One of the most intriguing questions in searching for life in the universe is the one asked by physicist Enrico Fermi in 1952. During a lunch break, Fermi reasoned our solar system is only a third of the universe’s age, leaving plenty of time for advanced alien civilisations to colonise the galaxy. He posed the question: “So where are they?”. It is known as the Fermi Paradox.
In 1998, economist Robin Hansen suggested one answer is what he coined as “The Great Filter” – that there are evolutionary steps from the formation of life from lifeless chemicals to intelligence capable of colonising the galaxy. On Earth, life made it through all the filters to intelligence.
We probably have around two billion more years to colonise before our Sun will no longer support life on Earth. Hansen, therefore, posits that the filter that prevents colonisation is in our future – and that no civilisations make it past that point and explains the Fermi Paradox.
There are certainly challenges to the continued existence of humanity, such as climate change, pandemics, and nuclear war. We are a society living on the edge, worried about the future for the generations to come.
There are many more answers to the Fermi Paradox – some have estimated up to 50 explanations, one being we are in a backwater of the galaxy and uninteresting since technological civilisation has only arisen in the past century or so.
When we look into the night sky, we are seeing the past. We don’t know what the sun looks like now – we can only see it from Earth as it was about eight minutes ago. Much of the Milky Way is as it was many thousands of years ago. Our best telescopes allow us to see the universe as it was billions of years ago. We cannot see it as it is now.
We are limited by the speed of light – which travels about 300,000 km per second. That is incredibly slow compared to distances in the universe, even to the nearest star system to our own, Alpha Proxima, about 4.3 light years away. An alien civilisation just 100 light years away would not see any signs of technology on Earth – Earth, as it is today, would be in their future.
There is another issue. If Fermi, Hansen, and others are wrong, and technological civilisations exist elsewhere in the galaxy, then they would have to co-exist in the same temporal framework. Too young, and no contact is possible. More than 200 or so years in advance of us, then they would be beyond our grasp as intelligence. It is another explanation of the Fermi Paradox.
Why use the world’s largest telescopes to undertake the Search for ExtraTerrestrial Intelligence (SETI) experiment? Morrison and Cocconi explained it in a 1959 Nature paper: If we have the tools to search but don’t use them, then we turn a low chance of success into a zero chance.
Astronomer Dr Carl Sagan said several decades later:
“In the deepest sense, the search for extraterrestrial intelligence is a search for ourselves.” It is about understanding our place in space and the future of life on Earth.”
What are the chances we are alone in the universe – that we are the first and only technological civilisation to ever arise in 13.8 billion years of our universe?
In 2016, a paper in Astrobiology calculated the possibility that extraterrestrials are not here because they are not there. Rather than asking how many technological civilisations might be out there, authors Frank and Sullivan asked a different question – what is the probability that we are the only ones in the cosmos?
Frank and Sullivan found that for humans to be alone, the odds of developing a technological civilisation on any planet in the universe would have to be about one in 10 billion trillion – a very low number. But even one chance in a trillion would produce ten billion other technological civilisations across the universe’s history.
So back to Fermi’s Paradox: Why aren’t they here? One possibility is that civilisations, including ours, don’t last very long. This is the L factor in the Drake Equation, the lifetime of a civilisation. Drake said it was unknowable because we have only one sample – our civilisation. If the lifetime of a civilisation is short – that intelligence arises and learns to self-destruct shortly after attaining a technological society – then there would be no chance of interstellar travel by any method, including generational spaceships, and no physical contact will ever be made. Technological civilisations pop into existence for a few hundred years or so and are then extinguished – Hansen’s Great Filter, but rather sooner than he suggests.
In the scenario that technological civilisations do not last long, it could still be possible to detect their past existence. A SETI signal will be necessarily old. Just as we see the sun as it was about eight minutes ago because of the relatively slow speed of light, so messages from any extraterrestrial intelligence will be hundreds or thousands of years old because of the time it took to reach us.