Question
How does Jewish law develop?
Answer
A Jewish law (Halacha) develops as decisions are made about questions that hadn’t previously been raised. This is generally a process of addition rather than change; Rabbis can’t overturn decisions made by earlier Rabbis, especially if these decisions have been widely accepted. (Customs, which weren’t the result of Rabbinic decrees, do sometimes change.)
The decrees issued by a Rabbinical court are effective only for those who accept the authority of that court. Some countries (e.g., Israel) have official Rabbinical structures, but others (e.g., the U.S.) don’t. In either case, Rabbinical courts don’t always have effective ways of enforcing their decisions.