Particle accelerators are used to generate beams of particles moving at very high speeds, which are typically then collided with matter or other particles to learn about fundamental interactions. Most of the time the particles in question are subatomic particles, though atoms can also be used. Such experiments are used to address fundamental questions in physics surrounding the structure of matter and space. Typical modern particle accelerators are several kilometers long, with some operating in a linear fashion and others in a large ring.