Parallel Worlds (45 page)

On a time scale of tens of thousands of years, there may be
an ice age, similar to the one that buried North America under almost a mile of
ice, making human civilization impossible. Before ten thousand years ago,
humans lived like wolves in packs, foraging for scraps of food in small,
isolated tribes. There was no accumulation of knowledge or science. There was
no written word. Humanity was preoccupied with one goal: survival. Then, for
reasons we still do not understand, the Ice Age ended, and humans began the
rapid rise from the ice to the stars. However, this brief interglacial period
cannot last forever. Perhaps in another ten thousand years, another Ice Age
will blanket most of the world. Geologists believe that the effects of tiny
variations in Earth's spin around its axis eventually build up, allowing the
jet stream from the ice caps to descend to lower latitudes, blanketing Earth
in freezing ice. At that point, we might have to go underground to keep warm.
Earth was once completely covered in ice. This might happen again.

On a time scale of thousands to millions of years, we must
prepare for meteor and comet impacts. Most likely a meteor or comet impact
destroyed the dinosaurs 65 million years ago. Scientists believe that an
extraterrestrial object, perhaps less than i0 miles across, plowed into the
Yucatan Peninsula of Mexico, gouging out a crater i80 miles across and shooting
enough debris into the atmosphere to cut off sunlight and darken Earth,
causing freezing temperatures that killed off vegetation and the dominant life
form on Earth at that time, the dinosaurs. Within less than a year, the dinosaurs
and most of the species on Earth perished.

Judging by the
rate of past impacts, there is a 1 in 100,000 chance over the next fifty years
of an asteroid impact that would cause worldwide damage. The chance of a major
impact over millions of years probably grows to nearly 100 percent.

(In the inner
solar system, where Earth resides, there are perhaps 1,000 to 1,500 asteroids
that are a kilometer across or greater, and a million asteroids 50 meters
across or larger. Asteroid observations pour into the Smithsonian
Astrophysical Observatory in Cambridge at the rate of about fifteen thousand
per day. Fortunately, only forty-two known asteroids have a small but finite
probability of impacting with Earth. In the past, there have been a number of
false alarms concerning these asteroids, the most famous involving the asteroid
1997XF11, which astronomers mistakenly said might hit Earth in thirty years,
generating worldwide headlines. But by carefully examining the orbit of one
asteroid called 1950DA, scientists have calculated that there is only a
tiny—but nonzero—probability that it may hit Earth on March 16, 2880. Computer
simulations done at the University of California at Santa Cruz show that, if
this asteroid hits the oceans, it will create a tidal wave 400 feet tall, which
would swamp most of the coastal areas in devastating floods.)

On a scale of
billions of years, we have to worry about the Sun swallowing up Earth. The Sun
is already 30 percent hotter today than it was in its infancy. Computer studies
have shown that, in 3.5 billion years, the Sun will be 40 percent brighter than
it is today, meaning that Earth will gradually heat up. The Sun will appear
larger and larger in the day sky, until it fills up most of the sky from
horizon to horizon. In the short term, living creatures, desperately trying to escape
the scorching heat of the Sun, may be forced back into the oceans, reversing
the historic march of evolution on this planet. Eventually, the oceans
themselves will boil, making life as we know it impossible. In about 5 billion
years, the Sun's core will exhaust its supply of hydrogen gas and mutate into a
red giant star. Some red giants are so large that they could gobble up Mars if
they were located at the position of our Sun. However, our Sun will probably
expand only to the size of the orbit of Earth, devouring Mercury and Venus and
melting the mountains of Earth. So it is likely our Earth will die in fire,
rather than ice, leaving a burnt-out cinder orbiting the Sun.

Some physicists
have argued that before this occurs, we should be able to use advanced
technology to move Earth to a larger orbit around the Sun, if we haven't
already migrated from Earth to other planets in gigantic space arks. "As
long as people get smarter faster than the Sun gets brighter, the Earth should
thrive," remarks astronomer and writer Ken Croswell.

Scientists have
proposed several ways to move Earth from its current orbit around the Sun. One
simple way would be to carefully divert a series of asteroids from the
asteroid belt so that they whip around Earth. This slingshot effect would give
a boost to Earth's orbit, increasing its distance from the Sun. Each boost
would move Earth only incrementally, but there would be plenty of time to divert
hundreds of asteroids to accomplish this feat. "During the several
billion years before the Sun bloats into a red giant, our descendants could
snare a passing star into an orbit around the Sun, then jettison the Earth from
its solar orbit into an orbit around the new star," adds Croswell.

Our Sun will
suffer a different fate from Earth; it will die in ice, rather than fire.
Eventually, after burning helium for 700 million years as a red giant, the Sun
will exhaust most of its nuclear fuel, and gravity will compress it into a
white dwarf about the size of Earth. Our Sun is too small to undergo the
catastrophe called a supernova and turn into a black hole. After our Sun turns
into a white dwarf star, eventually it will cool down, thereby glowing a faint
red color, then brown, and finally black. It will drift in the cosmic void as a
piece of dead nuclear ash. The future of almost all the atoms we see around us,
including the atoms of our bodies and our loved ones, is to wind up on a
burnt-out cinder orbiting a black dwarf star. Because this dwarf star will
weigh only 0.55 solar masses, what's left of Earth will settle into an orbit
about 70 percent farther out than it is today.

On this scale,
we see that the blossoming of plants and animals on Earth will only last a mere
billion years (and we are halfway through this golden era today). "Mother
Nature wasn't designed to make us happy," says astronomer Donald Brownlee.
Compared to the life span of the entire universe, the flowering of life lasts
only the briefest instant of time.

StagE 3: DEgEnEratE Era

In stage 3
(between 15 and 39), the energy of the stars in the universe will finally be
exhausted. The seemingly eternal process of burning hydrogen and then helium
finally comes to a halt, leaving behind lifeless hunks of dead nuclear matter
in the form of dwarf stars, neutron stars, and black holes. The stars in the
sky cease to shine; the universe is gradually plunged into darkness.

Temperatures
will fall dramatically in stage 3, as stars lose their nuclear engines. Any
planet circling around a dead star will freeze. Assuming that Earth is still
intact, what is left of its surface will become a frozen sheet of ice, forcing
intelligent life forms to seek a new home.

While giant
stars may last for a few million years and hydrogen- burning stars like our Sun
for billions of years, tiny red dwarf stars may actually burn for trillions of
years. This is why attempting to relocate the orbit of Earth around a red
dwarf star in theory makes sense. The closest stellar neighbor to Earth,
Promixa Centauri, is a red dwarf star that is only 4.3 light-years from Earth.
Our closest neighbor weighs only 15 percent of the Sun's mass and is four hundred
times dimmer than the Sun, so any planet orbiting it would have to be extremely
close to benefit from its faint starlight. Earth would have to orbit this star
twenty times closer than it currently is from the Sun to receive the same
amount of sunlight. But once in orbit around a red dwarf star, a planet would
have energy to last for trillions of years.

Eventually, the
only stars that will continue to burn nuclear fuel will be the red dwarfs. In
time, however, even they will turn dark. In a hundred trillion years, the
remaining red dwarfs will finally expire.

StagE 4: Black
HoIe
Era

In stage 4
(between 40 to 100), the only source of energy will be the slow evaporation of
energy from black holes. As shown by Jacob Bekenstein and Stephen Hawking,
black holes are not really black; they actually radiate a faint amount of
energy, called evaporation. (In practice, this black hole evaporation is too
small to be observed experimentally, but on long time scales evaporation
ultimately determines the fate of a black hole.)

Evaporating
black holes can have various lifetimes. A mini-black hole the size of a proton
might radiate i0 billion watts of power for the lifetime of the solar system. A
black hole weighing as much as the Sun will evaporate in 10
⁶⁶
years.
A black hole weighing as much as a galactic cluster will evaporate in 10
¹¹⁷
years. However, as a black hole nears the end of its lifespan, after slowly
oozing out radiation it suddenly explodes. It's possible that intelligent life,
like homeless people huddled next to the dying embers of dim fires, will congregate
around the faint heat emitted from evaporating black holes to extract a bit of
warmth from them, until they evaporate.

StagE 5: Dark Era

In stage 5
(beyond i0i), we enter the dark era of the universe, when all heat sources are
finally exhausted. In this stage, the universe drifts slowly toward the
ultimate heat death, as the temperature approaches absolute zero. At this
point, the atoms themselves almost come to a halt. Perhaps even the protons
themselves will have decayed, leaving a drifting sea of photons and a thin
soup of weakly interacting particles (neutrinos, electrons, and their
antiparticle, the positron). The universe may consist of a new type of
"atom" called positronium, consisting of electrons and positrons that
circulate around each other.

Some physicists
have speculated that these "atoms" of electrons and antielectrons
might be able to form new building blocks for intelligent life in this dark
era. However, the difficulties facing this idea are formidable. An atom of
positronium is comparable in size to an ordinary atom. But an atom of
positronium in the dark era would be about 10
¹²
megaparsecs across,
millions of times larger than the observable universe of today. So in this dark
era, while these "atoms" may form, they would be the size of an
entire universe. Since the universe during the dark era will have expanded to
enormous distances, it would easily be able to accommodate these gigantic
atoms of positronium. But since these positronium atoms are so large, it means
that any "chemistry" involving these "atoms" would be on
colossal time scales totally different from anything we know.

As cosmologist
Tony Rothman writes, "And so, finally, after 10
¹¹⁷
years, the
cosmos will consist of a few electrons and positrons locked in their ponderous
orbits, neutrinos and photons left over from baryon decay, and stray protons
remaining from positronium annihilation and black holes. For this too is
written in the Book of Destiny."

CAN INTELLIGENCE SURVIVE?

Given the
mind-numbing conditions found at the end of the big freeze, scientists have
debated whether any intelligent life form can possibly survive. At first, it
seems pointless to discuss intelligent life surviving in stage 5, when
temperatures plunge to near absolute zero. However, there is actually a
spirited debate among physicists about whether intelligent life can survive.

The debate
centers upon two key questions. The first is: can intelligent beings operate
their machines when temperatures approach absolute zero? By the laws of
thermodynamics, because energy flows from a higher temperature to a lower
temperature, this movement of energy can be used to do usable mechanical work.
For example, mechanical work can be extracted by a heat engine that connects
two regions at different temperatures. The greater the difference in
temperature, the greater the efficiency of the engine. This is the basis of the
machines that powered the Industrial Revolution, such as the steam engine and
the locomotive. At first, it seems impossible to extract any work from a heat
engine in stage 5, since all temperatures will be the same.

The second question
is: can an intelligent life form send and receive information? According to
information theory, the smallest unit that can be sent and received is
proportional to the temperature. As the temperature drops to near absolute
zero, the ability to process information is also severely impaired. Bits of
information that can be transmitted as the universe cools will have to be
smaller and smaller.

Physicist
Freeman Dyson and others have reanalyzed the physics of intelligent life coping
in a dying universe. Can ingenious ways, they ask, be found for intelligent
life to survive even as temperatures drop near absolute zero?

As the
temperature begins to drop throughout the universe, at first creatures may try
to lower their body temperature using genetic engineering. This way, they
could be much more efficient in using the dwindling energy supply. But
eventually, body temperatures will reach the freezing point of water. At this
time, intelligent beings may have to abandon their frail bodies of flesh and
blood and assume robotic bodies. Mechanical bodies can withstand the cold much
better than flesh. But machines also must obey the laws of information theory
and thermodynamics, making life extremely difficult, even for robots.