Improving Our Knowledge on the 229m Thorium Isomer: Toward a Test Bench for Time Variations of Fundamental Constants

For more than four decades, the lowest excitation in the whole landscape of atomic nuclei, the low‐lying, isomeric state of 229 Th, the so‐called thorium isomer, has challenged physicists of various disciplines. Being a solitaire with its uniquely low excitation energy of <10 eV, its predicted lifetime of a few hours results in an extremely sharp relative linewidth Δ E / E as low as 10 –20 . While until recently the indication of its existence was based only on indirect evidence, its unique properties inspired a multitude of potential applications, like the use of the thorium isomer as a nuclear frequency standard, potentially able to outperform even the best atomic clocks and a sensitivity‐enhanced access to potential temporal variations of fundamental constants. The various proposals to exploit the unique properties of 229m Th are presented herein, in particular focusing on its ability to serve as a test bench for time variations of fundamental constants like the fine structure constant.