>Wait for X time, then calculate 3/X. Do you now have an upper bound on the probability that an alien invasion will happen? Assuming we would observe an alien

>Wait for X time, then calculate 3/X. Do you now have an upper bound on the probability that an alien invasion will happen? Assuming we would observe an alien invasion and write about it, we have about 5000 years since humans started writing. So 3/5000a, so an upper bound on an alien invasion in a year (assuming p hasn't changed since we started writing) is 0,06% per year. It's an upper bound in any case, meaning that is this or less. That includes p = 0. You're just 95% confident that it won't be higher than that. But maybe the aliens brought us our writing system.