Fusion maybe. Off topic:
Many years ago a physicist told me that an engineer had told him, “If you scientists can make hydrogen fusion work we engineers will remake the world.” Some wag said, “Hydrogen fusion is the energy of the future and always will be.” So far that’s been true.
This is far from our topic of fertility and population except for one tiny point; if they can get hydrogen fusion working with any reasonable efficiency we will all become so rich and so mobile that the last traces of a structured gene pool will vanish. It’s the liberal’s wet dream. Hydrogen fusion is … well you know. Nobody talks about the atom bomb. It’s all hydrogen bombs. But the only functioning nuclear power generators are generally just atom bombs that don’t quite go off. But even such reactors are competitive with other sources of energy and although they produce a hideous amount of radioactive waste they in fact are said not to be as destructive to the environment as plain old coal or even natural gas.
Fusion power would be, it is thought, very cheap, very non-polluting and intrinsically safe; pull the plug and the thing stops, unlike a fission reactor where you may be in it for years trying to get control of a damaged installation.
There are two approaches that have been tried. (Nobody has asked me how to do it, and honestly the threat I mentioned earlier would give me pause anyway.) One approach is to contain the hydrogen in a magnetic bottle. There are the stellarator approach and the tokamac approach. They are two different approaches to the geometry of the bottle.
Another approach is to make a small hydrogen bomb, harvest its energy and then light up another. Obviously that is a move of desperation, but it seems to come the closest to working.
It has now been demonstrated (Tying Knots with Light Beams NATURE vol. 502 no. 7472 October 24, 2013 page 413 reviewing Phys. Rev. Lett. 111, 1050404(2013) describing work by Hridesh Kedia and team at the University of Chicago) that you can take beams of light produced by lasers and tie them in knots, and the knots are stable. The article is a bit coy but it certainly looks like this is a huge stride in the direction of making an effective magnetic bottle.
Prepare for the end of the world as you know it.
Again.
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