From the start, Mach’s principle was a controversial addendum to general relativity. Some of Einstein’s contemporaries, especially the Dutch mathematician Willem de Sitter, labored to show that his concept of inertia was inconsistent with other mathematical implications of general relativity. But it was the physicist Carl Brans who finally expelled the idea from respectable physics. In Brans’ PhD thesis, published in 1961, he used mathematics to demonstrate that inertia could not be explained by the gravitational influence of distant matter in the universe. After Brans’ paper, “everybody assumed that inertia à la Einstein was not contained in general relativity,” Woodward says. “That’s still the view of most general relativists.”

But as Woodward dug deeper into the history and science of general relativity, he couldn’t shake the feeling that Brans had gotten it all wrong. And as he discovered in the autumn of 1989, if you accepted Einstein’s view that inertia was inextricably linked to gravity, it opened up the possibility for propellantless propulsion.

Woodward’s views on gravity and inertia aren’t mainstream, but it’s not crazy to think Einstein might have been right all along. “I’m pretty comfortable with Jim’s take on it, because it’s very historically oriented,” says Daniel Kennefick, an astrophysicist and historian of science at the University of Arkansas, who has collaborated with Woodward. “He is very much motivated by Einstein’s understanding of Mach’s principle. It’s not at all unusual for an idea to be discovered, rejected, and then later make a comeback.”

In Einstein’s famous equation, E=mc², an object’s energy, E, is equal to its mass, m, multiplied by the speed of light squared. That means if you change an object’s energy, you will also change its mass. An object’s mass is a measure of its inertia—that’s why it takes greater force to push a more massive object than a less massive one—so changing its energy will also change its inertia. And if, per Mach’s principle, inertia and gravity are one and the same, then changing an object’s energy means messing with the very fabric of spacetime. In theory, anyway.

Woodward realized that if Einstein was right and inertia really is gravity in disguise, it should be possible to detect these brief changes in an object’s mass as its energy fluctuates. If part of an object accelerated at the exact moment when it became a little heavier, it would pull the rest of the object along with it. In other words, it would create thrust without propellant.

Woodward called these temporary changes in mass “Mach effects,” and the engine that could use them a Mach-effect thruster. By combining hundreds or thousands of these drives, they could conceivably produce enough thrust to send a spaceship to the stars in less than a human lifetime. How to keep a person alive in space for decades is still an enormous question. But it is a mere footnote to the more fundamental issue of figuring out how to cross a void trillions of miles wide in any reasonable amount of time.

By 1995, Woodward’s ideas about Mach effects had coalesced into a full theory, and he turned his attention to building a thruster to prove it. The design he settled on was simple and opportunistic. A local electronics manufacturer was relocating, and an employee had alerted the university it had some leftover materials on offer. Woodward swung by its old office and snapped up a pile of piezoelectric disks the company had left behind.

To build his interstellar engine, Woodward mounted the piezoelectric disks to a block of brass and put a cap on the other end to hold it all in place. When piezoelectric disks are hit with a pulse of electricity, they bulge slightly. This expansion causes them to push off of the brass block and accelerate in the opposite direction. According to Woodward’s theory of Mach effects, the electric current would also make the piezoelectric disks ever-so-slightly heavier. This causes them to pull the brass block toward them. When the electricity stops flowing, the whole ensemble will have scooted slightly forward. By repeating this process over and over, Woodward figured, the Mach-effect thruster should accelerate. Fearn, his closest collaborator, compares it to rowing a boat on the ocean of spacetime.

Over the next few years, he managed to coax a few hundred nanonewtons of thrust out of his Mach-effect drive. Most of Woodward’s peers dismissed his nearly imperceptible results as a measurement error. It is not hard to see why—when you blow out candles on a birthday cake, you produce around three orders of magnitude more force than what Woodward was reporting. Even if the device did work, it wouldn’t be enough to move a small satellite, much less a starship.