The Chirp Heard ’Round the Universe
An Achievement 1.3 Billion Years in the Making
At 4AM on September 14, 2015, a gravitational wave was detected for the first time in human history. Its source was two black holes, 1.3 billion light years away, whose violent collision created a release of gravitational-wave power ten times greater than the combined energy of all the stars in the universe.
The detection was one of the most notable scientific (and precision sensing) discoveries of our time, triggering a worldwide reaction that spanned scientific journals, mass media, late-night talk shows, and the award for the 2017 Nobel Prize in Physics.
Upon being announced in Physical Review Letters, one of the top physics journals in the world, the story has been covered by more news outlets than any other paper the journal has ever published. The New York Times said it was “destined to take its place among the great sound bites of science, ranking with Alexander Graham Bell’s ‘Mr. Watson, come here…’ ”
What was behind this ground-breaking event? Precision engineering at LIGO, the Laser Interferometer Gravitational-Wave Observatory – and innovative technology from Analog Devices.
Listening to the Universe
LIGO consists of a pair of facilities operating in unison – one in Hanford, Washington, the other in Livingston, Louisiana. Since gravitational waves don’t leave clues on the electromagnetic spectrum – they cannot be seen – the shared purpose of LIGO is to “listen” to the heavens for evidence of the waves’ existence.
Each LIGO facility places a laser within an ultra-high vacuum, splits the laser in two, and sends each beam down one of two 2.5-mile arms set perpendicular to one another. The laser beams are then reflected back from mirrors placed at the ends of the arms.
When a gravitational wave passes, space-time in the area is altered, causing a minute movement of the arms relative to each other, on the order of 1/10,000th the width of a proton. This changes the relative phases of the returned light incident upon the receiving optics, releasing light to an optical sensor and resulting in a measurable signal or “chirp.”
Analog Devices Performance Utilized by LIGO
LIGO utilizes a host of integrated Analog Devices technologies. All represent our commitment to create products that reach the pinnacle of precision engineering today, and can drive innovative, mission-critical applications well into the future.
Along with predicting and compensating for all other possible ambient noise sources, LIGO requires that their laser amplitude remain ultra-stable, with a variation of no more than 2x10-9 at about 100 Hz offset to the carrier.
As lasers can’t do this out of the box, the LIGO team needed a feedback system to measure light output and control the amplitude. This required an ultra-low-noise amplifier with particular performance. Upon extensive review from LIGO’s scientific panel as to which solution would be best, Analog Devices’ AD797 operational amplifier was chosen.
To stabilize laser frequency, the LIGO team uses Analog Devices’ AD590 high-precision temperature sensor to measure the average temperature of the glass vacuum chamber that houses the laser.
While the original output of the laser is standard, it quickly builds up to kilowatts within the resonant cavities of the arms. This can actually create a force sufficient to form acoustic resonances in the glass mirrors, so LIGO uses an ADA4700 high-voltage op amp to drive electrostatic actuators that actively damp the mirrors and keep them in alignment.
Another ADI part, the AD736 RMS chip, measures power delivered to the solenoids that are used to drive LIGO’s mirror suspension system and enable any required tilt, pitch and yaw.
Ahead of (What Einstein Thought) Possible
100 years ago, Albert Einstein proposed the existence of gravitational waves. But while mathematics led to his hypothesis that gravitational waves existed, he believed that actually detecting them would be virtually impossible – they were infinitesimally small.
Indeed, it took a precision instrument beyond measure to successfully achieve that goal; an instrument so precise that, metaphorically speaking, if you were to take all the sand from all the beaches on Earth, LIGO could detect the removal of a single grain.
Analog Devices is proud to have had its technology utilized by LIGO.