Module 109. History of Physics, Physicists, Branches of Physics, Fundamental Units

Physics, one of the oldest and most fundamental natural sciences, deals with the study of matter, energy, space, and time, and the interactions among them. It forms the foundation of all natural sciences and underpins technological progress. The term “physics” derives from the Greek word physis, meaning “nature”. Over centuries, the subject has evolved through observation, experimentation, and mathematical formulation, leading to a profound understanding of the universe.

Historical Development of Physics

The history of physics can be traced through several distinct stages, each marked by significant discoveries and theoretical revolutions.
1. Ancient and Classical PeriodIn ancient civilisations such as Greece, India, and Egypt, natural phenomena were studied philosophically rather than experimentally.

• Aristotle (384–322 BCE) proposed early ideas about motion and elements, though many were later proven incorrect.
• In India, texts like the Vaisheshika Sutra by Kanada described atomic theory long before its Western conception, suggesting that matter is composed of indivisible particles (anu).
• Archimedes developed principles of levers and buoyancy, laying the groundwork for mechanics.

2. Renaissance and the Birth of Modern PhysicsThe Renaissance period marked the transformation of physics into an experimental science.

• Nicolaus Copernicus (1473–1543) proposed the heliocentric theory, placing the Sun at the centre of the solar system.
• Galileo Galilei (1564–1642) conducted experiments on motion and inertia, using telescopic observations to support Copernican ideas.
• Johannes Kepler (1571–1630) formulated the laws of planetary motion.
• Isaac Newton (1643–1727) unified celestial and terrestrial mechanics in his Principia Mathematica (1687), introducing the laws of motion and universal gravitation. This era became known as Classical Physics.

3. Nineteenth-Century AdvancesThe 19th century saw remarkable developments in thermodynamics, electromagnetism, and atomic theory.

• James Clerk Maxwell (1831–1879) formulated Maxwell’s equations, uniting electricity and magnetism into a single theory of electromagnetism.
• Michael Faraday’s experiments on electromagnetic induction laid the foundation for electric power generation.
• Rudolf Clausius and Lord Kelvin developed the laws of thermodynamics, describing energy transformations.
• John Dalton revived atomic theory, explaining matter in terms of atoms.

4. Twentieth-Century RevolutionThe early 20th century transformed physics into its modern form through revolutionary ideas.

• Albert Einstein (1879–1955) proposed the theory of relativity, linking space, time, and gravity, and formulated the mass-energy equivalence (E = mc²).
• Max Planck (1858–1947) introduced quantum theory, explaining black-body radiation.
• Niels Bohr, Werner Heisenberg, and Erwin Schrödinger developed quantum mechanics, describing atomic and subatomic phenomena.
• Marie Curie, Henri Becquerel, and Ernest Rutherford advanced the understanding of radioactivity and nuclear structure.
• Later in the century, the Standard Model of Particle Physics was developed, explaining fundamental particles and forces except gravity.

5. Modern and Contemporary PhysicsContemporary physics integrates both classical and quantum ideas to explore complex systems and cosmic phenomena. Developments include:

• Quantum field theory, unifying quantum mechanics with relativity.
• Condensed matter physics, studying materials and superconductivity.
• Astrophysics and cosmology, exploring black holes, dark matter, and the origin of the universe.
• Particle physics, exemplified by discoveries from the Large Hadron Collider (LHC), including the Higgs boson in 2012.

Prominent Physicists and Their Contributions

Throughout history, numerous physicists have shaped the field with their discoveries and innovations:

• Galileo Galilei – Father of modern experimental physics; laws of motion and inertia.
• Isaac Newton – Laws of motion and gravitation; calculus; optics.
• James Clerk Maxwell – Theory of electromagnetism and light.
• Michael Faraday – Electromagnetic induction and field concept.
• Albert Einstein – Theory of relativity; photoelectric effect (Nobel Prize, 1921).
• Max Planck – Quantum theory pioneer.
• Niels Bohr – Bohr’s atomic model and quantum theory.
• Marie Curie – Research on radioactivity; discovery of polonium and radium.
• Ernest Rutherford – Nuclear model of the atom; discovered the nucleus.
• Werner Heisenberg – Uncertainty principle in quantum mechanics.
• Erwin Schrödinger – Wave equation in quantum mechanics.
• Enrico Fermi – Nuclear fission and reactor development.
• Stephen Hawking – Black hole thermodynamics and cosmology.
• Richard Feynman – Quantum electrodynamics and particle physics.

Indian physicists have also made remarkable contributions:

• C. V. Raman – Discovered the Raman Effect (Nobel Prize, 1930).
• Homi J. Bhabha – Developed India’s nuclear research programme.
• Satyendra Nath Bose – Developed Bose–Einstein statistics with Einstein.
• Meghnad Saha – Ionisation theory in astrophysics.
• Subrahmanyan Chandrasekhar – Chandrasekhar limit in stellar evolution (Nobel Prize, 1983).
• Jagadish Chandra Bose – Pioneer in radio and microwave optics.

Branches of Physics

Physics is a vast subject, divided into several major and applied branches based on the phenomena studied.
1. Classical Branches

• Mechanics – Study of motion, forces, and energy (e.g., Newton’s laws, dynamics).
• Thermodynamics – Study of heat, work, temperature, and energy transformations.
• Optics – Study of light and its behaviour, including reflection, refraction, and interference.
• Electromagnetism – Study of electric and magnetic fields and their interactions.
• Acoustics – Study of sound, its production, propagation, and reception.

2. Modern Branches

• Quantum Mechanics – Study of matter and energy at atomic and subatomic levels.
• Relativity – Deals with high-speed motion and gravitational effects on space and time.
• Nuclear Physics – Study of atomic nuclei and nuclear reactions.
• Atomic Physics – Examines atomic structure and electron behaviour.
• Solid State Physics – Focuses on the properties of solids, including semiconductors.
• Particle Physics – Studies fundamental particles like quarks and leptons.

3. Applied and Interdisciplinary Branches

• Astrophysics and Cosmology – Explore celestial bodies, the universe’s structure, and its origin.
• Biophysics – Application of physics to biological systems.
• Geophysics – Studies the physical properties of the Earth.
• Medical Physics – Applies physical principles in medical imaging and radiation therapy.
• Plasma Physics – Examines ionised gases used in fusion energy and space science.
• Computational Physics – Uses numerical models and algorithms to solve physical problems.

Each branch contributes to scientific and technological advancements, from space exploration and communication systems to renewable energy and medicine.

Fundamental Units in Physics

Measurement is fundamental to physics. Quantities are expressed using a system of units to maintain uniformity and precision. The International System of Units (SI) is the globally accepted standard, consisting of seven fundamental units and several derived units.

Derived units are combinations of the fundamental ones, used for quantities like:

• Force – newton (N) = kg·m/s²
• Energy – joule (J) = N·m
• Power – watt (W) = J/s
• Pressure – pascal (Pa) = N/m²
• Electric charge – coulomb (C) = A·s

Two other systems, the CGS system (centimetre–gram–second) and FPS system (foot–pound–second), were used historically but have largely been replaced by the SI system for scientific accuracy.

Significance and Integration

The history, branches, and measurement systems of physics collectively reflect humanity’s pursuit of understanding the natural world. From Newton’s mechanics to quantum field theory, each era has deepened insight into the laws governing the universe. The study of physics has led to transformative technologies—electricity, nuclear energy, electronics, and space exploration—shaping modern civilisation.