More than a hundred years have passed since Einstein formalized his theory of General Relativity (GR), the geometric theory of gravitation that revolutionized our understanding of the Universe. And yet, astronomers are still subjecting it to rigorous testing, hoping to find deviations from this established theory. The reason is simple: any hint of physics beyond GR would open new windows into the Universe and help solve some of the deepest mysteries of the cosmos.
One of the most rigorous tests ever carried out was recently carried out by an international team of astronomers led by Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany. Using seven radio telescopes around the world, Kramer and his colleagues observed a unique pair of pulsars for 16 years. In the process, they observed the effects predicted by GR for the first time, and with a precision at least 99.99%!
In addition to the MPIfR researchers, Kramer and his associates were joined by researchers from institutions in ten different countries – including the Jodrell Bank Center for Astrophysics (UK), the ARC Center of Excellence for Gravitational Wave Discovery (Australia ), the Perimeter Institute of Theoretical Physics (Canada), the Paris Observatory (France), the Osservatorio Astronomico di Cagliari (Italy), the South African Radio Astronomy Observatory (SARAO), the Netherlands Institute of Radio Astronomy ( ASTRON) and the Arecibo Observatory.
“Radio pulsars” are a special class of rapidly rotating, strongly magnetized neutron stars. These super dense objects emit powerful radio beams from their poles which (when combined with their rapid rotation) create a strobe effect that resembles a beacon. Astronomers are fascinated by pulsars because they provide a wealth of information about the physics governing ultra-compact objects, magnetic fields, the interstellar medium (ISM), planetary physics, and even cosmology.
Additionally, the extreme gravitational forces involved allow astronomers to test the predictions made by gravitational theories like GR and Modified Newtonian dynamics (MOND) in some of the most extreme conditions imaginable. For the purposes of their study, Kramer and his team examined PSR J0737-3039 A/B, the “Double Pulsar” system located 2,400 light-years from Earth in the constellation of puppies.
This system is the only radio pulsar binary ever observed and was discovered in 2003 by members of the research team. The two pulsars that make up this system spin rapidly – 44 times per second (A), once every 2.8 seconds (B) – and orbit each other with a period of just 147 minutes. While they are about 30% more massive than the Sun, they are only about 24 km (15 mi) in diameter. Hence their extreme gravitational pull and intense magnetic fields.
Besides these properties, the rapid orbital period of this system makes it an almost perfect laboratory for testing theories of gravity. As Professor Kramer stated in a recent MPIfR press release:
“We have studied a system of compact stars which is an unequaled laboratory for testing theories of gravity in the presence of very strong gravitational fields. To our delight, we were able to test a cornerstone of Einstein’s theory, the energy transported by gravitational waves, with 25 times better accuracy than with the Nobel Prize-winning Hulse-Taylor pulsar, and 1000 times better than what is currently possible with gravitational wave detectors.
Seven radio telescopes were used for the 16-year observation campaign, including the Parkes Radio Telescope (Australia), the Green Bank Telescope (USA), the Nançay Radio Telescope (France), the 100m Effelsberg Telescope ( Germany), the Lovell Radio Telescope (UK), the Westerbork Synthesis Radio Telescope (the Netherlands) and the Very Long Baseline Array (USA).
These observatories covered different parts of the radio spectrum, ranging from 334 MHz and 700 MHz to 1300 – 1700 MHz, 1484 MHz and 2520 MHz. By doing so, they were able to see how the photons coming from this binary pulsar were affected by its strong gravitational pull. As study co-author Professor Ingrid Stairs of the University of British Columbia (UBC) in Vancouver explained:
“We follow the propagation of radio photons emitted by a cosmic beacon, a pulsar, and follow their movement in the strong gravitational field of a companion pulsar. We see for the first time how light is not only delayed due to a strong curvature of spacetime around the companion, but also that light is deflected by a small angle of 0.04 degrees that we can detect. Never before had such an experiment been conducted at such a high space-time curvature.
As co-author Professor Dick Manchester of Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) added, the rapid orbital motion of compact objects like these allowed them to test seven different predictions. of GR. These include gravitational waves, light propagation (“Shapiro delay and light bending), time dilation, mass-energy equivalence (E = mc2), and what effect electromagnetic radiation has on the orbital motion of the pulsar.
“This radiation corresponds to a mass loss of 8 million tons per second! he said. “While that sounds like a lot, it’s only a tiny fraction – 3 parts out of a trillion trillion (!) – of the pulsar’s mass per second.” The researchers also made extremely precise measurements of changes in pulsar orbital orientation, a relativistic effect that was first observed with Mercury’s orbit – and one of the mysteries that the GR theory of Einstein helped solve.
Only here, the effect was 140,000 times stronger, which made the team realize that they also had to consider the impact of the pulsar’s rotation on the surrounding spacetime – aka. the Lense-Thirring effect, or “frame-dragging”. Like Dr. Norbert Wex of the MPIfR, another lead author of the study, this provided another breakthrough:
“In our experience, this means that we have to consider the internal structure of a pulsar as a neutron star. Therefore, our measurements allow us for the first time to use precision tracking of neutron star rotations, a technique we call pulsar synchronization to provide constraints on the extension of a neutron star. .
Another valuable lesson from this experience was how the team combined complementary observation techniques to achieve highly accurate distance measurements. Similar studies have often been hampered by poorly constrained distance estimates in the past. By combining the pulsar synchronization technique with careful interferometric measurements (and the effects of ISM), the team achieved a high-resolution result of 2,400 light-years with an 8% margin of error.
Ultimately, the team’s results not only agreed with GR, but they were also able to see effects that could not be studied before. As Paulo Freire, another co-author of the study (and also of MPIfR), put it:
“Our results are well complementary to other experimental studies that test gravity under other conditions or see different effects, such as gravitational wave detectors or the Event Horizon telescope. They also complement other pulsar experiments, such as our timing experiment with the pulsar in a triple star system, which provided an independent (and superb) test of the universality of free fall.
“We have achieved an unprecedented level of precision,” Professor Kramer concluded. “Future experiments with even larger telescopes can and will go even further. Our work has shown how such experiments should be conducted and what subtle effects now need to be taken into account. And, perhaps, we will find a a deviation from general relativity.
The article describing their research recently appeared in the journal Physical examination X,
Originally published on Universe today.
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Reference: “Strong-field Gravity Tests with the Double Pulsar” by M. Kramer et al., December 13, 2021, Physical examination X.