In quantum theory, virtual particles are particles that can briefly appear and disappear during particle interactions. They are often referred to as "virtual" because they do not correspond to particles that can be directly observed or detected like "real" particles. Instead, they are mathematical constructs that arise from the mathematical formalism of quantum field theory.
According to quantum field theory, all particles, whether they are elementary particles like electrons or quarks, or force-carrying particles like photons or gluons, are described as excitations of underlying quantum fields. These fields permeate all of space and time. Virtual particles emerge from the dynamic nature of these fields during particle interactions.
In quantum field theory, particles are represented as excitations of their respective fields. These excitations, or quanta, can be thought of as disturbances or ripples in the fields. When particles interact with each other, they can exchange virtual particles as carriers of the interaction.
For example, consider the electromagnetic force between two electrons. In quantum electrodynamics (QED), the theory that describes the electromagnetic interaction, the force is mediated by the exchange of virtual photons. One electron emits a virtual photon, which carries the electromagnetic force, and the other electron absorbs it. The virtual photon is then reabsorbed, and its energy is conserved.
These virtual particles exist only for a brief moment, typically within the constraints of the Heisenberg uncertainty principle. The uncertainty principle allows for temporary deviations from the conservation of energy and momentum, enabling the creation of virtual particles that violate the mass-energy relation for "real" particles. However, these deviations are short-lived, and the conservation laws are ultimately restored.
Virtual particles are an integral part of quantum field theory and play a crucial role in understanding particle interactions and the behavior of quantum systems. They contribute to phenomena such as quantum fluctuations, the Casimir effect, and the Lamb shift, among others. Although virtual particles cannot be directly observed, their effects are experimentally verifiable and have been confirmed through various high-precision experiments in quantum physics.