“After ruling out a range of potential experimental errors, we started to suspect that the interaction between the white dwarf and neutron star was not as simple as had been assumed to date,” concluded Willem van Straten, an astronomer at the Auckland University of Technology, about the detection of Lense-Thirring precession. This relativistic effect first hypothesized a century ago alters the orbit of two compact massive objects in a binary star system. The results of the twenty-year study confirm a prediction of Einstein’s general theory of relativity. When a massive object rotates, general relativity predicts that it pulls the surrounding spacetime around with it, a phenomenon known as frame-dragging.
This phenomenon causes precession of the orbital motion of gravitationally bound objects. While frame-dragging has been detected by satellite experiments in the gravitational field of the rotating Earth, its effect is tremendously small and challenging to measure. More massive objects, such as white dwarfs or neutron stars, provide a better opportunity to observe the phenomenon under much more intense gravitational fields.
Physicist Vivek Venkatraman Krishnan at the Max Planck Institute for Radio Astronomy and colleagues observed PSR J1141-6545, a young pulsar in a tight, fast orbit with a massive white dwarf. They measured the arrival times of the pulses to within 100 microseconds, over a period of nearly twenty years, which allowed them to identify a long-term drift in the orbital parameters.
After eliminating other possible causes of this drift, including other general relativistic effects, Venkatraman Krishnan et al. conclude that it is the result of Lense-Thirring precession due to the rapidly rotating white dwarf companion. The findings confirm the prediction of general relativity and allowed the authors to determine the white dwarf’s rotational speed.
Pulsars are Einstein’s Dream Laboratory
“At first, the stellar pair appeared to exhibit many of the classic effects that Einstein’s theory predicted. We then noticed a gradual change in the orientation of the plane of the orbit,” said lead author Krishnan. “Pulsars are cosmic clocks. Their high rotational stability means that any deviations to the expected arrival time of its pulses is probably due to the pulsar’s motion or due to the electrons and magnetic fields that the pulses encounter. Pulsar timing is a powerful technique where we use atomic clocks at radio telescopes to estimate the arrival time of the pulses from the pulsar to very high precision. The motion of the pulsar in its orbit modulates the arrival time, thereby enabling its measurement.”
“One of the first confirmations of frame-dragging used four gyroscopes in a satellite in orbit around the Earth, but in our system the effects are 100 million times stronger,” said Dr Norbert Wex, Max Planck Institute for Radio Astronomy.
“Pulsars are super clocks in space,” comments Evan Keane, Project Scientist for the Square Kilometer Array (SKA). “Super clocks in strong gravitational fields are Einstein’s dream laboratories. We have been studying one of the most unusual of these in this binary star system. 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. In this case we have seen Lens-Thirring precession, a prediction of General Relativity, for the first time in any stellar system.”
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Science/AAAS and The Australian Research Council Center of Excellence for Gravitational Wave Discovery