City Planets

I really like this Robin Hanson post about human (or other) population at a galactic level. His point:

If Earth were paved over with a city the density of Manhattan today (1.6 million in 60 square kilometers), Earth would have a population of 14 thousand billion. Since Manhattan now has an average building height of 25 meters, a two kilometer deep version could hold a million billion people, and a two thousand kilometer deep version (Earth’s radius is 6400km) could hold a billion billion people.

And his thesis:

Most science fiction seems to vastly underestimate the population that a single planet or star can hold, and the strength of the economic pressures to keep an economy close together, rather than spread across vast distances.

Before we get into this, let's just dismiss the "two thousand kilometer deep version" because 2,000km deep into the Earth it is between 3,000 and 5,000 degrees F and humans living there is pretty implausible. If he means building up into the sky 2,000 km, well...putting up buildings high enough to reach the ISS is just as implausible.
But a 2 km deep society is somewhat plausible, assuming you had engineering techniques to move the Earth that deep (you could dump it into the oceans to backfill them and make more land for building) and pump fresh air that deep. Or that you had the resources to build Burj Khalifa/Taipei 101 all over the place. So let's assume that Hanson's estimate of a world population of a million billion people (1,000 trillion aka 1 quadrillion) is a fair maximum. In that society, the average person needs 2,000 calories a day to stay healthy.
Now as I was getting wound up to go into some math about the natural resources required to sustain exawatts of calorie consumption, I noticed Hanson add the following update to his blog (as though to cut my argument off at the knees before I even wrote it!):

Even an unmodified sun radiates enough energy to cover the calorie consumption of over a hundred “galaxies” of humans, and far more ems.

But what he doesn't get after is what's to be done with the "waste heat" produced by our bodies. An oversimplified average amount of heat produced by a human body is 100 watts. So multiply that by the million billion people and you have a CRAPLOAD of waste heat...simply from the humans. Add in our bevy of electronic devices and you've essentially turned the surface of the Earth into a kiln.

Of course, if you did have 2,000 km high buildings, which Hanson suggested but I rejected, they'd essentially be going into LEO (low earth orbit) and you could use them (when they were on the night side of Earth) as massive heat sinks.

Nevertheless, I think the fundamental problem with Hanson's thesis that "science fiction authors consistently underestimate the population potential of a planet" is that Hanson is not cynical enough. With a mere 7 billion people, our planet is a torrid, violent, disease-ridden, famine-plagued, caste-driven, unsustainable mess. Multiply the human population by 1,000 and you multiply the problems, as well.


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