Back in February, the world watched in awe as physicists confirmed the existence of gravitational waves—the elusive ripples in spacetime first predicted by Albert Einstein over a century ago.
The discovery, made by the Laser Interferometer Gravitational-Wave Observatory (LIGO), was groundbreaking.
Not only did it provide direct evidence of gravitational waves, but it also confirmed that these waves were generated by one of the most extreme cosmic events imaginable: the collision of two black holes.
Since then, scientists have detected a second gravitational wave event, proving that these phenomena aren’t just rare, once-in-a-lifetime occurrences.
But now, researchers are taking it a step further. They believe these waves might permanently alter the fabric of space itself.
Even more astonishing?
We may have found a way to detect this permanent change—something known as gravitational-wave memory.
The Unseen Fingerprint of Gravitational Waves
“For so many years, people were simply concentrating on making that first detection of gravitational waves,” explains Paul Lasky, a physicist from Monash University in Australia.
“Once that first detection happened, our minds have become focused on the vast potential of this new field.”
But what exactly is gravitational-wave memory?
To understand it, let’s first revisit how gravitational waves work.
Gravitational waves are like ripples on a pond—they occur whenever a massive object moves, distorting the very fabric of space and time as it travels.
However, these distortions are almost imperceptibly small. The waves detected by LIGO in February were so minuscule that they measured just a billionth the diameter of an atom.
Despite their size, these waves carry vast amounts of energy.
When two black holes spiral toward each other and merge, the collision releases more energy than all the stars in the universe combined, sending out a ripple effect that extends across the cosmos.
But here’s the twist: scientists now believe these waves don’t just pass through space—they might leave a lasting imprint.
The Reality of Gravitational-Wave Memory
The idea of gravitational-wave memory isn’t new.
It was first proposed in 1974 by Russian physicists, but at the time, gravitational waves themselves were still theoretical, so the idea largely faded into obscurity.
But with LIGO’s recent detections, Lasky and his team have revived the theory, suggesting that gravitational waves might leave permanent scars in spacetime.
To visualize this effect, imagine two astronauts floating side by side near a binary black hole system—two black holes locked in an orbital dance.
Let’s say these astronauts start exactly 10 meters apart.
As the black holes spiral toward each other, their motion sends out gravitational waves, causing the astronauts’ distance to fluctuate slightly.
When the black holes finally merge, the waves stop—but the astronauts are no longer exactly 10 meters apart.
Their separation has permanently changed, meaning spacetime itself has been subtly altered.
This effect is what physicists call gravitational-wave memory.
Can We Actually Detect This Effect?
Here’s where things get tricky.
If gravitational waves are already difficult to detect, then gravitational-wave memory is even more elusive.
Lasky explains that these permanent distortions are expected to be 10 to 100 times weaker than the gravitational waves themselves.
For years, scientists believed LIGO would never be sensitive enough to detect this memory effect.
But Lasky and his colleagues have devised a workaround: volume detection.
“Our work has shown that the combination of all these mergers will enable us to measure the memory effect over time,” says Lasky.
“The key is being able to stack the signals from all of the events in a clever way.”
Instead of looking for gravitational-wave memory in a single event, researchers suggest that LIGO could combine data from multiple black hole mergers.
By stacking the signals together, they could amplify the memory effect enough to finally detect it.
Their calculations suggest that LIGO could confirm the presence of gravitational-wave memory after detecting 35 to 90 major black hole mergers.
And with LIGO’s sensitivity increasing, this could happen sooner than we think.
Why This Matters—And What It Could Reveal About Black Holes
Detecting gravitational-wave memory isn’t just a milestone in physics—it could help solve one of the biggest mysteries in modern science: the black hole information paradox.
Physicist Stephen Hawking spent decades pondering this paradox.
According to conventional physics, nothing, not even light, can escape a black hole’s event horizon.
But quantum mechanics tells us that information can never be truly destroyed.
So what happens to the information that falls into a black hole?
Hawking proposed a potential solution: ‘soft hairs’—zero-energy electromagnetic and gravitational radiation that might carry information out of a black hole.
If we can detect gravitational-wave memory, it could directly confirm the existence of these soft hairs, providing a clue about how information might escape a black hole.
LIGO co-founder Kip Thorne is among those impressed by this approach.
“This is a very clever way of measuring gravitational-wave memory and exploring it observationally,” he says.
“I never thought it’d be possible with LIGO.”
The Future of Gravitational Wave Astronomy
LIGO alone may not be able to confirm gravitational-wave memory just yet, but a new observatory may soon change that.
LISA (Laser Interferometer Space Antenna), a space-based gravitational wave detector set to launch in 2029, will be far more sensitive than anything on Earth.
With LISA, we may finally get a clear view of these lasting distortions in spacetime.
If confirmed, gravitational-wave memory would be one of the most profound discoveries in modern physics, rewriting our understanding of spacetime, black holes, and the fundamental nature of the universe itself.
For now, though, we wait.
But with LIGO’s continued discoveries and LISA’s upcoming launch, we might not have to wait another 100 years for answers.
One thing is certain: The universe has been sending us signals all along. Now, we just have to learn how to listen.
