It’s been more than a hundred years since Einstein formalized his theory of general relativity (GR), the geometric theory of gravity that has revolutionized our understanding of the universe. However, astronomers are still subjected to rigorous tests, hoping to find deviations from this well-established theory. The reason is simple: any indicator of physics beyond GR would open new windows on the universe and help solve some of the deeper mysteries about the universe.
One of the most stringent tests ever 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 from around the world, Kramer and his colleagues have observed a unique pair of pulsars for 16 years. In the process, they observed the effects predicted by GRs for the first time, and with
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In addition to the researchers from MPIfR, Kramer and his colleagues 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 Detection (Australia), and the Ocean Institute. For Theoretical Physics (Canada), Paris Observatory (France), Osservatorio Astronomico di Cagliari (Italy), South African Radio Astronomy Observatory (SARAO), Netherlands Institute for Radio Astronomy (ASTRON) and Arecibo Observatory.
Pulsars are rapidly rotating neutron stars that emit sweeping narrow beams of radio waves. Credit: NASA’s Goddard Space Flight Center
“Radio pulsars” are a special class of rapidly rotating, highly magnetic neutron stars. These ultra-dense objects emit powerful radio beams from their poles that (when combined with their rapid rotation) create a powerful beacon-like effect. Astronomers are fascinated by pulsars because they provide a wealth of information about the physics that govern ultra-small objects, magnetic fields, the interstellar medium (ISM), planetary physics, and even cosmology.
In addition, the intense gravitational forces allow astronomers to test predictions made by gravitational theories such as GR and Modified Newtonian Dynamics (MOND) under some of the harshest conditions imaginable. For their study, Kramer and his team examined PSR J0737-3039 A/B, a “double star” system located 2,400 light-years from Earth in Constellation Puppis.
This system is the only radio
“>pulsar binary ever and was discovered in 2003 by members of the research team. The two pulsars that make up this system have fast revolutions — 44 times per second (A), once every 2.8 seconds (B) — and orbit each other for just 147 minutes. While it is about 30% larger than the Sun, it is only about 24 km (15 mi) in diameter. Hence, its intense gravity and intense magnetic fields.
In addition to these properties, this system’s fast orbital period makes it a near-perfect laboratory for testing gravitational theories. As Professor Kramer said in a recent press release for MPIfR:
“We have studied a system of compressed stars and are an unrivaled laboratory for testing gravitational theories in the presence of very strong gravitational fields. To our delight, we were able to test the cornerstone of Einstein’s theory, the energy it carries
gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.“>gravitational waves, with an accuracy of 25 times better than that of the Nobel Prize-winning Hulse-Taylor pulsar, and 1,000 times better than what is currently possible with gravitational wave detectors.”
Artist’s impression of the trajectory of star S2 passing near Sagittarius A*, which also allows astronomers to test predictions made by general relativity under extreme conditions. Credit: ESO/M. Kornmeiser
Seven radio telescopes were used for the 16-year observation campaign, including Parkes Radio Telescope (Australia), Green Bank Telescope (US), Nansai Radio Telescope (France), Eiffelberg 100m Telescope (Germany), Lovell Radio Telescope (Kingdom United), Westerbork Synthesis Radio Telescope (Netherlands), and Very Long Core Array (US).
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. In doing so, they were able to see how the photons coming from this binary pulsar were affected by its strong gravity. As Professor Ingrid Stiers of the University of British Columbia (UBC) in Vancouver, a co-author of the study, explained:
“We follow the propagation of radio photons emitted by a cosmic beacon, a pulsar, and trace their motion in the strong gravitational field of a companion pulsar. We see for the first time how light is delayed not only by the strong curvature of space-time around a companion, but also that light is deflected by a small angle of 0.04 degrees that we can detect. Such an experiment had never been done before in such a high curvature of space-time.”
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 about GR. These include gravitational waves, light propagation (“Shapiro’s delay and light bending), time dilation, and the mass-energy equation (E = mc).2), and what is the effect of electromagnetic radiation on the orbital motion of a pulsar.
The Robert C. Bird Green Bank Telescope (GBT) in West Virginia. Credit: GBO/AUI/NSF
“This radiation is equivalent to a collective loss of 8 million tons per second!” He said. “Although that sounds like a lot, it’s a tiny fraction — 3 parts per thousand billion (!) — of the pulsar’s mass per second.” The researchers also made highly accurate measurements of changes in the orbital orientation of pulsars, a relativistic effect first observed with Mercury’s orbit — and one of the mysteries that Einstein’s theory of GR helped solve.
Only here, the effect was 140,000 times stronger, leading the team to realize that they also needed to consider the effect of the pulsar’s rotation on surrounding spacetime – aka. The Lense-Thirring effect, or ‘pull the frame’. Dr. Norbert Weeks of MPIfR, another lead author of the study, also allowed for another breakthrough:
“This means in our experience that we need to consider the internal structure of a pulsar as a
neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.“>neutron star. Hence, our measurements allow us for the first time to use precise tracking of neutron star cycles, a technique we call pulsar timing to provide constraints on the neutron star’s extension.”
Another valuable outcome from this experiment was how the team combined complementary monitoring techniques to obtain high-accuracy distance measurements. Similar studies have often been hampered by poor distance estimates in the past. By combining pulsar timing technology with precise interferometry measurements (and ISM effects), the team obtained a high-resolution result of 2,400 light-years with an 8% margin of error.
Artist’s illustration of two merging neutron stars. The narrow beams represent a gamma-ray burst, while the undulating spacetime lattice denotes the opposing gravitational waves that characterize the merger. Credit: NSF/LIGO/Sonoma State University/A. Simonet
In the end, not only were the team’s results consistent with GR, but they were also able to see effects that could not have been studied before. As Paulo Freire, another study co-author (also from MPIfR) expressed:
“Our results complement well other experimental studies that test gravity in other conditions or see different effects, such as gravitational wave detectors or the event horizon telescope. It also complements other pulsar experiments, such as our timing experiment with a pulsar in a triple star system, which provided an independent (and fascinating) test of the universality of free fall.”
“We have reached an unprecedented level of accuracy,” Professor Kramer concluded. Future experiments with larger telescopes could and will continue to go further. Our work has shown how such experiments should be conducted and which exact effects need to be taken into account now. Perhaps one day we will find a deviation from general relativity.”
The paper describing their research recently appeared in the journal X . physical reviewAnd
Originally Posted in universe today.
To find out more about this research:
Reference: “Strong-field gravity tests using the double star” by M. Kramer et al. December 13, 2021, X . physical review.
DOI: 10.1103 / PhysRevX.11.041050