“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.” – Arthur C. Clarke
It is a question humankind has asked since we first peered at the heavens – are we alone? We now know that our home galaxy is 100,000 light-years across, weighing in at 100 billion solar masses, and home to anywhere from 100 billion to 400 billion stars. The latest data from the Kepler mission suggests that there are roughly 40 billion Earth-sized planets in the so-called habitable zone. Of those 40 billion, 11 billion rocky, Earth-like planets are orbiting sun-like stars. Beyond that, the universe is an ancient 13.8 billion years old. If we were to compress the age of the universe into a year in the Gregorian calendar, one second is the equivalent of 440 years. Humanity achieved what archeologists call “behavioral modernity” 2 minutes before midnight on December 31st. The numbers are so large, even if intelligent life is exceedingly rare, that something strange seems to be going on. As the physicist Enrico Fermi put it, “Where is everybody?” In celebration of Halloween, let us look at one of the more macabre explanations for the deafening silence: The Great Filter.
The idea behind the Great Filter is a simple one – there is some step somewhere along the way from inorganic matter to galactic civilization that is either exceptionally improbable or insurmountable. To understand why, it helps to look back at the only example of (arguably) intelligent life capable of spaceflight we know of, Homo sapiens. Galileo created his first telescope in 1609, and in 2013 humanity became an interstellar species when Voyager 1 left our solar system. If we look back at our compressed time scale, that is about one second from midnight on December 31st. Admittedly, the Milky Way is a large place, but it has also been around for an incredibly long time. The Breakthrough Starshot project is an attempt to send a probe to Alpha Centauri at about 20% the speed of light. John Von Neumann proposed the idea of self-replicating machines back in the late 40’s. The idea of building a self-replicating probe to explore the cosmos does not appear to be against any known laws of physics. If in the next four centuries (or a second in our compressed time scale) we were to build such a probe, how long would it take to explore our home neighborhood? 500,000 years to cross the entire galaxy and let us allow double that time to replicate. All told, it would take 1.5 million years to explore the Milky Way, or about 1 hour in our compressed calendar. And yet we see no evidence of any extraterrestrial intelligence, so a natural question pops up – is the Great Filter ahead of us or behind us?
On our cosmic calendar, Earth formed sometime at the end of September or early October. Life appears very rapidly after – anywhere from five to twenty days after Earth’s formation. It might be that forming life from inorganic matter is just vanishing unlikely for a variety of reasons. We benefit from having a reasonably stable star, a relatively large moon to slow our orbit, and two giant guardians in the form of Jupiter and Saturn to suck up wayward asteroids. One other alternative is that the step from prokaryotes like bacteria to eukaryotes, cells with a nucleus containing their DNA within their membrane, is improbable. Eukaryotes are the building blocks for all multicellular life on Earth. As far as we know, eukaryotes only evolved once, and very late in comparison to how long life has been on the planet. In our conceptual calendar, Earth has had simple life for roughly 114 days, but eukaryotes only made their appearance 42 days ago.
But as for life, we only have a sample size of one to examine, so it can be hard to tell if we have passed the Great Filter. Fortunately, with the various missions to Mars, Europa, and Enceladus all planned, we may have the answer to whether life on our solar system is unique to Earth. Unfortunately, if the Great Filter is real, finding organisms unrelated to Earth life would be bad news. If we see evidence of life like prokaryotes, then we can assume that the formation of life is not a Great Filter, since it will have happened on at least two separate occasions out of a of the habitable planets within our Solar System. If we find evidence of complex, multicellular life, that means the jump from simple cells to complex multicellular organisms is also reasonably common. There may be more steps along the way, but there is one other foreboding conclusion – the Great Filter lies ahead of us, not behind us.
The misfortunes that can befall a nascent interstellar species to stop them from colonizing or at least exploring the galaxy are too numerous to count. Some are benign – perhaps we all upload our brains into some fancy virtual reality device. Others are depressing: there may be some physical law we have not discovered that stops us from exploration, or it may not be possible to overcome the cost. Other are apocalyptic, be it global warming, some version of nuclear war, a massive solar flare or asteroid, or some yet unforeseen tragedy from technological innovation. Ironically, the more life we find, and the more complex it is, the more likely we are getting ever closer to slamming into the Great Filter. Whether it be a few seconds or a minute before midnight, a reckoning may be coming.
We launched the Voyager missions with a message to any intelligent life out there with a record of human culture, science, and even Earth’s location. Some people criticize the message as incoherent to an extraterrestrial intelligence that evolved under different constraints or argue that the vastness of space is such that nothing would find it anyways. The Great Filter provides another argument – there is no one out there to receive it. If we are lucky, and the Great Filter is behind us, then maybe in time humanity can become the Great Old Ones about which future intelligent life tells stories. If the Great Filter is ahead of us, those missions may end up being the only monuments that we were ever here. Have a happy and spooky Halloween!