Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....


A World Ward II radar installation in Sitka, Alaska.

Everyone knows the story of how the nuclear bomb influenced the end of World War II. The Hiroshima and Nagasaki bombings, however, came when Germany was already defeated and Japan's surrender was only a matter of time.1) Far less well known, but more important to the outcome of the war, was a different contribution from physics: radar. This new technology, based on the work of theoretical physicists James Clerk Maxwell in the previous century, played a decisive role in the air war known as the Battle of Britain. If not for the English radar defenses, Hitler might have succeeded in conquering Britain and gone on to win the war.

In this chapter, we discuss the magnetic field, its intimate relationship to the electric field, and the existence of waves made of electric and magnetic fields linked to each other.

1)The bombings were a sad appearance of physics on the stage of history. Many of the physicists working on the Manhattan Project had seen it as a race against a Nazi bomb. After Germany surrendered, most had assumed the bombs would only be used in demonstrations, not in a first strike on cities, but criticism after the fact was muted, partly because of relief that there would not have to be an invasion of Japan. (On a personal note, both my grandfathers were in the Pacific during the war, and could have been killed during such an invasion.) Truman himself, however, believed that the same objective could have been accomplished by attacking a military target. A month earlier, he had written in his diary that he would "...use it so that the military objectives ... are the target and not women and children," but he apparently forgot his moral qualms once Secretary of State Byrnes pointed out that the bomb would be "a way to end the war before the Red Army" - America's ally - "entered Manchuria," tipping the postwar balance of power. Numerically, the number of people killed in the American firebombing of Tokyo was comparable to the number who died in the later nuclear attacks, and both were dwarfed by the toll of Chinese civilians raped or killed by the Japanese at Nanking, an atrocity that some have compared to the Holocaust. Nevertheless, one can't help feeling that unleashing nuclear weapons for the first time was a different thing, a breaking of a kind of taboo. Many physicists, even those born after 1945, feel a sense of collective guilt that their art led to an era in which nuclear weapons endanger our whole biosphere. That era did not end along with the Cold War, and only time will tell whether nuclear weapons will be used again - in Pakistan and India, or somewhere else.

Last Update: 2009-06-21