It was the summer of 1950 and physicist Enrico Fermi was walking to lunch with several colleagues. The men were talking about two stories that had been peppering the nation’s newspapers for months—the disappearance of trash cans lids and a spate of UFO sightings. Fermi joked that the two phenomena were connected. They talked about other things for a while and then, during a pause in the conversation, Fermi abruptly asked: "Where is everybody? " His colleagues immediately knew Fermi was talking about extraterrestrial visitors. They also suspected that the question was more profound than it appeared. Fermi was one of the most pre-eminent physicists in history. Winner of the Nobel Prize in 1938, he was skilled as both a theorist and an experimenter, and his judgment was so unerring that his colleagues called him "the Pope." Fermi addressed complex scientific issues by breaking the problem into pieces and doing swift calculations in his head to get a rough answer. These so-called "Fermi questions" form a vital part of the training of a scientist.
Fermi didn’t write down his thought process, but we can guess at it. The logic is as valid as it was over fifty years ago. Unless the Solar System is special, there are Earth-like planets near some of the billions of stars in the galaxy. A fraction of them host life. Given enough time, life on some of those worlds will evolve intelligence and technology. With a modest extrapolation of current technology, we will be able to travel at 1% of the speed of light, and at that speed it only takes ten million years to explore or colonize the galaxy if we assume that the travelers don’t linger and move swiftly on to new worlds. That’s a small fraction of the age of the galaxy and its oldest stars. There has been plenty of time and opportunity for alien civilizations to communicate with us or to visit us, yet we see no evidence of their existence. Where are they?
This conundrum has been called the Fermi paradox, because we fail to observe something that we might expect to. Most people, including many scientists, tend to have one of two gut reactions to Fermi’s paradox. They either think "Wow, there’s got to be tons of life out there, and there’s been time for many civilizations to evolve far past our capabilities, so the galaxy should be crawling with aliens" or "That’s crazy, intelligence is rare on Earth and will be rare elsewhere, and space is so vast that the chances of alien contact must be very low." What’s the more sensible reaction?
Before we consider possible answers to the Fermi’s question, let’s look at the nature of the argument. You can sense that it’s not a normal proposition. Science deals with observed phenomena and then tries to explain them. In this case, we are trying to account for the failure to observe something. The paradox stems from three statements, each of which seems reasonable. First, extraterrestrials capable of communication and space travel exist or have existed in the past. Second, if they’d visited our planet we would’ve seen them. Third, we haven’t seen them. (We’ll use the shorthand ET’s for space-faring aliens.) Everything hinges on the first statement. If ET’s have never existed then the contradiction formed by the second and third statements goes away — we are the first space-faring civilization. This conclusion accepts the Fermi paradox at face value, but to prove the first statement false would be a very tall order. If ET’s exist, then attention falls on the second statement. There are many plausible reasons why ET’s could exist yet not communicate or avoid our detection, so the second statement may be wrong, and the contradiction is avoided. If the third statement is wrong and we have actually seen ET’s then the paradox evaporates entirely.
In this tricky terrain, where evidence is absent, we have to be very careful with logic. To say "I haven’t seen ET’s, so they probably don’t exist" is invalid, because the bar is set very high to prove the absence of something in a large and complex universe. But saying "The argument above is invalid, so ET’s probably do exist" is also invalid! We’re not entitled to be surprised by the fact that ETs haven’t made contact unless we have a rational reason to believe they exist and a rational expectation that they would have made contact.