Field quantum
Field quantum is a fundamental concept of quantum field theory and describes the smallest, indivisible strength with which a physical field can exist or change its strength.
The term often only refers to the field quanta of the four fundamental interactions , i.e. their exchange particles , the gauge bosons .
properties
There are different types of field quanta, each belonging to a specific physical field. All field quanta can be created and destroyed, whereby the strength of the associated field increases or decreases. Field quanta of all kinds can exist:
- in real states; then the field quanta move freely through space and can be detected as individual particles. The corresponding field spreads in space and is often referred to as radiation or free radiation field of the respective type.
- in virtual states ; then the field quanta do not appear as particles or radiation, but cause the effects of a force field around the place of their generation ( source ) . In this context they are also referred to as exchange particles.
The absence of the radiation or force field, i.e. the field strength zero, corresponds to the state without a field quantum, which is called a vacuum . However, certain combinations of field quanta can spontaneously arise and disappear in a vacuum, regardless of any source, which causes observable effects in the form of vacuum polarization and vacuum fluctuation .
Examples
photon
The photon (also called light quantum) is the field quantum of the electromagnetic wave of a certain frequency . The total energy content of the wave field is an integral multiple of the energy of a photon (including zero).
Accordingly, in some processes an electromagnetic wave field appears as if it consists of a certain number of identical, mutually independent particles (e.g. in the case of the photo effect and statistical fluctuations in the cavity radiation ).
On the other hand, the total energy content of the field also corresponds to the strength with which the electric and magnetic fields are present, and these can continuously vary spatially and temporally. In particular, it is not possible to identify individual photons in an electromagnetic field and track them over time.
This contradiction between two illustrative modes of representation is characteristic of quantum physics and is called wave-particle dualism .
electron
The wave-particle dualism occurs in the same way when material particles such as electrons, for example , have to be described by matter waves , because they behave in this way in some processes. In quantum field theory, therefore, the fundamental distinction between particle and field is completely dispensed with. Instead, one speaks of an electron field (or neutrino field, etc.) and of the creation and destruction of the associated particles, just like the electromagnetic field.
Elementary particles
All types of elementary particles are field quanta of the field of the respective type. Their creation or annihilation corresponds to a stronger or weaker excitation of the field. As quanta of the same field, the particles are completely indistinguishable .
Compound systems
Occasionally, protons , neutrons , pions, etc., as well as other systems built up from elementary particles, sometimes even whole atoms , are treated as field quanta. A prerequisite for this is that they can not be distinguished by their quantum mechanical state .
Suggestions in many-body systems
In many-particle systems there are typical excited states that are described in the form of fields, e.g. B. sound waves , deformation vibrations , waves of the magnetic order or other collective excitations . Field quanta also exist for these forms of excitation, which show the character of a particle in certain processes . See e.g. Phonon , Magnon , Soliton .