``Wouldn't it be nice,'' most reasonable people would agree, ``if there were a way to obtain energy from fusion of some innocuous nuclei like deuterium without the enormous temperatures of nuclear explosions or the various `hot' controlled fusion reactors on the drawing boards.'' There certainly is a way to get deuterium nuclei close enough together to fuse without high temperatures - in fact, not too long ago I participated in an experiment that achieved D-D fusion at a temperature of 2.5 K: this involves forming a molecule of two deuterons and one negative muon - an unstable elementary particle which is more or less like an electron except that its mass is 207 times bigger. The heavy muon pulls the deuterons so close together that they fuse. This works. Unfortunately it doesn't work well enough to generate more energy than it took to make the muon in the first place! The closest anyone has come to ``break even'' using muons is more than a factor of ten too low in efficiency. Too bad. It is frustrating to come so close and then fail.
Perhaps because of this frustration, a few people recently deluded themselves into believeing they had coaxed deuterons into fusing by regular electrochemical means in a palladium metal matrix. Unfortunately this was bogus, as numerous more careful tests clearly showed. However, the dream of unlimited free energy easily generated with simple equipment is so seductive that many ``Cold Fusion" believers still hang onto it; moreover, they seem to have little trouble convincing sponsors to give them money to maintain their campaign to overcome experimental refutations with good press. For an interesting study in the sociology of science, see the cold fusion home page. For the other side of the story, talk to almost any physicist.