizations in our galaxy, but it is dependent on a set of seven variables or parameters.
(Image: Diversepixel/Shutterstock)

Before we can proceed to the Drake equation, we need to first understand what a Fermi question is, as it lies at the core of the development of the Drake equation.

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The Fermi Question

Italian physicist Enrico Fermi was a man of legendary intellect and, in my opinion, had a great approach to science. In addition to being an exceptional theoretical and experimental physicist, he was also known to probe the thinking ability of his students by asking them questions for which they couldn’t possibly know the answer. Now, remember that this was back in the day, long before Google and the Internet, so you had to actually think about things.

Photo of Enrico Fermi.
Italian physicist Enrico Fermi used to ask his students questions—which later came to be known as Fermi questions—for which they couldn’t possibly know the answer, in order to probe their thinking ability. (Image: Department of Energy. Office of Public Affairs/Public domain)

One of Fermi’s questions was, “How many piano tuners are there in Chicago?” To make an educated guess on how to estimate that, you have to start with how many people are in the Chicago area; then, how many houses will have a piano; how many of those households have their piano tuned each year; how long it takes to tune a piano; how long a piano tuner works each year; and so on.

Taking reasonable estimates for those quantities, you can come up with a decent estimate of how many piano tuners there are. When I do it, I get a number of about 225 piano tuners.

Now, cheating and using online resources like phonebooks and so forth, you find that there are about 290 piano tuners in Chicago. That’s a remarkable agreement, given that you probably had no idea what the answer should be.

Questions like this, answered with this sort of approach, are called Fermi questions. I used to put similar questions on some of my tests, when I taught introductory physics. The students would gripe and whine about why they couldn’t just look things up on their phone, but I got pretty good at asking questions for which Google wouldn’t help. The students didn’t always like it, but it taught them an invaluable lesson, which is the art of intelligent guesstimating.

There’s another interesting question, for which nobody knows the answer, which is “How many intelligent alien civilizations exist in the Milky Way right now?” Now we don’t know the answer to this, but it’s obviously a question that lends itself to being a Fermi question.

Learn more about the science of aliens.

The Creation of the Drake Equation

So, now let’s jump in a time machine and go back to 1961 and a conference aimed at trying to detect alien civilizations around nearby stars. The technical term for this is Search for ExtraTerrestrial Intelligence or SETI.

A few years before, physicists Giuseppe Cocconi and Philip Morrison published an article in the prestigious journal Nature, with the eye-catching title ‘Searching for Interstellar Communications’. Basically, they were thinking about how to look for radio transmissions from other civilizations, which, when you think about it, is really a very, very cool idea.

So, it was only half a year later or so that radio astronomer Francis Drake actually did just that. He used a 25-meter wide radio telescope in Green Bank, West Virginia and turned it to two nearby Sun-like stars, specifically Tau Ceti and Epsilon Eridani, and just listened.

It was a well-conceived and executed experiment, and it could have resulted in the discovery of the millennium. But it didn’t. Drake heard no signal. His experimental project, called Project Ozma was the first scientific attempt to find civilizations on planets around stars other than our own.

While Drake failed to find evidence of extraterrestrials, he certainly wasn’t dissuaded, and he decided to host a conference on the topic at the Green Bank facility. As he started preparing for other scientists to arrive, he realized that he needed to have a quantifying set of parameters to turn the question from an undisciplined one to a scientific one.

In the spirit of Enrico Fermi, he came up with what is now called the Drake equation. If N is the number of civilizations in our galaxy, he decided that it would be equal to R*.fp.ne.fl.fi.fc.L.

The parameters have the following meaning: R* is the average rate at which stars are formed in our galaxy; fp is the fraction of those stars that host planetary systems; ne is the average number of planets around the star that can support life; fl is the fraction of planets that could support life that actually do support life; fi is the fraction of planets that develop life that go on to develop intelligent life; fc is the fraction of planets with intelligent life that develop civilization and technology that we can detect; and, finally, L is the length of time each civilization emits radio waves or whatever it is we can detect.

If you multiply all of these, you get an estimate of the number of civilizations in our galaxy that we could detect, which is N.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

Limitation of the Drake Equation

In 1961, Drake suggested a set of parameters, and from that, he and his colleagues estimated that the number of civilizations in the Milky Way galaxy was somewhere in the 20 to 50 million range. That’s 20, not 20 million.

It’s worth noting that more modern parameters don’t help very much. We know much better details of how many planets exist and if they are in the orbital region where life can form. But we don’t know much about life forming, nor intelligence.

We do know from here on Earth that life formed almost immediately after the planet formed, suggesting that life is common. On the other hand, intelligence has arisen only once on Earth. That might mean that intelligence is rare. Also, over time, life has become more and more complex, so maybe intelligence is inevitable. The bottom line is that we really don’t know.

There are many people who think the Drake equation is a good estimator of the number of civilizations in our galaxy and that if we just figured out all of the parameters, we’d have it all nailed down. But that’s the misconception. The Drake equation is actually a very simplified equation and it neglects important parameters.

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A Single Alien Species Can Transmit Radio Signals From Multiple Locations

In a few hundred years, if we don’t destroy ourselves, we’ll figure out a way to go to distant stars. Now, I’m not talking about science fiction—not Star Trek, nor wormholes, hyper-drive, or anything that involves faster-than-light travel. I’m talking old-school, boring, slower-than-light, not-moving-very-fast travel.

Graphic representation of a spaceship traveling in deep space.
If humans don’t become extinct, then the colonization of the Milky Way could take only a million years. This means that an advanced alien civilization could have already passed that point, and could have traveled to many stars by now. So, the Drake equation should have been formulated to expect to hear radio transmissions from many more sources, which it wasn’t. (Image: Freestyle images/Shutterstock)

Suppose that we get technology to move at 1% the speed of light. That’s fast, to be sure, but it’s not breaking any laws of physics. If the stars are about five light-years apart, it would take 500 years to get to the next star. This is a long time, but not all that long.

Say it takes 500 years on the planet around the new star to build enough industry to launch two ships and they repeat the process. So, it’s about 1,000 years between launching two ships. But those colonies then make four ships, then eight, then 16, and so on. Depending on a lot of details, the colonization of the Milky Way could take only a million years. Maybe 10 million.

A million years is a long time, but remember that the Milky Way is over 10 billion years old. That is to say 10,000 times longer than that. Even our own Sun is 4.5 billion years old.

Now suppose that some alien species got a head start on us, and say their equivalent of Homo sapiens evolved 10 million years before we did. They’d have evolved to our level about 10 million years ago and then started colonizing. By now, they’d have colonized all of the galaxy. And that’s with just one species evolving before we did.

Getting back to the Drake equation, it doesn’t take into account the fact that a single species could travel to many stars, and, therefore, we would expect to hear radio transmissions from many more sources. That very likely possibility was completely missing in how he formulated his question.

The Reappearance Factor

There are other issues with the Drake equation as well. For instance, it assumes that when a civilization loses the ability to emit radio waves, it is gone forever. It is as though Drake assumed the only way a civilization would lose the ability to use radio is if everyone died. But I’ve watched enough dystopian science fiction movies to know that even if there were a dreadful World War III, presumably we’d rebuild and start over. Drake neglected what one might call the reappearance factor.

Fermi’s Colonization Criticism

Now, just because I’ve identified a couple of problems with the Drake equation doesn’t mean that it wasn’t a very good first step. It was. If nothing else, it identifies some of the key parameters that go into understanding if we are alone in the universe. Interestingly, Fermi had basically figured out the colonization criticism of the Drake equation long before others did.

It was in 1947 that the first UFO report was made that caught the attention of the American people. On June 24, 1947, Kenneth Arnold claimed to have seen what we would now call UFOs. The newspapers caught wind of it and soon it was on everyone’s mind, with many people making UFO reports. The Roswell Incident was just a few weeks later. For several years, UFO mania was a real thing.

In 1950, Enrico Fermi was having lunch with some of his colleagues and they were talking about recent UFO reports. Fermi fell silent as the conversation continued. Suddenly he erupted, “So where is everybody?”

He had thought it through and realized that if faster than light travel was possible, the galaxy should already be thoroughly teeming with species who evolved before we did. Earth should be deluged with visitors. And, as I have described, this would be true even if faster than light travel is impossible. And yet we aren’t. Just why that is so is the real question.

Taking the Drake equation to help us guide our thinking, one or more of those factors has to be much rarer than we imagine. Maybe intelligent life just doesn’t evolve very often. Maybe civilizations only emit radio for a short time and find other ways to communicate. Or maybe civilizations aren’t that long. Or maybe, once they develop technology involving nuclear weapons, civilizations simply wipe themselves out.

The fact is, we don’t know what it is that makes it seem like we are alone in the universe. Maybe we are. Maybe Carl Sagan was right, when he looked at the sky and said something along the lines of, “If we are the only intelligent species in the universe, then it’s a huge waste of space.”

Common Questions about the Drake Equation

Q: How many civilizations are there in the Milky Way?

In 1961, radio astronomer Francis Drake created an equation to calculate the number of intelligent civilizations in the Milky Way galaxy, and along with his colleagues came up with an estimated 20 to 50 million civilizations.

Q: What are the variables in the Drake equation?

The Drake equation comprises seven variables: R*, is the average rate at which stars are formed in our galaxy; fp, is the fraction of those stars that host planetary systems; ne, is the average number of planets around the star that can support life; fl, is the fraction of planets that could support life that actually do support life; fi, is the fraction of planets that develop life that go on to develop intelligent life; fc, is the fraction of planets with intelligent life that develop civilization and technology that we can detect; and L, is the length of time each civilization emits radio waves.

Q: What quantity can be calculated with the Drake Equation?

We don’t know if the Drake equation can actually calculate or not, but what it claims to do is provide an estimate of the number of civilizations in our galaxy. It is worth noting that the accuracy of the estimate is dependent on knowing beforehand the values of seven variables.

Q: Who made the Drake equation?

The Drake equation was created by the American radio astronomer Francis Drake in 1961. Incidentally, Drake is also responsible for the first modern Search for Extraterrestrial Intelligence (SETI) experiment called Project Ozma.

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