It’s the age old question: are we alone in the universe? It’s a topic we began wrestling as children, well before we even had basic knowledge of the solar system and galaxies. But it seems that even after taking astronomy classes, the answer isn’t much clearer. Instead, we now have a conflict between probability, which says there absolutely should be other intelligent life, and evidence, which insists there is not. I’m sure you know about the lack of evidence. Let’s explain why it’s so probable that more life exists.

There are 400 billion stars in our milky way galaxy. 20 billion of these are like our sun, and 1/5 of these have a planet that, like our Earth, is in the habitable not-too-hot and not-too-cold zone. Even if 0.1% of these planets had life, this means there should be 1 million planets with life just in the milky way!

Another thing to consider is that Earth, aged 4 billion years, is about a third of the age of the milky way, aged 13 billion years. While it’s true that the early years of our galaxy were chaotic and ridden with explosions and impacts, it would’ve quieted down enough after 2 billion years for other life-forming planets to develop. It’s also reasonable to assume that we would have seen these life forms by now. These other planets and their life forms would have had much more time than humans have to develop their space travel technology. If their spaceships could sustain populations across generations and started spreading these ships out to other planets, it would take only 2 million years to colonize the galaxy. Compare this to the 13 billion years that the milky way has been around and recall that there are potentially 1 million life-sustaining planets in the milky way that could do this colonizing.

As I’m sure you know, we’ve seen no sign of this. But why haven’t we? That is the Fermi paradox, named after physicist Enrico Fermi. There is no answer, but plenty of proposed explanations. The most interesting one to me is the great filter, so we’ll be focusing on that one.

The Great Filter

A lot of things have to fall into place for a life-form to develop the ability to colonize its galaxy. Perhaps one of these steps is a great filter, or barrier that life must overcome. It’s a challenge posed, that life forms either pass it or they don’t. Here’s the scary thing: we don’t know where it is.

It’s possible the great filter was something in the past that we already made it through. Perhaps it was the evolution of eukaryotic cells, or forming life at all. Maybe it was moving from chimpanzee-level intelligence to human-level intelligence. If this is the case, that means we somehow overcame something that everybody else couldn’t–this would make our kind of life rarer than we predicted. This is also the more exciting possibility. This makes the future, and the galaxy, ours to explore.

Here’s the scarier option: that the great filter is in our future. That means there’s a chance we won’t overcome it. Perhaps it will be nuclear war or climate change or a freak gamma ray explosion. If this is the case, there must have been lots of life like us before, but something kept it from developing enough to spread through the galaxy. Maybe this is what happened to life-forms on the older planets. This is the scarier of the options–that something lies ahead that nobody else managed to move past.

This is why philosopher Nick Bostrom says finding life on Mars would be devastating for us. If it’s simple life, it becomes less likely that the Great Filter is something already behind us. It would be even worse news if we found complex life on Mars–that would mean life more advanced than ours is not rare at all, and the Great Filter is still ahead of us.