An optical tweezer is a beam of light that holds and moves tiny objects like bacteria or dust grains. If you need to carry a particle from point A to point B, you could simply pull it straight there. But that would consume more energy than a cleverly chosen detour.
The physicists built on a method that uses mathematical curves to describe the path of the beam. Then they ran a computer search over possible routes to find the one that costs the least energy. Tests on simple cases confirmed that the method finds the correct solution.
The big surprise: if the particle is active, the most economical path in one direction is not the mirror image of the path back. It's as if a swimmer finds it easier to go with the current than against it. So when planning a route, you need to account not only for distance but also for the particle's "personality."
The method is named after the Swiss physicist Walter Ritz, who in 1908 proposed a similar calculation technique for a different problem, the vibrations of a string.