Throughout their glory days, their engineers worked entire star systems.

They dismantled planets and asteroids to construct an immense interstellar empire.

But now, in the twilight of their time, all of this is long gone.

All around them, the universe is dying as the last of the stars are going out.

Over countless millennia, the sky has continued to fade into an eternal night.

The aging universe gripped by desolation and decay,

And so, in the darkness, they wait for the end.

Long before they had realized it was coming

They knew that the universe was on a path of inevitable decline.

Methodically they hunted for a final place to wait out eternity.

And embarked on their last great feat of engineering.

Around a lost and lonely black hole, they built a new home. With demolished worlds as raw material, they

constructed a shell to completely enclose the darkness.

And within this thin shell, barely withstanding the gravitational grip of their savior,

They eked out their meagre lives.

The dwindling light of dying stars rained down upon their final home, Whilst the swirling black hole was harvested

to power their existence. But more than that, the black hole at their

heart gave them greatest gift of all, For the black hole gave them time.

No one remembered the name of the great scientist who had discovered the nature of time.

But the astro-engineers knew that time was not the same across the cosmos.

And here, within the immense gravity of the black hole, time trickled more slowly.

Whilst many years passed outside, mere moments flashed by within the immense sphere.

And so the last of the civilisation watched the future play out in front of them.

But they knew that they had only delayed, not averted, their ultimate demise.

And the darkness would inevitably envelop them forever and ever.

Of course, this story is little more than speculation.

But it is built on a scientific idea that changed our universe.

It’s been more than a century since Einstein’s relativity shook up our understanding of time

and space. But how does it really work? And what does

it actually mean? Both time and space seem so commonplace, so

obvious, so everyday. But beneath their ubiquity they hide a multitude

of unanswered questions - questions to which Einstein's theories only begin to answer.

What is space made of?

Does time exist?

And will hunting for their ultimate nature lead to sudden clarity, or will space and

time just become more elusive?

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As the lonely world lingered on,

Its beating heart warped the very fabric of the universe around it.

The civilisation had done everything they could to keep going, to put off the inevitable.

But try as they might, they could only bend reality. They could not break it.

"Behind it all is surely an idea so simple, so beautiful, that when we grasp it - in a

decade, a century, or a millennium - we will all say to each other, how could it have been

otherwise? How could we have been so stupid?"

What is space?

The question seems almost meaningless. As children we learn to describe our surroundings

as up-down, left-right, back-and-front. We call it three dimensional and are free

to explore each dimension. But just what is it, this universal platform

on which we play out our lives? It is a question that occupied the minds of

the earliest philosophers. In the fourth century BC, Plato declared that

space was the “The Nurse of Becoming”, a medium in which everything existed, but

with no qualities of its own - and his student Aristotle agreed that an empty void was impossible.

But it would be more than two thousand years before our concept of space was born.

By the coming of the seventeenth century, modern science was crystallizing.

The processes of the universe were being codified into physical laws.

And the understanding of these physical laws was evolving from myths and stories, to the

language of mathematics. Of course, Isaac Newton was at the forefront

of this revolution. But before he enters our stage, we must first

start with a boat. In 1632, Galileo published his seminal work

“Dialogue Concerning the Two Chief World Systems”.

He was in his mid 60s by this point, and had already had multiple run-ins with the Roman

Inquisition for his assertions that the earth rotated around the sun.

So he had decided to skirt controversy, and spend the intervening years quietly cementing

his myriad ideas on space and the cosmos into a book.

And at one point within this book, he muses on a boat.

More specifically - the life of a sailor locked below deck in a windowless cabin.

With plates and knives and a goldfish in a bowl on the table - a collection of birds,

flies, and butterflies. Just what does the sailor experience?

Tied up in port, the cabin is a picture of serenity, and all is calm as the goldfish

swims happily in its bowl. On the table, plates and cutlery remain in

their place, and the flying creatures happily flutter about.

But finding itself in rough seas, the cabin heaves and falls with the ship.

Plates and cutlery are wrenched off the table, water spills from the goldfish bowl.

On calm seas with wind-filled sails, the ship would speed up.

The sailor would feel this change, and see things sliding off the table.

But when the wind finally drops, the ship sails smoothly on the glassy sea.

Inside the cabin, all would be calm, serenity returning.

For the sailor, it would be as if the ship was still in port and was not moving at all.

A dropped plate would fall straight to the floor, and the sailor would sit comfortably

in their chair. And it was here Galileo realised something.

Without a window to reveal the truth, there are no experiments the sailor could do to

reveal whether the ship was moving or not. He concluded that there must be no absolute

concept of being at rest in space. Instead, everyone must experience any smooth,

uniform motion in the same way. All uniform motion must feel like simply being

still. Galileo declared, therefore, that any uniform

motion is simply relative to any other uniform motion.

And with this, the first theory of relativity had been born.

Galileo´s sailor floats gently on their sailboat, on seas near the earth's equator - rotating

at 1600 km an hour around the earth, which in turn orbits the sun at 67,000 km an hour,

which in turn orbits the milky way at 720,000 km an hour, which in turn is travelling towards

the Andromeda galaxy at 403,000 km an hour. And yet he feels nothing on his vast journey

millions of kilometres from his starting point. Unfortunately upon publishing the book, Galileo

once again fell foul of the Catholic Church - and was found guilty of heresy for his heliocentric

view of the cosmos. The work was banned, and would not be removed

from the church´s Index of Forbidden Books until 1835.

Within a few decades of Galileo´s downfall, two of Europe’s greatest minds were arguing

about the nature of space. One of them, Isaac Newton, was born in England

in 1642, within a year of Galileo’s death. He needs little introduction, and is known

now as one of the greatest thinkers of his age, perhaps one of the greatest of all time.

Whilst not forgotten, his opponent, Gottfried Leibniz, is somewhat less well known today.

Born in 1646 in what is present-day Germany, he was a leading thinker of his day, writing

on mathematics and philosophy. He pondered deep metaphysical questions, including one

that still haunts physicists and philosophers to this day - why there is something rather

than nothing. It was in the development of calculus that

the two men´s feud began. Whilst Leibniz published his work first, Newton

claimed that he had stolen his ideas. As president of the Royal Society at the time,

Newton set up a committee to investigate the dispute.

Unsurprisingly the committee found in favour of Newton.

And so this animosity carried over to their second disagreement.

A simple question: What happens to a spinning bucket of water?

Space, Newton declared, was a universal absolute, a rigid stage on which motion was played out.

And both would exist in a universe devoid of matter to experience any motion.

To argue his point, Newton asked us to think of a bucket of water.

If the bucket sits at rest, the surface of the water would be flat and level.

But if we spin the bucket, the water spins too and its surface becomes curved.

Newton asked “Just what is the water spinning with respect to?”

Newton claimed that the acceleration of the spin was relative to an absolute space - something

separate to the object itself - spinning a bucket in an empty universe would also curve

the surface of the water. But to Leibniz, space in an empty universe,

devoid of any matter, simply made no sense. The properties of objects, Leibniz claimed,

are essential in defining their meaning. Space only has meaning, in the relative locations

of objects. And similarly, time only had meaning when

discussing the relative motions of objects. Without matter Leibniz said, space and time

simply have no role, and hence no existence. Sadly, Liebniz died in 1716, with the argument

still in full swing - but it was Newton's ideas that stuck.

“Absolute space, in its own nature, without relation to anything external, remains always

similar and immovable, Absolute, true, and mathematical time, of itself, and from its

own nature, flows equally without relation to anything external."

This so-called “absolute space and time” would be the accepted science for nearly two

centuries - but with the caveat of Galileo´s rejection of absolute rest. Absolute space

may have won the debate - but absolute rest, a fixed point - was still an impossibility.

Relativity was still part of the argument. But that only applied to space.

Time was a totally different matter. With its implicit direction, time appeared

totally distinct. For Newton and Galileo, everyone’s clock

across the universe ticked with absolute synchronicity. A universal beat that ran through every event

in the cosmos. A second on Earth the same as a second everywhere

else. But is this true?

Is time malleable or an unswerving metronome that drags the cosmos forward? Does it itself

have properties or is it defined only by the events that run in its current?

To answer these questions we must begin not with physicists wondering about clocks, rulers

and motion. But with heat.

In the distant future universe, around the aging black hole, our dying civilization sits

and waits. For sitting and waiting is all they can do.

With the passing of the stars, raw energy had become the most precious thing.

To preserve what they had, they had slowed their very existence.

Every aspect was focused upon survival, as their sleepy eyes watched the ever darkening

skies. As total universal heat death crept across

the cosmos, They realised that time was their ultimate enemy.

"You may see a cup of tea fall off a table

and break into pieces on the floor… …but you will never see the cup gather itself

back together and jump back on the table."

What is time? Like space, the nature of time occupied the minds of many ancient thinkers.

In ancient Greece, Aristotle stated that time was simply the steps between before and after,

whilst Hindu philosophers saw time as cyclical, from creation to destruction over four billion

years. But it's true origin remained elusive.

Like space, time seems to be something obvious, something that is just present.

But it is clearly a different beast - we cannot freely travel through time.

Unlike space, time has a direction - a distinct past and coming future.

As with space, scientists can be pragmatic and not worry about the nature of time.

Coupled with a ruler, a clock completes the experimenter’s toolbox.

But it doesn’t mean we can ignore the question. And to understand time fully, we first have

to think about horses and steam engines. The coming of the industrial revolution presented

humanity with a problem. The original engines of civilization, draught

animals like horses and cattle, were relatively simple things.

Understanding how much to feed them and how much to work them was easy.

A certain number of bales of hay could guarantee a day’s work from a well-fed animal.

But what of new-fangled machines, such as a steam engine?

How much work can you get out of a heap of coal?

This was an important question from an economic standpoint.

Do you replace a horse with an engine if it is going to cost more to feed it?