If you lived through the eighties there are certain things you could not miss, and since this is a science blog I am of course not referring to fashion aberrations, like mullets and shoulder pads, but rather to what is widely regarded as one of the most notorious science scandals to date: Fleischmann and Pons Cold Fusion, the claim of tapping the ultimate energy source within a simple electrochemical cell.
For a short time it felt like humanity’s prayers to deliver us from fossil fuel had been answered (at least to those who believe in that sort of thing). Of course, paying the ever increasing price at the gas pump is a constant (painful) reminder that this euphoric moment at the end of eighties was but a short lived aberration. But back then it felt so real. After all, there already existed a well-known process that allowed for nuclear fusion at room temperature, catalyzed by the enigmatic muons. One of the first scientific articles that I read in English was on that phenomenon, and it was published just a couple of years earlier. So initial speculations abounded, that maybe muons in the cosmic background radiation could somehow help trigger the reported reaction (although there was no explanation given as to how this low muon flux density could possibly accomplish this). While my fringe blog focuses on the intrepid researchers who, despite the enormous blow back, still work on Fleischman Pons-style research, this post is about the former, the oft forgotten muon-catalyzed fusion.
It is a beautiful nuclear reaction, highlighting one of the most basic peculiarities of quantum mechanics: Quantum Tunnelling and Heisenberg uncertainty principle. Both of these are direct consequences of the manifest wave properties of matter at this scale. The former allows matter to seep into what should be impenetrable barriers, and the latter describes how a bound point particle is always “smeared out” over a volume – as if points are an abstraction that nature abhors. Last but not least, it showcases the mysterious muon, a particle that seems to be identical to electrons in every way but the mass and stability (about 200 times more mass and a pretty long half life of about 2 μs). Because it behaves just like a heavier twin of the electron, it can substitute the latter in atoms and molecules.
The Heisenberg uncertainty principle states that the product of momentum (mass times velocity) and position ‘certainty’ has a lower bound. Usually the term uncertainty is simply interpreted probabilistically in terms of the deviation of the expectation value. But this view, while formally entirely correct, obstructs the very real physical implication of trying to squeeze a particle into a small space, because the momentum uncertainty then becomes a very real physical effect of quantum matter. The particle’s velocity distribution will become ever broader, forcing the matter outwards and creating an orbital ‘cloud’ (e.g. specifically the spherical hydrogen s-orbital). There is really no good analogy in our everyday experience, they all sound silly: My version is that of a slippery soap in a spherical sink, the harder you try to grasp it the more powerful you send it flying. If you were to map all trajectories of the soap over time, you will find that on average it was anywhere in the sink with the probability decreasing towards the rim (that is unless you squeeze it so hard that it acquires enough speed to jump out of the sink – I guess that would be an analog to ionization). In the atomic and chemical realm, on the other hand, the very concept of a trajectory doesn’t hold up (unless you are dealing with Rydberg atoms). You may as well think of electron orbitals as charge distributions (as this is exactly how they behave in the chemical domain).
Because the momentum rather then the velocity enters into the equation, the orbitals for a heavier version of the electron will be considerably smaller, i.e. about 2oo times smaller for the muon, as this is the factor by which the particle’s velocity can be reduced in order to still get the same momentum. So muonic hydrogen is much smaller than the electron version. That’s already all that is needed to get fusion going, because if two heavy hydrogen nucleons are bound in a muonic μH2 molecule they are far too close for comfort. Usually the repellent force of the electrostatic Coulomb potential should be enough to keep them apart, but the quantum tunnel effect allows them to penetrate the ‘forbidden’ region. And at this distance, the probability that both nucleons occupy the same space becomes large enough to get measurable incidents of nuclear fusion i.e. μH2 → μHe.
The hydrogen used in the experimental realization is not the usual kind, but as with other fusion realizations, the heavier hydrogen isotopes deuterium and tritium are required, and since there is only one muon in the mix the d-t hydrogen is ionized. so that the equation looks more like this: (d-μ-t)+ → n + α (with the n indicating a fast neutron and the α a Helium-4 nucleus.)
The latter causes a lot of trouble as the muon ‘sticks’ to this alpha particle with a 1% chance (making it a muonic helium ion). If this happens, this muon is no longer available to catalyze more fusion events. This, in combination with the limited life time of the muons, and the ‘set-up’ required by the muons to bind to the hydrogen isotopes, is the limiting factor of this reaction.
Without a constant massive resupply of muons the reaction tempers off quickly. Despite decades of research this problem could never be surmounted. It takes pions to make muons, and the former are only produced in high energy particle collisions. This costs significantly more energy than the cold muon catalyzed fusion can recoup.
But there is one Australian company that claims that it has found a new, less costly way to make pions. They are certainly a very interesting ‘cold fusion’ start-up and at first glance seem far more credible than the outfits that my fringe blog covers. But on the other hand, this company treats their proprietary pion production process with a level of secrecy that is reminiscent of the worst players in the LENR world. I could not find any hint of how this process is supposed to work and why it supposedly can produce sufficient amounts of muons to make this commercially exploitable. (Pions could also be generated in two photon processes, but this would require even more input energy). So on second read the claims of Australian’s Star Scientific don’t really sound any less fantastic than the boasting of any other cold fusion outfit.
Any comments that could illuminate this mystery are more than welcome. Preliminary google searches on this company are certainly not encouraging.