All language subtitles for What Is Reality¿ (English)

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.