The quest to unite gravity with the other three forces has long plagued physicists. Whether we eventually devise a testable "unified" theory remains to be seen. (Image credit: koto_feja via Getty Images) When theoretical physicist David Gross was 13, he received a copy of a popular science book, "The Evolution of Physics" (Cambridge University Press, 1938), signed by Albert Einstein. The book, co-authored by Einstein himself, started Gross on a journey into the hearts of atoms, where he eventually helped answer a question that had bedeviled particle physicists for years: whether the constituent parts of protons and neutrons, called quarks, could be broken apart. The resulting principle of asymptotic freedom, which he developed in concert with Frank Wilczek and H. David Politzer, revealed that the forces between quarks waned as they got close to each other and strengthened as they moved apart.…