You glimpse a light in the night sky -- not a star, not an airplane, but something radically different. It moves with baffling speed, pulsates with radiance beyond anything you've witnessed. Three letters immediately enter your mind: U-F-O.
Technically an unidentified flying object can be anything when you get right down to it, but the term has become synonymous with extraterrestrial spacecraft. Alleged sightings began popping up in the 1950s and continue to this day throughout the world. Exact descriptions of alien spacecraft vary with each telling, but witnesses often describe a lighted object capable of hovering silently and zigzagging in midair.
The technology for such a craft and the ability for a living passenger to survive its g-forces are well beyond humanity's modern technology [source: Kaku]. Additionally, given the massive distance between habitable star systems, such craft would have to travel at impossible speeds or with patience that staggers the imagination.
What else does science have to say on the matter? Not much. From a scientific standpoint, there's insufficient evidence to make a case for alien visitation. Most UFO sightings depend on fallible human accounts, imperfect footage and conspiracy theory. All of this tends to crumble under the scrutiny of scientific method, humanity's best sieve for separating reality from fantasy.
After all, scientific inquiry hinges on something called the null hypothesis, which means the burden of proof is on anyone making a positive claim. A dog ate your homework? Great, where's the testable evidence? You saw an alien spacecraft? Excellent, let's test and validate the story.
In other words, it's up to so-called ufologists to convince the scientific world that UFOs are alien spacecraft, not for scientists to prove them wrong. Along the same lines, the scientific world doesn't go on the defensive every time someone sees a ghost. Even in the presence of testable evidence, perfectly terrestrial claims demand rigorous testing and a high degree of certainty in the results [source: Shermer].
Although the scientific world remains unconvinced, countless individuals continue to witness unexplainable things in the sky, sights that haunt or inspire them until their dying days. In rare cases, whole crowds glimpse such phenomena. What are we to make of such claims?