PARIS -- Physicists have dreamt about it for decades: harnessing the fusion process that powers the sun to make clean, safe and limitless energy. A multinational pact signed Tuesday may bring that dream a step closer to reality.

Seven partners representing half the world's population have agreed to build an experimental fusion reactor in southern France that could revolutionize global energy use for future generations.

Yet it is also just an experiment -- a bold, long-awaited, $12.8 billion experiment -- and it will be decades before scientists are even sure it works.

The ITER project by the United States, the European Union, China, India, Russia, Japan and South Korea will attempt to combat global warming by offering an alternative to fossil fuels. Controlling climate change and finding new energy sources are urgent goals for a growing global population.

"Worldwide demand for energy is expected to double in the next 25 years and we need to diversify our energy supply," said U.S. Secretary of Energy Samuel W. Bodman during a tour Tuesday of Princeton University's Plasma Physics Lab.

Experiments on two reactors at the Princeton lab -- one in use since 1999 and the other under construction -- will be crucial in helping scientists determine how to translate research at the ITER facility into a design for commercial fusion reactors, Bodman said.

The sun's power source

Physicists have been trying for half a century to create fusion, which replicates the sun's power source, produces no greenhouse gases and generates relatively little radioactive waste.

The International Thermonuclear Experimental Reactor project, or ITER, recognizes that no single country can afford the immense investment needed to move the science forward.

It is expected to take eight years to build the reactor in Cadarache in the southern region of Provence. A demonstration power plant may be ready by 2040, according to project organizers. If the prototype proves viable, it could point the way to designs for commercial power plants.

Officials involved in the project say 10 percent to 20 percent of the world's energy could come from fusion by the end of the century.

Some experts, though, question whether the project can keep all the promises being made on its behalf.

"I would be extremely surprised -- pleasantly surprised -- if there was a prototype" by 2040, said Matthew Bunn, nuclear expert at Harvard University.