The ancient Greek philosopher Socrates who lived between 469–399 BC is quoted in Plato’s Dialogues as saying: “for if anyone should come to the top of the air or should get wings and fly up, he could lift his head above it and see, as fishes lift their heads out of the water and see the things in our world, so he would see things in that upper world; and, if his nature were strong enough to bear the sight, he would recognize that that is the real heaven..”
Indeed, interstellar space might feel like heaven, given all the political upheaval we face on Earth. The practical question is how to reach interstellar space within our short lifespan? The nearest star system, Alpha Centaury, is 4.25–4.35 light-years years away. To reach it within a human lifetime requires a spaceship faster than a tenth of the speed of light. Rockets are limited by their exhaust speed, and require a fuel mass that grows exponentially in terminal speed beyond that. The fastest exhaust speed is the speed of light. Is there a fuel that can provide that? Yes, there is.
Antimatter annihilates with matter into pure radiation. The process is the ultimate manifestation of Albert Einstein’s finding that energy (E) equals mass (M) times the speed of light (c) squared. All other fuels convert mass to energy with a much smaller efficiency. Nuclear fuel which powers stars is at best a hundred times less efficient, not taking into account the mass of the nuclear reactor. Chemical fuel, which powers rockets and life, is billions of times less efficient, because it relies on molecular bonds.
Antimatter was discovered in a 1928 paper by Paul Dirac, where he realized that the relativistic version of the Schrödinger wave equation for electrons allows for anti-electrons with an opposite electric charge. These “positive electrons” were discovered by Carl Anderson in 1932 and named positrons. Similarly, the positively-charged protons were expected to have negatively-charged anti-protons, which make anti-hydrogen atoms when paired with positrons.
Fortunately, we are all made of matter and so we have no hesitation to shake hands in blind dates. But shortly after the Big-Bang, the Universe had nearly equal amounts of matter and antimatter. When the cosmic temperature was above ten billion degrees, positrons were produced by collisions of photons with more energy than the electron rest mass. Eventually, all the antimatter was drained by annihilation with matter, leaving behind a small residual surplus of matter. The small asymmetry between matter and antimatter amounted to merely two parts in a billion. The cosmic process that generated this small excess of matter over antimatter, commonly named baryogenesis, is unknown, but without it we would not exist. A symmetric Universe would have been full of radiation but not enough protons and electrons to fuse hydrogen in stellar interiors and produce carbon and oxygen to enable the chemistry of life.
Of course, if matter and antimatter were separated into distinct islands with no overlap, as speculated by the Nobel Laureate Hannes Alfvén, they could have survived to the present time, but the existence of such islands is ruled out by the gamma-ray background. If an island of antimatter existed within reach, its fuel reservoir would have served as a source of enormous wealth, because it would offer an energy supply per unit mass that is two billion times larger than crude oil on Earth.
Yes, you read it correctly. A barrel of antimatter would have been worth two billion times more than a barrel of oil in its energy output, worth two hundred billion dollars in today’s energy market. The entire net worth of Elon Musk is equivalent to the energy output of one barrel of antimatter.
For now, we have to face the harsh reality that no antimatter survived the early Universe. The only way to acquire antimatter in our arsenal of fuels is to produce it in our laboratories. Half a year ago, I visited the antimatter factory at CERN, which so far produced less than 10 nanograms of antimatter — enough to power a 100 Watt light bulb for 2.5 hours. Our current antimatter production process is highly inefficient. CERN requires a billion times more energy to make antimatter than the energy stored in the antimatter mass. The production of a gram of antimatter would cost more than a quadrillion dollars.
This is a pity because momentum conservation implies that a spacecraft which carries 20% of its mass in antimatter fuel could reach a third of the speed of light and reach Alpha Centauri within 13 years. The average acceleration could be an order of magnitude lower than 1-gee.
For now, we cannot fulfil Socrates’ dream of sailing to interstellar space in our lifespan, based on antimatter fuel.
Given this sober realization, let me close with two antimatter anecdotes. First, our body emits of order 100–200 positrons per hour as a result of the radioactive decay of the isotope potassium-40, which exists in our food and water. Second, the antihydrogen atoms produced at CERN were demonstrated last year to have the same response to Earth’s gravity as hydrogen atoms. Repulsive antigravity was ruled-out. The equivalence with matter is likely to extend also to other regimes. For example, antisemites made of antimatter probably have the same response to Semites as those made of matter. The main difference is that if they touch Semites made of matter, they will annihilate themselves as well.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
