Most people assume that the vacuum is empty. But according to quantum electrodynamics, the theory that describes the behaviour of the Universe at the very small scale, nothing could be further from the truth. The vacuum is actually seething with electromagnetic energy called zero-point energy and it's this that Maclay hopes to tap. The "zero" in zero-point refers to the fact that if you were to cool the Universe to absolute zero, its lowest possible energy state, some energy would remain. Actually, rather a lot of energy. Physicists disagree over just how much, but Maclay has calculated that a region of the vacuum the size of a proton could contain as much energy as all the matter in the entire Universe. 

In 1948, a Dutch physicist called Hendrick Casimir proposed a scheme to test for the presence of this energy. In theory, vacuum energy takes the form of particles that are constantly forming and disappearing on a tiny scale. Normally, the vacuum is filled with particles of almost any wavelength, but Casimir argued that if you were to place two thin uncharged metal plates very close together, longer wavelengths would be excluded. The extra waves outside the plates would then generate a force that tended to push them together, and the closer the plates were together, the stronger the attraction would be. In 1996, physicists measured the so-called Casimir effect for the first time. 

Maclay, a former professor of electrical engineering at the University of Illinois in Chicago, wants to go further and has formed a company called Quantum Fields in Richland Center, Wisconsin, to develop his ideas. He and others have calculated that the Casimir effect can produce repulsive forces as well as attractive ones. His analysis has focused not on metal plates but on tiny metal boxes, roughly 1 micrometre or less on each side, which he refers to as cavities(see diagram). 

It turns out that the Casimir force, and its direction, depend on the shape of the cavity. "If you have a cavity the shape of a pizza box, the pressure on the two large sides of the box pushes them together, but the force on the narrow sides pushes them apart," he says. 

The cavity Maclay finds most intriguing is long and thin, like the box a tube of toothpaste comes in, and about the size of an Escherichia coli bacterium. What's significant about this cavity is that one of its long sides is at perfect equilibrium: the inward and outward vacuum pressures are exactly equal. But it's a tenuous equilibrium. And that's what makes it interesting. 

Maclay plans to build a box in which the side at equilibrium--call it the lid--is free to move. If the lid moves inward slightly from the equilibrium point, the vacuum pressure inside the cavity goes down, and the lid is drawn farther in. If it moves outward the reverse happens and the lid is pushed away. The distances involved are tiny--less than 100 nanometres. The lid will be attached to a microscopic spring. So when the lid moves, the spring will be stretched or compressed and will tend to return to its original position. Maclay is hoping that by carefully balancing the vacuum pressure of the cavity and the elasticity of the spring, and by giving the lid just the right initial impulse, he can create a tiny oscillator driven by Casimir forces. 

That's the ideal scenario, at any rate. But it's easy to make things look good on paper. "From a theoretical viewpoint," says Maclay, "all kinds of things oscillate. But in the real world? Well, that's what we have to look at." 

Maclay plans to attack the problem in stages. Repulsive Casimir forces have never been measured so his first task will be to find out if he can even do this. Next he'll measure the inward and outward forces at the surfaces of cavities with different shapes, to see if they match predictions. And if all that goes well, he'll be ready to build a resonating cavity. 

The job of building the experimental setup falls to Rod Clark, a former nuclear engineer and president of MEMS Optical, a technology company based in Huntsville, Alabama, that manufactures microelectromechanical devices (MEMs). To build Maclay's cavities out of silicon, Clark hopes to use a combination of traditional lithographic etching and deposition techniques--the same techniques used to make integrated circuits. 

Clark is confident that he can produce the necessary structures. But he's also well aware of the challenges, the first of which is size. Maclay's specifications are at the limits of today's fabrication technology, says Clark. "We want to make it small in order to make the forces large. But we can't make it so small that we can't fabricate it." 

Maclay and Clark's current plan is to make an array of hundreds of topless cavities on a substrate, and then create a single lid that fits over the entire array. The lid will be suspended on springs above the array, which will be moved toward the lid in tiny steps. Initially the lid should remain still, but when the cavities get close enough, the difference in vacuum pressure should cause it to move and possibly even to oscillate. By peering across the surface of the lid through a microscope, it will be possible to measure its displacement with great precision. 

Neither Maclay nor Clark expects quick results. Over the course of the three-year period covered by NASA's grant, they hope to build three generations of devices. 

Still, Maclay is already dreaming of various types of "Casimir machines" that might be possible if his experiments prove successful. Microscopic vacuum-drive levers, pulleys and pistons come to mind, for example. Or perhaps a machine that contains cavities that generate different vacuum pressures and exploits that difference in much the same way that a heat engine exploits differences in temperature. "What we're looking at now are very simple things that ultimately will serve as components of more complicated systems," he says. "We've gotta kind of mess around to see what they can do." 

How does Maclay rate his chances of success? "If I thought the chance was zero," he says, "I wouldn't spend my time on it. I'm convinced we'll find some interesting things. Exactly what and what its utility will be, I don't know." 

Marc Millis, who heads NASA's Breakthrough Propulsion Physics programme, is also philosophical about the prospects. He'd be thrilled, of course, if Maclay handed him the keys to an interstellar propulsion drive. "I would be very surprised if there wasn't a potential breakthrough of some kind," he says of the entire propulsion project. If Maclay is able to reel in his fish, he could have a phenomenon to rival Faraday's in importance. And as Faraday once said: "Nothing is too wonderful to be true if it be consistent with the laws of nature."