My second post-doc. was at Southampton University, and one of the leading physical chemists there was Martin Fleischmann, who had an excellent record for clever and careful work. There would be no doubt that if he measured something, it would be accurate and very well done. In the academic world he was a rising star until he scored a career “own goal”. In 1989, he and Stanley Pons claimed to have observed nuclear fusion through a remarkably simple experiment: they passed electricity through samples of deuterium oxide (heavy water) using palladium electrodes. They reported the generation of net heat in significant excess of what would be expected from the power loss due to the resistance of the solution. Whatever else happened, I have no doubt that Fleischmann correctly measured and accounted for the heat. From then on, the story gets murky. Pons and Fleischmann claimed the heat had to come from nuclear fusion, but obviously there was not very much of it. According to “Physics World”, they also claimed the production of neutrons and tritium. I do not recall any actual detection of neutrons, and I doubt the equipment they had would have been at all suitable for that. Tritium might seem to imply neutron production, thus a neutron hitting deuterium might well make tritium, but tritium (even heavier hydrogen) could well have been a contaminant in their deuterium, or maybe they never detected it.
The significance, of course, was that deuterium fusion would be an inexhaustible source of clean energy. You may notice that the Earth has plenty of water, and while the fraction that is deuterium is small, it is nevertheless a very large amount in total, and the energy in going to 4-helium is huge. The physicists, quite rightly, did not believe this. The problem is the nuclei strongly repel each other due to the positive electric fields until they get to about 1,000 – 10,000 times closer than they are in molecules. Nuclear fusion usually works by either extreme pressure squeezing the nuclei together, or extreme temperature giving the nuclei sufficient energy that they overcome the repulsion, or both.
What happened next was that many people tried to reproduce the experiment, and failed, with the result this became considered an example of pathological science. Of course, the problem always was that if anything happened, it happened only very slightly, and while heat was supposedly obtained and measured by a calorimeter, that could happen from extremely minute amounts of fusion. Equally, if it were that minute, it might seem to be useless, however, experimental science doesn’t work that way either. As a general rule, if you can find an effect that occurs, quite often once you work out why, you can alter conditions and boost the effect. The problem occurs when you cannot get an effect.
The criticisms included there were no signs of neutrons. That in itself is, in my opinion, meaningless. In the usual high energy, and more importantly, high momentum reactions, if you react two deuterium nuclei, some of the time the energy is such that the helium isotope 3He is formed, plus a neutron. If you believe the catalyst is squeezing the atoms closer together in a matrix of metal, that neutron might strike another deuterium nucleus before it can get out and form tritium. Another reason might be that the mechanism in the catalyst was that the metal brought the nuclei together in some form of metal hydride complex, and the fusion occurred through quantum tunnelling, which, being a low momentum event, might not eject a neutron. 4He is very stable. True, getting the deuterium atoms close enough is highly improbable, but until you know the structure of the hydride complex, you cannot be absolutely sure. As it was, it was claimed that tritium was found, but it might well have been that the tritium was always there. As to why it was not reproducible, normally palladium absorbs about 0.7 hydrogen atoms per palladium atom in the metal lattice. The claim was that fusion required a minimum of 0.875 deuterium atoms per palladium atom. The defensive argument was the surface of the catalyst was not adequate, and the original claim included the warning that not all electrodes worked, and they only worked for so long. We now see a problem. If the electrode does not absorb and react with sufficient deuterium, you do not expect an effect. Worse, if a special form of palladium is required, that rectifying itself during hydridization could be the source of the heat, i.e.the heat is real, but it is of chemical origin and not nuclear.
I should add at this point I am not advocating that this worked, but merely that the criticisms aimed at it were not exactly valid. Very soon the debate degenerated into scoffing and personal insults rather than facts. Science was not working at all well then. Further, if we accept that there was heat generated, and I am convinced that Martin Fleischmann, whatever his other faults, was a very careful and honest chemist and would have measured that heat properly, then there is something we don’t understand. What it was is another matter, and it is an unfortunate human characteristic that the scientific community, rather than try to work out what had happened, preferred to scoff.
However, the issue is not entirely dead. It appears that Google put $10 million of its money to clear the issue up. Now, the research team that has been using that money still have not found fusion, but they have discovered that the absorption of hydrogen by palladium works in a way thus far unrecognised. At first that may not seem very exciting, nevertheless getting hydrogen in and out of metals could be an important aspect of a hydrogen fuel system as the hydrogen is stored at more moderate pressures than in a high-pressure vessel. The point here, of course, is that understanding what has happened, even in a failed experiment, can be critically important. Sure, the actual initial objective might never be reached, but sometimes it is the something else that leads to real benefits. Quite frequently, in science, success stories actually started out as something else although, through embarrassment, it is seldom admitted.
Finally, there is another form of cold fusion that really works. If the electrons around deuterium and tritium are replaced with muons, the nuclei in a molecule come very much closer together, and nuclear fusion does occur through quantum tunnelling and the full fusion energy is generated. There are, unfortunately, three problems. The first is to maintain a decent number of muons. These are made through the decay of pions, which in turn are made in colliders. This means very considerable amounts of energy are spent getting your muons. The second is that muons have a very short life – about 2 microseconds. The third is if they lose some energy they fall into the helium atom and stay there, thus taking themselves out of play. Apparently a muon can catalyse up to 150 fusions, which looks good, but the best so far is that to get 1 MW of net energy, you have to put 4 MW in to make the muons. Thus to get really large amounts of energy, extremely huge generators are required just to drive the generation. Yes, you get net power but the cost is far too great. For the moment, that is not a productive source.