Low hanging fruit can be fairly precisely defined. Human brains have finite memory and finite processing power. Relative to that, every year, a new scientist ha

Low hanging fruit can be fairly precisely defined. Human brains have finite memory and finite processing power. Relative to that, every year, a new scientist has to learn more about what others have done before him, before he can create something new. So, it becomes harder and harder every year to make a super contribution. What Einstein had to study before superseding Newton could probably be put into a 10-20K page book. What a scientist has to study before superseding quantum is probably closer to a 50-100K page book.