Einstein's Theory Of General Relativity Just Faced Its Toughest Test Yet. Know If It Passed

The scientists observed a pair of extreme stars called pulsars through seven radio telescopes across the globe, and used their observations to challenge Einstein's most famous theory.

New Delhi: An international team of researchers has helped conduct a 16-year long experiment to challenge Einstein's theory of general relativity, by looking to the stars. 

The study conducted by researchers from the University of East Anglia (UEA) was recently published in the journal Physical Review X. The scientists observed a pair of extreme stars called pulsars through seven radio telescopes across the globe, the study said.

They used their observations to challenge Einstein's most famous theory, and performed some of the most rigorous tests yet. According to the theory of general relativity, gravity is a curvature or distortion of space-time. Einstein explained how gravity affects space-time. 

The study reveals new relativistic effects which have been observed for the first time.

Dr Robert Ferdman from UEA's School of Physics, said in a statement that scientists, more than 100 years after Einstein published his theory, continue their efforts to find flaws in the theory. Ferdman said scientists know that the theory is not the final word in gravitational theory.

He explained that general relativity is not compatible with the other fundamental forces described by quantum mechanics. This makes it important to continue to place the most stringent tests upon general relativity, and discover how and when the theory breaks down, he said.

Deviation From General Relativity Would Open A Window On New Physics

He explained that finding any deviation from general relativity would be a major discovery that would open a window on new physics beyond the current theoretical understanding of the Universe.

He added that this may help scientists discover a unified theory of the fundamental forces of nature.

Michael Kramer from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and the lead author of the study, said the scientists studied a system of compact stars to test gravity theories in the presence of very strong gravitational fields. 

Ferdman explained that a pulsar is a highly magnetised rotating compact star that emits beams of electromagnetic radiation out of its magnetic poles, and weighs more than the Sun, but is only 15 miles across. Pulsars are incredibly dense objects that sweep the sky like a lighthouse, he added.

The scientists studied a double pulsar, which was discovered by members of the team in 2003. The double pulsar presents the most precise laboratory the team currently has, to test Einstein's theory, Ferdman said. 

Two pulsars, which orbit each other in just 147 minutes with velocities of about 1 million kilometres per hour, constitute the double pulsar. One pulsar spins at a rate of 44 times per second, while the companion star is young and has a rotation period of 2.8 seconds. The motion of these stars around each other can be used as a near perfect gravity laboratory, the study said.

Seven Radio Telescopes Were Used

The researchers used seven sensitive radio telescopes – in Australia, the US, France, Germany, the Netherlands, and in the UK, which has the Lovell Radio Telescope.

Professor Kramer explained that the researchers were able to test a cornerstone of Einstein's theory, which is the energy carried by gravitational waves. The researchers studied the energy 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, Kramer said.

He explained that the scientists were able to see effects that could not be studied before.

Professor Benjamin Strappers from the University of Manchester said the discovery of the double pulsar system presented the scientists with the only known instance of two cosmic clocks that allow precise measurement of the structure and evolution of an intense gravitational field.

Professor Ingrid Stairs from the University of British Columbia at Vancouver said the researchers follow the propagation of radio photons emitted from a pulsar, which is a cosmic lighthouse, and track their motion in the strong gravitational field of a companion pulsar.

 For the first time, scientists saw how the light is not only delayed due to a strong curvature of space-time around the companion star, but also that the light is deflected by a small angle of 0.04 degrees. Never before had such an experiment been conducted at such a high space-time curvature, Stairs said.

Total Of Seven Predictions Of General Relativity Tested

Professor Dick Manchester from CSIRO said that the fast orbital motion of compact objects like these allow scientists to test many different predictions of general relativity. These compact objects, which are about 30 per cent more massive than the Sun but only about 24 kilometres across, helped the scientists test a total of seven predictions of general relativity.

He said the precision not only allowed them to measure gravitational waves and light propagation, but also to measure the effect of "time dilation" that makes clocks run slower in gravitational fields.

He said the researchers had to take Einstein's famous equation, E = mc² into account when considering the effect of the electromagnetic radiation emitted by the fast-spinning pulsar on its orbital motion.

The radiation corresponds to a mass loss of eight million tonnes per second, and is only a tiny fraction of the mass of the pulsar per second.

The scientists observed with a precision of 1 part in a million that the orbit changes its orientation. This is a relativistic effect, which is 140,000 times stronger than the effect observed in Mercury's orbit, the study stated.

The impact of the pulsar's rotation on the surrounding spacetime is "dragged along" with the spinning pulsar.

Dr Norbert Wex, another main author of the study, said this phenomenon is termed the Lense-Thirring effect or frame-dragging. He said the researchers need to consider the internal structure of a pulsar as a neutron star.

The scientists used a technique known as pulsar timing to precisely track the rotations of the neutron star.

They combined it with other techniques such as interferometry (measurement method that uses the interference of waves) to determine its distance with high resolution imaging, the study said.

Professor Adam Deller, one of the researchers, said it is the combination of different complementary observing techniques that adds to the extreme value of the experiment. The team also took into account the effects of the interstellar medium on the pulsar.

Professor Bill Coles from the University of California San Diego said the researchers gathered all possible information on the system and derived a perfectly consistent picture involving physics from many different areas such as nuclear physics, gravity, interstellar medium, plasma physics, among others.

Paulo Freire, another researcher, said the results are complementary to other experimental studies which test gravity in other conditions, such as the Event Horizon Telescope.

Professor Kramer said their work has shown the way in which such experiments need to be conducted, and which subtle effects need to be taken into account. He concluded, "And maybe, we will find a deviation from general relativity one day."