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 tests, hoping to find deviations from this established theory. The reason is simple: any indication of physics beyond GR would open new windows onto the Universe and help resolve some of the deepest mysteries about the cosmos.

One of the most rigorous tests ever was recently conducted 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 across the world, Kramer and his colleagues observed a unique pair of pulsars for 16 years. In the process, they observed effects predicted by GR for the first time, and with an accuracy of at least 99.99%!

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. A new study identifies the origin of those radio waves. NASA’s Goddard Space Flight Center

“Radio pulsars” are a special class of rapidly rotating, highly magnetized neutron stars. These super-dense objects emit powerful radio beams from their poles that (when combined with their rapid rotation) create a strobing effect that resembles a lighthouse. Astronomers are fascinated by pulsars because they provide a wealth of information on the physics governing ultra-compact objects, magnetic fields, the interstellar medium (ISM), planetary physics, and even cosmology.

In addition, the extreme gravitational forces involved allow astronomers to test predictions made by gravitational theories like GR and Modified Newtonian Dynamics (MOND) under some of the most extreme conditions imaginable. For the sake 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 Puppis.

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 have rapid rotations – 44 times a 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 measure only about 24 km (15 mi) in diameter. Hence, their extreme gravitational pull and intense magnetic fields.

In addition to these properties, the rapid orbital period of this system makes it a near-perfect laboratory for testing theories of gravitation. As Prof. Kramer said in a recent MPIfR press release:

“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.”

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