GRAVITIC DRIVES


The interstellar Mothership from Close Encounters of the Third Kind glides effortlessly over Devil's Tower using an advanced gravity drive. Image (c) Columbia Pictures.

Gravity Pulse Projectile Drive
Tech Level: 17
Gravity Neutralizer
Tech Level: 17
Gravitic Repulsor
Tech Level: 18
Gravitic Enhancer
Tech Level: 18
Point Source Gravity Projector Drive
Tech Level: 19
Gravitic Displacer
Tech Level: 19
Combined Gravity Drive
Tech Level: 20

Gravitics is the name given to applied artificial gravity technology. Though artificial gravity generation and manipulation is still entirely theoretical, the term pops up occasionally in science fiction.

Artificial gravity technology saturates modern popular science-fiction, appearing in Star Trek, Star Wars, Stargate SG-1, Babylon 5, Asimov's Foundation and Robot stories, Brin's Uplift novels, Niven's Known Space stories, and a large host of others. For on-screen sources, this is mostly a financial consideration; its much cheaper and easier to show actors walking around in Earth-normal gravity on spaceships than to spend large percentages of the budget showing them floating about in simulated zero gee. Hence, gravitics in those fictional sources are a mature and ubiquitous technology to rationalize away this limitation.

In novels, games, and similar sources, many authors also take this route, to avoid the headache of realistically having to depict the complex effects of microgravity on all objects in their stories. Other authors, conversely, just love playing around with the concept of artificial gravity, making it one of the prize toys in their imaginary playgrounds. Also, because cheap gravitics is so prevalent in movie and TV sci-fi, many authors in the modern era just include it in their stories as one of the familiar signature technologies of the genre.

On the science side, gravity is the most well known and easily observed of the four fundamental forces of the universe, yet it is also by far the least understood. While gravity control has always been a popular feature of science fiction, it is also one of those speculative technologies whose exact workings in stories is always left conveniently obscure.

So far, the only way we know how to create a gravity field is the same way nature does it--by piling a lot of matter in one place. In order to produce a gravity field equivalent to the planet Earth's, one needs a mass equivalent to Earth. However, as that could be very detrimental to many practical space flight applications, other means of controlling this force would have to be found.

It would also be possible to create an 'artificial' gravity field by taking a mass of superdense matter and accelerating it to near-light speed, where it would accumulate more mass as it approached c. The gravity field would grow proportionately with its velocity, and if set in a ring or coil, like that of a particle accelerator, it would generate a gravity field that could be moderated by controlling the mass's velocity. Since it would be zipping through the coil so fast, the effects of its gravity would be felt all along the length of the coil without an appreciable drop off in performance. However, this scheme would require truly vast amounts of energy to achieve and sustain, usually at levels that would not be practical for most civilizations to apply.

A method that still relies on a great deal of still-fuzzy theory is to use quantum manipulation to create gravitons--the theoretical carrier particle of gravity. If a dense enough population of gravitons can be created for a given input of energy, practical artificial gravity generation may become possible.

Lacking the ability to do this, devices may instead concentrate on manipulating or enhancing existing gravity fields instead of generating their own from scratch. Again, this would probably rely on some advanced form of quantum particle manipulation we can only guess at with our humble twenty-first century understanding. Manipulating existing gravity fields could allow engineers to strengthen or weaken fields within a given radius or range, or even shuffle fields about, allowing the force to be felt at angles differing from where the field actually originated from.

Another aspect that could arise from the artificial manipulation of gravity is the emergence of technology that utilizes a true anti-gravity, or repulsive, force. There is emerging evidence that anti-gravity of a sort may be at work in the cosmos in the form of so-called "negative matter" or "dark energy", but its effects are very weak and can only be observed over very large--as in intergalactic--distances. An intricate understanding of gravity could also lead to the knowledge needed to understand and manipulate this new force of nature--if it is ever proven definitively to actually exist.

Antigravity would act like gravity, but would repel as opposed to attract. It could counteract gravity fields of like strength.

Another major consequence of gravitics is that gravity fields bend the fabric of space-time. By generating or manipulating gravitons, a gravitic drive may also be warping and stretching the fabric of the universe itself. This in itself would open the door to huge number of impressive and world-changing technologies we can only begin to guess at. In science fiction, gravity control is often seen as one of the major precursor technologies for producing FTL travel, as the ability to warp space for one superluminal scheme or another usually depends on expert manipulation of gravitic fields.

Gravitics technology, no matter what form it took, would have a profound affect on human civilization in a number of ways. This article is concerned only with its ability to act as a space drive; other articles dealing with other aspects of gravitics will be addressed in future articles. Links leading to more thorough and technical explanations on the science of gravity are in the Further Information section below.


GRAVITIC PULSE PROJECTILE DRIVE
Tech Level: 17

If the quantum manipulation of gravity becomes a reality, one of the most basic feats the technology would be capable of would be creating short-lived, artificial pinpoints of gravity using advanced particle accelerators. This leads the way to an odd but workable means of space propulsion. This scheme would use one-shot disposable particle accelerators--which by Tech Level 17 should be relatively cheap and easy to produce compared to today--mounted on projectiles which are shot ahead of the ship.

These accelerators would be pulsed to overload, creating the largest gravitic field they were capable of. The ship, trailing behind, would "fall" into this briefly-lived gravity field. As the field fades, another projectile is shot out, repeating the process until the ship builds up to the desired velocity by constantly falling in one gravity pinpoint after another. If timed exactly right, with one projectile's field taking up the slack just as the preceding one fades away, the ship can accelerate with no perceived discomfort experienced by the crew.

The efficiency of this particular drive scheme would depend on a lot of variables, the most significant of which is how powerful a gravity field the projectiles could create and how long each field would last. The gravity field created would have to exert a force on the ship greater than that exerted from shooting the projectile ahead of the ship, or else there will be no net gain in forward velocity.

Projectiles with weak fields would have to be pulsed close to the vehicle, while those with larger ones would need more distance to avoid pulping the crew. Unmanned vessels can of course use much more intense gravitic fields closer to the ship in order to induce more intense accelerations.

It should be noted that as the ship is drawn to the gravitic field, the gravitic field will also be drawn to the ship. The pinpoint source of gravitons will fall toward the ship even as the ship accelerates toward it. However, if the gravitic projectiles are engineered and employed properly, the artificial gravity field should fade to nothing before it "impacts" and affects the ship's forward motion.

Another hazard would be dealing with the debris of the projectile. The particle accelerator itself would still be in the path of the ship, held to its relative position by the very gravitic field it generated. It would have to be deflected in some way, or as an alternative, the ship can be canted at an angle to the accelerator's gravity field and "fall" into a curved trajectory toward it. The next projectile would be shot at a different angle to correct the forward motion back into the desired direction, allowing the vessel to scribe a languid spiral through space as opposed to a straight line. This might be a bit unpleasant for the crew, however, as being in the ship could feel like a long, extended roller-coaster ride.

In many ways, this is the converse of the Orion Nuclear Pulse Drive. Instead of pushing the vessel with nuclear explosions with projectiles from the stern, the ship is "pulled" with gravitic pulses with projectiles from the bow. Like the Orion scheme, the number of projectiles needed would depend on how intense the acceleration provided by the projectiles would be, and on the type of mission the ship is expected to undertake. For interplanetary hops, probably only a few hundred projectiles would be needed. For interstellar voyages, tens if not hundreds of thousands of gravitic pulses may be necessary.

Even though particle accelerators will likely be much easier to produce at higher tech levels, there is no guarantee that they will be significantly cheaper. This scheme could end up as quite expensive if a great many gravitic pulse projectiles are needed. Given the other, much more efficient and powerful drives that will be available at the tech levels when this propulsion scheme becomes available, gravitic pulse projectiles drives will likely be used only in specialized circumstances.


GRAVITY NEUTRALIZER
Tech Level: 17
An alien surface-attack saucer from the movie Independence Day floats mysteriously over a city using what may be a powerful gravity neutralizer. Image (c) Centropolis Entertainment and Twentieth Century Fox.

This device exerts a field that prevents gravitons from outside the field from interacting with mass inside of it. In other words, it cuts off the vehicle wholly or in part from any surrounding gravity field. A gravity neutralizer is also sometimes called a gravitic shield, as it "shields" the mass within from outside gravitational influence.

One method suggested for doing this is to surround the mass with a material that has anti-gravity properties, such as negative matter. However, generating or cultivating enough negative matter, still a theoretical substance, could prove very difficult. One would need as much negative matter for the shell as normal matter in the vehicle in order to completely shield it from surrounding gravity fields.

Another method may use advanced quantum manipulation to generate dense amounts of anti-gravitons along the outer shell of the mass to be lifted. The anti-gravitons cancel out the incoming gravitons from the surrounding masses. The more energy fed into the system, the more anti-gravitons are generated, and the more normal gravitons are intercepted and neutralized.

Gravity neutralizers need not totally cancel out the surrounding gravity field; it can do so partially in order to lighten the load of the vehicle and make it easier to move through other means.

Note that neutralizing local gravitational influence should not be confused with neutralizing the vehicle's mass. A 50-ton gravity-neutralized vehicle will still resist changes to its state of motion as any normal 50-ton vehicle, aside from any direct effects of the gravity shielding. By their nature, gravity neutralizers usually have to be coupled with another means of locomotion in order to get the vehicle to go where the pilot wants.

If used to completely neutralize the local gravity field, air pressure and wind will slowly push the vehicle upward. In many ways, a gravity-neutralized object will act like a hot air balloon, but with no upper ceiling to how far it can rise. If left completely on its own, a gravity neutralizer would eventually drift the vehicle up into orbit or beyond after a number of hours.

Masses can enter or leave the gravity-neutralizing field freely. More or less mass within the field may require more or less energy by the device to maintain its effect.

There seems to be a bit of a debate as to what happens to any object that's directly above a gravity-neutralizing field. The gravity field directly below it is cut off, so does that mean it will float off into space as well?

The answer is no. While the gravity directly below it is cut off, its assumed that close to the surface of a planet the planet's mass is still acting on it from other angles. Imagine yourself fifty feet above a gravity-shielded object anchored to the ground. The gravity neutralizing field is 10 feet in diameter. Directly below you there is no gravity, but gravity is still reaching you at an angle from the surrounding landscape. The amount of gee forces you experience may be less than normal, and you may certainly feel lighter, but there is little chance of you floating up unchecked.

The higher the gravity-neutralizing field is, however, the more of the planet's gravitational influence is blocked on an object above it by the field. So if you maintain your fifty-foot distance above the gravity-shielded object, you will feel gravity falling off more and more sharply as you and the vehicle ascend.

Usually a secondary means of locomotion will be employed with a gravity neutralizer in order to make the vehicle much more useful. The neutralizer will negate its weight, allowing its secondary means of propulsion to work much more efficiently without expending the energy it would normally need to fight gravity as well. A good metaphor for this type of vehicle would be a blimp--the gas envelop provides the lift, while a secondary source of propulsion--propellers--make the ship go where the pilot wants. Unlike a blimp, however, the neutralizer would work on any type of vehicle that can mount it, whether it be a car or an airplane or a spaceship.

Gravity neutralizers are ineffective as a means of locomotion away from large source of gravity, like a planet. They will prove useless in deep space, for instance. However, they should be able to work equally effective with natural as well as artificial sources of gravity.


GRAVITIC ENHANCER
Tech Level: 18

The opposite of a gravitic neutralizer, an enhancer multiplies and intensifies the effects of gravity on a mass, by creating many more gravitons around it than it could generate normally. This increase in graviton flux enhances the attraction between the gravitic-enhanced object and any other nearby mass or gravitic field.

Gravitic enhancers' role in propulsion is mostly to work in conjunction with more advanced artificial gravity propulsion schemes, allowing them to generate and use gravity fields of immense strength, but would sometimes be used singularly in specialized circumstances, such as in orbital bombardment by kinetic energy weapons.


GRAVITIC REPULSOR
Tech Level: 18
The Martian War Machines from the 1953 film version of The War of the Worlds used what appeared to be gravitic repulsors to stay aloft. Image (c) Paramount Pictures.

As our understanding of gravity grows sophisticated enough to allow its outright manipulation, it may lead to the discovery and understanding of a true anti-gravity force. Even though the existence of an anti-gravity force is currently postulated by many theorists, it has yet to be proven to actually exist. However, if it does, gravitic technology would seem to inevitably lead to its manipulation and artificial generation.

As previously stated, both "negative matter" and "dark energy" are theorized to have anti-gravity-like properties. A gravitic repulsor may need to have some quantity of one or the other, either with the exotic substance acting on its own or its effects intensified in some way. Alternately, quantum manipulation may produce carrier particles of the force in sufficient quantity to produce noticeable counter-gravity effects.

True anti-gravity will act like gravity, but in reverse. It will push away matter instead of pulling it closer. The intensity at which it pushes will also be dependent on the inverse square law, unless artificially intensified or redirected in some way. Also, it needs a mass to push against--like a gravity neutralizer, the effectiveness of a gravitic repulsor as a means of propulsion will fall away quickly the farther from a planet it gets, and will be useless in deep space.

In many ways, a gravitic repulsor will act like a gravity neutralizer, basically counteracting the effects of gravity under the vehicle to allow lift. Gravity is not completely neutralized, however; even though gravity directly beneath the repulsor is countered, gravity from the planet's surface will still be acting on it from other angles. Unlike a neutralizer, however, a repulsor can be used for lateral propulsion in its own right. By canting one or more repulsor units on a vehicle at an angle to the ground, the vehicle will gain a lateral acceleration as the repulsor "pushes" against the gravity field of the planet.

A good way to think of them is as a bit analogous to hovercraft. The turbofans on a hovercraft provide lift, and turned at an angle toward the ground, will provide part of their thrust for sideways motion as well. Of course this metaphor can only go so far. A repulsor-fitted vehicle could lift itself into orbit just like a neutralizer, whereas a hovercraft can only lift itself up a few meters. Also, repulsors need a mass to actually push against; angling one completely sideways so that its not pushing against the planet at all would negate its effect.


POINT SOURCE GRAVITY PROJECTOR DRIVE
Tech Level: 19

This drive scheme is also informally called the "push-me-pull-you" drive. It was one of the main concepts behind Michael P. Kube-Mcdowel's Trigon Disunity novel series.

In many ways, this is similar to the Gravitic Pulse Projectile Drive. But instead of shooting gravity-generating particle accelerators ahead of the ship, the ship's advanced gravity generators project a pinpoint field of gravity ahead of the ship. The ship falls into this artificial gravity well, and as the ship moves forward, the gravity point source moves with it and maintains a constant distance. A bit similar to a mule following a carrot on a stick tied to its back, a ship employing a Point Source Gravity Projector Drive is constantly "falling" into the gravity field it is projecting ahead of itself.

Like with the similar gravitic projectile scheme, it would probably not be one continuous field that the ship would maintain, but rather thousands of pulsed gravitic pinpoints that would begin fading as soon as they popped into existence. A quick succession of these pinpoint sources--maybe dozens or even hundreds a second--would ensure a smooth ride with no perceived discomfort to the crew. Otherwise the ship would have to be constantly pushing the gravity field ahead of it, which may or may not have pseudo-mass and therefore momentum and inertia of its own, requiring much more energy to move than the acceleration it would provide.

The pinpoint gravity sources would be drawn toward the ship even as the ship is drawn toward them, but ideally the pinpoint sources would fade back into the quantum foam quickly before they get close enough to adversely affect the velocity of the ship or any internal systems. The efficiency and power of the drive would depend on how powerful the pinpoint gravity fields would be, and how often it could pulse them. As with any other drive scheme, more powerful drives can be used by unmanned ships that do not have to worry about the fragility of a human crew.

This drive is able to eventually achieve significant fractions of lightspeed in velocity. If the gravity-point sources exert a continuous gravitational influence of one G, the ship will eventually approach the speed of light after one year or so. However, the drive would also seem to have an efficiency ceiling, where its effectiveness as a drive would begin to drop off significantly. The faster the ship goes, the more mass it would accumulate. Assuming the strength of the projected gravity point-sources have a constant strength, that means that as the ship becomes more massive, the less effective the point sources will be at pulling it forward. Rather, the ship will begin pulling much harder on them than they will on it, and its acceleration will drop off significantly and even disappear completely if the process is allowed to continue indefinitely.

The way around this limitation would be to use greater and more intense gravity point sources the faster the ship goes. This means that, like any other sub-light drive, it would have to expend more and more energy to move the ship the closer to luminal velocity the vessel got. The ultimate upper velocity a Point Source Gravity Projector Drive can go will depend on the strength of the point sources it can project. In the Trigon Disunity novels, ships using this drive were able to routinely achieve 50% to 80% light speed before the drive began dropping off in efficiency.

Another concern is the effect the gravity point sources will have on the interstellar medium the ship will be trying to plow through. The gravity point sources will actually accelerate particles in the ship's path toward the vessel, significantly increasing the hazard of both radiation and potential impacts on the vehicle. So even though the drive could theoretically achieve significant fractions of lightspeed, in most practical instances the drag and hazards of the interstellar medium at those velocities may limit it to 'merely' 10%-15% or so of lightspeed.

There is some concern about whether this drive could ever be made to work, even given the future reality of advanced gravitic technology, as it would seem to violate the Law of the Conservation of Momentum. However, the Point Source Gravity Projector Drive is a reaction drive, only here the reaction is a mutual pull--two gravity fields interacting with each other--instead of the mutual push of exhaust and projectile. The problem here is finding a way to create the pinpoint gravity sources outside of the ship that doesn't adversely affect the ship's forward momentum. The need for highly advanced quantum and gravitational field manipulation in order to do so is one of the reasons the Tech Level for this drive is as high as it is.


GRAVITIC DISPLACER
Tech Level: 19

An odd device found in some science fiction sources, a gravitic displacer takes an existing gravity field and redirects it effects, or "displaces" it, outside of its originating mass. In other words, if a gravitic displacer was directed at you, the gravity field the mass of your body generates would no longer be centered on you, but rather at a chosen spot near you, say six feet to your right. Your body, which still retains its original mass and inertia, would therefore be gravity-neutralized and would begin floating up.

As a means of locomotion, a gravitic displacer is usually used on a small section of planet a vehicle is resting on. It displaces the field acting under the ship, projecting it overhead instead of from below, allowing the ship to rise, as if it were 'falling' up. (In fact, this is exactly what it is doing--the gravity field that would normally make it fall down is simply being projected in a different location.) The displacer can then project the field ahead of the ship, propelling it forward as its pulled by the redirected gravity field. Used in conjunction with a gravitic enhancer, this can prove to be a very effective means of propulsion around a planet.

The true oddness of this concept comes from how some have proposed using it as a means of deep-space propulsion. The displacer would take the majority of the ship's own natural gravity field and project it ahead of the ship just like a Point Source Gravity Projection Drive. Combined with a gravitic enhancer that could intensify the field thousands-fold, it would provide a practical means of giving the ship a substantial amount of acceleration. All of the ship's natural gravity field can't be used, otherwise the ship would be completely gravity-neutralized and there would be nothing for the displaced field to "grab" onto.

Whether this could be ever made possible is unknown; the deep-space gravitic displacer drive would seem to severely violate the Law of the Conservation of Momentum, even more so than the previous drive. There is no outside force or energy acting on the ship at all--no star-hot exhaust to push it nor independent gravity field to pull on it--only its own gravity field split into two, literally the ship lifting itself up by its own bootstraps.


COMBINED GRAVITY DRIVE
Tech Level: 20
Tooling around the everglades with what appears to be a Combined Gravity Drive, from the the movie Flight of the Navigator. Image (c) Walt Disney Corporation.

The Combined Gravity Drive is representative of a number of advanced and incredibly versatile gravitic engines found in science fiction.

Basically, this is an all-in-one artificial gravity propulsion system. It can act as, and combine the effects of, a gravity neutralizer, a gravitic repulsor, a gravitic enhancer, and a point source gravity projector. Employing multiple such units allows the ships to perform some truly astounding feats of flight nearly impossible with other drive schemes. Computer control-systems read what the pilot wants of the vessel, and employs the various effect of its gravity drive to deliver. Need to hover mysteriously in place? The drive neutralizes 90%+ of the ship's gravity, just enough to counter the effects of air pressure pushing it up, so it can float effortlessly. Need to rapidly climb at 20 Gs? The drive creates a powerful repulsor field to push the vehicle rapidly against the planet surface below. Need to then stop on a dime? The drive creates a gravity-point source behind the vehicle strong enough--perhaps on the order of hundreds of G's--to suddenly stop it dead in mid-air. And so on and so forth. Of course protecting a biological pilot and crew from the effects of such rapid and potentially devastating accelerations and decelerations would be the responsibility of other systems.

Two on-screen sources show what appear to be advanced gravitic drives of this sort. The first is Close Encounters of the Third Kind, where the aliens' glowing subcraft zip about the landscape, pulling wild maneuvers effortlessly within meters of the ground. The second is Flight of the Navigator, as an alien ship uses its advanced propulsion system to fly rings around its human-created counterparts. In fiction, the alien Kzin in Niven's Known Space stories used miraculous-seeming "gravity polarizers" to conquer a large swath of interstellar space until they ran into those pesky humans.


FURTHER INFORMATION

In The Media

Star Trek, et al

Star Wars, et al

Babylon 5, et al

Close Encounters of the Third Kind

Flight of the Navigator


In Print

The Uplift novel series, by David Brin

The Foundation and Robot series, by Isaac Asimov

The Known Space series, by Larry Niven

The Ring of Charon and The Shattered Sphere by Roger McBride Allen

Emprise, Empery, and Enigma by Michael P. Kube-Mcdowel

Gravity Basics:

http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html


Advanced Gravity Theory:

http://relativity.livingreviews.org/Articles/lrr-1998-1/

http://web.archive.org/web/20060428001322/http://www.qgravity.org/

Gravity Drives:

http://www.thespacesite.com/space_future_propulsion_space_drive.html

Gravitic Drives in the Traveller Universe:

http://www.pocketempires.com/pe/drax000.htm


Article added 19 July 2007

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