Optically trapped nanospheres in high-vaccum experience little friction and\nhence are promising for ultra-sensitive force detection. Here we demonstrate\nmeasurement times exceeding $10^5$ seconds and zeptonewton force sensitivity\nwith laser-cooled silica nanospheres trapped in an optical lattice. The\nsensitivity achieved exceeds that of conventional room-temperature solid-state\nforce sensors, and enables a variety of applications including electric field\nsensing, inertial sensing, and gravimetry. The optical…