Time is flying by on this busy, crowded planet
as life changes and evolves from second to second. At the same time, the arc of the human lifespan
is getting longer: 67 years is the global average, up from just 20 years in the Stone
Age. Modern science provides a humbling perspective. Our lives, indeed even that of the human species,
are just a blip compared to the Earth, at 4.5 billion years and counting, and the universe,
at 13.7 billion years. It now appears the entire cosmos is living
on borrowed time. It may be a blip within a much grander sweep
of time. When, we now ask, will time end? Our lives are governed by cycles of waking
and sleeping, the seasons, birth and death. Understanding time in cyclical terms connects
us to the natural world, but it does not answer the questions of science. What explains Earth’s past, its geological
eras and its ancient creatures? And where did our world come from? How and when will it 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 2nd law 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 high
entropy or disorder. Nature fights this inevitable disintegration
by constantly reassembling matter and energy into lower states of entropy in cycles of
death and rebirth. Will entropy someday win the battle and put
the breaks on time’s arrow? Or will time, stubbornly, keep moving forward? We are observers, and pawns, in this cosmic
conflict. We seek mastery of time’s workings, even
as the clock ticks down to our own certain end. Our windows into the nature of time are the
mechanisms we use to chart and measure a 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 minutes
and 4.1 seconds if you’re judging by the stars, not the sun. Earth got its spin at the time of its birth,
from the bombardment of rocks and dust that formed it. But it’s gradually 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 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 our
retinas, 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. 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. Our days, and our lives, are short in cosmic
terms. But with our minds, we have learned to follow
time’s trail out to longer and longer intervals. We know from precise measurements that the
Earth goes around the sun every 365.256366 days. Much of the solar energy that hits our planet
is reflected back to space or absorbed by dust and clouds. The rest sets our planet in motion. You can see it in the ebb and flow of heat
in the tropical oceans, the annual melting and refreezing of ice at the poles, or seasonal
cycles of chlorophyll production in plants on land and at sea. 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 makes a series of interlocking wobbles called Milankovich cycles. They have been blamed for the onset of ice
ages about every one hundred thousand years. Then there’s the carbon cycle. Plants capture it from the air or the sea. It finds its way into soils or ocean sediments
as plants decay, or as waste passes out of the food chain. It can take a volcanic explosion, or a dramatic
lowering of sea levels, to release this carbon back into the air, often after millions of
years. The processes that shape a planet like ours
play only the smallest of roles in the evolution of the universe. So to glimpse time’s broader arcs, we must
look to cycles that govern the larger cosmos. The reigning theory is that the universe began
in a sudden expansion of space, the big bang. With entropy uniformly low, this was the time
of 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
dim and falling into what’s known as the cosmic dark ages. All the while, though, gravity pulled particles
together, fighting the expansion. After several hundred million years, larger
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 seeded
the rich cosmic landscapes we see in our telescopes. Trillions upon trillions of stars lit up galaxies
all across the cosmos. The arc of this era 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
raging in stars all around the universe. But in large stars, about ten million years
after their birth, gravity begins to gain the edge and tips the balance. When the mass concentrating in the core of
the star reaches a critical threshold, the core collapses in on itself. The energy released in the collapse 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 ignite, to form generations of
smaller stars like our Sun. A byproduct of star formation, solar systems
form in the collapse of the surrounding solar nebula. The life cycle of planets, especially those
in close, is tied to that of their parent stars. As stars like our sun age, they grow hotter
and more luminous. Billions of years from now, that will spell
the beginning of the end for our home planet. As raging solar winds begin to blast away
at our atmosphere, surface water will gradually disappear, rendering Earth uninhabitable. Finally, the sun will begin to swell, growing
so large that it actually envelops the Earth. Friction with the Sun’s outer edges will
cause this once blue world to gradually spiral inward. Unless they are large enough to go supernova,
most stars end their lives in more of a whimper than a bang, as shown in this gallery of dying
stars captured by the Hubble Space Telescope. In time, solar winds push their outer layers
so far out that they blossom in spectacular displays. That’s just what happened about 12,000 years
ago to the star that spawned the famed Helix Nebula. A vast glowing ring is the dying star’s
outer layers. On the inside, spokes of denser gas stubbornly
resist the star’s relentless winds. The star itself is now a dim, cooling remnant
called a white dwarf. It’s the size of Earth, but about two hundred
thousand times more dense. This is likely what’s in store for our sun. A distant civilization may scan it for planets,
but by then they won’t see Earth. This battle between energy and gravity repeats
in every corner of a galaxy like ours, with gravity drawing gas clouds into stars, and
stars burning themselves out on a variety of time scales, depending on their size. In time though, as the mass of the galaxy
collects in successive generations of small stars, it will grow dimmer and dimmer. Some galaxies will see a temporary rebirth,
if their mass gets stirred up and combined with another. That’s what’s destined to happen to our
Milky Way. At just about the time our sun begins to swallow
our planet, any remaining Earthlings 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. If it’s a direct hit, the stars in both
galaxies will gradually join together in a gigantic galactic puffball known as an elliptical
galaxy. All the turbulence of the merger could stimulate
a wave of new stars being born, reinvigorating the new larger galaxy. Dust-ups like this, in which galactic neighbors
merge, will be common as the era of stars moves into 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 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, the
universe has begun to accelerate outward. Gravity is losing its grip to an unseen force
called dark energy. You can see evidence of this now, out in the
huge voids of space between filaments of galaxies. These voids are like ever-expanding bubbles. Where the bubble walls touch you can see filaments
of galaxies. As the bubbles grow, the filaments will stretch
and 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 galaxies huddled together, with
little connection to anything else, and few clues to how they got there. At more distant reaches of time, tens of billions
of years from now, the sky will grow darker and darker as everything recedes away from
everything else. A good place to be, in those long twilight
years of the stellar era, is a place where gravity and energy have forged an extended
truce. Perhaps a place like this: not much larger
than 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 liquid water, and life. Brown dwarfs and red dwarfs form the vast
majority of stars in our galaxy. In fact, combined, their mass exceeds that
of all the large stars. 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 their host galaxies grow dim, another
process will begin to transform these small outposts. Over time, chance encounters between objects
will perturb their orbits, sending some toward the center of the galaxy, and others 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 the
universe is populated by red and white dwarf stars, steadily cooling, and by the charred
remains of supernova explosions: neutron stars. Even though these dead stars have used up
their nuclear fuels, they continue to produce small amounts of energy. They scoop up and annihilate dark matter particles
that manage to stray into their grasp. Here is where cosmic evolution 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 to
the early moments of the universe. Their eventual decay will mark the end of
the degenerate era, around a billion, billion, billion, billion years after the big bang. That’s a one followed by 40 zeros. Our picture of what happens after that depends
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 giant ring 27 kilometers
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, 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 centers of large galaxies today. As those galaxies age, over trillions of years
of time, much of their mass will spiral towards the center and into the jaws of ever more
ravenous black holes. Conceivably, these black holes could end up
weighing as much as a galaxy. But 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 charge 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 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 know
is part of a more complex cosmic reality, beyond the three spatial dimensions - plus
time - that we experience in our everyday lives. We may be like an insect living on the 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 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 the extra dimension. They’ll know a black hole is there when
they see the shower of particles predicted by Hawking’s theory. Its presence 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, a black hole
observed today will take it last gasp when the clock strikes 10 to the hundredth years
from now, a number known as a googol. That’s the end of 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 to
rest by then? Not if modern theories are correct. They hold that our universe is part of a much
larger cosmic cycle of birth and death, with whole new universes coming into being in the
space beyond our own. The time horizons of our universe may well
be a blip in this grander scheme of things. Back to Earth now. We are products of the great era of stars,
and witnesses to its great spectacles of gravity and energy. No doubt there are other beings somewhere
out there who are attempting to comprehend the universe. They too may invent the idea of “time”
and develop their own theories on where it’s all leading. Their discoveries - and ours – will not
survive the entropy at work in the universe, as we all go the way of the stars, and as
they give way to grand new eras in the life of the universe. And as our universe gives way to grand new
eras in the life of the cosmos. 6
The title is an oxymoron. "No one goes to that restaurant anymore. It's always too crowded." - Yogi Berra.