The Red Planet has been getting a lot of attention recently, between India's low-cost MOM probe successfully entering Mars's orbit, NASA's MAVEN doing the same, and the Curiosity rover reaching the most potentially fertile site yet for signs of ancient life. Elon Musk has even talked about building a city there. For most people who look out at the night sky and wonder whether we're alone, Mars holds our best hope for proving that we're not.
But not for Nick Bostrom. The famed futurist, author, and Director of the Future of Humanity Institute at Oxford University, thinks that any sign of life we find on Mars would be a bad sign, an argument he developed in a 2008 paper entitled "Where Are They?: Why I Hope The Search For Extraterrestrial Life Finds Nothing."
It would be good news if we find Mars to be completely sterile. Dead rocks and lifeless sands would lift my spirit.
Conversely, if we discovered traces of some simple extinct life form - some bacteria, some algae - it would be bad news. If we found fossils of something more advanced, perhaps something looking like the remnants of a trilobite or even the skeleton of a small mammal, it would be very bad news. The more complex the life we found, the more depressing the news of its existence would be. Scientifically interesting, certainly, but a bad omen for the future of the human race.
So why the apparent negativity?
Scientists around the world are hoping that NASA's Curiosity rover will find silicon in these rocks on Mount Sharp, an indication that this might have once been an environment for life. Nick Bostrom hopes the rover finds nothing of the kind.
Bostrom writes:
From these two facts it follows that there exists a "Great Filter." The Great Filter can be thought of as a probability barrier. It consists of exist one of more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth-like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology. You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be powerful enough - which is to say, the critical steps must be improbable enough - that even with many billions rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods.
In other words, if it's possible for life to evolve to the point that it can take to the stars, it has had plenty of opportunity to do so. The fact that we see no evidence for that whatsoever is an indication that it is extremely difficult to do. (Bostrom borrows the concept of the Great Filter from a paper by economist Robin Hansen.)
That difficulty could lie in the past - i.e., in the evolutionary steps that have led to us - or in the future, the steps between where we are now and where we'll need to be to go interstellar on the regular.
For example, perhaps it is very, very improbable that even simple self-replicators should emerge on any given Earth-like planet. Attempts to create life in the laboratory by mixing water and gases believed to have existed in the early atmosphere on Earth have failed to get much beyond the synthesis of a few simple amino acids. No instance of abiogenesis has ever been observed.
We don't really know how life got started in Earth, but our experiments so far suggest it's not as simple as sloshing a few amino acids around and waiting for them to come to life. There are theories that life was seeded onto Earth from elsewhere in space, perhaps a comet, but of course these theories wouldn't explain how living creatures spontaneously emerge from non-living matter.
So one possibility is that the Great Filter is behind us. This would explain the absence of observable aliens. Why? Because if the rise of intelligent life on any one planet is sufficiently improbable, then it follows that we are most likely the only such civilization in our galaxy or even in the entire observable universe.
Like Carl Sagan said, if you want to make an apple pie from scratch, you must first create the universe. You also need to invent human beings. The leap from a universe that allows for life to happen and a species of tool-using creatures that can conceive of delicious fruit-filled pastries might be daunting enough that we Homo sapiens are the only ones who have made the leap so far.
The other possibility is that the Great Filter is after us, in our future. This would mean that there is some great improbability that prevents almost all technological civilizations at our current human stage of development from progressing to the point where they engage in large-scale space-colonization and make their presence known to other technological civilizations. For example, it might be that any sufficiently technologically advanced civilization discovers some technology - perhaps some very powerful weapons technology - that causes its extinction.
This second possibility is, for Bostrom, the really frightening one.
If we find evidence of life on Mars, one theory is that it means that life as such isn't all that difficult to bring about - something that happens on two planets in the same solar system can't really be called a rare event.
Of course, if we find out that life somehow originated on Mars and was transferred to Earth, or vice versa, that would be a different story. In that case, the origination of life would still be an extremely improbable event that only happened once. It would thus still be a good candidate for the Great Filter to have come before current time.
Barring a single source of life for the two planets, the more complex the discovered Martian life turns out to be, the more likely it is that the Great Barrier lies in our future; whatever rocks other would-be starfaring civilizations ran aground on, it's that much likelier that we'll run aground of them too.
Even worse, whatever these catastrophes might entail, they could easily be awful enough to not merely keep us in low-Earth orbit, but to completely destroy humanity:
...the kind of societal collapse that merely delays the eventual emergence of a space-colonizing civilization by a few hundred or a few thousand years would not help explain why no such civilization has visited us from another planet. A thousand years may seem a long time to an individual, but in this context it's a sneeze. There are planets that are billions of years older than Earth. Any intelligent species on those planets would have had ample time to recover from repeated social or ecological collapses. Even if they failed a thousand times before they succeeded, they could still have arrived here hundreds of millions of years ago.
To constitute an effective Great Filter, we hypothesize a terminal global cataclysm: an existential catastrophe. An existential risk is one where an adverse outcome would annihilate Earth-originating intelligent life or permanently and drastically curtail its potential for future development.
In other words, if we advance technologically to the point that we might take to the stars, we might also be at a point where we can - and likely will - reduce ourselves to atoms. This could be the result of a terrible accident, warfare with the unthinkable weaponry of the future, or some other danger we haven't yet conceived of.
OK. So what's the good news? Or if not good, not frightening and depressing?
If - as I hope is the case - we are the only intelligent species that has ever evolved in our galaxy, and perhaps in the entire observable universe, it does not follow that our survival is not in danger. Nothing in the above reasoning precludes the Great Filter from being located both behind us and ahead of us. It might both be extremely improbable that intelligent life should arise on any given planet, and very improbable that intelligent life, once evolved, should succeed in becoming advanced enough to colonize space. But we would have some grounds for hope that all or most of the Great Filter is in our past if Mars is indeed found to be barren. In that case, we may have a significant chance& of one day growing into something almost unimaginably greater than we are today.
It's possible that we've overcome some absurd odds to exist in our present form and that incredible odds stand between us and interstellar travel. But, in a barren, lifeless Mars, Bostrom sees a ray of hope that the greater challenge is already behind us.