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. ▲
■ IR enters from left and is routed on right at
different frequencies. Actual device is the size of a
speck of dust. Photo courtesy of A. Piggott.
■ BY JEFF ECKERT TECHKNOWLEDGEY 2015
6 October 2015
■ 3D visualization of
gravitational waves
produced by two
orbiting black holes.
Photo courtesy of
Henze, NASA.