Imagine a world where electrical power plants don't pollute and fuel is cheap and plentiful. That's the vision of nuclear fusion -- the same primordial force that makes the sun shine and hydrogen bombs explode. For decades, scientists have pursued energy's holy grail. But after 40 years and $9 billion in U.S.-funded research, experts say commercial fusion energy is still at least a half-century away. Given recent advances in energy alternatives like wind and solar power, critics say it is time to abandon the fusion effort. But proponents argue that funding should be increased, not decreased, because the program is on the threshold of important breakthroughs. Global collaboration on fusion research, they add, could set a precedent for other “big-science” projects.