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+
m2 c4
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 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.
 |
 |
| A neutron decay to a
proton, an electron, and an anti-neutrino via a virtual
(mediating) W boson. This is neutron beta decay. |
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.
