By looking at layman's books on physics I have picked up the idea that "virtual" particle-antiparticle pairs continually pop out of the vacuum and then back into it again. Apparently according to the Heisenberg Uncertainty Principle the time that the particle pair can exist, [itex]\Delta t[/itex], is given by [itex] \Delta t \approx h / \Delta E [/itex] where [itex] \Delta E [/itex] is the energy of the particle pair. Is there any lower limit to [itex]\Delta E[/itex] like the neutrino mass? Or could the particle pair be a pair of photons with any energy? Could [itex]\Delta t[/itex] be billions of years if the particle-pair has a very very low energy ?