Spin had been added before it was explained
Electron spin and its magnetic coupling organized atomic doublets, but adding those properties to a nonrelativistic theory left a question: could they arise from a relativistic quantum equation itself? Dirac’s 1928 paper makes that theoretical demand explicit.
The target was not merely a correction for a faster classical orbit. Quantum evolution and relativity had to fit one mathematical structure. The resulting theory explained important electron properties while also exposing difficulties that its first paper did not resolve.
Read the evidence: P. A. M. Dirac (1928), The Quantum Theory of the Electron, Proc. R. Soc. A 117, 610–624 — UC San Diego scan
First-order evolution requires a richer state
The displayed equation is the modern free-particle form, with no electromagnetic field. The state has four components; the alpha and beta symbols denote dimensionless matrices, not additional coordinates. Their algebra makes the relativistic energy–momentum relation compatible with first-order time evolution.
Rest mass, the speed of light and reduced Planck constant set the units; the gradient acts on position. This equation alone is not a complete theory of electron creation, radiation or an interacting many-particle experiment. A four-component drawing also should not be read as four classical particles.
Read the evidence: P. A. M. Dirac (1928), The Quantum Theory of the Electron, Proc. R. Soc. A 117, 610–624 — UC San Diego scan
A success becomes a precision baseline
The minimally coupled Dirac electron gives a spin magnetic factor of two before radiative corrections. This connects intrinsic angular momentum and magnetic response more tightly than inserting an independent classical spinning charge. It provides a definite baseline that an experiment can challenge.
Kusch and Foley’s atomic comparisons later found a small departure from that baseline. Their 1948 abstract summarizes several atomic-state comparisons. The discrepancy is a measured magnetic-response problem, not a reason to discard every successful relativistic prediction.
Read the evidence: P. Kusch and H. M. Foley (1948), The Magnetic Moment of the Electron, Phys. Rev. 74, 250 — author abstract
Negative energies were a difficulty, not a finished discovery
The first 1928 paper explicitly postpones one of the difficulties associated with negative-energy solutions. It would be misleading to retell that paper as if the modern particle–antiparticle interpretation were already complete. A mathematical branch and a detected particle are different kinds of evidence.
Anderson’s 1933 report analyzes cosmic-ray tracks with positive charge and a mass much smaller than a proton’s. Curvature, ionization and energy-loss evidence constrain that identification. The experimental argument must be assessed on those records rather than supplied retrospectively by the existence of Dirac’s equation.
Read the evidence: P. A. M. Dirac (1928), The Quantum Theory of the Electron, Proc. R. Soc. A 117, 610–624 — UC San Diego scanC. D. Anderson (1933), The Positive Electron, Phys. Rev. 43, 491 — author abstract
The hydrogen comparison has a boundary
A nonrelativistic Coulomb model, a Dirac Coulomb model and a theory including radiative and nuclear corrections are distinct approximation levels. Their predicted coincidences and splittings need not agree. The experimental resolution determines which differences are actually tested.
The linked hydrogen Lab supplies a nonrelativistic reference; it does not solve the four-component equation. The coupled-spin Lab uses an effective Hamiltonian and is not a precision calculation of an electron anomaly. Those boundaries make the models useful comparisons rather than substitutes for the historical theory.
Read the evidence: P. A. M. Dirac (1928), The Quantum Theory of the Electron, Proc. R. Soc. A 117, 610–624 — UC San Diego scanP. Kusch and H. M. Foley (1948), The Magnetic Moment of the Electron, Phys. Rev. 74, 250 — author abstract
Follow the new questions opened by a theory
Read the electron-moment article for the comparison of magnetic factors, and the Lamb article for a different spectroscopic challenge to the Dirac baseline. These are separate observables. Agreement in one channel cannot be used as the measurement of another.
The theory’s significance lies both in the relations it successfully imposed and in the sharp questions it made possible. Later quantum electrodynamics and antiparticle experiments belong to their own evidence histories; this reading bridge does not claim to complete those subjects.
Read the evidence: P. Kusch and H. M. Foley (1948), The Magnetic Moment of the Electron, Phys. Rev. 74, 250 — author abstractC. D. Anderson (1933), The Positive Electron, Phys. Rev. 43, 491 — author abstract