In quantum field theory (QFT), real and virtual particles are concepts used to describe the behavior and interactions of particles within the framework of Feynman diagrams and propagators. Here's a breakdown of the distinction between real and virtual particles:
Real Particles: Real particles are the physical particles that exist in nature and can be directly detected and measured. They correspond to the particles we observe in experiments, such as electrons, photons, quarks, and so on. Real particles are associated with the poles of the propagator, which represents the probability amplitude for the propagation of particles between different spacetime points.
In Feynman diagrams, real particles are represented by solid lines with arrows, indicating their respective momenta and directions. These lines connect the interaction vertices in the diagram, reflecting the actual particle interactions observed in experiments. The lines may be straight or curved, depending on the type of particle and its momentum.
Virtual Particles: Virtual particles, on the other hand, are not directly observable or detected as individual particles. They are internal lines in Feynman diagrams and are associated with intermediate states or off-shell particles that arise during particle interactions. Virtual particles do not correspond to particles that exist as stable, on-shell states.
Virtual particles are represented by dashed lines in Feynman diagrams. They connect the interaction vertices and often have various momenta associated with them. The energy and momentum of virtual particles can take values that are not constrained by the on-shell mass-energy relationship (E² = p²c² + m²c⁴). These particles can be thought of as "borrowed" from the vacuum for a short time during the interaction process.
Propagator: The propagator is a mathematical tool used in QFT to calculate the probability amplitude for the propagation of particles between two spacetime points. It represents the possible paths a particle can take between the initial and final states.
The propagator can be decomposed into two parts: the real particle contribution and the virtual particle contribution. The real particle contribution corresponds to the poles of the propagator and describes the propagation of real, on-shell particles. The virtual particle contribution corresponds to the non-pole terms and involves integrals over all possible intermediate virtual particle states.
Physics of Virtual Particles: Virtual particles play a crucial role in understanding the behavior of particles and interactions in QFT. They mediate the forces between real particles and contribute to the overall dynamics of the system. Virtual particles can exchange momentum and energy, transferring these properties between interacting particles.
While virtual particles are not directly observable, their effects can be indirectly detected through measurable physical phenomena, such as scattering amplitudes or energy shifts. The concept of virtual particles provides a mathematical framework for calculating and understanding these observable effects within the context of QFT and Feynman diagrams.
It's important to note that the interpretation of virtual particles is a mathematical tool within the framework of perturbation theory and does not imply that particles are constantly popping in and out of existence. Virtual particles are a mathematical construct used to describe the behavior of quantum fields and their interactions.