The rule of cause and effect has a physical charge that can be flipped inside engineered materials.
Causality is usually treated as a simple yes-or-no fact of the universe where time only moves forward. This experiment proves that causality is actually a topological property that can be measured and manipulated in specific electronic systems. When a device hits a specific exceptional point, this causal charge jumps to a new value, altering how signals travel through the material. This discovery shifts causality from a philosophical constant to a tunable engineering tool. It opens the door to creating advanced circuits that can protect or reroute information in ways that were previously thought to be physically impossible.
Topological Charge of Causality at a PT-Symmetric Exceptional Point
arXiv · 2605.00117
Causality in linear response is conventionally treated as a binary property: a response function is either analytic in the upper half-plane or it is not. We show that in a PT-symmetric open dimer it instead carries a topological charge. As the gain-loss parameter crosses the exceptional point, a single pole of the reflection coefficient migrates into the upper half-plane, the Blaschke winding number jumps from 0 to 1, and standard Kramers-Kronig (KK) reconstruction acquires a Lorentzian residual