In physics, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional (2D) surface. If this surface is folded along a third dimension, it allows one to picture a wormhole "bridge". (Please note, though, that this is merely a visualization displayed to convey an essentially unvisualisable structure existing in 4 or more dimensions. The parts of the wormhole could be higher-dimensional analogues for the parts of the curved 2D surface; for example, instead of mouths which are circular holes in a 2D plane, a real wormhole's mouths could be spheres in 3D space.) A wormhole is, in theory, much like a tunnel with two ends each in separate points in spacetime. Read more

Title: A Star Harbouring a Wormhole at its Center Authors: V. Dzhunushaliev, V. Folomeev, B. Kleihaus, J. Kunz

We consider a configuration consisting of a wormhole filled by a perfect fluid. Such a model can be applied to describe stars as well as neutron stars with a nontrivial topology at their center. The presence of a tunnel allows for motion of the fluid, including oscillations near the center of the system. Choosing the polytropic equation of state for the perfect fluid, we obtain static regular solutions. Based on these solutions, we consider small radial oscillations and address the stability of the configurations.

Title: Can a wormhole generate electromagnetic field? Authors: Mubasher Jamil (Version v3)

We have considered the possibility of a slowly rotating wormhole surrounded by a cloud of charged particles. Due to slow rotation of the wormhole, the charged particles are dragged thereby producing an electromagnetic field. We have determined the strength of this electromagnetic field and the corresponding flux of radiation.

Two Russian scientists have claimed that they are close t to creating a time machine. Russian mathematicians Irina Arefeva and Igor Volovich have said the Large Hadron Collider (LHC) - a giant atom-smashing machine - could open the door to unexpected visitors from the future.

If there were a portal linking us to a parallel universe or some other region of space, how would we spot it? One suggestion is that it will give itself away by the curious way it bends light. The existence of wormholes linking different regions of space was suggested in 1916 by the Austrian physicist Ludwig Flamm as a possible solution to equations of general relativity, which Einstein had published that year. They have since become accepted as a natural consequence of general relativity, which predicts that matter entering one end of a wormhole would instantly emerge somewhere else, so long as the wormhole is somehow propped open. Though no direct evidence for wormholes has been observed, this could be because they are disguised as black holes. Now Alexander Shatskiy of the Lebedev Physical Institute in Moscow, Russia, is suggesting a possible way to tell the two kinds of object apart. His idea assumes the existence of a bizarre substance called phantom matter, which has been proposed to explain how wormholes might stay open. Phantom matter has negative energy and negative mass, so it creates a repulsive effect that prevents the wormhole closing. According to Shatskiys calculations, the way phantom matter deflects light would give the wormhole a signature that astronomers could look out for. The gravity of an object with a positive mass, such as an ordinary black hole, focuses light rays passing close to it as if it were a giant concave lens an effect known as gravitational lensing. Phantom matters negative mass would have the opposite gravitational lensing effect to normal matter, making any light passing through the wormhole from another universe or point in space-time diverge, and emerge from it as a bright ring. Meanwhile, any stars behind it would shine through the middle. Shatskiy suggests that his idea might offer a way for future space-based observatories such as Russias planned Millimetron Project to look for wormholes at the centre of large galaxies. Other researchers point out that the idea relies on several untested assumptions.

It is an interesting attempt to actually think of what a real signature for a wormhole would be, but it is more hypothetical than observational. Without any idea of what phantom matter is and its possible interactions with light, it is not clear one can provide a general argument - Lawrence Krauss at Case Western Reserve University in Cleveland, Ohio.

Critics also point out that even if phantom energy does exist, other objects might create a similar light signature.

The basic mechanism would not distinguish wormholes from negative energy stars - Don Marolf at the University of California, Santa Barbara.

Whats more, todays telescopes would struggle to see the signature in enough detail, says astronomer Daniel Holz at the University of Chicago, though he doesnt reject Shatskiys idea out of hand.

Its an interesting thing to think about, maybe after a few beers. Source New Scientist

Title: Electromagnetic Wormholes and Virtual Magnetic Monopoles from Metamaterials Authors: Allan Greenleaf, Yaroslav Kurylev, Matti Lassas, Gunther Uhlmann

We describe new configurations of electromagnetic (EM) material parameters, the electric permittivity epsilon and magnetic permeability µ, which allow one to construct devices that function as invisible tunnels. These allow EM wave propagation between the regions at the two ends of a tunnel, but the tunnels themselves and the regions they enclose are not detectable to lateral EM observations. Such devices act as wormholes with respect to Maxwell's equations and effectively change the topology of space vis-à-vis EM wave propagation. We suggest several applications, including devices behaving as virtual magnetic monopoles, invisible cables, and scopes for MRI-assisted surgery.

Title: A traversable wormhole Authors: S. Krasnikov (Version v5)

The paper has been withdrawn by the author, due to a fatal error. A horse stumbles that has four legs.

A class of static Lorentzian wormholes with arbitrarily wide throats is presented in which the source of the WEC violations required by the Einstein equations is the vacuum stress-energy of the neutrino, electromagnetic, or massless scalar field.

Title: Might EPR particles communicate through a wormhole? Author: E. Sergio Santini

We consider the two-particle wave function of an Einstein-Podolsky-Rosen system, given by a two-dimensional relativistic scalar field model. The Bohm-de Broglie interpretation is applied and the quantum potential is viewed as modifying the Minkowski geometry. In this way an effective metric, which is analogous to a black hole metric in some limited region, is obtained in one case and a particular metric with singularities appears in the other case, opening the possibility, following Holland, of interpreting the EPR correlations as being originated by an effective wormhole geometry, through which the physical signals can propagate.

The objects scientists think are black holes could instead be wormholes leading to other universes, a new study says. If so, it would help resolve a quantum conundrum known as the black hole information paradox, but critics say it would also raise new problems, such as how the wormholes would form in the first place. A black hole is an object with such a powerful gravitational field that nothing, not even light, can escape it if it strays within a boundary known as the event horizon. Einstein's theory of general relativity says black holes should form whenever matter is squeezed into a small enough space. Though black holes are not seen directly, astronomers have identified many objects that appear to be black holes based on observations of how matter swirls around them. But physicists Thibault Damour of the Institut des Hautes Etudes Scientifiques in Bures-sur-Yvette, France, and Sergey Solodukhin of International University Bremen in Germany now say that these objects could be structures called wormholes instead. Wormholes are warps in the fabric of space-time that connect one place to another. If you imagine the universe as a two-dimensional sheet, you can picture a wormhole as a "throat" connecting our sheet to another one. In this scenario, the other sheet could be a universe of its own, with its own stars, galaxies and planets. Damour and Solodukhin studied what such a wormhole might look like, and were surprised to discover that it would mimic a black hole so well that it would be virtually impossible to tell the difference

For budding time travellers, the future (or should that be the past?) is starting to look bleak. Hypothetical tunnels called wormholes once looked like the best bet for constructing a real time machine. These cosmic shortcuts, which link one point in the Universe to another, are favoured by science fiction writers as a means both of explaining time travel and of circumventing the limitations imposed by the speed of light. The concept of wormholes will be familiar to anyone who has watched the TV programmes Star trek or Dr Who.

The opening sequence of the BBC's new Doctor Who series shows the Tardis hurtling through a tunnel that suspiciously resembles a wormhole - although the Doctor's preferred method of travel is not known. But the idea of building these so-called traversable wormholes is looking increasingly shaky, according to two new scientific analyses. Remote connection A common analogy used to visualise these phenomena involves marking two holes at opposite ends of a sheet of paper, to represent distant points in the Universe. One can then bend the paper over so that the two remote points are positioned on top of each other. "(The wormholes) you would like to build - the predictable ones where you can say Mr Spock will land in New York at 2pm on this day - those look like they will fall apart" - Stephen Hsu, University of Oregon

If it were possible to contort space-time in this way, a person might step through a wormhole and emerge at a remote time or distant location. The person would pass through a region of the wormhole called the throat, which flares out on either side. According to one idea, a wormhole could be kept open by filling its throat with an ingredient called exotic matter. This is strange stuff indeed, and explaining it requires scientists to look beyond the laws of classical physics to the world of quantum mechanics. Exotic matter is repelled, rather than attracted, by gravity and is said to have negative energy - meaning it has even less than empty space. But according to a new study by Stephen Hsu and Roman Buniy, of the University of Oregon, US, this method of building a traversable wormhole may be fatally flawed. In a paper published on the arXiv pre-print server, the authors looked at a kind of wormhole in which the space-time "tube" shows only weak deviations from the laws of classical physics.

These "semi-classical" wormholes are the most desirable type for time travel because they potentially allow travellers to predict where and when they would emerge. Wormholes entirely governed by the laws of quantum mechanics, on the other hand, would likely transport their payloads to an undesired time and place. According to Leonard Susskind, of Stanford University in California, US, a time traveller would emerge "in the infinitely remote future or the infinitely remote past". Calculations by the Oregon researchers show a wormhole that combines exotic matter with semi-classical space-time would be fundamentally unstable. This result relies in part on a previous paper in which Hsu and Buniy argued that systems which violate a physical principle known as the null energy condition become unstable. "We aren't saying you can't build a wormhole. But the ones you would like to build - the predictable ones where you can say Mr Spock will land in New York at 2pm on this day - those look like they will fall apart," Dr Hsu said. (Although quantum theory allows the existence of negative energy, it also appears to place strong restrictions - known as quantum inequalities - on its magnitude and duration. These inequalities were first suggested in 1978. The inequalities bear some resemblance to the uncertainty principle. They say that a beam of negative energy cannot be arbitrarily intense for an arbitrarily long time. The permissible magnitude of the negative energy is inversely related to its temporal or spatial extent. An intense pulse of negative energy can last for a short time; a weak pulse can last longer. Furthermore, an initial negative energy pulse must be followed by a larger pulse of positive energy (see illustration). The larger the magnitude of the negative energy, the nearer must be its positive energy counterpart. These restrictions are independent of the details of how the negative energy is produced. One can think of negative energy as an energy loan. Just as a debt is negative money that has to be repaid, negative energy is an energy deficit. )

Pulses of negative energy are permitted by quantum theory but only under three conditions. First, the longer the pulse lasts, the weaker it must be (a, b). Second, a pulse of positive energy must follow. The magnitude of the positive pulse must exceed that of the initial negative one. Third, the longer the time interval between the two pulses, the larger the positive one must be - an effect known as quantum interest (c).

A separate study by Christopher J Fewster, of the University of York, UK, and Thomas Roman, of Central Connecticut State University, US, takes a different approach to tackling the question of wormholes. Amongst other things, their analysis deals with the proposal that wormhole throats could be kept open using arbitrarily small amounts of exotic matter. In the Casimir effect, the negative energy density between the plates can persist indefinitely, but large negative energy densities require a very small plate separation. The magnitude of the negative energy density is inversely proportional to the fourth power of the plate separation. Just as a pulse with a very negative energy density is limited in time, very negative Casimir energy density must be confined between closely spaced plates. According to the quantum inequalities, the energy density in the gap can be made more negative than the Casimir value, but only temporarily. In effect, the more one tries to depress the energy density below the Casimir value, the shorter the time over which this situation can be maintained. When applied to wormholes and warp drives, the quantum inequalities typically imply that such structures must either be limited to submicroscopic sizes, or if they are macroscopic the negative energy must be confined to incredibly thin bands. In 1996 we showed that a submicroscopic wormhole would have a throat radius of no more than about 10^-32 meter. This is only slightly larger than the Planck length, 10^-35 meter, the smallest distance that has definite meaning. We found that it is possible to have models of wormholes of macroscopic size but only at the price of confining the negative energy to an extremely thin band around the throat. For example, in one model a throat radius of 1 meter requires the negative energy to be a band no thicker than 10^-21 meter, a millionth the size of a proton. Visser has estimated that the negative energy required for this size of wormhole has a magnitude equivalent to the total energy generated by 10 billion stars in one year. The situation does not improve much for larger wormholes. For the same model, the maximum allowed thickness of the negative energy band is proportional to the cube root of the throat radius. Even if the throat radius is increased to a size of one light-year, the negative energy must still be confined to a region smaller than a proton radius, and the total amount required increases linearly with the throat size.

Fewster and Roman calculated that, even if it were possible to build such a wormhole, its throat would probably be too small for time travel. It might - in theory - be possible to carefully fine-tune the geometry of the wormhole so that the wormhole throat became big enough for a person to fit through, says Fewster. But building a wormhole with a throat radius big enough to just fit a proton would require fine-tuning to within one part in 10 to the power of 30. A human-sized wormhole would require fine-tuning to within one part in 10^60.

"Frankly no engineer is going to be able to do that," said the York researcher. It seems that wormhole engineers face daunting problems. They must find a mechanism for confining large amounts of negative energy to extremely thin volumes. So-called cosmic strings, hypothesized in some cosmological theories, involve very large energy densities in long, narrow lines. But all known physically reasonable cosmic-string models have positive energy densities. Warp drives are even more tightly constrained, as shown by Pfenning and Allen Everett of Tufts. In Alcubierre's model, a warp bubble travelling at 10 times light speed (warp factor 2) must have a wall thickness of no more than 10^-32 meter. A bubble large enough to enclose a star ship 200 meters across would require a total amount of negative energy equal to 10 billion times the mass of the observable universe. Similar constraints apply to Krasnikov's superluminal subway. A modification of Alcubierre's model was recently constructed by Chris Van Den Broeck of the Catholic University of Louvain in Belgium. It requires much less negative energy but places the starship in a curved space-time bottle whose neck is about 10^-32 meter across, a difficult feat. These results would seem to make it rather unlikely that one could construct wormholes and warp drives using negative energy generated by quantum effects.

The authors are currently preparing a manuscript for publication. However, there is still support for the idea of traversable wormholes in the scientific community. "Violations of the null energy condition are known to occur in a number of situations. And their argument would prohibit any violation of it". "If that's true, then don't worry about Hawking radiation from a black hole, the entire black hole vacuum becomes unstable." Cambridge astrophysicist Stephen Hawking is amongst those researchers that have pondered the question of wormholes. In the 1980s, he argued that even if a wormhole could be stabilised by exotic matter, something fundamental in the laws of physics would prevent them being used for time travel. This idea forms the basis of Hawking's Chronology Protection Conjecture.