At 4:18 a.m. Eastern time on 25 April, according to preliminary observations, a gravitational wave that had been traveling through deep space for many millions of years passed through the Earth. Like a patient spider sensitive to every jiggle in its web, a laser gravitational wave detector in the United States detected this subtle passing ripple in spacetime. Computer models of the event concluded the tiny wobbles were consistent with two neutron stars that co-orbited and then collided 500 million light-years away.
Next came scientific proof that when it rains it pours. The very next day at 11:22 am ET, the Laser Interferometer Gravitational-Wave Observatory (LIGO) picked up another gravitational wave signal. This time, computer models pointed to a potential first-ever observation of a black hole drawing in a neutron star and swallowing it whole. This second spacetime ripple, preliminary models suggest, crossed some 1.2 billion light years of intergalactic space before it arrived at Earth.
In both cases, LIGO could thank a recent series of enhancements to its detectors for such its ability to sense such groundbreaking science crossing its threshold.
LIGO’s laser facilities, in Louisiana and Washington State, are separated by 3002 kilometers (3,030 km over the earth’s surface). Each LIGO facility splits a laser beam in two, sending the twinned streams of light down two perpendicular arms 4 km long. The light in the interferometer arms bounces back and forth between carefully calibrated mirrors and optics that then recombine the rays, producing a delicate interference pattern.
The pattern is so distinct that even the tiniest warps in spacetime that occur along the light rays’ travel paths—the very warps of spacetime that a passing gravitational wave would produce—will produce a noticeable change. One problem: The interferometer is also extremely sensitive to thermal noise in the mirrors and optics, electronics noise in the equipment, and even seismic noise from nearby vehicle traffic and earthquakes around the globe.
Noise was so significant an issue that, from 2006 to 2014, LIGO researchers observed no gravitational waves. However, on September 14, 2015, LIGO detected its first black hole collision—which netted three of LIGO’s chief investigators the 2017 Physics Nobel Prize.
Over the ensuing 394 days of operations between September 2015 and August 2017, LIGO observed 11 gravitational wave events. That averages out to one detection every 35 days.
Then, after the latest round of enhancements to its instruments, LIGO’s current run of observations began at the start of this month. In April alone, it’s observed five likely gravitational wave events: three colliding black holes and now the latest two neutron star/neutron star-black hole collisions.
This once-per-week frequency may indeed represent the new normal for LIGO. (Readers can bookmark this page to follow LIGO’s up to the minute progress.)
Most promisingly, both of last week’s LIGO chirps involve one or two neutron stars. Because neutron stars don’t gobble up the light their collisions might otherwise emit, such an impact offers up the promise of Earth being bathed in detectible gravitational and electromagnetic radiation. (Such dual-pronged observations constitute what’s called “multi-messenger astronomy.”)
“Neutron stars also emit light, so a lot of telescopes around the world chimed in to look for that and locate it in the sky in all different wavelengths of light,” says Sheila Dwyer, staff scientist at LIGO in Richland, Wash. “One of the big goals and motivations for LIGO was to make that possible—to see something with both gravitational waves and light.”
The first such multi-messenger observation made by LIGO began in August 2017 with a gravitational wave detection. Soon thereafter came a stunning 84 scientific papers, examining the electromagnetic radiation from the collision across the spectrum from gamma rays to radio waves. The science spawned by this event, known as GW170817, led to precise timing of the speed of gravitational waves (the speed of light, as Einstein predicted), a solution to the mystery of gamma-ray bursts, and an overnight updating of models of the cosmic source of heavy elements on the periodic table. (Studies of the collision’s gravitational and electromagnetic radiation concluded that a large fraction of the universe’s elements heavier than iron originate from neutron star collisions just like GW170817.)
LIGO team members install in-vacuum equipment that is part of the squeezed-light upgrade. Photo: Matt Heintze/LIGO
When the S190425z and S190426c signals came in, telescopes around the world pointed to the regions of the sky that the gravitational wave observations suggested. As of press time, however, no companion source in the sky has yet been found for either.
Yet because of LIGO’s increased sensitivity, the promise of yet more observations increase the likelihood that another GW170817 multi-messenger watershed event is imminent.
Dwyer says LIGO’s latest incarnation uses high-efficiency mirrors that reflect light back with low mechanical or thermal energy transfer from the light ray to the mirror. This is especially significant because, on average, the laser light bounces back and forth along the interferometer arms 1000 times before recombining and forming the detector’s interference pattern.
“Right now we have a very low-absorption coating,” she says. “A very small absorption of that [laser light] can heat up the optics in a way that causes a distortion.”
If the LIGO team can design even lower-loss mirror coatings (which of course could have spinoff applications in photonics, communications and optics) they can increase the power of the laser light traveling through the interferometer arms from the current 200 kilowatts to a projected 3 megawatts.
And according to Daniel Sigg, a LIGO lead scientist in Richland, Wash., another enhancement involves “squeezing” the laser light so that the breadth of its amplitude is sharper than Heisenberg’s Uncertainty Principle would normally allow.
“We can't measure both the phase and the amplitude or intensity of photons [with high precision] simultaneously,” Sigg says. “But that gives you a loophole. Because we're only counting photons, we don't really care about their phase and frequency.”
So LIGO's lasers use “squeezed light” beams that have higher noise in one domain (amplitude) in order to narrow the uncertainty in the other (phase or frequency). So between these two photon observables, Heisenberg is kept happy.
And that keeps LIGO’s ear tuned to more and more of the most energetic collisions in the universe—and allows it to turn up new science and potential spinoff technologies each time a black hole or neutron star goes bump in the night.