SETI and the Fermi paradox: in search of ET


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This article originally appeared in The Skeptic, Volume 18, Issue 1, from 2005

Near the beginning of the film Contact (which is based on the novel of the same name by Carl Sagan) a young girl asks her father if he thinks there are people on other planets, to which he responds: “If it is just us, it seems like an awful waste of space”. It’s easy to sympathise with that comment, the unimaginable scale of the universe – billions of galaxies containing billions of stars – intuitively makes the Search for Extraterrestrial Intelligence (SETI) seem like a worthwhile endeavour. But is it?

Project Phoenix is the SETI Institute’s search for signs of alien life, using large telescopes to scan stars for transmissions in certain parts of the microwave spectrum. Specifically, they are looking for carrier type signals that would likely be used by aliens to call attention to their presence. Contrary to what people might imagine about SETI, any message being sent couldn’t necessarily be extracted from the signal until larger instruments were made to demodulate the signal’s content.

So far, if ET is out there, he’s been cosmically silent. Having said that, back in 1977 a candidate signal, ever since known as the ‘Wow!’ signal, was detected on a frequency prohibited for ground- or satellite-based transmitters, but unfortunately it was never picked up again. It’s not known if it was a genuine signal from an extraterrestrial intelligence or something else (maybe noise). To consider whether SETI is a waste of effort we need to examine the factors involved in the likelihood, or otherwise, of intelligent life existing elsewhere in our galaxy, and the universe as a whole.

The renowned physicist Enrico Fermi gave rise to the so-called Fermi Paradox by asking “Where are they?” after doing some analysis (apparently at a lunch table with colleagues!) and concluding that there should be many civilisations in the galaxy. Following on from this, there have been some models (for example, Crawford, 2000) which take certain assumptions and conclude that in just a few million years our galaxy could be colonised by a spacefaring civilisation.

Clearly the Fermi Paradox needs to be taken seriously, but the problem with models of how spacefaring populations could colonise a galaxy is the assumptions they are based on. Not surprisingly, counter arguments have been put forward to answer Fermi’s question, indeed, the matter has even received book-length treatment. In passing I should mention that UFO buffs would doubtless argue that extraterrestrials are already here. As there is no convincing evidence that UFOs are spacecraft from other worlds, and because UFOs can more plausibly be explained as natural phenomena, hoaxes, or optical illusions, there’s no need to take that idea seriously.

The first issue arising from the Fermi Paradox is the possible number of civilisations in the galaxy. There is no definitive answer to that, and probably never will be, but there have been attempts to come up with a figure. The seminal work in this regard is the Drake equation (Drake & Sobel, 1994), named after Frank Drake who founded SETI. The equation is:

            N = R x fp x Ne x fl x fi x fc x L

Where N is the number of detectable civilisations in space, R is the rate of star formation, fp is the fraction of stars that form planets, Ne is the number of planets hospitable to life, fl is the fraction of those planets on which life emerges, fi is the fraction of planets on which intelligent life evolves, fc is the fraction of planets with beings capable of interstellar communication, and L is how long that civilisation is detectable.

The question then is what values should be plugged into the Drake equation? Some of the figures will always be little more than educated guesses, typically based on what we know of life on Earth. Drake has derived an estimate of between one thousand and one hundred million advanced civilisations in our Milky Way galaxy (Drake & Sobel, 1994). Over time this estimate will likely change as detection of planets outside our solar system improves. Even if Drake’s estimate is far too high, if we are not alone in the universe, it would only take one planet to have harboured intelligent life in each galaxy to mean the universe has been home to billions of civilisations.

Over a hundred extrasolar planets have now been discovered, which demonstrates that planet formation is actually quite common. However, those extrasolar planets are mainly gas giants orbiting their star closer than Jupiter is to the sun – not the kind of planets on which we’d expect to find life. In time we may develop technology to detect smaller rocky planets which are more amenable to supporting life. At least we now know that planet formation is not a rare occurrence.

One answer to the Fermi Paradox could be that even though life is common in the galaxy, intelligent life isn’t. Here on Earth, life started fairly early on, around 3.5 to 4 billion years ago (White, 1999; Davies, 2003) with the Earth estimated to be about 4.5 billion years old. It is only in the last few million years that humans evolved, and much more recently that we’ve developed technology that can make our presence known across outer space.

The evolution of beings like ourselves was by no means inevitable; evolution is affected by many contingencies – such as the event(s) that led to the demise of the dinosaurs which had ‘dominated’ the Earth for over a hundred million years. The possibility of at least simple forms of life on other planets (or moons) is taken seriously now; there is even a scientific discipline called Astrobiology to study it. A hypothesis known as ‘Rare Earth’ (Cramer, 2000), suggests that complex intelligent life is very rare because on Earth a fortuitous combination of factors came together to allow the evolution of humans. This isn’t a totally new argument since for some time Creationists and others have argued that the Earth is special because many factors (such as the distance from the sun) are just right for it to support life.

We can look at factors influencing the lifespan of intelligent life here on Earth, although it’s a sample size of one and therefore extrapolating from it is possibly no better than divination. Until now, space exploration has been limited to our solar system, and manned missions haven’t been further than the moon. It’s early days for manned space exploration in particular, as the technology has only been around for a few decades. The costs may come down to a point in the future so that it is a more attractive proposition for governments faced with more pressing claims on their finances. As I write, the Chinese have just put their first man into space, and they may even send a manned mission to the moon, but it’s all probably more to do with national pride and geopolitics than serious space exploration.

Will humans be around long enough to progress to colonising outer space? That’s a difficult question to answer. However, there is a statistical argument known as the Doomsday argument (Bostrom, 2003) that claims we’re likely to be near the end of humanity’s lifespan. Using Bayesian statistics, the claim is that if you take your birth rank in comparison to all people who have ever lived, it is more likely to be the case that humanity will become extinct sooner rather than later. I’ll explain this by a commonly used example first. Imagine two urns containing numbered balls. One urn contains ten balls and the other a million. If you take a ball from one of the urns and it’s numbered six then it’s much more likely to have come from the urn containing ten balls. Now, take your birth order against all the people that have ever lived, this will be in the region of 60 billion. The probability of your birth order being 60 billion is much higher if humanity goes extinct soon than if the total number of people that ever lived reaches, say, trillions. I am not knowledgeable enough in statistics to give a strong opinion either way on the Doomsday argument, but it is taken seriously – academic papers have been published to refute it and defend it.

The Milkyway across the night sky

If other advanced civilisations exist in the galaxy there are many reasons why they won’t have colonies beyond their own planet. Space exploration is not a big priority for humans at the moment, though that might change in future, so why should it be for aliens? Also, the technology to send manned missions outside the solar system seems to be a long way off, and producing unmanned probes that could replicate themselves is only the stuff of science fiction at the moment. One day we might attempt to colonise planets orbiting stars other than the sun, but that is dependent on the vicissitudes of politics, economics, the environment etc.

The last variable in the Drake equation is the length of time a civilisation remains detectable. This will be influenced by the lifetime of the civilisation and how advanced their technology becomes. If the Doomsday argument has merit then we are entitled to wonder if other civilisations will also have a short lifespan. It is possible to come up with many good reasons to argue either way about the longevity of civilisations, but ultimately it’s little more than conjecture. We also need to bear in mind that the existence of suitably advanced extraterrestrials will need to coincide in certain ways with SETI – there’s a ‘window of opportunity’ for us to detect any signals sent not too long ago. Consider this: if a planet ten thousand light years away was home to an advanced civilisation that became extinct a million years ago, then their final signals would have passed Earth too long ago. Similarly, any signals being sent for the first time from anywhere more than a few tens of light years away wouldn’t have reached us yet.

Sadly we may never know if humankind is the only intelligent life to have inhabited the galaxy or universe. There are good grounds on both sides of the argument for the existence of extraterrestrial life, so after asking if SETI is a worthwhile project, the answer surely has to be yes, if we want to try for an answer to a most profound question: are we alone?


  • Bostrom, N. (n.d.). A Primer on the Doomsday argument. Retrieved 5 October 2003, from
  • Cramer, J.G. (2000). The “Rare Earth” Hypothesis. Retrieved 5 October 2003, from
  • Crawford, I. (2000). Where Are They? Scientific American. Retrieved 5 October 2003
  • Davies, P. (2003). Born Lucky. New Scientist, 179(2403), 33.
  • Drake, F. & Sobel, D. (1994). Is Anyone Out There? Glasgow: Simon & Schuster.
  • White, M. (1999). Life Out There. St Ives: Warner Books.
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