Einstein's theory of General Relativity, the geometric theory of gravitation, has been around for more than a hundred years. Astronomers are still trying to find deviations from the established theory. New windows onto the Universe would be opened by any indication of physics beyond GR.
A team of international scientists, led by Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, conducted one of the most rigorous tests ever. For 16 years, Kramer and his colleagues used seven radio telescopes from across the world to observe a pair of pulsars. They observed effects predicted by GR for the first time, and with an accuracy of at least 99.99%.
The Jodrell Bank Centre for Astrophysics (UK), the ARC Centre of excellence for Gravitational Wave Discovery (Australia), and the Theoretical Institute for physics were all part of the group.
The beams of radio waves from a pulsar are narrow and sweeping. The origin of those radio waves has been identified. NASA has a space flight center.
Radio pulsars are a class of rapidly rotating, highly magnetized neutron stars. The objects emit powerful radio beams from their poles that 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.
Astronomers can use the extreme forces involved to test predictions made by theories like GR and MOND under some of the most extreme conditions imaginable. The Double Pulsar system is located 2,400 light-years from Earth and was examined by the team.
This system is the only one ever observed and was discovered in 2003 by members of the research team. The two pulsars that make up this system have a rapid rotation of 44 times a second and a rapid rotation of 2.8 seconds. They measure only about 15 miles in diameter, which is less than the Sun. Their magnetic fields were very strong.
The rapid orbital period of this system makes it a perfect laboratory for testing theories of gravitation. Prof.Kramer said in a recent press release.
We studied a system of compact stars that is an unparalleled laboratory to test gravity theories. We were able to test the cornerstone of Einstein's theory, the energy carried by the waves, with a precision that is 25 times better than with the prize winning Hulse-Taylor pulsar, and 1000 times better than currently available with the detectors.
The path of the star S2 passes very close to Sagittarius A*, which allows the astronomer to test predictions made by General Relativity. Credit: M. Kornmesser
The Parkes radio telescope was used in Australia, the Green Bank Telescope was used in the US, the Effelsberg 100m telescope was used in Germany, and the Lovell Radio Telescope was used in the UK.
There were different parts of the radio spectrum covered by these observatories. They were able to see how the strong pull of the pulsar affected the photon coming from it. The study co-author was Prof. Ingrid Stairs from the University of British Columbia.
We follow the propagation of radio photons emitted from a lighthouse and track their motion in the strong field of a companion pulsar. We can see for the first time that the light is delayed due to a strong curvature of spacetime around the companion, and that it is not always reflected by a small angle. This is the first time that an experiment has been conducted at such a high spacetime curve.
The co-author Prof. Dick Manchester from Australia's Commonwealth Scientific and Industrial Research Organisation said that the fast orbital motion of compact objects allowed them to test seven different predictions of GR. Light propagation, time dilation, mass-energy equivalence, and what effect the radiation has on the pulsar's motion are included.
The Robert C. Byrd Green Bank Telescope is located in West Virginia.
He said that the radiation corresponds to a mass loss of 8 million tonnes per second. It is only a tiny fraction of the mass of the pulsar per second. One of the mysteries of Einstein's theory of GR was solved when the researchers made extremely precise measurements of changes to the pulsars' orbital orientation.
The team realized that they also needed to consider the impact of the pulsar's rotation on the surrounding spacetime, as the effect was 140,000 times stronger here. This allowed for another breakthrough, as Dr. Wex from the MPIfR was one of the main authors of the study.
It means that we need to consider the internal structure of a pulsar as a neutron star in our experiment. The first time we have used the precision tracking of the rotation of the neutron star is in our measurement, which allows us to use a technique called pulsar timing to provide constraints on the extension of a neutron star.
The team combined observing techniques to get highly-accurate distance measurements. The studies were often hampered by the poorly-constrained distance estimates. The team obtained a high-resolution result of 2,400 light-years with an 8% margin of error by combining the pulsar timing technique with interferometric measurements.
The artist is showing an illustration of two stars. The rippling spacetime grid shows the isotropic waves that are associated with the merger. The National Science Foundation is a credit.
The team was able to see effects that were not studied before, and they were in agreement with GR. Another co-author of the study expressed:
Our results are in line with other experimental studies which test gravity in other conditions or see different effects. The timing experiment with the pulsar in a stellar triple system has provided an independent test of the universality of free fall.
Prof. Kramer said that the level of precision was unprecedented. Future experiments with even bigger telescopes will go further. Our work has shown how the experiments need to be conducted and how the effects need to be taken into account. One day, we might find a deviation from general relativity.
The paper that describes their research was published in the journal Physical Review X.
The Physical Review X was read further.
