The 16-year study of the only known double pulsar system has made it possible to obtain data that is up to 25 times more precise. General relativity is 99.99% valid
For more than 16 years, an international team of astronomers and physicists has used telescopes around the world to observe a pair of pulsars, the only one discovered so far. The data obtained has been used to test Einstein’s general theory of relativity again but at unprecedented levels of precision. The results not only confirm the accuracy of the theory but also hint at subtle effects that until now had not been taken into account. The impressive work has just been published in ‘Physical Review X’.
Pulsars are a type of neutron star, extremely massive stellar corpses that rapidly rotate on themselves, emitting powerful beams of radiation.
Seen from Earth, they appear to flicker (or pulsate, hence their name) because their beams of radiation sweep across our planet at every turn, like a lighthouse beam, at regular intervals. Variations in the timing of these pulses can show in detail the effects predicted by general relativity. Although physicists have already found many individual pulsars, only one binary pulsar is known, with its two members orbiting each other. It’s called J0737-3039, it was discovered in 2003, and its discovery opened up a whole new world of possible ways to test general relativity.
One of the pair’s pulsars rotates about 44 times per second, while the other, slower, rotates once every 2.8 seconds. The slower pulsar dimmed in 2008, due to a quirk in general relativity that caused its rays to be out of sight. But the researchers kept monitoring the remaining visible pulsar, combining that new data with older observations to improve the precision of their measurements.
And now, under the direction of astrophysicist Michael Kramer of the Max Planck Institute for Radio Astronomy in Bonn, Germany, the team of researchers has published a comprehensive paper collecting all the observations.
Dick Manchester of Australia’s national science agency, CSIRO, and a member of the research team, explains how this result gives us a more precise understanding of our Universe: “General relativity theory describes how large-scale gravity works in the Universe, but it breaks down on the atomic scale, where quantum mechanics reigns.”
According to the researcher, “We needed to find ways to test Einstein’s theory on an intermediate scale to see if it is still true. Fortunately, the right cosmic laboratory, known as the ‘double pulsar’, was found using the Parkes telescope in 2003. Observations of the double pulsar over the past 16 years have been shown to be remarkably consistent with Einstein’s general theory of relativity — 99.99 percent, to be precise.”
General relativity, for example, says that extreme accelerations in the double-pulsar system should strain the fabric of space-time and send out gravitational waves that slow the system down. It has been calculated that the two pulsars will collide within 85 million years. And the study confirms it.
Like the ‘tick’ of a clock
However, with such a long time scale for this energy loss, its effects are difficult to detect. Fortunately, the precision of the rotations, which can be compared to the ‘ticks’ of a watch, are perfect tools for tracking those small disturbances.
As Adam Deller, from Swinburne University and also a member of the team, explains, those ‘tics’ take around 2,400 years to reach Earth, but “that still wasn’t enough to tell us how far away pulsars were, and we needed to know.” to prove general relativity”.
The researchers achieved this thanks to the Very Long Baseline Array, a network of telescopes around the world, which managed to detect a small annual oscillation in the position of the pulsars, which revealed the exact distance from Earth.
The observations, in effect, reveal that the orbit of the pulsars is shrinking. By measuring how long it takes the pulsars to complete each orbit, the researchers determined that the two members of the pair move closer to each other by about seven millimeters each day. That’s because, as they orbit, pulsars emit gravitational waves, ripples in space-time that expand and ‘steal’ some of the energy from the pulsars.
But there is a more subtle effect that also modifies that orbit. Pulsars gradually slow down over time, losing rotational energy. And because energy and mass are two sides of the same coin, that means the fastest pulsar is losing around 8 million metric tons per second.
“When I first realized that, it really blew me away,” says Kramer. Although it sounds like a lot, that loss of mass amounts to only a small adjustment of the orbit. Previously, scientists could ignore this effect in calculations because the adjustment was so small. But the orbit measurement is now precise enough to account for it.
“We will try again in the future,” says Deller, “using new radio telescopes and new data analysis in the hope of detecting some weakness in general relativity that will lead us to an even better gravitational theory.”
New Features of Pulsars
In short, the study has confirmed the validity of Einstein’s theory in several different ways. And it has allowed us to discover totally new characteristics of both pulsars.
For example, by studying the timing of the pulses when light from a pulsar passes by its companion, scientists were able to tell which way the pulsar is spinning the fastest. The results indicate that the pulsar rotates in the same direction as it orbits, and that, in turn, provides clues as to how the pair formed.
According to the study, the two pulsars started out as neighboring stars that exploded, one after the other. Often when a star explodes, the remnant it leaves behind is ejected, separating those pairs of stars. The fact that the faster pulsar spins in the same direction as it orbits means that the explosion that formed it didn’t rock it too hard, which helps explain how the junction stayed intact.
