Speculations about the Gravity Shield Engine.

Disclaimer: Students who read this are forewarned that it is informal speculation, not hard science. It deals with things such as gravity shields which may not even be possible in our world.

Warning! Do not try to build a gravity shield at home. Consider the unintended consequences if it should actually work, though that outcome is highly unlikely. H. G. Wells played with the idea (fictionally only) in his 1901 book The First Men in the Moon. It supposes that a gravity shielding material, called "cavorite" might be used in a flying machine, and even to lift a flying mchine to the moon. You can read about one unintended consequence in Chapter 2.

Elementary observations.

Most people look at this puzzle and respond quickly in several ways:

If the wheel were to experience a rotation through a small angle, it would be the same in every respect as it was before. So we have no reason to expect it to initiate rotation of its own accord.
Stevin's principle shows that it won't work.
The laws of thermodynamics show that it won't work.

These are true, of course. But they fail to tell us exactly which assumption of the proposal or its description was a fault, and which fundamental laws of physics it might violate.

Then there's these somewhat more interesting observations:

If the wheel is imagined to rotate from a stationary position, through a small angle, to a new stationary position, a small sector of mass near the top has been moved from zero potential energy on the left to a small amount of "new" potential energy at the right top. But at the same time an equal sector of mass has been moved from the bottom right into the shielded space, and loses that same amount of potential energy. So, whatever static position the wheel assumes, it has the same potential energy, and no energy has been gained simply by such displacement.
This same argument applies whichever direction the wheel is rotated, leading us to conclude that the arguments for "one-way" operation of the uniformly symmetric wheel version of this machine were flawed.

These observations are based on the fact that the wheel is solid and cylindrically symmetric, and all parts of it move at the same velocity. So moving the top of the wheel necessarily moves the bottom, and all the rest of the wheel, all at the same rotation speed.

The second observation recalls and refutes the original rationale/explanation of why this machine should work, which predicted only one-way operation of the machine.

But what about the other form of this engine: the eccentric weight? If this one starts at the top, falling in the unshielded region, it gains kinetic energy E = mgh (at the expense of potential energy) and attains a certain speed at the bottom. On entering the shielded region it loses potential energy P, but its speed is unchanged (constant) all the way up. It gains potential energy P+mgh going back into the unshielded region. Thus it begins the cycle with an additional amount of energy mgh more than when it started the previous cycle. This modified machine casts serious doubt that our previous "solution" can be applied to this version.

The gravitational potential energy must be zero in the shielded region. The gravitational potential energy is due to the force exerted by a gravitational field, and the shielded region is a field-free region.

We also observe that this rationale predicts that the wheel would gain kinetic energy when revolving in one direction, but lose kinetic energy when moving in the other direction.

All of this leaves nagging questions. What, exactly, does a gravity shield do, and is a gravity shield even a possibility?

More detailed observations.

Most folks who have commented to me about this engine say "Since you can't make a gravity shield, then you can't make such an engine." Others have said "This machine would work if there were such a thing as a gravity shield, and since the machine would violate laws of thermodynamics, this proves that gravity shields are impossible." In the process they may miss more fundamental flaws in the idea, and miss some fun in considering "what if" scenarios, straying into the realm of science-fiction.

We know that we can make electric field shields, and even partial magnetic shields. Is there any fundamental proof that gravity shields are impossible? There are experimenters right now (2002 CE) who claim to have built an apparatus which reduces the gravitational field slightly in a small region. But their experimental claims are suspect. Theoreticians produce very persuasive arguments why such a shield is impossible. We'll let the theoreticians and experimentalists argue about this one. In our usual spirit of generosity to inventors, let's grant that somehow they could obtain a gravity shield. Could they use it to make this engine work? Suppose that the region above the shield is really gravity-free or at least has significantly reduced gravitational field.

Just what would a gravitational shield do?

Since no one has yet made a gravitational shield, and we don't know whether anyone can, we should rephrase the question. Just what do we want a gravitational shield to do?

We want it to reduce (or increase) the effective gravitational force in a localized region of space, in particular, in the region encompassing just part of the path of a mass moving in a circle. This is a modest goal. We don't need a perfect or complete shield. One way we might imagine to do this is with a device that produces another gravitational field strictly localized in that finite localized region of space.

Shields must also obey the fundamental laws of physics. "Magical" shields are not allowed.

If the fundamental principle of vector addition of forces is still valid, then this field, together with the earth's gravitational field, would allow us to have one half of the path of the moving mass pass through a greater or lesser strength field than the other half.

The mass moves down in the earth's field, and up an equal distance in the earth's field, so the work done on it by the earth and the work done by it on the earth are equal in size. The net work done on the mass per cycle is therefore zero. But the "shield field" exists only over a portion of the path. If the mass moves up through this portion, the mass does work on the shield, losing kinetic energy. If the mass moves down through this portion, the shield does work on the mass, and the mass gains kinetic energy.

The shield is (1) a source of force, (2) it can do work on objects moving through its field, and (3) objects moving through its field can do work on the shield. We assume Newton's third law is still valid.

So, if the revolving mass gains kinetic energy, it does so because of work done on it by the shield. If the wheel loses kinetic energy, it does so by doing work on the shield.

A mass moving in a closed path in the earth's gravitational field does the same thing. As the mass moves down, the source of the gravitational field (the earth) does work on it. As the mass moves up, the mass does an equal amount of work on the source of the gravitational field. Over one cycle, the net work is zero.

So what's new in this PMM device? It's simply the idea of a "shield field" over a limited region of space. Such a boundary-limited gravitational field is unknown in nature, and we have good reasons to think it's not possible. But even if it were possible, and this machine were to gain speed continually, that energy increase is due to work done on it by the shield, and conservation of energy is still preserved.

For another take on this interesting machine, see Kevin Kilty's analysis.

—Donald E. Simanek

Return to The Museum of Unworkable Devices.