Investigatio
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Physics, in the order it was actually worked out

From a failed experiment in Cleveland
to the fields that make up everything.

Most explanations of relativity and quantum mechanics hand you the conclusions. This one walks the road: what people believed, which experiment broke it, and what had to be rebuilt afterwards. No physics background assumed. The mathematics is always optional.

Start with the experiment that broke classical physics46 of 46 lessons written

The path

  1. Act 1

    The classical picture

    By 1890 physics looked finished. What did it actually say?

    1831 – 1890
    1. Fields, not forcesFaraday could not do the mathematics, but he had the better idea: space itself carries the influence.Faraday's iron filings and induction ring, 1831
    2. Light is an electromagnetic waveMaxwell computed the speed of his waves and got the speed of light. Hertz then made some.Hertz's spark gap, 1887
    3. The aether problemEvery wave anyone knew of waved in something. So what does light wave in?Bradley's aberration 1729; Fizeau's moving water 1851
  2. Act 2

    The crack

    One experiment refused to give the answer everyone expected.

    1887 – 1904
    1. The Michelson–Morley experimentThe most famous null result in physics: they built an instrument sensitive enough, and found nothing.Michelson & Morley, Cleveland, 1887
    2. Saving the aetherFitzGerald and Lorentz proposed that moving objects contract. They were right about the formula and wrong about why.Trouton–Noble, 1903
  3. Act 3

    Special relativity

    What if the speed of light is simply the same for everyone?

    1905 – 1908
    1. Two postulatesEinstein threw away the aether and kept the constancy of c. Everything else follows.Einstein, Annalen der Physik, 1905
    2. The end of simultaneity"At the same time" turns out to depend on who is asking.Sagnac, 1913
    3. Time dilationA clock made of light, and why moving clocks must run slow.Langevin’s travelling twin, 1911
    4. Length contractionThe other side of the same coin.Heavy-ion collisions, RHIC & LHC
    5. E = mc²A footnote to the 1905 paper that turned out to run the stars.Cockcroft & Walton, Cavendish, 1932
    6. Spacetime and light conesMinkowski's geometry: not space and time, but one thing.Alväger et al., CERN, 1964
    7. Is any of it true?Muons that should not reach the ground, and atomic clocks flown around the world.Frisch & Smith, Mount Washington, 1963; Hafele–Keating, 1971
  4. Act 4

    General relativity

    What if gravity is not a force at all?

    1907 – 2015
    1. The happiest thoughtA falling man does not feel his own weight. Einstein called this the happiest thought of his life.Eötvös torsion balance, Budapest, 1889–1922
    2. Curved spacetimeMatter tells spacetime how to curve; spacetime tells matter how to move.Gravity Probe B, 2004–2011
    3. Mercury's perihelionA 43-arcsecond discrepancy that had been sitting unexplained since 1859.Le Verrier 1859; Newcomb 1882
    4. Light bendsAn eclipse expedition made Einstein famous overnight.Eddington & Dyson, Príncipe and Sobral, 1919
    5. Gravity slows timeMeasured up a lift shaft at Harvard, to one part in 10¹⁵.Pound & Rebka, 1959
    6. Black holesWhat happens when the curvature runs away with itself.Genzel & Ghez, Galactic Centre orbits, 1992–2020
    7. Gravitational wavesSpacetime itself ringing, detected a century after the prediction.LIGO, 2015
    8. Relativity in your pocketGPS would drift 10 km a day without both of Einstein’s theories.NTS-2, launched with the correction switched off, 1977
  5. Act 5

    The quantum break

    Why did classical physics predict infinite energy from a warm object?

    1900 – 1914
    1. The ultraviolet catastropheClassical physics predicts that a fireplace should kill you with ultraviolet light.Lummer & Pringsheim; Rubens & Kurlbaum, Berlin, 1898–1900
    2. Planck's desperate actHe assumed energy came in lumps to make the maths work, and spent years trying to undo it.Planck’s 1900 determination of h, k, N_A and e
    3. The photoelectric effectBrighter light does not give electrons more energy. Bluer light does.Lenard, Kiel, 1902
    4. The man who tried to disprove itMillikan spent ten years trying to refute Einstein and ended up confirming him to 0.5%.Millikan, 1916
    5. Atomic spectraAtoms emit only certain colours. Nobody could say why.Bohr derives the Rydberg constant, 1913
    6. Energy levels are realElectrons fired through mercury vapour lose energy only in fixed amounts.Franck & Hertz, 1914
  6. Act 6

    Quantum mechanics

    If light can be a particle, can a particle be a wave?

    1924 – 1935
    1. Matter wavesDe Broglie's PhD thesis was so strange his examiners wrote to Einstein.C₆₀ interference, Vienna, 1999
    2. Electrons diffractA laboratory accident produced the proof.Davisson & Germer, 1927
    3. The double slitFeynman called it the only mystery.Jönsson, Tübingen, 1961
    4. One electron at a timeSend them through singly and the interference pattern still builds up.Tonomura, Hitachi, 1989
    5. The wavefunctionSchrödinger's equation, and what the wave actually is.Born’s probability rule, 1926
    6. UncertaintyNot a limit on your instruments. A limit on what there is to know.Liquid helium’s refusal to freeze
    7. SpinSilver atoms split into exactly two beams, and the world got stranger.Stern & Gerlach, 1922
    8. MeasurementThe part nobody has finished arguing about.Searches for objective collapse, 2010s–present
  7. Act 7

    Quantum information

    Einstein thought entanglement proved the theory incomplete. Was he right?

    1935 – now
    1. Einstein's objectionThree years of trying to prove quantum mechanics inconsistent, and losing.Pound & Rebka, Harvard, 1959
    2. EntanglementTwo particles, one state, no matter how far apart.Wu & Shaknov, Columbia, 1950
    3. Bell's theoremA way to settle a philosophical argument with an experiment.Bell, 1964
    4. The experimentsForty years of closing loopholes. Einstein was wrong.Aspect 1982; loophole-free tests 2015
    5. QubitsInformation when the bits obey quantum rules.Nakamura, Pashkin & Tsai, NEC, 1999
    6. No cloning, and teleportationYou cannot copy a quantum state — which is exactly why you can teleport one.Micius satellite, 1203 km, 2017
    7. Why the world looks classicalDecoherence, and what makes a quantum computer hard to build.Haroche, ENS Paris, 1996
  8. Act 8

    Quantum fields

    What is everything actually made of?

    1927 – now
    1. Fields fill all spaceThe vacuum is not empty, and it is not quiet.Casimir force: Lamoreaux 1997, Mohideen & Roy 1998
    2. Particles are excitationsAn electron is a ripple in the electron field, and every ripple is identical.Anderson finds the positron, 1932; Hong–Ou–Mandel, 1987
    3. InteractionsWhat a Feynman diagram is really shorthand for.Electron g−2: Gabrielse et al., 1987–2023
    4. The Standard ModelSeventeen fields, and the most precisely tested theory ever written.Higgs discovery, ATLAS & CMS, CERN, 2012
    5. What is still missingGravity, dark matter, and why the theory cannot be the end of it.Rubin & Ford, flat rotation curves, 1970s