It used to be thought that the world was flat. That beyond the horizon lurked a bottomless void, measureless to humans. As our ships got better and our navigators more confident, vessels that disappeared over one horizon began returning triumphant from the other. The Earth was curved, a sphere with no edges. We didn’t fall off because a mysterious force called gravity kept everyone and everything stuck to the planet’s surface.
Then Albert Einstein came along and told us that space itself was curved. Now we realise that if we sail off into the sea of curved space we call the Universe, there are indeed bottomless voids that the unwary traveler can fall into – and they are measureless to humans. They lurk on the other side of their own dark horizons, and we call them Black Holes.
A black hole is an astronomical contradiction – a dark star, an invisible nothing, a prison of light. Its boundary is marked by the so-called Event Horizon, a sphere of darkness that shrouds the inside and defines the point of no return. There is no solid surface beyond, just a bottomless gravitational whirlpool so strong that it sucks everything – even light – relentlessly inward. Oblivion waits at the centre in the form of the Singularity, Gravity’s fatal attractor.
Hidden eternally from view, the Singularity marks the spot where an immense gravitational force has been concentrated. All the mass, light and energy that has ever fallen into the black hole is compressed by its own overwhelming gravity into a point that is infinitely small and infinitely dense. The more a black hole swallows, the heavier it gets, yet the singularity never changes. Space has been squelched out of existence and Time has been squeezed to a stop. Step over the event horizon and for all intents and purposes you’ve fallen off the edge of the universe.
A BRIEF HISTORY OF GRAVITY
The concept of Gravity, this unseen force that dominates our lives and pulls us eternally towards the ground, has long challenged the greatest human minds. Even in Galileo’s day, the tower at Pisa had a good lean to it – perfect for dropping things off. Galileo wondered why no matter how heavy or light objects were, they all took the same amount of time to fall to Earth. He puzzled too about why the planets moved they way they did. His conviction that they orbited around the sun led to house arrest for heresy. He was still trying to put the gravity puzzle together when he died in Florence in 1642.
On Christmas Day that same year, the gravity baton was handed to Isaac Newton, born weak and premature in a Lincolnshire farmhouse. Twenty three years later, Newton returned to Woolsthorpe Manor to sit out the plague sweeping southern England. With 18 months quiet thought in the countryside he discovered calculus, unravelled the nature of light, and began formulating laws for the motion of the planets: discoveries that still underpin most of modern physics. One day when he was having a short break with a cup of tea, a falling apple interrupted his thoughts and led him to ponder gravity itself.
The reason the apple fell straight down was that it was trying to fall to the centre of the earth where gravitational attraction was focused. And the Earth wasn’t the only object that had gravity, so did the moon, the sun and the planets. In fact, Newton reasoned, every object in the universe – including ourselves – has gravity. The bigger and heavier, the greater it’s gravitational force. We are glued to the surface of the Earth – and not the other way round – only because it is has so much more mass. The Earth orbits around the sun for the same reason. Finally, Newton had found a reason for the heavens to move the way they do.
In 1784, John Michell, Rector of Thornhill Church in Yorkshire and great forgotten 18th Century scientist, became intrigued with the idea of escape velocity – the minimum speed with which you need to travel upwards from a star or planet to escape its gravitational clutches. He knew that gravity depended upon mass and he knew the speed of light was fast but finite. How heavy, he wondered, would the sun have to become before its gravity would become so great that even light (which travels at 299,792 km/second) would be held back at its surface? The answer, Michell reasoned, was that if the sun was the same size but weighed 500 times more, the light from the Sun would not escape the Suns own gravity. The Sun would simply disappear from view. A few years later the great French mathematician Laplace came to the same conclusion independently. The concept of the Dark Star was born.
A UNIVERSE OF HOLES
Black holes remained an ignored theoretical curio until a young clerk in a Swiss patent office published his General Theory of Relativity in 1915. Albert Einstein realised that the universe was a fundamentally different place to the clockwork universe of Newton and commonsense. The three dimensions of space could not be separated from the fourth dimension of time. Together they form the ‘Spacetime’ continuum, a kind of invisible scaffolding that defines existence. Spacetime, though, is not an absolute, fixed thing. It can be warped, bent and curved.
Spacetime is ‘straight’ only when it doesn’t have anything in it. Wherever there is mass, there is gravity. Wherever there is gravity, space is curved. The curvature of space dictates how an object will move through it. The object will dictate how space bends around it. Gravity, according to Einstein, is the curvature of space.
Einstein’s thought experiment was to imagine space and time being squashed flat like a 2D rubber sheet. Put a massive object like the sun on the sheet and it will bend. The more dense an object is, the deeper the depression it makes in Spacetime and the stronger the gravity. Eventually a point is reached when the walls of the depression are stretched so steeply that nothing can climb out of it. It is, quite literally, a hole in the universe.
A STAR IS BORN
To understand the very large in the universe, you need to start with the very small. With the unlocking of the secrets of nuclear energy, scientists finally got a clue to how black holes might form in nature. Stars are born when enormous clouds of cosmic dust and hydrogen begin to clump and condense under their own gravitational weight. Gravity grows stronger by the hour as the increasing density of the protostar curves space more strongly. Faster and faster, the hydrogen gas falls in upon itself in the condensing core. The more it collides the hotter it grows. When the core reaches 10 million degrees, the hydrogen protons begin to fuse into helium. Some of the mass disappears, having being turned into energy and light. Like a giant cosmic light bulb, the star has switched itself on.
Every star we see in the heavens has a giant nuclear reaction raging at its core. It’s what makes a star like our sun shine so hot and bright. Gravity is still trying to pull the star’s gas tighter and tighter but is matched now by the energy pouring outwards from the nuclear reaction in the core. The star settles into a precarious balance that gravity will always win in the end.
THE BIG SQUEEZE
The ultimate fate of a star depends upon its mass. Our sun is middle-aged. It switched on 5 billion years ago and has enough fuel to burn for 5 billion more. But when, in that far distant future, the spent heart of the sun sheds its outer layers and shuts down, gravity will squeeze the core so tight it cannot be squeezed any more. It will become a ‘white dwarf’, a feeble ember the size of the earth but a hundred thousand times more dense.
The more massive the star is, the faster it burns its fuel and the shorter its life expectancy. A star 10 times as massive as the sun may survive only millions, not billions, of years. As it starts to collapse, the crush of in-falling matter slamming into the iron core sends the temperature rocketing to 50 billion degrees. The core has only seconds to respond – and it does so Supernova-style.
A supernova is a massive explosion. Huge quantities of material are blown into space, but only from the outer regions of the star. Most of the star has actually imploded, with the core being given a gravity bear hug so extreme that the protons and electrons have been squeezed into a ball of superdense subatomic particles called neutrons. The resulting ‘neutron star’ would weigh about one and a half times as much as the sun but would measure only about 20 kilometres across – about the size of Brisbane.
Astronomers can prove that neutron stars exist, because they give off a unique distress signal. Like a lighthouse warning of a dangerous shore, a neutron star sweeps space with a blinding beam of radiation, generated by a magnetic field more than a trillion times greater than the Earth’s. Such a neutron star is called a pulsar. To astronomers, the pulsing beam sweeping the darkness of space is an unmistakable warning that extreme gravity lurks nearby.
GRAVITY’S FINAL TRIUMPH
A neutron star resist the ongoing crush of gravity, only with its neutrons packed in like sardines in a tin. But if the remnants of the star after supernova weigh more than three times the mass of the sun, even neutrons cannot hold back the inexorable force of gravity. The neutrons are squashed into oblivion. The star’s core becomes so dense that gravity overwhelms space itself, distorting it so horribly that it, and time with it, is wrenched off from the outside universe. A darkness forms at the star’s heart and moves relentlessly outwards as the stars brilliance is sucked inwards. This is the hungry, growing maw of a black hole: gravity’s final triumph. There is no escape, no turning back, until the entire mass of the star has been swallowed and its brilliance completely extinguished.
A BLACKNESS BEYOND BLACK
Visible only by its invisibility, the margin of the black hole is marked by the event horizon, so-called because all events beyond are hidden from view. For a black hole like this the event horizon may be only a few kilometres in diameter but the void beyond impossibly deep to measure. The entire mass of the star has been reduced to a singularity – a point of infinite smallness and infinite density at the very centre of this black malevolence.
The singularity is where science ends and speculation begins. Space and time have ceased to exist, replaced by a seething chaotic mass we call quantum foam. This bizarre conjecture is where Einstein’s laws fail. This is where the laws of quantum mechanics fail. This is the realm of something called Quantum Gravity – one of the hottest areas of advanced mathematical research.
It is from a singularity that the Universe is believed to have begun. In many ways the collapse of a star to form a black hole singularity is the reverse of the Big Bang. Is this the way the Universe is going to end? Wilder speculation is that our entire universe might lurk inside someone else’s singularity. or even that universes can bud off from each other like this, like some sort of heavenly breeding organism.
It wasn’t until 1967 that John Archibald Wheeler slipped the term “Black Hole” into his paper at a scientific conference, and into the lexicon of the late 20th Century. They may have become a household name – but are they real?
BLACK HOLE OR WHITE ELEPHANT
Einstein himself couldn’t believe that such an invisible impossibility as a black hole could exist in the real universe beyond his theories. Today, his successors have no such problems. Astronomers not only think they have identified nearly 30 black hole candidates in our own Milky Way galaxy, they are now getting the proof that the holes behave in the relativistic way that Einstein’s theories predict.
A black hole is an elusive quarry with perfect camouflage: total blackness in the blackness of space. Searching for a black hole no bigger than Sydney’s CBD across hundreds or thousands of light years of space demands a sneaky approach. First you have to find a visible star that a black hole has trapped in orbit. Then you have to study how the star wobbles. John Wheeler described it as like looking for a pair of dancers on a dark dance floor. The heavy man dressed in black is invisible, but the bright white dress of a light women is an easy target as she is whirled around. Astronomers look for the bright stars that ‘orbit’ dark partners in the same way.
One of the best candidates is the star called V404 Cygni. Calculations shows V404’s dark partner is twelve times more massive than our Sun, yet totally invisible. But for every black hole orbiting another star, there must be many more solitary ones yet unseen. One of these could lurk quietly much closer to home.
THE COSMIC HOOVER
Although black holes have the power to hoover up anything and everything that strays too close, they can’t hunt. Contrary to popular belief, if you replaced our Sun with a black hole of the same mass the Earth wouldn’t get sucked in, there just wouldn’t be any sunlight. You could even orbit a black hole in a spacecraft just so long as you kept a safe distance.
Get too close though and strange things start happening. Space gets stretched longer and skinnier. You would find your feet being pulled miles away in front of you while your body is squeezed sideways. You will have become a piece of space spaghetti long before you reach the event horizon. Then you’d be ruptured into your own fundamental particles and disappear behind the veil of darkness.
It’d be a spectacular way to go but no-one would see it because time is being stretched as well. The photons carrying the image would struggle harder to leave your body the closer you fell. Even with a few million years to spare, an outside observer will see you slow to a halt above the event horizon, before slowly fading from view.
HEARTS OF DARKNESS
Most astronomers now concede that a black hole heavyweight lurks in the centre of our own Milky Way galaxy. Latest estimates are that it weighs in at a whopping 2 million times the mass of the sun – a dwarf in comparison to some of the truly supermassive black holes that may lurk in the cosmos.
By the 1950s, astronomers began turning optical telescopes towards some of the strongest signals that the new radio telescopes were picking up. Source number 3C 273 was found to be a bright star-like object with a ‘jet’ of intense radiation sticking out of it. It was the first of a number of similar objects given the name of ‘quasar’ or ‘quasi-stellar radio source’, but their real identity remained hidden for decades.
Quasars have now been revealed to be the energetic hearts of very active galaxies: brilliant discs of superheated gas and ruptured stars swirling at nearly the speed of light. Great jets of charged particles are blasted thousands of light years into space from above & below – like an axle through a wheel. The central engine that is driving all this activity, though, is hidden deep inside. It has to be small and it must be extraordinarily dense. The mathematics demand that the only beast that can drive such a display of raw power is a supermassive black hole. The heavier the hole, the faster the gases whirl in orbit. Astronomers have observed speeds which tally with black holes weighing up to five thousand million suns.
The theory goes like this: a galaxy evolves from a vast rotating cloud of gas that begins to clump and condense under its own weight into billions of stars arranged like an enormous Catherine wheel, a Mexican hat or a bee swarm. In the centre, where the gas is concentrated, enough matter to make millions or even billions stars has undergone titanic gravitational collapse to make a supermassive black hole. While the hole is still actively feeding on the inner part of the new galaxy it manifests itself as a quasar. Later, when all nearby food has been consumed, the black hole becomes quiescent, leaving a relatively quiet galactic core like the one in the Milky Way. If this theory is correct, then supermassive black holes are present in all but the smallest galaxies.
TOWARDS THE WITHIN
For all their ferocity, these supermassive black holes are surprisingly gentle giants up close. You can fall into one without turning to spaghetti.
Suppose you or I were an astronaut about to step into such an abyss at the edge of the universe. As I approach the event horizon, blackness spreads upwards around me. The Universe shrinks to a bright point directly overhead. As I meet and cross the horizon the universe above disappears in a blinding flash of photons trapped in orbit around the hole.
I am now inside the black hole and falling towards the Singularity. It’s not dark like I expected. I see a ring of dancing light where the singularity should be. It must be spinning so fast that the centrifugal force has balanced out gravity. Now it’s a naked glowing hula-hoop of indeterminate size. Around it I see glimpses of heavens unimaginable to humans, universes within universes, time within time…
But hey! No matter what I might see or experience inside the black hole, I could never send a message out. The secrets I discover will die with me as I achieve oneness with the Universe, at the central Singularity.
Also published on Medium.
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