The ball of fire in the sky that we call our Sun has been fusing hydrogen nuclei for 4.6 billion years. Sunlight enables the terrestrial chemistry of life-as-we-know-it, taking advantage also of the fusion products inside massive stars that predated the Sun and exploded as supernovae to enrich the material that made the Solar system with heavy elements. In the first ten million years of the Solar system, the leftover material from the accretion disk that fed the Sun made the planets, including our own bedrock — the Earth.
The heavy elements in the Solar disk first segregated to its midplane, coagulated chemically into dust particles, then clustered into large rocks by friction and vortices in the surrounding gas, and eventually grew big enough for gravity to hold them together as rocky planets. Consequently, the core of the Sun — which accreted early, must possess a higher abundance of heavy elements than its envelope — which accreted later after heavy elements were depleted into planets. Remarkably, the data on neutrinos emitted from fusion reactions in the core of the Sun was shown recently to confirm this expected onion shell structure.
When I entered astrophysics 35 years ago, I did not know how the Sun shines. This was embarrassing since John Bahcall, who generously offered me a five-year fellowship at the Institute for Advanced Study (IAS) in Princeton under the condition that I will switch to astrophysics, dedicated his career to solar neutrinos. John pioneered neutrino astrophysics which in addition to improving our understanding of the interior of the Sun, led to the discovery of neutrino masses and confirmed that supernovae make neutron stars, with a size of a city like Boston and a mass comparable to the Sun. My early papers with John were about element diffusion in the Sun, a sinking of heavy elements by gravity that is not strong enough to explain their inferred overabundance in the solar core.
A century ago, no astrophysicist knew how the Sun shines. Around 1920, Sir Arthur Eddington speculated that fusion of hydrogen into helium releases energy according to Albert Einstein’s equivalence of mass and energy. After the discovery of the neutron by James Chadwick in 1932, physicists were able to calculate the binding energy of each nucleus from the difference between the sum of the masses of the free neutrons and protons that made them and the actual mass of the nucleus. This nuclear binding energy fuels stars.
Iron-56 has 26 protons and 30 neutrons, and is the end product of nuclear burning in stars because it has the lowest mass per nucleon. Nuclei smaller than iron release energy as they grow in mass by nuclear fusion, whereas nuclei heavier than iron release energy when broken by nuclear fission. Whereas stars are powered by fusion, all electricity-producing nuclear reactors on Earth were based on fission so far. The natural fusion reactors with gravitational-confinement of the burning gas, otherwise known as stars, are stable. However, human-made fusion reactor concepts based on magnetic or inertial confinement are inflicted with violent instabilities.
Nuclear knowledge could be used for good and evil. During the second World War, the Manhattan Project led by the physicist Robert Oppenheimer (who later became the IAS director), resulted in the development of the first nuclear bombs. By now, nuclear weapons include fission bombs, like those dropped in 1945 on Hiroshima and Nagasaki, and thermonuclear bombs which use fission to trigger fusion between isotopes of hydrogen, deuterium and tritium.
In 1942, Enrico Fermi, constructed the first human-made, self-sustained fission reactor at the University of Chicago. Given this demonstration, he was recruited by Oppenheimer to the Manhattan Project. In 1950, while at Los Alamos, Fermi asked the famous question about extraterrestrial technological civilizations: “Where is everybody?” One possible answer is that they annihilated themselves within a century or two after discovering nuclear energy and are not around anymore. Most stars formed billions of years before the Sun. We are late; their party is over. In other words, Fermi could have found the answer to his question in his own actions.
Most recently, Russian President Vladimir Putin ratcheted up the nuclear rhetoric, saying he would use ‘all available means’ to defend Russian territory. US President Joe Biden warned that the world is at risk of a nuclear “Armageddon”. An escalation of the war in Ukraine to a world-wide conflict could pose an existential risk to humanity. It could provide an answer to Fermi’s question by demonstrating that technological species like ours may not be intelligent enough to avoid the use of nuclear weapons.
Are we on the brink of a Third World War of a nuclear nature?
Altogether, scientific knowledge has a destabilizing impact on a technological society, manifested by the potential for a nuclear war, a deadly pandemic from biological warfare or an accidental laboratory leak, an artificial intelligence instability, or a climate change driven by industrial pollution. We could gain a statistical perspective for our chances to survive by studying billions of rocky exo-planets. The technological scars on the surfaces or atmospheres of habitable Earth-like planets could educate us about the most common catastrophes that triggered the demise of other technological civilizations.
This echoes the biblical story, in which eating the forbidden fruit from the tree of knowledge of good and evil had led to the expulsion of Adam and Eve from the Garden of Eden. The expulsion was meant to prevent them from eating of the tree of life, and thus living forever. Indeed, if our technological knowledge will not destroy us, it could enable a self-repair mechanism for the human body, as employed by tardigrades, that would allow astronauts to survive in space forever.
Here’s hoping that we can stay in paradise for the long haul. Perhaps it is time to spend less time in the virtual realities of the metaverse, the multiverse or hypothetical extra dimensions, that were artificially constructed by humans, and more time in observing the beauty of the actual reality around us, shaped by nature.
In the words of Henry Thoreau, who lived near the path of my morning jog every day at sunrise: “I love Nature partly because she is not man, but a retreat from him. None of his institutions control or pervade her. There a different kind of right prevails. In her midst I can be glad with an entire gladness. If this world were all man, I could not stretch myself, I should lose all hope. He is constraint, she is freedom to me. He makes me wish for another world. She makes me content with this.”
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.
