The answer to life, the Universe and everything, according to Douglas Adams in The Hitchhiker’s Guide to the Galaxy, is 42. He was out by a factor of two: the solution to the Universe is 21. Or to be more precise, 21 centimetres, which is the wavelength of radiation emitted when a hydrogen atom shifts from one specific energy state to another. That’s why the plaque attached to NASA’s 1972 Pioneer 10 spacecraft pictures a woman whose height is shown as the binary representation of 8. (Eight of those 21 centimetres making 1.68 metres.)
In the twentieth century, a Who’s Who of physicists used hydrogen to predict and examine subatomic interactions. A paper this week describes a major advance in this effort — one that takes it into the mysterious realm of antimatter. It shows that antihydrogen, hydrogen’s antimatter counterpart, also produces the telltale 21-cm hydrogen emission line (M. Ahmadi et al. Nature 548, 66–69; 2017).
This latest experiment could answer some fundamental questions. One way to test for cracks in the standard model of physics — the rules that help to bind the Universe together — is to seek and find discrepancies in how matter and its antimatter counterparts behave. Decades of careful analysis of hydrogen atoms offer a benchmark that can now be tested against corresponding measurements of antihydrogen. Any divergence in the results could open a door to new physics: an answer to the Universe that somebody, somewhere, perhaps already knows.