Relativistic particle

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A relativistic particle is a particle which moves with a relativistic speed; that is, a speed comparable to the speed of light. This is achieved by photons and by tachyons to the extent that effects described by special relativity are able to describe those of such particles themselves. Several approaches exist as a means of describing the motion of single and multiple relativistic particles, with a prominent example being postulations through Dirac equations of single particle motion.

Massive particles are relativistic when their kinetic energy is comparable to or greater than the energy mc^2 corresponding to their rest mass. (This condition implies that their speed is close to the speed of light.) Such relativistic particles are generated in particle accelerators, and are naturally occurring in cosmic radiation. In astrophysics, jets of relativistic plasma are produced by the centers of active galaxies and quasars.

A charged relativistic particle crossing the interface of two media with different dielectric constants emits transition radiation. This is exploited in the transition radiation detectors of high-velocity particles.

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