Every wireless system rests on a model of how signals propagate through a physical space. Prof. Xinyu Zhang's group builds that model as a living replica. Their RF Digital Twin (RFDT) (MobiCom'26) enables fully differentiable RF ray tracing for the first time, maintaining a two-way link between a real environment and its digital copy: measured signals continuously calibrate the twin, and the calibrated twin is then optimized to design better wireless systems. Unlike conventional ray tracers or black-box neural models, RFDT stays physically grounded while remaining fully learnable, running faster than NVIDIA's Sionna and producing accurate gradients that match full-wave ground truth.