In 1935, while studying the behavior of particles in regions of extreme gravity, Einstein and Rosen introduced what they called a "bridge": a mathematical link between two perfectly symmetrical copies of spacetime. It was not intended as a passage for travel, but as a way to maintain consistency between gravity and quantum physics. Only later did Einstein–Rosen bridges become associated with wormholes, despite having little to do with the original idea.

But in new research published in Classical and Quantum Gravity, my colleagues and I show that the original Einstein–Rosen bridge points to something far stranger—and more fundamental—than a wormhole.

The puzzle Einstein and Rosen were addressing was never about space travel, but about how quantum fields behave in curved spacetime. Interpreted this way, the Einstein–Rosen bridge acts as a mirror in spacetime: a connection between two microscopic arrows of time.

Quantum mechanics governs nature at the smallest scales such as particles, while Einstein's theory of general relativity applies to gravity and spacetime. Reconciling the two remains one of physics' deepest challenges. And excitingly, our reinterpretation may offer a path to doing this.