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| Fig. 1: Antimatter Battery schematic showing torodial magnetic confinement. (Source: A. Madurowicz) |
In 1928, when Paul Dirac was attempting to unify quantum mechanics and special relativity, he was puzzled by peculiar solutions to his equations that allowed for electrons to have negative energy. A few years later, with the help of Robert Oppenheimer, Dirac was lead to conclude that such a negative energy solution was an anti-electron, a particle with the same mass but opposite charge to the regular electron. [1] Soon after, this particle was experimentally confirmed to exist and dubbed the positron as a portmanteau of positive and electron. This historical event marks the first confirmation for humankind that antimatter exists.
Generically, an antiparticle is identical to its standard particle twin, except with opposite charge, though most don't have special names like the positron, so an antiproton is simply an antimatter proton, etc. You can even make anti-hydrogen out of an antiproton and a positron and current research is investigating the question of might anti hydrogen fall upwards in a gravitational field. [2] Yet we are interested in antimatter because it provides the reactions with the highest energy efficiency per mass permissible by known physics. Antimatter annihilation has 100% mass to energy conversion ratio and cannot be superseded by anything obeying conservation of energy. A typical annihilation reaction could look like
where gamma represents a photon, or a particle of light. The energy of the photon is equal to the mass-energy of the electron or positron, through the mass-energy equivalence principle demonstrated by Einstein in special relativity, the famous E = mc2. If realized, the concept of an antimatter battery could be critical to fuel interstellar travel, where conservation of momentum for a rocket ship implies that one must carry around the fuel and propellant needed to navigate, and high efficiency becomes of utmost importance. [3]
An antimatter battery would likely use the reaction given above as its primary functioning principle for the fact that the electron/positron are the lightest known charged particles. While neutrinos are lighter (less massive), they lack any electric charge, and thus cannot be magnetically confined. If our battery is composed of normal matter, we will need to suspend the antimatter in a magnetic field in a vacuum else all of our fuel would touch the walls of its container, annihilating instantly in a rather unfortunate explosion. A schematic diagram of such a battery is shown in Fig. 1, which is using toroidal magnetic field to confine a donut shape mass of charged particles. The reason of importance for the lightness of the particle that the photons produced will obtain the mass energy of the particle being annihilated, which is a significant amount of energy. A photon with the energy of an electron 511 keV/c2 has wavelength around 2.5 picometers, which is smaller than the radius of a proton. [4] Such a photon would not be easily reflected by a mirror, which is what one would need to do to produce a directional impulse from an electron and positron annihilating at rest aboard a ship. (The reaction conserves momentum in the rest frame and so the two photons are emitted in opposite directions, not great for thrust unless one can be reflected, or absorbed and the energy used in some other way.)
Other challenges that the antimatter battery face are fuel production and storage. While the reaction is the most energy efficient to be released, it is far from the most efficient to be produced. While you could try and collect positrons from radioactive decays, this has many associated difficulties. [5] In addition, the requirement of magnetic confinement means that the battery consumes energy while it is simply storing antimatter by producing magnetic fields needed to suspend positrons in a vacuum. As a concept, the antimatter battery is great, but a practical implementation has to overcome significant challenges in order to be feasible. But these are challenges we must potentially overcome in order to send humans to other worlds, lest this one become no longer viable.
© Alexander Madurowicz. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. A. M. Dirac, "Quantised Singularities in the Electromagnetic Field," Proc. Roy. Soc. Lond. A 133, 60 (1931).
[2] The ALPHA Collaboration and A. E. Charman, "Description and First Application of a New Technique to Measure the Gravitational Mass of Antihydrogen," Nat. Commun. 4, 1785 (2013).
[3] M. C. V. Salgado, M. C. N. Belderrain, and T. C. Devezas, "Space Propulsion: A Survey Study About Current and Future Technologies," J. Aerosp. Technol. Manag. 10, e1118 (2018).
[4] W. Vassen, "The Proton Radius Revisited," Science 358, 39 (2017).
[5] T. D. Steiger et al., "Development of Intense, Long-Lived Positron Sources," Nucl. Instrum. Meth. A 299, 255 (1990).