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The other kind of wormhole, would be, one that can be crossed through in both directions. This kind of wormhole, is known as a traversable wormhole. However, a traversable wormhole, would only be possible, if, they were stabilized by a kind of exotic matter, with a negative energy density. There have been theoretical demonstrations, via quantum field theory, that, the energy density of certain regions of space can be negative. The Casimir effect, shows precisely this: that quantum field theory, can allow the energy density in certain regions of space, to be negative, relative to the energy of the vacuum. The Casimir effect represents a region of negative pressure density.
This is the closest real physical representation of what this kind of exotic matter would look like. These effects could potentially provide stability for a traversable wormhole. It could be possible, at least, in theory. In the context of general relativity, physicists are at a loss, to find a natural mechanism that may produce a traversable wormhole. However, quantum mechanics gives us an interesting possibility: the quantum foam. Quantum foam is a term made by John Archibald Wheeler in the 1950s. This was the proposal that, spacetime, at it’s most fundamental level, is not necessarily smooth. However, instead, at the scale of quantum gravity, spacetime could be fluctuating, in an ever-changing kind of foam. This hypothesis could suggest that very small wormholes could be appearing and disappearing (perhaps frequently) at the Planck scale (10^-35 meters).
In 1988, Kip Thorne, Michael Morris and Ulvi Yurtsever showed that it could be possible to build a time machine if someone could obtain some exotic negative matter (which has never been seen to exist) or energy (which only exists in small quantities). The idea behind negative matter and energy is that they can make a wormhole traversable. This means you can make a mild two-way trip through it. However, the problem with this kind of matter is that it is repelled by ordinary matter. It possesses antigravity. This makes it extremely difficult to find (if it even exists at all). That is not to say that there may not be some negative matter floating in deep space, that is possible. Negative energy, on the other hand, is possible, however, it is extremely rare.
Could a wormhole be used to build a working time machine? Kip Thorne from the California Institute of Technology, in 1988 asked that very question. Thorne proposed that time travel is possible by means of a wormhole under some specific conditions. There were several conditions that Thorne had to address when proposing his wormhole. These are the reasons that physicists didn’t take wormholes serious at the time:
1. The gravitational force at the center of a black hole is so strong that any time traveler would be ripped to shreds at the singularity.
2. Small disturbances could cause the Einstein-Rosen bridge to collapse, since these wormholes are in principle, unstable. Even the presence of the smallest of space crafts could be enough to close the mouth of this unstable wormhole.
3. To get to the other side of the wormhole, one would have to travel faster than the speed of light. This is impossible.
4. The large quantum effects may cause the wormhole to close in on itself. Intense radiation emitted by the black holes entrance would be sufficient to kill anyone who tries to enter or to even close the entrance.
5. Since time slows down as you approach a source of intense gravitational attraction, time would stop as one approached the center of the black hole.
The idea was that a wormhole could connect two distant points in time. A wormhole could connect to the present to the past. However, unlike in science fiction, this wormhole would require vast amounts of energy. This is perhaps reserved for advanced extraterrestrial civilizations. This is beyond what is technologically possible for centuries to come.
At any rate, this kind of wormhole is known as a transversable wormhole. This is too distinguish it from the non-transversable Schwarzschild wormhole or Einstein-Rosen bridge. Kip Thorne’s wormhole would not close up on itself and the traveler would experience the same amount of gravitational attraction that they feel on Earth. The trip would take about 200 days or less and would be about as comfortable as an airplane ride.
An example of a Thorne wormhole requires two chambers. These chambers were created, by means of two pairs of metal plates that run parallel to each other. There would be an intense electric force produced by these plates (larger than anything we are capable of by today’s technology). This force would be so strong that it produces a hole in the very fabric of spacetime that would connect the two chambers:
1. One chamber stays stationary on Earth.
2. The other chamber is accelerated by means of a high powered rocket to some near-light speed (relativistic) velocity and is
The chamber that has been accelerated (because of time dilation in special relativity: the faster you move the slower your clock ticks) ticks slower, thus, you can travel in time!
This kind of wormhole solution was titled, a "traversable wormhole." This is because, the one traveling through would not have to worry about being ripped apart by a collapsed star.
The Casimir effect is named for Hendrik Casimir.
However, the reason we don’t have these transversable wormhole time machines for sale today has to do with the very nature of the matter/energy necessary to construct them. What is required is a kind of exotic matter. This matter has some unusual properties, however, does not violate any of the laws of physics. If this kind of exotic matter exists, which it may, then it could be harnessed by some sufficiently advanced civilization.
In physics, all of the objects that we are aware of have a positive energy. However, if objects with negative energy can be produced, or energy less than the vacuum, then perhaps we can bend space in the kind of ways that we are talking about here. We could bend spacetime into a circle. What must be violated is known as the AWEC (averaged weak energy condition). For these kinds of time machines to be constructed, energy must become negative for some period of time. Although relativity theorists scoff at the idea of negative energy, as it would produce anti-gravity, which has never been observed, quantum mechanics can give us a glimmer of hope. Hendrik Casimir, in 1948, gave a quantum mechanical argument for negative energy. This demonstration involves two uncharged parallel metal plates. When we apply the Heisenberg Uncertainty principle to the vacuum or the “empty space” in between these plates we see that it is not so empty. In fact, there is a lot of activity. These are trillions of virtual particles: particle and antiparticle pairs that come in and out of existence. They are unobservable, however, do not violate the laws of physics. These virtual particles create a kind of negative attractive force between the two plates. Hendrik Casimir predicted that this force could be measured. This has been deemed the “Casimir effect”. Thus, perhaps we can place two parallel plates at the entrance and exit of the wormhole. This would produce negative energy at each end. We still do not know how plausible this idea is, as even Thorne admits. We will not know until we have a full theory of quantum gravity.
One must obtain large quantities of negative energy, which is rare indeed. Stephen Hawking has showed that, for all wormhole solutions, negative energy is required. To keep the mouth of the wormhole open and stable, large amounts of negative energy are necessary. Negative energy created using the Casimir effect would result in a wormhole too small to travel through. There are other places, besides using the Casimir effect, that we may find negative energy:
A rapidly moving mirror
High powered laser beams
Black hole event horizons
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