Four trips to the Moon a day? That’s one capability of a theoretical vehicle discussed in last January’s newsletter from the American Institute of Aeronautics and Astronautics. I hadn’t realized the AIAA was putting these newsletters online until I saw Adam Crowl’s post on Crowlspace discussing the above possibility. Adam notes that a vehicle powered by a so-called Mach-Lorentz Thruster (MLT) of the sort being studied by James Woodward (California State University, Fullerton) could not only make the four lunar trips a day but deliver almost 3000 tons of cargo a year.

The AIAA story, adapted by Paul March from his later presentation at the 2007 STAIF meeting (Space Technology and Applications International Forum) in Albuquerque, presents several startling scenarios, all of which come down to our understanding of inertia. Go back to the days of Isaac Newton and inertia is seen as an inherent property that causes a body to resist acceleration. Inertia means a body at rest will oppose anything that tries to get it into motion. And if it is already moving, inertia is that property that resists attempts to change the magnitude or direction of its velocity. [Addendum: Slightly changed from the original; see Jimmy Cone’s comment below].

But what causes inertia? Woodward, a professor of history as well as physics at Fullerton, sees inertia as the result of all objects in the universe — even the most distant — acting on an accelerated object. The concept is based on Mach’s Principle (named for 19th Century Austrian physicist Ernst Mach), and it may remind you a bit of some of our discussions about John Cramer’s Transactional Interpretation of quantum mechanics. Perhaps pushing on an object causes a gravitational disturbance that moves into the future, ultimately causing all other matter to move infinitesimally, creating a disturbance that moves backward in time and converges on the original object.

And thus you have one explanation for inertia. To say this is controversial is to belabor the obvious — among the scientists who abandoned Mach’s view was Einstein. But Woodward goes on, using Mach’s ideas, to show that objects undergoing acceleration experience transient fluctuations in their mass. Can these variations help us create spacecraft that expel no propellant? Woodward has been working on the concept since 1990, and the AIAA article offers a good introduction to his investigations. Here Paul March discusses the mass fluctuations under discussion:

The M-E [Mach Effect] is based on the idea that when a mass is accelerated through a local potential field gradient, its local rest mass is momentarily perturbed about its at-rest value. These resulting acceleration induced “mass fluctuations” used in conjunction with a secondary force rectification signal can then be used to generate an unbalanced force in a local mass system, which can accelerate a payload or generate energy. Local system energy and momentum conservation is maintained by interactions with all the distant mass in the universe. Therefore to accelerate a spacecraft here, the Machian interpretation of inertial reaction forces means that each star or other distant matter in the universe will move in the opposite direction of the locally accelerated mass in response here – even if only on an extremely small scale. Conservation of energy and momentum must be maintained globally, but nature doesn’t say how big the system box has to be, nor when the accounting has to be done.

Woodward’s continuing experiments at the ‘tabletop’ level have been provocative, and John Cramer investigated mass fluctuation under the auspices of the Breakthrough Propulsion Physics program in the late 1990s, although, as March notes, with inconclusive results. March goes on to the crux of things in describing a thruster built on these principles:

Assuming that mass fluctuations really do exist, in theory an M-E thruster can be built using externally applied forces that can push on the device’s “active” mass when it is lighter and then pull on this active mass when it is heaver in a cyclic manner, thus generating a net time-averaged force per Newton’s F=ma relationship.

Build a true Mach-Lorentz Thruster — assuming such a thing is possible — and if the technology scales the way Woodward believes it must, the outer Solar System is reachable in less than a month. In fact, the travel times are limited largely by the accelerations a human crew could endure. Clearly, the implications for interstellar missions are interesting indeed. But we’re a long way from building such devices. Indeed, conclusively verifying the viability of the thruster principle is still a work in progress, much less building larger MLTs to examine scaling issues.

Woodward’s ideas continue to be investigated. Peter Vandeventer has collected a number of non-published papers on his Woodward Effect site, while Woodward’s own home page offers useful background studies. Given the scope of the challenge of reaching the outer planets with human crews — much less the closest stars — it’s clear that major breakthroughs have to occur to replace conventional rockets and their bulky propellants. We’ll know one day if Woodward’s contribution to breakthrough propulsion physics can provide the answer. Right now we’re still trying to see if MLTs and the the Mach Effect itself make sense.