Virtual particle terms represent "particles" that are said to be 'off mass shell'. For example, they progress backwards in time, do not conserve energy, and travel faster than light.
According to my transforms in my paper " The Quantization of General Relativity: Photon Mediates Gravitation" http://dx.doi.org/10.14299/ijser.2014.10.002 by refusing objectivity and symmetry, it is resulted that the Lorentz transformation is vacuum energy dependent, and it leads to the wave-particle duality and Heisenberg uncertainty principle. According to my transforms, the Lorentz factor is equivalent to the refractive index in optics, that means according to quantum theory when the light incident inside the moving train with constant speed v, we get the Helmholtz equation for phenomena periodic in time, with a frequency of f=w/2pi inside the moving train for the observer on the ground. Now for the moving train in constant speed v (inertial frame) on the ground the wave that describe its motion is given by solving Schroedinger equation with potential equals to zero relative to the observer on the ground. According to my transforms, we can understand the relationship between the potential, vacuum and medium, they are one thing! Now when we are talking about fields, we are talking about the changing of the potential, and that leads to acceleration or deceleration and that equivalent according to my transforms to vacuum fluctuations,
Now by refusing the Lorentz symmetry in my transforms which is leading to disappearing all the paradoxes in SRT, thus, for the observer of the moving train, it is considered the difference of vacuum of the earth surface comparing to the vacuum of the moving train for the observer of the moving train is negative,(refractive index) to be purely imaginary. Then the solution of Helmholtz's equation is called an evanescent mode. In this case it is leading to measure a faster than light. But that required to understand how the vacuum fluctuations, and the Heisenberg uncertainty principle according to my transforms which is related to changing the potential (acceleration or deceleration in case of the moving train) which is leading to faster than light according to my transforms in the case of the train's velocity changed from v to zero for the observer of the moving train, and then how the twin in the moving train when he stops his train on the ground, he will find the twin on the ground older than him. In this case it is possible measuring faster than light without violation Lorentz invariance or causality according to my transforms, and the speed of light remains locally constant, and equals to the speed of light in vacuum according to my transforms. This concept is completely agreed with the concept of virtual particles.