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Relativistic particle

## Summary

A relativistic particle is a particle which moves with a relativistic speed; that is, a speed comparable to the speed of light.[1] This is achieved by photons 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 ${\displaystyle mc^{2}}$ corresponding to their rest mass. In other words, a massive particle is relativistic when its total mass-energy (rest mass + kinetic energy) is at least twice its rest mass. This condition implies that the particle's speed is close to the speed of light. According to the Lorentz factor formula, this requires the particle to move at roughly 85% of the speed of light. Such relativistic particles are generated in particle accelerators,[2] as well as naturally occurring in cosmic radiation.[3] 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.

## Notes

1. ^ Gibbons, Gary William. "Relativstic mechaniscs". Encyclopaedia Britannica. Retrieved June 6, 2021.
2. ^ For example, at the Large Hadron Collider operating with a collision energy of 13 TeV, a relativistic proton has a mass-energy 6,927 times greater than its rest mass and travels at 99.999998958160351322% of the speed of light.
3. ^ An example of this is the Oh-My-God particle.