In 1918, a pair of Austrian mathematicians named Josef Lense and Hans Thirring were thinking through the implications of Albert Einstein’s recently published general theory of relativity. If the fabric of space could be warped by gravity, they realized, it meant that rapidly spinning objects might actually drag the spacetime continuum around them as they rotate.
A century later, scientists have now witnessed this effect, known as Lense-Thirring frame-dragging, happening in a dramatic star system called PSR J1141–6545, according to a study published on Thursday in Science.
“This is the first evidence of frame-dragging in a binary star system,” said lead author Vivek Venkatraman Krishnan, a physicist at the Max Planck Institute for Radio Astronomy, in an email. “These are systems where there are two stars going around each other, unlike our Sun which is solitary.”
Astronomers discovered PSR J1141–6545 in the 1990s using Parkes radio telescope in Australia, and rapidly recognized that it was a useful natural laboratory for testing general relativity. While the theory predicts that all spinning objects drag spacetime around them, frame-dragging is far more detectable around more massive bodies that are spinning incredibly fast.
The system contains a pulsar and a white dwarf, two different types of dead star. The white dwarf is rotating incredibly fast due to past interactions with its companion, while the ultra-dense pulsar acts as a sort of gigantic "cosmic clock" that scientists can use to measure the frame drag of spacetime as the white dwarf spins.
“The rotation period of our Sun is about 25 days, which is too slow to cause a measurable drag,” Venkatraman Krishnan explained. “However, stars such as black holes, neutron stars, and white dwarfs—if sufficiently massive and fast-spinning in their own right—might provide a measurable effect.”
PSR J1141-6545 is particularly unique because the white dwarf in the system formed before the pulsar, which is a reversal of the normal sequence for these binaries. The star that created the pulsar was on its deathbed about a million years ago, but before it exploded into its current super-dense form, it shed much of its outer material.
Some that star stuff was dumped onto the white dwarf, which turbocharged its spin to a period of about three minutes, as opposed to the hour-scale day of more typical white dwarfs.
Fortuitously, the white dwarf’s companion emits precisely timed pulses of light—thus, the term pulsar—which is what makes these objects useful cosmic clocks in space. Over the past 20 years, astronomers have timed pulses from PSR J1141-6545 down to a tiny fraction of a second. That enabled them to witness a gradual drift in the system’s orbital plane of 0.0004 degrees per year, which this study confirmed is due to frame-dragging generated by the dizzying spin of the white dwarf.
“The reason we could do this is that there is a pulsar in the system,” Venkatraman Krishnan said. “Pulsars have extreme rotational stability and when one of their poles faces the Earth, they send a pulse to us for every rotation. This can be used to map the orbit of the pulsar with very high precision—something that is just not possible with other stars.”
While weak frame-dragging has been observed around our own planet using extremely sensitive satellites, this exotic binary system “induces frame-dragging that is 100 million times stronger than that of the Earth,” according to Venkatraman Krishnan.
The team hopes that its observation will spark other searches for extreme frame-dragging in the universe. This hunt will be bolstered by the next generation of radio observatories, such as the MeerKAT telescope in South Africa.
“The Southern Hemisphere has the richest portion of the Galactic plane of our Milky Way galaxy,” Venkatraman Krishnan said. “This new MeerKAT telescope has opened up several avenues for finding and observing other exotic binary systems” that can help scientists “understand fundamental physics.”