Alien spacecraft are strange. If decades of popular culture, eye-witness reports, and conspiracies are to be believed, most extraterrestrial whips are flying saucers. There is a dome on top and some landing gear below. There are pictures. The most famous may be the one above the words “I Want To Believe.”

These traditional UFOs clearly work in different ways than human aircraft. Though mankind has fiddled with saucer-shaped planes and rockets in the past, we’ve never succeeded at making them work. We don’t know how to do that within the bounds of physics. No, this doesn’t mean that flying saucers aren’t real. It simply means that they don’t make sense within a model of physics we’ve created. That said, neither does interstellar travel. So consistency, at least, is not a problem.

A lot of genuine scientific inquiry has gone into explaining the physics of flying saucers and other unidentified flying objects. Famously, William Markowitz made the case that UFO reports must be wrong in a 1967 article in Science. “Some people claim that nothing is impossible,” he wrote. “This is not so. The laws of mathematics and physics, if accepted, do provide limitations on what can be done.”

Markowitz detailed not only the difficulties of propelling a spacecraft through interstellar space, but also of gently landing and taking off again on the Earth’s surface, as UFOs are reported to do.

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