Real and Virtual Particles
Real Particles
Particles that can be observed either directly or indirectly in experiments are real particles.
Any isolated real particle satisfies the generalized Einstein relativistic relationship between its energy, E, its momentum. p, and its mass, m (c is the speed of light):
E2 = p2 c2+ m2c4
Notice that for a particle at rest, p=0, this becomes E = mc2. This is the minimum possible energy for an isolated real particle.
Virtual Particles
Virtual particles are a language invented by physicists in order to talk about processes in terms of the Feynman diagrams. These diagrams are a shorthand for a calculation that gives the probability of the process. The calculation is derived from quantum field theory.
Feynman diagrams have lines that represent mathematical expressions, but each line can also be viewed as representing a particle. However in the intermediate stages of a process the lines represent particles that can never be observed. These particles do not have the required Einstein relationship between their energy, momentum and mass. They are called "virtual" particles.
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A neutron decay to a proton, an electron, and an anti-neutrino via a virtual (mediating) W boson. This is neutron beta decay. |
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For example, in beta decay one can readily see that the energy available for the intermediate W boson cannot be greater than the mass-energy difference between a neutron and a proton, which is very much less than the mass-energy of a W boson. Thus, the W boson here cannot be observed, but the calculation based on this diagram correctly predicts the rate of the process.
Particle physicists talk about these processes as if the particles exchanged in the intermediate stages of a diagram are actually there, but they are really only part of a quantum probability calculation. It is meaningless to argue whether they are or are not there, as they cannot be observed. Any attempt to observe them changes the outcome of the process.
