In 1950, over a casual Los Alamos lunch, after laughing over a New Yorker cartoon about aliens, physicist Enrico Fermi wondered, “Where is everybody?” and there was a brief discussion about the difficulty of star travel. It had occurred to Fermi that the Milky Way was vast, that there was nothing special about the sun or the earth, and therefore that life comparable to ours elsewhere in the Galaxy should be common. This conclusion has only strengthened lately; NASA’s Kepler mission supports an occurrence rate around 20% and therefore an estimate of 40 billion rocky planets in the habitable zones of long-lived dwarf stars in the Milky Way.

If you get to roll the dice 40 billion times, you’re going to score double sixes quite a lot.

Despite the flying saucer craze that had popped up three years previously (the term flying saucer first appeared in 1947), the sober Fermi knew there was no credible evidence of visits from extraterrestrial visitors. This also has not changed. (Sorry, UFO fans.)

The discrepancy between 40 billion chances for intelligence life in the Milky Way versus one confirmed example (us) has come to be known as “the Fermi paradox.” This is slightly unfair. While Fermi certainly discussed the notion over lunch in 1950, the idea was put on much firmer grounds, and actually published, by others, much later. In 1975, Alan Hart computed colonization times for the Milky Way galaxy, finding them very short compared to the lifetime of the Galaxy. That implies that, if civilization had arisen even once in the entire history of the Milky Way, the civilization would have spread to the farthest corners of the Galaxy in a relative eyeblink of time. This specific concept is taught to college astronomy students under the name Fermi paradox.

Frank Tipler extended these ideas in 1980, and Robert H. Gray suggests we should really call the Fermi paradox the Hart-Tipler argument against the existence of extraterrestrial civilizations (Scientific American, Jan 29, 2016). As Gray notes, the latter just doesn’t have the same ring to it. In any case, it’s probably too late.


You, like Fermi, might be wondering, “Where is everybody?”

The list of possible answers is entertaining. I’ll even insert an evil one of my own.

  • Humanity is the very first civilization to ever arise in the Milky Way.
  • Star travel is too expensive.
  • Star travel is too dangerous. (This is nonsense, but I include it for completeness.)
  • Star travel takes too long.
  • They are here. They simply wish not to be seen. This is Ball’s “Zoo hypothesis,” which also resembles the “prime directive” of the fictional Star Trek Federation.
  • They are here, but they are in a form we do not recognize.
  • Biology and technology merge such that video games play directly in our brains and activate pleasure centers (search “brain stimulation reward”) and civilization loses interest in moving off the couch. This is Guy Worthey’s drooling wirehead hypothesis, just so you can properly assign credit.
  • Author Vernor Vinge extrapolated Moore’s Law of exponential computer power growth to all of technology and came up with the idea of a technological singularity, where humanity and technology grow so fast at some future date that they evolve together into some advanced, unrecognizable state.
  • “The Matrix has us.” That is, we are already living in an artificial, dreamlike environment, created for us by our ancestors, our real selves, or robots. Nothing is real.
  • One alien civilization does exist. It destroys all competition. When it detects us, it will come erase us.
  • Now, thank goodness, a less depressing one: Advanced civilizations can solve every problem except shedding waste heat from usingĀ  all their wondrous technological gadgetry. In order to radiate the waste heat away, they move to the outskirts of the Galaxy, and away from us.
  • Some postulate that advanced civilizations construct event horizons around themselves, disappearing forever into black holes of their own construction. While I agree that they could, I don’t see the reason for the stunt, but maybe it’s fun for them or something. Along similar lines, and additionally postulating parallel universes, the advanced civilizations might just move to “more interesting” places.

Now, while all of that is interesting fodder for your next casual coffee shop conversation, we must await the discovery of evidence to make much real progress in charting the generalized evolution of Galactic civilization. In the meantime, we are free to explore and work toward colonizing the Galaxy for ourselves. With 40 billion worlds to choose from, there’s a lot of prime real estate out there.

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