Cold fusion

Cold fusion cell at the US Navy Space and Naval Warfare Systems Center San Diego (2005)

For other uses, see Cold Fusion (disambiguation).

Cold fusion, sometimes called low energy nuclear reactions (LENR) or condensed matter nuclear science, is a set of effects reported in controversial laboratory experiments at ordinary temperatures and pressures; some researchers claim these effects are caused by nuclear reactions.

In 1989, Martin Fleischmann and Stanley Pons reported producing a tabletop nuclear fusion reaction at the University of Utah. In their press conferences and papers, they reported the observation of anomalous heating ("excess heat") of an electrolytic cell during electrolysis of heavy water using palladium (Pd) electrodes. Lacking an explanation for the source of such heat, they proposed the hypothesis that the heat came from nuclear fusion of deuterium (D). The report of their results raised hopes of a cheap and abundant source of energy. A review panel organized by the US Department of Energy (DOE) in 1989 found that the evidence for the discovery of a new nuclear process termed cold fusion was not persuasive.

Cold fusion gained a reputation as pathological science after critical books were published in the 1990-1993 time frame detailing the experiences of those who investigated the field early on. These books and the surrounding events led most scientists to conclude there was no substance to the reports of cold fusion. Numerous pro-cold fusion books have also been written from the 1990’s to present (see D. Britz Bibliography for a list and brief reviews.) As well, cold fusion proponents have reported what they contend are additional supporting results including some in peer-reviewed journals and at conferences, but most scientists have met these reports with skepticism.

Since 2004, additional supporting results have been reported in peer-reviewed journals and at conferences. Two peer-reviewed literature reviews in 2007 concluded that anomalous effects have been demonstrated by experiments that result in excess heat production and nuclear reaction products such as helium-4. Both reviews say that although many explanations have been proposed, several of which do not use new physics, none is yet satisfactory. The author of one of the reviews stated that the failure to attain replications was due to the inability to achieve sufficiently dense deuterium loading, and proposed a series of experiments to reveal the underlying mechanism(s) of the effect.

History

Early work

The special ability of palladium to absorb hydrogen was recognized as early as the nineteenth century by Thomas Graham. In the late nineteen-twenties, two German scientists, Friedrich Paneth and Kurt Peters, originally reported the transformation of hydrogen into helium by spontaneous nuclear catalysis when hydrogen was absorbed by finely divided palladium at room temperature. However, the authors later acknowledged that the helium they measured was due to background from the air.

In 1927, Swedish scientist J. Tandberg stated that he had fused hydrogen into helium in an electrolytic cell with palladium electrodes. On the basis of his work, he applied for a Swedish patent for "a method to produce helium and useful reaction energy". After deuterium was discovered in 1932, Tandberg continued his experiments with heavy water. Due to Paneth and Peters' retraction, Tandberg's patent application was eventually denied.

The term "cold fusion" was coined by E. Paul Palmer of Brigham Young University in 1986 in an investigation of "geo-fusion", or the possible existence of fusion in a planetary core.

Fleischmann-Pons announcement

Fleischmann said that he began investigating the possibility that chemical means could influence nuclear processes in the 1960s. He said that he explored whether collective effects, that would require quantum electrodynamics to calculate, might be more significant than the effects predicted by quantum mechanical calculations. He said that, by 1983, he had experimental evidence leading him to believe that condensed phase systems developed coherent structures up to 10^-7m in size. In 1984, Fleischmann and Pons began cold fusion experiments. In 1989, they reported that one of their experiments resulted in the melting and partial vaporization of the palladium cube used for their cathode, the partial destruction of their lab bench, a small hole in the concrete floor and damage to the fume hood.

Electrolysis cell schematic

In their original set-up, Fleischmann and Pons used a Dewar flask (a double-walled vacuum flask) for the electrolysis, so that heat conduction would be minimal on the side and the bottom of the cell (only 5 % of the heat loss in this experiment). The cell flask was then submerged in a bath maintained at constant temperature to eliminate the effect of external heat sources. They used an open cell, thus allowing the gaseous deuterium and oxygen resulting from the electrolysis reaction to leave the cell, along with some heat. It was necessary to replenish the cell with heavy water at regular intervals. The authors said that, since the cell was tall and narrow, the bubbling action of the gas kept the electrolyte well mixed and of a uniform temperature. Special attention was paid to the purity of the palladium cathode and electrolyte to prevent the build-up of material on its surface, especially after long periods of operation.

The cell was also instrumented with a thermistor to measure the temperature of the electrolyte, and an electrical heater to generate pulses of heat and calibrate the heat loss due to the gas outlet. After calibration, it was possible to compute the heat generated by the reaction.

A constant current was applied to the cell continuously for many weeks, and heavy water was added as necessary. For most of the time, the power input to the cell was equal to the power that went out of the cell within measuring accuracy, and the cell temperature was stable at around 30 °C. But then, at some point (and in some of the experiments), the temperature rose suddenly to about 50 °C without changes in the input power, for durations of 2 days or more. The generated power was calculated to be about 20 times the input power during the power bursts. Eventually the power bursts in any one cell would no longer occur and the cell was turned off.

In 1988, Fleischmann and Pons applied to the United States Department of Energy for funding towards a larger series of experiments. Up to this point they had been funding their experiments using a small device built with $100,000 out-of-pocket. The grant proposal was turned over for peer review, and one of the reviewers was Steven E. Jones of Brigham Young University. Jones had worked on muon-catalyzed fusion for some time, and had written an article on the topic entitled "Cold nuclear fusion" that had been published in Scientific American in July 1987. Fleischmann and Pons and co-workers met with Jones and co-workers on occasion in Utah to share research and techniques. During this time, Fleischmann and Pons described their experiments as generating considerable "excess energy", in the sense that it could not be explained by chemical reactions alone. They felt that such a discovery could bear significant commercial value and would be entitled to patent protection. Jones, however, was measuring neutron flux, which was not of commercial interest. In order to avoid problems in the future, the teams appeared to agree to simultaneously publish their results, although their accounts of their March 6 meeting differ.

In mid-March, both research teams were ready to publish their findings, and Fleischmann and Jones had agreed to meet at an airport on March 24 to send their papers to Nature via FedEx. Fleischmann and Pons, however, broke their apparent agreement, submitting their paper to the Journal of Electroanalytical Chemistry on March 11, and disclosing their work via a press conference on March 23. Jones, upset, faxed in his paper to Nature after the press conference.

Reaction to the announcement

The press initially reported on the experiments widely, and due to the surmised beneficial commercial applications of the Utah experiments, "scores of laboratories in the United States and abroad" attempted to repeat the experiments. The announcement raised hopes of a cheap and abundant source of energy.

On April 10, 1989, Fleischmann and Pons, who later suggested pressure from patent attorneys, published a rushed "preliminary note" in the Journal of Electroanalytical Chemistry. This paper notably contained a gamma peak without its corresponding Compton edge, a discrepancy that triggered accusations of fraud. Their "preliminary note" was followed up a year later in July 1990, when a much longer paper, going into details of calorimetry but without any nuclear measurements, was published in the same journal.

Also occurring on April 10, a team lead by John Bockris at Texas A&M University published their results of excess heat, followed up by a team at the Georgia Institute of Technology who observed production of neutrons. Both results were widely reported on in the press. The Georgia Institute of Technology team later retracted their announcement. For the next six weeks, additional competing claims, counterclaims and suggested explanations kept what was referred to as "cold fusion" or "fusion confusion" in the news.

On April 12, Pons received a standing ovation from about 7,000 chemists at the semi-annual meeting of the American Chemical Society. The University of Utah asked Congress to provide $25 million to pursue the research, and Pons was scheduled to meet with representatives of US President George H. W. Bush in early May.

Then on May 1, the American Physical Society held a session on cold fusion, which included several reports of experiments that failed to produce evidence of cold fusion. A second session began the next day with other negative reports, and eight of the nine leading speakers stated that they considered the initial Utah claim dead. Steven E. Koonin of Caltech described the Utah report as a result of "the incompetence and delusion of Pons and Fleischmann." Douglas R. O. Morrison, a physicist representing CERN, called the entire episode an example of pathological science.

In June 1989, John Bockris and his team at Texas A&M published their previously informal report of tritium generation. In September, they reported the sporadic observation of excess heat. In August, the state of Utah, which runs the University of Utah, invested $4.5 million to create the National Cold Fusion Institute. Nature published papers critical of cold fusion in July and November.

1989 DOE panel

In November, a special panel formed by the Energy Research Advisory Board, under a charge of the United States Department of Energy, said that it was not possible to state categorically that cold fusion has been convincingly either proved or disproved. The experimental results of excess heat from calorimetric cells reported to them did not present convincing evidence that useful sources of energy will result from the phenomena attributed to cold fusion. These experiments did not present convincing evidence to associate the reported anomalous heat with a nuclear process. Current understanding of hydrogen in solids gives no support for the occurrence of cold fusion in solids. Nuclear fusion at room temperature, of the type discussed in this report, would be contrary to all understanding gained of nuclear reactions in the last half century; it would require the invention of an entirely new nuclear process.

The panel "recommended against the establishment of special programs or research centers to develop cold fusion", but was "sympathetic toward modest support for carefully focused and cooperative experiments within the present funding system." The Panel recommended that "the cold fusion research efforts in the area of heat production focus primarily on confirming or disproving reports of excess heat" and stated that "investigations designed to check the reported observations of excess tritium in electrolytic cells are desirable.".

Further developments (1989-2004)

After the 1989 review by the DOE, cold fusion was generally seen as an example of pathological science. Science writers Robert L. Park and Gary Taubes have published books criticizing cold fusion experiments and researchers.

In June 1990, Gary Taubes wrote an editorial in Science suggesting that Texas A&M cells might have been spiked with tritiated water. A 3-professor panel of Texas A&M later found that none of the experiments were fraudulently conducted, saying that spiking was unlikely because scientists got different results when they tested the spiking theory by intentionally putting tritium in water. John Bockris later published his side of the controversy and a defense of academic freedom in "Accountability in Research".

In 1991, researcher Andrew Riley was killed when a cold fusion cell exploded, possibly due to accumulation of deuterium gas and the failure of a safety valve.

In September 1990, Fritz Will, Director of the National Cold Fusion Institute, compiled a list 92 groups of researchers from 10 different countries that had reported excess heat, tritium, helium4, neutrons or other nuclear effects. Proponents estimate that 3,000 cold fusion papers have been published, including over 1,000 journal papers and books, where the latter number includes both pro and con articles.^[α]

Fleischmann and Pons relocated their laboratory to France under a grant from the Toyota Motor Corporation. The laboratory, IMRA, was closed in 1998 after spending £12 million on cold fusion work.

A cold fusion calorimeter of the open type, used at the New Hydrogen Energy Institute in Japan. Source: SPAWAR/US Navy TR1862

Between 1992 and 1997, Japan's Ministry of International Trade and Industry sponsored a "New Hydrogen Energy Program" of US$20 million to research cold fusion. Announcing the end of the program, Hideo Ikegami stated in 1997 "We couldn't achieve what was first claimed in terms of cold fusion." He added, "We can't find any reason to propose more money for the coming year or for the future."

In 1994, David Goodstein described cold fusion as "a pariah field, cast out by the scientific establishment. Between [cold fusion] and respectable science there is virtually no communication at all. Cold fusion papers are almost never published in refereed scientific journals, with the result that those works don't receive the normal critical scrutiny that science requires. On the other hand, because the Cold-Fusioners see themselves as a community under siege, there is little internal criticism. Experiments and theories tend to be accepted at face value, for fear of providing even more fuel for external critics, if anyone outside the group was bothering to listen. In these circumstances, crackpots flourish, making matters worse for those who believe that there is serious science going on here."

Most people attempting to publish anything about the subject faced rejection of their papers. The late Nobel Laureate Julian Schwinger (1918 - 1994) was so outraged by the way the American Physical Society treated his papers that he resigned from that body in protest. Cold fusion researchers said that cold fusion was being suppressed, and that skeptics suffered from "pathological disbelief". They said that there was virtually no possibility for funding in cold fusion in the United States, and no possibility of getting published. They said that people in universities refused to work on it because they would be ridiculed by their colleagues.

To provide a forum for researchers to share their results, the first International Conference on Cold Fusion was held in 1990. The conference, recently renamed the International Conference on Condensed Matter Nuclear Science, is held every 12 to 18 months in various countries around the world. The periodicals Fusion Facts, Cold Fusion Magazine, Infinite Energy Magazine, and New Energy Times were established in the 1990s to cover developments in cold fusion and related new energy sciences. In 2004 The International Society for Condensed Matter Nuclear Science was formed "To promote the understanding, development and application of Condensed Matter Nuclear Science for the benefit of the public."

In February 2002, the U.S. Navy revealed that its researchers had been studying cold fusion on the quiet more or less continuously since 1989. Researchers at their Space and Naval Warfare Systems Center in San Diego, California released a two-volume report, entitled "Thermal and nuclear aspects of the Pd/D₂O system," with a plea for proper funding.

2004 DOE panel

In 2004, the DOE organized another panel to take a look at cold fusion developments since 1989 to determine if their policies towards cold fusion should be altered.

It concluded: "While significant progress has been made in the sophistication of calorimeters since the review of this subject in 1989, the conclusions reached by the reviewers today are similar to those found in the 1989 review." "The nearly unanimous opinion of the reviewers in the 2004 review was that funding agencies should entertain individual, well-designed proposals for experiments that address specific scientific issues relevant to the question of whether or not there is anomalous energy production in Pd/D systems, or whether or not D-D fusion reactions occur at energies on the order of a few electron volts (eV). These proposals should meet accepted scientific standards and undergo the rigors of peer review. No reviewer recommended a focused federally funded program for low energy nuclear reactions." "The reviewers believed that this field would benefit from the peer-review processes associated with proposal submission to agencies and paper submission to archival journals."

Recent developments

The reports of excess heat and anomalous tritium production^[α] have been met by most scientists with skepticism, although discussion in professional settings still continues. The American Chemical Society's (ACS) 2007 conference in Chicago held an "invited symposium" on cold fusion and low-energy nuclear reactions, and thirteen papers were presented at the "Cold Fusion" session of the March 2006 American Physical Society (APS) Meeting in Baltimore. Articles supporting cold fusion have been published in peer reviewed journals such as Naturwissenschaften, Japanese Journal of Applied Physics, European Physical Journal A, European Physical Journal C, International Journal of Hydrogen Energy, Journal of Solid State Phenomena, Journal of Electroanalytical Chemistry, and Journal of Fusion Energy.

In 2007, a United States Naval Research Laboratory researcher with no previous experience with cold fusion wrote a review of experiments with solid palladium cathodes and electrolytes with deuterium, or with D₂ gas loaded in palladium powders. The author said that more than 10 groups worldwide have reported the measurement of excess heat in 1/3 of their experiments and that most of the research groups have reported occasionally seeing 50-200% excess heat for hours to days. The positive results were difficult to reproduce, which the author attributed to variations in the loading ratios of deuterium to palladium achieved by different research teams.

In 2008, the government of India reviewed the field. M. R. Srinivasan, former chairman of the Atomic Energy Commission of India said: "There is some science here that needs to be understood. We should set some people to investigate these experiments. There is much to be commended for the progress in the work. The neglect should come to an end". On May 22, 2008, Arata and Zhang publicly demonstrated what they say is a cold fusion reactor at Osaka University.

Summary of evidence for cold fusion

Cold fusion experiments have been conducted with many types of apparatus. The main constituents are:

• a metal, such as palladium or nickel, in bulk, thin films or powder;
• deuterium and/or hydrogen, in the form of water, gas or plasma; and
• an excitation in the form of electricity, magnetism, temperature, pressure, laser beam(s), or of acoustic waves.

Cold fusion has remained controversial, but several experimenters have reported excess heat, X-rays, gamma rays, neutrons, protons, helium-4, helium-3 , and/or anomalous isotopic distributions. No experiment has unequivocally produced a particle emission spectrum matching that predicted by observations in nuclear science and high-energy physics. There is still no satisfactory theory explaining condensed matter nuclear science but many explanations have been proposed, several of which do not require new physics.

Excess heat

The excess power observed in some experiments is reported to be beyond that attributable to ordinary chemical or solid state sources; proponents attribute this excess power to nuclear fusion reactions.

In addition to Fleischmann and Pons, the generation of excess heat has been reported by others, including:

• Richard A. Oriani, while he was professor at University of Minnesota, in 1990,
• Ben Bush, of University of Texas at Austin, in 1991,
• Edmund Storms, while he was at Los Alamos National Laboratory, in 1993,
• Melvin Miles, at the Naval Air Warfare Center Weapons Division, China Lake, in 1993,
• Michael McKubre, of SRI International, in 1994,
• Yoshiaki Arata, while he was professor at Osaka University, in 1998,
• D. Gozzi, of Sapienza University of Rome, in 1998.

In 1993, Fleischmann reported "heat-after-death" experiments: he observed the continuing generation of excess heat after the electric current supplied to the electrolytic cell was turned off. Such observations have been reported by others.

The cold fusion researchers who presented their review document to the 2004 DOE panel said that "the hypothesis that the excess heat effect arises only as a consequence of errors in calorimetry was considered, studied, tested, and ultimately rejected".

Nuclear products

A CR-39 detector showing possible nuclear activity in cold fusion experiments at SSC San Diego.

The cold fusion researchers who presented their review document to the 2004 DOE panel on cold fusion said that there are insufficient chemical reaction products to account for the excess heat by several orders of magnitude. They reported a rate of helium production measured in the gas stream which varied linearly with excess power. They reported that ⁴He was produced at levels above that of the concentration in air. However, the amount of helium in the gas stream was about half of what would be expected for a heat source of the type D + D → ⁴He.

Cold fusion proponents have reported detection in their experiments of many kinds of radiation: alpha, beta, gamma, proton, triton. However, neutrons and other energetic emissions were never found in quantities commensurate with the excess heat, as would be expected by conventional fusion theory. This leads to the conjecture that the new process is somehow converting nuclear energy directly to heat.

In 2007, the Space and Naval Warfare Systems Center San Diego reported their observation of pits in CR-39 detectors during D/Pd codeposition experiments in the European Physical Journal. They said that those pits have features consistent with those observed for nuclear-generated tracks, that the Pd cathode is the source of those pits, and that they are not due to contamination or chemical reactions. They attributed some pits to knock-ons due to neutrons, and said that other pits are consistent with those obtained for alpha particles. Further analysis of "triple pits" suggests that Deuterium-Tritium reactions occurring inside the Pd lattice produce neutrons with an energy above 9.6 MeV.

Nuclear transmutations

In nuclear reactions, a chemical element may be transmuted into another. There are reports by cold fusion proponents of nuclear transmutations and isotope anomalies in cold fusion experiments.

Iwamura and associates published what they say is evidence of transmutations in the Japanese Journal of Applied Physics in 2002. Instead of using electrolysis, they forced deuterium gas to permeate through a thin layer of caesium or strontium deposited on calcium oxide and palladium, while periodically analyzing the nature of the surface through X-ray photoelectron spectroscopy. They said that as the deuterium gas permeated over a period of a week, cesium appeared to be progressively transmuted into praseodymium while strontium appeared to be transmuted into molybdenum with anomalous isotopic composition representing an addition of four deuterium nuclei to the original nuclide. When the deuterium gas was replaced by hydrogen in control experiments, no transmutation was reported to be observed. The authors said that they analyzed, and then rejected, the possibility of explaining these various observations by contaminations or migration of impurities from the palladium interior.

In 2001, Di Giulio reported transmutations in a similar gas-loading experiment with Palladium.

Criticism

In the original 1989 DOE review, skepticism towards cold fusion focused on four issues: the precision of calorimetry, the lack of consistently reproducible results, the absence of nuclear products in quantities consistent with the excess heat, and the lack of a mainstream theoretical mechanism. In the subsequent years considerable efforts have been made on these fronts, but today some issues still remain and some new ones have arisen.

Precision and accuracy of calorimetry

Main article: Calorimetry in cold fusion experiments

In the first years after the Fleishmann-Pons announcement various challenges were put forth. The efficacy of the stirring method in the Fleischmann-Pons experiment, and thus the validity of the temperature measurements was disputed by Browne. The experiment has also been criticized by Wilson. The possibility that electrochemically mediated deuterium-oxygen recombination can cause the appearance of excess heat was discussed by Shkedi and Jones.

The 2004 DOE panel noted that "significant progress has been made in the sophistication of calorimeters since ... 1989", and summarized that "Evaluations by the reviewers ranged from: 1) evidence for excess power is compelling, to 2) there is no convincing evidence that excess power is produced when integrated over the life of an experiment. The reviewers were split approximately evenly on this topic."

The panel continued, "Many reviewers noted that poor experiment design, documentation, background control and other similar issues hampered the understanding and interpretation of the results presented". The reviewers who did not find the production of excess power convincing said that excess power in the short term is not the same as net energy production over the entire time of an experiment, that such short-term excess power is only a few percent of the total external power applied and hence calibration and systematic effects could account for the purported effect, that all possible chemical and solid state causes of excess heat had not been investigated and eliminated as an explanation, that the magnitude of the effect had not increased after over a decade of work.

Kirk Shanahan suggested that a calibration constant shift could explain apparent excess heat signals, and that such a shift could occur by a redistribution of heat in a F&P cell. He further speculated that such a redistribution would occur if recombination at the electrode became active, but acknowledged that this is not experimentally proven. Cold fusion proponents say that such speculations are not supported by experimental results (in particular, that the measured volume of recombined output evolved gases does not allow for recombination within the cell), a statement that Shanahan later disputed.

Lack of reproducibility of excess heat

In 1989, the DOE panel noted that "Even a single short but valid cold fusion period would be revolutionary. As a result, it is difficult convincingly to resolve all cold fusion claims since, for example, any good experiment that fails to find cold fusion can be discounted as merely not working for unknown reasons." .

The cold fusion researchers who presented their review document to the 2004 DOE panel on cold fusion said that the observation of excess heat has been reproduced, that it can be reproduced at will under the proper conditions, and that many of the reasons for failure to reproduce it have been discovered. Contrary to these assertions, most reviewers stated that the effects are not repeatable, the magnitude of the effect has not increased in over a decade of work, and that many of the reported experiments were not well documented.

Missing nuclear products

The fusion of two deuterium nuclei usually produces either a tritium nucleus and a proton, or a helium-3 (³He) nucleus and a neutron. The level of neutrons, tritium and ³He actually observed in the Fleischmann-Pons experiments have been well below the level expected in view of the heat generated, implying that these fusion reactions cannot explain it. If the excess heat were generated by the fusion of two deuterium nuclei into helium (⁴He), a reaction which is normally extremely rare, gamma rays and helium (alpha particles) would be expected. In 1989, insufficient levels of helium (alpha particles) and gamma rays were observed to explain the excess heat.

New information was presented in 2004 to the DOE review panel regarding the production of ⁴He. When members of the panel were asked about the evidence of low energy nuclear reactions, twelve of the eighteen did not feel that there was any conclusive evidence, five found the evidence "somewhat convincing", and one was entirely convinced. The evidence of D+D fusion was taken as convincing or somewhat convincing by some reviewers; for others the lack of consistency was an indication that the overall hypothesis was not justified. Contamination of apparatus or samples by air containing 4He was cited as one possible cause for false positive results in some measurements.

An example of this was published by Clarke et al. in 2003. Their paper reported on the analysis of gases found in four ‘Case-type’ cells obtained from the McKubre group at SRI International, a primary cold fusion research group. The Abstract states: “One sample appears to be identical in composition to air, and the other three have been seriously affected by leak(s) into and from the SRI cells.” The Conclusions states: "The samples of gas from Case-type cells provided to us by the SRI workers do not show any evidence of production of ⁴He via cold fusion. Our analytical results can be explained by a combination of two factors: (a) severe leak(s) that allowed air into the cells, and also caused removal of gases including hydrogen from the cells to the atmosphere, and (b) the action of the Pd/C catalyst on O2 in the incoming air, which resulted in high CO and CO2 concentrations—telltale fingerprints of chemical combination of atmospheric O2 and C in the catalyst."

Insufficient theoretical explanations

Temperatures and pressures similar to those in stars are required for conventional nuclear fusion. The 1989 DOE panel said that such "nuclear fusion at room temperature [...] would be contrary to all understanding gained of nuclear reactions in the last half century" and "it would require the invention of an entirely new nuclear process." but it also recognized that "the failure of a theory to account for cold fusion can be discounted on the grounds that the correct explanation and theory has not been provided", that is, the lack of a satisfactory explanation could not be used to dismiss experimental evidence.

Cold fusion observations are contrary to the conventional physics of nuclear fusion in several ways :

• The average density of deuterium atoms in the palladium rod seems vastly insufficient to force pairs of nuclei close enough for fusion to occur according to mechanisms known to mainstream theories. The average distance is approximately 0.17 nanometers, a distance at which the attractive strong nuclear force cannot overcome the Coulomb repulsion. Deuterium atoms are closer together in D₂ gas molecules, which do not exhibit fusion.
• There is no known mechanism that would release fusion energy as heat instead of radiation within the relatively small metal lattice. The direct conversion of fusion energy into heat is not possible because of energy and momentum conservation and the laws of special relativity.

Cold fusion researchers acknowledge these issues and have proposed various speculative theories (for a full review, see Storms 2007) to explain the reported observations, but none has received mainstream acceptance.

Notes

• α.^{^} References to publications are listed in Storms 2007, pp. 52–61, 79–81 and in Hagelstein et al. 2004, pp. 25–29, to include Arata & Zhang 1998, Iwamura, Sakano & Itoh 2002, Mizuno et al. 2000, Miles et al. 1993 and Bush et al. 1991. Electrochemist Dieter Britz, who has remained neutral on the question of whether cold fusion exists, has compiled a list of 1385 papers published in scientific journals which includes numerous published scientific journal articles marked "res+", indicating positive research results or supportive theoretical calculations., and "res-", indicating negative research results or calculations.

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