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Morris-Thorne Wormhole / Space Tunnel

Sometimes it's called a "Morris-Thorne Wormhole"; sometimes they use "Space Tunnel" instead of "Wormhole". It was a concept developed by one of Kip Thorne's grad students, Michael Morris, to explore the idea of wormhole travel and side-stepping the speed of light barrier. To be more accurate, we should note physicists had already said that in theory wormholes could circumvent the speed of light. What Morris and Thorne did was present a scenario for making a wormhole stable enough and large enough that it was worth discussing practical issues of wormhole travel.

Their proposal was not for something humanity could seriously consider doing today. It would be a mega-engineering project, which they suggested could be carried out by "an advanced civilization". Physicist John Cramer in his column in Analog (AV 103 – see links below) said, "...a Morris-Throne wormhole would have to be of planetary dimensions, would require planet-mass quantities of negative mass-energy, and that the tidal forces created by the space curvature of the wormhole throat would be likely to destroy atoms (or people) attempting passage..." Later variants developed by other physicists may not have such extreme requirements or issues. However, the solutions remain theoretical, and if they could be implemented would require mega-engineering.

The Morris-Thorne model and variants derived from it do use a wormhole. A natural microscopic wormhole would somehow be identified. This would be a quantum level wormhole with two openings at opposite ends ("mouths") extremely close to each other. Both mouths would be artificially enlarged. The two mouths are enlarged to part of their full size. The mouths would then be transported by slower-than-light means to the locations the wormhole is intended to connect. For instance, if the wormhole is intended to connect two star systems, the wormhole would be extracted midway between the systems and the mouths would be transported to their respective star systems. Once at their intended locations, they would be enlarged to full size. A huge amount of mass-energy would be involved. The enlargement and stabilization of the wormhole mouths would also require massive engineering.

As is generally the case with wormholes, the mouths would tend to collapse and close off after a very brief amount of time. Physicists have identified scenarios in which wormholes could have naturally formed with stabilizing forces preventing such a collapse (see John Cramer's AV-53). However, we do not know that such wormholes exist. If they do, they would be much less common than the type that flashes into and out of existence.

Regardless of whether future wormhole engineers attempted to use naturally occurring unstable wormholes or were able to find naturally stable wormholes, they would have to deal with the following steps:

1) Locate a microscopic wormhole in the quantum foam
2) Somehow interact with it in order to stabilize it (if needed). This would require some degree of enlargement.
3) Transport at least one of the mouths to another location
4) Enlarge it and re-shape its topology to make it safely traversable

Variants of the Morris-Thorne model have been developed. These involve a "metric" for Einstein's relativity equations. In essence, they describe unusual space-time topologies that have extreme and complex curvatures. Their goal is to identify some type(s) of topologies that:

1) Can provide a shortcut to distant places
2) Can be widened enough to allow travel through its "tunnel"
3) Does not quickly close up again without requiring unreasonably large amounts of negative mass-energy to keep it open
4) The tidal forces and other stresses caused by the extreme space-time curvatures do not harm travelers.

This is not an easy combination of traits for a wormhole. Some interesting possibilities have been proposed. It's hard to be more definitive than that.

Orion's Arm Model

The feasibility of wormhole existence is accepted by standard physics theories, but at this time they are not something we have experimental familiarity with. As a result, most scientific discussions of wormholes, even of their practical use in the future, often don't get into specific aspects that would be the most apparent to a future person using one. To give some perspective of these, I will present some details from the Orion's Arm website. Orion's Arm is an internet "world building" project which tries to maintain consistency with known physics while envisioning a future galactic civilization. Orion's Arm describes a network of wormholes based on a variant of the Morris-Thorne model – a variant proposed by Peter Kuhfittig.

In this model, the topology of the wormhole is designed for various practical matters of safe travel through the wormhole. In order to meet those requirements, the practical distance between the "station" facility outside the wormhole and the wormhole's throat (the area halfway through the length of the wormhole) is 327 A.U. That is, 327 times the distance between the Sun and the Earth. Since that's from one station to the middle of the wormhole, the entire distance from outside one end to outside the other end of the wormhole would be twice that long. This has two major implications. First, travel through the wormhole will take quite a while unless done in a vehicle with speeds of the kind associated with spaceships. Orion's Arm even expects the spaceship to accelerate to its maximum velocity and then turn around for deceleration while in the wormhole. Therefore, the wormhole must be kept open large enough for such a vehicle to pass through it safely. Second, even at spaceship speeds, it would take a significant amount of time. The distance from outside one end to outside the other end is about 1% of a light year. That means a spaceship traveling at an average of 1% of light speed would take a year to traverse it. A spaceship traveling at light speed would require 1% of a year (3.65 days) to traverse it. Orion's Arm puts travelers into hibernation in order to allow 10 G acceleration, resulting in transit time of 32 days. This is certainly faster than interstellar travel through normal space, but it's not the instantaneous wormhole travel often portrayed in fiction.

(It is necessary to have a minimal velocity when at the wormhole's throat. So spaceships accelerate from when they enter the wormhole until it is about halfway to the throat, it then turns around and decelerates. After passing the throat, the ship accelerates until it is about halfway to the other end. It then turns around and decelerates. This limits the amount of speed the ship can build up during each acceleration. The ships never reach a speed where significant time dilation occurs for the travelers.)

For 3 A.U. on either side of the throat is a section of the wormhole's length called the Vortex. There the topology causes gravitational stresses and other factors to make this an increasingly dangerous region. Because of the extreme conditions in the throat, the ship is essentially limited to remaining on the previously established course, unable to respond to dangers.

In order for spaceships to pass through this difficult region, Orion's Arm allows a 100 meter radius for the throat. Given that size, an idealized wormhole would have a minimum mass of 1.36×1017 kilograms. Orion's Arm says, "In practice it is not ever achievable…" They mention a much more easily attainable solution as leading to a mass 10 billion times that of the Sun. The greater the mass, the more resources humans must put into the wormhole – so a lower mass is essential. In the Orion's Arm universe, they assume wormhole engineering has somehow progressed to the point of being within "orders of magnitude" of the ideal. (That's rather vague, but we can take that as meaning at least 100 times that of the ideal, which would be 1.36×1019 kilograms.)

According to physicists, if one transports a wormhole mouth at relativistic speeds, this results in the two wormhole mouths connecting two points in time. This arrangement can constitute a time machine. Many physicists believe this has consequences that will destabilize the wormhole. For just a single wormhole, the two mouths can be kept at the same time by extracting the wormhole halfway between the two star systems and transporting the two mouths to their respective destinations at the same speed. However, that does not solve issues that can arise in a network of various wormholes. In Orion's Arm, to avoid potential time machine-related problems, they place the transported mouths in an orbit far from the star. That results in additional travel time from the wormhole to the inner system.

Closing Note

As you can see from the Orion's Arm scenario, the traits a wormhole would need to have in order to be safely traversable could have far reaching implications. The tunnel might have to be quite long and more hazardous than traveling through interstellar space. The wormhole might have a mass many times that of the Sun – and that has a number of consequences. We can't say what solutions may or may not lie in the future, but there are clearly challenges between us and any solutions.

Links

John Cramer's Alternate View column from Analog magazine:
AV-33: Wormholes and Time Machines
AV-39 Wormholes II: Getting There in No Time
AV-53 Natural Wormholes: Squeezing the Vacuum
AV-69: NASA Goes FTL - Part 1: Wormhole Physics
AV-103 New Improved Wormholes

Wormhole engineering in Orion's Arm
http://www.orionsarm.com/tech/wormholes.pdf
http://insecure.ucdavis.edu/Members/adam/wormholes.pdf/view
http://www.orionsarm.com/intro/wormholes_and_exotic_matter_faq.html

A summary for a "Faster Than Light" course

For further info you might read:
http://emis.math.ecnu.edu.cn/journals/LRG/Articles/lrr-2004-9/articlesu21.html
http://www.uta.edu/physics/main/resources/ug_seminars/papers/RelativityandWormholes.doc
http://arxiv.org/pdf/gr-qc/0510079