A ball tossed into the air follows a path that classical physics can track with confidence. Shrink that ball down to the size of an atom, though, and the rules usually change. At that scale, particles can seem to pass through two openings at once. They can also tunnel through barriers and act more like waves than tiny objects. Now two MIT researchers say there is a tighter mathematical connection between those two worlds than physicists once thought. In a new paper, the team reports that a familiar idea from classical physics, known as least action, can be extended to reproduce the same answers as the Schrödinger equation for several standard quantum cases. Their method, they say, can handle examples such as the double-slit experiment and quantum tunneling. It does so while relying on a framework built from classical action and density. “Before, there was a very tenuous bridge that worked only for reasonably large [quantum] particles,” says study co-author Winfried Lohmiller, a research associate in the Nonlinear Systems Laboratory at MIT. “Now we have …