One morning, end of March, 1989, the World was shocked by the announcement of an experiment carried out by two unknown chemists in the University of Utah, Martin Fleischmann and Stanley Pons, in which they claimed having observed the nuclear fusion of deuterium in a simple electrochemical cell. Apparently, their experiment had been going on for several years already, completely self-funded. They used heavy (deuterated) water to run what looked like a normal electrolysis experiment with a palladium metal electrode, and observed a steady heat release in the cell, much larger than expected by usual electrochemical estimates. The announcement sparked huge attention worldwide, but also immediate and radical criticism. The result was hard to believe, and could not stand against the known laws of physics. It was long known that transition metals, such as Ti or Pd, can absorb huge quantities of hydrogen in the interstitial sites of their crystal lattice. For example, if all the octahedral sites of the Pd fcc lattice are occupied by one H atom (H2 molecules usually dissociate at metal surfaces, so atomic hydrogen enters the lattice), that makes four atoms in about 0.06 nm3, or a density of 0.1 g/cm3, that is a thousand times the gas density at room temperature (actually, it’s more like liquid hydrogen). However, as dense as it could be, the distance between lattice sites is way too large for nuclei to come close enough to fuse together. This was (and is) the key criticism to the F-P experiments: chemical energies are too small to match nuclear processes.
Moreover, the ethics of the two scientists had been far from adamant. The year before they had applied for a grant from the US DOE, and their proposal had been reviewed by Steven Jones, who was also doing experiments with muon-catalysed fusion, and had been running also a configuration very similar to the F-P electrochemical cell. The two teams met on March 6, and discussed the details of both experiments. To avoid priority problems, both teams agreed to publish their respective results simultaneously: they would meet again on March 24, and mail together their papers to Nature. However, apparently under pressure from the management of Utah university, Fleischmann and Pons broke the agreement and released a press conference on March 23, sending their paper to Journal of Electroanalytical Chemistry (261 (1989) 301). Jones, infuriated, sent his paper to Nature (338 (1989) 737) right after. While F-P placed all their bet on the excess heat, Jones observed neutron emission at 2,45 MeV, the typical signature of D-D fusion. However, the F-P paper was incomplete and full of strange omissions. For example, they claimed to have measured some nuclear emissions accompanying heat release, but the only data reported in the paper was a strange gamma-ray spectrum showing a broad, isolated peak at 2,2 MeV, without the corresponding Compton shoulder. This was later rejected as erroneous, and it turned out that F-P had made no nuclear measurements at all. In a subsequent paper they claimed to have measured neutron emission, but later also these data were discredited, because they did not take into account the temperature sensitivity of 10B detectors. Eventually, their problem was that as chemists they had no practice of nuclear measurements, and they did not allow anybody to collaborate with them. Already by the end of April, the New York Times declared the cold fusion a dead alley. Despite some groups occasionally claiming to observe similar results, the scientific community quickly reached the consensus that the F-P work was wrong, sloppy and inconsistent, if not a total hoax.
Indeed, the initial rumor of the “discovery” had also sparked some fun and humour about it. Back in 1989 the internet was just in its infancy, and scientists used to communicate by regular mail, for example exchanging papers and review reports with journals. With my friend Mario Carta at ENEA in Rome, we devised a very nasty joke behind the shoulders of our division director of the time, a stolid bureaucrat that everybody hated to some degree. We concocted a fake paper abstract for some NIST conference, that was being sent to him as a reviewer to evaluate. The fake paper was written by two fake Polish meteorologists, A. Ambrogy and W. Watko from the Krakow university, and reported stratospheric balloon measurements of cosmic rays in Antarctica, around the last weeks of March, 1989. According to them, due to some unspecified perturbation in Van Allen belts, the ozone hole had opened up even more (in those days, ozone crisis was a much-debated topic in the news), as it was clearly shown from their cosmic neutron spectra. The latter showed a sharp increase, exactly around March 23. With the help of a lab secretary (we even cleared and glued back US post stamps from another letter) we could make the whole thing to look like a genuine letter arriving from Gaithersburg. When our not-so-smart boss read the paper he jumped on his chair, immediately thought he had the solution to the cold-fusion mystery: Fleischmann and Pons had measured just some excess of cosmic rays, which also explained the impossibility of reproducing their experiment. So, he started calling around colleagues, seeking both confirmation and credit for his own “discovery”. I will not tell you about the obvious end of the story, but you can imagine the laughter and the shame.
A few months after, I moved to the Metal Physics Division at ENEA, where our group worked on defect properties and hydrogen storage in transition metal alloys. Therefore, it was the obvious thing to do for me and my other partner in crime, Vittorio Rosato, to put together a quick model of deuterium adsorption in palladium at supercritical densities. By April we had finished our calculations and the paper was published in the July issue of Journal of Materials Research (5 (1990) 2094). In retrospect, I am not happy with this paper. I think we were much too naïve in our setup, and we totally overlooked electronic structure effects. As a result, the paper did not attract many citations, however it was cited by both Storms and Hagelstein (see below) a few times. We stuffed a Pd fcc lattice with deuterium atoms in interstitial sites, up to very high concentrations of PdD1.2 , and studied the clustering of D with an energy functional parametrized to reproduce the DFT results of free atoms. We could never observe D-D distances shorter than 0.11 nm, even including dislocations, vacancies and high temperature up to 900 K. Hence, we saw no evidence of fusion-like D-D overlap, on a purely atomic basis.
However, crystal lattice effects (if any) could be more subtle. Low-energy enhancement of D-D and D-T fusion cross sections by electron screening has been known for a while (Salpeter, Austr. J .Phys 7 (1954) 373; Assenbaum et al, Zeit. Phys. A 327 (1987) 461). The F-P experiment itself, was not entirely a novelty. In 1927 John Tandberg reported that he fused hydrogen into helium with a Pd electrode, and despite being denied a patent, he continued his experiments with heavy water after the discovery of deuterium in 1932: that was practically the very same F-P experiment, sixty years earlier. Those (not many) people who kept working on cold fusion have rebranded the subject “low-energy nuclear reactions”, or LENR, probably to avoid the tarnishing of reputation that the etiquette “cold fusion” implied. LENR-based models lend support to recent results (see below) in which slow neutrons are the primary responsible for accelerating D-D fusion. The attention is now focussed on electroweak processes (e.g. Srivastava et al, Pramana J Phys 75 (2010) 617) in which electrons accelerated by strong electric fields produce neutrons (e– + p+ -> n + v). Bringing in a more sophisticated physics may help restore some credibility. Indeed, working on cold fusion had quickly become a showstopper for your career as a physicist, so the subject was dropped altogether in all laboratories by the 2000s. Already in the first half of the ‘90s no respectable journal would even consider a paper for publication. Nobel Laureate Julian Schwinger declared himself a supporter of cold fusion in the fall of 1989, after much of the initial excitement had already started to cool off. He tried to publish his theoretical paper “Cold Fusion: A Hypothesis” in PRL, but was rejected so harshly by two reviewers that he felt deeply insulted, and resigned from his honorary American Physical Society membership in protest.
Peter Hagelstein at MIT retried the F-P experiment in 2010, by adding laser pulses to the Pd-D cell. Despite the paper was published in the rather obscure Journal of Condensed Matter and Nuclear Science (IF 0,12), it contained a possibly interesting result: the heat generation appeared to increase by 2-3 orders of magnitude for particular pulse frequencies of the laser. This could imply a role of phonons in the Pd lattice. However, nobody could yet replicate their experiment. Another one who dedicated a large part of his scientific life to the cold case is Edmund Storms, in Santa Fe. He published a paper in 2016 (same journal), in which he repeated yet the F-P experiment but with specially machined Pd electrodes. He attributed the observed excess heat to the presence of lattice defects (microcracks) in the metal, which would explain why some F-P experiment replicas work and other do not. Again, no other lab has been able to reproduce Storms’ results. Two companies in Japan, Technova and Clean Planet, claim to have obtained excess heat from their respective cold-fusion equipment. The second company followed Storms’ idea of nanoscale defects, and in 2020 reported heat generation with electrodes made with nanoparticles of Cu-Ni-Zr oxide. Again, no other laboratory reproduced this result (maybe nobody tried, indeed…), nevertheless Clean Planet laid out a roadmap according to which a commercial reactor should be available by 2024. Neither company has disclosed the details of their machines, and no technical data of any kind have appeared yet.
Then, without much public rumor, Google funded a number of cold-fusion experiments (10 million dollars between 2015 and 2019) to explore three experimental set-ups involving metals and hydrogen. The about 30 scientists of the team tried deuterium pressure loading in Pd; Pd irradiation with deuterium beams; and high-temperature heating of Pd wires in D atmosphere. While they observed in some cases heat release about 100 times larger than expected, in no case they found evidence of D-D fusion. Their results led to a dozen of papers, and the final summary appeared in Nature 570 (2019) 45. However negative, such an earnest effort by a major player was instrumental in reviving the corpse of cold fusion research. In the past couple of years funding has received an unexpected boost, and the US DOE recently launched a call for projects on LENR. Research groups at NASA and US Navy have published new results in two joint papers, which reproduce previous data and provide new insight (Phys Rev C101 (2020) 44609, and 44610). In both papers, deuterium stored in metal samples reacts with deutons (D ions) accelerated by collisions with hot neutrons produced by different sources. This makes the link with the LENR concepts, about electroweak interactions generating neutrons by electron capture.
An arxiv preprint of sept. 2022 (https://arxiv.org/abs/2208.07245) makes a series of interesting points on this “cold fusion revival”. The authors are three very young scientists (Hunt is a postdoc) who met at MIT, where Metzler is research staff. While their paper is mostly a critical review of data and pertinent theoretical models, they put forward two different hypotheses that could explain why cold fusion happens in certain metals more readily than expected on the basis on known chemical physics. The first idea is that there could be unknown nuclear resonant states at very low energy (a domain totally unexplored by current nuclear physics), which could be excited and facilitate D-D tunneling across the Coulomb barrier. The other is that there could be some crystal lattice effect (again, the electronic screening) that facilitates D-Pd nuclear interactions, leaving behind excited Pd nuclei, which then decay with the “strange” neutron spectra sometimes observed.
It is true that nuclear physics in the strong confinement regime is still largely incomplete. The Standard Model of particle physics provides a clear picture of high-energy interactions in the asymptotic freedom regime, when the high-mass domain becomes accessible and we can see heavy bosons, the top quark, the Higgs, and so on. However, it does not give us a clue (and that’s a euphemism) about low-energy processes. We cannot calculate how three quarks and some gluons form a proton or a neutron, not to speak of how do they assemble into a nucleus, so that in the year 2022, about one hundred years after Bethe, Fermi and Gamow, we must still make recourse to fitted nucleon-nucleon potentials and the empirical mass-equation to model the nuclear matter. It is well possible that our knowledge of low-energy physics is still rich of surprises, and the conflicting results of these “cold fusion” experiments maybe are just warning us, to go look further and deeper. Moreover, these are relatively simple and inexpensive experiments, compared to other fields of physics and in particular to other routes to fusion energy research. Maybe worth a second try.