'In layman's terms I'd call them an energephagic transdimensional chameleoform, but seeing as this is the fourteenth century, let's just call them "nasty wooden beasties".'

I laughed, and it echoed around the church, for a moment drowning out the baby's first squeals. 'You are mad. We're the only people in here.'

Striding down the nave, the Doctor aimed his stylus first at the altar and then at the carved wooden skeletons.

'Doctor... They're made of wood.'

'Why do humans always a.s.sume aliens would disguise themselves as humans? You're not the only organic life on the planet, you know.'

Leaning in close, he tapped the stylus against one of the wooden skulls: Knock-knock-knock.

'h.e.l.lo. Wakey wakey. Rise and shine.'

Again he tapped the skull, which sounded, to my ears, exactly like a block of wood and nothing more.

I was beginning to lose my patience.

'Doctor, if, indeed, you are a doctor... Matilda just gave birth. Now isn't the time for childish games.'

And that, with diabolical timing, was when the skeleton moved; its head tilting to one side and its jaw falling open with a wooden squeak. I very nearly befouled myself, and could only splutter: 'Did it...? Did that thing just...? Can it...?'

'Yes, yes and yes,' said the Doctor. 'We should probably step away from them now. No sudden moves.'

'But they're wooden sculptures. What could they possibly do to us?'

'Oh, I don't know... Kill us and feast on our dying breaths? You see, that's why they were here in the first place. The plague. They didn't start it; they fed on those it killed. A b.u.mper harvest. Then nothing. But why didn't they just leave?'

As quietly as we could, the Doctor and I inched our way backwards down the nave; the skeleton never once taking its hollow eye sockets off us, not even for a second. Much to my horror, the thing detached itself slowly from its seat, standing to its full height, and the remaining skeletons followed suit. The creak and groan of wood separating from wood quickly alerted my fellow pilgrims.

'By the saints!' cried Isabel.

Looking up from her new-born son, Matilda screamed. Her husband Johannes was dumbstruck, and could only stare at the skeletons aghast as they made their way forward, two in the aisle and one in each nave, matching each other step for step.

'Listen, fellas,' said the Doctor. 'I don't suppose I could interest you in a game of chess? Or perhaps a quick round of Ker-Plunk?'

The skeletons took another creaking step forward, and in perfect synchronisation raised their scythes.

'OK,' said the Doctor. 'Everyone get behind Matilda.'

'Doctor?' I said, looking at him askance. 'Have you lost your mind?'

'Cower behind a woman?' said William of Bristol, and true to his nature he puffed up his chest and bellowed, 'Never! Have at you, vile demons!' Then, with fearsome bravery or foolishness he drew his dagger and gave charge.

'Don't!' yelled the Doctor, but it was too late. Our companion was within feet of the skeletons when one of them lowered its scythe, and with the curved wooden blade tapped the crown of his head. In an instant, the old warrior's eyes grew dim, and his lifeless body slumped to the ground. Then, looming over him, the skeleton opened its jaws and began to breathe in, and from William's corpse there rose a silver, ghost-like mist that hovered briefly in the air before vanishing into the skeleton's open mouth.

Johannes helped Matilda to stand, and our party retreated to the entrance of the church. Behind us, Hugo and Isabel had begun battering at the door, and from outside I thought I heard voices and the jangling of keys, but all seemed hopeless. Even if the villagers released us, there was surely little they could do to fend off our h.e.l.lish adversaries. I turned to the Doctor, hoping that he might have the answer, and saw only that same mischievous grin of antic.i.p.ation that I'd first witnessed in the tavern.

'Doctor?' I said. 'Do something!'

'Wait for it,' said the Doctor. 'Wait for it...'

The skeletons were now mere yards away. Cowering on the ground, Matilda held her baby to her chest and began to pray. Whether it was through hunger or fear, I know not, but the babe let out a cry that sang throughout the church, and all at once the skeletons paused and did something that none of us save, perhaps, the Doctor could possibly have expected: they screamed. Then, stretching out their gnarled and knotted fingers, they pointed not at Matilda, but at her baby, and as a chorus began to howl and wail in anguish. The Doctor's eyes grew wide, and a brief, ecstatic laugh escaped him.

'Yes!' he said, and then, turning to the others: 'I love it when I'm right. Which, granted, is most of the time. But still...'

Swiftly, the skeletons retreated to the church's altar and they began twisting and turning at details carved from wood, but to little effect. The altar itself glowed briefly, and from within it came a low and menacing hum, but both faded as soon as they began.

'That's it!' said the Doctor. 'They couldn't leave. Engine failure!'

He left us, and with cautious steps made his way towards the altar.

'What is he doing?' asked Johannes.

'I've no idea,' I replied.

As the Doctor came near, the skeletons raised their scythes again and hissed.

'No!' said the Doctor. 'I'm not here to harm you, or... you know... throw babies at you, or anything like that. I'm here to help.'

'Help them?' said Isabel. 'Why would he help those monsters?'

Once again, the Doctor held aloft his mysterious stylus, and placed it against the altar. It chirruped and it shone, and in the blinking of an eye the altar began to glow, and that low, throbbing sound grew louder and louder.

Suddenly, and with fortuitous timing, the church door was flung open and a grey beam of moonlight flooded the building, just in time for the villagers outside to see the events unfolding within.

Leaving the skeletons, the Doctor gestured to us that we should run (we needed little persuasion) and together we helped Matilda to her feet and bolted for the open door. Glancing back into the church, I saw what I'd thought was an altar now shining with a brilliant and blinding white light. From beneath us came a deep and sinister rumbling, and we were no sooner out of the church than the earth surrounding it began to crumble and crack.

Pilgrims and villagers alike fled down the hill, Matilda still clutching the baby to her chest. The noise was almost deafening, and slowly the whole structure twisted itself out of the earth and began rising up into the night sky. Then, with a colossal boom that stripped the leaves from nearby trees and rumbled across the neighbouring valleys like thunder, it was gone.

'It's a miracle!' I said.

'Why not?' said the Doctor, and together we sat, exhausted, on the ground.

We were on the outskirts of Bembibre. The sun had risen, and we were ready to move on to the next town, and the next. The Doctor, however, had given us his apologies. He would not be joining us for the rest of our journey.

'You knew,' I said. 'You knew those demons were in there. You knew the church was evil.'

'Not evil,' said the Doctor. 'Alien.'

'How can you say those things weren't evil? They killed William.'

'In order to eat. I don't remember reading anything about Geoffrey Chaucer being a vegetarian.'

'Another of your foreign words.'

'Precisely.'

'How?' I asked him. 'How could you possibly have known those creatures would be frightened away by a baby?'

'Call it an educated guess,' said the Doctor. 'But it wasn't the baby that scared them. Not really. It was life. New life. The opposite of death.'

'You mean it conquers death?' I said, in absent thought. 'That's a splendid idea for a story.'

'Not what I said,' said the Doctor. 'But go for it.'

Then, without saying another word, he walked away towards a small, blue, tomb-like edifice at the side of the path; an odd construction that none of us had noticed before. Through a narrow door in its side, the Doctor entered the blue box, and seconds later we witnessed one final miracle when, to our surprise, it vanished, leaving not a single trace that it was ever there.

'Four minutes [to live]? That's ages! What if I get bored? I need a television, couple of books. Anyone for chess? Bring me knitting.'

The Eighth Doctor, The Night of the Doctor (2013)

You are going to die. You might live for decades yet, but eventually you'll die. As the Ninth Doctor tells Ca.s.sandra in The End of the World (2005), 'Everything has its time and everything dies.' You, Ca.s.sandra and the whole universe have a limited existence. Yet, surprisingly, it was the development of steam engines during the Industrial Revolution which helped us to understand why everything must eventually end.

In The Mark of the Rani (1985), the Sixth Doctor resists helping English engineer George Stephenson with his design for a steam-powered engine that Stephenson hopes will be capable of extraordinary speed fifteen or perhaps even twenty miles an hour. 'You'll find the answer,' says the Doctor later, rather than interfere in the course of history. 'And when you do, your invention will take off like a rocket.'

The story is probably set sometime in the 1820s because in (the real) 1829 a steam locomotive called Rocket took part in compet.i.tion run by the Liverpool & Manchester Railway. Rocket won the compet.i.tion and its designer, Robert Stephenson, went on to provide the steam engines that worked the very first public transport system powered solely by machines. Robert Stephenson was helped in designing Rocket by his father, George the man the Doctor met in the story and for this and other work on the new method of transport, George Stephenson is often referred to as the 'father of railways'.

At the time, steam engines had a number of uses: they could pump water out of mines or drive machinery in factories, enabling people to extract raw materials and turn them into useful products more efficiently than ever before. In the early 1800s, the first engines using steam at high pressure were introduced meaning much more power could be produced by a smaller engine. Developments continued, making engines smaller, faster and more powerful.

Yet there had been little published science on the workings of steam engines until French physicist Nicolas Leonard Sadi Carnot published Reflections on the Motive Power of Fire (1824). Carnot's attempts to understand the laws of physics that dictated how steam power worked led to a whole new field of science known as thermodynamics, from the Greek for heat power. But thermodynamics doesn't just explain how heat is moved around inside steam engines. It can also show how the universe will end.

A steam engine works by transforming energy from one form into another, eventually converting it into something useful like the motion of a piston to drive a pump or the spinning of a wheel to move a train. Coal is burned with oxygen to make carbon dioxide, in the process liberating heat energy. This heat energy is used to boil water into steam, which then expands. The expanding steam pushes on a piston which begins to move and can be made to do useful work. The chemical energy stored inside the coal has been converted into heat energy and then, via the water, the steam and the piston, into energy of motion. Sadi Carnot reasoned that the process which ultimately drove the engine was the tendency for heat energy to flow from a hot region to a cold one, until the temperature of the whole system was the same everywhere.

On a molecular scale, heat is really just the random motion of all of the atoms and molecules which make up an object. The more heat energy the object contains, the higher the average speed of its particles and the higher its temperature. The atoms and molecules are constantly colliding with each other and transferring energy from one to another, so the energy is constantly being redistributed around the object rather than being concentrated in just a few particles. If two objects with different temperatures are placed in contact, their atoms and molecules will begin to knock against each other, transferring energy between them. Overall, the atoms and molecules from the hotter object will transfer more energy to those in the colder object and there will be a net flow of heat until it is evenly spread between them and the objects are at the same temperature. This transfer of heat from a warm object to a colder one is known as the First Law of Thermodynamics.

The design of a steam engine allows some of this heat flow to be harnessed to do useful work. But scientists soon realised that no engine could ever be 100 per cent efficient because at every stage a fraction of the energy would be wasted: some of the energy from the burning coal will always warm the container rather than the water, some of the energy of the expanding steam will always warm the piston rather than push it, and some of the energy of the moving piston will be turned back into heat by friction with its surroundings. By improving the design of the engine, the amount of waste could be reduced and the efficiency improved, but no matter how hard you try the waste can never be entirely eliminated.

This is not just true of steam engines: any process which involves the transfer of energy from one form to another, or from one object to another, will involve some wastage as heat. When we fire a gun, chemical energy from the gunpowder is converted into the energy of the motion of the bullet, but inevitably some of it also heats the barrel of the gun itself or is turned into the sound of the gun firing (which eventually heats the air). When we switch on an old-fashioned lightbulb, the energy carried by the electrical current is converted into light, causing the metal filament in the bulb to glow, but a fraction of the energy is also converted into heat, causing the filament to get hot. Modern energy-saving lightbulbs use other, more efficient, methods to convert electrical energy into light energy, but a certain amount of heat is still produced.

What's true of steam engines, bullets and lightbulbs is also true of the universe as a whole: in every process occurring across the cosmos, from the orbit of planets to the shining of stars and the turning of galaxies, some energy is inevitably being converted into the random motion of particles heat. In the long term, the tendency is for temperature differences across the universe to be evened out, as stars pour their heat and light into the frozen blackness of s.p.a.ce and clouds of gas expand, collide and cool. This led the nineteenth-century physicist Lord Kelvin to imagine what we now call the 'heat death' of the universe: a time in the far future when all available energy has been converted to the random motions of heat and this heat is spread evenly across the whole universe. With no differences in temperature to cause energy to flow from one place to another, no useful work could be done and engines, bullets, lightbulbs and even life itself would be impossible.

One way of thinking about this process is that the universe is slowly but inexorably changing from an 'ordered' state, in which matter and energy are concentrated into complex structures like galaxies, stars, planets and people, to a 'disordered' state where they are spread out more randomly. Scientists have given a name to this universal quality of disorder entropy. Its inevitable triumph is summarised by the Second Law of Thermodynamics, which states that entropy always increases.

We have an instinctive understanding of entropy because in some ways it defines our sense of the direction of time. If we put a dollop of strawberry jam in our porridge and stir it in, we expect the porridge to become pink as the molecules of the jam and the porridge are randomly mixed together. However long we stirred for, we would be extremely surprised to see the jam begin to separate out into a dollop again and if we saw a film of this happening we would guess that the film was being played backwards.

Technically, it's not impossible for stirring to cause the jam and the porridge molecules to separate out again, just very, very unlikely. We can think of it as a game of probabilities: if we imagine all the ways we could arrange the trillions of porridge molecules and the trillions of jam molecules in a bowl, the vast majority of these would involve a random mix of porridge and jam a disordered state. Only a small fraction of the possible arrangements would involve an ordered state, with all the porridge molecules neatly in one part of the bowl and all the jam molecules in another.

The same is true of any other system which involves a very specific arrangement of matter and energy. If we drop a cup of tea, it will go from an ordered state, in which silicate molecules are arranged in a cup shape and water, milk and tea molecules are confined to a cup-shaped volume, to a disordered state in which fragments of china and splashes of tea are spread randomly on the floor. There are simply far more ways to go from an ordered state to a disordered one than the other way round, so as time progresses teacups smash but shards of china and pools of liquid never spontaneously combine to form steaming cups of tea.

One intriguing exception to this rule of ever-increasing entropy is life. Living things are by definition highly ordered structures which depend on complex arrangements of molecules and carefully controlled flows of energy in order to function. Throughout their lifespans, living organisms seem to be able to keep entropy at bay, continuously maintaining their ordered state in apparent defiance of the Second Law of Thermodynamics. But this is only a partial victory: life maintains itself at the expense of its surroundings, pumping heat and excreting waste materials back into the environment and the total entropy of the universe is still increased.

'Entropy increases! The more you keep putting things together the more they keep falling apart!'

The Fourth Doctor, Logopolis (1981)

Ideas of entropy on a personal and a universal scale are explored in the Doctor Who story Logopolis (1981). Here it is revealed that the universe should have reached the point of heat death long ago but entropy has been kept at bay by the mathematicians of the planet Logopolis, who have found a way to siphon it off into neighbouring universes (what the inhabitants of these other universes think of the plan is never mentioned). Through the actions of the Doctor's arch enemy, the Master, the vital gateways are closed and entropy begins to claim the galaxies, stars and planets of our universe including the home world of the Doctor's companion, Nyssa.

In the story, the Fourth Doctor is able to halt the process and prevent the universe from collapsing into the ultimate disordered state, but only at the cost of his own life. However, this seems to be only a temporary if long-lasting reprieve: in Utopia (2007), the TARDIS travels forwards to the year 100 trillion, to an exhausted universe of fading stars and dying planets.

Just as the Fourth Doctor gave his live to save the universe, so all living things must eventually die. Life may be a process of holding entropy at bay but this can only be a temporary victory. Eventually all living systems begin to fail: ageing and death are, of course, examples of the inevitable trend towards increasing entropy.