Wilczek and Gross used particle accelerators to study quarks and the force that acts on them. The two scientists—and Politzer working independently—observed that quarks were so tightly bound together that they could not be separated as individual particles but that the closer quarks approached one another, the weaker the strong force became. When quarks were brought very close together, the force was so weak that the quarks acted almost as if they were free particles not bound together by any force. When the distance between two quarks increased, however, the force became greater—an effect analogous to the stretching of a rubber band. The discovery of this phenomenon, known as asymptotic freedom, led to a completely new physical theory, quantum chromodynamics (QCD), to describe the strong force. QCD put the finishing touches on the standard model of particle physics, which describes the fundamental particles in nature and how they interact with one another.