Massless particle

Massless particle

In particle physics, a massless particle is a particle whose invariant mass is zero. Currently, the only known massless particles are gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons.[1][2]

Neutrinos were, until recently, thought to be either massless or have a small mass; but, because neutrinos change flavour as they travel, at least two of them must have mass. (See below.)

Contents

Special relativity

The behavior of massless particles is understood by virtue of special relativity. For example, these particles must always move at the speed of light. In this context, they are sometimes called luxons to distinguish them from bradyons and tachyons.

Dynamics

Massless particles are known to experience the same gravitational acceleration as other particles (which provides empirical evidence for the equivalence principle) because they do have relativistic mass, which is what acts as the gravity charge. Thus, perpendicular components of forces acting on massless particles simply change their direction of motion, the angle change in radians being GM/rc2 with gravitational lensing, a result predicted by general relativity. The component of force parallel to the motion still affects the particle, but by changing the frequency rather than the speed. This is because the momentum of a massless particle depends only on frequency and direction (compare with the momentum of low speed massive objects, which depends on mass, speed, and direction). Massless particles move in straight lines in spacetime, called geodesics, and gravitational lensing relies on spacetime curvature. Gluon-gluon interaction is a little different: they exert forces on each other but, because the acceleration is parallel to the line connecting them (albeit not at simultaneous moments), the acceleration will be zero unless the gluons move in a direction perpendicular to the line connecting them (so that velocity is perpendicular to acceleration).

Gravitons

Theories which postulate that gravity is quantized introduce gravitons - massless tensor bosons (i.e. it has spin 2) which mediate gravitational interaction. So far there is no experimental evidence supporting their existence.

See also

References

  1. ^ G.Valencia(1992)"Anomalous Gauge-Boson Couplings At Hadron Supercolliders"
  2. ^ Bogdan A. Dobrescu(2004)"Massless Gauge Bosons Other Than The Photon"

Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • massless — adjective Date: 1879 having no mass < a massless particle > …   New Collegiate Dictionary

  • Particle detector — In experimental and applied particle physics and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high energy particles, such as those produced by nuclear decay,… …   Wikipedia

  • massless — masslessness, n. /mas lis/, adj. Physics. pertaining to an elementary particle having zero rest mass, as a photon. [1875 80; MASS + LESS] * * * …   Universalium

  • massless — mass•less [[t]ˈmæs lɪs[/t]] adj. phs pertaining to an elementary particle, as a photon, having zero rest mass • Etymology: 1875–80 mass′less•ness, n …   From formal English to slang

  • subatomic particle — or elementary particle Any of various self contained units of matter or energy. Discovery of the electron in 1897 and of the atomic nucleus in 1911 established that the atom is actually a composite of a cloud of electrons surrounding a tiny but… …   Universalium

  • Helicity (particle physics) — In particle physics, helicity is the projection of the spin vec S onto the direction of momentum, hat p ::h = vec Scdot hat p,qquad hat p = vec p / |vec p|Because the spin with respect to an axis has discrete values, helicity is discrete, too.… …   Wikipedia

  • Elementary particle — In particle physics, an elementary particle or fundamental particle is a not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic …   Wikipedia

  • Automatic calculation of particle interaction or decay — The automatic calculation of particle interaction or decay is part of the computational particle physics branch. It refers to computing tools that help calculating the complex particle interactions as studied in high energy physics, astroparticle …   Wikipedia

  • Flavour (particle physics) — In particle physics, flavour or flavor (see spelling differences) is a quantum number of elementary particles related to their weak interactions. In the electroweak theory this symmetry is gauged, and flavour changing processes exist. In quantum… …   Wikipedia

  • Drude particle — Drude particles are model oscillators used to simulate the effects of electronic polarizability in the context of a classical molecular mechanics force field. It is based on the Drude model of mobile electrons. Molecular mechanics models,… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”