How low can the US unemployment rate go?
https://www.nytimes.com/2018/02/28/upshot/how-low-can-unemployment-really-go-economists-have-no-idea.html
The Economist on the natural rate of unemployment:
https://www.economist.com/news/economics-brief/21727050-policymakers-have-spent-half-century-search-natural-rate-unemployment