The Dance that Spins Spacetime %

Vivek Venkatraman Krishnan and his team at the Max Planck Institute for Radio Astronomy in Bonn, Germany, detected relativistic frame dragging while studying a celestial laboratory located thousands of light-years away. The results were published in Science on January 30. Discovered in 1999, the laboratory, in the direction of the Southern Cross constellation, consists of two stellar heavyweights locked in an elaborate orbital dance.

The first is a white dwarf about the size of the Earth that experiences a “year” every five hours. The second is a pulsar called PSR J1141-6545, which is slightly heavier than the white dwarf, and spins around faster than two times per second. Such findings in the universe are extremely rare, and scientists studying general relativity see how Einstein’s “relativistic” assumptions could hold true even in extreme gravitational conditions.

Astronomers used radio telescopes to measure the effects of gravitation on the pulsar’s repeated, beat-like pulses. The “pulsar timing” helped scientists discover that PSR J1141-6545’s rotation rate was being affected by time-dilation, while the white dwarf’s orbit was decaying due to the emissions of gravitational waves.

These findings were in accordance to previous predictions, but the research team successfully detected another Einsteinian quirk: relativistic frame dragging, also known as the Lense-Thirring effect. The effect suggests that a fast-spinning object swirls the fabric of spacetime around it. “Imagine you have a bowl of honey, and you put a golf ball and some food coloring inside it,” says lead study author Vivek Venkatraman Krishnan. “If you twist the golf ball really fast, the honey swirls, too, dragging the food coloring along with it. In this case, the spinning ball is the white dwarf, the honey is spacetime curvature, and the food coloring is the pulsar.”

Researchers have studied the effects of relativistic frame dragging before, but this particular system exhibits frame dragging some 100 million times stronger than what could be studied around Earth. Even then, astronomers initially barely noticed it. In 2015, the timing of PSR J1141-6545’s pulsations revealed a small “drift” in the system’s orbital parameters that initially seemed to defy explanation; at least until Venkatraman allowed the pulsar’s orbital plane to alter its orientation.

“All of a sudden, it was clear the orbit was tumbling in space at a rate never seen before in such systems”, said Matthew Bailes, an astronomer at the Swinburne University of Technology in Australia, who led the intensive monitoring campaign since he first conceived it nearly 20 years ago.

The drift was partially because the pulsar tumbled as it was dragged along in the swirl of spacetime around its white dwarf companion, but this required the white dwarf to be spinning very fast. Based on standard white dwarf-pulsar binary models, such systems normally result in slow-spinning white dwarfs, but For PSR J1141-6545, the opposite must have taken place, with the white dwarf forming first and spinning up by stealing gas from the soon-to-go-supernova pulsar progenitor.

Similar timing studies of other binary systems composed of two pulsars could also reveal relativistic frame dragging, which could, in turn, help pin down those pulsars’ exact size—a crucial measurement that would reveal new information about their mysterious interior, but for now, the quest to test general relativity with ever greater scrutiny continues, with this latest astrophysical case being yet another confirmation of Einstein’s theory.