massless particles
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In
particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
, 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. Particles currently thought to be elementary include electrons, the fundamental fermions ( quarks, leptons, a ...
whose invariant mass is zero. There are two known gauge boson massless particles: 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, so they a ...
(carrier of
electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
) and the gluon (carrier of the
strong force The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the ...
). However,
gluons A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bind q ...
are never observed as free particles, since they are confined within
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the e ...
s. In addition the
Weyl semimetal Weyl equation, Weyl fermions are massless chiral fermions embodying the mathematical concept of a Weyl spinor. Weyl spinors in turn play an important role in quantum field theory and the Standard Model, where they are a building block for fermion ...
or
Weyl fermion Hermann Klaus Hugo Weyl, (; 9 November 1885 – 8 December 1955) was a German mathematician, theoretical physicist and philosopher. Although much of his working life was spent in Zürich, Switzerland, and then Princeton, New Jersey, he is ...
discovered in 2015 is also massless.
Neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s were originally thought to be massless. However, because neutrinos change flavor as they travel, at least two of the types of neutrinos must have mass. The discovery of this phenomenon, known as neutrino oscillation, led to Canadian scientist
Arthur B. McDonald Arthur Bruce McDonald, P.Eng (born August 29, 1943) is a Canadian astrophysicist. McDonald is the director of the Sudbury Neutrino Observatory Collaboration and held the Gordon and Patricia Gray Chair in Particle Astrophysics at Queen's Univer ...
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 ...
sharing the 2015
Nobel prize in physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
. *Note that although literal Weyl fermions have never been experimentally confirmed to physically exist, certain systems can act collectively such that they seem to contain Weyl fermion quasiparticles.


See also

* Relativistic particle *
Gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...


References

{{Particles Special relativity Particle physics