Sept. 14, 2015, was one of the most important days in science history. It marked the first-ever detection of gravitational waves, tiny ripples in space-time (the four-dimensional union of space and time), a milestone notched by the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Since that day, LIGO — composed of two highly sensitive laser interferometers located in Hanford, Washington and Livingston, Louisiana — has been joined by two smaller gravitational wave observatories: Virgo, which came online in Italy on Aug. 1, 2017, and the Kamioka Gravitational Wave Detector (KAGRA) located in Japan, in late 2019.
Over the course of four operating runs, separated by shutdowns to allow for improvements and upgrades, the LIGO-Virgo-KAGRA instruments have become so sensitive that they can now measure distortions in space-time caused by gravitational waves that are 1/10,000 the width of a proton, or 700 trillion times smaller than the width of a human hair. Together, the LIGO-Virgo-KAGRA collaboration has now detected over 300 gravitational wave signals, opening a completely new window to the universe that allows scientists to hear some of the most extreme and violent cosmic events.
Here, Space.com takes you through some of the most important gravitational wave breakthroughs that have occurred since 2015.
While these milestones come in no particular order, there is only one place we can start…
1. Proving Einstein right! The first gravitational wave detection
On Sept. 14, 2015, ripples in space-time washed over Earth that were generated by the merger of two black holes, each with a mass of around 30 times that of the sun. This signal, which would come to be known as GW150914 (GW for “gravitational wave” and the following numbers for the date of measurement), had been traveling to our planet for 1.4 billion years.
GW150914’s arrival and detection confirmed a theory that was first proposed a century earlier by arguably history’s most famous physicist, Albert Einstein, in his 1915 theory of gravity, general relativity.
General relativity predicts that objects with mass cause the very fabric of space-time to warp, with gravity arising from this warp. The larger the mass of an object, the greater the warp in space-time it generates, and thus the stronger its gravitational influence.
But general relativity also suggested that, when objects accelerate, they should generate ripples in space-time — gravitational waves. These would be significant enough to measure only for objects of truly massive status, such as black holes swirling around each other in a binary system and eventually merging.
Announced to the public on Feb. 11, 2016, GW150914 represented further validation of general relativity and confirmed that black hole mergers actually occur, creating more massive “daughter” black holes. The find also gave scientists a separate way to investigate the universe alongside “traditional” astronomy, which relies largely on the detection and study of light.
The achievement would earn Rainer Weiss, who passed away just last month, Kip Thorne and Barry Barish the 2017 Nobel Prize in Physics.
2. Heaviest black hole merger
On Nov. 23, 2023, LIGO-Virgo-KAGRA (LVK) detected the gravitational wave signal GW231123, which involved a clash between black holes with masses 100 and 140 times that of the sun. This collision created a daughter black hole with a mass around 225 times that of the sun, with the missing mass converted toa gravitational wave “screech” (which you can learn more about below).
This was the most massive black hole merger detected in gravitational waves to date, with the prior record holder being 2021’s GW190521, which was resulted in a daughter black hole with 140 solar masses.
“This is the most massive black hole binary we’ve observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation,” LVK collaboration member and Cardiff University researcher Mark Hannam said of GW231123. “Black holes this massive are forbidden through standard stellar evolution models.
“One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes.”
3. This neutron star merger was golden!
It’s not all black hole mergers for LKV. The gravitational wave detectors have also “heard” ripples in space-time from clashes between neutron stars. These are extreme stellar remnants composed of the densest matter in the known universe that, like stellar-mass black holes, are born when massive stars go supernova and die.
On Aug. 17, 2017, LIGO and Virgo detected a signal, GW170817, representing gravitational waves from a collision between neutron stars located around 130 million light-years from Earth. This was the first detection of gravitational waves from anything other than black holes.
This was an important scientific breakthrough, because it is thought that mergers between neutron stars generate the only environment that is extreme and violent enough to allow the fusion processes that can generate elements heavier than iron, like gold, silver and plutonium.
“It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see — and promising the world we would see,” David Shoemaker, spokesperson for the LIGO Scientific Collaboration and senior research scientist at the Massachusetts Institute of Technology’s (MIT) Kavli Institute for Astrophysics and Space Research, said in a statement at the time. “From informing detailed models of the inner workings of neutron stars and the emissions they produce to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving.”
GW170817 was humanity’s first step toward understanding how the gold in your jewelry box was forged. But this list isn’t done with this event just yet; its importance to science goes beyond the first detection of a neutron star merger.
4. Best of both worlds: Multimessenger astronomy is born!
As you might imagine, when stellar remnants as extreme as neutron stars collide, there is quite a burst of energy, and not just in gravitational waves, which can be considered gravitational radiation.
Neutron star mergers are also accompanied by flashes of light that astronomers have dubbed “kilonovas.” Thus, the first detection of a neutron star merger in gravitational waves offered scientists the unique opportunity to follow this up with “traditional astronomy,” which utilizes different wavelengths of the electromagnetic spectrum.
This led to GW170817 becoming one of the most widely studied astronomical events in history, with nearly one-third of the world’s electromagnetic astronomers chasing the gravitational wave detection via traditional astronomy.
Such work paid off, with NASA’s Fermi Gamma-ray spacecraft and Europe’s INTEGRAL (International Gamma-Ray Astrophysics Laboratory) both independently detecting a gamma-ray burst designated GRB 170817A erupting from this same merger.
This allowed astronomers to determine that the neutron star merger occurred in the galaxy NGC 4993, located about 140 million light-years away.
This was the first successful application of “multimessenger astronomy,” which observes cosmic events using more than one form of messenger — in this case, gravitational waves and electromagnetic radiation. The third spoke in this wheel is messengers in the form of high-energy particles, such as neutrinos or cosmic rays generated by cosmic events.
The fact that each of these “messengers” is created by a different astrophysical process means they have the potential to reveal different information about the same source. That makes multimessenger astronomy a powerful new tool in science.
To date, the event that generated GW170817 and launched GRB 170817A remains the only successful observation of an event in both gravitational waves and electromagnetic radiation.
“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe,” France Córdova, then the director of the U.S. National Science Foundation (NSF), which funds LIGO, said in a statement at the time. “This discovery realizes a long-standing goal many of us have had — that is, to simultaneously observe rare cosmic events using both traditional and gravitational-wave observatories.”
5. For whom the black hole tolls
The emission of gravitational waves from a binary black hole merger comes in three phases. As these orbiting black holes emit gravitational waves, their orbits tighten due to the loss of angular momentum from the system. This leads to the two black holes eventually colliding and merging, sending out a high-pitched gravitational wave “screech” followed by a diminishing “ringdown” of vibrations lasting for a fraction of a second.
“The [daughter] black hole is similar to a bell that rings, producing a spectrum of multiple fading tones that encode information about the bell,” Collin Capano from the Albert Einstein Institute said in a statement back in 2023, after he and his colleagues revealed that they had found strong observational evidence of at least two gravitational-wave frequencies existing in a binary black hole ringdown signal.
This ringdown signal, the aforementioned GW190521, can give details of the mass and spin of a resultant daughter black hole, to great precision.
“Achieving this multimode observation – in other words, the detection of two distinct vibration frequencies of a deformed black hole – has been a welcome surprise. It was widely assumed this would not be possible before the next generation of gravitational-wave detectors,” Capino said.
The GW190521 ringdown was also significant because it acted as a test of the idea that black holes can be described by just three characteristics: their mass, spin and electric charge. This theory is immortalized by physicist John Wheeler’s infamous phrase: “Black holes have no hair.”
“GW190521 passed the test and we found no signs of any black hole physics beyond Einstein’s general theory of relativity,” Capino’s colleague Julian Westerweck said back in 2023. “It is quite remarkable that a theory that is over 100 years old now continues to work so well.”
6. Mix it up! Detecting a black hole-neutron star ‘mixed merger’
Everybody loves chocolate, and most of us can’t get enough peanut butter, but it is when these two treats are mixed that they really come into their own. It turns out that black hole and neutron star mergers are the cosmic equivalent of chocolate peanut butter cups. No wonder scientists spent so long hunting for them.
On Jan. 5, 2020, LIGO/Virgo detected GW200105_162426, a signal from a neutron star with a mass 1.9 times that of the sun colliding with an 8.9-solar-mass black hole. It occurred five years after the detection of the first black hole-black hole merger, and three years after the first neutron star-neutron star merger.
This was the first evidence of a third kind of stellar remnant merger: a neutron star-black hole collision, or a “mixed merger.” With peanut butter cups, one is rarely enough, and that turns out to be true for mixed mergers, too.
The second neutron star-black hole collision event was spotted in the form of the signal GW200115_042309, detected just a few days later on Jan. 15, 2020. This neutron star had an estimated mass 1.5 times that of the sun, with its companion being a 5.7-solar-mass black hole.
“With this new discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary,” Astrid Lamberts, a scientist with the French national research agency CNRS at Observatoire de la Côte d’Azur, said in 2021. “We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way.”
To date, the LIGO-Virgo-KAGRA collaboration has detected and confirmed just two mixed mergers between a neutron star and a black hole, though there is another possible candidate that hasn’t been fully vetted yet.
7. The lightest black hole merger is a mixed mystery
On Aug. 14, 2019, LIGO and Virgo detected the gravitational wave signal GW190814 from a merger that occurred 790 million light-years away.
While one of the objects involved was a black hole of 22 to 24 solar masses, the identity of the second object isn’t as clear-cut as in the case of the mixed mergers above. That’s because its mass is right in the sweet spot between black holes and neutron stars.
With a mass 2.6 times that of the sun, the other component of this merger was either one of the lightest black holes ever seen or one of the heaviest neutron stars. As such, the fact that it was detected earlier than the two 2020 signals means that GW190814 could actually be the first recorded mixed merger.
The merger remains shrouded in mystery. Astronomers can find no electromagnetic counterpart, meaning this could be two merging black holes or a black hole that has completely devoured a neutron star. Solving this puzzle could help us better understand the cycle of life and death experienced by the most massive stars.
8. This one goes up to 11: The loudest gravitational wave ever!
Proving that the LIGO-Virgo-KAGRA collaboration is still at the cutting edge of gravitational wave science, this entry on our list comes from just this month!
On Sept. 10, 2025, LKV team members announced the detection of GW250114, the result of two merging black holes with masses around 32 times the mass of the sun.
What makes GW250114 remarkable is the fact that it is one of the clearest gravitational wave signals ever. So clear, in fact, that it not only further confirmed the theory of general relativity but also verified the theories of other black hole luminaries.
“GW250114 is the loudest gravitational wave event we have detected to date; it was like a whisper becoming a shout.” Geraint Pratten, member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham in England, said in a statement. “This gave us an unprecedented opportunity to put Einstein’s theories through some of the most rigorous tests possible — validating one of Stephen Hawking’s pioneering predictions that when black holes merge, the combined area of their event horizons can only grow, never shrink.”
GW250114 gets on the list because it demonstrates just how far LIGO-Virgo-KAGRA has come over the last 10 years.
Read More: Gravitational wave detector confirms theories of Einstein and Hawking: ‘This is the clearest view yet of the nature of black holes’
9. Hearing a cosmic symphony
This one isn’t LVK-related, but it is a gravitational wave discovery made during the last 10 years, so it still makes the list.
On June 28, 2023, it was revealed that the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) had detected low-frequency gravitational waves, a historic breakthrough that represents 15 years of searching. NANOGrav uses spinning neutron star pulsars as a timing array to detect the tiny fluctuations in space-time caused by gravitational waves.
The gravitational waves detected by LIGO and its collaborators represent a dramatic single “crash” of cymbals from violent events like collisions and mergers; the low-frequency gravitational wave signal NANOGrav heard is more akin to the gentle background harmony of violins.
The strength of the signal represents a gravitational wave orchestra of hundreds of thousands, maybe even millions, of supermassive black holes swirling around each other and eventually merging in the early universe.
“This finding opens up a new low-frequency window on the gravitational universe which will let us study how galaxies and their central black holes merge and grow with time,” National Radio Astronomy Observatory astronomer and NANOGrav researcher Scott Ransom told Space.com in 2023.
10. Proving Einstein … wrong!?!
This may come as a bit of surprise, but while every gravitational wave discovery made since 2015 has verified Einstein’s theory of general relativity, ironically, each has also proved the great physicist wrong, too.
That’s because Einstein believed that gravitational waves are so faint and so insubstantial, in terms of the displacement of space-time they cause as they wash through the cosmos at near light-speed, that we would never be able to detect them.
Even some of the scientists who were integral to the development of LIGO and the first detection of gravitational waves weren’t initially certain such a feat was possible, agreeing with Einstein.
“Rai Weiss proposed the concept of LIGO in 1972, and I thought, ‘This doesn’t have much chance at all of working,’” Kip Thorne, an expert on the theory of black holes, said in a statement earlier this month. “We had to invent a whole new technology.”
