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News Release

Einstein wins again

The theory of general relativity passes a range of precise tests set by pair of extreme stars

December 13, 2021 -- An international team of researchers, including electrical engineers at the University of California San Diego, has conducted a 16-year long experiment to challenge Einstein’s theory of general relativity with some of the most rigorous tests yet. Their study of a unique pair of extreme stars, so called pulsars, involved seven radio telescopes across the globe and revealed new relativistic effects that were expected and have now been observed for the first time. Einstein’s theory, which was conceived when neither these types of extreme stars nor the techniques used to study them could be imagined, agrees with the observation at a level of at least 99.99%.

The results are published in Physical Review X.

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Artistic impression of the Double Pulsar system, where two active pulsars orbit each other in just 147 min. The orbital motion of these extremely dense neutrons star causes a number of relativistic effects, including the creation of ripples in spacetime known as gravitational waves. The gravitational waves carry away energy from the systems which shrinks by about 7mm per days as a result. The corresponding measurement agrees with the prediction of general relativity within 0.013%. Credit: Michael Kramer/MPIfR

More than 100 years after Albert Einstein presented his theory of gravity, scientists around the world continue their efforts to find flaws in general relativity. The observation of any deviation from General Relativity would constitute a major discovery that would open a window on new physics beyond our current theoretical understanding of the Universe.

“We studied a system of compact stars that is an unrivalled laboratory to test gravity theories in the presence of very strong gravitational fields," said lead investigator Michael Kramer, a professor at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany. "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.” He explains that the observations are not only in agreement with the theory, “but we were also able to see effects that could not be studied before''.

“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," said Ingrid Stairs, a professor at the University of British Columbia at Vancouver. "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.”

This cosmic laboratory known as the “Double Pulsar” was discovered by members of the team in 2003. It consists of two radio pulsars which orbit each other in just 147 min with velocities of about 1 million km/h. One pulsar is spinning very fast, about 44 times a second. The companion is young and has a rotation period of 2.8 seconds. It is their motion around each other which can be used as a near perfect gravity laboratory.

“Such fast orbital motion of compact objects like these - they are about 30% more massive than the Sun but only about 24 km across - allows us to test many different predictions of general relativity - seven in total!" said Dick Manchester, a professor at Australia's national science agency, CSIRO. "Apart from gravitational waves, our precision allows us to probe the effects of light propagation, such as the so-called “Shapiro delay” and light-bending. We also measure the effect of  “time dilation” that makes clocks run slower in gravitational fields. We even need 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 the orbital motion. This radiation corresponds to a mass loss of 8 million tonnes per second! 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 measured - with a precision of 1 part in a million(!) - that the orbit changes its orientation, a relativistic effect also well known from the orbit of Mercury, but here 140,000 times stronger. They realized that at this level of precision they also need to consider the impact of the pulsar’s rotation on the surrounding spacetime, which is “dragged along” with the spinning pulsar. “Physicists call this the Lense-Thirring effect or frame-dragging. 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,” explained Norbert Wex from the MPIfR, another main author of the study. 

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The seven radio telescopes used for observations of the double pulsar PSR J0737-3039. Clockwise from upper left: Effelsberg Radio Telescope (Germany), Nançay Radio Telescope (NRT, France), Westerbork Synthesis Radio Telescope (WSRT, The Netherlands), Parkes Radio Telescope (Australia), Jodrell Bank Telescope (UK), Very Long Baseline Array (VLBA, U.S.), Green Bank Telescope (GBT, U.S.). Credit: Norbert Junkes/MPIfR (Effelsberg), Letourneur and Nançay Observatory (NRT), ASTRON (WSRT), ATNF/CSIRO (Parkes), Anthony Holloway (Jodrell Bank), NRAO/AUI/NSF (VLBA), NSF/AUI/Green Bank Observatory (GBT).

The technique of pulsar timing was combined with careful interferometric measurements of the system to determine its distance with high resolution imaging, resulting in a value of 2400 light years with only 8% error margin. “It is the combination of different complementary observing techniques that adds to the extreme value of the experiment," said Adam Deller, a professor at Swinburne University in Australia who was responsible for this part of the experiment. "In the past similar studies were often hampered by the limited knowledge of the distance of such systems.”

This is not the case here, where in addition to pulsar timing and interferometry also the information gained from effects due to the interstellar medium were carefully taken into account. “We gathered all possible information on the system and we derived a perfectly consistent picture, involving physics from many different areas, such as nuclear physics, gravity, interstellar medium, plasma physics and more," said William Coles, professor emeritus of electrical and computer engineering at the UC San Diego Jacobs School of Engineering. "This is quite extraordinary.”

“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”, said Paulo Freire, also from MPIfR.

“We have reached a level of precision that is unprecedented," said Kramer. "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.”

News release adapted from: Max Planck Institute for Radio Astronomy

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