Researchers have conducted a 16-year long experiment to challenge Einstein’s theory of general relativity. The international team looked to the stars — a pair of extreme stars called pulsars to be precise – through seven radio telescopes across the globe. Credit: Max Planck Institute for Radio Astronomy
The theory of general relativity passes a range of precise tests set by pair of extreme stars.
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.
The research team’s leader, Michael Kramer from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, says: “We studied a system of compact stars that is an unrivaled 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
Dance of pulsars. Animation of the double pulsar system PSR J0737-3039 A/B and its line of sight from Earth. The system — consisting of two active radio pulsars — is “edge-on” as seen from Earth, which means that the inclination of the orbital plane relative to our line of sight is only about 0.6 degrees.
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.
Dick Manchester from Australia’s national science agency, CSIRO, illustrates: “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! 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 Shapiro time delay. Animation of the measurement of the Shapiro time delay in the double pulsar. When a rapidly spinning pulsar orbits around the common center of mass, the emitted photons propagate along the curved spacetime of the trapped pulsar and are therefore delayed.
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. Norbert Wex from the MPIfR, another main author of the study, explains: “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 (function(d, s, id){
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