Module 116. Atomic, Nuclear and Modern Physics

Atomic, nuclear, and modern physics together constitute a central area of physical science concerned with the structure and behaviour of matter and energy at the smallest scales. These fields explore the nature of atoms, the composition of atomic nuclei, and the quantum phenomena that govern subatomic particles. Their development in the twentieth century transformed our understanding of the physical universe, giving rise to technological advancements in energy production, medicine, communication, and computation.

Atomic Physics

Atomic physics deals with the study of atoms as isolated systems consisting of a nucleus surrounded by electrons. It focuses primarily on the electronic structure, energy levels, and interactions of atoms with electromagnetic radiation.
The modern concept of the atom evolved through successive models:

• Dalton’s Atomic Theory (1808): Proposed that matter is composed of indivisible particles called atoms.
• J. J. Thomson’s Model (1898): The “plum pudding” model suggested that electrons were embedded in a positively charged sphere.
• Rutherford’s Nuclear Model (1911): Through the gold foil experiment, Rutherford demonstrated that atoms possess a dense, positively charged nucleus surrounded by electrons.
• Bohr’s Model (1913): Niels Bohr introduced quantised orbits, stating that electrons revolve in fixed energy levels without radiating energy and emit or absorb discrete quanta when transitioning between levels.

The Bohr model successfully explained the hydrogen spectrum but failed for multi-electron systems, leading to the development of quantum mechanics, which describes atomic structure through probabilistic wave functions.
The quantum mechanical model introduced the concept of atomic orbitals — regions around the nucleus where electrons are most likely to be found. These are characterised by quantum numbers that define the size, shape, and orientation of orbitals.
Spectroscopic studies reveal that atoms absorb or emit electromagnetic radiation in discrete amounts, leading to characteristic line spectra used for element identification. Applications include laser technology, spectroscopy, and atomic clocks.

Nuclear Physics

Nuclear physics focuses on the constituents and interactions of atomic nuclei. The nucleus, composed of protons and neutrons (collectively called nucleons), is bound together by the strong nuclear force, which counteracts the electrostatic repulsion between positively charged protons.
Key discoveries and concepts include:

• Radioactivity: Discovered by Henri Becquerel and further studied by Marie and Pierre Curie, radioactivity refers to the spontaneous disintegration of unstable nuclei emitting alpha (α), beta (β), or gamma (γ) radiation.
• Nuclear Reactions: Involve transformation of one nucleus into another, often accompanied by the release of vast amounts of energy. Two major types are:
  • Nuclear Fission: Splitting of a heavy nucleus (e.g., uranium-235 or plutonium-239) into smaller nuclei, releasing energy. This principle is used in nuclear reactors and atomic bombs.
  • Nuclear Fusion: Combining light nuclei (e.g., hydrogen isotopes) to form a heavier nucleus, releasing even greater energy. Fusion powers stars and is a focus of ongoing research for sustainable energy.
• Mass–Energy Equivalence: Proposed by Einstein in the equation E = mc², it shows that mass can be converted into energy, explaining the enormous energy released in nuclear processes.

Nuclear physics also underpins applications in medicine (radiotherapy, PET scans), agriculture (radiotracers, mutation breeding), and power generation (nuclear reactors). The study of nuclear forces and decay rates helps in dating archaeological samples through techniques such as carbon-14 dating.

Modern Physics and Quantum Theory

Modern physics refers to concepts developed after the discovery of the electron and the advent of relativity and quantum mechanics in the early twentieth century. It departs from classical physics by describing phenomena occurring at atomic and subatomic levels, where particles exhibit both wave-like and particle-like properties.

Quantum Theory

Quantum mechanics emerged from Max Planck’s work on black-body radiation (1900), introducing the concept of quantisation of energy. Einstein’s photoelectric effect (1905) provided further evidence, demonstrating that light consists of discrete packets of energy called photons.
According to Einstein, when light of sufficient frequency strikes a metal surface, electrons are ejected, confirming the particle nature of light. The effect supports the relation E = hf, where h is Planck’s constant and f the frequency of light.
Further developments by Schrödinger, Heisenberg, and Dirac established the framework of quantum mechanics:

• Heisenberg’s Uncertainty Principle: It is impossible to simultaneously determine the exact position and momentum of a particle.
• Schrödinger’s Wave Equation: Describes how the quantum state of a physical system changes with time.
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of quantum numbers.

These principles explain the stability of atoms, spectral lines, and chemical bonding at a microscopic level.

Relativity

Albert Einstein’s theories of special (1905) and general relativity (1915) revolutionised our understanding of space, time, and gravity. Special relativity introduced the concept that the laws of physics are identical for all observers in uniform motion and that the speed of light is constant. It led to time dilation, length contraction, and the equivalence of mass and energy.
General relativity extended these ideas to include gravitation as a curvature of spacetime caused by mass and energy, providing the foundation for modern cosmology and astrophysics.

Particle Physics

Particle physics, a branch of modern physics, investigates the fundamental particles that constitute matter and their interactions. Experiments using particle accelerators have revealed a wide range of subatomic particles beyond protons, neutrons, and electrons, including quarks, leptons, and bosons.
The Standard Model of Particle Physics classifies these particles:

• Fermions: Matter particles (quarks and leptons).
• Bosons: Force carriers (photon, gluon, W and Z bosons, Higgs boson).

The discovery of the Higgs boson at CERN in 2012 confirmed the mechanism by which particles acquire mass. Research in this field continues to explore phenomena beyond the Standard Model, including dark matter, neutrino oscillations, and unification theories.

Applications of Atomic and Modern Physics

The practical applications of atomic, nuclear, and modern physics are vast and transformative:

• Energy Production: Nuclear reactors (fission) and fusion research.
• Medicine: Diagnostic imaging (MRI, PET), radiation therapy, and radioisotope tracing.
• Electronics and Computing: Semiconductor technology, lasers, quantum computing, and nanotechnology.
• Communication: Fibre-optic transmission and satellite systems rely on quantum and electromagnetic principles.
• Space and Astrophysics: Nuclear fusion explains stellar energy; relativity governs space navigation and GPS accuracy.

Safety, Ethics and Environmental Considerations

While nuclear and modern physics have enabled remarkable progress, they also pose ethical and environmental concerns. Nuclear weapons proliferation, radioactive waste disposal, and radiation hazards require strict regulation and international cooperation. Advances in nuclear safety, waste reprocessing, and fusion technology aim to mitigate such risks.

Significance in Scientific Understanding

Atomic, nuclear, and modern physics collectively deepen humanity’s understanding of matter, energy, and the universe. They unify concepts from the microscopic quantum world to the macroscopic cosmic scale, revealing that all physical phenomena arise from fundamental interactions.