In the summer of 1950, the physicist Enrico Fermi and some colleagues at the Los Alamos Lab in New Mexico were walking to lunch, and casually discussing flying saucers – as you do – when Fermi blurted out “But where is everybody?” He was not the first to pose the question, and the precise phrasing is disputed, but the mystery he was referring to remains compelling.
We appear to live in a vast universe, with billions of galaxies, each with billions of stars, mostly surrounded by planets, including many like Earth. The universe appears to be 13.7 billion years old, and even if intelligent life requires an Earth-like planet, and it can only travel and communicate at the speed of light, we ought to see lots of evidence of intelligent life. But we don’t. No beams of light from stars intermittently occluded by artificial satellites so as to broadcast Pi. No signs of galactic-scale engineering. No clear evidence of little green men demanding to meet our leaders.
Numerous explanations have been advanced to explain this discrepancy, and one man who has spent more brainpower than most exploring them is Anders Sandberg, a computational neuroscientist who got waylaid by philosophy, which he pursues at Oxford University, where he is a senior research fellow. He joined the London Futurist Podcast to discuss the Fermi Paradox.
The Drake Equation
Frank Drake was an American astrophysicist who formulated an equation in 1961 to calculate the number of active and talkative alien civilisations in our galaxy. It contains some variables whose true value was unknown then, but have become known since, like the fraction of stars with Earth-like planets. It contains other variables whose values are still unknown, like the number of planets where communicative intelligent life originates – and how long they survive.
Depending on the variables you enter into the Drake equation, the Search for Extra-Terrestrial Intelligence (SETI) can seem obviously valuable, or a waste of resources.
Crossing the galaxy
It takes less time than you might think to cross the galaxy: 100,000 years travelling at light speed. This is a long time, of course, but less than the time that humans have been around, and that is only an eye-blink in the history of the Earth, never mind the universe. An advanced civilisation could send out self-replicating probes, travelling much more slowly than the speed of light, and pausing a while at each stop to build the next probe. We should be able to start doing this ourselves in a hundred years or so, if we don’t go extinct first. Colonising the galaxy like this would only take around ten million years or so, which again is a long time to us, but small in the context of the universe’s 13.7 billion-year history.
This observation has led some people to conclude that we are either alone in the galaxy – and perhaps the universe – or that we are among the very first technological civilisations. It is hard to fault the logic: the universe is so big and so old, and crossing it does look feasible given enough time, so the absence of evidence of intelligence does seem like evidence of absence.
Others argue that the technological hurdles to crossing the galaxy are higher than this supposes, but this is less credible. And even if travelling physically around the galaxy is harder than it seems, sending messages using light should not be.
The dark forest
Another explanation for the great silence is that everyone is hiding. If you are alone in a dark forest and you suspect the presence of dangerous animals, it is a good strategy to avoid drawing attention to yourself. We cannot know the disposition of alien intelligences, so maybe we should adopt this strategy. This seems a compelling argument that the endeavour known as METI – messaging extra-terrestrials – is a very bad idea.
However, the argument that every single intelligent civilisation would adopt this strategy is implausible. For one thing, forests on Earth are not silent. For another, it is highly improbable that billions of different civilisations would arrive at the same conclusion, and adhere strictly to the strategy. Some would surely be reckless, or over-confident, and some might want to let the universe know they existed if they were on the brink of inevitable extinction for some reason. In fact, we have already broken the rule ourselves by sending out the Voyager probe with instructions how to find us and destroy us.
Furthermore, concealing all traces of advanced technology is hard: the hot plasma produced by space rockets is detectable from far away, and even mere computation generates heat which may show up against the cold background of space.
Another explanation which depends on the implausible assumption that all advanced civilisations think and behave the same way is the idea that aliens are already everywhere, but they are concealing themselves from us so as not to “blow our minds”, as David Bowie put it in his 1972 single “Starman”. The same idea is found in the Star Trek series, where it is called the Prime Directive.
Virtual worlds are more fun
You don’t have to be hiding to be invisible. Advanced civilisations might discover that zooming around the galaxy is a lot less enjoyable than hanging out in virtual worlds. Perhaps they convert asteroids, planets, or even stars into computronium, and spend their time in alien versions of World of Warcraft. Once again this depends on a level of convergence in attitudes across the universe which is hard to credit. On Earth, greater wealth generates greater diversity, and there is no reason to think this will cease. More resources seem to beget more varied and weirder buildings and art projects, for example.
One way that the implausible convergence might come about is that beings in virtual worlds discover that no matter how much computronium they have, it is never enough. A joke from the 1990s stated that “hardware giveth, but software taketh away”. However much hardware you have, someone can devise software that will exhaust its capacity. Intelligent beings living in a simulation where they could create copies of themselves might have to be parsimonious. Interstellar travel is not that.
A related idea that appeals to Anders is the aestivation hypothesis, in which intelligent entities living inside computers decide to put themselves on pause for a few billion years until the universe cools down before they stage their main activities, because it will be easier to cool their processors then. Aestivation is the same as hibernation, except that it happens during summer instead of winter.
Of course, if the universe’s most advanced civilisations are waiting for the universe to cool down, they would surely take steps to ensure that no-one messes with their server farms while they sleep. In which case, when we start to travel between stars we might get a visit from a galactic AI policeman, telling us to stay put.
The great filter
If none of the explanations which require huge numbers of alien civilisations to think and behave uniformly are true, then one of the values of the Drake equation has to be very low, so as to make the number of intelligent civilisations a low one – perhaps just us. It could be that the formation of new life – or the formation of intelligent life – is very hard. Or it could be that intelligent life always goes extinct for some reason.
If such a filter exists, we should prefer it to be behind us rather than ahead of us. For this reason, Anders’ Oxford colleague Nick Bostrom has argued that we should hope not to find life on Mars, and certainly not traces of intelligent life. (Unless it originated on Earth and got flung to Mars by an asteroid strike – or vice versa.)
Sparse or simulated
Putting all this together, Anders concludes that the most likely explanation for Fermi’s paradox is that we are alone, or almost alone. In which case, we should pay heed to Bowie’s Starman when he tells us not to blow it, because we would be rare and maybe important.
Finally, the Fermi paradox can also be adduced as evidence for the simulation hypothesis, which states that we live in a virtual world created by an advanced intelligence – possibly our own descendants who are simulating their history. It is expensive enough in terms of computronium to simulate this world, or at least the parts of it whose existence you can be confident of. It would be far too expensive to simulate numerous alien civilisations as well.