Everything Flows [Editor's Comments]

Can you step twice in the same river? "Surely not!" would answer Heraclitus of Ephesus, the famous Ionian philosopher. If everything in nature flows, however, why should you trust your metamaterial to always stay the same? You shouldn't, indeed, as we learn in this issue of the magazine! Prof. Andrea Alu, in fact, is our guest editor for an incredibly interesting and informative special issue on time-varying metamaterials. I am very grateful to Prof. Alu for his successful efforts in collecting a great roster of astonishingly high-quality articles from top experts of the discipline, providing an impactful overview of the topic. Enjoy the reading!