Buckle up: gravitational waves
When Dr Lap-Ming Lin was in high school, he devoured popular science books on cosmology, the Big Bang, and black holes, “the fun stuff,” he says. Now a lecturer at the Chinese University of Hong Kong’s Department of Physics, his research seeks to understand how matter behaves in extreme environments, currently focusing on gravitational wave signals from compact stars. Asked what this means for the earthbound among us, his response is a simple, “Buckle up!”
Studying astrophysics is like hopscotching across a river.
Gravitational waves are energy emitted by all cosmological bodies as their mass interacts with the fabric of space-time. However, they are nearly undetectable given astronomical distances and time, and currently observed only from the strongest sources, such as colliding neutron stars or black holes. These waves penetrate regions of space where other electromagnetic rays cannot, and are therefore a coveted tool for scoping the inner workings of the universe. As is frequently the case in astrophysics, to learn about one thing, one must start somewhere else, “like hopscotching across a river,” Lin says.
For Lin, one stepping stone to gravitational waves is the study of neutron stars, the collapsed core of a large star which become one of the smallest, densest stellar objects in the universe with a radius of just 10 kilometres across but twice the mass of the sun. Working to see whispers of astrophysical activity, hedging the bets on something extreme gives scientists a clearer view of fundamental particles and microphysics at work.
Lin and his team study the properties and dynamics of neutron stars, hoping to use hints from the signals they dispatch. “We know little about the internal physics of neutron stars, and experiments for such high-density environments cannot be conducted on Earth,” Lin says, “So we look at only the neutron stars which can be somewhat easily detected, and only at the characteristics which will take us where we want to go.” Most commonly, this means studying pulsars (single, rapidly rotating neutron stars) or binary neutron stars (a pair of stars orbiting each other) which give off bursts of gamma ray or radio emissions. The energy given off as the orbit of a particular binary neutron star system slowly decreased gave the first indirect evidence of the existence of gravitational waves in the 1970s.
Among the most ambitious experiments chasing the unknown is the Laser Interferometer Gravitational-Wave Observatory a large-scale observatory for several decades and spent billions of dollars in the making. Built off early experiments testing Einstein’s theory of relativity, its aim is to detect gravitational waves and develop wave observations as an astronomical tool.
After a massive overhaul to quadruple its observational sensitivity, its researchers confirmed the detection of gravitational waves for the first time in 2016, caused by two black holes merging 1.3 billion light years from Earth. “It took us 100 years from Einstein’s hypothesis to prove it, but now this opens a whole new world of possibilities for models in gravity, nuclear, and particle physics, and our universe. Researchers will reach its peak sensitivity around 2021, so who knows what we’ll be able to see then,” Lin muses.
While general relativity is certainly the most popular theory and explains many phenomena, it is not theoretically perfect, particularly in modern cosmology and quantum mechanics. Testing gravity theory is possible by calculating gravitational waves from binary black hole and neutron star systems. This is then compared against the observed data, which theoretically should match, but the sensitivity of tools to gather the latter have yet to catch up for a definitive answer. “The evidence of gravitational waves agrees with general relativity theory so far, but more sensitive detectors in the future might help us differentiate between what the theory says we should be seeing and what we actually observe.”
Astrophysics is a more linear career path than other scientific fields, but the possibilities for research and collaboration are vast. Most research involves combining observations of one event from different angles to create an intricate composite understanding of the properties. Gravitational waves, which give the clearest observation of events and bodies that are undetectable by other means, are therefore a valuable addition. Lin has worked on neutron stars and gravitational waves since graduate school, including theoretical and computational work, using them to test extreme nuclear and particle physics, and working with colleagues from other branches of physics on bigger questions. “Because so much of what we do is unseen and hypothetical, we have to get really creative with the tools we do have, whether that’s scientific principles, telescopes, our eyes, even our universe—is a gamma ray just a gamma ray, or can it tell me something more?” Lin explains.
Lin cannot suppress a grin as he explains how detection of gravitational waves will move astrophysics to a new frontier, allowing scientists to see things that have never been visible before. “It’s like a magic window,” he remarks, “We’re quite sure that we’ll find black holes and neutron stars, but think of the theories we can prove and other phenomena that we haven’t even dream of yet.” Indeed, “we don’t know what will happen next, but we’ll give it our all trying to find out” seems to be the unofficial motto of physicists, but as Lin says, “We have to help science fiction writers find some new material so they inspire the future generation of young researchers!”
Dr Lap-Ming Lin completed his BSc and MPhil degrees at the Chinese University of Hong Kong and his doctoral research at Washington University in St. Louis, Missouri. He was a Croucher postdoctoral fellow at the Laboratory of the Universe and its Theories at the Observatory of Paris-Meudon in 2004 and is currently a lecturer at the Chinese University of Hong Kong. His research interests include relativistic astrophysics, gravitational wave sources, and computational general relativity.
To view Dr Lin’s Croucher profile, please click here.