Detect space-time distortions in a pair of pulsars

In 1915, Albert Einstein formulated the theory of general relativity, which radically changed the concept of gravity that had been held until then. The charismatic scientist explained gravity as the manifestation of the curvature of space and time. Einstein’s theory predicts that the flow of time is altered by mass. This effect, known as “gravitational time dilation,” slows down time near a massive object. It affects everything and everyone; in fact, people living on the ground floor of buildings age more slowly than their neighbors upstairs, about 10 nanoseconds each year. This tiny but real effect has been confirmed in many experiments with very precise clocks.

Now an international team including, among others, Michael Kramer from the Max Planck Institute for Radio Astronomy in Germany, Ingrid Stairs from the University of British Columbia in Canada, Robert Ferdman from the University of East Anglia in the UK, and Dick Manchester of CSIRO (the Australian national science agency), has presented its findings on the relativistic effects experienced by a pair of pulsars located about 2,400 light-years distant from Earth.

A pulsar is a neutron star that spins so fast that it usually takes much less than 1 second to make one complete revolution. The pulsar emits electromagnetic waves from its magnetic poles. The misalignment of the magnetic poles with the axis of rotation of the neutron star causes the radiation beams to rotate in the same way as the foci of a maritime lighthouse, sending beam pulses towards eventual distant observers. The period between each pulse corresponds to the rotation speed of the neutron star.

A neutron star is the dead core of a star that previously exploded as a supernova but, despite being highly compressed, has not become a black hole. Although it is not as tightly packed as a black hole, its density is so great that in atoms it forces electrons to “stick” against protons, giving rise to neutrons. Hence, these kinds of objects are called neutron stars.

The observed pair of pulsars was discovered in 2003 and is a magnificent natural laboratory for testing the theory of general relativity. The two pulsars orbit each other, taking only 147 minutes to make a full revolution and reaching speeds of around 1 million kilometers per hour. One of the pulsars rotates very fast on itself, executing about 44 complete rotations every second. His partner is young and has a rotation period of 2.8 seconds. The movement of each pulsar around its partner has been helpful in measuring relativistic distortions.

Artistic recreation of a pair of pulsars. (Image: Max Planck Institute for Radio Astronomy)

This fast orbital movement of objects as compact as these (they have 30% more mass than the Sun but only about 15 miles in diameter) has allowed to verify seven different predictions of the theory of general relativity.

The research team has seen for the first time how the light is not only delayed due to a strong curvature of space-time around the companion pulsar, but also that the light is deflected by a small angle of 0.04 degrees.

In addition to the emission of gravitational waves and the propagation of light, the team has managed to measure the time dilation effect that makes clocks run slower when the gravitational field is stronger.

The study authors have also managed to “weigh” the electromagnetic radiation emitted by the fast-spinning pulsar in orbital motion. This radiation corresponds to a loss of mass of 8 million tons per second. Although this sounds like a lot, it is only a tiny fraction of the pulsar’s mass.

The researchers also measured (with a precision of 1 part in a million) that the orbit changes orientation, a relativistic effect also known in the orbit of Mercury, but that in the pair of pulsars it becomes 140,000 times stronger.

The study is titled “Strong-field Gravity Tests with the Double Pulsar.” And it has been published in the academic journal Physical Review X. (Source: NCYT by Amazings)