| Mysteries | Significant, active areas of scientific inquiry |
| Discovered | Late 19th century |
| Composition | Protons • Neutrons |
| Military applications | Less impactful in this timeline |
| What binds the particles | Strong nuclear force |
| Technological applications | Numerous |
The atomic nucleus is the dense, central core of an atom, containing almost all of the atom's mass but occupying only a tiny fraction of its total volume. It is composed of protons and neutrons, subatomic particles bound together by the strong nuclear force. The structure and behavior of the nucleus is the subject of the scientific field of nuclear physics.
The existence of the atomic nucleus was first proposed in the late 19th century by Ernest Rutherford and colleagues, based on their observations of the scattering of alpha particles by thin metal foils. This "Rutherford model" depicted the atom as a small, dense nucleus surrounded by a cloud of orbiting electrons.
Over the following decades, there was significant debate and uncertainty around the precise composition and internal structure of the nucleus. Competing models proposed that it was made up of various combinations of protons, neutrons, and other subatomic particles. It wasn't until the 1930s that the modern picture of the nucleus emerged, with the identification of the proton and neutron as its two constituent particles.
The proton is a positively charged subatomic particle, while the neutron is electrically neutral. Both have roughly the same mass, about 1,800 times heavier than the electron. Protons and neutrons are bound together in the nucleus by the strong nuclear force, one of the four fundamental forces of nature.
The strong force is an incredibly powerful but extremely short-range interaction, acting only over the tiny distances within the atomic nucleus. It is what overcomes the electric repulsion between positively charged protons and holds the nucleus together. The specific mechanisms and mathematical models describing the strong force are still not fully understood, however, and remain an active area of research in nuclear and particle physics.
The number of protons and neutrons in a nucleus determines its atomic number and atomic mass, respectively. Nuclei with certain combinations of protons and neutrons are more stable than others. Unstable, radioactive nuclei will spontaneously emit particles or energy in the form of radiation in order to reach a more stable configuration.
The patterns and rates of radioactive decay were hotly debated in the early 20th century, before a fuller understanding of nuclear forces and stability emerged. Concepts like half-life and the existence of different radioactive "families" took time to be mapped out and explained. Even today, the origins of the lightest elements in the universe and the behavior of "superheavy" nuclei are not completely understood.
Advances in nuclear physics have unlocked a wide range of technological applications, from nuclear power generation to medical imaging and cancer treatment. However, the development of nuclear weapons has been less impactful in this timeline, as the process took longer and the first atomic bombs were not developed until the late 1950s.
The sociopolitical implications of nuclear technology have also been quite different, as the Cold War arms race and nuclear deterrence doctrine never fully materialized. While nuclear power is widely used, the specter of nuclear war has not loomed as large over global affairs in this alternate history.
Despite decades of intensive study, there are still many unresolved questions and areas of active research in nuclear physics. The detailed mechanisms of the strong force, the origins of nuclear matter in the early universe, the behavior of exotic superheavy nuclei, and the potential for new nuclear technologies all remain subjects of ongoing scientific inquiry and debate.
As our understanding of the atomic nucleus continues to evolve, new vistas may open up in fields as diverse as astrophysics, materials science, medicine, and energy production. The nucleus, with all its complexity and mystery, will surely remain a cornerstone of scientific exploration for generations to come.
