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page two - continuing the story of the lucky breaks....

the birth of substance

A hundred-thousandth of a second into the Big Bang, the cooling of the Universe had started in earnest and the temperature of the particle plasma was down to just a few trillion degrees. By the time the Universe was a full second old, it had dropped to barely  a billion degrees. This made everywhere in space as hot as the centre of a supernova, an exploding star. And the plasma was half a million times denser than water, its pressure an oppressive 10²¹ atmospheres.

Yet the Universe was now calm enough for its most important ingredients - the three basic particle families of the quarks, electrons and neutrinos - to start condensing out. Also along with them, the three fundamental forces of the strong force, the weak force and the electromagnetic force.

Exactly what is meant by the idea of a "force" is itself a bit of a riddle. As we shall see, it really describes something we are likely to see happen. Under the right conditions, a particle will appear to be pushed or pulled about in a certain characteristic way. The question then becomes what is causing this behaviour? We can either talk about it as something in the make-up of the particle - a property the particle possesses that makes it do particular things. Or we can treat it as something external, something in the world that is constraining the particle to behave in a certain way.

If the cause is deemed to lie outside the particle, there is the further question of whether that something is spread out like a field, a gradient, or whether it is more located - another particle in fact. Our notion of force gets mixed up with all these possible answers. An electron is said to possess the property of charge and yet interact via photons of electromagnetic energy. And a photon is sometimes treated as a particle of energy, sometimes a field.

It is confusing. But then even the notion of a proper particle – one with mass and a definite identity, like an electron or quark – does not bear too much examination either. Already we are suggesting such particles to be not much more than a twist, a knot, in the wider fabric of Reality.

But putting aside such concerns for the moment, both particles and their forces crystallised out of the extreme energies of the Big Bang in a quite particular way. The first particles to establish a crisp identity in the cooling Universe were the quarks, particles which experience the strong force.

The strong force, as its name suggests, is the most potent form of attractive charge. It is so powerful that even with the average temperature at every point of the Universe still in the region of 10²⁸ degrees, making everything want to jiggle about violently, quarks could start gluing themselves together into more permanent structures.

Some of the quarks were bound by the strong force into pairs known as mesons. Other quarks formed triplets to make particles like protons and neutrons. A proton is constructed of two up quarks and one down quark, a neutron of one up and two down quarks - up and down being rather fanciful names to distinguish quarks of opposite electrical charge.

One of the distinctive facts about quarks is that they have fractional electromagnetic charges. Whereas the electron and its anti-matter partner, the positron, have an electrical charge of -1 and +1 respectively (negative and positive charge), the up quark has a charge of + ⅔ and a down quark has one of - ⅓. Note here also that while an electron and positron are matter/anti-matter partners, the up and down quarks are both matter particles. They thus have their own anti-matter partners, the anti-up and anti-down, with reverse charges of - ⅔ and + ⅓.

The details get more complicated. The complete story on the fundamental particles is that they come in three energy levels - three generations for each of the three families. So electrons, up and down quarks, and neutrinos, are all the lowest weight, lowest energy, versions of their families.

In the early Universe, it would have been so hot that most electrons or quarks would actually have existed as their heftier mass brethren - muon and tau particles in the case of electrons; charm, strange, truth and beauty particles in the case of the more floridly-named quarks. Only as the Universe cooled did these particles manage to shed their puppy fat and tighten up into their lowest, most stable, incarnations.

The point, however, is that the slop of energy from the collapsing inflaton field did seem to have a variety of modes waiting to trap it. The energy could be knotted a number of ways to create particles that appeared differently oriented when it came to properties such as electric charge, the strong force, mass and spin. The particles then began reacting with each other, combining, repelling, or annihilating according to their found natures.

And in another apparently circular bit of logic - which we are coming to recognise as the hallmark of self-organising symmetry-breakings - the way the particles reacted fed back to stabilise their own existence. Particles popped out of the maelstrom of the Big Bang in a variety of types and started interacting with each other. And their interactions acted as a glue which prevented them from popping back out of existence.

Due to the overwhelming binding power of the strong force, the quarks were the first to sort themselves out in this way. As said, the strong force allowed quarks to join up as either doublet or triplet structures. And once bound into such structures, the quarks themselves became protected.

The random nature of this combining means that all possible configurations would have been explored during the early moments of the Universe. So there would have been some rather unbalanced arrangements such as the three up quark mixture known as a delta plus plus particle - three positive charges squeezed into much too tight a space. Or the negative charge mix of a delta minus, made purely of down quarks. The doublet of a meson is also an unstable structure and usually decays in split seconds. But eventually sound quark arrangements did emerge from the promiscious partner swapping.

The proton turned out to be the most balanced way of binding together three quarks. As far as we know, at the current low temperature of the Universe, protons will last forever. Their two up and one down quarks balance out all the mutual attractions and repulsions the best. Neutrons are almost as stable, an isolated neutron being able to survive about eleven minutes on average. Well, that's an age compared to the mesons and other exotic quark mixes which are lucky if they last 10^-10 seconds.

As the ambient temperature of the Universe fell to about 10¹⁵ degrees, the other two potential modes of particle interaction - the weak force and the electromagnetic force - could also start to be expressed.

At this stage, electrons and positrons could appear fleetingly as a result. They would congeal out of a bath of electromagnetic radiation – raw light energy. Or alternatively, just to demonstrate that all particles really are brothers under the skin, electrons and positrons could be formed from the mutation of quarks. At a high enough heat to disrupt the stabilising bonds of a quark structure like a proton, the quarks can convert into positrons (and, of course, anti-matter quarks into electrons).

Yet whether spawned by radiation or quark conversion, equal numbers of electrons and positrons were being produced at this stage, and so they were just as quickly annihilating each other, re-releasing their energy in a burst of electromagnetic radiation.

It was not until the Universe had existed for several whole seconds that things finally began to settle. The familiar modes of particles and particle interactions were all in place, even if the disordering effects of the great ambient heat meant that the results were not particularly stable and still easily reversed. It was at this point that a second surprising asymmetry made its appearance felt.

The first lucky break was the mysterious asymmetry that meant 300,000,001 matter particles were being produced for every 300,000,000 anti-matter particles, thus ensuring not all the energy of the Big Bang got frittered away in annihilations, leaving the Universe a featureless, ever-dimming, radiation bath. The second critical asymmetry was the way that electric charge just happened to get divided between the very different particle structures of electrons, protons and neutrons.

particle families

In a cool Universe, it is the electromagnetic force that really counts. The strong force is locked up in quark structures, curled in on itself to produce incredibly stable particles. The weak force is also short range in this way. It is only the electromagnetic force and gravity that remain free to reach across space and so stir things up on the broader scale. But this can only happen because the complementary faces of the electromagnetic force, positive and negative charge, are carried by different types of particles.

The negatively-charged electron is a low mass speck of a particle. It does not get tangled up with strong force interactions so survives as a solitary, irreducible, point of matter. But its positively-charged counterpart, the proton, is a combination of two ups and a down quark. Add together their three fractional charges of + ⅔, + ⅔ and - ⅓, and you get a total charge of +1 (and the three charges of a neutron’s quarks add up exactly to zero, of course, to make an electrically neutral particle).

The glue of the strong force makes the triplet structure of the proton irreducible in its own way. But it also makes it look like an elephant beside a flea compared to the electron. It is 600 times as massive.

How this measure of the structure’s energy content translate to physical size is a little trickier. An electron is a point and so has no extent, no real length or breadth, just a position. Or to put it more carefully – given what quantum theory has to say about the uncertainty of Reality at the Planck-scale – an electron is as small as something can be and still be considered located.

 But a proton, as a bag of particles, does occupy a definite region of space. And at about 10^-13 cm across, this volume is some 20 orders of magnitude larger than the Planck-scale. Moreover, a proton would also have a proper shape. As its quark contents shift about in response to outside forces, it expands and contracts a little. It may even get stretched out peanut shaped. 

So the two possible faces of electromagnetic charge are borne by quite dissimilar particles. A key symmetry is made actual by an asymmetry! Consider if only electrons and positrons, or protons and anti-protons, had existed as charge carriers. Mutual annihilation would soon have spent all their energy. No matter particle is stable in the face of its anti-particle.

But protons and electrons could co-exist, interacting without destructing - although even then it was fortunate that they ended up with opposite charges rather than both being positive, or both negative, which would have left all matter in a state of eternal repulsion.

Surprise at this apparently arbitrary way in which the surviving remnants of the Big Bang should dovetail so helpfully in their qualities is somewhat lessened by remembering that the Big Bang aftermath did indeed generate every possible kind of particle. Many of these perished precisely because they could not lock into any higher level productive relationships. But a great many more - and we may never know how many - might simply have become invisible to us because they don't interact. Or interact only vanishingly weakly.

The third major particle family after the quarks and electrons is the neutrino. The neutrino is a near massless particle with zero electric charge and no strong force. The only exchanges in which it takes part involve the weak force. So clearly the neutrino is not a top candidate for interesting interactions. If you were designing a world, its not a particle that offers much.

Yet neutrinos fly about the Universe in vast numbers. Estimates suggest that the total weight of neutrinos amounts to six times that of all the quarks and electrons combined! This is even though an individual neutrino weighs practically nothing, being less than a millionth the mass of an electron. So, objectively speaking, the neutrino dominates the Universe. Every other type of particle exists as a rare afterthought.

However the neutrino is only a ghostly presence to us because of its feeble interest in interactions. And who is to say that there are not a whole host of particles beyond the neutrino that don't interact at all with our world of mass and structure? Thus the prominence of quarks and electrons in the scheme of things may be largely self-selecting.

If a wide enough variety of particles were generated during the Big Bang, ranging from hot particles that had too many ways of interacting to survive to cold particles that barely wanted to interact at all, then the middling type of particles would be almost bound to have the right mix of keenness and stability, attachment and detachment, to be good building material.

It might all seem a stroke of great luck. But as they say, history is written by the winners. And given sufficient variety, some winners in the interesting-but-sufficiently-stable particle stakes were inevitable.

the first eleven minutes

We must not exaggerate the apparently fluky aspects of our existence. But nor do we need to as it already involves quite enough strange coincidences to be going with. The next few steps in the evolution of our Universe - the emergence of complex structure in the shape of atoms and stars – bring a few more.
 
An atom, as every schoolchild learns, is an aggregate of protons, neutrons and electrons. The simplest atomic material is hydrogen, a single proton teamed up with a single electron. But larger atoms are possible because the quarks in a proton slop just enough of their strong force to get gummed together into clusters. This forms a nucleus, an atomic core, with the positive charge to attract an equal number of electrons, and so build up from simple hydrogen to the heavier elements.

Yet building larger atoms also takes neutrons as more than a few protons clustered together concentrates too much positive electromagnetic charge in the one place. The electrically-neutral neutrons dilute the repulsive effect just enough to allow the binding power of the strong force to have the upper hand. 

Thus atoms depend on neutrons - which otherwise looked the spare part in a world of more interesting characters such as electrons and protons. And yet neutrons depend on atoms just as much.

As mentioned, a neutron standing alone in the open will decay in about eleven minutes. It will split three ways into the lower energy, thus more stable, structures of a proton, an electron and an anti-matter neutrino. But once a neutron is bound to a nucleus, it can’t decay. So the survival of neutrons turned out to depend entirely on the existence of atoms. And atoms could only form if there were neutrons available.

Which all meant it was a bit of a tight squeeze in the aftermath of the Big Bang. Neutrons could only be made from the Big Bang’s decaying energy. And large particle structures like atoms could only appear once the Universe had cooled considerably. It actually took about three and half minutes for the temperature to subside enough for the nuclei of the lighter elements such as hydrogen, helium, and lithium, to begin to form.

As said, hydrogen is really just a solitary proton, but it can exist also as a proton-neutron combo – either deuterium with one neutron or tritium with two. Helium is two protons with either one or two neutrons. Lithium is three protons with sometimes three, but more usually four, neutrons. The different mixes are known as isotopes and give the resulting atoms slightly different properties.

But anyway, luck was again with the Big Bang tale of creation and the eleven minute half-life of neutrons gave enough time for the majority to get mopped up into atomic clusters, so ensuring their preservation.

then the next 300,000 years

Phew! Another bottleneck scraped through. The next 300,000 years were to be somewhat less eventful. The Universe now needed time to expand and cool to the point where its many scattered atomic cores could gather stable complements of electrons about them and become proper atoms.

In the early days of the Universe, the boiling cauldron of heat meant that electrons and positrons materialised and dematerialised continuously. But after 300,000 years – by which time the visible Universe had expanded to 300,000 light years across, and by creating “empty” space rather than more hot inflaton terrain – the temperature was all the way down to 3,000 degrees. About as hot as the surface of sun, yet chilly enough for electromagnetic attraction at last to outweigh the general disordering effect of the background radiation.

So all of a sudden, at this magic temperature, the Universe underwent another phase transition. The mass of roaming electrons got swept up by the hydrogen, helium and lithium nuclei. The Universe had real elements, real substance.

It also became immediately transparent to light. While the electrons had run free, the Universe had been opaque as light waves could hardly travel a step before being absorbed. But the mopping up of electrons made the void more of a real void. Now electromagnetic radiation could pass reasonably unhindered through space. The last of the Universe's unbound energy could be quickly dissipated. Or at least it could be sent on interestingly long journeys – across intergalactic distances rather than trillionths of a metre.

This sudden lifting on travel restrictions is what created the abrupt burst of electromagnetic radiation that we now see as the cosmic background radiation, the faint afterglow that is our most direct evidence that the Big Bang actually happened. So this flash was not produced until some third of a million years following the actual event itself.

On the other hand, it does mark the true beginning of cold empty space – the kind of place where light particles can wander 13 billion years before striking some remote observer’s radio telescope. The creation of substance is certainly something to celebrate. But we must also learn to appreciate the subtler joy that is the creation of absence, the asymmetric partner to substance, if we are intending to tell the complete story of existence.

how to avoid a tedious fate

The next big step after the appearance of atomic structure was the development of stellar structure. The Universe was growing cool and stable. The wild energies of the inflaton field had drained away, leaving behind a flotsam of particles, a certain amount of waste heat, and an awful lot of empty space. Now was the time for gravity to begin to exert an organising effect on the Universe – to produce twinkling stars and wheeling galaxies.

What is gravity? Some theories in physics treat it as a force, and indeed suppose there are gravity waves and even gravity particles - the graviton. But gravity is different in being only attractive, whereas the other three forces are all charges that both attract and repel. Instead of the exact mirror symmetry of two complementary kinds of charge, gravity only offers the asymmetry of a constant pull.

Einstein's theory of relativity found it simpler to treat gravity as the shape of space. Mass bends the fabric of the Universe, dimpling spacetime itself, so objects must follow trajectories that curve them towards other objects. For Einstein, it is the generality of geometry rather than the particularity of little pushes and pulls that dictates motions under gravitational attraction.

So it is not at all clear how to place gravity in the scheme of things. But from the perspective of the origins of the Universe, at least we can say that gravity looks to be the symmetry partner of something – mass. The pair are somehow entwinned as the asymmetric face of each other, a negative quantity of one to pay for the positive quantity of the second. So gravity is some kind of anti-mass rather than just another property of mass like a different type of force or charge.

It may help to remember that mass – an array of located particles – was formed by the contraction of energy to knotted points. Like the twisting of a rubber sheet, this would have built in a tension between all the locations, a tug that would want to recollapse the space in-between. Gravity is the yearning that is the void, an emptied space that naturally wants to heal itself by closing up again.

Whatever the story, it took a while for gravity to assert its presence in the Universe as its pull is so remarkably feeble. The tug of gravity is some 10³⁶ times weaker than the attraction between two electromagnetic charges.

By now, you will have some feeling for the gap represented by 36 orders of magnitude. But it’s simple enough to demonstrate. Just use a magnet to make a paper-clip leap up off a table. A tiny magnetic field can easily defeat the entire gravitational pull of the earth.

However this relative weakness of gravity was not a problem once the Universe had been more or less cleared of all its other forces, with the strong and weak forces locked away inside clumps of protons and neutrons, and electromagnetic charge bound up with the formation of electrically-balanced atoms. The stage was empty enough for gravity to make its entrance.

The Universe had in fact become a fairly dull place - a vast emptiness dusted by hydrogen and helium atoms, with only a very occasional lithium atom to relieve the tedium. Of course, any act of creation ought to be admired. Something is always more amazing than nothing.

However if the development of the Universe had ended at this stage, would we be that wowed by a world amounting to little more than an ever-cooling, ever-thinning, cloud of gases? Thus it was lucky there was gravity to drive the next phase of its evolution – and to reveal a few more of the fortunate features concealed in the symmetries of the Big Bang.

Over the course of a few million years, gravity got to work on the gas clouds, causing them to break up into swirling nebulae and then to contract into increasingly dense balls. Atoms pulled on other atoms, clumps pulled on other clump. Empty space became ever emptier while the substance of the Universe grew ever more located.

This could have been a complete disaster. As clouds of hydrogen and helium collapsed into tight balls, the concentration of the mass would have increased the strength of the gravitational pull with exponential effect. The closer the atoms, the stronger the attraction. There would be a runaway process in which a knot of material was pulled tighter and tighter until the atoms themselves began to get crushed under their own weight.

First the empty space between the nuclei and the orbiting electrons would go. Then even space within the nuclei would disintegrate. A ball of gas would collapse entirely to become a black hole - a super-dense point with a gravitational field so strong that not even light escapes its pull. Atoms would go, even electrons and quarks would go, leaving just the black dot to mark the location where mass and gravity had been reunited in their raw unformed state.

So the Universe had yet another way of suffering a dull, tedious fate. It could have gone from being a featureless ever-widening sea of gas to an empty space littered with dark shrivelled shards of collapse. Yet instead, something else happened to the majority of the gas balls before they passed the point of gravitational no return and turned into black holes. They caught alight and became stars.

A star is powered by the process of nuclear fusion - the fusing of small atoms into larger ones. When hydrogen atoms are compressed to a certain pressure, they become heated enough to combine to form helium atoms. A little heat is needed to allow get them over this threshold, to allow them to get close enough to rearrange into a more complex structure. But once the shuffle has taken place, energy is actually released. In going from hydrogen to helium, the greater concentration of neutrons dilutes the repulsion felt by the positively-charged protons and so there is slightly less demand on the strong force glue.

This surplus - which amounts to 0.7 percent the mass of a hydrogen atom - is radiated away. Within the confines of a ball of gas, that has the effect of heating up other near-by atoms, pushing them too over the brink. So very quickly in any concentrated mass of hydrogen, there is a runaway chain-reaction. Like a thermonuclear bomb, the gas explodes.

Thus a star is the result of an amazing balancing act. Gravity wants to contract each ball of gas out of existence. But the balls catch fire, creating an internal pressure that blows the gas back out into surrounding space again.

The process is entirely self-regulating. As the fury of the explosion makes the ball of gas expand, the ball will cool and so the fusion rate will slow. As the fusion rate slows, gravity can start to contract the ball and this heats it up once more. Two awesomely destructive forces are at work - the black hole collapse of gravity and the raging heat of nuclear fusion. Yet because they operate in opposite directions, they fall into an equilibrium balance.

Remember the cunningness of this mechanism. There is symmetry in two opposed tendencies and asymmety in that the two tendencies look to be completely different kinds of things. Together an emergent balance is struck that allows the whole to persist. It is a very active way of looking at the creation of something – a system, a structure – whereas usually symmetry-breaking is treated as a simple, passive, affair. A snapping in two that then endures because to endure – to have continued existence – needs no further causal explanation.

Anyway, caught between two warring but self-adjusting tendencies, a star can hang suspended in the middle of nowhere, a fire without a hearth or vessel to contain it. At least until its stock of fuel is spent. Which fortunately takes quite a few billion years in most cases.

cosmic fine-tuning

We have been narrowly avoiding a succession of tedious fates – existences in which we humans don’t feature. But we are not quite out of the woods yet as we still risk being left with an endless black firmament punctuated by only the occasional burning globe. However good luck strikes again because - given the right starting point - a star can go on to be the forge for other chemical elements.

Hydrogen is just the first step for fusion type reactions. Hydrogen makes more helium. With sufficient pressure and temperature, helium atoms fuse to form still heavier atoms with larger nuclei. Step by step, fusion creates lithium with three protons, beryllium with four, boron with five, carbon with six, nitrogen with seven, oxygen with eight, and so on up the periodic table of elements.

Each of these step is self-fuelling in that it releases a little further energy. Thus the core of a star is always heating up, allowing it to move on to the next level of production. The cooler outer layers still burn the lighter elements while the furnace inside works all the way up to iron, the 26th element with a 26 proton structure.

After iron, a fresh problem arises because fusion has become a game of diminishing returns. The nuclei are growing so broad that the strong force glue, with its limited range, strains to reach across. So instead of further fusion steps releasing energy, they must consume it. Energy has to be injected into the reactions to beef up the strong force.

Fickle fate intervenes once more. In the final moments of a star's life, just when all its fuel is about gone, it does suddenly collapse. The bubble of fusion is burst. However as its outer layers cave in under gravity, the remaining material is crushed to much higher pressures than ever. There is a reheating to fantastic temperatures which prompts one last massive explosion - a supernova. The dying star flares with the light of a billion suns for a few weeks, splattering its contents across space.

The heat of this supernova is enough to generate all the heavier elements up to the rare earths like 92-proton uranium. After this, atoms really cannot get any bigger because the binding power of the strong force gives out completely. Even uranium is too large to be stable. Radioactive decay is the release of energy by fission - the splitting of an over-sized atom into two smaller and more stable ones.

The whole saga looks astoundingly fine-tuned. For example, if gravity had been just a touch stronger as a force, only very massive stars would have formed. Because size equals temperature, these would have fizzled through the Universe's entire supply of hydrogen and helium in just a few million years. The era of stellar evolution would have been over almost as soon as it started. Conversely, if gravity had been just a touch weaker, then the resulting stars would have been too small and cool to bake any elements heavier than helium.

Cosmologists have found that no matter which way they turn, the margin for error was alarmingly small. If electrons had been just a few times heavier than they are, they would have combined with protons to leave the Universe an inert sea of neutrons. If the strong force had been just fractionally weaker (or electromagnetic charge fractionally more powerful) then protons would never have been able to overcome their mutual repulsion and so clump together to make complex atoms.

Equally, if the balance had been tilted just slightly in the other direction, then protons would have clumped without needing neutrons as an intermediary. So there would have been no fusion reaction and again no complex chemistry – at least not as we know it.

As Goldilocks would have put it, the Universe turned out just right. Neither too hot nor too cold, too hard nor too soft. It was left rich in the variety of its interactions, and its ingredients had the emergent stability to endure long enough to make something of that variety.

the emergence of life

Roll the film forward some nine billion years after the Big Bang and we find that the Universe has burnt its way through its first generation of stars. The supernovae explosions that were the final act of many has left space littered with the full range of atomic elements. A second generation of stars has condensed out of the remaining gas and their gravitational fields are sweeping up the heavy dust, spinning it into orbiting planets.

The earth formed as a molten ball made up of 47 percent oxygen, 28 percent silicon, 8 percent aluminium, 4.5 percent iron, 3.5 percent calcium, and a healthy seasoning of the rest of the periodic table. The early years were rough as the solar system was an untidy place. Asteroids and other space junk rained down continuously. But gradually all the flying debris was mopped up in collisions with the sun and larger planets, or else it drifted away into deep space.

By about 3.8 billion years ago, the bombardment had ceased and the earth could grow its cool, stable crust of rock. Then with almost indecent haste, life erupted into being.

Life may well have begun earlier and been sterilised by one of the many asteroid strikes. But anyway, there is no particular evidence of delay in this event, no sign of a great struggle against the evolutionary odds.

Bacteria appeared almost overnight. The early bacteria were probably rock-eaters, literally, living in the fissures of the earth's surface and digesting the energy to be had in sulphur compounds and other mineral sources. They released oxygen and other gases as waste products, slowly giving the earth a proper atmosphere. After a billion years or so, the earth was transformed and ready for the next step, the emergence of complex animal life. Plants evolved efficient ways of harnessing the light of the sun and whole ecosystems grew rich on the back of this steady energy source.

The speed and ease with which life arose on earth suggests the Universe must be full of the living, the intelligent. Stars with planets are commonplace. Even if only earth-like planets are ripe for life - neither too hot, nor too cold; too big, nor too small - then our corner of the Universe may have witnessed a similar rush to complex biology many billions of times.

Indeed, that would be the conservative estimate. With the visible Universe holding 100 billion galaxies, each with 100 billion stars, it only assumes one earth-like planet for every 10,000 stars. A chain of ludicrously improbable coincidence might have been necessary to produce our Universe with its rich chemical and stellar structure. But our own existence as intelligent beings may in fact count as one of the most inevitable features of the whole affair. The specialness of creation doesn’t also have to be the specialness of human creation.

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