"Since after extinction no one will be present to take responsibility, we have to take full responsibility now." ― Jonathan Schell, The Fate of the Earth
When will the last person be born? The question is simple; the answer is difficult. J.R. Gott tried to answer this question using
Bayes' theorem and simple statistics. The number of people who will be born is likely to be large. But is it more likely that there will be one trillion people or two trillion? We expect larger numbers to be less likely than smaller numbers. We write $\text{Pr}(N) = k/N$ and $\text{Pr}(n) = k/n$, where
N is the number of people that will ever be born and
n is the number of people already born. Furthermore, we'll assume that there is nothing special about our position in human history. We were as likely to be born as the billionth or the hundred billionth person. Mathematically, this is written $\text{Pr}(n|N)=1/N$.
The likelihood that there will be
N people, given that we know there have been
n people already—$\text{Pr}(N|n)$—is obtained from Bayes' theorem:$$\text{Pr}(N|n)=\frac{\text{Pr}(n|N) \times \text{Pr}(N)}{\text{Pr}(n)} = \frac{n}{N^2}$$From this, we can calculate the probability for upper bounds on the number
N: $\text{Pr}(N \leq z)= \frac{z-n}{z}$. The first modern census was not conducted until the 18
th century, but we can estimate the number of people who have lived on Earth—it's on the order of
100 billion. We can be 95% confident that there will be fewer than 2 trillion people. Given that there are four births every second, this person will be born in 15 thousand years.
There are a number of objections that can be made against the doomsday argument on mathematical grounds; however, the biggest problem is that it fails to address the physical causes of extinction. Genetic mutations will continue to accumulate in human populations, perhaps causing
h. sapiens to differentiate into new species. Disasters may contribute to an early human extinction.
A 2008 report by the Future of Humanity Institute estimates a 1 in 5 chance of humans extinction before the year 2100. Though, the methodology of this study is questionable. The doomsday argument may be flawed, but it raises questions in mathematical inference, and has opened the door to managing the risks we face as a species.