Summer finds me in my garden, pulling crab grass. I’ve tried everything to get rid of it, but it comes down to sifting through the dirt, pulling out one root at a time, which allows room for reflection as my mind searches for something to occupy it.
I’m picking up worms with each handful of dirt, leaving me wondering how many worms are in my garden. I don’t have time to count them, given all the crab grass I need to eradicate, but is there some other way?
Turns out, there is.
If I make a box one foot on a side I can count the number of worms in that box down to about six inches deep without too much trouble. I do so, and find 20, give or take.
Glancing around the garden, I see the square foot I chose is about the same as any other, so I figure the likelihood of finding worms in each square foot of the garden is about the same.
I scan the boundary of the garden, and I see the fence has eight posts in one direction, and four in the other. Since the posts are about eight feet apart, I multiply the length of the garden (64 feet) by the width of the garden (32 feet) by the number of worms (20) by the chances I’ll find that many worms in each square foot (100% or 1). When I do, I get about 41,000 worms. If each worm weighs one-tenth of an once, that’s the weight of a medium-sized pig.
Shocking, isn’t it?
But is it right? I guessed on a lot of stuff. But for any reasonable estimate of the size of my plot or the number of worms, I get something like 10,000 to 50,000 worms. And if I want a better answer, I know exactly what to measure.
We do the same thing if we want to know how many intelligent civilizations might exist in the Milky Way Galaxy. We ask a bunch of smaller questions, like how many stars are born in our galaxy each year and how many earth-like planets do they have? What fraction of these planets evolve life, or intelligent life? Are any of these beings willing and able to talk to us? And, finally, how long do such civilizations last? We can numerically estimate answers to these questions and multiply them just like we did with the worms to get an idea of how many other planets might harbor intelligent life in our galaxy.
Estimates for most of these questions are a matter of some speculation, but not all of them.
Astronomers have counted the number of stars in the galaxy and estimated its age, so we have a good handle on the number of new stars born every year. We’ve also done a fair job of cataloging the number of stars with earth-sized planets. So far, it seems like nearly all stars have planets, and most have at least one or two earth-sized ones.
At the moment, we have no idea how many planets form life, how many evolve intelligent life, or how many are willing to communicate, but we do know there is at least one — us — and that puts some lower limits on what these fractions can be. They aren’t zero.
And we are working on measuring those, too. Astronomers are building telescopes to observe the atmosphere of planets around other stars, atmospheres that may contain the fingerprints of life. And we are also scanning the radio waves from nearby stars, looking to see if any such life may be trying to communicate with us.
Lastly, we are performing a very real experiment in how long an intelligent, communicating, technological civilization might last. The invention of radio astronomy, and thus our ability to communicate at interstellar distances, is only about 100 years old. How much longer we maintain that capability is currently an open question.
Reasonable estimates of these parameters put the number of civilizations in the galaxy at one — just us — to millions or billions. Only observations will resolve that discrepancy, but we have a clear roadmap of how to go about it.
Whether you are counting worms in your garden or galactic civilizations, the scientific process of asking the question, mapping out what you need to know and making the required measurements lets you put one foot in front of the other to get to the answer.