Forget the Pentagon Report, math can tell us if aliens exist

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The US military has released previously classified photos and films related to unidentified flying object (UFO) sightings, which mostly show something blurry moving strangely. Still, I hear that a friend of a friend has gone from thinking there’s a 1% chance that UFOs are aliens to now believing it is 50%. Is he rational?

People are constantly seeing things in the sky they don’t understand. The vast majority are aeroplanes, satellites, weather balloons, clouds, rocket launches, auroras, optical reflections and so on. But for some sightings, there’s no known explanation. The problem is that people jump to the conclusion “unknown = aliens”. And when you think about it, this is fairly odd. Why not angels?

Anyway, I like to do maths instead. The Bayes formula (below), a mainstay of statistics, gives the probability (Pr) of something, given some evidence.