particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

, a massless particle is an

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons. As a c ...

whose

invariant mass The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object or system of objects that is independent of the overall motion of the system. More precisely, ...

is zero. At present the only confirmed massless particle is the

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

Other particles and quasiparticles

Standard Model gauge bosons

The

(carrier of

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

) is one of two known

gauge boson In particle physics, a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions. Elementary particles whose interactions are described by a gauge theory interact with each other by the exchange of gauge ...

s thought to be massless. The photon is well-known from direct observation to exist and be massless. The other massless gauge boson is the

gluon A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...

(carrier of the

strong force In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, an ...

) whose existence has been inferred from

particle collision In particle physics, an event refers to the results just after a fundamental interaction takes place between subatomic particles, occurring in a very short time span, at a well-localized region of space. Because of the uncertainty principle, an eve ...

decay products; it is expected to be massless, but a zero mass has not been confirmed by experiment. Although there are compelling theoretical reasons to believe that

gluons A gluon ( ) is a type of massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a spin of 1. Through the s ...

are massless, they can never be observed as free particles due to being confined within

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

s, and hence their presumed lack of rest mass cannot be confirmed by any feasible experiment. The only other observed gauge bosons are the

W and Z bosons In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , an ...

, which are known from experiments to be extremely massive, even heavier than iron nuclei.

Hypothetical graviton

The

graviton In theories of quantum gravity, the graviton is the hypothetical elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with re ...

is a hypothetical tensor boson proposed to be the carrier of

gravitational force Newton's law of universal gravitation describes gravity as a force by stating that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the sq ...

in some quantum theories of gravity, but no such theory has been successfully incorporated into the

Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...

, so the Standard Model neither predicts any such particle nor requires it, and no gravitational quantum particle has been indicated by experiment. Whether a graviton would be massless if it existed is likewise an open question.

Quasiparticles

The

Weyl fermion In physics, particularly in quantum field theory, the Weyl equation is a relativistic wave equation for describing massless spin-1/2 particles called Weyl fermions. The equation is named after Hermann Weyl. The Weyl fermions are one of the three ...

discovered in 2015 is also expected to be massless, but these are not actual particles. At one time neutrinos were thought to perhaps be Weyl fermions, but when they were discovered to have mass, that left no fundamental particles of the Weyl type. The Weyl fermions discovered in 2015 are merely ''

quasiparticle In condensed matter physics, a quasiparticle is a concept used to describe a collective behavior of a group of particles that can be treated as if they were a single particle. Formally, quasiparticles and collective excitations are closely relate ...

s'' – composite motions found in the structure of molecular latices that have particle-like behavior, but are not themselves real particles. Weyl fermions in matter are like

phonon A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. In the context of optically trapped objects, the quantized vibration mode can be defined a ...

s, which are also quasiparticles. No real particle that is a Weyl fermion has been found to exist, and there is no compelling theoretical reason that requires them to exist.

Neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...

s were originally thought to be massless. However, because neutrinos change flavour as they travel, at least two of the types of neutrinos must have mass (and cannot be Weyl fermions). The discovery of this phenomenon, known as

neutrino oscillation Neutrino oscillation is a quantum mechanics, quantum mechanical phenomenon in which a neutrino created with a specific lepton lepton number, family number ("lepton flavor": electron, muon, or tau lepton, tau) can later be Quantum measurement, mea ...

, led to Canadian scientist Arthur B. McDonald and Japanese scientist

Takaaki Kajita is a Japanese physicist, known for neutrino experiments at the Kamioka Observatory – Kamiokande and its successor, Super-Kamiokande. In 2015, he was awarded the Nobel Prize in Physics jointly with Canadian physicist Arthur B. McDonald. On 1 O ...

sharing the 2015

Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

References

{{Particles Special relativity Particle physics

Other particles and quasiparticles

Standard Model gauge bosons

Hypothetical graviton

Quasiparticles

See also

References