For the past two decades, astronomers have been testing Einstein’s general theory of relativity using an exquisite celestial laboratory located thousands of light-years away, in the direction of the Southern Cross constellation.
Discovered in 1999, this laboratory consists of two stellar heavyweights locked in an elaborate orbital dance: a white dwarf—a slowly cooling Earth-sized cinder left behind by an evaporating star—twirling around a pulsar called PSR J1141-6545—a rapidly spinning, ultradense, city-sized neutron star produced by a cataclysmic supernova explosion. Each packs a bit more than the equivalent mass of our entire sun into its compact frame. Such couplings are unlikely, albeit relatively unremarkable throughout the galaxy, but this one is particularly special: the white dwarf resides in an exceedingly close orbit, experiencing a “year” of about five hours and speeds of up to a million kilometers per hour as it whips around its slightly heavier companion, which itself spins around faster than two times per second.
For scientists studying general relativity, the system is a literal match made in heaven. Various bizarre phenomena arising from Albert Einstein’s most successful theory rear their “relativistic” head in its extreme gravitational conditions. And astronomers using radio telescopes can precisely measure them, thanks to minuscule deviations those effects imprint on the pulsar’s metronomelike pulses. Such “pulsar timing” measurements show, for instance, that time dilation is distorting PSR J1141-6545’s apparent rotation rate and that the white dwarf’s orbit is gradually decaying because of the copious emission of gravitational waves—all in accordance with predictions. Now a research team has successfully detected another Einsteinian quirk: relativistic frame dragging—also known as the Lense-Thirring effect, after the theorists who predicted it more than a century ago—in which a fast-spinning object swirls the fabric of spacetime around it. The results were published in Science on January 30.
“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 of the Max Planck Institute for Radio Astronomy in Bonn, Germany. “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 detected relativistic frame dragging before, measuring its extremely small influence on satellite-borne experiments moving through Earth’s gravitational field as our planet spins. But this is the first time its subtle effect has been seen so clearly elsewhere in the cosmos—in this exotic system, the frame dragging is some 100 million times stronger than the effect would be around Earth. Even so, at first, astronomers barely noticed it. Charted across nearly two decades of observations using the Parkes Observatory and UTMOST radio telescopes in Australia, 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. Even after including all of the system’s previously detected relativistic effects that could tweak the motions of the white dwarf and pulsar, Venkatraman Krishnan and his colleagues failed to account for the drift. “We got really excited, because that meant either something was wrong with the data or our analysis—or it was signaling new physics beyond general relativity,” Venkatraman Krishnan says.
In this case—as in all others before it—Einstein’s theory ultimately won out over speculations about breakthrough physics. “[Venkatraman Krishnan] had a eureka moment when he allowed [the pulsar’s] orbital plane to alter its orientation—which we had previously assumed was fixed in space,” says Matthew Bailes, an astronomer at the Swinburne University of Technology in Australia, who has led the intensive monitoring campaign since he first conceived it nearly 20 years ago. “All of a sudden, it was clear the orbit was tumbling in space at a rate never seen before in such systems.”
That tumbling—technically called orbital precession—was from frame dragging (combined with the well-known classical effect of spin-induced deviations in the pulsar’s near-perfect spherical shape that slightly altered its gravitational field). In other words, the drift was partially because of the pulsar tumbling as it was dragged along in the swirl of spacetime surrounding its white dwarf companion.
But this scenario would require the white dwarf to be spinning very fast—remember the golf ball in honey— probably more than once per minute. That speed would be faster than could be explained in models of standard white dwarf–pulsar binary formation. Such systems begin as two normal stars. One of them first explodes as a supernova to form a pulsar, which then spins up to very high rotation rates by siphoning gas from its companion, transforming the companion into a slowly spinning white dwarf. 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. In a series of complex calculations culminating in 70 million simulations of binary star evolution, study co-author Thomas Tauris of Aarhus University in Denmark examined this process, finding a narrow-but-plausible range of masses and orbits for the two original stars that would result in the PSR J1141-6545 system.
“When the observers contacted me and asked if I could try to model this system, I was immediately hooked,” Tauris says. “I am extremely excited that testing Einstein’s theory of gravity goes hand in hand with state-of-the-art binary star modeling.”
As labyrinthine and circumstantial as this analysis may seem, it convincingly dovetails with earlier work done when PSR J1141-6545’s discoverers first sought to explain the system’s bizarre characteristics. The new study’s conclusions are “quite compelling,” says Victoria Kaspi, a McGill University astronomer, who was not involved with the paper and who discovered the system in 1999 using the Parkes radio telescope. “The pulsar-timing observations and data analysis are expertly done, and the team has also coupled that work with interesting binary-evolution simulations. Moreover they have found a nice confirmation of the [formation] scenario that we invoked way back when this unusual system was discovered. This is, of course, very gratifying—it’s nice to see one’s prediction verified!”
In the future, Venkatraman Krishnan says, 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. “There are many theories, but we don’t really know what happens to matter inside a neutron star. The density there is much greater than anything you could ever achieve in a lab. With further measurements [of binary pulsar systems], that’s something we might help deduce.”
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. “It was wonderful to have this come together after two decades of observing,” Bailes says. “Like many results in science, in hindsight, it wasn’t that surprising. But it is beautiful.”