Original subtitles

What happens if you just keep cutting in half?

Let’s start with an apple - and a really really sharp knife.

You take the knife in your hand and, with a flourish,

you slice the apple into two neat pieces.

The apple halves become quarters,

then the quarters become eighths.

In front of you are smaller and smaller pieces of pulpy white apple-

But by your fourteenth cut, a regular rectangular pattern emerges,

And you realise the smooth apple flesh is built out of an array of plant cells.

You keep on cutting, chopping the cells into smaller and smaller pieces.

Another eight cuts reduces the cells into complex molecules of carbon, hydrogen and oxygen,

And with a few more cuts, what was the apple is now ten to the twenty seven individual atoms.

And you stop. Is this it? Can you cut no more?

But your knife is incredibly sharp, and you begin to chop into the atom,

Peeling away the cloud of electrons, and revealing the seeming nothingness within.

Almost nothingness, for at the centre sits the tiny atomic nucleus,

A hundred trillion times smaller than the apple you started with.

But again you are undaunted and continue chopping, the nucleus splits into protons and neutrons,

Protons and neutrons into quarks and gluons, and you find yourself in the ethereal world

of the subatomic, a world governed by fuzzy probabilities, the apple

nothing but an ethereal wave upon a sea of fields. It feels like you have finally reached the limit,

the fundamental pieces of the universe. Though some think you can keep going.

After more than a hundred cuts, pointlike quarks and electrons might be revealed to be waving

strings, Barely a trillionth

of a trillionth of a trillionth of a metre across, vibrating across extra dimensions.

Down here, maybe even space and time are chunky, fundamental blocks that can be cut no

more, Or maybe any

semblance of reality will melt away, as we find that our universe is truly a simulated cosmos,

churning away on some super-dimensional computer. Or that our three dimensional existence is

nothing but an illusion, and everything we know and love is shown to be a hologram.

Or even more strangely, maybe it’s just raw mathematics -all the way to the bottom.

But which is true? And how can we be sure? To understand reality, the true nature of reality,

and how humanity over thousands of years has struggled to unravel it,

we are going to have to dissect the cosmos - and ask some very difficult questions.

Is the apple…real?

You can hold it, smell it, taste it. You can see the greenness of

its skin in the sunlight. Surely the apple is real.

But what does it mean to be real? Just what is reality?

Isaac Newton used to bookmark the pages in his books in a very specific way - he would

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To our truly ancient ancestors, reality must have been obvious.

From the planes of Africa to those wandering the snowy wastes of Northern Europe,

Reality was what you sensed with your eyes and ears, touch and tongue.

Reality was simply out there. The snap of a twig in a forest

might tell them that their next meal is at hand, Or warn them that they might be on the menu for

a beast with bigger teeth and sharper claws. Sensing reality was a matter of life or death.

Perhaps, in their day-to-day struggles, these truly distant ancestors had little time to wonder.

Little time to think deeply about the world that surrounded them.

But as the human mind developed and thoughts expanded,

The nature of reality surely changed too.

We know humans began to bury their dead almost one hundred thousand years ago.

Quite what they were thinking as they farewelled their loved ones we will never know.

But perhaps they were thinking of a world beyond our reality, an afterlife.

These ceremonies provide insight into human thought,

And that there was more to reality than meets the eye, or ear or nose.

Ideas around spirits and gods and invisible, yet powerful, influences began to emerge,

Along with questions of meaning and purpose.

The gods also gave us the apple, providing delicious fruit for us to eat.

There was no need to question the reality of the apple beyond this,

The answer was beyond the ponderings of the human mind - only found in the thoughts of the gods.

Plato was one of the greatest philosophers of the ancient world.

Living in Athens in the fourth century B.C, his real name was Aristocles, with Plato being

just a nickname - a nickname that meant broad and apparently referred to his stocky build.

The material world bothered Plato. Where do objects get their material properties?

Be it the greenness of the apple skin, Or the fact that dogs have four legs.

Our world, Plato contended, was not the real world.

And to understand this, we will have to explore his famous allegory of the cave.

Plato asked us to think about a deep cave.

In this cave, a group of prisoners are kept chained, staring at a wall.

The prisoners have always been there, chained in the cave,

And have never been outside and experienced the real world.

All the prisoners can see is the wall in the cave, in front of them.

And on the wall, they see shadows. These are formed by sunlight

streaming into the cave entrance, With the shadows formed as people

go about their daily business at the mouth of the cave.

images of experiences they could never have. What are they to make of this external

reality, one that they can barely see? How are they to understand the real world?

But we can go further. What if they are not

really seeing shadows of reality from beyond the mouth of the cave?

What if, instead, some mischievous people have blocked off the cave entrance,

And are illuminating the prisoner’s wall with a bright fire?

With this light, these people can now play mind games with the prisoners,

Using shadow puppets to play out stories on the wall.

No longer are the prisoners simply seeing people wander by the cave entrance,

Now they can see stories of monsters and battles, castles and kings.

What are the prisoners to make of this new reality? Is it any less real than before?

To them, this image on the wall is their reality, there is nothing else,

And so, these exciting new tales are as real as the previous shadow people milling about.

To them, the monsters and battles are truly real.

Plato then asks us to consider that we are like the prisoners.

And the reality we experience every moment of the day is a projection,

A projection of a place where true reality plays out,

With our imperfect world being a shimmering image of perfection.

To Plato, our experiences are just shadows of a true reality.

Considering what we know now, this was oddly prescient.

But for the story of reality, this was just the beginning.

To reveal to the secrets of the universe,

to unveil the ultimate truth of the nature of the cosmos,

The answers did not lie in the hands of the gods or ancient philosophy.

Instead, we would have to delve deep into the fundamental makeup of matter, time, and space.

We would have to turn to the insights into reality provided by scientific exploration.

And when we did, Things quickly got very strange indeed.

“…Philosophy will clip an Angel’s wings, Conquer all mysteries by rule and line,

…Unweave a rainbow…”

In the first few months of 1821, the poet John Keats died in Rome.

He was young, only twenty-five, but his body was wracked by the scourge of tuberculosis.

In his short time, he was prodigious, writing on the natural beauty of the world around us.

And like many before and after, he was only truly recognised after his death.

Keats was born almost seventy years after the death of Isaac Newton.

It was the time of the Industrial Revolution,

a revolution driven by Newton’s insights into the natural world.

The scientific revolution of Newton and his contemporaries had sought to peel back reality,

And reveal it as nothing more than a manifestation of physical law in action.

And finally it was Newton who had realised that the white light of

the Sun contained all the colours of the rainbow,

And that water droplets in the sky spread sunlight into a glorious display from red to violet.

The seemingly mystical appearance of one of nature’s true wonders had, Keats complained,

Been destroyed by Newton reducing it to little more than a prism.

In Keats words, Newton had tried to unweave the rainbow.

Newton’s triumph might have been Keat’s lament, But as Newton began to unweave the

nature of light, He kickstarted a

long tradition of science peeling away the very nature of reality to expose its bones.

Let’s think again about our apple, Of course, one of the defining characteristics

of a ripe apple is its colorful skin. But just where does this colour reside?

Apples are made of a complex mix of water, sugars, and more complex structures.

So why do we see the apple appear green?

To answer this question, we must delve deeper into the meaning of colour.

And the story begins with the last man who knew everything.

Thomas Young was born in the tiny Somerset town of Milverton in 1773.

Precocious from the very start, he learnt French, Greek, Italian,

Latin, Aramaic, Persian and more, And was instrumental in deciphering

Egyptian hieroglyphics soon after the discovery of the Rosetta Stone.

For some of you, Young’s name might already be familiar,

As it was Thomas Young who undertook the first of

what’s known as the double slit experiment. This experiment sits at the heart of many

We will come back to Young’s double slit experiment later in our story.

Artists knew that mixing coloured paints produces new colours,

And three colours – red, green, and blue – seemed to be particularly important.

Young proposed that the eye contains three kinds of fibres, fibres we now call cone cells,

One kind is sensitive to red, another to green and another to blue.

The amount that each kind of cell is stimulated is passed along the optic nerve and into the brain,

Where it is mixed like paint in the mind to give us the sensation of colour.

Things, however, get weird when we look at the eyes of other animals.

Our closest relatives - apes and monkeys of the old world - are similar to us,

sensitive to three colour bands,

But most mammals only have two - equivalent to our blue and green.

This must reduce their colour palette – would they see a banana as being yellow?

Bees have three colour receptors, one sensitive to blue, another to green,

and yet another to ultraviolet.

Reptiles, fish, and birds can have four or more colour receptors.

The small but violent mantis shrimp has a total of twelve receptors spread across the rainbow.

These animals must see the world in an array of colours that we can’t begin to imagine.

And so what are we to make from this confusing array of animal visions?

For humans, the yellow of the rainbow matches the stimulated signal from the skin of a banana,

But other creatures probably would not agree.

To them, maybe a strawberry or broccoli or human skin is the same colour as a banana.

This tells us something deep about the nature of reality.

Our sight is nothing but a shadow of reality created

by our brains – it is subjective, not objective. We live in a visual simulation of our own making,

Built from the stimulations being fed into our brains.

Newton told us about raindrops and sunlight, But the colours of the rainbow

exist only in our minds. The sounds that we hear are similarly illusions,

An interpretation in our minds of vibrating eardrums due to waves of pressure in the air.

Everything is subjective - we live in a virtual reality of our own making!

So if what we perceive is not truly reality, then what is?

How are we to build the nature of the universe if we can’t trust information delivered

by our senses? Well, space and time can at least be trusted.

At least the apple is here and now, that is one certainty.

Or is it?

For in the revolution that was to follow Newton and Young,

No part of reality was safe. ________________

Time ticked by slower in the past than today.

And in twenty twenty-three, astronomers measured by just how much.

Their focus was quasars from 12 billion years ago, black holes weighing billions

of times the mass of the Sun. Swirling in the intense gravity

of these black holes, matter orbits at close to the speed of light,

This matter is heated to billions of degrees and glows intensely, visible across the universe.

But in the turbulent maelstrom, before it vanishes into the hole, matter crashes and grinds,

And the intense light brightens and fades like a cosmic firework display.

Hidden in this flickering, astronomers found a subtle pattern, a special regularity,

and they realised they could use this quasar tick

to measure the passage of time when the universe was young.

These astronomers compared the tick of a clock today to the ticks of these quasars,

Finding that the farther away the quasar, the slower and slower these quasar ticks became.

And when the universe was one-tenth of its present age,

Relative to today, time was ticking at one-fifth its current rate.

This result was spectacular - but it was not unexpected.

For this cosmic dilation was written into the mathematical laws that govern the universe,

In a theory developed more than a century before - a theory that

forever changed the reality of space and time.

I think, therefore I am. It was French philosopher,

Rene Descartes, who uttered these famous words.

And this deep musing on the reality of being has reverberated through the centuries.

But Descartes stared into more than the meaning of being.

One night in 1619, Descartes shut himself away in a small room to avoid the cold.

As a mercenary fighting for the Dutch Protestant State,

he had studied engineering and mathematics. But this frigid night would be like no other.

The story goes that Descartes was confined to his sick bed, staring aimlessly about the room,

He spied a fly walking across the ceiling,

before launching into flight, darting about and settling on the wall.

As the fly buzzed about from ceiling to wall and back again,

a thought wandered across Descartes’s mind. How could he describe the journey of the fly?

He realised that he needed a reference point to anchor the motion of the fly,

And while this point is arbitrary, he must be consistent and not change it.

He also realised that talking about the distance from this point to the

fly is simply not sufficient, Because as well as the distance,

you need the direction of the fly to fully describe its motion.

Then a startling thought came to Descartes, a realisation that would

simplify the fly’s motion. For his point of reference,

Descartes took the corner of the room, where two walls meet the ceiling.

From this point, Descartes imagined lines running along the joins,

Defining axes that meet at the corner at right angles.

If you project the fly's location onto each of these axes,

Descartes reasoned, then you get numbers,

And it is these three numbers that you need to specify the location of the fly at any instant.

Three numbers to uniquely define a location in three-dimensional space.

Three numbers that change continuously with the ticking of the clock.

Descartes's coordinate

system seems obvious to us today, so normal. Children are taught to use it when they encounter

location and motion in elementary science, For professional scientists, laying down

a Cartesian coordinate system is the start of finding solutions,

Descartes's name continues to reverberate through the centuries.

Dying in sixteen fifty, Descartes was unaware of a young boy growing up in the English countryside.

This boy, Isaac Newton, would grow to use these notions of time and three-dimensional space,

Laying these down as the mathematical stage on which all motion takes place.

It was Isaac Newton who laid out the rules that we now call classical mechanics.

In Newton’s universe, time is a true absolute.

A single clock that ticks across the universe, a tick that everyone agrees on.

Space was slightly more complicated, as Descartes’s contemporary Galileo had discovered,

As each observer has the freedom to define the origin of their own coordinate system.

Galileo thought more in terms of stories than in the mathematics of Descartes or Newton,

And he laid out his notions on motions in thinking about being onboard a ship.

He realised that if the ship was in a harbour or sailing smoothly on a glassy sea,

Then the experience below deck would be identical.

However, if the wind picked up and the ship began to speed up,

Or if it sailed into choppy waters so the ship began to rock and roll,

Then those below deck would feel this change in motion,

As plates and cutlery slide off tables, and people become unsteady on their feet.

From this, Galileo declared that there must be no absolute rest in the universe,

All motion, he said, must be relative,

And the laws of motion must be the same for observers in a state of unaccelerated motion.

The first theory of relativity had arrived.

These ideas are reflected in Newton’s equations of classical mechanics.

Every observer in uniform motion can use Newton’s

three laws to calculate the outcomes of experiments.

It’s easy to test this yourself by dropping a ball on a train station

platform and seeing it fall to the floor. Repeat the experiment on a train travelling

at a uniform constant speed and again the ball will simply fall.

If, however, you drop the ball at the start or end of the train journey,

The train will be accelerating up to speed, or slowing down as it approaches its destination,

And dropping the ball at these times will see the ball veer towards the back or front of the train,

Seemingly defying Newton’s laws as no forces are pushing in these directions.

These fictitious forces only appear as your coordinate system is accelerating,

And they vanish as soon as you achieve uniform motion.

This picture of the background of reality existed for several centuries,

Until a Scottish physicist, James Clerk Maxwell,

finally unravelled the mystery of light, and with it, the bizarre reality of our universe.

Maxwell’s impact on science was enormous. By the mid eighteen hundreds, Maxwell had

mathematically united electricity and magnetism.

There were experimental clues that these strange phenomena were related,

But in a handful of equations, Maxwell showed these two things were one and the same.

This was already an immense achievement, one for which Maxwell would become scientifically famous,

But he looked deeper into his equations and realised that they implied something profound.

He realised that light itself must be an electromagnetic phenomenon,

Oscillating and intertwined fields of electricity and magnetism.

His equations also revealed the speed of light, a blistering

three hundred thousand kilometres per second, But curiously, not what this speed is relative

to – by just whose coordinates is this speed determined?

It seemed that a notion of absolute rest would be needed to save physics.

Some had proposed that there was an absolute background, an aether in which light propagated

throughout the universe, and had been creating experiments to test the idea.

This included experiments bouncing light rays off mirrors,

Or sending a current through an electrical circuit and lighting a bulb.

The outcomes of these experiments would depend on the motion relative to the aether,

And were built to demonstrate that the universe must possess an absolute state of rest.

However, this would be a serious headache for Galileo’s idea of relative motion.

Why would electromagnetism know about this state

of rest whilst the rest of physics didn’t seem to care?

Why would Galileo’s relativity hold for everything except electromagnetism?

Why would the physics of reality have to be so complicated?

In nineteen o’five, one physicist stated that it didn’t have to be so messy.

That the universe was not split into some physics that obeyed Galileo and some physics that didn’t.

No - electromagnetism had to respect relative motion also,

And so, at the start of the twentieth century,

Albert Einstein’s special theory of relativity was born.

At its core, the special theory of relativity reflects the dream of the lazy physicist,

The dream of explaining as much as possible with as little as possible,

Einstein demanded that all observers, irrespective of the relative motion,

Can use Maxwell’s equations to calculate the outcome of electromagnetic experiments.

But this comes with an uncomfortable consequence,

As all observers must measure an identical value for the speed of light.

And this was simply not possible with Newton’s notion of space and time.

And so space and time, Einstein claimed, are not rigid or absolute,

Instead, they must be flexible such that times and distances depend on who is observing them.

This sounds like lunacy - indeed this is not the experience of our everyday lives,

But experiments close to the speed of light reveal this distortion of space and time.

And it was in the nineteen sixties that irrefutable evidence

for the twisting of time was found, The focus was on a subatomic particle,

known as a muon, effectively a heavier version of the electron.

But, unlike the electron which lives forever, muons only last for two-millionths of a second,

After this, the muon disintegrates into electrons and neutrinos.

Physicists knew that high-energy muons were made in the upper atmosphere,

Created when particles from deep space smash into the gas molecules.

Travelling close to the speed of light, the muons should travel half a kilometre before decaying,

But physicists detected muons arriving at the ground, after travelling many tens of kilometres.

Why were these muons lasting so much longer than their lifetime said they should?

The answer, physicists realised, lay in Einstein’s special theory of relativity,

And, in particular, a quantity known as the Lorentz factor,

A factor that dictates the dilation of time.

To explore this, we must understand the immediate

consequence that sprang from the special theory of relativity.

The speed of light is an absolute speed limit to the universe,

No matter how hard we try, or how much energy we have available,

No object with mass could ever get up to the speed of light.

The Lorentz factor depends on the relative velocity,

But as speeds get closer to that of light, the Lorentz grows unbounded,

And so, for the muon, its life is over in just two-millionths of a second,

For the physicists here on Earth, the muon’s life appears much longer,

and it makes it to the ground.

There is a flip side to this story, the other side of Einstein’s strange relativistic coin.

The physicists see the long-lived muon travelling tens of kilometres before decaying,

But if you could ask the muon, it would say it had only covered a few hundred

metres in its short life, Space, as well as time,

can be distorted and dilated when speeds get close to that of light.

And Einstein also realised that this speed limit had profound implications for information,

As this could also not be transmitted faster than light.

A classic example is that we don’t see the Sun as it is now, but as it was eight minutes ago,

And the light we see from the Andromeda Galaxy set off before humans walked the Earth.

Through their telescopes, astronomers see galaxies as they were billions of years ago in the past,

The universe as it is now is completely hidden from us, we only spy as it was in the past,

Along what is known as our past light cone, and the further they are, the younger they appear.

Other consequences from Einstein’s view began to flow,

Where time and space mean different things to different observers.

Without the absoluteness of Newton’s time, there is no universally agreed now,

And the relative nature of time means that observers disagree on how much time has passed.

Einstein immediately realised that this doomed a cherished idea, the idea of synchronicity.

This is the notion that two events, at two different locations,

happen at exactly the same time. We experience this every day;

it is part of the rhythm of life where things happen at certain times.

But this is all an illusion - and to understand this,

let’s take a look at one of Einstein’s thought experiments.

Consider three people riding in a speeding train carriage.

One person stands at the back of the carriage, one at the front and one is in the middle.

As the train trundles along at uniform speed, these three people decide to do an experiment.

The person in the middle of the carriage has equipment that will release a pulse of light.

That pulse of light will travel to the back and the front of the carriage,

At either end of the carriage, each person has a mirror that

reflects the pulse back towards the middle, And at the instant the light is reflected,

the person at either end raises their hands in the air.

Of course, in any practical train carriage, this experiment would be over in a moment,

And the sluggish reaction time of the people would be the limiting factor,

But this is the beauty of thought experiments,

We can ignore all those day-to-day limitations and just concentrate on the physics.

Let’s think about the situation on the train. Everything seems symmetrical, from the

light pulse heading out and its reflection, To the pulse being received back in the middle of

the carriage and the experiment coming to an end. And it seems reasonable to say that both hands

were raised at the same time – it was synchronous!

Let’s now think of the view of an outside observer, not riding the train

but just standing on the platform. And for fun, let’s make the train

move very fast, close to the speed of light. Just what does this outside observer see as the

train roars by and the experimenters experiment? Well, this is where it gets a little odd.

Our outside observer sees the experiment begin and the pulses of light heading along the carriage.

But remember what Einstein’s special relativity demands,

That all observers see light travel at exactly the same speed – the speed of light.

What influence does this have on what our observer sees?

As the train is moving,

the back of the carriage is reducing the distance that the light has to travel,

While the front of the carriage is moving away, increasing the distance to the reflecting mirror.

The light eventually bounces off each of the mirrors and makes its way back

to the middle of the carriage. With the distances changing again,

eventually, both pulses make it back to the middle at the same instant.

When the outside observer is interrogated on what they saw, how will they describe the experiment?

They will say that they saw the pulses emitted at the same time,

And that they saw the pulses arrive back in the middle at the same time,

But they saw the reflections and hands being raised at different times!

Synchronicity depends on who is doing the observing!

Whilst Einstein was pleased with his theory of relativity, he immediately spotted a problem.

And that problem was gravity. Isaac Newton had given us his equations to

describe gravity back in the seventeenth century, And these worked well in explaining the rate at

which an apple falls from the tree or the orbit of the moon.

Newton’s equations, however, depended on the distance between masses.

In Newton’s universe, where time is absolute, we can talk about the separation at a precise now,

But in Einstein’s universe, where now is not universally defined,

Just which separation of the masses should we consider?

Which now is now?

The lack of synchronicity threw out the notion of a unique separation,

And Einstein could find no way to shoehorn Newton’s gravity into his relativity.

And so eventually, he had no choice but to reject Newton’s gravity in its entirety,

He had to go back to the drawing board and redefine gravity from the ground up.

But where in his equations could gravity be included?

The answer was even more bizarre than what had come before.

Both time and space were malleable.

Gravity could be encoded in the bending and stretching of the fabric of the cosmos.

It took a decade of effort, but in nineteen fifteen, Einstein had cracked the mathematics.

It is this general theory of relativity that we describe the action of gravity today.

No longer is gravity a force between two masses, now it is the curvature of spacetime.

It is this mathematics that we use to describe the expanding universe,

And even use in our daily lives to help run GPS.

The conclusions of Einstein’s relativity still hold across the cosmos.

There is no synchronicity, no universally agreed now.

Time and space can be immensely warped and distorted,

And whether things exist at the same time is simply in the eye of the observer.

And so now we can return to the remarkable quasar study from 2023.

Did time really run slower when the universe was young?

Well, if you were there, back in the infant cosmos, one second would feel like one second,

But as the universe expands, the light that travels through it is stretched.

This malleable, stretchy nature of spacetime leads to what we

observe in the distant cosmos being pulled thin across time.

And so it seems to us that the distant past ticked by at a slower pace.

And an even more extreme version of this concept

is shown in the time dilation experienced around black holes.

Spacetime is warped to impossible extremes in the centre of a black hole,

But it is also warped and stretched as you get closer to the horizon.

And so your experience of time slows down in comparison to distant observers.

And bizarrely, these two ideas,

time dilation around black holes and time dilation in the distant past,

despite the vastly different contexts in which they arise, are effectively the same thing.

And so from the earliest decades of the twentieth century, our vision of time was changed forever.

In Einstein’s revolutionary view of the universe, the passage of time is utterly personal.

Your clock, be it mechanical like a watch or biological like the beat of your heart,

Ticks and tocks at a regular one second per second.

Other people’s clocks can tick radically relative to yours,

If they are moving differently to you,

If they sit somewhere differently to you, in a different gravitational field,

Or, as we saw at the start, they are located in a different epoch of the life of the universe.

This bending of time is truly part of the cosmos,

The long-lived muons smashing into the surface of the Earth are a testament to this,

As are experiments where atomic clocks are taken for journeys around the Earth,

Ticking their own ticks so they are desynchronised when they are reunited.

And we might be able to use this relative nature of time to travel into the future.

All we need to is hang out in the intense gravitational field of a black hole,

A place where time, relative to the rest of the universe, effectively grinds to a halt,

So, whilst minutes or hours pass for you, when you emerge years and decades will have passed outside.

Pulling yourself away from the distant universe, you drag your attention back to the apple.

You are sure that the apple is here right now, no more than a couple of metres away.

But you now have to accept that your now is precisely that, the now that belongs to you.

Another way that your reality is yours and yours alone.

This thought makes you furrow your brow as the idea swirls in your mind.

When and where reality is seems to be a slippery question - but

what it is could perhaps be simpler? What is the world around you…made of?

How would you dismantle the universe?

THE ATOM SPLIT. HUGE FORCE RELEASED.

So screamed the headlines of the Melbourne Argus in May 1932.

Cambridge Achievement. Success of British Scientists.

The nature of reality had been smashed into pieces.

This headline was carried in a newspaper

twenty thousand kilometres from the laboratory in Cambridge,

A testament to the earth-shattering nature of the discovery.

A goal of ancient philosophers and seers had finally been achieved,

For the British scientists, Ernest Walton and John Cockcroft, were modern-day alchemists.

But instead of fire and brimstone, their laboratory gleamed with metal and wires.

With electricity, magnetism, and a beam of particles, they were able to transmute elements,

Lithium was spliced into two new atoms, atoms of helium.

Their discovery showed that the world of the subatomic was now in the physicists’ grasp.

The question of just what matter is had been on the minds of philosophers for a long time.

Some objects were clearly similar; stones of various shapes and sizes were hard,

while the solidness of stones contrasted with the squishiness of human flesh.

They seem quite different, even fundamentally different, at their core.

Philosophers had wondered if the stuff of nature was infinitely divisible,

Whether you could continue to slice it and dice it into smaller and smaller pieces.

But ancient Greek and Indian thinkers had a new thought,

that things are composed of ultimate pieces.

Tiny pieces we now know as atoms from the Greek for uncuttable.

To the ancient philosophers, these atoms were the fundamental pieces of reality.

The notion that all things are made of atoms remained

a philosophical musing for a few thousand years,

But by the eighteenth century, scientific evidence for the physical existence of atoms was emerging.

And not from the experimental laboratories of physicists, but from that of a botanist.

The botanist in question was Robert Brown, born in Scotland in 1773.

He had a distinguished career, travelling the world in the ship,

The Investigator, to chart plant life.

Many years later, in 1827, he was still studying the world of planets,

but his focus was tiny grains of pollen. This pollen was floating on the surface of water,

and Brown examined it under a microscope. But what Brown saw mystified him.

Instead of sitting restfully in the water, He saw them dance and jiggle.

Brown knew that pollen came from plants, so perhaps the jiggling was due to life in action.

That the pollen grains were able to propel themselves through the liquid.

But other materials, clearly dead material like chalk dust, showed the same kind of dance.

This Brownian motion had nothing to do with life.

Many people pondered the question of the jiggling pollen as seen by Brown,

But the answer would not be found for another seven decades - until 1905,

arguably the most famous year in all of science,

Albert Einstein´s annus mirabilis.

In this year of miracles, Einstein is generally remembered for one particular discovery,

His special theory of relativity and all the weirdness of space and time that conveys.

But it is also the year he revolutionized quantum physics through the photoelectric effect,

And he finally explained the weird motion found by Robert Brown.

Einstein’s insight is as simple as it is brilliant.

He realised that the jiggling of the pollen was not the pollen propelling itself,

But being propelled by external forces,

Forces delivered by the collision of individual water molecules.

But how can this possibly be? A pollen grain, whilst tiny,

is a billion billion billion times heavier than a water molecule.

How can the crash of a water molecule into pollen have any influence at all?

Imagine you are at a truly massive rock concert with music blasting from the stage.

You are deep in the crowd, a crowd of tens of thousands of people.

Now and again, the crowd surges to the left or right, front or back,

And in these surges, all you can do is go with the crowd.

Intermittently, the surges die away, as you stand shoulder to shoulder with your fellow revellers.

The crowd is restless, and you are jostled from all sides, a push from here and a push from there.

Your body moves with the pushes as you try and hold your space,

But you slowly start to jiggle about due to the relentless pushes.

The pushes are not even, sometimes more from the right, sometimes the left,

sometimes front or back, with you unable to predict when the next push will come.

No matter what you do, your entire body moves a little this way, and a little that way.

Einstein realised that Brownian motion was precisely this kind of drunkard’s walk.

Every second, he said, an uncountable number of water molecules collide with a grain.

The net result is that these fluctuations

in collisions back and forth lead to the random jiggling,

and the pollen grain dances along to the molecular tune.

From his calculations, Einstein was able to calculate the size

of the collisions and their fluctuations, Confirming that water was comprised of an immense

number of absolutely microscopic molecules. And so atomic hypothesis became atomic

reality, and atoms appeared to be the building blocks of our world.

And yet just as their existence was proved, these indivisible atoms began to divide.

It was in 1896, that Henri Becquerel and Marie Curie

found that the world of the atom was unstable, And that some elements could transmute into

others, spitting out other pieces as they did so - and in 1897 J.J. Thomson discovered the electron.

Something was living inside the atoms that made up reality,

And the race to split the atom, and reveal its contents, was on.

For his discovery, Thomson was awarded a Nobel Prize in 1906,

But it was his protégé that was to truly reveal the structure of matter,

The father of nuclear physics, Ernest Rutherford.

In the first decade of the twentieth century, Rutherford set two young

men to probe the structure of matter. In a darkened room at the University of

Manchester, they peered into inky blackness. They were looking for tiny flashes that

indicated a collision, as tiny pieces hit a fluorescent screen.

And the results so far had been as precisely expected.

The two men were Hans Geiger and Ernest Marsden. Geiger’s name would be immortalised in

the radiation detector named after him, But that still lay several years into his future.

For now, he patiently stared into the darkness, logging each of the tiny flashes that he spied.

The experiments continued, counting flash after flash, until one day,

Rutherford suggested that they look for flashes where none should be expected.

So, Geiger and Marsden dutifully modified their experiment and again stared into the darkness,

And it was then they changed our understanding of physical reality forever.

Rutherford, was stunned, saying it was the most incredible event of his life.

At forty years old, he was still quite young but had already been

awarded a Nobel Prize for his work. But with this new experiment,

the atomic world would never be the same again.

But why? What had they seen - and what had it meant?

Thomson’s discovery of the electron had provided new clues to the structure of atoms.

The electron clearly carried some negative electric charge,

So, inside the atom must be a mix of positive and negative charges, as overall they are neutral.

The question was just how this charge was distributed.

Thomson had proposed his model for the nature of the atom,

Something we now know as the plum pudding model.

Confusingly, a plum pudding doesn’t contain any plums,

but it is actually a fruit cake with raisins. And the electrons, Thomson said, were just

like these raisins, suspended in a positive mixture of cake.

It was precisely this model of the atom that Rutherford was testing in his experiment.

He created a beam of alpha particles, charged helium nuclei emitted from a radioactive source,

And directed them to crash into a foil of gold. It was the recoil of these collisions that

Geiger and Marsden were looking for as they peered for flashes in the dark.

As the alpha particles impacted the gold foil,

many of them sailed right through, missing the gold atoms entirely.

However, now and again, an alpha particle would crash directly into an individual gold atom.

Rutherford surmised that these should feel the push and pull of the charges within the atom,

And their paths would be deflected out of the beam.

If Thomson was correct, and the atom was a general mix of positive and negative charge,

Then these deflections would be relatively modest, flung only slightly from the beam.

And this is what Geiger and Marsden had spied as they sat in the darkness

of their Manchester laboratory. But following Rutherford’s suggestion,

they also found alpha particles scattered far, far from the beam.

Rutherford was astonished.

For an alpha particle to be so radically scattered was

simply impossible in Thomson’s vision of the atom.

Rutherford would go on to say:

“It was quite the most incredible event that has

ever happened to me in my life. It was almost as incredible as if you fired

a 15-inch shell at a piece of tissue paper and it came back and hit you.”

But the question remained - if Thomson’s atom

could not explain the scattering of alpha particles, what could?

Rutherford concluded that instead of being spread out, the positive charge

must be tightly concentrated, So that there is an immense

electromagnetic repulsion between the atomic charge and the alpha particle.

So immense, in fact, that the alpha particle can be flung in any direction.

This led to a rather confounding picture of what just an atom is.

Rutherford knew that the electrons in the atom carried a negative charge,

This charge must be broadly spread over the volume of the atom,

Spread so thinly that they barely influence the alpha particles.

The electrons, however, carried very little mass.

This meant the totality of the mass must sit in the positive charge at the nucleus of the atom.

But whilst massive, this nucleus is very small, tens of thousands of times smaller than the atom,

With this, the vast majority of the volume of an atom became nothing but empty space.

With Rutherford’s insights, the startling Pandora’s box of nuclear physics was open.

Rutherford’s atom fractures our view of reality,

of the apparently solid nature of the world around you.

Every chair, every wall, every tree, every apple is made of atoms,

Tied together into a myriad of molecules that define structure, colour and everything else.

But it seems that this is nothing more than an illusion.

apples are made of water, vitamins and butyl and hexyl acetate,

the molecules responsible for its taste,

But each atom in each molecule is really a cloud of electrons surrounding a miniscule nucleus.

And the vast majority of each atom is nothing but empty space.

The apple is mostly empty space, and its solid appearance is but an illusion.

Of course, it is not only the apple, but everything that

has the illusion of being solid. Everything, including yourself.

This is disconcerting, but maybe you can try

and comfort yourself with this image of yourself and reality.

Maybe it's ok that atoms are mostly nothing but empty space,

They are still made of something, with a nucleus and orbiting electrons.

Surely those pieces are real. Surely those pieces are really there.

But as they had with the molecule and atom before it, physicists would quickly

realise that the mysterious nucleus of an atom was also far from fundamental.

On the tenth of August nineteen fifteen, a sniper found his target.

His rifle trained on the enemy, He steadied his breathing and began

to apply gentle pressure to the trigger. Birds scattered as the shot rang out.

The sniper’s aim was true and his enemy crumpled to the ground.

The sniper moved on – there were plenty more to hunt.

Little did he know that his bullet had snuffed out

one of the greatest minds of the twentieth century.

World War One cost twenty million lives, both soldier and civilian,

with many more wounded and injured. The failed Gallipoli Campaign, seeking to

weaken the Ottoman Empire cost a hundred thousand. Every life lost is a tragedy, a missing loved

one whose potential goes unrealised.

But this is particularly true of Henry G.J. Moseley.

For before he joined up to be a soldier,

something he did out of duty and against the wishes of his family,

Moseley was a scientist who was unravelling the inner workings of the atom's nucleus.

Born in 1887 in Dorset, England, the son of an Oxford professor.

Moseley was a stellar student, studying at Oxford before joining Rutherford in Manchester in 1910.

He had started to use X-rays to bombard different elements,

With the hope of revealing the secrets of their atoms.

As he did this, he had spied X-rays being emitted at different energies,

and guessed something deep with the atom was causing them.

There was also a pattern - in heavier elements,

thought to contain heavier atoms, higher energy X-rays were emitted.

In Holland, the amateur physicist, Antonius Johannes van den Broek, presented a bold idea.

He knew that electrons, carrying the negative charge inside atoms, came in discrete chunks.

Each electron appeared to have the same mass and the same charge.

What if the nucleus was the same, built of separate but positive charges?

As atoms appear to be electrically neutral,

These positively charged chunks should exactly balance the negatively charged electrons.

And the only difference between the elements would be the number of positive charges in its nucleus,

As this would set the number of orbiting electrons and dictate how the atom interacts with others.

The simplest element would have one of these charges, the next two and so on.

Moseley realised that this was the pattern he was seeing with his x rays, and with this one insight,

he rearranged the periodic table based on what we now know as the

atomic number - still the form of the periodic table that we use today.

Speaking in 1962, long after Moseley’s untimely death,

the giant of quantum mechanics, Neils Bohr said

"You see actually the Rutherford work was not taken seriously. There

was no mention of it in any place. The great change came from Moseley."[

And who knows what else Moseley would have contributed to science,

Had not been cut down by that sniper’s bullet in nineteen fifteen.

Things moved quickly as the Great War ground towards its eventual end.

It was again Rutherford, always at the forefront who was deep in the discovery.

Through his investigations into radioactivity, he finally uncovered the true nature of the atom.

Hydrogen was the simplest element, Just one piece of positive charge

joined by one electron. Whereas helium had two

positive charges and two electrons. Then lithium with three of each,

beryllium with four, and so on and so on. Like the electron, each of these positive

charges were identical, Both in terms of the charge

they carried and the mass they were made of. Rutherford named these positively charged,

massive particles, protons. And eventually, in 1932 after

being theorised for years, the neutron was also unambiguously identified in experiments.

The makeup of the atom was complete.

It had started off as a mess of complexity, of different substances with different properties,

becoming simplified through the realisation that the world is

built from ninety-two natural elements. But with the discovery of the proton,

neutron and electron, things became simpler still, With atoms of all the elements and their isotopes

built of nothing but these three key pieces of reality.

In the outer reaches of the atom roam the electrons,

Negatively charged tiny particles that whizz around at close to the speed of light,

A million times smaller, at the very heart of the atom, sits the nucleus,

Positively charged and comprised of protons and neutrons, the nucleus is home to atomic mass.

But as Rutherford’s new view of atoms took hold, others realised it spelt doom

for physics as they knew it. It contained a fatal flaw:

There is, they said, no way an electron can remain in its orbit,

Rutherford´s version of the atom should not be able to exist at all!

The problem came from James Clerk Maxwell’s equations of electromagnetism,

These demanded that an accelerating charge must radiate away its energy as electromagnetic waves.

And as an orbiting electron is in a constant state of acceleration, it must constantly lose energy,

It should spiral into oblivion in the nucleus of the atom.

There was nothing in the physical laws of Newton or Maxwell that seemed to prevent this,

Nothing in the laws of reality as we understood

them could make matter stable. So, physicists made a decision.

They decided to change the rules, and in the process changed the nature

of reality forever ________________

In nineteen forty-three, the father of quantum mechanics

Neils Bohr found himself in a predicament. Germany had declared his family Jewish,

and his arrest was imminent, So, with the help of the

Dutch resistance, he fled to neutral Sweden, But his trials and tribulations were not over.

The British heard of his escape and decided they needed this quantum legend.

The Second World War had been raging for four long years,

And the Allies had decided that a new weapon, an atomic weapon,

must be developed to end the carnage. Bohr´s insights into the subatomic would

benefit them and stop the enemy from following the same path.

Retrieved from Sweden and travelling in disguise as James Baker, Bohr eventually made his way to

a secret town in New Mexico, and there, at Los Alamos,

he contributed to the Manhattan Project and the building of the atomic bomb.

Shyly, he always claimed that he wasn’t really needed amongst the bright young minds,

But project director, Robert Oppenheimer, disagreed:

“The early days of quantum mechanics were a heroic time. It was not the doing of any one man;

it involved the collaboration of scores of scientists from many different lands.

But from the first to last the deeply creative, subtle and critical spirit of

Niels Bohr guided, restrained, deepened and finally transmuted the enterprise.”

For three decades earlier, in nineteen thirteen, as the world

had been on the precipice of another World War,

Bohr’s mind had been focused not on how to unleash the power of the atom - but

the question of why atoms existed at all. If Rutherford was right on the structure of

the atom, why didn’t the electron simply spiral into the nucleus,

Just as the physical laws of Newton and Maxwell demanded?

Revolution had been brewing for decades.

By the close of the nineteenth century, physics had seemed pretty complete,

except for a few niggles - yet those niggles came to dominate and rewrite physics as we know it.

And it all began with the question of why hot objects glow.

Clearly, to make an object glow you have to supply it with energy,

This energy makes atoms vibrate, vibrations that generate light, the glow of a hot object.

We know that an iron poker in a fire glows with a beautiful cherry red,

And as we make it hotter, this red gives way to white and blue.

The question of why this particular choice of

colours became the focus of a forty-two-year-old Max Planck,

Who combined the rules of established physics to understand how matter and light interacted,

But his calculations failed time and time again and as he reached a moment of absolute despair,

He decided to rewrite the physics rule book - as many had done before him,

and would do after - but never again with quite such far reaching ramifications.

Electromagnetic energy, he said, came in distinct packets of light,

These discrete vibrations and light packets being quantised.

And suddenly, Planck’s strange new mathematics explained the glow of a hot poker.

Planck thought that all he had done was perform a little mathematical wizardry,

But by nineteen o’five, Einstein also said that light comes in discrete chunks, quanta of energy.

Explaining how electrons are ejected from metal when under a light source.

Suddenly the quantized world was popping up everywhere.

The quantum revolution was gathering pace as the subatomic world was laid bare.

And it was Rutherford’s insights that showed the atom itself would have to be quantized.

But just what rules governed this quantized atom eluded him,

And these answers came from Bohr.

Electrons, said Bohr, were only allowed to exist in distinct orbits about a nucleus.

These orbits are set by the momentum of the electron, and so each orbit has a specific energy.

Electrons can move between orbits only if the exact energy difference is absorbed or emitted,

And there would be the smallest orbit - that brought the electron no closer to the nucleus.

In demanding electrons could only exist in certain orbits with certain energies,

Bohr had quantized and saved the atom from collapsing in on itself,

with only chunks of energy able to change the atomic state.

This was a resounding success for Bohr, securing him the Nobel Prize

in Physics in nineteen twenty-two. But physicists still scratched

their heads as to why the atom was quantized.

This answer would come two years after Bohr received his Prize in Sweden from a radical

proposition by a French aristocrat and physicist, Louis de Broglie.

He had been watching the quantum revolution unfold,

And was perplexed by the notion that light, which was a wave in classical electromagnetism,

Sometimes behaved like a particle, delivering its energy in a single punch.

In this quantum world, thought de Broglie, why should matter not be subject to this duality?

So, he declared that an electron, which had been thought of as a distinct particle,

Also possessed wave-like properties in its existence.

Like light, de Broglie said, electrons can be refracted and can interfere.

And it was Schrodinger who applied this to the atom - asking us to think of the wave of the

electron as not at a precise point but spread over the entirety of the orbit around the atom.

And so Bohr's quantized atom finally made sense - the electrons were behaving as

waves - and could only have certain wavelengths.

Today, the notion that quantum objects can be particles and waves is common knowledge,

But in its time, de Broglie’s ideas shook

the foundation of physics to its core. Experiments soon followed to show that

beams of electrons showed distinctly wave-like, And in nineteen twenty-nine, de Broglie found

himself in Sweden receiving his own Nobel Prize.

By the nineteen-thirties, it seemed that reality was all sewn up.

The periodic table was built of just three particles, protons, neutrons and electrons,

Bound together in different ways by the mathematical

rules of electromagnetism and quantum mechanics.

But even this neat picture rapidly started to unravel.

In nineteen twenty-nine, Paul Dirac had been focused on the equations of quantum mechanics,

His goal was to make quantum consistent with the other great physics theory, Einstein’s relativity,

His new equations worked, but immediately Dirac was puzzled by what they told him.

For as well as describing the behaviour of electrons, something else was in the mathematics.

The equations also described a positively charged electron, a positron,

A particle that seemed to have no place in the orderly atomic world.

Perhaps, people thought,

it was nothing more than a mathematical oddity that has no place in reality,

But by nineteen thirty-two, it was unmistakably found in particle experiments.

The positron was found in cosmic ray experiments, high energy particles raining in from space.

These smash into atoms inside the particle detectors, generating new particles.

But soon after the discovery of the positron, a much stranger particle appeared in the collisions,

One that had less of a place in the atomic world than the positron.

It looked like an electron, with the same electric charge, except for one key difference.

This new particle appeared to be two hundred times as massive as the electron,

And unlike the electron, this new particle, the muon, decayed in two-millionths of a second.

Why does the muon exist when it seems that it’s not needed to build reality?

Indeed particle physicist Isodor Rabi quipped

“Who ordered that?” as the muon made a mess of atomic neatness.

But the discovery of the muon was the start of a cascade,

With many strange and differing particles popping out of experiments.

Into the nineteen sixties, chaos reigned in the world of the subatomic,

Until the next level of reality was laid bare in what became known as the standard model.

It was realised that protons and neutrons, and most other particles, are not truly fundamental,

But are made of smaller pieces known as quarks.

We now know there are six kinds of quarks, grouped into three families.

The lowest mass quarks are known as up and down, then charm and strange.

Most massive of all are top and bottom.

And the vast array of particles spat out in experiments are combinations of these quarks.

Protons and neutrons are built from up and down quarks,

Protons are two ups and a down while neutrons are two downs and an up.

Three up quarks combine to form a delta plus plus particle,

Whereas a D plus is a charm quark combined with an anti-down quark.

As well as the quarks, there are also the leptons.

The electron and the muon and the even more massive tauon.

Plus ghostly companions known as neutrinos, one for each kind of electron.

So, the majority of reality we experience is a combination of electrons and up and down quarks.

There are a few other pieces to the standard model, and we will meet those soon,

But it is important to note that these fundamental pieces appear to have no size at all.

These truly fundamental pieces of reality are truly point-like,

So, each of your protons and neutrons is effectively empty.

At the subatomic scale, your atoms are even emptier than Rutherford found.

Your brow furrows as you again think about the apple.

Knowing that it is mostly empty space was disturbing enough,

But at least the electron and the atomic nucleus seemed, well, solid.

In the quantum world, this is simply not the case.

Everything we thought was a particle is not. Quantum matter shows wave-like properties,

spread out over space, And not in any precise

location or with precise properties. Your apple is nothing more than

a mix of these bizarre waves, all combining to be the fruit you see.

You squint at the apple as the question of quantum waves washes over your mind.

Just what are these waves? And just what is doing the waving?

And is this what reality is – just a sea of subatomic waves?

________________ Imagine that you are in your kitchen,

mixing ingredients to bake a cake. As you read through the recipe,

you see that you need a couple of eggs, and so you head for the fridge.

In the fridge door, the eggs are neatly arranged in lines, with some holes filled and some empty.

You close the door and get back to your baking, thinking little of the remaining eggs.

In your mind, there are certainties.

When you open the fridge again, the eggs will still be there,

occupying the same holes as before. How could it be any other way?

But our modern understanding of reality suggests that the universe is actually much stranger.

Instead of eggs, let’s think of a collection of electrons similarly arranged in holes.

With the arrival of quantum mechanics in the early twentieth century,

It was realised that electrons cannot be described by the rigid rules of the physics of Newton,

Instead of exact positions and velocities, electrons were described by probabilities.

We can only talk about the chance of finding an electron here or there,

And if we observe an electron in a particular hole, it will

have a probability of being in the other holes,

So next time you check on the electron, it might have tunnelled into a new hole.

And all of the electrons might have hopped between the holes.

And so why don’t the eggs in your fridge similarly hop between the holes in your fridge door?

They too are governed by the rules of quantum mechanics and so by probabilities.

Well, in a sense, they do. For although the chance of an entire egg

quantumly jumping between holes is extremely tiny, Whilst small, it is not zero!

Though very unlikely, within the laws of physics everything is possible in the quantum world.

It is just a case of probability.

The quantum revolution dominated the physics of the early twentieth century.

Great minds from the globe turned their attention to the strange realm of the subatomic.

But not everyone was pleased with what this revolution had to say,

Including perhaps the greatest scientist of them all, Albert Einstein.

The problem started in nineteen twenty-six in a paper by the physicist Max Born.

A year earlier, Erwin Schrodinger published his eponymous equation,

This allowed physicists to calculate the properties of de Broglie’s matter waves,

But Schrodinger didn’t know just what these waves were.

Born, however, said that everything makes sense if you interpret the waves as waves of probability.

The amplitude of this wave function tells you the chances of an electron being here or there,

And as time ticks by, this wavefunction can wash back and forth,

As the probabilities for an electron being in a particular location change.

This answered one of the questions that had been nagging physicists when considering quantum waves,

Because when they experimented on electrons, they would see precise dings in their equipment,

They would detect individual electrons as distinct particles.

So how could they exist also as quantum waves?

To Born, the particles like electrons behave like waves as they travel from one place to another,

But when they interact with your detector, the wave collapses and

the electron is at a precise spot. Experimental evidence was quickly sought

to see if this picture was correct, And it didn’t take long to arrive.

The experimental setup was similar to Young’s famous double-slit experiment,

Where light was shone through two slits and showed the unmistakable patterns of interference.

This demonstrated that light travelled as a wave, as you need waves to construct interference.

And if quantum matter behaves like waves, it should show interference too!

The idea was to fire a beam of electrons onto a target of nickel,

with a regular pattern of atoms. If the electrons were little bullets,

we would expect them to randomly ricochet off the target atoms,

However, the experimenters found that there were certain directions

in which electrons preferred to recoil, And other recoil paths that they seemed to avoid.

This was the unmistakable pattern of interference, places where waves added and strengthened,

And others where the waves cancelled each other out.

To help understand this, imagine you are a sharpshooter at the Olympics.

Through your sights, you zero in on your target a hundred metres away.

You line up your crosshairs, take a deep breath and hold it.

But as you think about your glory yet to come, your mind starts pondering.

What if you were a sharpshooter in the quantum world?

In the quantum world, the certainty of everyday physics melts away.

Instead of exact locations and velocity,

your bullet is now governed by the rules of quantum probability,

And when you squeeze the trigger, the bullet emerges from the barrel as a quantum wave,

Spreading out in all directions like the ripples of water on a pond.

This quantum wave travels quite differently to the zipping path of the bullet,

Quantum waves can diffract around corners, and can interfere with other quantum waves,

A spray of quantum bullets would not result in a grouping of holes in the target,

Instead, a pattern of interference would emerge, with some bullets here and none there.

This strange mix of wavelike and particle-like properties in the

quantum world is known as a duality, And quite why we don’t experience

this in everyday life remains a mystery, Though physicists have been able to see

the effect of quantum waves even on the scale of large molecules,

Many tens of atoms travelling together as a quantum wave and showing interference.

And so experiments showed, precisely as Born had suggested,

the waves of the electrons were combining and cancelling,

But bizarrely their detectors were counting just individual electrons arriving.

Quantum objects were definitively travelling as waves but interacting with detectors as particles.

Indeed, as further proof, today we use this fact in our most powerful of microscopes.

Instead of a beam of light, these focus a beam of electrons onto a target,

And use the wave properties of the beam to reveal

details beyond that visible to light.

Born received his Nobel Prize in nineteen fifty-four,

one year before Einstein’s death. But in the decades in between,

Einstein had become a vocal critic of this probabilistic picture of nature,

convinced as he was that it could not be the way that the universe actually worked.

Whilst Einstein and Born were friends, they never saw eye to eye on reality.

Einstein was horrified by probability sitting at the heart of our world.

He felt that the universe must be deterministic at its heart.

Though he accepted that quantum mechanics could be probabilistic,

He believed there was something deeper,

something hidden beneath the quantum world that clung to determinism.

Indeed, in a now iconic letter to Born, Einstein outlined his sentiment:

"I, at any rate, am convinced that [God] does not throw dice.”

By nineteen thirty-five, Einstein had fled his native Germany and arrived in the USA.

And it was there he wrote about a paradox at the heart of quantum mechanics,

A paradox he was convinced would demonstrate the absurdity of some of its big ideas.

And this paradox, almost a century old,

is still key to our modern understanding of the quantum world.

It is known as the EPR paradox, after its authors, Einstein, Podolsky, and Rosen.

And it highlighted one of Einstein's biggest problems with quantum mechanics -

the apparently instantaneous collapse of the probability waves, through observation,

into particles. And to understand this,

Let’s start with thinking about two quantum particles.

One of the interesting things about the wavefunction is that when we have two particles,

The wavefunction for both is not simply the sum of the wavefunctions of the individual wavefunctions.

No - a new wavefunction can be written that encapsulates the properties of the two particles.

And it’s this combined wavefunction that gave Einstein a headache.

Imagine the two particles are brought into existence through some event.

The details of the event are unimportant,

but results in the particles having linked quantum values.

For example, imagine the particles are born with quantum spin, one spinning up, one spinning down.

This way the total spin is equal to zero, so spin has been conserved in their creation.

Except the wavefunction does not contain definite information, just probabilities.

So, each particle has a fifty per cent chance of spinning upwards,

and a fifty per cent chance of downwards. However an important rule of the universe

is that the total spin is conserved - so if you detect one is spinning upwards,

The other absolutely MUST be definitely spinning downwards.

In Einstein’s thought experiment, you can imagine the particles separating after their creation,

And moving an immense distance apart, maybe several light years.

They are travelling as waves, and you don’t know the orientation of either particle’s spin.

Everything is still a mix of probability.

Imagine one of the particles runs into a spin detector.

With a loud beep it registers the spin and the display reads SPIN UPWARDS.

You now know that the spin of the other particle is definitely spin downwards,

The wavefunction must have collapsed into the definite states of the particles.

But herein lies the problem.

How does this pair of widely separated particles know that one has had its spin determined?

Does the wavefunction, which is strung between the two particles, collapse instantaneously?

If it did, Einstein said, then this collapse would occur faster than the speed of light,

And that, of course, was forbidden by his special theory of relativity.

Nothing in the universe can travel faster than the speed of light, under any circumstances.

The solution, claimed Einstein, is that quantum mechanics simply could not be complete.

That underlying its apparently probabilistic façade lurked

something else, something definite and certain that we simply hadn't found yet.

These “hidden” variables would

be solid and deterministic values precisely describing a situation.

And it would be these hidden values that described the true nature of reality.

The hunt for these hidden variables was on, Einstein and de Broglie put new ideas on the

table, then eventually threw them in the bin. Others sought a solution to the EPR paradox which

brought determinism back to the heart of reality, But by the nineteen sixties, all attempts were

dashed by the insights of an Irish physicist in Switzerland.

John Stewart Bell was born in

Belfast in nineteen twenty eight. From a young age, he had decided

that he wanted to be a scientist, And in nineteen sixty, he found

himself as a theoretical physicist working on the Swiss French Border.

At the recently formed European Organization for Nuclear Research, or CERN for short.

Bell contributed many things to physics and the understanding of reality,

But he is probably best remembered for what is known as the Bell inequalities.

His thought experiment was similar to that laid out in the EPR paradox,

But with a subtle and insightful twist.

Just like in the EPR paradox,

we first make a pair of particles that are entangled in their wavefunction,

Then we allow them to separate to a large distance where each encounters a detector.

The detector can register spin up or spin down, but each can be oriented differently.

But let’s start by imagining the two detectors are perfectly aligned.

As we have seen before, if one detector registers an up, the other must register a down.

The readings on one detector might be up, up, down, up, down, down, down.

The other must read down, down, up, down, up, up, up.

The readings are perfectly correlated.

Bell’s twist was to consider what would happen if the two detectors were not aligned.

What if one was set to register up-down and the other was twisted to measure left-right?

This might sound like a strange thing to do, because

if a particle is spinning either up or down, How could it register any spin left or right?

If you are thinking like this, you are still considering the particles as classical things.

As if they are truly solid balls that contain a definite direction of spin.

But remember, they are not – they are purely quantum particles,

and their spins are probabilistic.

So, there will be some probability of spin up-down, and some probability of left-right.

Though thinking like this can start to bring on a headache,

it is the way the microscopic world works.

Bell started to wonder about the correlations of the outputs.

With one detector reading up, down, down, down, up, up.

And the other right, right, right, left, right.

He was able to conclude that if these choices are being made by hidden variables,

By definite quantities tied to each of the particles,

a particular pattern of correlations should be seen.

And if there are no hidden variables, so Einstein was wrong, a different pattern should emerge.

And so the rush to experimentally test Bell’s experiment was underway.

It wasn’t long until the first results were on the table.

Calcium atoms were excited in a hot oven, emitting two correlated photons as they cooled.

These photons zipped out of the oven and into the physicists’ detectors, who then

compared the polarizations of the photons, the vibrat ional direction of the electric field.

Bell said that if there were hidden variables defining the photons,

specific correlations would be measured. But if there were not, and the photons’

properties were spread over the quantum wave function,

Then a distinctly different set of correlations would be measured.

The results were conclusive, there could be no variables,

Reality was not a property of things but was non-local, extended over the quantum waves,

And as the many experiments undertaken ever since further confirmed this conclusion,

Einstein’s notion that the underlying universe was deterministic was quietly put to bed.

And so at this point, you might be wondering what all this means.

This seems like a minor academic argument about some exotic quantum stuff in weird experiments.

But the ramifications for reality are, however, huge,

Locality has always been a key concept in physics. What it means is that something responds purely

to its local environment. Proximity is important.

A mass responds directly to a force acting upon it,

But it does not know or care about a force acting on another mass far away.

It is the non-local nature of quantum mechanics that Einstein objected to,

That a particle detection in one part of the universe can influence the

properties of another elsewhere. In a letter to Max Born in 1947,

Einstein referred to this as spukhafte fernwirkungen - “spooky action at a

distance” and this was a step too far for Einstein’s view of the universe.

And what this means when extropolated is baffling.

An electron is not simply an electron, And neither is any other

fundamental particle in the universe. Particles must be entangled with others,

sharing their experiences across the universe, No individual thing is simply a thing.

Everything, universe wide, is truly connected.

You might be sharing Einstein’s feeling that this is a step too far.,

that there is no way your apple is entangled with other particles in the cosmos.

But we have to remember that the universe simply does not care about your feelings,

your intuition, what you can process in your brain.

It refuses to behave in a way that you think is sensible.

And the award of the 2022 Nobel Prize in Physics demonstrates this cosmic absurdity.

The laureates were Alan Aspect, John Clauser, and Anton Zellinger and the

subject was entangled photons. Experimenting over decades,

these physicists showed that widely separate particles behave as one,

The goal was to dismiss any prospect of there being any hidden variables defining reality.

And a recent example of these experiments, the Cosmic Bell Test, might be the most extreme.

The worry was that experiments to reveal Bell’s correlations might

not be quite telling us the truth. Experiments are built of atoms,

and what if all these particles had interacted throughout their long histories?

And worse than that, the experimenters themselves were made of similarly entangled particles too!

When physicists set up their experiments, they like to

think they are distinctly separate from them. They think they are but objective observers, just

looking at the outcomes of the laws of physics. The equipment in their lab is, in their minds,

effectively isolated from the rest of the universe,

And what they see is just the outcome of the physics in front of them.

However, this is not quite the case. The experimental apparatus is made of atoms,

Atoms that have a history much older than the experiment itself.

The physicists too are made of atoms, atoms with a history much older than them.

Atoms that have been around for billions and billions of years.

Over these billions of years, there have been countless opportunities for these atoms to tangle,

And in these interactions, any hidden variables could have been rewritten and correlated.

This would mean that the correlations the physicists measure could simply

be a result of the atoms' long history, Rather than the physics being non-local.

This had the potential to introduce quantum loopholes into the experiment.

And physicists wondered if hidden variables were colluding to make it look like they didn’t exist.

They needed to set the settings of the experiment in a way that could

not be influenced by Earthly ways, And so, they looked to the skies.

They set up their experiment as usual, and then they brought in the light of two distant stars,

The starlight fluctuated slightly, and these fluctuations became part of the experiment.

These flutterings were used to set the orientations at which the spins were measured.

And again, the experiment revealed that reality must be non-local.

Remarkably, these experiments were even repeated using cosmological quasars rather than stars,

Pushing the possibility of a hidden variable loophole occuring back almost 8 billion years to

the time of these extremely bright galaxies. Again, the spins of the particles were

explored at orientations set by the fluctuating light of the quasars,

And again, the results were unequivocal – reality must be non-local - it is not simply there.

Again, you look over and spy your apple, And again, you begin to wonder.

Your mind can handle the fact that your apple is, at heart, a collection of fundamental particles.

And it feels sensible that they are there, part of the fruit you can see.

But these particles are entangled with each other, and with others in the universe.

And it seems that the apple is actually spookily entwined with the cosmos.

And so now the question, is why?

Why exactly is this apple, and this universe, so connected?

What are the rules that run beneath everything? ________________

On the eighteenth of April, nineteen fifty-five, the old man died.

He was born in Germany almost eight decades earlier,

but his last moments were in an American hospital.

His final words were in his native tongue, but these are lost to posterity,

As his nurse who was caring for him was unfamiliar with the German language.

But in the hours before his death, the man had asked for three things - his glasses,

his writing equipment and his equations - to work on one final problem.

This man, Albert Einstein, is considered one of the greatest physicists of all time,

And his name is splashed across the pages of the textbooks of modern physics.

But later in his life, his work had become focused on a single question,

A question that still haunts physicists today.

Two central theories emerged from the revolution of the first decades of the twentieth century.

We have met them both in our story so far,

Einstein’s relativity described the actions of gravity across the cosmos,

While quantum mechanics explained the universe of the subatomic.

Quantum mechanics was refined throughout the twentieth century,

Evolving into what is now known as quantum field theory.

It is these fields that contain the rippling waves of probability in quantum mechanics,

And it is these fields that describe the physics of the fundamental pieces,

like electrons and quarks.

We have also already encountered the standard model of particle physics,

And this is really a theory about the interaction of these quantum fields.

When two charged particles, like electrons,

interact, they create vibrations in the electromagnetic field.

And these vibrations, the carriers of the electromagnetic force, are the photons.

And of course the electrons themselves are also just vibrations - vibrations in

the electron field - just as each type of quark that makes up neutrons and protons

are vibrations in their own quark fields, and so on and so on for other particles.

There are also other fields for other fundamental forces.

The strong force is responsible for holding the quarks in protons and neutrons together,

And for binding these particles in the nuclei of atoms.

Vibrations in the strong field are the gluons, the carriers of the strong force.

There are also fields associated with the weak force, responsible for aspects of radioactivity.

And their vibrations correspond to what are known as the W and Z bosons.

For every force, there is a vibration in a field, and that vibration is a quantum particle.

To understand this, let’s take a look at the life of a neutron.

Left on its own, a free neutron will decay in about fifteen minutes,

Transforming into a proton and spitting out an electron and a ghostly neutrino.

But just what is happening in terms of the quantum fields?

Remember that a neutron is a collection of three quarks, two down quarks and an up quark.

These are constantly generating vibrations in the strong field, with gluons flying

about and binding the neutron. But the weak field is still there,

and eventually one of the quarks will pluck a new vibration in it.

And one of the down quarks will transform into an up quark, turning the neutron into a proton.

The ripple in the weak field represents a W minus boson and flies from the newly formed proton.

But this particle lives for only a very short time,

less than a billionth of a second, And as its vibration vanishes,

it produces vibrations in new fields, creating an electron and neutrino.

All quantum particle interactions can be described in exactly the same way.

One other quantum field is necessary to round off the picture,

Perhaps the most famous quantum field of all, the Higgs field.

Interactions with the Higgs field give mass to the fundamental particles,

The vibrations represent the Higgs boson,

discovered in twenty twelve after a four-decade search.

It was in the early nineteen sixties that physicists realised that there was a problem.

As we know, the weak force has two associated particles, bosons known as the W and Z,

Quantum field theory, the theory that describes quantum forces, requires force

particles to be massless. For electromagnetism,

the force particle is the photon, a massless particle,

And for the strong force, the force particle is the gluon, again a massless particle.

So why, physicists asked, did the W and Z of the weak force have mass?

Simply adding mass to the W and Z in the equations proved a disaster,

As quickly the mathematical terms exploded to infinities,

So it was a new mechanism that would be needed to add mass to the W and Z,

And this mechanism was provided by the Higgs field - a field of energy that permeates all of space,

The W and Z particles feel the presence of the Higgs field,

coupling through a quantum interaction,

Whilst the photon and the gluon don’t see the Higgs field at all.

The problem of the masses of the W and Z bosons was solved,

But it was realised that the Higgs mechanism had more up its sleeve.

It was quickly seen that the mechanism that gave the weak force particles their mass,

Could give mass to all of the fundamental particles, to all the electrons and quarks,

As these particles travel through the seeming emptiness of space,

The presence of the Higgs field ensures they have just the right mass.

Quantum field theory is highly successful,

accurately predicting what is spat out of particle experiments.

Electrons, quarks, sprays of gluons and many other wonders can be found in its mathematics.

Einstein’s relativity is equally successful, predicting the motions of objects in the heavens.

But the superb accuracy of both these theories is where Einstein’s headaches began.

Whilst Einstein did not have the details of the fundamental forces available today,

He did realise the problem in the language of the two theories.

Relativity described gravity in terms of curved and warped spacetime,

Quantum field theory, on the other hand, spoke about probability and waves.

These two languages were completely at odds with each other.

You might wonder So what? Does it really matter if we need two sets of mathematics?

But to physicists like Einstein, this flew in the face of the dream of modern physics,

The dream of unification.

It started with James Clerk Maxwell and his work on electromagnetism.

In one set of equations, he showed that the two disparate phenomena, electricity and magnetism,

Are just two sides of a single coin.

With the advent of the quantum field theory, things became simpler again.

Three of the fundamental forces, electromagnetism, strong and weak, could be written in its language.

One mathematical formalism to encompass seemingly diverse phenomena,

It all worked so well – except for gravity.

Gravity simply did not fit.

Soon after the appearance of general relativity,

physicists wondered if extra dimensions were the answer.

What if spacetime had more dimensions, and these contained the electromagnetic force?

These ideas appeared to sort of work, but very imperfectly,

And of course left a big question, namely where are these extra dimensions?

Surely, we would notice if there were more than up-down, left-right and back-front in space.

But physicists offered a novel solution – what if these dimensions were curled up tightly,

So tightly in fact that we could have no experience

of their existence, no way to resolve them, Like using boxing gloves to put together lego.

Reality, some thought, could be built from a large number of these curled-up dimensions.

These ideas languished for several decades, but in the nineteen sixties saw a rebirth.

Physicists realised that these curled-up dimensions could encompass all of the forces,

But only if we treat fundamental particles as loops of string instead of points.

The string theory revolution had begun.

Into the nineteen eighties and nineties,

physicists were sure that they were on the right path.

Any day, someone could crack the formidable mathematics, and everything would fall into place.

But as they toiled in the more than twenty dimensions needed for their theory,

The fog of physics refused to lift, and unification seemed as distant as ever.

More recently, string theory has morphed into M-theory,

The M in the title left open by its originator Edward Witten for when

whatever M-theory truly is reveals itself. Strings have been replaced with membranes,

so that might be the answer - But in reality it is a loose

collection of mathematical ideas, themselves looking for unification.

Not everyone has been bitten by the string theory bug,

And others have hunted for mathematical structures that could underpin reality.

Loop quantum gravity is one such idea, knitting reality from

fundamental chunks of space and time. But it too has proven mathematically

challenging and has yet to prove itself as the unifying theory.

But physicists are resilient and the hunt for the ultimate theory of everything continues.

New mathematical ideas are proposed and tested, before usually being rejected,

And to guide their search, physicists rely on one of the key concepts in physics to guide them.

A concept that comes as close to the ultimate rule of the universe as anything else does:

The idea of symmetry.

We all have a passing notion of symmetry - things

like a vase or a building being pleasing to the eye.

But you can use mathematics to talk about symmetries,

And the existence of these symmetries has a profound impact on the nature of our reality.

Take, for example, a sphere, a shape we can define precisely in mathematics.

If we rotate a sphere, it still looks like a sphere.

The sphere has rotational symmetry, a change that does not alter its appearance,

And we can explore the mathematics of physics to search for similar symmetries.

Because, as Emmy Noether showed in nineteen fifteen, every

symmetry implies a conservation law.

But what does this mean?

Imagine you have a physics experiment, something like rolling balls down a slope.

And imagine you spend a while rolling balls and measuring their speeds.

Now imagine you move your physics experiment by a metre left or right and repeat the rolling.

Would you expect the outcome to be the same or different? Surely it would be the same.

So, moving the experiment didn’t change the outcome, and this is a symmetry.

But what is the conserved quantity that this implies?

The mathematics of Noether tell us that the conserved thing is momentum,

This property of motion, mass times velocity in Newton’s theory,

is conserved because of this symmetry.

Momentum cannot simply disappear in a physical interaction,

simply vanish - one billiard ball transfers its energy to another.

Other symmetries exist and imply other conservation laws.

If you rotate your experiment by ninety degrees and repeat the rolling of the balls,

Again, you expect the same outcome of your experiments,

Now the conserved quantity is angular momentum, the conservation of spin in the universe.

But perhaps the strangest symmetry is that associated with time,

The fact that if you perform your experiment today and then tomorrow, you expect the same results.

This time symmetry results in what seems like one of the most fundamental rules about the universe.

This time symmetry implies the conservation of energy.

Energy itself cannot be lost or created.

Symmetries run deep through our ideas of theoretical physics.

Einstein’s special relativity is built entirely on symmetries,

And it can be derived from what is known as the Lorentz group of symmetries.

Indeed it is these symmetries that demand that the speed of light is the fastest speed there is!

But it is in quantum mechanics that symmetries reign supreme.

It was realised that electromagnetism contained a symmetry, unexcitingly known as u one,

And that other symmetry groups, with more esoteric names, described the strong and weak forces.

With these symmetry groups come new conserved quantities.

For electromagnetism, the quantity that is conserved is charge.

But for the other forces, other quantum numbers appear, quantities that seem quite unfamiliar,

Isospin, lepton number, quark numbers and a host of others.

These all represent the rule book of reality, defining what can and cannot happen.

If the conservation laws hold, then interactions can occur.

If they are broken, then interactions are strictly forbidden.

Deep deep down - reality appears to be built on symmetries.

And in the search for even deeper reality,

physicists have tried to incorporate more and more proposed symmetries.

With supersymmetry, physicists try to hold up a mirror to the standard model,

Reflecting all existing fundamental particles with supersymmetric counterparts.

For electrons, there should also be selectrons, for photons there should be photinos and so on.

Supersymmetry improves the mathematical elegance of the standard model,

And for some, it indicates the true path to enlightenment,

But there has been no sign yet of any of the supersymmetric particles in any experiment.

This, many claim, shows that supersymmetry as an idea is dead.

Groups of symmetries also sit at the heart of string theories and M-theories,

And different symmetries are used to clean up and clarify loop quantum gravity.

Clues keep coming from geometry and symmetry, With many putting their faith in what is called

the anti-de Sitter/conformal field theory correspondence.

This mouthful, usually abbreviated as AdS/CFT, was found by Juan Maldacena in the late nineties.

A mathematical mapping between aspects of string theory containing gravity and particle theories,

It showed that the mathematics of one particular 4 dimensional universe without gravity,

a universe built on quantum mechanics alone, could match that of a 5 dimensional universe

with gravity. In short - that there was a clear mathematical link between the two - gravity and

Immense effort has been placed in uncovering

whether this correspondence tells us something deeper,

Perhaps a shadow of the true theory of everything that rules reality.

However the AdS/CFT correspondence could also have very bizarre implications.

You might be familiar with the notion of an optical hologram,

Where a three-dimensional image is encoded in a two-dimensional space.

One of the possible consequences of the Ads/CFT correspondence is

that this may be the case for reality. What we experience as reality really just

a projection of information stored in lower dimensions

And just like an optical hologram is nothing

but an illusion conjured up from a two-dimensional plane.

This notion, known as the holographic principle, grew out of our understanding of black holes.

Black holes are extremely simple objects,

completely described by only their mass, angular momentum, and charge.

A black hole carries no memory of the objects that have ever fallen in,

All information, other than those three properties, is seemingly forgotten.

But in physics, it is generally accepted that information cannot be created or destroyed,

Only processed from one form of information into another.

So, for black holes, the question of just where all of their information goes was problematic.

Originally, it was suggested that the information is still there, encased in the black hole’s heart.

Here, it would just be hidden from the rest of the universe, out of sight and out of mind.

But in the early nineteen seventies, this picture was completely demolished,

Destroyed by the insights of a young physicist named Stephen Hawking.

Hawking had wondered about the impact of quantum mechanics near a black hole.

He eventually showed that the one-way influence of the event horizon,

which allows things in but nothing out,

Conjured particles out of the foaming quantum background.

These particles escaped into the universe, carrying energy with them.

But from where did these particles’ energy arise?

Hawking’s solution was astounding.

He suggested that the energy was seemingly tapped from the mass of the black holes,

And as the particles escaped, the mass of the black hole steadily decreases,

Until it has completely evaporated away to nothing.

But that wasn't the end.

For if the black hole’s information was written inside it,

where did it go as the black hole evaporated? It didn’t appear to be written onto the

particles that were ejected, And physicists couldn't accept

that the information had been erased, So they hunted for another solution.

What if the information was not carried into the black hole,

but instead was left elsewhere? Perhaps, they suggested,

information was imprinted instead on the event horizon as things fell through.

This information could then be etched onto the escaping particles as the black hole evaporates,

With the total information released into the universe as the black hole totally disappeared.

In this picture, the black hole is a three-dimensional object,

filling three-dimensional space, But it stores its information on a

two-dimensional surface, the event horizon. In the same way,

a hologram holds a three-dimensional image, All encoded into a two-dimensional surface.

And some physicists have suggested the entire universe is structured the same way.

Before we leave the importance of symmetry in physics, there is one more thing to explore,

And that is the fact that the symmetries of our universe cannot be totally perfect.

Some of the symmetries have to be imperfect, have to be cracked,

For if they weren’t, we would not be here.

Take, for example, the asymmetry of matter in the cosmos.

The universe appears to be stuffed with electrons, many

orbiting atoms, some just floating out in space, But their anti-matter counterparts, the positrons,

are comparatively rare - extremely rare. In fact, within the Milky Way scientists

estimate that they make up no more than a quadrillionth the amount of matter.

And so this raises the question, where are all the positrons?

You might wonder why this is important, but it is really another question of symmetry.

Imagine the processes underway in the very early universe, at the time when matter was created.

If the processes were purely symmetric, they would have made equal amounts of matter and antimatter.

For every electron that was created, a corresponding positron would be created.

After the fiery birth of the Big Bang, these equal quantities of

matter and antimatter would annihilate, And this complete annihilation that would

leave nothing behind but a featureless soup of radiation.

With no matter, there would be no atoms, no stars, no planets, no people.

The universe would simply settle down into a featureless nothing.

So, there must have been a break in the physics of matter creation,

A tiny asymmetry of one part in a billion that left one electron for every billion annihilations,

As well as protons rather than antiprotons, neutrons rather than antineutrons,

Leaving behind the matter from which we are made.

Physicists are not quite sure where this crack is hidden in their theories,

And perhaps they will need their theory of everything for this to finally be revealed,

But other cracks in symmetry are apparent,

And these might be providing clues to the ultimate nature of these asymmetries.

The culprit is the weak force, the strangest member of the fundamental forces.

Unlike the other forces, the weak force seems to distinguish between matter and antimatter.

To understand this, let’s start by thinking about a single hydrogen atom.

This simplest of elements comprises of a positively charged proton at its nucleus,

Which is orbited by a negatively charged electron.

When this electron jumps in its orbits, it absorbs or emits a particular pattern of light.

What would happen if we considered instead an atom of anti-hydrogen,

This would be a negatively charged anti-proton orbited by a positively charged positron.

When the positron jumps in its orbit, again a pattern of light is absorbed or emitted,

And this pattern is precisely what is seen for everyday hydrogen.

This shows that electromagnetism is symmetric when considering matter and anti-matter,

And the same is true of the strong force and even gravity.

But not the weak force. The weak force is not symmetrical,

The outcome of reactions is different depending

on whether they take place with matter or anti-matter.

This is strangely tied to one of the universe’s most mysterious particles, the neutrino.

The neutrino only interacts via the weak force, so it’s always a player in weak interaction.

What makes the neutrino bizarre has something to do with one of its fundamental properties,

And that is that all neutrinos spin the same way.

This is a quantum property and quantum spin comes in quantized amounts - they do not literally spin.

Photons of light also possess spin, one unit of it in quantized amounts,

And you can think of spin being right or left-handed as the photon travels.

In our universe, we see both left-handed and right-handed photons,

But the same is not true for neutrinos.

All neutrinos are left-handed,

whilst their anti-matter cousins, anti-neutrinos, are right-handed,

And this differentiates the interactions between matter and anti-matter.

But why do the neutrino and the weak force differentiate

between left and right in the universe? Physicists still don’t know the answer,

but this crack in reality appears to have been essential for our very existence - without it

everything would have been completely annhialated in the opening seconds of the cosmos.

We have come a long way in this description of the rules of reality,

We’ve discussed quantum fields and broken symmetries, warped spaces and cracked symmetries,

But there is one final example that shows how truly bizarre the quantum world really is,

when you follow its rules to the very end.

Look up at the night sky.

The light you see from distant stars is detected through electrons in your eyes

exchanging photons with electrons from millions of light years away.

For two electrons to experience the electromagnetic force,

they excite a vibration in the photon field. This photon travels between the two electrons,

communicating the force. But how do the electrons

decide that they will exchange a photon and experience the electromagnetic force?

You might think that one electron generates the photon vibration which randomly impacts the other.

However, this is not how quantum mathematics works.

For the photon to be exchanged, the electrons have to agree – one to emit, one to receive.

But how can they agree as they can only communicate

at the speed of light, through photons?

So, what do the mathematics actually say?

Well, It all becomes a matter of interpretation.

In what is known as the transactional interpretation the electrons communicate,

A handshake between the electrons that is part of the photon exchange,

But to achieve this, the electrons exchange these handshakes through time!

One electron sends its handshake into the future, the other into the past,

And with a mutual agreement, the photon is exchanged,

even across billions of light years, and one day even trillions.

But this need for time travel as a fundamental piece of reality is too much for some,

who dismiss it as a mathematical convenience that masks something more fundamental.

And so where does this actually leave us in understanding reality?

From holographic universes to wild mathematical implications,

we now find ourselves at the philosophical edge of science and what science even is.

Is it a search for the truth, to uncover the ultimate nature of reality?

Or is the best it can ever do is give us a mathematical approximation, a representation,

Of a true reality that is forever hidden.

Your thoughts race as your mind tries to process this complicated picture.

But suddenly, the confusion is pushed aside,

As another thought snaps front and centre in your mind.

If nothing is real, then just what are you?

Just where do your thoughts reside?

Where are you? ________________

There is an ancient Greek legend about the ship of Theseus.

Over the centuries, time and travel takes its toll on the aging vessel,

And constant maintenance and repair are needed to keep it afloat.

Eventually, every plank and every nail,

every stick of thread and yard of sail, has been replaced.

Eventually, no original piece of the ship of Theseus remains.

The philosophers of Ancient Greece pondered whether this is still the ship of Theseus,

And if it wasn’t, when did Theseus’s ship stop being Theseus’s ship and become something else?

It might seem that discussions about an legendary ship are little more than philosophical musings,

But the ancient idea has some profound implications,

Implications about the reality of you.

Let’s start with a simple question – how old are you - really?

If you cut yourself,

vividly red blood will flow from the wound. The colour comes from red blood cells,

tiny cells suspended in a straw-coloured plasma. But is the blood that flows in your

veins the same as when you were a child? Just how long does a red blood cell survive?

Made deep in the dark marrow of our bones,

A red blood cell lives a mere four months before it is broken down and excreted from the body.

Every second, about five million red blood cells meet their ultimate demise,

As freshly made cells flow into your bloodstream to replace them.

Your body completely replaces your blood three times a year.

Indeed, almost all of your body is eventually broken down and replaced.

Your skin is replaced every few weeks,

and for your internal organs, it might take a decade, but they too are eventually replaced.

Whereas your bones regenerate over about fifteen years.

You are literally not the person you used to be.

Some mission-critical cells, such are nerves or in your heart, are effectively permanent,

But the rest come and go over your entire life.

And this is true of you and everyone you know, Every one of us, and all living creatures,

are biological ships of Theseus.

This can be hard to come to terms with. You can see the scars you gained in childhood.

And so - are you still the same you that was born? Are you the same you, even though your individual

cells have been replaced over and over? And like the ship of Theseus, if you are not

the original you, when did this transition occur? And if you are not you, then just what are you?

What makes you, you?

The location of the mind, the seat of consciousness, has long been mysterious.

To the Ancient Egyptians, the heart was the centre of all activity.

It was home to the soul, with thoughts radiating out to the bodily extremities.

This view was adopted by the great philosopher Aristotle,

Claiming the brain was too far from the centre of things to be important.

This confusion was born out of a problem of thinking about thinking.

Just what is a thought? Clearly dissecting a corpse

will reveal nothing physical, as consciousness will have departed,

In the 17th century Descartes decided that the brain and the mind must be separate things,

And Gottfried Wilhelm Leibniz supported Descartes's view on

the separation of brain and mind,

asking you to imagine him shrunk smaller than the size of a flea.

At this minute size, he then invited you to join him on a tour of the brain,

As you wander around, there would be no sight of thoughts within the fleshy material,

No ideas bubbling away inside all of the biological machinery.

No, he said, the mind is something separate, something distinct, from the brain.

But with the coming of the twentieth century, Descartes's dualism came under attack,

for advances in technology and biological understanding began to look at the working brain.

Real-time mapping revealed that Leibniz’s fanciful journey might be blind to seeing thoughts

Not due to anything mystical,

But because thoughts, whilst apparently physical, are invisible to the naked eye.

The secret to the mind was electricity.

With sensitive mapping, it was discovered that the brain pulses and bursts with electrical activity,

With certain sensations and feelings lighting up different parts of it.

It was realised that the brain is a highly convoluted network,

More than eighty billion neurons with a myriad of interconnections,

Signals trigger new signals that flow throughout the network,

Electrical activity surges and cycles - builds up and dies away.

But though it might be satisfying to think that the mind is messy electrical signals,

Many mysteries still remain as to just how it works.

Where is thought? Where is

love? Where is creativity and imagination? Where are memories stored? Why can I remember

the lyrics of songs I haven’t heard in years? Just how the brain works is a puzzle,

a clouded enigma, Indeed - even the great brain of

Einstein was remarkable for being unremarkable, stolen from its skull in his autopsy.

But focus has since grown on the complexity of the neural network inside our heads.

Maybe consciousness and thought are a result of this complexity,

An emergent phenomenon that can’t simply be revealed by exploring the network.

More than the sum of its parts.

Emergent phenomena have often confused and bemused scientists.

Indeed, emergent phenomena have often mystified many.

There is a famous story of a soviet aide visiting London in the nineteen eighties,

Wanting to seek out the person who organises the bread production in the city.

In the Soviet system, bread production was centralized, with every step controlled.

But in cities like Moscow, long queues at bread shops formed due to inefficiencies in the system.

In the West, there was no bread Czar managing production,

Yet an effective system had emerged from many, many workers performing their own tasks.

It was reminiscent of an ants’ nest.

If you watch such a nest, many thousands of ants scurry about, seemingly with purpose.

But no one ant controls the behaviour of the colony.

Instead, complex emergent phenomena arise from each ant following a simple set of rules.

Emergent phenomena can be found across the physical world.

Water is nothing but a collection of molecules attracting and repelling through electromagnetism,

But a body of water can flow and bubble, with turbulent eddies and swirls,

Emergent behaviour from simple rules.

The physical properties of matter and gases also emerge from simple rules,

And some now wonder if these simple rules emerge from even more fundamental phenomena.

Perhaps gravity emerges from truly fundamental pieces of space and time,

In the same way that the properties of water emerge from the interactions of molecules.

And so could it be true that the reality that you experience,

the world of wonders inside your head, is just the result of some complex wiring?

Some find this conclusion unsettling. But for others, this notion gives them great hope.

If consciousness is just the result of complexity, not something beyond the physical domain,

Then maybe we have the hope of recreating, if not exceeding, consciousness.

This has been the goal of artificial intelligence for many decades,

To build ever more complex networks,

both virtual and physical, in the hope that intelligence emerges.

There is a branch of physics, known as synthetic intelligence, that plans to put this to the test.

This is unlike artificial intelligence, where people are trying to make computers think.

Instead, in synthetic intelligence, physicists are building complex nano-wire networks,

Wires that act like the neurons and connections in our own minds.

They hope that electric currents pulsing through these wires would represent electronic thoughts,

And these nano-wires, that have the ability to reorganise, will have the ability to remember,

This, of course, sounds like science fiction,

But we might only be a few steps from the dream of thinking machines.

Of course, for those who cling to Descartes’s dualism, this is nothing but an empty quest,

As no physical mechanism will ever think in the way that humans do,

But for the believers, it is all just a question of time and complexity,

And one day their networks will become aware, become alive.

But if consciousness is purely physical, then deeper philosophical questions begin to emerge.

Are physical processes really playing out in fleshy pulp inside your skull?

What if you are a brain in a vat, being fed stimulation from a virtual reality?

What if you are not a physical brain at all, but just software executing on a computer?

And what if Descartes's dualism is right,

and that consciousness truly exists beyond the physical world?

A more ethereal plane where something like a soul resides.

This opens another philosophical question,

one that might be more challenging for the idea of reality.

Why should consciousness and thought be constrained to being human?

What if it was much more widespread than that? What if consciousness is everywhere?

It sounds again like we have entered the realm of science fiction,

But this notion of a ubiquitous consciousness, of panpsychism,

has been discussed by prominent philosophers, A consciousness that inhabits all corners

of the universe, That everything,

every particle, is, in some sense, alive.

People, trees, rocks, sand, dust, stars, galaxies, black holes, and plants – Everything conscious.

Even molecules, atoms, protons, quarks, and electrons - even the universe.

This idea, whilst beloved by some philosophers, is too much for physicists to stomach.

Philosophers tell them that it somehow links with the weirdness of quantum mechanics,

But physicists simply feel that this is well beyond the bounds of what they class as science,

And, as ever, both groups tend to ignore each other.

Could everything be conscious? Is reality alive?

Your scientific mind ponders on this for a moment, but concludes it is a step too far.,

And so you turn your head and look over, once again, at the apple.

It fills your focus, as your brain processes all the aspects

of reality that have crossed your mind, You wonder about the apple from its entirety

to its fundamental pieces, And as you think about

its existence and its reality, You wonder if the apple is contemplating yours!

You start to wonder if your thoughts on reality could get any weirder.

Of course they can. ________________

Picture a scientist - What picture pops into your mind?

For many, it will be the wiry-haired Albert Einstein,

late in life with a wrinkled face. Perhaps with his tongue sticking out.

Or maybe Isaac Newton in his formal regalia and eighteenth-century wig.

But these images are of these great scientists many years after their seminal works.

The alleged comedy show, The Big Bang Theory, has shown us that science is a young person’s game.

and although the awkwardness that is played for laughs is an unhelpful scientific stereotype,

it is correct that most scientists make their biggest breakthroughs when young.

Indeed, it is important to remember that Einstein was only twenty-six during his miraculous year.

Isaac Newton was also a young man when he had his greatest insights into nature.

And in sixteen sixty-six he was just twenty-three years old.

He had already proved himself as precocious, and had been studying at the University of Cambridge,

But this year was to be a year like no other.

The black death had returned again to Britain. The bubonic plague first raged across Europe

in the fourteenth century, but it had never really gone away.

For centuries it resurged and cut a swath through European populations.

With its arrival in 1666, people had fled the cities for the relative safety of the countryside.

And Isaac Newton left Cambridge for his family farm in the village of

Woolsthorpe in Lincolnshire. As death stalked the land,

Newton found solace under his famous apple tree,

And his mind turned to unravelling the secrets of the universe.

We have already met Newton’s ideas of space and time, but let’s look again at time.

With his equations of motion, Newton could calculate how things changed with time,

And time could be neatly cleaved into three distinct pieces,

A past that had gone, a future that is yet to come, and an instant that is now.

Reality, it appeared, existed at this unique instant of now, a precise and exact now,

Evolving from its state in the past to the present - and we

can calculate what will happen on into the future.

But Newton’s equations are purely deterministic, and if we had perfect knowledge at any time,

We could calculate the exact state of the universe at any other instant.

This raises an interesting question – in a Newtonian universe, has the past really gone?

Is the future really unknown and yet to come? In some sense, the realities of the past,

present and future all exist, And the instance of now is just

sliding through this already complete history, with us just experiencing a moment of reality.

This idea returned with a vengeance in Einstein’s theory of relativity,

Instead of just a background, space and time became central to the story,

With each fundamental particle tracing out a distinct path, its worldline,

through these four dimensions. And like Newton, Einstein’s

universe is purely deterministic, with past, present and future all existing.

This came to be known as the Block Universe.

And once again, the mathematics is pretty clear on something quite bizarre.

The equations of relativity are unequivocal – Einstein’s universe is a block universe.

With perfect knowledge, there is no mystery about the future as it can be calculated from now.

In fact, Einstein’s equations don’t define just when now is – you have to define when now is.

There is nothing special about now in Einstein’s universe.

Of course, the block universe raises some uncomfortable questions,

questions around free will. The ability to choose between

possible options seems innately human,

But in the block universe, this is just an illusion as the future is already written,

The block universe also has bigger consequences, consequences that stray back into science fiction.

If the past and future are out there in the block universe, could we ever reach them?

Out there is DaVinci and dinosaurs, and everything, every creature, that will ever be.

And maybe there are convoluted spacetime paths that will mean we will experience them in our now.

But the issue is that the block universe

just doesn’t feel real – surely the future is unknown and yet to come.

And so many tried to look beyond the raw theories of Einstein to provide the answer.

Eventually, the probabilistic nature of quantum

mechanics appeared to show that Einstein could not be correct,

the future being written from the possible outcomes of quantum questions.

And so the picture that emerges from this compromise is that the

universe is a block universe up to a point. And that point is now – now is constantly being

constructed as quantum questions are answered. Now adds another sheet of reality to the

block universe, unfolding an unknown future in front of us,

With the past solidified into the block behind us.

And interestingly, this is the picture offered by loop quantum gravity.

One of the contenders for a quantum theory of gravity,

Within this theory, space and time are constantly being weaved from fundamental loops,

Now is the edge of the weave, the past is the tapestry, and the future is the unknown ahead.

Our reality, our now, would be at the busy edge of the construction of space and time.

This gives you pause for thought. Is the apple you see now the same

apple you saw just a moment ago? Is its time, space and fundamental

constituents constantly being knitted from the ultimate fundamental?

Are you and the apple being rewritten at every single instant you experience?

After deconstruction after deconstruction, your grip on reality seems to be definitely slipping.

You settle on a simpler question - Where does reality exist?

Maybe reality is just the block universe,

and that’s all there is. Maybe free will is an illusion?

Or maybe loop quantum gravity is correct, and reality exists as an

instant of an unfolding universe. But what if there is something

much deeper underlying reality, something that is not physical?

What if at the bottom, reality is nothing by just numbers?

The notion isn’t as strange as you might expect,

Indeed, it reflects our understanding of the workings of the fundamental universe.

Written at the heart of all physical laws is the notion of information,

and information processing. Just what do the laws of physics do?

They take what we know about a particular situation, the information of that situation,

Such as the location and velocity of a ball flying through the air,

And process it into information of that scenario into the future.

This is probably most apparent in the rules of quantum mechanics,

Where the information of a scenario is encoded into the wavefunction.

The rules of the quantum evolve the wavefunction, the information, into the future,

Numbers are churned into new numbers as the laws of physics are played out.

This all might sound a little esoteric, But by the mid-twentieth century,

strong links had emerged between information and thermodynamics,

as it was realised that the effort to process a signal requires energy.

A computer has to expend energy to flip a bit from a one to a zero or vice versa,

As does your brain as you process even the simplest of thoughts.

All of this has led some to propose that everything is really information processing,

That there is no fundamental difference between the processing of information on a computer,

Or the processing of information in your mind,

Or the processing of physical information that underpins reality.

And this is the foundation of what is known as the simulation hypothesis.

This states that our thoughts, memories, and all of our experiences are nothing but computations.

This is deeper than the idea of a brain in a vat that we encountered previously,

As there is no physical brain. In fact, in our reality, there would be nothing physical at all.

Of course, the simulation hypothesis raises an immediate question – just

where are these computations taking place? Perhaps these exist in a higher reality,

a physical domain that we can know nothing about.

But what are the goals of our simulated reality? Why did our creators create us?

What if we are little more than characters in a game of Sims?

Indeed, what if the simulation was not created for us?

What if it was constructed to explore other aspects of synthetic universes,

And we are nothing but an unexpected side effect.

What if the creators know nothing about our existence, and care even less?

Of course, we can argue about all these questions for the answers might always elude us,

But here on Earth, physicists do generate synthetic universes

with the goal of understanding our reality,

And also explore other seemingly hypothetical universes where the laws of physics are different.

Maybe one day, with powerful computers, conscious creations may inhabit our synthetic universes.

At this point, an important question might have crossed your mind.

Even if we are living in a simulated universe so that our reality is completely synthetic,

Surely our creators inhabit a real reality, a physical reality.

Have we just pushed the problem upwards, or is it turtles all the way down?

We come to this conclusion because we imagine that there must be a physical computer,

A device that carries out the calculations that construct our reality and synthesize our universe.

But what if there is not? What if there is no physical basis for

reality? What if there are only calculations? This is the idea of the mathematical universe.

The lead proponent of the mathematical universe is Swedish-born Max Tegmark.

In his earlier research he focused on the challenges of modern cosmology.

But eventually he turned his attention to some of the biggest questions we have.

His mathematical universe is similar to that of a simulation hypothesis universe,

But without the need for a physical computer to do the computations.

But where would these calculations be performed.

What would be performing them? Surely you need a computer to compute.

But being truly fundamental means that these mathematics just exist,

And there would be no real answer to where they came from or where they exist.

Intriguingly, Tegmark’s mathematical universe actually a multiverse,

Containing not only our universe but all possible universes.

In the growing maelstrom of mathematical calculations,

there is not only our particular reality,

But all possible realities, each defined but distinct mathematical structures of their own.

Across this multiverse, there should be mathematics very similar to our own,

With multiple versions of you, whatever you really are, out there.

There will be other universes which are quite different to our own,

Different in terms of the strengths of the fundamental forces,

or dimensions of space and time.

The library of possible mathematics is immense, possibly infinite,

But it is definitely much larger than the subset

of mathematical structures we use to describe our cosmos.

So out there in the sea of fundamental mathematics, there could be much, much more,

Not only universes we have not dreamed of, but many we could never dream of.

And one last horror, one prospect of reality,

exists in the mathematical universe. And it is tied to infinity.

Every so often within the infinite sea of calculations,

the right calculations will randomly come together.

These calculations will look just like your mind, a mind filled with your memories and reality.

In an instant, you will be created, maybe considering an apple.

To this brand-new mind, with all of its memories, everything seems completely normal.

They remember the big events of their lives and picking their apple from the tree.

But the existence of this mind is only temporary, only a mathematical fluctuation.

As quickly as the mathematical structure of this mind came into being, it vanishes

back into the wash. This consciousness,

with experiences of reality as real as ours, is doomed to disappear in an instant.

Indeed, these relatively tiny fluctuations in the mathematics

would massively outweigh the likelihood of our entire cosmos.

How do you know that you are not one of these minds that will vanish in a moment?

How do you know that anything you think is real was there a moment ago?

How do you know that you are not about to wink out of existence?

By now you might be thinking, well, this is ridiculous.

I know that I am not a mathematical structure, and I remember what has happened to me.

There is no chance that I am a temporary existence,

I know that I am truly real.

But unfortunately getting rid of the mathematical universe does not fix this problem.

It is believed our cosmos has an infinite amount of time ahead of us.

And for this infinity, the universe is going to be cold and empty,

Long after the last star has died and the final black hole has evaporated into the background.

And so it might seem that we would reach the end of the story of the universe,

As not a lot will happen in our future empty cosmos – surely it

will be a quiet, uneventful place. But the future universe might not

be as dead and buried as it might seem, And to understand this, we need to explore

one of the most misunderstood concepts in physics – entropy.

Ludwig Boltzmann was one of the most insightful minds of nineteenth-century science.

Before it was widely accepted, he was a supporter of the atomic hypothesis,

The notion that at the smallest level, matter is made of indivisible atoms,

And he recast the laws of energy flow, thermodynamics,

in terms of the motion of these invisible atoms.

Boltzmann’s view attracted criticism from the scientific greats of his time,

But shortly before his death in nineteen o’six, direct evidence for atoms began to appear.

In pondering the realm of atoms, Boltzmann wondered about how they could be arranged.

Think about the air around you, atoms of nitrogen, oxygen and

carbon bound into a myriad of molecules. These molecules are whizzing about, colliding

with each other, with the walls and with you. But at every moment, every configuration

of atoms and molecules seems completely indistinguishable.

Experience tells us that the atoms are unlikely to spontaneously gather in one corner of the room,

But if we started with all the atoms of air in one corner,

they would rapidly spread out and fill the room.

Boltzmann showed that this difference is simply a matter of statistics,

That specific atomic configurations evolve into more general configurations.

This evolution of configurations, from specific to general, is known as the growth of entropy,

And it has become one of the most fundamental rules of the cosmos.

Because these notions of specific to general atomic

configurations can be thought of another way, That order eventually gives way to disorder.

Let’s try and understand this in terms of energy.

The total energy in any particular scenario must remain constant - this is another

of the rules of thermodynamics, But not all energy is the same,

some of it is useful, and some of it not - The growth of entropy means that useful

energy is degraded into useless energy.

Imagine you have a cylinder of compressed air. This scenario is a little like the situation where

you have all your air in the corner of the room. This highly specific case is said to have low

entropy as it has lots of useful energy. And to tap this energy, all you have to

do is open the valve. The rush of air from

the valve can be used to spin a wheel, And this spinning wheel can be used to do some

useful work, such as generate an electric current. But eventually, when all the pressures have

equalized, the rush of air stops, The wheel stops spinning,

and no more useful energy can be extracted. This is true of every physical process,

with useful energy constantly degraded into useless energy.

Indeed, your body is constantly fighting the growth of entropy,

But to do so, you need a supply of concentrated energy from the food you eat,

Which you process into more useless energy in the form of waste and an infrared glow,

You are doing your part in constantly increasing the entropy of the universe.

Entropy has been rising since the dawn of time, with useful energy constantly diminishing,

And the question of why the universe was born with extremely low entropy,

so lots of useful energy, is unanswered.

But eventually, in the far distant future, there will be no useful energy left in the cosmos,

And the universe will have entered the state of maximum entropy.

And so, as we have seen before, it appears that we have reached the end of our story,

For what can happen in this maximum entropy cosmos with no useful energy available?

But remember, the rules of thermodynamics laid out by Boltzmann are statistical in nature,

And in these statistics, bound by probabilities,

even the seemingly impossible can eventually happen.

Perhaps the air in your room could spontaneously gathering into a single corner,

or billiard balls could rearrange into their starting formation.

Boltzmann said that whilst this is highly improbable,

if we wait long enough, it must happen. How long might be a very long time,

much longer than the current age of the universe, But if there is one thing the future cosmos will

have in abundance, that is time. In the ultimate disordered state

of the future universe, order will sometimes spontaneously arise.

And so every-so-often the laws of thermodynamics and

quantum mechanics will fluctuate. These fluctuations will generally

bring some particles into being before they fluctuate away again.

the universe going back to being quiet and uneventful.

And even rarer than every-so-often, fluctuations will bring more substantial things into being.

Vanishingly rarely, the complexity of a human mind will fluctuate into existence.

Usually, these will be a blank slate, not thinking a single thought before vanishing again,

But rarer still, a mind with memories and experiences will occur.

These ethereal minds are known as Boltzmann Brains and are extremely rare events.

But with infinite time available they will appear again and again and again.

And over and over again, even your exact consciousness will pop into existence.

Your consciousness with all of your memories and experiences.

An infinite number of times.

Picking one of these at random, you are much

more likely to be a Boltzmann’s Brain in a cold, dead universe,

Than a physical, fleshy human enjoying a green, ripe apple.

You – the mind watching this video – temporary. Your existence simply a fluctuation,

in an instant, you will be gone.

Are you absolutely sure that you have not just popped into existence and are about to disappear?

Are you sure you are still here?

Yes.

At least for now.

And so for one final time your eyes settle on the solitary apple.

The apple that sent you on this journey looking for the nature of reality.

Your mind flashes across all the versions of reality you encountered in your dozing,

From the forms of Plato, through the weird world of quantum mechanics.

Across block universes, and in and out of holographic cosmoses.

But clarity returns as your stomach starts to rumble.

Again, you focus on the apple.

You reach out and pick it up, feeling the smoothness of its skin in your hand.

You take a bite.