Suicidesoldier#1
When you have half the population density of fargo north dakota all over the earth, and calculate how much farmland is needed, how much available arable land is available, and whatnot, and how much you could produce with the amount of water and fertilizer in the world, and including ranchland and cows n stuff, you get about oh 60 billion or so.
Metabolic efficiency is lower at low population densities, sprawl is bad for ecosystem health, and so is monoculture farming. You are never going to support 60B people on a Western diet with geoponic monocultures, we are already destroying the ecosystem with less than 10% of that population acting in the way you've described.
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With recycling and the awesome size of the earth there's not much of a problem with natural resources like iron and silicon.
It depends how often you are recycling them. There is such a thing as "entropy" for materials, they cannot be recycled infinitely.
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Since energy can be alleviated with Thorium, and in the future hopefully graphene solar panels, we'll be fine.
Uh, I think you have that a little backwards. Solar energy is what we can do now. Thorium is not. We already have the facilities to produce high-efficiency CIGS cells, and some people I know at a Dutch hackerspace called Labitat are building a 3D printer-like device that can produce CIGS cells with powder metallurgy in a small workshop. It's feasible that it could even produce the latest development in solar tech, which is a micropatterned metal screen on the surface of the panel that reduces its reflectivity by 're-capturing' light.
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Energy is your biggest concern since resources are abundant and what we need is transformation, say into cars or ammonia, instead of say, nitrogen and oxygen.
Not true. Solar energy will eventually reach a point where there is a surplus of energy for most of the day, with capacity only being reached at a modal or bimodal time each day. That surplus energy is going to have to be stored or used, or else it's going to waste. And since it's already falling from the sky, there is no reason not to store or use it, because the other possibilities for it are producing heat or being metabolized by autotrophs. At that point, we have a few choices:
1. Use the stored energy to gradually reduce the amount of solar energy captured
2. Build increasing amounts of storage and have ever-larger amounts of reserve energy
3. Use the excess generated energy for an "idle process", to do things like recycling, refining, computation, or scientific research.
Energy seems like it's scarce because fossil fuels are scarce. Fossil fuels are based on the same energy source as photovoltaic technology, which is the sun. But fossil fuels have a much lower conversion efficiency than PV, a very low replenishment rate, and by using them we are gradually reducing the effectiveness of all other forms of energy including fossil energy itself. The faster you extract and use it, the lower its efficiency.
Thorium has similar problems, except that it is created by thermonuclear processes that only occur near the end of a star's life, so it has an even lower replenishment rate than fossil fuels. It is a high-quality energy now, but anyone who studies energy, economics, or anything of the like knows that under the existing social paradigm, higher extraction efficiency (in this case, extraction of energy from the ground) just leads to higher consumption. Thus, the most likely outcome, if we switch the thorium energy, is that the economy will continue to be based on money flows decoupled from most observable measurements, and energy use will increase until thorium becomes a low-quality energy. Then, the people of the future will be having the exact same discussion we are having now, which is that solar energy will never be as efficient as thorium.
In reality, it's materials that are scarce, at least while we're confined to this planet. Energy falls on our heads daily, and will continue to do so for billions of years. Materials take billions of years to create, and cannot be recycled infinitely. Building long-lasting physical products that can be divided into small/standardized modules so their design can evolve over time, as opposed to being replaced completely, should be a very high priority for civilization, after the switch to ecologically/economically sustainable energy/agricultural technologies.