Author: Nick Bostrom
Future of Humanity Institute
Faculty of Philosophy & James Martin 21st Century School
Oxford University

Full paper (PDF)

Where Are They? Why I hope that the search for extraterrestrial life finds nothing, published in the MIT Technology Review, May/June issue (2008): pp. 72-77.


Around 2020, a Mars Sample Return Mission is expected to provide us with a solid answer to the question "Is there life on Mars?". If the answer were "Yes", mostly everyone would be quite happy. The discovery would certainly hold tremendous scientific significance. This paper demonstrates why we should actually hope that the answer will be "No", and why finding life on Mars, or anywhere in our solar system, would be very bad news. More precisely, the more advanced the life traces we find on Mars, the worse the news.

Currently, UFO-nuts notwithstanding, there is no evidence of extraterrestrial life. Yet, the universe is host to hundreds of billions of galaxies, each having on the order of 100 billions stars, some with planets that could develop life, of which a good fraction should develop intelligent life, of which a significant portion should develop space-travel technology, of which some would start colonizing other planets, in search for resources. Colonization would be an automated process, performed by self-replicating spacecraft (von Neumann probes) which would mine planets and asteroids, send some resources home, then build copies of themselves to further explore. Such probes have not yet been built, but are well within our reach in the next 50 years (see existing quasi-self-replicating machine such as the RepRap project). If the von Neumann probes traveled at no more than 1% the speed of light, our entire galaxy (100,000 light years across) would be colonized in about 20 million years, a spec compared to the age of the universe (15 billion years).

How come then, that we don't see any evidence of such civilizations? (This is known as the Fermi Paradox). Several hypotheses have been proposed.

  1. Civilizations choose to stay close to home and not colonize the universe.
  2. They do exist, but choose to hide themselves from our view, until we are evolved enough to "be admitted to the club".
  3. There aren't any other space-faring civilizations, because
    • We are the first one
    • All others have been destroyed before they started to colonize the universe.

There are several problems with the first hypothesis, that civilizations would stay close to home:

  • Life tends to spread wherever possible. We have found life in deep oceans, under the Antarctic ice shelf, or in radioactive waste.
  • Intelligent life tends to spread wherever possible. Humans cover all the continents, have a permanent presence in space, and are about to colonize oceans.
  • Resources on any planet are limited. Outer space is where most of the resources are. Space-faring civilizations need resources to survive and build whatever they value, be it temples or supercomputers.
  • Even if civilizations decide not to expand into space, they might change their minds after a few thousand years, or accidental expansion
  • Finally, it only takes one expansionist civilization for the entire galaxy to be colonized in a relatively short time.

The last argument also answers the second hypothesis, that technologically advanced civilizations exist around us but choose not to reveal themselves and hide all their artifacts: "It takes but one match to start a fire; only one expansionist civilization to launch the colonization of the universe".

The last hypotheses are that we are the first space-traveling civilization (highly unlikely, statistically), or that all others have been destroyed (or are being destroyed) before reaching space travel. Total civilization annihilation can happen due to the discovery of a technology that is necessary for space travel, and which can be used destructively. Nuclear weapons and molecular nanotechnology are two examples (see gray goo for the latter). Life might also not reach the point of developing technology (perhaps intelligence only arose on Earth due to a highly improbable combination of factors), or might not arise at all. Indeed, as of 2010, no one has yet synthesized a "protocell" (which would have the necessary properties of life) using basic components; abiogenesis has not been observed.

The author calls these causes that would prevent a planet from producing a space-faring civilization, "Great Filters": life being highly improbable, development of intelligent life being highly improbable, and intelligent life self-destructing before reaching space colonization. There may be other "Great Filters", but the idea is that if we do find life on Mars, it means that life is actually quite probable (since at least two planets in the same solar system developed it), and therefore we must explain the absence of other civilizations by the Great Filters happening further down the road from an empty planet to a civilized galaxy nexus. The more advanced life traces that we find on other planets, the more likely it is that a Great Filter of a self-destructive nature is what killed all other civilizations, and what's in store for ours.

Therefore, if we don't find any life on Mars, we should, actually, rejoice.

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