Nuclear fusion holds the promise of clean, endless power for everything from cars to homes to entire cities. Unfortunately, after decades of research, government-funded scientists all over the world have failed to come up with a way to harness the energy source in a scalable and safe way.

Now, sensing a business opportunity, tech startups are making their own attempts to produce the elusive fusion reaction. They've attracted a good deal of skepticism among scientists, but also a lot of funding from some big-name investors.

In case you need a quick primer on the science driving these ventures, Dino Grandoni of The New York Times, explains:

Nuclear fusion occurs when two atoms are squeezed together so tightly that they merge. That single, larger atom releases a tremendous amount of energy. This happens naturally at the center of the sun, where gravity easily crushes hydrogen into helium, spewing forth the sunlight that reaches Earth. But on Earth, making hydrogen hot and dense enough to sustain a controlled fusion reaction--one that does not detonate like a thermonuclear bomb--has been a challenge.

Here are three tech icons bankrolling the race to replicate the sun's power.

According to the Times, the Amazon founder has invested in General Fusion, a British Columbia-based startup, through his firm Bezos Expeditions. (As with the others below, the Times did not specify the amount of Bezos's investment.) General Fusion is trying a method that utilizes pistons, which will send shock waves through hydrogen gas in an effort to compress the hydrogen atoms to the point that they fuse. General Fusion has attracted $20 million from the Canadian government and $74 million from venture capitalists.

2. Peter Thiel

The famed PayPal co-founder invested in Helion Energy, a Washington state-based startup, through his firm Mithril Capital Management. Thiel has good company on this investment: The U.S. Energy Department gave Helion a $4 million grant from its $30 million pool for alternative fusion projects. Helion, which has a goal of providing cheap, carbon-free energy to the developing world, says the fusion engine it's working on will be 1,000 times smaller and more than 500 times cheaper than other projects.

3. Paul Allen