Luxon
Encyclopedia
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 hadron
s.
Neutrino
s were, until recently, thought to be either massless or have a small mass. Because neutrinos change flavour
as they travel, at least two of them must have mass.
. For example, these particles must always move at the speed of light
. In this context, they are sometimes called luxons to distinguish them from bradyon
s and tachyon
s.
) 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).
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, a massless particle is a particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle 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 building blocks of the universe from which...
whose invariant mass
Invariant mass
The invariant mass, rest mass, intrinsic mass, proper mass or just mass is a characteristic of the total energy and momentum of an object or a system of objects that is the same in all frames of reference related by Lorentz transformations...
is zero. Currently, the only known massless particles are gauge bosons: the photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
(carrier of electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
) and the gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
(carrier of the strong force). However, gluons are never observed as free particles, since they are confined
Colour confinement
Color confinement, often simply called confinement, is the physics phenomenon that color charged particles cannot be isolated singularly, and therefore cannot be directly observed. Quarks, by default, clump together to form groups, or hadrons. The two types of hadrons are the mesons and the baryons...
within hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s.
Neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s were, until recently, thought to be either massless or have a small mass. Because neutrinos change flavour
Flavour (particle physics)
In particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics, flavour is a global symmetry...
as they travel, at least two of them must have mass.
Special relativity
The behavior of massless particles is understood by virtue of special relativitySpecial relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
. For example, these particles must always move at the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. In this context, they are sometimes called luxons to distinguish them from bradyon
Bradyon
A bradyon, also known as a tardyon or ittyon, is a particle that travels slower than light. All known particles that have mass are bradyons...
s and tachyon
Tachyon
A tachyon is a hypothetical subatomic particle that always moves faster than light. In the language of special relativity, a tachyon would be a particle with space-like four-momentum and imaginary proper time. A tachyon would be constrained to the space-like portion of the energy-momentum graph...
s.
Dynamics
Massless particles are known to experience the same gravitational acceleration as other particles (which provides empirical evidence for the equivalence principleEquivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
) 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
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
. 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
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
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).