How one UT alumnus helped continue some of Einstein’s most ambitious work
The morning David Reitze, PhD ’90, vindicated Albert Einstein’s last remaining prediction about general relativity began like any other. The California Institute of Technology professor woke up and got dressed. He sat down to eat breakfast with his family, saw his daughter off to school, then headed to work. He arrived just before 8 a.m., logged onto his computer, and opened his email, expecting to see the usual.
Instead, something extraordinary sat in his inbox—a notification that had been one billion years in the making.
At 4:51 a.m. on Sept. 14, 2015, Reitze had received a message from LIGO, or Laser Interferometer Gravitational-Wave Observatory, where he splits his time as director. It was the signal of two black holes colliding in another galaxy, emitting the same amount of energy as a billion trillion suns throughout the universe. Spacetime itself had triggered the facility’s detectors, delivering proof long-awaited by decades of scientists.
Converted by researchers into sound waves, the signal’s tone can be described as a low, rising chirp. It was proof of the existence of gravitational waves—distortions that ripple through the fabric of space—which Einstein had predicted in 1915. For more than 100 years, researchers tried in vain to confirm their existence, as they remained one of the biggest mysteries in science.
“I thought to myself, this is it,” Reitze says.
But he had to be certain. Many scientists before him claimed to discover the same signals, only to be mistaken. “So there’s a number of things that had to happen,” he says. To start, he had to ensure the signal had not been man-made. There are two LIGO facilities, one located in Hanford, Washington, and the other in Livingston, Louisiana. At each base, there are people appointed to secretly “inject” fake signals into the LIGO systems as a test to keep the researchers and the computers sharp.
As messages about the signal came flooding in, Reitze reached out to check if anyone had run a test. After a unanimous ‘no,’ he began notifying his peers—a team of 1,000 global researchers, some of whom had been working toward this day for up to 40 years.
“From that day forward,” he says, “everything changed.”
The Relentless Pursuit
For five months, the team conducted a series of tests. They kept the discovery under wraps until publishing their research in Physical Review Letters.
In 1915, Einstein proposed his theory of relativity: The geometry of spacetime curves around matter and nothing travels faster than the speed of light. But to say information moves at the speed of light means that there has to be a way to carry it—and so comes the idea of a gravitational wave.
Gravitational waves occur when an object accelerates or decelerates, causing an emission of radiation (think of when a child bounces on a trampoline, sending a ripple through the material). But Einstein thought if gravitational waves existed, they would be so faint by the time they reached Earth, there’d be no hope of ever measuring them.
For the next 50 years, few paid the waves any mind. “They were just some mathematical theory you could play with,” Reitze says. “They had no practical consequence and you could never build an experiment to test them.”
Then in 1969, Joseph Weber, a professor at the University of Maryland, began experimenting with large aluminum bars. He believed if a gravitational wave hit Earth, it would ring through the cylinders. Weber claimed to detect gravitational waves on multiple occasions, but when no one else could replicate his results, he lost credibility in the gravitational waves community.
Around the same time, other scientists were working on similar projects around the world. At MIT, professor Rainer Weiss suggested using interferometry, a method of measuring tiny disruptions in the way waves interact with one another. He published an internal MIT report in 1972 detailing the physics behind interferometers and the hundreds of sources that could disrupt them, such as seismic tremors, radio noise, and motorcycle engines. In Scotland, former Caltech professor Ronald Drever was also busy leading groups in the use of interferometers.
By the ’80s, the U.S. National Science Foundation was funding both men’s research, bringing along Caltech professor Kip Thorne. The group of researchers began constructing prototypes, but after struggles over money, push-back from the science community, and internal disputes, it would be roughly another 20 years before things got moving. It wasn’t until the three researchers stepped down and Barry Barish of Caltech took over that the detectors were finally built in 1999.
Measuring The Wave
“Now the history is going to go pretty fast,” says Reitze, who joined the LIGO team in ’96. The first detectors constructed, called InitialLIGO, ran from 2006-10. The researchers had conducted six tests but came up with nothing.
They knew a discovery was unlikely but they had a long-term plan to use InitialLIGO as a stepping stone toward developing better instruments. By 2010, they began building AdvancedLIGO, which was made possible by the NSF and other contributions from countries like Scotland, Germany, and Australia—though it took four more years before the systems were running.
Luckily, timing was on their side. Just before AdvancedLIGO ran its first test, the gravitational wave passed through the interferometers serendipitously. From a bird’s eye view of both locations, two large pipes stretch out for 2.5 miles each, forming the observatories’ L-shaped structures. These cylindrical, four-foot diameter tubes are kept under a near-perfect vacuum, impenetrable by outside interference. At the ends of each arm, mirrors are precisely positioned inside the tubes. Laser light is split into two beams that travel back and forth down the arms, using light as a ruler.
When a gravitational wave travels throughout the universe, it stretches space in one dimension and compresses it in the other. So, if a gravitational wave passes through, you would get a little wider in one dimension and shorter in another. Then, the wave would cycle back and the opposite would happen—thinner in one dimension, taller in the other.
UT physics professor Richard Matzner, who served on the LIGO advisory committee in the early 2000s, was part of a team that developed computer codes used to detect black holes. By analyzing the nature of the wave, scientists were able to determine what it indicates. “If they’re not black holes, there would have to be something else that acted just like them,” Matzner says.
For instance, the low frequency of the wave that passed through LIGO indicated the binary black holes system. A higher frequency would have meant the emergence of a binary neutron star. Reitze says gravitational waves offer a whole new spectrum of emissions that explain astronomical objects.
“First, it was optical,” Reitze says. “Then astronomers realized there’s more to look at than just visible light—there are x-rays, infrared, gamma, and radio waves. Gravitational waves are the same.”
The Promised Land
The detection of the binary black holes marks a new era for the field of general relativity. Scientists are now seeing the universe, and actually hearing it, in a different way. “That’s really beautiful,” Reitze says. “We’re going to be doing the astronomy of the 21st century.”
Gravitational waves can go where light cannot, meaning scientists hope to better understand celestial elements like black holes, measuring their mass, momentum, and creation. By reaching into the darkest corners of the universe, the scientists could potentially study supernovas, neutron stars, and eventually, the Big Bang itself.
“The universe is putting signals out here for us to look at,” Matzner says. “Things we hadn’t thought to look at before. I don’t know what Einstein would say, but probably ‘holy something.’”
For now, the team at LIGO aims to see more signals come through. As they approach their next scientific run in July, the researchers are working on ways to improve the interferometers’ sensitivity, increasing their length and ability to pick up frequencies. They’re also thinking about designs for a new detector now that there is a real push from the scientific community. Additionally, observatories in Italy, Japan, and India have just been approved for construction.
Reitze remembers the morning the signal came in: receiving his colleagues’ emails with text in all caps, unable to contain their excitement. These past few months have been a series of firsts that have undeniably changed the game—the first observing run, the first binary black holes, the first gravitational wave. He is optimistic the team at LIGO will continue to find whatever they set their minds on.
“The analogy I’ll give is biblical,” Reitze says. “We’ve been wandering around the desert for 40 years now, and we just walked into the promised land.”
Photos from top:
The two merging black holes are each roughly 30 times the mass of the sun, with one slightly larger than the other. The event took place 1.3 billion years ago; The SXS (Simulating eXtreme Spacetimes) Project
The approximate location of the source of gravitational waves detected on Sept. 14, 2015, by the twin LIGO facilities, shown on a sky map of the Southern Hemisphere. The colored lines represent different probabilities for where the signal originated: the purple line defines the region where the signal is predicted to have come with a 90 percent confidence level; the inner yellow line defines the target region at a 10 percent confidence level; LIGO/Axel Mellinger Project
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