A translator retells a complex novel in another language, but only if the plot stands still. Once the characters start moving, the translator loses the thread. That's roughly how the 'double copy' worked — a method linking gravity and electricity. Until recently, it could only handle static black holes, where time plays no role.
In the new work, physicists peered inside a black hole — a trap from which even light cannot escape. There, beyond the event horizon (the boundary after which there is no turning back), space and time swap roles: falling toward the center becomes as inevitable as the flow of a river. It turns out that this inner dynamics can be translated into the language of an electric field. The translation is exact and does not require knowing what happens outside the hole.
The authors tested the method on two examples. The first is the classic Schwarzschild black hole, where the density at the center is infinite. Here, the electric field double grows without bound as the collapse proceeds, like the voltage in a bursting soap bubble. The second is the regular Bardeen black hole, which has a smooth core instead of a singularity. In this case, the field remains finite and passes calmly through the center, like a river flowing into a lake.
This approach turns the black hole's interior into a laboratory for studying the connection between gravity and other forces of nature. Perhaps this is a step toward understanding how to unify them into a single theory — without the need to peer into the singularity. And if the translator used to stumble over motion, now it has learned to translate even the most turbulent plot.
as written by the authors arXiv:2604.19920
The classical double copy provides a powerful bridge between gravity and gauge theory, but its most explicit realizations remain concentrated in stationary or highly symmetric settings. We show that trapped regions of black-hole geometries furnish an exact setting for time-dependent classical double copy. In the static, spherically symmetric case, each trapped interval admits a local single-copy description on the associated Kantowski--Sachs patch that is intrinsically time dependent, although it can be derived from static Kerr--Schild data and does not require knowledge of any exterior black-hole completion. We prove that this class is characterized intrinsically by a distinguished relation between the Kantowski--Sachs scale factors, equivalently by the longitudinal relation \(p_{\parallel}=-\rho\), and that the Kerr--Schild scalar and single-copy field are uniquely reconstructible from interior cosmological data. Schwarzschild provides the singular benchmark, for which the single-copy electric field diverges along the interior evolution, while the regular Bardeen solution yields a finite single-copy field throughout the trapped region and a smooth extension into a regular static core. The Bardeen core violates the strong energy condition in a compact region, whereas the corresponding single-copy Maxwell field remains regular and satisfies the standard classical energy conditions. We further show that the Bardeen horizon phase structure is encoded in the single-copy scalar. These results identify trapped Kerr--Schild interiors as an exact local laboratory for time-dependent classical double copy.
Inside a black hole, time flows toward the center: for an infalling observer, the past is the direction 'outward,' and the future is 'inward.'