A group of start-ups is promising a new and virtually unlimited source of power, one that produces none of the gases scientists say contribute to global warming.

The only problem? A way to harness the energy source, nuclear fusion — the reaction that gives birth to sunlight — still needs to be invented.

Such an achievement has long evaded government scientists and university researchers, despite decades of work and billions of dollars in research. But backed by hundreds of millions in venture capital and some of the wealthiest people in the technology industry, a handful of young companies say they can succeed where government has fallen short.

Nuclear fusion is one of many areas of science and energy now getting the backing of venture capitalists. The investor dollars coming into fusion start-ups, like those in many areas of science, still pale in comparison with the money spent by governments. But signs of progress, including some results that have eclipsed government projects, have generated hope among some scientists that the companies could help develop a fusion reactor within their lifetimes.