1895 » Wilhelm Roentgen discovered x-rays. While studying the luminescence (light) produced by cathode rays, Roentgen had placed a cathode ray tube in a box in a darkened room. (A cathode ray tube is a vacuum tube in which a cathode, or negatively charged electrode, sends out a stream of electrons.) A sheet of paper coated with a barium compound happened to be near the box. Roentgen noticed that when the tube was switched on in the closed box, the paper glowed brilliantly. He concluded that some sort of ray had penetrated the box and caused the paper to glow. Because he didn’t know what they were or where they came from, he called them x-rays (x for unknown). He also noticed the rays caused photographic plates, even when wrapped in paper, to darken or fog. This led him to take x-ray photographs of objects such as his hand. The photographs revealed the inner structure of the objects.

The world immediately appreciated the medical potential of x-rays. X-rays revolutionized medicine because they enabled doctors to see the interior of the body without surgery. Within five years of the discovery, for example, the British Army began using a mobile x-ray unit to locate bullets and shrapnel in wounded soldiers in the Sudan.

1898 » Marie and Pierre Curie discovered the radioactive elements radium and polonium. Before this discovery, uranium and thorium were the only known radioactive elements. While studying uranium minerals. Marie Curie noticed two minerals were much more radioactive than uranium itself. She and her husband, Pierre, chemically separated the compounds in the minerals and found a substance 400 times more radioactive than uranium. Marie named this substance polonium, after her native country of Poland. The Curies also found one of the uranium minerals contained another element more radioactive than uranium. They named this element radium. The Curies won the 1903 Nobel Prize in physics for these discoveries.

Marie Curie won the Nobel Prize in chemistry in 1910 for obtaining elemental radium in the metallic state. She was the first person to win the Nobel Prize twice. Soon after the Curies’ discoveries, other scientists discovered the radioactive element actinium. By 1912, thirty radioactive elements had been discovered.

1905 » Albert Einstein developed a theory about the relationship of mass and energy. The formula, E=mc[2], is probably the most famous outcome from Einstein’s special theory of relativity. The formula says energy (E) equals mass (m) times the speed of light (c) squared. In essence, it means mass is just one form of energy. Since the speed of light squared is an enormous number (186,000 miles per second)[2], a small amount of mass can be converted to a phenomenal amount of energy. Or, if there’s a lot of energy available, some energy can be converted to mass and a new particle can be created. Nuclear reactors, for instance, work because nuclear reactions convert small amounts of mass into large amounts of energy.

1927 » Herman Blumgart, a Boston physician, first used radioactive tracers to diagnose heart disease. Blumgart was a professor of medicine at Harvard University Medical School.

December 1938 » Two German scientists, Otto Hahn and Fritz Strassman, demonstrated nuclear fission. They found they could split the nucleus of a uranium atom by bombarding it with neutrons, the uncharged part of atoms. As the uranium nucleus split, some of its mass was converted to energy.

News of the discovery spread through the scientific community. Other physicists noticed the fission of one uranium atom gave off extra neutrons which could in turn split other uranium atoms, starting a chain reaction. In theory, this energy could be harnessed to make a powerful bomb.

August 1939 » on August 2, Albert Einstein sent a letter to President Roosevelt informing him of German atomic research and the potential for a bomb:
“Some recent work…leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future…that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated…. This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type…might very well destroy the whole port together with some of the surrounding territory.”

Einstein drafted the letter with the help of Hungarian physicist Leo Szilard, one of many scientists who had fled Europe to escape Nazi and Fascist repression. Szilard was a vocal advocate of a U.S. program to develop bombs based on the latest findings in nuclear physics. In the letter, they encouraged Roosevelt to fund American atomic research. This letter prompted Roosevelt to form a special committee to investigate the military implications of atomic research. Roosevelt approved uranium research in the United States in October 1939. This was the first decision among many that led to establishment of the Manhattan Project.