Life in the Universe – Probability and Perspective

Recommended Pre-Reading:
The Fermi Paradox

When we look up at the sky at night from a place without too much light, we can see thousands of stars. Telescopes and centuries of formal astronomy have shown us that there are billions of stars in our Milky Way galaxy, and billions of galaxies in the observable universe. Considering the mind-bogglingly high number of chances for civilizations to arise, and the incomprehensible amount of time that has passed since the big bang, it seems natural to ask why the sky is so silent. Where are all the aliens? This is famously called the Fermi Paradox. But is it actually a paradox? Do we really have good reason to believe there should be aliens everywhere?

Back in the days of Ancient Greece, Aristotle modeled the solar system—or rather the terrestrial system—with the Earth in the center, and the sun, moon, stars, and planets revolving around it in perfect circles. There is something in human nature that makes it easy to believe that the universe is centered around us. Aristotle’s model survived for over a thousand years into the middle ages, until a troublemaker by the name of Nicolaus Copernicus shook the world with the model we all know today, with the sun in the center and the Earth being a planet, just like all the other planets. Since then, the Copernican Principle has been the idea that there is nothing special about us. We don’t live at the center of the Universe, Earth is not a special planet, the sun is not a special star, and so on. We are an ordinary part of the Universe, just like everything else. Taking the Copernican Principle to its conclusion, it seems the cosmos should be positively teeming with civilizations’ phone calls, TV shows, and internet sites.

On the other hand, we must also consider the Anthropic Principle,* which basically says, “we should not be surprised to find ourselves in a place where intelligent life is possible.” For example, if you were to wake up in a room and find a letter which said, “There are a million rooms just like this one, and only one of them is occupied,” would you think to yourself, “what a miracle it is that I find myself in the only room out of a million that is occupied”? Of course not, because the fact that you are there is what makes it occupied. When applied to life in the Universe, the Anthropic Principle shows us that even if the chances of life arising around any particular star are a soul-crushing one in one septillion (1 in 1,000,000,000,000,000,000,000,000, or 0.000000000000000000000001) over 13 billion years, it is completely unsurprising that we find ourselves here on a planet that is perfectly suited for intelligent life. In fact, since we have no reason to believe the entire Universe ends anywhere near the edge of the observable universe, it doesn’t matter how infinitesimally small the odds are, there is really nothing interesting about the fact that we exist.

Taking the Copernican and Anthropic principles together, all we can really conclude is that we are probably normal as far as civilizations go. However, we still can make no reasonable guess as to how many civilizations should exist. There might be millions in our own galaxy that we are just on the brink of finding, or there might be no others in the entire observable universe. There are those who claim life should be everywhere, and those who claim Earth is unique. Who is right? We don’t know. What we do know is that neither has a good enough argument to win the debate, because there just isn’t enough data yet.

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Speaking of data, we can turn this question into a scientific one with the Drake Equation, which says the number of civilizations in a given volume of space is equal to a bunch of stuff multiplied together. Those variables are the rate of formation of stars in that volume that could potentially host life, the fraction of habitable stars with solar systems, the average number of habitable planets per solar system, the fraction of habitable planets where life begins, and the fraction of planets with life that evolve civilizations. I have taken liberties and condensed some of the variables together to make it easier to explain, but that is the gist of it. We know the first three pretty well. Almost all stars have solar systems, and it is quite common for stars to have rocky or ocean planets in their habitable zones. However, Earth is the only planet we know of that has life, and we don’t know how it got started, so we have no idea what the last two variables are. They could be anything from so small that ours is the only planet in the Universe with even microbial life, to so large that we might stumble upon our neighbors’ interstellar radio network any day now.

Talking about the Fermi Paradox has always bothered me. Not because it is a paradox, but because everyone calls it a paradox and I don’t see why. A paradox requires a valid theoretical prediction that either leads to two apparently contradictory conclusions, or disagrees with observational evidence. But the assumption that life should be everywhere in the Universe is not valid, hence no paradox. Regardless, whether or not we are alone is still one of the deepest, most awe-inspiring questions a human being can ask.

*The Anthropic Principle should not be confused with the Strong Anthropic Principle or the Weak Anthropic Principle, both of which apply the Anthropic Principle to the Universe. The Weak AP assumes there is a multiverse, while the Strong AP assumes there is not.