Two neutron stars – mind-bendingly dense remnants of stars long dead – are locked in an orbit so tight that gravity warps them into teardrop shapes.
The stars merge, and within seconds the newly combined star collapses on itself, driving a jet of hard radiation with such incredible ferocity it punches through the stellar carcass and begins tearing across the galaxy.
Gravitational waves, the kind of spacetime distortions created by the neutron star collision, were first predicted by Einstein in 1915 and first detected at LIGO 100 years later.
Central to Einstein’s picture of gravity is the idea that everything with mass warps the ‘fabric’ of space, so every planet, star or galaxy creates a kind of dent.
We already had evidence of this but the neutron star explosion was a direct confirmation, since the gravitational signal and the light travelled 130 million years and arrived at virtually the same time.
New theories of gravity that break the equivalence principle have been proposed to solve problems like dark matter and dark energy.
These theories often have light and gravity following different paths through space, to explain differences between what we see and what general relativity predicts without dark matter and dark energy.