Time is flying by on this busy, crowded planetas life changes and evolves from second to second.
At the same time, the arc of the humanlifespan is getting longer: 67 years is the global average, up from just 20 years in theStone Age.
Modern science provides a humbling perspective.
Our lives, indeed even that of the human species, are just a blip compared to the Earth, at4.
5 billion years and counting, and the universe, at 13.
7 billion years.
It now appears the entire cosmos is livingon borrowed time.
It may be a blip within a much grander sweep of time.
When, we nowask, will time end? Our lives are governed by cycles of wakingand sleeping, the seasons, birth and death.
Understanding time in cyclical terms connectsus to the natural world, but it does not answer the questions of science.
What explains Earth’s past, its geologicaleras and its ancient creatures? And where did our world come from? How and when willit end? In the revolutions spawned by Copernicus and Darwin, we began to see time as an arrow,in a universe that’s always changing.
The 19th century physicist, Ludwig Boltzmann,found a law he believed governed the flight of Time’s arrow.
Entropy, based on the 2ndlaw of thermodynamics, holds that states of disorder tend to increase.
From neat, orderly starting points, the elements,living things, the earth, the sun, the galaxy.
are all headed eventually to states of highentropy or disorder.
Nature fights this inevitable disintegration by constantly reassemblingmatter and energy into lower states of entropy in cycles of death and rebirth.
Will entropy someday win the battle and putthe breaks on time’s arrow? Or will time, stubbornly, keep moving forward? We are observers, and pawns, in this cosmicconflict.
We seek mastery of time’s workings, even as the clock ticks down to our own certainend.
Our windows into the nature of time are the mechanisms we use to chart and measurea changing universe, from the mechanical clocks of old, to the decay of radioactive elements,or telescopes that measure the speed of distant objects.
Our lives move in sync with the 24-hour day,the time it takes the Earth to rotate once.
Well, it’s actually 23 hours, 56 minutesand 4.
1 seconds if you’re judging by the stars, not the sun.
Earth got its spin atthe time of its birth, from the bombardment of rocks and dust that formed it.
But it’sgradually losing it to drag from the moon’s gravity.
That’s why, in the time of the dinosaurs,a year was 370 days, and why we have to add a leap second to our clocks about every 18months.
In a few hundred million years, we’ll gain a whole hour.
The day-night cycle is so reliable that ithas come to regulate our internal chemistry.
The fading rays of the sun, picked up by ourretinas, set our so-called “circadian rhythms” in motion.
That’s when our brains beginto secrete melatonin, a hormone that tells our bodies to get ready for sleep.
Finally, in the light of morning, the flowof melatonin stops.
Our blood pressure spikes… body temperature and heart rate rise as wemove out into the world.
Our days, and our lives, are short in cosmic terms.
But withour minds, we have learned to follow time’s trail out to longer and longer intervals.
We know from precise measurements that theEarth goes around the sun every 365.
Much of the solar energy that hits ourplanet is reflected back to space or absorbed by dust and clouds.
The rest sets our planetin motion.
You can see it in the ebb and flow of heatin the tropical oceans, the annual melting and refreezing of ice at the poles, or seasonalcycles of chlorophyll production in plants on land and at sea.
These cycles are embeddedin still longer Earth cycles.
Ocean currents, for example, are thought to make completecycles ranging from four to around sixteen centuries.
Moving out in time, as the Earth rotates onits axis, it makes a series of interlocking wobbles called Milankovich cycles.
They havebeen blamed for the onset of ice ages about every one hundred thousand years.
Then there’sthe carbon cycle.
Plants capture it from the air or the sea.
It finds its way into soilsor ocean sediments as plants decay, or as waste passes out of the food chain.
It can take a volcanic explosion, or a dramaticlowering of sea levels, to release this carbon back into the air, often after millions ofyears.
The processes that shape a planet like ours play only the smallest of roles in theevolution of the universe.
So to glimpse time’s broader arcs, we mustlook to cycles that govern the larger cosmos.
The reigning theory is that the universe beganin a sudden expansion of space, the big bang.
With entropy uniformly low, this was the timeof the tiny, subatomic particles like quarks and leptons stirred into a hot soup.
Within microseconds, they combined into atoms,setting in motion the primordial era.
The universe cooled as it ballooned, growing dimand falling into what’s known as the cosmic dark ages.
All the while, though, gravitypulled particles together, fighting the expansion.
After several hundred million years, largerclumps of matter had drawn together.
These isolated pockets of gas became dense enoughto heat up and ignite.
So began the era of stars.
In this glorious age, the universe seededthe rich cosmic landscapes we see in our telescopes.
Trillions upon trillions of stars lit up galaxiesall across the cosmos.
The arc of this era is defined by the life cycles of stars, whichvary according to their sizes.
Stars shine because gravity crushes matterinto their cores.
The energy released pushes outward and balances the inward force of gravity.
This battle between energy and gravity is raging in stars all around the universe.
Butin large stars, about ten million years after their birth, gravity begins to gain the edgeand tips the balance.
When the mass concentrating in the core ofthe star reaches a critical threshold, the core collapses in on itself.
The energy releasedin the collapse causes the star to explode in a blast of light and debris that’s visibleacross the cosmos.
In the wake of this supernova, shock wavescan cause nearby clouds of dust and gas to collapse and ignite, to form generations ofsmaller stars like our Sun.
A byproduct of star formation, solar systems form in thecollapse of the surrounding solar nebula.
The life cycle of planets, especially thosein close, is tied to that of their parent stars.
As stars like our sun age, they grow hotterand more luminous.
Billions of years from now, that will spell the beginning of theend for our home planet.
As raging solar winds begin to blast away at our atmosphere, surfacewater will gradually disappear, rendering Earth uninhabitable.
Finally, the sun will begin to swell, growingso large that it actually envelops the Earth.
Friction with the Sun’s outer edges willcause this once blue world to gradually spiral inward.
Unless they are large enough to gosupernova, most stars end their lives in more of a whimper than a bang, as shown in thisgallery of dying stars captured by the Hubble Space Telescope.
In time, solar winds push their outer layersso far out that they blossom in spectacular displays.
That’s just what happened about12,000 years ago to the star that spawned the famed Helix Nebula.
A vast glowing ringis the dying star’s outer layers.
On the inside, spokes of denser gas stubbornly resistthe star’s relentless winds.
The star itself is now a dim, cooling remnantcalled a white dwarf.
It’s the size of Earth, but about two hundred thousand times moredense.
This is likely what’s in store for our sun.
A distant civilization may scan itfor planets, but by then they won’t see Earth.
This battle between energy and gravity repeatsin every corner of a galaxy like ours, with gravity drawing gas clouds into stars, andstars burning themselves out on a variety of time scales, depending on their size.
In time though, as the mass of the galaxycollects in successive generations of small stars, it will grow dimmer and dimmer.
Somegalaxies will see a temporary rebirth, if their mass gets stirred up and combined withanother.
That’s what’s destined to happen to ourMilky Way.
At just about the time our sun begins to swallow our planet, any remainingEarthlings will see the stars of the Andromeda galaxy looming above the plane of our MilkyWay.
As shown in this simulation, the two are likelyto tear each other apart.
If it’s a direct hit, the stars in both galaxies will graduallyjoin together in a gigantic galactic puffball known as an elliptical galaxy.
All the turbulenceof the merger could stimulate a wave of new stars being born, reinvigorating the new largergalaxy.
Dust-ups like this, in which galactic neighborsmerge, will be common as the era of stars moves into its later stages.
But a wholesalethinning out of the universe is inevitable.
On a grand scale, recent studies of the cosmicexpansion rate show that the universe as a whole is in no danger of succumbing to gravity,or of ending in a Big Crunch.
In fact, over the last 6 billion years, theuniverse has begun to accelerate outward.
Gravity is losing its grip to an unseen forcecalled dark energy.
You can see evidence of this now, out in the huge voids of space betweenfilaments of galaxies.
These voids are like ever-expanding bubbles.
Where the bubble wallstouch you can see filaments of galaxies.
As the bubbles grow, the filaments will stretchand break.
The distance between galaxies will widen at a faster and faster pace.
Eventually,no matter where you are in the universe, you will see only a few isolated clusters of galaxieshuddled together, with little connection to anything else, and few clues to how they gotthere.
At more distant reaches of time, tens of billionsof years from now, the sky will grow darker and darker as everything recedes away fromeverything else.
A good place to be, in those long twilight years of the stellar era, isa place where gravity and energy have forged an extended truce.
Perhaps a place like this: not much largerthan our planet Jupiter, a Red Dwarf is one of the smallest and dimmest stars in our universe.
They have been shown to harbor planets close enough that their dim rays can sustain liquidwater, and life.
Brown dwarfs and red dwarfs form the vastmajority of stars in our galaxy.
In fact, combined, their mass exceeds that of all thelarge stars.
Because they burn so slowly, they’ll be the final beacons of the majesticage of stars, an era that will extend out to one hundred trillion years.
Even as their host galaxies grow dim, anotherprocess will begin to transform these small outposts.
Over time, chance encounters betweenobjects will perturb their orbits, sending some toward the center of the galaxy, andothers out into the void.
In this way, galaxies may gradually evaporate,with ever-denser concentrations of matter accumulating in their cores.
As that happens,the universe begins to take on a new character.
Welcome to the degenerate era, in which theuniverse is populated by red and white dwarf stars, steadily cooling, and by the charredremains of supernova explosions: neutron stars.
Even though these dead stars have used uptheir nuclear fuels, they continue to produce small amounts of energy.
They scoop up andannihilate dark matter particles that manage to stray into their grasp.
Here is where cosmicevolution slows to a crawl.
It’s expected that protons, the building blocks of all atoms,will slowly degrade, turning into sub-atomic particles that then decay into photons.
All the protons in existence date back tothe early moments of the universe.
Their eventual decay will mark the end of the degenerateera, around a billion, billion, billion, billion years after the big bang.
That’s a one followedby 40 zeros.
Our picture of what happens after that dependson what we learn in the coming years beneath the border of France and Switzerland, in oneof the largest physics experiments ever undertaken.
100 meters underground, the Large Hadron Colliderwas built to accelerate particles in opposite directions through a giant ring 27 kilometersaround.
When they reach nearly the speed of light, scientists will bring them into ferociouscollisions.
One goal: to define the final time horizonsof our universe, as well as the final moments of its most persistent objects.
Black holes,ranging from million to tens of billions of times the mass of our sun, occupy the centersof large galaxies today.
As those galaxies age, over trillions of years of time, muchof their mass will spiral towards the center and into the jaws of ever more ravenous blackholes.
Conceivably, these black holes could end upweighing as much as a galaxy.
But when they finally stop growing, will they too be subjectto the ravages of time? According to the physicist Stephen Hawking, the answer is yes.
He proposed a theoretical process of decaythat scientists are hoping to test in high-energy particle collisions at the Large Hadron Collider.
The idea is that, throughout our universe, particles of opposite charge constantly wellup in the vacuum of space.
They normally destroy each other.
But when this happens at the event horizonof a black hole, one particle can be pulled in while the other escapes.
That has the effectof slowly siphoning energy and mass from the hole.
If this is true, then even black holesare eventually doomed.
But finding out for sure is not easy.
Creatinga micro black hole, it seems, will take more energy than any Earth-bound collider can pack.
That is, unless there’s more to nature and to gravity than we’ve thought.
The key lies in whether the universe we knowis part of a more complex cosmic reality, beyond the three spatial dimensions – plustime – that we experience in our everyday lives.
We may be like an insect living onthe two-dimensional surface of a pond, unaware of the deep and complex reality below it.
It may be possible that an unseen extra dimension could intersect our world on an extremelytiny scale.
According to some scientists, when particlescollide at very high energies, the additional gravity needed to create a micro black holecould come from the extra dimension.
They’ll know a black hole is there when they see theshower of particles predicted by Hawking’s theory.
Its presence will open a brief window to adeeper cosmic reality, while shedding light on the ultimate future of our universe.
Basedon Hawking’s theory, a black hole observed today will take it last gasp when the clockstrikes 10 to the hundredth years from now, a number known as a googol.
That’s the endof the universe as we know it.
But look beyond that to, say, 10 to the googol,a googolplex, years? If you wrote all the zeroes in that number in tiny 1-point font,it would stretch beyond the observable universe.
Will the great arrow of time have come torest by then? Not if modern theories are correct.
They hold that our universe is part of a muchlarger cosmic cycle of birth and death, with whole new universes coming into being in thespace beyond our own.
The time horizons of our universe may wellbe a blip in this grander scheme of things.
Back to Earth now.
We are products of thegreat era of stars, and witnesses to its great spectacles of gravity and energy.
No doubt there are other beings somewhereout there who are attempting to comprehend the universe.
They too may invent the ideaof “time” and develop their own theories on where it’s all leading.
Their discoveries – and ours – will notsurvive the entropy at work in the universe, as we all go the way of the stars, and asthey give way to grand new eras in the life of the universe.
And as our universe givesway to grand new eras in the life of the cosmos.