Bohr solves mystery; invents bomb

Niels Bohr was frustrated—just a month earlier, his student Otto Frisch had been part of the dramatic discovery of uranium nucleus splitting, yet new experimental results left the situation more confused than ever.

At a boring faculty club dinner at Princeton, physicist George Placzek challenged Bohr’s understanding of fission results, claiming Bohr’s liquid drop model offered no possible explanation for uranium and thorium’s differential behavior.

Frisch’s January 1939 publication showed experimentally that uranium could be split by both high-speed and slow neutrons, while thorium (uranium’s periodic table neighbor) would only split under high-speed neutron bombardment.

As Placzek continued explaining the uranium-thorium puzzle, a fragment of solution came to Bohr—at risk of losing it, he stood up and quickly left the faculty club dinner without saying a word, trudging through snow in silence to Einstein’s office.

Bohr quickly drew two identical curves on Einstein’s blackboard, breaking chalk in his urgency—these curves represented nuclear cross-sections showing reaction probability as a function of incoming neutron energy.

Bohr labeled the first graph “thorium” but didn’t label the second “uranium”—instead he wrote “U-238,” then turned to Rosenfeld and said “now listen, I have it all”—realizing uranium-238 and uranium-235 would behave differently under neutron bombardment.

Four years earlier (1935), mass spectrometry showed that naturally occurring uranium wasn’t just one thing—it was primarily composed of two isotopes: uranium-238 and uranium-235, while thorium was essentially all one isotope.

According to Bohr’s liquid drop model, when absorbing an incoming neutron, U-238 ends up with an odd number of neutrons (239 total) while U-235 ends up with an even number (236 total)—creating crucially different binding energy configurations.

Bohr drew a third curve predicting how U-235 would behave under neutron bombardment—this time the whole curve represented only one outcome: splitting, even with zero-velocity neutrons.

Bohr wrote up his results and submitted to Physical Review—what he didn’t realize at the time was that he had found the perfect material to build the atomic bomb.