A particle accelerator is a device that uses electromagnetic fields to accelerate charged particles to high speeds and contain them in well-defined beams. They can be used for purposes like radiotherapy, ion implantation, and industrial and biomedical research. The largest particle accelerators in the world are the RHIC, the LHC at CERN, and the Tevatron, which are used for experimental particle physics research. Particle accelerators can be divided into low-energy machines like cathode ray tubes and X-ray generators, and high-energy machines capable of nuclear reactions like the LHC, which smashes particles together at high speeds to study the origins of the universe.