UFOs: how to calculate the odds that an alien spaceship has been spotted

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. Spelled out, it says that the probability that UFOs are aliens given some evidence is equal to how likely it is that the evidence would appear if UFOs really were aliens, times how likely it is that there are aliens. That needs to be divided by how likely the actual evidence is, which is notoriously difficult to work out. But what we are really interested in is…

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UFOs: how to calculate the odds that an alien spaceship has been spotted

Updated: October 12, 2022 — 10:27 am