When physicists say that virtual particles exist, they are referring to a concept within quantum field theory. Virtual particles are not particles in the usual sense of macroscopic objects with definite properties, but rather they are mathematical entities that arise as a consequence of certain calculations and interactions in quantum physics.
In quantum field theory, particles are understood as excitations or disturbances in underlying fields that permeate space. These fields are associated with specific types of particles, such as electrons, photons, or quarks. According to quantum theory, these fields can fluctuate and undergo temporary, short-lived changes due to the uncertainty principle.
Virtual particles are these temporary fluctuations or disturbances in the underlying fields. They are called "virtual" because they do not correspond to observable, long-lived particles that can be directly detected or measured. Instead, they are intermediate states that appear in mathematical calculations when describing the interactions between particles.
These virtual particles are associated with the exchange of energy and momentum during particle interactions. For example, in the case of the electromagnetic force, particles like photons are the exchange particles responsible for the force between charged particles. These photons can be treated as virtual particles during the interaction process.
It's important to note that the concept of virtual particles is a mathematical tool used to describe and calculate quantum interactions. They are not directly observable or measurable, and their existence is inherently tied to the framework of quantum field theory.