Short answer: The Heisenberg Uncertainty Principle
The Uncertainty Principle is not well understood and is generally taught improperly. The formulation is taken to mean that the position and momentum of a particle cannot be KNOWN to better than h-bar (h-bar is Planck's Constant divided by 2 pi).
delta X times delta P = h-bar
However, what the Uncertainty Principle really means is that a particle does not HAVE a momentum and a position to better than h-bar.
The difference is significant.
Consider a simple hydrogen atom with an electron around the nucleus. What keeps the negatively charged electron from spiraling in to the nucleus and combining with the proton??? It is a classic General Exam question. Indeed, if you calculate the answer from classical electrodynamics, the electron does spiral into the nucleus in something like 10^-31 seconds. The reason that it doesn't is that if the electron got too close to the nucleus, its position would be very small and therefore it would have to have a higher momentum. The momentum shoots the electron away from the nucleus.
It is the Heisenberg Uncertainty Principle that keeps the electrons in atoms from collapsing into the nucleus.
A reformulation of the HUP is delta Energy times delta Time = h-bar.
delta E * delta t <= h-bar
In a similar fashion, this formulation means that, for sufficiently short periods of time, the vacuum field (not vacuum space) must have signficant energies. And, if these energies are greater than the mass of a particle and its anti-particle, then the particles are formed. However, they recombine immediately and return the energy to the field, because this can only occur on time scales that are very short. That is why they are called virtual particles.
So virtual particles are a function of (t)?