All of the people using fitbits or other health monitors: congrats, your unique heart rhythm is now in a database. One that can be used to track or target you.
Seriously? That’s how this works?
All of the people using fitbits
IIRC none of the laser type monitors have the resolution for HRV.
All living things generate an electric/magnetic field. Some animals, like sharks, even have sensory organs to detect these fields. It would stand to reason that each magnetic field would be somewhat unique for each individual. Although these fields are probably too weak to be identified from any distance, suppose you were to scan a crowd with a microwave device. Each individual field would resonate with the microwave beam, producing different frequencies in return, if your equipment were sensitive enough. I have often wondered if such a method would not be useful in picking out or tracking individuals.