Physics

1. Overview

Physics is the fundamental scientific discipline that studies matter, energy, space, time, and the laws governing their interactions.

It seeks to explain phenomena across all scales of the universe, from:

  • subatomic particles
  • atoms and molecules
  • planets and stars
  • galaxies and the entire cosmos

Here are the 4 main branches of Phsyics which pretty much encompasses all the other sub-domains

  1. Classical Physics
  2. Relativity
  3. Quantum Mechanics
  4. Particle physics

Modern technologies such as semiconductors, lasers, GPS, nuclear energy, and quantum computing all originate from discoveries in physics.

Physics attempts to answer some of the deepest questions about reality:

  • What is the universe made of?
  • What laws govern nature?
  • How did the universe begin?
  • What is the ultimate structure of matter?

From the smallest quantum particles to the largest galaxies, physics provides the unifying framework for understanding the natural world.

2. Core Areas of Physics

This diagram shows the major areas of physics and their core concepts.

PHYSICS
|
+--- FOUNDATIONS OF PHYSICS
|    |
|    +--- scientific method
|    +--- measurement and units
|    |    +--- SI units
|    |    +--- dimensional analysis
|    |
|    +--- mathematical tools
|    |    +--- calculus
|    |    +--- linear algebra
|    |    +--- differential equations
|    |    +--- probability theory
|    |    +--- tensor calculus
|    |
|    +--- symmetry principles
|    +--- conservation laws
|         +--- conservation of energy
|         +--- conservation of momentum
|         +--- conservation of angular momentum
|
+--- CLASSICAL MECHANICS
|    |
|    +--- Newtonian mechanics
|    |    +--- Newton's laws
|    |    +--- projectile motion
|    |    +--- circular motion
|    |
|    +--- work and energy
|    |    +--- kinetic energy
|    |    +--- potential energy
|    |    +--- energy conservation
|    |
|    +--- momentum
|    |    +--- linear momentum
|    |    +--- impulse
|    |    +--- collisions
|    |
|    +--- rotational mechanics
|    |    +--- torque
|    |    +--- moment of inertia
|    |    +--- angular momentum
|    |
|    +--- Lagrangian mechanics
|    +--- Hamiltonian mechanics
|
+--- THERMODYNAMICS
|    |
|    +--- temperature
|    +--- heat transfer
|    |    +--- conduction
|    |    +--- convection
|    |    +--- radiation
|    |
|    +--- laws of thermodynamics
|    |    +--- zeroth law
|    |    +--- first law
|    |    +--- second law
|    |    +--- third law
|    |
|    +--- entropy
|    +--- statistical mechanics
|    |    +--- microstates
|    |    +--- partition functions
|    |
|    +--- phase transitions
|
+--- ELECTROMAGNETISM
|    |
|    +--- electric charge
|    +--- electric fields
|    +--- magnetic fields
|    +--- Maxwell's equations
|    |
|    +--- electromagnetic waves
|    +--- circuits
|    |    +--- Ohm's law
|    |    +--- Kirchhoff laws
|    |
|    +--- electromagnetic radiation
|         +--- radio waves
|         +--- microwaves
|         +--- visible light
|         +--- X-rays
|
+--- OPTICS
|    |
|    +--- geometrical optics
|    |    +--- reflection
|    |    +--- refraction
|    |
|    +--- wave optics
|    |    +--- interference
|    |    +--- diffraction
|    |
|    +--- polarization
|    +--- lasers
|    +--- fiber optics
|
+--- MODERN PHYSICS
|    |
|    +--- quantum mechanics
|    +--- quantum field theory
|    +--- general relativity
|
+--- QUANTUM MECHANICS
|    |
|    +--- wave particle duality
|    +--- Schrodinger equation
|    +--- uncertainty principle
|    +--- quantum states
|    +--- superposition
|    +--- quantum entanglement
|    |
|    +--- quantum systems
|         +--- hydrogen atom
|         +--- quantum harmonic oscillator
|
+--- RELATIVITY
|    |
|    +--- special relativity
|    |    +--- time dilation
|    |    +--- length contraction
|    |    +--- mass energy equivalence
|    |
|    +--- general relativity
|    |    +--- spacetime curvature
|    |    +--- gravitational waves
|    |    +--- black holes
|
+--- PARTICLE PHYSICS
|    |
|    +--- standard model
|    |    +--- quarks
|    |    +--- leptons
|    |    +--- bosons
|    |
|    +--- fundamental forces
|         +--- strong force
|         +--- weak force
|         +--- electromagnetic force
|         +--- gravity
|
+--- NUCLEAR PHYSICS
|    |
|    +--- atomic nuclei
|    +--- nuclear reactions
|    |    +--- fusion
|    |    +--- fission
|    |
|    +--- radioactive decay
|
+--- HIGH ENERGY THEORETICAL PHYSICS
|    |
|    +--- particle physics
|    |    +--- Standard Model
|    |
|    +--- quantum gravity
|         |
|         +--- string theory
|         +--- loop quantum gravity
|
+--- MATHEMATICAL PHYSICS
|    |
|    +--- differential geometry
|    +--- topology
|    +--- Calabi-Yau manifolds
|    +--- supersymmetry
|
+--- CONDENSED MATTER PHYSICS
|    |
|    +--- crystal structures
|    +--- semiconductors
|    +--- superconductivity
|    +--- magnetism
|    +--- nanomaterials
|
+--- ASTROPHYSICS
|    |
|    +--- stellar evolution
|    +--- galaxies
|    +--- black holes
|    +--- neutron stars
|
+--- COSMOLOGY
|    |
|    +--- Big Bang theory
|    +--- cosmic microwave background
|    +--- dark matter
|    +--- dark energy
|    +--- expansion of universe
|
+--- APPLIED PHYSICS
|    |
|    +--- electronics
|    +--- photonics
|    +--- materials science
|    +--- medical physics
|    +--- quantum computing
|
+--- FUTURE FRONTIERS
     |
     +--- quantum gravity
     +--- string theory
     +--- multiverse theories
     +--- dark energy research
     +--- AI driven scientific discovery

Physics is broadly divided into several major domains.

3. Classical mechanics (overview)

Classical mechanics studies motion and forces acting on objects.

Key concepts:

  • velocity
  • acceleration
  • force
  • momentum
  • energy
  • work
  • rotational motion

Major laws: Newton’s Laws of Motion

  1. Law of inertia
  2. Force = mass × acceleration
  3. Action–reaction principle

Example: Projectile motion of a football.

Applications: mechanical engineering, robotics, aerospace engineering, vehicle dynamics.

Questions to ask

  • What forces act on the system?
  • Is energy conserved?
  • How does momentum change during interaction?

4. Thermodynamics (overview)

Thermodynamics studies heat, energy transfer, and the behavior of systems at different temperatures.

Core concepts:

  • temperature
  • heat
  • entropy
  • energy conservation
  • equilibrium

The Four Laws of Thermodynamics:

  1. Zeroth Law – defines temperature equilibrium
  2. First Law – conservation of energy
  3. Second Law – entropy increases
  4. Third Law – entropy approaches zero at absolute zero

Example: Heat engines such as car engines convert thermal energy into mechanical work.

Applications: power plants, refrigeration, climate science, chemical engineering.

Questions to ask

  • How does energy flow through the system?
  • What processes increase entropy?
  • Is the system in equilibrium?

5. Electromagnetism (overview)

Electromagnetism studies electric and magnetic fields and their interactions with charged particles.

Core ideas:

  • electric charge
  • electric field
  • magnetic field
  • electromagnetic waves

Maxwell’s Equations unify electricity and magnetism.

Examples of electromagnetic phenomena: radio waves, light, X-rays.

Applications: telecommunications, electric motors, power generation, wireless technologies.

Questions to ask

  • What charges or currents generate fields?
  • How do fields interact with matter?
  • How does electromagnetic radiation propagate?

6. Optics (overview)

Optics studies light and its interaction with matter.

Subfields:

  • geometrical optics
  • wave optics
  • quantum optics

Core concepts: reflection, refraction, diffraction, interference, polarization.

Examples: rainbow formation, laser technology, optical fibers.

Applications: cameras, microscopes, telescopes, fiber-optic communication.

Questions to ask

  • Is light behaving as a wave or particle?
  • How does the medium affect propagation?

7. Quantum mechanics (overview)

Quantum mechanics describes the behavior of particles at atomic and subatomic scales.

Key ideas:

  • wave-particle duality
  • uncertainty principle
  • quantum superposition
  • quantum entanglement

Major equations: Schrödinger Equation

Example: electron behavior inside atoms.

Applications: semiconductors, lasers, MRI machines, quantum computing.

Questions to ask

  • What is the probability distribution of the particle?
  • How do measurement and observation affect the system?

8. Relativity (overview)

Relativity describes physics at very high speeds and strong gravitational fields.

Special Relativity (Einstein) describes physics at near-light speeds.

Key concept: mass–energy equivalence, E = mc².

General Relativity describes gravity as curvature of spacetime.

Example phenomena: black holes, gravitational waves, gravitational lensing.

Applications: GPS satellite systems, astrophysics, cosmology.

Questions to ask

  • How does gravity affect spacetime?
  • How does motion affect time and length?

9. Particle physics (overview)

Particle physics studies the fundamental constituents of matter.

Standard Model particles: quarks, leptons, bosons.

Important particles include: electron, photon, neutrino, Higgs boson.

Major experiments: CERN Large Hadron Collider; Higgs boson discovery (2012).

Questions

  • What are the fundamental building blocks of matter?
  • What forces govern particle interactions?

10. Astrophysics and cosmology (overview)

These fields study the structure and evolution of the universe.

Topics include: stars, galaxies, black holes, dark matter, dark energy, Big Bang theory.

Major discoveries: cosmic microwave background, expansion of the universe, gravitational waves.

Example: Hubble Space Telescope observations revolutionized cosmology.

Questions to ask

  • How did the universe originate?
  • What drives cosmic expansion?

11. Condensed matter physics (overview)

Condensed matter physics studies the physical properties of solids and liquids.

Examples: semiconductors, superconductors, magnetism, nanomaterials.

Applications: computer chips, materials engineering, quantum materials.

12. Major experiments in physics

Important experiments that shaped modern physics:

  • Michelson–Morley experiment
  • Photoelectric effect
  • Double slit experiment
  • Cavendish experiment
  • Higgs boson discovery

These experiments changed our understanding of reality.

13. Applications of physics

Physics drives many technologies.

Area Technology
Electromagnetismpower grids
Quantum mechanicssemiconductors
RelativityGPS
Opticsfiber internet
Thermodynamicsengines

14. Key mathematical tools in physics

Physics relies heavily on mathematics. Important areas:

  • calculus
  • linear algebra
  • differential equations
  • probability theory
  • tensor calculus

These tools allow physicists to model complex systems.

15. Knowledge map of physics

PHYSICS
|
+--- CLASSICAL PHYSICS
|    +--- mechanics
|    +--- thermodynamics
|    +--- electromagnetism
|    +--- optics
|
+--- MODERN PHYSICS
|    +--- quantum mechanics
|    +--- relativity
|
+--- PARTICLE PHYSICS
|    +--- standard model
|    +--- particle accelerators
|
+--- ASTROPHYSICS
|    +--- stars
|    +--- galaxies
|    +--- black holes
|
+--- CONDENSED MATTER
     +--- semiconductors
     +--- superconductors
     +--- nanomaterials

16. Important research papers

Foundational papers in physics include:

  • Einstein (1905) — On the Electrodynamics of Moving Bodies
  • Schrödinger (1926) — Quantization as an Eigenvalue Problem
  • Dirac (1928) — Quantum Theory of the Electron
  • Higgs (1964) — Broken Symmetries and the Mass of Gauge Bosons

These papers fundamentally shaped modern physics.

17. Bookshelf (starter list)

Important physics books include:

  • Stephen Hawking — A Brief History of Time
  • Richard Feynman — The Feynman Lectures on Physics
  • Brian Greene — The Elegant Universe
  • Leonard Susskind — The Theoretical Minimum
  • Sean Carroll — The Big Picture

These books explain complex ideas in accessible ways.

18. Learning resources

Top physics learning platforms:

  • MIT OpenCourseWare
  • Stanford Physics Lectures
  • Khan Academy Physics
  • Perimeter Institute Lectures
  • CERN Educational Resources

19. Classical mechanics

I enjoy classical mechanics as a way to sharpen intuition. Simple systems (pendulums, orbits, springs) are a good playground for thinking about invariants, symmetries and limits.

  • I like working problems where conservation laws do most of the work—momentum, energy, angular momentum.
  • Lagrangian and Hamiltonian formulations feel "closer to the code" of physics and make it easier to see symmetry.

20. Quantum mechanics

Quantum mechanics is where my intuition is constantly challenged—which is exactly why I like reading about it.

  • I am particularly interested in how different interpretations (Copenhagen, many-worlds, relational) try to make sense of the same math.
  • Thought experiments (double-slit, EPR, Bell tests) are a nice way to test my own understanding.

21. String theory and unification

I am not a physicist, but I enjoy reading about attempts to unify gravity with quantum mechanics, especially through string theory and related ideas.

  • Books and lectures from Brian Greene, Leonard Susskind and others help me build a high-level picture of extra dimensions, branes and dualities.
  • I am less concerned with the exact details than with the bigger question: what might a consistent quantum theory of gravity look like?

22. Black holes and spacetime

Black holes hit the sweet spot between relatively simple equations and extremely counterintuitive behavior.

  • I am fascinated by how general relativity predicts horizons and singularities, and how quantum effects lead to ideas like Hawking radiation.
  • I like following work on information, entanglement and the black hole information problem because it sits at the intersection of gravity, quantum theory and thermodynamics.

23. Miscellaneous notes

This section is reserved for things that do not fit neatly into classical or quantum: statistical mechanics, field theory curiosities, or links that I want to revisit.

24. Resources

Physics sources I find useful and trustworthy:

25. Bookshelf

A small physics bookshelf I track here, mainly to remember what to revisit:

  • The Feynman Lectures on Physics — Feynman, Leighton, Sands — Status: Reading
  • Six Easy Pieces — Richard Feynman — Status: Yet to Read
  • QED: The Strange Theory of Light and Matter — Richard Feynman — Status: Yet to Read
  • A Brief History of Time — Stephen Hawking — Status: Yet to Read
  • Cosmos — Carl Sagan — Status: Yet to Read
  • The Elegant Universe — Brian Greene — Status: Yet to Read
  • The Fabric of the Cosmos — Brian Greene — Status: Yet to Read
  • Seven Brief Lessons on Physics — Carlo Rovelli — Status: Yet to Read
  • Spacetime and Geometry — Sean Carroll — Status: Yet to Read
  • Our Mathematical Universe — Max Tegmark — Status: Yet to Read
  • In Search of Schrödinger's Cat — John Gribbin — Status: Yet to Read
  • Deep Down Things — Bruce Schumm — Status: Yet to Read

26. Domain Experts I follow

Physicists and science communicators whose work I follow to build intuition about fundamental physics: