Hawking radiation in a condensate of rubidium atoms

A sonic analogy for Hawking radiation was introduced nearly 40 years ago, motivated in part by the “trans-Planckian puzzle”. This has shed light on the puzzle (which, however, remains enigmatic), and it suggested that analog Hawking radiation could one day be observed in a laboratory. That day has come. In two recent papers, observations of Hawking radiation in rubidium condensates have been reported. The first attributed the observed features to the "black hole laser" effect, while the second reported measurements of the quantum entanglement of Hawking phonons with their partners. I'll explain this circle of ideas, describe the experiments, and report on theoretical analyses showing that, in fact, the laser effect was most likely not behind the observations, and that more work is needed to determine whether the measurements actually demonstrated entanglement.