At the center of attention is a black hole with two points of no return: the outer and inner horizons. Beyond the outer horizon, space and time swap roles, but the journey is not over. Further in, at the inner horizon, gravity has another surprise in store.
The authors peered into this world using the Wheeler–DeWitt equation — the master equation of quantum gravity. It describes the probability wave of the entire universe, but here it was applied to a miniature model of a black hole. Inside, they found a tachyonic region — a zone where the potential becomes negative, and the wave can behave unstably. The scientists built a numerical method to carefully guide the wave through this finicky spot.
The result was surprising. When quantum effects are weak, the wave packet obediently slides along the classical trajectory, barely spreading. But if you strengthen the parameter κ, which controls quantum corrections, the wave starts to smear out significantly along the way. Yet right at the inner horizon, it suddenly gathers back together, like a beam of light passing through a lens. The authors dubbed this effect refocusing.
The tachyonic region in the equation is named after hypothetical particles, tachyons, that always move faster than light. In quantum gravity, it signals instability, not actual superluminal speeds.
In Dan Simmons' novel 'Hyperion', the Time Tombs are surrounded by anti-entropic fields where past and future are tangled — somewhat like the tachyonic zone inside a black hole.