For many decades, a fantasy among space enthusiasts has been to invent a device that produces a net thrust in one direction, without any need for reaction mass. Of course, a reactionless space drive of this type is impossible. Or is it? By Charles Platt

Ever since I was old enough to read science fiction, I've wanted to visit Mars. Even the Moon would be better than nothing. Alas, rocket technology is unlikely to take me there within my lifetime.

The problem is that rockets are a poor tool for the job. Even if their safety record improves, they are inherently limited by the basic concept of reaction mass. Hot gases must blast out of the rear in order to move a space vehicle forward, and this entails carrying a fuel load that is hundreds of times heavier than the payload.

Ever since H. G. Wells imagined a gravity-shielding material in "The First Men in the Moon," space enthusiasts have fantasized about ways to achieve thrust without any need for reaction mass. Unfortunately, it seems impossible.

Or is it?


James Woodward's office, repurposed as a laboratory to investigate the reduction of inertial mass. Woodward's work bench is at bottom left, and the torsion balance is at top right.

Personally, I'm not so willing to use the word "impossible" anymore. In October of this year, at the laboratory of Dr. James Woodward in California State University at Fullerton (above), I watched a very small-scale experiment that was surprisingly persuasive. Unlike all the "free energy" scams that you see online, Woodward's device does not violate basic physical laws (it does not produce more energy than it consumes, and does not violate Newton's third law). Nor is Woodward withholding any information about his methods. He has written a book, published by Springer, that explains in relentless detail exactly how his equipment works–assuming that it does, indeed, work. He published his theory in Foundations of Physics Letters, vol. 3, no. 5, 1990, and he even managed to get a US patent — number 5,280,864, issued January 25, 1994.

I first heard about him in 1997, when I interviewed him for Wired magazine. His results were tentative, then, and he was cautious about making claims. "I have biweekly paranoia attacks," he told me, "and then I try something else to see if I can make this effect go away."

Almost twenty years later, the situation has changed. Dr. Heidi Fearn, a theoretical physicist who specializes in quantum optics at Fullerton, has done the math that she believes can justify Woodward's experimental evidence. Wikipedia now has a substantial entry about the Woodward Effect. The Space Studies Institute is championing the cause, inviting tax-deductible donations.

If a small amount of thrust really can be created using a power input but no reaction mass, the principle could be applied to correct orbital variations in satellites. If the effect turned out to be scalable, it would be a major game-changer for human spaceflight. Of course, this is a big "if"; but I think Woodward's idea shows more promise than any other alternate systems of propulsion. It would be infinitely more attractive than rocket motors.

The concept is based on the possibility of changing the mass of an object. Changing mass? How can that make sense? The answer is linked with the general theory of relativity.

Mass is Not Absolute

We tend to think of mass as a fixed quantity, but this is not necessarily so. We certainly know that mass is convertible into energy, as this happens on a daily basis in fission reactors. The general theory of relativity also describes an increase in mass that occurs with velocity, although the effect is negligible in everyday life.

More to the point, according to the late Ernst Mach (1838-1916), inertial mass depends on an interrelationship with other objects in the universe.

Mach was an Austrian physicist whose name is used as a measurement of speed, as in "Mach 1," the speed of sound at sea level. He was a contemporary of Einstein, to whom he suggested a thought experiment: What if there was only one object in the universe? Mach argued that it could not have a velocity, because according to the theory of relativity, you need at least two objects before you can measure their velocity relative to each other.

Taking this thought experiment a step further, if an object was alone in the universe, and it had no velocity, it could not have a measurable mass, because mass varies with velocity.

Mach concluded that inertial mass only exists because the universe contains multiple objects. When a gyroscope is spinning, it resists being pushed around because it is interacting with the Earth, the stars, and distant galaxies. If those objects didn't exist, the gyroscope would have no inertia.

Einstein was intrigued by this concept, and named it "Mach's principle." It has never been disproven, but it seemed to have no application until James Woodward became convinced that under certain circumstances, mass could change momentarily.

The Unconventional Professor

Woodward is an unusual character. Now in his seventies, his interest in finding an alternative to rocket-powered spaceflight extends all the way back to his youth. "I got an undergraduate degree in physics," he explains, "and went on to graduate school in physics, and at that point it was obvious I would never get a job as a journeyman physicist where somebody would let me work on something like this. So I changed my profession to the history of science, in which I could be employed while I pursued my goals in my spare time." Apparently he assumed that he could operate on a solo basis. "If you don't work on a problem, you don't solve it," he says laconically.

He received his doctorate in history at the University of Denver, where his thesis was on gravitation. He remarks reflectively: "I never imagined that I would end up in the fall of 1989, figuring out that the only thing that made sense was Mach's Principle, which says that action at a distance is the way that reality is."

He started buying components and equipment with his own money, gradually setting up a lab in his office at Fullerton. When he needed small parts that didn't exist, he learned how to build them himself, with help from a friendly machinist who ran the university workshop. After many iterations and a lot of painstaking fabrication work, he now has a benchtop demonstrator that he ran for me when I visited him.


James Woodward, Ph.D., and Heidi Fearn, Ph.D., in front of the data monitoring and acquisition systems.

Heidi Fearn — the physicist who checked his math — obtained her Ph.D. in physics at Essex University in England, and has been teaching physics at Fullerton since 1991, where she has known Woodward for more than twenty years. (The two of them are shown above.) She didn't take a serious interest in his work until she discovered that stacks of his equipment had been unexpectedly relocated in her back office while she was on vacation. As Woodward's project was now unavoidable, she found herself watching the experiments. "I saw that it wasn't just experimental noise," she recalls. "It was a very clear effect, on every run. It was a huge signal, relatively speaking. You can't get a signal like that from nothing. Something obviously was happening, and it wasn't something I could explain very easily."

Fearn still has a North Country British accent, and comes across as a very practical, pragmatic personality — not at all the type whom one expects be drawn to unconventional science. Indeed, she was skeptical about Woodward's ideas and was surprised when she found nothing wrong with the theoretical basis.

She became, as she puts it, "ninety-nine percent convinced," and started to collaborate informally on the project while still teaching physics at the university. She bought some test equipment with her own money, along with modeling software that she wants to use to design the next prototype. "Right now, I'm a theorist, but I find myself using trial-and-error," she says. "I'm not comfortable with that. Jim's been tinkering for more than twenty years. I want to get to the point where I can suggest something optimal."

Her goal is to scale up the effect by an order of magnitude.

The Method

And how, exactly, does it work? The idea is to accelerate a small object while varying its energy. For example, if the small object is a capacitor that is vibrating at a relatively high frequency, and the electrical charge on it fluctuates at twice that frequency, the mass of the capacitor should fluctuate, too. When Woodward told me about this in 1997, I asked him why such an easily demonstrated phenomenon had never been noticed by anyone else. "Probably because people don't normally go around weighing capacitors," he said.

In principle, you could try this yourself by using a stereo amplifier to power a speaker that has been repurposed to vibrate a capacitor at, say, 20kHz, while you would also charge and discharge the capacitor at 40kHz. This would be such an elementary experiment, you could set it up for maybe $50, but your challenge would be to measure the small mass variations that are supposedly occurring. You would also have a major problem excluding outside factors such as electromagnetic fields, vibration, air currents, thermal variations, and much more.


A stack of piezoelectric discs used in one of Woodward's experiments. The grid in the background is divided at 1/10" intervals.

To deal with these issues, Woodward used a vacuum chamber in which he placed piezoelectric elements about 3/4" in diameter, which flex when they are subjected to electric current. The elements are interleaved with metal discs, and the capacitance between the discs changes as they vibrate. The resulting force is tiny, but he is convinced that it is measurable. One of his thrusters is pictured above.

The Application

Why should this enable a space drive? Here's another thought experiment. If two boxes on wheels are connected with a rod, and one of the boxes contains a motor which turns a crank that pushes and pulls the rod, the boxes will move away from each other and then back toward each other. If they weigh an equal amount, the pushing and pulling will move the boxes equally, and they won't go anywhere. This sequence of events is shown here:


Inertia Thought Experiment 1: In this thought experiment, a motor in the box on the left turns a crank attached to the box on the right. As both boxes are of equal weight and are mounted on wheels, they move apart and then back together, ending up in the same place.

Suppose, however, that when the box containing the motor is pulling the other box, the weight of the box with the motor is briefly reduced. Now it is momentarily lighter, it moves farther toward the second box, and the pair of them end up slightly displaced from where they started. This is shown here:


Inertia Thought Experiment 1: If the thought experiment is modified so that the box on the left experiences a brief reduction in mass, the two boxes end up slightly displaced toward the right.

Finally, imagine that the boxes are inside a space ship, where the second box is bolted to the floor. The result is a drive that exerts an intermittent net force in one direction.

This doesn't violate Newton's Third Law; it simply adjusts the consequences by varying inertial mass. Nor does it violate the principle of conservation of energy, because the system requires power for its operation. It could acquire that power from solar panels or a small onboard nuclear reactor.

The Apparatus

The demonstration device that sits in Woodward's office contains a very sensitive torsion balance. The balance is a horizontal beam mounted on vertical E10 C-Flex flexual bearings. Any lateral force on one end of the beam will move it slightly, until the force is equally and oppositely resisted by the twisting of two thin strips of metal in the bearings.


The torsion balance is contained in an evacuated lucite cylinder. The hardwood frame is leveled and stabilized with lead weights (gray bricks, in the picture) to minimize vibration.

The torsion balance is completely enclosed in a cylindrical Lucite vacuum chamber about two feet long, as shown above. The chamber is supported on an elaborate wooden frame that is stabilized by brick-sized slabs of lead, to resist external vibration. The apparatus is so sensitive, the smallest seismic events can trigger it.

When I visited the lab, I watched Woodward set up the equipment for a demonstration. He is an amiable, unpretentious man who enjoys moments of self-deprecating humor but displays a quiet, implacable determination. He has, after all, been pursuing this project without any outside help for a couple of decades.

"The idea is to do a run with the force acting in one direction, then reverse the thruster, and run it again," he explains. "This eliminates almost all the most obvious external factors."

Because the thruster that he built by hand is so small, it exerts only a small amount of force. "People said I need 100 micronewtons, or a millinewton," he remarks as he settles himself in front of some video monitors and oscilloscopes. "I said, for a rational person, you just need a thrust level that you can see at the three to five sigma level." Still, he would like to create a more dramatic demo. "We're slowly transitioning to what is at least as much an engineering development as a continuing scientific investigation. The real problems now are more a matter of getting the engineering right, rather than verifying that the effects exist."

So, he is entirely confident about the theory?

"The theory has been sitting in peer-reviewed literature for more than fifteen years, so if there was something obviously wrong with it, the odds are, more than one person would have said that it's not right. It has attracted some critics, but not the kind who complain that something in particular is wrong, and can show that something in particular is in fact wrong."


Data monitoring and acquisition.

He triggers a pulse of voltage to the thruster. It has to be brief, because the current generates significant heat. An optical sensor detects movement of the balance. A trace on the screen in front of me jumps up, then falls back to its former level when the pulse ends. His stack of monitoring and acquisition equipment is shown above.

"Now," said Woodward, "I will reverse the thruster."


After one run, the balance is partially withdrawn from the cylinder so that the mu-metal box containing the thruster can be inverted. Then, then mu-metal box will be inverted to verify that the next run will generate a force approximately equal and opposite to that in the previous run.

This is a nontrivial procedure. He has to open the cylinder and turn the mu-metal box by hand, as shown above. Then he has to re-seal the cylinder and reestablish the vacuum, which requires quite a lot of pumping time.

While the pump is running, we go out to lunch for an hour, and he chats some more about his work. He laughs when I ask him if he has tried to get money from within the privately funded space community. Then he pauses to word his response carefully. "Those who are players in the field of advanced propulsion," he says, "feel obligated, for whatever reason, to dissuade people from considering seriously alternatives to their particular scheme. I think they may genuinely feel they are doing a public service when they say, 'Don't spend money on that, it can't possibly work.'" He gives me a wry smile. "But their own schemes have all failed."

By the time we get back to the office, enough air has been pumped out of the cylinder to permit another run, and sure enough, the effect is opposite and approximately equal. "The red trace is the voltage across the device," Woodward explains. "This here is the driving signal that goes into the power amplifier. The yellow trace is the accelerometer, the blue trace is the voltage waveform across the capacitor." He shrugs. He has seen this phenomenon thousands of times.

The Next Step

Obviously the Woodward Effect would be more immediately convincing if it could be scaled to the point where anyone can see the balance swing (or, better still, feel the force). This is why he and Heidi Fearn are adamant that the remaining problem is one of engineering refinement. The piezo discs that Woodward uses just happened to be cheaply available when he was hunting for surplus electronics. Are they the optimal size? He's not sure. Is the effect proportional to the frequency of the current squared, as theory suggests? Fearn thinks so, but until her modeling is complete, she doesn't really know.

What if they're merely measuring a side effect of the voltage pulse? "Well, it's not thermal, and it's not vibration," she says "We've done all kinds of damping on that balance. We've eliminated everything we can think of."

They have toyed with the concept of crowd funding. As a premium, they could give away some piezoelectric thrusters from previous iterations of the experiment. Maybe one day, if the Woodward Effect enables people to take off for other planets, those painstakingly hand-made modules would have significant historical value. The problem is, crowd funding requires a significant investment of time, which might be applied more productively to research.

Some independent efforts have been made to replicate Woodward's work, with preliminary results that seem to be encouraging. More are needed. This should not be too much of a challenge, since the fabrication process is described in such detail in his book. It's interesting that while rockets still entail high-level engineering and an unappealing amount of risk, an alternative principle can be tested by a single person in a small office, using parts that are either off-the-shelf or relatively easy to make. The costs are relatively modest. The potential benefits could be significant.