Correction to: Nature Physics https://doi.org/10.1038/s41567-018-0342-2 published online 15 November 2018.

In the version of this Perspective originally published, one of the authors’ names was incorrectly given as Kati Lassili-Perini; it should have been Kati Lassila-Perini. This has been corrected in all versions of the Perspective.