What Actually Are Space And Time? - VoiceTube: Learn English through videos!

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A hundred trillion years from now, the last of a great civilisation hides in the darkness.
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?
And it was out of this conundrum that thermodynamics was born.
Many minds wrestled over the question of the ultimate efficiency of engines.
Indeed - at the time of thermodynamics inception, a typical engine only worked at 3%.
In a physical steam engine, the heat of the fire is used to boil water,
But some of the fire’s heat just radiates into the air.
Metal scrapes against metal, screeching loud and hot to the touch - both forms of energy
loss. In any physical steam engine, this loss of
heat as waste is inevitable. Within the mathematics of thermodynamics,
perfect efficiency was found to be an illusion. Energy is always lost as heat flows from one
place to another. The concentrated energy released from burning
coal must be degraded as it flows through the engine and some must be lost into the
surroundings. And so a new measure was introduced to account
for this increase of decay and disorder. Entropy.
And it is probability that dictates how this happens.
As the early 20th century physicist George Gamow put it:
"For exactly the same reason the room in which you sit reading this book is filled uniformly
by air from wall to wall and from floor to ceiling, and it never even occurs to you that
the air in the room can unexpectedly collect itself in a far corner, leaving you to suffocate
in your chair. However, this horrifying event is not at all physically impossible, but only
highly improbable." Gamow goes on to give the waiting time of
such an event - trillions upon trillions times longer than the entire age of the universe.
Disorder is always statistically far more likely.
Through the new laws of thermodynamics physicists revealed an inexorable growth in entropy as
the universe marches on - a future universe destined to be more disordered and decayed
than today’s. Not only steam engines, but whole planets,
stars, galaxies, filaments - all marching from order to disorder.
It was in 1862 that the grim logical endpoint of these ideas was proposed, by Lord Kelvin,
for whom the measurement unit was named: "...although mechanical energy is indestructible,
there is a universal tendency to its dissipation, which produces…exhaustion of potential energy
through the material universe. The result would inevitably be a state of universal rest
and death, if the universe were finite and left to obey existing laws."
And so, was this time? A constantly dying universe heading for inevitable heat death?
Stars going out one by one in a steady march from potential energy to waste leaving the
trillion year old universe dark and spent? One of the great minds to occupy themselves
with entropy and the arrow of time was James Clerk Maxwell, the iconic Scottish nineteenth
century scientist. His views on thermodynamics shaped our understanding
of heat and gases - and he did all this with the assistance of a demon.
Maxwell understood the implications of entropy. He knew that if he mixed two gases, one hot
and one cold, the result would be warm gas. And he knew that the gas would stay warm and
mixed rather than separating into two halves, with hot gas in one and cold in the other.
But he wondered - what if we introduced a tiny demon who can sense each and every atom
in the gas. This demon can turn around atoms, directing
slow atoms to one side, and fast to the other. As the temperature of a gas is a reflection
of the average speed of its atoms, The demon has effectively separated the warm
gas into two unequal halves, one hot, and one cold.
The demon seems to have broken the laws of thermodynamics.
It has taken the disordered state, the warm gas, and created a more ordered state, hot
and cold gas. And whilst only a thought experiment - arguments
over the meaning of Maxwell’s demon have raged for over 150 years.
Some have stated that the demon must be expending energy to sort the gas atoms,
And so total entropy will continue to rise. However - some have proposed that it is not
energy that is important, But the fact that the demon uses information,
namely the speeds of the atoms, to do the sorting.
Linking energy, entropy and information might seem a little strange,
But over the last three-quarters of a century, This link has become stronger and stronger.
And as any touch of a computer will tell you, processing information generates a lot of
waste heat. But the situation is more complex than that.
It is not simply the processing of information that leads to waste heat, but the forgetting
of information. When we add three and two, the answer is,
of course, five. But if I told you an answer was five and ask
what two numbers are summed together, you cannot answer.
In a computer, logic gates combine electronic signals to do the addition -
Whilst two numbers are fed in, they are forgotten as the single answer is spat out.
The calculation is irreversible, the inputs lost to the universe.
And, in the action of forgetting, the logic gates heat up a little.
Thermodynamics therefore provides us with the ultimate limit for forgetting.
Called the Landauer limit, it is the inevitable release of energy from erasing a single bit
of information. And at room temperature it is just over 100th
of an electron volt. Proven experimentally in 2012, scientists
believe that at present computer chips produce thousands of times more heat than this limit
- but by 2035, they will most likely reach it.
that tiny bit of waste heat inescapably increasing the entropy of the universe.
Ultimately, across the universe, it is this irreversibility of calculations that drives
entropy to increase. forgetting information is therefore an essential
ingredient for defining an arrow of time. Does this mean that for yesterday and tomorrow
to have meaning, we must forget? Is the existence of the future implicitly
tied to our inability to remember? And it is now we can return to our lonely
civilization on the brink of universal heat death, in the far distant future…
When all useful energy is used up, and entropy is at maximum - would time even have any meaning?
Fundamental physics does not yet have a definitive answer, but it is an intriguing possibility.
But we have now reached a turning point. The foundations of time and space can only
get us so far - and though they are useful, there is a revolution coming.
A new order that will lead directly to the last days of our lonely black hole world.
As we continue in our journey, we are going to have to explore new time, and new space.
Within their black hole shell, many of the civilisation resigned themselves to their
fate and dozed their way to the end. But a few curious minds, with their dwindling
energy, still wondered about the universe. Great books that had existed for almost eternity
told them how space could bend and ripple, Central to these books was the story of light.
They knew that light’s speed was immense, and had used it to help measure their empire.
They knew that light was a limit they could never break, no matter how hard they had tried.
And they knew that the speed of light had been the first step in the long journey to
understand how the universe really worked.
"When you are next out of doors on a summer night, turn your head towards the zenith. Almost vertically above you will be shining
the brightest star of the northern skies—Vega of the Lyre, twenty-six years away at the
speed of light, near enough to the point of no return for us short-lived creatures…for
no man will ever turn homewards beyond Vega, to greet again those he knew and loved on
Earth." The speed of light has always been mysterious.
Early experiments in flashing lights back and forth had shown that it must be much faster
than sound. So scientists wondered - was it infinite in
speed? It was in 1676 that Danish astronomer Ole
Romer finally found the answer. Romer was observing the moons of Jupiter as
they circled the giant planet. And timing just when they entered the gas
giant´s planetary shadow. He had assumed that the orbits ticked like
clockwork, And so would be able to predict just when
the eclipses of the moons would begin and end.
But as he observed the moon Io throughout the year, his predictions got steadily worse,
and then better again. It became clear that the accuracy of his predictions
depended upon our distance to Jupiter, And he would need to include the extra time
taken by light having to travel further. And so with Romer´s data fellow astronomer
Christian Huygens calculated that light must move at more than 211,000 km every second,
not far off our modern estimate of about 300,000 km per second.
Romer’s observations confirmed that light was fast and finite - but precisely what light
was would have to wait for two centuries - for as well as the confusing implications of James
Clerk Maxwell's demon, he is also famous for intertwining electricity and magnetism into
a single idea - electromagnetism. Light, he found, was nothing more than a self-propagating
combination of the two - and written too into his equations was light’s blistering speed.
But there was still a problem. Just what was this speed relative to?
Maxwell’s equations gave no answers, so physicists began to search for a solution.
Perhaps, they hypothesised, light travelled in an invisible medium? A mysterious ether
permeating the entire cosmos? But that would also imply an ultimate state
of rest in the universe - a worrying thought, as that would break Galileo’s relativity.
The problem was severe - so whilst one group of physicists set out to measure the properties
of this supposed ether, others took the evidence in front of them and made an even larger leap.
And chief among them was a young Albert Einstein. Einstein wondered why electricity and magnetism
would not obey Galileo’s relativity. Why should experiments specifically using
the flow of electricity or spin of a magnet reveal absolute motion?
In a bold step he declared that they cannot. And with that, the special theory of relativity
was born. On Galileo’s ship, Einstein proposed, all
experiments would yield the same results, whether the ship was secured in port, or smoothly
sailing on a glassy sea. Throwing a ball would of course not reveal
whether the ship was moving But neither would measuring the speed of light!
The speed of light in a vacuum was constant - no matter the source.
This final statement seemed to fly in the face of the universe as laid out by Newton.
In Newtonian mechanics you could simply add speeds together.
And each observer would measure differing speeds dependent upon their own motion.
But according to Einstein, this was not the case for light.
Everyone would measure the same speed. Whether the ship was stationary, going at 50 knots,
or 50,000. However if this was true, something else had
to give - and the only freedom in the equations was the very nature of space and time themselves.
To work, each observer must have their own measurement of space.
And each observer must have their own measurement of time.
With special relativity, it was the speed of light that was absolute - not space and
time. Space and time were no longer the universal
stage on which physics played out. And just as Maxwell had combined electricity
and magnetism, Spacetime too was about to unite.
“Gentlemen! The views of space and time which I wish to lay before you … They are
radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere
shadows, and only a kind of union of the two will preserve an independent reality.”
In 1908 Herman Minkowski, Einstein´s former professor, came up with an idea.
In reaction to the revelations of special relativity in 1905, he had decided to explore
the geometry of these new equations. In Einstein’s formulation, space was space
and time was time, And to transform from one observer’s viewpoint
to another, you needed to mix the two together. But Minkowski pointed out that it was simpler
to mix space and time together - into spacetime. And to transform one observer’s spacetime
to another through geometry. And so finally, combined spacetime was born.
This new melding of the three dimensions of space and one dimension of time has come to
be known as "Minkowski Space" - though Minkowski himself tragically died in 1909 before his
idea had been fully embraced by the physics community.
Newtonian space and time had been completely upended - but Einstein was still not happy.
Though his ideas had revolutionized our ideas of space and time, they could not account
for gravity. Newton’s gravitational equations needed
the distance between masses And special relativity now told us that no
one could even agree on what these distances were.
So he went back to the drawing board and spent a decade thinking about gravity.
What eventually emerged from these ruminations in 1915 was a solution that shocked physics
to its core. Einstein took Minkowski’s geometric picture
of spacetime, and made both space and time bendy and stretchy, the presence of mass and
energy producing the curvature. Within his general theory, Einstein concluded
that gravity, as a force, simply did not exist - the effects of gravity were encoded within
the curvature of space and time. Newton’s picture of space and time was well
and truly dead, for not only were space and time relative, they were flexible as well.
The consequences of Einstein’s vision of relativity were quickly uncovered.
In the special theory the relative tick of clocks depended on motion.
And whilst everyone feels time passing at one second per second,
Different clocks will tick off different amounts of time.
With the coming of the general theory, time was shaken even more,
As where you are also influences the tick of your clock.
The presence of mass curves space and curves time,
And so gravity can dictate the relative ticking of a clock.
In 1916, Karl Schwarzschild solved the field equations of relativity for a spherical mass,
and written inside his equations was a completely collapsed mass, squeezed into a point,
Whilst it did not get its name for another fifty years, Schwarzschild had the mathematics
for a black hole. Schwarzschild’s solution showed that black
holes bend both space and time - and with this intense curvature comes intense gravitational
pulls - not even light able to escape. In the vicinity of a black hole, where gravitational
fields are immense, Time becomes more and more curved as you get
closer to the centre. Compared to clocks in the distant universe,
near the heart of darkness time ticks very slowly.
And it wasn't just black holes that sprung from the new equations.
In the century since Einstein’s gravitational insights, many more bizarre solutions have
been found. Throughout the relativistic literature there
are wormholes, warp drives and even entire curved universes.
All built from the malleable nature of space and time.
In 1919, observations of the deflection of starlight proved his theory and made Einstein
a scientific superstar - and so scientists turned their attention to measuring the effects
of general relativity exactly, to further cement the concept.
One of the weirdest of these experiments was undertaken by Joseph Hafele and Richard Keating
in 1971. Their equipment was a series of accurate caesium
clocks, and a set of jet plane journeys that completely encircled the Earth.
To begin the experiment, all the clocks were placed in the same location and synchronised.
Some of the clocks then headed off on a plane, some heading to the East, and others to the
West - some moving with the Earth’s rotation, others against it. $7600 was spent on flights,
with two seats on each plane going to “Mr Clock.”
And because they were flying, they were in a different gravitational field to the clocks
left behind on the ground. After the clocks had circled the world twice,
they were all brought together. If the universe was governed by Newton’s
absolute time, they should all have remained in sync.
But if Einstein was correct, relative motions and spacetime curvature would have desynced
them. The experiment was run, and the clocks were
reunited. They differed by a few hundred nanoseconds.
Einstein was declared the winner. But there is one more test of relativity that
has proven to be the most spectacular. In developing relativity, Einstein found that
stretchy spacetime can wobble and ring. Just as Maxwell found that electricity and
magnetism can ripple, so could gravity. But he could not decide if his mathematics
were correct or if he was fooling himself And struggled to conclude whether these gravitational
waves were part of reality. In 1974, Russell Hulse was a young astronomy
student who made a spectacular discovery. With his supervisor, Joseph Taylor, he was
peering at the universe with the 300m Arecibo Telescope, and he found a pulsar, a rapidly
spinning dead heart of a star that flashed radio waves.
This pulsar, PSR B1913+16, was spinning 17 times a second - and was not on its own, but
orbited another dead star heart, a neutron star.
And with the regular beeps of the pulsar, they were able to accurately chart out the
cosmic dance. What they found, however, was quite unexpected.
With Newtonian gravity, these dead stars would orbit each other for eternity,
But Taylor and Hulse found that the orbits were shrinking,
And the stars were slowly but steadily being drawn together.
Somehow the energy of their orbits was leaking out into the universe.
Taylor and Hulse realised Einstein’s gravitational waves were an ideal culprit.
They delved into the mathematics of general relativity,
And calculated how the orbiting stars form ripples in spacetime - showing how they carry
away precisely enough energy to explain the orbital demise.
In 1993, Taylor and Hulse received the Nobel prize for their discovery - and 24 years later,
the prize was awarded for the direct detection of gravitational waves.
The experiment was the Laser Interferometer Gravitational Wave Observatory, or simply
LIGO for short, which with unimagined sensitivity, can feel the tiny ripples of spacetime.
LIGO has opened a new and exciting window on the universe
They are uncovering merging black holes and the collisions between neutron stars.
And now astronomers even plan to hunt for the oldest gravitational waves, formed in
the birth of the universe. And so, in this new world ushered in by Einstein,
it is clear that the entire cosmos is written in the language of gravity, of curved and
warped space and time. But there was one more secret to uncover hidden
in the equations. First realised by Alexander Friedman in 1922
and later proved by Edwin Hubble, the expansion of the universe is the expansion of space
- expanding from an infinite point 13.8 billion years ago known today as the Big Bang.
Put simply: there was less space yesterday, and there will be more space tomorrow.
Every galaxy is moving further and further away from us, bar our local group, at an average
rate of 70 km/s/Mpc - which actually means that at the moment, for every 3.26 million
light-years distance from us a galaxy is, it is moving away from us at an extra 70 km/s/mpc.
So a galaxy 326 million light years from us is moving at 7000 km/s. And a galaxy 32.6
billion light years away? It recedes from us faster than the speed of light.
This may seem bizarre, after everything we have learnt up until this point - but the
universe´s speed limit only applies to objects moving through space - and these galaxies
do not move through space. Space simply gets between them.
This expanding universe makes curving and bending spacetime even more complex to understand.
As equations show that space is infinite, what is happening is that the universe is
actually becoming less dense. And clearly, this decrease in density is not
completely uniform across the universe. You, for example, are not slowly drifting
apart. Individual galaxies too hold themselves together
due to their mutual gravity, But as this gravity is a manifestation of
the curvature of space, What happens at the boundary between expanding
and non-expanding space? And that is not the only headache - as expanding
space makes the form of yesterday’s spacetime different to tomorrow’s spacetime - thus
breaking what was thought to be one of the key properties of the universe - conservation
of energy. The importance of symmetry in physics was
laid out in detail by mathematician Emmy Noether - in this case a symmetry meaning that when
you change your situation, the physics remains the same.
Changing location doesn’t change physics, meaning momentum is conserved.
And the fact that physics is the same today and tomorrow gives energy conservation.
But in an expanding universe, where spacetime is changing, this symmetry is shattered.
As space grows it doesn´t stretch - it doesn´t dilute.
There is just more of it. But as they travel across an expanding universe,
photons are stretched, and they lose energy - and galaxies are robbed of their speed as
their motion grinds to a halt. Energy is simply not conserved as the cosmos
grows, and this is a conundrum that causes problems for physicists to this day.
And so, it may now seem that space has finally become physical, real - it can bend, expand,
curve and ripple. But there is a final twist, one final rug
to be pulled out from beneath us. It can be summed up in the words of the Nobel
prize winner Steven Weinberg...
To the novice, this statement must seem almost
bizarre. How can a leading scientist make such a claim?
Well, because he is absolutely correct – in Einstein’s relativity, spacetime is truly
nothing. The mathematics look like bending and curving.
But in reality, relativity tells us space is nothing and has no properties.
But what of time in this new picture? How were seconds, hours and minutes affected
by the dawn of relativity?
To the future civilization, time meant many things. They knew that their time was unique, unshared
by any others. They understood that clocks ticked differently,
Dependent on where you are and what you are doing.
Their engineers had used this malleable nature of spacetime in shaping their civilization.
Great portals of distorted time and space allowed travel across the empire.
Whilst the slow ticks near the gravitational pull of a black hole had been used to slow
time and allow them to watch the end of everything.
"People like us who believe in physics know that the distinction between past, present, and future is only a stubbornly persistent
illusion." WIth the coming of Einstein’s general relativity,
physicists were presented with a new headache. They knew that every particle in the universe
had a past, present and future. And like a line drawn on a map, they could
chart the journey of a particle through the four dimensions of space and time, tracing
out its worldline from the past to the future through a series of nows.
Each particle in your body, each electron and quark, journeys on its own worldline.
Before you were conceived, the worldlines were dispersed.
But as you grew, many wordlines condensed into a bundle which is you.
And when you are gone, these worldlines will again scatter.
For a fleeting moment in the life of the universe, You exist as little more than a collection
of wordlines, a brief knot in the fabric of eternity.
Whilst unsettling, this appears to make sense, so where is the headache?
Firstly, we have to remember what the relativity of time really means.
With no absolute time, there is no uniform cosmic clock,
And this means that there is no such thing as a unique present, a true instant of now.
Without an absolute definition of a cosmic now, how do we define a unique notion of the
past? Without a now, just where does the future
begin? Headache.
Within the equations of relativity, all pasts, presents, and futures are already written.
The entire history of all things is already out there - somewhere.
This notion, known as the ‘block universe’, has bothered many physicists and philosophers,
as without a now, the cosmos cannot simply unfold from moment to moment.
All we can do as we trace out our worldline is follow our predefined path.
And concepts dear to us, such as free will, are lost.
But this cannot be correct. We clearly remember the past, and the future
is a mysterious door that has yet to be opened. They are clearly different.
Or are they? Consider two electrons hurtling towards each
other. Both carry an identical negative charge, and,
through electromagnetism, they repel. As they get closer, the repulsion grows and
their motion gradually slows, stops, and reverses. Eventually, the electrons hurtle away from
each other, back the way they came. There seems nothing strange about this.
But imagine we filmed the interaction between the two electrons.
And then showed the film to an audience of physicists - playing a mirrored version, the
left switched to right and vice versa. Your audience of physicists would still notice
nothing amiss with the movie on the screen. Switching left and right does not alter the
physics. On the screen, the electrons approach and
repel - all appearing to be completely normal. But what if you went one step further - what
if you were very clumsy and instead of switching left and right you switched past and future?
The film now runs backwards. Time has been reversed.
Your audience stares at the screen. What do they see?
In this time-reversed movie, two electrons hurtle towards each other.
They get closer and closer, with their repulsion growing.
Eventually, they halt in their motion and start to move away again.
With nothing out of the ordinary, the audience nods in approval at this simple display of
physics. But how is this possible?
If you had run the slapstick of Laurel and Hardy backwards, the viewers would have noticed
- and would immediately know that something was wrong with the arrow of time.
And herein lies the question: why is the electromagnetic interaction between electrons insensitive
to the direction of time? And not just electromagnetism, but gravity
and the strong nuclear force are also unaffected. The weak nuclear force does misbehave slightly
- but it is a very tiny effect. It seems that at their core, the universe’s
microscopic fundamental interactions do not possess an arrow of time.
Time could flow one way or the other, and they simply would not care.
But this leaves us with a disconnect. The macroscopic, large scale world we inhabit
certainly does know about time. Cooling coffee, burning wood, exploding supernovae
- these are not processes that simply can be run backwards. You cannot unscramble an
egg. With a little thought, this seems a little
bit strange. Our large scale world is nothing more than
the collective properties of an uncountable number of atoms.
And these atoms are interacting through a fundamental force, electromagnetism, each
of the myriad of electromagnetic interactions unaffected by the direction of time.
How can such an arrow emerge from the multitude of time ignorant interactions that take place
every second? How does time emerge?
Some have claimed there is a definitive arrow, an imprint of a cosmological arrow of time.
In the simple view of the block universe it stretches infinitely far into the past, and
into the future. But this block universe clearly doesn’t
appear to resemble our own. For we know that our universe didn’t stretch
infinitely into the past – it had a beginning. From observations, we know that the universe
was born almost fourteen billion years ago. We don’t know the process that brought it
into being, but it was born with both space and time.
Just where and how space and time came to be in the universe remains a mystery.
But they have remained an integral part of the cosmos over all of its history.
But there are other mysteries about the birth of the universe that we don’t understand.
And in particular, it appeared to be extremely special, being both hot and dense, and strangely
smooth. And this smoothness meant that the newborn
universe had a very peculiar property. The universe was born with very low entropy.
It might seem strange that smoothness implies low entropy.
As a gas spread throughout a room has higher entropy than gas all squeezed in one corner.
But for matter in the universe, this smoothness meant gravity could do its work, and fall
together and eventually clump into stars and galaxies.
And so as the universe expands, its entropy increases as the matter evolves.
Gravitational potential energy is steadily converted into stars, planets, and people.
Eventually, this energy is processed into waste heat that spreads throughout the universe.
And it is this change from low to higher entropy that imprints onto the cosmos its arrow of
time. Recent Nobel Prize winner, Sir Roger Penrose,
has thought hard about our universe’s initial entropy.
He concluded that the probability of this occurring by chance is one part in 10 to the
10 to the 123. Clearly, there must have been something special
about our universe’s birth. But what this was, we still don’t know.
And so would this mean that the block universe has no innate arrow of time?
Without the big bang would it be impossible to distinguish the past from the future?
Imagining how we would experience such a universe is very difficult to do.
But indeed, maybe our ability to imagine anything at all is ultimately because of the special
birth of the universe.
On the tenth of June 1944, a British Halifax bomber was flying over France. With four hundred other bombers, it was supporting
the D-Day landings in Normandy. But near the city of Laval, the aircraft was
struck by German flak. And crashed in flames into the French countryside.
The entire crew perished in the crash. Seven lives were lost, seven lives in a war
that eventually claimed millions. The pilot was a thirty-three-year-old Dutch
volunteer, Willem Jacob van Stockum. And whilst his name is not familiar today,
van Stockum was the man who discovered time travel.
Of course, by the 1940s, time travel was a staple of science fiction.
The Time Machine by H. G. Wells had been published half a century earlier.
But this was all fantasy and whimsy - and a firm impossibility in Newtonian space and
time. Yet within the new world of relativity - van
Stockum had discovered a scientific basis. Mathematically, relativity is notoriously
challenging. Einstein himself had wondered if his field
equations would yield any analytic solutions. But merely a year after presenting them to
the world, the first such solution was found, as Schwarzhild derived his black holes.
And so by the 1920s, the hunt was on for the mathematical form of the entire universe.
Along with the giants of relativity - Einstein, Friedmann, de Sitter and others - laying down
rules, finding expanding space which itself could
be curved and even space where parallel lines could converge and diverge - some scientists
explored the mathematics of hypothetical universes. And Hungarian mathematician Cornelius Lanczos
had found a rather peculiar solution. His equations described a universe of dust
that was rigidly rotating. And whilst it didn’t appear to describe
our actual universe, it was an intriguing result.
van Stockum began to wonder about the journey of particles through such a rotating universe.
As particles traveled from the past to the future, worldlines stretched around the universe.
But as the universe rotated, time and space were stretched and distorted.
And the nature of time itself became indistinct. Some worldlines stretched right around the
universe and met themselves, forming closed loops.
Along these, the future trod over the path of the past, over and over again.
Physicists call these closed-loop worldlines time-like paths.
But in everyday language, it was nothing less than time travel.
That space and time can be so warped as to allow time travel was shocking.
And whilst the rotating universe might not be physically realistic, it opened up the
question of whether there were other routes to the past
Or shortcuts to the future. van Stockum’s goal was to head to Princeton
to work directly with Einstein. But as the clouds of war were gathering, he
looked back to Europe. Once his homeland was occupied, his desire
was to get into the fight. And van Stockum’s own worldline ended in
a French field on a dark night in 1944. Whilst van Stockum’s name is now lost to
history, time travel and rotating universes are not - as they were rediscovered by the
eccentric mathematician Kurt Godel, in 1949. Godel is remembered today as one of the greatest
logicians of all time, and his famous incompleteness theorem still baffles today.
But his contribution to physics was equally shocking.
Escaping the turmoil in Europe as the storm cloud of the second world war gathered, unlike
van Stockum Godel did reach Princeton University. There he and Einstein became firm friends,
with Einstein supporting his application for American citizenship, specifically by distracting
him from pointing out flaws in the United States constitution to the judge seeing his
case. It was at Princeton that Godel turned his
remarkable mathematical mind to relativity, and the nature of spacetime - and in 1949,
Godel’s 70th birthday present to Einstein was a solution to the field equations of relativity.
Like Van Stocken, he had found the mathematics of a rotating universe - and closed time-like
curves that looped around his cosmos! On receiving his present, Einstein was, in
his own words, “disturbed” by the possibility. Godel´s wife had apparently knitted him a
sweater too, but it was not part of the final gift. History does not recall why.
Einstein died soon after, in 1955 and Godel followed him in 1978.
As an old man, Godel asked astronomers if they had found if the universe was truly rotating.
The answer was always “no it isn’t” - and that Godel’s universe is not our own.
But the possibility that Einstein’s relativity potentially allowed time-travel sent researchers
back to their equations. Could time and space really be bent back on
themselves so far to allow temporal exploration? Physicists have continued to find mathematical
shortcuts through space and time, and there are now many solutions to Einstein’s
equations in which space and time are extremely warped.
It would seem that in Einstein's relativity, time travel remains a stubborn theoretical
possibility. As an example, if you add spin to a black
hole, space and time twirl also. And if you dive right through the centre,
you might emerge somewhere and somewhen else. Another relativistic structure, a wormhole,
builds a spacetime bridge between two locations, And potentially between two different times
- but not necessarily a shortcut. So time travel appears to be written into
the equations of relativity. The reality of these solutions, whether they
can truly exist, remains unanswered. Perhaps we will never be able to focus enough
energy into a single place For spacetime to bend right back on itself.
We now understand how Einstein's space time works - but we still don’t know what it
is. Where can we turn next?
Well - it was not just Einstein who was charting a new path at the beginning of the 20th century.
It was a dramatic period for theoretical physics - and quantum mechanics was at the forefront
of the changing order. And so - perhaps, physicists thought - the
answer could lie at the smallest scales in the universe.
In the far future, the civilization had become desperate.
The stars had long died, and matter itself was starting to melt.
Very few remained now, almost frozen in the darkness.
The last of life grinding to a halt. But some eyes still stared into the skies,
To witness the last bursts of light in the universe.
The great books had told them this time would come,
Warning them that not even black holes would last forever.
Whilst the immense gravity of relativity held them together…
On the smallest scale, the action of the quantum world resulted in their ultimate decay.
For eons they had struggled to combine the two - the world of gravity had seemed so distant
from the quantum. And so too their black hole home was dissolving.
They could do nothing to stop it. Indeed, the last few were so very tired,
They didn’t even try.
"A university student attending lectures on general relativity in the morning and others on quantum mechanics in the afternoon might
be forgiven for thinking that his professors are fools, or have neglected to communicate
with each other for at least a century." There is a grave at Roselawn Cemetery in Tallahassee
Florida. Written on it is the name of a man who died
in 1984, aged 82. Unlike the others in the graveyard, the man
also has a plaque at Westminster Abbey, Not far from the mortal remains of Isaac Newton.
The plaque does not say much. It labels the man as a physicist and notes
his birth and death. But on the plaque is also an equation, a complex
mix of Latin and Greek letters. And this equation was the first unification
of Einstein’s relativity and quantum mechanics. The famous physicist Niels Bohr referred to
Paul Adrien Maurice Dirac as the strangest man to visit his institute.
Born in Bristol at the beginning of the twentieth century, he did not at first seem destined
for scientific greatness. In 1923, Dirac began his studies at the University
of Cambridge. famously focused on his science, He shunned
many human interactions, and his conversations were mainly silent. His colleagues named the
unit of one word per hour as a “dirac” in his honour.
But whilst speech was slow, his mind raced around the problems of physics.
It was a heady time to be a physicist, with both Einstein's new world of relativity and
the bizarre implications of quantum mechanics opening up - were the fundamental secrets
of the universe finally revealing themselves? When Dirac began his exploration of quantum
mechanics it was written in the past. The mathematics of Schrodinger and Heisenberg
played out on the stage of Newton. With the tick of an absolute clock, and Galileo’s
vision of space. But Dirac knew that this picture of space
and time was simply outdated. Surely the equations of quantum mechanics
should reflect Einstein's new visions of space and time.
This bothered Dirac, and he scrambled with the mathematics trying to make it work, spending
his Sundays walking alone turning over the equations in his mind.
And in December 1927, the fog began to clear. A relativistic quantum equation came into
view. An equation that obeyed Einstein’s demand
that there is no special rest in the universe. And Dirac used this equation to explain the
simplest of particles, the electron. Suddenly, various peculiar properties of the
electron made mathematical sense. Within Dirac’s equation, the electron spins
and behaves like a small bar magnet, Both properties had been difficult to explain,
but they were a natural consequence of relativity. But there was another property that was completely
unexpected. If you take the square root of one, there
are two solutions - plus one or minus one. In the same way, in explaining the electron
the Dirac equation has two solutions. One solution is negatively charged and clearly
represents the electron. But just what does the positive solution correspond
to? Dirac wondered if it could be the proton,
the positively charged particle within the nucleus.
But being almost two thousand times more massive, that could not be correct.
He eventually concluded his equation was predicting a new particle, the anti-electron.
This particle should have the same mass as the electron but have the opposite, positive,
charge. Antimatter.
The Dirac equation was the birth of quantum field theory, the most successful physical
theory. It is with these mathematics we describe all
of the fundamental particles and forces, the basis of the modern standard model.
And for each of the particles, there are anti-particles, electrons, positrons, quarks and anti-quarks.
All a consequence of Einstein’s view of relative space and relative time.
But quantum field theory is built on Einstein’s special theory of relativity.
What of gravity and the general theory of relativity?
What if we incorporate curved spacetime into the Dirac equation?
Unfortunately, after such incredible early success - the last century has brought us
no further in this quest. Within quantum field theory, the quantum wave
function that underlies existence still plays out within the arena of space and time.
Because of special relativity, this spacetime is more complex than Newton’s view, but
space and time are still the universal stage. And this stage is broken when considering
the curved spacetime of general relativity. Remember, in the general theory of relativity,
space and time are dynamic and evolving. They are not simply the stage - they are players
in the physics of the universe. Quantum mechanics was complicated enough,
but after many years of work its various infinities had ultimately been tamed.
With curving, bending, rippling spacetime - the infinities seemed uncontrollable.
With the failure to simply merge gravity and quantum mechanics, some physicists have searched
elsewhere. This has involved going back to the drawing
board, with new ideas for just what space and time are in the quest for the so-called
“theory of everything” - the so-far fruitless search to tie the microscopic quantum world
to the macroscopic world of general relativity and fully explain the universe.
And of course these theories of everything have not necessarily made things simpler.
In one of the leading contenders, string or m-theory, there might be 11 or even 26 dimensions.
But what do these ideas have to say about the fundamental nature of space and time?
Again it’s not so simple. In m-theory, space and time are part of the
fundamental structures of the universe. The strange, contorted shape of this structure
in multiple dimensions explains everything. Not just space and time, but all matter, all
radiation, and all of the forces. What are these fundamental structures made
of? M-theory doesn't tell us. Another contender for the theory of everything
is loop quantum gravity. On the face of it, this theory is even more
bizarre, with space and time being quantum phenomena.
At the tiny Planck scale, spacetime is chunky, fundamentally meshed together into a network.
And to us, this subatomic mesh has the appearance of smooth space and time.
Again, we can ask - what are these quantum grains made of?
And again, we are left disappointed as they just are.
But perhaps the solution is simpler than this. Perhaps - some have speculated - space and
time do not exist at all. Remember at the start of this story we heard
the disagreement between Newton and Leibniz. To Newton, space and time were part of reality
and existed independent of the matter in the universe.
Leibniz, however, said that it was the relationships between matter that defined space and time.
Without them, space and time would have no meaning.
And the relativistic vision of spacetime seemed to match this picture.
Einstein told us that matter defined the structure of spacetime,
And spacetime told matter what to do. We know that in the quantum picture, spacetime
appears to be lumpy. And that reality is possibly constructed of
these bits of spacetime as the universe grows. But what if space and time are not really
there? What if space and time are actually emergent
phenomena, something we experience only as macroscopic beings?
This might sound strange, but we know that we are sandwiched in the universe.
This means that we don’t feel the cosmological expansion that dominates the large-scale universe.
And similarly, we don’t feel the individual feel individual atoms as they collide with
our skin. Instead, we have a collective term, temperature
to describe what is happening. Perhaps space and time are the same?
In 1997, Juan Maldacena found a key relationship in the mathematics of string theory and gravity.
Known as the AdS/CFT correspondence, it could be accidental and of no consequence, but it
could also be pointing to something deeper, The path to uniting quantum mechanics and
gravity. But if this is the right path, something else
emerges. Through this relationship, space and time
become granular: pieces of fundamental length and fundamental time - Planck scale pixels
that set the smallest resolution of the universe. This would mean that at the smallest scales,
space and time would appear as nothing more than grains of sand on a beach.
And so, perhaps there is no space between the grains of reality - no time between one
grain and another. Perhaps to these grains, these are concepts
that make no sense - there are only their relations, how they interact.
For us, much larger than the scale of these grains, there is the concept of space.
And somehow through the relationships of the grains, we experience the experience of experience.
But underlying this, maybe ultimately, space and time simply don’t exist,
There are just fundamental bits and pieces, and their inter-relationships.
This may feel uncomfortable. Just where is the “you” in this relational
universe? Perhaps it is best to think of it like this:
Most of us have come to terms with the fact that we are physically a collection of atoms.
And somewhere in this collection, we, our consciousness, somehow emerges.
We seem to be able to live with this illusion of our being.
Maybe all we need to do is the same for the stage on which we play out our existence.
And so, we have come a long way and are approaching the end of our journey.
Space and time, our focus along our path, both seemed so natural, seemed so normal.
But we have seen that they are far more strange, more mysterious than they first appear.
Though the space and time of Newton were simple and absolute, they became more complex with
Einstein’s curving spacetime. And the quantum nature of spacetime attempts
to dice space and time into little pieces. But are we any closer to really understanding
its true nature? A lot of hope is pinned on our next fundamental
theories, That a theory of everything will eventually
shine a light on the universal stage. And maybe written into the theory will be
the true nature of space and time. Perhaps the block universe will be banished
as the universe unfolds. Perhaps quantum processes are constructing
a “now” one instant at a time. Or perhaps some process we have yet to imagine
is defining reality. But, of course, nature is not bound to reveal
its secrets. No matter how hard scientists work, they may
never reveal the fundamental truth. We must face the fact that some mysteries
might remain forever mysteries - indeed just what space and time actually are could forever
be beyond our grasp. And so, finally we return to the twilight
of the cosmos. WIthin their home, the last few watched their
black hole slowly evaporate. All they wanted was to eke out one more day,
one more moment. But eventually, the decay of the universe
could no longer be shut out. They had manipulated space, they had mastered
time, bent them to their will. But they could not defeat them.
And then, there was darkness.