A summit could bolster Xi Jinping’s stature. But talks about Ukraine’s future and limits on nuclear weapons pose uncomfortable choices for China.

© Haiyun Jiang/The New York Times

A summit could bolster Xi Jinping’s stature. But talks about Ukraine’s future and limits on nuclear weapons pose uncomfortable choices for China.

© Haiyun Jiang/The New York Times

As the popularity of the genre has surged, a dispute between a band and its label has put a spotlight on restrictive artist contracts.

© Lam Yik Fei for The New York Times

Iran’s willingness to re-engage on the issue suggests that Qatar-led diplomatic efforts to avert full-blown war between the U.S. and Iran have had some success.

© Morteza Nikoubazl/NurPhoto, via Getty Images

Iran’s willingness to re-engage on the issue suggests that Qatar-led diplomatic efforts to avert full-blown war between the U.S. and Iran have had some success.

© Morteza Nikoubazl/NurPhoto, via Getty Images

During World War II, physicists on the Manhattan Project worried that the Nazi German nuclear program might achieve the first nuclear bomb before the US and its allies. That's not what transpired, but historians have debated just how close the Germans came to achieving criticality in their last attempt at building a reactor: the B8 pile. Werner Heisenberg, who led that effort, claimed that they were close and had only needed a little more uranium. But according to a paper published in the Proceedings of the National Academy of Sciences Nexus, that was not the case: The project simply didn't have sufficient uranium and heavy water to build a working bomb, given the design they were using.

This particular research started in 2013, when University of Maryland physicist Timothy Koeth—a co-author on the new paper—received the gift of a mysterious heavy metal cube from a friend. He instantly recognized it as one of the uranium cubes used by German scientists in their unsuccessful attempt to build a working nuclear reactor. Only 14 such cubes are known to exist today.

In 2019, we reported on Koeth’s quest to track down the cube’s origins, as well as several other similar cubes that had somehow found their way across the Atlantic. At the time, Koeth and his partner in the quest, then-graduate student Miriam “Mimi” Hiebert, had tracked down 10 such cubes around the US. For instance, the Smithsonian Institute had a German uranium cube in storage. They tracked a third cube to Harvard University, where it was regularly passed around to students in introductory physics classes as a curiosity.

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© ohn T. Consoli, University of Maryland College Park

Some Iranian clerics and officials argue the country must rebuild deterrence by dropping its longstanding pledge not to build a nuclear bomb.

© Atta Kenare/Agence France-Presse — Getty Images

Some Iranian clerics and officials argue the country must rebuild deterrence by dropping its longstanding pledge not to build a nuclear bomb.

© Atta Kenare/Agence France-Presse — Getty Images

Some Iranian clerics and officials argue the country must rebuild deterrence by dropping its longstanding pledge not to build a nuclear bomb.

© Atta Kenare/Agence France-Presse — Getty Images

In his letter, the vice president suggested an immediate resumption of comprehensive talks, including about Iran’s nuclear program.

© Tierney L. Cross for The New York Times