Introduction to Quantum Mechanics

Back to Quantum Mechanics

Table of Contents

  1. Quantum Mechanics
  2. Feynman on Subatomic Particles
  3. Paradox of Quantum Mechanics
  4. Subatomic World
  5. Rutherford’s Planetary Atomic Model
  6. Failure of Rutherford’s Model

Quantum Mechanics

Quantum Mechanics is the theory of the interactions among atoms, subatomic particles, and electromagnetic radiation.

Feynman on Subatomic Particles

“Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.”

Richard Feynman

Paradox of Quantum Mechanics

  • Quantum Mechanics “has proved extraordinarily successful at predicting all of the observed behaviors of all physical systems under all circumstances.” (Britannica)
  • But at the theory’s core is a mystery which, in Feynman’s words, “we cannot make go away by ‘explaining’ how it works.”

Subatomic World

  • In 1897 the first subatomic particle was discovered, the electron.
  • It was soon apparent that the subatomic world was not governed by the laws of Classical Physics.
  • An example is Ernest Rutherford’s planetary model of the atom.

Rutherford’s Planetary Atomic Model

  • In 1911 Ernest Rutherford set forth his planetary model of the atom in which electrons orbit the nucleus like planets around the Sun.
  • Classical Mechanics predicts the orbits of the planets around the Sun using:
    • Newton’s Universal Law of Gravitation
      • F = Gm1m2/r2
    • Equation of Motion
      • F = MA
  • The idea was that Classical Physics would predict the orbits of electrons around the nucleus in the same way using:
    • Coulomb’s Law
      • F = Kq1q2/r2
    • Equation of Motion
      • F = MA
Rutherford Planetary Model, Britannica

Failure of Rutherford’s Model

  • An orbiting electron differs from an orbiting planet in a significant way: the electron has an electric charge.  Thus, as the Britannica puts it,
    • “The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.” 
  • Classical Physics thus incorrectly predicts that orbiting electrons spiral into the nucleus.
  • It wasn’t until the late 1920’s that physicists, led by Werner Heisenberg, Erwin Schrödinger, and Paul Dirac, developed the theory that correctly predicts how atoms and subatomic particles behave, Quantum Mechanics.