The Sun is a nuclear reactor made of mostly electrons and protons. Both fluids are heated by nuclear fusion reactions to a central temperature of about fifteen million degrees Kelvin. This creates an immense pressure that, left to its own devices, would have exploded the Sun in a few hours. We owe our existence to gravity. It keeps the Sun bound and maintains life-as-we-know it on Earth.
But here lies a puzzle that I considered in my first paper on astrophysics 35 years ago. The proton mass is 1836 times bigger than the electron mass. This means that gravity acts more strongly on protons than on electrons by a factor of 1836. If the pressure of protons is balanced by gravity, what binds electrons to the Sun?
The answer is that the Sun has a positive electric charge that repels the protons but attracts the electrons. One way to view this electric force is as a string that keeps the two fluids bound. The electric repulsion cancels half the solar gravitational attraction for the protons so that the electric attraction of the electrons is equal to the remaining net attraction for the protons and the two fluids stay together. This charge requires a tiny fraction of the electrons to be removed from the Sun. The repulsive electric force between two protons is 36 orders of magnitude stronger than their gravitational attraction, and so the required deficit of electrons is correspondingly smaller. The combination of the electrons and protons can be viewed as a nearly electrically-neutral fluid, with the total pressure of twice the pressure of each component, being balanced by gravity.
The electric potential drop from the center of the Sun to its outer surface is roughly the gravitational potential energy of a proton at the center of the Sun, of order a thousand Volts. In effect, the Sun is a thermal battery of that voltage.
Recognizing the natural thermal batteries near them, extraterrestrial technological civilizations may have devised a scheme to extract electric energy out of their host star. By drawing an electric current from the center of their star, they could supply the energy needs of their electric cars and spacecraft. This constitutes a fantastic source of clean energy.
Our accomplishment in drawing clean energy from the Sun pales in comparison. Solar panels take advantage of the tiny portion of the solar light that intercepts the Earth, which by itself is merely one part in a hundred million.
Olaf Stapledon imagined in his 1937 science fiction novel Star Maker that other civilizations might construct artificial megastructures to harvest most of the light generated by their star. Two years before I was born, Freeman Dyson discussed the possibility of searching for the infrared emission from such spheres in his 1960 paper, titled “Search for Artificial Stellar Sources of Infrared Radiation”. The concept of “Dyson Spheres”, was later defined as the signature of Type II civilizations on the Kardashev scale — which relates the technological level of a civilization to the amount of energy it taps. In my latest book, Interstellar, I instead rank civilizations based on their impact on their environment. Whereas our civilization behaves like a careless tourist who trashes the terrestrial scene, the most advanced civilization might have God-like abilities of creating life or even a baby universe in its laboratories. It creates more than it was given.
The most practical design for a Dyson sphere, suggested in a 1991 paper by Robert Forward, involves thin tiles which maintain a fixed distance from the star by balancing gravity against the star’s radiation pressure. In a recent paper, I noted that this delicate equilibrium will break down once the star evolves and brightens up towards the end of its life. This would push the megastructure tiles out to interstellar space because they were nearly unbound to start with. The interstellar pieces from broken Dyson spheres might therefore explain the anomalous non-gravitational acceleration of the interstellar object, `Oumuamua, without any cometary evaporation. With this interpretation, `Oumuamua was a piece from a broken Dyson sphere that drifted into the solar system and was pushed by reflecting sunlight.
The Rubin Observatory in Chile will likely detect many more interstellar objects like `Oumuamua in the coming years. Here’s hoping that these objects will inspire our engineers to design better schemes for harnessing clean energy from the Sun.
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 chairs the advisory board for the Breakthrough Starshot project, and 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.
