Oct 4 2019
Energy can be described as a quantity that should constantly remain positive—at least that is what people’s intuition tells them.
Vacuum is more complicated than we might think— according to quantum physics. (Image credit: © TU Wien)
A limit can be said to be reached when every single particle is eliminated from a specific volume until nothing is left there that could potentially carry energy. Or perhaps not. Can energy be still extracted even from empty space?
Quantum physics has repeatedly demonstrated that it contradicts one’s intuition—and this is also true in this example. Under specific circumstances, negative energies are enabled, at least in a specific range of time and space.
An international team of researchers at the Université libre de Bruxelles (Belgium), the TU Vienna, and the IIT Kanpur (India) has currently analyzed the extent to which negative energy could be realized.
However, it turns out that regardless of the kinds of quantum theories that are considered and irrespective of the kinds of symmetries that are believed to hold in the universe, there are constantly specific limits to “borrowing” energy. The energy can be below zero at the local level, but just like money borrowed from a bank, this energy has to be “paid back” ultimately.
Repulsive Gravity
In the theory of general relativity, we usually assume that the energy is greater than zero, at all times and everywhere in the universe.
Daniel Grumiller, Professor, Institute for Theoretical Physics, Vienna University of Technology
This holds a major outcome for gravity—that is, energy is associated with mass through the formula E = mc2. Hence, negative energy would also mean negative mass. Moreover, positive masses attract one another, but gravity can quickly become a repulsive force with a negative mass.
However, such negative energy is allowed by quantum theory.
According to quantum physics, it is possible to borrow energy from a vacuum at a certain location, like money from a bank. 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.
Daniel Grumiller, Professor, Institute for Theoretical Physics, Vienna University of Technology
Grumiller continued, “Various assumptions about this ‘interest’ (called ‘Quantum Interest’ in the literature) have been published, but no comprehensive result has been agreed upon.”
The supposed “Quantum Null Energy Condition” (QNEC), which was validated in 2017, proposes specific limits for the “borrowing” of energy by connecting quantum physics and relativity theory: A smaller-than-zero energy is therefore allowed, but only in a specific range and just for a specific time.
The quantum physical quantity, or what is known as entanglement entropy, governs the amount of energy that can be borrowed from a vacuum before the energetic credit limit can be said to be exhausted.
“In a certain sense, entanglement entropy is a measure of how strongly the behavior of a system is governed by quantum physics,” stated Daniel Grumiller. “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.”
Now, Grumiller has been able to generalize these unique calculations along with Max Riegler and Pulastya Parekh. Riegler completed his thesis in Daniel Grumiller’s research team at the TU Wien and is currently working as a postdoc at Harvard, while Parekh from the IIT in Kanpur (India) was a guest at the TU Wien and also at the Erwin Schrödinger Institute.
All previous considerations have always referred to quantum theories that follow the symmetries of Special Relativity. But we have now been able to show that this connection between negative energy and quantum entanglement is a much more general phenomenon.
Daniel Grumiller, Professor, Institute for Theoretical Physics, Vienna University of Technology
The energy conditions that vividly forbid the extraction of unlimited amounts of energy from a vacuum hold valid for extremely varied quantum theories, irrespective of symmetries.
The Law of Energy Conservation Cannot be Outwitted
Undoubtedly, this has no connection with the mystical “over unity machines” that supposedly produce energy out of nothing, since they are constantly presented in esoteric circles.
“The fact that nature allows an energy smaller than zero for a certain period of time at a certain place does not mean that the law of conservation of energy is violated,” emphasized Daniel Grumiller. “In order to enable negative energy flows at a certain location, there must be compensating positive energy flows in the immediate vicinity.”
It is not possible to obtain energy from nothing even if the matter is slightly more complex than previously believed, although it can turn out to be negative. At present, the latest study results place tight limits on negative energy, thus linking it with the typical characteristics of quantum mechanics.