Portland, Ore. - Empty space is not really empty. According to quantum-vacuum theory, at any instant it's composed of a finite amount of matter, an equal amount of antimatter and the simultaneous winking into and out of existence of a finite number of photons. The photons and particles that spontaneously appear and disappear balance to zero. Down at the nanoscale, however, that balance may someday be tipped in our favor by microelectromechanical systems (MEMS).
"When we start building at the nanoscale, we need to understand how [an effect known as the Casimir] force can hinder our efforts," said Ephraim Fischbach, a professor in Purdue University's School of Science. "If we learn enough, we can harness it to more precisely design mechanisms at the nanoscale."
The Casimir force measures a photon's ability to move small objects. As computer chips increasingly make use of photons (instead of electrons) to carry data, there will be a need for tiny on-chip mirrors that pivot to switch photons down different channels, Fischbach said. The Casimir force might enable MEMS designers to make the mirrors move more precisely.
The phenomenon is named for Dutch scientist Hendrick Casimir, who in 1948 proposed an experiment that would support the quantum electrodynamics part of Max Planck's and Werner Heisenberg's quantum vacuum fluctuation theory (i.e., that space is not empty but is filled with spontaneously appearing and disappearing particles and photons). Casimir reasoned that if the space between two parallel plates in a vacuum could be made small enough, then it would exclude the longer wavelengths of light. Hence, spontaneously appearing photons would "pile up" outside the plates, (since only short wavelengths could fit in between), exerting a force to drive the plates together.
It was not until 1997 that Steve Lamoreaux, now at Los Alamos National Laboratories, managed to create an experimental setup sensitive enough to measure the Casimir force. But he was only able to achieve results within 5 percent of Casimir's theoretical prediction.Case closed
Now Fischbach and colleagues have cinched the case by making more accurate measurements that verify within 0.5 percent that the Casimir effect does influence nanoscale devices. Fischbach's collaborators include Ricardo Decca, assistant professor at Indiana University-Purdue University Indianapolis; Daniel Lopez, staff scientist at Lucent Technologies; Dennis Krause, assistant professor at Wabash College; and professors Vladimir Mostepanenko and Galina Klimchitskaya from the Universidade Federal de Paraiba in Brazil.
"The Casimir effect is comparatively weak on most objects, but when you start building microscopic MEMS devices, it can profoundly influence how things work," Fischbach said.
Whenever two nanoscale objects are moved closer together, the "photonic pressure" on the outside of the pair exceeds the pressure in the space between them. Thus, "as the teeth of tiny MEMS gears get closer and closer together, the Casimir force gives them an extra push that, depending on their design, could freeze up the nanomachinery they are driving," said Fischbach. "Our accurate measurement of the force will help MEMS designers avoid such pitfalls."
Doomsayers have cited the Casimir force as an obstacle that could make nanoscale devices behave erratically as they continually scale to smaller sizes. But Fischbach counters that designers could harness the Casimir effect by factoring it into the MEMS design model. As a physicist, he is factoring his more accurate measurement of the Casimir force-which varied from 0.5 to 20 atmospheres-so that he can factor it out of the "quantum foam" of photons, matter and antimatter he theorizes is percolating out of the void.
"It's not often that a theoretical physicist gets the opportunity to unify theory and practice like this. Our discovery provides a new tool for nanotechnology; but for a theorist like me, it's also exciting because it could help me with my next experiments into whether extra dimensions exist at the nanoscale," said Fischbach.
Fischbach's team achieved its goal, despite the incredibly tiny forces at play, by using different materials for each of the opposing metals. A copper layer evaporated onto a MEMS torsional oscillator opposed a gold layer deposited on an aluminum sphere (Fischbach says one plate and a sphere were easier to set up in the lab than two plates).
The two metals were then varied in separation from 200 nanometers to 2 microns, with the integrated torsional oscillator providing a precise measurement. On average, the experimental setup was able to confirm Casimir's seminal calculations to within 0.5 percent of their expected theoretical value.
Still, the torsional oscillator should have been able to achieve more accuracy than that obtained by Fischbach's team. Even accounting for the finite conductivity and roughness of the two metals, the measurements should have been closer than 0.5 percent. "We believe the discrepancies we observed are the result of our less-than-complete characterization of the optical properties of the specific samples we used in the experiment," said Fischbach, who promises to make more precise measurements in the future.