In physics , a particle is a body that is small compared to the scale of the system under consideration. The internal structure of an individual particle does not matter, only its behavior as a whole towards other particles or external influences. Often the particles are then understood as point particles without extension (in the sense of point masses ). Particles are ideal objects . As a rule, one restricts oneself only to certain properties of the real physical object, such as the mass or the electrical charge , in order to study the interaction that is related to this property. Depending on the point of view, one and the same physical object can be viewed as a particle or a system of particles. This applies in particular to atoms, but also to atomic nuclei and also to protons and neutrons. According to the current understanding, the particles no longer composed of smaller components are called elementary particles and are described in the standard model of elementary particle physics ( see standard model ).
The word particle is also used as an abbreviation for elementary particles . These mean, on the one hand, “the smallest building blocks of matter”, which in turn are not composed of smaller particles, on the other hand, referring to “ exchange particles ” such as the photon , which convey the elementary forces .
In solid-state physics, one speaks both of the lattice atoms of particles and of the waves with which their excitations propagate over a ground state. This leads to the fact that a large number of phenomena are idealized as particles, the behavior of which can be described more clearly: In the quantum-physical description, the excitations of a crystal lattice are understood as particles, for example as polarons , excitons or phonons . Holes in the otherwise fully occupied energy bands of the electrons in a semiconductor have the characteristics of particles and are treated like positively charged particles.
In general, the term particle should not be used for particles. On the other hand, in certain areas these two terms are used completely synonymously:
- Composites of a few thousand atoms or molecules are synonymous with nanoparticles or nanoparticles.
- The terms particle therapy and particle therapy are used synonymously, although the protons and other ions used are only called particles in physics . The term particle used in medicine here goes back to the English particle .
In hydrodynamics , particle is sometimes a volume element of the fluid . Although this particle is “small”, it is macroscopic ; In other words , it contains so many molecules that, in addition to the mechanical properties of position and momentum, properties of thermodynamics such as pressure , temperature and entropy can also be ascribed to it.
The term 'corpuscle' for particles is out of date. It occurs, for example, in the historical debate between corpuscle theory and wave theory when describing light.
In the 5th century BC Chr. Democritus postulated that matter is composed of the smallest, indivisible units. Following this thought, John Dalton used the term atom (from ancient Greek ἄτομος átomos, “indivisible, indivisible”) for the smallest, in his opinion inseparable particles .
Viewing atoms as inseparable particles makes perfect sense in chemistry . They are used as objects, of which only the number of masses is initially considered as a property. If you order them according to the mass number (without knowing that this atomic number is also the atomic number!) And consider the chemical properties of the elements sorted in this way, you get the periodic table . This restriction to individual properties is essential for all uses of the term particle in physics.
It took another century from Dalton's time ( see the historical summary under atom ) until doubts arose about the indivisibility of atoms: Marie Curie recognized that one radioactive element can pass into another; Ernest Rutherford was able to show in his scattering experiment that the gold foil bombarded with alpha radiation is largely permeable. In considering the Rutherford experiment, both the incident alpha particles and the positively charged atomic nuclei stuck in the lattice are idealized as particles (it could just as easily be charged billiard balls), of which only a few properties are considered: the mass, the charge, the diameter and the speed. In this experiment it does not matter whether the atomic nuclei have any other structure or whether they are composed of other, smaller particles. These few properties of the observed particles are sufficient for the description of the experiment and the theoretical derivation of the scattering pattern.
When considering Bohr's model of the atom , the considered particles are an electron and an atomic core (consisting of the atomic nucleus and possibly other electrons). Again, the particles are reduced to their essential properties, charge and mass.
Otto Hahn , Lise Meitner and Fritz Straßmann succeeded in showing that when uranium atoms are bombarded with neutrons, transuranic elements (with a higher atomic number) are not only formed by increasing the mass number, as was previously assumed (see Enrico Fermi , 1934), but sometimes a nuclear fission into medium-sized atomic nuclei takes place. Here the nucleus can no longer be understood as a single particle, but only as composed of nucleons , i.e. protons and neutrons . Other important particles in nuclear physics are alpha particles , electrons and neutrinos . The question quickly arises as to what holds the protons and neutrons together in the nucleus, since the protons are all positively charged and have to repel each other. This strong interaction is explained by the fact that in quantum chromodynamics the nucleons are each made up of three quarks, which are held together by gluons (from English to glue “sticking together”). The residual interaction of this force outside of the nucleons holds them together in a similar way to the Van der Waals forces z. B. hold water molecules together.
Subatomic Particles and the Standard Model
The particle is different between the material particles and the interaction particles ( exchange particles ), as well as the material particles between the elementary particles and the composite particles.
The elementary particles are described by the standard model of elementary particle physics . Since this model is a quantum field theory, the particles are understood here as field quanta, ie as quantized amounts of energy from fields. The question of whether the particles or the fields are ultimately the “more fundamental” in nature is still controversially discussed today (2018). Most physicists, however, are of the quantum field theoretical view that there are no localized particles, but only fields (and their quanta, which are spatially as extensive as the field itself).
The elementary fields or their quanta are divided in the standard model into three families of leptons and three families of quarks . The leptons (from Greek λεπτος (leptos) "light, fine") are the electron and its neutrino , the muon and its neutrino, as well as the tau and its neutrino. The families of the quarks are called up and down , charm and strange , as well as top and bottom .
Quarks cannot occur individually in nature, which is called color confinement (see here ). Rather, they always form composite particles, which are called hadrons (from the Greek ἁδρός, hadrós , "thick") to distinguish them from the leptons . Hadrons are divided into mesons (from the Greek μεσος mesos "middle") and into baryons (from the Greek βαρύς barys "heavy"). Mesons consist of a quark and an antiquark, baryons consist of three quarks. The best known baryons are the proton and the neutron.
In the case of exchange particles , the Standard Model regards the photon as the exchange particle of the electromagnetic interaction . It is very closely related to the W bosons and the Z boson , which together with the photon are the exchange particles for the electroweak interaction . The exchange particles for the strong interaction are the gluons.
Of the four basic forces in physics , the standard model lacks gravitation and its exchange particle, the graviton . The results of the Standard Model agree very well with the results of accelerator experiments. However, it has not yet been possible to extend the same mathematical formalism to gravity. This is one of the great open questions in theoretical physics .
In the standard model, the particles get their mass through interaction with the Higgs field .
Quantum mechanical perspective
During the transition to quantum mechanics, particles become waves that describe their probabilities of location. If z. B. light (or an electron beam) on a double slit , this wave forms a diffraction pattern behind the slit. On a photo paper (or screen), the incident light (the electron beam) will only ever hit individual points. Only in the stochastic mean of many incident photons (electrons) does the diffraction pattern become visible again. This simultaneous interpretation as wave and particle is called wave-particle dualism .
In contrast to classical mechanics, in which the state of the particle is determined by position and momentum, position and momentum in quantum mechanics can never be measured exactly at the same time (see Heisenberg's uncertainty principle ).
In multi-particle systems, the particles are generated from a vacuum state by applying a generation operator . Such operators play a role especially in quantum field theory . Between the initial and final states of physical, interacting particles, virtual particles can arise and then disappear again, which do not satisfy any energy-momentum relationship and whose energy has no lower limit.
The concept of particles in mathematical physics extends from states in Hilbert spaces , on which one considers the algebras of operators, to waves, for which, for example, a certain scattering behavior can be calculated: this includes, among other things, solitons that are not diverging waves.
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