The Fermi paradox postulates that the human race is alone in the universe: if aliens are plentiful, why haven’t they come to visit yet? The Great Filter theory suggests that something – as yet undetermined – prevents or wipes out all life before it evolves to a complex technological civilisation capable of spreading throughout the universe. Adherents to the Great Filter theory would be greatly discouraged by discovering life on Mars: if the Great Filter is an event in our distant past (like the evolution of simple, single-celled life forms), then there is hope for the human race being the first life form to populate the cosmos. But if we discover complex multi-cellular life on Mars, then that evolutionary step, and all others before it, can be considered a galactic commonplace. The improbable step – the one before which most lifeforms are wiped out – would therefore be that much more likely to lie in our own future. Not a happy thought.
The Fermi Paradox, stated less glibly than above, does pre-suppose that if the universe were populated with an abundance of complex alien life, then we would be able to see some evidence of it. This is one of the potential flaws in the theory: what if the aliens are out there, but we cannot tell? One scenario for this is the “dangerous universe“: the idea that it is highly dangerous to let on that you exist, because if you do some other predator species will come and wipe you out; accordingly, noisy aliens are extinct, but quiet ones survive.
Another potential problem with the Fermi paradox is the question of whether we would even recognise alien life if we saw it. Some say that we the reason we haven’t seen any evidence of Dyson spheres is that we’ve only started looking relatively recently.
But there may be a simpler explanation to that last one. If there were a civilisation of Type II on the Kardashev scale (“”A civilization capable of harnessing the energy radiated by its own star (for example, the stage of successful construction of a Dyson sphere)”), what would their Dyson sphere look like from the outside? If the civilisation captured the entire energy of its star, wouldn’t that mean that no energy escaped its solar system?
Wouldn’t that look rather like a black hole, of which we have seen many already?