Evrim Yazgin
Cosmos science journalist
You couldn’t get much more different than the dry, warm subtropical climate of Eswatini than the cold, wet southern island of New Zealand.
But that didn’t bother Sebenele (Sebe) Thwala who left her home continent for New Zealand to peer into the farthest reaches of the cosmos. And her work as a second-year PhD scholarship student at Christchurch’s University of Canterbury (UC) is making waves.
She and her supervisors, Chris Stevens and Jörg Frauendiener, have been modelling how gravitational waves travel across time and space.
“When something dramatic happens in the universe – like 2 black holes merging – it sends out ripples in the fabric of spacetime called gravitational waves. We detect those ripples here on Earth, but we don’t get to see the event directly,” Thwala says.
Gravitational waves were predicted by Albert Einstein’s general theory of relativity. But it wasn’t until 2015 when instruments on Earth were powerful enough to confirm that these spacetime ripples exist.
And, in the 10 years since their discovery, gravitational waves are beginning to tell us all sorts of things about the nature of the universe.
There’s a lot of modelling involved. Astronomers can very rarely tell what the source of gravitational wave signals are. That means that they have to model backwards and forwards in time to come up with a good guess as to what could have caused the ripples measured by instruments on Earth.
Thwala says most simulations can lead to errors because they cut off the faraway parts of spacetime and make rough guesses about what happens there.
“To measure energy and momentum accurately, you actually have to look really far away from where the action is happening basically, at an infinite distance,” she says. “But if we simulate the whole thing properly, from the infinite past to the infinite future, we can avoid mistakes and get a clearer picture. This also helps us understand what happens when a gravitational wave hits a black hole, how much energy the black hole takes in, and how much energy is sent back out as more gravitational waves.”
Her research into developing a computer simulation that goes from the “absolute beginning” to the “absolute end” of a spacetime ripple has been published in the Physical Review Letters (PRL). The journal identified the paper as a “PRL Editors’ Suggestion” – a title given to papers that are particularly important, interesting and well written.
“This is an extraordinary milestone for a second-year PhD student, but to also receive the distinction of Editor’s Suggestion marks this work as truly exceptional,” says Stevens. “It reflects not only groundbreaking scientific insight, but also a level of impact, clarity, and originality that stands out internationally over a broad range of fields in physics.”
Thwala comes after a journey from the other side of the world. She is originally from Eswatini (formerly Swaziland) and completed her Master’s degree in South Africa. At the time, Stevens was also based in South Africa and was looking for students interested in pursuing research in gravitational physics.
The COVID epidemic got in the way initially, but Thwala was able to take up the invitation to do her PhD at UC.
She’s unsure what the future holds for her, saying “I’m being led by where the opportunities are”.
