by Marcus Chown
A black hole’s singularity is, however, surrounded by a ‘horizon’, an imaginary membrane that marks the point of no return for in-falling light and matter. But it is no mere passive boundary, and in 1974, British physicist Stephen Hawking discovered something remarkable about it.
To appreciate what Hawking discovered, it is necessary to understand what quantum theory says about empty space. Far from being empty, it is seething with energy; subatomic particles and their antiparticles are continually popping into existence in pairs, as permitted by the ‘Heisenberg Uncertainty Principle’. Nature turns a blind eye to these particles, not bothering about where the energy to create them comes from, as long as they meet and destroy, or ‘annihilate’, each other very quickly. It is a bit like a teenager borrowing a car from their dad overnight and returning it to the garage before he notices it is missing.
But Hawking realised that, near the horizon of a black hole, something very interesting happens. There is the possibility that one of the particles of a newly created pair falls through the horizon into the black hole. The remaining particle has no partner with which to annihilate and flies away from the hole, along with countless others in the same situation. Contrary to all expectations, therefore, black holes are not black but glow with emitted particles – ‘Hawking radiation’.
Hawking had earlier discovered that when black holes merge, the surface area of the horizon of the merged hole is always bigger than the sum of the areas of the two precursor black holes. The Israeli physicist Jacob Bekenstein had speculated that the surface area represents the ‘entropy’ of the black hole. This is a property that arises in the theory of thermodynamics – the theory of heat and motion that underpins physics, chemistry and many other fields – and which always increases. But it applies only to hot bodies, so how could it possibly apply to a black hole?
Hawking had found the answer: thermodynamics applied to black holes because they are hot. The proof is that they glow with heat – Hawking radiation. The significance of Hawking’s discovery is that at the horizon of a black hole, three of the great theories of physics meet – Einstein’s theory of gravity, quantum theory and thermodynamics. It was a first tentative step on the road to uniting them – the holy grail of physics.
However, Hawking radiation threw up a serious problem: its particles do not come from inside a black hole since nothing can escape its gravity. Instead, they are created just outside the horizon. The energy to create them has to come from somewhere, and it comes from the gravitational energy of the black hole itself. As it radiates Hawking radiation, it gradually shrinks away. Star-sized black holes have extremely weak Hawking radiation but, as a black hole gets smaller, the radiation gets brighter, until the hole finally explodes in a blinding flash. Since such ‘evaporation’ would take far longer than the current age of the universe, it might seem of no consequence, but nothing could be further from the truth.
It is a cornerstone of physics that information cannot be destroyed. A complete description of the star that initially collapsed to form a black hole would require recording the type and position of each of the huge numbers of subatomic particles that compose it. But once a hole has evaporated, there is literally nothing left, so where does the information go?
The current speculation is that the horizon of a black hole is not smooth and featureless, as Einstein’s theory of gravity suggests, but rough and irregular on the microscopic scale, and that it is in the lumps and bumps of its Lilliputian landscape that is stored the information that describes the star that gave birth to the black hole. Since Hawking radiation is born in the vacuum a hair’s breadth above a black hole’s event horizon, it stands to reason that it is influenced by the microscopic undulations of that membrane. Those undulations ‘modulate’ it in much the same way that pop music modulates the ‘carrier wave’ of a radio station. In this way, the information that described the precursor star is carried out into the universe, imprinted indelibly on the Hawking radiation. No information is lost, and one of the most precious laws of physics is left intact.
Since Murdin and Webster’s 1971 discovery of the first black hole in Cygnus X-1, more candidates have been found, though the total stands at fewer than twenty-five. But black holes – of a very different kind – had been stumbled on almost a decade earlier, in 1963.
‘Quasars’, discovered by Dutch–American astronomer Maarten Schmidt, are the super-bright cores of newborn galaxies. They typically pump out one hundred times the energy of a galaxy of stars, but from a volume smaller than the solar system. The only possible source of such prodigious luminosity is matter, heated to incandescence, as it swirls like water down a plughole into a black hole. But not a stellar-mass hole – one with a mass up to fifty billion times that of the Sun.
Initially, it was thought that ‘supermassive’ black holes power only ‘active galaxies’, the 1 per cent of unruly galaxies, of which quasars are the most striking example. But in the 1990s, astronomers using NASA’s Hubble Space Telescope in Earth’s orbit discovered that there is a supermassive black hole lurking at the heart of pretty much every galaxy. The one in the core of the Milky Way, known as Sagittarius A*, is a tiddler, weighing in at only 4.3 million times the mass of the Sun. Why there should be a supermassive black hole in every galaxy remains one of the great unsolved mysteries of cosmology.
But despite the observational evidence for the existence of black holes, it was always indirect. Astronomers observed stars or hot gas whirling fantastically quickly around an unseen compact object and inferred the existence of a black hole, but there was always the possibility it was instead some undreamt-of super-compact object, held up by some hitherto unknown force.
The definitive proof of the existence of black holes came, however, on 14 September 2015. That was the day that gravitational waves – ripples in the very fabric of space–time, predicted by Einstein in 1916 – were detected on Earth for the first time. The key thing was that the waveform detected was precisely what Einstein’s theory of gravity predicted for the merger of two black holes.
Black holes exist, beyond any doubt. Meanwhile, the quest continues to image them in space. The problem astronomers face is that stellar-mass black holes in the Milky Way are small and, well, black. Supermassive black holes, though big, are at cosmic distances and so also appear small. However, there are two black holes that are both relatively big and relatively nearby. One of them, Sagittarius A*, is 26,000 light years away in the centre of our galaxy, and the other, which is about one thousand times bigger, is in a nearby galaxy called M87.
Over the past few years, astronomers have been attempting to image the event horizons of these two supermassive black holes, using an array of co-operating radio telescopes scattered around the globe known as the ‘Event Horizon Telescope’ (EHT). The radio signals recorded at each site are combined on a computer at Haystack Observatory in Massachusetts, to simulate a giant dish the size of the Earth. The bigger the dish and the shorter the observing wavelength – EHT uses a wavelength of 1.3 millimetres – the more it can zoom in on details in the sky.
The hope is that the EHT will test a controversial recent claim by Hawking. Having shocked the world of physics in 1974 by claiming that black holes are not black but emit Hawking radiation, he did it again in 2014, when he claimed that event horizons do not exist, which means that, strictly speaking, neither do black holes.
The collapse of an object such as a star to form a black hole is violently chaotic and, rather than a horizon, all that forms, claimed Hawking, is a boundary of extreme space–time turbulence. Information can leak out through such an ‘apparent horizon’. Hawking’s conclusion was dramatic: ‘The absence of event horizons means that there are no black holes – in the sense of regimes from which light can’t escape to infinity,’ he wrote. ‘There are, however, apparent horizons which persist for a period of time.’ Black holes, in other words, are not what we thought they were.
So is the horizon around a black hole the point of no return
that everyone thought it was? Or is it merely an apparent horizon, as Hawking maintained, leaking stuff from inside the hole? The key thing is to observe the horizon and see whether it behaves as predicted by Einstein, or even whether it exists at all. ‘An image will allow us to test general relativity at the black hole boundary, where it has never been tested before,’ said Shep Doeleman of the Massachusetts Institute of Technology, leader of the EHT team. ‘It would symbolise a turning point in our understanding of black holes and gravity.’
That turning point has now been reached. On 10 April 2019, the EHT team revealed the first-ever picture of a black hole.27 It was not of Sagittarius A* but of M87, weighing in at seven billion times the mass of the Sun (Sagittarius A*, because it is smaller, was circled by matter many times while being observed, yielding a blurrier picture). The event horizon shows up as a dark ‘shadow’, backlit by intense radio waves emitted by matter heated to incandescence, as it swirls down through an ‘accretion disc’ onto the black hole.
‘The hole is a part of our universe permanently screened from view,’ says EHT physicist Feryal Özel of the University of Arizona in Tucson. ‘A place where our physics – at least, as currently formulated – cannot reach.’ Her Dutch colleague, Heino Falcke of Radboud University in Nijmegen, puts it more dramatically: ‘We have seen the gates of hell at the very end of space and time.’
Notes
1 The Hitchhiker’s Guide to the Galaxy by Douglas Adams (William Heinemann, London, 1995).
2 In 1974, Louise Webster returned to Australia to work at the Anglo-Australian Telescope at Siding Spring Observatory. She married a British radio astronomer called Tony Turtle, but sadly, despite having the first-ever liver transplant in Australia, she died at only forty-nine. See http://asa.astronomy.org.au/profiles/Webster.pdf.
3 ‘Optical Identification of Cygnus X-1’ by Paul Murdin and Louise Webster (Nature, vol. 233, 10 September 1971, p. 110).
4 The mass of HDE 226868 today is estimated to be twice the average mass estimated for such a star in 1971. Consequently, the black hole in Cygnus X-1 is known to be about fifteen times the mass of the Sun. Since black holes result from the implosion of the core of a massive star in a ‘supernova’ that blows 90 per cent of a star’s material into space, the precursor star must have been a monster of at least 150 solar masses.
5 ‘Oral Histories – Martin Schwarzschild’ (American Institute of Physics, 10 March 1977; https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4870-1).
6 Ibid.
7 Although pemphigus vulgaris is still incurable, its symptoms can be controlled with a combination of medicines that stop the immune system attacking the body. Most people start with high doses of steroids, which help stop new blisters forming and allow existing ones to heal. Gradually, the dose is reduced and another medication that reduces the activity of the immune system is used. If the symptoms do not return, it may be possible to stop taking the medication, but many people need ongoing treatment to prevent flare-ups.
8 From The Prelude, Book Three by William Wordsworth:
The antechapel where the statue stood
Of Newton with his prism and silent face,
The marble index of a mind for ever
Voyaging through strange seas of Thought, alone.
9 Masters of the Universe: Conversations with Cosmologists of the Past by Helge Kragh (Oxford University Press, Oxford, 2014).
10 In a way, it was good fortune that Erwin Freundlich did not get to observe the total eclipse of 21 August 1914 because Einstein’s prediction of the deflection of starlight by the Sun was wrong – only half the value that would be predicted by his final theory of gravity of November 1915.
11 Einstein’s special theory of relativity of 1905 had shown that space and time are aspects of the same seamless entity: space–time. As Einstein’s mathematics professor Hermann Minkowski said, addressing the eightieth Assembly of German Natural Scientists and Physicians on 21 September 1908: ‘The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.’
12 This succinct summary of Einstein’s theory of gravity is due to the American physicist John Wheeler.
13 ‘Oral Histories – Martin Schwarzschild’ (American Institute of Physics, 10 March 1977).
14 Einstein’s field equations of gravity actually contain 4 × 4 tables of numbers, which means there are sixteen equations. However, he was able to use ‘symmetry arguments’ to reduce the number of equations down to ten.
15 Karl Schwarzschild: Collected Works, edited by H. Voigt (Springer, Berlin, 1992).
16 Letter from Karl Schwarzschild to Albert Einstein, dated 22 December 1915 (‘Collected Papers of Albert Einstein’, vol. 8a, document 169).
17 Einstein and the History of General Relativity, edited by Don Howard and John Stachel (Birkhäuser, Boston, 1989, p. 213).
18 ‘Schwarzschild and Kerr Solutions of Einstein’s Field Equation – An Introduction’ by Christian Heinicke and Friedrich Hehl (7 March 2015: https://arxiv.org/pdf/1503.02172.pdf).
19 Masters of the Universe: Conversations with Cosmologists of the Past by Helge Kragh (Oxford University Press, Oxford, 2014).
20 ‘Cygnus X-1 – A Spectroscopic Binary with a Heavy Companion?’ by Louise Webster and Paul Murdin (Nature, vol. 235, 1972, p. 37).
21 The discovery of the black hole in Cygnus X-1 was made independently and pretty much simultaneously by American astronomer Tom Bolton at the University of Toronto’s David Dunlap Observatory. His paper was published a few weeks after that of Murdin and Webster. ‘Identification of Cygnus X-1 with HDE 226868’ by Tom Bolton (Nature, vol. 235, 4 February 1972, p. 271).
22 I am writing this account in large part because in 1972, aged twelve, I went with my dad to a Junior Astronomical Society meeting at Caxton Hall in London. The subject was the black hole candidate Cygnus X-1 and the speaker was Paul Murdin. It blew my mind!
23 In 1963, the New Zealand physicist Roy Kerr had found a solution of Einstein’s theory of gravity for the space–time warpage of a spinning black hole.
24 See Quantum Theory Cannot Hurt You: Understanding the Mind-Blowing Building Blocks of the Universe by Marcus Chown (Faber, London, 2008).
25 ‘Galactic Explorer Andrea Ghez’ by Susan Lewis (NOVA, 31 October 2006: http://www.pbs.org/wgbh/nova/space/andrea-ghez.html).
26 John Wheeler is often credited with coining the term ‘black hole’, but he merely popularised it. ‘In the fall of 1967, [I was invited] to a conference … on pulsars,’ he wrote. ‘In my talk, I argued that we should consider the possibility that the center of a pulsar is a gravitationally completely collapsed object. I remarked that one couldn’t keep saying “gravitationally completely collapsed object” over and over. One needed a shorter descriptive phrase. “How about black hole?” asked someone in the audience. I had been searching for the right term for months, mulling it over in bed, in the bathtub, in my car, whenever I had quiet moments. Suddenly this name seemed exactly right. When I gave a more formal Sigma Xi-Phi Beta Kappa lecture … on December 29, 1967, I used the term, and then included it in the written version of the lecture published in the spring of 1968’ (Geons, Black Holes and Quantum Foam by John Wheeler (W. W. Norton, New York, 2000, p. 296)).
27 ‘First M87 Event Horizon Telescope Results: The Shadow of the Supermassive Black Hole’ by the EHT Collaboration (Astrophysical Journal Letters, vol. 875, no. 1, 10 April 2019).
* Or, strictly speaking, mass-energy.
8
The god of small things
The discovery of the Higgs boson, just like the planet Neptune and the radio wave, was first predicted with a pencil, using mathematical equations.
MAX TEGMARK1
Tyger! Tyger! burning bright
; In the forests of the night,
What immortal hand or eye,
Could frame thy fearful symmetry?
WILLIAM BLAKE2
Methodist Central Hall, London, 4 July 2012
Jon Butterworth was pissed off. He was pissed off because on this day, which was destined to be one of the most remarkable of his forty-five years, he was stuck in London. He did not want to be in London. He wanted to be in Switzerland, where the action was. And, as if to rub salt into the wound, he could see the place where he would much rather be, on a giant video screen at the back of the stage of Methodist Central Hall in Westminster, London.
Butterworth’s mood improved, however, as he sat down beside his fellow panellists – his physicist colleague Jim Virdee and John Womersley, CEO of the UK’s Science and Technology Facilities Council – and surveyed the audience. Several hundred journalists, physicists and politicians, including the science minister David Willetts, had crowded into this one-hundred-year-old meeting place, a stone’s throw from the Houses of Parliament, and the excitement and anticipation were palpable. Butterworth was pleasantly surprised that the public and the politicians alike seemed fascinated by what he and thousands of other physicists had been doing over much of the past decade.
Butterworth and Virdee were at this press conference in London rather than in Switzerland because they were the respective UK heads of ATLAS and CMS. These giant experiments at CERN, the European laboratory for particle physics near Geneva, were two of the ‘eyes’ of the Large Hadron Collider. They were located where its super-high-energy beams of counter-rotating protons slammed into each other. Each experiment consisted of detectors wrapped like layers of an onion around the collision points. They measured the energy, electric charge and direction of the myriad pieces of subatomic shrapnel that exploded outwards.