In 1896 the French physicist Henri Becquerel left some uranium salts in a drawer, on top of a photographic plate wrapped in black paper. When he developed the plate, it was fogged, as if it had been exposed to light. Becquerel found that uranium gives off invisible rays on its own. Within half a century this discovery would light cities and destroy them.

Radioactivity

Marie and Pierre Curie took up Becquerel's work, coined the word radioactivity, and in 1898 discovered two new radioactive elements, polonium and radium. Radioactivity is the process by which unstable atomic nuclei break down on their own, giving off particles or energetic radiation.

The half-life of a radioactive substance is the time it takes for half of its nuclei to decay. It ranges from fractions of a second to billions of years, depending on the substance. Because heat, pressure and chemistry do not change the rate of decay, it serves as a reliable clock for dating rocks and ancient remains.

The early researchers did not understand the danger. Marie Curie carried radium samples in her pockets, and her laboratory notebooks are still radioactive today. In the 1920s young American women who painted watch dials with glowing radium paint, often shaping their brushes with their lips, suffered terrible illnesses. Their lawsuits helped establish workers' rights to safety and compensation.

Fission

In December 1938 the German chemists Otto Hahn and Fritz Strassmann found barium, a much lighter element, among the products of uranium bombarded with neutrons. Lise Meitner, an Austrian physicist of Jewish background who had fled Nazi Germany that summer, worked out the explanation with her nephew Otto Frisch. In nuclear fission a heavy nucleus such as uranium splits into two smaller nuclei, releasing a great deal of energy. The energy matches the loss of a tiny amount of mass, as E = mc² predicts. Hahn alone received the Nobel Prize for the discovery, an omission now widely seen as unjust.

Each fission also releases two or three neutrons. A chain reaction occurs when the neutrons from each fission go on to split further nuclei. Controlled, it can boil water to drive a power station; uncontrolled, it can destroy a city. Fully split, a kilogram of uranium 235 releases a few million times more energy than burning a kilogram of coal, because nuclear forces are far stronger than the chemical bonds broken in burning.

The bomb

Fearing that Nazi Germany would build such a weapon first, the physicist Leo Szilard drafted a letter, signed by Einstein in August 1939, warning President Franklin Roosevelt. The United States, with British and Canadian help, launched the Manhattan Project. The first nuclear test, code-named Trinity, took place in New Mexico in July 1945. In August the United States dropped atomic bombs on Hiroshima and Nagasaki. By the end of 1945 they had killed, by common estimates, somewhere between about 110,000 and more than 200,000 people, most of them civilians. Historians still debate whether the bombings were needed to end the war, weighing the deaths a planned invasion of Japan might have caused against the deliberate killing of civilians.

Some scientists who built the bomb opposed its use. In June 1945 a group in Chicago wrote the Franck Report, which urged that the bomb be shown in a demonstration rather than used on Japanese cities without warning. After the war many scientists became leading voices for arms control. The Nuclear Non-Proliferation Treaty of 1968 commits states without nuclear weapons not to acquire them, and the nuclear-armed states to work toward disarmament, while allowing peaceful nuclear energy.

Nuclear power

The first nuclear power station to feed an electric grid opened at Obninsk in the Soviet Union in 1954. Nuclear plants now supply around 9 percent of the world's electricity. Defenders point out that nuclear power releases almost no carbon dioxide while running and, per unit of energy, has caused far fewer deaths than fossil fuels, mostly because burning coal, oil and gas pollutes the air. Critics point to the 1986 Chernobyl disaster in Soviet Ukraine, the 2011 Fukushima accident in Japan after a tsunami, the high cost and long construction times of new plants, the links between civilian and weapons technology, and spent fuel that stays dangerous for thousands of years. Fusion, which powers the Sun, would join light nuclei instead of splitting heavy ones. In 2022 the National Ignition Facility in California got more energy out of a fusion reaction than its lasers put in, but practical fusion power remains years away.

Worked example: one discovery, two paths

The same chain reaction lights homes from Ontario to France and once leveled two cities. Physics cannot tell us which path to choose; it can tell us honestly what each path involves. That is why public understanding matters. A citizen who knows what a half-life is can weigh claims about waste storage. One who knows that reactor fuel is far too weakly enriched to explode like a bomb can judge fears about power plants more fairly. One who knows what a single warhead can do sees why treaties limiting them matter. The scientists of 1945 learned that discovery brings responsibility for every human life it touches. In a democracy, that responsibility is shared by everyone who votes on how knowledge is used.