Real cold fusion has never been tried
What the Callisto revelation teaches us.
Most people know the story of cold fusion, at least the beginning. Let me tell you how it ends.
If you combine two high school chemistry ideas—the Nernst equation and the ideal gas approximation—you get an expression relating gas pressure to the voltage at an electrode:
Since the pressure term is inside the logarithm, applying a small voltage difference to a gas-evolving reaction can create a huge pressure difference. Read the equation naively, and it seems like even half a volt should be able to make gas pressures higher than those at the center of the sun!
That naïve reading is exactly what motivated Martin Fleischmann and Stanley Pons in 1989. In a now-infamous experiment, they applied a few volts to an electrolysis cell, hoping to load deuterium from heavy water into palladium metal, which would hold the deuterium at such incredible pressures that it could undergo fusion.
Astoundingly, it seemed like it worked: their electrochemical cell appeared to produce copious “excess heat”, the result of a newly discovered fusion process. A breathless press release celebrated a clean and abundant new energy source for humankind.
The scientific community was skeptical. Conventional “hot” fusion uses tremendous energies to get nuclei close enough together so that nuclear attraction (which drops off exponentially with distance) overpowers electrostatic repulsion (which drops off more gradually as 1/r2), allowing nuclei to combine and release energy. No material can fully confine the hot plasmas they use—only exquisitely shaped electromagnetic fields can. Even if room-temperature electrochemistry really could generate such extreme conditions, wouldn’t a slender rod of palladium metal with an internal pressure of billions of atmospheres just break apart?
Obviously the 1989 announcement provoked an enormous hubbub and replication attempts from scientists all over the world. A majority of them failed to reproduce cold fusion, and over a few months the mainstream scientific community more or less dismissed the whole thing as pathological science[1].
The idea that wouldn't die
But a lot of people who tried to replicate the Fleischmann-Pons experiment did claim to measure excess heat from their electrochemical cells. These included some pretty respectable scientists like Robert Huggins at Stanford and John Bockris at Texas A&M, as well as researchers at some national labs. A scientific counter-consensus formed around cold fusion research, complete with star researchers, a journal, an international meeting, and hundreds of people who all claim to have observed “excess heat”.
Fast forward 35 years, and this is all ongoing and it’s surprisingly big and robust. There have been well-funded cold fusion efforts by Toyota, NASA, Google, SRI, the US Navy, the US Department of Energy, and lots of startups. Over 300 published replications claim evidence of cold fusion. New projects started at both Stanford and MIT in the last few years. Slight variants keep emerging. Hundreds of millions have been spent.
Over time, the feeling around cold fusion changed—less giant scientific blooper from the 80s and more like wow maybe there’s a chance this was real all along. The Where’s my Flying Car book has a representative take: “it’s not that cold fusion is a dead certain new energy panacea being suppressed by evil governments… [rather, the many reports of excess heat production are] a solid hint that there’s something in the physics that we don’t yet understand.”
Callisto
Project Callisto kicked off in 2015 as an effort to get to the bottom of all this. It was the biggest project ever in cold fusion—around a hundred total contributors working in 50,000sf of lab space over almost a decade. To my knowledge it was also the most scientifically rigorous, with successful technologists and engineers working alongside researchers in physics and materials science. I helped with it in 2016 and 2017 when I was in graduate school. A number of Orca contributors helped over those first few years.
Callisto replicated every cold fusion claim that seemed even a tiny bit credible. Unlike most replication efforts it didn’t end with “we did (or didn’t) find cold fusion”. Rather, Callisto tracked down the particular experimental configurations that Fleischmann and Pons and other researchers had used, and then in most cases identified specific non-nuclear explanations for why people had been fooled. Stuff like “your power supply from 1988 indicated it was supplying 3A at 1.8V but in that configuration it was supplying 3.12A, that’s why your cell got hotter than you thought it should” etc. Truly meticulous observational work.
Project Callisto got pretty fancy. Super sophisticated calorimetry, massively parallel experiments, every characterization and monitoring method to the point where they could have seen evidence of even a single fusion event. In addition to reproducing all the major “canonical” cold fusion claims, they also went into new regimes where cold fusion could be lurking, if anything like the proposed mechanism were true. They used femtosecond lasers to stimulate fusion in the metal lattices. They jolted the samples with heat and electric fields. They tried gas loading rather than electrochemical loading and in the process set a world record for static PxV compression for hydrogen. Truly virtuoso engineering.
Despite many individual anomalous measurements that initially seemed like evidence for cold fusion, the Callisto team always found that there was a prosaic explanation when they looked closely enough. The final verdict was clear: this is a tricky measurement and it’s easy to get confused, but there’s nothing remotely resembling nuclear fusion happening in these systems.
The full report is here for anyone curious.
The sociology of it all
Cold fusion is such a huge prize that it creates weird incentives for researchers and funders both. Since it promises near-infinite energy for all of humanity, it seems to justify giant investments even if the probability of it being real is extremely small [2].
It’s a little like the effective altruism stuff where (tiny probability) x (momentous outcome) creates extremes of opinion about what’s important, and people start having heated arguments about extinction risk or shrimp welfare or the urgency of colonizing the galaxy. It's simple and predictable. For fixed uncertainty in the assumptions, total uncertainty explodes as the terms in the expected value calculation diverge.
The variance in expected value that makes it hard to choose good moonshots also makes it hard to staff them. For funders who have lots of resources and make many bets, it makes sense to invest in long-shot projects with concatenated scientific uncertainties that amount to a series of double-or-nothing coin tosses. That maximizes global expected value. But from the perspective of the scientists working on these projects, even one failed coin toss can tank a career. The employee’s incentive might be to either find a lower-risk, lower-reward project, or else to subvert the moonshot so that it turns into a professional win even though the project does not succeed [3].
These dynamics explain why a lot of ambitious science projects like ITER or JCAP never end, but rather tail off into long-term jobs programs for the scientists involved. Thinking about Google X type projects, it seems like science-facing problems might be especially vulnerable to this type of subversion. If an engineering team keeps failing to build a computer chip or an electric vehicle or something, funders can just move on—there are known timelines for that sort of work. Not so for science, which can resist progress for decades before huge breakthroughs happen. You can justify going on forever, and they can justify funding you forever.
The Callisto reveal also shows us how science's Overton window has moved, or even split in two. A window with panes. Why do you think Callisto stayed secret for so long? Part of it was probably to secure a strong position in the case of success – IP around a cold fusion claim would be hard to protect. But the bigger reason was cultural. Cold fusion was untouchable in mainstream academic circles in 2015. To most "real scientists" it was an outgroup interest, socially-constructed madness, stigmatized to the point that many of the Callisto researchers insist on anonymity even years after the project concluded.
Maybe it’s useful that the world is developing different cultures of scientific inquiry that are somewhat siloed from one another, and that can ask different questions in different ways [4]. But for cold fusion the mainstream stigma was a headwind to getting to the truth of things [5]. It’s a testament to the good intentions of the team that this work ever got published at all.
Where this leaves us
Nobody will ever be able to fully disprove all the cold fusion claims to the satisfaction of the true believers. But Callisto should completely close the door on any further research on Fleischmann-Pons type cold fusion. This stuff now shares the epistemological status of ufology, lizard people, jet-fuel-can’t-melt-steel etc [6].
Nonetheless, there’s some other “cold fusion adjacent” stuff that might still deserve a closer look.
DARPA is supporting some ongoing work that’s pretty cool. They’re injecting energy into lattice-confined deuterium to activate possible fusion events—very similar to what the Callisto group was trying with femtosecond lasers. One group adding energy with an electron beam already claims to see fusion-like signals. I suspect they’ll find a nonmagical explanation for what they’re seeing (one colleague speculates a high-energy tail from their electron gun) but it is also possible that this is a real effect.
There is of course also muon-catalyzed fusion. Since fusion is all about getting small nuclei close enough together that the nuclei can “see” each other and bind, one way to get fusion in standard conditions is to shrink the electron cloud around the atom. Since a muon is basically a heavy electron, it takes up far less space than an electron orbiting an atomic nucleus [7]. Bombard hydrogenic atoms with muons then, and their charge clouds shrivel up, letting them get close enough to other atoms to fuse. One muon fired into heavy hydrogen can catalyze the fusion of hundreds of atoms this way. Traditionally it takes about 10x more energy to generate a muon than you get back from those fusion events—a losing proposition. But a few startups are betting that with better particle accelerators and improved muon dynamics they will be able to bend these numbers in their favor. The coolest of these IMO is Acceleron in Boston.
Anyways, real cold fusion has never been tried, and it’s a good bet it never will because cold fusion isn’t real. If anything like fusion in ambient conditions ends up working, it’ll probably still involve high energy stuff like muons or electron beams that aren’t really “cold” at all.
Footnotes
[1] Fleischmann and Pons didn’t help the whole situation by mistakenly claiming that they’d measured neutrons, gamma rays, and other easily falsifiable stuff that most others couldn’t replicate.
Both the Where’s my Flying Car book by J. Storrs Hall and Bad Science by Gary Taubes are great for more on this sordid history if you're interested.
[2] Cold fusion has lessons for "hot fusion" here, which has a lot of the same dynamics: sky high risks and rewards both, the ability to justify but also hide behind very high project costs and long timelines etc. It all makes hot D-T fusion controversial to almost the same degree. To some people it's civilization’s obvious next milestone, limitless energy from seawater, proof of our inevitable mastery of all four forces of nature, our only real hope for travel to deep space etc. To other people it’s the silliest possible way to boil water, pure hype and a bottomless money hole, energy much more expensive and less powerful than fission which society has rejected already, an academic Chichen Itza where billions of dollars and hundreds of promising careers are sacrificed for a beautiful science idea long ago proven to be an engineering dead end.
[3] “Beware the dreams of old men” is how I heard this from a mentor in grad school. For a young scientist, it’s dangerous to work with somebody older who’s already accomplished too much. Only something huge and improbable like fusion, room temperature superconductivity, gold from seawater, transmutation, wireless power transmission etc. can satisfy them.
[4] “Maybe it’s useful that the world has different cultures of scientific inquiry that can be helpful in different ways.”
The world already has parallel hard-tech cultures of progress (supersonic self-driving spaceplanes etc.) and atonement (climate-tech etc.) which are remarkably disconnected from one another. At the moment these are a little bit right/left or perhaps master/slave morality coded.
It’s still not clear which of these movements will accomplish more or help more over time. I suspect that the best topics to work on will as always be those few things endorsed both by those who want to be good and by those who want to be great.
[5] The Where’s my Flying Car chapter on cold fusion is mostly about the social and institutional pressures that prevented ‘respectable’ researchers from running cold fusion to ground for so long. These are the same pressures that made so many of the Callisto researchers insist on anonymity. Where’s my Flying Car bends this into commentary on how the academic establishment may be holding the world back.
There's a lot to dislike in mainstream university R&D. Science has gotten so self-absorbed and self-referential. It’s been so long since a university engineering department has invented anything useful in hard tech that most funders don't really believe in "translational research" any more. But I don’t think the cold fusion story has such a straightforward lesson about the usefulness of academia.
On one level, cold fusion shows us why some of the resistance to free thought in academia can be valuable and adaptive—there are too many conspiracy theories to possibly run them all to ground, it’s better to shut them out. At the same time, you might say that a lot of the fakest stuff in the nonacademic R&D world is so rampant now precisely because people can see that the academics are cloistered and aloof. It’s not clear whether the guardrails are too high or too low.
Whether or not he was right, Hall is getting what he wanted. Just as conspiracy type stuff has re-entered culture at large, fringe science like cold fusion is back in the Overton window for some academics. Most of the ‘respectable' groups now dabbling in cold fusion emerged post-Callisto and post-2016—ARPA-E, DARPA, Stanford etc.
[6] This was just an incorrect claim that turned out to be pretty hard to debunk in most labs. Climate change being fake, vaccines causing autism, and earth being flat are also in their own way hard to disprove for people who aren’t properly equipped.
Calorimetry in an electrochemical cell is just confusing. Electrical power is going in. Multiple gasses with different thermal characteristics are either leaving or swirling around and recombining. Hydrogen evolution, hydriding, dehydriding, oxygen evolution and H2/O2 recombination all have different enthalpic and also entropic heat signatures that you need to account for.
Eli Dourado wisely suggests that future work in cold-fusion adjacent topics should be required to offer up evidence other than calorimetry (e.g. alpha particles, helium production) before anyone pays any attention.
[7] The proper way to understand this is the effect of the reduced mass on the Bohr radius: 4pi eo hbar^2/m e^2.
But there’s a more intuitive way to understand. If electrons have negative charge and the nucleus has a positive charge, why don’t electrons fall into the nucleus? It’s basically the uncertainty principle—the electron takes up space Δx > hbar/2ΔP. So a muon, with a larger rest mass and therefore larger P, takes up less space than an electron in an atomic orbital. That lets nuclei get close enough together to fuse without crazy temperatures or pressures.