Marc Millis: Testing Possible Spacedrives

Marc Millis, former head of NASA’s Breakthrough Propulsion Physics project, recently returned from another trip to Germany, where he worked with Martin Tajmar’s SpaceDrive project at Germany’s Technische Universität Dresden. Recent coverage of the ongoing experimental work into spacedrives in both the popular and scientific press has raised public interest, leading Millis to explain in today’s essay why and how the techniques for studying these matters are improving, and how far we have to go before we have something definitive. Millis is in the midst of developing an interstellar propulsion study from a NASA grant even as he continues to examine advanced propulsion concepts and the methodologies with which to approach them.

by Marc Millis

Two recent articles, one in Scientific American [1] and the other in Acta Astronautica [2], prompted this update about the experimental tests of possible spacedrives. In short, the experimental methods are improving, but definitive results are not yet in hand. While this update is mostly on the “Mach Effect Thruster,” it also touches on the infamous “EmDrive,” as well as a refresher on the general quest for spacedrive physics.

First, what is a spacedrive? Presently, a spacedrive is still a goal rather than a proven device. The ambition is to find a fundamentally different way to propel spacecraft rather than rockets or sails. Rockets are limited by having to carry their entire journey’s reaction mass with them (propellant). Sails are limited by one-directional photons (or particles) from an external source. Imagine, instead, if there was some way for a spacecraft to interact with its surrounding spacetime to move in any direction and be limited only by the amount of available energy. That ambition is the essence of a spacedrive.

That detail – of interacting with spacetime to induce motion – is a matter of undiscovered physics. That makes it harder to grasp, harder to explain, and harder to solve. It’s easier to grasp engineering challenges that are based on known physics, since there are already operating principles to cite. With spacedrives, the operating principles are works-in-progress – more akin to lines of inquiry than having complete packages ready for scrutiny. Though theories for faster-than-light warp drives do exist (one type of spacedrive), the physics of the required negative energy is still debated – which itself is a prerequisite to devising how to engineer a warp drive. In addition, though there are experimental replications of thrusts from possible spacedrives, separating experimental artifacts from actual thrusts is also, still, a work in progress – and the main point of this update.

Before getting to the latest experiments, here is a bit more background behind the challenges of a spacedrive. At first blush, such wishful thinking might seem to violate conservation of momentum – a crucial detail. Conservation of momentum is easy to grasp for a rocket; the rearward-blasted propellant matches the forward momentum of the spacecraft. The situation is less obvious with spacedrives. There are a least 3 approaches to address conservation of momentum: 1) using a reaction mass indigenous to space or spacetime, 2) negative inertia, or 3) exploring the physics about inertial reference frames – the backdrop upon which the conservation laws are defined.

The majority of this update is related to the 3rd option – inertial frames. For new readers, a more complete introduction to various approaches and issues of both spacedrives and faster-than-light flight are spelled out in the book Frontiers of Propulsion Science [3]. If you’re curious about that broader coverage, that book and subsequent papers are one starting point.

Back to inertial frames and conservation laws: An inertial frame is such a ubiquitous property of spacetime that it is often taken for granted. It is what allows accelerated motion to be felt – the reference frame for Newton’s F=ma and the subsequent conservation laws. If you’ve never thought about it before, this can be hard to grasp because it’s so foundational. One useful book is