Quantum Vacuum: Less than Zero Energy

Is it possible to borrow energy from an empty space? And if yes, do we have to give it back? Energy values smaller than zero are allowed - at least within certain limits. Energy is a quantity that must always be positive - at least that's what our intuition tells us. If every single particle is removed from a certain volume until there is nothing left that could possibly carry energy, then a limit has been reached. Or has it? Is it still possible to extract energy even from empty space? Quantum physics has shown time and again that it contradicts our intuition - and this is also true in this case. Under certain conditions negative energies are allowed, at least in a certain range of space and time. An international research team at the TU Vienna, the Université libre de Bruxelles (Belgium) and the IIT Kanpur (India) have now investigated the extent to which negative energy is possible. It turns out that no matter which quantum theories are considered, no matter what symmetries are assumed to hold in the universe, there are always certain limits to "borrowing" energy.