Mostly Empty

Ernest Rutherford was born on August 30, 1871, on a farm in rural New Zealand, the fourth of twelve children. He would go on to discover the nucleus of the atom, establish the basic architecture of matter, and win the Nobel Prize in Chemistry for work he had done before he was 40. He described himself as a simple man who never lost his feel for a good experiment, and that directness ran through everything he did. The most consequential result of his career came from noticing that something was behaving in a way it absolutely should not.

The experiment that changed everything took place in Manchester between 1909 and 1911, conducted by his collaborators Hans Geiger and Ernest Marsden under his direction. The prevailing model of the atom at the time was J.J. Thomson's "plum pudding" model: a diffuse blob of positive charge with electrons embedded in it like raisins. Rutherford's team fired alpha particles, small, heavy, positively charged particles, at a thin sheet of gold foil. By the plum pudding model, the alpha particles should have passed straight through with only minor deflections, like bullets through a fog bank.

the apparatus from hans geiger's 1908 paper, used to observe the scattering of alpha particles by a thin metal foil. source: wikimedia commons

Most of them did. But roughly one in eight thousand bounced almost straight back. Rutherford described his reaction as about as incredible as if you fired a fifteen-inch shell at a sheet of tissue paper and it came back and hit you. Something small, dense, and positively charged at the center of the atom was repelling the alpha particles that came close enough to hit it. He called it the nucleus. The atom was not a diffuse blob but a tiny core surrounded by vast empty space and orbiting electrons. If the nucleus were the size of a marble, the atom would be the size of a football stadium.

The architectural implications of this were enormous. Matter as humans experience it, solid tables, hard walls, the ground underfoot, is almost entirely empty space. The apparent solidity of things comes from electromagnetic forces between electrons, not from any physical packing of substance. The chair you sit in does not support you because its atoms are dense; it supports you because the electrons at its surface repel the electrons at yours with enough force to counter gravity. Rutherford's atom was a revelation not just about chemistry but about the nature of physical reality.

He continued working. In 1917, he became the first person to deliberately transmute one element into another, firing alpha particles at nitrogen and producing oxygen. This was, by the medieval definition, alchemy: turning one substance into another. It happened to be real. His laboratory at the Cavendish in Cambridge, where he moved in 1919, became the most productive physics laboratory in the world. James Chadwick discovered the neutron there in 1932, completing the basic picture of the atom that Rutherford had begun outlining twenty years earlier.

Rutherford was famously dismissive of over-elaborate theory and insisted that experiments should be physically understandable. He distrusted equations that could not be translated back into pictures of what was actually happening. This put him at odds with the quantum mechanics revolution of the 1920s, which produced results that defied physical intuition entirely. He accepted the experimental results while remaining skeptical of their philosophical implications, a position that his younger colleagues found frustrating and that history has judged as charming but somewhat limited.

ernest rutherford, the new zealand physicist born august 30, 1871, who discovered the nucleus of the atom and established the basic architecture of matter. source: wikimedia commons

He died in 1937 after a brief illness, at the height of his powers, two years before the fission of uranium would put his discoveries at the center of the most consequential weapons program in history. Enrico Fermi's first controlled chain reaction in 1942 would not have been conceivable without the nuclear model Rutherford established. He had given the world a picture of matter's inner architecture. Others would decide what to do with it. The empty space inside the atom turned out to contain more energy than anyone had imagined.