⚛️ Modern Physics

What you’ll learn

1. Physics at Its PeakDescribe the confidence of classical physics around 1900 and the two anomalies that undermined it.By 1900 Newtonian mechanics, thermodynamics, and Maxwell's electromagnetism seemed to explain nearly everything, and Kelvin famously reduced the exceptions to two 'clouds.' Those clouds — the missing ether and the unexplained thermal glow — became relativity and quantum mechanics.
2. The Glow That Broke PhysicsExplain blackbody radiation, Wien's displacement law, and why classical theory failed with an 'ultraviolet catastrophe.'Hot objects glow with a universal, temperature-set spectrum whose peak shifts blueward as temperature rises. Classical physics predicted infinite emission at short wavelengths — plainly absurd — showing something fundamental about energy was misunderstood.
3. Planck's Desperate ActState Planck's quantum hypothesis E = hf and its significance and reluctance-filled origin.On 14 December 1900 Planck fit the blackbody spectrum by assuming energy is exchanged in discrete quanta proportional to frequency. He considered it a desperate formal trick, yet E = hf — with its tiny constant h — became the founding equation of quantum physics.
4. Einstein's Light QuantaExplain the photoelectric effect and how Einstein's photon hypothesis accounts for it.Light ejects electrons from metals only above a threshold frequency, regardless of intensity — inexplicable for waves. Einstein's 1905 proposal that light itself is quantized explains it one photon per electron; Millikan's measurements confirmed it and it earned Einstein the Nobel Prize.
5. Light: Wave or Particle?Weigh the wave evidence for light against the particle evidence and articulate wave-particle duality.A century of interference and diffraction results said light is a wave, yet the photoelectric and Compton effects proved photons carry energy and momentum like particles. Light is both — the duality quantum mechanics would eventually formalize.
6. Discovering the ElectronRecount Thomson's discovery of the electron and the plum pudding model it inspired.In 1897 J.J. Thomson showed cathode rays are subatomic particles nearly 2,000 times lighter than hydrogen, proving atoms have parts. His plum pudding model embedded electrons in diffuse positive charge, while Wilson's cloud chamber soon made particle tracks visible.
7. Rutherford's NucleusExplain the gold foil experiment and the nuclear model of the atom it forced.Alpha particles from radioactive sources mostly passed through gold foil, but rare back-bounces revealed a tiny, dense, positive nucleus holding nearly all the atom's mass. The nuclear atom was mostly empty space — and classically should have collapsed, setting up Bohr's intervention.
8. Spectral BarcodesDescribe atomic line spectra and Balmer's empirical formula for hydrogen.Energized gases emit sharp element-specific spectral lines rather than smooth rainbows. In 1885 Balmer captured hydrogen's visible lines in a simple integer-based formula — precise, unexplained, and the decisive test any atomic theory had to pass.
9. Bohr's AtomExplain Bohr's quantized energy levels and how quantum jumps produce spectral lines.Bohr's 1913 model restricts electrons to discrete energy levels, with photons emitted or absorbed only in jumps between them — deriving Balmer's formula and explaining spectra from lab tubes to auroras. Triumphant for hydrogen, it failed beyond it, exposing the need for a complete quantum mechanics.
10. Matter WavesState de Broglie's matter-wave hypothesis and the experiments confirming electron interference.De Broglie proposed in 1924 that all matter has wavelength λ = h/p, explaining Bohr's orbits as standing waves. Electron diffraction (1927) and single-electron double-slit experiments confirmed it, and electron microscopes exploit it daily.
11. The New MechanicsOutline matrix mechanics, the Schrödinger equation, and Born's probability interpretation.In 1925–26 Heisenberg (with Born and Jordan) and Schrödinger built two equivalent formulations of true quantum mechanics, deriving atomic behavior rather than postulating it. Born's rule made the wavefunction a map of probabilities — replacing certainty with exact statistics.
12. The Uncertainty PrincipleState Heisenberg's uncertainty principle and explain why it is intrinsic, not instrumental.Position and momentum obey Δx·Δp ≥ ħ/2 because localizing a wave requires blending many wavelengths. Measurement disturbs the system, and even ideal instruments cannot evade the trade-off — a fact made visible in Bose-Einstein condensates.
13. The Great DebateSummarize the Bohr-Einstein debate at Solvay 1927 and its long-term legacy.The 1927 Solvay Conference gathered the quantum theory's creators to argue over its meaning: Einstein rejected fundamental randomness while Bohr's Copenhagen view became the working consensus. Einstein's challenges, especially EPR, ultimately led to Bell tests confirming quantum predictions.
14. Relativity: The Other RevolutionTrace the path from the Michelson-Morley null result through special relativity to the 1919 confirmation of general relativity.The undetectable ether led Einstein to postulate a universal light speed, yielding time dilation, length contraction, and E = mc². General relativity recast gravity as spacetime curvature, confirmed when the 1919 eclipse expeditions measured starlight bending past the Sun.
15. The Quantum LegacyConnect quantum mechanics and relativity to the transistor, laser, atomic timekeeping, and GPS.Semiconductor band structure gave us the transistor (1947), Einstein's stimulated emission gave us the laser (1960), and cesium's quantum transition now defines the second. GPS works only with relativistic clock corrections — modern life runs on both 20th-century revolutions.

Questions this course answers

Grounded in trusted sources

• Helge Kragh — Quantum Generations: A History of Physics in the Twentieth Century (1999)
• Abraham Pais — Subtle Is the Lord: The Science and the Life of Albert Einstein (1982)
• Abraham Pais — Inward Bound: Of Matter and Forces in the Physical World (1986)
• Manjit Kumar — Quantum: Einstein, Bohr, and the Great Debate About the Nature of Reality (2008)
• NobelPrize.org — official Nobel Prize biographies and lectures
• NIST — cesium fountain clocks and the SI definition of the second
• Original papers: Planck (1900), Einstein (1905), Rutherford (1911), Bohr (1913), Heisenberg (1927)

Every Wunder lesson is built from real, reputable sources — never invented.

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