| HN Mirror

Because there are multiple paths and the virus really spreads like a wave frontier in a 10 dimensional space of human to human contacts graph. The virus also spreads in a non uniform way: it's not about the distance between two interacted persons, but about the nature of their interaction, whether they weared masks and so on. The virus also really likes to stick to surfaces, like door handles or plastic wraps, and this vector of transmission is very difficult to trace even manually. Think of credit cards. The virus floats in the air like smoke if someone coughed and others may catch it this way. An app can't account for that and instead builds a social graph of interactions. The app would notice a lot of people crowded in a parking lot and would assume the virus was transmitted between those 50 people, but it wouldn't know that all those people sit in their cars, so the app just made the transmission chain 50x less useless. A few more such gatherings and the relevance of tracing drops to those sub quantum levels of homeopathic medicine.