This plot shows gravitational-wave signals recorded by the LIGO Hanford detector almost ten years apart. The top shows data from LIGO's first-ever detection of gravitational waves, an event called GW150914, captured in 2015. The bottom shows the signal known as GW250114, captured in 2025. Both events involve colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times that of our Sun. The purple line shows the data, which are a combination of the signal plus background detector noise. The noise comes from a variety of sources, including seismic motions that jiggle giant mirrors inside LIGO. The green line shows the best-fit prediction from general relativity for each signal. The much lower noise seen today is thanks to cutting-edge improvements made to the LIGO detectors that hush unwanted noise. Credit: LIGO/J. Tissino (GSSI)/R. Hurt (Caltech-IPAC)
This video compares a newly detected gravitational-wave signal called GW250114 with the first gravitational-wave signal ever detected, GW150914, in 2015. Both signals came from colliding black holes, each between 30 to 40 times the mass of the Sun. The video converts the signals to sounds (called "chirps") and plays each detection twice. The first round is played at the original frequencies, in which the gravitational-wave frequencies have been converted directly into sound waves. In the second round, the pitch has been increased by 30 percent to make the chirps easier to hear.
A numerical relativity simulation of the recently observed GW250114 event. The blue and white surface shows a two-dimensional slice of the gravitational waves spiraling outward as the black holes orbit one another. Throughout this inspiral, the gravitational waves grow in magnitude, peaking as the black holes merge, and then decreasing rapidly as the newly formed remnant black hole settles. Credit: Deborah Ferguson, Derek Davis, Rob Coyne (URI) / LIGO / MAYA Collaboration. Simulation performed with NSF's TACC Frontera supercomputer.
This artwork imagines the ultimate front-row seat for GW250114, a powerful collision between two black holes observed in gravitational waves by the US National Science Foundation LIGO. It depicts the view from one of the black holes as it spirals toward its cosmic partner. Ten years after LIGO's landmark detection of gravitational waves, the observatory's improved detectors allowed it to "hear" this celestial collision with unprecedented clarity. The gravitational-wave data enabled scientists to distinguish multiple subtle tones ringing out like a cosmic bell across the universe (imagined here as intertwining musical threads spiraling toward the center). Credit: Aurore Simonnet (SSU/EdEon)/LVK/URI
Infographic explaining the significance of "overtones" detected by LIGO during a black hole merger.
Credit: Lucy Reading-Ikkanda/Simons Foundation
Caltech professors Barry Barish, Kip Thorne, and Fiona Harrison with Caltech President Tom Rosenbaum at a press conference for the 2017 Nobel Prize in Physics.
LIGO, Virgo, and KAGRA celebrate anniversary, announce verification of Stephen Hawking's Black Hole Area Theorem
On September 14, 2015, a signal
arrived on Earth, carrying information about a pair of remote black
holes that had spiraled together and merged. The signal had traveled
about 1.3 billion years to reach us at the speed of light—but it was not
made of light. It was a different kind of signal: a quivering of
space-time called gravitational waves first predicted by
Albert Einstein
100 years prior. On that day 10 years ago, the twin detectors of the US
National Science Foundation Laser Interferometer Gravitational-wave
Observatory (NSF LIGO) made the first-ever direct detection of
gravitational waves, whispers in the cosmos that had gone unheard until
that moment.
The
historic discovery
meant that researchers could now sense the universe through three
different means. Light waves, such as X-rays, optical, radio, and other
wavelengths of light, as well as high-energy particles called cosmic
rays and neutrinos, had been captured before, but this was the first
time anyone had witnessed a cosmic event through the gravitational
warping of space-time. For this achievement, first dreamed up more than
40 years prior, three of the team's founders won the
2017 Nobel Prize in Physics: MIT's
Rainer Weiss, professor of physics, emeritus (who
recently passed away
at age 92); Caltech's Barry Barish, the Ronald and Maxine Linde
Professor of Physics, Emeritus; and Caltech's Kip Thorne, the Richard P.
Feynman Professor of Theoretical Physics, Emeritus.
Today, LIGO, which consists of detectors in both Hanford, Washington and
Livingston, Louisiana, routinely observes roughly one black hole merger
every three days. LIGO now operates in coordination with two
international partners, the
Virgo gravitational-wave detector in Italy and
KAGRA
in Japan. Together, the gravitational-wave-hunting network, known as
the LVK (LIGO, Virgo, KAGRA), has captured a total of about 300 black
hole mergers, some of which are confirmed while others await further
analysis. During the network's current science run, the fourth since the
first run in 2015, the LVK has discovered more than 200 candidate black
hole mergers, more than double the number caught in the first three
runs.
The dramatic rise in the number of LVK discoveries over the past decade is owed to several improvements to
their detectors—some of which involve
cutting-edge quantum precision engineering.
The LVK detectors remain by far the most precise rulers for making
measurements ever created by humans. The space-time distortions induced
by gravitational waves are incredibly miniscule. For instance, LIGO
detects changes in space-time smaller than 1/10,000 the width of a
proton. That's 700 trillion times smaller than the width of a human
hair.
"Rai Weiss proposed the concept of
LIGO in 1972, and I thought, 'This doesn't have much chance at all of
working,'" recalls Thorne, an expert on the theory of black holes. "It
took me three years of thinking about it on and off and discussing ideas
with Rai and Vladimir Braginsky [a Russian physicist], to be convinced
this had a significant possibility of success. The technical difficulty
of reducing the unwanted noise that interferes with the desired signal
was enormous. We had to invent a whole new technology. NSF was just
superb at shepherding this project through technical reviews and
hurdles."
MIT's Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics and dean of the
School of Science, says that the challenges the team overcame to make
the first discovery are still very much at play. "From the exquisite
precision of the LIGO detectors to the astrophysical theories of
gravitational-wave sources, to the complex data analyses, all these
hurdles had to be overcome, and we continue to improve in all of these
areas," Mavalvala says. As the detectors get better, we hunger for
farther, fainter sources. LIGO continues to be a technological marvel."
This chart plots discoveries made by the LIGO-Virgo-KAGRA (LVK) network since LIGO's first detection, in 2015, of gravitational waves emanating from a pair of colliding black holes. The detections consist mainly of black hole mergers, but a handful involve neutron stars (either black hole-neutron star collisions or neutron star-neutron star collisions). Credit: LIGO/Caltech/MIT/R. Hurt (IPAC)
The Clearest Signal Yet
LIGO's improved sensitivity is exemplified in a
recent discovery of a black hole merger referred to as GW250114 (the
numbers denote the date the gravitational-wave signal arrived at Earth:
January 14, 2025). The event was not that different from LIGO's
first-ever detection (called GW150914)—both involve colliding black
holes about 1.3 billion light-years away with masses between 30 to 40
times that of our Sun. But thanks to 10 years of technological advances
reducing instrumental noise, the GW250114 signal is dramatically
clearer.
"We can hear it loud and clear, and that lets us test the fundamental laws of physics," says LIGO team
member Katerina Chatziioannou, Caltech assistant professor of physics
and William H. Hurt Scholar, and one of the authors of a
new study on GW250114 published in the
Physical Review Letters.
By analyzing the frequencies of gravitational waves emitted by the merger,
the LVK team provided the best observational evidence captured to date
for what is known as the
black hole area theorem, an idea put forth by Stephen Hawking in 1971 that says the total surface areas of black holes
cannot decrease. When black holes merge, their masses combine,
increasing the surface area. But they also lose energy in the form of
gravitational waves. Additionally, the merger can cause the combined
black hole to increase its spin, which leads to it having a smaller
area. The black hole area theorem states that despite these competing
factors, the total surface area must grow in size.
Later, Hawking and physicist
Jacob Bekenstein concluded that a black hole's area is proportional to its entropy, or degree of disorder. The findings paved the way for later groundbreaking work in the field of quantum gravity, which attempts to unite two pillars of modern physics: general relativity and quantum physics.
In essence, the LIGO detection allowed the team to "hear" two black holes growing as they merged into one, verifying Hawking's theorem. (Virgo and KAGRA were offline during this particular observation.) The initial black holes had a total surface area of 240,000 square kilometers (roughly the size of
Oregon), while the final area was about 400,000 square kilometers (roughly the size of
California)—a clear increase. This is the second test of the black hole area theorem; an
initial test was performed in 2021 using data from the first GW150914 signal, but because that data was not as clean, the results had a confidence level of 95 percent compared to 99.999 percent for the new data.
Thorne recalls Hawking phoning him to ask whether LIGO might be able to test
his theorem immediately after he learned of the 2015 gravitational-wave
detection. Hawking died in 2018 and sadly did not live to see his theory
observationally verified. "If Hawking were alive, he would have reveled
in seeing the area of the merged black holes increase," Thorne says.
The trickiest part of this type of analysis had to do with determining the
final surface area of the merged black hole. The surface areas of
pre-merger black holes can be more readily gleaned as the pair spiral
together, roiling space-time and producing gravitational waves. But
after the black holes coalesce, the signal is not as clear-cut. During
this so-called
ringdown phase, the final black hole vibrates like a struck bell.
In the new study, the researchers precisely measured the details of the ringdown phase, which
allowed them to calculate the mass and spin of the black hole and,
subsequently, determine its surface area. More specifically, they were
able, for the first time, to confidently pick out two distinct
gravitational-wave modes in the ringdown phase. The modes are like
characteristic sounds a bell would make when struck; they have somewhat
similar frequencies but die out at different rates, which makes them
hard to identify. The improved data for GW250114 meant that the team
could extract the modes, demonstrating that the black hole's ringdown
occurred exactly as predicted by math models based on the Teukolsky
formalism—devised in 1972 by
Saul Teukolsky, now a professor at Caltech and Cornell.
Another
study from the LVK, submitted to
Physical Review Letters
today, places limits on a predicted third, higher-pitched tone in the
GW250114 signal, and performs some of the most stringent tests yet of
general relativity's accuracy in describing merging black holes.
Visualization of the binary black hole merger called GW250114. The animation shows the inspiral and merger of the two black holes, then continues a few milliseconds into the ringdown phase. At that point the gravitational waves are separated into the two modes of the ringing remnant black hole that were identified in the observation. A predicted third tone (that the data place limits on) is also shown. Credit: H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), K. Mitman (Cornell University)
"A decade of improvements allowed us to make
this exquisite measurement," Chatziioannou says. "It took both of our
detectors, in Washington and Louisiana, to do this. I don't know what
will happen in 10 more years, but in the first 10 years, we have made
tremendous improvements to LIGO's sensitivity. This not only means we
are accelerating the rate at which we discover new black holes, but we
are also capturing detailed data that expand the scope of what we know
about the fundamental properties of black holes."
Jenne Driggers, detection lead senior scientist at LIGO Hanford, adds, "It
takes a global village to achieve our scientific goals. From our
exquisite instruments, to calibrating the data very precisely, vetting
and providing assurances about the fidelity of the data quality,
searching the data for astrophysical signals, and packaging all that
into something that telescopes can read and act upon quickly, there are a
lot of specialized tasks that come together to make LIGO the great
success that it is."
Pushing the Limits
LIGO and Virgo have also unveiled neutron stars over the past decade. Like
black holes, neutron stars form from the explosive deaths of massive
stars, but they weigh less and glow with light. Of note, in August 2017,
LIGO and Virgo witnessed
an epic collision between a pair of neutron stars—a
kilonova—that sent gold and other heavy elements flying into space and
drew the gaze of dozens of telescopes around the world, which captured
light ranging from high-energy gamma rays to low-energy radio waves. The
"multi-messenger" astronomy event marked the first time that both light
and gravitational waves had been captured in a single cosmic event.
Today, the LVK continues to alert the astronomical community to
potential neutron star collisions, who then use telescopes to search the
skies for signs of kilonovae.
"The LVK has made big strides in recent years to make sure we're getting high quality
data and alerts out to the public in under a minute, so that
astronomers can look for multi-messenger signatures from our
gravitational-wave candidates," Driggers says.
"The global LVK network is essential to gravitational-wave astronomy," says
Gianluca Gemme, Virgo spokesperson and director of research at the
National Institute of Nuclear Physics in Italy. "With three or more
detectors operating in unison, we can pinpoint cosmic events with
greater accuracy, extract richer astrophysical information, and enable
rapid alerts for multi-messenger follow-up. Virgo is proud to contribute
to this worldwide scientific endeavor."
Other LVK scientific discoveries include the first detection of collisions
between one neutron star and one black hole; asymmetrical mergers, in
which one black hole is significantly more massive than its partner
black hole; the discovery of the lightest black holes known, challenging
the idea that there is a
"mass gap" between neutron stars and black holes; and the
most massive black hole merger seen yet
with a merged mass of 225 solar masses. For reference, the previous
record holder for the most massive merger had a combined mass of 140
solar masses.
Even in the decades before LIGO began taking data, scientists were building foundations that made
the field of gravitational-wave science possible. Breakthroughs in
computer simulations of black hole mergers, for example, allow the team
to extract and analyze the feeble gravitational-wave signals generated
across the universe.
LIGO's technological achievements, beginning as far back as the 1980s, include several
far-reaching innovations, such as a new way to stabilize lasers using
the so-called Pound–Drever–Hall technique. Invented in 1983 and named
for contributing physicists Robert Vivian Pound, the late
Ronald Drever
of Caltech (a founder of LIGO), and John Lewis Hall, this technique is
widely used today in other fields, such as the development of atomic
clocks and quantum computers. Other innovations include cutting-edge
mirror coatings that almost perfectly reflect laser light; "quantum
squeezing" tools that enable LIGO to surpass sensitivity limits imposed
by quantum physics; and
new AI methods that could further hush certain types of unwanted noise.
"What we are ultimately doing inside LIGO is protecting quantum information
and making sure it doesn't get destroyed by external factors," Mavalvala
says. "The techniques we are developing are pillars of quantum
engineering and have applications across a broad range of devices, such
as quantum computers and quantum sensors."
In the coming years, the scientists and engineers of LVK hope to further
fine tune their machines, expanding their reach deeper and deeper into
space. They also plan to use the knowledge they have gained to build
another gravitational-wave detector,
LIGO India.
Having a third LIGO observatory would greatly improve the precision
with which the LVK network can localize gravitational-wave sources.
Looking farther into the future, the team is working on a concept for an even larger detector, called
Cosmic Explorer,
which would have arms 40 kilometers long (the twin LIGO observatories
have 4-kilometer arms). A European project, called Einstein Telescope,
also has plans to build one or two huge underground interferometers with
arms more than 10 kilometers long. Observatories on this scale would
allow scientists to hear the earliest black hole mergers in the
universe.
"Just 10 short years ago, LIGO opened our eyes for the first time to gravitational waves and changed
the way humanity sees the cosmos," says Aamir Ali, a program director in
the NSF Division of Physics, which has supported LIGO since its
inception. "There's a whole universe to explore through this completely
new lens and these latest discoveries show LIGO is just getting
started."
The LIGO-Virgo-KAGRA Collaboration
LIGO is funded by the US National Science Foundation and operated by Caltech
and MIT, which together conceived and built the project. Financial
support for the Advanced LIGO project was led by NSF with Germany (Max
Planck Society), the United Kingdom (Science and Technology Facilities
Council), and Australia (Australian Research Council) making significant
commitments and contributions to the project. More than 1,600
scientists from around the world participate in the effort through the
LIGO Scientific Collaboration, which includes the GEO Collaboration.
Additional partners are listed at
my.ligo.org/census.php.
The Virgo Collaboration is currently composed of approximately 1,000
members from 175 institutions in 20 different (mainly European)
countries. The European Gravitational Observatory (EGO) hosts the Virgo
detector near Pisa, Italy, and is funded by the French National Centre
for Scientific Research, the National Institute of Nuclear Physics in
Italy, the National Institute of Subatomic Physics in the Netherlands,
The Research Foundation – Flanders, and the Belgian Fund for Scientific
Research. A list of the Virgo Collaboration groups can be found at:
https://www.virgo-gw.eu/about/scientific-collaboration/. More information is available on the Virgo website at
https://www.virgo-gw.eu.
KAGRA is the laser interferometer with 3-kilometer arm length in Kamioka,
Gifu, Japan. The host institute is the Institute for Cosmic Ray Research
of the University of Tokyo, and the project is co-hosted by the
National Astronomical Observatory of Japan and the High Energy
Accelerator Research Organization. The KAGRA collaboration is composed
of more than 400 members from 128 institutes in 17 countries/regions.
KAGRA's information for general audiences is at the website
gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are accessible from
gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.
Contact:
Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu
