Nuclear Energy

What the Equation Explains

When a heavy nucleus such as uranium splits, the pieces together weigh slightly less than the original. The missing mass has become energy, in exactly the amount given by mass–energy equivalence. Because the conversion factor is the speed of light squared, a small loss of mass releases an enormous amount of energy: splitting the atoms in a kilogram of uranium yields millions of times more than burning a kilogram of coal.

When nuclear fission was discovered in Germany at the end of 1938, it was this equation that Lise Meitner and Otto Frisch used to work out how much energy each splitting nucleus releases.

What the Equation Does Not Do

It is an accounting rule. It tells you how much energy is available if a reaction happens. It says nothing about how to make one happen, or how to make it sustain itself. That required the discovery of the neutron in 1932, of fission in 1938, and of the chain reaction, all by other scientists. In 1905, and for three decades afterward, Einstein himself saw no practical way of releasing the energy.

The Letter

In 1939 the physicist Leo Szilard, who had conceived the idea of a nuclear chain reaction, feared that Germany would build a weapon. He needed a name the President would read. He visited Einstein, explained the physics, and drafted a letter. Einstein signed it on 2 August 1939. It warned that uranium could be made into extremely powerful bombs, and urged the government to support research. The economist Alexander Sachs delivered it to Franklin Roosevelt on 11 October.

What Einstein Did Not Do

He took almost no part in what followed. US Army Intelligence refused him a security clearance in July 1940, regarding his pacifism and political associations as a risk. He never visited Los Alamos and was kept out of the project. His only technical contribution was two days' work on an isotope-separation problem that Vannevar Bush sent him, without being told how it fitted into the wider effort.

He later said that had he known Germany would not succeed in building a bomb, he would never have lifted a finger. He spent his last decade campaigning for the control of nuclear weapons. See Convictions.

Nuclear Power and Medicine

The same physics runs the world's nuclear power stations. About 440 reactors supply roughly 9 percent of the world's electricity, with almost no carbon emissions in operation.

It also underlies nuclear medicine. Radioactive isotopes, most of them made in reactors, are used in more than 50 million procedures every year, about nine in ten of them diagnostic. See Medical Imaging.

How Direct Is the Link?

Einstein supplied the principle and, once, his signature. The equation is indispensable for understanding nuclear energy and was never a recipe for it. The popular image of him as the father of the bomb is wrong on the physics and wrong on the history.