The Big Picture,

Like death itself, it was once believed there was no escape from a black hole. Invisible and overwhelming, they are still one of the great mysteries of the cosmos, a point of no return that has given its name to anything in life that disappears without trace.

So strong is their gravitation pull that even light cannot escape. Science believes that they can be as small as a single atom, yet with the mass of a mountain. They are detectable only by the gravitational pull they exert on the stars and gasses around them.

The largest are called supermassive black holes, which have a mass equal to more than a million of our Suns. It is believed they are formed when a star collapses in on itself.

They are also irrevocably associated with Stephen Hawking, whose name is given to the radiation that black holes are predicted to emit, but which has not yet been observed.

Hawking, who has died aged 76, famously placed a bet with a rival theoretical physicist that black holes did not exist even though he did not believe this. His prize would be a magazine subscription – winning it would be compensation, he said, for the years of wasted work researching the subject.

It was once assumed that any object unlucky or foolish enough to approach a black hole would eventually reach what is known as the event horizon, a point at which the gravitational pull is so strong that nothing, not even even light, can move fast enough to escape.

Once trapped in a black hole, objects would be torn apart and then crushed to point where density and gravity become infinite, a point known as singularity and where the laws of physics no longer apply.

Hawking challenged the view that matter falling into a black hole would be trapped there for eternity. His theory was that black holes are slowly losing mass and will eventually disappear, leaving nothing.

This creates a paradox because the law of the conservation of mass says that in any given system, the mass of the chemical components before and after a reaction must be the same. It also apparently contradicts Einstein’s Theory of Relativity and the basics of quantum physics; the idea that no information can be lost for ever.

In his 2016 Reith Lecture for the BBC, Hawking observed that while it was impossible to see what is inside a black hole, it was not necessarily impossible to escape from one. Matter stored in a black hole would be released when it finally evaporated, he postulated.

“The message of this lecture,” he declared, “is that black holes ain’t as black as they are painted. They are not the eternal presence they were once thought. Things can get out of a black hole, both to the outside and possibly to another universe. So if you feel you are in a black hole, don’t give up. There is a way out.”