Harnessing the quantum power of empty space

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Harnessing the quantum power of empty space


20 February 2012 by David Harris

Harnessing the power of the vacuum (Image: Umar Mohideen, University of California at Riverside)

The elusive Casimir effect suggests we could use vacuum energy to move objects and make stuff – but can something really come from nothing?

"NOTHING will come of nothing." Shakespeare's epithet seems the kind of self-evident statement that only poets and philosophers would argue over. And physicists like Chris Wilson.

Last year, Wilson and his team at the Chalmers University of Technology in Gothenburg, Sweden, provided what seems a particularly egregious case of something for nothing. They claimed to have conjured up light from nowhere simply by squeezing down empty space (New Scientist, 19 November 2011, p 16). That would be the latest manifestation of a quantum quirk known as the Casimir effect: the notion that a perfect vacuum, the very definition of nothingness in the physical world, contains a latent power that can be harnessed to move objects and make stuff.

Sightings of this vacuum action have been mounting over the past decade or so, leading some physicists to propose a new generation of nanoscale machines to take advantage of it, and others even to suggest a leading role for vacuum energy in determining the origin and fate of the cosmos. Others remain to be convinced. So what's the true story?

The idea that a vacuum is a seething sea of something can be traced back to the early decades of quantum physics. In the late 1920s, the German physicist Werner Heisenberg came up with his famous uncertainty principle, which says that some pairs of measurable quantities are intimately connected: the more you know about the one, the less you know about the other.

Energy and time are one such pair. That means you cannot measure the energy of a physical system with perfect precision unless time itself is completely imprecisely defined - that is, you take infinite time to perform your measurement. It follows that the zero-energy nothingness of the vacuum can never be pinned down precisely. According to quantum theory, even a perfect vacuum is filled with wave-like fields that fluctuate constantly, producing a legion of ephemeral particles that continually pop up out of nowhere only to disappear again, filling the vacuum with a distinct, non-zero "zero-point energy".

This recasting of the vacuum gave fresh impetus to the centuries-old debate about the nature of nothingness (New Scientist, 19 November 2011, p 50). But evidence also began to accumulate that the newly lively vacuum had practical effects. Observe atoms carefully enough and you see a tiny effect known as the Lamb shift, in which vacuum fluctuations jostle an orbiting electron, subtly altering its energy. Something similar can be invoked to explain how electrons sometimes spontaneously jump between two atomic energy states, giving off photons of light.

But Hendrik Casimir's suggestion was the most eye-catching. In 1948, together with his colleague Dirk Polder, the Dutch physicist was trying to understand how colloids exist in a stable equilibrium. Colloids are mixtures in which one type of substance is dispersed through another, like fat globules in the watery solution of milk. Forces between the molecules in such a medium drop off more quickly with distance than basic calculations using the classical electromagnetic van der Waals force allow. It is as if something is pulling the constituent molecules closer together, giving the mixture extra stability.

Following a tip-off from the Danish quantum doyen Niels Bohr, Casimir calculated that this something could be vacuum action. Working out the effects of vacuum fluctuations in a colloid's complex molecular brew was impossibly involved. So Casimir considered a simple model system of two parallel metallic plates, and showed that the fluctuations could produce just the right enhanced attraction between them. His explanation was that the two plates limit the wavelength of vacuum fluctuations in the space between. Outside those confines, the fluctuations can have any wavelength they choose. With more waves outside than in, a pressure pushes inward on the plates (see diagram).

The effect is tiny: two plates 10 nanometres apart feel a force comparable to the gentle burden of the atmosphere on our heads. Such a minuscule contribution is easily washed out by a legion of other effects, such as residual electrostatic attractions between charges on the plates' surfaces. That makes confirming its existence extremely tough. "You need to know that you're really measuring the Casimir force," says experimentalist Hong Tang of Yale University. What's more, it is not easy to align plates to be perfectly parallel, while calculating the expected effect for other, more complex geometries takes some sophisticated mathematics.

It was only in 1996 that Steven Lamoreaux, a physicist then at the University of Washington in Seattle, made a breakthrough. Taking elaborate precautions to exclude all other effects, he found a tiny residual force pulling a metal plate and a spherical lens together (Physical Review Letters, vol 78, p 5). The Casimir effect, it seemed, was not a theorist's pipe dream: vacuum action was a real effect.

Since then, a steady trickle of results has confirmed other long-standing theoretical predictions. Soviet physicist Evgeny Lifshitz proposed in 1955 that the size of vacuum fluctuations would grow with rising temperature, resulting in a force that is more potent over longer distances. In February 2011, Lamoreaux, now at Yale University, and his team confirmed that this is indeed the case (Nature Physics, vol 7, p 230).

Nanoscale kick

As for the work of Wilson's team, their results, published last November, support a four-decade-old prediction that turns the logic of the original Casimir effect on its head. Rather than using the vacuum's pop-up particles to shift their surroundings, if you move a vacuum's surroundings fast enough, you can make real photons of light. In some quarters, this idea is controversial - but it is the most dramatic putative demonstration of the vacuum's powers to date (see "Light from speeding mirrors").

As sightings of such effects have multiplied, so have thoughts that we might harness them for our own devices. A popular proposal is to use the vacuum's energy to give nanoscale machines an additional kick. That requires something a little different from the original Casimir force, whose attractive effects are more likely to gum up the components of any mini-machine - a phenomenon referred to as static friction or "stiction".

By tweaking the geometries or material properties of the structures used to confine the vacuum, however, it should be possible to reverse the direction of the Casimir effect, creating an outward pressure to push two objects apart. In 2008, Steven Johnson and his colleagues at the Massachusetts Institute of Technology calculated that by adding a series of interleaving metal brackets, zipper-style, to the faces of the two metal plates you could in theory make the net force between them repulsive. A more recent study by Stanislav Maslovski and Mário Silveirinha of the University of Coimbra, Portugal, has indicated a similar effect using nanoscale metallic rods to create areas of repulsive force that can levitate a nanoscale metal bar (Physical Review A, vol 83, p 022508).

These forces could help nanoscale components such as switches, gears, bearings or motor parts to operate without jamming. Putting such devices into practice might not be easy, though. For a start, it would require components with atomic-scale polishing: look on a small enough scale - a thousand atoms or so - and metal surfaces usually thought of as smooth have patchy, crystal-like structures that would confine vacuum fluctuations in different ways, affecting the size of the Casimir force. For moving objects, things become even trickier.

Such complications are surmountable: in 2009 Federico Capasso and his group at Harvard University measured what appeared to be repulsive Casimir forces in a gold cantilever suspended in bromobenzene liquid above a silicon surface (Nature, vol 457, p 170). The forces generated were mere tens of piconewtons - but when you are trying to move nanoscale particles, a piconewton goes a long way. Nevertheless, there are still hurdles to be overcome before Casimir devices are everyday reality, says Johnson. "It is an experimental question - can we make devices this small and sensitive?" he says. "And it is also a theoretical question of whether we can design interesting uses for the Casimir force once the experimental capabilities arrive."

There is a more fundamental objection, however. The litany of theoretical predictions gradually being turned into experimental reality invites a simple conclusion: vacuum fluctuations are real, and they are what is responsible for what we call Casimir effects. But not all physicists buy that.

Their unease lies in calculations done by Casimir and Polder even before they settled on vacuum fluctuations as the explanation for the weakened van der Waals force. These showed that much the same weakening could be achieved simply by taking into account the finite time the force takes to be transmitted over large enough distances, such as between two plates separated by tens or hundreds of nanometres. That idea was revived and bolstered by calculations in the 1970s by the Nobel-prizewinning physicist Julian Schwinger. He never believed in the reality of vacuum fluctuations and developed a version of quantum field theory, which he called source theory, to do away with them. In this picture, the Casimir effect pops out just by taking into account the quantum interaction of charged matter, with no vacuum action at all.

Robert Jaffe, a particle theorist at the Massachusetts Institute of Technology, suggests the only reason the vacuum interpretation has gained such currency is because its mathematics happens to be a lot simpler. "There is a flippant way people refer to the Casimir effect as evidence for real vacuum fluctuations," he says. "But there is no evidence that the vacuum fluctuations exist in the absence of matter". Similarly, other effects invoked as proof of their reality - the Lamb shift and the spontaneous emission of photons from atoms - can be described purely as the result of charge interactions.

If this is so, it could have repercussions for more than our attempts to fine-tune the workings of nanomachines. The realisation in the past couple of decades that the universe's expansion is accelerating - a phenomenon ascribed to a mysterious "dark energy" - has fuelled a new interest in the power of the vacuum. At the moment, our best calculations of the vacuum's hidden energy come up with a figure some 120 orders of magnitude larger than the amount needed to bring about the cosmic acceleration, a mismatch that counts perhaps as the worst-ever prediction in physics. Yet observations of the Casimir effect are still eagerly seen as evidence for a power that might determine our cosmic fate.

Schwinger's original calculations were part of a wider attempt, ultimately unsuccessful, to banish vacuum fluctuations from quantum field theory. The truth may well lie uncomfortably in the middle: we might never be able to convince ourselves of the reality of vacuum energy, because any attempt to do so brings some form of matter into the equation. As philosophers of science Svend Rugh and Henrik Zinkernagel wrote in 2001, "It seems impossible to decide whether the effects result from the vacuum 'in itself'... or are generated by the introduction of the measurement arrangement."

Wilson hopes that the photons emerging from his apparatus in Sweden, if confirmed by other groups, will provide the final illumination to prove the reality of vacuum fluctuations. Equally, as our ability to construct filigree nanomachines and so test the Casimir effect increases over the coming years, perhaps some deviation from the predictions will give us a definitive handle on where the effects come from. Can nothing truly come of nothing? We might still have cause to speak again.


Light from speeding MIRRORS

In 1970, American physicist Gerald Moore proposed reversing the logic of the Casimir effect. He envisaged rapidly accelerating mirrors that would squeeze the vacuum fluctuations in the space between them so violently that they would give up some of their energy in the form of photons (Journal of Mathematical Physics, vol 11, p 2679).

In practice it is not possible to accelerate even a small macroscopic mirror fast enough to produce this "dynamical" Casimir effect, so last year Chris Wilson and his team from the Chalmers University of Technology in Gothenburg, Sweden, used rapidly varying electrical currents to simulate the effect of mirrors accelerating to something like a quarter of the speed of light. The result was the simultaneous production of pairs of photons from the vacuum, exactly as Moore had predicted (Nature, vol 479, p 376).

Wilson thinks there could be some exciting applications. During the era of inflation thought to have taken place right after the big bang, the boundary of the universe itself would have expanded at near the speed of light, leading to the creation of photons through the dynamical Casimir effect. "It is rather difficult to create your own big bang in the lab," says Wilson. "Our set-up or a similar one might be used to simulate these effects, essentially doing table-top cosmology."

Just as the original Casimir effect is disputed, however (see main story), not everyone is convinced that this interpretation of the experiment is right. One physicist, who preferred not to be named, says that as nothing in the experiment actually moves, it does not demonstrate the dynamical Casimir effect at all. Instead, it is just another "solid and interesting" example of a well-known effect in which some of a quantum circuit's electrical energy is emitted as light. The mathematical description of the two effects is very similar, he says, but "one should never mistake mathematics for reality".

Since the preliminary version of their paper was circulated, Wilson's team has carried out additional tests that Wilson thinks defuse such criticisms, although he acknowledges there are still dissenting voices.

"We did a number of sanity checks ruling out various spurious effects that could have masqueraded as the effect, including showing that we were starting from the vacuum state," he says. "But for some people, the dynamical Casimir effect will never be anything but a literal moving mirror."

David Harris is a writer based in Palo Alto, California


Source: www.newscientist.com