A recent article in the Economist alerted me to a recent paper by Thomas Hair and Andrew Hedman that profoundly reaffirms the conundrum that is the Fermi Paradox, an observational problem that is sometimes referred to as the Great Silence.
What's fascinating about the Hair and Hedman paper is that they are not cosmologists or astrobiologists, but rather mathematicians—and it is through the lens of number-cruching that they sought an answer to the question of how long it would take a civilization to colonize its local region given a specific set of parameters. And their findings are disturbing: No matter how they reworked the numbers, they came to the same conclusion: the Galaxy should be colonized by now:
To arrive at their conclusion Dr Hair and Mr Hedman assumed that outer space is dotted with solar systems, about five light years apart. They then asked how quickly a single civilisation armed with the requisite technology would spread its tentacles, depending on the degree of colonising zeal, expressed as the probability that intelligent beings decide to hop from one planet to the next in 1,000 years (500 years for the trip, at a modest one-tenth of the speed of light, and another 500 years to prepare for the next hop).So, the next time somebody smugly shrugs off the Fermi Pardox by suggesting that "it takes too long to colonize the Galaxy" or that "there hasn't been enough time," or that "the Galaxy is too big," tell them to shut-up and read this paper.
All these numbers are necessarily moot. If the vast majority of planets is not suitable, for instance, the average distance for a successful expedition might be much more than five light years. And advanced beings might not need five Earth centuries to get up to speed before they redeploy. However, Dr Hair and Mr Hedman can tweak their probabilities to reflect a range of possible conditions. Using what they believe to be conservative assumptions (as low as one chance in four of embarking on a colonising mission in 1,000 years), they calculated that any galactic empire would have spread outwards from its home planet at about 0.25% of the speed of light. The result is that after 50m years it would extend over 130,000 light years, with zealous colonisers moving in a relatively uniform cloud and more reticent ones protruding from a central blob. Since the Milky Way is estimated to be 100,000-120,000 light years across, outposts would be sprinkled throughout the galaxy, even if the home planet were, like Earth, located on the periphery.
Crucially, even in slow-expansion scenario, the protrusions eventually coalesce. After 250,000 years, which the model has so far had the time to simulate, the biggest gaps are no larger than 30 light years across. Dr Hair thinks they should grow no bigger as his virtual colonisation progresses. That is easily small enough for man's first sufficiently powerful radio transmissions (in the early 20th century) to have been detected and for a reply to have reached Earth (which has been actively listening out for such messages since the 1960s). And though 50m years may sound a lot, if intelligent life did evolve more than once, it could easily have done so billions of years before this happened on Earth. All this suggests, Dr Hair and Mr Hedman fear, that humans really do have the Milky Way to themselves. Either that or the neighbours are a particularly timid bunch.
Note: It doesn't appear that the Hair and Hedman paper is online; I have contacted them and asked for a copy or a link; stay tuned.