After years of discouraging setbacks, specialists in hydrogen fusion research are banding together to revive the effort to harness energy from thermonuclear fusion. Believers say fusion energy could become a reality in the mid-21st century. Next month, scientists and engineers from around the world will convene in Snowmass, Colo., at a first-of-a-kind meeting of fusion researchers, marking the beginning of a new era of cooperation rather than competition in the field. The main obstacle to accelerated fusion research is the disincentive created by low fossil fuel prices, according to many scientists.