From the July 2018 issue

Glimpsing gravitational waves

The screams of colliding black holes allow astronomers to study the universe in a whole new way.
By | Published: July 27, 2018 | Last updated on May 18, 2023
When two merging black holes collide, they produce gravitational waves that travel through the fabric of space-time at the speed of light. Now, astronomers can finally study these invisible cosmic ripples.
LIGO Collaboration/SXS (Simulating eXtreme Spacetimes) Project
Albert Einstein earned his lofty reputation for his many deep insights into physical reality. Among his greatest predictions was that massive, rapidly moving objects throw off ripples of energy at the speed of light, causing space-time itself to expand and contract. In papers published in 1916 and 1918, Einstein referred to this strange phenomenon as gravitationswellen. Today, we call them gravitational waves.

But for all his creative insights into the natural world, Einstein underestimated the ingenuity of humankind. He viewed his prediction of gravitational waves as a mere mathematical curiosity, and questioned whether they were physically real. Moreover, he doubted that future scientists could ever pick up these subtle space-time undulations. His calculations showed that these waves would be so feeble by the time they reached Earth that detecting them would require instruments of extraordinary precision far beyond the technology of his era.

We can excuse Einstein’s lack of faith; he was a product of his time. Early 20th-century astronomers had yet to discover black holes and neutron stars, which are incredibly dense cosmic objects that produce the strongest gravitational waves. And nobody back then was contemplating the possibility of multiple billion-dollar laser interferometers, each with arms 2.5 miles long.

The gravitational-wave event GW150914 was observed by the LIGO instruments in Hanford, Washington (H1, left panels) and Livingston, Louisiana (L1, right panels) on September 14, 2015, at 09:50:45 UTC. The upper panels show the observed and theoretically predicted signals; the lower panels show the signal’s characteristic chirp.
B.P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)
It wasn’t until nearly a century after his 1916 paper that an international team of more than 1,000 scientists proved that Einstein was indeed right in that gravitational waves exist, but wrong in assuming we could never detect them. At 5:50:45 a.m. EDT on September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) picked up the faint yet unmistakable rumbles of gravitational waves emanating from colliding black holes containing 36 and 29 solar masses. 

LIGO’s discovery opened a window on the universe, creating a revolutionary new branch of science that can obtain information on nature’s most extreme events — events that would otherwise be hidden from view. Since colliding black holes generate no form of light, we can discern these cosmic cataclysms only from the way they contort the fabric of space-time. 

In a mere fraction of a second, the black hole collision detected by LIGO converted about three times the mass of the Sun into gravitational-wave energy — with a peak power output of nearly 50 times that of the entire visible universe. But these waves diminished rapidly in intensity as they traversed space. By the time they reached Earth, after a journey that took about 1.3 billion years, they expanded and contracted space-time by 1/10,000 the width of a proton. It’s no wonder Rainer Weiss, Kip Thorne, and Barry Barish, three of LIGO’s key figures, shared the 2017 Nobel Prize in Physics: Detecting these minuscule fluctuations required a precision akin to measuring the 25 trillion-mile distance to the nearest stars to the width of a human hair.

Over many years, the LIGO collaboration met this formidable challenge by creating an engineering marvel. Scientists and engineers developed powerful and stable near-infrared lasers, tunnels evacuated to one-trillionth the density of air at sea level, mirrors polished so smooth that their roughness is measured at the scale of atoms, and sophisticated seismic isolation systems that can filter out the noise of passing trucks and howling wolves. Plus, the LIGO team built two of these facilities: one in Livingston, Louisiana, and a near-twin in Hanford, Washington. 

Since this initial detection, LIGO has heard the gravitational rumblings of four additional black hole mergers (and a likely fifth), involving objects ranging from 8 to 36 solar masses. The most massive collision, the first one, produced a black hole of 62 solar masses. Two others produced black holes with masses of about 50 Suns. These initial results have shone a light on a population of stellar-mass black holes previously undetected by astronomers. In the coming years, astronomers will gain deep insights into this group of relatively massive stellar-mass black holes, how they pair up in binary systems, and how their merger rate has evolved over cosmic history.

From left: Kip Thorne, Rainer Weiss, and Barry Barish shared the 2017 Nobel Prize in Physics for their “decisive contributions to the LIGO detector and the observation of gravitational waves,” according to the Nobel Prize committee.
Caltech Alumni Association; Bryce Vickmark; R. Hahn
Even better, the European Virgo detector near Pisa, Italy, joined the fray in August 2017. Just weeks after it commenced scientific operations, Virgo and both LIGOs picked up gravitational waves from merging neutron stars. And with three detectors in action, scientists could localize the source with sufficient precision to enable conventional telescopes to catch the aftermath. Over the following months, astronomers around the world studied the kilonova — a burst of electromagnetic radiation 1,000 times brighter than an erupting white dwarf (or nova), but not as bright as a supernova — as it brightened and faded across the entire electromagnetic spectrum. This ushered researchers into a new era of multi-messenger astronomy. 

Despite all the hoopla, accolades, and awards, the best is yet to come. Scientists and engineers are upgrading LIGO and Virgo, which will greatly extend their reach deeper into space and time. In the coming years, new laser interferometers in Japan and India will join the hunt, enabling even more precise localization of gravitational-wave sources for multi-messenger follow-up.

But to expand their search for gravitational waves, astronomers must move beyond ground-based interferometers. These Earth-bound instruments are tuned to hear gravitational waves coming from the inspiral and merger of stellar-mass black holes and neutron stars. In comparison, the gravitational waves produced by small black hole collisions are relatively high frequency, roughly akin to the glass-shattering, high-pitched sound waves produced by an opera singer. 

And Mother Nature isn’t just hitting the high notes; she is performing a veritable symphony with gravitational waves — though we cannot yet “hear” many of the instruments. However, by precisely timing the pulses of many dozens of millisecond pulsars sprinkled across the sky, radio astronomers will almost certainly be able to eventually pick out the bass notes coming from the inspiral of supermassive black holes, which contain millions or even billions of solar masses. Furthermore, astronomers combing through data from the European Space Agency’s (ESA) Gaia satellite are conducting a similar experiment by looking for slight changes in the positions of nearly a billion stars.

And in the early 2030s, ESA aims to launch its long-awaited Laser Interferometer Space Antenna mission — three spacecraft that will orbit the Sun as an equilateral triangle, firing lasers at one another to measure the distortions of space-time caused by low-frequency gravitational waves passing through our solar system. These waves will reveal countless orbiting stellar binaries in our Milky Way Galaxy, along with the mergers of monstrous black holes going nearly all the way back to the dawn of time.