Optional, Anonymous and Effective? Choose Two. At Most.
The media keeps hyping contact-tracing app as the miracle solution to the pandemic. They keep telling us how it will be optional and anonymous, when it really can be neither.
If it existed, the app would become a prerequisite to living normally again. You would like to dine in this restaurant? Show me your app. That would hardly be optional, else I could conveniently uninstall the app once it shows a potential exposure.
It can not be anonymous, because as the pandemic winds down, you really want to follow up on those contacts that were potentially exposed but that did not get tested. Once there are numbers like "x people had their status turn red but did not show up for testing" there will be enormous public pressure to de-anonymize those walking bio-terrorists.
At present, the app also just can't be effective. Living in a big city, just going out for a walk and shopping, we cross paths with hundreds of people, and with the current number of cases (symptomatic or not), there will be enormous numbers of false positives (i.e., you crossed path with a person that was later tested positive, but did not get infected) and false negatives (i.e., you crossed path with a Cov-positive person that was never tested because he was not symptomatic, or did not have a mobile phone with the app running at a time, but still got infected).