Longevity & Population [was Re: Wertheim on extropians]

Robert J. Bradbury (bradbury@www.aeiveos.com)
Fri, 17 Sep 1999 05:22:14 -0700 (PDT)

On Thu, 16 Sep 1999, Eliezer S. Yudkowsky wrote:

> Damien Broderick wrote:
> > 
> > More tricky ... is her claim that if we
> > double the human life span it will double the peak size of the global
> > population.  This strikes me as an obvious non sequitur, but I'd like some
> > clever mathematics to prove it.

Well, the best way to critisize something you can't logically critisize is to throw in things entirely unrelated to the topic. We've been through this all with the attempts to critisize nanotechnology.

> It's not a matter of math.  If population is determined by reproduction,
> it keeps expanding and expanding with no end in sight.  If population is
> determined by carrying capacity, it stops at the same number of
> long-lived humans as short-lived ones.  You're quite right, it doesn't sequit.

I've been dealing with the population question for years, since it is the first thing people come up with when you say we are going to extend the human lifespan.

I'll just throw out some thoughts on the topic.

This *isn't* news. Going back to Malthus and probably before there have always been concerns about this.

Dyson's 1960 paper that invented Dyson Shells pointed out the problem:
| The material factors which ultimately limit the expansion of a
| technically advanced species are the supply of matter and the
| supply of energy. At present the material resources being exploited
| by the human species are roughly limited to the biosphere of the earth,
| a mass of the order of 5x10^19 grams. Our present energy supply
| may be generously estimated at 10^20 ergs per second. The quantities
| of matter and energy which might conceivably become accessible to us
| within the solar system are 2x10^30 grams (the mass of Jupiter) and
| 4x10^33 ergs per second (the total energy output of the sun).


| The reader may well ask in what sense can anyone speak of the mass of
| Jupiter or the total radiation from the sun as being accessible to
| exploitation. The following argument is intended to show that an
| exploitation of this magnitude is not absurd. First of all, the time
| required for an expansion of population and industry by a factor of
| 10^12 is quite short, say 3000 years if an average growth rate of
| 1 percent per year is maintained. ...

So, the "growth" of "civilized" humanity is at *least* half-way complete. There are numerous articles that state quite clearly that the use of interstellar travel as an escape valve on population growth is *impossible* using known laws of physics.

So, sooner or later, we *will* hit the limits.

The carrying capacity of the planet or the solar system is entirely dependent on the technology at your disposal. Anyone making claims about limits without also clearly stating their assumptions about technology should be keelhauled until they agree to get it right.

The real limits on the number of "humans" are the mass & energy limits. Assuming around 3.4x10^24 kg of phosphorus in the solar system and about 1.2 kg of phosphorus per person you get about 2.7x10^24 people. Given the sun puts out 3.82x10^26 Watts, that gives you about 140 Watts/person. Since a human requires about ~100 W, we are all going to have to be on an energy diet unless we have *really* efficient energy conversion technology.

So, the *real* limit is an energy shortage around the time when you have a trillion trillion people. I suspect that you can bump that a little using thermonuclear fusion reactors burning the fuel in Jupiter & Saturn, but I don't believe don't have enough construction material to do much of that. Ultimately we have to solve the problem of how to efficiently mine metal out of the Sun.

So, the proper response to the longevity/population problem is