When physicists refer to forces being carried by virtual particles, they are employing a concept within quantum field theory to describe the exchange of energy and momentum between interacting particles. Virtual particles are not directly observable or detectable like "real" particles, but they are a useful mathematical tool for understanding and calculating the behavior of quantum systems.
In quantum field theory, forces between particles are understood as the result of the exchange of virtual particles. This concept is based on the idea that all fundamental forces are mediated by underlying force-carrying fields. These fields are associated with specific particles called gauge bosons, which are the virtual particles in question.
Here's a simplified explanation of how this works: When two particles interact, they exchange virtual particles that momentarily borrow energy from the vacuum. For example, in the electromagnetic force, photons are the virtual particles that mediate the interaction between charged particles. Similarly, the strong force is mediated by gluons, the weak force by W and Z bosons, and the gravitational force by hypothetical particles called gravitons (although a complete quantum theory of gravity is still under development).
Physically, this exchange of virtual particles affects the overall behavior of the interacting particles. The exchange alters the momentum and energy of the particles involved, leading to changes in their trajectories and interactions. These exchanges are depicted mathematically through Feynman diagrams, which provide a graphical representation of the particle interactions.
It is important to note that virtual particles do not exist as standalone particles with long lifetimes or independently observable properties. They are considered mathematical constructs that aid in understanding the behavior of quantum systems and calculating the probabilities of particle interactions.
In summary, when physicists say forces are carried by virtual particles, they are referring to the concept within quantum field theory that describes the exchange of energy and momentum between interacting particles. Virtual particles are a mathematical tool used to understand and calculate the behavior of quantum systems, but they are not directly observable as standalone particles.