Over a century after the publication of Albert Einstein’s general theory of relativity, scientists worldwide continue their efforts to find any weaknesses. An international team composed of researchers from ten countries attempted to challenge Einstein’s theory using a unique stellar system formed by two ‘radio pulsars.’ The coordinator of the international research team, Professor Michael Kramer of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, explains: ‘We studied a system of compact stars, which is an unrivaled laboratory for testing theories of gravity in the presence of strong gravitational fields. To our great delight, we were able to test a cornerstone of Einstein’s theory, the energy emitted in the form of gravitational wave emission, with a precision 25 times better than the Hulse and Taylor pulsar (which earned them the Nobel Prize in 1993) and a thousand times better than what has been done so far by gravitational wave detectors.’
Thanks to this new study, some of the effects resulting from Einstein’s theory have been observed for the first time ever. Professor Ingrid Stairs of the University of British Columbia in Vancouver provides an example: ‘We observed that the radio waves emitted by one of the two pulsars are not only delayed due to the strong curvature of spacetime around the companion, but they are also deflected by a small angle of 0.04 degrees. Never before has such an experiment been conducted in the presence of such a high curvature of spacetime.’
This system, known as the ‘double pulsar,’ is so far unique in the world of research. It was discovered by astrophysicist Marta Burgay in the distant 2003 during some observations with the Parkes telescope in Australia. Burgay, a researcher at the INAF-Osservatorio Astronomico di Cagliari, continues to be part of the team: ‘It was a discovery that promised a lot from the start and continues to produce science of primary importance. The two pulsars orbit each other in just 147 minutes at speeds of about 1 million kilometers per hour. One rotates very quickly, about 44 times per second. The younger companion has a rotation period of 2.8 seconds. Their mutual movement constitutes an irreplaceable laboratory of gravity. It is a great satisfaction to have been at the forefront for almost 20 years in using these two pulsars to question Einstein and see that his theory always responds brilliantly.’
Professor Dick Manchester of CSIRO (Australia) adds: ‘Such a rapid orbital movement of compact objects like these – they are about 30% more massive than the Sun, but with a diameter of only 24 km – allows us to test many predictions of general relativity – seven in total! In addition to gravitational waves and light propagation, our precision also allows us to measure the effect of time dilation that slows down clocks in gravitational fields. We must also consider Einstein’s famous equation E = mc2 when considering the effects on orbital motion due to the electromagnetic energy emitted by the faster rotating pulsar. The energy associated with this radiation corresponds to a mass loss of 8 million tons per second! Although it seems a lot, it is only a small fraction – 3 parts per trillion (!) – of the pulsar’s mass.’
The researchers also measured – with a precision of one part in a million (!) – that the orbit changes orientation, a relativistic effect already well known in Mercury’s orbit, but here 140 thousand times stronger. They realized that at this level of precision, the impact of the pulsar’s rotation on the surrounding spacetime, which is ‘dragged’ with the rotating pulsar, must also be considered. Dr. Norbert Wex of MPIfR, another lead author of the study, explains: ‘Physicists call this effect Lense-Thirring or frame-dragging. In our experiment, it means that we must consider the internal structure of a pulsar, that is, its being a neutron star. Therefore, our measurements allow us, for the first time, to use the precise tracking of the rotations of a neutron star, a technique we call timing, to provide constraints on the size of the faster rotating pulsar of the two present in the system.’
The pulsar timing technique was also combined with careful interferometric measurements of the system to determine its distance thanks to ultra-high-resolution radio maps. Professor Adam Deller of Swinburne University in Australia, responsible for this part of the experiment, emphasizes: ‘It is the combination of different complementary observation techniques that adds extreme value to the experiment. In the past, similar studies were often hampered by the limited knowledge of the distance of such systems.’ This is no longer the case for the double pulsar, for which, in addition to the pulsar timing study and radio interferometry (combining which yields a distance of about 2400 light-years), the information obtained from the effects due to the interstellar medium between the pulsar and Earth were also carefully considered. Professor Bill Coles of the University of California, San Diego, agrees: ‘We have gathered all possible information about the system and have derived a perfectly coherent picture, involving physics from many different areas, such as nuclear physics, gravity, the interstellar medium, plasma physics, and more. This is truly extraordinary.’
Michael Kramer specifies: ‘We have achieved an unprecedented level of precision. Future experiments with larger telescopes will go even further. Our work has shown how such experiments must be conducted and which subtle effects must now be considered. And, perhaps, one day we will find a deviation from general relativity…’ Andrea Possenti, a senior researcher at INAF-Osservatorio Astronomico di Cagliari and also a co-author of the study, concludes: ‘This work shows that in nature, the emission of gravitational waves behaves, at least 99.99%, as predicted by general relativity. The future observation of any deviation from this theory would, on the other hand, constitute a fundamental step forward towards a unified theory for all physics of the Universe, a theory capable of combining phenomena related to gravity with those related to quantum physics.’
Additional information
Radio pulsars, highly magnetized neutron stars that rotate rapidly, are fascinating objects. With a mass greater than that of our Sun, but only about 24 km in diameter, these incredibly dense objects produce beams of radio emission that sweep the sky like a lighthouse. Since their discovery by Jocelyn Bell-Burnell and Antony Hewish in 1967, more than 3000 pulsars have been found. Pulsars provide a wealth of information related to fundamental physics (such as theories of gravity, nuclear physics, electrodynamics, plasma physics) and astrophysics: gravitational potential and galactic magnetic field, interstellar medium, celestial mechanics, planetology, and even cosmology. They allow for the most accurate tests of gravity theories within extremely strong gravitational fields.
The double pulsar (PSR J0737-3039 A/B its official name) was discovered in 2003, in the direction of the constellation Puppis, to the left of the constellation Canis Minor, better known for hosting Sirius, the brightest star in the sky. The effect of the geodetic precession of the rotation axis (another effect of General Relativity, studied in previous publications) caused the radio pulses emitted by the slower rotating pulsar to disappear from our view starting in 2008: the date for their reappearance depends on some still unknown details about the shape of the emission beam and therefore could happen in a few months or a few years.
Seven high-sensitivity instruments were used for the observations. These include the Parkes telescope (now known as Murriyang) in Australia (observations centered around 700 MHz, 1400 MHz, and 3100 MHz), the Green Bank Telescope in the United States (observations at 820 MHz and 1400/1500 MHz), the Nançay Radio Telescope in France (observations in two bands with central frequencies of 1484 MHz and 2520 MHz respectively), the Effelsberg telescope in Germany (two different reception systems around the 20 cm wavelength), the Lovell Radio Telescope in the United Kingdom (with a frequency range of 1300-1700 MHz) and the Westerbork Synthesis Radio Telescope in the Netherlands (observations at 334 MHz). Additionally, observations were made with the Very Long Baseline Array (VLBA), with ten antennas distributed across the United States (operating at 1560 MHz).