Particle

For other uses, see Particle (disambiguation).

Arc welders need to protect themselves from welding sparks, which are heated metal particles that fly off the welding surface

In the physical sciences, a particle (or corpuscle in older texts) is a separate part of a larger system. Depending on the system they vary greatly in size or quantity, from subatomic particles like the electron, to microscopic particles like atoms and molecules, to macroscopic particles like powders and other granular materials. Particles can also be used to create scientific models of large objects within very large systems, such as humans moving in a crowd, celestial bodies in orbit, or galaxies in the expansion of the universe.

In some fields a particle is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.^[1][2] The term particle is rather general in meaning, and is refined as needed by various scientific fields. Anything that is composed of particles may be referred to as being particulate.^[3] However, the noun particulate is most frequently used to refer to pollutants in the Earth's atmosphere, which are a suspension of unconnected particles, rather than a connected particle aggregation.

Physics

Particles are often represented as dots. This figure could represent the movement of atoms in a gas, people in crowds, or stars in the night sky.

In models

Main article: Point particle

In physics, the concept of particles can be used to make simplifying assumptions when modelling nature. For example, to calculate the landing location and speed of a baseball thrown in the air, the baseball can be idealized as a rigid smooth sphere, ignoring its rotation, buoyancy, and friction, reducing the problem to the ballistics in classical mechanics.^[4]

Size

Galaxies are so large that stars can be considered particles relative to them.

See also: Particle size

The term "particle" is usually applied differently to three classes of sizes. The term macroscopic particle, usually refers to particles much larger than atoms and molecules. These are usually abstracted as point-like particles, even though they have volumes, shapes, structures, etc. Examples of macroscopic particles would include powder, dust, sand, pieces of debris during a car accident, or even objects as big as the stars of a galaxy.^[5][6]

Another type, microscopic particles usually refers to particles of sizes ranging from atoms to molecules, such as carbon dioxide, nanoparticles, and colloidal particles. These particles are studied in chemistry, as well as atomic and molecular physics.

The smallest particles are the subatomic particles, which refer to particles smaller than atoms.^[7] These would include particles such as the constituents of atoms – protons, neutrons, and electrons – as well as other types of particles which can only be produced in particle accelerators or cosmic rays. These particles are studied in particle physics.

Because of their extremely small size, the study of microscopic and subatomic particles falls in the realm of quantum mechanics. They will exhibit phenomena demonstrated in the particle in a box model,^[8][9] including wave–particle duality,^[10][11] and whether particles can be considered distinct or identical^[12][13] is an important question in many situations.

Particle physics

Main article: particle physics

Mass

See also: Mass and Higgs mechanism

In the context of particle physics, a massive particle is a particle with rest mass greater than zero while a massless particle has zero rest mass. Only a handful of elementary particles are massless: the photon, the gluon, and graviton. Every other known particle is massive as a result of the Higgs mechanism.^[14][15]

This also explains why ordinary matter, which is made from atoms, has mass. The mass of atoms is overwhelmingly concentrated in their nuclei, made of protons and neutrons. Electrons contribute a bit to the mass of atoms, but only a very small amount. Both the proton and neutron have over 1800 times more mass than electrons. Since protons and neutrons are made of quarks and gluons, the mass of matter can be ultimately traced to quarks and gluons and their interactions.^[14]

Massive particles must move at a speed less than the speed of light. Massless particles, in contrast, must move at the speed of light.^[15]

Rest mass is not conserved: a massive particle can decay into massless particles and massless particles can combine to produce massive particles. For example, a neutral pion is massive but can decay into two photons in about 10⁻¹⁶ seconds.^[16]: 435

Composition

A proton is composed of three quarks and held together with the strong interaction.

Particles can also be classified according to composition. Composite particles refer to particles that have composition – that is particles which are made of other particles.^[17] For example, a carbon-14 atom is made of six protons, eight neutrons, and six electrons. By contrast, elementary particles (also called fundamental particles) refer to particles that are not made of other particles.^[18] According to our current understanding of the world, only a very small number of these exist, such as leptons, quarks, and gluons. However it is possible that some of these might be composite particles after all, and merely appear to be elementary to scientists now.^[19][20][21] While composite particles can very often be considered point-like, although having internal structure, elementary particles have so far been found to have no structure.^[22]

Stability

Both elementary (such as muons) and composite particles (such as uranium nuclei), are known to undergo particle decay. Those that do not are called stable particles, such as electrons and helium-4 nuclei. The lifetime of stable particles can be either infinite or large enough to hinder attempts to observe such decays. In the latter case, those particles are called "observationally stable". In general, a particle decays from a high-energy state to a lower-energy state by emitting some form of radiation, such as the emission of photons.

Statistical mechanics

Main article: Statistical mechanics

In physical systems with vast numbers of interacting particles analysis of individual motions is impractical. For example, a single gram of O₂ contains ${\displaystyle 2\times 10^{22}}$ molecules. For these cases, statistical approaches have been developed which allow prediction of average quantities like the energy from the properties of the particles.^[23] The approach provides a theoretical basis for thermodynamics, the ideal gas laws, and understanding brownian motion.^[24]

Astrophysics

Astrophysics uses the idea of "particles" on many levels. Particle radiation is emitted by stars, cosmic rays are high energy particles (primarily bare atomic nuclei), components in planetary rings, components of solar wind, meteors, hydrogen nuclei inside the Sun, dust particles aggregating into planets and interstellar gas collapsing to form a star.^[25] Self-gravitating systems of point particles representing stars are used to model galaxy formation.^[26]: 85

N-body simulation

Main article: N-body simulation

In computational physics, N-body simulations (also called N-particle simulations) are simulations of dynamical systems of particles under the influence of certain conditions, such as being subject to gravity.^[27] These simulations are common in cosmology and computational fluid dynamics.

N refers to the number of particles considered. As simulations with higher N are more computationally intensive, systems with large numbers of actual particles will often be approximated to a smaller number of particles, and simulation algorithms need to be optimized through various methods.^[27]

Atmospheric science

In atmospheric science a particle is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.^[28] Dust, soot, and smoke are larger forms of particulate matter with smaller forms being atoms and molecules.^[29]

Soil science

Soil is considered composed of particles. The discrete particles in soil are solid; they cannot move relative to each other as easily as particles in a liquid. Force on the soil is transmitted by points of contact between the soil particles, with some of the force deforming the particles. The particles slide past each other in response to the force.^[30]

Distribution of particles

Main article: Colloid

Examples of a stable and of an unstable colloidal dispersion.

Colloidal particles are the components of a colloid. A colloid is a substance microscopically dispersed evenly throughout another substance.^[31] Such colloidal system can be solid, liquid, or gaseous; as well as continuous or dispersed. The dispersed-phase particles have a diameter of between approximately 5 and 200 nanometers.^[32] Soluble particles smaller than this will form a solution as opposed to a colloid. Colloidal systems (also called colloidal solutions or colloidal suspensions) are the subject of interface and colloid science. Suspended solids may be held in a liquid, while solid or liquid particles suspended in a gas together form an aerosol. Particles may also be suspended in the form of atmospheric particulate matter, which may constitute air pollution. Larger particles can similarly form marine debris or space debris. A conglomeration of discrete solid, macroscopic particles may be described as a granular material.

See also

• Antiparticle
• Brownian motion
• Corpuscularianism
• Fluid parcel
• List of particles
• Matter
• Mechanics
• Particle counter
• Particle detector
• Particle segregation
• Self-propelled particle
• Wave–particle duality

References

1. "particle - Glossary of Meteorology". glossary.ametsoc.org. Retrieved 2026-03-09.
2. "Particle". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.
3. Lambe, T. W.; Whitman, R. V. (1969). Soil Mechanics. John Wiley & Sons. p. 18. ISBN 978-0-471-51192-2. The word 'particulate' means 'of or pertaining to a system of particles'.
4. F. W. Sears; M. W. Zemansky (1964). "Equilibrium of a Particle". University Physics (3rd ed.). Addison-Wesley. p. 27. LCCN 63015265. A body whose rotation is ignored as irrelevant is called a particle. A particle may be so small that it is an approximation to a point, or it may be of any size, provided that the action lines of all the forces acting on it intersect in one point.
5. Dubinski, J. (2003). "Galaxy Dynamics and Cosmology on Mckenzie". Canadian Institute for Theoretical Astrophysics. Archived from the original on 2021-11-02. Retrieved 2011-02-24.
6. Coppola, G.; La Barbera, F.; Capaccioli, M. (2009). "Sérsic galaxy with Sérsic halo models of early-type galaxies: A tool for N-body simulations". Publications of the Astronomical Society of the Pacific. 121 (879): 437. arXiv:0903.4758. Bibcode:2009PASP..121..437C. doi:10.1086/599288.
7. "Subatomic particle". YourDictionary.com. Archived from the original on 2011-03-05. Retrieved 2010-02-08.
8. Eisberg, R.; Resnick, R. (1985). "Solutions of Time-Independent Schroedinger Equations". Quantum Physics of Atoms, Molecules, Solids, Nuclei, Ions, Compounds and Particles (2nd ed.). John Wiley & Sons. pp. 214–226. ISBN 978-0-471-87373-0.
9. Reif, F. (1965). "Quantum Statistics of Ideal Gases – Quantum States of a Single Particle". Fundamentals of Statistical and Thermal Physics. McGraw-Hill. pp. vii–x. ISBN 978-0-07-051800-1.
10. Eisberg, R.; Resnick, R. (1985). "Photons—Particlelike Properties of Radiation". Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.). John Wiley & Sons. pp. 26–54. ISBN 978-0-471-87373-0.
11. Eisberg, R.; Resnick, R. (1985). "de Broglie's Postulate—Wavelike Properties of Particles". Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.). John Wiley & Sons. pp. 55–84. ISBN 978-0-471-87373-0.
12. Reif, F. (1965). "Quantum Statistics of Ideal Gases – Identical Particles and Symmetry Requirements". Fundamentals of Statistical and Thermal Dynamics. McGraw-Hill. pp. 331ff. ISBN 978-0-07-051800-1.
13. Reif, F. (1965). "Quantum Statistics of Ideal Gases – Physical Implications of the Quantum-Mechanical Enumeration of States". Fundamentals of Statistical and Thermal Dynamics. McGraw-Hill. pp. 353–360. ISBN 978-0-07-051800-1.
14. Wilczek, Frank (2003). "The Origin of Mass" (PDF). The MIT Physics Annual. pp. 24–35.
15. O'Keefe, Madeleine (23 July 2019). "Massless particles can't be stopped". Symmetry Magazine.
16. Penrose, Roger (2005). The road to reality: a complete guide to the laws of the universe (1st ed.). New York: A.A. Knopf. ISBN 978-0-679-45443-4.
17. "Composite particle". YourDictionary.com. Archived from the original on 2010-11-15. Retrieved 2010-02-08.
18. "Elementary particle". YourDictionary.com. Archived from the original on 2010-10-14. Retrieved 2010-02-08.
19. D'Souza, I. A.; Kalman, C. S. (1992). Preons: Models of Leptons, Quarks and Gauge Bosons as Composite Objects. World Scientific. ISBN 978-981-02-1019-9.
20. Weinberg, Steven (1997). "What is an elementary particle?". Beam Line. 27 (1): 17–21.
21. Quigg, Chris (1997). "Elementary Particles: Yesterday, Today, and Tomorrow". Beam Line. 27 (1): 22–29.
22. US National Research Council (1990). "What is an elementary particle?". Elementary-Particle Physics. US National Research Council. p. 19. ISBN 0-309-03576-7.
23. Sachs, Ivo; Sen, Siddhartha; Sexton, James (May 11, 2006). Elements of Statistical Mechanics: With an Introduction to Quantum Field Theory and Numerical Simulation (1 ed.). Cambridge University Press. doi:10.1017/cbo9780511755620. ISBN 978-0-521-84198-6.
24. Reif, F. (1965). "Statistical Description of Systems of Particles". Fundamentals of Statistical and Thermal Physics. McGraw-Hill. pp. 47ff. ISBN 978-0-07-051800-1.
25. Demtröder, Wolfgang (2024). Astrophysics. Undergraduate lecture notes in physics. Cham: Springer. ISBN 978-3-031-22135-4.
26. Longair, Malcolm S. (2023). "Galaxy Formation". Astronomy and Astrophysics Library. doi:10.1007/978-3-662-65891-8. ISSN 0941-7834.
27. Graps, A. (20 March 2000). "N-Body / Particle Simulation Methods". Archived from the original on 5 April 2001. Retrieved 2019-04-18.
28. "particle - Glossary of Meteorology". glossary.ametsoc.org. Retrieved 2026-03-09.
29. US EPA, OAR (April 19, 2016). "Particulate Matter (PM) Basics". www.epa.gov. Retrieved 2026-03-09.
30. Lambe, T. W.; Whitman, R. V. (1969). Soil Mechanics. John Wiley & Sons. p. 18. ISBN 978-0-471-51192-2. The word 'particulate' means 'of or pertaining to a system of particles'.
31. "Colloid". Encyclopædia Britannica. 1 July 2014. Retrieved 2026-02-19.
32. Levine, I. N. (2001). Physical Chemistry (5th ed.). McGraw-Hill. p. 955. ISBN 978-0-07-231808-1.

Further reading

• "What is a particle?". University of Florida, Particle Engineering Research Center. 23 July 2010. Archived from the original on 23 September 2015. Retrieved 10 September 2015.
• Griffiths, D. J. (2008). Introduction to Particle Physics (2nd ed.). Wiley-VCH. ISBN 978-3-527-40601-2.
• Alonso, M.; Finn, E. J. (1967). "Dynamics of a particle". Fundamental University Physics, Volume 1. Addison-Wesley. LCCN 66010828.
• Alonso, M.; Finn, E. J. (1967). "Dynamics of a system of particles". Fundamental University Physics, Volume 1. Addison-Wesley. LCCN 66010828.
• Segal, S. (n.d.). "What is a Particle? – Definition & Theory". High School Chemistry: Help and Review. Study.com. Chapter 4, Lesson 6.
• "A basic guide to particle characterization" (PDF). Malvern Instruments. 2015. Archived from the original (PDF) on 2020-09-20. Retrieved 2016-11-04.