Paucity of physical evidence

"I've no doubt that UFOs are here to stay," said Seth Shostak, Senior Astronomer at the SETI Institute in Mountain View, California. "I'm just not convinced that alien craft are here to stay ... or for that matter, even here for brief visits.

"First, despite a torrent of sightings for more than a half-century, I can't think of a single, major science museum that has alien artifacts on display," Shostak said. "Contrast this paucity of physical evidence with what the American Indians could have shown you fifty years after Christopher Columbus first violated their sea-space. They could have shown you all sorts of stuff-including lots of smallpox-infested brethren-as proof that they were being 'visited,'" he said.

When it comes to extraterrestrial visitors in the 21st century, the evidence is anecdotal, ambiguous, or, in some cases, artifice, Shostak suggested.

Calling it "argument from ignorance", Shostak pointed to the claim that aliens must have careened out of control above the New Mexico desert simply because some classified government documents sport a bunch of blacked-out text. "How does the latter prove the former?"

Sure, the missing verbiage is consistent with a government cover-up of an alien crash landing, Shostak said. "But it's also consistent with an infinitude of other scenarios...not all of them involving sloppy alien pilots," he added.

Shostak said that it is not impossible that we could be visited. It doesn't violate physics to travel between the stars, although that's not easy to do.

"But really, if you're going to claim-or for that matter, believe-that extraterrestrials are strafing the cities, or occasionally assaulting the neighbors with an aggression inappropriate for a first date, then I urge you to find evidence that leaves little doubt among the professionally skeptical community known as the world of science."