Mordechai Segev

A struck tuning fork fades: its vibrations leak into the air. But there are systems where the oscillation does not decay, even though there are plenty of free waves around, ready to carry energy away. Physicists call such 'captured' states bound states in the continuum (BIC).

A pebble tossed into a pond sends ripples spreading in all directions. Now imagine the pond itself suddenly turning into jelly — instantly, faster than the wave can complete a single crest. Some ripples would keep spreading but change their spacing, while others would unexpectedly turn around and run back toward the point of impact, as if time were flowing backward for them.

A surfer glides across the ocean surface until a big wave comes, and then they're flying forward. Similarly, a free electron interacts with light: usually the energy exchange is barely noticeable, but if the surrounding medium begins to oscillate rhythmically, each wave crest pushes the particle harder and harder.