In quantum field theory (QFT), virtual particles are mathematical entities used as a tool to describe the interactions between particles. They are not directly observable and do not exist as physical particles outside their interaction points. This understanding is supported by several lines of evidence and reasoning:
Uncertainty Principle: The Heisenberg Uncertainty Principle states that there is a fundamental limit to the precision with which certain pairs of physical properties, such as position and momentum, can be known simultaneously. This principle implies that the concept of a well-defined trajectory or existence of particles outside their interaction points becomes fuzzy at small length and time scales. Virtual particles, being inherently short-lived and existing only during interactions, fall within this framework.
Conservation Laws: Virtual particles are often invoked to explain the exchange of momentum, energy, and other conserved quantities between interacting particles. Conservation laws dictate that the total momentum, energy, and other conserved quantities of a closed system remain constant. If virtual particles were to exist as real particles outside their interaction points, they would violate conservation laws by carrying away energy and momentum from the system.
Energy Considerations: Virtual particles are associated with temporary fluctuations in energy due to quantum uncertainties. These fluctuations have observable consequences, such as the Casimir effect, where virtual particles are thought to contribute to the forces between closely spaced conductive plates. However, these effects are localized and depend on the boundary conditions of the system. They do not imply the existence of persistent, independent virtual particles in the absence of interactions.
Experimental Confirmation: While virtual particles themselves are not directly observable, the effects of their interactions can be observed and measured experimentally. For example, the Lamb shift in atomic spectra, the anomalous magnetic moment of the electron, and the behavior of particle scattering processes have been successfully predicted and verified through QFT calculations, which incorporate the concept of virtual particles.
In summary, the evidence supports the understanding that virtual particles exist as mathematical tools within the framework of QFT to describe particle interactions. They do not exist as physical entities outside their interaction points and are not directly observable. Their effects can be detected and measured, providing confirmation of the underlying theory.