Dense stars spotted twisting and dragging the spacetime continuum
Even 100 years after Einstein formulated his theory of general relativity, new observations keep confirming its predictions. Now, Australian astronomers have found some of the best evidence of one component of the theory – a pair of high-density stars in close orbit appear to be dragging and twisting the fabric of spacetime itself.
The system in question is known as PSR J1141-6545, and it’s made up of two “dead” stars orbiting each other. One is a white dwarf roughly the size of Earth but 300,000 times denser, while the other is a pulsar that crams the equivalent of 100 billion Earth masses into a ball just 20 km (12.4 mi) wide. The pulsar orbits the white dwarf once every five hours, and both are spinning very fast.
Under general relativity, gravity is the result of huge amounts of mass bending the very fabric of spacetime. A few years after Einstein published the theory of general relativity, mathematicians Josef Lense and Hans Thirring proposed the concept of “frame dragging.” Essentially, general relativity suggests that rotating objects should drag spacetime around with them.
Most of the time that effect would be too minor to detect, so it’s remained unconfirmed for over 100 years. But now, the researchers on the new study claim to have found evidence of this long-sought side effect.
It’s not entirely surprising that PSR J1141-6545 would be the site of this breakthrough, given the crazy amounts of mass the system is throwing around. The astronomers have been studying the objects for almost 20 years using the Parkes telescope in Australia, watching how they interact over time. And in the long-term, they noticed a gradual change that could be attributed to frame dragging.
Pulsars are extremely regular, giving off bursts of energy like clockwork. If those pulses begin to arrive at a telescope at different times, it suggests interference somewhere along the path to those signals.
After untangling the complex physics around the system, the team determined that the pulsar signals were drifting slowly out of time over those 20 years. That indicated that the flat plane of its orbit was gradually tilting, which could be attributed to frame dragging (also known as Lens-Thirring precession).
“Treating the periodic pulses of light from the pulsar like the ticks of a clock we can see and disentangle many gravitational effects as they change the orbital configuration, and the arrival time of the clock-tick pulses,” says Evan Keane, an author of the study. “In this case we have seen Lens-Thirring precession, a prediction of general relativity, for the first time in any stellar system.”
Even after 100 years, the theory of general relativity continues to pass tests as its predictions are found to agree with new observations. Just a few years ago, one major aspect was finally confirmed – gravitational waves, which are ripples in spacetime caused by cataclysmic events like black holes merging.
The research was published in the journal Science and the idea is illustrated in an animated video below.