: by Vienna University of Technology
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, which 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. Locally, the energy can be less than zero, but like money borrowed from a bank, this energy must be "paid back" in the end...
From the interesting comments: RealityCheck
This is yet another context where the NULL (or ZERO) is misunderstood as to its meaning in REALITY PHYSICS terms. That is, the null/zero quantum energy state actually represents the UNIVERSAL *AVERAGE* ENERGY DENSITY STATE of the underlying energy-space 'fabric' which everything forms in/from and propagates along. Hence the "NEGATIVE ENERGY" state in Quantum Vacuum is merely a "below average" energy state in that particular location; and hence any "borrowing" merely transiently REDUCES that local quantum energy density while INCREASING the energy density of the adjacent location. Moreover, that also means that the Universe DOES have ABSOLUTE POINTS OF REFERENCE by consequence of ALL LOCATIONS being thus effectively manifested ALL THE TIME across infinite energy-space quantum vacuum totality of all locations. Just because our measurements/theories have so far not been able to fully encompass such an infinite totality of locations, it doesn't mean it is not there. :)