If there’s one thing I could change in the entirety of physics it would be the name for black holes. ‘But what’s in a name?’ you, or Juliet, might ask. A lot. While Tolkein might have argued that ‘Cellar Door’ were the two most beautiful words in the English language, I’d argue that no two words have ever caused more misunderstanding and misconceptions than ‘black hole’. Black hole gives people visions of a deep dark well you can fall down, a sink plug hole or even a cosmic whirlpool stealing away spaceships the same way sailors on the sea have been caught unaware.

Perhaps the most concerning thing of all is that the term black hole leads people to believe that black holes are the absence of something. That they are negative space. Something that takes away. Well, let me be the one to tell you that a black hole is the furthest thing from a hole you can get. A black hole isn’t the absence of something, it’s the presence of everything; matter in its densest possible form. I like to think of them more like mountains of matter than holes in the ground.

So where did this idea of a ‘hole’ come from? Well, in part we’ve got Einstein’s theory of general relativity to blame. General relativity is, first and foremost, a theory of gravity – it tells us how objects in space influence other objects and the paths they will take, either in orbit or with a quick deflection. Now you’re probably thinking, isn’t that what that guy Newton did when the apple fell on his head? Technically, yes. As told by many of his contemporaries, in the 1660s British physicist and mathematician Isaac Newton was inspired to think about what force causes things to fall after seeing an apple fall to the ground in his garden in Lincolnshire. He questioned why the apple always fell straight downwards and never varied by falling diagonally, or even upwards, figuring that the apple must therefore always be attracted to the very centre of the Earth. His notebooks from the time show how he puzzled over this idea for many years, wondering whether the force exerted by the Earth extended beyond its surface, perhaps even keeping the Moon in orbit.

It took Newton nearly two decades before he published arguably his most famous work, Principia, in 1687, in which he laid out his famous three laws of motion. The first: any object at rest will stay at rest, and any object in motion will stay in motion unless another force acts to slow it down. The second: the force applied to an object will be equal to its mass multiplied by its acceleration (usually people remember this from their high-school days as F = ma, after having it drilled into them repeatedly). The third: every action has an equal, opposite reaction²⁵ – which essentially means if you pull on something it will pull back.

But Newton didn’t stop there. He also defined his universal law of gravitation, which states that every single particle in the Universe attracts every other particle with a force that depends on how massive each one is, and dissipates as they get further away from each other (by their distance squared, so it weakens quickly). So, right now, you are attracted by gravity to this book you are holding and the book to you, but because you and the book are not, astrophysically speaking, that heavy, you barely even feel that pull (it’s a force of about 0.000000005 N; the force your back teeth generate when you chew is 1,000 N).

What Newton was suggesting in Principia was that there was an invisible force acting over great distances across the entire Universe. It was an idea that was met with huge scepticism by many scientists and philosophers at the time, who accused him of being drawn in by ‘occult’ ideas; they thought Newton was a crackpot. I like to remind people how you can’t see magnetic forces either, but you can still feel the magnetic attraction between two magnets. The effects of magnetism had been known since antiquity, and in 1600 British philosopher William Gilbert had published work outlining how Earth itself was a giant magnet. So the appreciation of invisible forces was already there in the scientific community, but perhaps in the heat of the moment Newton didn’t think of this microphone-dropping clap-back.

So while Newton’s work in Principia gave people a framework for describing gravity, and eventually propelled Newton to international scientific stardom, what Principia didn’t do was actually explain what gravity was and what caused it, much to the scientific community’s chagrin. It would be over 200 years (but not for lack of trying!) before another, different theory of gravity was proposed that actually explained the cause of gravity: Einstein’s theory of general relativity. Although Newton’s laws of motion and gravitation were eventually accepted by the scientific community, there was one problem with them. Although they could predict the positions of the planets in the Solar System as they orbited the Sun with great accuracy, for the closest planet to the Sun, Mercury, they always gave a slightly wrong answer.

No one knew quite why that was until way beyond Newton’s lifetime, when in 1859 French astronomer Urbain Le Verrier figured it out. Le Verrier was already a well-known and well-loved character in the astronomy community at the time, after he observed oddities in Uranus’s orbit in 1846. He predicted that they were caused by a large planet beyond the orbit of Uranus and sent a letter to the Berlin Observatory telling them where to look. That same evening, Neptune was discovered just 1° away from where Le Verrier predicted it would be (to understand how accurate that was, hold out your hand at arm’s length in front of the sky; your little finger is about 1° across at that distance from your face).

What do you do after predicting the existence of a planet in the Solar System that no one knew existed? Well, Le Verrier turned to predicting the motions and positions of all the planets in the Solar System, to ensure that nothing else had been missed. A mammoth task and one that kept him busy for the rest of his life. In that pursuit he studied the orbit of Mercury by observing its position for many years, and in 1859 published his data; a huge long list of the position of Mercury over a number of years. He noticed that what was happening to throw off his (and other’s) predictions for Mercury’s position was that its perihelion was ‘precessing’.

The precession of the perihelion of Mercury. The effect is exaggerated here to show the ‘spirograph’ shape the precession eventually makes over many millennia.

Planets don’t orbit the Sun in perfect circles. Instead, they orbit in ellipses, an oval-like shape described by two numbers: its furthest position from the centre (around the Sun this is known as aphelion; ap- away from, and -helion, from the Greek helios, meaning Sun) and the closest position to the centre (known as the perihelion).²⁶ For example, on 5 January every year the Earth is at perihelion 147.1 million kilometres away from the Sun, whereas on the 5 July it is at aphelion 152.1 million kilometres away – a difference of 5 million kilometres!

For the Earth’s orbit, aphelion and perihelion each occur in the same place. But what Le Verrier found for Mercury is that perihelion, the moment when it’s closest to the Sun, wasn’t in the same place every time Mercury came back around on its orbit. If you were actually to draw out Mercury’s orbit over a number of the planet’s years, it would look like a Spirograph pattern,²⁷ although the effect wouldn’t be noticeable over only a few orbits. Even though Mercury only takes eighty-eight days to go around the Sun, Le Verrier had to wait many orbits for this effect to become apparent to able to detect it.

In one sense, what was happening to Mercury’s orbit wasn’t that much of a surprise, as Newton himself had predicted it. When there’s a smaller object quite close to a massive object with other objects orbiting it, the smaller object is perturbed slightly by all the other objects in the system. So the main reason why Mercury’s orbit precesses is because it’s not just interacting with the Sun, but it’s also feeling the pull of all the other seven planets (plus all the dwarf planets, comets and asteroids littering the Solar System) that are also orbiting the Sun. But Le Verrier was the first to point out that if you use the equations in Newton’s theory of gravity to predict how much Mercury’s orbit should precess per century, it’s a smaller value than you observe.

Before declaring that there must be something wrong with a law of gravity that had been accepted for over 170 years, Le Verrier considered other explanations for the discrepancy. Including that the Sun is not perfectly round but is instead an oblate spheroid, meaning it’s a bit squashed at the poles. The same is true for Earth, and especially for Saturn because it’s rotating so quickly; matter at the equator bulges out a bit, in the same way you feel a force pushing you off a merry-go-round. It did turn out that that the Sun’s shape plays a small role in how much Mercury’s orbit precesses, but it still wasn’t enough to account for the discrepancy. So Le Verrier also suggested that there could be another planet inside the orbit of Mercury, orbiting the Sun much closer.

At the time, this extra planet was the most favoured hypothesis to explain the discrepancy, partly because, just thirteen years before Le Verrier had posited it, he had predicted the existence of Neptune due to the effect on Uranus’s orbit. So, as odd as the idea of an extra planet between the Sun and Mercury sounds to you and me, back then it wasn’t such an outlandish idea. Neptune had only just been discovered and there was a general feeling that there must be something else out there. So finding this hypothetical planet between the Sun and Mercury (dubbed Vulcan after the Roman god of volcanoes and fire and forges) became the focus of many astronomers during the rest of the nineteenth century.

The desire to be the person responsible for the discovery led to a lot of false claims, including people who were adamant that they’d observed a planet very close to the Sun during a solar eclipse, in a position where no known star was thought to be (in the background), despite no one else observing it during the same solar eclipse. All of these false claims gave rise to different descriptions of the properties of Vulcan and its orbit; if all the claims had agreed on the properties then perhaps the idea of a new planet inside Mercury’s orbit would have been quite convincing, but it became clear pretty quickly that this hypothetical planet was just that, hypothetical, and couldn’t explain the strange precession of Mercury’s perihelion.

So, with all other options exhausted, the only explanation was that Newton’s theory of gravity wasn’t quite right. This is where Einstein comes in. In the first decade of the twentieth century, Einstein announced his theory of special relativity to the world, which described what happened to your perception of time and space when you travelled close to the speed of light. It introduced the ideas of time dilation – the faster you travel the less time passes from your perspective – and length contraction – the faster you travel your length contracts in the direction you travel. Like most revolutionary theories, this was incredibly controversial and it left many unanswered questions. Trying to tie up all the loose ends, Einstein ended up coming up with a new way of explaining gravity: as the curvature of space itself. Massive objects curve the space around them and then anything travelling along that space, whether a planet or light, would follow a curved path. People often picture this as a sheet stretched taut, or a trampoline, with a basketball placed in the middle. If you then roll a ping pong ball along that surface it will follow a curved path, even if you set it off on a straight one. While that’s a great analogy, it doesn’t help us visualise the curvature of space in three dimensions, something the human brain can’t quite wrap its head around.

Einstein published his theory of general relativity in a series of papers between 1907 and 1915, and in them he proposed the equations that essentially describe the curvature of space that massive objects cause. It was a generic equation that could be applied to many different scenarios depending on different masses and, crucially, the different speeds the objects were travelling at: everyday speeds or close to the speed of light. Einstein found that when he applied his theory of general relativity to the problem of the Solar System, his equations reduced down to match Newton’s equations when objects weren’t moving at speeds close to the speed of light or close to very massive objects. So it wasn’t that Newton’s equations were wrong necessarily, it was just that they were a generalisation for a special case. Mercury, though, is close to the massive object of the Sun, and so Einstein’s equation for Mercury’s orbit was ever so slightly different from Newton’s. What Einstein did was work out how much of an effect this difference in the equation would have on the predicted position of a planet and in particular how much precession of Mercury’s perihelion was expected. He found that it was the same value measured by Le Verrier and used this as evidence for his newly proposed theory of gravity. He suggested two other phenomena that would also provide evidence for his new theory: massive objects should cause redshift of light (this stretching of the wavelength of light, ‘gravitational redshift’, was finally confirmed in 1954) and also the bending of light by massive objects.

In Einstein’s lifetime it was only possible to detect the latter: the bending of light from distant stars from behind the Sun during a solar eclipse. During an eclipse, it becomes dark enough to see the stars behind the Sun during the day that are usually only visible at night, six months earlier, when the Earth is on the other side of the Sun. You can compare the positions of the stars at night to the positions recorded during a solar eclipse, and see if the apparent positions of the stars change because their light has been deflected by the Sun curving the space around it. To do just that, British astronomers Frank Dyson and Arthur Eddington (who was already very well-known at the time for explaining general relativity to the English-speaking world after normal scientific lines of communication were disrupted during the First World War, but hadn’t quite reached his Big Name in Physics status yet with his work on the fuelling of stars) organised two expeditions to observe the solar eclipse of May 1919.²⁸ One expedition was to the Brazilian town of Sobral, led by Andrew Crommelin and Charles Rundle Davidson from the Royal Greenwich Observatory, and the other expedition was to the West African island of Príncipe, led by Eddington himself and Edwin Cottingham.

An image of the eclipse as observed by Eddington and Cottingham from Príncipe in 1919.

Despite some bad weather during the eclipse, Eddington obtained enough images to record the positions of stars and declare that the change in their position matched those predicted by general relativity. The results were announced at a meeting of the Royal Society in November 1919 and by the next day had made headlines all around the world. The most famous of which was the headline from The New York Times published on 10 November 1919, which read: ‘Lights all askew in the heavens . . . men of science more or less agog . . . nobody need worry.’²⁹ It made Einstein, as the man who ‘corrected’ Newton with his new theory of gravity, world famous, although acceptance of general relativity in the wider scientific community took some time.

First, because one experiment with one measurement is never enough for us scientists. It had to be repeated, but solar eclipses unfortunately don’t come along every day and the weather likes to get involved and ruin the party. Second, the understanding of general relativity among other scientists of the time wasn’t great. Einstein’s articles had been published in German, and not everyone could get an accurate translation in their own language, mainly because translators had to also be intimately familiar with physics and general relativity as well.

One thing Einstein never predicted with general relativity was black holes (it’s a common misconception that he did), although a rough draft of the idea of a black hole had been knocking around long before Einstein. In 1783, British clergyman by day, astronomer by night John Michell mused on the idea of objects so massive that light could not escape them, and dubbed them ‘dark stars’. He even went as far as saying that if they existed, we could still spot them by their gravitational pull on other visible objects.

It was German physicist and astronomer Karl Schwarzschild who, in 1915, just a few months after general relativity was published, unknowingly found the first mathematical description of a black hole by solving Einstein’s equations (more on that later). One possible scenario that Schwarzschild’s solutions described was all mass collapsing down to a single point. In this scenario, many terms in the equations became infinite. Even time itself would stop, which led to these objects being referred to as ‘frozen stars’. But if we think of this in terms of how Einstein described gravity, as the curvature of space and time, and go back to our analogy of the trampoline, we can imagine how putting an incredibly dense, heavy object on the trampoline would cause a very steep-sided depression. A hole, you might say. Yes, as much as we have Einstein to thank for, we perhaps also have him to grumble at for the idea of a ‘hole’ in space being planted into people’s brains.

Of course, the physicists of the day did not accept that Schwarzschild’s solutions were realistic, merely theoretical curiosities. What we now call ‘black holes’ were referred to as ‘gravitationally collapsed stars’ or just ‘collapsed stars’, which is also how prominent Swiss astronomer Fritz Zwicky referred to them in a paper in 1939. But by 1971, Stephen Hawking himself, in his paper ‘Gravitationally Collapsed Objects of Very Low Mass’, refers to them as ‘black holes’ in inverted commas. So where had this term come from in the time between the 1940s and the 1970s? What’s the etymology of the phrase ‘black hole’?

It seems we have famous American physicist Robert H. Dicke to blame for coining the phrase that eventually made its way around astronomy research circles. Unfortunately it’s a rather harrowing tale from a sad part of history that seems to have inspired Dicke. At the first Texas Symposium in Dallas, in 1961, attendees reported that in his presentation Dicke repeatedly compared ‘gravitationally completely collapsed stars’ to the ‘black hole of Calcutta’; a small prison cell in the dungeon of Fort William in Kolkata, India that measured just 4.30×5.50 metres (14×18 feet; about the size of three double beds).

Fort William was built to defend the British East India Company’s trade in Kolkata. However, the leader of the region, the Nawab of Bengal, Siraj ud-Daulah, ordered the construction be halted. The British carried on anyway, and in retaliation Siraj ud-Daulah’s forces laid siege to the fort. The majority of the British troops were ordered to abandon their posts and escape, except for 146 soldiers who were left behind as a last defence. The fort fell in June 1756 and the surviving British soldiers were all imprisoned in the ‘black hole’. The conditions were so cramped, with so many in such a small space, that overnight people died from suffocation and heat exhaustion. Reports vary on the number of lives lost, but historians estimate that sixty-four people were imprisoned and only twenty-one survived the night. There is a memorial at St John’s Church in Kolkata, which was erected in 1901 to those ‘who perished in the Black Hole prison of old Fort William’.

It was this historical event – of people being crushed in the prison – that rather morbidly led Dicke to use the term for when matter has been crushed and a star collapsed down due to gravity. One of his colleagues who picked up on the phrase was American physicist Hong-Yee Chiu (who is credited with inventing the word ‘quasar’ – a portmanteau of ‘quasi-stellar object’). He inspired science journalist Ann Ewing to write an article called ‘“Black Holes” in Space’ for the magazine Science News Letter in 1964, which marks the first time the term was ever used in print.

It was John Wheeler who is credited with truly popularising the name, though, turning the term from analogy into actual scientific jargon.³⁰ In 1968, he was giving a presentation at the NASA Goddard Institute in New York about his recent work studying ‘gravitationally completely collapsed objects’ when he jokingly complained that the term was too long and far too inconvenient to repeat all the time. According to Wheeler in his autobiography, someone in the audience at that point suggested ‘How about black hole?’, and he thought the term was perfect for its brevity and ‘advertising value’. He then adopted the term whole-heartedly, using it in an 1968 article for the American Scientist journal. The term quickly entered the scientific lexicon, with German astrophysicist Peter Kafka the first to use it in a scientific research article in 1969, with the likes of Stephen Hawking following suit by 1971. The term ‘black hole’ had stuck; much to my later annoyance.

I guess I should be grateful that the modern trope of shortening everything in astronomy to an acronym hadn’t quite gained traction in the 1960s, otherwise I’d probably be telling everyone that I study ‘GCCOs’ (gravitationally completely collapsed objects, d’uh). But what would I have named black holes instead if I’d had the chance? If I’d been there in the 1960s and had the same influence as Wheeler to dub these most spectacular of objects?

Honestly, I’m not sure, but if I had to choose, I think John Michell’s ‘dark stars’ is my favourite, and would cause less confusion over what black holes really are.³¹ Or perhaps ‘mountain’ might be a better word to describe the nature of black holes – because it’s not like the stuff that does ‘fall’ into a black hole just disappears. In fact, the material piles up and up, so much so that in some cases there can be over a trillion times the mass of the Sun squashed into a black hole. That is a literal mountain of matter. Just mountains that you can’t directly see because not even light can escape. I don’t want to have be the one to break it to Tammi Terrell and Marvin Gaye, but it turns out there are mountains high enough to keep me from getting to you.