Black Holes Are Even Stranger Than You Can Imagine

The Conversation

Our love of black holes continues to grow as our knowledge of these celestial bodies expands. The latest news is the discovery of a rare “middleweight” black hole, a relative newcomer to the black hole family.

We already knew that some black holes are just a few times the mass of our Sun, while others are more than a billion times as massive. But others with intermediate masses, such as the one 2,200 times the mass of our Sun recently discovered in the star cluster 47 Tucanae, are surprisingly elusive.

So what is it about black holes, these gravitational prisons that trap anything that gets too close to them, that captures the imagination of people of all ages and professions?

‘Dark stars’

As far back as 1783, within the framework of Newtonian dynamics, the concept of “dark stars” with sufficiently high density that not even light can escape their gravitational pull had been advanced by the English philosopher and mathematician John Michell.

Almost immediately after Albert Einstein presented his theory of general relativity in 1915, which supplanted Newton’s description of our Universe and revealed how space and time are intimately linked, fellow German Karl Schwarzschild and Dutchman Johannes Droste independently derived the new equations for a spherical or point mass.

Although at the time the issue was still something of a mathematical curiosity, over the ensuing quarter of a century nuclear physicists realised that sufficiently massive stars would collapse under their own weight to become these previously theorised black holes.

Their existence was eventually confirmed by astronomers using powerful telescopes, and more recently colliding black holes were the source of the gravitational waves detected with the LIGO instrumentation in the United States.

A dense object

The densities of such objects is mind-boggling. If our Sun were to become a black hole, it would need to collapse from its current size of 1.4 million km across to a radius of less than 3km (6km across). Its average density within this “Schwarzschild radius” would be nearly 20 billion tonnes per cubic centimetre.

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