A Scientist's Weird Hope About Extraterrestrials

In his new book Life 3.0 MIT physicist Max Tegmark confesses a strange hope he has about the search for extraterrestrial life: that it finds nothing. Below is his reasoning on page 245 of the book:

Although I'm a strong supporter of all the ongoing searches for extraterrestrial life, which are shedding light on one of the most fascinating questions in science, I'm secretly hoping that they'll all fail and find nothing! The apparent incompatibility between the abundance of habitable planets in our Galaxy and the lack of extraterrestrial visitors, known as the Fermi paradox, suggests the existence of what the economist Robin Hanson calls a “Great Filter,” an evolutionary/technological roadblock somewhere along the developmental path from the non-living matter to space-settling life. If we discover independently evolved life elsewhere, this would suggest primitive life isn't rare, and that the roadblock lies after our current human stage of development – perhaps because space settlement is impossible, or because almost all advanced civilizations self-destruct before they're able to go cosmic. I'm therefore crossing my fingers that all searches for extraterrestrial life find nothing: this is consistent with the scenario where evolving intelligent life is rare but we humans got lucky, so that we have the roadblock behind us and have extraordinary future potential.

Tegmark presents here rather concisely an argument similar to the argument presented by philosopher Nick Bostrom in this paper. The argument can be stated like this:

1. There must be some Great Filter which makes it very unlikely that planets produce civilizations that spread throughout the galaxy – something such as an unlikelihood of life originally appearing, an unlikelihood of intelligence ever appearing, an unlikelihood of interstellar travel, or an unlikelihood of a civilization surviving for long.
2. Such a Great Filter can either be in our past or our future (for example, if the Great Filter is the unlikelihood of life ever appearing on a planet, then the Great Filter is in our past, and we have already leaped over this hurdle).
3. If the Great Filter is in our future, we should be very sad, because it will mean our civilization will probably not last very long.
4. But if the Great Filter is in our past (some hurdle we have already jumped over), then we are in good shape, and our future is bright (the whole galaxy might be ours for the taking).
5. If we discover life on another planet, it is evidence that life commonly evolves in our galaxy, and this shows that the Great Filter must be in our future, and that we won't last very long.
6. Therefore, it is bad news if we discover any extraterrestrial life.

There are multiple problems involved in this line of reasoning. The first is the problem of assuming that extraterrestrials have not spread throughout the galaxy. We do not know that such a thing has not occurred. UFO sightings may provide evidence against such an assumption. The fact that Earth has not been colonized by extraterrestrials is no proof that some other civilization has not spread throughout the galaxy. It is quite possible that a civilization spreading throughout the galaxy would not colonize every habitable planet, but leave some planets as kind of zoos or nature reserves. When descendants of Europeans spread throughout North America, they did not fill the whole continent with settlements, but preserved quite a bit of North America as nature preserves. A similar conservation principle could be observed by some galactic civilization spreading across a galaxy. Or it might be impractical to colonize every habitable planet, for any number of technical or logistical reasons.

Another problem with the line of reasoning cited above is that it commits a fallacy along the lines of stating: if we discover extraterrestrial life, that would show the Great Filter is in the future, not our past. The discovery of any type of life other than civilized intelligent life would not at all do such a thing. Let's consider various possible candidates for a Great Filter:

Now after reviewing all of these possible candidates for a Great Filter, it should be clear that the mere discovery of some type of extraterrestrial life would not by any means show that this Great Filter is something that mankind has not yet overcome. For example, if some very simple prokaryotic microscopic life were to be discovered on Mars or a moon of Jupiter or Saturn, that would still leave standing the last seven of these candidates as possibilities, including the “Multi-cellular Life” filter that it is too hard for multi-cellular life to appear from microscopic life. And if we were to discover signs of oxygen in the atmosphere of a distant planet, suggesting that photosynthesis had occurred, that would still leave standing the filters such as the “Intelligence” filter and the “Language and Civilization” filter. It would still be perfectly possible that the Great Filter is that it is too hard for intelligent language-using tool-making life to appear.

So Tegmark is very much in error when he states, “If we discover independently evolved life elsewhere, this would suggest primitive life isn't rare, and that the roadblock lies after our current human stage of development – perhaps because space settlement is impossible or because all advanced civilizations self-destruct before they're able to go cosmic.” Discovering primitive life would do no such thing, as it would still leave standing as candidates for the Great Filter such possibilities as the “Intelligence” filter and the “Language and Civilization” filter. Discovering primitive life would still leave standing the possibility that it is very unlikely that life would ever evolve to a state of intelligence, language-use, tool-making and civilization, and that such a difficulty is the Great Filter.

And, similarly, Nick Bostrom committed the same error when he stated the following:

And if we discovered the fossils of some very complex life form, such as of some vertebrate-like creature, we would have to conclude that the probability is very great that the bulk of the Great Filter is ahead of us. Such a discovery would be a crushing blow. It would be by far the worst news ever printed on a newspaper cover.

This is completely wrong. From such a discovery, it would not by any means be true that “we would have to conclude that the probability is very great that the bulk of the Great Filter is ahead of us.” For there would still be standing the “Intelligence” filter and the “Language and Civilization” filter. It would still be perfectly possible that it is exceptionally rare for life to evolve to a state of intelligence, language-use, tool-making and civilization, and that such a difficulty is the Great Filter. If you think that intelligence, language-use, tool-making and civilization automatically follows once large-scale life exists, consider the fact that humans have a whole range of mental characteristics (including spirituality, artistic creativity, mathematical ability, philosophical reasoning, intellectual curiosity, imagination, and language use) that cannot be explained by appealing to natural selection, as these are things that do not increase an organism's chance of surviving until reproduction. Some evolution experts have argued that the appearance of something like humanity was extremely improbable. Until we discover some extraterrestrial civilization, it is still a very viable possibility that it is incredibly rare for life to evolve into civilized language-using life.

The only discovery that might allow us to reasonably say that the Great Filter is in our future and not our past would be the discovery of civilized life elsewhere in space. Would that be something that would be a cause for grief and sorrow, along the lines Tegmark and Bostrom have suggested? Would such a discovery tell us something negative about our future?

No, simply because the very moment such a discovery was made, the very idea of a Great Filter would be discredited. In our current state of knowledge, lacking any proof for extraterrestrials, this idea of a Great Filter has significant weight. But at the very moment we discovered proof of an extraterrestrial civilization, the idea of a Great Filter would be discounted and discredited. People would start saying: evidently this idea of a Great Filter isn't very sound, if we have already discovered an extraterrestrial civilization. They would also start saying: if we have already discovered one extraterrestrial civilization, there must be many others in our galaxy. The discovery of an extraterrestrial civilization would be a crippling blow to the very idea of a Great Filter.

We can therefore imagine no circumstances under which it would make sense to lament and grieve over a discovery about extraterrestrial life, on the grounds that it told us something negative about our future. Discovering anything less than civilized intelligent life would do nothing to prove the idea that the Great Filter is in our future and not our past. And if we did discover civilized intelligent life, the very idea of a Great Filter would instantly be so discredited that it would at that point not make sense to be drawing conclusions about our future based on such a concept.