Einstein-Rosen Bridges (ERBs), or wormholes, are a popular feature in science fiction. They were featured prominently in shows such as Star Trek: Voyager and Sliders. These portals have their basis in Einstein's Theory of General Relativity. A wormhole exists at the center of a black hole. A black hole is a star which has collapsed; its massive gravitational field sucks in everything which passes the boundary, called the event horizon.
Lorentzian wormholes known as Schwarzschild wormholes or Einstein-Rosen bridges are bridges between areas of space that can be modeled as vacuum solutions to the Einstein field equations by combining models of a black hole and a white hole. This solution was discovered by Albert Einstein and his colleague Nathan Rosen, who first published the result in 1935. However, in 1962 John A. Wheeler and Robert W. Fuller published a paper showing that this type of wormhole is unstable, and that it will pinch off instantly as soon as it forms, preventing even light from making it through. Before the stability problems of Schwarzschild wormholes were apparent, it was proposed that quasars were white holes forming the ends of wormholes of this type.
While Schwarzschild wormholes are not traversable, their existence inspired Kip Thorne to imagine traversable wormholes created by holding the 'throat' of a Schwarzschild wormhole open with exotic matter (material that has negative mass/energy).
In short, Hawking sums up his description of what happens within a black hole in this way:
…thus if light cannot escape, neither can anything else; everything is dragged back by the gravitational field. So one has a set of events from which it is not possible to escape to reach a distant observer. This region is what we now call a black hole. Its boundary is called the event horizon and it coincides with the paths of light rays that just fail to escape from the black hole.
Credit for the first theoretical musing regarding the principles behind black holes actually predates Einstein considerably, though. It was an eighteenth century geologist, of all people, named John Michell, who first considered the phenomenon.
Based on Newton's theory of gravitation, it was well accepted that every object possesses an "escape velocity," that is, the velocity at which an object has to be traveling in order to break free of the object's gravitational attraction. The escape velocity of the Earth is about 11.2 km/s, while the sun's is 615 km/s.
This simply means that if one was to fire a cannon straight into the air from the Earth's surface at a velocity of 10 km/s, it would not be moving fast enough to escape the Earth, and would eventually come crashing back down again. If fired at a speed of 12 km/s, however, it would have enough speed to finally break free and to fly into outer space, never to be seen or heard from again.
It was with this concept in mind that Michell pondedered this principle and wondered what kind of conclusions it could be led to. Eventually, Michell considered that if an object had sufficient mass and thus an escape velocity greater than the speed at which light travels (about 300,000 km/s), then even light wouldn't be moving fast enough to escape from such an object.
A white hole, the exit point to the ERB, is a black hole which runs backward in time. Black holes swallow whatever comes into contact with them, while white holes regurgitate them. White holes should not be able to exist, because they would spontaneously create order out of disorder, effectively violating the Second Law of Thermodynamics.
In 1963, mathematician Roy Kerr remedied a major difficulty of Einstein's theory by proving that, due to the principle of conservation of angular momentum, a star in the process of collapse would increase its rotation. It would have at its center a ring of neutrons manifesting extraordinary centrifugal force outward, countering the force of gravity on the inside.
A particle, object, or traveller would then not be crushed by the forces, but would travel through the point of singularity and exit the other side of the black hole, or white hole, perhaps within the same universe at a different location or time, or perhaps in a parallel universe.
However, any travel through this Kerr ring would be necessarily one-way. The gravity created by the collapse, though no longer sufficient to kill the individual, would prevent a return trip.
Physicists calculating the logistics of this form of travel find that a traveler may not survive travel through this medium. A light beam which followed an individual would acquire so much energy in the length of the ERB that it would kill the person and create such a high gravitational field that it would close the bridge.
John Wheeler, in 1962, (who first used the term "wormhole") found that an ERB is unstable in space, and if it opened, it would close again instantly, before a photon could travel through it.
An ERB would need a supply of negative energy in order to stay open, through a process such as the Casimir effect, which generates negative energy. Unfortunately, the Casimir effect only generates very small amounts of negative energy.
Physicists speculate that if large amounts of negative energy could be discovered, perhaps by an advanced society, then the creation of an ERB may be possible.
Lorentzian traversable wormholes would allow travel from one part of the universe to another part of that same universe very quickly or would allow travel from one universe to another. The possibility of traversable wormholes in general relativity was first demonstrated by Kip Thorne and his graduate student Mike Morris in a 1988 paper; for this reason, the type of traversable wormhole they proposed, held open by a spherical shell of exotic matter, is referred to as a Morris-Thorne wormhole. Later, other types of traversable wormholes were discovered as allowable solutions to the equations of general relativity, including a variety analyzed in a 1989 paper by Matt Visser, in which a path through the wormhole can be made in which the traversing path does not pass through a region of exotic matter. However in the pure Gauss-Bonnet theory exotic matter is not needed in order for wormholes to exist- they can exist even with no matter. A type held open by negative mass cosmic strings was put forth by Visser in collaboration with Cramer et al., in which it was proposed that such wormholes could have been naturally created in the early universe.
Wormholes connect two points in spacetime, which means that they would in principle allow travel in time, as well as in space. In 1988, Morris, Thorne and Yurtsever worked out explicitly how to convert a wormhole traversing space into one traversing time. However, it has been said a time traversing wormhole cannot take you back to before it was made but this is disputed.
Special relativity only applies locally. Wormholes allow superluminal (faster-than-light) travel by ensuring that the speed of light is not exceeded locally at any time. While traveling through a wormhole, subluminal (slower-than-light) speeds are used. If two points are connected by a wormhole, the time taken to traverse it would be less than the time it would take a light beam to make the journey if it took a path through the space outside the wormhole. However, a light beam traveling through the wormhole would always beat the traveler. As an analogy, running around to the opposite side of a mountain at maximum speed may take longer than walking through a tunnel crossing it. You can walk slowly while reaching your destination more quickly because the distance is smaller.
A wormhole could allow time travel. This could be accomplished by accelerating one end of the wormhole to a high velocity relative to the other, and then sometime later bringing it back; relativistic time dilation would result in the accelerated wormhole mouth aging less than the stationary one as seen by an external observer, similar to what is seen in the twin paradox. However, time connects differently through the wormhole than outside it, so that synchronized clocks at each mouth will remain synchronized to someone traveling through the wormhole itself, no matter how the mouths move around. This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a point in time prior to its entry.
For example, consider two clocks at both mouths both showing the date as 2000. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2005 while the stationary mouth's clock read 2010. A traveler who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2005, in the same region but now five years in the past. Such a configuration of wormholes would allow for a particle's world line to form a closed loop in spacetime, known as a closed timelike curve.
It is thought that it may not be possible to convert a wormhole into a time machine in this manner; some analyses using the semi-classical approach to incorporating quantum effects into general relativity indicate that a feedback loop of virtual particles would circulate through the wormhole with ever-increasing intensity, destroying it before any information could be passed through it, in keeping with the chronology protection conjecture. This has been called into question by the suggestion that radiation would disperse after traveling through the wormhole, therefore preventing infinite accumulation. The debate on this matter is described by Kip S. Thorne in the book Black Holes and Time Warps. There is also the Roman ring, which is a configuration of more than one wormhole. This ring seems to allow a closed time loop with stable wormholes when analyzed using semi-classical gravity, although without a full theory of quantum gravity it is uncertain whether the semi-classical approach is reliable in this case.
All around us are tiny doors that lead to the rest of the Universe. Predicted by Einstein's equations, these quantum wormholes offer a faster-than-light short cut to the rest of the cosmos - at least in principle. Now physicists believe they could open these doors wide enough to allow someone to travel through.
Quantum wormholes are thought to be much smaller than even protons and electrons, and until now no one has modelled what happens when something passes through one. So Sean Hayward at Ewha Womans University in Korea and Hisa-aki Shinkai at the Riken Institute of Physical and Chemical Research in Japan decided to do the sums.
They have found that any matter travelling through adds positive energy to the wormhole. That unexpectedly collapses it into a black hole, a supermassive region with a gravitational pull so strong not even light can escape.
But there's a way to stop any would-be traveller being crushed into oblivion. And it lies with a strange energy field nicknamed "ghost radiation". Predicted by quantum theory, ghost radiation is a negative energy field that dampens normal positive energy. Similar effects have been shown experimentally to exist.
Ghost radiation could therefore be used to offset the positive energy of the travelling matter, the researchers have found. Add just the right amount and it should be possible to prevent the wormhole collapsing - a lot more and the wormhole could be widened just enough for someone to pass through.
It would be a delicate operation, however. Add too much negative energy, the scientists discovered, and the wormhole will briefly explode into a new universe that expands at the speed of light, much as astrophysicists say ours did immediately after the big bang.
For now, such space travel remains in the realm of thought experiments. The CERN Large Hadron Collider in Switzerland is expected to generate one mini-black hole per second, a potential source of wormholes through which physicists could try to send quantum-sized particles.
But sending a person would be another thing. To keep the wormhole open wide enough would take a negative field equivalent to the energy that would be liberated by converting the mass of Jupiter.
IF YOU want to time-travel through wormholes - theoretical space-time "tunnels" that could act as short cuts through the universe - be prepared to choose between not knowing where you'll arrive or not arriving at all.
In theory, wormholes work when they are coated with a mysterious form of matter that exerts negative pressure. If a balloon were filled with the stuff, it would deflate. Physicists Roman Buniy and Stephen Hsu at the University of Oregon in Eugene calculated the properties of two types of wormholes with this exotic matter: one that follows the laws of classical physics and another that follows the laws of quantum mechanics. It turns out that with quantum-mechanical wormholes there is no guarantee about where exactly in space and time you will pop out. "The end point of the wormhole might be in a wall or under the Pacific Ocean," Hsu says. "Alternatively, you might exit a year before or after you thought you would."
Researchers had believed that classical wormholes, which are less uncertain, could serve as more practical portals through space-time. But Buniy and Hsu have found that these wormholes are inherently unstable. "If someone nudged the [wormhole] a little, it would cause the system to fall apart, the way a bridge would collapse," says Hsu. "It would probably not last long enough for you to get through to the other side."
That puts hopeful wormhole travellers between a rock and a hard place, says Hsu.