The concept of virtual particles is closely related to the idea of quantum fluctuations. Virtual particles can be thought of as coming into existence as quantities, such as the electric field, which fluctuate around their expectation values as required by quantum mechanics.
A little Note on Quantum Fluctuation: Quantum fluctuation occurs only in space-time. Some pseudoscientists claims that this occurs independently from space-time, which they use as argument for the Big Bang. But this people have a big loose screw in their mind if they believe that something could ever come from nothing.
Virtual particle are particles with a limited existence in time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle. The degree of uncertainty of each is inversely proportional to time duration (for energy) or to position span (for momentum).
Virtual particles exhibit some of the phenomena that real particles do, such as obedience to the conservation laws. If a single particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual. Virtual particles are viewed as the quanta that describe fields of the basic force interactions, which cannot be described in terms of real particles. Examples of these are static force fields, such as a simple electric or magnetic field, or the components of any field that do not carry information from place to place at the speed of light (information radiated by means of a field must be composed of real particles). Virtual photons are also a major component of antenna near field phenomena and induction fields, which have shorter-range effects, and do not radiate through space with the same range-properties as do electromagnetic wave photons. For example, the energy carried from one winding of a transformer to another, or to and from a patient in an MRI scanner, in quantum terms is carried by virtual photons, not real photons.
The virtual particle forms of massless particles, such as photons, do have mass (which may be either positive or negative) and are said to be off mass shell. They are allowed to have mass (which consists of “borrowed energy“) because they exist for only a temporary time, which in turn gives them a limited “range”. This is in accordance with the uncertainty principle, which allows existence of such particles of borrowed energy, so long as their energy, multiplied by the time they exist, is a fraction of Planck’s constant. Possession of mass also allows single virtual photons to be more easily created and emitted from single charged elementary particles, something that cannot happen for massless photons, without violating conservation of momentum and energy (single real photons are always created and emitted from systems of two or more particles).
The concept of virtual particles arises in the perturbation theory of quantum field theory, an approximation scheme in which interactions (in essence, forces) between real particles are calculated in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram, which facilitates the understanding of calculations.
A virtual particle is one that does not precisely obey the m2c4 = E2 − p2c2 relationship for a short time. In other words, its kinetic energy may not have the usual relationship to velocity–indeed, it can be negative. The probability amplitude for it to exist tends to be canceled out by destructive interference over longer distances and times. A virtual particle can be considered a manifestation of quantum tunnelling. The range of forces carried by virtual particles is limited by the uncertainty principle, which regards energy and time as conjugate variables; thus, virtual particles of larger mass have more limited range.
There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence, whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, “real particles” are viewed as being detectable excitations of underlying quantum fields. As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are “temporary” in the sense that they appear in calculations, but are not detected as single particles. Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled. In this sense, virtual particles are an artifact of perturbation theory, and do not appear in a non-perturbative treatment.
There are two principal ways in which the notion of virtual particles appears in modern physics. They appear as intermediate terms in Feynman diagrams; that is, as terms in a perturbative calculation. They also appear as an infinite set of states to be summed or integrated over in the calculation of a semi-non-perturbative effect. In the latter case, it is sometimes said that virtual particles cause the effect, or that the effect occurs because of the existence of virtual particles.
There are many observable physical phenomena resulting from interactions involving virtual particles. For bosonic particles that exhibit rest mass when they are free and “real,” virtual interactions are characterized by the relatively short range of the force interaction produced by particle exchange. Examples of such short-range interactions are the strong and weak forces, and their associated field bosons. For the gravitational and electromagnetic forces, the zero rest-mass of the associated boson particle permits long-range forces to be mediated by virtual particles. However, in the case of photons, power and information transfer by virtual particles is a relatively short-range phenomenon (existing only within a few wavelengths of the field-disturbance, which carries information or transferred power), as for example seen in the characteristically short range of inductive and capacitative effects in the near field zone of coils and antennas.
Some field interactions which may be seen in terms of virtual particles are:
- The Coulomb force (static electric force) between electric charges. It is caused by the exchange of virtual photons. In symmetric 3-dimensional space this exchange results in the inverse square law for electric force. Since the photon has no mass, the coulomb potential has an infinite range.
- The magnetic field between magnetic dipoles. It is caused by the exchange of virtual photons. In symmetric 3-dimensional space this exchange results in the inverse square law for magnetic force. Since the photon has no mass, the magnetic potential has an infinite range.
- Much of the so-called near-field of radio antennas, where the magnetic and electric effects of the changing current in the antenna wire and the charge effects of the wire’s capacitive charge may be (and usually are) important contributors to the total EM field close to the source, but both of which effects are dipole effects that decay with increasing distance from the antenna much more quickly than do the influence of “conventional” electromagnetic waves that are “far” from the source. (“Far” in terms of terms of ratio of antenna length or diameter, to wavelength). These far-field waves, for which E is (in the limit of long distance) equal to cB, are composed of real photons. It should be noted that real and virtual photons are mixed near an antenna, with the virtual photons responsible only for the “extra” magnetic-inductive and transient electric-dipole effects, which cause any imbalance between E and cB. As distance from the antenna grows, the near-field effects (as dipole fields) die out more quickly, and only the “radiative” effects that are due to real photons remain as important effects. Although virtual effects extend to infinity, they drop off in field strength as 1/r2 rather than the field of EM waves composed of real photons, which drop 1/r (the powers, respectively, decrease as 1/r4 and 1/r2). See near and far field for a more detailed discussion. See near field communication for practical communications applications of near fields.
- Electromagnetic induction. This phenomenon transferring energy to and from a magnetic coil via a changing (electro)magnetic field can be viewed as a near-field effect. It is the basis for power transfer in transformers and electric generators and motors, and also signal transfer in metal detectors, magnetic and magnetoacustic anti theft electronic tags, and even signals between patient and machine in an MRI scanner. Some confusion about the use of “radio waves” results when these devices are used at conventional RF frequencies, as they are in an MRI scanner. See resonant inductive coupling and wireless energy transfer for other practical examples.
- The strong nuclear force between quarks is the result of interaction of virtual gluons. The residual of this force outside of quark triplets (neutron and proton) holds neutrons and protons together in nuclei, and is due to virtual mesons such as the pi meson and rho meson.
- The weak nuclear force - it is the result of exchange by virtual W and Z bosons.
- The spontaneous emission of a photon during the decay of an excited atom or excited nucleus; such a decay is prohibited by ordinary quantum mechanics and requires the quantization of the electromagnetic field for its explanation.
- The Casimir effect, where the ground state of the quantized electromagnetic field causes attraction between a pair of electrically neutral metal plates.
- The van der Waals force, which is partly due to the Casimir effect between two atoms.
- Vacuum polarization, which involves pair production or the decay of the vacuum, which is the spontaneous production of particle-antiparticle pairs (such as electron-positron).
- Lamb shift of positions of atomic levels.
- Hawking radiation, where the gravitational field is so strong that it causes the spontaneous production of photon pairs (with black body energy distribution) and even of particle pairs.
Most of these have analogous effects in solid-state physics; indeed, one can often gain a better intuitive understanding by examining these cases. In semiconductors, the roles of electrons, positrons and photons in field theory are replaced by electrons in the conduction band, holes in the valence band, and phonons or vibrations of the crystal lattice. A virtual particle is in a virtual state where the probability amplitude is not conserved. Examples of macroscopic virtual phonons, photons, and electrons in the case of the tunneling process were presented by Günter Nimtz in and.