Completely agree about the headline and lead, but if you slog through it, the article did an ok job explaining the concept:
> The trick is to create a certain kind of reflection. First, imagine a regular spatial reflection, like one you see in a silver-backed glass mirror. Here reflection occurs because for a ray of light, silver is a very different transmission medium than air; the sudden change in optical properties causes the light to bounce back, like a Ping-Pong ball hitting a wall. Now imagine that instead of changing at particular points in space, the optical properties all along the ray’s path change sharply at a specific moment in time. Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it. This is a “time reflection.”
> To do so, physicist Andrea Alù and his colleagues devised a “metamaterial” with adjustable optical properties that they could tweak within fractions of a nanosecond to halve or double how quickly light passes through.
So you have some volume of material and wave of light is passing through it. You alter the material everywhere in that volume so that the light does a 180 at every point along the wave at the same instant, reversing the stream of light. Any signal encoded in the light comes back in the opposite order you put it in, like you played a recording backward, or popped a stack until it was empty.
You’re a more generous reader than me if you think this is illuminating:
> Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it.
Because to my mind, a ping pong ball returning to the player who last hit it seems like something I would describe as ‘recoiling in space’, not ‘recoiling in time’.
Returning stuff back the way it came is actually a thing people associate with ordinary mirrors - so if you’re trying to tell me how a time mirror differs from a normal mirror this ping pong ball metaphor is not exactly helpful.
Yes, but typically the different optical properties are strictly localized in space. Air here and water there. In this paper it's basically like you have a pool full of water with a light beam traversing halfway to the ground and then you instantly turn the air in the pool into water and the light goes, "oh hell no" and backs out of there
> The trick is to create a certain kind of reflection. First, imagine a regular spatial reflection, like one you see in a silver-backed glass mirror. Here reflection occurs because for a ray of light, silver is a very different transmission medium than air; the sudden change in optical properties causes the light to bounce back, like a Ping-Pong ball hitting a wall. Now imagine that instead of changing at particular points in space, the optical properties all along the ray’s path change sharply at a specific moment in time. Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it. This is a “time reflection.”
> To do so, physicist Andrea Alù and his colleagues devised a “metamaterial” with adjustable optical properties that they could tweak within fractions of a nanosecond to halve or double how quickly light passes through.
So you have some volume of material and wave of light is passing through it. You alter the material everywhere in that volume so that the light does a 180 at every point along the wave at the same instant, reversing the stream of light. Any signal encoded in the light comes back in the opposite order you put it in, like you played a recording backward, or popped a stack until it was empty.