Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


17.4.2  Antigravity and Reactionless Field Drives

Over the years the theme of antigravity and inertialess propulsion systems have been widely discussed, mostly in the fictional or pseudoscientific literature. From H.G. Well’s cavorite gravity screen in his First Men in the Moon, to such questionable propositions as the Biefield-Brown effect,137 Blackett’s Spin-Magnetic Coupling Theory1194 (which apparently inspired the late science fictioneer James Blish to write Cities in Flight with its "spindizzy drive"2809), F. B. Hli’s Theory of Electrogravitics (see the lively debate in Cashmore and Gordon,1197,1199 Hli,1196 Hli and Okress,1201 Johnson and Hli,1198 and Okress1202), Leonard G. Cramp’s G Field Theory,755 and of course the infamous Dean Drive (see Adams,2859 Campbell,1361 Cuff,2773 Davis,1371 Jueneman,2807 Klotz et al,2772 Pournelle,2806 and Stine2771), the idea has enjoyed a colorful and vituperative history. A number of U.S. patents have been issued on supposed antigravity machines.2773 Even Einstein himself spent the last thirty years of his life searching for a unified field theory that would relate gravity and electromagnetism, and the search continues apace today.2774

To construct a gravity screen would theoretically require the ability to achieve gravitational polarization of matter.16 This would imply the existence of two very different kinds of matter -- positive mass, which is attracted towards the Earth, and negative mass, which is repelled. At one time it was believed that antimatter might turn out to have negative gravitational mass,2692 but most physicists would dispute this today.1314,2952

Yet the search for negative mass continues. So far as we know there is no experimental evidence for negative matter, although it does appear in several solutions to the field equations in General Relativity. Papers by A.K. Raychaudhuri1521 and R. Mignani and E. Recami1519 suggest that tachyons may experience a gravitational repulsion to ordinary mass and thus may be interpretable as "negative matter," although because of their imaginary masses they will still fall towards a positive mass.

Besides gravity shields, negative masses, if they exist, could be employed directly for propulsion. A negative mass at rest beside a positive mass would begin to accelerate. Why should this be so? The negative mass (which repels all matter) would push on the positive mass, but the positive mass (which attracts all matter) would pull on the negative mass1190 If the two objects "weigh" the same, they will chase each other and will neither separate nor collide. Energy and momentum is conserved, since they each sum to zero.

Presumably the negative mass could be created out of empty space if a positive mass of equal "weight" was created simultaneously. The net energy cost would be zero, since (-m)c2 + (+m)c2 = 0. To achieve reasonable starship accelerations, compact masses with densities like black holes should be used. Dr. Robert Forward elaborates:

What we really want to do is make a dense negative mass and a dense positive mass down in the engine room. We’d just pull them out of empty space, put a charge on the positive one and couple it with the spacecraft with electric fields. Now we have the two masses down in the engine room; they’re probably about 10-23 cm across and they weigh a little more than the spacecraft. The positive one is coupled to the spacecraft and the negative one pushes the positive one which pushes the spacecraft. Our vehicle’s acceleration can be as high as we can tolerate.2014

A related concept is the idea of inertia control. Gravitational mass represents the force of gravity, and inertial mass represents the force of physical acceleration. The Eotvos experiment demonstrated that the two kinds of mass are identical out to eleven decimal places, under normal terrestrial conditions. But suppose we (or clever aliens) could arrange abnormal conditions which would allow the inertial mass of a chunk of matter to vary. When inertia is decreased, the same force imparts a higher acceleration; as inertial mass is brought close to zero, tiny forces would be able to produce huge accelerations. Lowering the inertial mass of fusion rocket propellant tanks would eliminate most of the normal constraints on lengthy interstellar missions for such vehicles.

Three other antigravity machines have been discussed by Robert Forward which involve no violations of the basic and established laws of physics (Figure 17.5).


Figure 17.5 Three Possible Antigravity Machines2014
Image for Special Relativistic Antigravity Machine

Newton’s law of gravity does not obey Special Relativity. However, a "linearized" version of Einstein’s gravity theory (General Relativity) does. In this version, a mass flow gives rise to a "protational field" much as a charge flow gives rise to a "magnetic field" in conventional physics. This is called the "Lense-Thirring Effect."

Image of Torus for Special Relativity Antigravity Machine
The Special Relativistic Antigravity Machine is a torus wrapped in helical tubing. A dense mass flow is rapidly circulated through this tubing, causing a protational field to arise along the circumferential axis of the torus.

Just as a changing magnetic field generates an electric field, a changing protational field should generate a unidirectional gravity field. This field, directed upwards, would cancel planetary gravity and provide vertical propulsion.


According to Einstein’s General Relativity theory, the presence of mass in a flat spacetime will cause that space to become curved. Mass is intimately connected with space -- a rotating mass causes spacetime to "rotate" too.

The General Relativistic Antigravity Machine consists of a torus of dense mass which is turning inside-out like a smoke ring. A mass with this motion, according to Einstein’s theory, "drags the metric" through the center of the torus. Unidirectional general relativistic forces arise which are equivalent to gravity, and may be used to neutralize planetary surface gravity or as a space propulsion system.

Image of General Relativistic Antigravity Machine
Image for Inertia Redistribution Antigravity Machine

While not widely accepted by theoretical physicists, there is a theory that the inertia possessed by all matter is a "tensor" quantity -- that is, a multidimensional vector. In other words, inertia is not just a simple quantity but rather must be measured in the three separate directions of normal space.

The Inertia Redistribution Antigravity Machine does not destroy inertia or convert it into energy, but merely redistributes it in different directions. The starship to which the device is attached becomes "heavier" in the horizontal plane (thus imparting navigational stability) and "lighter" in the vertical direction. An object with minuscule inertial mass in one direction can be accelerated to fantastic velocities with negligible force.


The first of these, which he calls the Special Relativistic Antigravity Machine, involves a mathematical analogy between gravitational and electric fields.2740 A "linearization" of General Relativity gives a version of Newtonian mechanics which obeys Special Relativity. (Classical Newtonian mechanics does not.) In electromagnetism, something called charge is surrounded by a spherically symmetric electric field. In gravitation, something called mass is surrounded by a spherically symmetric gravity field. It may be said that the simple Newtonian gravity field is the gravitational analogue to the electric field.

The linearized General Relativity theory provides a similar analogy to magnetic fields. Much as a magnetic field is due to the motion of an electric charge or current flow, the linearized theory suggests that a moving mass, or "mass current," will give rise to a new kind of gravity field by a mechanism known as the "Lense-Thirring Effect."2890 Scientists plan to try to measure this field as it is produced by the rotating Earth in future satellite experiments.3320

Forward calls this new field a "protational field." He claims that, based on the existence of the field, an antigravity machine might theoretically be constructed in the shape of

...a torus with a tube wrapped around it, filled with very dense matter. If we started accelerating that mass flow through the tube around the torus, we would get a constantly increasing protational field, inside the torus. A changing protational field will create a gravity field just as a changing magnetic field will create an electric field. If we did it right, we would have an upward gravity field that could be used to cancel the field of the Earth.2014

A second kind of antigravity machine suggested by Dr. Forward couples directly to "the fabric of space-time." His General Relativistic Antigravity Machine makes use of the notion that the presence of mass in a flat space-time causes a curvature, and that a rotating mass causes space-time to rotate too:

Imagine a rotating torus of dense mass, turning inside out like a smoke ring. An inside-out turning ring of very dense mass will create a force in the direction of the motion -- a "dragging of the metric" as it is sometimes called. There will be general relativistic forces in the direction of the velocity of the mass. These forces are equivalent to a gravity field which again, theoretically, can be used to cancel the gravity field of the Earth.2014

And, as Willy Ley once pointed out, such a weightless body "would be squeezed out of the atmosphere by the weight of the air around it."2808

Finally, there is the Inertia Redistribution Antigravity Machine. The main principle behind inertia redistribution is the idea that inertia is a "tensor" quantity.2740 A tensor is just a multidimensional vector, so all this means is that we are accepting for the sake of argument the hypothesis that inertia may be a quality of matter that can be resolved into distinct directional components. That is, in our normal three-dimensional world, inertial mass becomes a three-dimensional quantity. While current experimental evidence does not support the tensor theory of inertia, if it is correct it leads to an interesting possibility for propulsion.

The Redistribution Machine does not get rid of inertial mass, but rather redistributes it so that some of it is pointing in new directions. If the machine makes the starship’s mass heavier in the horizontal plane and proportionately "lighter" in the vertical direction, a relatively tiny amount of force applied vertically would cause relatively large accelerations in that direction. The benefits are similar to those achieved using inertia control, discussed above, with the added advantage that inertia is conserved. (Note that since gravitational mass is unchanged, spacecraft will still feel the same attraction to planets and other massive bodies.)

Inertialess starships would have a number of interesting performance characteristics. Such a system must necessarily act upon every atom of the vessel in order to be effective. Far from any planet (so gravitational mass can be ignored), the inertialess craft could start and stop almost instantaneously. Since passengers have almost no inertia in the direction of flight, hideous accelerations can be tolerated (in that direction) with equanimity. For instance, if vertical/forward inertia is cut to 1% of normal and the ship accelerates at 100 gees, passengers would feel only an effective 1 gee of force. Inertialess starcraft would be virtually crashproof, since with no forward inertia people would not be thrown from their seats if an obstacle was struck. And, depending on how fast inertia can be suddenly redistributed, right-angle turns and hairpin bends should also be quite possible.


Last updated on 6 December 2008