Virtual particles

In physics, a virtual particle is a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum due to the Heisenberg Uncertainty Principle. (Indeed, because energy and momentum in quantum mechanics are time and space derivative operators, then due to Fourier transforms their spans are inversely proportional to time duration and position spans respectively).
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These are particles of matter (together with their antimatter counterparts) that spontaneously spring into existence and then annihilate one another in accordance with Heisenberg's Uncertainty Principle , (E x T < h) and Einstein's Special Relativity relation, (E=mc2). See Hawking Radiation .
Virtual particles are the carriers of the four fundamental forces, e.g.virtual photons carry the electromagnetic force and virtual mesons (mostly pions) transmit the strong nuclear force. In this way action-at-a-distance is understood. The range of nuclear forces was first explained by Yukawa, 1932 . He assumed the proton lost mass (M) , that is energy E (=Mc2), for a time T which was too short to detect because the product E x T was less than approximately Planck's constant (h). The proton created and emitted a pi-meson of rest mass energy 140 MeV for a time of 10-22 second, during which time, travelling close to the speed of light, it could go about 1 fermi i.e. 10-15 m. This distance is approximately the diameter of a proton and neutron and is the known range of nuclear forces. The more massive the particles are, the shorter the length of time during which they can exist, and heavier short lived particles , such as the kaon , moderate the nuclear force.

A continuous search is always in progress to see if there is a graviton which would be the exchange particle of the gravitational interaction.
The particles that are assumed to carry the weak nuclear force are the W and Z particles.