Chapter 1

Jerome has been in this cell for eleven weeks since his arrest. The weather has turned cold and damp, and he struggles to keep himself warm. Until yesterday, he had no idea why he was in prison; no one would tell him anything. The hunched man who brings fresh straw every day refuses even to look at him. The tall, thin boy who brings the food smiles as he sets the bowl down on the writing desk, but has nothing to offer but a shrug in response to Jerome’s questions. Yesterday, though, a new face entered the cell. When the guard turned the key and opened up the door, the stranger took one step in, threw down the yellow robe, smirked, turned, and walked away. And then Jerome knew.

He puts down his pen and turns his attention to the robe that now smothers his emaciated frame, pulling at it as if it burns his skin. It is embroidered with demons who are using forks and fiery flames to torture hapless, grimacing men. Jerome knows its significance: it is what the heretics wear when led to the stake.

We are in December now. Christmas is coming and the last light of the day is fading ever earlier. The cell is small and dark, with a window big enough only for a small boy to climb through. It has been many decades since Jerome was a small boy. He is sat behind a rickety desk — a small mercy afforded to him by the authorities. Perhaps they hope he will write more blasphemies and make their case easier to prosecute.

Jerome looks up from the robe and stares at me through the gloom. He is not sure if I am an apparition. I am not sure, either. Eventually, without shifting his gaze from my eyes, he puts his fingers to his mouth and pulls out a small emerald. It is held on a chain around his neck. He lets the stone fall to his chest.

‘Do I know you?’ he says. His voice is thin and high pitched — reedy — entirely unbecoming to a man in his seventh decade.

‘I don’t think so,’ I say.

‘Did they send you to spy on me?’

‘They?’

‘My Inquisitors.’ He pulls at the yellow robe again.

‘No.’ I break from his gaze and survey the cell again. ‘I’m just here. As far as I can tell.’ It is all I have to offer.

It seems to satisfy. ‘Oh,’ he says. He picks up the pen again. ‘Well, welcome, then.’

‘Are you writing to Archbishop Hamilton?’ I say.

He stares at me. ‘Why would I do that?’

‘To ask for his help.’

Jerome shakes his head. ‘He will be dead by now,’ he says. ‘I’m sure of it.’

I have read the history books and I know the truth. ‘He’s not. Your treatment was more successful than even you might have hoped.’ I hesitate, unsure whether this constitutes some breach of the rules of engagement. I decide I don’t care. Nobody has told me the rules. ‘You should write to him,’ I say.

And that is how, I like to think, I came to save the life of Jerome Cardano.

It is going to be difficult to convince anyone that I saved Jerome. Jerome was born in 1501 and I in 1970. Bound as we all seem to be by time’s arrow, I can see that there are problems with the concept. I may already be coming across as an unreliable narrator. But, before you judge, first learn something about where I’m coming from and — more importantly — familiarise yourself at least a little with the ideas of quantum theory.

According to our best description of the atomic and subatomic world, atoms and their constituent particles are able to exist simultaneously in two places at once. It’s there in the theory and we’ve seen it in experiments. They can even exist at two different moments simultaneously. So, even as they gather to form my body, their notion of time and space is utterly different to the one I experience. And so, I ask, why shouldn’t I be in two places and epochs at once?

I’m playing with you, of course. I am an unreliable narrator. That’s the whole premise here. But aren’t we all? After all, I’ve already mentioned my ‘experience’ of time as if I know what that means. All I can tell you about that particular phenomenon is that my experience involves my consciousness — something that scientists can’t even define, let alone explain. If quantum physics is slippery, it’s nothing compared to the minimal friction you’ll encounter when you try to pin down a neuroscientist on consciousness.

One of the problems is that consciousness is entirely subjective. I believe I am conscious; I have no way of telling whether you are. You, to me, are therefore an unreliable narrator. A narrator is only reliable when we can corroborate their version of events. We take the view that if several people agree on a narrative arc, it is probably a trustworthy description of how things happened. But how can I trust anyone else if I don’t know what’s going on — if anything — in their head? What’s more, it doesn’t mean that other things — things no one mentioned — didn’t happen. Even the agreed narrative may not tell the whole truth.

I certainly cannot corroborate Jerome’s version of events. I can only go by what he says — and what he says is sometimes odd. I first came across him some years ago now, when researching a book about how science works. I was writing a chapter about the origins of scientific creativity and was seeking out examples of strange sources of inspiration: hallucinogenic and dream states, daydreams or poetry-inspired visions, and so on. Most scientists chose to hide these questionable sources. But not Jerome.

He invented the mechanical gimbal that was to make the printing press possible. His idea led to the ‘Cardan joint’ that takes the rotary power in the driveshaft of your car’s engine and allows it to be transmitted to the front and rear axles. We have already mentioned the mind-bending imaginary numbers that are multiples of the square root of -1 and the original mathematics of probability. He pioneered the experimental method of research in areas as diverse as medical cures for deafness and hernia, cryptography, and speaking with the dead (forgive him, these are not strictly scientific times). Jerome’s autobiography details some of these achievements, yet when he documents their source he says they came from ‘the ministrations of my attendant spirit’.

Here we would say he is indulging in unreliable narration. We tend not to believe in visiting spirits, especially those that impart scientific insights. So this is surely a lie, or the raving of a disturbed mind? Jerome’s father also had a spirit visitor, as it happens. As a trained scientist, I should probably put all this down to a genetic predisposition to psychosis or schizophrenic delusions. Yet despite, or perhaps because of, this I became quietly fascinated. I read everything on Jerome that I could find. The vast majority of his four million words of writings (four million!) are only available in Latin (not my strong suit), but there are a few biographies in English. A couple were written in the nineteenth century. A Norwegian mathematician called Øystein Ore published another in 1953, focused on Jerome’s probability work. There’s a more general biography from 1969, written by a jobbing journalist called Alan Wykes. More recently, some academic scholars have dissected Jerome’s astrological studies and his medical work. All this seeped into me, permeating my thinking and my imagination, and mixing itself with my experience and my knowledge. Then it set hard in my brain as thoughts and imaginings about the possible, the probable, and the unlikely. It became a new narrative for me, as compelling as quantum theory and just as unreliable. Jerome and I are now inextricably entangled across space and time, just like the photons that spooked Einstein so badly.

Photons, I should explain, are the fundamental particles of light and other radiation. They travel at — unsurprisingly — the speed of light, which is the maximum speed of anything in the universe. In his special theory of relativity, Einstein showed that travelling at the speed of light is equivalent to halting time. That means photons do not experience time, as such. However, that didn’t stop him balking at entanglement, arguably still quantum theory’s most shocking revelation.

This is the discovery that you can cause two photons (or any other quantum particles) to interact so that their properties become shared between them. You can then separate the pair by half a universe, do something to one and immediately see the effects of your action in the properties of the other. Einstein dismissed entanglement as proof that quantum theory must be somehow incomplete, deriding it as ‘spooky action at a distance’. We know now that entanglement works across time as well as space. We’ll get into this later. All I’m saying is, given that Jerome is spooking me now, perhaps I spooked him then.

For the moment, let’s go back and visit the beginning.

It is astonishing — a testimony to his tenacity — that Jerome is born at all. This is the beginning of the sixteenth century and Italy is in its Renaissance period. As far as most of its inhabitants are concerned, this is not the Renaissance Italy you are thinking of, with its glorious legacy of art and culture. True, Leonardo’s creativity is in full flow, and in a few months Michelangelo will return to Florence and begin work on his statue of David. But this Italy is a patchwork of regional states, broken by centuries of petty internal conflict and civil war, and rotten with plague, poverty, and superstition. Its rulers are in thrall to a succession of bloated, self-aggrandising popes. Living through the harsh reality of everyday life in Italy’s Renaissance is not glamorous, and that is why, when Chiara Micheria, a fat little widow with a short temper, realises she is pregnant again, she seeks out an apothecary.

Having gained herself a consultation, she asks the apothecary how best to induce an abortion. The church’s newly published witch-hunting manual, Malleus Maleficarum, has pronounced abortions the work of the devil, and midwives inducing them are to be treated as witches. But, however dangerous the move might be, Chiara feels herself in no position to deal with yet another bastard child. After all, she already has three brats of dubious parentage.

Truth be told, Chiara is a woman of somewhat loose morals and everyone around knows her brood may not have been her husband’s progeny. Not that he is around to protest. A few years ago he was murdered in a bar brawl, the result of being caught cheating at the card table. To Chiara’s credit, the father of this latest baby is known, at least — and almost respectable, too. Fazio Cardano is a mathematician and jurisconsult. He is not much to look at, though: fifty-six years old and toothless, with shoulders that are stooped and rounded from hours hunched over Euclid’s books on geometry. And even if love were blind, marriage is not an enticing prospect for these two. The couple argue constantly and neither wants to bind themselves to the other just because of a child. What’s more, Milan is beset by new cases of the plague. Chiara is a practical woman, which is why she has left Fazio to take his chances, gathered up her children, and headed twenty miles south to plague-free Pavia.

It is here that the apothecary recommends she drink a poison to bring on a miscarriage. She follows his advice to the letter, but it doesn’t work. Neither do the next two doses. Despite all her efforts and expenditure, on 24 September 1501, after a three day labour, a nursemaid violently pulls a child with a head of curly black hair from between Chiara’s thighs. At first, it seems the reluctant mother is in luck: the child appears to be stillborn. However, the nurse puts him in a bath of warm wine and Jerome Cardano kicks into life.

This is not the last time that Jerome stares death in the face. Less than two years later, while he is still being suckled by a wet nurse, an outbreak of plague kills his half-brothers and half-sister. The nurse, too, catches the plague and dies. Her infected milk brings him out in five carbuncles, including one on the end of his nose, which scar and mark him for the rest of his life. But he survives, growing into a sickly boy who is prone to long bouts of illness and is often confined to his room in the family home in Milan’s Via del Rovelli.

The illnesses are frequently severe, but they never defeat him. Perhaps he is made hardier by the rough treatment he receives at the hands of his parents, both of whom beat and whip him. From the age of five, Jerome accompanies his father on visits to clients, acting as his book-carrying page. Occasionally, when Fazio needs to consult a volume en route, he orders Jerome to stand still and places the book on his crown. If Jerome moves, disturbing his father’s concentration, he receives a blow to the head. It is dark testimony to the treatment he gets from his mother that Jerome recalls his father as the ‘more loving’ of the two.

It’s not all hardship though. Such journeys give Jerome experience of the best his world has to offer. He sits in on discussions between his father and Leonardo da Vinci. He sees Leonardo’s The Last Supper on the refectory wall at the convent of Santa Maria delle Grazie, long before it decays. The artist experimented with paints that lasted only a couple of decades before they began to flake and fade. Seeing it later in life, Jerome called it ‘blurred and colourless’ in comparison with the glorious painting he had seen as a boy. I daren’t tell him about Napoleon’s troops peppering it with stones and horse dung.

For all the occasional glamour, serving his father fixes Jerome’s gaze on a different path. Fazio is grooming his son for a legal career, but in vain: by the time he is eight years old, the boy has decided that legal books are too heavy and legal arguments too dull and inconclusive. Numbers, geometry, and medicine are much more attractive. By the age of twelve, he has enough understanding in arithmetic to read the works of the Arabic alchemist Jabir ibn Hayyan and has developed a theorem that allows him to calculate the distance between stars from their latitude and longitude. The theorem is almost certainly riddled with errors. Nonetheless, the young Jerome’s desire to explore the cosmos is making itself clear, as is his lust for lasting fame. ‘I desire to defend myself from obscurity,’ the precocious twelve-year-old writes in a letter to a friend.

‘Five hundred years later, people still know your name,’ I tell him. He looks unconvinced.

He steeples his hands together, laying his elbows on the splintered wood of the desk. The apex of the steeple covers the pox scar on his nose — the pose looks practised, useful for hiding this small disfigurement.

‘It’s what you wanted, isn’t it?’

He gives me no response. Just a continued fixed stare. He appears to be sucking on the emerald again. It is hard to know how to take this forward. I want to flatter him, to convey my admiration. I don’t want to tell him that he is actually overwhelmingly unknown. Really, it is only a few mathematicians and a smattering of historians who have any familiarity with his work. Every day, hundreds of millions of people drive around in cars whose power transmission depends upon a Cardan joint, but they don’t know anything about the man himself. Also, I’m wary of opening up a discussion of cars and internal combustion engines. That might be a lot to take in for a man who has just learned he is to face the Inquisition.

‘What are they accusing you of?’ I ask, nodding to the door that leads out of the cell. I ask as if I don’t know.

Jerome’s eyebrows lower and his eyes narrow. He stares at me in open suspicion. Finally, he takes the stone from his mouth and breaks his silence.

‘My father told me he was visited by a familiar spirit the whole of his life,’ he says. ‘Are you the same one?’

‘Is that what you think I am?’ The idea makes me laugh out loud. ‘A spirit? Your guardian angel?’

He stares at me, confused, then begins to smile, an endearing but ugly sight that lifts the pox scars on his face out from behind the hairs of his beard. ‘It was you,’ he beams. ‘You sent me the warnings. About my mother’s death, and Giovanni’s marriage, and …’

He stops because I am still laughing. This is the trouble with us human beings. We have no ability to confine ourselves to the realities of the natural universe. As soon as anything remotely inexplicable happens, we reach for the supernatural.

Perhaps this is a good time to tackle the astrology question. Here’s the simple truth: I could not be such good friends with Jerome if I did not consider him a rational man. He is not one to attribute supernatural causes to everything that he can’t explain. In fact, he celebrates the strange and wonderful abilities of the human mind, its seemingly endless capacity to reason, imagine, and invent. He records his encounters with a variety of technologies and innovations that have impressed him over the years, things such as a hinged, hooked hand that would close on a thief’s fingers if he tried to take money from a purse. ‘Our age is prolific of distinguished and very great inventions,’ he says in De Subtilitate (On Subtlety), a book he published in 1550, aged forty-nine.

Nothing impressed him more than the skills of a great conjuror. He marvels at the card tricks that confuse, then amaze, the onlooker. He doesn’t understand how the conjuror can cause his audience to choose specific cards from the pack. ‘It was too extraordinary for us to follow by human deliberation … unless he had sometimes asked us to take out a number of different cards, I would have suspected that he had supplied a pack consisting of cards of the same kind,’ he says. For all the mystical thinking of his epoch and his own belief in the reality of the supernatural, he sees no dark influence at work. ‘The evidence was that it all was the work of a conjurer rather than demons,’ he writes.

If the card sharps were entertaining, the Renaissance showmen were astonishing:

There is no end to the inventions of this skill — shifting things, hiding them, swallowing them, emitting floods of fluid from one’s eyes or forehead, producing nails and string from one’s mouth, chewing glass, piercing one’s arms and hands with a spike, uniting iron chains while the links remain intact — indeed, a greater feat, I have seen three rings thrown up and coming down interlinked, though they were unbroken and separate before and while being thrown up … They show a child without a head, and the head without the child — but they are all alive, and the child comes to no harm in the meantime.

Then there are the rope walkers, who would climb steep lines from the ground up into a tower, often while balancing something — or someone — on their shoulders. Jerome assures us in On Subtlety that there is no occult power at work. It is, he says, simply a demonstration of the extraordinary feats to be achieved when we gain an understanding of how nature is. ‘It is in fact magic, when something extraordinary is done on natural principles, concealed though these are,’ he says.

I am telling you all this because I want you to understand he is no different to you and me, even if he does believe in the influence of the stars and planets over the lives of humans. Such a belief is a completely tolerable idea at his stage in history and it is still surprisingly inviting to an intelligent human mind. I know this because I tried it out.

I decided quite early on that, if I am to understand Jerome, to know him a little, I should dabble in astrology. To this end, I had a couple of readings done. One was by an Indian astrologer called Vishal. He knew only my name, and date and place of birth, and didn’t tell me anything terribly profound until I asked him about the car I had just bought. His astrological charts were all on his laptop computer (a pleasing conjunction of ancient and modern) and it didn’t take him long to tap something car-related into the program.

‘I see two cars in your future,’ he said.

I laughed. ‘Does that mean I’ve bought a dud?’

‘I can’t tell you that,’ he said.

Apparently, Vishal just saw two cars. And two weeks later, my mechanic was working on the one I had bought when he found a significant problem. He informed me I had indeed bought a dud. I wish I could tell you it was an issue with the Cardan joint, but it was a simple case of hidden but chronic chassis corrosion. He advised me to return the car, get my money back, and buy another — preferably from a reputable dealer.

My other reading was by a woman called Sue. Perhaps I had been primed by Vishal, but some of her pronouncements seemed uncanny. Then, however, I had a psychologist listen to a recording of the session and he was gloriously unimpressed. Sue was performing the classic generalisations designed to connect with almost anyone of my age and gender, he said. Vishal, he added, had just been lucky with a stab in the dark about the car.

I felt suitably chastised. But it made me realise how easy it is to believe that hidden causes lie behind everyday events.

Jerome’s investigations of astrology were, to him, entirely rational. In a worldview formed by the biblical scriptures, heavenly portents are a given. The book of Revelation declares that the heavens are like a scroll of parchment — so why not see what’s written on it? Genesis tells us that the stars and the moon were given ‘for signs and for seasons’. There are moments in Israelite history when we are told the sun stood still in the sky. The prophets saw all kinds of celestial signs associated with divine judgement: Isaiah tells us the sun will be ‘dark at its rising’; Joel tells us ‘the stars withdraw their shining’. True, there are passages condemning astrology used for divination, but that stricture is about attempting to know the mind of God. The writers clearly want us to be aware of God’s control over nature and its betrayal of his mood in signs written on the sky.

Even within this paradigm of reasonable astrology, Jerome is something of a progressive, imbued with a rising scepticism. Take that as a mark of his fine mind: the scepticism arose despite his conservative upbringing. Jerome’s father taught him the principles of casting horoscopes and he was often in the company of men — always men — discussing the art of interpreting the celestial sphere. Which makes it all the more remarkable that the young Jerome writes this to a friend in 1519, at the age of eighteen:

It was very well for my father to wring his hands at the issue of fortune determined by the heavenly bodies on the houses where we set our homes; but I recall thinking that the distance was very great for such an issue to be made, for might not the sun shine on Cathay when rain fell upon Padua? And if the biggest star of all could not stretch its warmth thus far, how could the smaller ones seem to malign from such a height? Now I know I was in error; but it was a childish curiosity and not without the gleanings of a cynic.

Centuries later, we science lovers consider ourselves to have a more sophisticated perspective. There is no physical force — and certainly not one associated with the stars or planets — that might influence our personalities or life events. Yes, there are forces that act at a distance — gravity and electromagnetism, for instance — but they are weak and ineffectual, even over the kinds of distances that stretch between Earth’s countries and continents. What force could possibly exert influence on us from the planets or stars?

But even in Jerome’s time, it was not a modern thing to question astrology. Cicero and Augustine had offered criticisms. Logical thinkers had pointed out that the time and date of birth could not determine character and destiny, as astrologers claimed, because twins often had very different characters and destinies. The Genesis account of Jacob and Esau makes that abundantly clear. What’s more, astrologers were adept at self-examination. Ptolemy, the Greek-Egyptian mathematician, astronomer, and astrologer, was a vicious critic of his own predictive powers and their limitations. It was acknowledged that, in a world created only a few thousand years before, as the contemporary cosmology had it, there hadn’t been time for some of the conjunctions and other phenomena to occur repeatedly. Thinkers acknowledge that if the celestial events had happened only once in recorded history, any claims about their significance relied on entirely spurious single-point observations.

Jerome remained conflicted. The stars and planets are not too distant to exert some influence, he suggested. It is clear from his other writings that he is convinced an influence does exist. However, he admits he has only the vaguest idea of what is going on and that it may not be exerted over everything in life. ‘A man is a fool who attaches too much meaning to insignificant events,’ he says in his autobiography, The Book of My Life. He was seventy-five years old then and clearly still believed the heavenly portents were signs of something — they were just incredibly difficult to read. ‘There were stars which threatened, from every aspect, my death, which all declared would be before my forty-fifth year,’ he writes, ‘all vain findings, for I live, and I am in my seventy-fifth year! It is not the fallibility of the art; it is the inexperience of the artificer.’

Jerome’s views on astrology mirror our own on quantum physics. In quantum experiments we see things appear in two different places at once, or exert an instantaneous influence over something that is half a world away. We cannot make sense of it, but we don’t dismiss it as ridiculous. We have the evidence of our experiments, after all, just as the astrologers have the ‘evidence’ of experience.

For three years, I worked in a research laboratory studying quantum physics. At the end of those three years, I produced a report of my research: my PhD thesis. It sits on the shelf above my desk, bound in blue with the title embossed in gold: ‘Quantum mechanical behaviour of superconducting weak link capacitor circuits in the range from 9K to 0.3K.’ It describes a project to take a ring of niobium metal and make the electrical current circulating within it travel in two directions at once. Let me be clear: this is not two different currents, but one, doing two things simultaneously.

The phenomenon is known as superposition. I was not the first to create a superposition in a niobium ring — far from it (though I was the first to do it at the particularly low temperatures to which my niobium ring was cooled). Nor was I the last; others have followed up the work. But we still don’t really understand it.

The explanation of superposition — such as it is — is more commonly laid out in terms of the ‘double slit experiment’. It demonstrates that matter can simultaneously exist as discrete objects, like an archer’s arrow, and as waves, such as those that travel through a large, continuous body of water.

Imagine an archer trying to fire arrows into the watchtower of a fortified medieval castle. His only hope of getting his arrows to the inside is by firing at one of the two narrow arrow slits set two metres apart in the wall. Assuming he is good at his job, his arrows will hit the wall behind the arrow slits. There will be a cluster of arrows at two distinct points on that wall.

Now imagine a trebuchet that throws a huge bucketful of water at the wall. The water will go through both slits and the two portions that emerge into the room will hit each other as they travel towards the far wall. That means they affect each others’ paths, so there won’t be two distinct splashes on the wall, but a messy wet patch that stretches across its extent. It might be a bit wetter at the points directly behind the arrow slits, but much of the rest of the wall will be wet, too.

We can imagine this because we know how single objects or particles behave and we know how things like large bodies of liquid behave. What we also know now is that there are things that behave like both — and neither.

Physicists generally describe the double slit experiment in terms of light. The light is fired at two very narrow slits in an opaque barrier. The far wall on the other side of the barrier is a light-detecting screen.

When English scientist Thomas Young shone light on a pair of slits in 1803, he found that the light hit the screen giving a peculiar pattern of bright and dark areas. He explained this in terms of two light beams emerging from the slits (one from each slit) and ‘interfering’ with each other. Interference was a known property of waves. When two water waves, for example, travel in the same direction, they will spread out and overlap. If the crest of one wave coincides with the crest of the other, their strength is doubled, and a larger crest is created. Similarly, if the wave troughs coincide, the trough is deepened. Where a crest meets a trough, the wave is destroyed and the water is, effectively, flat.

The pattern of light and dark ‘fringes’, to Young, was long-sought proof that light is a wave, not a particle. An old debate was finally settled.

Except that it wasn’t. Early in the nineteenth century, Einstein showed that light exists as particles we now call photons. He won a Nobel Prize for his efforts and a few decades later we learned to control lasers that could emit one photon at a time. That changed the face of physics forever.

Repeat Young’s experiment with just a single photon fired at the slits and something utterly extraordinary happens. You have to replace the light-detecting screen with one that permanently records the impact site of each photon by changing colour, say from black to white, but it is worth the effort. You also have to be patient: with just one photon hitting the screen at a time, it takes a little while for the miracle to emerge. But emerge it does.

Eventually, the screen has turned from black to white in exactly the same places as it would if two beams of light were emerging from the slits. There is still interference, despite the fact that there is only one photon in the apparatus at any one time. The only explanation is that the single photon has somehow travelled through both slits simultaneously and then interfered with itself. This experiment — which has been conducted countless times without any failures — contains what the celebrated physicist Richard Feynman called ‘the only mystery’ of quantum physics. The mystery is that, given a choice of two paths, a quantum particle takes both. It happens with single electrons (Claus Jönsson of the University of Tübingen was the first to observe that, in 1961) and with larger particles, such as atoms and molecules. It happens with the large body of quantum material (known as a Bose–Einstein condensate) that makes up the electrical current within my ring of niobium. Yet none of these occurrences make sense.

What makes it even more confounding is the fact that, if you try to see which path the particle took by placing some kind of detector on one of the slits, the interference pattern goes away and you go back to two distinct clumps. Under observation, continuous waves turn back into discrete particles. It is as if watching the water emerge from the arrow slit makes the water change its fundamental character and suddenly behave like the archer’s arrows. In the quantum world, it seems, the weirdness doesn’t like to be watched.

This is entirely within the theory’s predictions. Quantum theory is our most successful mathematical framework and its predictions have never been wrong. However, it is utterly bankrupt when it comes to explaining the physical world. It tells us what we will see in any given quantum experiment, but leaves the ‘interpretation’ — a description of what is actually going on — entirely up to us.

So, what happens between the time when the photon enters the apparatus and is detected at the other side? When does the photon begin its double existence? If those slits could talk, what would they tell us? We don’t yet know.

The interpretations of quantum physics are the tales we have constructed around this simple experiment, and any one of them might be true. One interpretation — explanation, if you like — says the photon simply isn’t real until you detect it at its final resting place. That means you can’t actually say that it went through both slits; in the absence of detection, it didn’t have any existence in the experiment. Another interpretation says there is an undetectable ‘guiding wave’ that determines the photon’s path through one slit, while leaving a false trail that suggests it went through both. Another is that there is a branching of worlds where, in one world, the photon went through one slit and, simultaneously, in another world, it went through the other. The interference pattern we see is the ‘crosstalk’ between these worlds.

What do you make of this? Are you comfortable with things pinging into existence only when they hit a detector? Can you stretch to a false trail created by an undetectable wave? And what about these parallel worlds? Does that seem like it could ever be the simplest, most plausible explanation for the outcome of an experiment?

Of course not. So we delve deeper, convinced that the fault lies with our poor grasp of reality. ‘Quantum theory does not trouble me at all. It is just the way the world works,’ the great physicist John Archibald Wheeler once wrote. ‘What eats me, gets me, drives me, pushes me, is to understand how it got that way … the quantum is the crack in the armor that covers the secret of existence.’

For the young Jerome, curious and sharp-minded, the only way to learn the secret of existence was to practise the art of astrology. This he did throughout his life, quickly becoming expert at getting under the skin of his subject — and gaining the attention of paying customers.

The speed of his advances is illustrated in the difference between his first and second astrological publications. The first, published at his own expense when he was thirty-three, was a ‘Prognostico’. Typical of the genre, it comprised a series of short- and long-term predictions about developments in anything from religion to politics to the weather. Such pamphlets were commonplace — Europe was crawling with astrologers. No amount of official derision would dampen public enthusiasm for foretellings. The Archbishop of Canterbury dismissed Michel de Nostredamus’s 1558 prognostication as a ‘fantastical hotch-potch’, for example, but a vast swathe of Europeans, from merchants in the market to princes in their palaces, acted on Nostradamus’s words.

Jerome was keen to divert some of this attention (and money) his way. To distinguish himself from other, lesser practitioners who were ‘defiling this noble science’, as Jerome put it, he announced himself as relatively unknown. Not, Jerome added, because he was a poor practitioner of the art, but because he was not one of those people who is happy to achieve fame by saying what noblemen want to hear.

Having set out his stall with this humblebrag, he blinds the reader of his Prognostico with details of offset axes in the celestial spheres and the precession of equinoxes. There is no shortage of technical detail by which he establishes his scientific credentials. He stops short of predictions about wars because ‘there is no part of astrology harder than this one, and yet the bulk of these crazy diviners speak more boldly about it, in their bestiality, than about anything else.’

Jerome later refers to his works of this time as ‘astronomical tables’, but however scientific Jerome wanted his astrological works to appear, they were primarily published to make money and create a reputation. As a backup, he was careful to record himself as ‘a doctor from Milan’ on the title page — an advertisement for his other line of business. He was, at this stage, not yet licensed to practise medicine by the Milanese College of Physicians and, without a licence, he couldn’t officially see patients within the city. Yet he needed to scrape together a living and was open to offers. Jerome knew he had his work cut out making it as an astrologer in such a controversial and overcrowded space. That’s why he followed a different path with his second publication: Supplement to the Almanach. Published four years later, in 1538, its unique selling point is its inclusion of a primer on astronomy. ‘One who wishes to attain knowledge of the stars must begin with knowledge of the planets,’ Jerome explains. And so he lays out the movements of the known planets and instructs the reader on how to find each of them in the sky. He explains how to find the pole and all the constellations of the Zodiac. He gives tips on remembering all the various positions. It is an unprecedented popularisation of astrology, removing the barrier between amateur and professional; Jerome is extending an open invitation to anyone wanting to understand the secrets of the stars. He is, according to the twenty-first-century scholar Anthony Grafton, ‘a sixteenth-century counterpart to Patrick Moore or Carl Sagan’.

All he needs from his client is humility and tolerance. ‘This alone I ask you, O reader, that when you peruse the account of these marvels that you do not set up for yourself as a standard human intellectual pride but rather the great size and vastness of earth and sky.’ It is Jerome’s way of excusing his approximations and errors: astronomer and astrologer cannot hope for absolute precision when dealing with the near infinite phenomena of the celestial sphere. That said, Jerome considers himself well within the limits of the plausible. ‘And, comparing with that Infinity these slender shadows in which miserably and anxiously we are enveloped, you will easily know that I have related nothing which is beyond belief,’ he says.

A second volume of this publication — Corrections to Errors of Time and Motion — explains the latest thinking about the motions of the planets, offering corrections to errors in accepted texts. These revisions were drawn from Jerome’s own observations of the sky. It is a triumph: he paints himself as a hands-on astronomer, a trustworthy guide who has done more than just read the texts of the ancient sages. Two centuries later, the great Tycho Brahe will cite Jerome’s works with respect. Having laid out his scientific credentials, Jerome then explains how science can improve on traditional astrology. And having done that, he ensures its popularity by going on to offer ten celebrity horoscopes.

Some were bigger celebrities than others, it has to be said. His father, Fazio, for example, was on the lesser-known side. But he also drew up a chart for Suleiman the Magnificent; for the Holy Roman Emperor Charles V; for King Francis I of France and — perhaps most importantly — for Pope Paul III, himself a great believer in astrology.

This isn’t just about catching public interest. It is a calculated bid for influence. Jerome fully believes he is worth listening to. He believes himself possessed of gifts and insights that he wants to share widely. He is approaching his fourth decade and keen to get on in the world. And so he dedicates the book to an old acquaintance of whom we’ll hear more very soon: Filippo Archinto.

At that time, Archinto was the governor of Rome. In return for Jerome’s dedication, Archinto told Jerome that Pope Paul might be open to a gift or two, which could establish the astrologer as a major public figure. So Jerome went all-out and created a horoscope of Christ to be presented to the Pope. And nearly forty years later, in 1570, that is what has landed him here, awaiting trial at the Inquisition and convinced that God has been protecting him through a guardian angel.

It seems odd that Jerome would think of me as any kind of useful guardian — he is in prison, after all. And this angel certainly did nothing to protect him from the machinations of Nicolo Tartaglia.