Even most scientists in the field had no idea how to build a nuclear bomb until the Hiroshima headlines.

Judged as of 2018-02-28 against the historical record of the Manhattan Project era (1939–August 1945). True: historical sources show that most physicists and nuclear scientists working at the time (outside the classified Manhattan Project and comparable programs) did not know how to build a nuclear bomb, or doubted one could be built soon, until the Hiroshima bombing was announced in August 1945. Mostly true: the general point holds but needs a small qualification, for example that many scientists knew fission bombs were possible in principle but not how to make one in practice. Mixed: the evidence is split, for example a large share of the relevant scientific community was aware of the bomb's feasibility or of the project, so 'most had no idea' is roughly half right. Mostly false: the claim overstates a real point, for example most scientists in the field knew the basic route to a bomb (critical mass, enrichment or plutonium) from published pre-1940 work and only lacked the engineering details. False: historical evidence shows that most scientists in the field did know how a nuclear bomb could be built before Hiroshima.

Set before the research began.

1. E1 Statement by the President Announcing the Use of the A-Bomb at Hiroshima Harry S. Truman Library (official US government record of the presidential statement) · 6 August 1945

   “Before 1939, it was the accepted belief of scientists that it was theoretically possible to release atomic energy. But no one knew any practical method of doing it.”

2. E2 How German physicists reacted to the Hiroshima bomb kottke.org · 3 October 2016

   “WEIZSÄCKER: I would say that, at the rate we were going, we would not have succeeded during this war.”

3. E3 How German physicists reacted to the Hiroshima bomb kottke.org · 3 October 2016

   “when they said at one time one could make bombs, I was shattered.”

4. E4 How understanding nature made the atomic bomb inevitable Science News · 6 August 2020

   “In early 1939, physicists meeting to discuss fission concurred that a fission bomb was thinkable. “Everybody agreed that it was perfectly possible to make a nuclear explosive,” Bethe remembered.”

5. E5 Manhattan Project: Early Government Support, 1939-1942 U.S. Department of Energy, History Program (OpenNet) · 1 November 2025

   “As the news of the fission breakthrough spread from Berlin in early 1939, many physicists within the United States (and elsewhere) immediately realized the potential danger posed by atomic energy.”

6. E6 the science of war: nuclear history Scientific American (Special Online Issue no. 3, July 2002) · July 2002

   “It was certainly not statesmen or military leaders who first promoted the wartime potential of the fission process, but physicists in all these countries.”

7. E7 Transcript of Surreptitiously Taped Conversations among German Nuclear Physicists at Farm Hall (August 6-7, 1945) German History in Documents and Images (GHDI) · 3 November 2026

   “The point is that in Germany very few people believed in it.”

E4 and E5 show that after the fission news in early 1939, many physicists understood that a fission bomb was possible in principle; Bethe recalled that "everybody agreed" a nuclear explosive was possible. None of the evidence shows that most scientists outside the secret programmes knew a practical route to building one. E1 says no one knew a practical method before 1939, and E7 records that in Germany "very few people believed in it," which fits a field that saw the bomb as possible in theory but did not know how to make one. That matches the criteria's "mostly true" case: the general point holds, with the qualification that feasibility in principle was widely known.