A Propulsion Overview
Speaking at last fall’s International Astronautical Congress in Prague, Tau Zero founder Marc Millis offered a condensed summary of the present state of the art in advanced propulsion physics, summarizing a variety of approaches and next-step questions from the book he co-edited with Eric Davis called Frontiers of Propulsion Science (2009). He’s now written a paper based on the presentation. It’s a useful distillation of an extremely detailed work (739 pages) and well worth scanning now that Millis has made it available on the arXiv site.
Quite a few propulsion concepts have gone through the early stages of the scientific process, with problems defined, data being collected and hypotheses formulated, and Millis also refers to those cases where ideas have progressed into the testing stage. He’s fascinated with the idea of using investigations into broad issues of cosmology to focus in on something far more utilitarian, the possible relevance of new observations for spaceflight. From the paper:
> While general science continues to assess cosmological data regarding its implications for the birth and fate of the universe, a spaceflight focus will cast these observations in different contexts, offering insights that might otherwise be overlooked from the curiosity-driven inquiries alone. Homework problems to help teach general relativity now include warp drives and traversable wormholes. Even if there are no spaceflight breakthroughs to be found, adding the inquiry of spaceflight expands our ability to decipher the lingering mysteries of the universe.
Focus on the ‘Space Drive’
The approaches gathered in Frontiers of Propulsion Science are too numerous to list here, but let’s focus for a moment on the issue of space drive physics. A space drive is the umbrella term used to describe the interactions between a vehicle and surrounding space to induce motion, the key point being that such a technology, if ever developed, would eliminate the need for propellant. That’s a big issue — if we could move a craft in this way, we would be dropping the energy requirements from exponential to squared functions of trip velocity, opening up a wide range of mission possibilities we simply cannot achieve with rocketry or space sails.
Notice that the space drive is fundamentally different from what is more and more known as a ‘warp’ drive, the point being that the space drive interacts with spacetime rather than warping spacetime. Quoting Millis again:
> Warp drives and wormholes are rooted in the Riemannian geometry of general relativity, where sufficient energy densities can warp spacetime analogously to how a huge gravitational mass bends spacetime. In contrast, most space drive concepts begin with Newtonian representations where the operative goal is to interact with reaction mass embedded in the properties of spacetime. This also implies, therefore, that space drive concepts will be light-speed limited since they operate within spacetime.
In other words, a warping of spacetime, if possible, would allow the craft to take advantage of the fact that there is no speed-of-light restriction when it comes to the expansion of spacetime itself (a notion that draws on cosmic inflation in the Big Bang era). The space drive is a different animal, and it compels a different kind of research. The primary issues are conservation of momentum and net external thrust, with the concept raising huge questions about the sources of inertial frames, the nature of the quantum vacuum energy, and the physics of photon momentum in media.
Our understanding of inertial frames is not complete, and various versions of Mach’s principle exist in the literature — speculating that inertial frames are the result of surrounding matter — with issues that remain unresolved. Nor is our understanding of vacuum energy complete. In fact, says Millis, “Depending on factors chosen for the calculation, the equivalent mass density of the quantum vacuum energy can span from the insignificant 10-26 kg/m3 to the enormous value of 1098 kg/m3.”
From Quantum Physics to Cosmology
The extension of quantum physics to cosmological scales is obviously a work in progress, with numerous approaches still in play. Will practical applications one day flow from their reconciliation? We can’t know, but Millis believes that adding the desired propulsion and power goals to these studies offers a useful additional perspective.
Over three dozen concepts toward breakthrough propulsion and power are addressed in this paper in a complex table that reflects the content of Frontiers of Propulsion Science, categorized by the challenges they address and their respective methods, and drawing on the ‘grand challenges’ addressed by NASA’s Breakthrough Propulsion Physics project, which Millis ran from 1996 to 2002. His paper goes on to offer up a challenging set of approaches matched with ideas about the unfinished physics they provoke, along with suggestions for next-step research. Many of these are general themes rather than specific tasks — to cite a few from his lengthy list:
* Explore vacuum energy experiments using negative index of refraction materials, ultra-high electrical carrier density materials and superconductors * Determine if measurable effects are possible using ultra-high intensity tabletop lasers to test the space-warping assertions from general relativity * Independently repeat or devise new experiments to explore Martin Tajmar’s unconfirmed observations of inertial frame dragging related to rotations of ultra-cold matter * Revisit prior theoretical attempts to mature Mach’s principle into testable theories, but now including the following natural observations that were not known at the time of those earlier attempts: the absolute frame reference from the Cosmic Microwave Background radiation, anomalous trajectories of deep space probes, and the anomalous observations that lead to the Dark Matter and Dark Energy hypotheses.
And so on. Systematic and rigorous research can flow out of all this, but we have no way of knowing whether future discoveries in these areas will reveal new methods to help us cross interstellar distances at speeds faster than we can create through presently understood methods. The challenge is open-ended and energizing. Adds Millis: “Progress is not made by conceding defeat. With a combination of risk-taking vision and impartial rigor, useful, reliable results will accumulate.” Those results may or may not offer us a route to the stars, but they will help us discover things about the universe we need to know, a worthwhile outcome in the best tradition of scientific research.
The paper is Millis, “Progress in Revolutionary Propulsion Physics.” Preprint available.
Related: My article “Tau Zero Takes Aim at Interstellar Propulsion” is now available on the Discovery News site.