Listening to the Universe

The 2015 discovery of gravitational waves is one of the biggest science stories of recent decades. Penn State astrophysicist Chad Hanna, a member of the international research team that studies gravitational waves, reflected on the group’s achievements and the birth of “multi-messenger astronomy.” This piece is adapted from two articles he wrote for the online news site The Conversation.

Like many of my colleagues working for the Laser Interferometer Gravitational-Wave Observatory (LIGO), the morning of Monday, September 14, 2015, caught me completely off guard. LIGO had just started listening for gravitational waves, one of the last unproven predictions of Einstein’s theory of general relativity—and within its first few days of gathering data, it had found one.

More than 100 years ago, Einstein hypothesized that gravitational waves are formed when matter and energy warp space and time. The effects he predicted sound bizarre: As a gravitational wave passes by, the distance between objects changes ever so slightly. All around us space is oscillating, distances are changing, and we are being stretched and squeezed by passing gravitational waves. Only the most extreme objects in the universe can bend space enough to produce ripples that are measurable here on Earth. The effect is so tiny that until now we had no instruments that could detect it. Advanced LIGO was designed to change all of that by directly measuring tiny ripples in space itself.

This was LIGO’s first day on the job. We had worked toward this moment for over a decade, but it was so early in LIGO’s first official observing run that I hadn’t even had a chance to enable my text message alerts! Instead, I read about the event on my phone as I walked to campus hours after it had been observed. Like others on the team, I thought this signal was just a test of the system, nothing to get excited about. Then, just before 2 p.m., we received word that no tests had been performed. The signal was real!

At first it was unclear which of many possibilities could be responsible. It would have to be a major astronomical event that released immense amounts of energy, such as a binary merger, a nearby supernova, or some unforeseen occurrence. Over the next several weeks, the LIGO team verified that the signal, which we labeled GW150914, could only have been caused by a gravitational wave generated by the merger of two black holes that released energy as they smashed together. Einstein had been right. Again.

Last fall, the Nobel Prize in physics was awarded to three of the founders of our international collaborative effort—Rainer Weiss, Kip Thorne, and Barry Barish—in recognition of this first observation that confirmed Einstein’s revolutionary theory.

LIGO logistics

The LIGO Scientific Collaboration, of which I am a member, includes more than 1,000 people from dozens of institutions and 15 countries. There are two LIGO instruments, one in Louisiana and one in Washington state. We also work with the Virgo Collaboration that operates a detector in Italy and the GEO600 detector team in Germany. And it took all of us to detect and analyze that first gravitational wave.

GW150914 stretched and squeezed our nearby space by about 1 part in 1021. This is equivalent to squeezing the entire Milky Way galaxy by a typical person’s height. As you might imagine, it is nearly impossible to measure such a small change. To do so, LIGO uses high-power lasers, ultra-high vacuum, and some of the most advanced optics ever built.

The basic idea is simple: LIGO has two 4-km-long arms, built at 90 degrees with respect to one another. A high-power laser beam is split in two to travel down each arm separately. When each beam gets to the end of an arm, it’s reflected back by a mirror. If one arm momentarily becomes longer than another, due to the change in space caused by a gravitational wave, then the laser light from the two arms will get out of sync.

We continuously record the recombined laser light, which encodes how the gravitational wave causes space to stretch and squeeze at frequencies that are very similar to what the human ear can hear. That’s why we often think of LIGO as “listening” to the universe. In fact, you can literally listen to the gravitational waves detected with LIGO using headphones.

The LIGO detectors, for the most part, are sensitive to sources all over the sky, which means a single detector can’t tell from which direction a gravitational wave arrived. However, using multiple detectors around the globe, we can localize the source of a given signal.

Our aim was to be able to know within seconds that a gravitational wave had reached the Earth. Then we could immediately inform other astronomers, who could point their telescopes in the direction of the event in the hope that the gravitational wave would have an electromagnetic counterpart. Having information from multiple channels would be a bit like having both sound and picture when watching a film—it would give us a more complete impression of the event, a better idea of what happened.

We knew that gravitational waves had the potential to provide something like a soundtrack for our universe, but the 2015 breakthrough and four subsequent gravitational wave observations were never associated with electromagnetic counterparts because they all sprang from binary black holes.

A new era

That all changed last year. My phone now enabled to receive LIGO alerts, on August 17, 2017 at 8:47 a.m. EDT, I received a text message that indicated a gravitational wave candidate had been identified. Not only that, but this new observation, GW170817, had a coincident gamma-ray burst.

The burst was detected by the Fermi Gamma-ray Space Telescope in low Earth orbit, that happened to be pointing in the direction of the new gravitational wave when it arrived at Earth. That was a stroke of luck, but for all that had gone right that morning, a few things were bound to go wrong.

Around the time that the new signal arrived at the LIGO Washington detector, the LIGO Louisiana detector suffered from a burst of instrument noise. Data from the Virgo detector in Italy was clean, but the transatlantic data transfer had stopped due to a network connection outage.

Despite these problems, the LIGO rapid response team quickly notified our over 70 observing partners all over the world. It turned out that the instrumental noise in the LIGO Louisiana data affected only the very end of the detected signal, which lasted more than 100 seconds. Eventually, we were able to analyze all three gravitational wave detector data streams to figure out when the signal arrived at each one. Then we triangulated the gravitational wave source to a sufficiently small area on the sky that astronomers could survey the entire region.

We had the gamma-ray data, but astronomers using ground-based telescopes had to wait for nighttime to look for other kinds of signals. About 10 hours after the initial alert, we got the first news of a visible light counterpart: A new bright spot that hadn’t been there previously was spotted in a galaxy in the direction of the gravitational wave. Over the coming weeks, we learned that there were ultraviolet and X-ray counterparts and even radio waves that together allowed us to confirm that the gravitational wave was due to the collision of two neutron stars. Each observation revealed a new part of the story. Astronomy had entered the era of talkies!

To be able to observe and document the merger of two neutron stars was a tremendous achievement. We were lucky to pin down the location of the gravitational wave quickly enough to identify the observational counterparts in time to capture their views of the event.

Next time around, we hope that gravitational wave identification can happen even sooner and at a more favorable time of day so we don’t miss out on the earliest optical emission. Perhaps one day we’ll even be able to use the early gravitational wave emission leading up to a neutron star collision to predict where on the sky they’ll merge, and have telescopes already pointed in that direction, ready for the show. 

This story appeared in the Spring 2018 issue of Research/Penn State magazine.