Gravity Waves detected in Historic finding by LIGO Observatory
An interview with NASA scientist John Cannizzo of Goddard Space Flight Center
By Catherine Asaro
On February 11, 2016, scientists announced they had achieved one of the great scientific goals of recent history–the first direct detection of gravitational waves. The detection was made by the Laser Interferometer Gravitational-wave Observatory, more commonly known as LIGO.
Einstein predicted the existence of gravitational waves with his General Theory of Relativity, but in the century since he first proposed the idea, no one had ever seen direct evidence that the waves exist. In September of last year, a LIGO scientist noticed a blip in the data stream from the instrument and sent notice to the LIGO team that “a very interesting event” may have occurred. Initially, little excitement accompanied the announcement. For the past twenty years, since LIGO first went online, “interesting events” have occurred at a rate of roughly one a month–and they all turned out be false alarms.
That all changed on September 14, 2015.
The event turned out to be far more than merely interesting—LIGO had recorded a cosmically huge event, the merger of two large black holes in a cataclysmic maelstrom of energy. That event sent gravitational waves propagating across space. Such dramatic events have undoubtedly occurred before, unknown to humans, but this time we were watching—and astronomy will never be the same.
Doctor John Cannizzo, a research scientist at the Goddard Space Flight Center in Greenbelt, Maryland says, “It’s like when Galileo first turned a telescope on the heavens. He changed forever how we, as humans, view the universe. So LIGO represents a new way of observing the cosmos.”
The first event detected by LIGO shows what sea change this represents. Black holes are impossible to see. Their gravity is so great that no light can escape; hence the name black holes. Humans have always looked at the sky, yet up until now, we couldn’t see an event even as fierce as two black holes spiraling into each other because our methods of observation relied on electromagnetic radiation, such as light. Now we have a new way to “see,” and even hear the sky—by using gravitational waves. Such a wave distorts space. It changes the size of the objects it passes through, only slightly, but enough to affect their shape and size. Now that we can detect those changes, it will alter how we observe the universe.
In the following interview, John Cannizzo talks about LIGO. Cannizzo received his Ph.D. from the University of Texas at Austin in 1984 with a thesis on the theoretical astrophysics of accretion disks. Prior to joining the Laboratory for High Energy Astrophysics at Goddard, he worked at Harvard College Observatory in Massachusetts, McMaster University in Ontario, Canada, Kenyon College in Ohio, the Max Planck Institute for Astrophysics in Bavaria, Germany, and the Laboratory for Astronomy and Solar Physics at Goddard. His research interests include interacting binary stars, active galactic nuclei, accretion disk theory, N-body simulations, and time series analysis. He joined the LIGO collaboration in 2002 when he had the opportunity to work with Doctor Jordan Camp at Goddard on the analysis of data produced by LIGO.
Asaro: What exactly is LIGO?
Cannizzo: The acronym stands for Laser Interferometer Gravitational-wave Observatory. It’s basically a very big detection instrument. It consists of two facilities, one in Livingston, Louisiana and the other in Hanford, Washington. The collaboration is an international project. It began in 1992 on the initiative of Kip Thorne, Ron Drever, and Rainer Weiss.
Asaro: That’s quite an impressive lineup. Kip Thorne is the Emeritus Feynman Professor of Theoretical Physics at the California Institute of Technology (Caltech) and one of the world’s leading experts on the astrophysical implications of general relativity. Ron Drever is a Professor Emeritus at Caltech and in 2007 he won the Einstein Prize, shared with Rainer Weiss, for outstanding work in the field of gravitational physics. Rainer Weiss, an emeritus professor at the Massachusetts Institute of Technology (MIT), invented the interferometric gravitational wave detector, which led to the development of LIGO.
The Executive Director of LIGO is David Reitze, a professor from the University of Florida and the spokesperson is Gabriela Gonzáles at Louisiana State University. LIGO is funded by the National Science Foundation, whose current Director is France Córdova.
Cannizzo: The success of LIGO is a tribute to their work. The NSF really put their support into LIGO. In fact, I think it’s the largest project they’ve ever sponsored.
Asaro: The detection of this event occurred on September 14, 2015, but the announcement didn’t come until February 11, 2016. Why the long wait?
Cannizzo: The LIGO team had to verify that the instrument had detected a genuine event. They’ve worked diligently for the past months make that verification. They were incredibly thorough and took the time to do all the necessary checks.
Asaro: When you first heard about it, did you believe it was real?
Cannizzo: I was skeptical at first, and for a while afterward, because we had seen that sort of blip many times before and it always turned out to be a false alarm. Over the weeks and months following, as I began to accept this historic event, my excitement grew. I see this as the dawn of a new era in science. We’ve opened a new window on the universe that we never had before.
Asaro: I understand LIGO is the most precise detector ever built. Tell us more about the instrument.
Cannizzo: It’s big! Each facility consists of two chambers set at right angles to each other, each of them two and a half miles long and several feet in diameter, with a vacuum inside. A powerful laser shines along the length of each, reflecting many times off of mirrors at each end. If a gravity wave passes through the chamber, it changes by a tiny amount how those reflected waves interact with each other. The chamber needs that great length because the differences accumulate over a larger distance, until they become large enough for us to detect.
Asaro: Can you describe more about gravitational waves?
Cannizzo: Einstein proposed them. His theory of General Relativity predicts that gravitational waves propagate through space. They are produced by great masses moving very close to each other. Imagine these objects–many times more massive that our own sun—passing within just kilometers of each other. That’s what happens as black holes merge. Their interaction sets off gravity waves that are larger than normal. The wave is a fluctuation in the fabric of space that causes objects to distort when it passes through them.
Asaro: That sounds like they could make the Earth fluctuate!
Cannizzo: Well, it’s true, but it isn’t a problem for Earth. Even powerful gravity waves are incredibly tiny on human scales. If a strong signal passes through a meter stick, it changes the length of the stick by 0.000000000000000000001 meter. That is about a million times smaller than the diameter of a proton, a particle in the nucleus of an atom. And this change lasts only about a second.
Asaro: If those waves are so tiny, how did we know they existed. What spurred the NSF to fund a huge project like LIGO?
Cannizzo: That story goes back to the early 1970’s. That was when two radio astronomers—Russell Hulse and Joseph Taylor Jr.—discovered the first pulsar, what we call a neutron star. It was emitting periodic radio waves in a binary star system with an orbital period of 7.75 hours. A neutron star is very compact. It has between one and two times the mass of our entire sun, yet all of that material is packed into a sphere only ten miles wide. It is the most dense form of matter known after a black hole, which is a star collapsed to a single point.
By carefully timing of the signals created by the pulsar, they determined that its orbit was shrinking ever so slightly with time. Why? One possibility: gravitational waves were carrying off energy. It took several decades of observations, but in the end they determined that the decrease in the binary period matched Einstein’s predicted rate to within about one tenth of one percent. Their work was an indirect detection of gravitational waves, a triumph of physics that earned Hulse and Taylor the 1993 Nobel Prize in Physics.
Asaro: You first became involved in LIGO in 2002 and did monitoring runs at the observatory in Livingston, Louisiana. What was that like?
Cannizzo: We called that version of the instrument Initial LIGO. We didn’t see any detections then. We had a lot of down time because many natural events were enough to knock the instrument offline, for example an earthquake anywhere in the world. When that happened, it could take anywhere from minutes to hours for the disturbances to subside. Yet even with that incredibly sensitivity, the detector still wasn’t sensitive enough to detect astronomical events.
With all that down time, we didn’t always have much to do. I’d pick up movies from Blockbuster, and the night shift operator would project them onto the wall while we waited for the instrument to come back online. We watched the Incredible Hulk while waiting to detect gravity waves from the incredible hulks of the universe.
LIGO then had a major and considerable upgrade, which took several years to complete. When that was done, around the middle of 2015, the instrument was substantially more sensitive. They called the new and improved version Advanced LIGO. In September of 2015, they turned on Advanced LIGO for some checks, preparing the instrument for the first official science run—and during that check, when no one expected it, we witnessed the first LIGO event, that dramatic merging of two black holes.
Asaro: So the universe had a surprise in store for you all.
Cannizzo: A welcome surprise! If I’d had to guess a year ago about when the first detection would happen, I would have predicted sometime around 2018 to 2020. We knew it was coming, but not many expected it this soon. The engineers did a fantastic job. The hats are off to the people who built the interferometer and did all the upgrades on this instrument.
Asaro: When the instrument makes a detection, what do you actually see?
Cannizzo: The distortion of space creates a signal called a chirp. It’s an oscillation that increases in both frequency and amplitude to a certain point, and then abruptly stops. The signal on September 14, 2015 lasted about two tenths of a second. It consisted of about eight oscillations of increasing amplitude and a frequency that went from 35 cycles per second to 150 cycles per second.
Asaro: Why did you need two observatories to pick up the chirp? Wouldn’t one be enough?
Cannizzo: To record a chirp, yes, but we need two observatories to verify we are seeing a real astronomical event. That’s because other events can produce signals resembling the chirps, such as a plane passing overhead or a bolt of lightning. However, those spurious signals show up at only one observatory. For us to believe the chirp represents a real event, it must show up in exactly the same pattern at both observatories.
The signal on September 14, 2016 came in strong and same at both detectors. It was slightly delayed at the Hanford location compared to Livingston, indicating it came from a direction in the sky closer to Louisiana than Washington. The frequency of the oscillations and the way the frequency changed indicated one of the black holes was probably about 29 times the mass of the sun and the other about 36 solar masses. The distance of the event from Earth was huge, roughly about one billion three hundred million light years, where a light year is the distance light travels in one year.
Asaro: Was this what you expected to see as a first detection by LIGO?
Cannizzo: Actually, it was unexpected in several ways. First, double black holes are thought to be quite rare. A black hole is a collapsed object formed at the very end of the life for a massive star. Having two stars with that much mass together in a binary system is unusual. Such stars also lose a lot of mass during their brief lives. You need a star about two hundred times the mass of our own sun to end up with a black hole having “only” thirty to forty solar masses. The bias before The Event was that we would see a double neutron star merger first because such binaries are a lot more common. So why did we see the merger of two black holes instead? It may be because black hole binaries are much more massive than neutron stars and therefore produce significantly stronger gravity wave signals.
Asaro: What’s next? What effect will this have on the scientific community?
Cannizzo: The success of LIGO will probably provide great stimulus for similar projects all over the world. Many other facilities have already begun similar efforts. LIGO is set for another run in five to six months, this time lasting for about half a year. If current upgrade efforts are successful, they will continue to improve the sensitivity of LIGO, and we hope bring in many more detections. LIGO will usher in an exciting new era of scientific discovery.
Asaro: Thank you, Doctor Cannizzo. It sounds like a wonderful year for science.
Cannizzo: My pleasure. It’s a great time to be an astronomer.
For more information, see http://www.ligo.caltech.edu
About the author: Catherine Asaro received her Ph.D. in theoretical Chemical Physics from Harvard. She currently directs the Chesapeake Math Program, including top-ranked students in contests such as the USA Mathematical Olympiad and the American Regional Mathematics League. She has appeared as a speaker at various institutions, including Harvard, the National Academy of Sciences, Georgetown, the US Naval Academy, and a Guest of Honor at science fiction conventions in the US and overseas. She is also a member of SIGMA, a think tank of writers and scientists who advise the government as to future trends affecting national security. For the past twenty years, she has worked as a novelist, with over twenty-six books in science fiction, fantasy, and near future thrillers.