what good is the casimir effect?

I've been culling through my bulging fodder file, discarding all those things I was sure would make for awesome blog posts, but never got around to writing them, and now, well, I probably never will. But there's also bits and pieces worth salvaging, such as the items related to the Casimir effect. Since just last week, Matt over at Starts With a Bang graced the blogosphere with a fantastic post detailing the basics of this unusual quantum phenomenon, I figured the timing was right to highlight some recent findings that make use of it, in some way.

Just what is this "Casimir effect"? Basically, it refers to the attraction between two objects should they come within, say, 1/5000 of an inch of each other. It's related to the energy inherent in the quantum vacuum. Empty space isn’t really empty. It roils and boils with quantum fluctuations, occasionally spitting out pairs of “virtual” elementary particles and antiparticles. These virtual particles annihilate and disappear back into the quantum vacuum so quickly that the apparent violation of energy conservation incurred by their creation can’t be observed directly.

So how do we know they exist? There is indirect evidence in the Casimir effect, named after Henrik Casimir, the Dutch physicist who discovered it in 1933. Normally two uncharged parallel metal plates would remain stationary because there is no electromagnetic charge to exert a force to pull them together (or push them apart). But Casimir found that if the plates are close enough, there is still a tiny attractive force between them.

 Because the parallel plates are so close together, virtual particle pairs can’t easily come between the plates, so there are more pairs popping into existence around the exterior of plates than there are between them. The imbalance creates an inward force from the outside that pushes the plates together slightly. The smaller the separation between the plates, the fewer virtual pairs can get between them, and the greater the force of the inward attraction. The Casimir effect is quite small, equal to the weight of 1/30,000 of an ant.

So the Casimir effect is pretty cool, but it's fair to ask whether it has any relevance on the macroscale — where most of us live our daily lives.  Back in 1996, a Dutch scientist named Sipko Boersma claimed one could see the Casimir effect between two ships moored close together in a strong swell. He based that upon a reading of French nautical writer P.C. Caussee's 1836 book, entitled The Album of the Mariner, which supposedly warned of this effect. It's an oft-repeated story that, alas, is probably not true, according to a 2006 Nature article reporting on the investigation by physicist Fabrizio Pinto.

"A former NASA scientist, [Pinto] is both a keen sailor and president of InterStellar Technologies, a company that researchers practical applications of the Casimir force," Nature reported. And when he tracked down a copy of Caussee's book to verify the claim for himself, he found that Boersma misread the original text: "Caussee never claimed that two ships attract in a heavy swell. Rather, he said this happens when the sea is completely calm." Nor could Pinto find any experimental evidence for the Casimir effect between two closely moored ships.

But that doesn't mean the Casimir effect is worthless when it comes to practical applications! The Casimir force was finally measured accurately in 1997. And in 2009, MIT's Alejandro Rodriguez and some of his fellow physicists started playing around with combinations of different materials in different shapes, and hypothesized that some of those combinations should generate repulsive forces, akin to the Casimir Effect. Choose those combinations carefully enough, and you can devise a kind of stable "Casimir molecule," where the attractive and repulsive forces generated balance out.

These are complex calculations, so it's impressive that Rodriguez et al. managed to complete calculations for "combinations of infinite slabs made alternatively of silicon and silicon dioxide, for nanoparticles, and for alternating slabs and spheres." The team is most excited about their findings on the forces generated between Teflon and silicon nanospheres immersed in ethanol. Per an article in Technology Review:

"By choosing the radii of these spheres carefully they can be suspended against the force of gravity above an infinite slab. It turns out that the force between the particles is repulsive at separations closer than 100 nm but becomes attractive as the distance increases."

And voila! Under those conditions, you should get a stable Casimir molecule! Full Disclosure: I have no idea what is meant here by an "infinite slab," but right now, these calculations are purely theoretical, and infinities — while mainstays of higher mathematics — generally don't translate well into reality. That might be one reason why Rodriguez et al. have yet to measure experimentally the repulsive forces they claim should be generated by some of these combinations. The article dithers a bit on that score, claiming that doing the experiment should be easy, "provided the size of the nanoparticles can be controlled with the required difficulty," while backtracking in the very next sentence by claiming "these experiments will be fraught with difficulty." I'm guessing we won't be seeing practical applications for these structures any time soon.

But no worries, because Rodriguez was back last summer with another possible use for the new tool they devised for calculating the various forces that create the Casimir effect: as a kind of WD-40 (oil) to reduce "stiction" (a combination of stick and friction caused by the Casimir effect) in accelerometers, gyroscopes and other micro-electro-mechanical system (MEMS) chips. Rodriguez & Company discovered it should be possible to arrange all those tiny moving parts in such a way that the forces that normally attract, can be made to repulse, thereby greatly reducing "stiction." Per Smarter Technology:

The researchers proved their technique works by designing a prototype consisting of an ellipsoid plunger that gets inserted into a complementary hole in a flat plate. The shapes of the hole and the plunger ensure that the Casimir force is balanced until the plunger is moved, at which point the Casimir force causes the parts to repel, thus overcoming the forces of stiction.

MEMS are in everything these days, by the way. So don't say the Casimir effect never did anything for you.