Even after the raging furnace of the first few minutes had died away,

Temperatures universe-wide were more than a hundred million degrees.

For thousands of years, this primal heat burned - a cosmos of plasma, a super-hot mix of particles

and radiation.

Until one day it changed forever.

That day arrived when the universe was almost four hundred thousand years old, and had cooled

to about three thousand kelvin.

In this now comparatively tepid soup lone electrons met lone protons, and finally could

stick together - forming the first atoms.

But this was not all.

For as each electron and proton bound together, a small amount of energy was released.

A packet of energy that raced away at the cosmic speed limit, the speed of light.

A particle of energy born in the formation of a hydrogen atom.

A particle we call a photon, a particle of light itself.

This photon was far from the first.

But as the universe began transitioning from plasma into neutral gas, light could then,

for the first time, stream freely through its reaches.

And so, our photon's long, long journey began.

It headed out first into the universe’s dark ages

A time before the first stars burned, a time before the first galaxies formed.

In the eerie darkness, gravity pulled on mass to mold the first seeds of cosmic structure.

But the photon sped on, and noticed nothing.

Eventually, the first stars burst into life around it,

Massive and bloated, these ancient suns burned themselves out in a cosmic blink of an eye

as the first supergiant black holes grew rapidly between them as they eagerly devoured mass.

But the photon sped on, and noticed nothing.

The first galaxies began to assemble The sky lit up with the fires of uncountable

young stars across the cosmos as they began to fuse the initial hydrogen and helium atoms

into heavier elements.

But the photon sped on, and noticed nothing.

Millions steadily turned into billions of years,

And as galaxies grew and matured, eventually, the intense light of young stars began to

settle.

The photon’s journey could have potentially lasted forever into eternity

But after almost 14 billion light years of travel, a large spiral galaxy steadily came

into view Its destiny was set.

Near a small blue dot orbiting a small white star.

After crossing the last few thousand years, the photon collided with a piece of metal

Part of a telescope built by humans and orbiting near the planet Earth

The photon’s energy was completely absorbed, energising electrons, and registering on detectors.

But as the photon vanished from existence, its billions-year long journey complete - it

simply did not notice.

Because to the photon itself, the journey never took place.

13.8 billion years of cosmic time disappeared in an instant.

Yet how can this be?

Light has existed in the universe from its earliest moments,

And will continue to exist long after humanity and the stars are shred to dust.

But just how does it work?

And how can it seemingly last forever?

And perhaps most importantly - what even is it?

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Light had played a pivotal role since the cosmos´ very beginning.

In these earlier times, it had only existed for the very briefest moments,

Slamming into speeding particles before it had a chance to travel anywhere, one piece

of light dying as another was born However our parcel of light was born into

a very different, very transparent, universe.

Forged with the first true atoms from featureless plasma.

With the cosmic maelstrom of the Big Bang finally abated,

Our photon could begin its immense journey unhindered.

As it travelled, many generations of stars lead to our Sun, forming in the collapse of

an immense gas cloud, and billions of years after, humans began to walk on the surface

of our small rocky planet.

And so eventually, as our photon was a couple of thousand light years distant from Earth

- these humans began to wonder.

“All men by nature desire to know.

An indication of this is the delight we take in our senses…(and) above all others the

sense of sight…The reason is that this, most of all the senses, makes us know and

brings to light many differences between things.”

The ancient Greeks wondered if light emanated from the eyes, touching and feeling the world

around us.

But clearly, there are times when it is dark when we can’t see anything.

So, they concluded light must be something external, something captured by our eyes.

Islamic scientists began to unravel the properties of light,

Finding rules of reflection and the magnification properties of glass lenses.

Light was clearly a natural part of the universe around us.

But it took the coming of the scientific revolution, and a fight between two giants of science,

for light’s deep secrets to be finally uncovered.

The year was 1652, and Dutch physicist, astronomer, mathematician and all round genius Christiaan

Huygens was exploring optical phenomena.

He had noted how light travelled through lenses and bounced off mirrored surfaces, and was

particularly interested in the phenomenon of refraction - where the path of light is

bent when it passes from one medium to another.

Huygens noticed that light was often split into the colours of the rainbow by his instruments,

And sometimes strange patterns of dark and light would be produced.

Clear evidence, to him, that light was some sort of wave.

Some sort of travelling, oscillating phenomenon.

Oscillations can be found throughout nature, from planetary orbits to vibrating electrons,

But let’s start with a simple picture - think of a child on a swing.

As they swing, their position oscillates from one position to the next, then back again,

Just like a pendulum that drives the regular ticking and tocking of a grandfather clock.

When oscillations act in unison, but slightly out of step, waves are formed.

A stone thrown into a flat pond pulls the water down with it, but the water bounces

back.

This splash of water pulls on its neighbours, inducing them to oscillate, which, in turn,

pulls on their own neighbours.

These oscillations fan out across the pond as a steadily rippling pattern of waves.

Waves are everywhere, from sound waves coursing in the air, to ocean water waves driven by

the wind and moon.

Seismic shifts can generate violent and destructive earthquakes in our planetary crust,

Whilst similar waves ripple in the atmosphere of the Sun and other stars.

And so - light seemed to be a wave.

But this left a question.

If light is a wave, just what was doing the waving?

In Britain, Robert Hooke also reached a similar conclusion about the nature of light.

Hooke had observed light travelling through glass, and he realised that this picture of

wavey light could explain a lot of the phenomenon he had seen.

This was cutting edge science at the time.

But Hooke had a problem.

And that problem was a man.

There are no surviving portraits of Robert Hooke, and over the years a rumour passed

down the generations that this powerful man was to blame - having conveniently lost the

painting when taking over as head of the Royal Society in London.

For Robert Hooke had a powerful enemy, and that enemy’s name was Isaac Newton.

Since cleared of any wrongdoing in the absence of contemporary images of Hooke, there is

still little question that the two men were not friends.

For as well as his far reaching discoveries in mathematics and gravity,

Newton also had an interest in optics and the nature of light itself,

And he did not like what Hooke or Huygens had to say.

It was Newton who discovered that white light could be split into a rainbow by passing it

through a prism, And like Hooke he had kept musing on this.

But unlike Hooke, Newton did not conclude that light was some sort of wave.

To Newton, light consisted of “corpuscles”.

To Newton, light was made of tiny individual particles.

Newton’s focus was the phenomenon of diffraction, the fact that waves bend around a sharp edge.

He knew that sound, a wave in the air, bent as it travelled past sharp edges.

It was clear that you could eavesdrop on a conversation from around a corner without

being able to see the gossipers.

You could hear from behind an object but you could not see.

So, he reasoned, light simply could not be a wave.

And he didn't stop there.

Newton went further – much, much further.

He reasoned that light, as a stream of particles, would even feel the pull of gravity

In his book Opticks, published in 1704, he wrote

“Do not Bodies act upon Light at a distance, and by their action bend its Rays?”

And though on this he was right - he was not proved right for centuries.

So it was Newton's corpuscular theory of light that reigned supreme,

Due more to his weight of personality and scientific standing, as opposed to its ability

to explain the complex observations of light.

Over the years however, steadily, the tide began to turn away from Newton.

In 1800, polymath Thomas Young shone light through a pair of narrow slits,

And observed a pattern of interference on a background screen.

This wasn’t the first demonstration of interference, but it was the clearest.

How could Newton’s picture of light as particles explain Young’s observation of interference?

How could light, as tiny bullets passing through either one slit or the other, produce the

observed pattern?

Simply throwing a couple of pebbles into a still pond reveals that interference is naturally

produced by waves, either in water or in light.

Other observations of light supported its wave-like nature, including light´s polarisation

through a material called calcite.

But for centuries a big secondary question remained unanswered.

If light was a wave, what was doing the waving?

Our cosmic parcel of light was born when a proton captured an electron.

It sped out into the universe, powerful and energetic.

But as it travelled, and the universe expanded, it started to lose some of that energy.

The light, originally blue to our eyes, steadily morphed through the colours of the rainbow,

and into the red.

Soon it was joined by other energetic light, shining from countless billions of newly formed

stars.

There were different types of light too, light of exceptionally high energy - and light with

barely any energy at all.

The universe was awash.

Our light did not know that these would be invisible to human eyes,

For it would be many billions of years until eyes existed,

And indeed - these X-rays and radio waves, as we call them,

were unknown to us until the very end of the nineteenth century.

“In this new era, thought itself will be transmitted by radio.”

Guglielmo Marconi was at his father’s estate near Bologna in Italy.

He was still a young man, aged only 20, but his education had opened his eyes to an invisible

world.

In the decades before, the nature of light had steadily been unravelled

And Marconi was ready to use this new-found knowledge to change everything.

Staring at his equipment, Marconi was waiting to see a faint spark in the darkness.

With its bundle of wires and coils of his workshop, such a spark would not be surprising,

But the impetus for this spark was not in the equipment before him,

It was in similar equipment located several miles away.

Of course, the nineteenth century had seen the arrival of the telegraph,

Where electronic pulses are sent along wires that cross entire countries and continents.

But this needed wires to be strung through the air and under the oceans.

Marconi had no need for such wires connecting his equipment.

He would be sending messages not along bits of copper.

His messages would simply fly, completely unseen, through the air.

But how?

The answer lies with one of science's greatest geniuses.

The answer lies with James Clerk Maxwell.

When a young James Clerk Maxwell arrived at the University of Cambridge in 1850,

He was told that attendance at the 6 am church service was compulsory for all students.

The Scottish-born prodigy had long been a night owl and simply responded

“Aye, I suppose I could stay up that late”.