We spend billions of dollars on military equipment, yet most of it will likely never see battle. According to philosopher Thomas Hobbes, we do this because humans are motivated primarily by fear and a desire to survive – we’d like to think other states won’t threaten us, but history shows that sometimes they do.
The same logic yields interesting results when applied to potential interstellar relations. Mathematically speaking, we’re unlikely to be the only intelligent species around. Should we be as afraid of them as we are of each other?
NASA aims to put the first human boot on Martian soil sometime in the late 2030s, meaning homo sapiens will have taken just 350,000 years to become a multi-planetary species. If the history of Earth was shortened to a single day, our ascent from ape to space-farers has taken place in the final minute and 17 seconds.
We also know there are plenty of planets in our galaxy with conditions that could support life. We’ve already discovered 3,200 in the small areas we’ve looked at. The final tally could be in the billions. The principle of non-exclusivity states that diversity tends to prevail unless there is a systemic mechanism preventing it, and mathematical probability also says we’re not alone: according to the Drake Equation, which uses a number of variables to estimate the density of cosmic life, we should be one of 20 neighbouring civilisations.
So if it doesn’t take long for intelligent species to spread beyond their home planet, there’s billions of home planets out there, and cosmic diversity tends to prevail - where is everyone?
This question is known as Fermi’s Paradox, and it has puzzled academics for decades. There are a number of explanations. Perhaps we haven’t met any other forms of intelligent life because they don’t exist in our galaxy. Space is prohibitively enormous, so it's possible intelligent life exists in nearby star systems but, like us, hasn’t yet figured out how to travel through interstellar space. There is also the possibility that they have and we just don’t know it, or that they could figure it out if they wanted to but just can’t be bothered.
Other studies argue that the distribution of life through the galaxy is not dependent on propulsion technology. Researchers have run simulations of interstellar settlement, including variables such as spacecraft velocity, and found that the natural movement of stars over a long enough time period is itself enough to create a galactic civilisation. Yet the simulations also found a large natural variability: their galaxies were either teeming with life or almost totally silent. Perhaps ours is silent; perhaps intelligent life rarely becomes space-faring; perhaps it self-destructs before it can.
Nonetheless, it seems remarkable that we have yet to gain even a hint of another civilisation. Humanity has been inadvertently broadcasting signals into space since the invention of the radio, and stronger TV signals have reached well beyond nearby habitable planets. We’re spending millions looking for similar signs of modulated radiation from other civilisations. Why haven’t we at least heard something?
There is a darker explanation, one that follows the Hobbesian logic of spending billions on defence capability to protect ourselves from each other. Maybe civilisations that give away their location don’t survive.
As Chinese author Liu Cixin puts it: the galaxy is a dark forest, each civilisation a quiet hunter eliminating those it encounters before they can do the same. This explanation may seem far-fetched. Yet it is grounded in game theory, and according to a paper from the Royal Astronomical Society, is one of the only plausible hypotheses explaining the ‘Great Silence’; it provides a mechanism to systematically prevent all contact over vast time periods, is wholly consistent with all the factors in the Drake Equation, and fits our observations of the galaxy’s electromagnetic environment.
This was the view taken by Stephen Hawking, who once compared possible contact with interstellar intelligent life to the arrival of Europeans in North America. Only one civilisation of the many possibly out there would need to follow this path for the theory to work, destroying all other civilisations soon after they invent the radio.
Yet Europeans didn’t go to North America simply to eradicate Native American nations or because they were afraid: they were motivated by ideology and perceptions of prestige and glory. These may be our vices, the traits that push humans towards violence, but our species is actually less violent today than we have ever been, increasingly pulled towards peace by what Steven Pinker called the ‘better angels of our nature’ – empathy, reason, self-control. Pinker argues this trend is a product of the same Hobbesian logic used in the ‘Dark Forest’ theory.
If humans have become less violent as we’ve advanced, despite outbursts of war and what the news leads us to believe, then perhaps the same is true of other space-faring civilisations. As scientist David Brin wrote: “It might turn out that the Great Silence is like that of a child’s nursery, wherein adults speak softly, lest they disturb the infant’s extravagant and colourful time of dreaming.”