According to general relativity, when a massive object rotates, it pulls the inertial frame—the surrounding spacetime—around with it. The strength of that drag is proportional to the object’s intrinsic angular momentum. In 1918, just two years after Albert Einstein published the theory, Austrian mathematicians Joseph Lense and Hans Thirring predicted that in a binary system, the frame dragging would force the orbital motion of two gravitationally bound objects to precess.

Credit: Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

Researchers first measured the effect—albeit exceedingly weak—in 2004 with satellites orbiting Earth (see Physics Today, July 2011, page 14). But binary pulsar systems are the more natural, gravitationally intense setting in which to explore the phenomenon. Nearly 20 years ago, researchers led by Matthew Bailes of Australia’s Swinburne University spotted one 10 000 light-years from Earth and started tracking it using Australia’s Parkes 64 m radio telescope. The pulsar system consists of a 20-km-diameter neutron star locked in a tight, five-hour orbit around a spinning white dwarf the size of Earth, as depicted in the figure.

Pulsars emit a steady beat of radio waves as they spin. By recording the arrival times of those beats, the researchers were able to discern the pulsar’s motion relative to Earth. They have now published an account of the work. Vivek Venkatraman Krishnan, lead author and a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Germany, had nearly two decades of data to process. He and his colleagues noticed a gradual and slight change in the orientation of the plane of the orbit—a shift of just 1.7 arc seconds per year.

The results are the first demonstration of frame dragging measured in any astrophysical setting. But making sense of the data was complicated. Of the observed tilt in orbital inclination, Krishnan, Bailes, and coworkers estimate that more than 80% comes from a combination of a (Newtonian) quadrupole moment induced by the rotation of the white dwarf and the relativisitic Lense–Thirring frame dragging. Although the researchers cannot yet disentangle the two contributions, they find statistically that regardless of geometry, frame dragging is always at play in the system and is the dominant term if the spin period of the white dwarf exceeds 270 seconds. (V. Venkatraman Krishnan et al., Science 367, 577, 2020.)