The strength of the gravitational attraction between two objects represents the amount of gravitational energy in the field which attracts them towards each other. When they are infinitely far apart, the gravitational attraction and hence energy approaches zero. As two such massive objects move towards each other, the motion accelerates under gravity causing an increase in the positive kinetic energy of the system. At the same time the gravitational attraction - and hence energy - also increase in magnitude. But the law of energy conservation requires that the net energy of the system does not change. This can only be resolved if the change in gravitational energy is negative, thus cancelling out the positive change in kinetic energy. Since the gravitational energy is getting stronger, this decrease can only mean that it is negative.
A universe in which positive energy dominates will eventually collapse in a "big crunch", while an "open" universe in which negative energy dominates will either expand indefinitely or eventually disintegrate in a "big rip". In the zero-energy universe model ("flat" or "Euclidean"), the total amount of energy in the universe is exactly zero: its amount of positive energy in the form of matter is exactly canceled out by its negative energy in the form of gravity.
Quantum field effects
In the Casimir effect, two flat plates placed very close together restrict the wavelengths of quanta which can exist between them. This in turn restricts the types and hence number and density of virtual particle pairs which can form in the intervening vacuum and can result in a negative energy density. This causes an attractive force between the plates, which has been measured.
It is possible to arrange multiple beams of laser light such that destructive quantum interference suppresses the vacuum fluctuations. Such a squeezed vacuum state involves negative energy. The repetitive waveform of light leads to alternating regions of positive and negative energy.
Negative energy appears in the speculative theory of wormholes. A wormhole directly connects two locations which may be separated arbitrarily far apart in both space and time, and in principle allows near-instantaneous travel between them.
A theoretical principle for a faster-than-light (FTL) warp drive for spaceships has been suggested, involving negative energy. It comprises a solution to Einstein's equations of general relativity, in which a bubble of spacetime is moved rapidly by expanding space behind it and shrinking space in front of it.
- Alan Guth The Inflationary Universe: The Quest for a New Theory of Cosmic Origins (1997), Random House, ISBN 0-224-04448-6 Appendix A: Gravitational Energy demonstrates the negativity of gravitational energy.
- Stephen Hawking; The Grand Design, 2010, Page 180.
- Everett, Allen; Roman, Thomas (2012). Time Travel and Warp Drives. University of Chicago Press. p. 167. ISBN 0-226-22498-8.
- Ford and Roman 2000
- Stephen Hawking; A Brief History of Time, Bantam 1988, Pages 105-107. ISBN 0-593-01518-5
- Stephen Hawking; "How to build a time machine", Mail Online 27 April 2010(retrieved 4 November 2014)
- Lawrence H. Ford and Thomas A. Roman; "Negative energy, wormholes and warp drive", Scientific American January 2000, 282, Pages 46–53.