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in physics, a disturbance, in a medium, that behaves as a particle and that may conveniently be regarded as one. A rudimentary analogy is that of a bubble in a glass of beer: the bubble is not really an independent object but a phenomenon, the displacement of a volume of beer by carbon dioxide gas, but, because of the characteristics of the surface of liquid in contact with the gas, the bubble retains a certain identity as it rises and floats. It, like a quasiparticle, carries properties characteristic of objects, such as size, shape, energy, and momentum. Two bubbles can bounce off each other; quasiparticles, too, undergo collisions. Some specific quasiparticles are the exciton, phonon, magnon, and polaron (qq.v.).