By Perimeter Institute
On February 11, 2016, a “chirp” heard around the world reached Sizheng Ma, an undergraduate physics student in China at the time.
Scientists from the Laser Interferometer Gravitational-wave Observatory (LIGO) played that sound as they announced the first ever detection of gravitational waves, caused by the distant merger of two black holes.
These “ripples” in spacetime, caused by the collisions of massive objects, were predicted by Albert Einstein in 1916. It took a century of technological development to finally detect them.
That changed the course of Ma’s life. He decided to study these fascinating signals, pursuing his PhD at California Institute of Technology (Caltech), which had been involved in LIGO’s discovery. Then he came to Perimeter Institute, where he is currently a postdoctoral researcher in the strong gravity group.
Now, Ma along with his colleagues — Neil Lu , Ornella J. Piccinni, Yanbei Chen and Ling Sun — have made a significant new contribution to our knowledge of these black hole mergers and their aftermath, revealing that gravitational waves carry direct imprints of the remnant object’s event horizon and its properties. The results were published in Nature on June 24.
The paper describes the detection of a distinctive gravitational wave signature that comes from the “frame dragging” of spacetime near the horizon of a binary black hole merger, captured by LIGO on January 14, 2025.
“By measuring this specific feature in a recent gravitational wave event, we were able to touch, in a sense, this strong gravity environment, a very distorted spacetime region,” Ma says.
Although nothing can escape from the centre of a black hole, the space around it is tempestuous. A black hole can generate a vortex that stirs and disrupts the spacetime around it. Imagine a stirring machine, such as a rapidly spinning spindle in a vat of honey, dragging the honey around it. Except in this case, the honey is spacetime itself, being dragged along.
When two such monsters collide, it’s like two hurricanes merging. As the black holes come closer and closer to one another, like ice dancers twirling around each other spinning faster and faster, the twisting of the spacetime becomes stronger.
“The closer they get the stronger the swirling is, and at the end stage, the orbital motion is basically dominated by this frame dragging effect,” Ma explains.
The GW250114 event from 2025 was not too much different from LIGO’s first detection of gravitational waves in 2016. Both involved black holes about 1.3 billion light years away and in both cases, the black holes were around 30 to 40 times the mass of our sun.
But the signal that was picked up in the recent event was dramatically louder and clearer, thanks to ten years of technological advances that have reduced signal noise. That enabled Ma and his colleagues to test their theoretical predictions describing the distinctive wave signature of frame dragging against an actual merger event.
Their theory matched nicely with what was seen in the actual event.
“Prior to this paper, we put out a theoretical paper discussing how to interpret this signal, and then, with this theoretical understanding, we went on to search for this signal from the recent gravitational wave event. We were quite lucky to see it because this event was the loudest to date,” Ma says.
Ma has been fascinated by the theories of Albert Einstein ever since he was a schoolchild growing up in Xi’an, in north central China, home to the famous collection of clay statues known as the Terracotta Army. That inspired a passion for physics and led him to want to study the strong gravity regime of black holes.
What Ma and his colleagues found in the black hole merger signature confirmed Einstein’s theories. “We confirmed once again that general relativity is really accurate. You can see this gravitational wave signal and you can overlay that with the prediction from Einstein and they agree very accurately,” Ma says. “If you think about it, that is amazing.”
What is equally important is that Ma and his colleagues have developed a “tool” for probing these strong gravity regions near the horizon of black hole mergers. Ma says that the theoretical work, which involved how to interpret this signal, was the difficult part.
“You have to know how to extract the physics in the signal, understand the physics that triggers the oscillation,” he says. In step one, the theoretical step, we made the connection between the physics of the merger and the gravitational wave. Then, with this understanding, we could move on to step two, and search for that specific signal. We found it.”
In a few months, Ma will move on to his second postdoctoral position, at Johns Hopkins University.
He recently became one of the winners of the highly competitive NASA Hubble Fellowship Program (NHFP) that enables outstanding postdoctoral scientists to pursue independent research in any area of NASA Astrophysics, using theory, observations, simulations, experimentation, or instrument development.
Ma hopes to continue to improve this tool for understanding the signals of gravitational waves from black hole mergers.
“Right now I am preparing a follow-up paper to lay a solid theoretical foundation. I think we will get to the point where we can show how these signals evolve over time. The first model was like a toy model, but now we can solidify the whole calculation to make everything exact.”
Although Einstein’s general relativity was confirmed in this recent work, many scientists hope to find cracks in the theory that will lead to new physics that can take them beyond Einstein, and better understand how gravity fits with quantum theory.
The horizons of black hole mergers, where gravity is at its strongest, are good candidates for that, and the tool Ma and his colleagues have developed gives scientists a better probe for those regions.