ET Hypotheses

Physicist Enrico Fermi famously asked "Where are they?" when discussing the possibility of extraterrestrial civilizations. His point was simple. If a civilization in the Milky Way had achieved spaceflight even a million years ago-- not long in cosmic terms-- it would have had time to spread throughout the galaxy. They should be here. Since Fermi had no evidence of that, he concluded there likely were no mature, spacefaring civilizations in the galaxy.

UFO researchers disagree there is no evidence, but they have yet to produce a single smoking gun. SETI researchers are also looking for evidence in the form of electronagnetic transmissions that can be picked up across interstellar space. A few interesting possibilities have been found, but nothing has been confirmed.

There have also been hypotheses to counter Fermi's question. The Zoo Hypothesis holds that Earth is being watched by ETs much as we watch animals in a zoo. A similar idea, the Quarantine Hypothesis, argues that Earth has been put off-limits by ETs to allow humanity to develop as we will. A third, the Jungle Hypothesis, suggests that ETs are around and are not particularly trying to hide from us, but they operate at such a different level that we are unlikely to come in contact with them, much as a given ant in a jungle is unlikely to ever come across a human even though humans don't try to hide from ants.

All these attempts to deal with Fermi's simple question suffer from the same basic weakness. They lack evidence to support them. They are, therefore, at least for the moment, more philosophical constructs than scientific hypotheses. That could change in a moment, of course, perhaps especially in the case of SETI, where researchers are pursuing plausible strategies in a systematic way. Until that new moment, however, all we can really say about the possibility of other civilizations in this galaxy is absence of evidence is not evidence of absence.