Dr. Kip Thorne: “This really is a whole new way of observing aspects of the universe that you can’t see in any other manner.”
Sep. 14, 2025
Ten years ago, on Sept. 14, 2015 at 5:51 AM EDT, the Laser Interferometer Gravitational-Wave Observatory (LIGO) installations in Hanford, Washington and Livingston, Louisiana detected a 0.2-second long signal in their data. Later referred to as a “chirp,” this signal would be scrutinized for five months before scientists at LIGO announced that it was the first ever direct observation of gravitational waves.
First theorized in the 20th century, gravitational waves can be intuitively described as ripples in a four-dimensional pond. Though humans live in three-dimensions, the physics of large objects in outer space is so complex that scientists prefer to look at it in four-dimensional spacetime: three dimensions for space, and one dimension for time. Just like boats on the Charles make waves as they speed by, large objects with strong gravitational pulls will warp spacetime and produce gravitational waves. The data collected by LIGO in 2015 confirmed the existence of an additional method to observe the universe and confirmed Einstein’s prediction of gravitational waves from his theory of general relativity.
Since then, new gravitational wave detectors such as the Virgo Interferometer in Italy and the KAGRA detector in Japan have joined in the effort of looking for invisible waves. From its conception in the 1970s, LIGO has been an ambitious attempt to transform the theoretical prediction of gravitational waves into experimental reality.
Born in MIT
Before LIGO, the only detections of gravitation waves were indirect, but these indirect observations provided crucial evidence that such waves existed and could be measured. In 1974, Russell Hulse and Joseph Taylor published their observations on a binary pulsar star system (named the Hulse-Taylor Pulsar) in which two neutron stars slowly spiral towards one another at a rate agreeable with the energy loss caused by gravitational waves. Although Hulse and Taylor would go on to win the Nobel Prize in Physics for their work in 1993, the scientific community still lacked a method to directly observe the effects of gravitational waves.
The first person to propose building LIGO was former MIT professor and physicist Rainer Weiss ’55, PhD ’62, in 1972. Weiss hypothesized that interferometry, a technique that uses superimposed light waves from lasers to detect any potential phase differences caused by interferences, could be a viable way to detect such waves. Building an extremely sensitive laser system, Weiss hoped, would be enough to detect and measure the microscopic vibrations caused by gravitational waves.
While interferometry to detect variations had been performed, a system on the scale to detect vibrations via gravity left many experts in the physics community skeptical about the success of LIGO.
“We had [strength of the signals] pinned down, but the distance between what the technology of that era could do and what was required was so enormous that I was very skeptical,” renowned theoretical physicist and emeritus professor at California Institute of Technology (Caltech) Dr. Kip Thorne recalled. “It just seemed to me that it would never be possible in my lifetime to achieve the sensitivities that are required.”
Despite this, by 1976, discussions with Weiss as well as with the Russian experimental and theoretical physicist Vladimir Braginsky eventually convinced Thorne to devote a large portion of his career to LIGO. Weiss and Thorne would go on to submit a proposal to Caltech for the LIGO project in 1976, which would be approved in 1977. By 1979, the U.S. National Science Foundation approved funding for the joint MIT/CalTech LIGO project.
Silence and Patience
Even though LIGO sites would complete construction in 1999, it would take an additional seven years for LIGO to reach the desired design sensitivity. LIGO’s sensitivity would be one of the key focuses throughout LIGO’s operational history, striving continuously to lower the threshold of detection until it reached the range of the predicted signal strength of gravitational waves. LIGO’s detectors need to be enormous, with each arm stretching 4 kilometers (2.5 miles) in length, because gravitational waves cause incredibly tiny distortions in space. When a gravitational wave passes through Earth, it causes a strain, or a fractional change in length. For the strongest gravitational waves LIGO has detected, this strain is on the order of 10-21, which means LIGO’s 4-kilometer arms stretch and compress by roughly 10-18 meters – still unimaginably small, but just large enough for LIGO’s ultra-precise laser interferometry system to detect above the noise. The longer the detector arms of LIGO, the larger the absolute change in distance becomes, even though the fractional change remains the same.
The first round of the detectors were not specific enough. “I wasn’t necessarily surprised,” said LIGO Chief Director Scientist and MIT Kavli Institute Senior Research Scientist Dr. Peter Fritschel. However, for Fritschel, this was not discouraging. “We were all pretty confident that advanced Lego would make detections, but it wasn’t clear exactly when,” he explained. “It’s not as if we installed the detectors and we turned them on and they work at their design sensitivity right away. It’s always an incremental progress.”
Thorne himself had expected that the first generation of detectors would not be able to have the capabilities to detect the gravitational waves. “When we submitted our construction proposal, we said that this is very difficult, and we will likely have to build two generations of detectors in order to succeed,” Thorne explained. “The first generation of detectors, so called initial LIGO, will be at a level of sensitivity where if nature is really kind, we’ll see something. If nature is not kind, we won’t and we must be prepared to not see anything.” Through LIGO’s first 15 years of operationability, LIGO would undergo another upgrade of its system to the Advanced LIGO system, finally allowing LIGO to reach the sensitivity necessary for gravitational wave detection.
A Chirp Forty Years in the Making
Soon after the installation of advanced LIGO machines in both the Washington and Louisiana locations, both LIGO installations detected the now famous “chirp.” Created by the merging of two black holes 1.3 billion light years away, the chirp marked the first direct observation of a gravitational wave in history.
Despite the discovery, many scientists at LIGO were initially wary. “The concern was that we could have been hacked. Somebody could have put the pulse signal in there somehow,” Thorne explained. However, once the signal was confirmed to have been authentic the attitude quickly shifted. “It’d been very exciting, like the previous six to nine months, because of the progress we were making before the first detectors,” said Fristchel.
“My own emotional reaction was the deep satisfaction that we had made the right choices at many places along the way to be able to pull this off,” recalled Thorne. “Ray Weiss, what I observed in him was primarily an emotional reaction of extreme relief. He seemed to be feeling quite guilty about having convinced hundreds of young people to come into this field to work on this and they didn’t have any gravity detections yet, having spent over a billion dollars of taxpayer money.”
Shortly after, LIGO would go on to detect a second binary black hole merging event on December 26th of the same year. Three more gravitational wave events would be recorded over the next half year, with one being the first detected neutron star collision.
The Future of Gravitational Wave Detection
Since the initial detection, LIGO has continued to conduct observing runs to observe gravitational waves. However, the LIGO observatories in Louisiana and Washington are now joined by other gravitational wave detectors around the globe. Completed in 2003 and upgraded in 2017, Europe’s Virgo interferometer works together with the two stations in the U.S. to help aid in gravitational wave origin location.
When a single gravitational wave observatory detects a signal, it can determine when the wave arrived but cannot pinpoint where it came from. It creates a ring of potential source locations in the sky, all equidistant from the detector. With two observatories, the situation improves dramatically. Since gravitational waves travel at the speed of light, a source that’s closer to one detector than the other will reach the nearer detector first. This time difference narrows down the possible source locations to two arc-shaped regions in the sky where the circles from each detector intersect, but there’s still ambiguity about which of the two regions contains the actual source.
“Because we want to localize the sources, we need to basically triangulate. So you need at least three detectors to be able to get an idea of where in the sky the source comes from, and LIGO alone can’t do that with just two detectors,” explained LIGO Laboratory Deputy Director Dr. Albert Lazzarini ’70 PhD ’74.
This is why the global network of detectors is so powerful. The network can often localize gravitational wave sources to within tens of square degrees.
With the new detectors like Japan’s KAGRA and Italy’s Virgo joining the hunt for gravitational waves, newer and more advanced gravitational wave detectors will soon outperform LIGO in its sensitivity capabilities. Moreover, with President Trump’s proposal of eliminating one of the antennas and decreasing LIGO’s operating budget in 2026 by 40 percent, research at LIGO could get more difficult. Despite this, plans to construct new observatories with arms up to 40 kilometers in length – ten times longer than existing LIGO observatories – are in the works. “If you consider the whole universe, even with our detectors right now, as sensitive as they are, we’re still probing what people would call the local universe,” Fritschel explained. With the Cosmic Explorer, a planned, next-generation gravitational-wave observatory, scientists will be able to explore events “across the whole visible universe, so it’s a big game changer.”
This expanded reach could unlock some of the most profound mysteries in cosmology. According to Lazzarini, it is hypothesized that gravitational waves were formed in the early universe. Being able to detect such waves would allow greater insight into the birth of the universe and formation shortly after the Big Bang.
As LIGO approaches its next decade of discovery, it remains both a testament to decades of persistence and global collaboration, as well as a foundation for the future of gravitational wave science. From the first faint “chirp” in 2015 to the ambitious projects now on the horizon, LIGO’s story reflects a broader truth: the universe still has much to reveal, and humanity has only just begun to listen.
