Alcubierre Warp Drive

 An idea for achieving faster-than-light travel suggested by the Mexican theoretical physicist Miguel Alcubierre in 1994.¹ It starts from the notion, implicit in Einstein’s general theory of relativity, that matter causes the surface of space-time around it to curve. Alcubierre was interested in the possibility of whether Star Trek’s fictional “warp drive” could ever be realized. This led him to search for a valid mathematical description of the gravitational field that would allow a kind of space-time warp to serve as a means of superluminal propulsion. Alcubierre concluded that a warp drive would be feasible if matter could be arranged so as to expand the space-time behind a starship (thus pushing the departure point many light-years back) and contract the space-time in front (bringing the destination closer), while leaving the starship itself in a locally flat region of space-time bounded by a “warp bubble” that lay between the two distortions. The ship would then surf along in its bubble at an arbitrarily high velocity, pushed forward by the expansion of space at its rear and the contraction of space in front. It could travel faster than light without breaking any physical law because, with respect to the space-time in its warp bubble, it would be at rest. Also, being locally stationary, the starship and its crew would be immune from any devastatingly high accelerations and decelerations (obviating the need for “inertial dampers”), and from relativistic effects such as time dilation (since the passage of time inside the warp bubble would be the same as that outside).

Could such a warp drive be built? It would require, as Alcubierre pointed out, the manipulation of matter with a negative energy density. Such matter, known as exotic matter, is the same kind of peculiar stuff apparently needed to maintain stable wormholes-another proposed means of circumventing the light barrier. Quantum mechanics allows the existence of regions of negative energy density under special circumstances, such as in the Casimir effect.

Further analysis of Alubierre’s Warp Drive concept by Chris Van Den Broeck of the Catholic University in Leuven, Belgium,² has perhaps brought the construction of the starship Enterprise a little closer. Van Den Broeck’s calculations put the amount of energy required much lower than that quoted in Alcubierre’s paper. But this is not to say we are on the verge of warp capability. As Van Den Broeck concludes: “The first warp drive is still a long way off but maybe it has now become slightly less improbable."

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Superluminal: A Scientific Look at Science Fiction Stardrives

by Paul Lucas

Almost five decades of practical space experience has shown that just getting into low orbit is expensive and difficult. Sending missions to nearby planets takes years of planning and preparation; an interstellar mission probably won't be practical for many decades, if not centuries. The stars seem very far away indeed.

Unless someone finds a shortcut.

Faster than light (FTL) starship drives are the favored short cut used by science fiction writers when they weave stories about humanity's future among the stars. Though FTL travel remains a fantasy in real life, the hunger of many creators to get their characters out into the wilds of the Great Dark have impelled them to imagine many different means of beating a beam of light to its destination.

Some creators use FTL travel simply as a convenient plot device - an obscure means of getting Our Heroes from Situation A to Adventure B. Many others, however, use as much real-life science and theory as possible when constructing their superluminal spaceship engines. They think through the consequences of the technology and struggle to maintain an internally consistent logic.

In this article, we'll look at some of the more well-conceived means of FTL travel in science fiction, and see if and how they intersect with real world science.

 

HYPERDRIVES

The word hyperdrive takes its name from the term "hyperspace." Hyperspace is a theoretical realm that exists beyond our three-dimensional world, which permeates our universe and into which our space/time can curve.

Take a one dimensional object, a line. Turn the line in on itself at right angles and you get a two dimensional object - a square. Now take the square, turned it on itself at right angles, and we go up a dimension to the third - a cube. Now take the cube, turn it on itself at right angles and you get...

What?

According to theory, you get an object that projects into a fourth physical dimension (not to be confused with Time, which is also sometimes called the fourth dimension), called a hypercube. Trying to actually visualize this with our analog 3D brains can be difficult; however, the mathematics and logic of it work out very neatly, and leads to a number of interesting implications. Think of a 2D object, like an infinitely thin piece of paper. You can "warp" it into a third dimension by folding or rolling it. We know from Einstein that 3D space can also be warped; observational evidence for this abounds in the form of gravitational lensing effects in astronomy. But what does it warp into?

The answer is hyperspace, a fourth physical dimension beyond our three-dimensional senses' perception, which surrounds and permeates our 3D universe much like the 3D universe surrounds and permeates our 2D piece of paper.

But how does this help one travel between the stars? There is some evidence that our 3D cosmos as a whole may be curved, just as a 2D object (our piece of paper) can be curved into a sphere or parabola. Instead of travelling the entire distance over the curved surface of the paper, you could "jump" through the empty space inside the sphere, shortening the distance you have to cover. The hyperdrive uses this principle taken up a dimensional level; it allows a ship to take a more direct 4D "shortcut" to bypass the 3D "surface area" in between two points.

In order to take advantage of higher dimensions, one has to warp 3D space. As natural-occurring gravity sources already warp space, most hyperdrive schemes envision artificial, concentrated gravity fields to warp or tear the fabric of space/time in order to insert the ship into higher-dimensional space. These may be enhanced by quantum mechanical manipulation.

In several authors' versions of the hyperdrive, the drive imparts a higher dimensional "momentum" to the starship, meaning the ship can't alter course or come out of hyperspace until it comes to the end of its 4D trajectory and precipitates back into normal space. This is the case of the hyperdrives in Asimov's Foundation series, the world of the Traveller RPG, and the Star Wars Universe. In other versions, the hyperdrive opens a "portal" or undergoes a phase shift to enter hyperspace, and the ship can maneuver freely in the higher-dimensional realm. The ship then must open another portal or phase shift again to exit it. This is similar to the scheme used in Babylon 5 and the Known Space stories of Larry Niven.

However, Hyperspace is almost certain to be far more complicated than just a void with an extra physical dimension. It may be prone to odd curvatures and anomalies, and may be host to different storms, currents, hyperobjects, and perhaps even life. This is the case in Jeffrey Carver's Star Rigger series of novels, where pilots ("riggers") translate the chaotic dangers of hyperspace into visual metaphors they can more easily grasp, and literally have to "sail" through hyperdimensional storms, whirlpools, tidal waves, and other dangers.

Hyperdrive travel can also be complicated by the fact that more than one level of hyperspace can theoretically exist. After all, if there's a fourth dimensional aspect to the cosmos, why not a fifth or sixth? Each level may be subject to physical quirks we can only guess at, and would have its own dangers and advantages. Even if 3D space is flat, the 4D space that permeates it may itself be curved, allowing ships that can access the 5D level of hyperspace FTL travel where a lower level 4D hyperdrive would get the pilot nowhere. It is generally assumed in science fiction stories that accessing higher levels of hyperspace require more powerful and sophisticated drives than lower levels do.

 

WARP DRIVES

Warp drives come to us from the many incarnations of Star Trek. Instead of shunting the ship into higher-dimensional continuum, the "classic" version of the warp drive bends the space-time of our own universe around the ship. This creates a bubble of subjective space-time that is out of synch with the rest of the universe. Because it is "detached" from normal space, the bubble - and the ship within it - can be accelerated faster than lightspeed. Since the bubble is itself not a material object, this does not violate relativity. Moving is accomplished by creating a wavefront of warped space-time in front of the ship, a distortion into which the space-time bubble is constantly trying to "fall."

To visualize this, think of space as a stretched sheet of rubber, and the warp bubble as a marble resting on its surface. Instead of moving the marble directly, you push your finger into the rubber sheet, directly in front of the marble. By moving your finger at a fast enough rate, you can move the marble all the way across the sheet by creating a constantly-moving depression the marble is continually trying to fall into. Extend this analogy into four-dimensional space and you have the basis for the warp drive.

Warp bubbles can be expanded and contracted; in principle a ship in warp would be able to "tow" almost any mass smaller than itself as long as it was within the warp bubble. Warp bubbles can be made synchronous and can attach and/or merge to others like strings of soap bubbles.

Since warp bubbles are detached from normal space, they should generally be unaffected by things outside the bubble. Remember the name of the drive; the fabric of space is warped around the vessel. If an enemy outside the bubble fired a pair of laser beams right at the warp ship, the beams would bend around the ship and then resume their normal course once on the other side of the warp bubble. However, significantly large amounts of outside energy or mass can disrupt the bubble and precipitate the vessel back into normal space.

The converse is also true; a ship inside a warp bubble should not be able to affect things outside of it. However, it can absorb things in nearby space by expanding or contracting the bubble, and if two bubbles are close enough, they may merge, making activities like exchanging cargo and weapons fire possible, but extremely tricky, at translight speeds.

This FTL scheme was given a bit of a boost in scientific credibility in 1994 when Miguel Alcubierre, a young physicist at the time, wrote a paper exploring the possibility of a warp-drive like system within the framework of Einstein's Theories of Relativity. Alcubierre's version of the warp drive is similar to Star Trek's, but instead of creating a distortion to fall into, the ship contracts the fabric of space/time in front of it, compressing a small part of the distance to its destination, and expands it toward the aft, increasing the amount of space between it and its origin point. It would, in a sense, be like "surfing" on a ship-created wave of manipulated space/time. Unfortunately, Alcubierre's scheme requires an enormous amount of negative energy, something that has only been tentatively detected in laboratories in exceedingly small amounts. Exactly how to acquire and use the vast amounts needed remains unknown, and keeps the Alcubierre's version of the warp drive from being taken more seriously than Star Trek's.

 

TELEPORT DRIVES

Teleportation was one of the great Macguffins of science fiction - an incredibly neat idea that once seemed to have little real scientific basis. However, Recent experiments with quantum teleportation in laboratories has shown this may not be the case. Wholesale teleportation of objects larger than a photon still seem extremely unlikely, but cannot be completely ruled out. Cheap and easy teleportation would have a number of profound effects on human civilization. Among other things, it could make for a means of superluminal travel, depending exactly on how the technology worked.

The matter-to-energy-and-back-to-matter transfer seen in Star Trek wouldn't work as an FTL drive; it's limited to lightspeed, and signal degradation would almost certainly occur over distances of light years. It wouldn't be too fun to have only three-quarters of your original molecules show up at the destination point. The same is true of quantum teleportation currently being investigated in the real world; it too is limited to the speed of information exchange. It also has the rather disturbing feature of having to destroy the teleported object at the transmission site before it can be reassembled on a quantum level at the destination.

Instead, the type of teleportation most useful to star travelers would perhaps be based on a well known phenomenon: quantum tunneling, also called the tunnel-diode effect. Thanks to a quirk in the way quantum physics works, it's entirely possible for a particle to disappear in one spot and appear in another almost instantaneously. The particle doesn't actually "go" anywhere in between; it simply fades from existence in one position and fades back into existence in another, as if the universe is using it as a variable in a strange kind of existential bookkeeping. This has been seen in the lab many times, and the effect has been integrated into modern electronic components, including some of the integrated circuits in the computer you're using to read these words. If this effect could be duplicated for macroscopic objects, the world would have a true means of wholesale teleportation.

Teleport systems come in two types: open and closed. Open systems require only a transmitter, while a closed system requires both a transmitter and receiver. Each gives birth to different types of stardrives.

Closed systems could be used to create an end-teleport drive. First proposed by Larry Niven in an essay over three decades ago, this consists of a ship with a transmitter built into its aft and a receiver built into its bow. This drive assumes teleportation is near-instantaneous; both transmitter and receiver execute their functions at almost the exact same instant in time. The ship is built in such a way that everything within it, including the teleport machinery and the receiver, is built over the transmitter. When activated, the transmitter teleports the ship to the receiver, which performs its function in the split-second before it too is teleported along with the rest of the vessel. The craft is therefore teleported exactly one ship-length. By doing this over and over, the vessel can fly through space. By cycling through teleports very quickly (on the order of many millions of times per second) the ship can seem to accelerate faster than light.

It is important to remember that the ship only seems to accelerate that fast. The craft isn't accelerating at all; its popping up at one point after another without actually moving in a conventional sense. Its momentum can therefore be independent of its direction of travel. For example, the ship can be constantly accelerating using a rocket motor in a direction opposite the one its end-teleport drive is taking it. Once the ship arrives at its destination, it shuts its teleport drive off and shoots back into the direction from which it came, using all the velocity it independently built up while in FTL flight.

End-teleport drives could be used in this manner to build fearsome, planet-cracking weapons. Build an end-teleport drive around a solid mass, say a small asteroid weighing several hundred tons. Position the weight above a planet so it will fall into the atmosphere, and set the drive to teleport it back into its original position once it falls one ship-length. The dead weight would constantly fall, as gravity would constantly be affecting it, but wouldn't move in position. When it achieves the desired velocity, the teleport drive is turned off and the weight shoots at the planet with its new, very deadly speed. After only a few hours of accelerating and teleporting, it would have enough kinetic energy built up to wipe out a county with its impact; after a few days, a medium-sized state. If the attacker were to eventually let it accelerate up to near-light-speed (this would take well over six months) he could blow the entire planet surface to rubble. However, this weapon would need a very rugged and advanced drive system in order to handle the vast kinetic energies constantly being cycled through it.

An open teleport drive is similar to the end-teleport drive, but it doesn't need a receiver in order to work. It simply jumps itself and the surrounding ship to a pre-determined spot. Its range may be rather limited - somewhere between a few hundred meters and a few kilometers - but like its cousin, it could cycle through teleports at many millions of times per second and thus achieve apparent translight speeds.

A lot of the characteristics of the end-teleport drive would be shared by an open teleport drive - with one major difference. An end-teleport drive could only send a ship in one direction: straight ahead. In order to maneuver, it would need to turn off the drive for a few seconds and reorient itself using conventional sublight thrusters. An open drive would have no such limitation. The drive could send the ship anywhere within its range, regardless of direction, with a single jump. The ship could jump in one direction at apparent translight speed, then, in a split second, change direction at any angle with no loss of apparent velocity. It could even reverse direction instantly.

A version of the open teleport drive, called a stutterwarp, was a primary feature in GDW's late, great science fiction RPG 2300 AD. In order to make the drive less overwhelming, the designers of the game built several limitations into the stutterwarp. The drive wouldn't work anywhere close to a gravitational mass like a planet or star, and it could only travel 7.7 light years before the drive built up a charge of radiation lethal to the crew. In order to discharge the radiation, the ship had to spend a few days in a gravity well, where, of course, the drive couldn't work, forcing the ship to depend on more conventional sublight drives for maneuvering.

 

WORMHOLES

Wormholes are more properly called Einstein-Rosen Bridges, after the two men whose work in the early 20th century led to the first theories involving the extreme warping of space. However, the theory was considered too fantastic to be taken seriously at the time and was little more than an aberrant curiosity for decades. Then, in the 80s, renowned physicist Kip Thorne, trying to help his pal Carl Sagan create a believable means of interstellar travel for the novel Contact, looked hard at the old equations and, with the help of a number of graduate students, found that wormholes perhaps weren't that far-fetched after all. In the years since, the theoretical study of wormholes has been expanded considerably by a number of leading physicists.

In their simplest definition, wormholes are tunnels bored through the fabric of space-time itself. They can be created by warping space with very unlikely tools - either with singularities, which lie at the heart of those fear-inducing objects called black holes, or by tapping into the basic, Planck-scale firmament of the universe itself.

A black hole is a spherical area in space that absorbs all light, energy, and matter that encounters it; a singularity is the pinpoint of super-collapsed matter that lies at a black hole's center - the remnant of a massive, dead star. A singularity has infinite density and no physical dimensions - no length, no width, and no height. It is quite literally an infinitesimally tiny point with the mass of an entire star packed into it. Its gravity field is so immense that its escape velocity exceeds lightspeed - hence its radius of utter darkness as seen from the outside. This gravity field also warps space to such a degree that physical laws as we know them break down.

A sufficiently advanced species could use this warping of space to their own benefit, to drill shortcuts through the fabric of the universe and reach across the cosmos.

Despite what has been depicted in a number of sci-fi sources, naturally-occurring black holes do not give birth to wormholes - at least, not wormholes that a spaceship could survive passage through. These wormholes form only at the birth of certain rare singularities, go nowhere except to space-time dead ends, and last only microseconds. Any ship unlucky enough to be caught in one would be crushed instantly by the immense gravitational forces crashing in on them as the wormhole collapsed - assuming the ship could survive passage that close to a black hole to begin with.

But black holes are not all alike; they come in a number of different varieties. It is only the artificial combination of very specific black hole characteristics that can give birth to a type of traversible wormhole called a Kerr-Newman Ringwarp.

A Kerr-Newman black hole must possess two important factors: an electrical charge and a high spin rate. These two factors affect the way the singularity warps space. Manipulating these two factors as the original mass collapses allows the newly-born singularity to punch a hole through the fabric of space to desired coordinates. Black holes sit at both ends of the new wormhole, with the same singularity at their center.

Because the singularity has no physical dimensions, it does not actually "spin." Rather, the angular momentum oblates the singularity - "flattens" it - into a ring. The more spin it has, the larger the ring will be. Inside the ring is the tunnel that the singularity has bored through the fabric of space - the Ringwarp. It is possible to expand the ring so its diameter exceeds the black hole's event horizon, the distance from the singularity at which its escape velocity exceeds light speed. At this point, the event horizon reconfigures itself around the singularity ring, allowing a ship to enter the Ringwarp through its center without being trapped by the black hole's gravity. And thus a traversible wormhole is born.

Instead of creating a tunnel to another portion of our universe, a Kerr-Newman Ringwarp could also be used to punch a hole into hyperspace or perhaps even into another universe. In the novel Starquake, an alien named Cheela used a Kerr-Newman Ringwarp to save several human astronauts stranded in orbit around a neutron star. In the novel Ring, by Stephen Baxter, the alien Xeelee constructed this type of wormhole using cosmic strings, in order to create an "escape hatch" out of the universe.

Another method of accessing wormholes involves not so much creating the tunnels through space/time as mining them from the subatomic quantum foam. According to theory, wormholes are constantly forming and collapsing at the Planck-scale level of existence, the point where physical measurements become meaningless, around 10^-33 meters. The vast majority of the wormholes found there lead to only a few Planck-lengths away. However, some can stretch many light years, and some may even lead to the other side of the cosmos. We can imagine that a species with spectacularly advanced technology might possess the means to detect, stabilize, and expand these brief-lived quantum wormholes for macroscopic use.

However, expanding the wormhole mouth to usable dimensions and keeping it open would take enormous energies. The gravitational forces at work in the expanded wormhole would constantly try to collapse its openings, and would require a strong counter-force to keep it open. To think of it another way, the "ocean" of space-time would keep trying to rush in and fill the "drainage" hole created by the wormhole. This would create unbelievable pressure at the wormhole mouth, far exceeding a billion quadrillion tons per square inch for a wormhole with an opening large enough to accommodate most spaceships - say, several kilometers wide. This level of pressure is akin to having millions of earth-sized planets balanced on your thumb.

This problem of pressure could be counteracted in several ways. The first is the use of copious amounts of an as-yet theoretical substance called negative matter. Negative matter would have negative mass and would therefore possesses anti-gravity properties. Negative matter could line both wormhole mouths, counteracting the crushing wave of space-time trying to force its way in. Another way to keep the openings apart would be to use force fields or powerful artificial gravity fields, but keeping the wormhole mouth from collapsing with these methods would require the constant energy output of an entire star. Interstellar species might be forced to build Dyson spheres or similar mega-artifacts in order to power their wormholes.

There is no theoretical limit to how far a wormhole could stretch. A wormhole could span the entire length of our universe as easily as it could the length of a football field. Also, wormholes' mouths are not static fixtures; they could be moved. Because they can be electrically charged, a sufficiently powerful fleet of ships could shift them using immense magnetic fields. The process would be slow, but one wormhole mouth could thus be "dragged" anywhere in the universe while its other opening remained at its point of origin, allowing its builders to slowly explore the universe one far-flung star system at a time.

Passage through a wormhole would be as simple as flying through a large tunnel; a vessel no more advanced than John Glenn's original Mercury capsule could transit from one star system to the next once the wormhole was built and stabilized. Passage time would be mere minutes at most. How a wormhole tunnel would look would be anyone's guess. Who knows what the inside of a singularity looks like?

However, wormhole travel would have its hazards. Brushing up against the sides of the wormhole throat would mean instant destruction for a ship as it was shredded and crushed by the immense gravitational forces barely held in check. In addition, too large a mass passing through a wormhole might disrupt the delicate balance keeping it open, causing it to spontaneously collapse; any ship caught in the mouth of a collapsing wormhole would be instantly annihilated. The fate of a ship trapped inside a collapsing wormhole throat is unknown. It might be crushed; it might precipitate out somewhere in normal space; it might be launched into a different universe altogether; or it might be forever trapped in an isolated bubble of space/time, with no hope of escape.

A wormhole is a central feature of the TV series Star Trek: Deep Space Nine. An account of a journey through a wormhole can also be read about in the aforementioned novel Contact, by Carl Sagan. In the novel The Ring of Charon, by Roger MacBride Allen, an immensely powerful alien species called the Charonians use wormhole technology to steal the planet Earth itself.
 

CLOSING THOUGHTS

Faster Than Light travel will likely remain in the realm of imagination for quite some time, and may never be made a reality at all. But that doesn't stop science fiction creators, and their many fans, from dreaming that the next quantum theory or cosmological find just might lead to a superluminal shortcut to the stars.


References 1. Alcubierre, M. "The Warp Drive: Hyper-fast travel within general relativity," Classical and Quantum Gravity, 11, L73-77 (1994).
2. Broeck, C. van den. "A 'warp drive' with more reasonable energy requirements," Classical and Quantum Gravity, 16, 3973-79 (1999).


© David Darling
 

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