A century after it was theorized, astronomers detected the effects of the Lense-Thirring precession – a drag effect of relativistic references – on the movement of a binary star system, composed of a white dwarf and a pulsar.
Vivek Krishnan and colleagues from four countries analyzed twenty years of observational data from the binary to finally confirm this prediction, made by Einstein’s general theory of relativity.
Artistic representation of the “reference drag”: two stars orbiting each other twisting space and time
When a massive object spins, general relativity predicts that it pulls space-time around it, a phenomenon known as frame drag.
This phenomenon causes the precession of the orbital movement of gravitationally coupled objects, such as the two bodies of a binary system – precession is the change in the axis of rotation of an object induced by another star, a very subtle gyroscopic effect, but one that can be imagined like a clumsy top that threatens to fall.
Although the trail of references has already been detected by artificial satellite experiments in the Earth’s gravitational field, in these cases the effect is tremendously small and difficult to measure. More massive objects, such as white dwarfs or neutron stars, offer a better opportunity to observe the phenomenon under much more intense gravitational fields.
Artistic representation of a rapidly rotating neutron star and a white dwarf dragging the fabric of space-time around its orbit.
Precession
Vivek Krishnan and his colleagues observed PSR J1141-6545, a young pulsar spinning rapidly in a tight orbit around a huge white dwarf.The pulsar is located 10,000 to 25,000 light-years from Earth in the constellation Musca (the fly), which is near the famous Southern Cross constellation.
A pulsar is a fast-spinning neutron star that emits radio waves along its magnetic poles. (Neutron stars are corpses of stars that died in catastrophic explosions known as supernovas; the gravity of these remnants is powerful enough to crush protons together with electrons to form neutrons.)
PSR J1141-6545 circles a white dwarf with a mass about the same as the sun’s. White dwarfs are the superdense Earth-size cores of dead stars that are left behind after average-size stars have exhausted their fuel and shed their outer layers. Our sun will end up as a white dwarf one day, as will more than 90% of all stars in our galaxy.
The pulsar orbits the white dwarf in a tight, fast orbit less than 5 hours long, hurtling through space at about 620,000 mph (1 million km/h), with a maximum separation between the stars barely larger than the size of our sun,
They measured the arrival times of the pulses – a pulsar flashes as if it were a cosmic beacon – with an accuracy of 100 microseconds, over a period of almost twenty years, which allowed them to identify a long-term deviation in orbital parameters.
After eliminating other possible causes of this orbital drift, the team concluded that it is the result of the Lense-Thirring precession (Josef Lense [1890-1985] and Hans Thirring [1888-1976]) due to the rapid rotation of the white dwarf’s companion.
These results confirm the prediction of general relativity and allowed the authors to improve the accuracy of the calculations of the speed of rotation of the white dwarf.
