Going Interstellar (collection)

But in the publicity and excitement accompanying Einstein’s meteoric rise, a seemingly minor aspect of special relativity was generally ignored by non-physicists. From the imaginary vantage point of his light-speed trolley car, Einstein considered the total energy of a stationary object on Earth’s surface. Since the object was not moving, it had no kinetic energy (or energy of motion). Since it was at the same level as the Earth-surface reference frame, it had no potential energy (or energy of position). But it did posses “rest energy.” The quantity of rest energy is dependent upon the speed of light in vacuum (c) and the object’s mass (m). Rest energy is defined in that awesome expression:

Rest Energy = mc²

Appearing in a footnote in one of Einstein’s special relativity papers, this definition of rest energy indicated that mass could be converted into energy and energy could be converted into mass. Physicists could no longer talk about the conservation of mass or the conservation of energy, but nature would now conserve “mass-energy.”

Specialists in the 1920s began to utilize mass-energy conversion and conservation in their research. Physical chemists such as Marie and Pierre Curie had pondered the question of how decay particles in radioactive processes obtained their energy. The obvious answer was that a small fraction of the mass of the decaying nucleus was converted into a particle’s kinetic energy.

Astrophysicists such as Sir Arthur Eddington had wondered how the Sun and other stars could maintain stability for the immense durations required by the fossil record. Once again, the answer required mass-energy conversion in the stellar interior.

But could humans ever tame this process or derive benefit from it? The answer came as war clouds were gathering once again in Europe. Fortunately for all of us, the censors in Nazi Germany were not well trained in nuclear physics or appreciative of its potential. As the Second World War approached, a group of German physicists solved the problem of tapping nuclear fission energy—and published their results in the open literature!

In 1938, it was known that one particular isotope of uranium—Uranium 235—was radioactive. When it decays by nuclear fission (splitting), this massive nucleus splits spontaneously into several less massive (daughter) nuclei and fast-moving (thermal) neutrons. It was also known that the fission of this nucleus could be induced by bombarding it with thermal neutrons. In their epochal paper, Otto Hahn, Lisa Meitner and Fritz Strassmann calculated the density of uranium required to trap emitted neutrons within the U-235 sample. The rapid reaction of uranium in the sample would produce enormous energy. It became known as the chain reaction.

Few realized it at the time, but this simple calculation would provide the basis of both the atomic bomb and the fission reactor. One who recognized the potential immediately was our old friend Albert Einstein.

If we could go back in time a few decades to observe any historical event, one choice might be Einstein in his office at the Institute of Advanced Studies opening the German physics journal containing the epochal paper. Perhaps he was wearing his baggy sweater and smoking his pipe as he opened the journal and read the paper. Perhaps he did a few calculations to check the result.

Einstein knew what the Nazis planned. He had been fortunate to escape Europe and had worked to save family members and colleagues. As a non-native English speaker with a good knowledge of German and Yiddish, he may first have dropped the pencil on his desk and removed his glasses. Then he may have muttered “Oy Mein Gott,” as the terrible reality sank in.

An ordinary mortal may have visited a Princeton pub and drunk himself into oblivion. But Einstein was far from ordinary. He crafted a letter describing his concerns and posted it to President Roosevelt.

If one of us writes a concerned letter to the President of the United States (or any other world leader) we might expect a response from a low-level intern. But Roosevelt realized that Einstein was no ordinary mortal. And he knew that war clouds were thickening. He responded by convening a conclave of the best American nuclear experts to check the validity of Einstein’s concerns and the German team’s calculations. The Manhattan Project, which would result in the atomic bombs dropped on Japan in the final days of World War II, had started!

Even Einstein was amazed (and saddened) by the power of his mass-energy footnote. When he was interviewed after the Hiroshima bombing, he implied that perhaps he should have been a plumber!

After the war, nuclear experts in both the US and USSR realized that the atomic bomb—which works by the fission, or splitting, of heavy atomic nuclei—was not the final answer to humanity’s destructive quest. Work would be devoted to the more powerful thermonuclear bomb—which operates by fusing or combining light atomic nuclei in a manner analogous to the Sun.