If you let heavy cream go sour you get smetana. Sour cream is made with a heat treatment and the addition of Lactococcus cremoris. Almost all American sour cream is actually more like smetana rather than proper sour cream. Proper sour cream is runnier, more like kefir, with a higher fat content and more sour than the spreadable and milder American product. Crème fraîche and Mexican cream are similar to smetana. I fully endorse culinary lab experimentation.