Physicists may have found evidence of echoes in the gravitational waves emanating from two colliding black holes. If confirmed, the discovery would show Einstein's general theory of relativity breaks down at the event horizon – the edge of a black hole that serves as the point of no return.

There are three different types of black hole, the most common being stellar black holes. These form when the centre of massive star collapses in on itself. It then becomes a region in space that has such a strong gravitational force that nothing – not even light – is able to escape.

At the centre is a gravitational singularity. This is a one-dimensional point where density and gravity become infinite and the laws of physics no longer exist. What happens at the black hole boundary – the event horizon – is not known, but several theories have been put forward.

Earlier this year, scientists announced the discovery of gravitational waves – ripples in the fabric of spacetime first predicted by Einstein 100 years ago. The waves are generated by massive accelerating objects in space – like black holes – and scientists can trace them back to their origin.

gravitational waves
LIGO data showing gravitational wavesNSF
Using the Laser Interferometer Gravitational-Wave Observatory (LIGO), scientists identified gravitational waves from two colliding black holes. Since then, they have observed three gravitational wave events.

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