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). They were predicted back in 1929 and first observed only in 2011. Until now, they were studied in space, as a wave trapped in a cage. Now scientists have done the same, but in time.
Imagine a track of conductors along which a wave runs — if you rapidly change the resistance of this track, the wave does not scatter but gathers into a stable blob: a bright peak in the center, with decaying oscillations at the edges. Such a blob is a temporal bound state. Curiously, it turned out to be asymmetric, although the driving influence was symmetric. It's as if you shook a swing equally to the left and right, and it suddenly froze, tilted to one side only.
This experiment is a first step into a new field. Previously, it was thought that time flows uniformly and does not affect waves. Now it is clear: time can be played with, just like space. Perhaps in the future, optical computers will emerge where light is delayed in time rather than in space. Or lasers that emit strictly defined pulses without losses.
A tuning fork that never fades is no longer a fantasy. Scientists have caught a wave in time and hold it until they themselves let go.
Bound states in the continuum are like sound in a perfect resonator: energy does not escape, even though the exits are open. In nature, this is rare, but engineers have learned to create them artificially.
In the series 'Doctor Who', the TARDIS is locked in time and space — temporal BICs resemble this trick, but for waves.