Are black holes a threat to Earth?

What is a black hole?

Let’s begin at the beginning. The stuff of the most primal science-fiction nightmares, a black hole is a gargantuan, all-devouring celestial body whose gravitational field is so powerful that nothing can escape its void. Not even light.

This is why - prior to the Event Horizon Telescope Project returning our first photograph of one on 10 April 2019, a major breakthrough - no human being had ever actually set eyes on a black hole, in spite of their phenomenal size.

There are three types: primordial, stellar and supermassive.

The first is the smallest, the second the most common around our Milky Way like the Sagittarius A* and the third, the largest: looming monstrosities lurking at the centre of galaxies, their origins unknown.

A black hole’s event horizon is the point of no return beyond which anything gets swallowed into oblivion. Stars, planets, gas, dust, all forms of electromagnetic radiation, you name it.

Albert Einstein is credited with first predicting the existence of black holes through his General Theory of Relativity in 1916 but the name for the phenomenon was only coined by American astronomer John Wheeler as recently as 1967, three years after one was actually located for the first time. That was the Cygnus X-1, tracked down through the advent of X-ray astronomy.

Do they pose a threat to life as we know it?

To put things into perspective, the picture circulated by excited astronomers two years ago proclaiming that they had “seen the unseeable” shows a black hole at the centre of the Messier 87 galaxy, which is situated around 54 million light years from Earth in the Virgo cluster.

Having said that, scientists were this summer astonished to find a vast collection of more than 100 black holes loitering inside our own galaxy, nestled in among the Palomar 5 star cluster a mere 80,000 light years from Earth. Actually, that’s still a lot.

While so much about black holes remains a mystery, many of our ideas about them amount to little more than theoretical speculation.

The consensus certainly seems to be that they are extremely dangerous, with the thought of their ability to rip stars apart a deeply unsettling one.

The chances of survival for a human astronaut attempting to pass through one and use it as a “wormhole” to travel to another galaxy, as seen so often in Hollywood blockbusters like Christopher Nolan’s Interstellar (2013), are thought not to be too promising.

The fearless space adventurer is more likely to be stretched like spaghetti before being torn apart or immediately incinerated amid a maelstrom of burning particles, depending on who you talk to. That’s even assuming they don’t collide with a whirling spiral of space debris beforehand.

As for whether our world is in danger of being swallowed up by a black hole anytime soon, it’s probably best to just ask Nasa.

“The universe is a big place,” writes the US space agency’s Elizabeth Landau. “In particular, the size of a region where a particular black hole has significant gravitational influence is quite limited compared to the size of a galaxy. This applies even to supermassive black holes like the one found in the middle of the Milky Way. This black hole has probably already ‘eaten’ most or all of the stars that formed nearby, and stars further out are mostly safe from being pulled in. Since this black hole already weighs a few million times the mass of the Sun, there will only be small increases in its mass if it swallows a few more Sun-like stars.

“There is no danger of the Earth (located 26,000 light years away from the Milky Way’s black hole) being pulled in.

She continues: “Future galaxy collisions will cause black holes to grow in size, for example by the merging of two black holes. But collisions won’t happen indefinitely because the universe is big and because it’s expanding, and so it’s very unlikely that any sort of black hole runaway effect will occur.”

Landau does note a strange kinship between these monstrosities and our own planet, however.

“Stellar-mass black holes are left behind when a massive star explodes, she explains. “These explosions distribute elements such as carbon, nitrogen and oxygen that are necessary for life into space. Mergers between two neutron stars, two black holes, or a neutron star and black hole, similarly spread heavy elements around that may someday become part of new planets. The shock waves from stellar explosions may also trigger the formation of new stars and new solar systems.

“So, in some sense, we owe our existence on Earth to long-ago explosions and collision events that formed black holes.”

Why are we asking this now?

Our knowledge of black holes was expanded again recently when scientists observed light from the other side of one for the first time ever, a prospect previously considered impossible because of their ravenous appetite for light.

Astrophysicists from Stanford University studying X-rays being thrown out into the universe by a supermassive black hole sitting in the middle of another galaxy 800 million light years away reported evidence of light “echoes” reverberating.

Publishing their findings in the academic journal Nature, the scientists said the X-rays under observation kept to a strange pattern: after the bright, initials flarings, there would be smaller and later ones, which appeared in different “colours” than those more obvious ones, defying expectations.

“The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself,” astrophysicist Dan Wilkins explained.

Their research project had begun with a slightly different aim of investigating a more common light formed by a black hole: the corona that wraps around the outside of it, formed as material falls in.

That corona is among the brightest continuous sources in the universe, flinging out X-ray light that can then be used to analyse the black hole itself, a subject that continues to attract the undying fascination of stargazers the world over.