Is Einstein’s theory of gravity still relevant — we put it to its toughest test yet

For more than 100 years, Albert Einstein’s general theory of relativity has been our best description of how the force of gravity acts throughout the Universe.

General relativity is not only very accurate but ask any astrophysicist about the theory and they’ll probably also describe it as “beautiful”. But it has a dark side too: a fundamental conflict with our other great physical theory, quantum mechanics.

General relativity works extremely well at large scales in the Universe, but quantum mechanics rules the microscopic realm of atoms and fundamental particles. To resolve this conflict, we need to see general relativity pushed to its limits: extremely intense gravitational forces at work on small scales.

We studied a pair of stars called the Double Pulsar which provide just such a situation. After 16 years of observations, we have found no cracks in Einstein’s theory.

Pulsars: nature’s gravity labs

In 2003, astronomers at CSIRO’s Parkes radio telescope, Murriyang, in New South Wales discovered a double pulsar system 2,400 light years away that offers a perfect opportunity to study general relativity under extreme conditions.

To understand what makes this system so special, imagine a star 500,000 times as heavy as Earth, yet only 20 kilometers across. This ultra-dense “neutron star” spins 50 times a second, blasting out an intense beam of radio waves that our telescopes register as a faint blip every time it sweeps over Earth. There are more than 3,000 such “pulsars” in the Milky Way, but this one is unique because it whirls in an orbit around a similarly extreme companion star every 2.5 hours.

According to general relativity, the colossal accelerations in the Double Pulsar system strain the fabric of space-time, sending gravitational ripples away at the speed of light that slowly sap the system of orbital energy.

This slow loss of energy makes the stars’ orbit drift ever closer together. In 85 million years time, they are doomed to merge in a spectacular cosmic pile-up that will enrich the surroundings with a heady dose of precious metals.