Negative energy

If a wormhole could exist, it would appear as a spherical openingto an otherwise distant part of the cosmos. In this doctored photograph of Times Square, the wormhole allows New Yorkers to walk to the Sahara with a single step, rather than spending hours on the plane to Tamanrasset. although such a wormhole does not break any known laws of physics, it would require the production of unrealistic amounts of negative energy.Can a region of space contain less than nothing?Common sense would say no; the most one could do is remove all matter and radiation and be left with vacuum. Butquantum physicshas a proven ability to confound intuition, and this case is no exception. A region of space, it turns out, can contain less thanw nothing. Its energy per unit volume - the energy density - can be less than zero.Needless to say, the implications are bizarre. According toEinstein’s theory of gravity, general relativity, the presence of matter and energy warps the geometric fabric of space and time.What we perceive as gravity is the space-time distortion produced by normal, positive energy or mass.But when negative energy or mass - so-called exotic matter - bends space-time, all sorts of amazing phenomena might become possible: traversable wormholes, which could act as tunnels to otherwise distant parts of the universe; warp drive, which would allow for faster-than-light travel; andtime machines, which might permit journeys into the past.Negative energy could even be used to make perpetual-motion machines or to destroyblack holes. AStar Trekepisode could not ask for more.For physicists, these ramifications set off alarm bells. The potential paradoxes of backward time travel–such as killing yourgrandfather before your father is conceived–have long been explored in science fiction, and the other consequences of exotic matter are also problematic.They raise a question of fundamental importance: Do the laws ofphysics that permit negative energy place any limits on its behavior?We and others have discovered that nature imposes stringent constraints on the magnitude and duration of negative energy, which (unfortunately, some would say) appear to render the construction of wormholes and warp drives very unlikely.
Double Negative:

Before proceeding further, we should draw the reader’s attention to what negative energy is not.It should not be confused with antimatter, which has positive energy. When an electron and its antiparticle, a positron, collide, they annihilate. The end products are gamma rays, which carry positive energy. If antiparticles were composed of negative energy, such an interaction would result in a final energy of zero.One should also not confuse negative energy with the energy associated with the cosmological constant, postulated in inflationary models of the universe [see "Cosmological Antigravity," byLawrence M. Krauss; SCIENTIFIC AMERICAN, January 1999]. Such a constant represents negative pressure but positive energy. (Some authors call this exotic matter; we reserve the term for negative energy densities.)The concept of negative energy is not pure fantasy; some of its effects have even been produced in the laboratory. They arise fromHeisenberg’s uncertainty principle, which requires that the energy density of any electric, magnetic or other field fluctuate randomly. Even when the energy density is zero on average, as ina vacuum, it fluctuates.Thus, the quantum vacuum can never remain empty in the classical sense of the term; it is a roiling sea of "virtual" particles spontaneously popping in and out of existence [see "Exploiting Zero-Point Energy," by Philip Yam; SCIENTIFIC AMERICAN, December 1997]. In quantum theory, the usual notion of zero energy corresponds to the vacuum with all these fluctuations.So if one can somehow contrive to dampen the undulations, the vacuum will have less energy than it normally does–that is, less than zero energy.

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