A chemical reaction can be divided into three stages: the initial encounter of the reactants, the final emergence of the products, and everything in between. The middle stage—which involves all the making and breaking of chemical bonds, the rearrangement of atoms, and the surmounting of energy barriers—is difficult (although not impossible) to directly observe. Much of it can be reconstructed, however, through a combination of theoretical calculations and a careful look at the speeds, directions, and quantum states of the reaction products. (See, for example, Physics Today, February 2019, page 14.)

In the submicrokelvin regime, the nature of the experiments means that that product information is usually inaccessible. Researchers prepare a gas of ultracold molecules—for example, potassium–rubidium, or KRb—in an optical dipole trap, monitor the rate at which the molecules disappear, and infer that the disappearance must be due to a chemical reaction. The trap has an energy...

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