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If civilization still exists on planet earth in a billion years, what materials can it use?

Lasting civilizations can use only materials

  1. that form closed cycles or loops (such as carbon, water, oxygen, hydrogen, hydrocarbons, nitrogen), or
  2. that can be regained from the environment economically to form forced cycle before they reach harmful concentrations, or
  3. that (if available from outside) can accumulate in ever increasing concentrations in the environment/biosphere without negative effect on life and civilization, and
  4. that exist in the biosphere or can be brought there by economical means

Carbon and hydrocarbon based materials, for which cycles exist in nature, have amazing properties, ranging from diamond, nanotubes and materials that can be turned into semiconductors and transistors or plastics as well as living organisms. Those that nature recycles before dangerous concentrations accumulate could be used indefinitely in civilizations.

For materials such as metals or doted semiconductors no natural cycles are known. Recycling in a current sense is incomplete. Even a modest wear or oxidation of such materials would eventually result in high concentrations in the environment. Lasting civilizations could use such materials only if they posess and employ economical ways to remove these traces from the environment to keep the concentrations from exceeding harmful levels. In this way new, technically closed, but complete cycles would exist. For many materials we widely use today, such as aluminum or nickel, possibly even iron, we do not yet posess economical removal technologies from the environment at low concentrations. Such cycles may or may not be feasible. E.g., nanotechnologies may be able to close the loops. For a lot of materials this issue is not a concern on a horizon of centuries and would need to be solved in the long run, but for many substances it is unknown which levels are harmless and it is safe to switch to materials that can continue sooner rather than later.