A surprising result in quantum mechanics is that a vacuum is not empty. Particles can appear out of nothing for very short periods of time. This phenomenon can be understood as a consequence of the energy–time uncertainty principle, whereby restriction of a measurement to an extremely short time interval leads to large fluctuations in energy in the interval. Although indirect effects of these ‘virtual’ particles are well studied, it is only by probing a vacuum on very short timescales that the particles become ‘real’ and can be directly observed1. But do these particles appear completely randomly, or are they correlated in space and time? Writing in Nature, Benea-Chelmus et al.2 provide an answer to this question by finding evidence for correlations between fluctuations in the electric field of a vacuum.To read more, click here.