Overview

Quantum Mechanics (QM) is the physics of the very small. It reveals a world that is fundamentally weird, probabilistic, and counter-intuitive. It shattered the deterministic clockwork universe of Newton.

Core Idea

The core idea is wave-particle duality. Matter (electrons) acts like waves, and light (photons) acts like particles. Nothing has a definite position until you measure it.

Formal Definition

The state of a system is described by a Wave Function ($\Psi$). The Schrödinger Equation describes how this wave evolves.

  • Born Rule: The square of the wave function ($|\Psi|^2$) gives the probability of finding the particle at a certain spot.

Intuition

  • Superposition: Schrödinger’s Cat is both dead and alive until you open the box. An electron is in many places at once.
  • Entanglement: “Spooky action at a distance.” Two particles can be linked so that measuring one instantly affects the other, even light-years away.
  • Uncertainty Principle: You can’t know both position and momentum perfectly. The more you know one, the less you know the other (Heisenberg).

Examples

  • Double Slit Experiment: Electrons fired at two slits create an interference pattern (like waves), even if fired one by one. But if you watch which slit they go through, they act like particles.
  • Tunneling: A particle can pass through a solid wall if it borrows energy for a short enough time.

Common Misconceptions

  • Misconception: Quantum means “magic” or “consciousness.”
    • Correction: It is a precise mathematical theory. “Observation” usually means interaction with a particle, not a conscious human looking.
  • Misconception: It only applies to atoms.
    • Correction: It applies to everything, but the effects wash out at large scales (decoherence).

Applications

  • Electronics: Transistors and lasers rely entirely on quantum effects.
  • MRI: Uses nuclear magnetic resonance (quantum spin).

Criticism and Limitations

  • Incompatibility: QM and General Relativity (Gravity) don’t get along. We still lack a “Theory of Everything” (Quantum Gravity).

Further Reading

  • Quantum Mechanics: The Theoretical Minimum by Leonard Susskind
  • In Search of Schrödinger’s Cat by John Gribbin