Energy is generally perceived as something that must be positive at all times. Yet this particular question begs to be answered: "Is it possible to extract energy from empty space?"

A team of researchers from TU Vienna in Austria, the Université libre de Bruxelles in Belgium, and IIT Kanpur in India have combined forces to find out if negative energy is a possibility.

According to their study, it turns out that it is possible — it can be borrowed, much like money from a bank, but it also has to be returned.

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## Negative energy as a repulsive force

The theory of general relativity makes us assume that "the energy is greater than zero, at all times and everywhere in the Universe," said Professor Daniel Grumiller from the Institute for Theoretical Physics at the TU Wien (Vienna).

If we follow the theory of general relativity, this would mean that negative energy means negative mass. Positive masses attract each other, and if a negative mass arises, gravity will become a repulsive force.

Quantum theory, however, allows for negative energy.

Grumiller continued, "According to quantum physics, it is possible to borrow energy from a vacuum at a certain location, like money from a bank."

He carried on, "For a long time, we did not know about the maximum amount of this kind of energy credit and about possible interest rates that have to be paid. Various assumptions about this "interest" (known in the literature as "Quantum Interest") have been published, but no comprehensive result has been agreed upon."

By linking relative theory and quantum physics, it was found that energy that is lower than zero is then permissible, but only in a specific range, and for a specific time limit.

The amount of energy that can be borrowed from a vacuum before it hits its 'credit limit' depends on a quantum physical quantity, also known as entanglement entropy.

"In a certain sense, entanglement entropy is a measure of how strongly the behavior of a system is governed by quantum physics," said Grumiller.

"If quantum entanglement plays a crucial role at some point in space, for example, close to the edge of a black hole, then negative energy flow can occur for a certain time, and negative energies become possible in that region."

Grumiller closed off by saying: "In a certain sense, entanglement entropy is a measure of how strongly the behavior of a system is governed by quantum physics."

"If quantum entanglement plays a crucial role at some point in space, for example, close to the edge of a black hole, then a negative energy flow can occur for a certain time, and negative energies become possible in that region."