Particle physics experiments

10 physics experiments that changed our view of the universe

The concept of an atom – the point at which matter can no longer be cut – dates back to the ancient Greeks, with the word itself derived from the Greek word “atomos” meaning “indivisible”. Until 1897, scientists believed that atoms had no internal structure and were the smallest units of matter. This was before the discovery of a small negatively charged particle – the electron – by Joseph John Thomson.

In 1904, JJ Thomspon suggested that these particles were embedded in a positively charged substance much like scattered fruit in plum pudding in his aptly named plum pudding atomic model.

This model was overturned by the Geiger-Marsden experiment, also known as the gold foil experiment or the α-particle scattering experiments, pioneered by Ernest Rutherford and conducted by his proteges, Ernest Marsden and Hans Geiger.

Shooting α particles – which we now know are identical to a helium-4 nucleus – emitted from a radioactive source onto a thin sheet of gold foil, Rutherford reasoned that if the plum-pudding model of the atom was correct, these moving particles would suffer the tiniest of deflections. This is because an α particle is about 7,000 times more massive than an electron.

The 1911 experiments showed that α particles sometimes undergo a large deflection. While only one in 20,000 alpha particles had been deflected by 45° or more, this was enough to trigger a major redesign of the atom and unveiled the presence of the atomic nucleus.

Rutherford likened the results to firing a 15-inch shell at a sheet of tissue paper and bouncing it directly at you!

This revealed that the majority of matter in an atom is concentrated at its center. Rutherford proposed a model of the atom with electrons orbiting a massive positively charged nucleus.

This model would be reversed over time, but it represented an essential step in the discovery of the proton and the neutron and in the unveiling of the atomic structure.