Gravitational Waves Were Discovered 10 Years Ago – Here’s How They Changed Everything

Ten years ago, on September 14, 2015, at 9:50 am UTC, our planet was compressed and then stretched by a tiny fraction of a nanometer for a handful of milliseconds. Earth has experienced this since its birth, 4.5 billion years ago; gravitational waves have existed since the beginning of the universe. This one, though, was special. For the first time, the hominins that now occupy every landmass on its surface saw it happen.

The discovery of gravitational waves started long before that historic September day, but it is in that moment that the astronomical world changed forever. GW150914, as the event was called, was caused by two black holes colliding. One was 36 times the mass of our Sun, the other 29 times, and they merged into a single 62 solar mass black hole.

As the two objects spiraled and merged, they released an incredible amount of energy. In the last few milliseconds, the power output of the event was more than all the light emitted by all the stars in the visible universe. Those ripples in space-time traveled for 1.4 billion light-years to us.

On Earth, the two detectors that make up LIGO (Laser Interferometer Gravitational-Wave Observatory), based in Washington State and Louisiana in the US, were observing. They are L-shaped, and each arm is 4 kilometers long. A laser is sent down the vacuumed arms and bounced off a mirror, designed to be as still and as unaffected by the rest of the world as possible. If no ripples in space-time are detected across those arms, the lasers will come back together and cancel each other out. That morning, both detectors registered a signal. Astronomy and physics would never be the same again.

Check The Tick-Tocks Of The Cosmic Clocks

The idea that gravitational waves were a thing came from Einstein’s theory of general relativity, published in 1915. This places a full century between the first theoretical idea of them and the first direct observation of them. For about half of that time, nobody believed them to actually exist.

“Einstein didn’t believe that the waves were actually natural and could exist in nature. They were thought of more as a mathematical artifact,” Professor Vicky Kalogera, from Northwestern University, told IFLScience. Kalogera is one of LIGO’s leading astrophysicists. “For the second half of that century, people got convinced that these things may exist in nature, and efforts to actually detect them started developing over many decades.”

The measurements we are making now remain the most precise measurements humans have ever achieved in any field of science and engineering.

Prof Vicky Kalogera

Crucial to that effort was the discovery of the Hulse-Taylor Pulsar. Pulsars are a special type of neutron star that emit pulses of radio waves as they rotate. Some of them rotate hundreds of times a second, making them exceptional cosmic clocks. But there is something special about the Hulse-Taylor Pulsar. The pulsar is the first ever binary pulsar discovered, and by measuring the beat of the clock, researchers discovered that the two objects are getting closer.

Their orbit decays over time in precise agreement with the emission of gravitational waves. We could not see them, but their effect was there. The first iteration of LIGO operated from 2002 to 2010. In Europe, a slightly smaller interferometer called Virgo, based in Italy, operated between 2003 and 2011. Neither was sensitive enough to detect the elusive waves. All three of these observatories underwent major upgrades in the early 2010s, pushing not just what the instruments can do, but getting to the very limits of precision measurement in all scientific fields.

The Unparalleled Precision Of Quantum Stillness

The LIGO and Virgo facilities are sensitive to compact objects, such as neutron stars or stellar mass black holes, spiraling and merging together. There are plenty of other sources of gravitational waves in the universe. Some are in different frequencies so can’t be discovered by these instruments, others are just difficult to see among the noise, but should be there. The approach seems simple, but it is not.

“The way the gravitational waves manifest themselves on Earth is they change the lengths of our arms as measured by light. That’s kind of amazing,” Professor Joseph Giaime, Director of the LIGO Livingston Observatory, told IFLScience.

                          

The change in length is 10-19 meters, which is about 1 billion times smaller than the diameter of a hydrogen atom. This is how small we are talking about. The mirrors that reflect the laser are held by superattenuators, which can reduce the shaking of the planet by one trillion times. They are so sensitive that researchers were able to measure quantum fluctuation on them – the first for a macroscopic object. They were also placed in an almost quantum standstill.

“The measurements we are making now remain the most precise measurements humans have ever achieved in any field of science and engineering,” Professor Kalogera told IFLScience. “It is beyond what anybody could ever believe was possible. If you’re going to identify a breakthrough, that is an unimaginable breakthrough.”

Do You Hear The Universe Sing?

By a complete fluke of nature, the frequencies of gravitational waves seen by LIGO and Virgo are in the same range as the frequency of audible sounds for humans. We can’t hear the gravitational waves, but this easy translation made it possible to turn the gravitational wave signal into something audible: the chirp. The first chirp, in 2015, became iconic. I even had it as my phone ringtone before it went into never-not-on-silent mode.

“The kinds of experiences we went through with the first one were never repeated with any other. The intensity of that feeling of discovery was unique,” Professor Kalogera told IFLScience. “That’s a favorite beyond words.”

The announcement came five months later. Time required to make sure, beyond any doubt, that they had indeed found a genuine signal. There have been previous claims, and the LIGO-Virgo collaboration did not want to be the physicists who cried wolf. Even in the collaboration, there was skepticism.

“Like many others, I simply didn’t believe it. The reason is that many of us thought it was a test: an ‘injection.’ We used to do many tests where we inject fake signals into the data of our detector,” Professor Pia Astone, from La Sapienza University, told IFLScience during our interview for CURIOUS Live last year. “And then in the end, we had a meeting and the spokesperson of the collaboration said: No, this is not a test.”

                           

The data was thoroughly tested, in multiple ways, against mundane and even sinister possibilities.

It was just gravity wave people who saw [GW150914]. But just a couple of years later, we saw a pair of neutron stars that merged and lit up in all manner of ways: that changed astronomy forever, really. We are now part of the multi-messenger team!

Prof Joseph Giaime

“The second step was even more complicated. We thought: ‘OK, but is it possible that someone cheated? In the sense that they introduced the signals having in mind to do something bad,” Professor Astone explained. “And so we got together a group of people. They were charged to really try to see whether it was possible to enter the labs and inject a signal without leaving any clues. After one week, they gave us a report, and the conclusion was: It is very difficult, but not impossible.”

The paranoia seems almost an overreaction, 10 years and multiple detections after the fact. But at the time, the team did not want to leave any stone unturned. It was the beginning of an incredible change, but it needed a little bit more to redefine astronomy as we know it.

“It was just gravity wave people who saw [GW150914],” Professor Giaime told IFLScience. “But just a couple of years later, we saw a pair of neutron stars that merged and lit up in all manner of ways: that changed astronomy forever, really. We are now part of the multi-messenger team!”

The Paradigm Shift

Multi-messenger astronomy is the name of this revolution we are experiencing. We might not fully appreciate its potential yet, but it is on par with Galileo’s pointing the telescope at the heavens for the first time or Henrietta Swann Leavitt working out the relation that will be used to measure cosmic distances.

For centuries, astronomy was all about what humans could see with their naked eyes. Then came photographic plates, but still in visible light. In the last century, we have incorporated all the possible wavelengths of light. Now, we are adding non-light to this astronomical arsenal: neutrinos and gravitational waves.

GW170817 was a neutron star collision in galaxy NGC 4993, located 130 million light-years from Earth. It was the strongest gravitational signal observed at the time, and it produced a gamma-ray burst. It confirmed a bunch of theories about the universe and challenged others. It led to the production of dozens of papers in the announcement, with 3,500 astronomers working on them.

The science was revolutionary, but it is also a very human story. It involved a large fraction of all working astronomers in the world. A sad but statistically natural fact, three scientists on those papers passed away between the discovery and the publication two months later. 

It was also a time of exciting frenzy in the field. While details were strictly embargoed, the epochal discovery was practically an open secret. In a way, it demonstrated that astronomers could never keep the discovery of alien life a secret if they had to. Scientists can’t wait to tell you what they have found.

                           

“The first detections were not the destination. The first detection was like jumping on the train for a long trip because we knew if you can detect one, there should be many more,” Professor Kalogera told IFLScience.

Gravitational waves were now a thing humanity could observe. With the genie truly out of the bottle, the only way was up.

A Very Noisy Universe

Ten years on, the LIGO and Virgo observatories have a fourth sibling, KAGRA in Japan, and they are currently in the midst of the fourth run (O4). In the first three runs alone, 90 events were recorded. The first data from the first nine months of O4, from May 2023 to January 2024, reports 128 confirmed events. It is expected that the full run, which will continue until November 18, 2025, will easily pass the 400 detection and maybe reach 500. In a single decade, we have gone from seeing single events every few months to having one every other day.

In 10 years, not only have we made the first detection with our first observatories, we’ve gone to making hundreds of these.

Dr Daniel Williams

And it’s not just the number of events that have grown. The papers describing the initial O4 data are behemoths. 

“These papers have got so large that the introduction has its own paper now the methods section has its own paper, and the results section has its own paper. In total, this is like 130 pages worth of paper,” Dr Daniel Williams, from the University of Glasgow, who worked on the O4 first nine months report, told IFLScience. 

Dr Williams serendipitously joined the collaboration as a Ph.D. student on September 14, 2015. Together, we discussed some of the most intriguing events in the last run, such as GW230814, which is the strongest gravitational-wave observation to date.

Events such as those that will allow researchers to precisely test Einstein’s general relativity, to see if there are discrepancies between theory and observation. Once there are enough clear signals, gravitational waves would allow an independent way to measure the expansion rate of the universe, currently a vexing problem in cosmology: a crisis known as the Hubble Tension. Things have changed in a decade, and they are going to change a lot more.

“In 10 years, not only have we made the first detection with our first observatories, we’ve gone to making hundreds of these,” Dr Williams told IFLScience. “Soon, we’ll be starting to be able to measure things like the expansion rate of the universe with the detections. I don’t know what other circumstance there is where you can be there and see a field developing not only so fast but so broadly.”

The Unprecedented View Of The Unknown 

What does the future hold for gravitational wave observations? It is bad form to try to make broad sweeping predictions, but the LIGO-Virgo-KAGRA Collaboration is working on the plans for the current decade and beyond. When O4 ends, the LIGO will get some upgrades before the next run, which is expected to be conducted for several years, and which will hopefully push the number of detections past 1,000.

“There are a number of improvements that are kind of on the docket.  One set of them is for a run that we call O5, the fifth observational run, which will happen over the next four or five years,” Professor Giaime told IFLScience.

There are not just physical upgrades for the detectors; there are analytical tools that will help, too. Researchers at Caltech and Gran Sasso Science Institute in Italy teamed up with Google DeepMind to develop a machine learning algorithm, commonly referred to as AI, to get clearer signals in the data. The approach is called Deep Loop Shaping.

“I am looking forward to seeing Deep Loop Shaping deployed in daily operation on LIGO and future gravitational wave observatories. And, I am particularly looking forward to the scientific impact that this new method will have. To all the mind-blowing new insights about this universe we live [in] that we will gain thanks to this method,” Jonas Buchli, Research Scientist at Google DeepMind, told IFLScience.

Research Engineer Brendan Tracey added, “I hope that Deep Loop Shaping can remove the control system as a key source of noise, both to provide direct benefit, but also so that improvements to other systems will benefit LIGO sensitivity, rather than being drowned out by control noise.”

Within a decade, the quantity and quality of the data is expected to have massively increased. All going well, by 2035, we will be entering a whole new era of gravitational wave astronomy thanks to the European Space Agency’s LISA mission, set to be the largest ever space-based gravitational wave observatory.

“LISA is meant to launch in 2035; fingers crossed, there are no more delays. That pushes us into a completely new part of the gravitational wave spectrum,” Dr Williams stated.

LISA will push us into a whole new range of gravitational waves.

Image Credit: ESA

LISA is a system of three spacecraft that can travel in formation over 1 million kilometers (600,000 miles) with lasers connecting them. It will be able to detect white dwarf binaries, binaries between black holes of extremely unequal masses, and the collisions of some supermassive black holes, as well as possibly probe the gravitational signature from the Big Bang.

“Ground-based detectors opened the era of gravitational-wave astronomy, but LISA, the first gravitational-wave observatory in space, will take us to an entirely new frontier,” Dr Nora Lützgendorf, LISA Lead Project Scientist, told IFLScience. “With its million-kilometre arms, LISA will detect the low-frequency waves from merging supermassive black holes, something we have never seen before, and reveal how galaxies and black holes grow across cosmic history.”

On supermassive black holes and more, it will come to help the Pulsar Timing Array. As we have seen, pulsars are excellent clocks, and if you have enough of them and know them very well, you can use them as a massive gravitational waves observatory, one the size of our galaxy. Some preliminary results show extreme promise to deliver on that potential. There are even proposals to turn the Moon into a gravitational wave detector. 

The LIGO, Virgo, and KAGRA observatories have decades more ahead of them. However, the Trump Administration has proposed such a dramatic cut to the national science budget that one of the two LIGO observatories would have to close. Currently, Congress is pushing back on those cuts.

While funding is uncertain, researchers are working on a proposal to massively improve the LIGO detectors, an approach dubbed Sharp ‘#’. India is planning to have LIGO India open by 2030, and next-generation observatories such as the Einstein Telescope, part of the European Gravitational Observatory consortium, and the US’s Cosmic Explorer will be bigger and even more sensitive. They will be needed.

“No area of exploration in science ever stayed with its first instrument,” Professor Kalogera told IFLScience. “We didn’t stay with Hubble’s telescope. I don’t mean the space telescope. I mean the telescope Edwin Hubble used. We will need next-generation gravitational wave detectors.”

We might want to say that the future for gravitational wave research is bright, but given their penchant for acoustic analogies, we can certainly say that a path of success for gravitational wave astronomy is loud and clear.