I often hear or read of virtual particles appearing because they borrow energy from the vacuum fluctuations, but they "return" the energy back to the vacuum so that there are no imbalances. Wait a minute. Doesn't the insights of relativity tell us there is no such thing as just energy or mass, but mass-energy. This mass-energy entity can manifest as either mass or energy, but it is the same beast. So, to say that particles have to exist only for a fleeting moment because they have borrowed energy from the vacuum seems redundant. If they are partices, their energy content is exactly mc sq, and so nothing has "disappeared" out of the vacuum, it's just mass-energy changed its form.