Einstein wins again

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, as part of the established scientific method researchers around the world continue their efforts to find flaws in general relativity (GR). The observation of any deviation from GR would constitute a major discovery that would open a window on new physics beyond our current theoretical understanding of the Universe. 

Prof. Ingrid Stairs from the University of British Columbia at Vancouver gives an example: “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. 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 in a regime with such a high spacetime curvature.”

This cosmic laboratory is known as the “Double Pulsar”. 

Both McLaughlin and Lorimer were part of the team that discovered the double pulsar system in 2003. Since the discovery, they and their students have been involved in studies of numerous aspects of the system ever since.  A focal point of research in the Center for Gravitational Waves and Cosmology, pulsar astronomy research continues to expand our knowledge into the Universe. McLaughlin explains “Our results are nicely complementary to other experimental studies, including gravitational-wave detectors such as LIGO, which test gravity in other conditions or see different effects. Over the very long timeline of our experiments, we’ve been able to measure many effects not measurable in any other system or through any other technique.”

Video credit: Josh White/Maura McLaughlin/Nihan Pol

Over the very long timeline of our experiments, we’ve been able to measure many effects not measurable in any other system or through any other technique.

Maura McLaughlin

A Double Pulsar: what does it mean?

The double pulsar system 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 (in astronomical terms) relatively young and has a rotation period of 2.8 seconds. The motion of the two pulsars around each other makes them an almost perfect laboratory for tests of gravity.

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 neutron stars 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

Seven radio telescopes from around the world were used as key observational tools in the study.

Among the seven radio telescopes was the National Science Foundation’s Green Bank Telescope (GBT) in Green Bank, West Virginia (below).

The GBT has provided monthly observations of the system for roughly 14 years.  With its 100 meter diameter and state-of-the-art receivers, it is the cornerstone of the observational campaign, as it provides more sensitive timing observations than any other telescope. Lorimer emphasizes that “since there is not enough time available on a single telescope to carry out the necessary observations, it is vital that different telescopes contribute to the study, however this makes the analysis extremely complex as the data must be combined with nanosecond precision.”

The results are published in APS Physical Review: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.041050