|Fig. 1: Feynman diagram demonstrating mutual annihilation of an electron and a positron. (Source: Wikimedia Commons)|
A staple of science fiction that tries to explain interstellar travel is use of matter-antimatter reactors as a power source. Does antimatter make a good fuel to power the starships of tomorrow? What about more earthbound uses? The short answer is no. We will see that, although there are patents and proposals for long distance interplanetary or even interstellar travel making use of antimatter fuel in some distant future, more earthbound energy generation from antimatter does not make sense.
There are several esoteric subatomic antimatter particles, with the existence of more of them a subject of on-going experiments. Perhaps the simplest antimatter particles for the educated non-scientist to understand are positrons and antiprotons, corresponding to their (non-anti) matter counterparts, respectively, electrons and protons. The antimatter particles have opposite charge from their matter counterparts, positive charge for positrons and negative for antiprotons.  Aside from their charges and potentially explosive properties when contacting regular matter, antimatter particles such as positrons and antiprotons are expected to obey the same physical laws as regular matter. CERN is testing this matter-antimatter symmetry in experiments with captured anti-hydrogen atoms. [2,3]
Compared to even a thermonuclear engine, an equal mass of matter plus antimatter will produce two orders of magnitude more energy. When antimatter meets matter an annihilation reaction results. The meeting of a proton and an anti-proton for example produces 9 × 1016 Joules/kilogram vs. a nuclear reaction using D + He-3 which produces hydrogen, helium, and some neutrons, yielding 3.52 × 1014 Joules/kilogram. 
One of the more ambitious proposals for collecting antimatter involves stimulating our sun to produce it for us. Orbiting Solar Observatory‐7 based spectrogram analysis of solar flare interaction with the upper atmosphere showed that cosmic rays from the sun produce some positrons.  Based on this observation, this speculative proposal suggests using a space-based nuclear pumped X-ray laser "to induce a pattern of excited plasma in the solar atmosphere" to stimulate more positron generation via "synthetic solar flare."  Aside from the proposal's author's confusion of a DARPA non-nuclear program with a nuclear-based program of the same name, one problem with the proposal, even assuming the difficulties in harvesting positrons out there could be overcome, is that the Excalibur nuclear pumped X-ray laser program cited in the proposal was a failure. [7,8] Several United States government patents have been granted for the capture and handling of antimatter. [9-12]
Antimatter clearly is something that you can't just have in casual storage. Since contact with normal matter results in high-energy annihilation, antimatter must be stored in a vacuum and contained by an electromagnetic field.  An accident at a facility that produced antimatter in sufficient quantity for spacecraft propulsion, for example, would be subject to disastrous explosions should containment of the latest batch of antiprotons fail. Given previous unfounded public hysteria surrounding possible black hole production at CERN, any industrial-scale antimatter production facility should be located at an isolated location, perhaps on the moon. [14,15]
Once a given quantity of antimatter is available, it is nanogram-per-nanogram the most powerful fuel available. The trouble is that current methods of producing antimatter require much more energy than the antimatter in turn will yield. Storing the antimatter so that it is safe from annihilation also requires a great deal of energy since conventional containers cannot be used and the antimatter is contained using magnetic fields.
All around us, here on Earth, all is matter. Antimatter cannot last long without the special containment fields and techniques. Upon contact between matter and antimatter, there is annihilation. But what if there are regions out there in space that are all antimatter, just as our region of space is all matter? This question touches on theories of the "big bang" from which the universe is believed to have originated. If the universe started out concentrated in one place, any antimatter would have annihilated itself and a corresponding amount of matter, so some modification of the big bang theory would be required that allowed antimatter regions to form without being annihilated.  Current predominant theories make it unlikely that space trawlers will harvest antimatter for purposes of energy production.
Antimatter is currently only generated on purpose by people and stored in laboratories for use in scientific studies, generally of the properties of antimatter.  Some medical diagnostic equipment makes use of positrons, as in PET (Positron Emission Tomography), for example, but these are not captured or stored. The lab methods currently in use for production of antimatter are not efficient enough for any practical propulsion or other energy applications, with one unit of antimatter energy requiring 10 billion units of energy to produce the antimatter. 
Of all of the potential energy-from-antimatter applications, propulsion from space travel is the most prominent, even receiving attention and funding from NASA.  Although pure antimatter-matter reaction propulsion systems would require more antimatter than we can produce in the foreseeable future, hybrid systems that utilize small quantities of antimatter as "catalysts" to generate higher-than-normal-energy fusion reactions would be more feasible. These would still require considerable development of propulsion technology, industrial standard antimatter production, and reliably safe antimatter containment and direction techniques. If perfected, however, such "antimatter-catalyzed" fusion propulsion systems would make possible round trips to Mars and Jupiter in the range of 1.5 and 3 years, respectively. 
There are many minds at work on the long range goal of effectively limitless energy. Physicists and crackpots alike are working with theoretical equations, supercomputers, high-energy colliders, and deceptively simple Feynman diagrams (Fig. 1).  As suggested in the Containment section above, even if practically useful quantities of antimatter were obtainable and even if the production or storage facilities were located on the moon, there is still a considerable leaky container problem to solve. There are decades of experience in attempting to magnetically contain more conventional (non-antimatter) plasmas for purposes of nuclear fusion, but even these have not yet advanced to the point that there are commercially viable fusion reactors. The stakes with antimatter energy generation would be much higher. Containment leakage from a plasma fusion reactor would mean that the fusion would stop rather quickly, as the plasma immediately cooled. Leakage from an antimatter reactor would result in a much larger explosion, with the antimatter annihilating an equal quantity of matter with virtually all of the matter and antimatter converted into energy (see the 9x1016 Joules/kilogram number above).  It would be premature to abandon any of our current energy sources in favor of antimatter just yet, but it is OK to watch your favorite science fiction drama and dream about the future.
© Genevieve Payzer. 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.
 G. B. Andresen, et al., "Trapped Antihydrogen," Nature 468, 673 (2010).
 C. Smorra, et al., "A Parts-per-Billion Measurement of the Antiproton Magnetic Moment," Nature 550, 371 (2017).
 M. Ahmadi et al., "Observation of the Hyperfine Spectrum of Antihydrogen," Nature 548, 66 (2017).
 S. K. Borowski, "Comparison of Fusion/Antiproton Propulsion Systems for Interplanetary Travel," in Fusion Energy in Space Propulsion, ed. by T. Kammash (AIAA, 1995) pp. 89-127, 90.
 S. Gaidos, "Cosmic Mystery: High-Hyphen's Energy Invaders From Space Could Signal a Nearby Pulsar, or Perhaps Dark Matter," Science News, 175, No. 5, 16 (2009).
 W. Mook, "Industrial Production of Positronium and Its Uses," J. Space Philosophy 4, No. 2, 105 (2015).
 R. L. Park, Voodoo Science: The Road From Foolishness to Fraud (Oxford University Press, 2000) pp. 186-188.
 T. E. Repetti, "Application of Reactor-Pumped Lasers to Power Beaming," Idaho National Engineering Laboratory, October 1991, p. 17.
 G. A. Smith, "Apparatus and Method for Long-Term Storage of Antimatter," U.S. Patent 7,709,819. 4 May 10.
 G. A, Smith, R. A. Lewis, and S. D.Howe, "Container for Transporting Antiprotons," U.S. Patent 6,160,263, 12 Dec 00.
 B. I. Deutsch, "Process for Preparing Antihydrogen," U.S. Patent 4,867,939, 19 Sep 89.
 L. Kasprowicz, "System for the Storage and Transportation of Anti-Matter," U.S. Patent 6,606,370, 12 Aug 03.
 M. G. Millis and Eric W. Davis, Frontiers of Propulsion Science (AIAA, 2009) pp. 564, 73, 75, 78.
 E. Harrell, "Collider Triggers End-of-World Fears," Time Magazine, 4 Sep 08.
 A. Selk, "Newly Discovered Moon Tunnel Could Be the Perfect Place for a Colony, Scientists Say, Chicago Tribune, 21 Oct 17.
 A. I. Sanda and I. I. Bigi, CP Violation (Cambridge University Press, 2009).
 C. O'Connell, "Antimatter to Ion Drives: NASA's Plans for Deep Space Propulsion, Cosmos, 18 Mar 16.
 D. Kaiser "Physics and Feynman's Diagrams," Am. Sci. 93, 156 (2005).