10 years of gravitational waves

12 September 2025

On September 14, 2015, at 11:50:45 Italian time, a signal generated by a pair of black holes, with masses equivalent to approximately 29 and 36 solar masses reached Earth. After spiraling around each other at incredible speeds, they merged, releasing an enormous amount of energy. It was a gravitational wave that had traveled for about 1.3 billion years at the speed of light.
What made this event unique in the history of science is that the signal GW150914 (the numbers indicate the date it arrived on our planet, September 14, 2015) is the first gravitational wave ever directly observed.

The gravitational waves were detected by both of the twin instruments of the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States, located in Livingston, Louisiana, and in Hanford, Washington state.
This important result, published in the scientific journal Physical Review Letters, was achieved thanks to the data from the two LIGO detectors by the LIGO and Virgo Scientific Collaborations. Virgo is the European detector located in Italy, near Pisa, at the European Gravitational Observatory (EGO), founded by the Italian National Institute for Nuclear Physics (INFN) and the French National Centre for Scientific Research (CNRS).

immagine artistica di onde gravitazionali

Artistic illustration of gravitational waves

The first direct detection of gravitational waves was announced by the LIGO and Virgo collaborations on February 11, 2016, after many months of analysis and verification.

First theorized by Albert Einstein a hundred years ago, gravitational waves are a key prediction of the theory of General Relativity, which Einstein formulated in 1915. This theory describes gravity as a manifestation of the curvature of spacetime. Spacetime describes the structure of the universe and can be imagined as a kind of fabric, but in four dimensions: the three spatial ones, plus time.

According to General Relativity, the fabric of spacetime pervades the entire universe, is warped by massive bodies, and is disturbed by moving masses, such as black holes or neutron stars that eventually merge. These disturbances are what we call gravitational waves, which spread out from their source similarly to ripples on the surface of a pond, traveling at the speed of light.

The Virgo Collaboration ( © Virgo Collaboration, M. Perciballi)

Born in the depths of space and generated by cosmic cataclysms such as the merger of black holes or neutron stars, gravitational waves reach Earth with effects so faint that even Einstein doubted they could ever be detected.
It took years of investment, study, research, development, and the work of many people to turn what once seemed like an impossible challenge into a success made possible through the development of innovative technologies in cryogenics, optics, vacuum systems, and data analysis.

We talk about the search for gravitational waves and their discovery in the interview with Fulvio Ricci, who at the time was the coordinator of the Virgo Scientific Collaboration, and that, together with David Reitze, then director of LIGO, announced a long-awaited discovery on February 11, 2016, during an emotional joint Italian-American press conference, broadcast live simultaneously from Washington, D.C. and Cascina (Pisa) at the EGO headquarters.

Virgo is a project conceived, built, and led by INFN and CNRS, later joined by Nikhef (Netherlands), with other European institutions also becoming part of the Scientific Collaboration.

The interview is published in the latest issue of Particle Chronicle, INFN’s newsletter.

The discovery of gravitational waves has opened a new era in the study of the universe, offering a completely new and complementary way of observing the cosmos, alongside traditional exploration techniques, which rely mainly on the detection of electromagnetic radiation and particles such as neutrinos and cosmic rays, in the quest to answer key questions in astrophysics, cosmology, and fundamental physics.

The result led, the following year (2017), to the awarding of the Nobel Prize in Physics to three of the founders of LIGO: Rainer Weiss, Barry Barish, and Kip Thorne of Caltech

Artistic illustration of two neutron stars merging. (© NSF/LIGO/Sonoma State University/A. Simonnet)"

In 2017, the European detector Virgo joined LIGO in data collection, enabling much more precise localization of gravitational wave sources.
In August 2017, LIGO and Virgo witnessed the extraordinary collision of two neutron stars, which occurred 130 million light-years away from us. This collision gave rise to a kilonova, dispersing gold and other heavy elements into space.
The event was immediately reported to dozens of telescopes on Earth and in space, which were able to capture the electromagnetic signals generated by the same event — from high-energy gamma rays to low-energy radio waves.
This observation marked a historic milestone: the birth of multimessenger astronomy — a new approach to exploring the universe that allows scientists to study the same astrophysical event through different cosmic messengers, each carrying distinct and often complementary information.

The search for further neutron star collisions remains one of today’s most promising frontiers, and the LIGO-Virgo-KAGRA interferometer network is at the core of an alert system that enables electromagnetic telescopes to scan the skies for signs of a new potential kilonova.

The Search for Gravitational Waves Today

Today, the global gravitational wave detector network (LVK), made up of the LIGO interferometers, Virgo, and KAGRA in Japan (which joined in 2020), operates in a coordinated manner and observes about one black hole merger every three days.
Since September 14, 2015, a total of around 300 black hole mergers have been observed. Some of these have been confirmed, while others are still under further analysis. Of these, about 80 were detected up to 2020, and about 230 since June 2023, during the current observation run — more than double the number recorded during the first three runs.
This significant increase in the number of detected events over the past decade is due to various improvements made to the detectors, some of which employ cutting-edge quantum precision engineering.

Artistic illustration of black holes (© LIGO)

Gravitational wave interferometers are instruments of extraordinary precision: the spacetime distortions caused by gravitational waves are incredibly tiny.
To detect them, LIGO, Virgo, and KAGRA must measure changes in spacetime smaller than one ten-thousandth the size of a proton that is, about 700 trillion times smaller than the thickness of a human hair.
The most recent detection, the signal GW250114, was announced few days ago, on September 10, 2025.
The event, a collision between two black holes approximately 1.3 billion light-years away, with masses between 30 and 40 times that of our Sun is similar to the first signal observed in 2015. However, thanks to ten years of technological advances that have significantly reduced instrumental noise, the GW250114 signal is much clearer and richer in information.

Rendering of Einstein Telescope

The Future of Gravitational Wave Research

Looking to the future, the gravitational wave scientific community is already working on next-generation detectors.
The European project, called the Einstein Telescope (ET), envisions the construction of one or two giant underground interferometers with arms more than 10 kilometers long, while the U.S. project, named Cosmic Explorer, would be similar to the current LIGO detectors but with 40-kilometer-long arms.
Observatories of this scale would allow us to detect the earliest black hole mergers in the universe, taking us back to the distant past of our cosmos and forward into its future, helping us understand what its ultimate fate might be.

The Einstein Telescope is thus the major research infrastructure for the next-generation gravitational wave detector to be built in Europe. It is a world-class scientific and technological endeavor, and Italy is a candidate to host it in Sardinia, in the area of the disused Sos Enattos mine, in the province of Nuoro.
Sardinia’s candidacy is strongly supported by the Italian government, the Ministry of University and Research, the national scientific community, and civil society.
ET is considered a flagship project at the international level and has been included in the 2021 Roadmap of the European Strategy Forum on Research Infrastructures (ESFRI).
The project involves building a large underground facility to host the gravitational wave detector at a depth of 100 to 300 meters, to ensure it remains in a condition of “silence”, isolated from all sources of noise, both natural and human-made, that could interfere with measurements.
Its construction will represent a major scientific, technological, and engineering challenge, in which Italy will play a leading role.
website: https://www.einstein-telescope.it/en/home-en/