Researchers conducted a 16-year experiment to challenge Einstein’s general theory of relativity. The international team looked at the star – a pair of extreme stars called pulsars – through seven radio telescopes around the world. Credit: Max Planck Institute for Radio Astronomy
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General relativity goes through a series of rigorous tests set by a pair of extreme stars.
More than 100 years after Albert Einstein presented his theory of gravity, scientists around the world are continuing their efforts to find flaws in general relativity. Observing any deviation from general relativity would be a major discovery that would open windows to new physics beyond our current theoretical understanding of the universe.
“We have studied compressed star systems and are an unrivaled laboratory for testing theories of gravity in the presence of very strong gravitational fields,” said research team leader Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany. The foundation of Einstein’s theory is the energy it carries
Ingrid Stears of the University of British Columbia in Vancouver provides an example: “We followed the propagation of radio photons emitted by cosmic flares, pulsars, and tracked their motion in the strong gravitational field of companion pulsars.
We see for the first time how light is delayed not only by the strong curvature of space-time around the companion, but also that light is deflected by the tiny 0.04 degree angle we can detect. Such an experiment has never been done before in such a high curvature of space-time.”
Star dance. Animation of the PSR J0737-3039 A/B dual pulsar system and its line of sight from Earth. The system – which consists of two radioactive pulsars – is “on the edge” as seen from Earth, meaning that the tilt of the orbital plane relative to our line of sight is only 0.6 degrees.
Known as the “double star”, this cosmic laboratory was discovered by team members in 2003. It consists of two radio pulsars that orbit each other in just 147 minutes at a speed of about one million km/h. One of the pulsars spins very fast, about 44 times per second. The companion is young and has a rotation period of 2.8 seconds. Their movement around each other can be used as a near-perfect gravity test.
Dick Manchester of Australia’s national science agency, CSIRO, explains: “The fast orbital motion of solid bodies like this – which is about 30% larger than the Sun, but only about 24 km in diameter – allows us to test many different predictions from general. relativity – Seven in total! In addition to gravitational waves, our accuracy allowed us to investigate the effects of light propagation, such as the so-called “Shapiro delay” and bending of light. We also measured the “time widening” effect that makes the clock run slower in gravitational fields.
We even need to take Einstein’s famous equation E = mc2 taken into account when considering the effect of electromagnetic radiation emitted by rapidly rotating pulsars on orbital motion. This radiation is equivalent to a mass loss of 8 million tons per second! While that sounds like a lot, it’s a tiny fraction – 3 parts per thousand billion (!) – of the pulsar’s mass per second.”
Shapiro’s time delay. Animation of Shapiro’s time delay measurement in a dual pulsar. As the rapidly rotating pulsar orbits around a common center of mass, the emitted photons are scattered along the curved space-time of the trapped pulsar and thus delayed.
The researchers also measured – with an accuracy of one part per million (!) – that the orbit changes direction, a relativistic effect is also known from Mercury’s orbit, but here it is 140 thousand times stronger. They realized that at this level of precision they also needed to consider the effect of the pulsar’s rotation on the surrounding spacetime, which the pulsar “pulls” in. Norbert Weeks of MPIfR, another lead author of the study, explains: “Physicists call this the Lense-Thirring effect or frame drag. In our experiments, this means we need to consider the internal structure of an a . pulsar
The pulsar timing technique combined with precise system interferometry measurements to determine distance with high-resolution imaging, resulted in a value of 2,400 light-years with a margin of error of only 8%. Team member Adam Fiedler, from Swinburne University in Australia who was in charge of this part of the experiment, highlights: “It is the combination of various complementary monitoring techniques that adds maximum value to the experiment. Similar studies have often been hampered in the past by limited knowledge of the distances of these systems.” This is not the case here, where, apart from pulsar timing and interferometry, information derived from the forces of the interstellar medium is also carefully considered. Bill Coles of the University of California San Diego agrees: “We gathered all possible information about the system and extracted a completely consistent picture, involving physics from various fields, such as nuclear physics, gravity, interstellar,
“Our results complement other experimental studies that test gravity under other conditions or look at different effects, such as the gravitational wave detector or the Event Horizon Telescope. They also complement other pulsar experiments, such as our time experiment with pulsars in three-star systems. , which provides an independent (and great) test of the universality of free fall,” said Paulo Freire, also of MPIfR.
Michael Kramer concludes: “We have achieved an unprecedented level of accuracy. Future experiments with larger telescopes can and will continue. Our work has shown how such experiments should be carried out and which exact effects should be taken into account now. . maybe we’ll find deviations from general relativity someday…”
For more information on the research, see Einstein’s Biggest Theoretical Challenge in a 16-Year Experiment – Testing General Relativity with Extreme Stars.
Reference: “Strong field gravity test using multiple stars” by M. Kramer et al. December 13, 2021, X. physical review.
DOI: 10.1103 / PhysRevX.11.041050
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