To us, vacuums appear to contain nothing at all. But, it you were to look closely, very, very closely (to the order of 10^-35m), space is actually a foaming mass of quantum activity. This quantum foam is made of particles and micro-black holes popping in and out of existence, apparently in contravention of the second law of thermodynamics, they appear out of nothing with energy, then disappear again just as quickly. The key to this is the uncertainty principle. The disturbance is permitted to ?borrow? a tiny amount of energy and exist for a very short length of time, and then it must return the energy and disappear again. But, the more energy it borrows, the less time it is allowed to exist. These ?temporary? particles, called virtual particles, are not just theoretical, they have been proven to have real effects on scientific experiment.