Where is Everybody?
I did a little reading on a NASA website. And then I read a bit more on Wikipedia. Beware! Even a little reading, when done by a philosopher, can produce hours of speculation.
A few facts, before the speculation. First, the Milky Way galaxy, our cosmological home, has about 300 billion stars. The galaxy is shaped like a disk, bulging at the middle and tapering at the edges, with stars strung out on four spiral arms. The starry, visible portion of the galaxy is about 120,000 light years across, but scientists think dark matter extends the size of the Milky Way much further. There are satellite galaxies in our “near neighborhood” that cluster around the Milky Way. Our own star, the sun, is located about 27,000 light years from the center of the galaxy, out on one of the spiral arms. We’re not way out on the edge, but we’re definitely not in the middle.
So far, astronomers have found several thousand planets orbiting other stars in the galaxy. These discoveries have all occurred in the last three decades. Astronomers have long theorized that many stars will have multiple planets, just as our sun does, so the total number of planets in the galaxy could be 500 billion or even more. Astronomers will be announcing discovery of new planets for many decades to come.
There is a Fermi problem associated with that number (500 billion planets), the most famous of all the Fermi problems.
Background: a “Fermi problem” seeks to estimate some unknown quantity about which you have no direct information. For example, how many piano tuners are there in Chicago? You don’t need to consult the phone book or interview piano owners to make a reasonable estimate. First, you guess how many people live in Chicago; 5 million, let’s say. Then you guess how many family units that would be; nowadays, the typical household has three or four persons, so Chicago has somewhere between 1 and 2 million. Then you guess which percentage of families would own pianos. Let’s guess one in ten, so that gives us 100,000 to 200,000 pianos in Chicago. How often do people have their pianos tuned? Let’s say once every two years; that means 50,000-100,000 need to be tuned every year. If a piano tuner tunes a piano in 2 hours and works 40 hours a week, that’s 20 pianos a week. If the tuner works 50 weeks a year, he tunes 1000 pianos. So there would be 50-100 piano tuners in Chicago.
Notice that a Fermi problem does not expect a close answer, only a plausible one. The number of piano tuners in Chicago could be 10 times as many as our guess (500-1000 tuners) and still be useful. We can be confident there are more than 20 piano tuners in Chicago, but less than 5000.
Now, the most famous Fermi problem, also called the “Fermi Paradox,” traces back to Enrico Fermi himself. In 1950, while working with Edward Teller and other scientists at Los Alamos, New Mexico, Fermi asked: “Where is everybody?” (At least, that’s one version of the story. It was reconstructed later from memories, after Fermi died in 1954. Every version of the tale gives Fermi credit for the question.)
Fermi and his friends had been discussing UFO stories, which were in the news at the time. (Of course, UFO sightings have not ceased, but they were new then.) None of the men believed in UFOs, but Fermi’s question pointed to a problem. Given reasonable guesses about relevant questions, Fermi said we should have received visits from alien beings. How many alien visits should Earth have received?
First, assume that the percentage of planets that can support life is fairly small, say 5% of all planets.
Second, assume that the percentage of planets that could support life and actually do support life is small, say 10% of the first group. So only 0.5% of planets would be habited.
Third, assume that the percentage of habited planets that develop intelligent, possible space-faring life (comparable to humanity in 1950) is very small, say 1%. So only 0.005% of planets would be habited by possibly space-faring life.
Fourth, assume that faster than light travel is impossible, so that space-faring populations can only travel space at rocket ship speeds (circa 1950), so travel across the galaxy would take as much as 5 million years.
Now 0.005% of 500,000,000,000 (500 billion) planets means there would be 25,000,000 space-faring civilizations in the galaxy. In cosmological terms, 5 million years is a very short time; in geological terms it’s still short. The Earth should have been visited many, many times by alien species. Yet we see no evidence on Earth of their visits. Where is everybody?
Obviously, the estimates could be too high. Let’s say we overestimated the likelihood of habitable planets, the percentage of actually inhabited planets, and inhabited planets with space-faring life by a factor of 100 in each case. That would reduce the likelihood of alien space-farers by a factor of 1 million. We still end up with 25 space-faring civilizations in our galaxy. The 5 million year estimate for spreading across the galaxy wouldn’t change. Where is everybody?
Maybe the aliens visited Earth a long time ago, found it boring, and left. But we see no evidence of their visits.
Actually, the Fermi paradox is worse than that, because of radio. Fermi asked his question in terms of alien visits to Earth. But alien species don’t need to visit Earth and leave evidence; all they need to do is use radio (which humans began using in Fermi’s lifetime). Radio transmissions move at the speed of light. A radio program from the most distant part of our galaxy would reach Earth in less than 100,000 years. (Our own radio and television broadcasts have been spreading the news of intelligent life on Earth for more than 100 years. Alien species within 100 light years of Earth could access our radio signature.). In cosmological time, 100,000 years is almost nothing. If there have been space-faring civilizations in our galaxy any time before the last few thousand years, our radio telescopes should be able to hear them.
The Fermi paradox, particularly in its radio signature form, is well-known among astronomers and cosmologists. That’s why SETI, the search for extra-terrestrial intelligence, has been going strong for 50 years. We have been using radio telescopes, deep-space probes, and space station telescopes to look for indications of intelligent life in our galaxy. But scientists have not reported any such evidence. Where is everybody?
50 years of searching is not very long. It’s possible that we’ll find evidence of intelligent ETs next year. But every year that passes without finding any positive evidence—especially since we are continually refining and strengthening our search—increases the power of the paradox. Perhaps we need to re-examine our estimates.
Maybe life, particularly intelligent life, is much rarer than we thought. After all, despite the confident assertions of philosophical naturalists, we do not know how life first formed on Earth. Too often, the Chris Hitchenses and Richard Dawkinses of the world simply assert that life had to begin in a certain way, because they are committed a priori to naturalism.
(By the way, it’s fun to make plurals of names that end in s, like Hitchens and Dawkins.)
One depressing suggestion needs to be mentioned. In the twentieth century humanity discovered thermonuclear weapons only four decades after building radios—in cosmological terms, almost simultaneously. Maybe that has been true of ET civilizations too. Maybe all of them discovered weapons of extinction. And maybe they all used them to destroy themselves. That’s a lot of speculation, but it could explain the lack of radio evidence of ETs, because the radio signature of such a civilization would be a band of radio waves only 50 or 100 light years across. If our SETI researchers weren’t looking in the few years those radio signals pass the earth, we would never notice them.
More and more, the possibility begins to dawn on us. Maybe we really are alone in the universe.
Or . . . maybe there is intelligent life in the universe, but it looks very different from us. Perhaps there are intelligent beings who don’t use radio to communicate and whose natural movement is so swift they wouldn’t bother to build spaceships. Maybe they wouldn’t even have physical bodies. How would we know?
But SETI researchers are not looking for angels.
