The scientific study of the basic constituents of matter and the forces acting upon and between them. Physicists use accelerators to accelerate charged particles in an electric field in order to have them collide at high energy, freeing subatomic particles that can be detected, identified, and studied. Accelerators may be linear (that at Stanford University, California, is 3.2 km long), or, more commonly, circular, the largest having a diameter of 27 km.