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.
Inside a black hole, time flows toward the center: for an infalling observer, the past is the direction 'outward,' and the future is 'inward.'