UNIVERSITY OF EAST ANGLIA
CREDIT: MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY
Researchers at the University of East Anglia and the University of Manchester helped carry out a 16-year experiment to challenge Einstein’s theory of general relativity.
The international team searched for stars – a pair of extremely precise stars known as pulsars – through seven radio telescopes around the globe.
And they used them to challenge Einstein’s most famous theory with some of the most rigorous tests to date.
The study, published today in the journal Physical Review X, revealed relativistic effects that, although expected, were observed for the first time.
Dr Robert Ferdman, from the UEA’s School of Physics, said: “As spectacular success as Einstein’s general relativity has been demonstrated, we know it’s not the last word in the theory of gravity.
“More than 100 years later, scientists around the world are still trying to find holes in his theory.
“General relativity is incompatible with the other fundamental forces described by quantum mechanics. It is therefore important to continue to perform the most rigorous tests possible on general relativity, to discover how and when the theory breaks.
“Finding any deviation from general relativity would constitute a major discovery that opens the door to new physics beyond our current theoretical understanding of the Universe.
“And it could help us eventually discover a unified theory of the fundamental forces of nature.”
Led by Michael Kramer from the Max Planck Institute for Radio Astronomy in Bonn, Germany, an international team of researchers from ten countries has put Einstein’s theory to the most rigorous tests.
Dr. Ferdman said: “A pulsar is a highly magnetic rotating compact star that emits beams of electromagnetic radiation from its magnetic poles.
“They are heavier than our sun but they are only about 15 miles across – so they are extremely dense objects that generate radio beams that sweep the sky like a lighthouse.
“We studied a double pulsar, discovered by team members in 2003 and presented in the most precise laboratory we currently have to test Einstein’s theory. His theory, of course, was conceived when neither these types of extreme stars, nor the techniques used to study them, were conceivable.”
The binary pulsar consists of two stars orbiting each other in just 147 minutes at a speed of about 1 million km/h. A pulsar is spinning very fast, about 44 times a second. The companion is young and has a spin cycle of 2.8 seconds. It is their movement around each other that can be used as a near-perfect gravity laboratory.
Seven sensitive radio telescopes were used to observe this double pulsar – in Australia, the US, France, Germany, the Netherlands and in the UK (Lovell Radio Telescope).
“We have studied a system of compact stars that is an unparalleled laboratory for testing gravity theory under conditions of very strong gravitational fields,” said Professor Kramer.
“To our delight, we were able to test the foundation of Einstein’s theory, the energy carried by gravitational waves, with an accuracy 25 times higher than the Nobel Prize-winning Hulse-Taylor pulsar and 1000 times better. than current possible gravitational wave detector. ”
Not only were the observations consistent with the theory, he explained, “but we were also able to see effects that weren’t possible to study before.”
Professor Benjamin Stappers, from the University of Manchester, said: “The discovery of the double pulsar system was made as part of a survey co-led by the University of Manchester and presented us with the only known case. known about two cosmological clocks that allow for precise measurements of the structure and development of intense gravitational fields.
“The Lovell Telescope at the Jodrell Bank Observatory has been monitoring it every few weeks since. These long baselines of high quality and frequent observations provide an excellent data set to combine with data from observatories around the world. ”
Professor Ingrid Stairs from the University of British Columbia in Vancouver, said: “We tracked the propagation of radio photons emitted by a cosmic lighthouse, a pulsar, and tracked their motion during strong gravitational field of a companion pulsar.
“For the first time, we see that light is not only delayed by the strong curvature of spacetime around the companion, but it is also deflected by a small 0.04 degree angle that we can detect. . Never before has such an experiment been performed at such a high spacetime curvature”.
Professor Dick Manchester from Australia’s national science agency, CSIRO, said: “Such rapid orbital motion of compact objects like this – they are about 30% more massive than the Sun but only about as wide as they are. 24 km – allowing us to test many different predictions of general relativity – seven in total!
“In addition to gravitational waves and the propagation of light, our accuracy also allows us to measure the effect of ‘time dilation’ that causes clocks to run slower in the gravitational field.
“We even need to take Einstein’s famous equation E = mc2 Attention should be paid when considering the effect of electromagnetic radiation emitted by a rapidly rotating pulsar on orbital motion.
“This radiation corresponds to a mass loss of 8 million tons per second! While this may seem like a lot, it is only a tiny fraction – 3 parts in a trillion trillion billion (!) – the mass of the pulsar per second”.
The researchers also measured – with an accuracy of 1 part per million (!) – its orbital changes in direction, a relativistic effect well known from Mercury’s orbit, but here 140,000 times stronger.
They realized that to this degree of precision, they also needed to consider the effect of the pulsar’s rotation on the surrounding spacetime, which is “pulled” with the rotating pulsar.
Dr Norbert Wex from MPIfR, another lead author of the study, said: “Physicists call this the Lense-Thirring or frame drag effect. In our experiment, that means we need to treat the internal structure of a pulsar as a neutron star.
“Our measurements thus allow us to use, for the first time, precise tracking of the rotations of neutron stars, a technique we call pulsar time to provide constraints on the elongation of a neutron star.”
The pulsar timing technique was combined with the system’s careful interferometric measurements to determine its distance from high-resolution images, resulting in a value of 2400 light-years with an error of just 8 %.
Research team member Professor Adam Deller, from Swinburne University in Australia and responsible for this part of the experiment, said: “It was the combination of different complementary observational techniques that added up. extreme value of the experiment. In the past, similar studies were often hindered by the limited knowledge of the distances of such systems”.
This is not the case here, where in addition to pulsar time and interferometry, the information obtained from effects due to the interstellar medium is also carefully taken into account.
Professor Bill Coles from the University of California San Diego agrees: “We collect all the information that is possible on the system, and we give a completely consistent, physics-related picture from many disciplines. various, such as nuclear physics, gravity, the interstellar medium, plasma physics, and more. This is quite extraordinary.”
Paulo Freire, also from MPIfR, said: “Our results nicely complement other experimental studies that have tried to test gravity under other conditions or see different effects, like wave detectors. gravity or Event Horizon Telescope.
“They also complement other pulsar experiments, such as our timing experiment with pulsars in a triple star system, which have provided an excellent and independent test of the universality of free fall. do.”
Professor Kramer added: “We have reached an unprecedented level of accuracy. Future experiments with even larger telescopes can and will go even further.
“Our work has shown how such experiments are conducted and what subtle effects now need to be taken into account. And, maybe, one day, we’ll find a deviation from general relativity. ”
“Testing Strong Field Gravity with Double Pulsar” is published in Physical Review X on December 13, 2021.
JOURNEYS
Physical Review X
RESEARCH METHODS
Observational research
RESEARCH SUBJECTS
Do not apply
ARTICLE TITLE
Test strong gravity with Double Pulsar
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