What is a particle
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Understanding the Concept of a Particle
Definition of a Particle in Physics
In physics, the term "particle" can refer to various small objects, ranging from protons and electrons to dust particles. However, in the context of particle physics, it specifically refers to the irreducibly smallest detectable particles and the fundamental interactions that explain their behavior . These particles include electrons, protons, photons, mesons, and more, collectively known as elementary particles .
Particle States in Quantum Field Theory
The concept of a particle becomes more complex when examined through the lens of quantum field theory (QFT). In QFT, especially on curved spacetime, uniquely defined particle states do not generally exist. This has led to the suggestion that QFT should be interpreted in terms of eigenstates of local operators rather than particle states . Despite this, the particle-like behavior of quantum fields is empirically observed in high-energy physics experiments, indicating that particles are still a useful concept in certain contexts .
Local vs. Global Particle States
In flat spacetime, there are two distinct notions of particles: globally defined n-particle Fock states and local particle states. Local particle states are the physical objects detected by finite-size particle detectors and are eigenstates of local field operators. When particle detectors are sufficiently large, global and local particle states converge in a weak topology, though not in norm . This distinction is crucial for understanding particle states in the context of gravity and quantum gravity, where conventional global particle states may not be well-defined .
Particle Physics and Fundamental Interactions
Particle physics, also known as high-energy physics, studies the nature of these fundamental particles and their interactions. This field encompasses the study of gauge bosons, the Higgs boson, leptons, quarks, mesons, and baryons, as well as hypothetical particles like supersymmetric particles and dark photons . The properties and interactions of these particles are summarized in extensive reviews and tables, providing a comprehensive overview of the current understanding in particle physics .
Particle Methods in Fluid Mechanics
Beyond high-energy physics, particle methods are also used in fluid mechanics and continuum mechanics. These numerical methods involve particles that possess finite extension, can rotate, and have spin. The interaction forces between these particles are derived from the force laws of continuum mechanics, ensuring the conservation of mass, momentum, angular momentum, and energy .
Conclusion
The concept of a particle is multifaceted, varying from the smallest detectable entities in particle physics to the more complex states in quantum field theory and practical applications in fluid mechanics. Understanding these different perspectives helps reconcile the theoretical and empirical observations of particle behavior across various fields of physics.
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