Can we turn back time? Ask a savvy physicist, and the answer will be “it depends.”
Schemes for retrograde time travel abound but usually involve irreconcilable paradoxes and rely on outlandish theoretical constructs such as wormholes (which may not actually exist). Yet when it comes to simply turning back the clock—akin to stirring a scrambled raw egg and seeing the yolk and white reseparate—a rich and growing subfield of wave physics shows that such “time reversal” is possible.
Reversing time would seem to fundamentally clash with one of the most sacred tenets of physics, the second law of thermodynamics, which essentially states that disorder—more specifically “entropy”—is always increasing, as humbly demonstrated in the incessant work needed to keep things tidy. This inexorable slide toward mess and decay is what tends to make unscrambling eggs impossibly difficult—and what propels time’s arrow on a one-way trip through our day-to-day experiences. And although so far there’s no way to unscramble an egg, in certain carefully controlled scenarios within relatively simple systems, researchers have managed to turn back time.
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.”
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