Particle

In physics is called a body which is small compared to the scale of the system in a particle. The internal structure of a single particle is not important, but only its behavior as a whole in relation to other particles or external influences. Insofar particles are ideal objects. Often restricted to certain properties of the real physical object, such as the mass or electrical charge to study the interaction that is related to this property. Depending on the approach, ie, the same physical object can be regarded as a particle or system of particles a and. This is especially true for atoms, just as well as for atomic nuclei and also for the protons and neutrons ( see chemistry to nuclear physics ). The no longer composed of smaller components according to current understanding, particles are called elementary particles and described in the Standard Model of elementary particle physics (see standard model).

The word particle is used as short for elementary particles. This means on the one hand " the smallest building blocks of matter " that are not in turn composed of smaller particles, on the other hand, based on " exchange particles " such as the photon, which mediate the elemental forces.

Overview

In quantum mechanics, a particle is represented by a wave function, the amplitude of the probability density of the particle ( see particles in quantum mechanics ).

In solid state physics one speaks both the lattice atoms of particles, as well as in the waves with which their ideas spread throughout a ground state. This means that while a variety of phenomena are idealized as particles whose behavior can be described as illustrative: So in the quantum physical description of the excitations of a crystal lattice can be regarded 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 to be treated and how positively charged particles.

Related terms

The term particle is not to be used generally for particles. In certain areas, these two terms are on the other hand completely synonymous:

• Composites of a few thousands of atoms or molecules are interchangeably referred to as nanoparticles or nanoparticles.
• In hydrodynamics particles denote the smallest possible number of molecules. It should be large enough to addition to the mechanical properties and momentum to possess properties of thermodynamics, such as pressure, temperature and entropy.
• Particle therapy and particle therapy are used interchangeably, although the protons used therein and other ions in physics are always referred to only as particles. The used here in medicine label particle is the English particle borrowed.

The term corpuscle ' for particles is outdated. The historical debate between corpuscular and wave theory in the description of light and its solution in the wave -particle duality is a possible starting point for the discussion of the quantum mechanics.

From chemistry to nuclear physics

In the 5th century BC Democritus postulated that matter from smallest indivisible units is composed. Following this idea used John Dalton in 1803 for the smallest, in his opinion, indivisible particles, the term atom ( from Ancient Greek ἄτομος átomos " not zerschneidbar, indivisible ").

To consider atoms as indivisible particles, resulting in the chemical sense. They are used as objects from which you initially viewed as a property of only the mass number. One arranges them according to the mass number ( without knowing that this order number at the same time is the atomic number! ) And looks at the chemical properties of the ordered elements, then we obtain the periodic table. This restriction on individual properties is quite essential for all uses of the term particles in physics.

It took another century of Dalton (see the historical outline below Atom) until doubts arose at this indivisibility of atoms: Marie Curie realized that a radioactive element can change to another; Ernest Rutherford was able to show in his scattering experiment that bombarded with alpha rays gold film is substantially permeable. In considering the Rutherford experiment, both the incident alpha particles, as well as the fixed in the lattice positively charged atomic nuclei as particles idealized be ( it might as well be charged billiard balls ), of which one considers only a few properties: mass, charge, the diameter and the velocity. It does not matter in this experiment whether the atomic nuclei have any further structure, or whether they are composed of other, smaller particles: these few properties of the particles under consideration are sufficient for the description of the experiment and the theoretical derivation of the scattering pattern from.

When considering the Bohr model of the atom the particles under consideration are an electron and an atomic core (consisting of the nucleus and possibly inner electrons). Again, the particles are reduced to their essential characteristics: charge and mass. More properties are not required of the particles.

Otto Hahn, Lise Meitner and Fritz Strassmann were able to demonstrate that at bombarding uranium atoms not by increasing the mass number Transuranium Elements ( higher atomic number ) are produced by neutrons, as was assumed until then (see Enrico Fermi, 1934), but a nuclear fission takes place in mid-sized elements. Here, the core can not be more than a single particle understand, but only as a composed of protons and neutrons ( nucleons together called from Latin nucleus " core "). Other important particles in nuclear physics are alpha particles, electrons, and neutrinos. It turns quickly the question of what holds together the protons and neutrons in the nucleus, since the protons are all positively charged and would repel. This strong interaction is explained by the fact that in quantum chromodynamics the nucleons sees each composed of three quarks than that (from English to glue " stick together " ) are held together by gluons. The residual interaction of this force outside of the nucleons holds them together similar to the van der Waals forces hold the water molecules.

The Standard Model of elementary particle physics

Particle physics is different between the particles of matter and the interaction particles ( exchange particles ), and in the particles of matter between elementary and composite particles.

The elementary particles are described by the Standard Model of elementary particle physics. They are divided into three families of leptons and three families of quarks. The leptons (from the Greek λεπτος ( leptos ) " light, fine " ) are the electron and its neutrino, the muon and its neutrino, and the tau neutrino and his. The families of quarks are denoted by up and down, charm and strange, and top and bottom.

Quarks can not occur separately in nature, which is called color confinement (see also). Rather, they always form composite particles, which distinguish them from the leptons as hadrons ( ἁδρός of Greek, hadrós, "thick" ) are referred to. Hadrons are subdivided into mesons (from Greek mesos μεσος " Central " ) and baryons (from Greek βαρύς barys "heavy" ). Mesons consist of a quark and an antiquark, while baryons consist of three quarks. The best known representatives of baryons are the proton and the neutron.

The exchange particles, the standard model considers the photon as the exchange particle of the electromagnetic interaction. It is very closely related to the W - boson and the Z boson, which jointly with the photon, the exchange particles for the electroweak interaction. The exchange particle for the strong interaction, the gluons.

Of the four fundamental forces of physics while the gravitational and her exchange particle, the graviton is absent in the standard model. So exactly the results of the standard model is also consistent with the experiments at the accelerators, it has not yet been able to extend the same mathematical formalism on gravitation. This is one of the great open questions of theoretical physics.

In the Standard Model particles are assumed to be massless, but also giving them an apparent mass that they, the Higgs boson interact with a not yet experimentally proven particles.

Quantum mechanical perspective

The transition to the quantum mechanics of particles are waves that describe their probabilities. Meets eg light (or electron beam) on a double slit, so that wave forms behind the slit, a diffraction pattern. On a photo paper (or screen ) will always take only a few points, the incident light ( the electron ). Only in the stochastic means many of incident photons ( electrons), the diffraction pattern becomes visible again. This simultaneous interpretation as a wave and a particle is called the wave -particle duality.

In contrast to classical mechanics, in which the state of the particle is determined by the position and momentum, position and momentum in quantum mechanics can never be simultaneously measured accurately (see Heisenberg uncertainty principle ).

Mehrteilchensystemen in the particles produced by the application of an operator of generating a vacuum condition. Such operators play in particular in quantum field theory a role. Virtual particles may appear and disappear again that satisfy no energy - momentum relation and their energy has no lower bound between the initial and final states of physical, interacting particles.

The particle concept in mathematical physics extends from states in Hilbert spaces, where one considers algebras of operators, to waves, in which for example a particular scattering behavior can be calculated: this does not include solitons, which are to be diverging waves are.