An artist's impression of the experiment. (University of Southampton) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Acting on a tiny particle levitating in a magnetic trap, physicists have just measured the smallest gravitational pull ever recorded. The particle weighed just 0.43 grams. And the strength of the gravitational force at play was on the scale of attonewtons (10-18 newtons). That's small enough to be right on the verge of the quantum realm, teasing the possibility of finally figuring out how classical physics and quantum mechanics interact. "For a century, scientists have tried and failed to understand how gravity and quantum mechanics work together," says physicist Tim...