Behind the blackout curtains, physicists got their work orders. A few, horrified by what was planned, refused the summons. But most went to work, knowing that discovery could not be stopped, that the U.S. and its scientific allies must make it first. Many hoped that they would fail and that their failure would prove forever irrevocable.
Last week the War Department told the story of their success. Professor H. D. Smyth, chairman of Princeton's physics department, who wrote the report, could not tell it all. But what he could tell, even in the prim language of the scientific laboratory, made the most fantastic and meaningful story to come out of the war.
Partnership Formed. The U.S. entered the atom race in the fall of 1939 when Franklin Roosevelt appointed an informal "Advisory Committee on Uranium." It was a small project until the Nazi panzers roared over France. Then the world was struck by a terrible urgency. On Oct. 11, 1941, nearly two months before Pearl Harbor, President Roosevelt wrote to Winston Churchill, offering British nuclear physicists a plan to work in the U.S. Churchill accepted. The U.S. and Britain were partners.
All through the perilous spring of 1942, the scientists worked. In numerous guarded laboratories, their strange apparatus glowed and hummed. By June they had made progress. The program mushroomed, was transferred to the War Department.
"The Manhattan Engineer District" was the purposely deceptive name given the project. Its centers were full of G-men. Its couriers were Army officers, brief cases chained to their wrists. It rated highest priorities for men and materials. From dozens of universities and industrial plants physicists, chemists and mathematicians vanished into thin air; the Manhattan District had snatched them.
Explosive Calculations. Before the war it was discovered that slow-moving neutrons could split the atoms of the uranium isotope, U-235, giving a mighty gush of energy. Besides energy, their "fission" produced more flying neutrons. If enough of these in turn split uranium atoms, the reaction would maintain itself, gain momentum. It would flash through all the uranium, like the flame of a match through excelsior.
This "chain reaction," which the Manhattan District now had to develop, did not happen naturally, chiefly because only one part in 140 of ordinary uranium is U-235. Most of the rest is another isotope, U-238—which, instead of splitting like U-235, absorbs the newborn neutrons with the result that the atomic flame goes out like a match in wet excelsior.
Obviously, the remedy was to separate the active U-235 from natural uranium, getting rid of the U-238. It was simple in principle, like drying the water content out of a sodden fuel. But the physicists shuddered when they finished their calculations. No chain reaction, they found, could take place in a small bit of U-235, but a large enough chunk would surely explode.
The problem, once they had the big chunk, might be to keep it from exploding whenever it was struck by any wandering neutron. The explosion, they calculated, would certainly be more violent than anything yet seen on earth.