An emergency arrest device for a zip line is disclosed. The device uses a rope wound around the zip line, e.g., in a double helix, as the “core reactor” that provides stopping force. The device receives the ends of the wound rope and controls the position of the rope and tension within the rope to create and modulate the stopping force. Specifically, the device may comprise a front plate, a rear plate assembly, connecting members connecting the front plate and the rear plate assembly, and a movable carriage. The rear plate assembly includes a first receiving structure adapted to receive and secure one end of the rope. The moveable carriage is mounted within the emergency arrest device between the front plate and the rear plate assembly and carries a second receiving structure adapted to receive and secure the other end of the rope.