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The Doomsday Argument is the idea that we can estimate the total number of humans that will ever exist, given the number that have lived so far. This in turn tells us how likely it is that human civilisation will survive far into the future.

The numbers are not optimistic. Anthropologists think some 70 billion humans have so far lived on Earth.  If we assume that we have no special status in  human history, then simple probabilistic arguments suggest that there is a 95 per cent chance that we are among the last 95 per cent of humans that will ever be born. And this means there is a 95 per cent chance that the total number of humans that will ever exist will be less than 20 x 70 billion or 1.4 trillion. 

Now suppose that the world population stabilises at 10 billion and our life expectancy is 80 years, then the remaining humans will be born in the next 10,000 years. That’s not a long future for humanity. Today, Austin Gerig at the University of Oxford and a couple of pals put forward a new argument with a (slightly) happier ending.  

These guys look at the scenario in which many civilisations have evolved throughout the universe, the so-called “universal doomsday” argument. “In that case, we should consider ourselves to be randomly chosen from all individuals in that universe or multiverse,” they say.

In the past, these universal arguments have been no more optimistic than the ordinary ones. They generally state that long-lived civilizations must be rare because if they were not, we would be living in one. What’s more, because  long-lived civilizations are rare, the prospects for our civilisation ever becoming long-lived are  poor.

One problem with these conclusions is that they are based on very general arguments. So the new work that Gerig and co have done is to develop a more detailed analysis that takes into account factors such as the number of existential threats that civilisations will face—things like nuclear wars, asteroid impacts and global pandemics, not to mention the many threats we have not yet thought of.

This new approach approach allows Gerig and co to take a more fine-grained look at the odds that humanity will survive for much longer in future than it has existed in the past.

The results are complex but their main conclusion gives some reason for hope. “If [the number of existential threats]  is not too large, the probability of long-term survival is about a few percent,” they say.

Although this can hardly be called optimistic, it is nowhere near as gloomy as previous calculations.

Gerig and co say their calculations suggest some obvious actions humanity could take to significantly improve its chances of long term survival. “If there is a message here for our own civilization, it is that it would be wise to devote considerable resources (i) for developing methods of diverting known existential threats and (ii) for space exploration and colonization,” say Gerig and buddies. “Civilizations that adopt this policy are more likely to be among the lucky few that beat the odds.”

Scientists have only recently has begun to study existential risk in a systematic way but this work is only beginiing to feed through into the public arena in the form, for example, of an increased focus on Earth-crossing asteroids. Perhaps it’s time to take existential threats much more seriously.

Ref: arxiv.org/abs/1303.4676 : Universal Doomsday: Analyzing Our Prospects for Survival

 

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