Continuity equation for the flow of Fisher information in wave scattering

Using waves to explore our environment is a widely used paradigm, ranging from seismology to radar technology, and from biomedical imaging to precision measurements. In all these fields, the central aim is to gather as much information as possible about an object of interest by sending a probing wave at it and processing the information delivered back to a detector. Here we demonstrate that an electromagnetic wave scattered at an object carries locally defined and conserved information about all of the object’s constitutive parameters. Specifically, we introduce the density and flux of Fisher information for general types of wave fields and identify the corresponding sources and sinks of information through which all these new quantities satisfy a fundamental continuity equation. We experimentally verify our theoretical predictions by studying a movable object embedded in a disordered environment and by measuring the corresponding Fisher information flux at microwave frequencies. Our results improve the understanding of the generation and propagation of information and open up possibilities for tracking and designing the flow of information even in complex environments.