Now there ’s a look in your eyes, Like black holes in the sky.
PINK FLOYD, ‘Shine on you crazy diamond’
The black holes of nature are the most perfect macroscopic objects there are in the Universe: the only elements in their construction are our concepts of space and time.
SUBRAHMANYAN CHANDRASEKHAR
Black holes are regions of space–time where gravity is so enormously strong that nothing, not even light, can escape. Probably, you think these celestial objects are esoteric and have no bearing on your everyday life. Nothing could be further from the truth. The birth of the Milky Way Galaxy, without which you would not be reading these words, might have been triggered by a black hole. Not only that but black holes reveal something about everyday reality that is so startling it is scarcely believable. Our Universe might be a giant hologram. You might be a hologram.
A black hole is a testament to the irresistible force of gravity, which, like the German football team, can be halted temporarily but always wins in the end. It triumphs because it is an attractive force between every piece of matter in the Universe and every other piece and nothing can nullify it. By contrast, the electromagnetic force – which holds together the atoms in your body – can be both attractive and repulsive and, on the large-scale at least, it is pretty much always cancelled out.
A black hole is a prediction of Einstein’s theory of gravity, the general theory of relativity. It is cloaked by an event horizon, an imaginary membrane that marks the point of no return for in-falling matter and light. If an astronaut were able to hover just outside the event horizon, his time would slow down so much, as a consequence of Einstein’s theory, that, in principle, it would be possible for him to look outwards and watch the entire future history of the Universe flash past his eyes like a movie in fast-forward.1
Inside the horizon, the distortion of time is so great that time and space actually swap places. This is why the singularity, the point at which in-falling matter is crushed out of existence at the centre of the hole, is unavoidable. It is exists not across space but across time so can no more be avoided than you can avoid tomorrow.
At the singularity, all physical quantities such as density sky-rocket to infinity. ‘Black holes are where God divided by zero,’ said the American comedian Stephen Wright. The singularity is an indication that Einstein’s theory of gravity has been stretched beyond its limits and no longer has anything sensible to say. Almost certainly, a better theory – a quantum theory of gravity – will show that the singularity is not a singularity but instead just a super-high-density knot of mass energy.
‘The black hole teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as “sacred”, as immutable, are anything but,’2 said John Wheeler, the American physicist who popularised the term black hole.3
Although the general theory of relativity predicted black holes, Einstein never believed in their existence. This is not uncommon in physics. Theorists often have difficulty overcoming their sheer disbelief that nature really dances to the tune of the arcane symbols they scrawl across a whiteboard. ‘Our mistake is not that we take our theories too seriously,’ as Nobel Prize-winning physicist Steven Weinberg observed, ‘but that we do not take them seriously enough.’
Stellar black holes – which form from the catastrophic shrinkage of massive stars at the end of their lives – are, by their very nature, hard to spot. After all, they are very small and, well, very black. However, if a black hole is in a binary star system, we may see the telltale X-rays emitted by matter ripped from a companion star and heated to incandescence as it is sucked down into the hole. The first stellar-mass black hole, Cygnus X-1, was discovered by the Uhuru satellite in 1971. But, actually, something that would turn out to be far more significant in the black-hole story – and, crucially, more significant for us - was found eight years earlier.
Quasars, discovered by Dutch-American astronomer Maarten Schmidt in 1963, are the super-bright cores of galaxies, blazing like beacons at the edge of the Universe. Because their light has taken most of the age of the Universe to reach us, they also blaze at the beginning of time. Typically, a quasar pumps out the energy of 100 normal galaxies such as the Milky Way but from a region smaller even than our Solar System. Nuclear energy – the power source of the stars – is woefully inadequate. The only process that can explain the prodigious energy output of quasars is matter heated to incandescence as it swirls down into a black hole. But not a mere stellar-mass black hole – a black hole with a mass of billions of suns.
For a long time after Schmidt’s discovery, astronomers thought that such supermassive black holes were cosmic anomalies. They believed that such monsters powered only the badly behaved 1 per cent of galaxies of which the most extreme examples were quasars. But, over the past few decades, it has become clear that there is a supermassive black hole not only in the heart of these active galaxies but in the heart of pretty much every galaxy, including our own Milky Way. Most are quiescent, having gorged on and utterly exhausted their feedstuff of interstellar gas and ripped-apart stars.
The origin of supermassive black holes – unlike their stellar-mass cousins – is a mystery. Perhaps they are born when stellar-mass black holes collide and coalesce in the crowded heart of a galaxy. Or perhaps they form directly from the shrinkage of a giant pre-stellar cloud of gas. One thing is for sure: they grow extremely big extremely quickly. By the time the Universe was 500 million years old – a mere 5 per cent of its current age – there were supermassive black holes in existence that had already reached billions of solar masses.
But, although supermassive black holes are impressive on a human scale, they are minuscule on a cosmic scale. Not only are they tiny compared with their parent galaxies – even the biggest would fit easily within our Solar System – but they also have very small masses compared with the mass of the stars in their galaxies.
Despite this, they appear to control the stellar content and structure of their parent galaxies. For instance, the mass of the stars in the core of a galaxy is invariably about 1,000 times the mass of the central black hole, hinting that there is an intimate connection between a supermassive black hole and its parent galaxy. Consider for a moment how surprising this is. It is as if the growth of a mega-city such as Los Angeles were controlled by something as small as a single mosquito.
The means by which tiny supermassive black holes project their power over vast reaches of space are jets. Propelled by twisted magnetic fields in the gas swirling down to oblivion, these channels of super-fast matter stab outwards from the poles of the spinning black hole. They punch their way through the galaxy’s stars and out into intergalactic space, where they puff up titanic balloons of hot gas – some of the largest structures in the known Universe.
In fact, such balloons of gas were the first hint that science got of the existence of supermassive black holes. In the 1950s, radio astronomers, using equipment adapted from war-time radar, discovered that the radio emission observed from some galaxies came not, as expected, from the central knot of stars but, mysteriously, from giant, radio-emitting lobe, on either side of the galaxy.
In the early 1980s, the thread-thin jets that are feeding the lobes were imaged for the first time by the 27 radio dishes of the Very Large Array in New Mexico. They mock our puny attempts at accelerating matter. Whereas the Large Hadron Collider near Geneva can whip a nanogram or so of matter to within a whisker of the speed of light, nature can boost many times the mass of the Sun each year to similar speeds along cosmic jets.
The jets control the structure of their parent galaxies because, in the inner regions where the jets are still fast and powerful, they drive out all the gaseous raw material of stars, snuffing out star formation. In the outer regions of galaxies, however, where the jets are slower, the jets have the opposite effect. As they slam into gas clouds, the concussion may trigger them to collapse under gravity to give birth to new clutches of stars.
But supermassive black holes, by starting and stopping star formation, do not merely sculpt galaxies. They might also determine the very character of the stars that form in them. Galaxies that contain the biggest supermassive black holes – so-called giant elliptical galaxies – appear to contain a much greater proportion of cool, red, long-lived stars, and there is evidence that the black hole might be responsible. Such red dwarfs spawn planets with few heavy elements such as carbon and magnesium and iron. Crucially, these are essential for life.
This has implications for our own Galaxy because, 27,000 light years away in the dark heart of our Milky Way, lurks a supermassive black hole 4.3 million times the mass of the Sun. Sagittarius A* might sound impressive but, actually, it is an insignificant tiddler compared with its 30-billion-solar-mass cousins in the cores of some quasars. Until recently, it was believed to be a mere coincidence that our Galaxy contains only a relatively small supermassive black hole. But is it? The giant elliptical galaxies that litter the cosmos might be chock-a-block full of planets but every last one of them might be a desert world, sterile and lifeless. The benign black hole in the heart of our Milky Way might be a large part of why we find ourselves here and not somewhere else. It might be a large part of why you are at this moment reading these words.
Actually, we might be even more beholden to supermassive black holes than this. Most astronomers believe that galaxies give birth to supermassive black holes. But there is a contrary view and that is that supermassive black holes give birth to galaxies.
In this view, a giant gas cloud out in space shrinks catastrophically under its own gravity and, without forming any stars first, spawns a supermassive black hole. When its jets switch on, they stab outwards across space. If they happen to slam into an inert gas cloud floating in the void, the concussion causes the cloud to collapse, fragmenting into stars. In other words, it makes a galaxy.
This is no idle theoretical speculation. Astronomers know of a supermassive black hole floating in the void without a discernible galaxy of stars surrounding it. Extending from this naked quasar are two oppositely directed jets. And, at the end of one, is a newborn galaxy about the size of our Milky Way. It appears to have been triggered to form about 200 million years ago when the jet stabbed like a laser beam into a sleeping gas cloud. In the future, the supermassive black hole will fall into the heart of the galaxy it created and the galaxy birth process will be complete.4
If the idea that supermassive black holes zap galaxies into being is right, it is an extraordinary story how these objects have come in from the cold. Once they were thought to power only a tiny minority of anomalous active galaxies. Then it was discovered they exist in the heart of pretty much every galaxy. Now it appears they may actually create galaxies. You and I might owe our very existence to a supermassive black hole.
But black holes, in addition to being essential to our existence, might also have something extraordinary to tell us about the Universe we live in – and indeed the nature of everyday reality. The Universe might be a hologram – a 3D representation of an underlying 2D reality. You, without knowing it, might be a hologram.
Recall that a black hole is born when a massive star reaches the end of its life and shrinks catastrophically, crushing the star to a point-like singularity. The vanishing of a star in such a dramatic way was not a problem for physics until, in 1974, Stephen Hawking showed that, paradoxically, black holes are not completely black. They radiate into space so-called Hawking radiation.
Hawking imagined quantum processes going on just outside the event horizon. All the time, in the vacuum around us, subatomic particles and their antiparticles are popping into existence along with their antiparticles and then popping out of existence again. The energy to create such virtual particles is paid back quickly and so nature turns a blind eye. However, sometimes one particle of a pair falls into the black hole. The remaining partner, with no twin with which to annihilate, cannot pop back out of existence. No longer a fleetingly real particle, it now has a permanent existence. The energy to create it has to come from somewhere. And it comes from the gravitational energy of the black hole. Bit by bit, as the energy of countless particles of Hawking radiation has to be paid for, the hole loses its mass energy until, eventually, it vanishes, or evaporates.
The trouble with Hawking’s discovery is that it implies that, when a black hole evaporates, all information about the star that initially shrunk to create the black hole – the type and location of all its atoms, for instance – will disappear too. This contradicts a fundamental edict of physics that information can never be created or destroyed.5
A clue to the resolution of the black-hole information paradox came from Israeli physicist Jacob Bekenstein. He discovered something profound about the event horizon: its surface area is related to the entropy of the black hole. In physics, a body’s entropy is synonymous with its microscopic disorder.6 But you do not need to know this. The crucial thing to know is that entropy is intimately related to information. A billion-digit number in which each digit is unrelated to the next has a high degree of disorder, or entropy; simultaneously, it contains a lot of information since the only way to convey it to someone is to tell them all billion digits.
This is the key clue to resolving the black-hole information paradox. In 1997, string theorist Juan Maldacena of Princeton’s Institute for Advanced Study showed that it is in the horizon that the information that describes the star may be stored – as microscopic lumps and bumps. So, when the black hole radiates Hawking radiation from the vicinity of its horizon, the radiation has impressed on it information about the star, just as the radio waves from BBC Radio One have pop music impressed on them. So, when the black hole disappears, the song of the star is not lost at all. It is broadcast to the Universe as Hawking radiation. No information is ever lost.
But all this implies, incredibly, that a 2D surface – the horizon of a black hole – can store sufficient information to describe a 3D object – a star. This is exactly what the hologram on your credit card does.
This might seem an esoteric speculation about an esoteric type of celestial body. But, in the late 1990s, Leonard Susskind of Stanford University in California made a surprising and mind-blowing connection. The Universe, in common with a black hole, is surrounded by a horizon. It is a horizon in time rather than in space but it is a horizon none the less. So, reasoned Susskind, he information that describes the 3D Universe might be stored in the horizon of the Universe.
What this means is open to a wide range of interpretations. A conservative interpretation is simply that the Universe contains a lot less information than we imagined, meaning that the Universe is more like a crudely drawn sketch than a fine oil painting. A more extreme interpretation is that the Universe is truly a hologram – a 2D object stored on the cosmic horizon that creates the illusion of a 3D Universe. So, either we are living on that 2D surface, believing we are 3D, or our Universe is some kind of 3D projection of that 2D surface. You and I and everyone else might be living in a giant hologram. Black holes, far from being esoteric celestial objects, have the most profound implications for you and your everyday life. Black holes are indeed masters of the Universe.
1 The first galaxies to form were actually relatively small. But, over the past 10 billion years or so, they have repeatedly merged and cannibalised each other, growing ever bigger until finally creating the galaxies we see around us today.
2 John Haines, ‘Little Cosmic Dust Poem’ (1983); http://tinyurl. com/crwo3y4.
3 Strictly speaking, the cosmic background radiation is brightest at a far-infrared wavelength of about 1 millimetre. Historically, however, it was first spotted at the easier-to-detect microwave wavelength of a few centimetres.
4 Strictly speaking, it is necessary to be at high altitude or in space to see the Universe glowing with the relic heat of the big bang. This is because water vapour in the atmosphere strongly absorbs the far infrared of the cosmic background radiation. At altitude, this water vapour is frozen out.
5 At one time there was a rival of the big bang. In 1948, Fred Hoyle, Hermann Bondi and Thomas Gold proposed that, although the Universe is expanding, new material continually pops into existence out of nothing to make new galaxies, so the Universe never gets more dilute but always looks the same. The steady state was dealt a killer blow by the discovery that the distant, and therefore ancient Universe, looks very different from today, and by the discovery of the cosmic background radiation in 1965.
6 When hydrogen nuclei approach each other close enough, they come under the influence of the powerful nuclear force. Like pieces of shrapnel in an explosion in reverse, they begin to fall towards each other. Faster and faster they fall until, finally, they collide. By the time this happens, however, they have acquired a tremendous energy of motion, which they must somehow get rid off if they are to stick together rather than rebound outwards. The surplus energy might be lost in the form of a high-energy particle or gamma ray. The details are unimportant. The key thing is that the formation of a nucleus of helium out of hydrogen nuclei is accompanied by a loss of a large amount of energy. This is the ultimate origin of sunlight. See my The Magic Furnace.
7 Absolute zero, equivalent to -273.15 °C, is the lowest possible temperature. Classical, or pre-quantum, physics predicts that, as the temperature falls, the jiggling of atoms gets ever more sluggish. At absolute zero, it stops altogether.
8 The cosmic background radiation broke free of matter about 379,000 years after the birth of the Universe. It had existed before but its photons could barely travel across space without being redirected, or scattered, by free electrons. About 379,000 years old, the Universe had cooled sufficiently for electrons to combine with atomic nuclei to make the first atoms. Without free electrons to hinder them, the photons of the fireball were suddenly free to travel across space unhindered. We detect them today as the cosmic background radiation. They have come directly to us from this epoch of last scattering.
9 The speed of light is the cosmic speed limit only in Einstein’s special theory of relativity of 1905. In his general theory of relativity of 1915, space can expand at any speed it likes. Evidence for this faster-than-light expansion comes from the size of the observable Universe. Although the Universe has existed for only 13.8 billion years, it is 84 billion light years across.
10 According to quantum theory, the vacuum is not empty. Far from it. Whereas in the everyday world the law of conservation of energy forbids energy being created from nothing, in the subatomic world, nature overlooks this edict. Energy can be conjured out of nothing, as long as it is paid back quickly. Think of a teenager who gets away with borrowing his dad’s car overnight as long he returns it to the garage the following morning before his dad notices its absence. In the same way, nature turns a blind eye to energy being conjured out of nothing as long as it is for only an ultra-short time. Consequently, the quantum vacuum, far from being empty, seethes with restless energy.
11 According to Einstein’s equations of gravity, the source of gravity is u + 3P, where u is energy density and P is the pressure. Usually, the second term is ignored because, in normal circumstances, the pressure of matter – due to its microscopic components buzzing about – is negligible compared with the energy density of matter. But it is always possible that there exists some hitherto unknown type of material in which the pressure is not negligible. And, if the pressure P is negative and less than – u/3, this reverses the sign of u + 3P, making gravity repulsive – that is, gravity blows rather than sucks. This is the case with the inflationary, false, vacuum. Incidentally, negative pressure means that, instead of pushing outwards, the vacuum is trying to shrink everywhere. Yet, bizarrely, it has repulsive gravity, and inflates. The reason for this is that the pressure has no direct effect. Every chunk of shrinking vacuum is surrounded by other chunks of shrinking vacuum so, overall, the negative pressure cancels out. Instead, the negative pressure has an indirect effect entirely through Einstein’s equations, which endow it with repulsive gravity.
12 This is typical of anything quantum. Its behaviour – for instance, whether it decays – is totally random, totally unpredictable. The Universe was a quantum object in its first split second of existence because it was smaller than an atom.
13 To be fair, nobody has yet managed to unite quantum theory with Einstein’s theory of gravity. A quantum theory of gravity, for all we know, may predict the exact energy density observed for the dark energy. Uniting quantum theory – a theory of the very small – and the general theory of relativity – a theory of the very big – is essential to understanding the birth of the Universe. After all, at that time, something very big was also very small.
14 The clumping together of matter to form galaxies could begin only when the fireball had cooled enough for electrons to combine with nuclei to form the Universe ’s first atoms. The reason is that free electrons interact strongly with, or scatter, photons, and there were about 10 billion for every electron in the big-bang fireball. They blasted apart any matter trying to clump together under gravity. Once electrons were mopped up by atoms, however, gravity gained control of the Universe. The time when galaxies began to form, about 379,000 years after the birth of the Universe, is known as the epoch of last scattering. Priceless information about this period is imprinted on the cosmic background radiation.
15 The evidence for dark matter also comes from within galaxies. The stars in the outer regions of spiral galaxies such as our Milky Way, for instance, are orbiting too fast. Like children on a speeded-up roundabout, they should fly off into intergalactic space. The reason they do not, astronomers maintain, is that they are in the grip of the gravity of a huge mass of dark matter. This dark matter, which greatly outweighs the visible stars, is believed to form a spherical halo in which is embedded the flattened disc of the spiral galaxy.
16 Earlier, it was mentioned that a crucial piece of evidence for the big bang is that 25 per cent of the mass of the Universe is helium. That is 25 per cent of the ordinary matter.
17 See ‘The Holes in the Sky’, Chapter 6 of my book The Universe Next Door.
18 Cosmological parameters such as the age and expansion rate of the Universe were once very badly known. Everything changed with the launch in 2001 of NASA’s Wilkinson Microwave Anisotropy Probe to observe the afterglow of the big bang. It ushered in the age of precision cosmology.
19 Apologies for using the image of a bubble in two different contexts. Each bubble that forms in the inflationary vacuum actually contains an infinite number of big-bang regions (smaller bubbles), each like our observable Universe. If you are wondering how something can be bounded yet infinite, it is because the inflationary vacuum is expanding so incredibly fast that, to observers inside the bubble, the boundary is unreachable. Effectively, therefore, the bubble is infinite.
20 Arthur C. Clarke, ‘The Wall of Darkness’, The Other Side of the Sky.