08 Sep '07 03:43

We want to predict how long the human species will continue to exist. This seems like a hard thing to predict, but let's try to attack the problem with probability.

Imagine all the humans who were ever born or ever will be born, stretched out in one long line, in order of their birth. Label the front of the line 0 and the end of the line 100. Where are we on the line? There's no way of telling, as we don't know how many people come after us. We can, however, estimate how many people come before us. There are various estimates of the total number of humans who have ever been up to the present; let's use 100 billion as our number.

OK, so now imagine picking a random human from our line. On average, where will this human be on the line. The mean value will be 50, the exact center. There's only 10% chance that our random human will be at a point less than 10, that is, in the first 10% of all humans ever born.

But here's the important part: since we have no way of knowing where we fall on the line, we might as well imagine ourselves as being located in a random point on it. *That means that there is only a 10% that we are among the first 10% of humans born.* There is a 90% chance that we are among the last 90% of humans. So, let's suppose we are at the location on the line numbered 10. That means that, there will be 800 billion humans born, ever. Let's say 100 million babies are born each year until the species dies out. That gives us a mere 7,000 years of future existence, if we're at position "10" on the line.

But there's a 90% probability that our position comes later than 10! In that case, we even have less than 7,000 years left!

Basically, we have shown that there is a 90% chance that the human species will die out in the next 7,000 years.

Imagine all the humans who were ever born or ever will be born, stretched out in one long line, in order of their birth. Label the front of the line 0 and the end of the line 100. Where are we on the line? There's no way of telling, as we don't know how many people come after us. We can, however, estimate how many people come before us. There are various estimates of the total number of humans who have ever been up to the present; let's use 100 billion as our number.

OK, so now imagine picking a random human from our line. On average, where will this human be on the line. The mean value will be 50, the exact center. There's only 10% chance that our random human will be at a point less than 10, that is, in the first 10% of all humans ever born.

But here's the important part: since we have no way of knowing where we fall on the line, we might as well imagine ourselves as being located in a random point on it. *That means that there is only a 10% that we are among the first 10% of humans born.* There is a 90% chance that we are among the last 90% of humans. So, let's suppose we are at the location on the line numbered 10. That means that, there will be 800 billion humans born, ever. Let's say 100 million babies are born each year until the species dies out. That gives us a mere 7,000 years of future existence, if we're at position "10" on the line.

But there's a 90% probability that our position comes later than 10! In that case, we even have less than 7,000 years left!

Basically, we have shown that there is a 90% chance that the human species will die out in the next 7,000 years.