Nuts and Volts - October 2015

Another Stab at GW Detection

Back in 1916 — as derived from the theory of General Relativity — Einstein predicted the existence of gravitational waves, which are defined as ripples in the curvature of space-time caused by asymmetric (i.e., not spherically symmetric) accelerations of large masses — e.g., supernovae and colliding black holes. Their existence is widely accepted and — as far back as 1993 — a Nobel Prize in Physics was awarded related to observations of binary pulsars that precisely matched predictions of their behavior related to the emission of such waves. The snag is that even though several gravitational-wave detection facilities have been built, none have actually directly detected them so far. The problem is that the amplitude of gravitational waves drops off as the inverse of your distance from the source, so we're talking about extremely tiny perturbations by the time they reach Earth. Your detection device has to be very, very sensitive.

The latest attempt involves two National Science Foundation-funded Advanced Laser Gravitational Wave Observatories (Advanced LIGO, www.ligo.org) in Louisiana and Washington. The facilities are basically laser interferometers in which laser beams are shot down 2. 4 mi ( 4 km) tubes and reflected by mirrors. Any existing gravitational waves should create an interference pattern when the two beams meet, thus proving that they exist. The original LIGO went into operation in 2002 with the capability of detecting changes equivalent to 1/1000th of the size of the diameter of a proton. That didn't do the trick, though, so the Advanced LIGO — with ten times the sensitivity — was dedicated back in May and could be operational by the time you read this.

So, what good is all this? Well, if it pans out, the program will allow scientists to look at the last minutes of life of black holes as they spiral toward each other, vibrate, and merge into one larger hole. It will also allow us to test theories of how the universe developed as far back as a nanosecond or so after the Big Bang. If that doesn't float your boat (perfectly understandable), consider that LIGO detects waves in the audio frequency range of 10 to 1,000

Hz, so by feeding the signals into a speaker, you should be able to "hear" them. They are expected to sound something like the audio file located at www.ligo.org/science/GW-Overview/sounds /chirp40-1300Hz.wav. ▲

ADVANCED TECHNOLOGY Algorithm to "Revolutionize"

Computing?

Inside your computer, it takes a lot of power to push electrons from one chip to the next. Not only does that waste about 80 percent of your microprocessor's energy consumption (thus generating all that heat), it slows down data transfer through the interconnects. It's not breaking news that photons provide more efficient data transfer, which is why the Internet runs over fiber optic threads. Scaling the concept down to the chip level has been impractical, as optical interconnects are designed one at a time and thousands of them are required for an electronic system. However, a recent Nature Photonics article described a new design algorithm developed at the Stanford School of Engineering ( engineering.stanford.edu) that could help replace wires in practical systems. The engineers believe that they have broken the design bottleneck with their "inverse design algorithm," which is pretty much descriptive of how it works. The engineers specify what they want the optical circuit to do, and the software provides the details of how to fabricate a silicon structure to perform the task.

"We used the algorithm to design a working optical circuit and made several copies in our lab," noted engineer, Jelena Vuckovic, reporting that the devices functioned flawlessly despite being fabricated in a relatively primitive facility. The Stanford work relies on the fact that silicon is easily penetrated by IR light, and different silicon structures can bend IR in useful ways.

The Stanford structures are so slender that more than 20 of them could fit inside the diameter of a human hair. These silicon interconnects can direct a specific frequency of infrared light to a specific location to replace a wire. The algorithm can create switches or conduits or whatever is required for the task. The Stanford team believes they have set the stage for the next generation of even faster and more energy-efficient computers that use light rather than electricity for internal data transport. ▲