How Early Experiments Unveiled the First Fundamental Particles

Introduction – Yang Zhenning’s book Lectures on Physics – The Journey of Discovering Fundamental Particles reviews the revolutionary era of physics from the late 1800s to the early 1900s, highlighting key experiments and discoveries that built the foundations of particle physics.

At the turn of the 19th century, classical mechanics and Faraday‑Maxwell electromagnetic theory had reached their limits, and new phenomena such as cathode rays, the photo‑electric effect, radioactivity, and the Zeeman effect emerged.

Lord Kelvin still described electricity as a continuous fluid, a view later challenged by J. J. Thomson’s experiments.

In 1897 Thomson measured the charge‑to‑mass ratio (e/m) of cathode rays, proving they were particles much lighter than ions. His apparatus (Fig. 1) and schematic (Fig. 2) showed a narrow beam deflected by electric and magnetic fields, allowing calculation of e/m.

Thomson’s results led him to propose that an atom contains Z electrons of charge –e embedded in a uniformly distributed positive charge +Ze, forming a neutral atom.

In 1911 Lord Rutherford’s α‑particle scattering experiments revealed that most of an atom’s mass and positive charge is concentrated in a tiny nucleus, contradicting Thomson’s model. Rutherford’s nuclear model (Fig. 3) showed a small positively charged core surrounded by electrons.

Rutherford and Bohr’s work, presented in Bohr’s 1930 Faraday Lectures, emphasized the distinction between ordinary chemical properties (determined by electron configuration) and nuclear properties (determined by charge and mass).

Bohr’s hydrogen atom theory and the emerging quantum concepts of Planck (quantum of radiation), Einstein (photo‑electric effect), and de Broglie (matter waves) laid the groundwork for quantum mechanics.

Schrödinger’s wave equation (1926) formalized quantum mechanics, explaining the wave‑particle duality of electrons.

Table 1 (not reproduced) illustrated why higher‑energy particle accelerators are needed to probe smaller distances.

In the 1930s, the neutron was discovered by Joliot‑Curie’s team (1932) and confirmed by Chadwick, showing that nuclei consist of nearly equal numbers of protons and neutrons.

Beta decay required a neutral particle to conserve energy; Fermi introduced the neutrino (v) to account for the missing energy.

Anderson’s cloud‑chamber photographs (Fig. 10) revealed the positron, the electron’s antiparticle, confirming Dirac’s prediction of charge‑conjugate partners.

By 1933 the known fundamental particles included electrons (e⁻), positrons (e⁺), protons (p⁺), neutrons (n⁰), photons (γ), neutrinos (ν) and antineutrinos ( ν̄ ), as shown in Fig. 11.

Later refinements distinguished the neutrino emitted in neutron decay (anti‑neutrino) from the original neutrino concept.

All images are retained with only src and alt attributes to illustrate the historical experiments and particle charts.