Observation of double pulsar system shows that general relativity still can’t be beat.
An international team have put Einstein’s theory through the wringer with the help of a double pulsar system and 16 years of rigorous testing.
Using telescopes from around the world, including Australia with the CSIRO’s Murriyang radio telescope, at Parkes, they found that Einstein’s general theory of relativity – originally published back in 1915 – still holds true today.
But what is general relativity anyway? In short, it’s the description of gravity used in modern physics; a geometric property of space and time also known as four-dimensional spacetime. However, while being the simplest theory consistent with experimental data at space-sized measures, general relativity cannot be reconciled with the laws of quantum physics at the very smallest scales of our universe.
According to research team member Dr Dick Manchester – a fellow at Australia’s national space agency and CSIRO – this result helps to better refine our understanding of the universe.
‘The theory of general relativity describes how gravity works at large scales in the universe, but it breaks down at the atomic scale where quantum mechanics reigns supreme,’ says Manchester.
‘We needed to find ways of testing Einstein’s theory at an intermediate scale, to see if it still holds true. Fortunately, just the right cosmic laboratory, known as the double pulsar, was found using the Parkes telescope in 2003.
‘Our observations of the double pulsar over the past 16 years proved to be amazingly consistent with Einstein’s general theory of relativity – within 99.99 per cent to be precise,’ he says.
But how did they do it?
The first binary pulsar system – SR B1913+16 – was identified in 1975, but both it and those found subsequently comprised a pulsar and a star orbiting each other. The double pulsar system – PSR J0737-3039A/B – was spotted in 2003 and remains the only system yet found that contains two pulsars in a binary orbit, which offers a rare opportunity to test general relativity.
A double pulsar system acts kind of like a clock with a ticking second hand. The two orbiting pulsars – dense neutron stars – create very strong gravitational fields and emit radio waves at a regular time interval (the second hand, in this analogy). They also have very stable – and very speedy – rotation times.
The stars in the double pulsar system complete an orbit every 2.5 hours, with one pulsar rotating 45 times each second while the other spins just 2.8 times per second.
The two pulsars are predicted to collide in 85 million years’ time as, according to general relativity, the extreme accelerations in the system strain the fabric of space-time and send out ripples that will slow it down. But with such a long time scale this energy loss is difficult to detect.
Fortunately, the clock-like ticks of the radio waves coming from the spinning pulsars are perfect tools to trace these tiny changes. Pulsars with a stable rotation enable the measurement of miniscule variations in the arrival times of those ticks to test for gravitational theories.
Member of the research team Associate Professor Adam Deller, from Swinburne University of Technology and the ARC Centre of Excellence for Gravitational Waves (OzGrav), explains that these ticks take around 2400 years to reach Earth.
‘We modelled the precise arrival times of more than 20 billion of these clock ticks over 16 years,’ says Deller.
‘That still wasn’t enough to tell us how far away the stars are, and we needed to know that to test general relativity.’
By adding in data from the Very Long Baseline Array – a network of telescopes spread across the globe – the research team was able to spot a tiny wobble in the star’s positions every year, which could be used to determine their distance from Earth.
Perhaps to the disappointment of the researchers, the end result showed that Einstein’s theory held: results were 99.99 per cent in accordance with the predictions of general relativity.
The double pulsar system remains a unique tool for testing gravitational theories, and the team plans to continue to use it to poke at Einstein’s theory.
‘We’ll be back in the future using new radio telescopes and new data analysis hoping to spot a weakness in general relativity that will lead us to an even better gravitational theory,’ says Deller.