Here are some numbers that are worth knowing. There are about 7.5 billion humans who are currently alive. That’s quite a lot: certainly more than have ever been alive at any point in the past. But it’s probably only about 7% of all the modern humans, Homo sapiens, who have ever lived; there have been about 108 billion of those in the last 50,000 years.
(Depending where you draw the arbitrary line around “modern human”, of course.)
That’s just the beginning, though. What we really want to know is: how many will come after us?
The philosopher Nick Bostrom, building on work by the great Derek Parfit, had a go at answering this question; you can also read about it in my own book. The planet Earth will be able to support complex life for about a billion years. Say that the human population stabilises at a roomy one billion in the next thousand years or so; Bostrom works out that there will be about 10,000,000,000,000,000 people who follow us. That’s 10 quadrillion; about 1.3 million times as many people are currently alive.
Of course, that’s assuming we stay on Earth. If we leave the planet – if, at some point in the next million years or even more, we develop genuine space capability – it gets much more dramatic. If we can build craft that travel at 50% of lightspeed, we could reach about six quintillion stellar systems before the expansion of the universe puts the rest out of our reach; if we can reach 99% of lightspeed, that figure is more like 100 quintillion.
If, says Bostrom, 10% of those stars have habitable planets, each capable of supporting one billion humans for one billion years, then the number of human lives that could be supported is 1 followed by 35 zeroes. I could tell you how many times the current human population that is, but it would involve writing out some very long strings of noughts.
(Where it gets really weird, of course, is if we eventually learn how to build our own habitats from space rocks, so we’re not limited to existing planets; or upload human brains into computers, so we each take up a few picometres of circuitry rather than some number of thousands of square metres of planetary surface. Then you get some seriously huge numbers. But I don’t want to stretch your credulity here.)
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