source: Where Are They? by Nick Bostrom

The Great Filter

The Great Filter one or more highly improbable evolutionary steps that would be required to produce an intelligent civilization that would be visible to us

This filter could be in our future or in our past

  • if a Great Filter is in our future, we’ll probably never colonize the galaxy
  • so we better hope the Great Filter is in our past

Note: the Great Filter could be in our future and our past

  • Even if it’s behind us, we’ll probably be some sort of post-human by the time we colonize the galaxy

A past Great Filter

If the Great Filter is in our past, then we might be the only intelligent civilization in our galaxy. This would explain the absence of observable aliens

If the great filter is in our past, that means it’s extremely unlikely that an Earth-like planet will produce a space-faring civilization

We can’t calculate how probable this is based on our understanding of evolutionary biology, but we can identify several possible Great Filters

  • It took hundreds of millions of years between the formation of Earth and the first known life. This could be a great filter
  • Another potential great filter: the transition from prokaryotes to eukaryotes (this took 1.8 billion years)
  • Some evolutionary transitions aren’t good candidates for great filters. Examples: flight, sight and photosynthesis all developed multiple times on Earth. This indicates that they’re not unlikely events

A future Great Filter

There could also be a Great Filter that prevents civilizations that are at our level from progressing to the point where they colonize space

Nick Bostrom is defining an effective great filter as an existential catastrophe

It’s possible that there’s some destructive tendency common to almost all sufficiently advanced civilizations. And it would have to be really destructive and really probable

  • if a civilization failed a thousand times before succeeding, they could have still arrived here millions of years ago
  • some technological discovery could constitute a great filter

This is extremely related to Bostrom’s Vulnerable World Hypothesis

Other possibilities

There might be aliens that we can’t detect. Bostrom thinks this is unlikely

  • If aliens at this level exist, they would have expanded to us by now

Aliens might have decided not to colonize the universe. Bostrom thinks this is unlikely too

  • there are resources in space
  • all life on earth spreads whenever it can, so it would be reasonable to assume that alien life does too
  • the civilization would have to never change its mind about non-expansion
  • it only takes one. Even if nine alien races decide not to take over the galaxy, a tenth alien race might

How might a species colonize space?

Von Neumann probes unmanned self-replicating spacecraft capable of interstellar travel

  • named after Jon von Neumann
  • if a probe were to travel at 1/10 the speed of light, aliens could colonize the galaxy within a couple million years
  • this is very short in comparison to the amount of time it takes for life to evolve (billions of years)

Life on Mars

We’re sending probes to Mars now. Bostrom hopes they don’t discover any signs of life

  • if there was life on Mars, it’s more unlikely that the Great Filter was behind us
  • the more complex the life we find, the less hope we have of colonizing the galaxy

We should still be investigating Mars. The more information we have, the better

The Observation Selection Effect

Whether intelligent life is common or rare, every observer is guaranteed to find themselves

  • this means that if Earth is the only place in the galaxy with intelligent life, it’s not a coincidence that we live here