The 28 Years That Broke Physics: Photons and Matter Waves

Light is a wave, yet it arrives in lumps. Electrons are particles, yet sent one at a time through two slits, they build up the stripes that only waves can make. So how can the same thing be a wave and a particle? In October 1927, 29 physicists posed for a photograph in Brussels. Seventeen of them won Nobel Prizes, and in the years before it they had taken apart the two most solid ideas in physics: that light is a wave, and that matter is made of particles. We follow those 28 years, from 1900 to 1928, one experiment at a time, and derive every result: Planck's glowing cavity and the ultraviolet catastrophe, Einstein's photons and the photoelectric effect, Taylor's needle and single-photon fringes, Millikan's measurement of Planck's constant, photon momentum and stimulated emission, Compton scattering, light as a probability wave, de Broglie's matter waves, Schrödinger's equation, electron diffraction, Heisenberg's uncertainty principle, and quantum tunnelling. After every discovery we jump to today: thermal cameras and the cosmic microwave background, the grain in night-time photos, photomultipliers and photon-counting CT, the kilogram defined by Planck's constant, solar sails, optical tweezers and laser cooling, Compton cameras, quantum random numbers and quantum keys, electron microscopes and cryo-EM, why atoms don't collapse, LIGO's squeezed light, the scanning tunnelling microscope, the flash memory in your phone, why the Sun shines, and the superconducting qubits behind the 2025 Nobel Prize in physics. Along the way, 12 worked problems. Chapters 0:00 Brussels, October 1927 0:42 The 2025 Nobel Prize: a circuit that tunnels 1:08 A street lamp's hailstorm of photons 1:34 Wave or particle? The double slit 2:02 The plan: 1900 to 1928 2:37 1900: the glow nobody could explain 4:24 Rayleigh-Jeans and the ultraviolet catastrophe 6:48 Planck's steps and his radiation law 8:38 Wien's law and the Stefan-Boltzmann law 9:53 Today: thermal cameras, light bulbs and the CMB 10:50 1905: Einstein's photons, E = hν 13:09 Problem 1: photons from a sodium lamp 13:43 The photoelectric effect: three puzzles 15:02 Einstein's photoelectric equation 15:59 Problem 2: matching blackbody temperatures 17:08 Today: camera noise, sunburn and solar cells 18:36 1909: Taylor's needle, one photon at a time 19:50 1916: Millikan tries to prove Einstein wrong 20:36 The stopping potential 22:18 Measuring Planck's constant 23:48 Problems 3 to 6: the photoelectric effect 27:01 Today: photomultipliers, photon-counting CT, the kilogram 28:45 1916: photon momentum and radiation pressure 29:47 1917: stimulated emission, the idea behind the laser 30:36 Problem 7: a helium atom's recoil 31:12 Today: solar sails 32:07 Today: optical tweezers and laser cooling 33:04 1923: Compton's X-rays hit electrons 34:54 Deriving the Compton shift 36:52 The Compton wavelength and the unshifted peak 37:56 Problem 8: the Compton shift 38:51 Today: CT scans, radiotherapy and Compton cameras 39:57 What is a photon? Light as a probability wave 41:44 One photon, two opposite paths (Lai and Diels) 42:26 The three rules for light 42:53 Today: quantum random numbers and quantum keys 44:00 1924: de Broglie's matter waves, λ = h/p 46:02 Problems 9 and 10: de Broglie wavelengths 47:51 Electron vs proton 48:23 1926: Schrödinger's wave function 49:29 Born: the wave function as probability 50:09 Schrödinger's equation 51:20 Solving it for a free particle 53:32 1927: Davisson and Germer's lucky accident 54:35 Checking de Broglie: electron diffraction 55:30 G. P. Thomson: electron rings 56:11 Electrons one at a time: the double slit 56:58 How big can a matter wave be? 57:47 Why particle tracks are straight 58:31 Today: the electron microscope 59:21 Today: cryo-EM, neutrons and atom interferometers 60:18 1927: Heisenberg's uncertainty principle 61:12 Heisenberg's microscope 62:18 Problem 11: the uncertainty principle 63:00 Today: why atoms don't collapse 65:49 Today: LIGO and squeezed light 67:32 Back to the 1927 photograph: the answer 68:17 1928: Gamow and tunnelling 68:47 Electrons at a potential step 70:51 Tunnelling through a barrier 71:53 Problem 12: electron vs proton tunnelling 72:48 Alpha decay 73:34 Today: the scanning tunnelling microscope 74:36 Today: flash memory 75:21 Today: leaky transistors, and why the Sun shines 76:11 Today: the 2025 Nobel Prize and quantum computers 76:57 Notes Photographs: the 1927 Solvay photograph by Benjamin Couprie, and other public domain and CC0 images from Wikimedia Commons. Earth / Terra by murilo.kleine (CC BY 4.0): https://sketchfab.com/3d-models/30fd7e9c4e... Textures and lighting: Poly Haven (CC0). Human base meshes: Blender Studio (CC0). The double slits, Tonomura's build-up, the diffraction rings and scattering, wave packets, tunnelling and Millikan's data points are our own simulations, not original photographs or data. #physics #quantumphysics #science