The Fermi Paradox: If our galaxy is crowded with habitable planets, where are all our neighbours?

(Credit: NASA/JPL-Caltech)

Saturday, June 28, 2014, 9:00 AM - If you're up at the cottage for the long weekend, and you're gazing up into the splendor of the twinkling stars of the night sky, give a thought to this little problem: our galaxy alone is supposed to have billions of habitable worlds, and even the most conservative estimates produce thousands to possibly millions of alien civilizations out there. Yet, despite decades of listening in for signals from these civilizations, and only one brief sliver of a possibility, all has been quiet and there's been no credible evidence that we've been visited by aliens. So, if all that intelligent life is supposed to be out there, where is everyone?

This 'little problem' is known as the Fermi Paradox, as this exact question was asked back in 1950 by physicist Enrico Fermi, as he contemplated that same set of circumstances. In the vastness of the galaxy (or even the universe), how is it that we have no evidence of there being other intelligent lifeforms? Is life actually so rare that Earth is the only one where it developed? Is intelligent life an anomaly, so although there are plenty of worlds that are teeming with life, we're truly alone as a species that can contemplate the universe around us? Does intelligent life develop, but then get snuffed out by disaster, internal conflicts, or war with other intelligent life? Or are they out there, but are unwilling or unable to communicate with us?

The distances involved in space travel or even space communication shouldn't really be an issue in this, as there are plenty of star systems that are a lot older than our own, even in our own neighbourhood. Kapteyn's Star, only 13 light years away, with its potentially-habitable planet that's 7 billion years older than Earth, is an excellent example. That would be plenty of time for any alien civilization that developed there to expand out into the galaxy. For communication, radio signals travel at the speed of light, so faster than anything else can through space (we can leave aside any ideas of warp ships for now). So, signals from older civilizations would have reached quite far. Even for a young species, like ourselves, there'd be some hints in the local neighbourhood. Our radio signals have expanded out to form a 'bubble' roughly 200 light year across by now. As the image below shows, compared to the whole galaxy, it's really not that far at all. Still, there's an estimated 15,000 stars in that region of space, so surely we'd turn up at least one neighbour. Right?

Credit: Adam Grossman / Nick Risinger c/o The Planetary Society

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Well, there's a few ideas about this.

One thought is that there is a 'Great Filter' in place when it comes to evolution. All the stages of evolution before the filter are relatively easy, but the Filter is the really hard one, so only a few (if any) species can get past it. With only our own planet and the life on it as an example, there's no telling where the Filter would be, though. It might be the development of of DNA or multi-cellular life. It could be the development of a self-aware brain that's capable of using tools. It may even be a stage we haven't reached yet. If it's either of the first two, we could be the first species to achieve this level of development, or we may be part of a bunch reaching it at roughly the same time. If the Filter is still ahead of us, though, we could be in for some rough times.

Another thought is that there are alien civilizations out there, but there's some other reason for us neither seeing nor hearing any evidence of them. The timing could be off, meaning that when they were sending out signals or passing through this section of space, our distant ancestors were still scratching grubs from the dirt, or still lived in the oceans. Our region of space, out here between the two major spiral arms of our galaxy, could simply be too 'rural' for them, so they don't get out this way and they don't even know we exist. There's the 'Zookeeper Hypothesis' that says the aliens are there and they know about us and they're watching, but that's it. If they visit, it's only akin to us going to the zoo to see the exotic animals (look but don't touch). It could be that our technology is just too primitive to detect any signals, spaceships or structures the aliens have produced. Another possibility is that it's quiet because many civilizations have come to the conclusion that the risk of contact is just too great, and they actively avoid calling attention to themselves. And they could be right, since there could be someone going around silencing everyone - perhaps a xenophobic alien race that wants to remove any potential competition before it can develop to their level.

If the Great Filter isn't there to spoil the fun, so to speak, and the aliens really are out there, what should we do about any potential contact? 

There has been one hint of an alien signal that we know about. In 1977, astronomer Jerry Ehman was listening to a section of space with the Big Ear radio telescope, specifically as part of the Search for Extraterrestrial Intelligence (SETI), when he detected what is now known as The Wow Signal. It was a radio burst that lasted for 72 seconds, rising to a peak and then falling off, apparently just as it should if we intercepted a signal as the dish of the telescope rotated into the beam and then out of it again as the Earth rotated. Although no Earthly source has been found for this, astronomers haven't been able to pinpoint an alien source for it either, and it has never been repeated. It might be alien, and then again, it might not.

With our own signals, there's nothing we can do about the radio and television signals we've emitted so far. They're already out there. We've sent a reply message back to the exact same region of space the Wow Signal came from. More recently, we've beamed a collection of Tweets at nearby exoplanet Gliese 667C c, one of the best habitable-planet candidates we know about. However, some of our best minds believe that actively advertising our presence is a bad idea, at least until we're able to advance more, and thus be better prepared (technologically and psychologically) to receive interstellar guests. There's a counter to that, though, in an interstellar version of the 'prisoner's dilemma' - a scenario where each civilization could stay quiet and safe, but there is a bigger payoff for initiating contact, because the exchange of information - science, art, culture - could yield benefits for both and push the limits for their development.

When it comes down to it, though, even with the possibility that we're alone and the potential threats, the idea of contacting aliens is just too tantalizing to let go. Imagine if we managed to tap into a vast communications network going on all around us (perhaps using laser light), simultaneously discovering that we're not alone and we're able to listen in on the conversations of the civilizations around us (once we interpreted the messages, that is). It would certainly make for some exciting times.