Virtual photons are a fundamental concept in

particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

and

quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines Field theory (physics), field theory and the principle of relativity with ideas behind quantum mechanics. QFT is used in particle physics to construct phy ...

that play a crucial role in describing the interactions between electrically charged particles. Virtual photons are referred to as " virtual" because they do not exist as free particles in the traditional sense but instead serve as intermediate particles in the exchange of

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

between other particles. They are responsible for the

electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interac ...

that holds matter together, making them a key component in our understanding of the physical world. Virtual photons are thought of as fluctuations in the

electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...

, characterized by their

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

, and polarization. These fluctuations allow electrically charged particles to interact with each other by exchanging virtual photons. The electromagnetic force between two charged particles can be understood as the exchange of virtual photons between them. These photons are constantly being created and destroyed, and the exchange of these virtual photons creates the electromagnetic force that is responsible for interaction between charged particles. Virtual photons can be classified into positive and negative virtual photons. These classifications are based on the direction of their energy and momentum and their contribution to the electromagnetic force. If virtual photons exchanged between particles have a positive energy, they contribute to the electromagnetic force as a repulsive force. This means that the two charged particles are repelled from each other and the electromagnetic force pushes them apart. On the other hand, if the virtual photons have a negative energy, they contribute to the electromagnetic force as an attractive force. This means that the two charged particles are attracted to each other and the electromagnetic force pulls them towards each other. It is important to note that positive and negative virtual photons are not separate particles, but rather a way of classifying the virtual photons that exist in the electromagnetic field. These classifications are based on the direction of the energy and momentum of the virtual photons and their contribution to the electromagnetic force. Virtual photons can have a range of polarizations, which can be described as the orientation of the electric and magnetic fields that make up the photon. The polarization of a virtual photon is determined by the direction of its momentum and its interaction with the charges that emit or absorb it. The range of polarizations for virtual photons can be compared to the range of colors for visible light, with each polarization corresponding to a specific orientation of the electric and magnetic fields. Virtual photons are said to be " off-shell", which means that they do not obey the usual relationship between energy and momentum that applies to real particles. Real photons must always have energy equal to the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

times their momentum, but virtual photons can have any energy that is consistent with the

uncertainty principle The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...

. This allows virtual photons to carry a wide range of energies, even if they are not physically real. Virtual photons are responsible for

Lamb shift In physics, the Lamb shift, named after Willis Lamb, is an anomalous difference in energy between two electron orbitals in a hydrogen atom. The difference was not predicted by theory and it cannot be derived from the Dirac equation, which pre ...

, which is a small shift in the energy levels of hydrogen atoms caused by the interaction of the atom with virtual photons in the vacuum. They are also responsible for the

Casimir effect In quantum field theory, the Casimir effect (or Casimir force) is a physical force (physics), force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of a field (physics), field. The term Casim ...

, which is the phenomenon of two uncharged metallic plates being attracted to each other due to the presence of virtual photons in the vacuum between them. The attractive force between the plates is caused by a difference in the density of virtual photons on either side of the plates, which creates a net force that pulls them together.

References

{{reflist Quantum field theory Photons