Again, interest from a StephenBaxter and ArthurCeeClarke story. It has been thought for a long time that space is just a state of complete emptiness, as an absolute void, a vacuum. Actually, this does not even exist. Rather the laws of quantum mechanics predict the real vacuum to be a seething sea of particle pairs, energy fluctuations and force perturbations popping in and out of existence and thereby capable of both quantum mischief and, veritable technological magic. The quantum vacuum is in reality a plenum.

If you bring the temperature down to absolute zero (T=0), the electromagnetic radiation left is called the electro-magnetic zero-point field. There are four aspects of the zero-point field that may someday prove to be relevant to interstellar propulsion: the possibilities of generating forces, of tapping even a tiny fraction of the enormous zero-point energy, of manipulating inertia, and of manipulating the gravitation of objects. There now exist theoretical justifications for investigating all of these possibilities. Indeed, the CasimirForce is more than a theoretical possibility. It has now been carefully measured in the laboratory.

Thanks to Einstein's famous equation E = mc2, Heisenberg's uncertainty principle also implies that particles can flit into and out of existence, their duration dictated only by their mass. This leads to the astonishing realization that all around us "virtual" subatomic particles are perpetually popping up out of nothing, and then disappearing again within about 10-23 seconds. "Empty space" is thus not really empty at all, but a seething sea of activity that pervades the entire Universe.

Such an image is worryingly reminiscent of the ether - a discredited idea that bedevilled physics until the beginning of this century. But Einstein's special theory of relativity showed that physics works perfectly well without this peculiar, all-pervasive fluid, which was supposed to be the medium through which light and other interactions travelled from place to place. This does not mean that a universal fluid cannot exist, but it does mean that such a fluid must conform to the dictates of special relativity. The vacuum is not forced to be mere quantum fluctuations around an average state of true nothingness. It can be a permanent, nonzero source of energy in the Universe.

This has cosmic consequences. Special relativity demands that the vacuum's properties must appear the same for all observers, whatever their speed. For this to be true, it turns out that the pressure of the vacuum "sea" must exactly cancel out its energy density. It is a condition that sounds harmless enough, but it has some astounding consequences. It means, for example, that a given region of vacuum energy retains the same energy density, no matter how much the region expands. This is odd, to say the least. Compare it with the behaviour of an ordinary gas, whose energy density decreases as its volume increases. It is as if the vacuum can draw on a constant reservoir of energy.

But there is more. One of the key features of Einstein's general relativity (GR) theory is that mass is not the only source of gravitation. In particular, pressure, both positive and negative, can also give rise to gravitational effects. If the vacuum has a permanent (positive) energy density, it must be balanced by a negative pressure (a tension). According to GR, this must give rise to a repulsive gravitational effect. This feature of the vacuum lies at the heart of perhaps the most important new concept in cosmology of the past decade: cosmic inflation. Developed principally by Alan Guth at MIT and Andrei Linde, now at Stanford, the idea of cosmic inflation arises from the assumption that the very early Universe was packed with unstable vacuum energy whose "antigravitational" effect expanded the Universe by a factor of perhaps 1050 in just 10-32 seconds. Then the vacuum energy died away, leaving random fluctuations whose energy turned into heat. Because energy and matter are interchangeable, the result was the matter creation we now call the big bang.

At a stroke, inflation solves a number of problems that had troubled cosmologists. For example, it explains the apparent coincidence that the Universe we see today seems to be teetering between expanding for ever and collapsing. Cosmic inflation would have "flattened out" the initially highly curved surface of the Universe, and according to calculations based on GR this would have led to the amount of mass-energy that was formed being just enough to allow the Universe to escape from its own gravity and expand for ever. The behaviour of the vacuum 15 billion years or so ago thus holds the key to the future fate of the Universe.

One way out of this bind would be a vacuum state that did not vanish after inflating the Universe. Perhaps a tiny remnant of it persists, providing a gentle unseen "push" to the contents of the Universe. This would boost the speed at which galaxies race away from each other, and give the impression that the Universe as it is now is nearer to the big bang state - and thus younger - than it really is.

Vacuum energy can do more, however. Though inflation predicts that the density of mass-energy in the Universe is right on the borderline between expansion and collapse, astronomers have only found between 10 and 20 per cent of the required mass. So where is the rest? This is another problem that a remnant nonzero vacuum may solve. By Einstein's equation, an energy density is equivalent to a mass density, so vacuum energy could account for some - perhaps most - of the missing mass.

But where does the inertia come from? Einstein believed that it was somehow induced in objects whenever they accelerate relative to the rest of the Universe, though quite how this interaction worked he never made clear. Now a group of American researchers has put a new gloss on Einstein's idea: instead of acceleration relative to the distant stars, they believe that inertia is generated by acceleration through the vacuum.

They base their idea on an esoteric quantum vacuum effect first discovered in the mid-1970s by Paul Davies, now at the University of Adelaide, and independently by William Unruh of the University of British Columbia. The Davies-Unruh effect predicts that if you accelerate through it, the usually uniform vacuum state turns into a tepid sea of heat radiation from your point of view if you accelerate through it. Two years ago, this triggered a thought in the minds of Bernhard Haisch of the Lockheed Solar and Astrophysics Laboratory in Palo Alto and, independently, Hal Puthoff of the Institute for Advanced Studies at Austin, Texas. Both wondered if, like the heat radiation, inertia is a product of acceleration through the vacuum.

Joining forces with Alfonso Rueda, a theorist at California State University, Long Beach, Haisch and Puthoff last year came up with a new version of Newton's second law. Again, it has F for force on the left-hand side, and a for acceleration on the right. But in place of M, their version featured a complex mathematical expression tying inertia to the properties of the vacuum. It implies that fluctuations in the vacuum give rise to a magnetic field through which all objects move. If the object accelerates, its constituent particles feel the grip of this magnetic field, whose resistance manifests itself as inertia. The larger the object, the more particles it contains and the stronger the reluctance to undergo acceleration.

It is a neat idea, though it is not without its critics. Haisch and his colleagues had to deal with a problem familiar to every theorist trying to understand the vacuum, which is that estimates of the effects of vacuum energy inevitably end up having to add together all the frequencies of fluctuation that contribute to the total vacuum energy. The trouble is that some frequency limit has to be imposed, otherwise the result is an infinitely energetic vacuum. Worse still, all sensible guesses as to what the frequency cutoff might be still lead to ludicrously high values, as much as 120 orders of magnitude out of kilter with the limits set by observations of distant galaxies. As NobelPrize winning physicist Steven Weinberg of the University of Texas puts it, "This must be the worst failure of an order of magnitude estimate in the history of science."

This problem has prompted some theorists to search for a mechanism that forces the vacuum energy to be precisely zero, while Haisch and his colleagues have tried resorting to a rather obscure theory of gravity which was sketched out in the late 1960s by the Russian physicist Andrei Sakharov. According to Sakharov's theory, the vacuum has no gravitational effects. But Milonni, for one, is not impressed with the way Haisch has applied the theory to vacuum energy.

Important as this wrangle is, it pales into insignificance compared with another consequence of the link between the vacuum state and inertia. By altering the vacuum state, it might be possible to alter the inertia of objects. This is the stuff of science fiction, though as Haisch points out, "History is full of impossibilities turned into technologies, from flying to splitting atoms". He stops short of talk about spacecraft powered by vacuum energy, which "switch off" their inertia when they want to move on. "It might only prove possible to modify inertia on the atomic scale, but not the macroscopic scale," he says.

In the meantime, Haisch and his colleagues are concentrating on building up solid observational support for their theory. Later this year, The Astrophysical Journal will publish research by Rueda, Haisch and Daniel Cole of IBM in Vermont that suggests that the vacuum plays a key role in creating structure in the Universe. (Ap. J. article now online) They claim that the vacuum accelerates charged particles, sweeping them up to form concentrations of matter surrounded by vast cosmic voids. The formation of structure in the Universe is one of the oldest mysteries of cosmology, so it would be a feather in the cap of the theorists if the vacuum proved to be the missing ingredient.

But the most tantalizing idea to emerge from these developments remains the prospect of manipulating the vacuum. The idea originated in 1948, when Hendrick Casimir of the Philips Laboratory in Eindhoven, Holland, made a startling prediction. Bring two perfectly conducting flat plates close to each other, he claimed, and a force will appear between them, pushing them closer together. That force, he said, was the result of the flat plates cutting off the space between them from the seething sea of the vacuum around them. It was as if the rest of the vacuum was hammering on the plates, trying to get in and thus forcing them together.

Nine years later, M. J. Sparnaay, also at Philips, verified Casimir's startling prediction. The effect is, however, incredibly feeble, amounting to a pressure of just one hundred-millionth of an atmosphere on plates held a thousandth of a millimetre apart. It may be unfamiliar, but it can be seen in the forces within liquids and gases (see "A brief history of the vacuum").

Though no-one has the faintest idea how to boost the Casimir effect to a useful size, its existence has prompted some theorists, including Cole and Puthoff, to look at ways of putting the vacuum to technological use. In research published 18 months ago in Physical Review, they pointed out that as plates are drawn together by the Casimir effect they develop kinetic energy that turns into heat when the plates finally collide. They went on to look at exploiting this effect by imagining a vacuum "engine" consisting of large numbers of colliding plates. Astonishingly, their calculations showed that such an engine could indeed extract energy from the bottomless well of the vacuum. There wouldn't be much energy to play with. "Optically polished square-metre plates collapsing to one micron spacing would yield half a nanojoule, and even if the collapse took place in a millisecond, that's only half a microwatt - not much to write home about," admits Puthoff. "That's why you would need microscopic, throwaway systems running at high rate." Quite what form they would take, no-one yet knows.

-- sg ''(written in 2003)''