An international team has discovered an exotic double object that consists of a tiny, but unusually heavy neutron star that spins 25 times each second, orbited every two and a half hours by a white dwarf star. The neutron star is a pulsar that is giving off radio waves that can be picked up on Earth by radio telescopes. Although this unusual pair is very interesting in its own right it is also a unique laboratory for testing the limits of physical theories.
This pulsar is named PSR J0348+0432 and is the remains of a supernova explosion. It is twice as heavy as the Sun, but just 20 kilometres across. The gravity at its surface is more than 300 billion times stronger than that on Earth and at its centre every sugar-cubed-sized volume has more than one billion tonnes of matter squeezed into it. Its companion white dwarf star is only slightly less exotic; it is the glowing remains of a much lighter star that has lost its atmosphere and is slowly cooling.
Einstein’s general theory of relativity, which explains gravity as a consequence of the curvature of spacetime created by the presence of mass and energy, has withstood all tests since it was first published almost a century ago. But it cannot be the final explanation and must ultimately break down.