Edited by Marc G. Millis and Eric W. Davis

- reviewed by Pat Galea, Galea Research -

Copyright © 2009 AIAA
ISBN 978-1-56347-956-4
 739 pp

Marc Millis and Eric Davis of the Tau Zero Foundation have released Frontiers of Propulsion Science, a compilation of emerging science relevant to interstellar propulsion technologies.

While decades of science fiction have accustomed us to the idea that the human race is destined to live on in distant solar systems, the harsh reality is that travel to the stars is really very very hard. Ridiculously hard. Even the closest stars to the sun are so far away that it would take thousands of years for a small probe to arrive using our current propulsion systems. Our technology is not even remotely close to being able to send humans on such a voyage.

But should we let our present limitations bind our conception of what is possible? Can we establish the limits of what physics will allow, even if the engineering challenges are beyond daunting at this stage? And having sketched out the theoretically plausible, can we work out a plan of action for tackling these problems both experimentally and theoretically so we can, in the near term, make progress toward our goals?

Marc Millis headed NASA’s Breakthrough Propulsion Physics Project from 1996 to 2002. This project examined various technologies to see if they could be developed into plausible propulsion systems. When the project was wound up, Marc went on to set up the Tau Zero Foundation, wholly independent of the government. The foundation is pursuing similar aims, but has a much wider scope and is free of many of the limitations of hte government-funded project.

The Frontiers of Propulsion Science (FOPS) book is the first major publication to summarize the wide variety of science that is being considered right now as the basis for possible future development into starship technology. It is published by the American Institute of Aeronautics and Astronautics (AIAA) as part of their Progress in Aeronautics and Astronautics series.

This book is ambitious. Millis acknowledges right from the start that while some of the science is well-accepted, the ability to turn it into actual technology is a very difficult engineering problem; and other science topics examined are extremely speculative, in that we simply have no idea yet how nature works in these areas.

FOPS provides a wide-ranging reference for researchers who are starting out in interstellar propulsion. It ranges from background material on how to formulate the rocket equation for different types of engines, to advanced rockets using plausible (but difficult) technologies such as fusion and antimatter, through to methods of exploiting the Zero Point Field to extract energy from the vacuum, and creation of warp bubbles. It also contains chapters showing negative analysis of techniques that have been widely discussed, but have not demonstrated any useful results in the lab.

This is not a book for the non-technical layman. For a start, it’s US$130! It also contains a lot of math and technical language. A reader with a solid technical background who enjoys this kind of detail will find a lot of interest within. However, the book is really aimed at managers, scientists, engineers and graduate students, to give them a good grounding in the status of the research. Millis has discussed plans to create a less technical version of the book for a more general audience.

From a personal perspective, I found two parts of the book most interesting. First, discussion of technologies that are not too far out. These are the kind of systems that we could plausibly build within the next twenty to fifty years. Despite the fact that we haven’t yet managed to achieve break-even fusion on Earth, the physics is sound, and I do not see any reason to believe that we won’t be able to achieve this in a starship. Someday.

The second aspect I find most interesting is discussion of the really way off technologies, such as wormholes and warp bubbles. Even if the physics ultimately turns out to allow such things to exist theoretically, they will be unbelievably hard to create, and it may even turn out to be technically impossible.

FOPS provides good background for both ends of the propulsion technology spectrum.

There is far too much material in the book for me to give more than a taste, so if you want to see a list of the topics covered, have a look at the Table of Contents.

After considering the limits of interstellar flight technology, examining the capabilities of various types of rockets and sails, Millis considers the prerequisites for space drive science. A “space drive” is a hypothetical propulsion system which apparently violates Newton’s third law of motion; it propels the ship forward without chucking stuff out of the back. Well, scientists are somewhat reluctant to throw out Newton III, so in fact we’re really talking about interactions with stuff such as space itself. This is one of the first ideas considered in FOPS that is truly speculative. We truly do not know whether this kind of drive is possible at all. Among the methods considered are : pushing on something in space, such as dark energy, dark matter or quantum vacuum fluctuations. Other ideas include inertial modification, somehow reducing the intertial mass of the ship, and field drives, exploiting properties of the gravitational field to propel the ship.

Eric Davis considers the possibility of manipulating the gravitational field. (The physical possibility, remember; we are constantly reminded throughout the book that engineering is a different game.) It appears that there are a large number of different techniques being considered for achieving this manipulation and creating antigravity. If this were simply a numbers game, one of these would be bound to work! Unfortunately, nature is not that kind to us.

Millis reminds us of some mechnical “antigravity” devices that have been proposed over the years. You may have seen such systems which are generally composed of gyroscopes on rods which are swung about in various fashions. If you’ve played with a decent sized gyro for a while, you’ll appreciate how easy it is to convince yourself that an antigravity drive can’t be too hard to build. It really does feel like there’s some strange force which could be exploited here. Alas, gyros do not violate Newton’s laws. In fact, we can explain how they work by using Newton. Millis takes us through a variety of such devices and explains why they don’t actually do what the inventors think they are doing. Lest you worry that null chapters are a waste of space, the point here is to show the kinds of errors that are easy to fall into, and how to avoid these traps.

Other chapters provide background of further technologies that have not demonstrated positive results, but continue to be promoted by some as potential propulsion devices. These include Yamishita’s electrogravitational machine, which was supposed to decrease in weight when activated, and the asymmetric capacitor thrusters of Biefeld and Brown.

G. Jordan Maclay’s chapter on Thrusting Against the Quantum Vacuum gives us our first major introduction to the Casimir effect. Maclay explains the theoretical possibility of pushing against the quantum vacuum using vibrating mirrors. Yes, it turns out that the force produced is truly pitiful, but a lot of the techniques in the speculative category are like this. We may yet find ways to increase the thrust by orders of magnitude to make it genuinely useful.

Jean-Luc Cambier introduces Stochastic Electrodynamics, and the idea that we might be able to manipulate intertial mass by manipulating the zero point field.

Eric Davis leaps into the really speculative stuff when he talks about Faster-than-Light Approaches in General Relativity. These ideas have become quite well known over the years with the popularity of Thorne’s wormholes and Alcubierre’s warp bubble. It seems that Einstein’s general theory of relativity does allow for these strange solutions that permit rapid transit faster than light. We have to be careful what we mean by “faster than light”, however. With these techniques, the ship is always moving at sub-light speed in its local frame. We are just exploiting some strange effects, such as dragging the frame along with us (in the case of the warp bubble) or taking a short cut (in the case of the wormhole). The net result is that the ship arrives at the destination sooner than a light beam travelling through normal space. Although these constructs are compatible with general relativity, we don’t know whether they are possible in reality. Firstly, we haven’t unified general relativity with quantum mechanics yet; such a unification may help or hinder us, depending on the details. Secondly, these constructs seem to demand use of “exotic” stuff, such as negative energy. Thirdly, according to Einstein’s special relativity, faster than light travel can be used effectively as a time machine, allowing violations of causality. Some loopholes are presented in FOPS, but I find them somewhat unconvincing at this stage. Right now, I think that physics will either allow the existence of time machines, or it will not allow faster than light travel. Of course, developments in physics are welcome to change my mind!

Energy sources are a major concern for any starship. Gary Bennett considers the more conventional power plants, such as radioisotope thermoelectric generators and fusion systems. Davis and H. Puthoff look at methods for extracting energy from the vacuum itself, using our old friend the Casimir effect.

C. Maccone (of whom I’ll write more at a future date) presents a chapter on the computational tools for performing general relativity calculations. He also introduces a set of well-thought out conventions that should be adopted by all propulsion researchers so that they are all talking the same language. This covers such aspects as the units of measurement, and sign convention for the metric.

Finally, Millis presents his chapter on Prioritizing Pioneering Research, borne of his experience in the Breakthrough Propulsion Physics Project. He covers important topics such as how breakthroughs happen, how we can prioritize our work to encourage “vision with rigor”, and what criteria should be used in the evaluation of projects.

FOPS is a fascinating book covering a vast range of topics. It is definitely the book to read for anyone embarking on research in the exciting field of interstellar propulsion.

This review first appeared on Galea Research online, 27 May 2009, and is used here by the kind permission of the author.