A million to one chance

The writer and philosopher Terry Pratchett (1948-2015) wrote that 'Scientists have calculated that the chances of something so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.'[1].

At first glance, you think this quote is ludicrous and could not possibly be true.

But suppose one day a dinosaur would say to his friend (everybody needs a friend, right?) that there was a million to one chance that a massive meteorite would hit the earth and wipe out all dinosaurs. While the probability is indeed a million to one, the chances of a meteorite hitting the planet are actually one in two. That is because if it doesn't impact the earth today, it will probably do so tomorrow.

Another example is the massive earthquake that devastated the San Francisco area in 1906. The quake had an estimated magnitude of 7.9 and was the result of a rupture of the San Andreas Fault. Seismologists estimated that a major earthquake like that of 1906 would happen once every 100 years or so. In 2023, 'The Big One' seems long overdue.

Let us calculate the possibilities of a large earthquake happening in or around San Francisco. One would think that an earthquake that happens every 100 year would have a chance of happening once every (100 x 365 days =) 36,500 days. As it is actually already 17 years too late, the chances are now once every (117 x 365 days =) 42,705 days. That doesn't look like much, but the chances of a major earthquake that devastates San Francisco are actually one in two. That is because if it doesn't hit today, it will probably do so tomorrow.

And yet, The philosopher Didactylos has summed up an alternative hypothesis as 'Things just happen. What the hell.'[2].

[1] Terry Pratchett: Mort – 1987
[2] Terry Pratchett: Hogfather - 1996