The earthlings' guide to a black hole

NEW YORK • Welcome, earthlings, to the place of no return: A region in space where the gravitational pull is so strong that not even light can escape it.

This is a black hole. It is okay to feel lost here. Even Albert Einstein, whose theory of general relativity made it possible to conceive of such a place, thought the concept was too bizarre to exist.

But Einstein was wrong, and here you are. You should not be here. Surely you will be pulled in. But fear not, dear earthling: Your brain has taken millions of years to get here, and it is ready for this gaze into the darkness. So, let us get started.

A BLACK HOLE IS A HUNGRY BEAST

It swallows up everything too close, too slow or too small to fight its gravitational force. With every planet, gas, star or bit of mass consumed, the black hole grows.

The edge of a black hole, its event horizon, is the point of no return. At the event horizon, light is drawn into a black hole, never to escape. And nothing is faster than light.

IF YOU FALL INTO A BLACK HOLE, IT IS NOT CLEAR HOW YOU WILL DIE

Will gravity rip you apart and crush you into the black hole's core? Or will a firewall of energy sizzle you into oblivion? Could some essence of you ever emerge from a black hole?

The question of how you would die inside a black hole is one of the biggest debates in physics. Called the firewall paradox, it was posited in March 2012 by a group of theorists, including Dr Donald Marolf, Dr Ahmed Almheiri, Dr James Sully and Dr Joseph Polchinski.

Based on the mathematics in Einstein's general theory of relativity of 1915, you would fall through the event horizon unscathed, then the force of gravity would pull you into a noodle and ultimately cram you into singularity, the black hole's infinitely dense core.

However, Dr Polchinski and his team pitted Einstein against quantum theory, which posited that an event horizon is a blazing firewall of energy that would torch your body to smithereens. However, the presence of a firewall would violate the precious principles of relativity, which decreed the existence of black holes. And so physics is stuck.

ASTRONOMERS HAVE EVIDENCE OF BLACK HOLES IN NEARLY EVERY GALAXY IN THE UNIVERSE

Nearly every galaxy - our own Milky Way, as well as the 100 billion or so other galaxies visible from Earth - shows signs of a super-massive black hole in its centre.

Moreover, the bigger a galaxy is, the more massive is its central black hole. Nobody knows why.

Of the billions of stars in the Milky Way, about one in every 1,000 new stars is massive enough to become a black hole. Our Sun is not. But a star 25 times heavier is.

'A BLACK HOLE HAS NO HAIR'

On March 28, 2011, astronomers detected a long gamma ray burst coming from the centre of a galaxy four billion light-years away.

This was the first time humans observed what might have been a dormant black hole eating a star.

No matter what a black hole eats, it is all the same to the black hole.

"A black hole has no hair," physicist John Archibald Wheeler once said, meaning that a black hole remembers only the mass, spin and charge of its dinner.

The more a black hole eats, the more it grows. In 2011, scientists discovered one of the biggest black holes ever, more than 300 million light-years away. It weighs enough to have gobbled up 21 billion suns.

Scientists want to know if the biggest black holes are the result of two holes merging or one hole eating a lot. But scientists do not know how they grew so large.

TO FIND THE DARKNESS, FOLLOW THE LIGHT

Light cannot escape a black hole, so seeing what is inside one is impossible. Getting a picture of a black hole's edge is difficult, and getting a clear picture is something else entirely.

And until now, it has never been done. So far, scientists have detected black holes only indirectly, by their signatures, such as a gamma ray burst, supernova or, perhaps, an object on the brink of a black hole's event horizon.

Typically, if tremendous energy is emanating from a massive core at the centre of a galaxy, the core is probably a black hole.

The Event Horizon Telescope, the one Dr Sheperd Doeleman and his colleagues used to photograph the black hole in the galaxy M87, features a cast of more than 100 scientists on four continents and one very important crystal used to calibrate atomic clocks.

In April 2017, scientists staked out eight telescopes atop mountains on four continents, synchronised them, pointed them at the sky and waited. And so they brought Einstein's monster, the black hole, into view for the first time.

A BLACK HOLE IS NOT FOR EVER

Quantum effects suggest that, as Hawking radiation leaks into the universe, a black hole will eventually dissipate. It would take many times the age of the universe for a black hole to evaporate fully.

Like Einstein, physicist Stephen Hawking did not believe his own theory at first. But the numbers were right. Physicists now view his result as the backbone for whatever future theory will bring together gravity and quantum theory.

A GIANT MAGNET IN EUROPE WILL NOT DESTROY THE PLANET

Before the European Organisation for Nuclear Research fired up the Large Hadron Collider in 2008, critics worried that smashing together protons in a 27.4km ring underground would create a black hole that would swallow the Earth.

The centre's scientists squelched the concerns by pointing out that, according to their calculations, ultrahigh-energy cosmic rays were already penetrating the atmosphere and would have created about 100 tiny black holes on Earth every year. If tiny black holes were a genuine problem, Earth would have collapsed into infinity long ago.

NYTIMES

A version of this article appeared in the print edition of The Straits Times on April 12, 2019, with the headline 'The earthlings' guide to a black hole'. Print Edition | Subscribe