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  • Time is flying by on this busy, crowded planet... as life changes and evolves from second to

  • second.

  • And yet the arc of human lifespan is getting longer: 65 years is the global average ...

  • way up from just 20 in the Stone Age.

  • Modern science, however, provides a humbling perspective. Our lives... indeed the life

  • span of the human species... is just a blip compared to the age of the universe, at 13.7

  • billion years and counting.

  • It now seems that our entire universe is living on borrowed time...

  • And that even it may be just a blip within the grand sweep of deep time.

  • Scholars debate whether time is a property of the universe... or a human invention.

  • What's certain is that we use the ticking of all kinds of clocks... from the decay of

  • radioactive elements to the oscillation of light beams... to chart and measure a changing

  • universe... to understand how it works and what drives it.

  • Our own major reference for the passage of time is the 24-hour day... the time it takes

  • the Earth to rotate once. Well, it's actually 23 hours, 56 minutes and 4.1 seconds... approximately...

  • if you're judging by the stars, not the sun.

  • Earth acquired its spin during its birth, from the bombardment of rocks and dust that

  • formed it.

  • But it's gradually losing that rotation 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 18 months.

  • In a few hundred million years, we'll gain a whole hour.

  • The day-night cycle is so reliable that it has come to regulate our internal chemistry.

  • The fading rays of the sun, picked up by the retinas in our eyes, set our so-called "circadian

  • rhythms" in motion.

  • That's when our brains begin to secrete melatonin, a hormone that tells our bodies to get ready

  • for sleep. Long ago, this may have been an adaptation to keep us quiet and clear of night-time

  • predators.

  • Finally, in the light of morning, the flow of melatonin stops. Our blood pressure spikes...

  • body temperature and heart rate rise as we move out into the world.

  • Over the days ... and years... we march to the beat of our biology.

  • But with our minds, we have learned to follow time's trail out to longer and longer intervals.

  • Philosophers have wondered... does time move like an arrow... with all the phenomena in

  • nature pushing toward an inevitable end?

  • Or perhaps, it moves in cycles that endlessly repeat... and even perhaps restore what is

  • there?

  • We know from precise measurements that the Earth goes around the sun once every 365.256366

  • days.

  • As the Earth orbits, with each hemisphere tilting toward and away from its parent star,

  • the seasons bring on cycles of life... birth and reproduction... decay and death.

  • Only about one billionth of the Sun's energy actually hits the Earth. And much of that

  • gets absorbed by dust and water vapor in the upper atmosphere.

  • What does make it down to the surface sets many planetary processes in motion.

  • You can see it in the annual melting and refreezing of ice at the poles... the ebb and flow of

  • heat in the tropical oceans...

  • The seasonal cycles of chlorophyll production in plants on land and at sea... and in the

  • biosphere at large.

  • These cycles are embedded in still longer Earth cycles.

  • Ocean currents, for example, are thought to make complete cycles ranging from four to

  • around sixteen centuries.

  • Moving out in time, as the Earth rotates on its axis, it completes a series of interlocking

  • wobbles called Milankovic cycles every 23 to 41,000 years.

  • They have been blamed for the onset of ice ages about every one hundred thousand years.

  • Then there's the carbon cycle. It begins with rainfall over the oceans and coastal waves

  • that pull carbon dioxide into the sea.

  • There, it's captured by ocean plants... including tiny organisms called plankton. They are eaten

  • by fish and other creatures, and the carbon is passed on up the food chain.

  • Eventually, when plants and animals die or expel waste, the carbon falls to the ocean

  • bottom where it's impounded in layers of sediment.

  • Without people, it takes a volcanic explosion... or a dramatic lowering of sea levels... to

  • send the carbon back into the air, often after millions of years.

  • The idea that Earth and life changes on deep time scales emerged in revolutions of thought

  • associated with Copernicus and Darwin.

  • But the processes that shape a planet like ours play only the smallest of roles in the

  • evolution of the cosmos. So to glimpse time's broadest arcs we must look to the universe

  • beyond.

  • The reigning theory is that it all began in a sudden expansion of space... the big bang.

  • This was the time of the tiny: The first microseconds set in motion the primordial era. Atoms formed

  • within a hot soup of subatomic particles.

  • The universe cooled as it ballooned outward... growing dim... and falling into what's known

  • as the cosmic dark ages. But gravity was always at work; particles pulling on one another.

  • And after several hundred million years, considerable clumps of matter had drawn together.

  • These isolated pockets of gas became dense enough to heat up... and ignite.

  • So began the era of stars...

  • In this glorious age, the universe planted the rich cosmic landscapes we see in our telescopes...

  • where hundreds of billions of stars light up galaxies all across the universe.

  • The arc of this great era of stars is defined by the life cycles of stars, which vary according

  • to their sizes.

  • Stars shine because gravity crushes matter into their cores. The energy released pushes

  • outward and balances the inward force of gravity.

  • This battle between energy and gravity is still raging in stars all around the universe.

  • But in large stars, about ten million years after their birth, gravity gains the edge...

  • and tips the balance.

  • When the core of such a star crosses a critical mass threshold, it collapses in on itself.

  • The energy released causes the star to explode in a blast of light and debris that's visible

  • across the cosmos.

  • In the wake of this supernova, shock waves can cause nearby clouds of dust and gas to

  • collapse... and catch fire... to form a generation of smaller stars like our Sun.

  • When sun-like stars go, the end will be more of a whimper than a bang, as shown in this

  • gallery of dying stars captured by the Hubble Space Telescope.

  • As their cores gets heavier and heavier, over billions of years time, fierce winds will

  • begin to push on their outer layers... causing them to blossom out in spectacular displays.

  • That's just what happened about 12,000 years ago to the star that's become the famed Helix

  • Nebula. Today we see the dying star's outer layers form a vast glowing ring. On the inside

  • of the ring, spokes of more dense gas are being exposed by its winds.

  • The star itself is now a dim, cooling remnant called a white dwarf. It's the size of the

  • Earth, but about 3 million times more dense.

  • This is likely what's in store for our sun. A civilization distant in time may scan the

  • Sun for planets, but they won't see Earth.

  • Our home planet's end will have begun long before that, as rising solar luminosity gradually

  • blasts away its atmosphere, rendering it uninhabitable. Surface water will disappear, evaporated by

  • all that heat and stripped off by solar winds.

  • Finally, as the sun blows off its outer layers, they will envelope the Earth. Friction with

  • those gases will cause this once blue world to gradually spiral home, melting into its

  • mother sun.

  • This battle between energy and gravity repeats in every corner of a galaxy like ours... with

  • gravity drawing gas clouds into stars...

  • The stars will burn themselves out on wide variety of time scales... depending mostly

  • on how large they are.

  • They'll leave remnants that slowly grow cooler.

  • As a result, galaxies like ours will grow dimmer over time... unless they get seriously

  • stirred up.

  • That's what's going to happen to our galaxy. At just about the time our sun begins setting

  • its sights on swallowing Earth, any remaining inhabitants here will see the stars of the

  • Andromeda galaxy looming above the plane of our Milky Way.

  • As shown in this simulation, the two are likely to tear each other apart...or at least severely

  • jumble each other up. If it's a direct hit, all the stars in both galaxies will gradually

  • join together in a gigantic galactic puffball known as an elliptical galaxy.

  • All this turbulence of the merger could stimulate a wave of new stars being born, reinvigorating

  • the new larger galaxy.

  • Dust-ups like this - where galactic neighbors come together - will be common as the epoch

  • of stars grows toward its later stages. But a wholesale thinning out of the universe is

  • inevitable, on a grand scale.

  • Recent studies of the cosmic expansion rate show that the universe is in no danger of

  • succumbing to gravity...it won't all end in a Big Crunch.

  • In fact, over the last 6 billion years, it's begun to accelerate outward... as gravity

  • loses its grip on the universe to an unseen force called: dark energy.

  • You can see evidence of this now, out in the huge voids of space between clusters of galaxies.

  • Think of the voids as ever-expanding bubbles ... where the bubble walls touch are filaments

  • of galaxies.

  • As the bubbles grow, these filaments will stretch and break. The distances between galaxies

  • will widen at a faster and faster pace. Eventually, most observers will see only a few isolated

  • clusters of galaxies huddled together... with little connection to anything else ... and

  • few clues as to how they got there.

  • A good place to be, in those long twilight years of the stellar era, would be a place

  • where gravity and energy have forged an extended truce.

  • Perhaps a place like this:

  • It's one of the smallest and dimmest stars in our universe. And yet brown dwarfs like

  • this have been shown to harbor planets close enough to bask in their dim rays.

  • Brown dwarfs - and their hotter cousins the red dwarfs - form the vast majority of stars

  • in our galaxy.

  • Because they burn so slowly, they'll be the final beacons of the majestic age of stars...

  • an era that will extend out to one hundred trillion years.

  • Even as galaxies like ours grow dim, another process will begin to transform them. Over

  • time, chance encounters between objects will perturb their orbits... sending some towards

  • the center of those galaxy, and others out into the void.

  • In

  • this way, galaxies may gradually evaporate, with ever-denser concentrations of matter

  • accumulating in their cores.

  • The universe now begins to take on a new character.

  • Welcome to the degenerate era... in which the cosmos is populated by red and white dwarf

  • stars... steadily cooling... and by the charred remains of supernova explosions: the neutron

  • stars and black holes... slowly spinning down.

  • Even though these dead stars have used up their nuclear fuels, the universe continues

  • to produce small amounts of energy. They scoop up and annihilate dark matter particles that

  • manage to stray into their grasp.

  • Now here is where change slows to a crawl. It's expected that protons, the building blocks

  • of all atoms, will slowly degrade... turning back into sub-atomic particles that then decay

  • into photons, particles of light.

  • All the protons in the universe date back to the earliest moments. Their decay marks

  • the end of the degenerate era... around a billion, billion, billion, billion years after

  • it all began. That's a one followed by 40 zeros.

  • Our picture of what happens after that will come to depend on what we learn in the coming

  • years beneath the border of France and Switzerland... in one of the largest physics experiments

  • ever undertaken.

  • 100 meters underground, the Large Hadron Collider was built to accelerate particles in opposite

  • directions through a ring of tubes, 27 miles around. When they reach nearly the speed of

  • light, scientists will bring them into ferocious collisions.

  • One goal: to define the final time horizons of our universe.

  • And the final moments of its most persistent objects.

  • Black holes, ranging from millions to billions of times the mass of our sun, occupy the centers

  • of most large galaxies today. As those galaxies age, much of their mass will spiral towards

  • the center... and over trillions of years that mass will fall into ever more ravenous

  • black holes.

  • Conceivably, these ultra-massive black holes could end up weighing as much as a galaxy.

  • When they finally stop growing, will they too be subject to the ravages of time?

  • According to the physicist Stephen Hawking, the answer is yes.

  • He proposed a theoretical process of decay that 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 charges constantly well

  • up in the vacuum of space. They normally destroy each other.

  • But when this happens at the event horizon of a black hole, one particle can be pulled

  • in while the other escapes. That has the effect of slowly siphoning energy and mass from the

  • hole.

  • If this is true, then even black holes are eventually doomed. But finding out for sure

  • is not easy.

  • Creating a micro black hole, it seems, will take more energy than any Earth-bound collider

  • yet conceived can pack.

  • That is, unless there's more to our universe and to gravity than we've thought.

  • The key lies in whether the world we know is part of a more complex cosmic reality,

  • beyond the three spatial dimensions plus time that we experience in our daily lives.

  • If so, we would be like insects living on the two-dimensional surface of a pond, completely

  • unaware of the deep and complex reality below it.

  • It may be possible that one of these of extra dimensions could intersect our world on an

  • extremely tiny scale.

  • According to some scientists, when particles collide at very high energies the additional

  • gravity needed to create a micro black hole could come from this extra dimension.

  • They'll know a black hole is there when they see the shower of particles predicted by Hawking's

  • theory.

  • That shower will open a brief window to a deeper cosmic reality... while shedding light

  • on the ultimate future of our universe.

  • Based on Hawking's theory, the last black holes will disappear when the cosmic clock

  • strikes 10 to the hundredth years from now... that's a number known as a googol.

  • That's the end of our universe. And yet, it's still far short of forever.

  • What will happen, say, in 10 to the googol, a googolplex years? If you wrote all those

  • zeroes out in tiny 1 point font, it would stretch beyond the observable universe.

  • Will the great arrow of time ever come to rest? Or does that arrow fly a curved path,

  • destined to cycle back again and again as whole new universes come into being in a way

  • similar to our own?

  • The numbers that describe the time horizons of our universe are incomprehensible, yet

  • they may well be relatively insignificant in the grand scheme of things.

  • Earth and humanity are products of the great era of stars, and we have been witness to

  • its great spectacles of gravity and energy. Yet it's fair to say that we occupy a truly

  • tiny period within the vast cosmic sweep of deep time.

  • We can easily imagine there are others out there somewhere who also look out and attempt

  • to comprehend the changes they see. They too may invent the idea of "time"...and develop

  • their own theories on where it's all leading.

  • Will their discoveries - and ours - somehow survive... as we all gradually go the way

  • of the stars that made life possible...?

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Time is flying by on this busy, crowded planet... as life changes and evolves from second to

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時はいつ終わるのか?(バージョン1) (When Will Time End? (Version 1))

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    Wonderful に公開 2021 年 01 月 14 日
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