Sybil attacks exploit cheap digital identities. Physical Proof of Proximity (PoPI) introduces real-world constraints to counter this. Instead of relying on cryptography alone, PoPI uses physical signals like radio strength and time-of-flight to verify presence.

Unlike Proof of Work or Proof of Stake, PoPI makes attacks costly in physical terms. Attackers must deploy actual hardware nearby rather than spinning up cloud nodes. This shifts the challenge from computation to logistics and spatial coordination.

PoPI isn’t suited for global blockchains but works well in local mesh networks, IoT clusters, and offline systems. Challenges remain around scalability, calibration, and hardware variance. It’s a research-driven approach, not a ready-made solution.

Experts see potential in niche applications where physical presence is already assumed. Nation-state threats and device theft remain outside its threat model. Implementation requires clear trade-off recognition.