What is a black hole? A guide for earthlings

A black hole swallows up everything too close, too slow or too small to fight its gravitational force. PHOTO: REUTERS

NEW YORK (NYTIMES) - Welcome, earthlings, to the place of no return: a region in space where the gravitational pull is so strong, not even light can escape it. This is a black hole.

It's OK 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 shouldn't 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's ready for this gaze into the darkness. So let's get started.


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.


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 Donald Marolf, Ahmed Almheiri, James Sully and 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.

But 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.

Pretty much everything we know about how the universe works depends on black holes.

Either someone is wrong, or we have to admit that earthlings still aren't equipped to understand the universe. The firewall paradox calls into question the most definitive theories of science. The insights and wisdom of Einstein, Dr Polchinski or Professor Stephen Hawking notwithstanding, everything we know about the universe could change if we could know for certain what happens to information inside a black hole.


In 2003, an international team led by the X-ray astronomer Andrew Fabian discovered the longest, oldest, lowest note in the universe - a black hole's song - using Nasa's Chandra X-ray Observatory. The B-flat note, 57 octaves below middle C, appeared as sound waves that emanated from explosive events at the edge of a supermassive black hole in the galaxy NGC 1275.

The notes stayed in the galaxy and never reached us, but we couldn't have heard them anyway. The lowest note the human ear can detect has an oscillation period of one-twentieth of a second. This B-flat's period was 10 million years.

The "songs" of black holes may be responsible for a declining birth rate of stars in the universe. In clusters of galaxies such as Perseus, the home of NGC 1275, the energy these notes carry is thought to keep the gases too hot to condense and form stars.

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Although no black hole is close enough to Earth to pull the planet to its doom, there are so many black holes in the universe that counting them is impossible. Nearly every galaxy - our own Milky Way, as well as the 100 billion or so other galaxies visible from Earth - shows signs of a supermassive black hole in its centre.

Moreover, the bigger a galaxy is, the more massive 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. Stellar-mass black holes result from the death of these stars, and can exist anywhere in the galaxy.


On July 2, 1967, a network of satellites recorded an explosion of gamma rays coming from outer space. In retrospect, it was one of the first indications that black holes are real. Today, scientists believe that a gamma ray burst is the final breath of a dying star and the birth of a stellar-mass black hole.

The dramatic transformation starts when a massive star runs out of fuel. As the star begins to collapse, it explodes. The star's outer layers spew out into space, but the inside implodes, becoming denser and denser, until there is too much matter in too little space. The core succumbs to its own gravitational pull and collapses into itself, in extreme cases, forming a black hole.

Theoretically, if you shrank any mass down into a certain amount of space, it could become a black hole. Our planet would be one if you tried to cram Earth into a pea.


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 - a star, a donkey, an iPhone, your grammar teacher - it's 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 don't know how they grew so large.


Light can't escape a black hole, so seeing what's 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 Prof 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.


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

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


Before the European Organisation for Nuclear Research fired up the Large Hadron Collider in 2008, critics worried that smashing together protons in a 27km ring underground would create a black hole that would swallow the earth.

Scientists had smaller ones in mind. In theory, the search for the smallest particles in the universe might kick up mini black holes in the collider's underground tubes, enabling researchers to observe general relativity and quantum mechanics in action, and perhaps open the door to solving the firewall paradox.

A decade earlier, similarly apocalyptic worries arose over Brookhaven National Laboratory's Relativistic Heavy Ion Collider. The centre's scientists squelched these 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.

Still, in June 2008, a safety review proclaimed the Large Hadron Collider safe. Experiments commenced, the Higgs boson was found and Earth survived after all.

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