There are a variety of phenomena under the heading of “cold fusion”, but for now I’m primarily thinking about the oldest, most famous, and most-widely-tested aspect: Heat produced in palladium-deuteride systems, which is (allegedly) due to the D + D → He⁴ nuclear reaction.

If D + D → He⁴ is really what’s going on, it has a number of properties which are awfully hard to explain. The cold-fusion skeptic John Huizenga described these as the “miracles” of cold fusion, in the sense that they have no possible explanation. Anyway, everyone agrees that a plausible theory of cold fusion would at minimum need to answer the following two questions:

• Why doesn’t the Coulomb barrier prevent fusion from occurring in the first place? Since the two nuclei are positively charged, they repel very strongly until they get so close that they can fuse. It can happen at extremely high temperatures or pressures, as in a thermonuclear bomb, or a star, or a tokamak, or using a laser the size of a football stadium. It can also happen if you accelerate a beam of deuterons to a high speed, and shoot it into other deuterons, as in a Farnsworth Fusor (try it at home!). It can also happen in muon-catalyzed fusion, for well-understood reasons. But it is difficult to see how the Coulomb barrier could be overcome in a cold-fusion experiment.
• If D+D fusion is occurring, why does it only create helium-4, and why doesn’t it create comparable quantities of helium-3, tritium, neutrons, and gamma-rays? That’s what normally happens in conventional “hot” D-D fusion. In fact, if cold fusion produced neutrons at the same “branching ratio” as you expect from “hot” D-D fusion, it would be easily detected in the experiments … by the radiation-poisoning death of everyone in the room! Actually, neutrons and tritium are sometimes seen in tiny tiny amounts (if I understand correctly), but it’s such a low level that it could only be a “side-channel” at best, as opposed to the main event producing all that heat. So, obviously the reaction is proceeding in a different way than hot fusion. What is it, and why? (The constraints will be discussed more in the next post.)

Cold-fusion skeptics think that there is no theory that answers these questions. Proponents have offered a variety of theories that they claim DO answer these questions. Should we believe them? We shall find out! Stay tuned!