Ask An Expert – The Woodward Effect And General Relativity

John P. Millis, Ph.D. for — Your Universe Online

This article is the latest installment in a new series where redOrbit´s in-house experts will answer questions submitted by you, the reader. Got a science or space question that´s stumping you? Each week we´ll select a handful of the wiliest questions you can whip up to tease the brains of our resident gurus (we call them ℠geeks´).


The Woodward effect is a weird result of General Relativity. It states that by rapidly changing the energy density of an object, you can actually cause transient fluctuations in its inertial mass. This means that if you arrange things such that you push on the object when it’s more massive and pull on it when it’s less massive, you’ll produce a net force, seemingly violating Newton’s Third. The result of this is that it’s possible to create a propellant-less spaceship engine. Is the Woodward Effect real or is it just too good to be true, like electro-gravity and cold fusion? – Gordon


The Woodward effect is one of many effects that can be derived from the framework of general relativity. The mathematics can be somewhat opaque and so it can be difficult to determine if the resulting effect is a physical, or purely mathematical construct.

On the most basic level, the premise that fluctuations of an object´s inertial mass are possible is not a problem. Depending on how you define the system, Newton´s laws are not violated because the total momentum is conserved.

Some experiments suggest that mass fluctuations may exist at levels more than two orders of magnitude greater than those suggested by the Woodward effect. While other experiments have yielded inconclusive results.

In short, whether the Woodward effect is physical remains to be seen. From a mathematical perspective there have been challenges to Woodward´s derivation, such as his reliance on Mach´s Principle — itself an ill-defined statement relating mass distribution and inertia — as well as his application of the principle to general relativity.

Space Craft Design

The greater question is whether the Woodward effect, or any effect of general relativity that would result in variable inertial mass in an accelerating reference frame, could be used as a means to motivate a space ship.

Certainly, the engineering that would be required to create an engine based on the Woodward effect would be a considerable hurdle. I am currently unaware of any design studies that have been done that could take advantage of the effect, though that does not mean it is impossible.

On the plus side, an engine based on the Woodward effect would not require any sort of exotic matter — a problem that plagues other proposed propulsion designs, such as warp-drive technology which would require large amounts of negative energy.

So is the Woodward effect pie-in-the-sky science? The underlying principle may be physical, though more experiments need to be done to verify the effect. However, creating a propulsion device based on the Woodward effect is likely far off in the future, if it is possible at all.

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