Radio Pulsar Binary Proves Einstein at Least 99.99% Right
NASA

Radio Pulsar Binary Proves Einstein at Least 99.99% Right

A 16-year experiment was carried out by researchers to test Einstein’s theory of general relativity. The worldwide team used seven radio telescopes around the world to look to the stars — specifically, a pair of extreme stars known as pulsars. Credit: Max Planck Institute for Radio Astronomy

More than a century has gone since Einstein developed his General Theory of Relativity (GR), the geometric theory of gravitation that transformed our view of the Universe. Even so, astronomers continue to put it through rigorous tests in the hopes of finding variations from this well-established hypothesis. The reason is simple: any evidence of physics beyond GR would provide fresh insights into the Universe and help in the resolution of some of the universe’ most fundamental mysteries.

An multinational team of astronomers led by Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, has completed one of the most rigorous tests yet. Kramer and his colleagues observed a unique pair of pulsars for 16 years using seven radio telescopes from around the world. They witnessed effects predicted by GR for the first time, and with at least 99.99 percent accuracy!

An multinational team of astronomers led by Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, has completed one of the most rigorous tests yet. Kramer and his colleagues observed a unique pair of pulsars for 16 years using seven radio telescopes from around the world. They witnessed effects predicted by GR for the first time, and with at least 99.99 percent accuracy!

In addition to researchers from the MPIfR, Kramer and his associates were joined by researchers from institutions in ten different countries – including the Jodrell Bank Centre for Astrophysics (UK), the ARC Centre of Excellence for Gravitational Wave Discovery (Australia), the Perimeter Institute for Theoretical Physics (Canada), the Observatoire de Paris (France), the Osservatorio Astronomico di Cagliari (Italy), the South African Radio Astronomy Observatory (SARAO), the Netherlands Institute for Radio Astronomy (ASTRON), and the Arecibo Observatory.

Pulsars are fast-spinning neutron stars that emit narrow, sweeping beams of radio waves. Credit: NASA’s Goddard Space Flight Center

“Radio Pulsar” is a type of rapidly rotating, strongly magnetic neutron star. These super-dense structures emit powerful radio beams from their poles, which, when combined with their rapid rotation, produce a strobing effect resembles a lighthouse. Pulsars excite astronomers because they reveal so much about the physics governing ultra-compact objects, magnetic fields, the interstellar medium (ISM), planetary physics, and even cosmology.

Furthermore, the enormous gravitational forces involved allow astronomers to put gravitational theories like GR and Modified Newtonian Dynamics (MOND) to the test under some of the most extreme conditions imaginable. Kramer and his colleagues analyzed PSR J0737-3039 A/B, the “Double Pulsar” system located 2,400 light-years from Earth in the constellation Puppis, for their research.

Members of the research team identified the only radio pulsar binary system ever seen in 2003. The two pulsars that comprise this system rotate rapidly – 44 times per second (A), once per 2.8 seconds (B) – and orbit each other in just 147 minutes. While they are almost 30% more massive than the Sun, their circumference is just about 24 km (15 mi). As a result, they have a strong gravitational pull and strong magnetic fields.

In addition to these characteristics, the quick orbital period of this system offers it a near-perfect laboratory for testing gravitational theories. In a recent MPIfR press release, Prof. Kramer said:

“We studied a system of compact stars that is an unrivalled laboratory to test gravity theories in the presence of very strong gravitational fields. To our delight we were able to test a cornerstone of Einstein’s theory, the energy carried by gravitational waves, with a precision that is 25 times better than with the Nobel-Prize winning Hulse-Taylor pulsar, and 1000 times better than currently possible with gravitational wave detectors.”

Artist’s impression of the path of the star S2 passing very close to Sagittarius A*, which also allows astronomers to test predictions made by General Relativity under extreme conditions. Credit: ESO/M. Kornmesser

The Parkes radio telescope in Australia, the Green Bank Telescope in the United States, the Nançay Radio Telescope in France, the Effelsberg 100-m telescope in Germany, the Lovell Radio Telescope in the United Kingdom, the Westerbork Synthesis Radio Telescope in the Netherlands, and the Very Long Baseline Array in the Netherlands were used for the 16-year observation campaign (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. They were able to see how photons from this binary pulsar were influenced by its tremendous gravitational attraction by doing so. According to research co-author Prof. Ingrid Stairs of the University of British Columbia (UBC) at Vancouver:

“We follow the propagation of radio photons emitted from a cosmic lighthouse, a pulsar, and track their motion in the strong gravitational field of a companion pulsar. We see for the first time how the light is not only delayed due to a strong curvature of spacetime around the companion, but also that the light is deflected by a small angle of 0.04 degrees that we can detect. Never before has such an experiment been conducted at such a high spacetime curvature.”

According to co-author Prof. Dick Manchester of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), the quick orbital motion of compact objects such as these allowed them to test seven different GR predictions. These include gravitational waves, light propagation (“Shapiro delay and light bending”), time dilation, mass-energy equivalence (E=mc2), and the effect of electromagnetic radiation on the orbital motion of the pulsar.

The Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. Credit: GBO/AUI/NSF

“This radiation corresponds to a mass loss of 8 million tonnes per second!” he said. “While this seems a lot, it is only a tiny fraction – 3 parts in a thousand billion billion(!) – of the mass of the pulsar per second.” The researchers also made extremely precise measurements of changes to the pulsars’ orbital orientation, a relativistic effect that was first observed with the orbit of Mercury – and one of the mysteries Einstein’s theory of GR helped resolve.

Only here was the effect 140,000 times stronger, leading the scientists to realize that they also needed to consider the effects of the pulsar’s rotation on the surrounding spacetime — a phenomenon known as the Lense-Thirring effect, or “frame-dragging.” According to Dr. Norbert Wex of the MPIfR, another primary author of the work, this enabled another breakthrough:

“In our experiment it means that we need to consider the internal structure of a pulsar as a neutron star. Hence, our measurements allow us for the first time to use the precision tracking of the rotations of the neutron star, a technique that we call pulsar timing to provide constraints on the extension of a neutron star.”

Another important takeaway from this experiment was the team’s use of complementing observing approaches to acquire highly accurate distance measurements. In the past, such research were frequently hampered by unconstrained distance estimations. The team got a high-resolution result of 2,400 light-years with an 8 percent margin of error by combining the pulsar timing technique with careful interferometric observations (and the effects of the ISM).

Artist’s illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst, while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Credit: NSF/LIGO/Sonoma State University/A. Simonnet

Finally, the team’s findings were not only consistent with GR, but they were also able to observe impacts that had previously been overlooked. As Paulo Freire, another research co-author (and also from MPIfR), expressed :

“Our results are nicely complementary to other experimental studies which test gravity in other conditions or see different effects, like gravitational wave detectors or the Event Horizon Telescope. They also complement other pulsar experiments, like our timing experiment with the pulsar in a stellar triple system, which has provided an independent (and superb) test of the universality of free fall.”

“We have reached a level of precision that is unprecedented,” Prof. Kramer concluded. “Future experiments with even bigger telescopes can and will go still further. Our work has shown the way such experiments need to be conducted and which subtle effects now need to be taken into account. And, maybe, we will find a deviation from general relativity one day.”

The paper describing their research was recently published in the journal Physical Review X.

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